Felipe Guzman

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• Collaboration, T. L., Collaboration, T. V., Collaboration, T. K., Abac, A. G., Abbott, R., Abouelfettouh, I., Acernese, F., Ackley, K., Adhicary, S., Adhikari, N., Adhikari, R. X., Adkins, V. K., Agarwal, D., Agathos, M., Abchouyeh, A. M., Aguiar, O. D., Aguilar, I., Aiello, L., Ain, A., , Ajith, P., et al. (2025).

  Search for continuous gravitational waves from known pulsars in the first part of the fourth LIGO-Virgo-KAGRA observing run

  . Astrophysical Journal, 983 99. doi:https://doi.org/10.48550/arXiv.2501.01495
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  Continuous gravitational waves (CWs) emission from neutron stars carries information about their internal structure and equation of state, and it can provide tests of General Relativity. We present a search for CWs from a set of 45 known pulsars in the first part of the fourth LIGO--Virgo--KAGRA observing run, known as O4a. We conducted a targeted search for each pulsar using three independent analysis methods considering the single-harmonic and the dual-harmonic emission models. We find no evidence of a CW signal in O4a data for both models and set upper limits on the signal amplitude and on the ellipticity, which quantifies the asymmetry in the neutron star mass distribution. For the single-harmonic emission model, 29 targets have the upper limit on the amplitude below the theoretical spin-down limit. The lowest upper limit on the amplitude is $6.4\!\times\!10^{-27}$ for the young energetic pulsar J0537-6910, while the lowest constraint on the ellipticity is $8.8\!\times\!10^{-9}$ for the bright nearby millisecond pulsar J0437-4715. Additionally, for a subset of 16 targets we performed a narrowband search that is more robust regarding the emission model, with no evidence of a signal. We also found no evidence of non-standard polarizations as predicted by the Brans-Dicke theory. [Journal_ref: Astrophys.J. 983 (2025) 2, 99]
• Collaboration, T. L., Collaboration, T. V., Collaboration, T. K., Abac, A. G., Abbott, R., Abouelfettouh, I., Acernese, F., Ackley, K., Adhicary, S., Adhikari, N., Adhikari, R. X., Adkins, V. K., Agarwal, D., Agathos, M., Abchouyeh, A. M., Aguiar, O. D., Aguilar, I., Aiello, L., Ain, A., , Akutsu, T., et al. (2025).

  Search for gravitational waves emitted from SN 2023ixf

  . Astrophysical Journal, 985 183. doi:https://doi.org/10.48550/arXiv.2410.16565
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  We present the results of a search for gravitational-wave transients associated with core-collapse supernova SN 2023ixf, which was observed in the galaxy Messier 101 via optical emission on 2023 May 19th, during the LIGO-Virgo-KAGRA 15th Engineering Run. We define a five-day on-source window during which an accompanying gravitational-wave signal may have occurred. No gravitational waves have been identified in data when at least two gravitational-wave observatories were operating, which covered $\sim 14\%$ of this five-day window. We report the search detection efficiency for various possible gravitational-wave emission models. Considering the distance to M101 (6.7 Mpc), we derive constraints on the gravitational-wave emission mechanism of core-collapse supernovae across a broad frequency spectrum, ranging from 50 Hz to 2 kHz where we assume the gravitational-wave emission occurred when coincident data are available in the on-source window. Considering an ellipsoid model for a rotating proto-neutron star, our search is sensitive to gravitational-wave energy $1 \times 10^{-4} M_{\odot} c^2$ and luminosity $2.6 \times 10^{-4} M_{\odot} c^2/s$ for a source emitting at 82 Hz. These constraints are around an order of magnitude more stringent than those obtained so far with gravitational-wave data. The constraint on the ellipticity of the proto-neutron star that is formed is as low as 1.08, at frequencies above 1200 Hz, surpassing past results. [Journal_ref: ApJ 985 183 (2025)]
• Guzman, F., Mehmet, M., Sanjuan Munoz, J., & Wang, P. (2025).

  Demonstration of 3.5 × 10⁻¹³ laser frequency stability at 1000 s using an iodine-filled hollow-core fiber photonic microcell

  . Optics Express, 33(Issue 26), 55068-55080. doi:https://doi.org/10.48550/arXiv.2502.00513
• Jersey, K., Hollis, H., Chia, H. Y., Sanjuan, J., Fulda, P., Mueller, G., & Guzman, F. (2025). Picometer sensitive prototype of the optical truss interferometer for LISA. Classical and Quantum Gravity, 42(Issue 3). doi:10.1088/1361-6382/ad9f13
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  The optical truss interferometer (OTI) is a contingent subsystem proposed for the LISA telescopes to aid in the verification of a 1 pm Hz optical path length stability. Each telescope would be equipped with three pairs of compact fiber-coupled units, each forming an optical cavity with a baseline proportional to the telescope length at different points around the aperture. Employing a Pound-Drever-Hall approach to maintain a modulated laser field on resonance with each cavity, the dimensional stability of the telescope can be measured and verified. We have designed and developed prototype OTI units to demonstrate the capability of measuring stable structures, such as the LISA telescope, with a 1 pm Hz sensitivity using a set of freely mountable fiber-injected cavities. Aside from its initial motivation for the telescope, the OTI can also be readily integrated with other systems to aid in ground testing experiments. In this paper, we outline our experimental setup, measurement results, and analyses of the noise limitations.
• Jersey, K., Hollis, H., Chia, H., Sanjuan, J., Fulda, P., Mueller, G., & Guzman, F. (2025).

  Picometer Sensitive Prototype of the Optical Truss Interferometer for LISA

  . Classical and Quantum Gravity, 42(3), 035008 (2025).
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  The optical truss interferometer (OTI) is a contingent subsystem proposed for the LISA telescopes to aid in the verification of a $1 \frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ optical path length stability. Each telescope would be equipped with three pairs of compact fiber-coupled units, each forming an optical cavity with a baseline proportional to the telescope length at different points around the aperture. Employing a Pound-Drever-Hall approach to maintain a modulated laser field on resonance with each cavity, the dimensional stability of the telescope can be measured and verified. We have designed and developed prototype OTI units to demonstrate the capability of measuring stable structures, such as the LISA telescope, with a $1 \frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ sensitivity using a set of freely mountable fiber-injected cavities. Aside from its initial motivation for the telescope, the OTI can also be readily integrated with other systems to aid in ground testing experiments. In this paper, we outline our experimental setup, measurement results, and analyses of the noise limitations. [Journal_ref: Classical and Quantum Gravity, 42(3), 035008 (2025)]
• Raman, G., Ronchini, S., Delaunay, J., Tohuvavohu, A., Kennea, J. A., Parsotan, T., Ambrosi, E., Bernardini, M. G., Campana, S., Cusumano, G., D'Ai, A., D'Avanzo, P., D'Elia, V., de Pasquale, M., Dichiara, S., Evans, P., Hartmann, D., Kuin, P., Melandri, A., , O'Brien, P., et al. (2025).

  Swift-BAT GUANO follow-up of gravitational-wave triggers in the third LIGO-Virgo-KAGRA observing run

  . Astrophysical Journal, 980(2). doi:10.3847/1538-4357/ad9749
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  We present results from a search for X-ray/gamma-ray counterparts of gravitational-wave (GW) candidates from the third observing run (O3) of the LIGO-Virgo-KAGRA (LVK) network using the Swift Burst Alert Telescope (Swift-BAT). The search includes 636 GW candidates received in low latency, 86 of which have been confirmed by the offline analysis and included in the third cumulative Gravitational-Wave Transient Catalogs (GWTC-3). Targeted searches were carried out on the entire GW sample using the maximum--likelihood NITRATES pipeline on the BAT data made available via the GUANO infrastructure. We do not detect any significant electromagnetic emission that is temporally and spatially coincident with any of the GW candidates. We report flux upper limits in the 15-350 keV band as a function of sky position for all the catalog candidates. For GW candidates where the Swift-BAT false alarm rate is less than 10$^{-3}$ Hz, we compute the GW--BAT joint false alarm rate. Finally, the derived Swift-BAT upper limits are used to infer constraints on the putative electromagnetic emission associated with binary black hole mergers. [Journal_ref: ApJ, Volume 980, 2025, 207]
• Yamamoto, K., Kelly, P., Majji, M., & Guzman, F. (2025).

  Covariance Analysis of Attitude and Angular Rate Estimation using Accelerometers

  . arvix:250200513, 10. doi:https://doi.org/10.48550/arXiv.2502.00513
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  In this work a method for using accelerometers for the determination of angular velocity and acceleration is presented. Minimum sensor requirements and insights into how an array of accelerometers can be configured to maximize estimator performance are considered. The framework presented utilizes linear least squares to estimate functions that are quadratic in angular velocity. Simple methods for determining the sign of the spin axis and the linearized covariance approximation are presented and found to perform quite effectively when compared to results obtained by Monte Carlo. [Journal_ref: ]
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2024). GWTC-2.1: Deep extended catalog of compact binary coalescences observed by LIGO and Virgo during the first half of the third observing run. Physical Review D, 109(2). doi:10.1103/PhysRevD.109.022001
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  The second Gravitational-Wave Transient Catalog, GWTC-2, reported on 39 compact binary coalescences observed by the Advanced LIGO and Advanced Virgo detectors between 1 April 2019 15∶00 UTC and 1 October 2019 15∶00 UTC. Here, we present GWTC-2.1, which reports on a deeper list of candidate events observed over the same period. We analyze the final version of the strain data over this period with improved calibration and better subtraction of excess noise, which has been publicly released. We employ three matched-filter search pipelines for candidate identification, and estimate the probability of astrophysical origin for each candidate event. While GWTC-2 used a false alarm rate threshold of 2 per year, we include in GWTC-2.1, 1201 candidates that pass a false alarm rate threshold of 2 per day. We calculate the source properties of a subset of 44 high-significance candidates that have a probability of astrophysical origin greater than 0.5. Of these candidates, 36 have been reported in GWTC-2. We also calculate updated source properties for all binary black hole events previously reported in GWTC-1. If the eight additional high-significance candidates presented here are astrophysical, the mass range of events that are unambiguously identified as binary black holes (both objects ≥3M⊙) is increased compared to GWTC-2, with total masses from ∼14M⊙ for GW190924_021846 to ∼182M⊙ for GW190426_190642. Source properties calculated using our default prior suggest that the primary components of two new candidate events (GW190403_051519 and GW190426_190642) fall in the mass gap predicted by pair-instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (the mass ratio is less than 0.65 and 0.44 at 90% probability for GW190403_051519 and GW190917_114630 respectively), and find that two of the eight new events have effective inspiral spins χeff>0 (at 90% credibility), while no binary is consistent with χeff
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2024). Search for Gravitational-wave Transients Associated with Magnetar Bursts in Advanced LIGO and Advanced Virgo Data from the Third Observing Run. Astrophysical Journal, 966(1). doi:10.3847/1538-4357/ad27d3
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  Gravitational waves are expected to be produced from neutron star oscillations associated with magnetar giant flares and short bursts. We present the results of a search for short-duration (milliseconds to seconds) and long-duration (∼100 s) transient gravitational waves from 13 magnetar short bursts observed during Advanced LIGO, Advanced Virgo, and KAGRA’s third observation run. These 13 bursts come from two magnetars, SGR 1935+2154 and Swift J1818.0−1607. We also include three other electromagnetic burst events detected by Fermi-GBM which were identified as likely coming from one or more magnetars, but they have no association with a known magnetar. No magnetar giant flares were detected during the analysis period. We find no evidence of gravitational waves associated with any of these 16 bursts. We place upper limits on the rms of the integrated incident gravitational-wave strain that reach 3.6 × 10−23 / Hz at 100 Hz for the short-duration search and 1.1 × 10−22 / Hz at 450 Hz for the long-duration search. For a ringdown signal at 1590 Hz targeted by the short-duration search the limit is set to 2.3 × 10−22 / Hz . Using the estimated distance to each magnetar, we derive upper limits on the emitted gravitational-wave energy of 1.5 × 1044 erg (1.0 × 1044 erg) for SGR 1935+2154 and 9.4 × 1043 erg (1.3 × 1044 erg) for Swift J1818.0−1607, for the short-duration (long-duration) search. Assuming isotropic emission of electromagnetic radiation of the burst fluences, we constrain the ratio of gravitational-wave energy to electromagnetic energy for bursts from SGR 1935+2154 with the available fluence information. The lowest of these ratios is 4.5 × 103.
• Armano, M., Audley, H., Baird, J., Binetruy, P., Born, M., Bortoluzzi, D., Brandt, N., Castelli, E., Cavalleri, A., Cesarini, A., Cruise, A., Danzmann, K., De Deus Silva, M., Diepholz, I., Dixon, G., Dolesi, R., Ferraioli, L., Ferroni, V., Fitzsimons, E., , Flatscher, R., et al. (2024). Sensor noise in LISA Pathfinder: Laser frequency noise and its coupling to the optical test mass readout. Physical Review D, 109(4). doi:10.1103/physrevd.109.042003
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  The LISA Pathfinder (LPF) mission successfully demonstrated the feasibility of the technology needed for the future space borne gravitational wave observatory LISA. A key subsystem under study was the laser interferometer, which measured the changes in relative distance in between two test masses (TMs). It achieved a sensitivity of 32.0-1.7+2.4 fm/Hz, which was significantly better than the prelaunch tests. This improved performance allowed direct observation of the influence of laser frequency noise in the readout. The differences in optical path lengths between the measurement and reference beams in the individual interferometers of our setup determined the level of this undesired readout noise. Here, we discuss the dedicated experiments performed on LPF to measure these differences with high precision. We reached differences in path length difference between (368±5) μm and (329.6±0.9) μm which are significantly below the required level of 1 mm or 1000 μm. These results are an important contribution to our understanding of the overall sensor performance. Moreover, we observed varying levels of laser frequency noise over the course of the mission. We provide evidence that these do not originate from the laser frequency stabilization scheme which worked as expected. Therefore, this frequency stabilization would be applicable to other missions with similar laser frequency stability requirements.
• Bhaskara, R. R., Majji, M., & Guzmán, F. (2024). Quantized State Estimation for Linear Dynamical Systems. Sensors, 24(19).
• Collaboration, T. L., Collaboration, T. V., Collaboration, T. K., Abac, A. G., Abbott, R., Abouelfettouh, I., Acernese, F., Ackley, K., Adhicary, S., Adhikari, N., Adhikari, R. X., Adkins, V. K., Agarwal, D., Agathos, M., Abchouyeh, A. M., Aguiar, O. D., Aguilar, I., Aiello, L., Ain, A., , Ajith, P., et al. (2024).

  A search using GEO600 for gravitational waves coincident with fast radio bursts from SGR 1935+2154

  . Astrophysical Journal, 977(2), 27. doi:10.3847/1538-4357/ad8de0
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  The magnetar SGR 1935+2154 is the only known Galactic source of fast radio bursts (FRBs). FRBs from SGR 1935+2154 were first detected by CHIME/FRB and STARE2 in 2020 April, after the conclusion of the LIGO, Virgo, and KAGRA Collaborations' O3 observing run. Here we analyze four periods of gravitational wave (GW) data from the GEO600 detector coincident with four periods of FRB activity detected by CHIME/FRB, as well as X-ray glitches and X-ray bursts detected by NICER and NuSTAR close to the time of one of the FRBs. We do not detect any significant GW emission from any of the events. Instead, using a short-duration GW search (for bursts $\leq$ 1 s) we derive 50\% (90\%) upper limits of $10^{48}$ ($10^{49}$) erg for GWs at 300 Hz and $10^{49}$ ($10^{50}$) erg at 2 kHz, and constrain the GW-to-radio energy ratio to $\leq 10^{14} - 10^{16}$. We also derive upper limits from a long-duration search for bursts with durations between 1 and 10 s. These represent the strictest upper limits on concurrent GW emission from FRBs. [Journal_ref: ApJ 977 255 (2024)]
• Nelson, A., Sanjuan, J., & Guzmán, F. (2024). 1/f Noise Mitigation in an Opto-Mechanical Sensor with a Fabry–Pérot Interferometer. Sensors, 24(6), 1696. doi:10.3390/s24061969
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  Low-frequency and 1/f noise are common measurement limitations that arise in a variety of physical processes. Mitigation methods for these noises are dependent on their source. Here, we present a method for removing (Formula presented.) noise of optical origin using a micro-cavity Fabry–Pérot (FP) interferometer. A mechanical modulation of the FP cavity length was applied to a previously studied opto-mechanical sensor. It effectively mimics an up-conversion of the laser frequency, shifting signals to a region where lower white-noise sources dominate and (Formula presented.) noise is not present. Demodulation of this signal shifts the results back to the desired frequency range of observation with the reduced noise floor of the higher frequencies. This method was found to improve sensitivities by nearly two orders of magnitude at 1 Hz and eliminated 1/f noise in the range from 1 Hz to 4 kHz. A mathematical model for low-finesse FP cavities is presented to support these results. This study suggests a relatively simple and efficient method for (Formula presented.) noise suppression and improving the device sensitivity of systems with an FP interferometer readout.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2023). Constraints on the Cosmic Expansion History from GWTC-3. Astrophysical Journal, 949(2). doi:10.3847/1538-4357/ac74bb
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  We use 47 gravitational wave sources from the Third LIGO-Virgo-Kamioka Gravitational Wave Detector Gravitational Wave Transient Catalog (GWTC-3) to estimate the Hubble parameter H(z), including its current value, the Hubble constant H0. Each gravitational wave (GW) signal provides the luminosity distance to the source, and we estimate the corresponding redshift using two methods: the redshifted masses and a galaxy catalog. Using the binary black hole (BBH) redshifted masses, we simultaneously infer the source mass distribution and H(z). The source mass distribution displays a peak around 34Me, followed by a drop-off. Assuming this mass scale does not evolve with the redshift results in a H(z) measurement, yielding H0 = 68+12-8 km s-1 Mpc-1 (68% credible interval) when combined with the H0 measurement from GW170817 and its electromagnetic counterpart. This represents an improvement of 17% with respect to the H0 estimate from GWTC-1. The second method associates each GW event with its probable host galaxy in the catalog GLADE+, statistically marginalizing over the redshifts of each event's potential hosts. Assuming a fixed BBH population, we estimate a value of H0 = 68+8-6 km s-1 Mpc-1 with the galaxy catalog method, an improvement of 42% with respect to our GWTC-1 result and 20% with respect to recent H0 studies using GWTC-2 events. However, we show that this result is strongly impacted by assumptions about the BBH source mass distribution; the only event which is not strongly impacted by such assumptions (and is thus informative about H0) is the well-localized event GW190814.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2023). GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo during the Second Part of the Third Observing Run. Physical Review X, 13(4). doi:10.1103/physrevx.13.041039
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  The third Gravitational-Wave Transient Catalog (GWTC-3) describes signals detected with Advanced LIGO and Advanced Virgo up to the end of their third observing run. Updating the previous GWTC-2.1, we present candidate gravitational waves from compact binary coalescences during the second half of the third observing run (O3b) between 1 November 2019, 15∶00 Coordinated Universal Time (UTC) and 27 March 2020, 17∶00 UTC. There are 35 compact binary coalescence candidates identified by at least one of our search algorithms with a probability of astrophysical origin pastro>0.5. Of these, 18 were previously reported as low-latency public alerts, and 17 are reported here for the first time. Based upon estimates for the component masses, our O3b candidates with pastro>0.5 are consistent with gravitational-wave signals from binary black holes or neutron-star-black-hole binaries, and we identify none from binary neutron stars. However, from the gravitational-wave data alone, we are not able to measure matter effects that distinguish whether the binary components are neutron stars or black holes. The range of inferred component masses is similar to that found with previous catalogs, but the O3b candidates include the first confident observations of neutron-star-black-hole binaries. Including the 35 candidates from O3b in addition to those from GWTC-2.1, GWTC-3 contains 90 candidates found by our analysis with pastro>0.5 across the first three observing runs. These observations of compact binary coalescences present an unprecedented view of the properties of black holes and neutron stars.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2023). Population of Merging Compact Binaries Inferred Using Gravitational Waves through GWTC-3. Physical Review X, 13(1). doi:10.1103/PhysRevX.13.011048
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  We report on the population properties of compact binary mergers inferred from gravitational-wave observations of these systems during the first three LIGO-Virgo observing runs. The Gravitational-Wave Transient Catalog 3 (GWTC-3) contains signals consistent with three classes of binary mergers: binary black hole, binary neutron star, and neutron star-black hole mergers. We infer the binary neutron star merger rate to be between 10 and 1700 Gpc-3 yr-1 and the neutron star-black hole merger rate to be between 7.8 and 140 Gpc-3 yr-1, assuming a constant rate density in the comoving frame and taking the union of 90% credible intervals for methods used in this work. We infer the binary black hole merger rate, allowing for evolution with redshift, to be between 17.9 and 44 Gpc-3 yr-1 at a fiducial redshift (z=0.2). The rate of binary black hole mergers is observed to increase with redshift at a rate proportional to (1+z)κ with κ=2.9-1.8+1.7 for z≲1. Using both binary neutron star and neutron star-black hole binaries, we obtain a broad, relatively flat neutron star mass distribution extending from 1.2-0.2+0.1 to 2.0-0.3+0.3M⊙. We confidently determine that the merger rate as a function of mass sharply declines after the expected maximum neutron star mass, but cannot yet confirm or rule out the existence of a lower mass gap between neutron stars and black holes. We also find the binary black hole mass distribution has localized over- and underdensities relative to a power-law distribution, with peaks emerging at chirp masses of 8.3-0.5+0.3 and 27.9-1.8+1.9M⊙. While we continue to find that the mass distribution of a binary's more massive component strongly decreases as a function of primary mass, we observe no evidence of a strongly suppressed merger rate above approximately 60M⊙, which would indicate the presence of a upper mass gap. Observed black hole spins are small, with half of spin magnitudes below χi≈0.25. While the majority of spins are preferentially aligned with the orbital angular momentum, we infer evidence of antialigned spins among the binary population. We observe an increase in spin magnitude for systems with more unequal-mass ratio. We also observe evidence of misalignment of spins relative to the orbital angular momentum.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2023). Search for Gravitational Waves Associated with Fast Radio Bursts Detected by CHIME/FRB during the LIGO-Virgo Observing Run O3a. Astrophysical Journal, 955(2). doi:10.3847/1538-4357/acd770
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  We search for gravitational-wave (GW) transients associated with fast radio bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project, during the first part of the third observing run of Advanced LIGO and Advanced Virgo (2019 April 1 15:00 UTC-2019 October 1 15:00 UTC). Triggers from 22 FRBs were analyzed with a search that targets both binary neutron star (BNS) and neutron star-black hole (NSBH) mergers. A targeted search for generic GW transients was conducted on 40 FRBs. We find no significant evidence for a GW association in either search. Given the large uncertainties in the distances of our FRB sample, we are unable to exclude the possibility of a GW association. Assessing the volumetric event rates of both FRB and binary mergers, an association is limited to 15% of the FRB population for BNS mergers or 1% for NSBH mergers. We report 90% confidence lower bounds on the distance to each FRB for a range of GW progenitor models and set upper limits on the energy emitted through GWs for a range of emission scenarios. We find values of order 1051-1057 erg for models with central GW frequencies in the range 70-3560 Hz. At the sensitivity of this search, we find these limits to be above the predicted GW emissions for the models considered. We also find no significant coincident detection of GWs with the repeater, FRB 20200120E, which is the closest known extragalactic FRB.
• Hines, A., Nelson, A., Zhang, Y., Valdes, G., Sanjuan, J., & Guzman, F. (2023). Compact optomechanical accelerometers for use in gravitational wave detectors. Appl. Phys. Lett, 122(9). doi:10.1063/5.0142108
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  We present measurements of an optomechanical accelerometer for monitoring low-frequency noise in gravitational wave detectors, such as ground motion. Our device measures accelerations by tracking the test-mass motion of a 4.7 Hz mechanical resonator using a heterodyne interferometer. This resonator is etched from monolithic fused silica, an under-explored design in low-frequency sensors, allowing a device with a noise floor competitive with existing technologies but with a lighter and more compact form. In addition, our heterodyne interferometer is a compact optical assembly that can be integrated directly into the mechanical resonator wafer to further reduce the overall size of our accelerometer. We anticipate this accelerometer to perform competitively with commercial seismometers, and benchtop measurements show a noise floor reaching 82 pico-g Hz-1/2 sensitivities at 0.4 Hz. Furthermore, we present the effects of air pressure, laser fluctuations, and temperature to determine the stability requirements needed to achieve thermally limited measurements.
• Jersey, K., Harley-Trochimczyk, I., Zhang, Y., & Guzman, F. (2023). Optical truss interferometer for the LISA telescope. Applied Optics, 62(20). doi:10.1364/ao.493108
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  The Laser Interferometer Space Antenna telescopes must exhibit an optical path length stability of pm/√Hz in the mHz observation band to meet mission requirements. The optical truss interferometer is a proposed method to aid in the ground testing of the telescopes, as well as a risk-mitigation plan for flight units. This consists of three Fabry-Perot cavities mounted to the telescope, which are used to monitor structural displacements. We have designed and developed a fiber-based cavity injection system that integrates fiber components, mode matching optics, and a cavity input mirror into a compact input stage. The input stages, paired with return mirror stages, can be mounted to the telescope to form optical truss cavities. We performed a thorough sensitivity analysis using various simulation methods to support the fabrication and assembly of three first-generation prototype cavities, each of which exhibited satisfactory performance based on our models.
• Jersey, K., Harley-Trochimczyk, I., Zhang, Y., & Guzman, F. (2023). Optical truss interferometer for the LISA telescope. Applied Optics, 62(21), 5675. doi:10.1364/ao.493108
• Sanjuan, J., Sanjuan, J., Sinyukov, A., Sinyukov, A., Warrayat, M., Warrayat, M. F., Guzman, F., & Guzman, F. (2023). Gyro-Free Inertial Navigation Systems Based on Linear Opto-Mechanical Accelerometers. Sensors, 23(8).
• Zhang, Y., Hines, A., Wilson, D., & Guzman, F. (2023). Optomechanical Cooling and Inertial Sensing at Low Frequencies. Physical Review Applied, 19(5). doi:10.1103/PhysRevApplied.19.054004
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  An inertial sensor design is proposed in this paper to achieve high sensitivity and large dynamic range in the subhertz-frequency regime. High acceleration sensitivity is obtained by combining optical cavity readout systems with monolithically fabricated mechanical resonators. A high-sensitivity heterodyne interferometer simultaneously monitors the test mass with an extensive dynamic range for low-stiffness resonators. The bandwidth is tuned by optical feedback cooling to the test mass via radiation pressure interaction using an intensity-modulated laser. The transfer gain of the feedback system is analyzed to optimize system parameters towards the minimum cooling temperature that can be achieved. To practically implement the inertial sensor, we propose a dynamic cooling mechanism to improve cooling efficiency while operating at low optical power levels. The overall system layout presents an integrated design that is compact and lightweight.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). All-sky search for continuous gravitational waves from isolated neutron stars using Advanced LIGO and Advanced Virgo O3 data. Physical Review D, 106(10). doi:10.1103/PhysRevD.106.102008
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  We present results of an all-sky search for continuous gravitational waves which can be produced by spinning neutron stars with an asymmetry around their rotation axis, using data from the third observing run of the Advanced LIGO and Advanced Virgo detectors. Four different analysis methods are used to search in a gravitational-wave frequency band from 10 to 2048 Hz and a first frequency derivative from -10-8 to 10-9 Hz/s. No statistically significant periodic gravitational-wave signal is observed by any of the four searches. As a result, upper limits on the gravitational-wave strain amplitude h0 are calculated. The best upper limits are obtained in the frequency range of 100 to 200 Hz and they are ∼1.1×10-25 at 95% confidence level. The minimum upper limit of 1.10×10-25 is achieved at a frequency 111.5 Hz. We also place constraints on the rates and abundances of nearby planetary- and asteroid-mass primordial black holes that could give rise to continuous gravitational-wave signals.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). All-sky search for gravitational wave emission from scalar boson clouds around spinning black holes in LIGO O3 data. Physical Review Applied, 105(10). doi:10.1103/PhysRevD.105.102001
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  This paper describes the first all-sky search for long-duration, quasimonochromatic gravitational-wave signals emitted by ultralight scalar boson clouds around spinning black holes using data from the third observing run of Advanced LIGO. We analyze the frequency range from 20 to 610 Hz, over a small frequency derivative range around zero, and use multiple frequency resolutions to be robust towards possible signal frequency wanderings. Outliers from this search are followed up using two different methods, one more suitable for nearly monochromatic signals, and the other more robust towards frequency fluctuations. We do not find any evidence for such signals and set upper limits on the signal strain amplitude, the most stringent being ≈10-25 at around 130 Hz. We interpret these upper limits as both an "exclusion region"in the boson mass/black hole mass plane and the maximum detectable distance for a given boson mass, based on an assumption of the age of the black hole/boson cloud system.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). All-sky, all-frequency directional search for persistent gravitational waves from Advanced LIGO's and Advanced Virgo's first three observing runs. Physical Review D, 105(12). doi:10.1103/PhysRevD.105.122001
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  We present the first results from an all-sky all-frequency (ASAF) search for an anisotropic stochastic gravitational-wave background using the data from the first three observing runs of the Advanced LIGO and Advanced Virgo detectors. Upper limit maps on broadband anisotropies of a persistent stochastic background were published for all observing runs of the LIGO-Virgo detectors. However, a broadband analysis is likely to miss narrowband signals as the signal-to-noise ratio of a narrowband signal can be significantly reduced when combined with detector output from other frequencies. Data folding and the computationally efficient analysis pipeline, PyStoch, enable us to perform the radiometer map-making at every frequency bin. We perform the search at 3072 HEALPix equal area pixels uniformly tiling the sky and in every frequency bin of width 1/32 Hz in the range 20-1726 Hz, except for bins that are likely to contain instrumental artefacts and hence are notched. We do not find any statistically significant evidence for the existence of narrowband gravitational-wave signals in the analyzed frequency bins. Therefore, we place 95% confidence upper limits on the gravitational-wave strain for each pixel-frequency pair, the limits are in the range (0.030-9.6)×10-24. In addition, we outline a method to identify candidate pixel-frequency pairs that could be followed up by a more sensitive (and potentially computationally expensive) search, e.g., a matched-filtering-based analysis, to look for fainter nearly monochromatic coherent signals. The ASAF analysis is inherently independent of models describing any spectral or spatial distribution of power. We demonstrate that the ASAF results can be appropriately combined over frequencies and sky directions to successfully recover the broadband directional and isotropic results.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Constraints on dark photon dark matter using data from LIGO's and Virgo's third observing run. Physical Review Applied, 105(6). doi:10.1103/PhysRevD.105.063030
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  We present a search for dark photon dark matter that could couple to gravitational-wave interferometers using data from Advanced LIGO and Virgo's third observing run. To perform this analysis, we use two methods, one based on cross-correlation of the strain channels in the two nearly aligned LIGO detectors, and one that looks for excess power in the strain channels of the LIGO and Virgo detectors. The excess power method optimizes the Fourier transform coherence time as a function of frequency, to account for the expected signal width due to Doppler modulations. We do not find any evidence of dark photon dark matter with a mass between mA∼10-14-10-11 eV/c2, which corresponds to frequencies between 10-2000 Hz, and therefore provide upper limits on the square of the minimum coupling of dark photons to baryons, i.e., U(1)B dark matter. For the cross-correlation method, the best median constraint on the squared coupling is ∼1.31×10-47 at mA∼4.2×10-13 eV/c2; for the other analysis, the best constraint is ∼2.4×10-47 at mA∼5.7×10-13 eV/c2. These limits improve upon those obtained in direct dark matter detection experiments by a factor of ∼100 for mA∼[2-4]×10-13 eV/c2, and are, in absolute terms, the most stringent constraint so far in a large mass range mA∼2×10-13-8×10-12 eV/c2.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). First joint observation by the underground gravitational-wave detector KAGRA with GEO 600. Progress of Theor. and Exp. Phys., 2022(6). doi:10.1093/ptep/ptac073
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  We report the results of the first joint observation of the KAGRA detector with GEO 600. KAGRA is a cryogenic and underground gravitational-wave detector consisting of a laser interferometer with 3 km arms, located in Kamioka, Gifu, Japan. GEO 600 is a British-German laser interferometer with 600 m arms, located near Hannover, Germany. GEO 600 and KAGRA performed a joint observing run from April 7 to 20, 2020. We present the results of the joint analysis of the GEO-KAGRA data for transient gravitational-wave signals, including the coalescence of neutron-star binaries and generic unmodeled transients. We also perform dedicated searches for binary coalescence signals and generic transients associated with gamma-ray burst events observed during the joint run. No gravitational-wave events were identified. We evaluate the minimum detectable amplitude for various types of transient signals and the spacetime volume for which the network is sensitive to binary neutron-star coalescences. We also place lower limits on the distances to the gamma-ray bursts analyzed based on the non-detection of an associated gravitational-wave signal for several signal models, including binary coalescences. These analyses demonstrate the feasibility and utility of KAGRA as a member of the global gravitational-wave detector network.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). First joint observation by the underground gravitational-wave detector KAGRA with GEO 600. Progress of Theor. and Exp. Phys, 2022(6). doi:10.1093/ptep/ptac073
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  We report the results of the first joint observation of the KAGRA detector with GEO 600. KAGRA is a cryogenic and underground gravitational-wave detector consisting of a laser interferometer with 3 km arms, located in Kamioka, Gifu, Japan. GEO 600 is a British-German laser interferometer with 600 m arms, located near Hannover, Germany. GEO 600 and KAGRA performed a joint observing run from April 7 to 20, 2020. We present the results of the joint analysis of the GEO-KAGRA data for transient gravitational-wave signals, including the coalescence of neutron-star binaries and generic unmodeled transients. We also perform dedicated searches for binary coalescence signals and generic transients associated with gamma-ray burst events observed during the joint run. No gravitational-wave events were identified. We evaluate the minimum detectable amplitude for various types of transient signals and the spacetime volume for which the network is sensitive to binary neutron-star coalescences. We also place lower limits on the distances to the gamma-ray bursts analyzed based on the non-detection of an associated gravitational-wave signal for several signal models, including binary coalescences. These analyses demonstrate the feasibility and utility of KAGRA as a member of the global gravitational-wave detector network.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Narrowband Searches for Continuous and Long-duration Transient Gravitational Waves from Known Pulsars in the LIGO-Virgo Third Observing Run. Astrophysical Journal, 932(2). doi:10.3847/1538-4357/ac6ad0
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  Isolated neutron stars that are asymmetric with respect to their spin axis are possible sources of detectable continuous gravitational waves. This paper presents a fully coherent search for such signals from eighteen pulsars in data from LIGO and Virgo's third observing run (O3). For known pulsars, efficient and sensitive matched-filter searches can be carried out if one assumes the gravitational radiation is phase-locked to the electromagnetic emission. In the search presented here, we relax this assumption and allow both the frequency and the time derivative of the frequency of the gravitational waves to vary in a small range around those inferred from electromagnetic observations. We find no evidence for continuous gravitational waves, and set upper limits on the strain amplitude for each target. These limits are more constraining for seven of the targets than the spin-down limit defined by ascribing all rotational energy loss to gravitational radiation. In an additional search, we look in O3 data for long-duration (hours-months) transient gravitational waves in the aftermath of pulsar glitches for six targets with a total of nine glitches. We report two marginal outliers from this search, but find no clear evidence for such emission either. The resulting duration-dependent strain upper limits do not surpass indirect energy constraints for any of these targets.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Search for Subsolar-Mass Binaries in the First Half of Advanced LIGO's and Advanced Virgo's Third Observing Run. Physical Review Letters, 129(6). doi:10.1103/physrevlett.129.061104
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  We report on a search for compact binary coalescences where at least one binary component has a mass between 0.2 M and 1.0 M in Advanced LIGO and Advanced Virgo data collected between 1 April 2019 1500 UTC and 1 October 2019 1500 UTC. We extend our previous analyses in two main ways: we include data from the Virgo detector and we allow for more unequal mass systems, with mass ratio q≥0.1. We do not report any gravitational-wave candidates. The most significant trigger has a false alarm rate of 0.14 yr-1. This implies an upper limit on the merger rate of subsolar binaries in the range [220-24200] Gpc-3 yr-1, depending on the chirp mass of the binary. We use this upper limit to derive astrophysical constraints on two phenomenological models that could produce subsolar-mass compact objects. One is an isotropic distribution of equal-mass primordial black holes. Using this model, we find that the fraction of dark matter in primordial black holes in the mass range 0.2 M
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Search for continuous gravitational wave emission from the Milky Way center in O3 LIGO-Virgo data. Physical Review D, 106(4). doi:10.1103/physrevd.106.042003
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  We present a directed search for continuous gravitational wave (CW) signals emitted by spinning neutron stars located in the inner parsecs of the Galactic Center (GC). Compelling evidence for the presence of a numerous population of neutron stars has been reported in the literature, turning this region into a very interesting place to look for CWs. In this search, data from the full O3 LIGO-Virgo run in the detector frequency band [10,2000] Hz have been used. No significant detection was found and 95% confidence level upper limits on the signal strain amplitude were computed, over the full search band, with the deepest limit of about 7.6×10-26 at ≃142 Hz. These results are significantly more constraining than those reported in previous searches. We use these limits to put constraints on the fiducial neutron star ellipticity and r-mode amplitude. These limits can be also translated into constraints in the black hole mass-boson mass plane for a hypothetical population of boson clouds around spinning black holes located in the GC.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Search for gravitational waves from Scorpius X-1 with a hidden Markov model in O3 LIGO data. Physical Review D, 106(6). doi:10.1103/physrevd.106.062002
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  Results are presented for a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to allow for spin wandering. This search improves on previous HMM-based searches of Laser Interferometer Gravitational-Wave Observatory data by including the orbital period in the search template grid, and by analyzing data from the latest (third) observing run. In the frequency range searched, from 60 to 500 Hz, we find no evidence of gravitational radiation. This is the most sensitive search for Scorpius X-1 using a HMM to date. For the most sensitive subband, starting at 256.06 Hz, we report an upper limit on gravitational wave strain (at 95% confidence) of h095%=6.16×10-26, assuming the orbital inclination angle takes its electromagnetically restricted value ι=44°. The upper limits on gravitational wave strain reported here are on average a factor of ∼3 lower than in the second observing run HMM search. This is the first Scorpius X-1 HMM search with upper limits that reach below the indirect torque-balance limit for certain subbands, assuming ι=44°.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Search of the early O3 LIGO data for continuous gravitational waves from the Cassiopeia A and Vela Jr. supernova remnants. Physical Review D, 105(8). doi:10.1103/physrevd.105.082005
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  We present directed searches for continuous gravitational waves from the neutron stars in the Cassiopeia A (Cas A) and Vela Jr. supernova remnants. We carry out the searches in the LIGO detector data from the first six months of the third Advanced LIGO and Virgo observing run using the weave semicoherent method, which sums matched-filter detection-statistic values over many time segments spanning the observation period. No gravitational wave signal is detected in the search band of 20-976 Hz for assumed source ages greater than 300 years for Cas A and greater than 700 years for Vela Jr. Estimates from simulated continuous wave signals indicate we achieve the most sensitive results to date across the explored parameter space volume, probing to strain magnitudes as low as ∼6.3×10-26 for Cas A and ∼5.6×10-26 for Vela Jr. at frequencies near 166 Hz at 95% efficiency.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2022). Searches for Gravitational Waves from Known Pulsars at Two Harmonics in the Second and Third LIGO-Virgo Observing Runs. Astrophysical Journal, 935(1). doi:10.3847/1538-4357/ac6acf
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  We present a targeted search for continuous gravitational waves (GWs) from 236 pulsars using data from the third observing run of LIGO and Virgo (O3) combined with data from the second observing run (O2). Searches were for emission from the l = m = 2 mass quadrupole mode with a frequency at only twice the pulsar rotation frequency (single harmonic) and the l = 2, m = 1, 2 modes with a frequency of both once and twice the rotation frequency (dual harmonic). No evidence of GWs was found, so we present 95% credible upper limits on the strain amplitudes h0 for the single-harmonic search along with limits on the pulsars' mass quadrupole moments Q22 and ellipticities ε. Of the pulsars studied, 23 have strain amplitudes that are lower than the limits calculated from their electromagnetically measured spin-down rates. These pulsars include the millisecond pulsars J0437-4715 and J0711-6830, which have spin-down ratios of 0.87 and 0.57, respectively. For nine pulsars, their spin-down limits have been surpassed for the first time. For the Crab and Vela pulsars, our limits are factors of ∼100 and ∼20 more constraining than their spin-down limits, respectively. For the dual-harmonic searches, new limits are placed on the strain amplitudes C21 and C22. For 23 pulsars, we also present limits on the emission amplitude assuming dipole radiation as predicted by Brans-Dicke theory.
• Armano, M., Audley, H., Baird, J., Binetruy, P., Born, M., Bortoluzzi, D., Brandt, N., Castelli, E., Cavalleri, A., Cesarini, A., Cruise, A., Danzmann, K., De Deus Silva, M., Diepholz, I., Dixon, G., Dolesi, R., Ferraioli, L., Ferroni, V., Fitzsimons, E., , Flatscher, R., et al. (2022). Sensor noise in LISA Pathfinder: An extensive in-flight review of the angular and longitudinal interferometric measurement system. Physical Review Applied, 106(8). doi:10.1103/PhysRevD.106.082001
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  In a previous article [1], we have reported on the first subpicometer interferometer flown in space as part of ESA's LISA Pathfinder mission, and have shown the residual sensor noise to be on the level of 32.0-1.7+2.4 fm/Hz. This review provides a deeper and more complete overview of the full system and its interferometric mission performance under varying operational conditions, allowing a much more detailed view on the noise model. We also include the optical measurements of rotations through differential wave front sensing (DWS), which reached a sensitivity of as good as 100 prad/Hz. We present more evidence for the long-term stability of the interferometric performance and components. This proves a solid foundation for future interferometry in space such as the LISA mission.
• Hines, A., Nelson, A., Zhang, Y., Valdes, G., Sanjuan, J., Stoddart, J., & Guzmán, F. (2022). Optomechanical Accelerometers for Geodesy. Remote Sensing, 14(17). doi:10.3390/rs14174389
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  We present a novel optomechanical inertial sensor for low-frequency applications and corresponding acceleration measurements. This sensor has a resonant frequency of 4.715 (1) Hz, a mechanical quality factor of 4.76(3) × 105, a test mass of 2.6 g, and a projected noise floor of approximately 5 × 10−11 ms−2/ (Formula presented.) at 1 Hz. Such performance, together with its small size, low weight, reduced power consumption, and low susceptibility to environmental variables such as magnetic field or drag conditions makes it an attractive technology for future space geodesy missions. In this paper, we present an experimental demonstration of low-frequency ground seismic noise detection by direct comparison with a commercial seismometer, and data analysis algorithms for the identification, characterization, and correction of several noise sources.
• Valdes, G., Hines, A., Nelson, A., Zhang, Y., & Guzman, F. (2022). A characterization method for low-frequency seismic noise in LIGO. Appl. Phys. Lett, 121(23). doi:10.1063/5.0122495
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  We present a method to characterize the noise in ground-based gravitational-wave observatories such as the Laser Gravitational-Wave Observatory (LIGO). This method uses linear regression algorithms such as the least absolute shrinkage and selection operator to identify noise sources and analyzes the detector output vs noise witness sensors to quantify the coupling of such noise. Our method can be implemented with currently available resources at LIGO, which avoids extra coding or direct experimentation at the LIGO sites. We present two examples to validate and estimate the coupling of elevated ground motion at frequencies below 10 Hz with noise in the detector output.
• Zhang, Y., & Guzman, F. (2022). Fiber-based two-wavelength heterodyne laser interferometer. Optics Express, 30(21). doi:10.1364/OE.466332
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  Displacement measuring interferometry is a crucial component in metrology applications. In this paper, we propose a fiber-based two-wavelength heterodyne interferometer as a compact and highly sensitive displacement sensor that can be used in inertial sensing applications. In the proposed design, two individual heterodyne interferometers are constructed using two different wavelengths, 1064 nm and 1055 nm; one of which measures the target displacement and the other monitors the common-mode noise in the fiber system. A narrow-bandwidth spectral filter separates the beam paths of the two interferometers, which are highly common and provide a high rejection ratio to the environmental noise. The preliminary test shows a sensitivity floor of 7.5pm/√Hz at 1 Hz when tested in an enclosed chamber. We also investigated the effects of periodic errors due to imperfect spectral separation on the displacement measurement and propose algorithms to mitigate these effects.
• Zhang, Y., & Guzman, F. (2022). Quasi-monolithic heterodyne laser interferometer for inertial sensing. Optics Letters, 47(19), 5120-5123. doi:10.1364/OL.473476
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  We present a compact heterodyne laser interferometer developed for high-sensitivity displacement sensing applications. This interferometer consists of customized prisms and wave plates assembled as a quasi-monolithic unit to realize a miniaturized system. The interferometer design adopts a common-mode rejection scheme to provide a high rejection ratio to common environmental noise. Experimental tests in vacuum show a displacement sensitivity level of 11 pm/√Hz at 100 mHz and as low as 0.6 pm/√Hz above 1 pm. The prototype unit is 20 mm × 20 mm × 10 mm in size and weighs 4.5 g, allowing subsequent integration in compact systems.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). All-sky search for continuous gravitational waves from isolated neutron stars in the early O3 LIGO data. Physical Review D, 104(8). doi:10.1103/physrevd.104.082004
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  We report on an all-sky search for continuous gravitational waves in the frequency band 20-2000 Hz and with a frequency time derivative in the range of [-1.0,+0.1]×10-8 Hz/s. Such a signal could be produced by a nearby, spinning and slightly nonaxisymmetric isolated neutron star in our Galaxy. This search uses the LIGO data from the first six months of Advanced LIGO's and Advanced Virgo's third observational run, O3. No periodic gravitational wave signals are observed, and 95% confidence-level (C.L.) frequentist upper limits are placed on their strengths. The lowest upper limits on worst-case (linearly polarized) strain amplitude h0 are ∼1.7×10-25 near 200 Hz. For a circularly polarized source (most favorable orientation), the lowest upper limits are ∼6.3×10-26. These strict frequentist upper limits refer to all sky locations and the entire range of frequency derivative values. For a population-averaged ensemble of sky locations and stellar orientations, the lowest 95% C.L. upper limits on the strain amplitude are ∼1.4×10-25. These upper limits improve upon our previously published all-sky results, with the greatest improvement (factor of ∼2) seen at higher frequencies, in part because quantum squeezing has dramatically improved the detector noise level relative to the second observational run, O2. These limits are the most constraining to date over most of the parameter space searched.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). All-sky search for long-duration gravitational-wave bursts in the third Advanced LIGO and Advanced Virgo run. Phys Rev D, 104(10). doi:10.1103/PhysRevD.104.102001
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  After the detection of gravitational waves from compact binary coalescences, the search for transient gravitational-wave signals with less well-defined waveforms for which matched filtering is not well suited is one of the frontiers for gravitational-wave astronomy. Broadly classified into "short"1 s and "long"1 s duration signals, these signals are expected from a variety of astrophysical processes, including non-axisymmetric deformations in magnetars or eccentric binary black hole coalescences. In this work, we present a search for long-duration gravitational-wave transients from Advanced LIGO and Advanced Virgo's third observing run from April 2019 to March 2020. For this search, we use minimal assumptions for the sky location, event time, waveform morphology, and duration of the source. The search covers the range of 2-500 s in duration and a frequency band of 24-2048 Hz. We find no significant triggers within this parameter space; we report sensitivity limits on the signal strength of gravitational waves characterized by the root-sum-square amplitude hrss as a function of waveform morphology. These hrss limits improve upon the results from the second observing run by an average factor of 1.8.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). All-sky search for short gravitational-wave bursts in the third Advanced LIGO and Advanced Virgo run. Phys. Rev. D., 104(12). doi:10.1103/PhysRevD.104.122004
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  This paper presents the results of a search for generic short-duration gravitational-wave transients in data from the third observing run of Advanced LIGO and Advanced Virgo. Transients with durations of milliseconds to a few seconds in the 24-4096 Hz frequency band are targeted by the search, with no assumptions made regarding the incoming signal direction, polarization, or morphology. Gravitational waves from compact binary coalescences that have been identified by other targeted analyses are detected, but no statistically significant evidence for other gravitational wave bursts is found. Sensitivities to a variety of signals are presented. These include updated upper limits on the source rate density as a function of the characteristic frequency of the signal, which are roughly an order of magnitude better than previous upper limits. This search is sensitive to sources radiating as little as ∼10-10 Mc2 in gravitational waves at ∼70 Hz from a distance of 10 kpc, with 50% detection efficiency at a false alarm rate of one per century. The sensitivity of this search to two plausible astrophysical sources is estimated: neutron star f modes, which may be excited by pulsar glitches, as well as selected core-collapse supernova models.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). All-sky search in early O3 LIGO data for continuous gravitational-wave signals from unknown neutron stars in binary systems. Physical Review D, 103(6). doi:10.1103/physrevd.103.064017
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  Rapidly spinning neutron stars are promising sources of continuous gravitational waves. Detecting such a signal would allow probing of the physical properties of matter under extreme conditions. A significant fraction of the known pulsar population belongs to binary systems. Searching for unknown neutron stars in binary systems requires specialized algorithms to address unknown orbital frequency modulations. We present a search for continuous gravitational waves emitted by neutron stars in binary systems in early data from the third observing run of the Advanced LIGO and Advanced Virgo detectors using the semicoherent, GPU-accelerated, binaryskyhough pipeline. The search analyzes the most sensitive frequency band of the LIGO detectors, 50-300 Hz. Binary orbital parameters are split into four regions, comprising orbital periods of three to 45 days and projected semimajor axes of two to 40 light seconds. No detections are reported. We estimate the sensitivity of the search using simulated continuous wave signals, achieving the most sensitive results to date across the analyzed parameter space.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Constraints from LIGO O3 Data on Gravitational-wave Emission Due to R-modes in the Glitching Pulsar PSR J0537-6910. The Astrophysical Journal, 922(1). doi:10.3847/1538-4357/ac0d52
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  We present a search for continuous gravitational-wave emission due to r-modes in the pulsar PSR J0537-6910 using data from the LIGO-Virgo Collaboration observing run O3. PSR J0537-6910 is a young energetic X-ray pulsar and is the most frequent glitcher known. The inter-glitch braking index of the pulsar suggests that gravitational-wave emission due to r-mode oscillations may play an important role in the spin evolution of this pulsar. Theoretical models confirm this possibility and predict emission at a level that can be probed by ground-based detectors. In order to explore this scenario, we search for r-mode emission in the epochs between glitches by using a contemporaneous timing ephemeris obtained from NICER data. We do not detect any signals in the theoretically expected band of 86-97 Hz, and report upper limits on the amplitude of the gravitational waves. Our results improve on previous amplitude upper limits from r-modes in J0537-6910 by a factor of up to 3 and place stringent constraints on theoretical models for r-mode-driven spin-down in PSR J0537-6910, especially for higher frequencies at which our results reach below the spin-down limit defined by energy conservation.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Constraints on Cosmic Strings Using Data from the Third Advanced LIGO-Virgo Observing Run. Physical Review Applied, 126(24). doi:10.1103/PhysRevLett.126.241102
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  We search for gravitational-wave signals produced by cosmic strings in the Advanced LIGO and Virgo full O3 dataset. Search results are presented for gravitational waves produced by cosmic string loop features such as cusps, kinks, and, for the first time, kink-kink collisions. A template-based search for short-duration transient signals does not yield a detection. We also use the stochastic gravitational-wave background energy density upper limits derived from the O3 data to constrain the cosmic string tension Gμ as a function of the number of kinks, or the number of cusps, for two cosmic string loop distribution models. Additionally, we develop and test a third model that interpolates between these two models. Our results improve upon the previous LIGO-Virgo constraints on Gμ by 1 to 2 orders of magnitude depending on the model that is tested. In particular, for the one-loop distribution model, we set the most competitive constraints to date: Gμ≲4×10-15. In the case of cosmic strings formed at the end of inflation in the context of grand unified theories, these results challenge simple inflationary models.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Diving below the Spin-down Limit: Constraints on Gravitational Waves from the Energetic Young Pulsar PSR J0537-6910. Astrophysical Journal Letters, 913(2). doi:10.3847/2041-8213/abffcd
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  We present a search for quasi-monochromatic gravitational-wave signals from the young, energetic X-ray pulsar PSR J0537-6910 using data from the second and third observing runs of LIGO and Virgo. The search is enabled by a contemporaneous timing ephemeris obtained using Neutron star Interior Composition Explorer (NICER) data. The NICER ephemeris has also been extended through 2020 October and includes three new glitches. PSR J0537-6910 has the largest spin-down luminosity of any pulsar and exhibits fRequent and strong glitches. Analyses of its long-term and interglitch braking indices provide intriguing evidence that its spin-down energy budget may include gravitational-wave emission from a time-varying mass quadrupole moment. Its 62 Hz rotation frequency also puts its possible gravitational-wave emission in the most sensitive band of the LIGO/Virgo detectors. Motivated by these considerations, we search for gravitational-wave emission at both once and twice the rotation frequency from PSR J0537-6910. We find no signal, however, and report upper limits. Assuming a rigidly rotating triaxial star, our constraints reach below the gravitational-wave spin-down limit for this star for the first time by more than a factor of 2 and limit gravitational waves from the l = m = 2 mode to account for less than 14% of the spin-down energy budget. The fiducial equatorial ellipticity is constrained to less than about 3 ×10-5, which is the third best constraint for any young pulsar.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo during the First Half of the Third Observing Run. Phys. Rev. X, 11(2). doi:10.1103/PhysRevX.11.021053
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  We report on gravitational-wave discoveries from compact binary coalescences detected by Advanced LIGO and Advanced Virgo in the first half of the third observing run (O3a) between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. By imposing a false-alarm-rate threshold of two per year in each of the four search pipelines that constitute our search, we present 39 candidate gravitational-wave events. At this threshold, we expect a contamination fraction of less than 10%. Of these, 26 candidate events were reported previously in near-real time through gamma-ray coordinates network notices and circulars; 13 are reported here for the first time. The catalog contains events whose sources are black hole binary mergers up to a redshift of approximately 0.8, as well as events whose components cannot be unambiguously identified as black holes or neutron stars. For the latter group, we are unable to determine the nature based on estimates of the component masses and spins from gravitational-wave data alone. The range of candidate event masses which are unambiguously identified as binary black holes (both objects ≥3 Mo˙) is increased compared to GWTC-1, with total masses from approximately 14 Mo˙ for GW190924_021846 to approximately 150 Mo˙ for GW190521. For the first time, this catalog includes binary systems with significantly asymmetric mass ratios, which had not been observed in data taken before April 2019. We also find that 11 of the 39 events detected since April 2019 have positive effective inspiral spins under our default prior (at 90% credibility), while none exhibit negative effective inspiral spin. Given the increased sensitivity of Advanced LIGO and Advanced Virgo, the detection of 39 candidate events in approximately 26 weeks of data (approximately 1.5 per week) is consistent with GWTC-1.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Observation of Gravitational Waves from Two Neutron Star-Black Hole Coalescences. Astrophysical Journal Letters, 915(1). doi:10.3847/2041-8213/ac082e
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  We report the observation of gravitational waves from two compact binary coalescences in LIGO's and Virgo's third observing run with properties consistent with neutron star-black hole (NSBH) binaries. The two events are named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and GW200115; the first was observed by LIGO Livingston and Virgo and the second by all three LIGO-Virgo detectors. The source of GW200105 has component masses, whereas the source of GW200115 has component masses and (all measurements quoted at the 90% credible level). The probability that the secondary's mass is below the maximal mass of a neutron star is 89%-96% and 87%-98%, respectively, for GW200105 and GW200115, with the ranges arising from different astrophysical assumptions. The source luminosity distances are and, respectively. The magnitude of the primary spin of GW200105 is less than 0.23 at the 90% credible level, and its orientation is unconstrained. For GW200115, the primary spin has a negative spin projection onto the orbital angular momentum at 88% probability. We are unable to constrain the spin or tidal deformation of the secondary component for either event. We infer an NSBH merger rate density of when assuming that GW200105 and GW200115 are representative of the NSBH population or under the assumption of a broader distribution of component masses.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Population properties of compact objects from the second LIGO-Virgo gravitational-wave transient catalog. Astrophysical Journal Letters, 913(1). doi:10.3847/2041-8213/abe949
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  We report on the population of 47 compact binary mergers detected with a false-alarm rate of 0.01 are dynamically assembled. Third, we estimate merger rates, finding RBBH = 23.9-+8.614.3 Gpc-3 yr-1 for BBHs and RBNS = 320-+240490 Gpc-3 yr-1 for binary neutron stars. We find that the BBH rate likely increases with redshift (85% credibility) but not faster than the star formation rate (86% credibility). Additionally, we examine recent exceptional events in the context of our population models, finding that the asymmetric masses of GW190412 and the high component masses of GW190521 are consistent with our models, but the low secondary mass of GW190814 makes it an outlier.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Search for Lensing Signatures in the Gravitational-Wave Observations from the First Half of LIGO-Virgo's Third Observing Run. ApJ, 923(1). doi:10.3847/1538-4357/ac23db
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  We search for signatures of gravitational lensing in the gravitational-wave signals from compact binary coalescences detected by Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Advanced Virgo during O3a, the first half of their third observing run. We study: (1) the expected rate of lensing at current detector sensitivity and the implications of a non-observation of strong lensing or a stochastic gravitational-wave background on the merger-rate density at high redshift; (2) how the interpretation of individual high-mass events would change if they were found to be lensed; (3) the possibility of multiple images due to strong lensing by galaxies or galaxy clusters; and (4) possible wave-optics effects due to point-mass microlenses. Several pairs of signals in the multiple-image analysis show similar parameters and, in this sense, are nominally consistent with the strong lensing hypothesis. However, taking into account population priors, selection effects, and the prior odds against lensing, these events do not provide sufficient evidence for lensing. Overall, we find no compelling evidence for lensing in the observed gravitational-wave signals from any of these analyses.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Search for anisotropic gravitational-wave backgrounds using data from Advanced LIGO and Advanced Virgo's first three observing runs. Physical Review D, 104(2). doi:10.1103/physrevd.104.022005
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  We report results from searches for anisotropic stochastic gravitational-wave backgrounds using data from the first three observing runs of the Advanced LIGO and Advanced Virgo detectors. For the first time, we include Virgo data in our analysis and run our search with a new efficient pipeline called pystoch on data folded over one sidereal day. We use gravitational-wave radiometry (broadband and narrow band) to produce sky maps of stochastic gravitational-wave backgrounds and to search for gravitational waves from point sources. A spherical harmonic decomposition method is employed to look for gravitational-wave emission from spatially-extended sources. Neither technique found evidence of gravitational-wave signals. Hence we derive 95% confidence-level upper limit sky maps on the gravitational-wave energy flux from broadband point sources, ranging from Fα,Θ
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Searches for continuous gravitational waves from young supernova remnants in the early third observing run of advanced LIGO and Virgo. Astrophysical Journal, 921(1). doi:10.3847/1538-4357/ac17ea
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  We present results of three wide-band directed searches for continuous gravitational waves from 15 young supernova remnants in the first half of the third Advanced LIGO and Virgo observing run. We use three search pipelines with distinct signal models and methods of identifying noise artifacts. Without ephemerides of these sources, the searches are conducted over a fRequency band spanning from 10 to 2 kHz. We find no evidence of continuous gravitational radiation from these sources. We set upper limits on the intrinsic signal strain at 95% confidence level in sample subbands, estimate the sensitivity in the full band, and derive the corresponding constraints on the fiducial neutron star ellipticity and r-mode amplitude. The best 95% confidence constraints placed on the signal strain are 7.7 × 10-26 and 7.8 × 10-26 near 200 Hz for the supernova remnants G39.2-0.3 and G65.7+1.2, respectively. The most stringent constraints on the ellipticity and r-mode amplitude reach ≲10-7 and ≲ 10-5, respectively, at frequencies above ∼400 Hz for the closest supernova remnant G266.2-1.2/Vela Jr.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Tests of general relativity with binary black holes from the second LIGO-Virgo gravitational-wave transient catalog. Physical Review D, 103(12). doi:10.1103/physrevd.103.122002
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  Gravitational waves enable tests of general relativity in the highly dynamical and strong-field regime. Using events detected by LIGO-Virgo up to 1 October 2019, we evaluate the consistency of the data with predictions from the theory. We first establish that residuals from the best-fit waveform are consistent with detector noise, and that the low- and high-frequency parts of the signals are in agreement. We then consider parametrized modifications to the waveform by varying post-Newtonian and phenomenological coefficients, improving past constraints by factors of ∼2; we also find consistency with Kerr black holes when we specifically target signatures of the spin-induced quadrupole moment. Looking for gravitational-wave dispersion, we tighten constraints on Lorentz-violating coefficients by a factor of ∼2.6 and bound the mass of the graviton to mg≤1.76×10-23 eV/c2 with 90% credibility. We also analyze the properties of the merger remnants by measuring ringdown frequencies and damping times, constraining fractional deviations away from the Kerr frequency to δf^220=0.03-0.35+0.38 for the fundamental quadrupolar mode, and δf^221=0.04-0.32+0.27 for the first overtone; additionally, we find no evidence for postmerger echoes. Finally, we determine that our data are consistent with tensorial polarizations through a template-independent method. When possible, we assess the validity of general relativity based on collections of events analyzed jointly. We find no evidence for new physics beyond general relativity, for black hole mimickers, or for any unaccounted systematics.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2021). Upper limits on the isotropic gravitational-wave background from Advanced LIGO and Advanced Virgo's third observing run. Physical Review D, 104(2). doi:10.1103/physrevd.104.022004
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  We report results of a search for an isotropic gravitational-wave background (GWB) using data from Advanced LIGO's and Advanced Virgo's third observing run (O3) combined with upper limits from the earlier O1 and O2 runs. Unlike in previous observing runs in the advanced detector era, we include Virgo in the search for the GWB. The results of the search are consistent with uncorrelated noise, and therefore we place upper limits on the strength of the GWB. We find that the dimensionless energy density ωGW≤5.8×10-9 at the 95% credible level for a flat (frequency-independent) GWB, using a prior which is uniform in the log of the strength of the GWB, with 99% of the sensitivity coming from the band 20-76.6 Hz; ωGW(f)≤3.4×10-9 at 25 Hz for a power-law GWB with a spectral index of 2/3 (consistent with expectations for compact binary coalescences), in the band 20-90.6 Hz; and ωGW(f)≤3.9×10-10 at 25 Hz for a spectral index of 3, in the band 20-291.6 Hz. These upper limits improve over our previous results by a factor of 6.0 for a flat GWB, 8.8 for a spectral index of 2/3, and 13.1 for a spectral index of 3. We also search for a GWB arising from scalar and vector modes, which are predicted by alternative theories of gravity; we do not find evidence of these, and place upper limits on the strength of GWBs with these polarizations. We demonstrate that there is no evidence of correlated noise of magnetic origin by performing a Bayesian analysis that allows for the presence of both a GWB and an effective magnetic background arising from geophysical Schumann resonances. We compare our upper limits to a fiducial model for the GWB from the merger of compact binaries, updating the model to use the most recent data-driven population inference from the systems detected during O3a. Finally, we combine our results with observations of individual mergers and show that, at design sensitivity, this joint approach may yield stronger constraints on the merger rate of binary black holes at z2 than can be achieved with individually resolved mergers alone.
• Armano, M., Audley, H., Baird, J., Binetruy, P., Born, M., Bortoluzzi, D., Brandt, N., Castelli, E., Cavalleri, A., Cesarini, A., Cruise, A., Danzmann, K., De Deus Silva, M., Diepholz, I., Dixon, G., Dolesi, R., Ferraioli, L., Ferroni, V., Fitzsimons, E., , Flatscher, R., et al. (2021). Sensor Noise in LISA Pathfinder: In-Flight Performance of the Optical Test Mass Readout. Physical Review Letters, 126(13). doi:10.1103/PhysRevLett.126.131103
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  We report on the first subpicometer interferometer flown in space. It was part of ESA's Laser Interferometer Space Antenna (LISA) Pathfinder mission and performed the fundamental measurement of the positional and angular motion of two free-falling test masses. The interferometer worked immediately, stably, and reliably from switch on until the end of the mission with exceptionally low residual noise of 32.0-1.7+2.4 fm/Hz, significantly better than required. We present an upper limit for the sensor performance at millihertz frequencies and a model for the measured sensitivity above 200 mHz.
• Kelly, P., Majji, M., & Guzmán, F. (2021). Estimation and error analysis for optomechanical inertial sensors. Sensors 21, 21(18). doi:10.3390/s21186101
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  A sensor model and methodology to estimate the forcing accelerations measured using a novel optomechanical inertial sensor with the inclusion of stochastic bias and measurement noise processes is presented. A Kalman filter for the estimation of instantaneous sensor bias is developed; the outputs from this calibration step are then employed in two different approaches for the estimation of external accelerations applied to the sensor. The performance of the system is demonstrated using simulated measurements and representative values corresponding to a bench-tested 3.76 Hz oscillator. It is shown that the developed methods produce accurate estimates of the bias over a short calibration step. This information enables precise estimates of acceleration over an extended operation period. These results establish the feasibility of reliably precise acceleration estimates using the presented methods in conjunction with state of the art optomechanical sensing technology.
• Zhang, Y., Hines, A., Valdes, G., & Guzman, F. (2021). Investigation and mitigation of noise contributions in a compact heterodyne interferometer. Sensors, 21(17), 5788. doi:10.3390/s21175788
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  We present a noise estimation and subtraction algorithm capable of increasing the sensitivity of heterodyne laser interferometers by one order of magnitude. The heterodyne interferometer is specially designed for dynamic measurements of a test mass in the application of sub-Hz inertial sensing. A noise floor of 3.31 × 10−11 m/√Hz at 100 mHz is achieved after applying our noise subtraction algorithm to a benchtop prototype interferometer that showed a noise level of 2.76 × 10−10 m/√Hz at 100 mHz when tested in vacuum at levels of 3 × 10−5 Torr. Based on the previous results, we investigated noise estimation and subtraction techniques of non-linear optical pathlength noise, laser frequency noise, and temperature fluctuations in heterodyne laser interferometers. For each noise source, we identified its contribution and removed it from the measurement by linear fitting or a spectral analysis algorithm. The noise correction algorithm we present in this article can be generally applied to heterodyne laser interferometers.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2020). Gravitational-wave Constraints on the Equatorial Ellipticity of Millisecond Pulsars. Astrophysical Journal Letters, 902(1). doi:10.3847/2041-8213/abb655
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  We present a search for continuous gravitational waves from five radio pulsars, comprising three recycled pulsars (PSR J0437-4715, PSR J0711-6830, and PSR J0737-3039A) and two young pulsars: the Crab pulsar (J0534+2200) and the Vela pulsar (J0835-4510). We use data from the third observing run of Advanced LIGO and Virgo combined with data from their first and second observing runs. For the first time, we are able to match (for PSR J0437-4715) or surpass (for PSR J0711-6830) the indirect limits on gravitational-wave emission from recycled pulsars inferred from their observed spin-downs, and constrain their equatorial ellipticities to be less than 10-8. For each of the five pulsars, we perform targeted searches that assume a tight coupling between the gravitational-wave and electromagnetic signal phase evolution. We also present constraints on PSR J0711-6830, the Crab pulsar, and the Vela pulsar from a search that relaxes this assumption, allowing the gravitational-wave signal to vary from the electromagnetic expectation within a narrow band of frequencies and frequency derivatives.
• Guzman, F., Hines, A. S., Richardson, L., & Wisniewski, H. (2020). Optomechanical inertial sensors.. Applied optics, 59(22), G167-G174. doi:10.1364/ao.393061
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  We present a performance analysis of compact monolithic optomechanical inertial sensors that describes their key fundamental limits and overall acceleration noise floor. Performance simulations for low-frequency gravity-sensitive inertial sensors show attainable acceleration noise floors on the order of 1×10-11m/s2Hz. Furthermore, from our performance models, we devised an optimization approach for our sensor designs, sensitivity, and bandwidth trade space. We conducted characterization measurements of these compact mechanical resonators, demonstrating mQ-products at levels of 250 kg, which highlight their exquisite acceleration sensitivity.
• Hines, A., Richardson, L., Wisniewski, H., & Guzman, F. (2020). Optomechanical inertial sensors. Applied Optics, 59(22). doi:10.1364/AO.393061
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  We present a performance analysis of compact monolithic optomechanical inertial sensors that describes their key fundamental limits and overall acceleration noise floor. Performance simulations for low-frequency gravity-sensitive inertial sensors show attainable acceleration noise floors on the order of 1 × 10−11 m/s2√Hz. Furthermore, from our performance models, we devised an optimization approach for our sensor designs, sensitivity, and bandwidth trade space. We conducted characterization measurements of these compact mechanical resonators, demonstrating mQ-products at levels of 250 kg, which highlight their exquisite acceleration sensitivity.
• Joo, K., Clark, E., Zhang, Y., Ellis, J., & Guzman, F. (2020). A compact high-precision periodic-error-free heterodyne interferometer. Journal of the Optical Society of America, 37(9), B11-B18. doi:10.1364/JOSAA.396298
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  We present the design, bench-top setup, and experimental results of a compact heterodyne interferometer that achieves picometer-level displacement sensitivities in air over frequencies above 100 MHz. The optical configuration with spatially separated beams prevents frequency and polarization mixing, and therefore eliminates periodic errors. The interferometer is designed to maximize common-mode optical laser beam paths to obtain high rejection of environmental disturbances, such as temperature fluctuations and acoustics. The results of our experiments demonstrate the short- and long-term stabilities of the system during stationary and dynamic measurements. In addition, we provide measurements that compare our interferometer prototype with a commercial system, verifying our higher sensitivity of 3 pm, higher thermal stability by a factor of two, and periodic-error-free performance.
• Richardson, L., Hines, A., Schaffer, A., Anderson, B. P., & Guzman, F. (2020). Quantum hybrid optomechanical inertial sensing. Applied Optics, 59(22), G160.
• Richardson, L., Rajagopalan, A., Albers, H., Meiners, C., Nath, D., Schubert, C., Tell, D., Wodey, É., Abend, S., Gersemann, M., Ertmer, W., Rasel, E., Schlippert, D., Mehmet, M., Kumanchik, L., Colmenero, L., Spannagel, R., Braxmaier, C., & Guzmán, F. (2020). Optomechanical resonator-enhanced atom interferometry. Communications Physics, 3(1). doi:10.1038/s42005-020-00473-4
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  Matter-wave interferometry and spectroscopy of optomechanical resonators offer complementary advantages. Interferometry with cold atoms is employed for accurate and long-term stable measurements, yet it is challenged by its dynamic range and cyclic acquisition. Spectroscopy of optomechanical resonators features continuous signals with large dynamic range, however it is generally subject to drifts. In this work, we combine the advantages of both devices. Measuring the motion of a mirror and matter waves interferometrically with respect to a joint reference allows us to operate an atomic gravimeter in a seismically noisy environment otherwise inhibiting readout of its phase. Our method is applicable to a variety of quantum sensors and shows large potential for improvements of both elements by quantum engineering.
• Wisniewski, H., Richardson, L., Hines, A., Laurain, A., & Guzmán, F. (2020). Optomechanical lasers for inertial sensing. Journal of the Optical Society of America A: Optics and Image Science, and Vision, 37(9). doi:10.1364/JOSAA.396774
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  We have developed an inertially sensitive optomechanical laser by combining a vertical-external-cavity surface-emitting laser (VECSEL) with a monolithic fused silica resonator. By placing the external cavity mirror of the VECSEL onto the optomechanical resonator test mass, we create a sensor where external accelerations are directly transcribed onto the lasing frequency. We developed a proof-of-principle laboratory prototype and observe test mass oscillations at the resonance frequency of the sensor through the VECSEL lasing frequency, 4.18 ± 0.03 Hz. In addition, we set up an ancillary heterodyne interferometer to track the motion of the mechanical oscillator’s test mass, observing a resonance of 4.194 ± 0.004 Hz. The interferometer measurements validate the VECSEL results, confirming the feasibility of using optomechanical lasers for inertial sensing. © 2020 Optical Society of America
• Hamann, I., Sanjuan, J., Spannagel, R., Gohlke, M., Wanner, G., Schuster, S., Guzman, F., & Braxmaier, C. (2019). Laser-dilatometer calibration using a single-crystal silicon sample. International Journal of Optomechanatronics, 13(1). doi:10.1080/15599612.2019.1587117
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  Marginal changes in geometrical dimensions due to temperature changes affect the performance of optical instruments. Highly dimensionally stable materials can minimize these effects since they offer low coefficients of thermal expansion (CTE). Our dilatometer, based on heterodyne interferometry, is able to determine the CTE in 10-8 K-1 range. Here, we present the improved interferometer performance using angular measurements via differential wavefront sensing to correct for tilt-to-length coupling. The setup was tested by measuring the CTE of a single-crystal silicon at 285 K. Results are in good agreement with the reported values and show a bias of less than 1%.
• Lundquist, M., Paterson, K., Fong, W., Sand, D., Andrews, J., Shivaei, I., Daly, P., Valenti, S., Yang, S., Christensen, E., Gibbs, A., Shelly, F., Wyatt, S., Eskandari, O., Kuhn, O., Amaro, R., Arcavi, I., Behroozi, P., Butler, N., , Chomiuk, L., et al. (2019). Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo's Third Observing Run. Astrophysical Journal Letters, 881(2). doi:10.3847/2041-8213/ab32f2
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  We present Searches After Gravitational-waves Using ARizona Observatories (SAGUARO), a comprehensive effort dedicated to the discovery and characterization of optical counterparts to gravitational-wave (GW) events. SAGUARO utilizes ground-based facilities ranging from 1.5 to 10 m in diameter, located primarily in the Northern Hemisphere. We provide an overview of SAGUARO's telescopic resources, its pipeline for transient detection, and its database for candidate visualization. We describe SAGUARO's discovery component, which utilizes the 5deg2 field of view optical imager on the Mt. Lemmon 1.5 m telescope, reaching limits of ≈21.3 AB mag while rapidly tiling large areas. We also describe the follow-up component of SAGUARO, used for rapid vetting and monitoring of optical candidates. With the onset of Advanced LIGO/Virgo's third observing run, we present results from the first three SAGUARO searches following the GW events S190408an, S190425z and S190426c, which serve as a valuable proof-of-concept of SAGUARO. We triggered and searched 15, 60, and 60 deg2 respectively, 17.6, 1.4, and 41.8 hr after the initial GW alerts. We covered 7.8%, 3.0%, and 5.1% of the total probability within the GW event localizations, reaching 3σ limits of 19.8, 21.3, and 20.8 AB mag, respectively. Although no viable counterparts associated with these events were found, we recovered six known transients and ruled out five potential candidates. We also present Large Binocular Telescope spectroscopy of PS19eq/SN2019ebq, a promising kilonova candidate that was later determined to be a supernova. With the ability to tile large areas and conduct detailed follow-up, SAGUARO represents a significant addition to GW counterpart searches.
• Ryan, W. (2018). Towards a photonic quantum standard for mass and force. IEEE Conference on Precision Electromagnetic Measurements (CPEM).
• Wagner, R., Guzman, F., Chijioke, A., Gulati, G., Keller, M., & Shaw, G. (2018). Direct measurement of radiation pressure and circulating power inside a passive optical cavity. Optics Express, 26(18). doi:10.1364/OE.26.023492
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  A mechanical force sensor coupled to two optical cavities is developed as a metrological tool. This system is used to generate a calibrated circulating optical power and to create a transfer standard for externally coupled optical power. The variability of the sensor as a transfer standard for optical power is less than 2%. The uncertainty in using the sensor to measure the circulating power inside the cavity is less than 3%. The force measured from the mechanical response of the sensor is compared to the force predicted from characterizing the optical spectrum of the cavity. These two forces are approximately 20% different. Potential sources for this disagreement are analyzed and discussed. The sensor is compact, portable, and can operate in ambient and vacuum environments. This device provides a pathway to novel nanonewton scale force and milliwatt scale laser power calibrations, enables direct measurement of the circulating power inside an optical cavity, and enhances the sensitivity of radiation pressure-based optical power transfer standards.
• Armano, M., Audley, H., Auger, G., Baird, J. T., Bassan, M., Binetruy, P., Born, M., Bortoluzzi, D., Brandt, N., Caleno, M., Carbone, L., Cavalleri, A., Cesarini, A., Ciani, G., Congedo, G., Cruise, A. M., Danzmann, K., Deus Silva, M., De Rosa, R., , Diaz-Aguil\'o, M., et al. (2016). Sub-Femto-$g$ Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results. Phys. Rev. Lett., 116, 231101.
• Gordon, A. (2016). Using small mass and force metrology for laser power measurement. IEEE Conference on Precision Electromagnetic Measurements (CPEM).
• Katharina-Sofie, I. (2016). Experimental demonstration of deep frequency modulation interferometry. Optics Express, 24(2), 1676--1684.
• Yiliang, B. (2016). An optomechanical accelerometer with a high-finesse hemispherical optical cavity. IEEE International Symposium on Inertial Sensors and Systems.
• Ferran, G., & Danzmann, K. (2015). Thermo-elastic induced phase noise in the LISA Pathfinder spacecraft. Classical and Quantum Gravity, 32.
• Guzm\'an, C. (2015). MEMS optomechanical accelerometry standards. American Society of Precision Engineering, Conference on Precision Interferometry.
• Guzm\'an, C., & Taylor, J. M. (2015). Optomechanical motion sensors. American Society of Precision Engineering, Conference on Precision Interferometry.
• M., T. (2015). Towards a FPGA-controlled deep phase modulation interferometer. Journal of Physics: Conference Series, 610, 012042.
• Oliver, G. (2015). Optomechanical reference accelerometer. Metrologia, 52.
• Guzm\'an, C. (2014). High sensitivity optomechanical reference accelerometer over 10 kHz. Applied Physics Letters, 104.
• Melcher, J., Stirling, J., Cervantes, ., Pratt, J., & Shaw, G. (2014). A self-calibrating optomechanical force sensor with femtonewton resolution. Applied Physics Letters, 105.
• Schwarze, T., Gerberding, O., Cervantes, ., Heinzel, G., & Danzmann, K. (2014). Advanced phasemeter for deep phase modulation interferometry. Optics Express, 22.
• Yicheng, W. (2014). Alignment and testing of the NIST Calculable Capacitor. IEEE CPEM 2014, Conference on Precision Electromagnetic Measurements.
• Cervantes, F., Flatscher, R., Gerardi, D., Burkhardt, J., Gerndt, R., Nofrarias, M., Reiche, J., Heinzel, G., Danzmann, K., Bot\'e, L., Mart\'in, V., Mateos, I., & Lobo, A. (2013). LISA Technology Package Flight Hardware Test Campaign. Astronomical Society of the Pacific Conference Series, 467, 141.
• M, N., & Zweifel, P. (2013). State Space Modelling and Data Analysis Exercises in LISA Pathfinder. Astronomical Society of the Pacific Conference Series, 467, 161.
• F, A. (2012). The LISA Pathfinder Mission. Classical and Quantum Gravity, 29(12), 124014.
• Joshi, A., Datta, S., Rue, J., Livas, J., Silverberg, R., & Cervantes, F. (2012). Ultra-Low Noise, Large-Area InGaAs Quad Photoreceiver with Low Crosstalk for Laser Interferometry Space Antenna. SPIE Astronomical Telescopes and Instrumentation, 84532G--84532G.
• Audley, H., Danzmann, K., Mar\'in, A. G., Heinzel, G., Monsky, A., Nofrarias, M., Steier, F., Gerardi, D., Gerndt, R., Hechenblaikner, G., Johann, U., Luetzow-Wentzky, P., Wand, V., Antonucci, F., Armano, M., Auger, G., Benedetti, M., Binetruy, P., Boatella, C., , Bogenstahl, J., et al. (2011). The LISA Pathfinder interferometry - hardware and system testing. Class. Quantum Grav., 28(9), 094003.
• Esteban, J. J., Garc\'ia, A. F., Barke, S., Peinado, A. M., Cervantes}, ., Bykov, I., Heinzel, G., & Danzmann, K. (2011). Experimental demonstration of weak-light laser ranging and data communication for LISA. Optics Express, 19(17), 15937--15946.
• F., A. (2011). From laboratory experiments to LISA Pathfinder: achieving LISA geodesic motion.. Class. Quantum Grav., 28(9), 094002.
• F., A. (2011). LISA Pathfinder Data Analysis. Class. Quantum Grav., 28(9), 094006.
• F., A. (2011). LISA Pathfinder: mission and status. Class. Quantum Grav., 28(9), 094001.
• Guzm\'an, C. (2011). Characterization of photoreceivers for LISA. Classical and Quantum Gravity, 28(9), 094010.
• Gerhard, H. (2010). Deep phase modulation interferometry. Opt. Express, 18(18), 19076--19086.
• A., M., & Zweifel, P. (2009). The first mock data challenge for LISA Pathfinder. Class. Quantum Grav., 26.
• F., S., & Danzmann, K. (2009). The end-to-end testbed of the optical metrology system on-board LISA Pathfinder. Class. Quantum Grav., 26.
• M, A., & Zweifel, P. (2009). LISA Pathfinder: the experiment and the route to LISA. Class. Quantum Grav., 26.
• M., H., & Zweifel, P. (2009). Data analysis for the LISA Technology Package. Class. Quantum Grav., 26.
• Marina, D., & Danzmann, K. (2009). Laser interferometer for spaceborne mapping of the Earth's gravity field. J. Phys.: Conf. Ser., 154.
• Guzm\'an, C. (2008). Subtraction of test mass angular noise in the LISA technology package interferometer. Applied Physics B, 90.
• Guzm\'an, C., & Danzmann, K. (2007). Real-time phasefront detector for heterodyne interferometers. Applied Optics, 46.
• A., G. M., & Danzmann, K. (2006). Interferometric characterization of the optical window for LISA Pathfinder and LISA. AIP Conference Proceedings, 873.
• A., G., & Danzmann, K. (2006). On-orbit alignment and diagnostics for the LISA Technology Package. Classical and Quantum Gravity, 23.
• C., K., & Ward, H. (2006). Construction of the LTP Optical Bench Interferometer. AIP Conference Proceedings, 873.
• G., H., & Wand, V. (2006). Interferometry for the LISA technology package LTP: an update. Journal of Physics: Conference Series, 32.
• V., W. (2006). LISA Phasemeter development. AIP Conference Proceedings, 873.
• V., W., & Ward, H. (2006). Noise sources in the LTP heterodyne interferometer. Classical and Quantum Gravity, 23.
• A., G., & Danzmann, K. (2005). Phase locking to a LISA arm: first results on a hardware model. Classical and Quantum Gravity, 22.

Proceedings Publications

• D., J., Martinez, B., George, D., Nelson, A., Mehmet, M., Muñoz, J. S., & Guzman, F. (2025, 2025/9/16). High sensitivity, high bandwidth photonic integrated optomechanical accelerometer: a numerical approach. In High sensitivity, high bandwidth photonic integrated optomechanical accelerometer: a numerical approach, 13594, 1359409.
• Dahn, J., Nelson, A., Sanjuan, J., & Guzman, F. (2025, 2025/9/18).

  Optomechanical gravity gradiometer for remote sensing

  . In Optomechanical gravity gradiometer for remote sensing, 13599, 1359902.
• Harley-Trochimczyk, I., Guo, X., & Guzmán, F. (2024). Development of a compact adjustable fiber collimator mount for optomechanical accelerometers. In Novel Optical Systems, Methods, and Applications XXVII 2024, 13130.
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  Precise alignment of laser beams used in heterodyne interferometry is vital and necessary to the precision, accuracy, and quality of the measurement, but off the shelf-based breadboard setups have a large physical footprint and many components that can introduce unwanted noise. Our lab creates optomechanical accelerometer devices including a fused silica resonator with a 5Hz natural frequency and uses a heterodyne displacement interferometer to readout the position of the test mass, which can then be used to determine the acceleration of the device. A novel compact fiber injector system design is presented here that reduces the footprint of the fiber collimator input of the heterodyne interferometer by an order of magnitude from a breadboard setup, down to 24 x 16 x 19 mm. This new injector system integrates both fibers of different frequencies directly onto the mount with the resonator, increasing stability and reducing entry points for vibrational noise while minimizing the optical path length difference between beams. Each beam can then be independently tilted and decentered to maximize the fringe visibility at the output of the interferometer, using spring-loaded adjustment screws and secured in place with locking screws. An accelerometer using these injectors measured a displacement of 10-9 m/√Hz at 10-2 Hz in air with the test mass anchored, nearly identical to the previous breadboard setup while being much more compact and portable. I will present the design, integration onto an accelerometer, and the initial acceleration noise measurements taken using these fiber injector systems.
• Capistran, L., Hines, A., Carter, J., Guo, X., Valdes, G., Sanjuan, J., & Guzman, F. (2023). Wide-band dual optomechanical resonator for inertial navigation. In Quantum Sensing, Imaging, and Precision Metrology 2023, 12447.
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  Accelerometers are key sensors in many fields and applications such as precision metrology, gravimetry measurements, gravitational wave observatories, and navigation where position and attitude need to be determined accurately. A combination of six accelerometers provides all the necessary information to estimate position and orientation of a rigid body and thus serves as an inertial navigation system for autonomous navigation. Fused-silica based mechanical resonators paired with laser interferometric read-outs enable compact high-accuracy accelerometers. In this talk, we will present a wide-band accelerometer based on a double resonator with two test masses of different sizes in a single frame. One of the resonators has a resonance frequency of about 50 Hz, while the other is optimized for lower frequencies and has a nominal frequency of about 10 Hz. The combination of the two resonators allows for excellent long-term precision while maintaining good measurement bandwidth. We will show the experimental characterization in air and in vacuum of the double-resonator using a heterodyne laser interferometer and a fiber interferometer and its expected performance as an inertial sensor.
• Martinez, B., Nelson, A., Hines, A., Mock, J., Sanjuan, J., Guzmán, F., & Valdés, G. (2023). Modeling, fabrication, and readout of compact optomechanical accelerometers. In Proc. SPIE 12434, MOEMS and Miniaturized Systems XXII, 1243404.
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  High-sensitivity accelerometers are key for many applications including ground-based gravitational wave (GW) detectors, in-situ or satellite gravimetry measurements, and inertial navigation systems. We will present our work on the development of optomechanical accelerometers based on the micro-fabrication of mechanical resonators and their integration with laser interferometers to read out their test mass dynamics under the presence of external accelerations. We will discuss the latest developments on compact millimeter-scale resonators made of fused silica and silicon, optimized for frequencies below 1 kHz and exhibiting low mechanical losses. While fused silica has demonstrated high mechanical quality factors at room temperature, silicon devices perform significantly better at very low temperatures, which is particularly relevant for future ground-based gravitational wave detectors where cryogenic environments will be used to improve the sensitivity of the observatories. We will report on our design, modeling, and fabrication process for the silicon-based resonators and present their characterization by means of highly compact fiber-based Fabry-Perot cavities.
• Nelson, A., Hines, A., Valdes, G., Sanjuan, J., & Guzman, F. (2023). Low frequency inertial sensing. In 10th IEEE International Symposium on Inertial Sensors and Systems, INERTIAL 2023.
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  We present an optomechanical inertial sensor for low frequency applications. This accelerometer is readout optically instead of capacitively, limiting electrostatic noise in the system. It consists of a 5 Hz monolithic fused-silica resonator with an oscillating test mass and a heterodyne interferometer readout. It is designed to be a compact, portable, and cost-effective alternative for highly sensitive inertial sensors at low frequencies. Potential applications include but are not limited to gravimetry, geodesy, and hydrology. The resonator has a measured mechanical quality factor (Q) of 477,000 and an mQ -product of 1200 kg. This high Q factor reduces thermal motion, allowing for a device with a competitive acceleration noise floor.
• Wang, P., Sanjuan, J., & Guzman, F. (2023). Laser frequency stabilization using HCN gas cell. In Proc. SPIE 12665 Novel Optical Systems, Methods, and Applications.
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  Laser frequency fluctuations are one of the limiting factors for many laser-involved precision measurements such as interferometry. Laser frequency locks with the Pound-Drever-Hall (PDH) method use typically an optical cavity as a reference, which are very sensitive to environmental noises. In contrast, spectroscopy methods using atom or molecular transitions and phase modulation spectroscopy behave better in the long term. A well-sealed fiber-based Hydrogen Cyanide (HCN) gas cell that is very compact and light-weighted is chosen. And We investigate laser frequency stabilization using the absorption line of an HCN gas cell instead of a cavity to provide better frequency stability in the low-frequency regime. In our lab, a fiber-coupled HCN gas cell laser frequency lock was built and thermally stabilized to provide better long-term stability. It is designed to work with our heterodyne interferometer around 1550 nm wavelength. The HCN gas cell locking setup using phase modulation (PM) spectroscopy shows less than 0.5 MHz frequency drift over 12 hours measurement and stability levels of 1 kHz/√Hz for frequencies above 0.2 Hz.
• Guzman, F. (2022). Displacement interferometry for inertial sensing. In Latin America Optics and Photonics Conference, LAOP 2022.
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  We present here recent results of our research work on novel optomechanical inertial sensing technologies, and the corresponding laser-interferometric displacement sensors we are developing for integrated portable units, as well as other interesting sensing schemes.
• Nelson, A., & Guzmán, F. (2022, 2 March 2022). Compact optomechanical inertial sensors with fused silica and Si-based resonators. In Proc. SPIE 12016, Optical and Quantum Sensing and Precision Metrology II.
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  Inertial sensors are used in a variety of applications including inertial navigation and precision measurements. Optical measurement of test mass displacement in a resonator allows for the creation of compact accelerometer systems. Fused silica resonators allow for excellent acceleration sensitivities due to their high mechanical quality factor, Q, at room temperature, but this changes significantly at lower temperatures. The Q factor of crystalline silicon, however, remains high at low temperatures. We work with compact fused silica resonators that operate at room temperature and aim to fabricate compact comparable mechanical resonators from Si wafers. We will report on the fabrication progress of these resonators and results from ringdown and sensitivity measurements.
• Nelson, A., Hines, A., Zhang, Y., & Guzman, F. (2022). ADVANCES IN OPTOMECHANICAL INERTIAL SENSING. In 37th Annual Meeting of the American Society for Precision Engineering, ASPE 2022.
• Nelson, A., Hines, A., Zhang, Y., & Guzmán, F. (2022). Optomechanical technologies for broadband inertial sensing. In Proc. SPIE 12223, Interferometry XXI.
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  Accelerometers are a vital component in inertial sensing and geodesy, gravitational physics, seismic noise detection, hydrology, and other fields requiring precision measurements. Our group develops compact low and high frequency optomechanical inertial sensors to measure acceleration for various applications. Our sensors measure the linear displacement of an oscillating test mass with displacement laser interferometers that are fiber-coupled or free space. The observed external acceleration is recovered from the displacement of the test mass. Our compact 5 Hz resonator will operate as a relative gravimeter and be read out by a compact, highly sensitive free-space heterodyne laser interferometer. It has demonstrated low mechanical losses with quality factors above 4.77 × 105 and mQ-products greater than 1200 kg. Our millimeter scale higher frequency resonators are made of fused silica for operation at room temperature and Si for operation at cryogenic temperatures. They will be readout with fiber based Fabry-Perot cavities or waveguide ring resonators that are currently under development. We are working to fabricate the Si resonators and are optimizing the process using Bosch and cryo-Si DRIE etching. Here, we report our progress on design and fabrication along with preliminary measurement results for all resonator prototypes.
• Zhang, Y., Joo, K., & Guzman, F. (2022, 5 March 2022). Fiber-based two-wavelength heterodyne displacement interferometer. In Proc. SPIE 12008, Protonic Instrumentation Engineering IX.
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  Precision displacement laser interferometry is crucial in various applications such as microlithography, high-performance profilometry, and gravitational wave detection. We are currently developing a fiber-based heterodyne laser interferometer that features compact size and low noise floor. Laser beams at two different wavelengths are utilized to construct a fiberbased interferometer system. Narrow band spectral filters are used to separate the beams of different wavelengths and to control their optical paths. The highly common optical paths between the two interferometers provide a high commonmode rejection ratio to instrument and environmental noise sources. In this paper we present the interferometer design, benchtop prototype system, and preliminary measurement results obtained in the lab environments. A benchtop prototype shows sub-nm/Hz displacement sensitivities in air at frequencies above 100 mHz in our lab.
• Guzman, F. (2021). Optical metrology for gravitational wave observatories and geophysics. In 2021 Conference on Lasers and Electro-Optics, CLEO 2021.
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  Decades of efforts by the scientific community in advancing optical metrology enable unprecedented displacement sensitivities that make our scientific endeavors possible. Examples from the gravitational-wave and geodesy communities are briefly presented in this paper.
• Hines, A., Nelson, A., Richardson, L., Valdes, G., & Guzman, F. (2021, August 2021). Advancements in optomechanical resonators for novel inertial sensors. In Proceeding SPIE 11816 Optomechanics and Optical Alignment.
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  Our work in the Laboratory of Space Systems and Optomechanics (LASSO) at Texas A&M University involves using optomechanical resonators coupled with compact, high-precision interferometers to create novel inertial sensors. These resonators are etched from monolithic fused silica, which is known to have very low internal losses, allowing for high mechanical quality factors and low thermal acceleration noise in the test mass. Previous measurements at mTorr pressures have demonstrated Q's of 1.91 x 105, corresponding to estimated thermal acceleration noise floor on the order of 10-10 m s- 2/√Hz for frequencies above 30 mHz. In this pressure regime, gas damping is still the dominant loss mechanism. At sufficiently low pressures such that gas damping is negligible, we anticipate mechanical quality factors of the order of 106 and thermal acceleration noise at levels of 10-11 m s-2/√Hz in the sub-Hz regime. As expected, previous measurements have shown significant ambient vibrations that limit our ability to observe the noise floor of the resonator. Hence, we have developed a dedicated vibration isolation platform to mitigate external disturbances, which consists of a pendulum with a magnetic anti-spring to lower the resonant frequency. Sensors constructed with these resonators would be lightweight and cost-effective, making them promising candidates for field applications in geophysics, navigation, and site exploration.
• Jersey, K., Zhang, Y., Harley-Trochimczyk, I., & Guzman, F. (2021). Design, fabrication, and testing of an optical truss interferometer for the LISA telescope. In Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems III 2021, 11820.
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  LISA is a space-based gravitational wave observatory aimed at detecting gravitational waves in the frequency range of 0.1 mHz to 0.1 Hz. The observatory is composed of three spacecraft, each separated by 2.5 million km in an equilateral triangle formation, trailing the Earth in a heliocentric orbit. One of the many crucial components to the mission is the LISA telescope, a bidirectional component used to expand an outgoing laser beam to the far spacecraft as well as compress a large incoming beam to a diameter of a few mm at the optical bench. Since the telescope is in the path of the long-baseline interferometer, its structure must be dimensionally stable at the pm/√Hz level at mHz frequencies. A way to measure the stability of the LISA telescope is with a compact optical truss interferometer (OTI), consisting of three Fabry-Perot cavities mounted along the telescope to monitor structural distortions over time. All three cavities are operated with a common laser source, and each cavity is equipped with an acousto-optic modulator to shift the nominal laser frequency as well as an electro-optical modulator to modulate the laser phase for Pound-Drever-Hall locking. Variations in each cavity’s length create variations in their corresponding laser frequency, which can be measured against a reference frequency that is locked to an external ultra-stable cavity. We will present the design and preliminary results in the fabrication and testing of first-generation OTI prototypes.
• Nelson, A., & Guzman, F. (2021). Micro-fabrication of Si-based optomechanical inertial sensors for cryogenic temperatures. In Applied Optical Metrology IV 2021, 11817.
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  The design of next generation gravitational wave observatories considers operation at cryogenic temperatures to enhance their sensitivity by reducing thermal noise fluctuations. Inertial sensors are used on the observatory platforms to measure local seismic noise and counteract its effects by active control or subtraction in post-processing. Measuring the displacement of a test mass in a resonator system allows for creation of a compact accelerometer system. Currently, there are no commercial inertial sensors available that are capable of operating at cryogenic temperatures and providing the required sensitivities for gravitational wave observatories. Materials such as fused silica exhibit very low losses at room temperature. However, this changes significantly at lower temperatures. Unlike fused silica, the Q factor of crystalline silicon structures is expected to remain high at low temperatures, making it a likely candidate for use in these types of inertial sensors. We are working to fabricate compact mechanical resonators from Si wafers to test their mechanical response. Micro-fabrication consists of optimizing the photolithography and Bosch etching processes for through-wafer Si etching on a 280 μm, 500 μm, and 1 mm wafer. Successful etching on 280 μm wafers has been achieved. We report on the design, model, and fabrication progress of these resonators.
• Zhang, Y., & Guzman, F. (2021). Noise suppression methods in picometer heterodyne displacement interferometer. In Applied Optical Metrology IV 2021, 11817.
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  High sensitivity displacement interferometer has wide applications in gravitational wave detection area, performing as crucial part in test mass dynamics measurement and seismic motion monitoring for low-noise observatory operation. With advances in heterodyne laser interferometry, sensitivities at levels of sub-nm/šHz over sub-Hz frequencies can be achieved. However, the breakthrough towards picometer level still needs various techniques in noise characterization and suppression. In this article, a compact heterodyne laser interferometer design as well as benchtop prototype system is presented. Common noise sources and their effects are investigated, including laser frequency noise, non-linear OPD noise, thermo-elastic noise, as well as readout noise from phasemeters and photoreceivers to determine the sensitivity limits in our system. Furthermore, each individual noise source is characterized with dedicated instruments and the coupling coefficients are determined respectively. By subtracting the individual noise contributions, the interferometer sensitivity reaches a sensitivity at the picometer level above 100 mHz frequency. We will present our progress and current results.
• Wisniewski, H., Richardson, L., Laurain, A., Hines, A., & Guzman, F. (2020). Optomechanical laser for inertial sensing. In Advanced Solid State Lasers, ASSL 2020 - Part of Laser Congress, LAC 2020.
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  We present proof-of-concept results in the development of inertially sensitive optomechanical lasers. These devices consist of Vertical-External-Cavity Surface-Emitting Lasers that are combined with monolithic fused silica mechanical resonators. External accelerations are directly transcribed onto corresponding changes in the laser frequency, thus providing direct frequency-based inertial measurements.
• Wagner, R., Guzman, F., & Shaw, G. (2018). Towards a Photonic Quantum Standard for Mass and Force. In 2018 Conference on Precision Electromagnetic Measurements, CPEM 2018.
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  A fundamental standard for measuring small forces can potentially be realized using single photons to transfer a known number of quantized photon momenta using the process of reflection. For such an experiment, a resonant optical cavity will maximize the number of momentum transfers per photon, making it possible to scale the total force generated. We have developed and tested such a system that operates in the many photon regime as a stepping stone towards a photonic quantum force standard.
• Bassan, M., Armano, M., Audley, H., Auger, G., Baird, J. T., Binetruy, P., Born, M., Bortoluzzi, D., Brandt, N., Caleno, M., Carbone, L., Cavalleri, A., Cesarini, A., Ciani, G., Congedo, G., Cruise, A. M., Danzmann, K., de Deus Silva, M., De Rosa, R., , Di Fiore, L., et al. (2017). LISA PATHFINDER FIRST STEP TOWARD A GRAVITATIONAL WAVE SPACE OBSERVATORY. In 16th Vulcano Workshop on Frontier objects in Astrophysics and Particle Physics, Vulcano 2016, 64.
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  We briefly review the concept of a space-based gravitational wave interferometer, and the science it can explore in the milliHertz frequency region. Then we discuss the LISA Pathfinder technology demostrator mission that is currently flying and will soon deliver the first results.
• Wang, Y., Cervantes, F., Stambaugh, C., Smid, R., Calvo, H., Koffman, A., Pratt, J., & Lawall, J. (2014, August 2014). Alignment and testing of the NIST calculable capacitor. In Conference on Precision Electromagnetic Measurements (CPEM).
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  This paper reports progress on the NIST effort to develop a new calculable capacitor, focusing on improvement of the guard electrode motion control as well as issues associated with the overall electrode alignment. Design of a multi-wavelength Fabry-Perot interferometer which may facilitate testing the calculable capacitor in air is also discussed.
• Joshi, A., Datta, S., Rue, J., Livas, J., Silverberg, R., & Guzman Cervantes, F. (2012). Ultra-low noise, large-area InGaAs Quad photoreceiver with low crosstalk for laser interferometry space antenna. In High Energy, Optical, and Infrared Detectors for Astronomy V, 8453.
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  Quad photoreceivers, namely a 2 × 2 array of p-i-n photodiodes followed by a transimpedance amplifier (TIA) per diode, are required as the front-end photonic sensors in several applications relying on free-space propagation with position and direction sensing capability, such as long baseline interferometry, free-space optical communication, and biomedical imaging. It is desirable to increase the active area of quad photoreceivers (and photodiodes) to enhance the link gain, and therefore sensitivity, of the system. However, the resulting increase in the photodiode capacitance reduces the photoreceiver's bandwidth and adds to the excess system noise. As a result, the noise performance of the front-end quad photoreceiver has a direct impact on the sensitivity of the overall system. One such particularly challenging application is the space-based detection of gravitational waves by measuring distance at 1064 nm wavelength with ∼ 10 pm/√Hz accuracy over a baseline of millions of kilometers. We present a 1 mm diameter quad photoreceiver having an equivalent input current noise density of < 1.7 pA/√Hz per quadrant in 2 MHz to 20 MHz frequency range. This performance is primarily enabled by a rad-hard-by-design dualdepletion region InGaAs quad photodiode having 2.5 pF capacitance per quadrant. Moreover, the quad photoreceiver demonstrates a crosstalk of < -45 dB between the neighboring quadrants, which ensures an uncorrected direction sensing resolution of < 50 nrad. The sources of this primarily capacitive crosstalk are presented. © 2012 SPIE.
• Killow, C., Bogenstahl, J., Perreur-Lloyd, M., Robertson, D., Ward, H., Cervantes, F., & Steier, F. (2008). Design and construction of the LISA technology package optical bench interferometer. In 11th Marcel Grossmann Meeting on Recent Developments in Theoretical and Experimental General Relativity, Gravitation and Relativistic Field Theories, MG 2006.
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  The LISA Technology Package (LTP) is an experiment that will fly on board the space based gravitational wave demonstrator mission, LISA Pathfinder. The LTP optical bench interferometer will be used to monitor the changes in separation between two test masses with a sensitivity of 10pm/√Hz in the measurement band of 3mHz to 30mHz. The precision alignment processes required to manufacture this ultra-stable, space-worthy optical bench are described and the design and construction status presented. © 2008 World Scientific Publishing Co. Pte. Ltd.
• García Marín, A. F., Bogenstahl, J., Guzmán Cervantes, F., Steier, F., Reiche, J., Skorupka, S., Wand, V., Nofrarias, M., Sanjuan, J., Jennrich, O., Heinzel, G., & Danzmann, K. (2006). Interferometric characterization of the optical window for LISA Pathfinder and LISA. In AIP Conference Proceedings.
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  In LISA Pathfinder and LISA the position fluctuations of drag free test masses will be determined interferometrically to picometer precision. To this end, laser light is brought to interference on an ultra stable optical bench after being reflected on the test mass, which needs to be in an ultra-high vacuum. The present baseline for both missions includes a separate vacuum enclosure for each test mass, so that the sensing laser beam has to pass through an optical window. This window is therefore a transmissive element in the interferometer and its associated pathlength fluctuations are potentially significant. We have selected an athermal glass that should minimize the thermally induced pathlength changes. Several prototype windows, both mounted and unmounted, have been produced and characterized. The pathlength sensitivity to both temperature fluctuations and temperature gradients has been measured with a dedicated interferometer prototype. We have also compared the long-term stability of the LISA Technology Package interferometer when an optical window is present in the optical path to the situation without window. Finally, glass samples have been radiated and the absorption in the glass after the radiation tests has been measured to be negligible at the wavelength of interest (1064 nm). We present here the results of our measurements, which indicate that using a window does not influence the interferometer performance. © 2006 American Institute of Physics.
• Heinzel, G., Bogenstahl, J., Braxmaier, C., Danzmann, K., Garcia, A., Guzman, F., Hough, J., Hoyland, D., Jennrich, O., Killow, C., Robertson, D., Sodnik, Z., Steier, F., Ward, H., & Wand, V. (2006). Interferometry for the LISA technology package LTP: An update. In Journal of Physics: Conference Series.
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  This paper gives an update on the status of the LISA technology package (LTP) which is to be launched in 2009 by ESA as a technology demonstration mission for the spaceborne gravitational wave observatory LISA. The dominant noise source in the interferometer prototype has been investigated and improved such that it is now comfortably below its budget at all frequencies. © 2006 IOP Publishing Ltd.
• Killow, C., Bogenstahl, J., Perreur-Lloyd, M., Robertson, D., Steier, F., Ward, H., & Guzmán Cervantes, F. (2006). Construction of the LTP optical bench interferometer. In AIP Conference Proceedings, 873.
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  The LISA Technology Package is an experiment that will fly onboard the LISA demonstrator mission, LISA Pathfinder. Integral to the LISA Technology Package and to LISA are ultra-stable optical benches capable of measuring inertial test mass positions to below 10pm/Hz over 1000 second timescales. Aspects of the current design and construction of the LISA Technology Package optical bench interferometer are described. © 2006 American Institute of Physics.
• Wand, V., Heinzel, G., Danzmann, K., & Guzmán, F. (2006). LISA phasemeter development. In AIP Conference Proceedings.
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  The baseline concept of LISA had been developed within an industrial investigation by Astrium/EADS and is being reviewed since 2005 in an ongoing formulation phase. One of the most important key technology development issues remains the demonstration of the interferometric readout of the main science measurement. This includes the hardware development of an appropriate Phase Measurement System (PMS). We present the status of our work concerning the design and implementation of a LISA-like PMS with particular emphasis on the hardware development based on Field Programmable Gate Arrays (FPGA's) as main technology platform and we report on our first results demonstrating the performance of the PMS with synthetic signals. © 2006 American Institute of Physics.

Creative Productions

• Guzman, F. (2016. First direct detection of gravitational waves. Televised interview (in Spanish) to the international news channel NTN24(www.ntn24.com), Washington, DC, USA, February 11th, 2016.. International news channel NTN24. Washington DC, USA.
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  First direct detection of gravitational waves. Televised interview (in Spanish) to the international news channel NTN24(www.ntn24.com), Washington, DC, USA, February 11th, 2016.