P. R. Saulson

Thermal noise in interferometric gravitational wave detectors due to dielectric optical coatings

We report on thermal noise from the internal friction of dielectric coatings made from alternating layers of Ta2O5 and SiO2 deposited on fused silica substrates. We present calculations of the thermal noise in gravitational wave interferometers due to optical coatings, when the material properties of the coating are different from those of the substrate and the mechanical loss angle in the coating is anisotropic. The loss angle in the coatings for strains parallel to the substrate surface was determined from ringdown experiments. We measured the mechanical quality factor of three fused silica samples with coatings deposited on them. The loss angle, ||(f), of the coating material for strains parallel to the coated surface was found to be 4.2 ± 0.3 × 10−4 for coatings deposited on commercially polished slides, and 1.0 ± 0.3 × 10−4 for a coating deposited on a superpolished disc. Using these numbers, we estimate the effect of coatings on thermal noise in the initial LIGO and Advanced LIGO interferometers. We also find that the corresponding prediction for thermal noise in the 40 m LIGO prototype at Caltech is consistent with the noise data. These results are complemented by results for a different type of coating, presented in a companion paper.

Brownian motion of a mass suspended by an anelastic wire

The theory of elasticity and the fluctuation‐dissipation theorem were used to calculate the thermal noise power spectrum of an extended mass suspended by an anelastic wire. The implications for interferometric detectors of gravitational waves are discussed.

The PyCBC search for gravitational waves from compact binary coalescence

We describe the PyCBC search for gravitational waves from compactobject binary coalescences in advanced gravitational-wave detector data. The search was used in the first Advanced LIGO observing run and unambiguously identified two black hole binary mergers, GW150914 and GW151226. At its core, the PyCBC search performs a matched-filter search for binary merger signals using a bank of gravitational-wave template waveforms. We provide a complete description of the search pipeline including the steps used to mitigate the effects of noise transients in the data, identify candidate events and measure their statistical significance. The analysis is able to measure false-alarm rates as low as one per million years, required for confident detection of signals. Using data from initial LIGO’s sixth science run, we show that the new analysis reduces the background noise in the search, giving a 30% increase in sensitive volume for binary neutron star systems over previous searches.

Brownian motion of a torsion pendulum with internal friction

Abstract We constructed a torsion pendulum in which the dissipation is dominated by internal friction in the fiber. We compare the measured thermal noise spectrum with the prediction from the fluctuation-dissipation theorem. The agreement is excellent. The spectrum exhibits an approximately 1 f slope below resonance. We discuss the implications for interferometric gravitational wave detectors.

High quality factor measured in fused silica

We have measured the mechanical dissipation in a sample of fused silica drawn into a rod. The sample was hung from a multiple-bob suspension, which isolated it from rubbing against its support, from recoil in the support structure, and from seismic noise. The quality factor, Q, was measured for several modes with a high value of 57.1±0.1 million found for the mode number 2 at 726 Hz. This result is about a factor of 2 higher than previous room temperature measurements. The measured Q was strongly dependent on handling, with a pristine flame-polished surface yielding a Q three to four times higher than a surface which had been knocked several times against a copper tube.

Mechanical quality factors of fused silica resonators

We have measured the room temperature resonant quality factors Q for a large set of resonant modes for each of several fused silica blocks and one fused silica cylinder. The modal frequencies ranged from 7.8 to 50 kHz. Our best results were obtained when the samples were suspended in a single loop of fine wire. For every sample tested we have measured at least one mode with Q⩾107. Our highest Q for any sample measured was (1.86±0.06)×107 for a block resonating at 26.4 kHz. Our tests included several different types of high purity fused silica, but could not detect any significant differences in mechanical loss. We believe our Q measurements are limited by losses due to our suspensions, as Q depends strongly on the shape of each resonant mode and the proximity of each mode’s nodal regions to the wire suspension points.

Very high quality factor measured in annealed fused silica

We present the results of quality factor measurements for rod samples made of fused silica. To decrease the dissipation, we annealed our samples. The highest quality factor that we observed was Q = (2.03 ± 0.01) × 108 for a mode at 384 Hz. This is the highest published value of Q in fused silica measured to date.

The LSC glitch group: monitoring noise transients during the fifth LIGO science run

The LIGO Scientific Collaboration (LSC) glitch group is part of the LIGO detector characterization effort. It consists of data analysts and detector experts who, during and after science runs, collaborate for a better understanding of noise transients in the detectors. Goals of the glitch group during the fifth LIGO science run (S5) included (1) offline assessment of the detector data quality, with focus on noise transients, (2) veto recommendations for astrophysical analysis and (3) feedback to the commissioning team on anomalies seen in gravitational wave and auxiliary data channels. Other activities included the study of auto-correlation of triggers from burst searches, stationarity of the detector noise and veto studies. The group identified causes for several noise transients that triggered false alarms in the gravitational wave searches; the times of such transients were identified and vetoed from the data generating the LSC astrophysical results.

Pendulum mode thermal noise in advanced interferometers: A comparison of fused silica fibers and ribbons in the presence of surface loss

The use of fused-silica ribbons as suspensions in gravitational wave interferometers can result in significant improvements in pendulum mode thermal noise. Surface loss sets a lower bound to the level of noise achievable, at what level depends on the dissipation depth and other physical parameters. For LIGO II, the high breaking strength of pristine fused silica filaments, the correct choice of ribbon aspect ratio (to minimize thermoelastic damping), and low dissipation depth combined with the other achievable parameters can reduce the pendulum mode thermal noise in a ribbon suspension well below the radiation pressure noise. Despite producing higher levels of pendulum mode thermal noise, cylindrical fiber suspensions provide an acceptable alternative for LIGO II, should unforeseen problems with ribbon suspensions arise.

Dissipation mechanisms in pendulums and their implications for gravitational wave interferometers

We present a precise test of the dissipation dilution effect in the transverse wire mode quality factors in pendulums. Several excess loss mechanisms are diagnosed in detail, with special attention paid to dissipation from sliding friction.