Hanford Observatory Research Articles

Particle swarming of sensor correction filters

Reducing the impact of seismic activity on the motion of suspended optics is essential for the operation of ground-based gravitational wave detectors. During periods of increased seismic activity, low-frequency ground translation and tilt cause the Advanced LIGO observatories to lose ‘lock’, reducing their duty cycles. This paper applies modern global-optimisation algorithms to aid in the design of the ‘sensor correction’ filter, used in the control of the active platforms. It is shown that a particle swarm algorithm that minimises a cost-function approximating the differential root mean squared velocity between platforms can produce control filters that perform better across most frequencies in the control bandwidth than those currently installed. These tests were conducted using training data from the LIGO Hanford Observatory seismic instruments and simulations of the Horizontal Access Module Internal Seismic Isolation platforms. These results show that new methods of producing control filters are ready for use at LIGO. The filters were implemented at LIGO’s Hanford Observatory, and use the resulting data to refine the cost function.

Towards windproofing LIGO: reducing the effect of wind-driven floor tilt by using rotation sensors in active seismic isolation

Modern gravitational-wave observatories require robust low-frequency active seismic isolation in order to keep the interferometer at its ideal operating conditions. Seismometers are used to measure both the motion of the ground and isolated platform. These devices are susceptible to contamination from ground tilt at frequencies below 0.1 Hz, particularly arising from wind-pressure acting on building walls. Consequently, during LIGO’s first observing run both observatories suffered significant downtime when wind-speeds were above 7 m s−1. We describe the use of ground rotation sensors at the LIGO Hanford Observatory to correct nearby ground seismometers to produce tilt-free ground translation signals. The use of these signals for sensor correction control improved low-frequency seismic isolation and allowed the observatory to operate under wind speeds as high as 15–20 m s−1.

Improving astrophysical parameter estimation via offline noise subtraction for Advanced LIGO

The Advanced LIGO detectors have recently completed their second observation run successfully. The run lasted for approximately 10 months and lead to multiple new discoveries. The sensitivity to gravitational waves was partially limited by correlated noise. Here, we utilize auxiliary sensors that witness these correlated noise sources, and use them for noise subtraction in the time domain data. This noise and line removal is particularly significant for the LIGO Hanford Observatory, where the improvement in sensitivity is greater than 20%. Consequently, we were also able to improve the astrophysical estimation for the location, masses, spins and orbital parameters of the gravitational wave progenitors.

Suppression of thermal transients in advanced LIGO interferometers using CO2 laser preheating

In high optical power interferometric gravitational wave detectors, such as Advanced LIGO, the thermal effects due to optical absorption in the mirror coatings and the slow thermal response of fused silica substrate cause time dependent changes in the mirror profile. After locking, high optical power builds up in the arm cavities. Absorption induced heating causes optical cavity transverse mode frequencies to drift over a period of hours, relative to the fundamental mode. At high optical power this can cause time dependent transient parametric instability, which can lead to interferometer disfunction. In this paper, we model the use of CO2 laser heating designed to enable the interferometer to be maintained in a thermal condition such that transient changes in the mirrors are greatly reduced. This can minimize transient parametric instability and compensate dark port power fluctuations. Modeling results are presented for both single compensation where a CO2 laser acting on one test mass per cavity, and double compensation using one CO2 laser for each test mass. Using parameters of the LIGO Hanford Observatory X-arm as an example, single compensation allows the maximum mode frequency shift to be limited to 6% of its uncompensated value. However, single compensation causes transient degradation of the contrast defect. Double compensation minimise contrast defect degradation and reduces transients to less than 1% if the CO2 laser spot is positioned within 2 mm of the cavity beam position.

Identification and mitigation of Advanced LIGO noise sources

In order to increase the reach of the astrophysical searches, various sources of instrumental and environmental noise must be identified and ameliorated. Here we discuss efforts to understand the origin of noise manifested as short-duration bursts (glitches) and/or range-impacting features at LIGO Hanford. Several examples found at LIGO Hanford Observatory in O1 and O2 were identified including glitches due to an air compressor, ringing phone, airplanes, and an incorrect servo setting, and a decrease in detector sensitivity due to truck traffic.

Low‐Frequency Tilt Seismology with a Precision Ground‐Rotation Sensor

We describe measurements of the rotational component of teleseismic surface waves using an inertial high‐precision ground‐rotation sensor installed at the Laser Interferometer Gravitational‐Wave Observatory (LIGO) Hanford Observatory (LHO). The sensor has a noise floor of 0.4 nrad/√Hz at 50 mHz and a translational coupling of less than 1 μrad/m enabling translation‐free measurement of small rotations. We present observations of the rotational motion from Rayleigh waves of six teleseismic events from varied locations and with magnitudes ranging from M 6.7 to 7.9. These events were used to estimate phase dispersion curves that show agreement with a similar analysis done with an array of three STS‐2 seismometers also located at LHO.

Subtracting Tilt from a Horizontal Seismometer Using a Ground‐Rotation Sensor

Abstract We demonstrate the use of a high‐precision ground‐rotation sensor to subtract wind‐induced tilt noise in a horizontal broadband seismometer at frequencies above 10 mHz. The measurement was carried out at the LIGO Hanford Observatory using a low‐frequency flexure‐beam‐balance with an autocollimator readout and a T240 seismometer, located in close proximity to each other. Along their common horizontal axis, the two instruments show significant coherence below 100 mHz, which increases as a function of wind speed due to floor tilt induced by wind pressure on the walls of the building and the ground outside. Under wind speeds of 10–15 m/s, correcting the seismometer for measured ground rotation lowered the signal by a factor of ∼10, between 10 and 100 mHz. This article describes the instruments used, shows representative data for low and high wind speeds, and discusses the tilt subtraction and possible limitations.

Hydraulic external pre-isolator system for LIGO

The hydraulic external pre-isolator (HEPI) is the first six degrees of freedom active seismic isolation system implemented at the Laser Interferometer Gravitational Wave Observatory (LIGO). Implementation was first completed at the LIGO Livingston Observatory (LLO) prior to LIGOʼs fifth science run, successfully cutting down the disturbance seen by LLOʼs suspended optics in the two most prominent seismic disturbance bands, the microseism (0.1–0.3 Hz) and the anthropogenic (1–3 Hz) bands, by a factor of a few to tens. The improvement in seismic isolation contributed directly to LLOʼs much improved duty cycle of 66.7% and LIGOʼs triple coincident duty cycle of 53%. We report the design, control scheme, and isolation performance of HEPI at LLO in this paper. Aided by this success, funding for incorporating HEPI into the LIGO Hanford Observatory was approved and installation is currently underway.

Angular instability due to radiation pressure in the LIGO gravitational-wave detector

We observed the effect of radiation pressure on the angular sensing and control system of the Laser Interferometer Gravitational-Wave Observatory (LIGO) interferometer's core optics at LIGO Hanford Observatory. This is the first measurement of this effect in a complete gravitational-wave interferometer. Only one of the two angular modes survives with feedback control, because the other mode is suppressed when the control gain is sufficiently large. We developed a mathematical model to understand the physics of the system. This model matches well with the dynamics that we observe.

Accurate measurement of the time delay in the response of the LIGO gravitational wave detectors

We present a method to precisely calibrate the time delay in a long baseline gravitational-wave interferometer. An accurate time stamp is crucial for data analysis of gravitational wave detectors, especially when performing coincidence and correlation analyses between multiple detectors. Our method uses an intensity-modulated radiation pressure force to actuate on the mirrors. The time delay is measured by comparing the phase of the signal at the actuation point with the phase of the recorded signal within the calibrated data stream used for gravitational wave searches. Because the signal-injection path is independent of the interferometer's control system, which is used for the standard calibration, this method can be an independent verification of the timing error in the system. A measurement performed with the 4 km interferometer at the LIGO Hanford Observatory shows a 1 µs relative accuracy when averaging over 50 min. Our understanding of the systematic time delay in the detector response has reached the level of 10 µs.

Search for gravitational waves associated with the gamma ray burst GRB030329 using the LIGO detectors

We have performed a search for bursts of gravitational waves associated with the very bright Gamma Ray Burst GRB030329, using the two detectors at the LIGO Hanford Observatory. Our search covered the most sensitive frequency range of the LIGO detectors (approximately 80-2048 Hz), and we specifically targeted signals shorter than 150 ms. Our search algorithm looks for excess correlated power between the two interferometers and thus makes minimal assumptions about the gravitational waveform. We observed no candidates with gravitational wave signal strength larger than a pre-determined threshold. We report frequency dependent upper limits on the strength of the gravitational waves associated with GRB030329. Near the most sensitive frequency region, around 250 Hz, our root-sum-square (RSS) gravitational wave strain sensitivity for optimally polarized bursts was better than h_RSS = 6E-21 Hz^{-1/2}. Our result is comparable to the best published results searching for association between gravitational waves and GRBs.

Optimal combination of signals from colocated gravitational wave interferometers for use in searches for a stochastic background

This article derives an optimal (i.e., unbiased, minimum variance) estimator for the pseudo-detector strain for a pair of co-located gravitational wave interferometers (such as the pair of LIGO interferometers at its Hanford Observatory), allowing for possible instrumental correlations between the two detectors. The technique is robust and does not involve any assumptions or approximations regarding the relative strength of gravitational wave signals in the detector pair with respect to other sources of correlated instrumental or environmental noise. An expression is given for the effective power spectral density of the combined noise in the pseudo-detector. This can then be introduced into the standard optimal Wiener filter used to cross-correlate detector data streams in order to obtain an optimal estimate of the stochastic gravitational wave background. In addition, a dual to the optimal estimate of strain is derived. This dual is constructed to contain no gravitational wave signature and can thus be used as on off-source measurement to test algorithms used in the on-source observation.

Characterization of the LIGO 4 km Fabry–Perot cavities via their high-frequency dynamic responses to length and laser frequency variations

Recent measurements at the LIGO Hanford Observatory have confirmed the predicted high-frequency dynamic response of km scale Fabry–Perot cavities to length and laser frequency variations. The dynamic response functions have been exploited to determine a number of cavity parameters including the cavity length and the resonance width. A new technique based on a variation of these measurements has been utilized to measure the interferometer arm cavity lengths with a precision of 80 µm. We present an overview of these measurements and discuss how the dynamic field responses could be used to measure the cavity g factors which are related to the mirror radii of curvature.