Robert E. Spero

LIGO: The Laser Interferometer Gravitational-Wave Observatory

The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project is to detect and study astrophysical gravitational waves and use data from them for research in physics and astronomy. LIGO will support studies concerning the nature and nonlinear dynamics of gravity, the structures of black holes, and the equation of state of nuclear matter. It will also measure the masses, birth rates, collisions, and distributions of black holes and neutron stars in the universe and probe the cores of supernovae and the very early universe. The technology for LIGO has been developed during the past 20 years. Construction will begin in 1992, and under the present schedule, LIGOs gravitational-wave searches will begin in 1998.

IMPROVED SENSITIVITY IN A GRAVITATIONAL WAVE INTERFEROMETER AND IMPLICATIONS FOR LIGO

Sensitivity enhancements in the laser interferometer gravitational wave observatory (LIGO) projects 40 m interferometer have been achieved through two major instrumental improvements. Improved vibration isolation has reduced the noise due to ground motion. New test masses with less mechanical dissipation were installed to lower the thermal noise associated with mirror vibrations. The minimum interferometer noise (square root of the spectral density of apparent differential displacement) reached 3 x 10^(-19) m/Hz^(1/2) near 450 Hz.

Overview of the LISA Phasemeter

The LISA phasemeter is required to measure the phase of an electrical signal with an error less than 3 μcycles/Hz over times scales from 1 to 1000 seconds. This phase sensitivity must be achieved in the presence of laser phase fluctuations 108 times larger than the target sensitivity. Other challenging aspects of the measurement are that the heterodyne frequency varies from 2 to 20 MHz and the signal contains multiple frequency tones that must be measured. The phasemeter architecture uses high‐speed analog to digital conversion followed by a digital phase locked loop. An overview of the phasemeter architecture is presented along with results for the breadboard LISA Phasemeter demonstrating that critical requirements are met.

Postprocessed time-delay interferometry for LISA

High-precision interpolation of LISA phase measurements allows signal reconstruction and formulation of time-delay interferometry (TDI) combinations to be conducted in postprocessing. The reconstruction is based on phase measurements made at approximately 10 Hz (for a 1 Hz signal bandwidth) at regular intervals independent of the TDI delay times. Interpolation introduces an error less than 1x10{sup -8} with continuous data segments as short as 2 s in duration. The 10 Hz sampling rate represents an increase from the 2 Hz sampling rate needed for the original implementation of TDI. The advantages of this technique include increased flexibility of the data analysis and significantly simplified hardware.

Experimental Demonstration of Time-Delay Interferometry for the Laser Interferometer Space Antenna

We report on the first demonstration of time-delay interferometry (TDI) for LISA, the Laser Interferometer Space Antenna. TDI was implemented in a laboratory experiment designed to mimic the noise couplings that will occur in LISA. TDI suppressed laser frequency noise by approximately 10(9) and clock phase noise by 6×10(4), recovering the intrinsic displacement noise floor of our laboratory test bed. This removal of laser frequency noise and clock phase noise in postprocessing marks the first experimental validation of the LISA measurement scheme.

Gravitational Wave Detectors Using Laser Interferometers and Optical Cavities: Ideas, Principles and Prospects

In these two lectures we will discuss principles underlying the development of some laser interferometer gravitational radiation detectors, some of the techniques being devised to overcome the many experimental problems, and results obtained so far in experimental work at the University of Glasgow and the California Institute of Technology. We also hope to give some indication about possible long-term prospects, as well as presenting some new ideas bearing on these problems.

Laser link acquisition demonstration for the GRACE Follow-On mission

We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 μrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 μrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.

Progress in interferometry for LISA at JPL

Recent advances at JPL in experimentation and design for LISA interferometry include the demonstration of time delay interferometry using electronically separated end stations, a new arm-locking design with improved gain and stability, and progress in flight readiness of digital and analog electronics for phase measurements.

Optical Cavity Laser Interferometers for Gravitational Wave Detection

Most of the techniques being developed for detection of gravitational radiation involve sensing the small strains in space associated with the gravitational waves by looking for changes in the apparent distance between two (or more) test masses. In many of the experimental searches performed so far the detectors consisted of massive aluminium bars, the metal near the ends of the bars acting as the test masses, and impulsive strains induced in the bars were searched for. Thetrain sensitivity of such experiments has been in the range 10−16 to 10−18 for pulses of duration of order 1 millisecond, the limits usually being set by thermal noise in the bar, and transducer and amplifier sensitivity. Current predictions of gravitational waves to be expected from various types of astrophysical sources suggest that strain sensitivities some three orders of magnitude better than these are likely to be required for detection of gravitational wave bursts from known types of sources at a useful rate, although indecd signals may be present over a wide frequency range — from 10 kHz to 10−4 Hz or lower. (A good summary is given in the proceedings of a conference on “Sources of Gravitational Radiation” [1]). Work on bar gravity wave detectors is continuing; but an alternative approach is to use widely separated and nearly free test masses, and monitor changes in their separation by optical interferometry techniques. This method shows considerable promise for both high sensitivity and wide bandwidth and frequency coverage. At the sensitivity levels required absolute length measurements would be difficult, but a comparison of two baselines perpendicular to one another, which may be affected in opposite senses by a gravitational wave travelling in a suitable direction, provides a practical alternative. Early experiments of this type were carried out at Hughes Laboratories [2] using a simple Michelson interferometer to monitor separations between ree test masses suspended in vacuum. The displace- ment sensitivity of such an arrangement may be improved by causing the light in each arm of the interferometer to travel back and forth many times between mirrors attached to the test masses, and a multireflection system of this type using Herriott delay lines was proposed by R. Weiss [3]. Experimental work on multireflection Michelson interferometers for gravity wave detection has been carried out at MIT, the Max-Planck Institute at Munich, and the University of Glasgow.

A flight-like optical reference cavity for GRACE follow-on laser frequency stabilization

We describe a prototype optical cavity and associated optics that has been developed to provide a stable frequency reference for a future space-based laser ranging system. This instrument is being considered for inclusion as a technology demonstration on the recently announced GRACE follow-on mission, which will monitor variations in the Earths gravity field.