Optical Cavity Laser Interferometers for Gravitational Wave Detection

Abstract

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. The strain 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 indeed 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 three test masses suspended in vacuum. The displacement 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.

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