Introduction

All laser interferometers rely on measuring the strain in space caused by a gravitational wave, sensitivities of the order of 10–22 over millisecond timescales being required to allow a good probability of detection.

In principle the strain as monitored by the change in separation of two test masses hung as pendulums can be measured against the wavelength of light from a stable source, but the degree of wavelength or frequency stability required of the source is unreasonably high. It is much more conceivable to measure the distance between test masses along an arm with respect to the distance between similar masses along a perpendicular arm. This is particularly appropriate since the interaction of a gravitational wave is quadrupole in nature and so can cause an opposite sign of length change in the two arms. The measurement of a differential length change of this type when performed by interferometry puts much less demand in principle on the frequency stability of the illuminating laser light – since a Michelson interferometer is insensitive to changes in the wavelength of the light used if the path lengths are equal. However, in practice a fairly high degree of frequency stability is required. In the case of optical delay lines in the arms of a Michelson interferometer this is a result of the difficulty in achieving equal path lengths and of some light being scattered back early without completing the full number of reflections (Billing et al, 1983).