DEMANDS ON LASER PURITY IN AN INTERFEROMETRIC GRAVITATIONAL WAVE DETECTOR

Abstract
In the signal frequency window (500 to 3000 Hz) inside which the Michelson interferometer is to detect gravitational radiation, it must at least be able to resolve 10^-9 of a fringe. This requires a laser illumination of extremely high purity. Noise contributions due to laser fluctuations in power and in beam geometry can be kept sufficiently small by proper control of the operating point, and by the use of a mode-selective resonator. The strong demands on frequency stability arise from two deficiencies of the interferometer : (i) a residual path difference between the two arms requires δ[MATH] < 5.10^-2 Hz/√Hz ; (ii) the interference of scattered light with the main beam, with large path differences ƊL, may require δ[MATH] < 3.10^-3 Hz/√Hz. A rather conventional frequency control (with feed back to the laser cavity) is expected to allow δ[MATH] < 3.10^-2 Hz/√Hz. A totally independent second control loop uses the interferometer arms as reference, and corrects the light phase (and thus the frequency) with a Pockels cell, after the light has left the laser. A stability of δ[MATH] < 10^-4 Hz/√Hz may eventually be possible. The noise due to scattered light may be reduced further by a scheme of breaking the coherence between scattered light and main beam.