A non-linear error signal extraction technique for length control of a Fabry-Perot cavity

We present a non-linear signal extraction technique which provides a signal proportional to the length of a FP cavity with an rms motion larger than o ne laser wavelength. This method, based on a mechanical or frequency modula tion plus a suitable fringe counting technique, can be used in a closed loo p configuration allowing to reduce the length variation within one resonanc e peak of the cavity. This is a starting point for usual linear servo-loops . The principle of operation is described. The results of a computer simula tion are confirmed by an experimental test on a bench top interferometer. A possible application is the guidance into lock of a suspended FP cavity, s uch as those used for long baseline interferometric antennas like VIRGO or LIGO. (C) 1999 Published by Elsevier Science B.V. All rights reserved.