Effect of laser phase noise in Sagnac interferometers

A theoretical analysis of the responsivity and the noise caused by backscattering in a Sagnac interferometer used as a sensor for reciprocal measurands, such as acoustic waves, is presented. Both Rayleigh backscattering and reflections from splices are taken into account. The noise power is found to increase proportionally to the source coherence time, and a noise equivalent phase shift in the range of 0.1*10/sup -6/ rad r.m.s./Hz/sup 1/2/ is predicted for typical fibers and diode lasers. Experimentally, a noise equivalent phase shift of 2.5*10/sup -7/ rad r.m.s./Hz/sup 1/2/ at 10 kHz was observed, with a detector current of 3 mu A. >

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