THERMODYNAMIC PHASE NOISE IN FIBER INTERFEROMETERS

Phase noise due to thermodynamic fluctuations in the optical path leng th is evaluated in this paper for basic fibre interferometers. In Mach -Zehnder and Michelson interferometers, where the temperature phase fl uctuation (TPF) is that intrinsic to the fibre, this noise has been re ported to be comparable to shot noise and a possible limit to sensor s ensitivity in practical cases. We show that in Sagnac interferometers, used in fibre gyro and in Faraday current sensors, the TPF noise is d ecreased with respect to that intrinsic to the fiber because propagati on in the same optical path leads to a correlation of the phase fluctu ations. In addition, we show that in Fabry-Perot and ring resonators, as multiple reflections increase the effective path length, TPF noise is enhanced and can be dominant over shot noise even for moderate fibr e lengths.