Thermal noise in a fiber optic sensor

The mean intrinsic thermal noise in a fiber optic interferometric sensor is calculated from the equipartition theorem. The spectral density of the fluctuating force is the mechanical equivalent of the well-known thermally induced voltage noise (Johnson noise) in electrical resistors. The consequent phase noise generated by this fluctuating force in a high-sensitivity fiber optic interferometric seismic sensor is found to dominate the phase noise of present opto-electronic interferometric demodulators, thus setting the limit for minimum detectable vibration amplitudes. This effect is identical to that found to limit the sensitivity of ‘‘mirrored galvanometers’’ as the optical leverage of the transducer was increased. It is shown that this contribution to the sensor noise is reduced when the sensor is used above resonance as a seismometer rather than below resonance as an accelerometer. The further increase in noise-limited detection thresholds introduced by aliasing of the noise concentrated about the resonance frequency in time-sampled accelerometers is emphasized.