Scattering Matrix Analysis on the Use of a Wide-Band Laser Source in a Passive Fiber Rate Sensor

Summary

Recent results obtained by various laboratories experimenting with passive fiber optic rotation sensors have suggested that a wide-band laser source, provided it is not so broad that dispersion effects in sensor elements external to the fiber ring, such as the input-output coupler, become significant, can appreciably reduce low frequency random noise at the sensor output. This low-frequency noise is due to the time variation of the elements comprising the fiber ring’s scattering matrix model at a given source wavelength.

A question has arisen, additionally, as to whether the use of a fluctuating, or random, state of polarization (SOP) at each such source mode may further reduce the low-frequency noise. In this theoretical analysis we show that the use of a random SOP for each mode is not only unnecessary but, in general, gives rise to a nonreciprocal phase (NRP) shift between the coun-tertraveling beams in the fiber ring at each mode. We find instead that the use of a nonrandom SOP at each source wavelength via suitably oriented polarizers and analyzers eliminates the NRP.

Finally, we show that the resultant intensity signal at the sensor photo-detector exhibits a $1/\sqrt N$ suppression of the noise amplitude when a wide-band source with N modes is employed. This will occur provided there is sufficient spacing between the various modes so that the scattering matrix elements become uncorrelated with each other from one wavelength to the next.