Noise and Bias Error Due to Polarization Coupling in a Fiber Optic Gyroscope

This paper reports a comprehensive model of the noise and drift induced by polarization coupling in a fiber optic gyroscope (FOG) interrogated with a laser of arbitrary linewidth. It includes the effects of dynamic phase biasing, a realistic description of the laser phase noise and polarization-dependent loss. This model yields concise analytical expressions for the noise and drift dependencies on the laser linewidth, the fiber length and holding parameter h, and the fractional power launched into the unwanted polarization at the input to the sensing coil. For all realistic FOG parameter sets, the polarization-coupling noise is found to be insignificant. For a 1-km coil, a typical holding parameter (h = 10-5) and push-pull modulation, the drift is only ~1 μ rad up to a linewidth of~100 MHz, which is far lower than previously believed. The drift decreases to even lower values for larger linewidths. Experimental measurements of the noise and drift in a 150-m FOG and their dependence on laser linewidth support these predictions. Birefringence modulation can be brought to bear to reduce this residual drift enough to meet the requirement for aircraft inertial navigation. Ultimately, this analysis shows that low-polarization coupling error can be obtained without the use of a broadband source.