Pseudo-random-bit-sequence phase modulation for reduced errors in a fiber optic gyroscope.
Low noise and drift in a laser-driven fiber optic gyroscope (FOG) are demonstrated by interrogating the sensor with a low-coherence laser. The laser coherence was reduced by broadening its optical spectrum using an external electro-optic phase modulator driven by either a sinusoidal or a pseudo-random bit sequence (PRBS) waveform. The noise reduction measured in a FOG driven by a modulated laser agrees with the calculations based on the broadened laser spectrum. Using PRBS modulation, the linewidth of a laser was broadened from 10 MHz to more than 10 GHz, leading to a measured FOG noise of only 0.00073  deg/√h and a drift of 0.023  deg/h. To the best of our knowledge, these are the lowest noise and drift reported in a laser-driven FOG, and this noise is below the requirement for the inertial navigation of aircraft.
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.
Reducing polarization-fluctuation induced drift in resonant fiber optic gyro by using single-polarization fiber.
A novel hybrid single-polarization (SP) fiber ring resonator is demonstrated by using a polarization-maintaining coupler formed by splicing a section of SP fiber into the resonator. The SP fiber selectively eliminates the unwanted resonance by introducing high loss for the unwanted eigenstates of polarization in the resonator. The calculated result shows that this hybrid SP resonator is a good candidate for a tactical-grade performance gyro with a high environmental temperature stability. The experiment shows that the desired resonance in the resonator can keep an excellent stability in a wide temperature range, thus the temperature-dependent polarization-fluctuation drift in the resonant fiber optic gyro is sufficiently suppressed. As a result, a random walk coefficient of 0.08°/√h and a typical bias stability below 0.3°/h for an integration time of 300 s have been carried out.