Optimization of the Angular Random Walk in Laser-Driven Fiber-Optic Gyroscopes

This paper investigates whether the angular random walk (ARW) of a fiber-optic gyroscope (FOG) driven by a broadened laser can be minimized, or equivalently the signal-to-noise ratio maximized, by optimizing the phase bias of the FOG, and by how much. Expressions of the ARW's dependence on the phase bias and detected power are derived that take into account the main sources of noise of a FOG (namely detector noise, shot noise, relative intensity noise, and backscattering noise) for both sine-wave and square-wave modulation and demodulation. For backscattering noise, which dominates at usual detected powers, these hitherto unknown dependences are calculated using a published numerical model. For completeness and comparison, the more common case of a FOG interrogated with broadband light is also treated. In a FOG driven by a broadened laser, the answer is that for either modulation waveform the reduction in ARW afforded by biasing optimally is small (~1 dB). With broadband light, optimum biasing improves the ARW also only slightly with sine-wave modulation (~1 dB), but significantly with square-wave modulation, by up to ~10 dB (this value increases as the square root of the detected power). These predictions are in excellent agreement with the dependences measured in a 3.24-km FOG biased with square-wave modulation. At the maximum available detected power, an ARW reduction of 3 dB was measured with a broadband light source, and 0.7 dB with a broadened laser.