Moisture-Induced Bias Drift in a Fiber-Optic Gyroscope

In a recent publication we reported a theoretical analysis and experimental validation of the strain induced by moisture along the fiber of the sensing coil in a fiber-optic gyroscope (FOG). This strain induces a slow nonreciprocal phase shift in the coil, which results in an undesirable drift in the FOG output. This model accounted for the mechanical effect of the fiber-coating expansion under moisture, as well as the slow diffusion of this moisture from the environment into the coil. In this new study, we extend this earlier work and present a comprehensive model of the drift generated in the FOG output by this moisture-induced strain. Measurements were also performed on a gyro-triad utilizing three compact sensing coils potted in epoxy and unprotected from moisture (unlike under normal operation, where they would be protected by suitable packaging). Under normal operating conditions (dry coils and constant temperature), each gyro had a bias drift of ∼4 mdeg/h at an integration time of 17-22 h. The coils were subjected to a relative humidity that was swept linearly from 25% to 80% and back to 25% over a period of ∼4 hours, four times in a row. These moisture ramps produced a measured peak-to-peak bias drift of 0.35 deg/h, in excellent agreement with the model prediction. This result implies that to reduce this drift contribution to the normal value, the moisture variations must be kept below about ±0.3%. It is clear that the effect of humidity cannot be underestimated, and that gyro coils require protection from humidity.