Bias Errors in Fiber-Optic Gyroscopes Caused by Lyot Depolarizers

摘要

Fiber optic gyroscopes (FOG) are by now commercially produced, nevertheless these sensors sometimes show bias errors, i.e. the offset rotation rate varies with temperature and other environmental parameters. An optical bias error source will be described, which may occur in every FOG with a Lyot depolarizer and an integrated optics circuit (IOC). Simulations deliver insight into the processes and illustrate the dependencies. Experimental results confirm the theoretical predictions very well. Usually low coherence light sources like superluminescent diodes are used in FOGs to avoid undesirable interferences between error signals from reflections etc.. Multipath propagation in the sensor takes place. The resulting path differences between the occurring light waves and their polarization determine whether they interfere or not. The analysis of the standard FOG design leads to the identification of a bias error source which has not been described yet. Although high precision FOGs usually use a polarization maintaining coil, the part in front of the IOC is in most cases made of single mode fiber to avoid a costly polarization maintaining coupler. The remaining birefringence of the single mode fiber is temperature dependent and can lead to signal fading at the IOC which acts as a polarizer. For this reason a Lyot depolarizer is introduced behind the light source to reduce the degree of polarization. The index of refraction of the IOC is different from that of the fiber so that Fresnel reflections at the interfaces occur. To reduce those the front and back face are angled polished. Due to the angle the already reduced reflections from the two divided beams additionally possess an optical path difference. But if this path difference matches the path differences gained in the depolarizer within the coherence length a rotation rate equivalent modulated phase is produced. The simulations have been carried out with an advanced simulation tool for modeling the light propagation in interferometric fiber optic sensors with low coherence light sources. Underlying is a refined method which takes polarization and coherence effects into account. The light source parameters are represented by the power spectrum resp. the coherence function. All signals and perturbations and interferences between them are considered with the corresponding degree of coherence. Active phase modulation in the FOG is modeled, too. The parameters of the incorporated materials and components like the fiber are taken into account as a function of temperature so that a full temperature dependent model of the FOG can be built. Simulation results of the errors to expect will be shown. Measurements with our fiber optic gyro show temperature dependent bias errors of up to 20/h compared to bias variations of less than 0.1/h without depolarizer. Methods to avoid this problem are addressed.