Phase-Conjugate Fiber-Optic Gyros

Abstract

Phase-conjugate fiber-optic gyros (PCFOG’s) use phase conjugation to compensate for reciprocal phase changes due to thermal and mechanical effects on the fiber, while at the same time. allowing for the measurement of the nonreciprocal phase shift produced by rotation.1,2 Where the best standard fiber-optic gyros require polarization-preserving fibers and couplers to avoid polarization scrambling that is a source of noise and signal fading, the PCFOG can avoid this problem by using polarization-preserving phase conjugation.3 This has the advantage of allowing for the use of inexpensive nonpolarization preserving, and even multimode fibers and components.3-5 Our first objective was to demonstrate that the PCFOG is sensitive to the nonreciprocal phase shift produced by the Sagnac effect and can be used to sense rotation. A proof of concept experiment was set up for this objective using an externally-pumped crystal of barium titanate as the phase conjugator. This experiment, reported in Ref. 6, provided the first demonstration of rotation sensing with a PCFOG. In this proof of concept demonstration the length of the fiber-optic coil, and therefore the sensitivity of the gyro, was limited by the coherence length of the laser. To solve this problem we set up a PCFOG consisting of a Michelson interferometer in which the light beams from two arms travel as clockwise and counterclokwise beams respectively, in the same fiber optic coil and reflect from the same self-pumped phase-conjugator. We reported the demonstration of rotation sensing with this PCFOG in Ref. 7. Again, since phase conjugation can correct for modal scrambling, a PCFOG can use multimode fibers. However, complete correction of modal scrambling requires a polarization-preserving conjugator,5 and the corresponding experimental setup of a PCFOG is complicated. To solve this problem we set up a PCFOG using a multimode fiber coil, a nonpolarization-preserving conjugator,4 and a spatial filter to discriminate against the portion of the light reflected by the conjugator that does not correct for modal scrambling. This experiment, reported in Ref. 8, provided the first demonstration of rotation sensing with a PCFOG using multimode fiber.