Polarization dependence of response functions in 3×3 Sagnac optical fiber current sensors

We show theoretically that the response functions of a lossless Sagnac optical fiber current sensor based on a 3×3 coupler fundamentally depend on the polarization state of light entering the coupler even for systems with no linear birefringence. For a lossless, zero-birefringence system the desired response functions, sinusoids separated by 120° phase shifts, are obtained only for circularly polarized input light. The response functions for linearly polarized and depolarized inputs are sinusoids separated by 180° and yield zero-slope small-signal responses; in addition, two outputs are degenerate, so the responses are similar to those observed in 2×2 systems. Thus, 3×3 Sagnac systems offer no advantage over 2×2 systems for linearly polarized input light. The predicted polarization dependence of 3×3 Sagnac response functions is experimentally confirmed. This result establishes the need for increased system complexity in 3×3 Sagnac current sensors since polarization control optics are required to provide the proper input polarization