Fiber-optic wave retarders made from a short section of polarization-maintaining (pm) fiber are crucial components of interferometric fiber-optic current sensors for high-voltage substations [1]. Another potential application of such retarders is in chiral spectroscopy, e. g. for the detection of specific molecules. In a fiber-optic current sensor, as considered here, the magnetic field of the current introduces a differential optical phase shift between left and right circularly polarized light waves during their round trip through a fiber coil that encloses the current conductor (Faraday-effect). Closed-loop fiber gyroscope technology is used to measure the current-induced optical phase shift (Fig. 1). The phase retardation of the retarder which generates the circular waves and its temperature dependence affect the sensor scale factor and are therefore critical to the sensor performance. Using an appropriately designed retarder (Fig. 2) it is possible to compensate for the temperature dependence of the Faraday-effect [1] that is 0.7% per 100°C. The overall retardation can be substantially influenced by splice joints - especially if short beat length fiber is used as fusion splicing modifies the fiber birefringence in the proximity of the joints. Conventional methods to determine the phase and/or group birefringence of pm fibers [2–3] are generally applied to long and homogeneous fiber segments. The alternative method presented here is ideally suited for short fiber sections (of a few millimeters in length) acting as wave retarders and also accounts for the influence of splice joints.
展开▼
