High-Birefringent Fiber Loop Mirror Sensors: New Developments

The fiber loop mirror based on a ring configuration is one of the devices in optical fibers most used in communications and sensors (Mortimore, 1988). The device is formed when the two output ports of a directional coupler are spliced. In this configuration, the two waves at the coupler outputs travel in opposite directions but following the same optical paths in an optical fiber ring, which assures constructive interference as the waves re-enter the coupler. Thus, all light is reflected to the input port containing losses essentially in the fiber, the splice region and the coupler. Due to this “mirror” characteristic, the device is frequently used in the formation of resonant cavities in optical fiber lasers (Urquhart, 1989). A structure of this kind, with interesting properties, occurs when the ring contains a section of fiber with high-birefringence. When this happens, an interference pattern is generated. It depends only on the fiber birefringence and length, being independent of the remaining ring extension (Fang & Claus, 1995).