In-line optical fiber-sensors based on low-loss semi-reflective in-fiber mirrors

This paper presents the design and fabrication of semi-reflective in-fiber mirrors and their usage for the realization of different miniature fiber sensors. The fabrication of a semi-reflective in-fiber mirror is based on the selective chemical etching and splicing of standard single-mode fibers (SMF). The mirror's reflectance can be set precisely in between any range between 0.1% and 9.5 %. The practical usability of the produced in-fiber mirrors was evaluated by the fabrication of an in-line temperature sensor and evanescent field refractive index (RI) sensor. A temperature sensor is an intrinsic type of in-line FPI, formed between an in-fiber mirror and a flat-cleaved optical fiber tip. As an example, a temperature sensor that was optimized within a range from 0 to 100 degrees C showed a temperature resolution better than 0.1 degrees C, and a repeatability better than 0.2 degrees C. The evanescent RI sensor was created by the splicing of a small-diameter SMF between two in-fiber mirrors and removing of the intermediate fiber-cladding by chemical etching. The effective index of the fundamental mode depended on the surrounding-medium RI, which was interrogated by a spectrally-resolved technique. A high sensitivity of 830 nm/RIU was measured at RI of 1.444.