Ultrastable in reflection photonic crystal fiber modal interferometer for accurate refractive index sensing

A compact in reflection modal interferometer consisting of a stub of large-mode area photonic crystal fiber (PCF) spliced to standard fiber is presented. In the splice, the voids of the PCF are fully collapsed allowing so coupling and recombining PCF core and cladding modes. The interferometer is highly stable over time and can be used for different applications. The measuring of refractive index in the 1.33-1.45 range with high sensitivity is demonstrated. Sensing applications based on refractive index changes are also feasible.