In-fiber Mach–Zehnder interferometer formed by large lateral offset fusion splicing for gases refractive index measurement with high sensitivity

A versatile, compact and sensitive in-fiber open cavity Mach–Zehnder interferometer (MZI) gas refractive index (RI) sensor formed by fusion splicing a short section of single-mode fiber (SMF) between two sections of single-mode fibers with a large intentional lateral offset is demonstrated. The fabrication is easy, safe, and cost effective. Gas or other materials can contact with the light signal in the totally open cavity of sensor directly and easily, thus the ambient refractive index (RI) change can be detected very quickly. We discussed the fabrication and operation of the sensor, and we successfully used the sensor to monitor changes of refractive index of air as a function of pressure and obtained a sensitivity of over 3402 nm/RIU (refractive index unit) with good linearity and repeatability.