Asymmetrical in-fiber Mach-Zehnder interferometer for curvature measurement.

We demonstrated a compact and highly-sensitive curvature sensor based on a Mach-Zehnder interferometer created in a photonic crystal fiber. Such a Mach-Zehnder interferometer consisted of a peanut-like section and an abrupt taper achieved by use of an optimized electrical arc discharge technique, where only one dominating cladding mode was excited and interfered with the fundamental mode. The unique structure exhibited a high curvature sensitivity of 50.5 nm/m^-1 within a range from 0 to 2.8 m^-1, which made it suitable for high-sensitivity curvature sensing in harsh environments. Moreover, it also exhibited a temperature sensitivity of 11.7 pm/°C.