In-fiber Mach–Zehnder interferometric refractive index sensors with guided and leaky modes

Refractometry based on a tapered bend-insensitive fiber Mach–Zehnder interferometer operating in both guided and leaky mode domains is investigated for the first time to our knowledge. We propose a novel approach that allows for refractive index measurement by monitoring power changes in leaky outer-cladding modes, and simultaneous phase-based temperature detection using inner-cladding modes, which remain guided. The proposed scheme is used to extend the operating range of a fiber-optic MZI refractive index sensor into the regime in which outer cladding modes become leaky, a domain previously inaccessible to similar sensors given the complex nature of the phase dependence exhibited by leaky modes on external refractive index. The experimental results agree well with theoretical predictions, and demonstrate the potential of in-fiber refractometers for high-sensitivity sensing over large measuring ranges.