▎ 摘　　要

A robust graphene-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a pair of 3-dB long-period fiber gratings is proposed and demonstrated for ammonia (NH3) gas sensing. The sensor operates on the principle of changing the phase of the cladding mode of a fiber through changing the conductivity of the graphene coating by adsorbed NH3 molecules, which gives rise to a shift in the interference spectrum with an amount that depends on the gas concentration. Our experimental sensor shows a sensitivity of similar to 3 pm/ppm for a gas concentration from similar to 10 to similar to 180 ppm. The proposed in-fiber MZI can serve as a generic platform for the development of fiber sensors and devices that incorporate two-dimensional materials.