An optical fiber in-line Mach-Zehnder interferometer based on a pair of inner microholes in the fiber core is proposed and demonstrated for refractive index and temperature sensing. The microhole is fabricated by firstly inscribing a short line structure by femtosecond, and then discharging the short line area by using a fusion splicer. The incident light beam on the first microhole is divided into two parts, one remains traveling in the fiber core and the other is directed into the cladding and then collected by the second microhole after multiple reflections at the air/cladding interface. The two parts recombines in the fiber core, thus forming a Mach-Zehnder interferometer. By controlling the separation of the two microholes, transmission spectrum with different free spectral range can be obtained. The sensitivities achieved for refractive index and temperature are similar to 68.2 nm/ RIU and 57.8 pm/degrees C, respectively. Such a device is robust in structure, easy in fabrication, and convenient in operation.
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