We propose a refractive index insensitive temperature sensor based on hollow annular core fiber (HACF) Mach-Zehnder interferometer (MZI). The HACF is composed of a large-diameter air hole, an annular core around the air hole and a cladding. The MZI is fabricated by inserting a short section of the HACF between two short multimode fibers, and the interference occurs between the light beams transmitting along the air hole and the annular core of the HACF. Experimental results show that the MZI is insensitive to external refractive index and has temperature sensitivity of 30 pm/°C.
A refractive index insensitive temperature sensor based on coaxial dual-waveguide optical fiber was proposed and demonstrated. The coaxial fiber contains a central core along the fiber axis and an annular core between the inner/outer claddings. By inserting the coaxial fiber in between two single mode fibers through core-offset splicing, cladding modes are excited at the splice point and therefore a modal Mach-Zehnder interferometer is achieved. The effective refractive index of the inner cladding mode is independent of the external refractive index due to the existence of the annular core. Owing to the large thermo-optic coefficient difference between the coaxial fiber's core and cladding, the modal interferometer has high temperature sensitivity. Such an interferometer is extremely suitable for temperature measurement in wet or liquid environment.
A novel integrated fiber-optic sensor based on a symmetrical twin-core fiber was proposed and demonstrated as a strain
sensor. By heating and tapering the symmetrical twin-core fiber, the light from one core can be partly or completely
coupled into the other core in the coupled zone as the twin-core fiber was pulling. By detecting the output power of any
one core, the strain applied on the twin-core fiber can be detected. The output power of twin core fiber versus the applied
strain was experimentally investigated, and the results showed that the twin-core fiber can be used as a strain sensor.
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