Ultrasensitive temperature sensor and mode converter based on a modal interferometer in a two-mode fiber.

This paper presents an ultrasensitive temperature sensor and tunable mode converter based on an isopropanol-sealed modal interferometer in a two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers. The sensor provides high-sensitivity temperature sensing by taking advantages of TTMF, isopropanol and the Vernier-like effect. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as strong evanescent field on its surface. The temperature sensitivity of the sensor can be improved due to the high thermo-optic coefficient of isopropanol. The Vernier-like effect based on the overlap of two interference spectra is applied to magnify the sensing capabilities with a sensitivity magnification factor of 58.5. The temperature sensor is implemented by inserting the modal interferometer into an isopropanol-sealed capillary. The experimental and calculated results show the transmission spectrum exhibit blue shift with increasing ambient temperature. Experimental results show that the isopropanol-sealed modal interferometer provides a temperature sensitivity up to -140.5 nm/°C. The interference spectrum has multiple dips at which the input LP01 mode is converted to the LP11 mode. This modal interferometer acts as a tunable multi-channel mode converter. The mode converter that can be tuned by varying temperature and mode switch is realized.