TCF-MMF-TCF fiber structure based interferometer for refractive index sensing

•New structure: TCF-MMF-TCF.•Low cost: the special optical fiber is not used in the experiments.•High sensitivity: in the use of ordinary fiber case, we get high sensitivity.•The sensor keeps low dependence on temperature before and after etching.

A liquid refractive index (RI) sensor based on in-fiber Mach–Zehnder interferometer (MZI) by sandwiching multi-mode fiber (MMF) between two short sections of thinned core fiber (TCF) is proposed in this paper. The first section of TCF excites the high-order modes and the second section TCF couples the core mode and high-order modes into lead-out SMF to form intermodal interference. The sensor with MMF length of 20 mm and TCFs length of 1 mm was fabricated. The transmission spectrum variation of the sensor with respect to surrounding refractive index (SRI) has been studied by experiment. The results show that the central wavelength of dips/peaks shifting had a good linearity with SRI. The RI sensitivity of the sensor is 130.00 nm/RIU over the RI range of 1.3333–1.4182. The RI sensitivity increase to 433.60 nm/RIU after etching the MMF cladding of the sensor. The sensor keeps low dependence on temperature before and after etching.