Tip Fiber-Optic Intermodal Interferometer for Refractive Index Sensing

We propose an all-fiber-optic multimode interferometer for measuring liquid refractive index. The interferometer was manufactured by using the arc splice technique between standard single-mode fiber and optical microfiber. A maximum refractive index sensitivity of 112 dB/RIU is experimentally demonstrated. In addition, thermal effects exhibit low phase modulation (12 pm/°C) with minimal intensity variations (-10.7 × 10 -3 dB/°C); as a result, cross sensitivity around 0.05 × 10 -6 RIU/°C and 0.9 × 10 -6 RIU/°C is obtained for intensity and phase modulation. The stability analysis shows minimal wavelength and power variations, 0.01 nm and 0.27 dB respectively, for constant temperature and refractive index. Spatial frequency analysis was explored as an alternative method to measure liquid refractive index avoiding thermal effects.