High-sensitivity refractive index and temperature sensor based on cascaded dual-wavelength fiber laser and SNHNS interferometer

A novel differential intensity-measurement high-sensitivity refractive index (RI) sensor based on cascaded dual-wavelength fiber laser and single-mode-no-core-hollow-core-no-core-single-mode (SNHNS) structure is proposed and demonstrated. The sensing unit consists of one uniform fiber Bragg grating (FBG) and an SNHNS structure as all-fiber interferometer filter. The dual-wavelength fiber laser has a ring cavity composed of two FBGs with central wavelengths of 1550.10nm and 1553.61nm. Through monitoring the wavelength shift and the output power difference of the dual-wavelength fiber laser, the simultaneous measurement for RI and temperature is realized. In our experiment, the proposed fiber laser sensor exhibits high RI sensitivities of -193.1dB/RIU and 174.8dB/RIU in the range of 1.334-1.384. The relative variation of output power at the two FBG wavelengths shows a higher RI sensitivity of -367.9dB/RIU with better stability, which is greater than the traditional modal interferometer structure. Meanwhile, the temperature sensitivity of the proposed sensor is 8.53 × 10-3nm/°C, and the changes of laser output power caused by temperature are -0.223dB/°C and 0.215dB/°C.