All-glass extrinsic Fabry-Perot interferometer thermo-optic coefficient sensor based on a capillary bridged two fiber ends.

An all-glass extrinsic Fabry-Perot interferometer (EFPI) is demonstrated for thermal-optic coefficient (TOC) of water, glycerol, and their mixture (volume ratio of 1:1). The compensation for the thermal expansion of Fabry-Perot (FP) cavity is realized by assembling a glass capillary and optical fibers through a CO₂ laser welding. The thermal responses of EFPIs are tested in air at different cavity lengths of 578.6 μm, 911.7 μm, and 1520.3 μm, respectively. The corresponding refractive index errors induced by thermal expansion of FP cavity are negligible, which are demonstrated to be 4.33×10^-6  RIU/°C, 4.13×10^-6  RIU/°C, and 3.45×10^-6  RIU/°C when temperature increases from 20.03°C to 60.78°C. The thermal-optic coefficients of water, glycerol, and their mixture are measured to be -1.5×10^-4  RIU/°C, -2.3×10^-4  RIU/°C, and -2.0×10^-4  RIU/°C, respectively. Our study suggests a potential use of this sensor for TOC measurements of liquids with the advantages of low costs and robustness.