Ultra-sensitive temperature sensor based on PDMS filled Fabry-Perot cavity and air-bubble Fabry-Perot cavity in parallel

Highly sensitive fiber optic temperature sensors are designed and fabricated using the Vernier Effect (VE) and First-order Harmonic Vernier Effect (FHVE). The sensor comprises two Fabry-Perot cavities (F-P) in parallel. Single-mode fiber is fused together with a small segment of capillary, and then polydimethylsiloxane (PDMS) is filled in the capillary to form a sensing F-P cavity. The reference F-P cavity is composed of air-bubble inside the single-mode fiber etched by femtosecond laser. Two reference interferometers FPI1 and FPI2, as well as two sensing interferometers FPI3 and FPI4, were prepared using this method. The free spectral ranges (FSR) of FPI1, FPI2, FPI3, and FPI4 are 14.09 nm, 7.80 nm, 7.01 nm, and 6.48 nm, respectively. The temperature sensitivities of FPI3 and FPI4 are 2.2622 nm/degrees C and 2.4958 nm/degrees C, respectively. FPI1 and FPI2 are almost insensitive to temperature. FPI3 and FPI2 have similar FSR, and they generate traditional VE sensor in parallel, with a temperature sensitivity of 20.41 nm/degrees C, which is about 9.02 times that of FPI3. The FSR1 of FPI1 is approximately twice that of FPI3, and they generate FHVE1 in parallel. The temperature sensitivity of FHVE1 reaches -99.66 nm/degrees C, which is 44.05 times that of FPI3 and 4.88 times that of VE1. This is currently the highest known temperature sensitivity. In addition, the FSR1 of FPI1 is also approximately twice that of FPI4. Parallel connection between FPI4 and FPI1 can also generate FHVE2, but the sensitivity is only 22.17 nm/degrees C, which is 4.5 times lower than the sensitivity of FHVE1. From this, it can be seen that in FHVE, the smaller the FSR detuning (FSRr - (i+1) FSRs) between the sensing cavity and the reference cavity, the greater the amplification factor of sensitivity. The materials of the proposed sensor are cheap, easy to fabricate, and has high sensitivity. It can be used to measure temperature in environments with high sensitivity requirements.