Fiber-optic temperature sensors with light waves as the carrier and optical fibers as the medium are used to transmit and sense temperature signals. Compared with traditional sensors, fiber-optic temperature sensors have high information capacity, anti-electromagnetic interference, anticorrosion properties, high measurement accuracy, and safety, along with being explosion proof. They are applied in various fields, such as national defense, military, civil engineering, energy, environmental protection, and medical health. Compared with fiber-optic temperature sensors, such as fiber gratings and Fabry-Perot interferometers, micro-cavity temperature sensors have the advantages of small size, high resolution, fast response time, and low cost. For pure SiO2 micro-cavities, the improvement in temperature sensitivity is limited by the poor thermal sensitivity of the quartz material. Results of previous studies demonstrate that combining SiO2 micro-cavities with thermally sensitive materials is effective for improving temperature sensitivity.
In this study, a highly sensitive temperature sensor based on a polydimethylsiloxane (PDMS) sensitized hollow-core micro-cavity resonator (PS-HCMR) is developed and implemented. Based on the thermal sensitivity of PS-HCMR wall mode resonance spectrum and the high thermal optical effect and thermal expansion effect of PDMS, the high sensitivity perception and measurement of temperature are achieved. The temperature sensitivity of the HCMR is measured at 27?33 ℃ by coating 50-μm and 150-μm thick PDMS films on the HCMR using a coating method. An experimental comparison with the temperature sensitivities of a solid-core micro-cavity resonator (SCMR) and pure SiO2 HCMR (Table 1) is performed to verify the high-sensitivity temperature-sensing performance of the PS-HCMR and the effect of PDMS film thickness on the HCMR. The thermal sensitivity effects of higher-order-mode whispering gallery mode (WGM) in micro-cavities with different PDMS film thicknesses are compared theoretically and experimentally.
The use of PDMS plated on an HCMR is proposed to achieve high-order-mode high-sensitivity sensing with a fast response, good stability, and high sensitivity because of the high coefficient of thermal expansion. The simulation results (Fig. 2) indicate that, when the coating thickness is 0 μm, the temperature increases and the spectral lines move toward the long wavelength (red shift) owing to the positive coefficient (1.1×10-5 K-1) of thermal expansion and thermal-optical coefficient (5.5×10-7 K-1) of SiO2. When the PDMS is very thin (thickness of 2 μm), the positive thermal-optical coefficient of Si and the negative thermal-optical coefficient (-4.5×10-4 K-1) of PDMS compensate for each other, and the PDMS thermal expansion coefficient (9.6×10-4 K-1) dominates at this time, resulting in a response to temperature increase that still shifts the spectral lines in the long wavelength direction. When the PDMS thickness is increased further (from 2 μm to 10 μm), the resonance response shifts the WGM resonance spectrum in the short wavelength direction (blue shift) owing to overcompensation because the negative thermo-optical coefficient of PDMS is much larger than the positive thermo-optical coefficient of SiO2, and the blue shift increases with increasing PDMS thickness. When the thickness is significantly larger than 10 μm, the spectral line shifts to longer wavelength direction as the PDMS thickness increases. As shown in Figs. 8 and 9, the experimental results indicate that the combination of the HCMR with the high Q value and the polymer PDMS with a high thermal expansion coefficient achieves a stable structure with a temperature sensitivity of 0.127 nm/℃, which is 2.87 times higher than PS-SCMR (44.16 pm/℃) and 32 times higher than the conventional pure SiO2-based HCMR (3.93 pm/℃).
A highly sensitive temperature sensor based on the PS-HCMR is proposed. High-Q PS-HCMRs with 200-μm-diameter PDMS with film layer thicknesses of 150 μm and 50 μm are prepared, and the temperature sensitivity is greatly enhanced by taking advantage of the high thermal expansion coefficient and high thermo-optical coefficient of PDMS material, as well as the high-order mode resonance in the hollow-core structure of the HCMR. The experimental results show that, when the film layer thickness is 150 μm, the temperature sensitivity of the proposed PS-HCMR can reach 0.127 nm/℃, which is 2.87 times better than that of the PS-SCMR and 32 times better than that of the pure SiO2 HCMR. The PS-HCMR temperature sensor proposed has good application prospects in the fields of industrialized control, health monitoring, environmental monitoring, and biochemical reaction control.
