Ultra sensitive fiber optic temperature sensor based upon Fabry-Perot cavity directly fabricated from PMMA slab

A novel ultrasensitive optical fiber temperature sensor is proposed and experimentally validated. Temperature sensing is achieved using a Fabry-Perot interferometer (FPI) manufactured directly from the thermosensitive material polymethyl methacrylate (PMMA) slab. Firstly, we used a CO₂ laser to ablate a micro groove on the PMMA slab, and then two single-mode fibers (SMF) with flat cut end faces are placed in the micro groove from both sides, aligning their two reflective surfaces, then the SMF on both sides are firmly fixed using ultraviolet glue to form an FPI, named FPI₁. When heating FPI₁, the expansion of the PMMA slab elongates the F-P cavity length in FPI₁, resulting in the variation of the optical path difference of light transmitted in FPI₁, causing the dip wavelength of FPI₁ spectrum to drift, thus achieving temperature measurement. Due to the high coefficient of thermal expansion and contraction of PMMA slab, the temperature of a single FPI₁ reaches 13.35 nm/°C. To further amplify the temperature sensitivity of FPI₁, we introduced optical vernier effect technology to construct sensor S₁. FPI₁ and FPI₂ are paralleled to form S₁. FPI₂ is fabricated by sequentially fusion splicing SMF - capillary - SMF, and its free spectral range is close to that of FPI₁. FPI₂ has low temperature sensitivity and is acted as the reference interferometer in S₁. The experimental findings display that the sensitivity of S₁ reaches 115.67 nm/°C, which is the highest temperature sensitivity currently known. It has developed the temperature sensitivity of FPI₁ by nearly 8.7 times. In summary, the proposed sensor has a simple structure, easy producing, low cost, robustness, and highly sensitive, making it one of the optimal choices for temperature measurement in industrial applications.