Label-Free Temperature Sensor Based on Critical-Wavelength Detection in Modified Vernier Envelope

This article proposes a novel detection scheme to measure temperature by detecting a unique and easy-identifiable critical wavelength (CWL) in a modified Vernier envelope. The Vernier envelope is produced by cascading two critical-wavelength existing single mode fiber (SMF)–few-mode fiber (FMF)–SMF (SFS) structures and has varying free spectrum range (FSR) with a CWL in the operational wavelength planned. The FSR of the Vernier envelope gets maximum at the CWL, which makes the CWL distinguishable and entirely different from the peaks or dips in the Vernier envelope. Both experimental and simulation results show that the CWL in the Vernier envelope shifts monotonously with temperature, and the wavelength sensitivity is improved compared with that of the single SFS structure. A temperature measurement range from 25 °C to 330 °C is demonstrated in the experiments with a maximum temperature sensitivity of 0.622 nm/°C. By reducing the difference between the physical length of the FMFs employed in the sensing and the reference SFS structure, the maximum temperature sensitivity of the CWL in the Vernier envelope can be further enhanced to 0.822 nm/°C. The proposed temperature sensor can be optimized to apply in high-sensitivity measurement or large temperature-range measurement to satisfy the requirements in practical applications.