Sensitivity-Enhanced Temperature Sensor of Cascaded Lyot Filter and Mach–Zehnder Interferometer Based on the Vernier Effect

The compact structure and resistance to electromagnetic interference of optical fiber temperature sensors have garnered significant interest. Nevertheless, it is imperative to enhance the detecting sensitivity in order to adequately fulfill the existing requirements. This study proposes and demonstrates a sensitivity-enhanced optical fiber temperature sensor of cascaded Lyot filter and Mach-Zehnder interferometer (MZI) based on the Vernier effect. The MZI, which has two consecutive bubble-type upper tapers, is employed as a reference interferometer owing to the exceptional stability of its totally polarization-maintaining fiber (PMF) construction. A Lyot filter is created by fusion splicing a segment of PMF at a 45° angle between two linear polarizers, functioning as a sensing interferometer. The sensitivity of the cascaded sensor improves through the Vernier effect, which is produced when the length of the sensing PMF in the Lyot filter is modified to closely match the free spectral range (FSR) of the MZI. The experimental results reveal the Lyot filter’s sensitivity to be −1.4 nm/°C, while the cascaded sensor achieves an increased sensitivity of −28.3 nm/°C, with a sensitivity amplification factor of 20.2. The proposed sensor structure, composed of PMF, leverages the controllable polarization characteristics of PMF to enhance the system performance, effectively addressing the issue of detection error that arises from the fluctuating polarization state of the optical signal being transmitted throughout the sensitivity test. This proposed optical fiber temperature sensor, with its robust polarization stability, simple fabrication, and high sensitivity, is suitable for high-precision industrial applications.