Vernier sensitization based on optical fiber LP₀₁-LP₁₁ modal interference temperature rise negative response

This thesis investigates the modal characteristics of the LP₀₁ and LP₁₁ modes in a two-mode fiber (TMF), their modal interference behavior, and the corresponding vernier effect-based sensitivity enhancement characteristics. The dispersion and temperature sensitivity of the two modes were simulated, revealing that the LP₀₁-LP₁₁ modal interference exhibits a negative temperature response, specifically, the interference spectrum undergoes a blue shift with increasing temperature, with a sensitivity of −0.068 nm/°C. A TMF-based Mach-Zehnder interferometer (MZI) was fabricated using this interference mechanism, and its measured temperature sensitivity was determined to be −0.058 nm/°C. This TMF MZI was cascaded separately with two types of interferometers featuring positive temperature responses to construct enhanced vernier structures: one is an ultraviolet (UV)-cured adhesive Fabry-Pérot interferometer (FPI), and the other is a single-mode fiber (SMF) MZI. Within the temperature range of 30 °C to 80 °C, the cascaded structure of the TMF MZI and UV-cured adhesive FPI exhibited a positive temperature response with a sensitivity of 1.450 nm/°C, which is 25 times that of the standalone TMF MZI and 8 times that of the standalone UV-cured adhesive FPI. In contrast, the cascaded structure of the TMF MZI and SMF MZI showed a negative temperature response with a sensitivity of −0.794 nm/°C, which is 13.7 times that of the standalone TMF MZI and 18.5 times that of the standalone SMF MZI. The underlying reason for the opposite temperature response trends of the two cascaded structures was analyzed: the drift direction of the spectral envelope of the cascaded structure is consistent with that of the individual interferometer with a smaller free spectral range. The findings of this study provide solid theoretical and experimental foundations for the further research and practical application of few-mode fibers.