Theoretical Analysis of Vernier-Effect-Induced Sensitivity Enhancement of Dual Fiber Fabry-Pérot Cavities in OFDRs

The vernier-effect-based sensitivity enhancement of two kinds of sensing units consisting of dual fiber Fabry-Pérot (FP) cavities in the Optical Frequency Domain Reflectometry (OFDR) is analyzed in this paper. Theoretical analysis reveals that significant differences exist in the sensitivity enhancement between the cascaded and parallel dual fiber FP cavties when demodulated by an OFDR system. When the conditions of the vernier effect are satisfied, the sensing unit with cascaded FP cavities does not exhibit a sensitivity enhancement compared to a single FP sensor, whereas the sensing unit with parallel FP cavities can achieve an enhanced sensitivity. This phenomenon differs from that observed in direct wavelength interrogation systems. The results are further verified with numerical simulations on the temperature sensing. When the vernier-effect conditions are met, the sensitivity of the sensing unit with cascaded FP sensors is 9.99 pm/°C, while the sensitivity of the sensing unit with parallel FP sensors can reach up to 128.97 pm/°C. The findings of this paper provide valuable insights for the design of high-sensitive distributed optical fiber sensing systems.