Optical path design for improving fringe finesse in a Fabry–Perot sensing system based on 2 × 1 and 1 × 2 couplers: simulation and experiment

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Traditional Fabry–Perot (F–P) sensors are limited by fixed reflectivity and a finite number of interference orders, which restrict their detection sensitivity and resolution to some extent. To improve the fringe finesse (F) of the F–P sensing system, this study proposes a dynamic loop optical design based on 2 × 1 and 1 × 2 fiber-optic couplers combined with an erbium-doped fiber amplifier (EDFA). This design utilizes the phase-accumulation effect within the loop and EDFA gains compensation to extend dual-beam interference into multi-beam interference. Numerical simulations and experimental results show that, under optimized parameters (P1 = P2 = 10%, G = 20 dB), the system's fringe finesse increases from 2.03 to 66.25, a more than 30-fold increase. The full width at half maximum (FWHM) sharpens from 2.63 to 0.08 nm, and the peak normalized intensity variation rate increases from 0.75 to 29.49, nearly a 40-fold increase, significantly enhancing the system's sensitivity and resolution. This approach, through non-invasive optical loop dynamic feedback and parameter optimization, overcomes the performance limitations of traditional F–P sensors, providing a new, to the best of our knowledge, solution for high-precision optical sensing in complex environments such as high temperatures and high pressures. It also demonstrates potential applications in fields like distributed acoustic pressure sensing, high-resolution spectral analysis, and narrow-linewidth laser design.