Simulation and optimization of high finesse concave fiber Fabry-Perot cavity

As a critical component in optical sensing and filtering, optimizing finesse and insertion loss in fiber Fabry-Perot (FP) cavities is essential for achieving high-performance applications. Although previous studies have realized high finesse in fiber FP cavities through concave cavity structures, the insertion loss at high finesse remains poorly suppressed, restricting the practical implementation of high-finesse fiber FP cavities. This work focuses on concave fiber FP cavities and reveals the collaborative influence mechanism of end-face reflectivity and intracavity loss on the peak transmittance and finesse through theoretical analysis. Finite element simulations are employed to demonstrate how input-output coupling losses caused by different structural parameter errors affect the mode field distribution and peak transmittance, indicating that optimizing cavity geometric parameters can effectively mitigate mode mismatch and reduce overall insertion loss. Following the theoretical and simulation investigations, we demonstrated a concave fiber FP cavity with a finesse of 700 and insertion loss of ∼ 4 dB using our real-time observable CO₂ laser processing system. This study provides critical insights into the relationship between finesse and loss, offering guidance for simultaneously enhancing the finesse and peak transmittance of concave fiber FP cavities. These findings facilitate the widespread application of fiber FP cavities in high-precision optical filtering and sensing technologies.