A theoretical model is developed to address the fringe visibility and additional phase in the interference spectra of low-finesse extrinsic optical fiber excited Fabry-Pérot interferometers. The model described in the paper applies to both single-mode and multimode fiber excitations; according to the theory, the fringe visibility and additional phase term are primarily determined by the working wavelength and angular power density distribution outputting from the excitation fiber, rather than based on spatial and temporal degree of coherence. Under certain approximations, the output interference intensity and the spatial power density distribution projected onto the fiber axis form a Fourier-transform pair, which potentially provides a tool for spatial density distribution analysis of fiber output. With excellent agreement with experiments, the theory presented in this paper leads to design guidelines for Fabry-Pérot interferometric sensors and insightful physical understanding of such devices. Interferometric Optical Fiber Sensor Using Kirchhoff's Diffraction Formalism," Opt. Fiber Technol. 1(4), 380-384 (1995). 14. F. Pérennès, P. C. Beard, and T. N. Mills, "Analysis of a low-finesse Fabry-Perot sensing interferometer illuminated by a multimode optical fiber," Appl. Opt. 38(34), 7026-7034 (1999 Optical Society of America. Cheng Ma, Bo Dong, Jianmin Gong, and Anbo Wang, "Decoding the spectra of low-finesse extrinsic optical fiber Fabry-Perot interferometers," Opt. Express 19, 23727-23742 (2011
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