Matching optimization for SFS-structured interferometers with step-index fibers

Axial-aligned SFS-structured interferometers with step-index fibers hold advantages of easy fabrication, high stability, and extremely low cost, while low extinction ratio of the interferometer remains challenging. Here, we investigate the influence of core radius and refractive index of the fibers adopted in the interferometer on its extinction ratio and coupling loss, aiming to achieve the extinction ratio above 15dB – this criterion is applicable for practical use. The improvement of extinction ratio values presented in experiment was from 2dB to 7dB, which match perfectly with theoretical values, therefore demonstrates the effectiveness of the theoretical conclusion.