Numerical analysis of DP-MZI–SI comb filter improvement using polarisation-maintaining fibre

Abstract In this paper, we present a numerical analysis of a novel comb filter in a double-pass Mach–Zehnder interferometer and a Sagnac interferometer (DP-MZI–SI) configuration and verify it by partial experimental demonstration. The proposed filter comprises an in-line fibre polarisation controller (PC), positioned in one arm of the Mach–Zehnder interferometer, and a polarisation-maintaining fibre (PMF) with a three-component fibre-coil PC in the Sagnac loop. The transmission of the DP-MZI–SI filter was determined using the Jones matrix of each component. This filter allows control of the free spectral range (Δλ) via two alternative modes of operation: by changing the length of the PMF or by increasing the arm length of the filter. In addition, the contrast of the interference fringes and their position can be controlled by adjusting the PCs. The possibility to generate stable patterns with narrow fringe separation was also demonstrated. Simulation results for the DP-MZI–SI filter were validated through experimental measurements. The experimental and numerical results agreed closely in terms of both Δλ and fringe visibility. The proposed DP-MZI–SI filter has characteristics suitable for application to multi-wavelength, tunable, and switchable fibre laser development, as well as to optical sensors, communication systems, and microwave generation.