All-fiber mode selective comb filter based on Mach–Zehnder interferometer

A mode selective comb filter based on the Mach–Zehnder interferometer (MZI) is proposed and theoretically analyzed. The proposed filter consists of two vector mode selective couplers (MSCs). The vector MSCs are used to convert the fundamental mode to the second order vector modes. Both the wavelength selection and the mode selection functions with a broad bandwidth can be obtained in the proposed filter. Meanwhile, the proposed filter shows excellent comb filtering effects for both the fundamental mode and the second order mode in the C-band. Especially, the coupling efficiency of the second order mode is larger than 96% in the wavelength range from 1540 nm to 1560 nm. The proposed filter can find applications in the fields of the advanced optical fiber lasers, mode division multiplexing (MDM) systems and wavelength division multiplexing (WDM) systems.