Ultra-Broadband Mode Filter Based on Phase-Shifted Long-Period Grating

We propose an ultra-broadband mode filter based on the structure of a phase-shifted long-period grating formed along a few-mode waveguide terminated with two linear tapers. To demonstrate the operation principle, we design a mode filter to reject the fundamental mode of a three-mode waveguide that supports the E11, E21, and E12 modes. Our typical experimental device fabricated with polymer material, which has a length of 26 mm, provides polarization-insensitive 10-dB rejection of the E11 mode against the E21 and E12 modes with bandwidths of ~190nm (1440-1630 nm) and ~140nm (1450-1590 nm), respectively. The proposed mode filter could serve as a general platform for the development of mode filters and mode-dependent-loss compensators for mode-division-multiplexing applications.