Liquid-crystal clad tunable wavelength de-multiplexer and coupler/filter in silicon nitride rib waveguide structure

We report long-period waveguide grating (LPWG) based tunable wavelength de-multiplexer and tunable coupler/filter in liquid crystal (LC) clad rib waveguides on silicon nitride on insulator platform. High birefringence and electrically tunable refractive index of the MLC-9200-000 LC have been used to achieve a tuning range covering most of the S, C and L bands of the optical communication system. The structure consists of two parallel rib waveguides with an elevated slab. We have considered a new concept of injecting power into a higher-order slab mode. The power from the slab mode is coupled to the rib modes using LPWGs inscribed on the sidewalls of the ribs. The structure works as a tunable wavelength de-multiplexer when the grating periods in both ribs are different. The wavelength tunability of the de-multiplexer is achieved by an external voltage applied across the liquid crystal cell. Our numerical results show wavelength tuning of the de-multiplexer in the range 1.437 - 1.627 μm. The structure works as an electrically tunable coupler/filter when the grating period of both the gratings are the same. We numerically demonstrate the tuning of the coupler in the wavelength range 1.487 - 1.572 μm and that of the filter in the range 1.484 - 1.577 μm with 20-dB rejection.