Tunable broadband filter based on lithium-niobate asymmetric Mach–Zehnder interferometer for coarse wavelength-division multiplexing applications

Channel-select filtering technology is a very promising solution for realizing reconfigurable demultiplexing components required in a flexible coarse wavelength-division multiplexing system. Here, we propose and demonstrate an electro-optic tunable broadband filter based on an asymmetric Mach–Zehnder interferometer formed on lithium niobate for this application. The counter-tapered directional coupler is introduced to replace the conventional one formed with uniform waveguides so that a splitting ratio of 50%:50% is much easier to be realized over a wide wavelength range and, hence, increase bandwidth of the filter. In addition, to reduce the bending loss in the long-wavelength region, the waveguide width and the height of the involved S-bend waveguides are optimized experimentally. Our fabricated filter, which has a total length of ∼30  mm, exhibits an extinction ratio of ∼15  dB and a wide tuning range of >40  nm with an electrical wavelength tuning sensitivity of ∼4.5  nm  /  V.