Design and fabrication of liquid crystal microring resonator-based tunable optical add-drop filters

In fiber optic communication systems, tunable optical add-drop filters are needed for wavelength selection and wavelength channel (de)multiplexing. Ideally filters should possess wide dynamic tuning range, narrow bandwidth and high sidelobe-suppression capabilities. In this contribution, we show that such filters can be obtained with microring resonators incorporating liquid crystals as core materials. Special thermotropic liquid crystal blends were developed such that, in their isotropic phase, to be able to provide large Kerr constants, low loss from visible to infrared and sub-microsecond response time. These blends were used in the design of Vernier type filters with 2 to 4 serially coupled ring resonators. The filters were designed by means of FEM simulations, to operate in C-band. Parameters such as the number of rings, ring geometry and light coupling geometry were optimized to obtain large free spectral range and wide tuning range, narrow bandwidth, high sidelobe-suppression and low insertion loss. Moreover, the influence of fabrication tolerances on the device optical loss was considered in the design. The designed filters were fabricated on silicon wafers by micromachining processes. In particular, a special wafer assembly process was developed to ensure improved wafer bonding and low insertion loss. Experimental results revealed loss optimized tunable wavelength reconfigurable filters with add / drop functions of large free spectral range (FSR) of about 50 nm, wide dynamic tuning range of about 1 FSR and fast (sub-microsecond) tuning capabilities. Additionally, we show that these devices can be useful for other WDM applications, e.g., switching in C-band or visible range.