Directional Coupler Based on Single-Crystal Diamond Waveguides

We report on the design, fabrication, and characterization of a 1550 nm/1950 nm directional coupler based on single-crystal diamond waveguides. The input and output ports of the waveguides are connected with 1550 nm and 1950 nm grating couplers to allow out-of-plane light coupling. The devices are fabricated using electron-beam lithography and reactive ion etching technologies. We obtain a good performance for both the grating couplers (efficiency of -10.9 dB at 1550 nm and -11.6 dB at 1950 nm) and waveguides (attenuation of 2.3 dB/mm at 1550 nm and 4.5 dB/mm at 1950 nm). Furthermore, the actual directional coupler component is found to be quasi-lossless. By means of directional coupler calibration devices, we measure high extinction ratios between the "cross" and "through" ports (22 dB at 1565 nm and 21 dB at 1949 nm), hence showing a high-efficiency wave-combining functionality. Taking into account that integrated directional couplers are essential for allowing light routing between different on-chip devices, our results pave the way for larger-scale and more complex photonic integrated circuits in single-crystal diamond thin films.