High Efficiency Blazed Fiber-Chip Grating Coupler With Interleaved Trenches

We demonstrate a fiber-chip surface grating coupler that interleaves standard full and shallow-etched trenches to maximize directionality in the upward direction. The coupler is implemented in a regular SOT substrate with 220 nm silicon thickness and etch depths of 220 nm (full etch) and 70 nm (shallow etch), as offered by silicon photonic foundries. The blazing effect is controlled by adjusting the separation between the two sets of trenches. This way, grating directionality exceeding 95% is achieved independently of the bottom oxide (BOX) thickness. Couplers have been fabricated at LETI using 193 nm DUV lithography on 200 mm SOT wafers with 2 pm BOX. The measured coupling efficiency is -2.1 dB with a 3 dB bandwidth of 52 nm.