Ultra-broadband 2 × 2 adiabatic 3 dB coupler using subwavelength-grating-assisted silicon-on-insulator strip waveguides.

We report on a compact, ultra-broadband, 2×2 adiabatic 3 dB coupler using silicon-on-insulator (SOI) strip waveguides assisted by sub-wavelength gratings (SWGs). In our device, two tapered SWG-assisted SOI strip waveguides achieve an adiabatic mode evolution of the two lowest-order transverse electric modes, in a two-waveguide system, for broadband 3 dB power splitting. Theory predicts that the proposed coupler will operate from 1200 nm to 1700 nm. We have been able to measure the performance of a device with a 15 μm long mode evolution region that achieves even, broadband power splitting over the 185 nm wavelength range of our tunable laser with an imbalance of less than ±0.3  dB and with low average excess losses of <0.11  dB.