A Taper to Reduce the Straight-to-Bend Transition Loss in Compact Silicon Waveguides

Strong confinement of light in silicon waveguides allows for sharp bends and, as a result, high-density integration. However, the mode transition loss between the straight and bent portions of a silicon waveguide begins to affect the device performance when the bending radius becomes small. In this letter, we show that a transition region with a step taper between the straight and bent portions of the waveguide can effectively reduce this transition loss. This is demonstrated by measuring the intrinsic round-trip losses of micro-racetrack resonators, where ultralow loss can be precisely characterized according to the quality (Q)-factor change. The results show that the taper can suppress the transition loss from 0.016 to 0.0022 dB for a 4.5-μ m bend radius. Consequently, we improve the Q-factor of such a racetrack resonator from 31 000 to 87 000.