Hybrid laser diode with ultrawide wavelength-tunable range using curved directional couplers

Wavelength-tunable laser diode with a wide tuning range is required for optical communication systems and optical sensing. External cavity laser diodes with silicon-photonic wire waveguides and ring resonators have small footprint because of high refractive index contrast between Si. However, power coupling efficiency K of conventional straight directional coupler between ring and bus waveguides have large wavelength dependence, which lowers tunable range. In this study, we demonstrate a hybrid wavelength-tunable laser diode using curved directional couplers, whose wavelength dependence on K is low. The wavelength-tunable range record of 120.9 nm has been achieved. In addition, curved directional couplers are tolerant of waveguide width fabrication error.