External Cavity 637-nm Laser with Increased RSOA-to-PIC Alignment Tolerance and a Filtered Sagnac-Loop Reflector with Single Output Waveguide

The design of a 637-nm wavelength, photonic-integrated-circuit-based external cavity laser (PIC-based ECL) aimed at quantum technology applications is presented together with first experimental results. The PIC is designed to provide relaxed alignment tolerance for coupling to a reflective semiconductor optical amplifier (RSOA) gain chip. This is achieved by using a multi-mode edge coupler (MMEC) in place of the usually employed single-mode coupling schemes. A 1-dB-penalty misalignment tolerance of up to +/- 2.4 um can be achieved in the plane of the chip, creating a path towards reliable flip-chip integration at short wavelengths. The power coupled to the PIC is fed to a Sagnac-loop reflector, filtered by a pair of ring resonators operated in Vernier configuration for providing the required frequency selective optical feedback. The ring resonators are designed to have different loaded Q-factors and they are asymmetrically coupled to bus and drop waveguides with suitably engineered directional couplers to provide single output waveguide emission. Moreover, requirements for high output power and narrow linewidths are balanced. Finally, preliminary measurements strongly suggest lasing in the fabricated devices, with further performance optimization being currently carried out.