  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.
