Silicon nitride PIC-based ECL at 637nm with flip-chip-bonded RSOA

We demonstrate, to the best of our knowledge, the first flip-chip integrated extended-cavity laser (ECL) operating at 637 nm on a silicon nitride (SiN) photonic integrated circuit (PIC) platform. The device combines a reflective semiconductor optical amplifier (RSOA) with a wavelength-selective feedback circuit implemented on the PIC. The gain chip is integrated through a passive flip-chip bonding process, forming a compact external cavity defined by the highly reflective back facet of the RSOA and the PIC-based reflector. Light emitted from the RSOA is coupled into the PIC through a multimode edge coupler (MMEC) that provides an enlarged acceptance aperture and maintains efficient coupling under lateral misalignments of ±2.4 µm with only 1 dB excess coupling loss. Vertical alignment is achieved using etched pedestals fabricated in a cavity fitting the RSOA, ensuring accurate positioning of the gain chip relative to the PIC waveguide interface. The coupled light is split evenly and guided toward a Sagnac-loop reflector, where it is spectrally filtered by a pair of ring resonators operating in Vernier configuration. This feedback circuit enables single-mode laser operation and wavelength tuning over 4 nm, with an on-chip output power of approximately −5 dBm. The presented architecture provides compact integration of ECLs in SiN-based PICs at short wavelengths, for applications such as spectroscopy, sensing, imaging, metrology, quantum photonics, and life-science applications.