Fabrication of self-aligned ridge waveguide lasers with spot-size converter for quantum well intermixed superluminescent diodes

We fabricated InGaAs/GaAs multiple quantum well ridge waveguide lasers with a spot-size converter (SSC) in a single epitaxial growth step by using conventional lithography and reactive ion etching. The SSC was based on the laterally tapered rib adiabatic waveguide to expand the mode size. The narrow tapered ridge top of ~0.7 µm was opened by a self-aligned method associated with benzocyclobutene planarization and overall etch-back processes. For an uncoated 1.5 mm long cavity, the far-field beam divergence angle from the SSC facet was about 15° (parallel) and 27° (perpendicular) at 20 °C under continuous-wave (CW) operation, indicating a significant reduction compared to that from the laser facet (i.e., 32° × 41°). The device exhibited a maximum output power of 62 mW/facet with a threshold current of 73 mA and a slope efficiency of 0.34 mW mA−1. A selective intermixing technique using impurity-free vacancy diffusion was applied to the SSC laser structure to achieve a superluminescent light source via an optimized anti-reflectivity coating. The CW output power up to 1.4 mW at 20 °C was observed and the spectral bandwidth was approximately 24 nm.