Loss Characterization of High-Index-Contrast Ridge Waveguide Oxide-Confined InAlGaAs Quantum Well Racetrack Ring-Resonator Lasers

摘要

The use of the non-selective, O_2-enhanced wet thermal oxidation of deep-etched sidewalls in GaAs-based heterostructures has enabled the fabrication of low-loss, high-index-contrast ridge waveguides suitable for ring resonator laser devices. In a self-aligned process, the grown native oxide simultaneously provides excellent electrical insulation, passivation of the etch-exposed bipolar active region, and smoothing of etch roughness. The resulting strong lateral optical confinement at the semiconductor/oxide interface has enabled half-racetrack ring resonator (R~3) lasers with bend radii r as small as 6 μm. In this work we have experimentally characterized the loss due to the mode mismatch at the straight to curved waveguide transition from analysis of efficiency data of half-R~3 lasers with multiple cavity lengths. Using an 808 nm InAlGaAs graded-index separate confinement heterostructure, the transition losses are extracted from an inverse efficiency 1/η_d vs. length L plot for half-R~3 lasers with r=150, 100, 50, 25 and 10 μm and 3 different ridge widths, w. The round trip transition loss ranges from 11.5 to 37.0 dB (for w=7.3 μm), 6.7 to 27.0 dB (w=4.2 μm), and 1.8 to 16.2 dB (w=2.1 μm) with decreasing radii, showing a clear decrease with width and corresponding improved mode overlap in the transition region. Simulation results elucidate the role of mode mismatch vs. radiative bend loss in high-index-contrast racetrack ring resonator lasers. We demonstrate a full-ring laser having a tangential stripe output coupler guide fabricated via e-beam lithography and non-selective oxidation with a threshold current density of 719 A/cm~2 for an r=150 μm, w=6 μm ring.