The use of the non-selective, O<sub>2</sub>-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<sup>3</sup>) 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<sup>3</sup> 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<sub>d</sub> vs. length L plot for half-R<sup>3</sup> 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<sup>2</sup> for an r=150 μm, w=6 μm ring.