We report the development of racetrack-cavity semiconductor ring laser
s that operate unidirectionally by means of an active crossover wavegu
ide. These racetrack-cavity lasers show improved performance over prev
ious circular-cavity unidirectional ring lasers with continuous-wave s
ingle-frequency output powers of greater-than-or-equal-to 10 mW, and w
ith up to 99% of the lasing output emitted from the preferred ring dir
ection.