A unidirectional quantum cascade ring laser

We report on the design, fabrication, and characterization of a unidirectional quantum cascade ring laser operating at a wavelength of around 3.4 mu m at 200 K. A unidirectional operation is achieved by incorporating an S-shaped crossover waveguide in a manner that it couples light from the counterclockwise direction to the preferred clockwise direction. The ring laser unidirectionality is confirmed by measuring the counterpropagating wave suppression ratio (CWSR) as a function of injection current. At 1.5 times the threshold current, the CWSR is 9 that is 90% of the light is emitted in the favored (clockwise) direction. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3524200]