Bistability and unidirectionality in triangular-ring diode lasers.

摘要

The triangular ring diode laser is a compact, efficient semiconductor laser, made by using dry-etched turning mirrors to connect three ridge waveguides. In general, such a laser is bidirectional: Equal amounts of light circulate in the clockwise and counterclockwise directions. This dissertation describes the design, fabrication, and testing of unidirectional ring lasers, in which up to 98% of the output light emerges from a single direction. Unidirectional rings were expected to have lower relative intensity noise (RIN), higher spectral purity, and greater resistance to optical feedback than either bidirectional ring lasers or Fabry-Perot lasers.;After the failure of the first fabrication runs, the ring laser process recipe was revised extensively. Reliability was improved until the lasers could operate continuously at dc currents of 100 mA, up to nine times threshold. Also, an increase in the waveguide etch depth allowed higher order lateral modes to propagate. The revised process produced unidirectional ring lasers with unusual behavior: the preferred direction would switch from clockwise to counterclockwise and back as the drive current was varied. Some devices were bistable, able to lase in either direction at a given drive current. This beam-switching behavior, which had not been seen before, occurred in ring lasers of all shapes and sizes, and for lasers made from three different gallium arsenide-based quantum-well materials. An explanation is proposed, based on the theory of two-mode competition via gain saturation.;Beam ratios as large as 79 to 1 were observed for triangular ring lasers without any intentionally designed asymmetric cavity elements. In light of this discovery, a reexamination of the established methods for making unidirectional devices is presented, along with suggestions for future unidirectional laser designs.;When switched into a stable unidirectional mode, one device exhibited RIN of -150 dB/Hz at 250 MHz, the lowest RIN ever seen in a triangular ring laser. A study of the effect of external feedback on threshold current, beam ratio, spectrum and RIN showed that the typical ring laser is no less sensitive to feedback than a cleaved-facet Fabry-Perot laser of equivalent size and waveguide structure.