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2 May 1994 Semiconductor ring lasers
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Semiconductor ring lasers suitable for monolithic integration in optoelectronic circuits may be formed of curved waveguide (or by a ring of etched mirrors), and fabrication depends on high- quality lithography and etching. Very small devices may be made if the r.i. contrast is sufficiently high, and devices down to 10 micrometers in diameter have been realized. The quality of the dry-etched sidewalls is more critical in curved than in straight waveguides, and wall roughness, together with high r.i. contrast, leads to severe optical scattering loss. Wall roughness has been reduced markedly by high-definition e-beam lithography and optimized dry etching, and the effect of damage has been reduced by nitride passivation. In small ring lasers, directional output couplers are not practicable, and Y-junctions have been used, which cause additional scattering loss. MMI couplers show promise. If strip-loaded or buried rib waveguiding is employed to avoid degradation of the exposed active layer, much larger bend radii must be accepted. Larger structures show promise as actively or passively modelocked sources operating around 10 GHz. Feedback into the laser through the output coupler remains a problem, although a reduced coupling fraction can provide some degree of isolation.
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Peter J. R. Laybourn and Thomas F. Krauss "Semiconductor ring lasers", Proc. SPIE 2150, Design, Simulation, and Fabrication of Optoelectronic Devices and Circuits, (2 May 1994);

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