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PDF of thesis.
Thesis (Ph. D.)--University of Rochester. Institute of Optics, 1999.
The concept of controlling the spatial emission patterns from concentric-circle-grating distributed-feedback semiconductor lasers is an intriguing and important idea. The concentric-circle-grating structure provides a unique two-dimensional mechanism for providing feedback within an optical waveguide cavity as well as for coupling light out normal to the surface. Although the azimuthally polarized, circularly symmetric fundamental mode has previously been observed from optically pumped circular-grating lasers, single-spatial-mode lasing in radially polarized, higher-azimuthal-order modes has also been observed from these devices, leading to the question of deterministic spatial mode control. These higher-azimuthal-order spatial modes have been predicted theoretically using a vector-scattering approach. Inherent to the concept of mode control is the fabrication technology of circular gratings. Electron-beam lithography has been shown to be the only method to date capable of producing the small-period (~250 nm) circular gratings that are necessary for circular emission patterns; however, such systems are complex and inevitable errors arise during the course of fabrication. An investigation of electron-beam system errors coupled with dry etching requirements has yielded improved understanding into the processes required to produce high-quality circular gratings. Using such fabrication techniques and understanding their limitations, controlling the output of a circular-grating laser is deemed feasible. Varying parameters such as grating depth and gain distribution for optically pumped semiconductor lasers shows that coarse control is possible using these simple methods. Alternative mechanisms that may require tighter tolerances on grating fabrication are also evaluated theoretically, including chirped and tapered gratings as well as spiral and elliptical gratings. These inhomogeneous gratings are shown to have varying effects on the mode spectrum and mode intensity profiles inside the laser cavity as well as to improve mode discrimination of the concentric-circlegrating surface-emitting semiconductor laser.
