Circularly symmetric, distributed feedback structures for surface-emitting semiconductor lasers.

摘要

This thesis explores a fundamentally unique, two-dimensional optical resonator which confines the propagation of circular waves through a distributed-feedback (DFB) process. In addition to an investigation of the unique physics associated with the interaction of circular waves with radially periodic structures, a theoretical and experimental analysis of a novel, circularly symmetric surface-emitting semiconductor laser is carried out. The intent of this analysis is to show that it is possible for a semiconductor laser to directly produce low-divergence, spectrally narrow emission that is circularly symmetric in cross-section. Such emission requires lasing in a single radial and azimuthal mode in a resonator that is much larger than an optical wavelength in two dimensions.;A coupled-amplitude theory is developed that describes the coupling of both cylindrical waves and circular guided modes along the radial direction. The angular nature of propagation is characterized by a discrete set of azimuthal modes, which remain uncoupled in a circularly symmetric periodic structure. The theory is applied to the classical problem of a radiating source surrounded by a circular, periodic structure. It is found that a substantial region near the center of the structure can exist, wherein the spontaneous emission from sources at all possible locations is either strongly enhanced or inhibited. Furthermore, the theory is applied to the problem of a circular DFB laser. It is shown that the behavior of the modes associated with the radial direction is very similar to that of a linear DFB laser, while the modes associated with the azimuthal dimension are very densely spaced in frequency and threshold gain. The results suggest that it should be difficult to achieve lasing in a single azimuthal mode.;The experimental analysis emphasizes the possibility of exciting circular modes in optically pumped semiconductor lasers. An analysis of the fabrication of concentric-circle gratings by electron-beam lithography and of the resulting lasers shows that lasing in spoke-like filaments can dominate in some cases. However, it is found that under certain conditions lasing in two-dimensional, circular modes can be achieved. In fact, lasing in a single azimuthal mode is observed. These lasers produce low-divergence (