Abstracts

In an effort to improve the performance ofhigh-power semiconductor lasers to meet the demands ofapplications, this thesis contains work studying the issues whichlimit their performance: beam filamentation and spatial feedbackeffects.
Through computer simulations, weinvestigate the role of three nonlinear mechanisms which can leadto filamentation, and determine the stability boundaries of thematerial parameters for which the device will not exhibitfilamentary tendencies. We use an analytic theory to verify thesefindings, and to predict the spatio-temporal nature of thefilaments through an analytic expression for the gain, in whichcontributions of the various mechanisms can clearly be seen. Weexperimentally verify the spatiotemporal characteristics of thefilaments, discover effects of the stripe width and transitions tochaos, and discuss how to compare the relative severity offilamentation among different devices.
Wepropose a new method of controlling filamentation usingbelow-bandgap semiconductor nonlinearities. With simulations, wedetermine under what conditions this imposed nonlinearity cancounteract the carrier-induced self-focusing inside the activeregion. We fabricate a prototype device using new epitaxial layerscontaining the below-bandgap nonlinearities, and compare theperformance of these new devices to a control set.
In studying the spatial effects of opticalfeedback, we use Fresnel diffraction theory to derive an expressionfor the field that is reflected back into the laser. This result isapplied to our computer model and used to explore the effects offeedback on narrow-stripe, broad-area, and tapered-stripesemiconductor lasers. Re-examining feedback in narrow-stripedevices through experiments and analytic theory, we investigate thecoupling effects between the narrow waveguide and the feedbackfield, and the changes in the operating characteristics of thelaser due to this coupling. We experimentally examine the beamquality of high-power tapered-stripe devices under feedback, payingparticular attention to the degradation of the far-field intensitypattern and the issue of filamentation. Finally, we measure theeffects of feedback on the spatial distribution of the laser fieldin broad-area semiconductor lasers, specifically investigating themodification of filamentation due to a lateral displacement in thefeedback field with respect to the laser facet.