Spatial and temporal stability of high power semiconductor lasers.

Abstract

High power compact laser sources are a necessity in today's technical world. While laser sources have been either compact, such as semiconductor lasers, or high power, such as solid state lasers, the presence of power in excess of 1 W in a compact source has remained an elusive goal. Recently several novel approaches for obtaining sufficient laser power in a semiconductor material have become available. These methods, antiguided arrays, monolithically integrated master oscillator power amplifiers (M-MOPA), and external feed-back induced polarization control, show great promise but have not been sufficiently analyzed to instill comfort in large scale marketing. In response to this need, we present in this thesis an analysis theoretical as well as experimental examining the stability of these devices. The first device, the antiguided laser array utilizes carefully spaced index depressions to confine the lateral field and promote coupling between all the elements in the array. The lateral spacing of the antiguides, or index depressions, is determined by the resonance of the laterally propagating waves. The result is strong coupling and low loss between elements. While this method can indeed produce well in excess of 1 W of coherent radiation, temporal observations show self-pulsing at a 4 GHz repetition rate. This pulsing can be controlled by reduction of interelement loss in the region between waveguides. The second device, the M-MOPA, couples the output of a distributed feedback laser into a flared amplifier. While this method too produces high power, the tapered amplifier filaments at powers greater than 1 W. One source of filamentation is shown to arise from the background spontaneous emission of the amplifier. Furthermore, because the filamentation requires in excess of one nanosecond to arise, pulsed operation can significantly reduce the distortion. If continuous wave operation is necessary, antiguided substructures eliminate filamentation. The final device is a simple Fabry-Perot laser with an antireflection coating on one facet. By rotating the polarization state of the feedback, the polarization emission can be controlled. The significant discovery is the existence of elliptical polarization states when the laser is switching from the Transverse Electric to the Transverse Magnetic mode. From these measurements, we conclude that an array for TM lasers is a possible means by which to provide high power TM operation.