WARF: P94186US
Single-Lobe Surface Emitting Complex Coupled Distributed Feedback Semiconductor Laser

INVENTORS

•

Dan Botez, Masoud Kasraian

Semiconductor lasers may be constructed to be either edge emitting or surface emitting. In edge-emitting semiconductor lasers, crystal facet mirrors are located at opposite edges of the multilayer structure (typically an n-type layer, a p-type layer and an undoped active region) to provide reflection of emitted light back and forth in a longitudinal direction, eventually resulting in lasing action and the emission of laser light from one of the facets. Another type of device, which may be either edge emitting or surface emitting, utilizes distributed feedback (DFB) structures rather than conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index and/or the gain (i.e. value of the optical mode gain or loss at a particular location) of the semiconductor laser structure. Experimental studies of distributed feedback structures have included edge-emitting distributed feedback lasers and surface emitting index coupled distributed feedback lasers (SE-IC-DFB). Second order SE-DFB lasers provide reflection of guided waves by means of second order diffraction as well as radiation of free waves away from the film surface as a result of first-order diffraction. These lasers have many attractive features and are good candidates for the realization of reliable blue/green lasing. However, in SE-IC-DFB devices, radiation loss is the mode discriminator, and the spatial mode of least radiation loss, and thus favored to lase, is invariably an asymmetric one. Thus, all SE- IC-DFB lasers operate in a two-peaked anti-symmetric near-field pattern (vulnerable to gain spatial hole burning (GSHB) which causes in turn a multimode operation), and a corresponding double-lobed far-field beam pattern (which allows utilization of only half the emission, lending it undesirable for laser applications). Several methods proposed for making the far-field pattern approach that of a single-lobed profile have proved fundamentally unreliable and/or vulnerable to GSHB. Recently, edge-emitting complex-coupled (CC) DFB lasers consisting of periodic variations of both index and gain have received much attention. Such devices have several advantages over the conventional IC-DFB lasers, including high yield of single-longitudinal-mode operation, large gain threshold difference, reduced spatial hole burning effects, facet-reflectivity-independent characteristics, relatively low sensitivity to feedback, and a relatively small linewidth enhancement factor. The semiconductor laser of the present invention is formed with a second-order surface-emitting complex-coupled distributed feedback (SE-CC-DFB) structure which includes regions of periodic variation of both refractive index and gain. A typical semiconductor laser structure in accordance with the invention includes a substrate, an active region layer, cladding layers surrounding the active region layer, outer faces, electrodes by which voltage can be applied across the semiconductor structure, an opening in one electrode by which the light is emitted, and a region of periodically alternating elements of metal and/or semiconductor material formed on or in the semiconductor structure which defines an even-order distributed feedback grating. The adjacent elements of the grating differ from one another in both index of refraction and gain; each pair of adjacent elements together have a width selected to be substantially equal to an integer multiple of the effective wavelength in the semiconductor structure of the favored mode of light emission.

• provides a high power monolithic surface emitting semiconductor laser with a stable single lobe far-field radiation profile which is normal to the plane of the surface, and a nearly uniform near-field intensity pattern;
• single lobe, single frequency operation of the semiconductor laser can be achieved at high power levels with high efficiency;
• can be constructed without need to etch the grating into the semiconductor material and thus it is ideally suited for material systems which are very difficult to etch and/or cleave (e.g. gallium nitride) thus permitting selection of desired laser light wavelengths, including light in the blue/green wavelength region.

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