Tech Briefs

Novel techniques for improving the efficiency and functionality of high-powered laser diodes

Laser diodes are an integral part of everyday life, incorporated into commonplace items as diverse in function as laser pointers, fiber-optic communications systems, and DVD players. Manufacturers make most laser diodes by layering specially doped semiconductor materials on a wafer. By slicing tiny chips from these wafers to attain two perfectly smooth, parallel edges, they create very thin (tens of microns) waveguides. These waveguides define a resonating cavity that causes stimulated light to combine in a way that embodies a "laser" and propagates its lasing action. Although this process represents a highly successful and wellengineered means for producing semiconductor lasers, the lasers do not produce an optimum beam. Beam emission occurs from the small rectangular opening at the end of the chip, a configuration that results in an elliptically distorted beam as well as the loss of output efficiency. In addition, the output aperture's relatively small size can lead to destruction of the cleaved and polished end facet during the laser's high-power operation. Laser diodes produced using this process are also susceptible to substantial fluctuations in output wavelength and beam quality as a function of temperature. Furthermore, since the chip emits beam output from an edge instead of its top or bottom surface, manufacturers experience difficulty both in packaging various diode configurations and in combining the output beams of multiple laser diodes.

ImageLaser diodes have a number of military, medical, and industrial uses, including applications wherein highpower operation over wide temperature ranges or at a specific frequency is critical. Laser detection and ranging (LADAR) is one such application relying heavily on laser diode technology. Itself a crucial tool in remote sensing, aerial surveying, three-dimensional (3-D) profiling, automated process control, target recognition, autonomous machine guidance, and collision avoidance applications, LADAR technology is also well-suited to homeland security, law enforcement, and antiterrorism activities. Semiconductor laser diodes are perfect candidates for direct use in low-power applications or as optical pumps for higher-power, solidstate lasers. While there is a great need for low-cost, high-powered lasers capable of operating at a number of different near-infrared wavelengths, a laser must operate in the infrared at wavelengths >1500 nm (beyond the transmission band of the human eye) to prevent retinal damage.

ImageAccordingly, AFRL scientists initiated key project efforts to address three critical requirements associated with nearinfrared laser sources. The first of these requirements was to create high-density arrays consisting of rapid pulse frequency, thermally stable laser diodes to generate high-power combined output. The second requirement was to modify the semiconductor chip; by changing the chip's configuration to achieve a surface-emitting output beam, the scientists sought to alleviate the extreme difficulties associated with projecting or coupling the highly asymmetric beams produced by multiple edge-emitting laser diodes. The Gaussian "round" output produced by surface-emitting lasers is nearly optimum for coupling, via lenses or optical fiber, to telescopes and other laser devices. AFRL's third requirement was that the developments associated with the near-infrared laser technology ultimately lead to devices capable of operating at the longer, infrared wavelengths needed both for eye safety and for safe use of the laser in infrared camera/sensor applications such as illuminators or test system projectors.