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.
Laser diodes have a number of military, medical, and industrial uses, including applications wherein high-power 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.
Accordingly, AFRL scientists initiated key project efforts to address three critical requirements associated with near-infrared 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.
AFRL and the Air Armament Center collaborated in sponsoring Small Business Innovation Research (SBIR) efforts with Aerius Photonics, LLC (Ventura, California), and Photodigm, Inc. (Richardson, Texas), to develop a higher-output semiconductor laser meeting the three preestablished requirements. Each company approached the problem differently. During its SBIR Phase I effort, Aerius Photonics successfully applied a new semiconductor laser technology, developed originally for telecommunications applications, to pulsed LADAR and range finders.1,2 The Vertical-Cavity Surface-Emitting Laser (VCSEL) is grown vertically, with laser end mirrors (defined by Bragg grating layers) on the top and bottom surfaces (see Figure 1). Fabricated from layered semiconductor materials, a VCSEL's structure is analogous to a stack of pancakes (see Figure 2). As the technology's name indicates, the VCSEL emits light from one of its surfaces and, since its output aperture is round, typically produces a good beam structure for coupling via fibers or lenses (see Figure 3). However, due to the small size of the VCSEL's laser cavity, the highest power ever generated by the commercial forms of these devices is approximately 1-2 W. In many commercial and military applications, power requirements may reach hundreds of watts.
Like most semiconductor lasers, the Aerius Photonics VCSELs rely on semiconductor wafers fabricated using common volume manufacturing techniques. VCSELs offer other advantages as well. For example, they require no mechanical packaging assembly to produce the laser, and technicians can test them on the wafer prior to additional processing. In addition, VCSELs produce a circular output beam that designers can easily integrate into devices without using complex lenses. This same output characteristic enables the assembly of beams into dense 2-D arrays (see Figure 4 on next page). Finally, VCSELs are energy-efficient and demonstrate greater wavelength stability over varying temperatures than do other semiconductor lasers.
Aerius Photonics engineers successfully employed VCSEL technology to produce 60 W of peak pulse power from a single VCSEL, achieving six times more output than previously reported for any other diode laser. They were also able to produce 30 W peak power pulses at 67 kHz, with more than 0.5 μJ per pulse at 15 ns pulse widths. These lasers are capable of operating in dense 2-D arrays with hundreds of elements, generating hundreds of times more output power per chip. Dr. Jon Geske, the Aerius Photonics principal investigator, summarizes the technology's potential: "Partnering with the Air Force in the SBIR program, Aerius Photonics has been able to develop an entirely new source-laser approach for reduced costs in LADAR and laser range-finder applications. This is a real advancement in the field and will make a major impact on how future semiconductorbased LADAR systems are built for military and civilian applications."
SBIR Phase I funding not only allowed Aerius Photonics to create the technology for a new generation of compact, high-performance, low-cost LADAR transmitters applicable to aviation, automotive, law enforcement, recreational equipment, and other civilian markets, but also enabled the company to address the key risk factors (e.g., operational temperature range, power output, and beam quality) related to VCSEL usage in such varied applications. The company's VCSEL approach therefore offers a promising cost/performance solution to the challenges of fabricating semiconductorbased LADAR systems.
Photodigm, Inc., devised the second SBIR-funded approach, which exploited a mature waveguide semiconductor laser technology—known as the Grating-Coupled Surface-Emitting Laser (GCSEL)—to develop a viable method for combining multiple output beams and directing output from the chip's surface rather than its edge. Photodigm investigated the application of a high-power, eye-safe, 1550 nm GCSEL array with integrated lenses in military detection and ranging systems. GCSELs have the desirable traits of both conventional edge-emitting lasers and vertical-cavity semiconductor lasers.3 Their arrays exhibit a large active gain volume and wafer-level scalability favorable for high-power operation.4 Additionally, the GCSEL's surface-emitting nature simplifies the process of combining output beams from multiple lasers to achieve either a higher-power beam or a multiple-wavelength beam (see Figure 5). Foundries can easily form 1-D and 2-D high-power GCSEL arrays to accommodate a broad range of applications in LADAR, proximity sensing, machining, materials processing, and medical diagnostics and therapeutics. Ongoing improvements in output beam quality also make this device ideal for use in diode-pumped solid-state laser applications.
During Photodigm's SBIR Phase I project, researchers led by Dr. S. David Roh successfully designed, fabricated, and characterized eye-safe, high-power GCSEL arrays with >2.5 W output power and lasing at 1535 nm (see Figure 6). The specific GCSEL under investigation also exhibits superior output beam quality compared to traditional, edge-emitting lasers. Photodigm researchers are now focused on improving the GCSEL's power efficiency and output beam quality and exploring ways to achieve even higher power levels by combining multiple GCSEL arrays. This technology will enable the rapid development of low-cost, high-power, longwavelength, surface-emitting lasers, while circumventing major technical issues associated with conventional edge-emitting lasers and longwavelength VCSELs. Furthermore, the GCSEL architecture is extendable to both shorter and longer wavelengths (630-1900 nm and beyond), the large optical aperture area is less susceptible to catastrophic optical mirror damage, and the large gain volume allows higher-power operation compared to VCSELs. Most importantly, the GCSEL architecture provides a single-device technology platform versatile enough to be adapted for different applications.
GCSELs have enormous promise in revolutionizing the use of diode-based lasers in applications where light quality is paramount. Because all other high-power diode laser technologies produce multimode beams, their primary use is to pump solid-state lasers to produce high-quality laser beams. These two-component units reduce an optical system's efficiency, while increasing its complexity. Because the GCSELs under development operate in eye-safe wavelengths, they meet military requirements for atmospheric propagation applications, including LADAR, target designation, and range finding. These high-brightness laser emitters will likewise find commercial application in high-resolution materials processing and high-coherence optical systems.
As Dr. John E. Spencer, Photodigm's president and chief executive officer, maintains, "The fundamental requirement for the success of any technology-based small business is to find product applications that can develop into major businesses. The SBIR program introduces us to those first-mover military customers who are able to absorb development risk, while simultaneously directing us to seek the wider customer base that ensures both commercial viability and military affordability. Photodigm is pleased to support the efforts of the US Department of Defense in ensuring the technological superiority of our military forces."
Mr. Don Snyder, Dr. Jon Geske (Aerius Photonics, LLC), and Dr. S. David Roh (formerly of Photodigm, Inc.), of the Air Force Research Laboratory's Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document MN-H-05-12.
- Geske, J., et al. "CWDM Vertical-Cavity Surface-Emitting Laser Array Spanning 140 nm of the C, S, and L Fiber Transmission Bands." IEEE Photonics Technology Letters, vol 16 (May 04): 1227-1229.
- Bjorlin, E. S., Geske, J., and Bowers, J. E. "10 Gb/s Optically Preamplified Receiver Using a Vertical-Cavity Amplifying Optical Filter." 2002 Optical Fiber Communications Conference, Postconference Technical Digest, vol 1. Washington DC (2002): 153-155.
- Evans, G. A., et al. "Grating-Outcoupled Surface-Emitting Semiconductor Lasers." Surface-Emitting Semiconductor Lasers and Arrays, Chapter 4. eds. Evans, G. A., and Hammer, J. M. Academic Press (1993): 119-216.
- Roh, S. D., et al. "High-Power, Single-Frequency, Surface-Emitting Laser With a Lens for Lateral Mode Control." Presented at Solid-State and Diode Laser Technology Review, Tech. Digest, P-12. (2004).