The urge to obtain more power from a new laser technology is fundamental and does not need a specific application to motivate the research. Extracting higher power from early diode lasers went hand in hand with improving their reliability, and the technical challenges were fundamentally the same—to reduce the threshold current and control the mechanisms that led to catastrophic failure, particularly facet damage.
Convenient packages, dependable lifetimes, and a choice of manufacturer are indications that high-power diode lasers have become a mature technology—to be sure, intensive research and continuing advances are still the rule, but in the marketplace, price is a primary consideration in the selection of a device. It has been estimated that within the next few years prices could fall to $1/W for some high-power devices.
Developments in high-power diode lasers have been at the center of revolutionary advances in the telecommunications and printing industries. In applications for which output beams of the highest quality are not required, advantages in size, cost, reliability, and efficiency have led increasingly to the choice of diodes over solid-state and gas lasers. [Diode lasers for telecommunications will be discussed in a future article in this series -Ed.]
Pumps for solid-state lasers
The first major application for a high-power diode laser was as an 808-nm optical pump source for Nd:YAG. First introduced in 1985, the main advantages of diode-pumped solid-state lasers (DPSSLs) over flashlamp-pumped devices were obvious. They were at least ten times more efficient and, consequently, smaller in size as a result of the decreased cooling requirement. Flashlamp lifetimes are measured in hundreds of hours, as opposed to tens of thousands of hours for the diode pump arrays. The main hurdle was the cost of diode-laser arrays—roughly half the total cost of DPSSLs.
Reductions in the cost of diode-laser pumps led directly to widespread use of DPSSLs, which today have replaced virtually all flashlamp-pumped lasers with less than 100-W output. Depending upon the application, a DPSSL can be pumped by a single-stripe diode laser, a fiber-coupled array, or a two-dimensional stack of arrays. DPSSLs with more than 1-W output are available in packages a few centimeters on a side, frequency-doubled DPSSLs produce reliable high-quality visible beams, and high-power DPSSLs are in the kilowatt range.
In addition to the 808-nm pump for Nd:YAG (which is also now used to pump Nd:YLF and Nd:YVO4), 915-nm sources are produced for fiber-laser pumps, 940-nm arrays pump Yb:YAG, and 980-nm single-mode sources pump erbium-doped fiber amplifiers. Ironically, by providing the market force to reduce the cost of high-power diode lasers, DPSSLs may have helped lay the foundation for their own eventual obsolescence by the direct output of diode lasers themselves.
Medical applications
High-power diode lasers are finding their way into virtually all medical applications, from hair removal to ophthalmic surgery. Recent promising developments in photodynamic therapy (PDT) for cancer has led SDL (San Jose, CA) to develop a 3-W, 630-nm diode-laser system specifically for this application. The system has been approved for PDT in Europe and is undergoing Food and Drug Administration tests in the USA.
Photodynamic therapy originated 100 years ago when German researchers injected themselves with derivatives of hemoglobin (which carries oxygen in red blood cells) and observed sunburn-like reactions in their skin. Exposure to red light caused the hemoglobin derivative to produce highly reactive forms of oxygen that damaged the tissues where the derivative had accumulated.
As a treatment for cancer, a light-activated hemoglobin derivative is combined with another blood component that is attracted to dividing cells. A characteristic of cancer cells is their rapid reproduction, so the result is a selective concentration of the PDT chemical in tumor cells. The chemical produces toxic species of oxygen in cancerous lung cells when exposed for several minutes to fiber-delivered red laser light.
The reactive oxygen has a half-life of only 5 ms, so once the laser is turned off, the toxic effects cease. The tumor is destroyed by the disintegration of the tiny blood vessels that nourish it.
Noble-gas MRI
A new technique for magnetic resonance imaging (MRI) makes use of diode lasers to produce high-resolution images of lungs. Conventional MRI technology relies on the resonant signal from protons in the hydrogen of water molecules in the body. However, most patients' lungs are filled with air, not water. By mixing spin-polarized noble gases with inhaled air, an image of the lungs can be formed with unprecedented resolution. These spin-polarized nuclei resonate extremely efficiently with the MRI signal.
Developed at Princeton (Princeton, NJ) and other universities, hyperpolarized-noble-gas MRI makes use of high-power diode lasers to orient the spins of the valence electrons of helium or xenon. By circularly polarizing the 795-nm output of the laser, electronic spin polarizations of nearly 100% can be achieved in microseconds. The electrons transfer their spin polarity to nuclei in collisions with neighboring gas molecules.
The weak nature of these interactions combined with competing mechanisms, which do not transfer spin to nuclei, result in production rates of roughly a liter of gas per 50 W-hours of absorbed light. Thus, multiple watt levels of light are needed to produce sufficient quantities of the hyperpolarized noble gas. The simplicity and reduced cost of new high-power diode systems has led to their selection over Ti:sapphire in this emerging application.
Improvements in beam quality and spectral control have led to the preference for high-power diode systems over complex dye and Ti:sapphire lasers in other laboratory applications, including Raman spectroscopy and the cooling and trapping of atoms. Diode systems are particularly well suited to airborne applications, such as pollution monitoring, or even space-borne applications. The Columbia and Discovery shuttle missions in 1997 and 1998 carried Opto Power Corp. (Tucson, AZ) lasers in a system to guide the docking of unmanned spacecraft.
Printing
Conventional printing processes use silver-halide films, which produce large amounts of chemical waste that is expensive and detrimental to the environment. In the early 1990s, thermal plates and films were developed that eliminated chemical processing. Coupled with innovations in the use of computers to control prepress processing, the new technology has greatly reduced preparation time and costs and, because no film is involved, simplified corrections.
The new thermal technology, however, is 10-4 to 10-5 times less sensitive to light than older films. The main factors in choosing the required high-power light source are operating costs, performance, and reliability. High-power diode lasers have enabled the design of innovative new printing systems. SDL, for example, produces an 830-nm diode laser that couples a 0.6-W CW beam into a 100-µm fiber for small spot sizes and long working distance, allowing high image production and lower demands on print-system mechanical tolerances.
Materials processing
As prices continue to fall, materials processing may prove to be the largest nontelecommunications-related market for high-power diode lasers. Direct output from diode lasers can replace traditional sources for microheat-treating and microwelding and compete with more-expensive DPSSLs. The small size and modest power requirements of these systems are leading to a potential "bench-top production" revolution in some manufacturing environments.
For example, a 30-W, fiber-coupled diode laser from Opto Power is being used to solder components in the sensor system of the new F-22 jet fighter. Although the sensitive components require low-temperature solder joints and have a center-to-center pin spacing of only a millimeter, the bench-top system can perform 300 solder joints per minute (see figure). Other bench-top thermal applications include welding, sintering, and annealing.
Expanding opportunities
The impact of direct diode lasers is just beginning to be felt in scientific and medical research. As new types of arrays (such as vertical-cavity surface-emitting lasers) and high-power single-stripe devices are developed with spatial and spectral characteristics that compete with existing lasers, this impact will grow. And given the apparent robustness of gallium nitride (GaN) as a laser material, the development of high-power direct blue sources may be more rapid than that of red diode lasers.
For applications in industry and medicine, a high-power diode laser is largely a directed thermal source. Even today, however, many customers buy arrays as components and build their own integrated systems, including optics, power supplies, and coolers. In the future, diode-laser manufacturers will simplify the use of their products by offering a wider but standardized range of integrated systems. These direct diode-laser systems will find a broader range of potential users and continue to erode the market share of existing lasers.
ACKNOWLEDGMENTS
We would like to thank the following individuals and organizations for providing images used in the Semiconductor Lasers 2000 series timeline: Michael W. Davidson, Florida State University; GE Research and Development Center; Zhores I. Alferov; Nick Holonyak, University of Illinois; Dan Botez, University of Wisconsin-Madison; Lucent Technologies, Bell Labs; Connie Chang-Hasnain, University of California Berkeley; Nichia Chemical Corporation; Jack Jewell, Picolight Corp. -Ed.
Next month the series will highlight developments that have led to high-power diode lasers.