KRISTA MCEUEN
The continuing development of semiconductor laser technology is leading to higher output powers, improved efficiency, and better reliability from devices with improving price-to-performance ratios. Such advances are opening up many previously unavailable commercial markets for diode lasers—both for specialized applications and for more widely used processes requiring a concentrated source of heat (see photo). Not only are new markets appearing, but established applications are also benefiting from these developments. High electrical-to-optical power conversion efficiency combined with low maintenance and compactness make diode lasers ideal for industrial, medical, graphics, and illumination applications, as well as the pumping of both conventional and waveguide solid-state lasers. Recent improvements in output power and power density mean that diode lasers are increasingly replacing both conventional and laser sources in many applications.
Commercially available 1-cm diode laser bars emitting at 870, 915, and 940 nm, such as those recently introduced by Opto Power Corp. (OPC; Tucson, AZ), are based on advanced device structures that allow these monolithic arrays of individual emitters to operate with a continuous-wave (CW) output power of up to 40 W and to produce a peak output power of 100 W without compromising lifetime (see Fig. 1).1 Such power levels open up applications in industrial, medical, and other markets that were previously beyond the performance characteristics of diode lasers.
The OPC high-power semiconductor bars deliver twice the power of conventional bars with increased overall efficiency. They also offer improved lifetime, the ability to operate at elevated temperatures, and reduced beam-divergence angles (see Fig. 2). The combination of more power and less beam divergence provides an end user with higher brightness, which can lead to increased throughput in process-related applications as well as reducing the overall system cost and complexity. In many such applications, overall operating costs are reduced by the faster process time, which also minimizes device on-time and consequent power consumption costs.
While raw output power is the primary concern in many uses of diode lasers, the characteristic elliptical spatial mode of diode-laser bars presents special challenges in common industrial applications such as marking, soldering, epoxy curing, sintering, cutting, and microwelding. These tasks often have demanding requirements on power, spot size, and working distance—or depth of field—so a minimal loss of brightness can be tolerated in the beam-imaging system. Hence, novel imaging techniques may have to be combined with the high-power bars in order to achieve the required beam profile. At OPC we have, therefore, developed diode-laser products based on a beam-shaping technique licensed from the University of Southampton (Southampton, England) that produces a beam with symmetrical spatial profile.2
One area in which the increased brightness of high-power bars is enabling dramatic advances is that of fiber lasers and other fiber-coupled, diode-laser components. With the OPC 40-W bars, more than 30 W of output power can be delivered through a 1.16-mm-diameter fiber bundle with a numerical aperture (NA) of less than 0.1—measured at the 1/e2 power envelope. With a pair of diode-laser bars, 60 W of power can be delivered through a 1.55-mm-diameter bundle with the same NA. And more than 20 W of power from a single high-power bar with the beam shaper can be coupled into the 0.25-mm-diameter pump core of a double-core, single-mode fiber laser.3
High-power applications
Microwelding and cutting are two industrial processes benefiting from higher-power bars. The bars are capable of providing the necessary thermal impulse rapidly enough to avoid collateral damage caused by localized heating. Typical laser requirements are 25 W of power in a 0.35-mm-diameter spot—corresponding to a power density of 6.5 kW/cm2. High-power diode lasers are being used to microweld miniature stainless-steel components for medical instruments and automotive parts, as well as stainless-steel wire.
Diode lasers are particularly well suited to welding and cutting of plastics. More power allows faster processing of thicker materials—a key requirement, for example, in printed-circuit-board manufacturing. Processing with a diode laser allows elimination of the irregularities characteristic of traditional cutting tools that dull with use.
In soldering applications, a crucial factor is the repeatable application of heat in a small, well-controlled spot. Diode lasers produce a repeatable amount of heat with reasonably uniform distribution. This reduces the required dwell time on critical components, thereby reducing the potential for collateral thermal damage while increasing processing speeds. Another important advantage to soldering with diode lasers is that, in contrast to soldering irons and other contact tools, diode lasers virtually eliminate risk of damage to electronic components due to electrostatic discharge.
Medical applications of high-power bars include a variety of thermal treatments, as well as contact and noncontact surgery. In all of these procedures, output power and fiber size are critical. Smaller fibers provide greater flexibility in traveling through the human body. A single 40-W bar used with the production beam shaper is capable of coupling more than 10 W into a 250-µm-diameter single-core fiber. A set of four bars with individual beam shapers is currently being used to couple 50 W of power into a 600-µm-diameter, single-core fiber.
Rare Earth Medical Inc. (REM; West Yarmouth, MA) is evaluating the use of an OPC 10-W, high-brightness, fiber-coupled diode laser—consisting of a 40-W bar coupled through a beam shaper and launched into a 250-µm-diameter single-core fiber—in a several medical applications, including the excision of tumors and treatment of cardiac arrhythmia. The company has obtained Small Business Innovation Research (SBIR) funding from the National Cancer Institute and the National Heart, Lung, and Blood Institute of the National Institutes of Health (Bethesda, MD) to develop a Teflon diffusing tip that emits light uniformly from the side of a fiber instead of the tip. The Lightstic is capable of destroying tumors found in the breast, liver, and other areas. It is also being used in photodynamic therapy trials and has been placed remotely into the ventricle of the heart in preclinical experiments to "ablate" arrhythmia.
"In these photothermal and photodynamic therapy applications, fiber flexibility and size are key issues," says Ed Sinofsky, founder and CEO of REM. "To deliver therapeutic power levels, conventional diode lasers require 400-µm- or 600-µm-core fibers. High-power, high-brightness technology produces better power density, affording use of a 250-µm-core fiber. This means smaller, less-invasive access and greater flexibility to deliver our devices remotely through guiding catheters into the heart or other places where bigger fibers can't go."
Although the core technology has matured, innovation continues to drive diode lasers into new applications. The development of higher-power diode lasers, along with advanced imaging techniques, will ultimately result in increased use of diode lasers in diverse industrial applications, clinical and diagnostic medicine, and, ultimately, in yet-to-be-discovered processes requiring a precise application of heat.
REFERENCES
1. S. Gupta et al., "High power diode lasers arrays and 2-D stacks," 1997 Diode Laser Tech. Rev., (1997).
2. W. A. Clarkson et al., "Novel beam shaping technique for high-power diode bars," CLEO '94 Tech. Digest, paper CTHL2, p. 360 (1994).
3. D. Inniss et al., "Ultrahigh-power single-mode fiber lasers from 1.065 to 1.472 mm using Yb-doped cladding-pumped and cascaded Raman lasers," CLEO '97 postdeadline paper CPD-31-2 (1997).