A high-power diode laser primer

Auto production, market scanners, and space elevators

Silke Pflueger

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The majority of industrial lasers in use today are CO2, solid state, or fiber lasers. But slowly, diode lasers are making inroads into many of today's applications as the power and beam quality of these lasers is increased. Keep on reading for a look at the history of diode lasers, their technology, and their many uses, with a special focus on laser material processing.

The early years

The first laser diodes, discovered shortly after LEDs in 1962, had to be cooled with liquid nitrogen to generate continuous-wave light. A breakthrough occurred in 1970, when Alferov and Kroemer independently discovered the double heterostructure laser diode (http://www.nobelprize.org/nobel_prizes/physics/laureates/2000/alferov-lecture.pdf), which enabled operation at room temperature, in continuous wave (CW) mode, and at a potentially wide range of wavelengths. The remainder of the 1970s was put to good use developing materials and fabrication methods for these diodes, leading to the development of reliable, relatively low power devices.

Figure 1. Beam shaping optics for high power diode laser stacks, with fast axis collimators, beam reformatting, and focusing optics. (Courtesy of Laserline)

The 1980s saw the wide deployment of diode lasers replacing HeNe-lasers in grocery store scanners and enabling CD players, prices of which dropped rapidly, driven by falling prices of the laser diodes.

Aside from reducing cost, another big push of the mid-1980s was to increase the output power of diodes. In 1990, Lawrence Livermore National Labs presented a 1.45 kW stack-to-pump solid-state slab laser.

From "dumb" high power to focusable energy

Optically pumping a solid state slab laser was all about meeting the right wavelength band with as much power as possible, but not about focusing the light to a small spot.

High power diode lasers, on the other hand, have a very distinct beam, and consist of a large number of individual laser diodes, each with very high beam quality in one axis (fast axis) and low beam quality in the other (slow axis). To get to high power, rather than separating the semiconductor wafer into individual diodes, 19 or more emitters form an array on one piece of semiconductor, called a diode bar, which typically is 1 cm wide. These are then mounted on thin coolers and stacked -- resulting in hundreds of beamlets, not one single beam as with conventional lasers.

To preserve beam quality, the fast axis of the entire bar is first collimated with a micro-cylinder lens. The resulting beam is rectangular, with vastly different beam qualities in the two axes, requiring beam shaping optics that will square the beam quality.

Figure 2. Ultra-high brightness diode laser cutting steel. (Courtesy of TeraDiode)

Dilas and Laserline, both located in Germany, as well as Nuvonyx in St. Louis, MO, were the first companies to develop high power diode lasers specifically for the industrial laser market. It all started with low-power, low-beam-quality applications such as plastic welding, heat treatment, and paint stripping, the latter two applications also lending themselves to line focusing. These companies subsequently drove the development by refining the mechanical and optical designs and by taking advantage of the ever-increasing power levels out of diode lasers.

Expanding into mainstream applications

Fiber coupling, and increasing the output power to several kilowatts, turned high power diode lasers from a niche product to a major player in the market. Brazing automobile body-in-white parts became the breakthrough application, showing that diodes were reliable and energy efficient competitors, able to vie with established laser technologies and also with traditional welding technologies because of easier energy accessibility to the weld joint and improved stiffness of the welded components.

With higher power and efficiency, 6 to 10 kW diode lasers became the choice for cladding applications in energy generation, which were being fueled by increasing oil prices and improvements in laser cladding technology. The water walls in coal-fired boilers and oil drilling equipment are laser coated to withstand corrosive atmospheres and abrasive forces. Competing with conventional technologies such as thermal spraying, diode lasers were able to produce better coatings with a metallurgical bond to the base material and less heat input, which reduces or eliminates secondary post-processing operations, with similar or improved cost structure.

Ultra-high brightness diodes

Getting to the largest and most lucrative of laser applications, laser cutting, requires a shift in the optical design of the diode. Conventional stack based architectures are limited in brightness by the dark space between the emitters and between the bars. The key to ultra-high brightness diodes is to design an optical system that allows access to the brightness of the emitters and combines the beams while preserving the brightness.

Several companies are working on ultra-high brightness diode lasers. The key technology they all share is dense wavelength multiplexing (FIGURE 1). Individual laser diodes or banks of them are lasing at slightly different wavelengths, typically in the 900–1000 nm range, and combined with filters or gratings. TeraDiode, a Massachusetts-based company, reaches 2 kW with 3.1 mm*mrad. DirectPhotonics (Berlin, Germany) is targeting 2 kW with 7.5 mm*mrad. Two large government programs, the European BRIDLE and the German BrightLas, are aiming to reach similar performance levels, while at the same time investigating additional technologies surrounding high power diode lasers. Initial cutting tests have shown TeraDiode's lasers producing results that are equivalent, if not superior, to fiber lasers (FIGURE 2).

Why the drive to diode lasers instead of the existing technology, especially fiber lasers? One reason, in this writer's perhaps-biased opinion, is that all high-power diode lasers only use passive, free space optics. Fewer components, often automatically assembled, leads to higher efficiency, smaller size, better reliability, and ultimately a better cost structure.

And finally: Space elevators

Space elevators are essentially long cables, anchored on a planet or a moon, with the center of mass located in geostationary orbit. Climbers would be used to transport goods up the cable into orbit. One challenge among many others is powering the climbers, and this is where diode lasers come into play. Functioning as a "cordless extension cord," they send energy in the form of light to the climber, which converts it into electrical energy. However, even though a Japanese company plans to build a space elevator by 2050, laser cutting is certainly going to be the short term market for all ultra-high brightness diodes currently in development.

Between improvements in semiconductor laser material, mounting technologies, and the new optical combination schemes, it is foreseeable that the diode laser will revolutionize the CW material processing market for cutting and welding. While fibers and disks will continue to have a role in energy storage for pulsed systems, their days as brightness enhancers for low brightness diodes could likely numbered.

Silke Pflueger is general manager of DirectPhotonics (silke.pflueger@directphotonics.com). She is also an Editorial Advisor to Industrial Laser Solutions.

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