All the benefits of laser diodes apply to direct-diode systems: small size, long life, and high wall-plug efficiency.
Although sometimes grouped with fiber lasers, so-called direct-diode lasers are actually very different beasts. While a fiber laser consists of a gain fiber that is end- or side-pumped, a direct-diode laser is actually made up of a non-gain optical fiber that is simply filled with light that originates from a multitude of laser diodes coupled to the fiber.
Both fiber lasers and direct-diode lasers in high-power forms are used for materials processing. While high-power fiber lasers can be had with either single-mode or multimode outputs, high-power direct-diode systems are always multimode, due to the fact that the etendue of optics for combining light from many laser diodes will be substantially above the single-mode limit. Nevertheless, direct-diode makers are continually improving the brightness of their products—in addition, many materials-processing applications do not require a single-mode beam. Most, but not all, direct-diode lasers emit in the near-infrared (near-IR).
While this article cannot include more than a portion of the direct-diode products available in today’s market, a representative sampling of what can be found is presented here.
Flexible beam characteristics
In the near-IR around 1 µm, a single-mode laser beam would have a beam-parameter product (BPP) of about 0.3 mm-mrad, which is unachievable by direct-diode lasers, which have a BPP that can range from a few to a few tens of mm-mrad. For example, II-VI (Kleinmachnow, Germany) produces a variety of direct-diode laser products with power levels from a few watts to multikilowatt systems, as well as many types of single laser-diode emitters and bars that it can use in its direct-diode laser products. “All systems in the lower- to medium-power-level range are based on II-VI’s proprietary single-emitter laser diodes,” says Haro Fritsche, manager of business development and technologies at II-VI. “By vertically stacking multimode, high-power single emitters and using polarization beam combining, a BPP of about 6–15 mm-mrad is achieved in a small, conductively cooled assembly with power levels of up to 300 W. II-VI integrates these assemblies into a broad portfolio of fiber-pigtailed modules for fiber-laser pumping. These assemblies are also integrated into turnkey direct-diode laser systems with detachable fibers, such as II-VI’s DirectLight900 product line, which can achieve very high power and is ideal for materials-processing applications.”
In the DirectProcess900, the high-power beam is achieved by wavelength-multiplexing the laser stacks and polarization-combining submodules, says Fritsche. This approach enables the DirectProcess900 to achieve a BPP of 6 mm-mrad over a broad range of power levels from 100 W to beyond 1000 W. The flat-top beam profile can then be delivered to the workpiece in free space or through fibers with a core size of 100 or 200 µm. Due to the wavelength multiplexing, these systems emit within a wavelength range of 900 to 980 nm.
For further power scaling, II-VI recently extended the DirectProcess900 product line with a high-power, wavelength-stabilized bar-based module featuring a flat-top beam profile and a BPP of 5 mm-mrad, notes Fritsche (see Fig. 1). The base module achieves a bandwidth of less than 25 nm and an output power of 1.5 kW. These basic modules can be used for direct applications like cutting and welding as well as for high-energy laser pumping. Wavelength multiplexing extends the power levels of this platform far into the multikilowatt range without any change in beam characteristics. Cooling systems and driver electronics will soon be available for these products, says Fritsche. II-VI also produces beam-delivery products such as high-power fiber-optic cables and laser processing heads.
The DirectProcess900 FlexShape light engine has a free-space rectangular BPP of 6 mm-mrad and is equipped with a remote-controlled variable beam shaper to generate rectangular beam profiles with length and width individually adjustable on the fly from 0.2 mm to more than 50 mm. This allows it to excel in applications that require rectangular beams of adjustable length and width while maintaining a highly uniform beam intensity profile, says Fritsche. “The system is ideal for soldering microelectronic parts, where relatively small spot sizes ranging from 0.2 to 3 mm are adjusted over a fairly long working distance,” he says. “These long working distances are enabled by the low BPP of the laser system. The workpiece area is uniformly illuminated by the flat-top beam profile. By utilizing a flexible spot size, the FlexShape option offers customers the ability to use one laser system for all soldering geometries.”
Power levels between 800 and 1200 W are offered in rack-mounted assemblies that are 2 RU in height and 19 in. in width. “Another key feature of the system is its very short rise- and fall-times of less than 10 µs, while maintaining a very uniform power distribution across the beam, making this laser system ideal for numerous heat treatment applications,” says Fritsche. The DirectProcess900 can also be had with linear polarization, which is achieved by replacing the polarization coupling with wavelength multiplexing. By keeping the laser beam polarization aligned with the cutting direction, cutting quality and speed increase significantly.
“Another benefit of direct-diode laser systems such as the DirectProcess900, compared with pumped laser systems such as fiber lasers, is the ability to operate at lower wavelengths and a broader bandwidth, typically between 900 nm and 980 nm, to achieve better absorption in some materials such as silicon and aluminum,” Fritsche explains.