Ultra-high brightness, kilowatt-class fiber-coupled diode lasers

A long-standing, unmet objective of laser technologists has been to produce high-power direct-diode lasers with very high brightness. Brightness is defined as the ratio of power to laser beam quality and, for a given power level, it is maximized when beam quality approaches the diffraction limit.

A large potential market exists for kilowatt-class direct-diode lasers that are bright enough for industrial processes including cutting and welding of sheet metal. Ultra-high brightness lasers with narrow spectral bandwidth (less than 4nm) are useful for pumping high-performance fiber and solid-state lasers. Applications in directed-energy weapons are the most demanding, with requirements of nearly diffraction-limited operation at power levels in excess of 10kW. Conventional direct-diode lasers offer competitive price and high reliability but are not bright enough to carry out the most challenging materials processing or pumping applications.

We have developed fiber-coupled diode lasers that use wavelength beam combining (WBC) to achieve brightness levels orders of magnitude higher than those of existing commercial products. The WBC external cavity serves two purposes: wavelength stabilization and brightness scaling.¹ An array of diode lasers is placed in this external cavity consisting of a transform lens, a diffraction grating, and an output coupler: see Figure 1. Each of the three emitters in the example array is wavelength-locked, as illustrated by the three colors, red, green, and blue. The brightness scaling property is illustrated by two black rays that represent the spatial superposition at the output coupler of the three element beams. The output beam quality is ideally that of a single element in the array. The output power is scaled by the number of elements in the array, while preserving the beam quality of a single emitter.

Figure 1. Schematic diagram of an example three-emitter external-cavity diode laser that uses wavelength beam combining.

We also built and demonstrated a fiber-coupled direct-diode laser with a power level of 1,040W from a 200μm core diameter, 0.18 numerical aperture (NA) output fiber (90% power content): see Figure 2.² The measured beam parameter product is 18mm-mrad with a brightness of 31MW/cm²-str. The power conversion efficiency (PCE) peak is 25% at 50A (500W), reducing to 22% at 100A (1,040W): see Figure 3.

Figure 2. The 1,040W direct-diode laser system coupled to a 200μm, 0.18 numerical aperture fiber. The laser module is shown at the center of the 19-inch rack on the left of the photograph. A laser cutting and welding workstation is shown on the right.

Figure 3. Power conversion efficiency (PCE), output power (P), and voltage versus current for the laser shown in Figure 2.

We carried out demonstrations of sheet metal cutting at 1,040W of laser output power using the 200μm, 0.18 fiber coupled to a processing head. We cut a 1.6mm-thick 304 stainless steel with nitrogen assist gas and found the edge quality to be comparable or better than that obtained with other types of industrial lasers, including CO₂ and fiber lasers. In addition, we cut a 6.65mm-thick mild steel with oxygen assist. We have been able to cut mild steel at sheet thickness up to 12mm, although with reduced edge quality as compared with the 6.65mm-thick sheets. Keyhole welding in stainless steel was also demonstrated.

Furthermore, we have extended TeraDiode's direct-diode laser technology to smaller core-diameter fibers with a corresponding increase in brightness. A 418W laser module coupled to a 100μm fiber with an output NA of 0.15 has been shown: see Figure 4. The PCE at the maximum rated current of 110A (418W) is 29%: see Figure 5.

Figure 4. 418W fiber-coupled direct-diode laser module with 100μm, 0.15-numerical-aperture output fiber.

Figure 5. Power conversion efficiency, output power and voltage vs. current for the laser shown in Figure 4.

We have also demonstrated a laser module with 370W coupled to a 50μm fiber with an output NA of 0.15, which corresponds to a brightness level of 257MW/cm²-str. We found this system to be over one order of magnitude brighter than that of state-of-the-art fiber-coupled diode lasers.

In summary, our direct-diode technology based on WBC can be applied to brightness scaling of diode lasers of any wavelength for defense and commercial applications. We anticipate that output power levels in the range of approximately 1000W can be achieved with lasers with 50μm, 0.15 numerical aperture output fibers. Some potential applications at different wavelengths include eye-safe fiber-coupled direct-diode lasers for industrial applications and eye-safe high-brightness direct-diode lasers for directed energy applications.

More information on TeraDiode can be found at www.TeraDiode.com.