LASER-BEAM SHAPING: Diode-laser system yields 11 kW homogenized output
Homogenization techniques to produce structured light (uniform lines or two-dimensional illumination in a square array, for example) are becoming more important for applications such as machine vision, liquid-crystal-display technology, the printing industry, and other surface applications like hardening or thermal annealing.
Homogenization techniques to produce structured light (uniform lines or two-dimensional illumination in a square array, for example) are becoming more important for applications such as machine vision, liquid-crystal-display technology, the printing industry, and other surface applications like hardening or thermal annealing (see “Laser projections assist machine-vision applications,” p. 131). Engineers at Dilas (Mainz-Hechtsheim, Germany, and Tucson, AZ) have developed a diode-laser system with an 11 kW output power at 940 nm-to their knowledge, the highest output power reported for a direct-diode system-that can deliver 1 kW/cm2 at a working distance of 400 mm with better than 90% homogeneity in the intensity distribution over a 55 >× 20 mm area.1
There are several known techniques for laser-beam homogenization, including overlapping the beams of several diode lasers (or laser bars) arranged in a line or array, using a waveguide structure to mix the intensity distribution from a single diode input, or by using a micro-optical lens array to create overlapping beamlets from a single diode source that simulates the overlapping effect from multiple sources. However, each of these methods can suffer from nonuniformity among the multiple source lasers and/or micro-optic diffraction effects that limit the output beam homogeneity.
To overcome these issues, Dilas uses an imaging microlens array. In this system, two microlens arrays are separated by a distance d, which is identical to the focal length of the second microlens array. Light leaves the diode-laser source and passes through collimation optics onto the first microlens array. This first array creates a series of beamlets that enter the second array and a field lens that images the overlapped beamlets onto the workpiece. In this imaging system, the demand on initial laser-beam quality is not as high and diffraction effects can be minimized. In addition, imaging microlens arrays provide better homogeneity and edge steepness. Furthermore, changing the distance d will change the size of the homogenized output beam over a small range and will affect the edge steepness of the intensity profile.
To develop the diode system for a nominal 55 × 20 mm homogenized output, the engineers used Zemax (Bellevue, WA) software to model the resulting intensity distribution based on laser and optical parameters (see figure). To adjust edge steepness, the ratio of the divergence of the incident beam in the x and y directions, as well as the numerical aperture of the microlens array, need to be controlled in a nonimaging setup. In the imaging setup used by Dilas, slight misalignment of the two microlens arrays allows the designer to adapt the edge steepness of the resulting laser-beam intensity profile. And, to minimize diffraction effects, calculations show that microlens elements with a Fresnel number well above 10 (well into the near-field diffraction regime) are needed in the optical setup.
The final diode-laser system consists of five 940 nm diode lasers-each delivering 2.4 kW of laser power-and beam-homogenization optics. The output beam-intensity profile in the focal plane was experimentally analyzed at low power using a screen that converts infrared light into visible light for analysis, as well as by illuminating a plastic panel. For both methods, experimental results agreed with predicted simulations in terms of spot size and intensity distribution. For beam analysis at full operating power, a commercial beam monitor from Primes (Pfungstadt, Germany) was used. The measured homogeneity was found to be ±5%, in line with predicted results.
“The modular approach of the laser system allows the customization of the total output power as well as the customization of intensity profile and focus dimensions,” says engineer Bernd Köhler. “The total output power can be varied from several watts up to more than 11 kW. The modularity of the homogenization optics allows homogenization in one or two directions resulting in line, rectangular, or quadratic illumination profiles. We believe that, in the future, further applications will benefit from the advantages of high-power diode-laser systems with homogenized intensity distributions.”
1. B. Köhler et al., Proc. SPIE 6456, paper 6456-22 (Feb. 7, 2007).