Beam-quality Measurements for Materials Processing Lasers and the Proposed ISO Standard

Book excerpt: Laser beam-quality can indicate the maximum power density that can be obtained with a specific laser and serves as a figure-of-merit when comparing lasers, calibrating lasers, and in assessing a laser's operating condition. Measurements of laser beam-quality for a 1000 watt continuous wave CO2 laser and a 400 watt pulsed Nd:YAG laser have been completed. These measurements were made with two different type instruments: (1) a specially constructed apparatus that uses an integrating sphere and diamond apertures and (2) a commercially available instrument that incorporates a scanning and spinning hollow needle. Laser beam-quality and the propagation constants have been determined using a non-linear curve-fitting technique and the new proposed ISO standard for measuring laser beam-quality. The curve fitting analysis and the ISO analysis were found to produce approximately the same values for beam-quality and the propagation constants. However, a comparison of the experimental procedures required with the two techniques showed significant differences. The utility of the ISO technique was limited by it's experimental procedural restrictions. The importance of spherical aberration in distorting laser beam-quality measurements and in limiting the application of beam-quality to laser spotsize prediction is also detailed.