Diffractive Optical Elements in Infrared Systems

Diffractive Optical Elements

Infrared laser systems are the most widely used in the material processing industry. The emission wavelength of these laser systems goes from 1 to 10.6 micrometers, depending on the exact type of lasing process within the system. These infrared laser systems are capable of reaching light densities high enough to be used in either marking, ablation or welding processes.

More often than not, these infrared laser systems work much more efficiently with the inclusion of a diffractive optical element that serves one or more specific purposes. Such a diffractive optical element has to be made from a material that is transparent to the infrared wavelength that is going to be used. The most common application is perhaps to transform the initial Gaussian radiance pattern of a laser system into a Top Hat radiance distribution. This transformation entails removing the smooth edges and turning them into clear-defined boundaries. The immediate effect of this transformation is that all the light lands exactly on a predefined area with the consequent improvement in process efficiency and quality.

For infrared laser systems there are other, and quite useful as well, optical transformations that can be performed to enhance the process in question. One type of transformation is referred to as the flexishaper and its purpose is to distribute the irradiance from a fibre laser between two areas: a central spot and a ring surrounding it. To this effect, it uses two different diffractive optical elements in tandem and a rotation of one with respect to the other produces a change in irradiance between these two areas of interest. This is useful for rapid laser welding, as it prevents the formation of humps and discontinuities in the weld. 

Another useful transformation can be referred to as the C-shaper transformation which involves creating a ring with an opening. This type of irradiance distribution is useful during some welding processes in which the appearance of bubbles and inclusions can hamper the process. Thus, the opening in the C provides a way out for these defects, leaving a smooth weld behind. There is another similar transformation called the T-shaper . In this case the irradiance distribution is characterised by a hot spot located in the upper part of a letter T like illumination pattern. The non-zero irradiance in the rest of the letter T is useful in a welding process referred to as butt welding because it helps reduce the weld seam height and thus obtaining a more homogenous process.

There are other applications for diffractive optical elements in infrared laser systems. In fact, there can be as many as the number of different welding processes. 

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