Use of a diffractive optic for high power laser cutting

There is a current interest in using laser cutting for nuclear decommissioning applications. The benefits of using lasers for this application include the high speeds available, the tolerance of the process, the lightness of the cutting head, the lack of a reaction force with the part being cut, and the ease of automation of the laser cutting process. Of course laser cutting is a thermal process and a potential detriment, is that the residual laser beam, passing through the kerf, might damage or indeed set fire to something positioned behind the part being cut. This paper describes the use of a diffractive optical element in the laser beam forming optics, designed to extend the depth of focus of the system, without increasing the focal length of the focusing optic. In this way, for cutting thick materials, the goal is to achieve the cutting performance of a long focal length lens, with the beam divergence of a short focal length lens. A design of diffractive optical element is presented, which when used with a 5 kW fiber laser, in conjunction with a set of conventional beam forming optics, including a 250 mm focal length focusing lens, is able to cut 40 mm thickness C-Mn steel plate at 50 mm/min, while varying the nozzle tip to plate stand-off distance by 100 mm. For the same laser power in the focusing beam, this performance is equaled by removing the diffractive optic and 250 mm lens and replacing these with just a 500 mm lens. However, the power density in the residual beam, when using the diffractive optical element, has been reduced by a factor four when compared to that from the optical arrangement using the 500 mm lens.