Holographic beam shaping of high power lasers for additive manufacturing

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The modern world has an ever increasing demand for consumer products and technology which has a direct impact on the manufacturing industry who are always trying to minimise costs and increase efficiency whilst minimising the impact on this production on both society and the environment. The manufacturing industry has recently seen a massive rise in direct fabrication processes such as 3D printing which require minimal tooling and can produce high quality items at low costs and low environmental impact. Most recently there have been a series of highly adventurous manufacturing techniques developed such as additive manufacture (AM) which has greatly expanded the range of materials that can be processed from plastics to metals to other more exotic materials.One of the most important AM processes has resulted from the use of high powered lasers to deliver high energy light waves which can be used to melt a metallic powder and gradually add layer up on layer of material to make up a final manufactured part or product. These systems use a scanning mirror to control the positions of a high powered laser spot onto a bed of powder. This process creates a very intense region of heat at the focus of the laser which fuses the powder, however the control of this thermal energy is very difficult due to the highly localised nature of the laser spot in the process. This leads to thermal stresses and distortions in the part being made which are very difficult to predict and even more difficult to control reliably.Computer generated holograms use optical diffraction as a means of controlling the distribution of energy in three dimensions based on a two dimensional pattern which is displayed on a liquid crystal display. The diffraction process can then be used to control the distribution of the write laser on the powder to give a more controlled area of powder melt, minimising the thermal impact of the writing process. The hologram can also be used to compensate for imperfections in both the laser and the optics, hence it can deliver near diffraction limited performance. More importantly, the melt process and be monitored in real time and the hologram can be recalculated to mitigate these effects as well as control the shape, quality and material of the AM process. One of the biggest limitations tot his holographic control is the amount of optical energy that can be controlled by the liquid crystals display. This limit will be investigated fully and a new generation of displays will be produced as a result of this research which are designed specifically for high power laser illumination in AM processes.