Holographic beam shaping of high power lasers for additive manufacturing

现代世界对消费品和技术的需求不断增加，这对制造业有着直接的影响，制造业总是试图将成本降至最低，提高效率，同时将对社会和环境的影响降至最低。制造业最近见证了3D打印等直接制造工艺的大幅增长，这种工艺只需最少的工具，可以低成本和低环境影响生产高质量的产品。最近发展了一系列极具冒险精神的制造技术，如添加剂制造(AM)，它极大地扩大了可加工材料的范围，从塑料到金属，再到其他更奇异的材料。最重要的AM工艺之一是使用高功率激光发射高能光波，用于熔化金属粉末，并逐渐在材料层上添加，以形成最终的制造部件或产品。这些系统使用扫描镜来控制高功率激光光斑在粉末床上的位置。这一过程在熔化粉末的激光焦点处产生了非常强烈的热区，然而，由于过程中激光光斑的高度局部化性质，这种热能的控制非常困难。这导致制造部分中的热应力和变形非常难以预测，甚至更难可靠地控制。计算生成的全息图使用光学衍射作为一种手段来控制基于在液晶显示器上显示的二维图案的三维能量分布。然后，可以使用衍射过程来控制写入激光在粉末上的分布，以获得更可控的粉末熔化区域，从而将写入过程的热影响降至最低。全息图还可以用来补偿激光和光学中的缺陷，因此它可以提供接近衍射限制的性能。更重要的是，熔化过程并被实时监测，全息图可以被重新计算，以减轻这些影响，并控制AM工艺的形状、质量和材料。他的全息控制的最大限制之一是液晶显示器可以控制的光能总量。这一极限将被充分研究，作为这项研究的结果，将生产出专门为AM工艺中的高功率激光照明而设计的新一代显示器。

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.