A holographic diffuser is an optical component that is commonly used as a beam homogenizer. The input beam of many common multimode lasers might have some ripples, speckle or radiance variations across its cross-sectional area that can harm laser process quality and uniformity. Such a laser beam must be homogenized, or made smoothers, to achieve the required laser process quality.
This beam homogenization process is kind of a common process of all optical diffusers. A holographic diffuser goes one step further as it can be used also to transform the radiance shape of the input beam beyond a simple homogenization. Thus, for example, if the beam has a round shape, the output beam can be square, or vice versa. In fact, any geometrical shape can be obtained when using a holographic diffuser, provided that a sound optimization algorithm is employed when designing the structure of the holographic diffuser.
The holographic diffuser is a type of diffractive optical element and thus it works by utilizing the wave nature of light. Compared to the design of conventional lenses, the design of the holographic diffuser is of a completely different nature. Indeed, to design a holographic diffuser the use of certain optimization techniques that make use of the diffractive integral in one way or another, has to be employed. Although this may seem to add more complications than benefits, the truth is that it permits the design of diffractive structures that in turn allow the user to perform very complex transformations on input beams.
Considering all of the above, the holographic diffuser can be used to obtain complex radiance patterns that will enhance the performance of the laser process as a whole. A holographic diffuser can be used to increase the process stability of a laser surface annealing machine, for example. In this application any non-uniformity, such as ripples on the irradiance pattern of the laser, will be clearly visible in the Annealed surface . Thus, the use of a holographic diffuser can be used to improve the appearance of the final, annealed, surface.
Holographic diffusers can also be used to fine-tune the irradiance pattern of the beam in applications where a non-conventional pattern is required. For example, it may be the case that an irradiance pattern with zones of different energy levels is needed, such as in laser wire brazing to bond two different metals. Or maybe a pattern exhibiting a linear variation can yield a benefit to the user, such as in welding of different thickness materials. All these complex operations can be accomplished with the use of a holographic diffuser.
