Top Hat beams are a particular family of beams that have many benefits for industrial processes that require laser beams for material processing.
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Top Hat laser beams stand out from the rest of the laser beams as they have an area of uniform or constant energy that is sharply bounded. In fact, according to the theory of Top Hat beams, the radiance falls abruptly outside the energy area. Within this area, there are almost no ripples or any other type of non-uniformity.
The characteristics of top hat beams
To appreciate the benefits of the Top Hat beam compared to other types of beams, it is best to start with the characteristics of the most common type of radiance for commercial or industrial laser beams. This is the Gaussian beam which gets its name from the fact that the beam’s profile can be described mathematically by the Gaussian equation, the same equation used to describe the normal distribution in statistics. Thus, the Gaussian beam will have a peak at the center, which is smooth, and then the radiance starts to fall gradually toward the edges of the beam. In this case, there are no hard edges, and radiance behaves asymptotically, which means that, in theory, the radiance never reaches the zero value.
These characteristics of Gaussian beams, which in many cases is the type of beam inherent to the laser system that will be used, are detrimental to the application. To start with, the beam being unbounded will result in the light spilling into the neighboring areas of the intended zone for the laser treatment. This will induce losses in the available light, and the system could be inefficient. Moreover, as a consequence of the smoothness of the central part of the beam, even within the desired treated area, the irradiance will not be uniform.
All these drawbacks or setbacks can be overcome by substituting the Gaussian beam with a Top hat beam. With a Top Hat beam, all the light energy will land on the treated area with excellent uniformity. This will naturally increase the throughput of the laser process.
Sadly, when a Top Hat laser beam is focused on the work plane using a focus lens, the Top Hat energy distribution is not maintained and converted to a gaussian-like sync distribution at the focal plane. This can be overcome by using an imaging approach instead of focus. However, reaching the small spots needed for material processing requires imaging at high ratios sensitive to tolerance and complicated. A simpler approach is to use a diffractive top hat spot generator that creates a top hat beam only at the focus plane instead of generating a propagating top hat beam that cannot be focused.
