Top Hat Application
Most laser systems that are now being used in any industry sector exhibit a radiance pattern that is not uniform in angular space. When the beam comes into focus, the focal spot also displays this non-uniformity. In the simplest of cases, this irradiance pattern can be very accurately described by a Gaussian distribution. This distribution is characterized at a maximum at the center with a very smooth decay towards the edges. There is a convention to define the extent of the beam as the point at which the radiance has fallen to a certain percentage of the central maximum. In theory, however, the beam’s radiance, or irradiance, has no bounds, just as the equation for a Gaussian distribution predicts.
This behavior may be harmless or inconsequential for some applications but for many others it is undesirable. For example, in material processing with laser light, using a Gaussian beam may result in a non-uniform irradiance on the treated part. It will also cause some creeping of the light into surrounding areas that were meant to be left untouched.
For these reasons, it is beneficial to transform the inherent Gaussian beam from a laser into a Top Hat beam. This other type of beam is characterized by a plateau of uniform irradiance bounded by hard, abrupt edges. Thus, a plot of the radiance profile will resemble the erstwhile Top Hat that was used in past centuries. On the other hand, the 2D shape of the plateau can be either circular, rectangular, or any other geometrical shape that is deemed adequate for the application at hand.
In reference to the application mentioned above, this type of beam will ensure that the treated area will be uniformly illuminated and that there is no light leaking outside of it. As a result, the efficiency of the process will be optimized and the finished work on the material will be more precise.
To transform a Gaussian beam into a Top Hat beam a series of special lenses in tandem can be placed along the optical path of the laser. This approach will consume space and the design of the lenses can become complicated and hold many production deviations and tolerances on the way. Thus, a much more practical approach is to use a diffractive optical element, or DOE, that is a single, flat component that is also placed along the path of the beam and before any focusing element. The DOE exploits the wave nature of light and it works very well with coherent light and single-mode Gaussian beams.
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