There is a myriad of industries now benefiting from the use of laser systems. They can be used in material processing, in medicine, and in metrology applications, just to name a few. Below in this article, we will cover the Top Hat Beam Shaper.
Most laser systems emit a beam that is said to have a Gaussian profile. Such beam profile is characterized by a smooth peak at the center that then slowly decays towards the edges of the beam. Strictly speaking, the tails of the beam expand beyond limits and so the beam is said to be unbounded.
It can be expected that such a beam profile is not ideal for most laser applications. For instance, in laser marking, the beam irradiance hitting the treated area may creep to neighboring areas which will substantially reduce the quality of the laser process. If the beam, instead of being a Gaussian with its smooth edges, were to be a beam characterized by hard edges, then no leaking outside the treated area would occur and the laser process would be cleaner. The same rationale can be applied in lasers for medical treatments. Furthermore, the ablation of a tissue area can be uneven if the laser spot is not uniform. Thus, aside from the hard edges, it is necessary that the irradiance within those edges is a plateau of uniform irradiance.
A beam with these characteristics, hard edges and uniform plateau, is referred to as a Top Hat beam, in resemblance to the erstwhile hat now only worn in costumes or by magicians. To obtain a Top Hat from the inherent Gaussian beam of the laser system, a Top Hat Beam Shaper has to be used. In the context of laser light that is single-mode, coherent, and monochromatic, a Top Hat Beam Shaper made from a diffractive optical element is the optimum choice. A Diffractive Top Hat Beam Shaper will transform the input Gaussian radiance into a Top Hat radiance and this will increase the efficiency of the laser system process for the reasons explained above.
In the case that the laser system is multimode, there are other alternatives for Top Hat beam shapers. The most common one is to use a diffuser, such as a diffractive diffuser or a microlens array, to modulate the input Gaussian beam. Given that each lenslet in the array samples a small portion of the beam, it can be assumed that each lenslet sees a uniform region. After some propagation the many sub-beams get scrambled, giving rise to a more uniform radiance pattern. Diffractive diffusers operate in a similar manner, but have sharper boundaries as they do not clip the light at each sub-beam but instead work on the entire beam.
