Flat Top Beams

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Flat Top Beams

Flat Top Beams are beams with a radiance distribution pattern that can be described as having a zone of constant radiance with very abrupt edges along which the radiance falls to zero.  In many cases of practical interest this beam radiance distribution is very convenient and sought after but, on the other hand, a flat top beam is certainly not the most common distribution that can be found in laser beams.  Indeed, the most common type of beam distribution is the Gaussian distribution or a close approximation to it. A Gaussian beam is a beam that can be described by a combination of any of the Laguerre-Gaussian polynomials. More often than not, it is the first polynomial that is more commonly used when describing a Gaussian beam. That basic Laguerre-Gaussian polynomial or mode depicts a radiance distribution that has a peak at the centre that then slowly falls asymptotically as the radial coordinate increases. In principle, then, the radiance extends beyond any desired boundaries.

These traits of being unbounded and non -uniform make Gaussian beams not completely adequate for many laser applications, be it in industry or in the medical sector. The main reason is that when a Gaussian beam is brought to a focus, the area of the focal spot is then also unbounded, and it can extend well beyond the treated area. In contrast, a Flat Top Beam, when brought to focus, is well bounded and the treated area has a constant irradiance. It can be inferred that this beam behaviour is beneficial for laser ablation as it will increase the efficiency and fineness of the process. The same conclusion can be applied, for example, for a laser system used in dermatology or surgery. In these applications it is imperative that there is no light leaking to the areas surrounding the area being treated.

Flat Top Beams are sometimes also referred to as Top Hat Beams but a Flat Top Beam is a more general case as the shape of the area with uniform radiance can be either circular, rectangular or any other closed shape. In fact, It can also be a shape with some added complexity like lines or rings. The final shape will depend on the application.

To obtain a Flat Top Beam from the more commonly found Gaussian beam, it is often necessary to add a diffractive optical element along the optical path.