The radiance profile emitted by lasers can be described, in almost all cases, with a type of mathematical formulation referred to as Gaussian profile. This radiance profile is characterised by a peak of maximum radiance at the centre and smooth fall-out with tails that in theory never reach the zero value. Consequently, a Gaussian irradiance profile has no clear-defined boundaries and non-zero irradiance areas can still be found far from the centre.
There are many laser applications in which the smoothness and absence of clear boundaries on the beam are more of a hindrance. In these cases, it would be more desirable to have a homogenous distribution of the light power and with clear beam delimitations. A Top Hat laser beam fits this description.
An ideal Top Hat Laser beam is characterised by a level of constant irradiance bounded by sharp edges. A cross sectional view of the beam gives an indication of the origin of the name as it resembles the old gentleman’s hat. In practice, the edges will not be completely straight lines as there will be some transfer area from the plateau to the ground zero level.
Top Hat Laser beam generation
The most common method to obtain a Top Hat laser is to use a diffractive optical element. The diffractive optical element, DOE, modulates the phase of the input beam. Then, as a result of diffraction propagation, the beam irradiance profile is changed in a predefined manner.
This method is the preferred option when the input beam is a Gaussian TEM 00 mode with high coherence and low M2 value. This will be the case for almost all of the applications that require a Top Hat laser beam. Still, there are some other applications in which the input beam is multi-mode Gaussian beam. In this case, the alternative is to use a flat top homogeniser diffuser.
Applications of Top Hat laser beam
Top Hat laser beams are being harnessed in many industry sectors where a laser beam needs to be focused into a fixed size area and shape. Processing of materials with lasers, for instance, is a clear example in which the energy of a laser needs to be well localised and uniform across the treated area. The plateau of uniform irradiance can be either circular, rectangular or any other desired shape. The important aspect is that the irradiance falls sharply after crossing the edges. In this way most of the light power can be used, and no processing of undesired areas at the edges occurs.