A flat-top beam (or top-hat beam) is a light beam (often a transformed laser beam) having an intensity profile which is flat over most of the covered area. This is in contrast to Gaussian beams, for example, where the intensity smoothly decays from its maximum on the beam axis to zero. Such beam profiles are required for some laser applications. For example, one requires a constant intensity over some area in some techniques for the processing of semiconductor wafers and other materials. Also, nonlinear frequency conversion at very high power levels can be more efficient when performed with flat-top beams.
Typically, however, a flat-top beam profile still has some smooth edges, so that it can be approximated with a supergaussian profile, rather than a rectangular profile. A supergaussian intensity profile of order n is defined by the following equation:
The higher the order, the steeper are the edges of the profile.
Flat-top beams can be spatially coherent, having smooth phase profiles. However, flat-top beams made with certain beam homogenizers are spatially incoherent, having rather complicated phase profiles. In such cases, a flat intensity profile is achieved only with a superposition of many spectral components, each of which may have a quite structured intensity profile.
Figure 2 shows the beam quality factor M2 of supergaussian beams as a function of the supergaussian order. The more we approximate a rectangular profile with a high beam order, the worse is the beam quality.
Propagation of Coherent Flat-top Beams
Note that in contrast to a Gaussian beam, a flat-top beam is not a free-space mode. This means that during propagation in free space, the shape of the intensity profile will change. The steeper the edges of the intensity profile are, the more rapidly will such changes occur. Figure 3 shows a simulated example for an initially supergaussian beam profile with supergaussian order 8 and flat wavefronts.
Of course, that change of beam profile may be negligible within the distance to the application. For beams with larger diameter and not too steep edges of the intensity profile, the beam size and shape may stay approximately constant.
Generation of Flat-top Beams
In many cases, a flat-top beam is obtained by first generating a Gaussian beam from a laser and then transforming its intensity profile with a suitable optical element. There are different kinds of beam homogenizers to do that transformation, using different operation principles; some are based on diffractive optics. Different types of beam shapers can differ a lot concerning spatially coherent or incoherent beam profiles, length of the usable top-hat region, sensitivity to input beam parameters etc.
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