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Flat-top Beams

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Definition: a light beam with a flat intensity profile

Category: general optics

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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.

flat-top and other intensity profiles

Figure 1: A flat-top beam profile (red) in comparison to a Gaussian (green) and super-Gaussian (blue) intensity profile. All three beams have the same optical power and the same effective mode area.

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:

super-Gaussian profile

The higher the order, the steeper are the edges of the profile.

Propagation of 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 2 shows a simulated example for an initially supergaussian beam profile.

beam evolution

Figure 2: Evolution of an initially supergaussian beam in free space. The beam profile first contracts and then expands again, now getting smooth edges. Note that the color scale of each profile is adjusted such that the same color saturation is achieved on the beam axis; in reality, the intensity decreases for the expanding beam.

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 shapers to do that transformation. For example, certain combinations of aspheric lenses can be used, or some diffractive optics. With aspheric lenses, one can obtain a good beam flatness (with low residual ripple) and a high power efficiency, also a high damage threshold, which is particularly important when working with Q-switched lasers.

See also: Gaussian beams, beam shapers, Spotlight article 2010-04-08
and other articles in the category general optics

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