定义：
光束具有平的强度截面。

平顶光束是指激光光束（通常为经过转换的激光光束）强度截面很大区域内是平的。这与高斯光束不同，高斯光束的强度是从最大值沿着光束轴向外平缓的减小为0. 在有些激光器应用中需要采用这种类型的光束。
例如，在处理半导体晶圆或者其它材料时在某些区域需要强度为常数。另外，在高功率时的非线性频率转换过程中，如果采用平顶光束效率会更高。通常，平顶光束边沿比较平滑，因此可近似看做超高斯型，而不是矩形。 

图1：平顶光束（红色）与高斯光束（绿色）和超高斯强度分布（蓝色）的对比。三束光的功率相同，有效模式面积相等。 

平顶光束的传播 
与高斯光束不同的是，平顶光束不是自由空间的模式。也就是说光束在自由空间中传播时，强度形状会发生变化。
强度曲线的边缘越陡，变化越大。图2是模拟的初始为超高斯光束形状的光束的传播情况。 

图2：初始为超高斯形状的光束在自由空间中传播的演化情况。光束首先被压缩，后来又展开，边缘非常平滑。
以上情况下的颜色选取是基于光束轴向饱和时颜色是相同的这一前提。实际上，随着光束的扩展，强度变小。 

实际应用时光束形状的变化可以忽略不计。对于具有更大直径以及边缘变化不剧烈的光束，其大小和形状近似为不变。 

产生平顶光束 
通常需要先从激光器中得到一束高斯光束，然后采用一些合适的光学元件改变其强度形状得到平顶光束。有许多种光束整形器可以改变光束形状。
例如，可以采用多个非球面透镜，或者一些衍射光学器件。采用非球面透镜，得到的光束形状非常平坦并且功率效率很高，以及很高的损伤阈值，这在调Q激光器中非常重要。

Definition: a light beam with a flat intensity profile

Alternative term: top-hat beams

More general terms: light beams, laser beams

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.

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:

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: Beam quality factor M² of supergaussian beams as a function of the supergaussian order. (2 corresponds to a Gaussian beam.)

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.

Figure 3: 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 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.