- Flat-Top Beams

While many lasers will output a Gaussian mode, this is not always the most desirable beam profile for a particular application. As we have seen, Gaussian beams have an M2 = 1 and consequently have a low divergence for a given waist size; this makes them suitable for applications where the laser beam must travel a long distance and still maintain some degree of energy concentration. Gaussian beams are also easy to understand in terms of their propagation since they remain invariant in shape, only changing in size following simple analytical rules (see Chapter 1). However, in many applications a flat-top beam profile, sometimes called a top-hat beam, would be more advantageous, for example, in materials processing, lithography, micromachining, and medical applications. In many of these applications, a beamwith a near uniform distribution of energy is desirable: while a Gaussian beam would have a peak energy density of double the average, a flat-top beam would ideally have the same energy density across the entire active area of the beam. In Via drilling this would mean a drill rate that is the same across the area of the hole. In medical applications, say eye surgery, it would mean that the rate of material removal would be equal across the beam and would avoid potential damage due to the large peak energy density at the center of a Gaussian beam. Unfortunately, flat-top beams suffer from some disadvantages: they are not part of the mode set of standard laser resonators and so require custom optics either inside or outside the laser in order to create them; they do not propagate in a shape invariant manner, and so passing them through optical elements (e.g., lenses) would not only change the size but also the profile of the beam. In fact as a general rule, flat-top beams tend not to remain flat-top for very long. However, this can be overcome through a better understanding of the propagation properties of such beams.