A top hat laser beam profile has a uniform intensity across its cross section, with sharp edges and a flat top. It’s often used in various applications such as micromachining, material processing, and scientific experiments. The flat top area is where the laser energy is concentrated, creating a highly uniform and tightly focused spot. Such a flat top profile cannot freely propagate in a laser cavity, thus Gaussian beam profiles are a more common laser output shape, with a bell-shaped intensity profile. The gaussian is the shape of the basic mode that can build up in the laser cavity and is the typical output of high quality single mode lasers.
Unfortunately, Gaussian beams have a bell-shaped intensity profile, with the highest intensity in the center and decreasing towards the edges. This results in a less uniform intensity across the cross-section of the beam, with lower intensity near the edges.
The difference between top hat and Gaussian beam shapes becomes important when considering the requirements of different applications. For example, when micromachining, the uniformity and tight focus of top hot spots makes them ideal for creating precise cuts or drilling holes. On the other hand, Gaussian spots can result in non-uniform processing, with deeper ablation in the center vs the edges of the spot and broad edges or heat- affected zone (HAZ)
Another factor to consider is the stability in the size of the processed area. For a given process threshold energy, once a top hat spot achieves this threshold for processing (ablation for example) , the size of the processed area is almost independent of the laser pulse energy. This is in stark contrast to a gaussian spot, where the width of the processed area is directly proportional to pulse energy, resulting in instability in processed area/ line width.
Additionally, top hat beams are often used in scientific experiments that require highly uniform laser illumination, such as laser cooling and trapping of atoms, laser-based particle acceleration, and laser spectroscopy. In these applications, the flat top profile of top hat beams provides a well-defined and uniform light source, which is essential for the accurate measurement of physical phenomena.
In conclusion, top hat beam shapes and Gaussian beam shapes are two different types of laser beam profiles with distinct characteristics. They both have their advantages and disadvantages. The choice between the two depends on the specific requirements of the application, available resources, and budget. Top hat beams are ideal for applications that require uniform intensity, tight focus, and precise control, while Gaussian beams are more appropriate for applications where the stability, uniformity and edge sharpness of the processed area is less critical.
