Flat Top Laser beams
The vast majority of laser beams exhibit a Gaussian irradiance profile. This means that their irradiance decays smoothly from the centre point towards the edges. In theory, a Gaussian irradiance profile is not bounded and thus it can expand indefinitely. As a result, in certain processes involving laser beams, a finite amount of light energy is always wasted. Conversely, a Top Hat laser beam is characterised by a plateau of uniform irradiance with sharp edges. These beams are also referred to as Flat Top Laser beams.
Beam Shaping with DOEs
Although a Top Hat, or Flat Top, laser beam would be the preferred for many laser applications, lasers do not naturally emit this type of beam profiles. What is needed then is an extra component that can convert a Gaussian beam into a Flat Top Beam. In this scenario, the most practical option is to use a purposely designed diffractive optical element.
Diffractive Optical Elements (DOE) , also known as computer generated holograms (CGH), are window-like optical components that exploit the wave nature of light. They can be designed to perform complex transformations on coherent light beams such as laser beams.
The diffractive optical element imparts a localised phase change to the beam that, after propagation, gives rise to a smooth flat-top irradiance profile with a desired shape such as circle, square, rectangle or line.
This type of beam shaper is also referred to as an Analytical Top Hat beam shaper and it works perfectly well with coherent, TEM00 Gaussian beams with low M2 values. This type of input beam is common in certain applications in the laser material processing industry. The other type of beam shaper is based on diffusing elements and this type works best with low coherent, multi-mode laser beams.
Typical System Layout
The most common setup used with a beam shaper is a laser beam that illuminates the DOE, followed by a focusing lens (often an F-Theta lens). The far field of the DOE is reproduced at the focal plane.
For best performance of the beam shaper, the lens must be at least twice the size of the DOE and be aberration-free, as well as having a flat field over the top hat size at the focal plane. Even given these constraints, a Top Hat laser beam shaper can be integrated into very small laser processing systems.
Applications of Top Hat Laser Beams
This type of beam irradiance profile is harnessed in many industry sectors where a laser beam needs to be focused into an area of precise shape and size. These include:
microelectronics and semiconductor, where such profiles are used for copper removal, hole drilling, scribing of contacts and other processes.
Laser metal processing (Automotive and other industries) for cutting and welding with improved edge quality
Green energy industries for battery contact production using laser processes
and a whole plethora of other processes in hi tech manufacturing.
