Diffractive beam shaping is a common type of laser beam shaping with wide applications in various fields, including laser cutting, scribing, and micromachining. Diffractive beam shapers form laser beams into specific irradiance distributions. Exploiting the wave nature of light requires specific optical components known as Diffractive Optical Elements (DOE). These optical components excel at transforming single mode TEM00 light beams, such as many laser beams, to smooth shapes with sharp, well defined edges.
The transformation of a Gaussian profile into a Top Hat laser beam profile is a vital application of beam shapers. In this transformation, the broad transition region with gradually fading edges of the Gaussian beam becomes a uniform intensity distribution with a very narrow transition region and extremely sharp edges. In a Gaussian beam, the spot width increases proportionally with the growth in pulse energy. However, after this transformation, the area above a specific progression threshold remains unchanged regardless of variations in laser pulse energy.
Top Hat Laser Beams and Top Hat Beam Shapers
A Top Hat laser beam can be described as a level of consistent irradiance confined by very sharp edges. Viewed cross-sectionally, this beam resembles a gentleman’s hat. In reality, the edges of this beam are not straight lines.
Diffractive Optical Elements (DOEs) are employed as Top Hat beam shapers to transform a Gaussian beam profile into a uniform intensity spot. The DOE is followed by a lens system. The focusing lens can be placed at any suitable location after the DOE to replicate the far field of the optical component at the lens’ focal plane. The diameter of the DOE aperture and other optics along the path should be at least twice the diameter of the input laser beam. The lens and all other optics, including folding mirrors and scanners, must have diffraction limited performance at all system fields. With the correct design, a Top Hat laser beam shaper can be integrated into any compact laser system.
Applications of Top Hat Laser Beams
In many industrial sectors where high-quality, coherent laser beams are needed to focus on a specific area of distinct shape and size, a Top Hat laser beam profile is essential. Using a tailored DOE, such as rectangular, square, round, line, or exotic structures, one can shape the Top Hat laser beam into various distributions. The application area determines the intensity distribution. Some common applications of Top Hat laser beams include laser scribing, such as dicing, electrode exposure in Back side passivated solar panels , etc., laser cutting, laser micromachining – such as micro-drilling, ablation, and kerf removal, lithography, light-sheet and microscopy cytometry, surface and annealing treatments, and metrology of thin semiconductors.
The common objective of these applications is to maintain a clear boundary between treated and untreated areas. In these applications, the treated area remains constant despite changes in laser energy. This makes it possible to utilise nearly all the energy, unlike in a Gaussian beam, where a significant percentage of the energy is unexploited, resulting in a blurred edge or large HAZ (heat affected zone).
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