THE LASER USER


ISSUE 115 MARCH 2025 BEAM SHAPING


These beams can be useful for applications such as bulk chemical laser sensing or other specialised applications, as they maintain their flat top profile over distances that can be 100 times the size of the top hat (for example, for a 3 mm flat top beam, a collimation range of 300 mm is reasonable). However, they are rarely useful for most industrial beam shaping applications, as they cannot be focused directly to a small flat top spot. To achieve a small spot, such beams must be imaged at large ratios (often 1:10-1:30) with the result that for typical F-Theta EFL=100 mm, such an optical system would have a >3 m optical path.


Figure 2: Typical industrial laser beam shaping optical setups


shaping methods must be made for an optical system with the same DL value. i.e, the same wavelength, NA and laser quality.


Industrial laser beam shaping characteristics


Industrial laser applications requiring shaping are wide-ranging, including welding, micro-grooving, scribing, drilling, surface treatments, wafer inspection and many others. As such, it is hard to say that there is any single set of characteristics common to all industrial shaping applications. However, based on our experience in the field, the following often apply to many industrial laser beam shaping tasks:


• Small shaped spots (line/rectangle/circle), often in the range of 2-6 DL. This requires precise shaping with very little optical aberrations.


• Use of Galvo scanners with F-theta lenses to scan the spot over the work surface (Figure 2).


• High power levels that require large beam diameters to avoid damage to optical components (typically >3 mm, in some cases up to 30 mm).


• Space constraints: while optical paths of 1-2 m can be found within industrial machines, a more compact optical setup is often preferred for ease of integration.


These characteristics determine the type of beam shaping suitable for most industrial laser systems.


Beam shaping by single element


Single element beam shapers are freeform optics that operate by modifying the phase front of the incoming laser beam, creating a uniform intensity distribution at the far field. Both refractive and diffractive optical elements (DOEs and ROEs) can serve as single element beam shapers, with the main difference being the effect of manufacturing tolerances and sensitivity to wavelength.


In general, DOEs are suitable for cases of shaping to a small number of diffraction limits (<15DL in x, y), where the high accuracy of DOE lithographic production methods is required to achieve good uniformity. ROEs are more suitable for cases where the shape is larger, or for polychromatic laser illumination, as surface error


effects on shaping in such cases are weaker.


All types of single element beam shapers typically operate at the lens system’s waist (focal plane). This lens can be external, such as an F-theta lens or a laser objective, or alternatively, it can be incorporated into the active surface as a superposition of the shaping optical function and a lens, making the shaper a focal beam shaper.


Multi element beam shaping


While single element beam shapers can achieve a flat top intensity pattern at focus or far field, the phase front at this plane is not flat. This is important for certain applications where defocus behaviour must be symmetric around the waist.


By using more than a single element, more complex manipulations of phase and intensity can be performed, including generating a flat top beam with a flat phase profile. The simplest such configuration is what is known as “collimated top hat” and is composed of a beam shaper element that generates a flat top beam at a certain plane, where a second phase element flattens the phase, generating a collimated top hat beam (Figure 3).


More complex multi-surface configurations do exist but are generally not required for simple shaping tasks such as shaping a TEM00


Gaussian


into a flat top beam of any given shape or size. Typically, the more surfaces that are used for beam shaping, the higher the cost and the more losses, scattering and aberration are added to the beam.


Conclusion


Beam shaping is widely used in many industrial laser applications. Beam shaping by single element can generate a precise flat top spot that can be a few diffraction limits in size. Single element beam shapers are easy to integrate into optical setups, can handle high power including ultra-short pulses, and generate the flat top at the workplace by direct focusing.


Multi element beam shaping is more suitable for specialised applications where the beam is not focused. For most industrial systems, there is no advantage in utilising more than a single surface for beam shaping, and there are many disadvantages. These include increased costs, reduction in LDT and a larger bulk of the shaping system.


Contact: Natan Kaplan natan@holoor.co.il www.holoor.com


Figure 3: Imaging setup for scaling a collimated flat top beam (typically 1-3 mm) to an industrially relevant micro spot (typically 50-200 µm). The imaging arm is roughly equal to the required magnification times EFL


Natan Kaplan is Holo/Or’s CTO and R&D Manager, with 10 years of experience in micro optics and beam shaping.


SEE OBSERVATIONS P30 29


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