ISSUE 115 MARCH 2025 BEAM SHAPING


INDUSTRIAL BEAM SHAPING:


SINGLE ELEMENT VS MULTI ELEMENT NATAN KAPLAN Laser beam shaping is a well-established


practice in most industrial laser applications, enabling improved processing quality, increase in processing rate and other laser process benefits. One well- established beam shaping method is using a single, passive optical element to achieve the desired intensity at the system focus. Other methods, such as using multiple elements, have recently gained much publicity, with strong claims regarding shaping quality.


In this article, we review the relative merits of beam shaping by a single optical element compared to multi-element approaches, for industrial laser applications.


The need for beam shaping in industrial applications


Laser beam shaping is an envelope term for manipulations that change the shape of the


laser spot at the work plane. Single mode TEM00 laser beams have a Gaussian profile, which is often sub-optimal for many industrial processes. Often, a laser process has an energy density threshold above which the process occurs, thus fluctuations in pulse energy will change the processed area size if the spot is Gaussian. A way of mitigating this phenomenon is using flat top (top hat) illumination, where changes in laser energy have much less effect on the size of the processed zone. Other advantages of flat top beam shaping include HAZ reduction and steeper side wall angles.


These advantages of beam shaping are especially relevant for industrial applications, where processed area stability and edge sharpness are often critical.


Beam shaping theory and quality parameters


Beam shaping can be carried out using many methods, but for clarity we will group beam shaping into two groups: input dependent beam shapers vs phase-randomising beam shaping diffusers.


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Beam shaping diffusers operate by overlaying the beam on itself, thus creating a uniform intensity envelope with internal speckles. The level of speckling is a function of the laser temporal coherence. Thus, multimode fibre lasers and excimer lasers generate shapes with good internal uniformity, while single mode lasers will create a shape with strong internal speckling.


Diffusers are almost always single elements and are typically either refractive (lens array based) or diffractive, with performance mostly determined by the required angles and the system parameters. Diffusers typically do not require an exact input beam diameter or laser M2


value.


In this article our focus is on the more precise and industrially relevant type of beam shaping, i.e. input-dependent beam shaping. This relies on knowing the exact laser profile going into the shaper to allow the output beam to maintain a flat top profile without scrambling the phase and generating internal speckles. Thus, such shaping can achieve flat top spots or beams with good uniformity and sharp edges, but it is highly sensitive to input parameters such as beam diameter, laser quality (M2


) and the lens system used to focus the beam to the work plane.


Input-dependent top hat beam shaper quality can be quantified by several parameters:


� Efficiency – the fraction of input laser energy within the shape, typically defined at exp-2 of the flat top level. Most beam flat to beam shapers have >90% efficiency, regardless of shaping method.


� Edge sharpness/transfer region width – the width of transfer region characterised in mrad for angular shaping or µm at the focal


plane. This is a function of both design and system parameters.


� Uniformity – the internal variation in the flat top energy level, characterised as peak-to- valley or peak-to-average. Uniformity is mostly a function of system parameters, although beam shaper production tolerances do play a role.


These beam shaping quality parameters are all dependent on a single system parameter, regardless of the specific beam shaping method. This parameter is the diffraction limit, or in focal systems, the diffraction limited spot size. A single mode TEM00


Gaussian beam with a


certain diameter and wavelength has an angle of divergence due to diffraction, even when perfectly collimated. This divergence, together with the EFL of the focusing system, determines the spot size when the beam is focused, called the diffraction limited spot size (DL).


It is physically impossible to shape a beam into a shape that is smaller than the DL, making the DL the basic scale unit of beam shaping. The ratio of DL to spot size or beam divergence determines the quality of the shaping. i.e. how “flat top” is the flat top illumination.


The more DLs there are in the flat top shape, the better the flat top quality (See Figure 1). Accordingly, meaningful comparisons between


THE LASER USER


Figure 1: Shaping a 20x20 µm flat-top spot, for two cases: DL 4 µm vs 9 µm. Both systems have the same EFL=100 mm with different input beam diameter (2 mm vs 5 mm)


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