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CREATING ROUND and SQUARE FLATTOP LASER SPOTS in MICROPROCESSING SYSTEMS with SCANNING OPTICS Paper M305

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CREATING ROUND and SQUARE FLATTOP LASER SPOTS in MICROPROCESSING SYSTEMS with SCANNING OPTICS Paper M305 CREATING ROUND AND SQUARE FLATTOP LASER SPOTS IN MICROPROCESSING SYSTEMS WITH SCANNING OPTICS Paper M305 Alexander Laskin, Vadim Laskin AdlOptica Optical Systems GmbH, Rudower Chaussee 29, 12489 Berlin, Germany Abstract performance can be improved by applying of beam Performance of various modern laser based shaping optics. micromachining techniques can be improved by Depending on an application either round or square applying square laser spots with uniform intensity laser spots are required, therefore optics of beam distribution. And providing possibility of scanning of shaping systems should provide possibility to realize such a spot over whole working field with using not only variable intensity distributions but also popular 2- and 3-axis galvo mirror scanners is of various spot shapes. great importance for many laser microprocessing The essential feature of the refractive field mapping technologies, like scribing, drilling vias in PCB, flat beam shapers is that they transform the laser beam panel display repair. These tasks can be successfully profile in a control manner by accurate introducing solved with using the field mapping refractive beam and further compensation of wave aberration, shaping optics like Shaper and Focal-Shaper. Due therefore the resulting collimated output beam has to their unique features, such as low output low divergence and there is no deterioration of the divergence, high transmittance as well as extended beam consistency. On the other hand this allows to depth of field these beam shapers provide a freedom adapt the beam shapers to create the final laser spots in building an optimum optical system. Depending on of required shape and intensity profile. the conditions of a particular technique it is possible In this paper there will be considered two approaches to apply either a Shaper with imaging optics or a of building industrial optical systems on the base of Focal-Shaper with focusing lenses. And important refractive beam shapers: imaging of output of a feature of these approaches is in easy adaptability to Shaper, and focusing of output beams of Focal- optical design of already existing material processing Shaper’s being optimized to generate round and systems. There will be considered several optical square focused spots. Examples of real layouts based on various refractive beam shapers implementations will be presented as well. Shaper/Focal-Shaper to generate square shaped laser spots of uniform intensity which sizes span from Focusing and Imaging optical approaches several tens of microns to millimetres. Examples of real implementations will be presented as well. Focusing of a laser beam The issues to choosing an appropriate beam shaping Introduction optical system with scanners when focusing of a laser Applying of refractive beam shapers with laser beam with using F-theta lenses were discussed scanning optics is important in realizing various thoroughly in the papers9,10, let’s describe those of industrial laser technologies as well as techniques them that are important for further considerations. used in scientific and medical applications. Today the Typical optical systems with using mirror scanners galvo mirror scanners with F-theta, telecentric or consist of following components: laser, beam- other lenses as well as gantry systems are widely expander, scanning head and focusing lens. used in different applications like micromachining, From the point of view of optics the 2-axis scanning solar cell manufacturing, microwelding, drilling head presents a pair of flat mirrors turning the holes, selective laser melting and others which direction of optical path. The beam-expander is used Fig. 1 Focusing of various laser beams by a lens. when necessary to correct the beam size for further that its intensity profile stays just Gaussian after the optical system, for example, to expand the beam for lens, only its size is varying! The spot in the focal smaller final laser spot. To provide the spots, which plane of the focusing lens has intensity distribution size is comparable with wavelengths, the F-theta described by Gaussian function, Fig. 1 a), this is a focusing lenses should have diffraction limited image well-known feature widely used in laser technics. quality over entire working field. But this remarkable feature of stable intensity Details of behaviour of laser beam profile in zone of distribution is valid for Gaussian beams only! focal plane of a lens can be found in papers2,5,9, here In case of a flattop initial beam, Fig. 1 b), the we emphasize on some important features. intensity profile If in focal plane is described by the According to the diffraction theory, the intensity function called as Airy Disk, distribution in a plane of analysis is result of I () = I [J (2)/(2)]2 (1) interference of secondary wavelets. Since the f f0 1 conditions for interference change while a beam where J1 is the Bessel function of 1st kind, 1st order, propagation the intensity profile is variable and is polar radius in the focal plane, If0 is a constant. depends on the initial beam profile at the entrance of In the space between the lens and its focal plane the the focusing lens. interference pattern gets strong variation both in size Some important for practice examples of intensity and in intensity distribution. The focusing of a profile behavior by focusing a beam is shown in flattop beam never leads to creating a spot with Fig. 1: uniform intensity, neither in focal plane nor in - focusing of a TEM00 (Gaussian) laser beam, intermediate planes. In other words: If a particular - focusing of a flattop beam, application needs a laser spot of uniform intensity - creating a flattop spot in zone of focus of a lens, (flattop) there is no sense to focus a flattop beam! Essential feature of focusing the Gaussian beam is The technique of creating a flattop laser spot in focal plane of a lens is illustrated in Fig. 1 c). The the beam with Airy Disk intensity distribution is the intensity distribution at the input of a lens required function of the field mapping beam shaper Focal- to create a flattop round spot in the focal plane can Shaper. As a rule this system is recommended to be be found by mathematical computations based on applied when the final spot size is several microns or the inverse Fourier-transform technique, described tens of microns, typically below 100 micron. Then it is for example in book2. The solution is just Airy Disk reasonable to realize a focusing layout with setting up function, analogous to one in formula (1). This the Focal-Shaper ahead of the lens and scanning means: a flattop laser spot in the focal plane of a mirrors. This approach will be illustrated by examples lens is produced when the input beam has intensity later. distribution described by the Airy Disk function, so essentially non-uniform. More detailed analysis9 Imaging layouts shows that in the space between the lens and its Another approach of generating the flattop laser spots focal plane there are variations of both the size and is just creating an image of a certain uniformly the intensity distribution, and optimum, from the illuminated aperture with using an imaging optical point of view of practice, working planes are shifted system. Let’s consider a couple of optical layouts, from the focal plane towards the lens. Creating of Fig. 2, that are used in industrial equipment. Fig. 2 Imaging layouts: a) Imaging with a single lens, example – microobjective lens, b) 2-lens system, example - Collimator + F-theta lens, c) Intensity profiles when imaging a low divergent laser beam. The first layout, Fig. 2a), presents an ordinary image between the lenses 1 and 2, the distance between formation with using a lens. Here the lens is just a those lenses isn’t critical and can be chosen from the singlet, sure, for high quality imaging a more point of view of design requirements of a particular sophisticated optical systems should be applied, for industrial system. Here can, for example, a galvo- example aplanats (with correction of spherical mirror scanning system be locating. aberration and coma), microobjective lenses. The last layout, Fig. 2 c), demonstrates the behavior Calculation of parameters of a particular imaging of intensity profile of a low divergent laser beam in setup can be done with using well-known formulas of the above considered 2-lens imaging optical system. geometrical optics, described, for example in book6. It is presumed here that the object is uniformly Let us note several important for practice issues: illuminated and a beam from each point of the object - approximation of geometrical optics presumes that plane has low divergence, near the same like a laser each point of the image is created by a beam of rays beam of the similar size, 2u = 2, – these features emitted by a corresponding point of the object, are typical for output beam of a refractive field - the object and image are located in optically mapping beam shaping system like Shaper which conjugated planes, that means there is a same optical will be in more details described later. path length for all rays of a particular beam, Let’s consider the transformation of the beam intensity - the real image is always created after the lens focus, distribution. In the optically conjugated image plane it - the transverse magnification is defined as a ratio will be similar to one in the object plane, this means if between distances from principal planes of the lens the intensity distribution is uniform in object plane it to, correspondingly, image and object: will be uniform in image plane as well; sure, the image size will be defined by the transverse magnification . = - h’ / h = - s’ / s , (2) The beam in object space has low divergence, hence, - the product of object size h and aperture angle u as discussed earlier (see comments to Fig.1 b), the (exactly sinu) is constant all over the optical system: intensity profile in the common focus F’1+2 will be described just by Airy disk function (1).
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