Micro-lens arrays for laser beam homogenization and transformation V. Sinhoff, S. Hambuecker, K. Kleine, O. Ruebenach, C. Wessling INGENERIC GmbH, Dennewartstr. 25-27, 52068 Aachen, Germany ABSTRACT For many innovative applications a significant improvement in the homogeneity of the laser beam is a critical requirement when using semiconductor lasers. There are several different methods for the homogenization of laser radiation. Homogenization using micro-cylinder lens arrays is a considerably elegant and compact solution. In this case the incident laser beam is separated into partial beams by one or more micro-lens arrays. These partial beams are then overlaid in the homogenization plane by the downstream optics. Depending on the arrangement and geometry of the micro-lenses, this enables homogeneously illuminated lines, rectangles or squares to be generated. The major advantage of this solution lies in the increased freedom of adjustment to account for the initial beam profile, as well as the extremely compact design. In addition to a comparison of different homogenization principles the paper describes new approaches of homogenization via micro-lens arrays and compares the impact on the array performance by different manufacturing approaches. Keywords: micro-lens arrays, micro-optics, high-power laser, homogenization, transformation, fiber coupling, BTS, aspheres

1. INTRODUCTION For a long time, micro-optics have been an indispensable component of the system design for shaping the beam of high power semiconductor lasers – whether in the form of fast and slow axis collimation optics in order to collimate or symmetrize the laser beam, or in the form of beam transformation optics for efficient fiber coupling. For many innovative applications a significant improvement in the homogeneity of the laser beam is a critical requirement when using semiconductor lasers. Likewise, for the next generation of laser diode types, appropriate collimation optics need to be developed and be manufacturable on a large scale. INGENERIC’s background is the manufacture of micro-optics for the efficient beam shaping of semiconductor lasers. For more than 10 years the company develops and manufactures fast-axis and slow-axis collimation optics as well as beam transformation systems. With this product experience and manufacturing knowledge INGENERIC is now broadening the spectrum of solutions based on high-end micro-lens arrays.

2. MICRO-CYLINDER LENS ARRAYS Micro-cylinder lens arrays fulfill different functions in different applications. On the one hand they might be used for the homogenization of laser light, on the other hand they might be used to efficiently transform the light into a fiber. In both cases form accuracy, surface quality and minimization of transition zones between the lens elements is of decisive importance. 2.1 Homogenization There are several different methods for the homogenization of laser radiation [1, 2] (Fig. 1). For example, homogenization of a specified beam profile, for example a Gaussian beam, can be achieved using specially shaped aspheres. The disadvantage of this solution is the high sensitivity of the homogenization result in terms of fluctuations in the initial beam profile and the assembly accuracy. Another method for homogenization of laser radiation is the use of waveguides or optical fibers. However, this method requires a sophisticated assembly process and sufficient installation space.

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Homogenization using micro-cylinder lens arrays is a considerably more elegant and compact solution. In this case the incident laser beam is separated into partial beams by one or more micro-lens arrays. These partial beams are then overlaid in the homogenization plane by the downstream optics. Depending on the arrangement and geometry of the micro-lenses, this enables homogeneous illuminated lines, rectangles or squares to be generated. The major advantage of this solution lies in the huge scope for adjustment to account for the initial beam profile, as well as the extremely compact design.

Figure 1. Principles for homogenization based on refractive optics

The crucial requirements for the fault-free performance of the micro-cylinder lens array are high contour accuracy, and the minimum possible dead zone between the lens segments. In terms of contour accuracy, it is not just the shape of the individual lens segments that is crucial, but particularly the repeat accuracy and the pitch accuracy (the spacing and positioning of the lens segments relative to one another). To guarantee fault-free imaging, the required accuracies fall into the sub-micrometer range. The dead zone describes the area immediately between the lens segments and, because of technical production limitations, cannot typically be described deterministically. The larger the dead zone, the higher the radiation losses and so too the variations from the desired homogeneity. The exact manufacturing method has a crucial influence on the size of the dead zone. Depending on the selected manufacturing process the values for the dead zones vary between +/- 20 µm up to +/- 60 µm. Figure 2 shows the principal difference between arrays manufactured with alternative production technologies. To visualize the different properties of the arrays the light is traced through the micro-cylinder lens array using a stereo microscope and a plane parallel plate. On the right hand the superior property is recognizable through the extremely thin line width. Because of the excellent contour accuracy, the arrays also have an extremely constant focal position. Looking at the left hand of figure 2 the quality differences become obvious. The non-parallel lines hint at the varying focal position and the wider line between the lens elements is a sign for wider dead zones. The difference in quality becomes obvious when the cross section of the lenses is investigated. The cross section of the etched array (Fig. 3, left hand side) shows a well formed radius between the adjacent lens elements. This zone is apparently 30 … 40 µm wide. On the right hand side the lens arrays show a very sharp transition zone. In this case the width of the transition zone is 5 µm. Figure 4 shows the impact of the laser source on the performance of the array. The left hand sided picture shows the homogenization result when the micro-lens array is transmitted by light generated by a single-mode laser, the right hand sided picture shows the homogenization result when a multi-mode source is used. In both displayed cases the optical setup is identical. In case of the single-mode laser the homogeneity of the plateau is in concurrence with a modulation generated by interference.

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Figure 2. Homogenization results with micro-lens arrays based on different manufacturing techniques

Figure 3. Cross sections of transition zones between adjacent lens elements of micro-lens arrays

When a multi-mode laser source is used the beam shows a homogenous plateau. Nevertheless also in this case high frequency modulations can be observed which are caused by interference effects of partly coherent beam fractions. Using incoherent multiple laser bar stacks this effect can fully be avoided. All arrays are produced from highly refractive and highly transparent optical glass. The laser damage threshold is 2 kW/cm². Typical coatings range from 400nm up to 2000nm. Aside standard specifications INGENERIC offers the

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production of customer-specific designs. Furthermore INGENERIC micro-lens arrays offer design flexibility with respect to a large height profile relative to the spacing of the lens segments.

Figure 4. Homogenization via micro-lens arrays with different beam sources

2.2 Collimation While crossed cylinder lens arrays generate a rectangular flat top profile, the output beam profile when using arrays with a rotationally symmetrical aperture is circular. The general conditions with regard to the design and production requirements for the arrays are comparable with those for a cylindrical structure. However, as well as being able to homogenize laser beams, this array structure is of particular interest for beam shaping of VCSELs and LEDs. Arranging VCSELs in a row enables optical powers ranging from several 100W and up into the kilowatt range to be achieved. The advantage over edge emitting diodes is the cost effectiveness of VCSELs, as they can be completely processed at wafer level all the way along the production chain and are very durable in use. The design freedom of the VCSEL arrays therefore results in tough requirements for the lens arrays, which are used for collimation of the laser light. As well as varying the distance between the VCSELs, the shape of the emitting region can also vary from circular. In terms of the appropriate optics, there may thus be a future demand for micro-freeform optics arranged as high performance arrays (Fig 5). When it comes to the design, the advantage of INGENERIC arrays lies in the use of highly refractive optical glass. For example, complex designs with tough demands on the apertures can be translated into structures suitable for manufacture. Typical apertures for manufactured lens arrays range from 0.5 to 3.0mm, while focal lengths scale from 0.7mm for extremely short focal length requirements on up to 25.0mm. The standard pitch is between 0.2 and 3.0mm. To ensure a high fill ratio for hexagonal arrangements, the selected pitch between the lenses can be smaller than the aperture, in which case the lenses literally overlap.

Figure 5. Wafer with micro-lens arrays for VCSEL-beam shaping

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2.3 Fiber coupling There are different methods for the coupling of diode lasers into fibers [3]. Figure 5 shows the advantages and disadvantages of different optical solutions for fiber coupling diode bars. The basic principle of the displayed solutions is identical: To adjust spatial and angular distribution within fast and slow-axis and to achieve thus an identical beam parameter product in both axis of the semiconductor laser, the beams of the single emitters are rotated and subsequently re-arranged to an entire beam. In this comparison it becomes evident that the cylindrical lens array offers some advantages with respect to space and limited complexity, but that on the other hand the request to couple light into fibers with 200µm and smaller core diameter impose some limits on the allowable tolerances and thus require advanced manufacturing technology.

Figure 6. Comparison of different beam transformation optics and details on BeamROT

The impact of the quality of micro-lens array becomes obvious when fast-axis collimation quality is displayed versus coupling efficiency. A small deterioration of the fast-axis collimation quality caused by diode bar smile, poorer collimation quality of the FAC or inferior alignment quality of the FAC results in a heavy drop of the coupling efficiency (Fig. 7, left graph). The other way around a micro-lens array with larger aperture or rather smaller transition zones between the lens elements reacts more forgiving in case of a deteriorated fast-axis collimation quality (Fig. 7, right bottom) and guarantees thus a better coupling efficiency.

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Figure 7. Coupling efficiency versus fast-axis collimation quality

The diode bar parameters for the graph shown in Figure 6 are: wavelength 808nm, FA-divergence 60° (1/e²), bar smile < 1 µm, number of emitters 19, emitter width 150 µm, emitter pitch 500 µm and SA-divergence 11.2 ° (1/e²). The achievable result can be seen in Figure 8. After the collimation with an INGENERIC FAC07-300-XXB a divergence of 3.5mrad is measured. No side lobes can be observed at that stage of the optical path. The micro-lens array (trade name: BeamROT) is placed approx. 180µm in front of the FAC and rotates the beam. On the right hand side of figure 8 you can see a perfect rotation of the single emitter and in the image of the far field camera a perfect fit when the single beams are overlayed by a collimating lens. The divergence is in the same order of magnitude as after the FAC. Further investigations were conducted with regard to increased diode divergence, diode smile and poorer FAC collimation quality. Aberrations could be observed, but due to the small transition zones between the lens elements they had only limited impact on the coupling efficiency.

Figure 8. Beam profile after fast-axis collimation and after BeamROT

3. CONCLUSION Micro-lens arrays are critical components for an effective beam shaping of high power semiconductor lasers. Collimation, homogenization and effective fiber coupling are applications discussed within this paper. Aside the principal design of micro-lens arrays their function depends heavily on the underlying manufacturing technology. The manufacturing principle defines the achievable form accuracy and surface roughness. This becomes obvious looking at the transition zones. While current manufacturing techniques allow transition zones in the region of +/- 20 µm up to +/- 60 µm, the displayed investigations show that with a specific conditioned production technology values below 5 µm can be achieved. This opens up advantages with respect to the transmission and beam quality in homogenization applications as well as increased coupling efficiencies when these array types are used for fiber coupling. In the future more applications of micro-lens arrays will be seen with respect to the beam shaping of VCSELs. To increase cost effectiveness of VCSELs, the diode and optic wafer will be bond together and then completely processed at wafer level all the way along the production chain.

ACKNOWLEDGEMENTS The authors thank the German government for the support under contract number 13N10851.

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REFERENCES [1] Koenning, T, Alegria, K., Wang, Z., Stapleton, D., Patterson, S., Koehler, B., Biesenbach, J., “kW-class line sources for direct applications”, Proc. SPIE 8241 (2012) [2] Wessling, C., Ruebenach, O., Hambücker, S., Sinhoff, V., Banerjeeab, S., Ertel, K., Mason, P., ”Efficient Pumping of Inertial Fusion Energy Lasers”, Proc. SPIE 8602-17 (2013) [3] Yamaguchi; S., Daimon, M., Chiba, K., Kobayashi, T., Saito, Y., “Optical path rotating device used with linear array laser diode and laser apparatus applied therewith”, U.S. Patent 5,513,201 (1994)

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