CANUNDA. Application note. Flexible high-power laser beam shaping.

Transcription

1 CANUNDA Application note Flexible high-power laser beam shaping

2 CONTENT

3 INTRODUCTION LASER BEAM SHAPING SOLUTIONS 5 7 Multimode laser beam shaping Singlemode laser beam shaping and combining Ultrafast laser Non-coherent beam combining APPLICATIONS 15 Improving sheet cutting speed Pre-joining surface ablation Surface patterning High aspect-ratio single-shot drilling Adaptive beam shaping and combining CANUNDA 19 Canunda-HP Canunda-MP CAILabs GLOSSARY

4 INTRODUCTION

5 Laser material processing is one of the most common application of laser technology. It was introduced in the 1970s in the microelectronic component industry and, in the 1990s, CO2 lasers entered the automotive production plants for metal welding. Nowadays gas, crystal, fiber and diode lasers are widely used in a broad range of applications from micro-processing to thick metal sheet cutting. Laser material processing benefits from high productivity, good flexibility and reliability with an unrivaled precision. Additionally, laser processes, like metal joining used in automotive industry, are more robust than regular electric welding. Laser material processing takes advantage of the light capacity to focus energy on a very narrow surface. For CW lasers, the absorbed energy creates a local temperature increase which melts or vaporize the material (thermal machining). In the case of short pulse laser, the high energy flux can sublimate and ionize (make a plasma) the material (athermal machining). The first effect is used for metal cutting and welding and the second for micro- and nano-scale processing. The laser beam intensity distribution depends on the laser technology and can affect the interaction between the laser spot and the material (Table 1: commercial 1kW laser outputs for different technologies). Currently, the available laser power delivery is sufficient for most applications. The main limitation to the processing speed and quality improvement is the beam profile or beam intensity distribution. CO2 laser SM fiber laser Nd: YAG laser MM fiber laser MM disk laser MM Diode lasers Type Coherent Diamond E-1000 IPG YLR WC JK Laser JK1002SM IPG YLS-1000 Trumpf Tru- Disk 1000 Lumentum CORELIGHT YLE2100 CW power 1.0 kw 1.0 kw 1.0 kw 1.0 kw 1.0 kw 2.1 kw M 2 < BPP (mm*mrad) - - ~ Fiber core diameter - SMF 600 µm 50 µm 50 µm 450 µm Beam profile Gaussian Gaussian Step edges Step edges Step edges Step edges Table 1: Commercial 1kW laser outputs for different technologies 5

6 LASER BEAM SHAPING SOLUTIONS

7 Laser material processing is used in a large number of industries including large scale ones such as automotive or microelectronics where tools productivity and flexibility are a permanent challenge. Laser processing speed is related to the light intensity (power concentration) on the sample but also the way energy and matter interacts (heat conduction, melting pools ) and direct throughput improvement could be achieved by a laser power increase. However, this simple approach is not always realistic because the laser power is limited by the kw cost and using a non-optimal beam shape would lead to a waste of energy. For example, in a simple Gaussian beam, only the beam intensity above the processing threshold is used. All the remaining energy causes unwanted material heating (Figure 1: Gaussian and flat-top beam profile comparison). An alternative method is beam-shaping. Beam shaping involves transverse beam intensity tailoring in order to maximize the laser energy available for the process and optimize the local effect. It allows to increase the processing speed at a given laser power but also to improve the process quality, reducing, for example, the amount of raw material used in an assembly, therefore the product cost. A basic laser beam reshaping application is the Gaussian to flattop transverse intensity profile transformation which optimizes the available laser power usage and improves the processing quality. Unlike the Gaussian beam, the flat top beam has a constant transverse intensity and steeper edges avoiding the waste of the laser energy below the material processing threshold. Gaussian intensity profile Flat top profile intensity processing threshold intensity processing threshold wasted energy wasted energy spot diameter transverse position spot diameter transverse position Figure 1: Gaussian and flat-top beam profile comparaison 7

8 Several beam shaping solutions have been developed so far, with different level of complexity, total efficiency and cost. Total efficiency means the ratio of total input laser power and output power converted into the target shape. There are basically two categories of laser reshapers which depends on the type of incident beam: ones are singlemode beams (generally described as Gaussian intensity profile) and others multimode beams. Multimode lasers generally offer higher output power with up to 16kW. These lasers are dedicated to heavy processing work such as thick metal sheet cutting and joining. Several technologies can be used for beam reshaping, each with pros and cons. We can divide these technologies in four groups: -- Diffractive optical elements (DOE) are transmissive or reflective plates micro-structured with a complex diffraction pattern. This pattern changes the incident beam phase profile to generate output beam profiles by means of light waves interferences. DOEs are compact components, easy to install in the laser beam path and are well adapted to reshape Gaussian input profiles and homogenize and reshape multimode incident beams. Their efficiency is relatively high (~90%) as long as they use multilevel patterning. Alternatively, a binary DOE shows an efficiency generally higher than 70%. High power handling DOEs are quite expensive due to the photolithography process in use. -- Refractive phase shifters are single refractive optical elements with tailored curvature (aspheric, cylindrical, conical ). It applies a phase shift allowing the energy redistribution from a Gaussian beam into a flat-top distributed beam. The optical element curvature and alignment highly depends on the incident beam parameters. -- Refractive micro-lenses (or Refractive Optical Elements - ROEs) are micro-lenses arrays that can perform homogenization function and beam shaping. They are adapted to noncoherent light or highly multimode beams. Micro-lenses arrays are compact and can handle high power levels. Micro-lenses can be made by photolithography or, more conveniently, directly molded helping to keep low volume production costs. - - Multi-plane light conversion (MPLC) is an advanced light reshaping technique based on tailored multireflective phase elements. The MPLC component is a versatile setup that allows to reshape almost any kind of incident beam with specially generated elements including singlemode or highly multimode beams. Due to its unique ability to model both light phase and amplitude it can generate complex shapes with non-uniform intensities (e.g. intensity gradient) without affecting the beam profile (BPP) or depth of field. The complete setup can be adapted as an add-on on the focusing optics. Additionally, a single MPLC can support several independent input beams. 8

9 Multimode laser beam shaping The following table shows a comparison of three laser beam reshaping technologies for multimode high-power CW application. These three technologies are also compatible with a wide range of laser and wavelength. Refractive lens arrays (ROE) 1 Diffractive Optical Element (DOE) homogenizers 2 MPLC 3 Canunda-HP platform Input beam Multimode Multimode Singlemode or Multimode Transformation Homogenized shape Homogenized shape Intensity and shape Power handling (CW, >60 sec.) Output beam quality vs. incident beam Depth of field (vs in. beam) Up to 10kW Up to 8 kw Up to 12kW Degraded Degraded Similar 4 Degraded Degraded Optimal 4 Misalignment sensivity Low High Low Multi-Beam shaping and combining? No No Yes MM Complex shapes No Yes, reduced efficiency Yes, almost free form Efficiency > 90 % < 90 % multi-level > 95 % Cost $ $$$ (multi-level) $$ Table 2: 3 high-power multimode laser beam reshaping technologies comparaison 1 Beam shaping and homogenisation of high-power fibre lasers using a concave toroidal micro lens array, Paul Bair, PowerPhotonic, Laser Munich Intensity-adapted laser welding (IALW) of aluminium alloys, Stefan Liebl, IWB Munchen (Holo-Or DOE) Diffraction limited 5 For a given shape 9

10 Single laser beam shaping and combining Ultrafast laser The following table shows a comparison of four laser reshaping technologies adapted to singlemode ultrafast laser beam inputs. These technologies are compatible with a wide range of operating regime from CW to ultrafast femtosecond pulse duration and up to several tens of watt per beam. Diffractive Optical Element (DOE) 6 Spatial Light Modulator (SLM) 7 Refractive optics (aspheres, axicons...) Canunda-MP MPLC 3 Input beam Singlemode Singlemode Singlemode Singlemode or Few-modes Multi-beam reshaping and combining No No No Yes Transformation Intensity and shape Intensity and shape Intensity and shape Intensity, shape and phase Misalignment sensitivity High High High Low Adaptive control No Yes No Yes 8 Power handling (CW) N/A Up to 18 W > 100 W Up to 100 W Pulse energy handling (@1.03 µm, 0.5 ps) > 1 mj > 0.6 mj > 2.5 mj > 1.0 mj 9 Tailored shaping Yes, with reduced efficiency Yes No Yes: almost freeform Efficiency < 90 % (multilevel) < 85 % > 95 % > 95 % Cost/availability $$$ (multilevel, DOE only) $$$$ $ (lens only) $$ (complet setup) Table 2: Comparaison of three high-power multimode laser beam reshapers Hamamatsu LCOS-SLM X10468 Technical information 8 With multiple inputs 9 With HR coating

11 Non-coherent beam combining Non-coherent laser power combining is an interesting alternative to the single laser raw power increase. Combining several kw singlemode sources to achieve a single multi-kw laser output is generally more robust and more cost effective than a single multi-kw laser. Fused fiber combiners and free space combiners can achieve non-coherent combining with a relatively good efficiency and control of the beam quality. Figure 2: non-coherent laser combining solutions comparison DTU 10 ; OFS 11 ; Furukawa 12 ; Fraunhofer IAOF 13 ; SPI Lasers 14 ; Theory 15 Unlike coherent combiners, these technologies do not require wavelength, polarization and phase control and are directly compatible with high power lasers. 1 2 n Figure 3: Non-coherent combiner principle 10 All-fiber 7x1 signal combiner for incoherent laser beam combining, Noordegraaf et al., DTU, SPIE Fiber Laser Building Blocks, TrueM2 Beam Combiners, OFS, technical datasheet Furukawa Review, No Demonstration of >5kW emissions with good beam quality from two different 7:1 all-glass fiber coupler-types, Marco Plötner et al., Fraunhofer IAOF, PhotonicsWest redpower Multi kw OEM Building blocks for high power Fiber Lasers, SPI Lasers, technical datasheet Beam quality of multimode Laguerre-Gaussian beams, Olivier Pinel, CAILabs S.A.S., whitepaper

12 As for transverse mode multiplexing in telecommunication fibers, the possibility to simultaneously reshape both phase and amplitude of several lasers allows incoherent combining of multiple input laser beams. The output laser beam is then formed by the N lowest order modes of the output base (for instance a Laguerre-Gaussian base of modes) keeping the resulting M² and divergence as low as possible. Measurements performed on a CAILabs beam combiner shows combined beam quality, measured by the M², of 2.7 and 3.5 respectively for 5 and 9 combined lasers (Figure 2: noncoherent laser combining solutions comparison). Figure 4: left: single Gaussian and 9 combined LG modes beams right: intensity vs. width plot for single Gaussian and 9 combined LG modes beams (simulation) 12

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14 APPLICATIONS The following applications are laser beam reshaping use cases applied to metal sheet processing and fine material processing. It is a limited introduction to the improvements and flexibility permitted by an efficient laser reshaping and combining tool.

15 Improving sheet cutting speed This tailored laser beam allows to double the cutting speed of a laser machine by optimizing the melted metal evacuation from the melt pool and cut edge trimming. Two several kw multimode laser beams are shaped and combined to achieve this fast cutting process. melt pool melting beam trimmed cutting edge melt ejection and edge trimming beam cut kerf cutting direction Figure 5: Improved cutting, beam target (left) and result (right, using MPLC) Pre-joining surface ablation This double beam pattern associates a pulsed laser line ablative function to the CW spot welding. These laser shapes allow to sand off oxide or protective layers from the metal surface before performing the laser joining. This essential step avoids the junction zone to be contaminated ensuring a robust joining without complicating the process with a preparation pass. Two several kw multimode laser beams are shaped and combined to achieve this advanced welding process. Figure 6 : Pre-joining ablation, beam target (left) and result (right with MPLC) 16 Olsen et al. J. Laser Appl. 21, 133 (2009), project DOEFLAC 17 Efficient, mode-selective spatial multiplexer based on Multi-Plane Light Conversion, Olivier Pinel, CAILabs, JNPLI

16 Surface patterning This multiple-singlemode transformations allows to create entangled complex beam shapes. Such patterns are used as a laser stamp in direct surface patterning or micro-processing (marking, layer removal) reducing the dependence to scanner speed and increasing throughput. Additionally, the resulting beam have an optimal depth of field as their divergence is diffraction limited. Figure 7: Surface patterning, target shape (left) and MPLC simulation (a) and measurement (b) High aspect-radio single-shot drilling Several applications including through-silicon VIA drilling (TSV) or transparent sheet cutting (glass or sapphire) require a narrow spot size in conjunction with a long depth of field. Figure 8 : Measured circle beam profile (left) and Bessel beam at far field (right) 16

17 While the lowest beam divergence (and longest DOF) is generally achieved by a Gaussian mode, the Bessel beam features much longer central intensity rounded by side-lobes. Bessellike beams central lobes exhibit much longer depth of field than an equivalent Gaussian beam (with equal useful width) and shows self-healing properties. Figure 9 : Bessel beam longitudinal intensity profile (simulation) These elongated features are created by interferences of two plane waves generally implemented with a cylindrical geometry as the optical Fourier transform of a ring shaped beam s amplitude. The MPLC technology is a flexible way to generate high-quality Bessel beams. Its reflective design ensures high pulse energy handling with low dispersion. We measured a MPLC generated Bessel beam characteristics with a 126 µm DOF for a 2 µm center diameter (FWHM). Adaptive beam shaping and combining An interesting evolution of the non-coherent beam combining is the capacity to combine custom tailored beam in order to achieve a precise output intensity distribution. This shaping technique gives the availability to change the intensity profile by simply adjusting the input power of the lasers. The achievable profiles are pre-defined by the single channel shapes, allowing a direct, fast and high-power handling adaptive laser transverse intensity control. Though, laser power change the global system s cost is also positively impacted as this beam shaper replaces a single high power laser by several lower power sources. This adaptive beam shaper benefits to all laser material processing applications which requires fast adaptive laser intensity profile. 17

18 CANUNDA

19 Canunda-HP Canunda-HP is specially designed for high-power beam reshaping including strongly multimode beams up to 10 kw CW. It is carefully designed to minimize optical losses and manage thermal effects. Canunda-MP Canunda-MP is a versatile mid-power beam-shaper based on the CAILabs R&D multiplexer platform. It can reshape up to ten singlemode beams from lasers operating either in pulsed or continuous regime with up to 100 W of total average power. Canunda-MP can reshape beams with different wavelengths with minimal beam quality degradations. Canunda-HP Canunda-MP Maximum handling power (total) 10 kw 100 W CW or pulsed Wavelength (µm) , 1030, Maximum input pulse energy µj Multi wavelength operation No Yes Input Up to 10 free space inputs Up to 10 single mode fibers or free space beams Beam type Singlemode and multimode Singlemode Outputs Single beam, diffraction limited Single beam, diffraction limited Dimensions (mm 3 ) 350 x 160 x x 100 x 52 Table 3: Canunda-HP and Canunda-MP specifications 19

20 CAILabs CAILabs is a French start-up, based in Rennes, providing innovative photonics solutions to harness the full potential of optical fibers. We develop and manufacture a large range of light shaping components based on our patented, efficient and flexible technology of Multi-Plane Light Conversion.

21 GLOSSARY CW: Continuous Wave DOE: Diffractive Optical Element DOF: Depth Of Field HP: High-Power HR: High Reflectivity MM, MMF: Multi Mode Fiber MP: Mid-Power MPLC: Multi Plane Light Conversion PL: PuLsed ROE: Refractive Optical Element SM, SMF: Single Mode Fiber CONTACT Feel free to contact us for any other information about our products or our technology at : contact@cailabs.com 21

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