CANUNDA Application note Version 06/10/2015
2
TABLE OF CONTENTS INTRODUCTION LASER BEAM SHAPING SOLUTIONS APPLICATIONS Sheet cutting improved speed Hardened welding Pre-joining surface ablation Slow cooling heating CAILabs CANUNDA CAILabs GLOSSARY p.4 p.6 p.10 p.10 p.10 p.11 p.11 p.12 p.13 p.14 3
INTRODUCTION 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, CO 2 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, some 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 can be used for metal cutting and welding and the second for fine surface processing. The laser beam intensity distribution depends on the laser technology (Table 1) and can affect the interaction between the laser spot and the material. 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. 4
CO2 laser SM fiber laser Nd: YAG laser MM fiber laser MM disk laser MM Diode lasers Type Coherent Diamond E-1000 IPG YLR- 1000-WC JK Laser JK1002SM IPG YLS-1000 Trumpf TruDisk 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 < 1.2 1.1 - - - - BPP (mm*mrad) - - ~36 2.0 5.9 1.4 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
LASER BEAM SHAPING SOLUTIONS Laser material processing is used in a large number of industries including large scale industries such as automotive or microelectronics where tools productivity and fl exibility 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 ). 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). 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 and the assembly robustness, 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 fl at-top transverse intensity profi le transformation which optimizes the available laser power usage and improves the processing quality. Unlike the Gaussian beam, the fl at 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 6
Several beam shaping solutions have been developed so far, with different level of complexity, total efficiency and cost, total efficiency meaning the ratio of total input laser power and output power converted into the target shape. There are basically two categories of laser reshapers depending on the type of incident beam: ones are singlemode beams (generally described as Gaussian intensity profile) and others multimode beams. Multimode output lasers offer generally 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 aspheric (or a-cylindrical) curvature. 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. 7
-- Refractive micro-lenses (or Refractive Optical Elements - ROEs) are micro-lenses arrays that can perform homogenization function and beam shaping. They are adapted to non-coherent 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 up to two independently reshaped beams. The following table shows a comparison of three multimode laser beam reshapers for high-power CW application. These three technologies are also compatible with a wide range of pulsed laser regimes and wavelength. 8
Refractive lens arrays 1 DOE homogenizers 2 MPLC 3 Design wavelength 1.075 µm 1.060 µm 1.060 µm Power handling (CW, >60 sec.) Up to 10kW Up to 8 kw Up to 12kW BPP vs. incident beam Degraded Degraded Similar Misalignment sensitivity Low High Low Wavelength operating range ~100 nm ~nm ~10 nm Multi-Beam combining No No Yes MM Complex shapes No Flat top profiles Efficiency ~ 95 % Cost $ Yes with reduced efficiency Flat top profiles ~ 75 % binary ~ 90 % multilevel $ binary $$$ multilevel Yes : almost free form Any profile > 95 % $$ Table 2: Comparaison of three high-power multimode laser beam reshapers 1 Beamshaping and Homogenisation of High-Power Fibre Lasers using a Concave Toroidal MicrolensArray, Paul Bair, PowerPhotonic, Laser Munich 2015 2 Intensity-Adapted Laser Welding (IALW) of Aluminum Alloys, Stefan Liebl, IWB Munchen 3 Efficient, mode-selective spatial multiplexer based on Multi-Plane Light Conversion, Olivier Pinel, CAILabs, JNPLI 2015 9
APPLICATIONS The following applications are laser beam reshaping use cases applied to the metal sheet processing industry. It is a limited introduction to the improvements and fl exibility permitted by an effi cient laser reshaping and combining tool. Sheet cutting improved speed: This tailored laser beam allows to double the cutting speed 4 of a laser machine by optimizing the melted metal evacuation from the melt pool and cut edge trimming. melt pool melting beam trimmed cutting edge melt ejection and edge trimming beam cut kerf cutting direction Figure 2: Improved cutting, beam target (left) and result (right, using MPLC) Hardened welding: This double spot pattern improves metal joining by adding a post-welding laser quenching in a single pass. The quenching helps to reduce the strain-age cracking 5. quenching laser welding laser 4 Olsen et al. J. Laser Appl. 21, 133 (2009), project DOEFLAC 5 Profi les from: United States Patent 6639173, 2003 GE (use of electron beams) 10 joining edge melted zone joining direction Figure 3: post-welding quenching, 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 function. 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 6. contaminated surface ablation laser 1 kw-pl welding laser 4 kw-cw melted zone cleaned surface joining direction Figure 4: Pre-joining ablation, beam target (left) and result (right, using MPLC) Slow cooling heating: This tailored beam adapts the laser power to the optimal material cooling ramp. A slow cooling can help to improve mechanical properties of the treated zone and avoid fast cooling defects 6. fast temperature increase slow cooling gradient heating direction Figure 5: Intensity gradient, beam target (left) and result (right, using MPLC) 6 Efficient, mode-selective spatial multiplexer based on Multi-Plane Light Conversion, Olivier Pinel, CAILabs, JNPLI 2015 11
CAILabs CANUNDA Canunda versatile beam reshaper, based on MPLC technology, is available in two platforms: a high-power (HP) and a mid-power (MP) one. Canunda-HP is specially designed for high-power beam reshaping including strongly multimode beams up to 12 kw CW. It is carefully designed to minimize optical losses and manage the thermal effects. 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 10 W of total average power. Canunda-MP can reshape beams with different wavelengths in the NIR range. Canunda-HP Canunda-MP Maximum handling power (total) 12 kw (60 sec.) 10 W CW or pulsed Wavelength (µm) 1.03 1.08 0.90 1.65 Max peak power (total) N/A 1 kw (pulse > 20ns), 20 MW (pulse > 500fs) Wavelength 1060nm Multi wavelength operation No Yes Input 2x 200/400µm MMF Up to 10 single mode fi bers or free space beams Outputs Single beam, diffraction limited Single beam, diffraction limited Dimensions (cm 3 ) 61.2 x 61.2 x 26.0 (w/o connectors or focusing optics) 15 x 10 x 5.2 12 Table 3: Canunda-HP and Canunda-MP specifications
CAILabs CAILabs is a leading provider of light shaping components. We develop and manufacture a large range of products based on our patented, efficient, and flexible technology of Multi-Plane Light Conversion. CAILabs develop and manufacture spatial multiplexers for telecommunication networks, LAN network upgrade, and multibeam shapers and combiners for industrial laser shaping and fiber amplifiers. 13
GLOSSARY CW: Continuous Wave DOE: Diffractive Optical Element HP: High-Power MM, MMF: Multi Mode Fiber MP: Mid-Power MPLC: Multi Plane Light Conversion NIR: Near InfraRed PL: PuLsed ROE: Refractive Optical Element SM, SMF: Single Mode Fiber 14
15
CAILabs 8 rue du 7 ème d Artillerie 35000 RENNES FRANCE