Scanning optical coherence topography (OCT) is a 3D imaging technique based on low-coherence interferometry. In recent days, it became a key technology in laser processing. The OCT probe beam is coupled co-axially to the laser beam into the processing optics and provides depth information of the probe. Additional information is obtained when the OCT beam is deflected using a small field scanner attached to the processing optics. This report will present manifold applications for OCT process control, ranging from monitoring the weld depth during the welding process, tracking joints in laser remote fillet welding, or localizing the position of pins in three dimensions for precise positioning of the laser beam.
In laser welding applications optical coherence tomography (OCT) is used to measure the capillary depth for process monitoring and process control. A controlled constant weld depth is expected to run applications closer to their process limits and reduce the number of destructive sample inspections. An essential premise is a reliable weld depth measurement independent from influencing factors. This work analyzes the influence of laser power, beam diameter, feed rate, and work piece material on the weld depth measured using the OCT technology. The results obtained by using fixed laser optics are compared to the corresponding results from scanner optics.
We have integrated an imaging thermographic sensor into commercial welding optics for observation of the weld zone. Key element of the sensor is an InGaAs-camera that detects the thermal radiation of the welding process in the wavelength range of 1,200 to 1,700 nm. This is well suited to record images of the keyhole, the melt pool and the thermal trace. The sensor was integrated to the welding heads for on-axis observation to minimize the interfering contour to ensure easy adaption to industrial processes. The welding heads used were established industrial-grade TRUMPF optics: a BEO fixed optics with 280 mm focal length, or a TRUMPF PFO-3D scanner optics with 450 mm focal length. We used a TRUMPF TruDisk 16002 16kW-thin disk laser that transmits its power through a 200 μm core diameter light cable. The images were recorded and features of the various process zones were evaluated by image processing. It turns out that almost all weld faults can be clearly detected in the NIR images. Quantitative features like the dimension of the melt pool and the thermal trace can be derived from the captured images. They are correlated to process input parameters as well as to process results. In contrast to observation in the visible spectrum the NIR camera records the melt pool without auxiliary illumination. As an unrivaled attribute of the NIR sensor it supports an online heat flow thermography of the seam and allows identifying missing fusion (“false friends”) of lap joints virtually during the welding process. Automated weld fault detection and documentation is possible by online image processing which sets the basis for comprehensive data documentation for quality assurance and traceability.
We have investigated process monitoring of laser beam welding with a TruDisk disk laser to detect process faults. Additionally to monitoring laser beam welding processes by a conventional VIS camera an NIR (near-infrared) camera reveals new information. Our sensor detects thermal radiation between 1100 and 1700 nm from the weld zone, which represents surface temperatures above 1000 K. Using the thermal radiation from the process we can observe all major weld defects without auxiliary illumination. The camera is integrated in a standard TRUMPF welding optics for on-axis observation. A real-time image processing system analyzes the camera images regarding welding irregularities and delivers information to characterize the weld process and its result. Actually, we perform an online passive heat-flow thermography that uses the process itself as the heat source and that probes the thermal attributes of the seam. By this means we can detect regions of bad fusion (“false friends”) virtually during the welding process. In addition to conventional thermography we have investigated the use of ratio pyrometry by using to NIR-cameras that observe the process in two different spectral bands. By considering the pixel-per-pixel ratio the influence of surface effects it greatly reduces and we obtain images of the weld zone with an absolute temperature scale. We have compared ratio pyrometry measurements with conventional thermography.
We have developed an on-axis camera-based online sensor system for laser beam welding diagnostics that detects the
thermal radiation in the near-infrared (NIR) spectral range between 1200 and 1700 nm. In addition to a sensor in the
visible (VIS) range, our camera detects the thermal radiation of the weld pool more clearly, and it is also sensible to the
radiation of the solidified weld seam. The NIR images are analyzed by real-time image processing. Features are
extracted from the images and evaluated to characterize the welding process. Keyhole and weld pool analysis
complement VIS diagnostics, whereas the observation of the weld seam and heat affected zone with an NIR camera
allows online heat flux thermography. By this means we are able to detect bad joints in overlap weldings ("false
friends") online during the welding process.
A long-standing challenge for semiconductor lasers is scaling the optical power and brightness of many diode lasers by
coherent beam combination. Because single-mode semiconductor lasers have limited power available from a single
element, there is a strong motivation to coherently combine the outputs of many elements for applications including
industrial lasers for materials processing, free space optical communications, and defense. Despite the fact that such a
coherently-combined source is potentially the most efficient laser, coherent combination of semiconductor lasers is
generally considered to be difficult, since precise phase control is required between elements.
We describe our approach to coherent combination of semiconductor lasers. The Slab-Coupled Optical Waveguide
Laser (SCOWL), invented at Lincoln Laboratory, is used as the single-mode diode laser element for coherent
combination. With a 10-element SCOWL array, coherently combined output power as high as 7 W in continuous wave
using an external cavity has been demonstrated, which is the highest output level achieved using a coherent array of
semiconductor lasers. We are currently working on a related approach to scale the coherent power up to 100 W.
Diode-pumped solid-state lasers are gaining acceptance as the desired laser source for materials processing as well as a
host of new applications that are expanding rapidly. Because of this, the performance, stability and lifetime of the diode-pump
source face unprecedented scrutiny. Increasing the lifetime of the diode, while increasing power, remains a
primary focus of the industry. One lifetime limiting issue is that of a voltage potential in the water cooling channels
which can cause cooler degradation and lower efficiency over time. Studies have been carried out that explore different
cooling approaches based on passive schemes where insulation layers are present to shield the voltage from the water
channels. However, with the introduction of insulation layers, a reduction of the deployable power from that of
microchannel coolers is seen. This report explores the effects of passive cooling approaches on the power and
divergence of 1 cm AuSn/CuW mounted bars with fill factors ranging from 10% to 50%. It is shown that a 150 W array
can be realized on a passive cooler and multiplexed to give a 1600 W stack. Thermal modeling is presented along with
life-test data for passively cooled devices.
KEYWORDS: Polarization, Resistance, Quantum wells, Indium, Temperature metrology, High power lasers, Data modeling, Thermal modeling, Diodes, Laser welding
As diode pumped solid state lasers gain more market share, the performance, stability and lifetime of the diode pump
source faces unprecedented scrutiny. Lifetimes of diode pumps in excess of 35,000 hrs are sought with no intervention
or maintenance from the end user. One lifetime and power limiting phenomena for arrays is that of solder creep typical
with traditional mounting using soft solders such as Indium. Harder solders such as Gold/Tin on Copper-Tungsten
submounts provide a more robust and stable mounting system for long term high power pump sources. Furthermore,
beam multiplexing of laser bars require tight wavelength and polarization purity which are affected by mounting induced
strain. In this investigation, high power 940 nm laser bars, operating in the 100 to 200 W power range, were mounted
using AuSn/CuW and In soldering schemes. The differences in thermal and strain characteristics are investigated
through the examination of the emitter wavelength, nearfield measurements, polarization and smile. The measurements
are correlated with finite element modeling to predict the 3-dimensional thermal distributions within the laser bars.
JENOPTIK Laser, Optik, Systeme GmbH has developed for the first industrial all-solid-state Red-Green-Blue laser system for large image projection systems. Compact in design (0.75 m3, 180 kg, 3 kW power consumption), the system consists of a modelocked oscillator amplifier subsystem with 7 ps pulse duration and 85 MHz pulse repetition frequency, an optical parametric oscillator, and several non-linear stages to generate radiation at 628 nm, 532 nm and 446 nm with an average output power above 18 W. Each of the three colors is modulated with the video signal in a contrast ratio of 1000:1 and coupled into a common low order multi mode fiber. The system architecture relies on efficiently manufacturable components. With the help of FEM analysis, new engineering design principles and subsequent climatic and mechanical tests, a length stability below 50 micrometers and an angle stability below 10 (mu) rad have been achieved. The design includes efficient laser diodes with integrated thermo- electric cooler and a lifetime above 10000 hours. The stability of the output power is better than +/- 2% in a temperature range from 5 degree(s)C to 40 degree(s)C. The system operates reliably for more than 10000 hours under field conditions. The design is based (among others) on work by Laser-Display-Technologie KG and the University of Kaiserslautern.
We have developed a high-power laser system that is based on actively cooled GaAs diode laser stacks. Fast axis collimation and subsequent beam rearrangement generates a symmetric laser beam in respect to the beam parameter product along the two main axes. By polarization and wavelength coupling 100 diode laser elements can be coupled into one fiber at a beam parameter product of less than 200 mm*mrad in both directions and more than 2 kW cw output power at the workpiece. At a spot diameter of less than 1 mm the power density exceeds 250 kW/cm2. First material processing experiments show that deep welding at working speeds that meet industrial requirements in steel can be observed. High-power diode lasers show that they become suitable for industrial work.
High power diode laser bars and stacks are of great interest in industrial applications due to their high electro-optical efficiency, their small type of construction and maintenance free operation. With highly sophisticated beam shaping optics diode lasers can be used as pumping sources for solid state and fiber lasers and direct for material processing, e.g. welding, soldering and marking metals. We have developed different fiber coupled diode laser systems with output power up to greater than 2 kW cw into a spot 0 1.0 mm (power density greater than 250 kW/cm2) and systems with output power 170 W cw into a spot 0 0.38 mm (power density about 150 kW/cm2). The 2 kW system operates with a 0 1.5 mm fiber (N.A. 0.32) and consists of polarization and wavelength coupled stacks with an overall electro-optical efficiency of 23%. The smaller system operates either with a 0 0.6 mm (N.A. 0.22) or 0 0.4 mm (N.A. 0.33) fiber and consists of a single stack. Polarization and wavelength coupling will be realized in future. The overall electro-optical efficiency is about 27%.
High-power diode lasers have reached output power and reliability to meet requirements for industrial applications. Stacking of laser elements to modules increases the output power, beam shaping techniques allow to focus the radiation of a module to a single spot. An integrated diode laser systems with totally 50 laser bars is shown, that includes cooling, power supply and control unit. The laser radiation is transmitted by an optical fiber and an objective focuses the radiation onto the workpiece with a round spot of less than 1 mm diameter and cw power of more than 1 kW.
KEYWORDS: Semiconductor lasers, High power diode lasers, High power lasers, Diodes, Heatsinks, Resistance, Copper, Collimation, Materials processing, Solid state lasers
High power laser diodes and especially high power laser diode modules made enormous progress in the last few years. Different aspects of high power laser diodes are treated starting from general description of high power laser diodes and their mounting techniques, characterizing the electro- optical behavior of single laser bars and finally presenting beamshaping optics for the collimation of large modules. The later technique allows for symmetrical focal spots in the kilowatt range with a beam quality of about 170 mm*mrad. Different aspects of current applications of high power laser diodes are presented.
Based on a pair of step-mirrors for beam rearranging we coupled the emission of three high-power diode laser arrays into an optical fiber of 800 micrometer diameter. We compressed the fast axis collimated beams of three diode lasers in respect to their fast axes by means of a step prism and symmetrized the beam parameter product by reordering the radiation which is focused into a fiber then. By simple optimization a coupling efficiency of 70% can be obtained.
AlGaAs/GaAs high power diode lasers with a nominal output power of 15W were aged at different conditions. At a heatsink temperature of 25 degrees C aging at constant current (CC) and constant power (CP) mode is compared for aging times of 6000 hours. We derived an end-of-life criteria that results in the same lifetime for CC and CP operation assuming identical degradation mechanisms in both cases. The degradation observed differs only significantly beyond 3000-4000 hours of aging with increasing degradation for CP operation. In constant current mode the heatsink temperature is increased resulting in a junction temperature of about 80 degrees C. Assuming an Arrhenius relation the activation energy is estimated. It turns out that different activation energies can be derived either by taking the degradation of the output power at the elevated temperature or at the reference temperature respectively.
Higher power laser diodes have been aged under various operating conditions to gain information on the long-term output power behavior. In particular, the degradation of cw diode lasers that are operated at constant output power or at a constant driving current are compared to each other. It turns out that the lifetime results and their comparability strongly depend on the 'end-of-life' criterion. Especially for constant power operation extrapolation of the lifetime for longer intervals might give inconsistent results if the degradation of threshold current and slope efficiency are not known. Aging tests at increased temperature have been performed to investigate whether accelerated lifetime tests give reliable results to estimate lifetime and degradation rate at nominal operating conditions.
H. Bartels, Peter Boerger, Friedhelm Dorsch, Bernd Eppich, Uwe Habich, Ruediger Maestle, Holger Muentz, Wilfried Plass, R. Pohl, U. Quattelbaum, Marcus Scholl, Laura Serri, H. Staubach, Emanuele Stucchi, C. Wild, Norbert Wolf
Within the EUREKA-project 'CHOCLAB' a workshop was organized, where several partners analyzed the beam of an industrial carbon-dioxide laser device. The goal was to qualify the ISO 11146 standard by using different measurement techniques. This paper compares the results of the determination of the beam propagation factor and beam widths. The following test methods for measuring the beam width were applied: (1) determination of the power density distribution with a rotating pinhole device and subsequent evaluation of beam widths, (2) determination of the power density with a pyroelectric camera and subsequent evaluation of beam widths, (3) moving knife edge method. For the measured data various methods of evaluation were used. These include data processing with different filters, evaluation of averaged power density distributions as well as different algorithms for curve fitting. Despite the different methods of measurement and evaluation the results showed reasonable agreement. This proves the applicability of the ISO 11146 standard to an industrial carbon-dioxide laser device.
The combination of a scanning monochromator, a digitizing oscilloscope and a computer, allows time-resolved spectral measurements on laser diodes. The flexible system described covers a wide range of timescale, well below 1 microsecond(s) ec at a spectral resolution of down to 0.05 nm. With this system the authors have investigated quasi-cw laser diode bars that are designed for solid-state laser pumping, at various operating conditions. The time evolution of the center wavelength is quite well fitted by a square root function according to non-stationary thermodynamics. The measuring system enables judgement of the quality of mounting technique and helps in choosing the right heatsink to minimize the wavelength chirp during a light pulse.
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