Stefan Heinemann

Compact high brightness diode laser emitting 500W from a 100μm fiber

High power, high brightness diode lasers are beginning to compete with solid state lasers, i.e. disk and fiber lasers. The core technologies for brightness scaling of diode lasers are optical stacking and dense spectral combining (DSC), as well as improvements of the diode material. Diode lasers have the lowest cost of ownership, highest efficiency and most compact design among all lasers. Multiple Single Emitter (MSE) modules allow highest power and highest brightness diode lasers based on standard broad area diodes. Multiple single emitters, each rated at 12 W, are stacked in the fast axis with a monolithic slow axis collimator (SAC) array. Volume Bragg Gratings (VBG) stabilizes the wavelength and narrow the linewidth to less than 1 nm. Dichroic mirrors are used for dense wavelength multiplexing of 4 channels within 12 nm. Subsequently polarization multiplexing generates 450 W with a beam quality of 4.5 mm*mrad. Fast control electronics and miniaturized switched power supplies enable pulse rise times of less than 10 μs, with pulse widths continuously adjustable from 20 μs to cw. Further power scaling up to multi-kilowatts can be achieved by multiplexing up to 16 channels. The power and brightness of these systems enables the use of direct diode lasers for cutting and welding. The technologies can be transferred to other wavelengths to include 793 nm and 1530 nm. Optimized spectral combining enables further improvements in spectral brightness and power.

Very high brightness diode laser

Multiple Single Emitter (MSE) modules allow highest power and highest brightness diode lasers based on standard broad area diodes. 12 single emitters, each rated at 11 W, are stacked in fast axis and with polarization multiplexing 200W are achieved in a fully collimated beam with a beam quality of 7mm*mrad in both axes. Volume Bragg Gratings (VBG) stabilize the wavelength and narrow the linewidth to less than 2nm. Dichroic mirrors are used for dense wavelength multiplexing of 4 channels within 12 nm. 400W are measured from a 0.2 mm fiber, 0.1 NA. Control and drive electronics are integrated into the 200 W platform and represent a basic building block for a variety of applications, such as a flexible turn key system comprising 12 MSE modules. An integrated beam switch directs the light in six 100 μm, or in one 0.2 mm and one 0.1 mm fiber. 800W are measured from the six 0.1 mm fibers and 700W from the 0.2 mm fiber. The technologies can be transferred to other wavelengths to include 793 nm and 1530 nm. Narrow line gratings and optimized spectral combining enable further improvements in spectral brightness and power.

Hardening and welding with high-power diode lasers

Commercially available high power diode lasers (HPDLs) with output powers of up to 6 kW have been recognized as an interesting tool for industrial applications. In certain fields of application they offer many advantages over Nd:YAG and CO2 lasers because of their low maintenance, compact design and low capital costs. Examples of successful industrial implementation of HPDLs include plastic welding, surface hardening and heat conduction welding of stainless steel and aluminum. The joining of plastics with an HPDL offers the advantages of producing a weld seam with high strength, high consistency and superior appearance. One example is the keyless entry system introduced with the Mercedes E-class where the microelectronic circuits are embedded in a plastic housing. Other applications include instrument panels, cell phones, headlights and tail lights. Applications in the field of surface treatment of metals profit from the HPDLs inherent line-shaped focus and the homogeneous intensity distribution across this focus. An HPDL system is used within the industry to harden rails for coordinate measurement machines. This system contains a customized zoom optic to focus the laser light onto the rails. With the addition of a temperature control, even complex shapes can be hardened with a constant depth and minimum distortion.

Single emitter based diode lasers with high brightness high power and narrow linewidth

Multiple Single Emitter (MSE) modules allow highest power and highest brightness fiber coupled diode lasers based on standard broad area diodes. 12 single emitters, each rated at 11W, can be stacked in fast axis and yield more than 100W in a fully collimated beam with a beam quality of 7mm*mrad in both axes. Optical transfer efficiencies of >88% from diode facet to after the fiber are achieved resulting in efficient and compact fiber coupled modules. Volume Bragg Gratings (VBG) stabilize the wavelength over a tuning range of >10nm and narrow the linewidth of individual diodes to less than 2nm. The brightness is scaled by polarization multiplexing and optical stacking is deployed for larger fibers resulting in 700W delivered from a 200μm fiber, 0.2 NA. Wall plug efficiencies of 35% are achieved. The challenge of MSE fiber coupled diode lasers lies in high precision, high yield manufacturing and not so much the optical design of the device, since only collimating lenses and a focusing optic are used. However, a large number of individual components must be handled and consistently aligned with high precision. The 100W module comprises 12 single emitters and the 700W/200μm/0.2 laser comprises 120 single emitters with 85% optical fill factor. Pointing tolerances and collimation errors of all emitters cannot exceed 10% of the spot size to realize the benefits of highest brightness from single emitters compared to bars. The two major assembly processes of MSE fiber coupled diode lasers are the precision diode reflow process and the accurate 5 axis alignment of the fast axis collimation lens (FAC). The reflow process enables positioning of 12 single emitter diodes on submounts within +/-5μm on a common heatsink. Special image processing software performs automated precision alignment and fixation of the FAC with a consistent accuracy of better 0.3um and 0.12mrad. It is also deployed for automated alignment of the external VBG. Wavelength stabilization in an external resonator aims to maximize the locking range and to minimize the drop of output power as well as linewidth. Front facet reflectivity of the diode laser, reflectivity of the volume Bragg grating (VBG) and different resonator designs are investigated.

Aging properties of AlGaAs/GaAs high-power diode lasers

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.

Efficient Er:YAG lasers at 1645.55 nm, resonantly pumped with narrow bandwidth diode laser modules at 1532 nm, for methane detection

Eye safe laser operation at 1645.55 nm (6077 cm-1) of resonantly pumped Er:YAG laser systems is demonstrated in cw and Q-switched operation. High brightness diode laser modules emitting at 1532 nm have been utilized as pump sources providing an absorption efficiency of up to 96%. This leads to an overall efficiency of the Er:YAG laser of 30%. For cw operation, 9 W output power is possible at pump power of 30 W while Q-switching results in generation of more than 7 mJ pulses with duration of 60 ns and repetition rate of 500 Hz. The Er:YAG laser systems have been applied for methane detection measurements demonstrating their feasibility for CH4-DIAL applications

Welding head for ‘self guided’ laser welding

Precise positioning of the laser beam on the work piece is crucial for high quality laser welds; e.g. for butt welding the focal point of the laser beam with respect to the joint must be maintained within an accuracy better than 20µm - 150µm, depending on the focused beam radius. These stringent accuracy requirements call for high precision robots, a repeatable work piece profile and precise clamping. To compensate for insufficient repeatability of work piece or clamping, seam-tracking devices are used. A sensor measures the joint position and computes a correction vector to follow the actual joint trajectory. The deviation is compensated either by robot trajectory adjustment or by an additional tracking axis. Disadvantages of this approach are complex installation of the devices due to interfacing with the robot control, the need of teaching and calibrating the sensor and principle based accuracy restriction that limit the usability in more complex 2d contours and with low accuracy robots.We recently introduced a more flexible and precise approach that utilizes an advanced camera-based sensor that is capable of measuring seam position, relative displacement between work piece and sensor and melt pool of the process with one single device. This paper describes a realized ‘self guided’ welding head, which uses this approach in combination with an integrated high power scanner. The result is a welding head that follows a curved or linear butt weld with high precision and independent of the actual robot trajectory; without the need of calibration, robot interfacing and alignment.Precise positioning of the laser beam on the work piece is crucial for high quality laser welds; e.g. for butt welding the focal point of the laser beam with respect to the joint must be maintained within an accuracy better than 20µm - 150µm, depending on the focused beam radius. These stringent accuracy requirements call for high precision robots, a repeatable work piece profile and precise clamping. To compensate for insufficient repeatability of work piece or clamping, seam-tracking devices are used. A sensor measures the joint position and computes a correction vector to follow the actual joint trajectory. The deviation is compensated either by robot trajectory adjustment or by an additional tracking axis. Disadvantages of this approach are complex installation of the devices due to interfacing with the robot control, the need of teaching and calibrating the sensor and principle based accuracy restriction that limit the usability in more complex 2d contours and with low accuracy robots.We recently int...

Fiber-coupled high brightness high power diode laser for solid-state laser pumping and material processing

Two applications emerge as drivers for higher brightness fiber-coupled diode lasers: advanced solid-state pumping schemes and materials processing. In contrast to the well-established side-pumping schemes of laser rods, advanced pumping schemes for todays solid-state lasers make use of the high brightness of the pump sources to increase the performance and efficiency of the solid-state laser. Materials processing applications such as metal welding and cutting are commonly served with solid-state lasers or CO2 lasers. Lately, the increased lifespan, reduced systems costs and increased brightness of fiber-coupled diode laser systems make them a new alternative. In this work, a diode laser system is described that yields 250 wats in a 600 micrometer spot with a numerical aperture 0.2 of the focused beam, corresponding to a F/# of 2.4. The system is based on a single 15 bar stack that operates in cw-mode. For brightness enhancement, it incorporates a measure to increase the fill factor of the emitting aperture and polarization multiplexing. The brightness in the focus spot is 105W/cm2 with a F# of 2.4 focusing optic and 3x105W/cm2 with a high speed of F/# of 1.4. To achieve the required symmetry for fiber coupling, the system incorporates a beam transformer that assimilates the beam quality along the two main axes of the beam profile. A monolithic design is chosen to reduce alignment tolerances and to increase ruggedness.

Laser-based facet inspection system

We developed a laser based inspection system which was used to monitor defect distributions in optoelectronic devices such as diode lasers. Basically the system works as the well-known laser beam induced current (LBIC) technique. Various lasers emitting in the 633-1300 nm wavelength range were employed as excitation source. A number of high power laser diode arrays (LDA) aged under different aging conditions (parameters: injection current, heat sink temperature, time) were inspected with the system. The scans obtained revealed significant differences for different aging levels of arrays. This finding allows us to determine the aging status of LDA and contributes to find methods for giving failure predictions for individual devices.

Packaging of high-power bars for optical pumping and direct applications

Continuous cost reduction, improved reliability and modular platform guide the design of our next generation heatsink and packaging process. Power scaling from a single device effectively lowers the cost, while electrical insulation of the heatsink, low junction temperature and hard solder enable high reliability. We report on the latest results for scaling the output power of bars for optical pumping and materials processing. The epitaxial design and geometric structures are specific for the application, while packaging with minimum thermal impedance, low stress and low smile are generic features. The isolated heatsink shows a thermal impedance of 0.2 K/W and the maximum output power is limited by the requirement of a junction temperature of less than 68oC for high reliability. Low contact impedance are addressed for drive currents of 300 A. For pumping applications, bars with a fill factor of 60% are deployed emitting more than 300 W of output power with an efficiency of about 55% and 8 bars are arranged in a compact pump module emitting 2 kW of collimated power suitable for pumping disk lasers. For direct applications we target coupling kilowatts of output powers into fibers of 100 μm diameter with 0.1 NA based on dense wavelength multiplexing. Low fill factor bars with large optical waveguide and specialized coating also emit 300 W.