Now showing 1 - 10 of 15
  • Publication
    Laser drilling of micro-hole arrays in tantalum
    ( 2015)
    Patwa, R.
    ;
    Herfurth, H.
    ;
    Bratt, C.
    ;
    Christophersen, M.
    ;
    Phlips, B.F.
    X ray collimator optics for space application require an array of high aspect ratio holes of 60:1 with a minimal tantalum (Ta) thickness of >= 2 mm and a very high open area fraction (hole versus wall fraction) of 70% to achieve high collimator efficiency. Each collimator with a drilled area of 110 mm x 70 mm contains several million holes and need a fast drilling process. Laser percussion drilling was performed using an IR pulsed disk laser in a 1 and 2 mm thick Ta plate. A tightly spaced hexagonal closed packed pattern was used to maximize open area fraction with hole-to-hole spacing as small as 80 mu m. However, this resulted in a high concentration of debris and a thick recast layer on the remaining walls between the holes. Different process gases were investigated to minimize debris formation and reduce the recast layer thickness. Ramping of pulse energy during the drill cycle was investigated to minimize the adhesion between the substrate and recast layer. Chemical etching was used to remove the debris and recast from the top surface and the inside of the laser-drilled holes. Hole cross sections showed that a high aspect ratio was achieved with a hole diameter of about empty set50 mu m in 2 mm thick Ta. To achieve the shortest drilling time of 200 ms per hole, the process parameters were optimized and a hybrid nozzle, with both horizontal and vertical gas flow, was developed and implemented.
  • Publication
    Multi-beam laser additive manufacturing
    ( 2013)
    Patwa, R.
    ;
    Herfurth, H.
    ;
    Chae, J.
    ;
    Mazumder, J.
    Today, Laser Additive Manufacturing (LAM) is typically performed using a single beam with power up to multiple-kilowatts. The associated high heat input and limited process control hampers tight manufacturing tolerances and the applicable material spectrum. This paper highlights the development of Multi-beam LAM technology to address the shortfalls of today's technology and to broaden the applicability to many industries. Multi-beam LAM deploys several low power beams, each precisely controllable with a minimum heat input thus providing the capability to tailor the applied energy to the specific needs of the application. The single beams either work in parallel to scale productivity without sacrificing precision or in close proximity creating desired heat profiles. This new approach is scalable in productivity through multiplication and is expected to allow deposition of difficult to coat materials through tailored heat profiles. Advances are expected in near net shape manufacturing of complex structures with fine features and high dimensional accuracy. A compact prototype processing head for Multi-beam LAM was designed and built to investigate the capability of the new technology. The head incorporates latest high-brightness diode laser technology and a compact powder nozzle design. Two laser beams are being emitted, a stationary beam with fixed position on the work piece and a movable beam that can be positioned relative to the stationary beam. A very effective solution with high spatial resolution and fast actuation was developed for steering the movable laser beam. The movable beam cannot only be set to a fixed position but it can also be scanned at high frequencies. The power of both beams is individually controlled. Ongoing process investigations and future MB-LAM target specific applications for vehicles, jet engines and medical devices serving the automotive, aerospace, medical and defence sector. Initial results are being presented.
  • Publication
    High speed remote laser cutting of electrodes for lithium-ion batteries: Anode
    ( 2013)
    Lee, D.
    ;
    Patwa, R.
    ;
    Herfurth, H.
    ;
    Mazumder, J.
    Lithium-ion battery performance is affected by cut surface quality during the electrodes' cutting process. Currently, die cutting and rotary knife slitting have been used to cut prismatic and cylindrical electrodes, respectively. Both techniques, which require expensive tooling that wear out over time, result in process instability and poor cut quality. These will cause an internal short circuit and significant heat generation in the cells. Laser cutting, proved and widely utilized in the industry, can solve the abovementioned problems by improving cut surface quality due to many advantages, such as no tool wear, high energy concentration, fast processing speed, very narrow Heat Affected Zone, applicability to nearly all materials, and flexibility of laser power. Investigating underlying physical phenomena with numerical analysis provides significant advantages to fully utilize the remote laser cutting of electrodes for lithium-ion batteries. In this paper, a mathematic al model of three-dimensional self-consistent remote laser cutting is presented for anode (graphite-coated copper) of lithium-ion batteries. Computational and experimental results, obtained by using laser power of 450 W and scanning speed of 5 m s-1, show a two-step melt pool geometry and copper composition increase on the material interface.
  • Publication
    Laser drilling up to15,000 holes/sec in silicon wafer for PV solar cells
    ( 2013)
    Patwa, R.
    ;
    Herfurth, H.
    ;
    Mueller, G.
    ;
    Bui, K.
    One approach to realize a back contact solar cell design is to 'wrap'' the front contacts to the backside of the cell [1]. This results in significantly reduced shadowing losses, possibility of simplified module assembly process and reduced resistance losses in the module; a combination of measures, which are ultimately expected to lower the cost per watt of PV modules. A large number of micro-vias must be drilled in a silicon wafer to connect the front and rear contacts. Laser drilling was investigated using a pulsed disk laser which provided independent adjustment of pulse width, repetition rate and laser power. To achieve very high drilling rates, synchronization of the laser pulses with the two-axis galvanometer scanner was established using a FPGA controller. A design of experiments (DOE) was developed and executed to understand the key process drivers that impact the average hole size, hole taper angle, drilling rate and hole quality. Laser drilling tests were per formed on wafers with different thicknesses between 120 m and 190 m. The primary process parameters included the average laser power, pulse length and pulse repetition rate. The impact of different laser spot sizes (34 m and 80 m) on the drilling results was compared. The results show that average hole sizes between 30 - 100 m can be varied by changing processing parameters such as laser power, pulse length, repetition rate and spot size. In addition, this study shows the effect of such parameters on the hole taper angle, hole quality and drilling rate. Using optimized settings, 15,000 holes per second are achieved for a 120 m thick wafer with an average hole diameter of 40m.
  • Publication
    Compact high brightness diode laser emitting 500W from a 100µm fiber
    ( 2013)
    Heinemann, S.
    ;
    Fritsche, H.
    ;
    Kruschke, B.
    ;
    Schmidt, T.
    ;
    Gries, W.
    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.
  • Publication
    Very high brightness diode laser
    ( 2012)
    Heinemann, S.
    ;
    Lewis, B.
    ;
    Michaelis, K.
    ;
    Schmidt, T.
    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 m, 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 t o include 793 nm and 1530 nm. Narrow line gratings and optimized spectral combining enable further improvements in spectral brightness and power.
  • Publication
    Dopant uniformity and concentration in boron doped single crystal diamond films
    ( 2012)
    Demlow, S.N.
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    Berkun, I.
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    Becker, M.
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    Hogan, T.
    ;
    Grotjohn, T.A.
    High quality single crystal boron-doped diamond films are deposited in a microwave plasma-assisted CVD reactor with feedgas mixtures including hydrogen, methane, diborane, and carbon dioxide at reactor pressures of 160 Torr. The effect of diborane levels and other growth parameters on the incorporated boron levels are investigated, and the doping efficiency is calculated over a wide range of boron concentrations. The boron level is investigated using infrared absorption, and compared to SIMS measurements, and defects are shown to affect the doping uniformity.
  • Publication
    Investigation of different laser cutting strategies for sizing of Li-ion battery electrodes
    ( 2012)
    Patwa, R.
    ;
    Herfurth, H.
    ;
    Heinemann, S.
    ;
    Mazumder, J.
    ;
    Lee, D.
    Lithium-ion batteries are currently considered to be the most promising advanced battery technology for electric vehicles that require high energy capacity. A lot of research and development activities have been focused on their development to achieve efficient mass production capabilities and to successfully commercialize the technology. This paper discusses the laser cutting process for coated anode/cathode and uncoated copper/aluminum tabs for both cylindrical and prismatic Li-ion cell designs. A number of different cutting strategies have been investigated using IR (fiber/disc) and UV laser sources in pulsed/cw configurations. An in-depth development study has been performed to understand the effect of different processing parameters on the maximum cutting speed, cut edge quality and overall energy efficiency of the process. Results show that excellent cut quality can be achieved using optimal processing parameters with cutting speeds ranging from several m/min up t o 300 m/min depending on the processing requirements and the corresponding cutting approach.
  • Publication
    High brightness, frequency stabilized diode laser at 1530nm
    ( 2012)
    Heinemann, S.
    ;
    Lewis, B.
    ;
    Sczepansky, S.
    ;
    Schmidt, T.
    We report on high brightness diode laser at 1.5 m with wavelength stabilized output. 22W are delivered from a uncoated 100m fiber with 0.15 NA at 1532nm with a bandwidth of 2 nm. InP diode lasers emitting at 1.5 m show much lower power than GaAs based diodes emitting around 900 nm due to the low electro-optical efficiency of 1.5 m diodes of about 35%, compared to about 65% of GaAs diodes. Single emitters allow the highest power from given size broad area emitter due to optimized cooling. Up to 6W (15W) are available from a 95 m broad area single emitter at 1.5 m (9xx nm). At 1.5 m the maximum power is typically limited by thermal roll over and efficient heat dissipation from the diode is essential for power scaling. Optical stacking is deployed for power scaling thus symmetrizing the beam quality in fast and slow axis for efficient fiber coupling. Typically, 65% efficiency for optical stacking and fiber coupling are achieved resulting in more than 22W from a 100 m fiber of 0.15 NA. Resonant pumping of Er lasers requires a 2nm linewidth centered at 1530nm. The free running diodes show about 10nm linewidth (96% power content) and about 2.5nm/A tuning coefficient with varying drive current depending on heatsinking. Frequency stabilization is achieved with external Volume Bragg Gratings. More than 85% power is confined within a 2nm bandwidth up to 8A drive current resulting in 17W from the uncoated 100 m fiber. The diodes are emitting at 1546nm at 8A without VBG and 20W from the fiber are possible with the proper lower wavelength diode.
  • Publication
    Diodes for resonantly pumped CW and pulsed Er:YAG lasers
    ( 2012)
    Fritsche, H.
    ;
    Kruschke, B.
    ;
    Heinemann, S.
    ;
    Wang, X.
    ;
    Zhao, Z.
    ;
    Eichler, H.J.
    Eye safe laser radiation at 1.6 m is realized by a resonantly pumped Er:YAG laser operating in cw and q-switched mode employing broad and narrow spectrum diodes intended for medical applications and trace gas detection.