Juergen Jandeleit

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toFthres = 172 mJ/cm2. The ablation rate per pulse is measured as a function of intensity in the range of 5 × 109 to 2 × 1011 W/cm2 and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.

Fundamental investigations of micromachining by nano- and picosecond laser radiation

The removal processes of ceramics and metals were investigated using pico- and nanosecond laser radiation produced by diode-pumped Nd:YAG lasers. The laser radiation was focused to spot diameters smaller than 10 mm, yielding power densities up to 5 = 10 12 Wrcm 2 . The threshold fluence for removal and the removal depth per pulse were determined for 40 pico- and 10 nanosecond laser pulses using the fundamental wavelength, the second harmonic and the third harmonic laser radiation of the laser system. For 40 ps laser pulses pump and probe investigations were used to study the interaction of intense ultrashort laser beams with matter. By this technique ultrashort processes can be photographed with a time resolution determined by the pulse length of pump and probe pulses. The measurements allow a detailed characterization of the material removal including melting, vaporization and fast resolidification as well as the feedback of the surrounding atmosphere to the processed microstructures. The threshold fluences for material removal and the removal rates per pulse were determined for Si N , SiC and WC as a function of laser pulse length and laser wavelength. Using picosecond laser 34 radiation microstructures were produced in different ceramics and metals demonstrating the suitability of short laser pulses for the production of microstructures with dimensions smaller than 10 mm and for ultra-precise removal of thin layers. q 1998 Elsevier Science B.V.

Micromachining of metals and ceramics by nano- and picosecond laser radiation

The removal processes of Si3N4- and SiC-ceramics and tungstencarbide were investigated using 40 ps and 10 ns laser pulses. The threshold fluence for removal and the removal rate per pulse were determined. Changes in the chemical composition of the processed surfaces are described and the influence of the removal strategy on the processing results are discussed. Micro-structures were produced in combination with high- resolution optics and precision motion control systems. In SiC-ceramics grooves were produced with geometries smaller than 30 micrometer. In Si3N4-ceramics holes were drilled with diameters smaller than 6 micrometer. The influence of scanning velocity, overlap of the laser pulses and pulse energy on material removal and surface finish were discussed.

Picosecond-laser-pulse-induced heat and mass transfer

The interaction of picosecond and sub-picosecond laser pulses with metals is investigated both, theoretically and experimentally. Analyzing the Boltzmann equation for electrons and phonons the hyperbolic two-temperature model of heat conduction in metals is obtained. In particular the parameter range for which the hyperbolic effects are significant is analyzed. For calculations a numerical algorithm based on the method of lines is developed. Experimentally laser pulses with a duration of 40 ps are used to remove thin metal films. The removal process is analyzed by pump and probe measurements with the time resolution of 40 ps. The single-shot removal threshold and the removal rate per pulse are determined for copper. By this technique the existence and the propagation of shock waves in the ambient atmosphere induced by the removal process are detected. Theoretical calculations are compared with experiments and the results from the literature.

Packaging and characterization equipment for high-power diode laser bars and VCSELs

High-power diode lasers serve as small and highly efficient laser-beam sources. Their output power has been increased steadily over the recent years. Nowadays, the output power of one diode laser bar is sufficient for many different applications such as pumping of solid state lasers, direct material processing, medical applications or printing. The successful use of high-power diode lasers depends on their high reliability in combination with a long lifetime. For a further increase in the quality of high-power diode lasers the properties of semiconductor lasers have to be improved as well as the cooling and mounting techniques for these bars onto specially designed heat sinks.

267-W cw AlGaAs/GaInAs diode laser bars

High-power 980 nm-diode laser bars have been fabricated in the AlGaAs/GaInAs material system. The bars are 1 cm wide and comprise 25 broad area lasers with 200 micrometer aperture and 2 mm resonator length. Hence, the fill factor is 50%. To reduce the power density at the facet, we used an LOC structure with low modal gain, which also helps to prevent filamentation. The measured threshold current was 14 A and a record output power of 267 W cw was achieved at 333 A with an electro-optical conversion efficiency of 40%. With less thermal load, at 150 W output power the conversion efficiency was as high as 50% and the corresponding slope efficiency was 0.9 W/A. Microchannel copper heat sinks with a thermal resistance of less than 0.29 K/W were used for mounting the bars. The coolant temperature was set for all measurements to 22 degrees Celsius and the flux was 0.9 l/min. Additionally, the top electrode of the p-side down mounted bars was cooled by a second heat sink, which was pressed gently on the top electrode.

Packaging and characterization of high-power diode lasers

High power diode lasers can be used for a lot of applications such as pumping of solid state lasers, direct material processing (for example welding, soldering, annealing) and printing. The successful use of high power diode lasers depends on their high efficiency and reliability in combination with a long lifetime. For a further increase in the quality of high power diode lasers the properties of semiconductor laser bars have to be improved as well as the mounting techniques for these bars onto specially designed heat sinks. For most applications the electro-optical properties of the high power diode lasers have to be known exactly. Detailed information on the propagation characteristics and the transverse mode distribution of diode laser beams is necessary for the optimization of the overall performance. In addition the electro-optical characterization is a first test for the quality of high power diode lasers. An automated test set-up developed at the Fraunhofer-Institut fur Lasertechnik will be presented. The electro-optical data such as threshold current, slope efficiency, center wavelength, spectral width, total conversion efficiency and cw-output power can be measured as a function of driving current as well as the beam divergence angles in fast and slow direction of the high power diode lasers. Different methods to determine the exact divergence angles will be discussed. A high power test set-up is developed which allows driving currents up to 360 A. Using diode laser bars developed at the Fraunhofer- Institute fur Angewandte Festkorperphysik a record cw-output power of 267 W could be achieved at 333 A.

Reliability and degradation mechanisms of high-power diode lasers

The degradation of GaAlAs/GaAs laser diode bars mounted on copper micro channel heat sinks was investigated using optical microscopy, scanning electron microscopy and EDX- spectroscopy. The high power diode lasers were characterized before and after burn-in and after long term lifetests. Changes in surface morphology and surface composition of the facets were detected as well as changes in threshold current, slope efficiency and wavelength. Due to the degradation threshold current increases and slope efficiency decreases while the wavelengths were shifted to higher values showing a broader spectral width. The influence of these changes on performance and lifetime of the high power diode lasers will be discussed.

Fabrication and characterization of high-power diode lasers

High power diode lasers can be used for a lot of applications such as pumping of solid state lasers, direct material processing and printing. The successful use of high power diode lasers depends on their high efficiency and reliability in combination with a long lifetime. For a further increase in the quality of high power diode lasers the properties of semiconductor laser bars have to be improved as well as the mounting techniques for these bars onto specially designed heat sinks. For most applications the electro-optical properties of the high power diode lasers have to be known exactly. Detailed information on the propagation characteristics and the transverse mode distribution of diode laser beams is necessary for the optimization of the overall performance. In addition the electro-optical characterization is a first test for the quality of high power diode lasers. An automated test set-up developed at the Fraunhofer-Institut für Lasertechnik will be presented. The electro-optical data such as threshold current, slope efficiency, center wavelength, spectral width, total conversion efficiency and cw-output power can be measured as a function of driving current as well as the beam divergence angles in fast and slow direction of the high power diode lasers. Different methods to determine the exact divergence angles will be discussed.

Improved high-temperature operation of InGaAs/AlGaAs LOC SQW diode lasers by incorporation of short-period superlattice quantum-well barriers

InGaAs/AlGaAs large optical cavity (LOC) single quantum well (SQW) lasers emitting at 980nm were grown incorporating an AlGaAs/GaAs short-period superlattice layer next to the quantum well in order to improve the carrier confinement and thus high-temperature operation. Symmetric and asymmetric structures have been realized. High characteristic temperatures T0 above 300K between 20#C and 50#C operating temperature were measured for the symmetric structures without deterioration of the internal quantum efficiencies (> 90%) and low intrinsic losses (about 1cm-1). The improvement in the characteristic temperature is mainly attributed to the reduced thermionic emission of the carriers out of the quantum well due to the large effective barrier height of the short-period superlattice. Caused by the incorporation of the short-period superlattice the devices showed a higher series resistance, which could be lowered by switching to asymmetric structures. These asymmetric devices had unchanged high internal quantum efficiencies and low intrinsic losses but lower characteristic temperatures of about 250K.