Ultrafast lasers, which generate optical pulses in the picosecond and femtosecond range, have progressed over the past decade from complicated and specialized laboratory systems to compact, reliable instruments. Semiconductor lasers for optical pumping and fast optical saturable absorbers, based on either semiconductor devices or the optical nonlinear Kerr effect, have dramatically improved these lasers and opened up new frontiers for applications with extremely short temporal resolution (much smaller than 10 fs), extremely high peak optical intensities (greater than 10 TW/cm2) and extremely fast pulse repetition rates (greater than 100 GHz).

Recent developments in compact ultrafast lasers

In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

Considering that the proton is a basic subatomic component of all ordinary matter — as well as being ubiquitous in its solo role as the hydrogen ion H+ — there are some surprising gaps in our knowledge of its structure and behaviour. A collaborative project to determine the root-mean-square charge radius of the proton to better than the 1% accuracy of the current 'best' value suggests that those knowledge gaps may be greater than was thought. The new determination comes from a technically challenging spectroscopic experiment — the measurement of the Lamb shift (the energy difference between a specific pair of energy states) in 'muonic hydrogen', an exotic atom in which the electron is replaced by its heavier twin, the muon. The result is unexpected: a charge radius about 4% smaller than the previous value. The discrepancy remains unexplained. Possible implications are that the value of the most accurately determined fundamental constant, the Rydberg constant, will need to be revised — or that the validity of quantum electrodynamics theory is called into question. Here, a technically challenging spectroscopic experiment is described: the measurement of the muonic Lamb shift. The results lead to a new determination of the charge radius of the proton. The new value is 5.0 standard deviations smaller than the previous world average, a large discrepancy that remains unexplained. Possible implications of the new finding are that the value of the Rydberg constant will need to be revised, or that the validity of quantum electrodynamics theory is called into question. The proton is the primary building block of the visible Universe, but many of its properties—such as its charge radius and its anomalous magnetic moment—are not well understood. The root-mean-square charge radius, rp, has been determined with an accuracy of 2 per cent (at best) by electron–proton scattering experiments1,2. The present most accurate value of rp (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants3. This value is based mainly on precision spectroscopy of atomic hydrogen4,5,6,7 and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of rp as deduced from electron–proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant3). An attractive means to improve the accuracy in the measurement of rp is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift10 (the energy difference between the 2S1/2 and 2P1/2 states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations11,12,13,14,15 of fine and hyperfine splittings and QED terms, we find rp = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value3 of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

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The size of the proton

The principal ideas of the thin-disk laser design will be illustrated and the advantages for operating different laser materials will be explained. The results for continuous-wave (CW) and Q-switched operation as well as for amplification of short (nanosecond) and ultrashort (picosecond, femtosecond) pulses demonstrate the potential of the thin-disk laser design. The scaling laws for this laser design show that the power limit for CW operation is far beyond 40 kW for one single disk and the energy limit is higher than 3 J from one disk in pulsed operation. Also, the applicability of the thin-disk laser concept to optically pumped semiconductor structures will be discussed. When pumping directly into the quantum wells, the energy defect between the pump photon and the laser photon can be smaller than 5%, thus reducing the waste heat generated inside the semiconductor structure. First results demonstrate the potential of this new concept. Finally, a short overview of the industrial realization of the thin-disk laser technology will be given.

Fifteen Years of Work on Thin-Disk Lasers: Results and Scaling Laws

Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry

A new, scalable concept for diode-pumped high-power solid-state lasers is presented The basic idea of our approach is a very thin laser crystal disc with one face mounted on a heat sink This allows very high pump power densities without high temperature rises within the crystal Together with a flat-top pump-beam profile this geometry leads to an almost homogeneous and one-dimensional heat flux perpendicular to the surface This design dramatically reduces thermal distortions compared to conventional cooling schemes and is particularly suited for quasi-three-level systems which need high pump power densities Starting from the results obtained with a Ti:Sapphire-pumped Yb:YAG laser at various temperatures, the design was proved by operating a diode-pumped Yb:YAG laser with an output power of 44 W and a maximum slope efficiency of 68% From these first results we predict an exctracted cw power of 100 W at 300 K (140 W at 200 K) with high beam quality from a single longitudinally pumped Yb: YAG crystal with an active volume of 2 mm3 Compact diode-pumped solid-state lasers in the kilowatt range seem to be possible by increasing the pump-beam diameter and/or by using several crystal discs

Scalable concept for diode-pumped high-power solid-state lasers

Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Ultrashort pulses appear very promising for material removal with ultra high precision. Initial investigations showed, however, several unexpected quality problems such as formation of recast, ripples and irregular hole shapes even in the femtosecond pulse regime. After describing the problems, this contribution will present some progress in fundamental understanding of the ultrafast ablation process. On the basis of this knowledge technical means have been developed allowing to achieve an unprecedented level of accuracy at acceptable expenses. The latter being strongly influenced by the shortness of the laser pulse, a comparison between pico- and femto-second regime will be presented.

Helmut Hügel

Papers

Scalable concept for diode-pumped high-power solid-state lasers

Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Micromachining with ultrashort laser pulses: from basic understanding to technical applications

Laser in der Fertigung

Understanding Pore Formation in Laser Beam Welding