Scalable 30 fs laser source with 530 W average power.

We present a power-scalable laser source with 30 fs pulse duration, 530 W average power at 500 kHz repetition rate, and beam quality M2<1.2. The compact and efficient setup consists of ytterbium-based Innoslab amplifiers and subsequent nonlinear pulse compression with an argon-filled Herriott cell.

In situ investigation of production processes in a large chamber scanning electron microscope.

A large-chamber scanning electron microscope (LC-SEM) provides an ideal platform for the installation of large-scale in situ experiments. Our LC-SEM has internal chamber dimensions of 1,2 × 1,3 × 1,4 m3 (W × H × D) (Fig.1) and makes it possible to incorporate novel in situ experimental devices, which are reported on here. The present manuscript describes in detail the development of in situ test equipment for the study of a broad range of processes in production engineering. Direct observation of the materials modification mechanisms provides fundamental insight into the underlying process characteristics. An in situ turning device was developed, tested and used to observe the chip formation on the microstructure scale of a 43CrMo4-sample. Laser beam micro welding was integrated into the LC-SEM to achieve in situ analysis of the welding process on stainless steel 1.4310. A heating module was employed for in situ wetting experiments to observe the formation and solidification of the melt of a tin-copper brazing filler on an aluminium cast alloy.

Diode-pumped alexandrite ring laser in single-longitudinal mode operation for atmospheric lidar measurements.

We present, to the best of our knowledge, design and performance data of the first diode-pumped Alexandrite ring laser in Q-switched single-longitudinal mode (SLM) operation. The laser resonator contains two Alexandrite crystals, which are pumped longitudinally by means of two laser diode-bar modules emitting at 636 nm. Single-longitudinal mode operation is achieved by seeding the laser with a diode laser operating in SLM and actively stabilizing the cavity, yielding a linewidth of < 10 MHz at the potassium resonance line at 770 nm. The pulse energy is 1 mJ at a repetition rate of 150 Hz and 0.65 mJ at 320 Hz. The beam quality of M2 < 1.2 in both directions remains unchanged for the different repetition rates. After characterization in the laboratory, the laser was implemented in a novel mobile lidar system and first atmospheric measurements were conducted successfully.

A New Laser-Processing Strategy for Improving Enamel Erosion Resistance.

In the present study, a new automatic laser-processing strategy allowing standardized irradiation of natural tooth areas was investigated. The objective was to find a combination of laser parameters that could cause over a 600°C temperature increase at the enamel surface while not damaging enamel, avoiding temperature change above 5.5°C in the pulp and increasing enamel erosion resistance. Seventy-seven bovine enamel samples were randomly divided into 6 laser groups and 1 negative control (C/no treatment/ n = 11). A scanning strategy (7 × 3 mm) was used for the CO2 laser treatment (λ = 10.6 µm, 0.1-18 J/cm2) with different pulse durations-namely, 20 µs (G20), 30 µs (G30), 55 µs (G55), and 490 µs (G490), as well as 2 modified pulse distances (G33d, G40d). Measurements of temperature change were performed at the surface (thermal camera/50 Hz), at the underside (thermocouples), and at the pulp chamber using a thermobath and human molars ( n = 10). In addition, histology and X-ray diffraction (XRD/ n = 10) were performed. Erosion was tested using an erosive cycling over 6 d, including immersion in citric acid (2 min/0.05 M/pH = 2.3) 6 times daily. Surface loss was measured using a profilometer and statistical analysis with a 2-way repeated-measures analysis of variance (α = 0.05). Only G20 fulfilled the temperature requirements at the surface (619 ± 21.8°C), at the underside (5.3 ± 1.4°C), and at the pulp (2.0 ± 1.0°C), and it caused no mineral phase change and significant reduction of enamel surface loss (-13.2 ± 4.0 µm) compared to C (-37.0 ± 10.1 µm, P < 0.05). A laser-scanning strategy (20 µs/2 kHz/1.25 J/cm2, 3.4 mm/s) has been established that fulfilled the criteria for biological safety and significantly increased enamel erosion resistance (64%) in vitro.

Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier.

We demonstrate a compact diode-pumped Yb:KGW femtosecond oscillator-Yb:YAG Innoslab amplifier master oscillator power amplifier (MOPA) with nearly transform-limited 636 fs pulses at 620 W average output power, 20 MHz repetition rate, and beam quality of M(x)(2) = 1.43 and M(y)(2) = 1.35. By cascading two amplifiers, we attain an average output power of 1.1 kW, a peak power of 80 MW, and a 615 fs pulse width in a single linearly polarized beam. The power-scalable MOPA is operated at room temperature, and no chirped-pulse amplification technique is used.

400W Yb:YAG Innoslab fs-Amplifier.

The Innoslab design, already established for neodymium doped laser crystals, was applied to ytterbium doped laser materials. Recent progresses in brightness of high power diode lasers facilitate efficient pumping of quasi-three-level laser materials. Innoslab amplifiers are compared to competing thin-disk and fiber fs-amplifiers. A compact diode-pumped Yb:YAG Innoslab fs-oscillator-amplifier system, scalable to the kilowatt range, was realized. Numerical simulations result in conditions for high efficiency and beam quality. Nearly transform and diffraction limited 680 fs pulses at 400 W average output power and 76 MHz repetition rate without using CPA technology have been achieved at room temperature so far.

White-light frequency comb generation with a diode-pumped Cr:LiSAF laser.

We have created a broad spectrum spanning more than an optical octave by launching femtosecond pulses from a battery operated Cr:LiSAF laser into a photonic crystal fiber. Despite the massive broadening in the fiber, the comb structure of the spectrum is preserved, and this frequency comb is perfectly suited for applications in optical frequency metrology.

Partially end-pumped Nd:YAG slab laser with a hybrid resonator.

A Nd:YAG slab is partially end pumped by a diode laser stack with three diode laser bars. The pumped volume has a rectangular cross section. A hybrid resonator, which is stable in the plane of small dimension and is off-axis unstable in the plane of large dimension of the gain cross section, was used to yield highly efficient laser operation at diffraction-limited beam quality. The laser design and experimental results are reported.

Neodymium:YAG 30-W cw Laser Side Pumped by Three Diode Laser Bars.

A theoretical calculation of pump power deposition in a direct water-cooled Nd:YAG laser rod, side pumped by three diode laser bars is presented. The pumping cavity design provides a nearly uniform pump profile. More than 30-W cw output power with optical-to-optical efficiencies of more than 30% are obtained.

Highly efficient diode-stack, end-pumped Nd:YAG slab laser with symmetrized beam quality.

An efficient high-power cw Nd:YAG slab laser, partially end pumped by diode-laser stacks, and a novel beam-shaping technique are reported. The optical efficiency amounted to 44 %, and the slope efficiency amounted to 55 %. Introducing an intracavity Brewster plate to polarize the laser beam, we obtained an optical efficiency of 35 % and a slope efficiency of 41 %. The output beam was rectangular and the beam quality asymmetric in two orthogonal directions. To equalize the beam quality, we introduced a step-mirror beam-shaping technique. The beam-shaping technique and the results obtained are discussed.

Dynamics and stability of a laser system with second-order nonlinearity.

We use a multimode rate-equation model to investigate the stability of a laser system with second-order nonlinearity and compare the results with the literature on intracavity frequency-doubled lasers. The emphasis is on the Hopf bifurcation between the asymptotically stable state and the stable oscillation that occurs with variation of the conversion efficiency from the fundamental frequencies to their sum. This bifurcation results in two simple expressions that describe the stability curve, depending on material and cavity parameters. We also report on an experimental confirmation of the predicted temporal behavior of sum-frequency generation.