Ralf Knappe

High-efficiency 60 W TEM 00 Nd:YVO 4 oscillator pumped at 888 nm

We propose a technique for pumping Nd:YVO(4) with high optical power at 888nm while making absorption independent of the pump light polarization state. This is especially suitable for systems end pumped by high-power, high-brightness fiber-coupled diode sources associated with long vanadate crystals to effectively spread the heat load in a large volume. A compact 60 W output, 55% optical efficiency cw TEM(00) oscillator was demonstrated.

47 W, 6 ns constant pulse duration, high-repetition-rate cavity-dumped Q-switched TEM(00) Nd:YVO(4) oscillator.

We report on a cavity-dumped Q-switched TEM(00) Nd:YVO(4) oscillator offering a unique combination of high power, constant short pulse duration, and high repetition rate, suppressing the gain dependence of pulse duration in classical Q-switched oscillators. Its performance is compared with that of the same oscillator operated in a classical Q-switched regime, demonstrating the much higher peak powers achievable with this technique, especially at high repetition rates. Up to 31 W of 532 nm green light was generated by frequency doubling in a noncritical phase matched LBO crystal, corresponding to 70% conversion efficiency.

Scaling ablation rates for picosecond lasers using burst micromachining

High-precision micromachining with picosecond lasers became an established process. Power scaling led to industrial lasers, generating average power levels well above 50 W for applications like structuring turbine blades, micro moulds, and solar cells. In this paper we report, how a smart distribution of energy into groups of pulses can significantly improve ablation rates for some materials, also providing a better surface quality. Machining micro moulds in stainless steel, a net ablation rate of ~1 mm3/min is routinely achieved, e.g. using pulse energy of 200 μJ at a repetition rate of 200 kHz. This is industrial standard, and demonstrates an improvement by two orders of magnitude over the recent years. When the energy was distributed to a burst of 10 pulses (25 μJ), repeated with 200 kHz, the ablation rate of stainless steel was 5 times higher with the same 50 W average power. Bursts of 10 pulses repeated with 1 MHz (5 μJ) even resulted in an ablation rate as high as 12 mm3/min. In addition, optimized pulse delays achieved a reduction of the surface roughness by one order of magnitude, providing Ra values as low as 200 nm. Similar results were performed machining silicon, scaling the ablation rate from 1.2 mm3/min (1 pulse, 250 μJ, 200 kHz) to 15 mm3/min (6 pulses, 8 μJ, 1 MHz). Burst machining of ceramics, copper and glass did not change ablation rates, only improved surface quality. For glass machining, we achieved record-high ablation rates of >50 mm3/min, using a new state-of-the-art laser which could generate >70 W of average power and repetition rates as high as 2 MHz.

Fast micromachining using picosecond lasers

Laser micromachining is mostly based, so far, on Q-switched laser sources. Their nanosecond pulse width often limits the accuracy and quality of laser processes by thermally initiated effects. Precision micromachining benefits from ultra short laser pulses. Up to now mostly amplified fs lasers with low repetition rates were used, with the result of low processing speed. New diode pumped solid-state picosecond lasers can also meet the demands of precise micro-machining. Their pulse duration of about 10 picoseconds provide the optimum performance e.g. for metal processing. These lasers also provide high average powers and much higher repetition rates of more then 100 kHz to maximize throughput. New potentials of picosecond lasers for the processing of different materials with high precision and increased speed will be discussed.

Single-mode continuous-wave Cr 3+ :LiSAF ring laser pumped by an injection-locked 670-nm broad-area diode laser

We report on an efficient continuous-wave Cr(3+):LiSrAlF(6) ring laser pumped by the near-diffraction-limited output of an injection-locked 665-nm broad-area diode laser. With an absorbed pump power of 120 mW the ring laser generates 22 mW of single-mode 855-nm radiation with a spectral linewidth of less than 10 MHz. Unidirectional operation and longitudinal-mode control of this ring laser is achieved by injection of 855-nm diode-laser radiation into the ring cavity. Optical feedback of ring-laser radiation into the diode laser narrows the diode lasers linewidth and locks its frequency to a resonance of the ring resonator.

Generation of tailored picosecond-pulse-trains for micro-machining

Novel solid-state picosecond lasers provide a strong benefit for high precision micro-machining. Pulse repetition rates as high as > 500 kHz with pulse energies of > 4 μJ enable fast machining with the precision of low fluence ablation. In addition, new potentials for these lasers are given by advanced modulators with digital timing control that allow the user to generate sequences or groups of pulses: E.g. a sequence of two pulses can be generated and repeated up to 300 kHz. The amplitude of these two pulses can be adjusted independently and the delay is selectable in 20 ns steps. This kind of pulse-strategies with picosecond lasers can support higher ablation rates, similar to the machining results that were demonstrated with double ns-pulses, recently. In another application, groups of > 20 pulses were repeated with > 50 kHz for ultra-precise machining. The distribution of the energy yields a few hundred nJ per pulse and results in an ablation depth per pulse in the range of several nm. Therefore the ablation depth formed by a group can be digitally controlled by the number of pulses in group. Samples for high quality drilling, cutting and structuring of several materials will be presented and the new potentials of this kind of picosecond laser processing with improved precision and speed will be discussed.

Applications of picosecond lasers and pulse-bursts in precision manufacturing

Just as CW and quasi-CW lasers have revolutionized the materials processing world, picosecond lasers are poised to change the world of micromachining, where lasers outperform mechanical tools due to their flexibility, reliability, reproducibility, ease of programming, and lack of mechanical force or contamination to the part. Picosecond lasers are established as powerful tools for micromachining. Industrial processes like micro drilling, surface structuring and thin film ablation benefit from a process, which provides highest precision and minimal thermal impact for all materials. Applications such as microelectronics, semiconductor, and photovoltaic industries use picosecond lasers for maximum quality, flexibility, and cost efficiency. The range of parts, manufactured with ps lasers spans from microscopic diamond tools over large printing cylinders with square feet of structured surface. Cutting glass for display and PV is a large application, as well. With a smart distribution of energy into groups of ps-pulses at ns-scale separation (known as burst mode) ablation rates can be increased by one order of magnitude or more for some materials, also providing a better surface quality under certain conditions. The paper reports on the latest results of the laser technology, scaling of ablation rates, and various applications in ps-laser micromachining.

888 nm pumping of Nd:YVO4 for high-power high-efficiency TEM00 lasers

Nd:YVO4 is a widely used gain medium in commercial lasers providing up to several tens of watts in a diffraction limited beam. Its high gain favors high repetition rates and short pulses in nanosecond Q-switched and picosecond mode-locked regimes. However, output power is limited by strong thermo-optical effects leading to an aberrated thermal lens and ultimately the crystals fracture. In this contribution, we present the optimized pumping of vanadate at 888 nm, benefiting from polarization-independent absorption, reduced quantum defect and very low absorption coefficients compared to the common pump wavelengths of 808 and 880 nm. After a presentation of the principle and the key characteristics of a high-power fiber-coupled end-pumped multimode oscillator, a series of systems based on this pumping technique are presented. A compact 60W high-efficiency TEM00 CW oscillator first proves the potential for high-power high-beam-quality systems. A CW intracavity-doubled system provided 62 W of power at 532 nm. A cavity-dumped Q-switched oscillator providing up to 47 W of average power with 6 ns long pulses at all repetition rates was investigated. Passive mode-locking of an oscillator providing 56 W of output power was achieved with a saturable absorber mirror. Finally, a high-power oscillator was amplified with high efficiency in a power amplifier based on the same pump/crystal configuration. The wide range of systems demonstrated illustrates the simplicity and flexibility of 888 nm pumping for extending the benefits of vanadate in the higher power range.

Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed feedback (DFB) diode lasers operated with a plane external cavity (XC) mirror positioned as close as possible to the diode-laser front facet. These lasers generate single-frequency near IR radiation at wavelengths of 1392, 1580, 1602, and 1653 nm. A piezoelectric variation of the XC length provided continuous single-frequency tuning to as high as 19 GHz. A further benefit of XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscillations to less than 60 dB. The lasers total intensity noise is close to the shot noise limit. The laser linewidth (measured in a beat note experiment) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2nu(3) overtone spectra of methane by single-scan single-pass absorption or frequency-modulation spectroscopy.

High-average power Nd:YVO4 regenerative amplifier seeded by a gain switched diode laser

We report on a Nd:YVO4 regenerative amplifier (RA), end pumped by 888 nm-diode lasers. The output power was about 46W at repetition rates from 150 to 833kHz with an M2-factor of 1.2. The amplifier was seeded by a gain switched diode laser, generating pulses with a duration of 65 ps and a pulse energy of ≈ 5 pJ. The high gain of the RA of more than 70 dB provides amplified pulse energies as high as 180μJ. Bifurcations of the pulse energy could be avoided. Pulse amplitude fluctuations of only 1.2% for 10,000 consecutive pulses were measured. The long term output power stability of the laboratory setup was 0.3%.