Diodengepumpte Ultrakurzpuls-Strahlquellen zur Erzeugung von Pikosekunden-Impulszügen hoher mittlerer Leistung und Energie durch gütegeschaltete Modenkopplung (English shop)

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Recent investigations into the precision machining of metals have shown that very good results in terms of machining quality can be achieved by using laser systems with pulse durations in the range of 10 ps. Compared with the fs systems used to date, ps lasers have the advantage of delivering considerably higher average output powers. In addition, they can be built to be substantially simpler, more robust and more cost-effective. In order to meet the very high demands placed on the quality of the machined surfaces and edges for certain applications, the energy of the laser pulses should lie only slightly above the ablation threshold of the metals. Owing to the good focusability of the radiation, pulse energies of a few μJ are sufficient for this. As a result, the ablation depth achievable per pulse, and hence the machining speed, is greatly reduced. By increasing the repetition rate of the ultrashort pulses into the range of a few hundred kHz, an economically acceptable process speed can be achieved. A further possibility for significantly increasing the ablation rates is the use of laser systems that emit groups of mode-locked pulses, so-called bursts. Initial investigations into micromachining with bursts additionally show an improvement in machining quality compared with the use of single pulses.

A promising method for generating bursts of ultrashort pulses is the construction of an oscillator with stable Q-switched mode locking (QML). Owing to the amplitude modulation of the ultrashort pulses, such a laser moreover delivers higher pulse energies compared with continuously mode-locked lasers and higher average output powers than ps oscillator–amplifier systems. Until now, QML has been regarded as an undesirable disturbance of continuous mode-locked operation, since the high peak intensities generally led to the destruction of intracavity optical components. The realisation of a laser with defined stable QML requires the exact identification and control of the parameters that enable a targeted transition from continuous to Q-switched mode locking.

The aim of the present work was the development of a picosecond beam source with stable Q-switched mode locking delivering high average power, energy and repetition rate. In order to realise a QML laser, a suitable passive mode-locking technique first had to be selected. To this end, mode locking with a semiconductor saturable absorber and the phase-self-adjusting mode-locking (PSM) technique were investigated. For each of the two techniques, a continuously mode-locked oscillator was set up and characterised as a basis for QML. By adapting the laser parameters, the transition into Q-switched operation was accomplished. The oscillator with PSM could be brought into QML operation by detuning the coupled resonator. The stability range of the detuning was very small, so that small fluctuations of the resonator lengths, due to the interferometric principle of the PSM technique, already led to strong disturbances of QML operation. For the oscillator with a semiconductor saturable absorber, Q-switched mode locking could be achieved by reducing the pulse energy density on the absorber. In the experiment, this was implemented by adapting the transmission of the output coupler, the pump power and the beam diameter on the absorber. Defined QML operation could be realised for a very small parameter range. The average output power of the laser was 17.2 W with very good beam quality (M2<1.1). At 290 kHz, the repetition rate of the Q-switch pulses lay in the range of the relaxation frequency of the laser. The maximum energy of the ultrashort pulses could be increased by about a factor of six to 0.74 μJ compared with the energy in continuous operation. The duration of the mode-locked pulses was determined to be 22 ps. In this first realisation, the long-term stability was still limited by damage to the absorber as a result of high non-saturable losses. The experimental results showed, however, that overall mode locking with semiconductor saturable absorbers is better suited for the construction of a laser with Q-switched mode locking.

On the basis of the experimental experience gathered, a new oscillator with a semiconductor saturable absorber was designed for the realisation of a long-term stable QML laser. To avoid damage to the absorber and thus to increase the long-term stability, the intracavity power was reduced by lowering the pump power. In addition, absorbers with lower non-saturable losses and thus a higher damage threshold were available for the further work. In order to achieve shorter pulse durations, a gain-at-the-end configuration is used. The generation of Q-switched mode locking was achieved here by the use of an additional loss modulator. Starting from continuous mode-locked operation, the Q factor of the resonator is varied by means of an acousto-optic modulator (AOM). Adapting the losses and the modulation frequency of the AOM allowed the realisation of stable QML operation without damage to the absorber. The average output power was 7.2 W with a nearly diffraction-limited beam (M2 <1.1). The value for the maximum pulse energy was 0.5 μJ. This corresponds to an enhancement of 5.5 compared with continuous mode locking. The repetition rate of the Q-switch envelopes could be varied between 180 and 200 kHz by means of the modulation frequency of the AOM. The duration of the ultrashort pulses, with a repetition rate of about 80 MHz, was determined to be 12.8 ps.

The theoretical description of Q-switched mode locking was possible only by means of numerical simulation. For this purpose, the master equation for a laser with a slow saturable absorber was solved using the split-step Fourier method. The results of the simulations agree well with those from the experiment. Furthermore, the numerical model confirms that a significant increase in the enhancement by adapting the properties of the absorber in conjunction with the laser parameters is not possible.

Finally, demonstration experiments on the micromachining of metals were carried out with the realised laser with Q-switched mode locking. The results of the investigations show that the QML laser is suitable for micromachining. For the industrial application of the Q-switched mode-locking concept, further optimisation of the process parameters is necessary.

The work carried out demonstrates that the realisation of a beam source with stable Q-switched mode locking is possible. The maximum achievable increase in pulse energy through Q-switching is, however, limited by disturbance of the mode locking.