High average power femtosecond thin disc Yb:YAG laser

Only recently, it became possible to build power scaleable directly diode pumped high-power solid-state lasers by employing the new thin disc laser concept. This approach allows to use quasi-three level systems like Yb:YAG as laser active media yielding very high efficiency and diffraction limited beam quality at the same time. Additionally, the YB:YAG fluorescence bandwidth supports about 100 fs pulse duration in an ideal modelocking setup. Kerr- lens modelocking with mirror dispersion control seems to be the only compatible technique with respect to power scalability. Hence, 30 W and 60 W diode pumped resonator setups are proposed yielding pulses of ~ 150 fs at ~ 100 MHz repetition rate with average power of 10 and 20 W, pulse energies of 0.1 and 0.2 mu-J, respectively, and peak powers of the order of 1 MW, representing a first power scaling step towards real high-power femtosecond lasers, emitting already at this stage about 30 times more average power than currently employed resonators at such pulse durations. The diffraction limited beam quality will allow focusing to highest intensities of the order of TW/cm. Due to its modular concept, the proposed setup can be extended to a master oscillator regenerative amplifier scheme using the same pump modules, aimed at achieving ~ 0,5 mJ pulse energies of ~ 200 fs duration at ~ 10 kHz repetition rate, approaching the PW/cm regime. Such a diode pumped femtosecond laser design being compact, efficient and in the long perspective of reasonable cost will offer various new application possibilities in a wide range from science to the newly established femtosecond materials processing field, and a number of them will be tested in the second part of the project.

The FWF project "Femtosecond Yb:YAG thin disk laser" was aimed towards the development of a new, innovative type of laser for the generation of femtosecond laser pulses with high average power. At the time of submission of the proposal it was among the first ones recognizing the rising importance of femtosecond laser pulses in materials processing and should yield a new laser for this task. The advantages of this technology are based on the possibility of precision manufacturing of any material, even of outstanding hardness or highest melting point. Furthermore, in contactless treatment being specifically important for biological soft tissue, and finally, in materials processing associated with least thermal or acoustical shock for the remaining material being important in both cases, anorganic as well as organic materials. An example would be the treatment of dental hard tissue ("laser drilling" using femtosecond pulses where least heat impact together with highest possible speed of ablation is desired, not being satisfied with existing dental lasers). The realisation of this project was based on a cooperation with the University of Stuttgart, where Dr. Giesen had successfully developed a continous wave high power laser based on the Yb:YAG thin disk concept. The author was the first in the world who recognized the combination of this laser with the concept of femtosecond pulse generation to be highly useful. Unfortunately, during realization of the various technological steps a lot of delay occurred being out of control of the project management so that progress lagged although useful and recognized improvements in some detail could be established. In case of laser development, there are also alternative methods, e.g. for the modulation to yield femtosecond pulses. In this respect collegues of ETH Zurich took over cooperating with the same Stuttgart group on basis of another method of modulation, the so-called semiconductor saturable absorber mirror (SESAM). Eventually they could succeed in demonstrating the real break through when publishing a 730fs, 16,3 W laser! The international resonance was impressing. As seen from today the Zurich variant represents the better, more versatile setup for an Yb:YAG femtosecond laser having a lot of capacity for successful implementation. This project had to be terminated before reaching the decisive milestones. Nevertheless, noteworthy success can be claimed: 1. The proposed type of laser is found to be the most successful in this new category of high-average power ultra- short pulses. 2. Details of improvement of the resonator have been internationally recognized and have been published in a number of review papers. 3. In many (invited) papers the author could successfully advertise for the advantageous aspects of femtosecond laser technology. This lead even to specific developments in Austrian industry in this direction (e.g. dental lasers). Small and young Austrian laser enterprises have been motivated to develop further in this direction.