Verstärkung ultrakurzer Pulse mit Colquiriiten

Femtosecond laser systems find their application in science since the first generation of laser pulses with durations below 1ps and more and more also in industrial applications. For this development the change from laser active dyes to solid state media has been crucial. Titan saphir (TiSa) is the mostly used solid state medium in commercial femtosecond laser systems. These TiSa laser systems exhibit the highest pulse energies for pulse durations below 100 fs so far but still need significant operating expense which limits their applicability in a broad range of possible uses. Main reason for complexity and costs of these systems is the pump laser source which has to be in the green spectral range due to the specific absorption spectrum of TiSa. In this spectral range no laser diodes are available and therefore frequency converted solid state lasers have to be used as pump sources. Starting point for the development of the laser system described in this work was the absence of directly diodepumped laser systems with pulse durations below 100 fs and peak powers above 1 GW. While these properties can in principle also be achieved by other solid state laser media which can be excited with laser diodes the herein used colquiriite crystals have a unique property in terms of their amplification bandwidth. In this work a directly diode pumped regenerative amplifier on the basis of colquiriites is presented. The amplification properties have been investigated theoretically and experimentally; their occured no significant differences in this analysis. A solution of the rate equations under consideration of energy transfer upconversion from literature has been augmented by a correcting factor to reduce the numerical error of the known solution by more than 10%. For the first time a closed solution for the maximum extractable energy in a regenerative amplifier has been deduced. To excite the colquiriit crystals efficiently a pumping source in the red spectral range (685 nm) has been set up. The achieved total pump power exceeded 20 W and the radiation had symmetrical beam qualities of M2 almost equal to 30 in four beam lines which is the highest pump power for colquiriites on the basis of laser diodes demonstrated so far. For the first time a stretcher-free CPA with manipulation of the spectral phase by chirped mirrors during the intracavity amplification has been applied. The spectral amplitude has been manipulated during intracavity amplification by specially designed mirrors to succesfully reduce the effect of gain narrowing. Nonlinear effects during amplification have not been observed. However the theoretical calculations of intracavity pulse propagation suggest that the use of the demonstrated stretcher compressor concept can not be done at much higher pulse energies. Upconversion processes were not limiting in the deposable pulse energy. Thermal Quenching limited the operation to repetition rates of 2,5 kHz. The homogenity of the laser crystals did not allow the use of thinner crystals to achieve higher average powers. The achieved pulse duration of 75 fs is a factor of two shorter than the previous demonstrated values of diode pumped colquiriite systems; the usable pulse energies (160 µJ) are more than an order of magnitude larger compared to the current state of technology. The developed femtosecond laser system has been succesfully applied to microstructuring of biological tissue and direct generation of ultrashort pulsed x-ray radiation