FEW-CYCLE LASERS

Ultrashort laser pulses have found numerous applications in industry and research over the last years. However, there is still a great demand for keep on improving the pulse parameters to increase the performance of the application, or even to enable the investigation of newly discovered phenomena. Especially the increase of the spectral bandwidth of ultra short pulses to reach pulse durations of only a few femtoseconds is of great interest for many current research topics. Such few-cycle laser pulses enable for example time-resolved studies of ultrafast processes in atoms and molecules. If their pulse energy is high enough, it becomes possible to drive highly-nonlinear processes in gases with few-cycle pulses, which can generate coherent pulses in the extreme ultra-violet region (high-harmonic generation - HHG), opening the door for a whole variety of new applications.

fewcycle1 
 

Chirped-mirror compressor at the output of an OPCPA system.

Our group's goal is to develop schemes that can generate and amplify few-cycle laser pulses and to enhance their performance to achieve record values in pulse duration and average power. One promising technique for the generation of huge spectral bandwidths is the nonlinear pulse propagation in micro-structured fibers, which recently has shown the generation of pulses containing less than two optical cycles.

The amplification of such pulses is very challenging due to the large spectral bandwidth. However, the optical parametric amplification (OPA) is known to provide large amplification bandwidths, depending on the crystal and the geometry. Beta-barium borate (BBO) can amplify nearly a whole optical octave, if the signal and the pump interact in a non-collinear manner. Since the OPA is an instantaneous nonlinear process, no energy is stored in the amplification medium. This leads to the possibility to amplify the signals to extremely high average powers without damaging the crystal and without decreasing the beam quality due to thermal lensing, enabling the usage of high-power state-of-the-art fiber chirped-pulse amplification systems as pump sources for the OPA.

fewcycle2

Spectrum and pulse shape of an amplified few-cycle pulse.

After the amplification process the pulses need to be temporally compressed to their fourier-limit. Since large spectral bandwidths are generated, all the dispersion introduced by the system has to be compensated. Even the dispersion of the air plays a significant role and has to be considered to reach the shortest possible pulses. Compression is usually realized with a chirped-mirror compressor to remove second order dispersion, while a spatial-light modulator is used to compensate all higher-order terms.

Selected publications:

[1] S. Hädrich, S. Demmler, J. Rothhardt, C. Jocher, J. Limpert, and A. Tünnermann, "High-repetition-rate sub-5-fs pulses with 12 GW peak power from fiber-amplifier-pumped optical parametric chirped-pulse amplification," Opt. Lett. 36, 313-315 (2011)

[2] S. Demmler, J. Rothhardt, A. M. Heidt, A. Hartung, E. G. Rohwer, H. Bartelt, J. Limpert, and A. Tünnermann, "Generation of high quality, 1.3 cycle pulses by active phase control of an octave spanning supercontinuum," Opt. Express 19, 20151-20158 (2011)

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