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Saclay Laser Interaction Center
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Laser and Nonlinear Optic R&D

Second and Third Harmonic "Achromatic" Generator

Most of the pump-probe experiments made at the LUCA or PLFA facilities use fundamental, second harmonic and third harmonic pulses provided by the TiS lasers. In the previous home-made 2ω and 3ω generators, crystals’ thicknesses were chosen to obtain a compromise between efficiency and temporal broadening due to group velocity mismatch between the pulses. In this configuration, the pulse durations are 100fs typically.
In order to obtain shorter pulses with sufficiently high energy, we designed an “achromatic” Second Harmonic Generator based on a noncolinear geometry and pulse front tilt. The pulses obtained with this configuration were much shorter: the pulse duration was 20 fs at 400nm after recompression. For this experiment, SPIDER measurements at 400nm were made for the first time in the laboratory. The full potential and limits of this system have still to be tested experimentally with very short pulses. This should be done in mid 2008 on the SOFOCKLE facility, using 10 fs pulses generated by post-compression after spectral broadening of infrared pulses in hollow core fibres filled with Argon gas. The third harmonic generator based on the same principles has been designed and is presently under construction.

Repetition Rate (frep) stabilization

Standard synchronisation systems of CPA lasers generally suffer from several limitations which, while acceptable in most cases, make them not compatible with many of our applications. For instance, using standard synchronisation system, it is not possible to synchronize precisely 2 different lasers, as requested for the EUROFEL program or for NOPCPA laser chains. Also, for CEP stabilisation ,a standard synchronisation scheme does not guarantee the amplification of pulses leaving the oscillator with the same CEP (successive pulses leaving the oscillator have a relative CEP shift of λ/2).
To circumvent part of these limitations, we have achieved the stabilization of the repetition rate of a TiS oscillator through the use of an Rb atomic clock, RF and electronic chains and a mirror mounted on a piezo actuator in the oscillator cavity. The jitter, estimated through phase noise measurement, is presently of about 2 ps and should be reduced with the suppression of mechanical resonances in the laser oscillator cavity. This work was done in close collaboration with Daniele Rovera from the Observatoire de Paris (SYRTE).
In parallel, we have worked with Amplitude Technologies in order to modify the commercial synchronization module (Genpulse) used on PLFA. This led to a new synchronisation module, now commercial and routinely used on PLFA, which enables achieving amplification of only pulses leaving the oscillator with the same CEP.

CEP stabilisation and high energy ultra-short pulses generation

A fundamental feature concerning short pulses, especially in the sub-15fs timescale, is the position of the carrier wave compared to the envelope of the pulse. This position is linked to the Carrier Envelope Phase (CEP).
Since the end of 2005, we have been working on a project concerning the CEP stabilisation and the reduction of the pulse duration of high energy pulses (>10mJ) generated by CPA TiS lasers. Unlike most of the commercial systems, the scheme we are proposing is based on a CPA laser using large stretching and compression ratios, thus enabling amplification to higher pulse energy. This work is funded by the ANR agency (Attoscience program) and by the Conseil Général de l’ESSONNE (LIPS program). It is carried out in collaboration with the Amplitude-Technologies company on the PLFA laser.
The first step to stabilize the CEP of PLFA is to efficiently stabilize its oscillator. Rather than developing our own system, we have bought a commercial stabilized oscillator from Femtolaser Gmbh (Synergy system). The CEP stabilisation of this oscillator has been maintained over several hours in our laboratory.
We have then carried out first tests of CPA amplification of pulses from the stabilised oscillator. In order to check if the stabilisation is maintained after the amplification, we have developed an f-2f interferometer which measures the CEP drift of the amplified pulses. Its general design follows the scheme proposed by [Jones et al. SCIENCE 288, 635 (2000)]. In opposition to most of the systems reported in the literature, our f-2f interferometer performs shot-to-shot measurements up to 10 kHz repetition rate.
Figure 4 displays a typical f-2f fringe pattern obtained for 100 successive laser shots. It can be observed that the fringe position significantly evolves from shot to shot, indicating that the CEP stabilisation of the oscillator is not yet conserved after amplification. We will now start a thorough study aiming at identifying and correcting the main drift sources. We will especially concentrate on the regenerative amplifier and the influence of beam pointing in the grating based stretcher-compressor.

table characteristics
Concurrently, we have performed preliminary tests for reducing the pulse duration by post compression based on filamentation in a gas cell. Our objective is to produce 10fs pulses with energies up to 5mJ. The spectra we obtained were 100nm broad (FWHM) corresponding to a Fourier transformed pulse of 8fs. The SPIDER recently developed in our group will now enable us to better characterize the generated pulse and optimise the correction of the spectral phase distortions. This work is also supported by theoretical calculations made in collaboration with T. Auguste from the AttoPhysics group.

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Saclay Laser-matter Interaction Center

Up date: 22-04-2009
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