Carrier- Envelope Phase stabilization of a grating based chirped pulse amplified laser at 17 mJ, 1

reflection grating with multi-stages amplifier [3-7] (pulse energy higher than 10 mJ) are scalable to higher energies but more difficult to stabilize. Our CEP stable laser system has been developed by Amplitude Technologies in collaboration with the SLIC Group in CEA Saclay, within our joint laboratory "Impulse". The laser pulses from the CEP stabilized oscillator are stretched by an Offner type Stretcher. The stretching factor is about 6 ps/nm leading to a pulse duration around 360 ps. The laser pulses are amplified at a 1 kHz repetition rate by 3 amplifiers: a regenerative amplifier, a pre-amplifier and a cryogenically cooled amplifier producing respectively 0.5, 3 and 17 mJ pulses. Pulse compression is achieved with a standard two-grating compressor in a double pass configuration. The 70% compressor transmission allows us to obtain, at the output of the system, pulses of 12 mJ and 40 fs pulse duration. We measured the CEP variation at 3 mJ by bypassing the last amplifier and at full power (17 mJ) when stabilizing the oscillator and applying a slow control loop. This feedback can be applied either on the oscillator or through a new CEP control technique using the linear Electro-Optic (EO) effect [7]. Measurements are performed using a fast multi-kHz f-2f collinear interferometer developed at CEA-Saclay. CEP control by EO device is illustrated on Figure 1a. Figure 2b shows the shot to shot and 10 ms averaged measurements for 3 mJ energy/pulse over a period of 20 minutes. The distribution is Gaussian with a standard deviation of 270 mrad (shot to shot measurements) and 110 mrad if calculated from averaged measurements (over 10 pulses). At 17 mJ, using the cryo­ cooled amplifier, CEP exhibits higher shot to shot noise close to 420 mrad. The noise increase is mainly due to vibrations induced by the cooling system which was verified by a CEP noise measurement in the 300 mrad range when stopping the cryo-cooler for a short period of time. Thus we report to our knowledge, one of the best CEP stabilization value in the 20W range using a new original method to control the CEP of CPA laser systems .