High spatio-temporal quality, CEP-controlled, sub-10fs front-end light source based on XPW

Summary form only given. We present the current development of an injector for a high-contrast, ultrashort laser system devoted to relativistic laser-plasma interactions at kHz repetition rate [1]. The front-end is based on a 1 kHz, CEP-stabilized Ti:Sa CPA, delivering 1.4 mJ, 28 fs pulses, followed by a cross-polarized wave (XPW) filter for temporal cleaning and shortening. This filter is an optimized version of the waveguided XPW device described in [2]. 300 μJ pulses are routinely generated, corresponding to a XPW internal efficiency as high as 33% and a global energy transmission of 22%. As shown in Fig. 1(a), the XPW filter broadens and shapes the initial amplifier spectrum into a perfectly Gaussian spectrum with 110 nm FWHM, which supports sub-10 fs pulse duration (8.5 fs FTL) This corresponds to a 2.8 temporal pulse shortening factor. The significant spectral broadening and reshaping during XPW arise from the combination of high efficiency and optimized input spectral phase. Accurate characterization highlights the fidelity of the proposed injector. Despite the fact that the spectral broadening is due to nonlinear effects, short-term spectral stability is not degraded after XPW and remains below 2% rms across the entire spectrum (acquisition of 500 consecutive spectra). The long-term spectral stability was monitored by recording the spectrum at regular intervals over 90 min, showing only minor variations. We also measured the excellent spatial quality of the output beam, with a shot-to-shot beam pointing stability of 30 μrad (8 μrad rms). Furthermore, long-term energy stability is monitored continuously and is below 3% rms after the XPW stage. Finally, the measured CEP drift is 170 mrad rms (Fig. 1d). Although slightly higher than the amplifier (typically 110 mrad rms), this value confirms the robustness of the XPW filter. We also study the complex spatio-temporal dynamics of pulse shortening and spectral broadening during the XPW process. This intensity-dependent feature first affects the spectral homogeneity of the XPW beam (Fig. 1b). We highlight the role of self-phase modulation and self-focusing undergone by the fundamental beam and the mechanisms leading to spectral homogeneity of the XPW beam in the far-field (Fig. 1c). The experimental results are supported by 3D calculations. To conclude, our work shows that XPW can be configured to act as a robust pulse cleaning and shortening device, producing sub-10 fs pulses with exceptional spatio-temporal fidelity.