Fig. 1. Schematic of the experimental stretcher setup with three configurations. First from the top, the laser pulse bypasses the stretcher using the bypass mirror; second from the top, the beam enters the stretcher, is incident on an optical grating (G) () and a spherical mirror (), before being reflected from a folding mirror; third from the top, the pulse enters the stretcher and transverses through an unfolded design with two gratings and two spherical mirrors. In all three configurations the beam path ends in the third-order cross-correlator (Sequoia, Amplitude) with which the temporal profile is measured. The very bottom depicts a side view of the general setup.

Fig. 2. Measured temporal profile of a laser pulse that was amplified in a uOPA stage (bypass), after transversing through a folded stretcher (a) or an unfolded stretcher (b). The measurements were executed for a smaller beam size, indicated as blue in the plot (FWHM = 1.1 mm) and a larger beam size, indicated as red in the plot (FWHM = 5.8 mm). Shaded areas indicate uncertainties of the alignment procedure. The black lines indicate the slope of the rising edge, with the steeper line corresponding to the smaller beam.

Fig. 3. PSD of the spherical mirror and the flat folding mirror used in the stretcher setup, as commonly expressed in variance over frequency interval. Since the variance of the height distribution is approximately of the order of nm and the relevant spatial frequency interval of the order of mm⁻¹, we chose nm² mm.