Fig. 1. Configuration of the ELI-NP high-power laser system for 1 PW delay measurements. Two pulses split from the same seed pulse passed through the active amplifiers highlighted in red (A1.1 and A1.2, and A2), which are the ones used to produce the 1 PW output. An optical delay line provided a coarse compensation of the path length difference between the two arms. The measurements were performed after the output of the 1 PW optical compressors.

Fig. 2. Principle of the temporal delay measurement. A temporally compressed pulse (1) and a temporally stretched chirped pulse (2) are focused by lenses (L1 and L2) on a transparent target. The short pulse generates a plasma mirror on the target, which reflects part of the chirped pulse. The spectrum of the transmitted part of the chirped pulse (3) is collected by an optical fiber and measured by a spectrometer. This spectrum has a cutoff that encodes the arrival time of the short pulse.

Fig. 3. Key components of the femtosecond delay measurement setup. 1, pump pulse (8 mJ, D = 50 mm); 2, stretched probe pulse (25 μJ, D = 20 mm); 3, probe after spectral cut; DL, motorized delay stage; PBS, polarizing beamsplitter cube; QWP, quarter-wave plate, L1, pump focusing lens (f = 1 m, D = 76.2 mm); L2, probe focusing lens (f = 0.6 m, D = 25.4 mm).

Fig. 4. Time delay between the 1 PW outputs at 1 Hz repetition rate. (a) Free-running operation. (b) Operation with closed feedback loop. (c) Response of the feedback loop after a large jump in the delay.