Working principles of the LWS-20 relativistic sub-two-cycle system. (a) Schematic of the light source. Pulses coming from a common broadband Ti:Sa oscillator are divided into a seed and a pump arm. The seed is generated by first amplifying the oscillator pulses in a 9-pass CPA system and then broadening them in a neon-filled HCF. After broadening, an XPW setup is optionally used for temporal contrast enhancement. The seed is then stretched in a grism stretcher and afterwards sent into the OPS. The pump for the four NOPA stages is generated in a Nd:YAG laser amplifier, providing a low-energy and a high-energy arm at both the second harmonic (532 nm) and the third harmonic (355 nm). After the OPS, the pulses are compressed and sent to the experimental chambers. A single-shot CEP meter is optionally used for electric-field-sensitive experiments. (b) Schematic of the OPS, depicting the temporal evolution of the pulses at the different amplification stages. The AOM first shapes the spectral amplitude and phase of the seed pulses which are then fed into the four NOPA stages. The first two NOPA stages are pumped by the two low-energy pump beams at 532 nm and 355 nm, respectively, and the latter two are pumped by the two high-energy pump beams. (c) Spectral intensity evolution in the synthesizer. Optical spectrum of seed (blue line), and after the first (orange dashed line), second (yellow dotted line), third (purple dash-dotted line), and fourth (green thick line) amplification stage. Consecutive energy enhancements in the visible and near-infrared parts of the seed provide a broadband spectrum beyond what is achievable in a single stage. (d) Spectral intensity after amplification, but before compression, for different configuration of the NOPA stages depicting the tuning capabilities of the OPS. The corresponding Fourier-limited pulse durations are indicated in the legend.