Fig. 1. Schematic of the 4 μm OPCPA system and the HCF-based CBC system. KTA, KTiOAsO₄ crystal; DL, delay line; HWP, half-wave plate; TFP, thin film polarizer; DM, dichroic mirror; T, telescope; W, CaF₂ window; IR, image relay; BS, beam splitter; RM, roof mirror; G, grating; KEPM, knife edge prism mirror; L, lens; Kr, krypton.

Fig. 2. (a) Spectra measured by MOZZA and (b) corresponding temporal profiles measured by SHG-FROG at different krypton pressures. (c) Spectral evolution throughout increasing gas pressures output from the HCF measured by SHG-FROG. (d) The measured compressed pulse durations with 2 mm CaF₂ (blue curve), corresponding FTL duration (green curve), and estimated FTL duration (red curve) for different krypton pressures. Dashed line represents 26 fs, corresponding to two optical cycles for 4 μm laser pulse.

Fig. 3. Temporal and spectral characterizations of the compressed pulse. (a) Measured and (b) reconstructed SHG-FROG traces. (c) Blue curve, pulse temporal profile of 22.9 fs duration (FWHM); green curve, retrieved phase. (d) Blue curve, reconstructed spectrum of SHG-FROG; red curve, directly measured spectrum by MOZZA; green curve, retrieved phase.

Fig. 4. (a) Beam profiles before the lens L7. Beam profiles at the focal plane of L7 with the relative phase of the individual pulses (b) Δϕ=π and (c) Δϕ=0. (d) Beam profiles at the HCF output window. The beam profile is measured by a pyroelectric thermal camera (PyroCAM IV) with a resolution of 80 μm. (e) Michelson-type interferometer for the measurement of temporal coherence of the compressed pulse and (f) the measured interference fringes. (g) Michelson-type interferometer for the measurement of spatial coherence of the compressed pulse and (h) the measured interference fringes. D, D-shape mirror; DL, delay line; BS, beam splitter.