Fig. 1. (a) Schematic diagram of the cascaded nonlinear compressor through filamentation in air. HWP, half-wave plate; L, lens; C, chirped mirror; F, filamentation; HR, high reflection mirror; PM, power meter. Photos of the generated filamentation in air in the first (b) and second (c) stages.

Fig. 2. Spectra of the input and the output pulses of each nonlinear compression stage in the linear (a) and logarithm (b) scales, respectively. The black, blue and red curves are the pump spectra, the spectra after the first and the second nonlinear compression stages, respectively.

Fig. 3. SHG-FROG measurements of the 160, 65 and 32 fs pulses from the Yb:CALGO regenerative amplifier, after the first and second compression stages, respectively. Input pulse: the measured (a) and retrieved (b) FROG traces. The FROG error is measured as 0.93%. (c) The retrieved spectral intensity and phase, compared to the spectrum, independently measured using a spectral analyzer. (d) The retrieved and TL temporal profiles indicating a measured pulse width of 160 fs. The pulse from the first compression stage: (e) and (f) are the measured and retrieved FROG traces, respectively. The FROG error is measured as 0.94%. (g) The retrieved spectral intensity and phase, compared to the spectrum independently measured using a spectral analyzer. (h) The retrieved and TL temporal profiles indicating a measured pulse width of 65 fs. The pulse from the second compression stage: (i) and (j) are the measured and retrieved FROG traces, respectively. The FROG error is measured as 1.09%. (k) The retrieved FROG spectral intensity and phase, compared to the spectrum, independently measured using a spectral analyzer. (l) The retrieved and TL temporal profiles indicating a measured pulse width of 32 fs.

Fig. 4. (a) The measured spectra at different spatial positions, which are indicated by the five stars across the beam as shown in the inset. (b) Beam quality measurement after nonlinear compression. (c) The measured output power over 1 hour of continuous operation.