Fig. 1. (a) Schematic of the experimental setup: SESAM, semiconductor saturable absorber mirror; OC, output coupler; λ/2, half-wave plate. (b) Output characteristics of the mode-locked Yb:CaF2 oscillator: autocorrelation trace with a corresponding fit and (inset) optical spectrum. (c) Dispersion of the PCF with indicated pump spectrum and (inset) scanning electron microscope cross-section image of the PCF.

Fig. 2. Electric field profiles of the cross sections of TEM00 and TEM01 modes (dashed lines) with the resulting normalized intensity of the total electric field (solid lines) at (a), (c) Δφtot=0 and (b), (d) Δφtot=π. The ratio of the mode amplitudes α is equal to (a), (b) 0.2 in the upper row and (c), (d) 0.7 in the lower row. For animation, see Visualization 1 (α=0.7).

Fig. 3. Averaged beam profiles at different regimes with corresponding pulse trains and spectra (in logarithmic scale) registered at the PCF output. The set (a) corresponds to the pump laser operating at a stable, pure Gaussian regime; in (b) we started the modulation by exciting the TEM01 mode. The modulation effect is more profound in (c), where the contribution of the higher transverse mode is stronger.

Fig. 4. Pulse trains after the PCF at the modulation frequency of (a) 7 MHz and (b) 21.5 MHz.