Fig. 1. Schematic diagram of the Ho:YAG SCF amplification system. L1/L2, optical collimation/focusing system; M1, M2, M3, concave dichroic mirrors; CM1, CM2, plane dispersive mirrors; OC, output coupler; BF, birefringent filter; DM, plane dichroic mirror; LD, laser diode.

Fig. 2. Output performance of the Tm:LuScO₃ laser. (a) Tunable mode-locked operation: spectra and average output power (circles and dashed line) and (b) the corresponding autocorrelation traces where τ is the pulse duration (full width at half maximum, FWHM) assuming sech²-temporal shapes. (c) Optical spectrum, (d) autocorrelation trace with the inset showing the recorded near-field spatial beam profile, (e) radio frequency (RF) spectrum of the fundamental beat note measured with a resolution bandwidth (RBW) of 100 kHz and (f) RF spectrum on a 1 GHz span (RBW = 300 kHz) at the selected seed wavelength of 2091.6 nm. SNR, signal-to-noise ratio.

Fig. 3. Performance of the Ho:YAG SCF pre-amplifier. (a) Optical spectrum and (b) autocorrelation trace measured after the pre-amplifier. (c) Beam caustics and near-field intensity profile (inset) with fitted M² propagation factors. (d) Simulated pump light spatial intensity distribution in the Ho:YAG SCF of the pre-amplifier pumped by the 1907 nm LD with Z indicating the distance from the input pump face.

Fig. 4. Input–output characteristics of the power amplifier. (a) Measured average output power, (b) net gain and (c) optical extraction efficiency versus absorbed pump power for different input levels; η_slope, slope efficiency with respect to absorbed pump power. (d) M²-factors of the multi-stage Ho:YAG SCF power amplifier versus absorbed pump power measured at P_in = 0.8 W; the insets show near-field output beam profiles recorded at absorbed pump power levels of 10 W (left) and 90 W (right) with x and y designating the horizontal and vertical directions, respectively.

Fig. 5. Spectral and temporal performance of the Ho:YAG SCF power amplifier. (a) Spectra and (b) autocorrelation traces measured at the output of the power amplifier chain at the maximum pump level for the three different seed average powers; τ is the derived pulse duration (FWHM intensity). The gray spectrum in (a) and the right-hand scale show the stimulated emission cross-section σ_SE of Ho:YAG. Dynamic evolution of (c) the optical spectrum and (d) the autocorrelation trace from the seed laser through the pre- and power amplifier at the maximum seed level.

Fig. 6. Overview of 2-μm ultrafast amplifiers based on Ho-doped bulk gain media and OPCPA^{[15,18,19,34,37,48-58]}. Note that the results of direct amplification of femtosecond pulses in conventional fibers are not described here due to their much lower power level.