The optical properties of Yb:YAG ceramic doped with different Yb concentrations are presented. The absorption
coefficient at peak absorption wavelength of 940 nm increases linearly with Yb concentration in Yb:YAG ceramics.
Low-threshold and highly-efficient continuous-wave (cw) laser-diode end-pumped Yb:YAG microchip ceramic laser
with near-diffraction-limited beam quality was demonstrated at room temperature. Slope efficiencies of 79%, 67% and
optical-to-optical efficiency of 60%, 53% at 1030 nm and 1049 nm, respectively were achieved for 1-mm-thick Yb:YAG
ceramic plate (C<sub>Yb</sub> = 9.8 at.%) under cw laser-diode pumping. Dual-wavelength operation at 1030 nm and 1049 nm with
5% transmission of the output coupler was achieved by varying pump power intensity. 1049 nm laser operation was
automatically obtained by using 5% transmission output coupler when absorbed pump power is higher than 1 W. The
lasers operate in multi-longitudinal-mode; the effect of pump power on the laser emission spectra for both wavelengths is
addressed. The laser wavelength around 1030 nm shifts to short wavelength at low pump power region and then to red
with increase of the absorbed pump power, while the laser wavelength around 1049 nm does not change with the pump
power. Excellent laser performance indicates Yb:YAG ceramic laser materials could be potentially used in high-power
solid-state lasers operating at 1030 nm, 1049 nm, or both wavelengths simultaneously. Laser-diode pumped lowthreshold
and highly-efficient passively Q-switched Yb:YAG ceramic microchip laser with Cr<sup>4+</sup>:YAG ceramic as
saturable absorber has also been demonstrated. The slope efficiency is as high as 37%, and the optical-to-optical
efficiency is as high as 29% for 89% initial transmission of Cr<sup>4+</sup>:YAG ceramic. The pulse width of 380 ps and peak
power of over 82 kW at repetition rate of 12.4 kHz was obtained. Single-longitudinal-mode oscillation and wide-separated
multi-longitudinal-mode oscillation due to etalon effect of Cr<sup>4+</sup>:YAG thin plate was achieved depending on the
pump power level.
The antiphase states were observed experimentally in a laser-diode pumped sub-nanosecond microchip Cr,Yb:YAG self-Q-switched multimode laser which is resulted from the spatial hole burning effect. The stable two-mode, and three-mode oscillation are obtained with the increase of the pump power ratio. The modified multimode rate equations including the spatial hole-burning effect in the active medium and the non-linear absorption of the saturable absorber is proposed. The numerical simulations of the antiphase dynamics of such laser are in good agreement with the experimental data and the antiphase dynamics was explained by the evolution of the inversion population and the bleaching and recovery of the inversion population of the saturable absorber.