High energy lasers are important for many applications ranging from micro-machining to surgery to nonlinear optics. In regenerative amplifiers reaching mJ energies, the intracavity non-linearities become important. To minimize those, high stretching factor and increased spot sizes are needed in the crystal, the latter reducing gain at fixed pump power. To keep up the gain and increase the extractable energy, the pump power is increased, leading to higher thermal load in the laser medium. The host CALGO, with a thermal conductivity of 6.3 W/m/K [1], and a lifetime of 420μs, is better qualified for an amplifier crystal than KYW or YAG. The emission spectrum is broadband and lasing was demonstrated between 1018nm and 1052nm [2]. High power with good beam quality, as well as 192fs short pulses and 1.3μJ energy, were obtained from a thin-disk oscillator [3], and 12.5W with sub-100fs pulses have been extracted out of a single-crystal cavity at 80MHz repetition rate [4].