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A ceramic ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk laser is investigated at 15°C (288 K) and also at 80 K, where it behaves as a four-level laser. We introduce a new two-phase spray cooling method to cool the Yb:YAG. One system relies on R134a refrigerant while the other uses liquid nitrogen (LN2). The use of two systems allows the same disk to be tested at the two temperatures. When the Yb:YAG is cooled from room to cryogenic temperatures, the lasing threshold drops from 155 W to near 10 W, while the slope efficiency increases from 54% to a 63%. A 277 W laser with 520 W of pump is demonstrated. We also model the thermal and structural properties at these two temperatures and estimate the beam quality.

A comparison is made between geometric and diffractive parameters of interest for intermediate spherical aberration. The purpose is to identify the transition region where the validity of one gives way to the other so that a best image plane can always be defined by using one of the criteria. In the process a new criterion, first power peak, was discovered that is applicable over the range of interest.

Experimental and theoretical diffraction intensities are studied for large spherical aberrations ranging from 5X to 50X. In addition, diffractive and geometric rms spot sizes are compared. The experiments and analysis show that for large spherical aberrations there is a core near-axis intensity region that is independent of the aperture radius. This is verified by a stationary phase expansion of the diffraction intearals.

The classical spherical aberration structural coefficient for a plano-convex lens is compared to one derived via Fourier optics. The effects of these differences on diffraction are then studied in the maximum Strehl planes.

Detail in the spot diagrams of Fig. 12 was lost in reproduction of the published paper. The figure is reproduced here in its entirety with improved detail in the spot diagrams. The following discussion further explains the figure.