To analyze optical damage of germanium (Ge) induced by a continuous wave (CW) laser, numerical and experimental studies were carried out. Temperature and solid-liquid phase transition with laser conditions were estimated by numerical simulation. In our experiments, we examined morphological changes with hillocks, material changes in the GeO2 layer by oxidation, and new crystal domains formed by recrystallization. The material damage process was explained. Transmittance reduction was also observed in the mid-infrared region. We confirmed that hillock formation, oxidation, and recrystallization through resolidification are critical factors in damaging the optical performance of Ge with a CW laser.

We have developed a slab-type Nd:YAG gain module based on the techniques of conduction cooling and end pumping. The Nd:YAG slab is end-capped on both ends by undoped pure YAG and is pumped through the end-caps by stacked arrays of laser diode bars. The slab's surfaces of total internal reflection are in contact on both sides with microchannel cooling blocks which are cooled by water circulation. The power oscillator based on the gain module generates more than 400 W at 1-kW pumping with a slope efficiency of 55%. The small-signal gain of the gain module is 10 in a single zig-zag pass, and the amplified beam shows a near diffraction-limited beam quality.

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To achieve high output energy by generating a large gain mode volume, a Nd:phosphate glass ring regenerative amplifier with a built-in 1:1 telescope was proposed as a part of a terawatt chirped-pulse amplification Nd:glass laser, and its performance was characterized. In this ring regenerative amplifier, an input energy of 80 pJ could be amplified up to 33 mJ, and the output beam size was 2.3 mm in width and 2.0 mm in height. The amplified spectrum showed that high amplification could be obtained without a noticeable gain narrowing from the injected spectrum of 6 Å bandwidth.

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