repetition rate and a pulse width of 10 ns. The input pulse propagates through a poLarizer P,, a Faraday rotator and a polarizer P,. The pulse then passes through the slab amplifier eight times and is extracted by using polarization rotation. The Nd: YAG slab (4 mm X 8 mm X 35 mm) is pumped from both sides by four quasiCW (200 k s ) 300W LD arrays at 50 Hz repetition rate. The LD arrays were fabricated by Hamamatsu Photonics K.K. The slab is cut for normal incidence to eliminate polarization dependent loss and the input faces (four in total) are AR coated at 1064 nm. The thermal birefringence in the slab is compensated by means of a 90 degree quartz rotator. Figure 2 shows energy extraction efficiency of the amplifier plotted as a function of the initial small signal gain. From Fig. 2, it is seen that the extraction efficiency increased after thermal birefringence compensation especially at higher small signal gain values. A maximum extraction efficiency of up to 58% was obtained at a small signal gain of 2.01. The stored energy was estimated to be 19 mJ while the output energy of the 8-pass amplifier was 13 mJ at the maximum energy of the LD arrays. The successful operation of this system demonstrates that it is applicable and scalable to the design of higher power laser systems with high efficiency and high beam quality. *Central Research Laboratory, Hamamatsu Photonics K.K. 5000, Hirakuchi, Hamakita, Shizuoka 434, JAPAN