Demonstration of gain saturation and double-pass amplification of a 69.8 nm laser pumped by capillary discharge.

With a 45-cm-long capillary, we obtained a saturated 69.8 nm laser with a gain coefficient of 0.4  cm and a gain length product of 18. In order to increase the laser energy further, a double-pass amplification of the 69.8 nm laser was first realized with a SiC mirror without a coating. With a half cavity, the effective plasma column length and the effective gain length product can reach 84 cm and 33.7, respectively. The amplitude of a laser pulse for double-pass amplification is 9 times larger than that for single-pass amplification. In addition, the full width at half-maximum (FWHM) pulse width of a laser pulse and FWHM divergence for single-pass amplification are 1.4 ns and 0.5 mrad, respectively, which increase to 2.2 ns and 3.4 mrad for double-pass amplification.