Diffraction‐Limited High‐Radiance Nd‐Glass Laser System

A Nd‐glass laser system is described which emits a 4 GW diffraction‐limited laser pulse, at a radiance of 2×1017 W/cm2‐sr. The device consists of a spatially mode‐selected Q‐switched oscillator, operating in the lowest‐order HE11 transverse mode, followed by a series of amplifiers that are coupled with appropriate beam‐expanding optics. A beam with a far‐field pattern corresponding to Fraunhofer diffraction, is approached through use of the proper gain distribution in the first amplifier and a 50% over‐expanded beam to allow uniform illumination of a physical aperture placed in front of the second amplifier. This system requires high optical‐quality laser rods capable of transmitting a diffraction‐limited beam. The induced optical distortions of the amplifiers during the pumping are minimized by appropriate Nd‐doping concentration, surface treatment, flashtube arrangement, and filtering of the pump light, yielding uniformity of pumping over almost the full cross section. Some experimental results on long‐...