A first amplification structure uses a single pass external diffusion amplifier wherein the picosecond beam cross-sectional area is matched to the cross-sectional area of the gain medium. A half waveplate between the gain medium and the incoming beam optimizes the polarization of the beam diameter to the polarization of the gain medium. A second amplification structure uses a double pass external diffusion amplifier wherein the beam cross-sectional area is matched to the cross-sectional area of the gain medium and passed twice therethrough. A half waveplate and a rotator create a right circular polarized beam through the gain medium and a maximum "R" coated reflector resides beyond the external diffusion amplifier and reflects a left circular polarized beam back through the gain medium, the rotator and the half waveplate where it becomes horizontally polarized and is then transmitted out of the amplification structure by the polarization sensitive beam splitter.