Calculation on 3-Dimensional Propagation of Amplified Spontaneous Emission in a Cassegrain Type 3-Pass Amplifier

The multi-pass laser amplifier has been often used as the final power amplification stage in the high peakpower laser systems. The Amplified Spontaneous Emission (ASE) at such a strongly pumped laser generally results in a parasitic oscillation that limits a high gain operation of the amplifier. The three dimensional propagation of the finite-band-width ASE from laser glasses has been calculated for a 30-m long Cassegranian 3-pass disk amplifiers of 35-cm clear aperture. The computational code includes the temporal behavier of flashlamp pumping, the gain reduction in a laser disk due to ASE generation, the long-path back-and-force propagation of ASE between Cassegranian mirrors, and the reduction effect on ASE of spatial filter installed after the amplifier. The ASE energy and its profile have been compared for the on-axis and off-axis arrangement of Cassegranian optics, and also z-pass geometries for three pass configuration. The off-axis alignment of the mirror pairs is very preferable to reduce ASE to one hundredth level than that appeared in the on-axis geometry.