Argus laser

Argus was a two-beam high power infrared neodymium doped silica glass laser with a 20 cm (7.9 in) output aperture built at Lawrence Livermore National Laboratory in 1976 for the study of inertial confinement fusion. Argus advanced the study of laser-target interaction and paved the way for the construction of its successor, the 20 beam Shiva laser.

It was known from some of the earlier experiments in ICF that when large laser systems amplified their beams beyond a certain point (typically around the gigawatt level), nonlinear optical effects would begin to appear due to the very intense nature of the light. The most serious effect among these was "Kerr lensing", where, because the beam is so intense, that during its passage through either air or glass the electric field of the light actually alters the index of refraction of the material and causes the beam at the most intense points to "self focus" down to filament like structures of extremely high intensity. When a beam collapses into extremely high intensity filaments like this, it can easily exceed the optical damage threshold of laser glass and other optics, severely damaging them by creating pits, cracks and grey tracks through the glass. These effects became so severe after just the first few amplification stages of early lasers, that it was seen as essentially impossible to exceed the gigawatt level for ICF lasers without destroying the laser itself after just a few shots.

In order to improve the quality of the amplified beams, LLNL had started experimenting with the use of spatial filters in the single-beam Cyclops laser, built the previous year. The basic idea was to extend the laser device into a very long "beamline", over which any imperfections that accumulated in the beam would be successively removed after every amplification stage. A series of tubes with lenses on either end would focus the light down to a point (the focal point) where it would pass through a pinhole which would reject stray unfocused light, smoothing the beam and eliminating the high intensity spots which would have otherwise been further amplified causing damage to down-beam optics. The technique was so successful on Argus it was often referred to as being "the savior of laser ICF".

...
Wikipedia