Cyclops laser

Cyclops was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) in 1975. It was the second laser constructed in the lab's Laser program, which aimed to study inertial confinement fusion (ICF).

The Cyclops was a single-beam Neodymium glass (Nd:glass) laser. The Janus laser, a two-beam version of it, was also completed in 1975. The main scientific aims of its construction were for the study of nonlinear focusing effects in high power laser beams, novel amplification techniques (disks of Nd:glass at the brewster angle), spatial filtering techniques which would be used on subsequent higher powered lasers such as the Argus and Shiva lasers and for inertial confinement fusion (ICF) research.

Even the earliest ICF laser experiments demonstrated that one of the main problems which needed to be addressed was poor focusing of the beams and damage caused to optics due to the beam's extreme intensities caused by the optical Kerr effect, 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.

This novel problem only became obvious as the lasers were scaled up in power to where nonlinear phenomena occur with very intense beams of light. LLNL's Krupke stated:

If the intensity of the light gets high enough —as in fusion lasers— the electric field in the light perturbs the atoms of the glass so strongly that the glass responds in a nonlinear way.

...
Wikipedia