High power glass laser systems for laser fusion use many BK 7 glasses, and the laser induced damage of this glass is a serious problem. Since the BK 7 glass is generally melted in platinum (Pt) crucibles, the laser damage due to metallic Pt particles occurs inside the glass. The Pt particles cause fractures when irradiated at a fluence higher than 3 J/cm2 with 1064 nm, 15 ns laser pulse. The size of the fracture increases with irradiation fluence, and the damaged glass has to be replaced.

In special cases, BK 7 glass is melted in ceramic crucibles, but there is a problem of cristobalite production inside the BK 7 glass. The laser damage threshold of cristobalite is about 4.5-5.5 J/cm2 with 1064 nm, 15 ns laser pulse.

We have found average damaged particles in BK 7 glasses melted in Pt and ceramic crucibles to be 200-400 and ̃ 10 particles per liter, respectively. At present, laser damage in BK 7 glass is being improved by changing a composition ratio of melting materials.