Chemical Durability of Phosphate Laser Glasses Polished with Pitch, Pads, or MRF

LLE Review, Volume 100 257 Introduction Large, high-power laser systems are currently under construction; such systems include the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL),1 the LMJ laser at CEA in France, and the OMEGA EP at LLE.2 These new lasers will require large amounts of neodymium phosphate laser glass, which is known to be sensitive to water.3 When improperly handled or exposed to too much humidity, phosphate glass surfaces may cloud—a result of increased surface roughness due to chemical reactions. Smooth surfaces are required for such lasers (for example, a 2to 10-Å-rms roughness level is specified for the NIF1); rougher surfaces cause scatter, which can result in intensity modulation in the laser beam, leading to damage to downstream optics and “(increased) fluence on the spatial-filter pinholes.”1 Transmission loss also causes output energy loss, significantly reducing performance. Thus the chemical durability of the laser glass used is of great importance to its fabrication, storage, cleaning, and handling.