Investigations on laser mirrors of differing coating structure

Laser mirror reflectivity and integral backscattering were investigated. Mirrors were industrially fabricated, or were developed at DFVLR and have ZnS/MgF2 or Ag/ZnS/MgF2 interference coatings. The experimental mirror-coating machine was modified to get reproducible mirrors. The substrate holder was reconstructed so that privileged directions in the layer combination are suppressed. The measurement of the transmissibility during the coating process was carried out at an angle of incidence of 30 deg arc with a built in deflection device. An IR heating device mounted into the machine enabled temperature of the substrates and the substrate holder to rise to 130C. The effects limiting the mirror parameters, such as substrate temperature, impurities on the substrate surfaces and evaporation speed, are discussed. The measurements show that substrate heating of ZnS/MF2 coatings provide hard and stable layers; the use of Ag/ZnS/MgF2 layer combinations increases the reflectivity by 0.1%.