Study on the feasibility of double stack high reflector coating at 355 nm

The feasibility of the idea of double stack HR coating was discussed in this paper both in theory and experiment. The theoretical simulation was made by employing optical coating design software. The analysis results showed that the design of double stack HR coating was feasible, which made the HR coating have ascendancy not only at reflectance but also at laser damage resistance. Then, the LaF₃/MgF₂ HR coating, the HfO₂/SiO₂ HR coating and the double stack HR coating were prepared for comparison, respectively. Transmittance spectra, surface morphologies and damage morphologies of these coatings were measured. Measurement of laser-induced damage threshold (LIDT) of S polarized light of the samples was performed at 355 nm, 45° incidence. The measurement results showed that the LIDT value of the LaF₃/MgF₂ HR coating with 30 layers was very high, but the reflectance was low. When the layer number was increased up to 36, lots of cracks appeared on the surface of the LaF₃/MgF₂ HR coating, with the LIDT badly declining. It was thought that the residual stress resulted in the cracks and the decline of the LIDT. The spectra measurement showed the double stack HR coatings could provide higher reflectance and wider reflection band than LaF₃/MgF₂ HR coating with less layer pairs. Any crack was also not found on the surface of the double stack coatings. Meanwhile, the double stack HR coatings possessed greater laser damage resistances than the HfO₂/SiO₂ HR coating. The damage morphologies showed that the damage of the double stack coating was even milder than that of the HfO₂/SiO₂ coating. Therefore, the double stack design was effective to gain high reflectance and great UV laser radiation resistance simultaneously.