Meeting thin film design and production challenges for laser damage resistant optical coatings at the Sandia Large Optics Coating Operation

Abstract

Sandia's Large Optics Coating Operation provides laser damage resistant optical coatings on meter-class optics required for the ZBacklighter Terawatt and Petawatt lasers. Deposition is by electron beam evaporation in a 2.3 m × 2.3 m × 1.8 m temperature controlled vacuum chamber. Ion assisted deposition (IAD) is optional. Coating types range from antireflection (AR) to high reflection (HR) at S and P polarizations for angle of incidence (AOI) from 0° to 47°. This paper reports progress in meeting challenges in design and deposition of these high laser induced damage threshold (LIDT) coatings. Numerous LIDT tests (NIF-MEL protocol, 3.5 ns laser pulses at 1064 nm and 532 nm) on the coatings confirm that they are robust against laser damage. Typical LIDTs are: at 1064 nm, 45° AOI, Ppol, 79 J/cm2 (IAD 32 layer HR coating) and 73 J/cm2 (non-IAD 32 layer HR coating); at 1064 nm, 32° AOI, 82 J/cm2 (Ppol) and 55 J/cm2 (Spol ) (non-IAD 32 layer HR coating); and at 532 nm, Ppol, 16 J/cm2 (25° AOI) and 19 J/cm2 (45° AOI) (IAD 50 layer HR coating). The demands of meeting challenging spectral, AOI and LIDT performances are highlighted by an HR coating required to provide R > 99.6% reflectivity in Ppol and Spol over AOIs from 24° to 47° within ~ 1% bandwidth at both 527 nm and 1054 nm. Another issue is coating surface roughness. For IAD of HR coatings, elevating the chamber temperature to ~ 120 °C and turning the ion beam off during the pause in deposition between layers reduce the coating surface roughness compared to runs at lower temperatures with the ion beam on continuously. Atomic force microscopy and optical profilometry confirm the reduced surface roughness for these IAD coatings, and tests show that their LIDTs remain high.