Interference optics for lasers and parametric oscillators emitting in the middle-IR range

A wide range of film materials, suitable for the fabrication of interference optics for the middle-IR range, was investigated. The selected film pair (YF₃—ZnS) had minimal internal porosity, low mechanical stresses, and good adhesion to substrates made of quartz, fluorite, barium fluoride, and leucosapphire. Experiments were used to determine the optimal film deposition procedures so as to minimise the residual microporosity, mechanical stresses, and losses resulting from diffuse scattering and absorption. Highly reflecting and output mirrors were made for the middle-IR range, as well as mirrors for an optical parametric oscillator (λ = 3.5 — 12 mm) and antireflection coatings for a nonlinear GaSe crystal. The optical strength of such coatings was 25 — 30 J cm^–2. The interference optics developed in this way was used in an optical parametric oscillator with a GaSe crystal emitting at 4.2 mm when pumped with λ = 2.12 μm radiation.