Calcium Fluoride Window
Calcium Fluoride (CaF₂) Window is a flat optical component made from crystalline calcium fluoride, used as a protective or transmissive element in laser and photonic systems. It allows light to pass while isolating environments (e.g., protecting optics from contaminants, vacuum, or pressure differentials) with minimal optical distortion or loss.
Technical Information:
CaF₂ is a cubic (fluorite-structure) single crystal (or sometimes polycrystalline), optically isotropic (no birefringence in ideal form), with these key properties:
Transmission range: Approximately 0.13–0.18 μm (deep UV/VUV) to 8–10 μm (MWIR). High transmission across UV, visible, and IR, with low absorption.
Refractive index: Low, ~1.43 at visible wavelengths (e.g., ~1.433 at 589 nm), dropping to ~1.399 at 5 μm. Fresnel reflection loss is modest (~5.4% for two surfaces at 5 μm), so uncoated windows are often usable.
Dispersion: Very low (high Abbe number ~95); lowest group-velocity dispersion (GVD) among common IR materials, beneficial for ultrashort pulses.
Absorption and laser damage: Very low absorption coefficient; high laser-induced damage threshold (LIDT), especially in the UV. Popular for high-fluence applications. Thermo-optic coefficient dn/dT is negative (~–10.6 × 10⁻⁶ /°C).
Other physical properties: Density ~3.18 g/cm³; melting point ~1360–1420 °C; thermal conductivity ~9.7 W/m·K; coefficient of thermal expansion ~18–19 × 10⁻⁶ /°C; relatively soft (Knoop hardness ~158) and susceptible to scratches and thermal shock. Chemically stable and resistant to water under normal conditions.
Windows are available uncoated (broadest range) or with AR coatings (e.g., for 1.65–3 μm or other bands) to further reduce reflections. They come in various diameters/thicknesses, with high surface quality, flatness, and parallelism options. Specialized “laser-durable” or “excimer-grade” material minimizes defects, color centers, and subsurface damage for longer lifetime under intense UV exposure.
Applications:
Excimer lasers (ArF 193 nm, KrF 248 nm, XeCl 308 nm): Industry-standard windows, resonator optics, and beam-delivery components due to high UV transmission, low absorption, and high damage threshold. Critical in semiconductor microlithography and micromachining.
High-power and ultrafast lasers: Free-space laser windows, vacuum windows (e.g., in compressors or target chambers), and optics for femtosecond systems (Ti:Sapphire, OPAs) where low dispersion and nonlinear index minimize pulse distortion and B-integral. Also used with chemical lasers and other high-energy systems.
Spectroscopy and imaging: FTIR, UV/IR spectroscopy windows and beamsplitters; cryogenically cooled thermal imaging systems; broadband continuum generation.
Other photonic uses: Protective windows for gas detectors (VUV/MWIR), substrates for specialized coatings, prisms/lenses in precision optics, and components in photolithography steppers.
CaF₂ is preferred over many alternatives (e.g., fused silica, MgF₂, BaF₂, ZnSe) when broad spectral coverage, UV performance, low dispersion, or high LIDT is required, though it is softer and more thermally shock-sensitive than some glasses or sapphire. Proper handling, polishing (subsurface-damage-free), and optional protective coatings are important for maximizing performance and lifetime.