BK7 Optical Glass
BK7 (or N-BK7) Optical Glass is a high-quality borosilicate crown glass. It serves as one of the most widely used standard optical materials in photonics and laser systems, particularly for visible and near-infrared (NIR) applications.
Key Technical Properties:
Composition: Borosilicate crown glass (primarily SiO₂ with B₂O₃ and alkali/alkaline earth oxides). Modern N-BK7 is lead- and arsenic-free for environmental compliance.
Refractive Index: nd=1.51680 (at 587.6 nm, helium d-line). It exhibits low dispersion.
Abbe Number (vd): ≈64.17 (high value indicates low chromatic dispersion, beneficial for color correction).
Transmission: Excellent in the visible spectrum (internal transmittance >99% for 10–25 mm thickness from ~400–1060 nm). Usable range roughly 350–2100 nm (or 380–2100 nm), with decreasing performance in UV below ~350–380 nm and increasing absorption in deeper IR.
Homogeneity: Very high refractive index homogeneity (down to ~1×10⁻⁶), with low bubble/inclusion content.
Thermal/Mechanical:
Density: ~2.51 g/cm³
Coefficient of Thermal Expansion (CTE): ~7.1 × 10⁻⁶ /K (20–300°C)
Good scratch resistance and chemical stability, but softer than fused silica.
Dispersion formula (Sellmeier coefficients) and detailed refractive index data across wavelengths are readily available from manufacturers for precise ray-tracing and design (e.g., via ABCD matrix or Zemax modeling).
Relation to Lasers and Photonics:
BK7 offers an excellent balance of optical performance, manufacturability, and cost for precision components. Its low dispersion supports achromatic designs, while high homogeneity minimizes wavefront distortion.
Limitations in high-power lasers:
Higher absorption and scattering compared to fused silica, leading to more thermal lensing and lower laser-induced damage threshold (LIDT).
Not ideal for high-power or UV applications; fused silica is often preferred for those due to lower CTE (~0.55 × 10⁻⁶ /K), broader transmission (down to ~195 nm), and better thermal properties.
Applications in Lasers and Photonics:
Lenses and Windows: Protective windows, focusing/collimating lenses, and substrates for visible/NIR laser systems (e.g., HeNe, diode lasers).
Prisms and Beam Splitters: For beam steering, dispersion compensation, or interferometry, leveraging low dispersion.
Mirrors and Substrates: Coated substrates for dielectric mirrors or partial reflectors in moderate-power laser cavities.
Precision Optics: Camera lenses, microscope objectives, telescopes, and metrology instruments where cost-effective visible performance is needed.
Laser Systems: Entry-level or visible laser optics, laser guide stars (in astronomy), LiDAR, and biomedical imaging; less suited for high-energy pulsed or UV lasers.
Other: Micro-optics, fiber coupling elements, and educational/scientific setups.
BK7 remains a go-to "workhorse" material for cost-sensitive, high-volume precision optics in the visible range. For demanding laser applications involving high power, thermal stability, or UV, alternatives like fused silica or specialized glasses are typically chosen.