Schott Optical Glass
SCHOTT optical glass is a family of precision optical glasses made by SCHOTT Advanced Optics (Germany). In lasers and photonics it is used both as passive substrate and lens material (windows, lenses, prisms, beam splitters) and as active laser glass (rare-earth-doped gain media). The name most often encountered on drawings is N-BK7, SCHOTT’s lead-free borosilicate crown that is the industry workhorse for visible and near-IR optics.
SCHOTT also produces fused silica (LITHOSIL), radiation-hardened glasses, solarization-resistant variants, low-expansion ZERODUR, and a well-known line of Nd- and Er-doped laser glasses used in high-energy and medical lasers.
Passive optical glasses — basic properties:
Optical glasses are specified by refractive index nd (at 587.6 nm) and Abbe number νd (dispersion). SCHOTT types span crowns (low index, low dispersion) to dense flints (high index, high dispersion). Modern “N-” types are RoHS-compliant (lead- and arsenic-free).
N-BK7 (glass code 517642) typical values:
nd=1.51680, νd=64.17
Useful transmission roughly 350 nm–2 µm (internal transmittance of a 10 mm path is ~0.997 near 400–1060 nm)
Density 2.51 g/cm³, CTE ≈7.1×10−6/K (–30 to +70 °C)
dn/dT modest (~3 × 10⁻⁶/K class at visible wavelengths)
Good homogeneity and low stress birefringence when fine-annealed
High-transmission grades (N-BK7HT, N-BK7HTSultra) improve blue/UV edge transmission and resistance to solarization under intense blue light—relevant for RGB laser projectors and manufacturing lasers.
Other frequently used SCHOTT types in photonics:
Fluor crowns and extra-low-dispersion glasses (N-FK, N-PK) for color-corrected lenses.
Dense flints (N-SF, N-LASF) for high-NA focusing.
Radiation-resistant “G” glasses (e.g. BK7G18) with cerium doping for space and nuclear environments.
Fused silica for UV, high laser-damage threshold, and low thermal expansion.
Bulk laser-induced damage thresholds of crown glasses such as N-BK7 are high in the near-IR (nanosecond pulses at 1064 nm can exceed 1 kJ/cm² in bulk); surface LIDT and coatings usually limit the optic first. Flint glasses generally damage more easily, especially in the green.
Active laser glasses:
SCHOTT melts platinum-inclusion-free phosphate and silicate laser glasses doped with Nd³⁺ (~1053–1060 nm) or Er³⁺/Yb³⁺ (~1.53–1.54 µm, “eye-safe”). Examples: LG-750 / LG-760 / LG-770 (high-energy phosphate), APG-1 / APG-760 (better thermo-mechanical figure of merit for high average power), LG-680 (silicate), and LG-940/950/960 (Er:Yb). These are supplied as rods, slabs, and meter-class plates. Nd:phosphate glass from SCHOTT is the gain medium in large inertial-confinement-fusion lasers (NIF, LMJ).
Applications:
Laser optics (passive)
N-BK7 and fused silica are standard substrates for windows, folding mirrors, lenses, polarizers, and beam splitters from the visible through ~1.5 µm. High-homogeneity N-BK7HT is used for large prisms and interferometer optics. Solarization-stable grades serve blue-laser and projector optics. Radiation-hard glasses fly in space instruments.
Imaging and beam delivery
Camera, microscope, and machine-vision lenses; collimators and focusing optics for industrial lasers; endoscope and medical-device optics (lanthanum crowns for compact high-index designs).
High-energy and high-power lasers
Nd:glass slabs and rods in scientific petawatt and fusion lasers; Er:glass in rangefinders and medical lasers. Passive SCHOTT glasses (S7000 series and others) are used as cladding, flow tubes, and windows around the gain medium.
Precision and harsh environments
ZERODUR for laser gyroscopes and telescope mirrors; radiation-resistant glasses for nuclear and space; high-LIDT coated components for pulsed lasers.
In practice, “SCHOTT optical glass” on a laser-optics drawing usually means a catalog type (most often N-BK7 or a named HT/radiation-hard variant) chosen for known index, dispersion, homogeneity, and transmission. For UV, ultrafast, or multi-kW IR work, fused silica or other specialty SCHOTT materials are specified instead.