A borosilicate glass substrate is a foundational base material used in the manufacturing of various optical components. Characterized by the addition of boron trioxide B(2)O(3) to the standard silicate glass mixture, this material is highly valued in the optics industry for its exceptionally low coefficient of thermal expansion (CTE), high chemical durability, and excellent optical clarity across the visible and near-infrared (NIR) spectrums.
In optical engineering, these substrates serve as the physical foundation upon which complex thin-film coatings are deposited to create filters, mirrors, and beamsplitters.
Key Optical Specifications
| Optical Property | N-BK7 |
| Refractive Index (nd at 587.6 nm) | 1.5168 |
| Abbe Number (V d) | 64.17 |
| Dispersion (nf - nc) | 80.5 × 10-4 |
| Transmission Range | 350 nm to 2100 nm |
| Coefficient of Thermal Expansion | 7.1 × 10-6 / K |
Composition and Key Characteristics
Standard silicate glass (like typical window glass) is prone to cracking under rapid temperature changes. The addition of boron trioxide alters the atomic structure of the glass, significantly reducing its thermal expansion.
- Low Thermal Expansion: Borosilicate glass can withstand severe temperature gradients without fracturing. This dimensional stability is critical for optical components that must maintain their exact shape and surface figure when exposed to high-intensity light sources or fluctuating environmental temperatures.
- Optical Transmission: Borosilicate substrates offer high, flat transmission from the near-ultraviolet (approx. 310 nm) through the visible spectrum and into the near-infrared (up to roughly 2.7 µm).
- Chemical Durability: It is highly resistant to water, neutral and acid solutions, and potent chemical mixtures, making it ideal for environments where degradation is a concern.
- Cost-Effectiveness: While it does not offer the extreme deep-UV transmission or absolute thermal stability of UV Fused Silica, borosilicate provides a highly economical alternative for applications that do not operate in the deep ultraviolet range.
Common Types in Optics
Several proprietary and standard formulations of borosilicate glass are heavily utilized in the photonics industry:
N-BK7 (Borosilicate Crown Glass): Arguably the most common optical glass for high-quality visible and NIR applications. It is relatively hard, takes high-quality polish easily, and is used extensively for lenses, prisms, and thick substrates.
Applications in Optical Components
Because of its balance of optical clarity, thermal stability, and cost, borosilicate glass is heavily utilized as a substrate material across a variety of optical systems:
- Optical Filters: Borosilicate serves as the rigid, transparent base for dielectric thin-film coatings. It is the standard substrate for many bandpass filters, shortpass/longpass edge filters, and Neutral Density (ND) filters operating in the visible and NIR ranges.
- Dichroic Mirrors and Beamsplitters: The thermal stability of borosilicate ensures that the delicate angle-of-incidence (AOI) requirements of dichroic coatings are maintained, preventing environmentally induced "blue shift" or structural warping.
- Imaging Systems: It is frequently used as the protective cover glass for imaging sensors (including those used in standard visible and some SWIR applications) and as the base material for precision optical windows.
- Illumination Systems: Due to its resistance to thermal shock, borosilicate substrates are ideal for use in front of high-intensity light sources, such as Xenon or Mercury arc lamps, where standard glass would shatter from the heat.