BK7 vs UV Fused Silica: When to Use Which Material

Introduction

BK7 and UV fused silica are the two most widely used optical glass materials in precision optics manufacturing. While both are excellent choices for many applications, they have fundamentally different optical, thermal, and mechanical properties that make each material better suited for specific use cases. Understanding these differences is essential for making cost-effective and performance-optimal material selections.

Material Properties Comparison

The table below summarizes the key optical and physical properties of BK7 and UV fused silica:

Property	BK7 (SCHOTT N-BK7)	UV Fused Silica
Transmission Range	350 nm – 2.0 μm	185 nm – 2.5 μm
Refractive Index (n_d)	1.5168	1.4585
Abbe Number (V_d)	64.17	67.82
Density (g/cm³)	2.51	2.20
CTE (10^-6/°C, 20–100°C)	7.1	0.55
Knoop Hardness	610	480
LIDT (1064 nm, 10 ns)	~10 J/cm²	~20 J/cm²
Homogeneity (typical)	±2 × 10^-6	±1 × 10^-6
Relative Cost (same geometry)	1× (baseline)	2–4×

Transmission Characteristics

BK7 Transmission

BK7 transmits well from about 350 nm in the near-UV through the visible and into the near-IR to approximately 2.0 μm. Internal transmission exceeds 99.5% per 10 mm path length from 400 nm to 1.8 μm. Below 350 nm, absorption increases rapidly due to the iron oxide content in BK7. BK7 is not suitable for excimer laser applications (193 nm ArF, 248 nm KrF) or deep-UV lithography.

UV Fused Silica Transmission

UV fused silica transmits from approximately 185 nm in the deep-UV through the visible and near-IR to about 2.5 μm. The extended UV transmission comes from the amorphous SiO₂ structure without metallic impurities. This makes UV fused silica the standard material for UV laser optics, excimer laser applications, and UV spectroscopy. In the IR, fused silica has an OH absorption band near 2.7 μm, so for IR applications beyond 2.2 μm, IR-grade fused silica (low-OH) should be used instead.

Thermal Properties

The most dramatic difference between these materials is the coefficient of thermal expansion (CTE). BK7 has a CTE of 7.1 × 10^-6/°C, while UV fused silica has a CTE of only 0.55 × 10^-6/°C — roughly 13 times lower. This has profound implications:

Thermal lensing: In high-power laser applications, absorbed power causes local heating and refractive index change (dn/dT). Fused silica's low CTE means less thermal deformation of the optical surface, maintaining wavefront quality under high thermal load.
Temperature stability: In environments with large temperature swings (outdoor optics, space applications), fused silica maintains its shape and optical performance far better than BK7.
Thermal shock resistance: Fused silica can withstand rapid temperature changes without cracking, making it suitable for high-temperature processes and cryogenic applications.

Laser Damage Threshold

Laser-induced damage threshold (LIDT) is a critical specification for high-power laser optics. UV fused silica consistently outperforms BK7 by a factor of approximately 2× at common laser wavelengths. This is due to fused silica's higher purity, lack of metallic impurities, and better UV absorption characteristics. For pulsed laser applications at 1064 nm (Nd:YAG), bulk damage thresholds are approximately 10 J/cm² for BK7 and 20 J/cm² for UV fused silica (10 ns pulse, single shot). The actual LIDT of a coated optic depends heavily on the coating quality and design.

When to Choose BK7

Visible and near-IR imaging systems (400 nm – 1.8 μm) where cost is a primary concern
Low-to-moderate power laser applications (CW or low-energy pulsed)
Large optics where the cost difference becomes substantial (diameters > 50 mm)
Applications not exposed to significant thermal loads or temperature extremes
Standard catalog optics where BK7 is the default substrate

PhotonEdge's BK7 Plano-Convex Lenses and BK7 Optical Windows offer excellent value for these applications.

When to Choose UV Fused Silica

UV laser applications (excimer lasers at 193 nm, 248 nm, 355 nm frequency-doubled Nd:YAG)
High-power pulsed laser systems where LIDT is critical
Applications requiring thermal stability (space optics, outdoor systems, interferometry)
Any application below 350 nm wavelength
Fluorescence microscopy where UV excitation is used

For these demanding applications, choose PhotonEdge's UV Fused Silica Plano-Convex Lenses and UV Fused Silica Optical Windows.

Cost Considerations

BK7 is significantly less expensive than UV fused silica for equivalent geometries. The raw material cost difference is approximately 2–4×, and this ratio increases for larger apertures where UV fused silica blanks are harder to source in high homogeneity grades. For a typical 25 mm diameter, 5 mm thick plano-convex lens, a BK7 version costs roughly one-third to one-half of the UV fused silica equivalent. This cost difference means BK7 should always be the default choice unless your application specifically requires the UV transmission, thermal stability, or higher LIDT of fused silica.

Conclusion

BK7 and UV fused silica are both excellent optical materials, but they serve different application spaces. BK7 is the cost-effective workhorse for visible and near-IR applications with moderate power levels. UV fused silica is the premium choice for UV transmission, high-power lasers, and thermally demanding environments. By matching material selection to your specific wavelength, power, and environmental requirements, you can optimize both performance and cost. PhotonEdge offers precision optics in both materials with full coating options — contact us for guidance on your next project.

引言

BK7和紫外熔融石英是精密光学制造中最广泛使用的两种光学玻璃材料。虽然两者都是许多应用的优秀选择，但它们具有根本不同的光学、热学和机械特性，使每种材料更适合特定用途。了解这些差异对于做出具有成本效益和性能最优的材料选择至关重要。

材料特性对比

下表总结了BK7和紫外熔融石英的关键光学和物理特性：

特性	BK7（SCHOTT N-BK7）	紫外熔融石英
透过范围	350 nm – 2.0 μm	185 nm – 2.5 μm
折射率 (n_d)	1.5168	1.4585
阿贝数 (V_d)	64.17	67.82
密度 (g/cm³)	2.51	2.20
CTE (10⁻⁶/°C, 20–100°C)	7.1	0.55
努氏硬度	610	480
LIDT (1064 nm, 10 ns)	~10 J/cm²	~20 J/cm²
均匀性（典型值）	±2 × 10⁻⁶	±1 × 10⁻⁶
相对成本（相同几何）	1×（基准）	2–4×

透过特性

BK7透过率

BK7从近紫外约350 nm透过可见光到近红外约2.0 μm具有良好的透过率。在400 nm至1.8 μm范围内，10 mm光程的内部透过率超过99.5%。在350 nm以下，由于BK7中的氧化铁含量，吸收迅速增加。BK7不适用于准分子激光应用（193 nm ArF、248 nm KrF）或深紫外光刻。

紫外熔融石英透过率

紫外熔融石英从深紫外约185 nm透过可见光和近红外到约2.5 μm。延展的紫外透过率来自不含金属杂质的非晶SiO₂结构。这使紫外熔融石英成为紫外激光光学、准分子激光应用和紫外光谱的标准材料。在红外区域，熔融石英在2.7 μm附近有OH吸收带，因此对于2.2 μm以上的红外应用，应使用红外级熔融石英（低OH）。

热学特性

这两种材料最显著的差异是热膨胀系数（CTE）。BK7的CTE为7.1 × 10⁻⁶/°C，而紫外熔融石英仅为0.55 × 10⁻⁶/°C——低约13倍。这具有深远影响：

热透镜效应：在高功率激光应用中，吸收的功率引起局部加热和折射率变化（dn/dT）。熔融石英的低CTE意味着光学表面热变形更小，在高热负荷下保持波前质量。
温度稳定性：在温度波动大的环境中（户外光学、太空应用），熔融石英比BK7更好地保持形状和光学性能。
抗热冲击性：熔融石英能承受快速温度变化而不开裂，适用于高温工艺和低温应用。

激光损伤阈值

激光损伤阈值（LIDT）是高功率激光光学的关键规格。紫外熔融石英在常见激光波长下始终优于BK7约2倍。这是因为熔融石英纯度更高、不含金属杂质且紫外吸收特性更好。对于1064 nm（Nd:YAG）脉冲激光应用，体损伤阈值BK7约10 J/cm²，紫外熔融石英约20 J/cm²（10 ns脉冲，单次发射）。镀膜光学的实际LIDT在很大程度上取决于镀膜质量和设计。

何时选择BK7

可见光和近红外成像系统（400 nm – 1.8 μm），成本是主要考虑因素
低到中等功率激光应用（连续或低能量脉冲）
大口径光学，成本差异显著（直径 > 50 mm）
不受显著热负荷或温度极端影响的应用
BK7作为默认基材的标准目录光学元件

恒鼎光的BK7平凸透镜和BK7光学窗口为这些应用提供出色价值。

何时选择紫外熔融石英

紫外激光应用（193 nm、248 nm准分子激光、355 nm三倍频Nd:YAG）
LIDT关键的高功率脉冲激光系统
需要热稳定性的应用（太空光学、户外系统、干涉测量）
350 nm波长以下的任何应用
使用紫外激发的荧光显微镜

对于这些苛刻应用，选择恒鼎光的紫外熔融石英平凸透镜和紫外熔融石英光学窗口。

成本考量

对于相同几何尺寸，BK7比紫外熔融石英便宜得多。原材料成本差异约2–4倍，对于大口径高均匀性级别的紫外熔融石英毛坯更难采购，该比例更大。对于典型的25 mm直径、5 mm厚平凸透镜，BK7版本的成本约为紫外熔融石英等效产品的三分之一到一半。这种成本差异意味着BK7应始终作为默认选择，除非您的应用特别需要熔融石英的紫外透过率、热稳定性或更高LIDT。

结论

BK7和紫外熔融石英都是优秀的光学材料，但服务于不同的应用领域。BK7是可见光和近红外中低功率应用的成本效益型主力材料。紫外熔融石英是紫外透过、高功率激光和热苛刻环境的优选材料。通过将材料选择与您的特定波长、功率和环境要求匹配，您可以同时优化性能和成本。恒鼎光提供两种材料的精密光学元件及完整镀膜选项——请联系我们获取项目指导。