As the “transparent barrier” of the optoelectronic system, the optical window undertakes the key task of efficient transmission of optical signals while protecting the internal precision optical components. As a professional optical component manufacturer, MOK Optics has built a full-spectrum window solution covering ultraviolet to far infrared based on years of experience in optical material research and development. This article will systematically analyze the technical characteristics of core window materials such as H-K9L/N-BK7, fused quartz, sapphire, and calcium fluoride/magnesium fluoride, and provide professional guidance for selection in different application scenarios.
Ⅰ. Basic technical parameter system of optical windows
1. Key performance indicators
Transmittance characteristics: including average transmittance, cutoff band, and smoothness of transmittance curve
Optical uniformity: refractive index uniformity (Δn≤±2×10⁻⁶)
Mechanical strength: Knoop hardness (HK), Young’s modulus (E) and fracture toughness (KIC)
Environmental stability: coefficient of thermal expansion (CTE), moisture resistance (g/m²·day) and acid and alkali resistance (pH tolerance range)
Surface quality: surface roughness (Ra), scratch-pitting standard (MIL-PRF-13830B)
2. Coating technology support
Broadband anti-reflection film (BBAR): reflectivity in the 400-700nm band <0.5%
Hydrophobic and oleophobic film: water contact angle>110°
Conductive ITO film: square resistance <10Ω/□
Laser protection film: damage threshold>10J/cm² @1064nm
II. H-K9L/N-BK7 optical window: economical visible light solution
1. Material essential characteristics
H-K9L (Chinese label) and Schott N-BK7 are borosilicate crown glasses with equivalent optical performance, with:
Abbe number: Vd=64.2, a typical low-dispersion material
Internal quality: bubble degree meets Class 0 (ISO10110-3)
Stress birefringence: <5nm/cm (better than national standard GB/T 7962.1)
2. Precision machining process
MOK Optics adopts a unique “three-step polishing method”:
Diamond rough grinding: removal amount 50-100μm, surface roughness Ra<0.1μm
Cerium oxide fine grinding: surface shape correction to λ/2@632.8nm
Ion beam shaping: final surface shape reaches λ/10, roughness Ra<1nm
3. Typical application scenarios
Industrial vision system: wavefront distortion <λ/4 when used with a 5-megapixel lens
Laser alignment device: absorption coefficient <0.1%/cm at 1064nm
Biological detection equipment: meets USP Class VI biocompatibility standards
III. Fused quartz window: an all-round player from ultraviolet to near-infrared
1. Analysis of material advantages
Key characteristics of synthetic fused quartz (Fused Silica) that distinguish it from natural quartz:
Hydroxyl content: UV absorption difference between <1ppm (Type III) and ~200ppm (Type II)
Metal impurities: Fe content <0.1ppm, ensuring transmittance in the deep ultraviolet region
Radiation resistance: transmittance drops by <3% after irradiation with 10⁶ Rad γ rays
2. Extreme environment adaptability
Temperature stability: working range -200°C to +1000°C (instantaneous)
Thermal shock performance: ΔT>800°C water quenching without cracking
Corrosion resistance: erosion rate in 48% HF acid <0.1μm/h
3. High-end application cases
DUV lithography machine: transmittance at 193nm>99.5%/10mm
Space optics: meet ISO 21348 proton radiation standards
High-power laser system: damage threshold reaches 15J/cm²@355nm, 10ns
IV. Sapphire window: the best choice in extreme environments
1. In-depth analysis of crystal characteristics
C-axis oriented sapphire window has anisotropic characteristics:
Hardness performance: Mohs hardness 9 (second only to diamond)
Thermal conductivity: 25.1W/(m·K) @25°C (better than most optical materials)
Dielectric strength: >40kV/mm (preferred for high-voltage observation window)
2. Special processing technology
Laser-assisted cutting: reduce the subsurface damage layer to <5μm
Plasma polishing: achieve atomic-level surface (RMS <0.2nm)
Special-shaped window molding: can process complex surfaces such as spherical and aspherical surfaces
3. Military-grade application verification
Missile fairing: maintain optical performance under Mach number 5 conditions
Deep-sea observation window: no deformation under 10,000 meters of water pressure (100MPa)
High-temperature window: continuous operation in an oxidizing environment of 1800°C for >500h
V. CaF₂/MgF₂ window: key material from deep ultraviolet to mid-infrared
1. Precision growth technology
MOK uses the Bridgman-Stockbarger method to grow CaF₂:
Raw material purity: 6N grade (99.9999%)
Crystal orientation: <111> Direction deviation <0.5°
Annealing process: gradient annealing to eliminate internal stress (ΔT=1°C/h)
2. Spectral application examples
Excimer laser system: ArF laser (193nm) transmittance >99%/5mm
Fourier spectrometer: No absorption peak in the 1-7μm band
Astronomical observation: No risk of cracking as a low-temperature (-196°C) window
Summary
As optoelectronic systems develop towards multi-spectral fusion and intelligence, optical windows have evolved from simple protection components to intelligent optical interfaces with functions such as signal modulation and environmental perception. Correct selection of window materials and coating solutions can increase the system signal-to-noise ratio by more than 30% and extend the MTBF (mean time between failures) by 2-3 times, which requires comprehensive consideration of up to 27 technical parameters such as optics, mechanics, and the environment. MOK Optics provides full life cycle management services for optical windows, from material selection, reliability verification to on-site maintenance, to provide “zero-defect” window solutions for various optoelectronic systems.