Quartz Glass Selection for Laser Processing: Key Parameters and Material Selection Guidelines

In laser processing systems, optical components such as protective windows, focusing lenses, beam expanders, scanners, and high-power optical assemblies rely heavily on one critical material: fused quartz glass (Fused Silica / Fused Quartz). Compared with conventional optical glass, fused quartz offers superior optical transmission, exceptional thermal stability, extremely low thermal expansion, and excellent chemical resistance, making it indispensable in laser processing, precision optics, semiconductor manufacturing, aerospace, and scientific applications.

However, many engineers face the same challenge during material selection: Why are there large cost differences among quartz grades? What do low-OH, high-OH, laser-grade, and DUV-grade actually mean? Which materials are suitable for 1030 nm, 1064 nm, or 193 nm laser systems?

This article explains the key properties, performance parameters, and practical selection principles for quartz glass in laser processing applications.

Why 石英玻璃 Is Essential in Laser Processing

Fused quartz is an amorphous material made from ultra-high-purity silicon dioxide (SiO₂). Compared with standard optical glass, it provides several significant advantages:

• Wide transmission spectrum (approximately 180–3500 nm)
• Extremely low thermal expansion coefficient
• Excellent high-temperature resistance
• High laser damage resistance
• Superior chemical stability
• Outstanding corrosion resistance

In high-power laser environments, materials must not only transmit light efficiently but also withstand thermal loads and intense pulse energy.

Manufacturing methods—including electric fusion, flame hydrolysis, and plasma chemical vapor deposition (PCVD)—directly affect hydroxyl (OH) concentration and metallic impurity levels. These trace components strongly influence optical performance across different wavelengths.

Therefore, quartz glass selection is not simply determined by SiO₂ purity; impurity control is often the decisive factor.

Four Critical Parameters of Quartz Glass for Laser Applications

1. Optical Transmission

Transmission determines how much laser energy passes through the material.

Manufacturers generally specify two transmission values:

Total Transmission

Includes reflection losses at air-glass interfaces.

Typical value:

92–93%

Coating quality significantly affects this value.

Internal Transmission

Measures only material absorption, excluding surface reflections.

Typical value:

Greater than 99.95%

Internal transmission better reflects the intrinsic purity of quartz glass.

For common infrared laser wavelengths such as 1064 nm, standard grades often provide sufficient performance. However, deep ultraviolet applications require extremely high-purity materials.

2. Optical Homogeneity

An ideal optical material should maintain a uniform refractive index throughout its volume.

In practice, variations can arise from:

• Uneven melting processes
• Cooling rate fluctuations
• Impurity distribution
• Internal stress

These factors produce refractive index variations that affect optical quality.

For applications including:

• Laser scanning systems
• Semiconductor lithography optics
• High-precision focusing systems

Optical homogeneity directly impacts beam quality and machining accuracy.

3. Striae

Striae are localized refractive index variations within glass.

They usually result from uneven flow behavior during melting.

Under optical inspection, they often appear as thread-like patterns.

For high-power laser systems, striae can cause:

• Wavefront distortion
• Beam deviation
• Focus instability

As a result, high-end laser optics impose strict requirements on striae quality.

4. Coefficient of Thermal Expansion (CTE)

Laser operation inevitably generates heat.

Thermal expansion determines dimensional stability under temperature changes.

The relationship can be expressed as:

Where:

• ΔL = dimensional change
• α = thermal expansion coefficient
• L = original length
• ΔT = temperature change

A lower α indicates greater dimensional stability.

Typical fused silica value:

~0.5 × 10⁻⁶ /K

This is significantly lower than conventional optical glass, making quartz highly suitable for precision and high-power laser systems.

Laser Wavelength Determines Material Selection

Different wavelengths require dramatically different material characteristics.

Deep Ultraviolet (DUV) Applications

Typical wavelengths:

193 nm, 248 nm

應用：

• Semiconductor lithography
• Precision micromachining
• Advanced optical systems

Requirements:

• Ultra-low metallic impurities
• Extremely low fluorescence
• Exceptional homogeneity
• 高雷射損害閾值

Even trace levels of Fe, Cu, and Cr can create absorption centers and degrade DUV performance.

Recommended materials:

• Corning 8655
• Corning 7979
• High-end DUV-grade fused silica

UV–Visible Applications

Typical wavelength range:

250–800 nm

應用：

• UV optics
• Precision lenses
• 光學窗
• 影像系統

Requirements:

• High transmission
• Stable optical properties
• Good homogeneity

High-OH materials often exhibit better short-wavelength UV transmission performance.

Recommended materials:

• Corning 7980
• Ohara SK1310
• General-purpose synthetic fused silica

Infrared Laser Applications

Typical wavelengths:

1030 nm, 1064 nm, 1070 nm

應用：

• Fiber laser cutting
• Laser welding
• Beam delivery systems
• Protective windows

Requirements:

• Low absorption
• Thermal stability
• High laser resistance

OH groups produce infrared absorption peaks near:

• 945 nm
• 1245 nm
• 1385 nm
• 2210 nm
• 2700–2800 nm

For high-power infrared systems, low-OH grades are preferred.

Recommended materials:

• Corning 7980
• Corning 7979
• Ohara SK1300

Mainstream Quartz Glass Grades and Their Characteristics

Corning 7980

Type:

Standard synthetic fused silica

Features:

• Low impurity concentration
• Excellent UV to IR transmission
• Broad application range
• Cost-effective performance

應用：

• General laser processing
• 光學窗
• Medium-power systems
• Semiconductor equipment

Corning 7979

Type:

Ultra-low thermal expansion fused silica

Features:

• Extremely low OH content
• Outstanding thermal stability
• Superior infrared performance

應用：

• High-power laser systems
• Precision optical assemblies
• 科學儀器

Corning 8655

Type:

DUV / Laser-grade fused silica

Features:

• Ultra-low metallic impurities
• Extremely high laser damage threshold
• Optimized for deep UV wavelengths

應用：

• ArF 193 nm lithography
• KrF 248 nm optical systems
• High-end semiconductor optics

Practical Quartz Glass Selection Workflow

A simple selection process can significantly reduce engineering risks:

Step 1: Identify Laser Wavelength

• DUV (<250 nm): choose DUV-grade materials
• UV–Visible (250–800 nm): standard synthetic grades are sufficient
• Infrared (>800 nm): prioritize low-OH materials

Step 2: Evaluate Transmission Requirements

For standard laser processing applications, transmission differences often originate from coating quality rather than substrate materials.

For ultra-high-power or DUV applications, substrate purity becomes critical.

Step 3: Consider Laser Power Conditions

For pulsed and high-energy laser systems:

Laser-Induced Damage Threshold (LIDT) becomes a key factor.

Special laser-grade materials should always be considered.

Step 4: Assess Thermal Stability Requirements

Applications with severe thermal cycling or extreme dimensional stability requirements should prioritize:

• Ultra-low expansion fused silica
• Low-absorption grades
• Precision optical materials

總結

Selecting quartz glass for laser processing is not simply a matter of choosing the highest purity material. The optimal choice depends on laser wavelength, power density, thermal conditions, and optical performance requirements.

For standard fiber laser systems, materials equivalent to Corning 7980 are often sufficient. However, for high-power infrared lasers, DUV systems, and precision optical applications, premium grades such as Corning 7979 or 8655 become essential.

Understanding the relationship between OH content, impurities, wavelength, and optical performance can significantly improve system reliability and reduce long-term operational costs.

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