Fused silica has a long history of industrial optical application, with its first commercial use dating back to ultrasonic delay lines in 1952. Benefiting from its superior optical transparency, high structural stability, excellent laser damage resistance and scalable manufacturing capability, modern fused silica has been widely deployed in diversified commercial and high-end scientific optical scenarios, including optical windows, mirrors, lenses and optical fibers. Representative industrial and scientific applications of fused silica are displayed in Figure 1.
 
Figure 1. Typical application scenarios of fused silica optical components.

1. Schematic diagram of UV lithography equipment with fused silica illumination and projection optics highlighted in blue;
2. Fused silica focusing lenses of the National Ignition Facility (NIF) under cleaning ;
3. Nadir observation window of the Destiny Laboratory on the International Space Station (ISS), fabricated from high-purity fused silica ;

(d) Optical camera window of NASA’s Curiosity Mars rover.

1 UV Microlithography

Optical microlithography serves as the core manufacturing process for integrated circuits (ICs). In projection lithography systems, the mask pattern is precisely projected onto the photoresist layer through a series of high-precision optical lenses, and semiconductor micro-nano patterning is realized via subsequent development and etching processes. As shown in Figure 1(a), the incident laser beam enters from the bottom left of the lithography tool, where the key illumination and projection optical modules composed of fused silica are marked in blue.
The minimum printable feature size of projection lithography follows the classical formula: R = Kλ/NA, where  represents the system process factor,  denotes the operating wavelength, and λ is the numerical aperture of the imaging system. Driven by Moore’s Law, which predicts a doubling of on-chip transistor density every two years, lithography technology has continuously advanced toward shorter wavelengths and higher resolution.
The technological iteration from traditional lithography to deep-UV lithography in the 1980s and 1990s relied heavily on high-performance fused silica materials. Customized fused silica with ultra-high refractive index homogeneity, extremely low birefringence and excellent laser damage resistance enabled the stable operation of KrF (248 nm) and ArF (193 nm) excimer laser lithography systems . For next-generation extreme ultraviolet (EUV) lithography operating at 13 nm, reflective optical systems are adopted, and the core optical components are made of Corning EUV-grade ULE titania-doped silica glass with near-zero thermal expansion, realizing further breakthroughs in ultra-fine chip manufacturing.

2 High Energy Laser Systems

High-energy laser systems are core experimental platforms for inertial confinement fusion (ICF) research and advanced laser physics exploration. Such high-precision laser devices require large-diameter, high-uniformity fused silica components, including focusing lenses, protective windows, phase plates and debris shields, to ensure beam quality and long-term stable operation.
World-class high-energy laser facilities represented by the U.S. National Ignition Facility (NIF), University of Rochester OMEGA Laser Facility, U.S. Naval Research Laboratory NIKE Laser, and France’s Laser Mégajoule all take high-purity fused silica as the primary optical material. In August 2021, the NIF achieved a landmark breakthrough, outputting 1.9 MJ laser energy and generating a record 1.3 MJ fusion energy output, which greatly promoted the development of controllable nuclear fusion new energy technology.
Figure 1(b) shows the wedge-shaped fused silica focusing lenses used in the NIF system. These lenses are responsible for converging 192 UV laser beams to the target chamber center. Fused silica is uniquely suitable for this extreme working condition due to its outstanding deep-UV transmittance, ultra-high laser damage threshold and ultra-smooth surface processing performance.

3 Space Exploration and Astronomical Observation

Fused silica has become a core optical material for astronomical observation and aerospace exploration due to its excellent thermal stability, super polishing performance and large-size forming capability. Since the 1960s, fused silica has been applied to ground-based telescope mirrors. Starting from 1964, Corning has supplied fused silica mirror blanks with a diameter of no less than 2.6 m, including the 3.6 m large-aperture mirror for the European Southern Observatory (ESO) La Silla Observatory. In addition, the COSMO Large Coronagraph Telescope, equipped with a 1.5 m fused silica primary lens, will become the world’s largest refracting telescope upon completion. The Laser Interferometer Gravitational-Wave Observatory (LIGO), which first detected gravitational waves in 2015, also adopts low-hydroxyl high-purity fused silica as its core mirror material.
Benefiting from excellent radiation resistance and optical stability in extreme space environments, fused silica has been widely used in manned spacecraft and deep-space exploration equipment. All manned space vehicles of the U.S. space program, including the Apollo 11 lunar module and space shuttles, are equipped with Corning HPFS 7940/7980 fused silica observation windows. The nadir observation window of the International Space Station’s Window Observational Research Facility (WORF), known as the highest-precision optical window in manned spacecraft, is customized with distortion-free fused silica, as shown in Figure 1(c).
Moreover, fused silica plays an indispensable role in deep-space exploration missions. In 1993, fused silica corrective lenses were installed on the Hubble Space Telescope to correct optical aberration. In August 2012, NASA’s Curiosity rover successfully landed on Mars, and its core camera optical system was also equipped with fused silica optical components (Figure 1(d)).

4 Optical Fiber Communication

Optical fiber communication is one of the most influential and revolutionary applications derived from fused silica and flame hydrolysis manufacturing technology. A standard optical fiber consists of three layers: core glass, cladding glass and outer polymer coating (Figure 2). The fiber core has a higher refractive index than the cladding, and optical signals are confined to propagate within the core through total internal reflection, realizing long-distance low-loss optical transmission. The outer polymer coating protects the fiber from mechanical damage and environmental erosion.
 
Figure 2. Structural schematic of a commercial optical fiber, including polymer coating, cladding glass and core glass.
Telecommunication-grade optical fibers typically adopt doped fused silica as the core material and pure high-purity fused silica as the cladding material. To meet the demand for long-distance infrared signal transmission, optical fiber preforms are mainly fabricated via the soot-to-glass process, which effectively eliminates hydroxyl and trace metal impurities and suppresses optical absorption loss.
The first low-loss fused silica optical fiber was successfully prepared in 1970 using improved flame hydrolysis technology, with a TiO₂-doped silica core and pure silica cladding, achieving an attenuation of 16 dB/km at 632.3 nm. With continuous technological optimization, modern commercial optical fibers mainly adopt GeO₂-doped silica cores. Compared with titanium-doped fibers, germanium-doped fibers have lower infrared absorption, enabling stable operation in the low-scattering 1310 nm and 1550 nm communication bands.
Combined with the Rayleigh scattering suppression strategies mentioned in previous sections, the performance of ultra-low-loss optical fibers has been further improved. At present, optimized fused silica optical fibers with relaxation-enhancing dopants and post-annealing treatment have achieved an ultra-low attenuation of less than 0.15 dB/km at 1550 nm, supporting the development of high-speed and long-distance optical communication networks.

Summary

With ultra-high material purity, controllable large-scale manufacturing capability, unique optical properties and excellent thermal stability, fused silica has supported a series of landmark technological breakthroughs in modern industry and science, covering semiconductor micro-nano manufacturing, high-energy laser physics, aerospace exploration and global optical communication. As a typical single-component amorphous material, fused silica is regarded as a classic model for glass structure research. However, its simple chemical composition conceals anomalous structural evolution and performance characteristics that differ from conventional multi-component silicate glasses, which has attracted sustained research attention from material scientists for centuries.
 
 
The unique performance advantages and adjustable structural characteristics of fused silica ensure its irreplaceable value in high-end optical fields. With the continuous upgrading of semiconductor lithography, laser equipment, aerospace technology and communication systems, the demand for high-purity, low-defect and large-size fused silica optical components will continue to grow. In-depth exploration of fused silica structural regulation mechanisms and innovative application scenarios will remain an important research direction in the field of optical materials in the future.
 

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