Attached to the tool belt of every optical designer is a staggering variety of materials from which to choose.  However, from high and low index optical glasses, absorptive filters, various crystals, composites and plastics, very few can boast the wide ranging appeal of fused silica.  In terms of application, this high purity, non-crystalline material is often the go-to substrate of choice, and for good reason.  If you’re in the market for an all-purpose, high performing lens, window, mirror substrate or even a simple, stable tool, then consider the following:

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Range of Transmission:  Through various manufacturing processes, producers of fused silica have enabled it to achieve amazingly high throughput over a wide range of the spectrum.  A standard grade ultraviolet fused silica will transmit >90% from 200nm to 2 microns with only slight deviation at the 1.4 micron wavelength.  Switch to an IR grade fused silica, which removes the OH absorption bands, and you’ll achieve >90% from 250nm to 3.5 microns.  Still other fused silica varieties have low metallic impurities which allow for transmission in the high-power semiconductor, extreme UV regions of 248nm and 193nm.  As most optical glasses “turn on” at 350nm and begin to “tail-off” near the 2 micron range, fused silica is simply a superior, more versatile optical material.

Low Coefficient of Thermal Expansion:  A hallmark of fused silica is its thermal stability.  Whereas other optical materials will lose their surface accuracy when subjected to large changes in temperature, fused silica is known for its ability to resist thermal shock and expansion.  This important property makes it a perfect choice for mirror substrates when considering the application of either a metal or dielectric coating.  The low CTE also helps maintain the precision of an optics transmitted wavefront distortion over a range of temperatures.

Minimal/No Fluorescence: In the design of every optical system, the signal-to-noise ratio is an important factor.  When exposed to high intensity radiation such as UV light, many materials will absorb the energy and re-emit it, thereby fluorescing.  This effect introduces unwanted noise into the system which degrades the overall signal and reduces the effectiveness of the instrument.  Fused silica provides extremely low, and in many cases, no fluorescence in the presence of such radiation.  This specific characteristic has earned fused silica the reputation as the material of choice for laser applications.

High Chemical Resistance: Fused silica is chemically inert and will not react with a wide array compounds, including most acids in very high concentrations, with the exception of hydrofluoric acid.  Such durability is useful when fused silica is employed as a window in harsh environments or when used as a laboratory tool in contact with caustic chemicals.  This resistance also protects the integrity of an optic’s surface polish assisting both the flatness and transmitted wavefront distortion.

Fused Quartz

One of the most common optical material questions is the difference between fused silica and fused quartz.  From a manufacturing standpoint, fused quartz is made by the melting of highly pure, crushed natural quartz.  On the other hand, fused silica is made by melting highly pure silica through a flame hydrolysis process where it oxidizes and forms an amorphous (crystal-free) structure.  Both materials share all the above described properties with the exception that quartz, due to metallic impurities in the crushed precursor material, does not transmit well in the ultraviolet spectrum.

Esco Optics performs shaping, lapping, polishing and thin-film coating on all grades of fused silica and quartz.  We maintain stock of material from numerous manufacturers and welcome the opportunity to assist with your optical requirements.  We also encourage all customers to view our selection of stock fused silica windows, lenses, cylinders and mirror substrates.  For all non-stock items, our technical sales team is ready to answer your questions and help custom-tailor optics to meet your specific needs.

Fused Silica

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: amorphous silicon dioxide

Alternative terms: fused quartz, quartz glass, silica glass

More general term: optical materials

More specific terms: infrared grade silica, UV-grade silica

Category: optical materials

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Fused silica is amorphous silicon dioxide. It can be obtained e.g. by melting silica powder such that the grains are fused together, and cooling it down fast enough to avoid crystallization. Fused silica in a purified form belongs to the most important optical glasses, or more generally optical materials, both for a wide range of bulk optical components and in fiber optics.

There is also a variety of silicate glasses, which have fused silica as their main component but contain additional substances such as soda, alumina, germania or lime. They generally have much lower glass transition temperatures and also differ from pure silica in many other respects, e.g. in terms of the transparency range and the thermal expansion coefficient.

Fused silica is sometimes called fused quartz or quartz glass. However, it should be kept in mind that it is an amorphous material, while quartz is crystalline. (When lamps are said to have a quartz envelope, it is always fused silica; the same holds for most “quartz tubes”.) Other common names are silica glass and vitreous silica.

While silica has a very wide range of industrial and other applications, this article focuses on optical properties and applications in optics.

Key Properties of Fused Silica

Fused silica has several remarkable features both concerning its mechanical, thermal, chemical and optical properties:

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• It is hard and robust, and not too difficult to machine and polish. (One may also apply laser micromachining.)
• The high glass transition temperature makes it more difficult to melt than other optical glasses, but it also implies that relatively high operation temperatures are possible. However, fused silica may exhibit devitrification (local crystallization in the form of cristobalite) above °C, particularly under the influence of certain trace impurities, and this would spoil the optical properties.
• The thermal expansion coefficient is very low – about 0.5 · 10−6 K−1. This is several times lower than for typical glasses. Even far weaker thermal expansion around 10−8 K−1 is possible with a modified form of fused silica with some titanium dioxide, introduced by Corning [4] and called ultra low expansion glass.
• The high thermal shock resistance is a result of the weak thermal expansion; there is only moderate mechanical stress even when high temperature gradients occur due to rapid cooling.
• Silica can be chemically very pure, depending on the fabrication method (see below).
• Silica is chemically quite inert, with the exception of hydrofluoric acid and strongly alkaline solutions. At elevated temperatures, it is also somewhat soluble in water (substantially more than crystalline quartz).
• The transparency region is quite wide (about 0.18 μm to 3 μm), allowing the use of fused silica not only throughout the complete visible spectral region, but also in the ultraviolet and infrared. However, the limits substantially depend on the material quality. For example, strong infrared absorption bands can be caused by OH content, and UV absorption from metallic impurities (see below).
• As an amorphous material, fused silica is optically isotropic – in contrast to crystalline quartz. This implies that it has no birefringence, and its refractive index (see Figure 1) can be characterized with a single Sellmeier formula.

• The nonlinear index of fused silica is one of the lowest of all optical materials.
• It also exhibits relatively low chromatic dispersion and thus belongs to the crown glasses.
• For some applications, the high radiation resistance of pure fused silica is relevant.
• The high phonon energies (resulting from the light elements Si and O) lead to strong non-radiative transitions of integrated rare earth ions, which is beneficial in some cases but excludes the use of fused silica in other cases.

Fabrication of Fused Silica

Fused silica can made by melting some solid form of silica and cooling the melt sufficiently fast to avoid crystallization. A quite high temperature around  to °C is needed – far above the glass transition temperature of many common optical glasses. The required heat can be provided by an electrically heated furnace or by a flame (Verneuille process) obtained with some combustion gas mixed with pure oxygen. For obtaining high quality material as needed in optics, contamination with unwanted impurities, which is particularly likely due to the high temperature, should be minimized with a suitable choice of materials, e.g. for crucibles.

One can use natural quartz crystals as the raw material, but this will generally lead to a relatively low material quality because quartz can contain a range of impurities (e.g. aluminum and sodium), which affect the optical properties, in particular the transmissivity in certain spectral regions. Therefore, one normally uses some chemically refined silica, which can exhibit a very low concentration of impurities.

Highly purified silica, e.g. for fiber fabrication (more precisely, the fabrication of fiber preforms), can be obtained in a chemical reaction. For example, one can burn silica tetrachloride (SiCl4) in a hydrogen–oxygen flame, where the oxygen combines with the silicon and the chlorine escapes in the form of HCl. The resulting synthetic silica is deposited in the form of a very fine powder (dust), which then can be fused to obtain solid material. It may exhibit substantial OH content, but a very low level of metallic impurities. In order to minimize OH content of the obtained silica for application in infrared optics, one needs to avoid hydrogen by using a vapor-free plasma flame.

Fused Silica Grades

There is not simply fused silica of higher or lower quality; it depends on the intended application (see below) what aspect of quality is relevant:

• Trapped air bubbles and other inclusions of course need to be avoided for any optical applications, except if one wanted to fabricate an optical diffuser. High optical homogeneity is also usually required.

• For application in infrared optics, it is essential to have a low content of hydroxyl (OH) – often somewhat inappropriately called water content, since it is wrong to assume that the material would contain H2O molecules. An OH content below 10 ppm is typically required for IR-grade fused silica. Substantial absorption bands related to hydroxyl content are around 2.2 μm and 2.7 μm wavelength, but there are also overtone bands e.g. in the 1.4-μm region, which are relevant for the 1.5-μm telecom wavelength band.
• For applications in the ultraviolet spectral region, other properties are relevant. The UV transmission can be limited by various metallic impurities, which therefore need to be carefully minimized for UV-grade fused silica (while they do not matter much for IR applications). Also, it is important that the material is not substantially degraded by UV irradiation; one requires good solarization resistance, which implies low radiation-induced absorption through color centers. Another possibly important feature is to have low UV-induced fluorescence and phosphorescence.

The fabrication method (see above) must be chosen accordingly. For example, ordinary flame processing would often lead to a too high hydroxyl content for UV applications.

Various trade names are related to the type and application area. For example, Herasil, Homosil, Optosil and Vitreosil are fabricated with flame fusion, exhibit high OH content (around 150 to 400 ppm) and are thus suitable for visible and ultraviolet applications, but usually not in the infrared. Suprasil and Spectrosil are variants made with flame hydrolyzation of SiCl4, having a much lower content of metallic impurities, but also having a high OH content. Very low OH content (possibly well below 1 ppm) is achieved for materials like Infrasil, Suprasil W and Spectrosil WF, made with a water vapor-free plasma flame. Often, such trade names comes with additional numbers for different variants which are optimized for specific applications.

Of course, the surface preparation is another aspect of quality, as generally in optics. Various kinds of specifications can be relevant, e.g. surface flatness and scratch–dig specifications.

Applications of Fused Silica

Fused silica is used for a wide range of optical components, such as lenses, prisms, optical flats, mirror substrates and diffraction gratings. Key advantages are the broad spectral transmission range, the hardness and low thermal expansion – e.g. for large telescope mirrors, where the possibility to fabricate large pieces is also vital. Fused silica is also used for optical windows, when a high pressure difference between both sides and/or a limited window thickness leads to the requirement of high mechanical strength. For photomasks, the high UV resistance can be important.

Fused silica is also widely used for the envelopes of various kinds of lamps, if those are exposed to high temperatures or high temperature gradients. For example, halogen lamps and various kinds of gas discharge lamps (particularly high intensity discharge lamps) need to be operated with a very hot envelope to avoid depositions which would diminish the light output. In some cases, the high ultraviolet transmissivity of fused silica is required; that is particularly the case for excimer lamps. In the case of halogen lamps, the high UV transmission is actually often unwanted, and makes necessary the use of additional filter glasses.

Acousto-optic modulators are often based on a piece of fused silica, particularly for high-power laser applications.

In dielectric coatings, fused silica is often used as the low-index material. It can be deposited in a vacuum chamber with electron beam evaporation or ion beam sputtering, for example.

Another important application area is fiber optics. Most optical fibers, including nearly all telecom fibers, are silica fibers. Here, one usually does normally not use pure silica throughout because an optical fiber usually contains a waveguide structure. A common option is to use pure fused silica for the fiber cladding while having some kind of silicate glass (e.g. germanosilicate) for the fiber core. Particularly for large-core multimode fibers, one may alternatively have a pure-silica core (exhibiting particularly low propagation losses) and a “depressed cladding”, which is typically doped with fluorine to obtain a reduced refractive index. Most photonic crystal fibers are made from pure silica.

Because the transmission distances in fibers are often very long (e.g. dozens of kilometers), sufficiently low propagation losses are generally needed, and this requires highly purified forms of silica. Indeed, the development of low-loss fibers, suitable for example for optical fiber communications, first required the identification of relevant impurities and the careful optimization of fabrication processes. See the article on silica fibers for more details.

One can fabricate various other types of waveguides on silica surfaces (or somewhat below). This is important in the context of photonic integrated circuits.

Related Articles

Bibliography

[1]I. H. Malitson, “Interspecimen comparison of the refractive index of fused silica”, J. Opt. Soc. Am. 55 (10), (); https://doi.org/10./JOSA.55. [2]R. Brückner, “Properties of structure of vitreous silica. I”, J. Non-Crystalline Solids 5, 123 (); https://doi.org/10./-(70)-0 [3]R. Brückner, “Properties of structure of vitreous silica. II”, J. Non-Crystalline Solids 5, 123 (); https://doi.org/10./-(71)-9 [4]C. L. Rathmann, G. H. Mann and M. E. Nordberg, “A new ultralow-expansion, modified fused-silica glass”, Appl. Opt. 7 (5), 819 (); https://doi.org/10./AO.7. [5]T. Olivier et al., “Nanosecond Z-scan measurements of the nonlinear refractive index of fused silica”, Opt. Express 12 (7), (); https://doi.org/10./OPEX.12. [6]Q. Feng et al., “Strong UV laser absorption source near 355 nm in fused silica and its origination”, Opt. Express 29 (20), (); https://doi.org/10./OE. [7]R. Schiek, “Nonlinear refractive index in silica glass”, Opt. Mater. Express 13 (6), (); https://doi.org/10./OME.

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