Optical Material: Fused Silica

Optical Material: Fused Silica is the fourth in a series of blog posts that aims to highlight the various optical materials used to design and manufacture optical components for the photonics industry.

Although float glass [featured in a recent blog post] is the most common material used in optical components, there are several demanding applications that require superior material characteristics such as high chemical purity, high-temperature resistance, high transmission, chemical inertness and low co-efficient of expansion. In such cases, alternative material choices are needed, and fused silica (SiO2) is a predominant candidate that satisfies these strict optical requirements.

Precision Fused Silica Optical Blanks
Precision Fused Silica Optical Blanks

Fused silica is an ultra-pure, amorphous form of glass that is made from high purity silica. Typically, the concentration of silica is greater than 99.9999% with miniscule quantities of other metal oxides. Due to its extremely high purity, it finds many applications in optical windows, optical components for UV transmission, IR optics (with OH ions removed), semiconductors, metrology and more.

Fabrication

Fused silica is fabricated from a very highly refined precursor such as silicon tetrachloride (SiCl4), sometimes branded as STC. Since the final purity depends on the purity of the precursor, it is ensured that the concentration of metal oxide impurities is as low as possible (under 1%).  Unlike conventional glass that is fabricated by melt processing, fused silica is made using Chemical Vapor Deposition (CVD).  A heat source, such as a furnace or plasma, burns the precursor in the presence of hydrogen or oxygen to form a ‘soot’ that is deposited on a desired substrate. The substrate can be rotated or translated to obtain the desired shape of the components, such as optical fibers, tubes, lenses, etc.

Optical Properties

Transmission Characteristics

Unlike conventional glass which is not suited for ultraviolet (UV) applications, fused silica offers high transmission in the UV spectrum due to its high purity. It typically has an operational range from around 180nm to 2μm. Infrared (IR) transmission can be further enhanced and extended to 3.5μm by removing OH ions. These features make it an excellent choice for components used in UV, Visible and SWIR imaging.

Fused Silica - UV and IR Transmission Characteristics
Fused Silica – UV and IR Transmission Characteristics

Refractive Index

The refractive index of fused silica is highest in the UV region – 1.54 at 200nm. It decreases to 1.45 at 600nm and 1.4 at 2400nm. The optical dispersion can be obtained using the following Sellmeier equation:

 n^2 (\lambda) = 1 + [0.6961663 \lambda^2 / (\lambda^2 - 0.0046791)] + [0.4079426 \lambda^2 / (\lambda^2 - 0.0135120)] + [0.8974794 \lambda^2 / (\lambda^2 - 97.9340025)]

Here, λ is the wavelength of light and n is the refractive index.

Fused Silica - Refractive Index
Fused Silica – Refractive Index

Applications of Fused Silica

UV Optical Components

As mentioned earlier, the high purity of fused silica enables UV applications from 180nm onwards. This gives it a major advantage over conventional glass such as BK7 which is not UV compatible. Silica-based lenses, mirrors, beamsplitters, microscope slides and optical windows can be found in imaging devices, lasers and optoelectronic devices that operate at UV wavelengths in the range of 180-400nm. They can be coated with metallic, dielectric and anti-reflection coatings, and can be achieved in large format configurations.

Excimer Laser Components

UV lasers based on ArF and XrF lasing materials have laser wavelengths of 193nm and 248nm, respectively. They are extensively used in semiconductor fabrication, lithography, and eye surgery. Due to its excellent UV transmission fused silica is the material of choice for optical components such as mirrors and lenses in UV laser cavities.  Dynasil offers custom ArF and XrF grade Corning HPFS® with a transmission of 99.8%/cm at the lasing wavelengths.

Microlithography

Fused silica wafers are routinely used as substrates for microlithography and lithographic techniques including nanoimprint lithography, electron beam lithography and direct laser writing. They have high temperature resistance, low thermal expansion, and chemical inertness, which makes them suitable for lithography applications. Most importantly, UV lasers are used in photolithography due to their high-resolution micro/nano features and fused silica is an excellent candidate for UV applications. They are also used as masters for replicating micro-optical components such as lens arrays and gratings.

MEMS and Sensors

In recent years, fused silica has been used for radiofrequency MEMS (microelectromechanical systems) applications. Resonators made of fused silica with piezoelectric electrodes have high Q-factors and find applications in gyroscopes used in inertial navigation. Due to the compact nature of MEMS sensors, they are used in aerospace systems where lightweight and robust sensors are desirable.

Fused Silica Optical Blanks

Fused silica blanks are a foundational element used to manufacture optical components for lasers, semiconductors, telecom equipment, electronics, scientific instruments, and an assortment of energy and aerospace applications. For instance, a lens maker or an eye glass manufacturer would grind and polish blanks to achieve the desired radius of curvature. This would determine the function and power of the lens.

Dynasil Fused Silica (DFS) provides the world’s purest glass, and fused quartz products to enable next generation photonics applications. DFS is a US-based authorized distributor of Corning’s High Purity Fused Silica – HPFS® 7980, 7979 IR and Ultra-Low Expansion – ULE® 7972. DFS offers round and rectangular fused silica optical blanks with a wide range of lateral dimensions and thicknesses. Additionally, various types of surface finishing are supported along with custom fabrications options.

Dynasil Fused Silica is an Authorized Distributor of:

Also in this series…

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