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:
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.
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.