Quartz glass is made from the purest form of silica – silicon dioxide – to produce ultra-clear glass for optical fibres. These in turn can be used to transmit light over many kilometres. These fibres are also used widely in medical applications to provide accurate diagnostics while ensuring a minimal surgical intervention.
The challenge now is to create a quartz glass fibre that can work in the deep ultraviolet part of the electromagnetic spectrum at wavelengths below 190 nano metres. Many diagnoses need a true illumination using the entire light spectrum to be able to distinguish between different types of tissues. This in turn not only improves the diagnosis, but also a patient’s safety.
Silica is the most common compound to be found on Earth, but only a small fraction of it is suitable for making quartz glass. Beach sands that include salt and organic fragments are not suitable.
Conventional glass is impermeable to the ultraviolet part of the electromagnetic spectrum – wavelengths smaller than 350 nanometres – but quartz glass is permeable to these wavelengths. It is also physically and chemically stable at high temperatures. This strength comes from the material’s string silicon – oxygen chemical bonds that combine to give an open crystalline structure. As a result, this glass has a high gas permeability and a low coefficient of thermal expansion compared with conventional glasses.
The high purity of the glass improves the transmission of white light that provides a better illumination of the point of examination.
However, the purity of the silica used in this glass creates its own problems at mid to deep ultraviolet wavelengths. For wavelengths smaller than 275 nano metres, the glass becomes susceptible to image distortion and attenuation. At these wavelengths, the ultraviolet light induces surface defects in the material. These defects depend on how the glass is manufactured and on any trace contaminants that may have been introduced during the manufacturing process.
Solarisation resistant fibre can be produced using glass with a high hydroxide anion content or by loading the glass with dissolved hydrogen at a high pressure and temperature. Hydrogen loading makes the fibre completely resistant to solarisation.
Unfortunately, the effect only lasts a few weeks as the hydrogen will diffuse out from the material. But if the high hydroxide anion content is placed into the core of the glass fibre, the resulting optic can perform at deep ultraviolet wavelengths.
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