Multimode Silica-silica
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Silica-silica fiber, also known as Quartz-Quartz fiber, is the widest used optical fiber type for low loss light transmission over the widest range of wavelengths. A very high-purity silica core is surrounded by a doped silica cladding of lower refractive index. Additionally all silica-silica fibers must have an additional coating to ensure adequate strength.
Optical fibers have become widely used for telecommunications. For this application fiber users require low loss at a single operating wavelength and very high bandwidth so that the maximum number of telephone calls can be multiplexed down a single fiber. These fibers have small cores that are doped with other materials to raise their refractive index.
In other fields such as spectroscopy or laser delivery, other parameters are more important such as a wide range of operating wavelengths, high power transmission, maximum light transmission, radiation resistance etc. In these cases pure silica core fibers have many advantages.
To ensure maximum light transmission for a given fiber diameter, the core diameter is maximized and the cladding is chosen to be relatively thin compared to telecommunications fibers. A typical ratio for core to cladding diameter is 1:1.06 meaning a 200 µm core diameter fiber has a cladding diameter of 212 µm.
These silica fibers are recommended for spectroscopy applications and laser delivery. The polyimide-coated fiber has an extremely low fluorescence.
Silica-silica fibers are generally available in two varieties, the UV to visible types (180 nm to 1200 nm) and the near-IR types (500-2500 nm). Although both types have a pure silica core the OH content of the silica is typically a thousand parts per million in the UV grade, and 1ppm for the IR grade.
Coatings
The most common coating for silica fibers is acrylate. This is a transparent plastic coating which allows use over a temperature range of -40°C to +85°C. It is available in all fiber diameters and is typically 100 µm thick for smaller diameter fibers.Polyimide coating allows use from -190°C to +350°C. It must be applied in a very thin layer typically only 5 µm thick. This is very useful for fiber bundles where it is desired to maximize the core area of the bundle but this thin layer has less abrasion resistance than acrylate. It is currently only available in fibers of core diameter up to 300 µm.
For very high performance coatings Oxford Electronics also offers a range of metal-coated fibers. Metal coatings give very high tensile strength, hermetic sealing against many chemicals and high temperature performance.
