In order to give protection to a silica optical fiber it must be coated with some material, usually a polymer. Acrylate is the most commonly used and this restricts the maximum temperature of use to about 100 °C. Polyimide coated fibers can operate at temperatures up to 300 °C. For higher performance, metal-coated fibers using pure metals of copper, aluminium and gold have been available for some time.
These give higher temperature use, better hermeticity, and higher strength than polymer coated fibers.
The copper alloy coating allows use at temperatures up to 600 °C or higher for short periods and 500 °C for long periods. This compares well with gold-coated fibers but at a fraction of the cost. In addition the fibers can be made in long lengths (several kilometres) and in a wide variety of fiber types such as single mode, graded index, and step index large core fibers in UV and IR grades of silica. The graded index types will be of particular interest to users of Distributed Sensors (e.g. Temperature and Strain) where the fiber is used as the sensing element.
The new fibers will allow higher temperature use over long lengths at a price only slightly higher than polymer coated fibers. Other applications include UHV use where outgassing from polymer coated fibers causes problems. The high temperature operation, improved hermeticity and availability in long lengths will be of interest to those involved with deep well or mining applications.