15.1 INTRODUCTION

An optical fiber has to be defended by some protective coating from mechanical damage during handling and from environmental factors during its use. In many cases, a polymer coating is appropriate for enough protection. However, there are a number of special applications of optical fibers in which ordinary polymer-coated fibers cannot be used. These applications can be divided into several groups:

1. Increased reliability (hermeticity of the coating is important)

2. High vacuum (when outgassing from the coating is undesirable)

3. Possibility of soldering (embedded fibers, pigtails, inlets to high vacuum)

4. Delivery of high-power laser radiation (polymer can inflame by scattered light)

5. Medical applications (metal-coated fibers can be sterilized using ETO, steam, e-beam, or ?-radiation)

6. Harsh environments

   • High-temperature environments (>350 C)

   • Nuclear radiation (polymer coating decays under radiation)

   • Chemicals (if they do not cause corrosion of the metal)

Real-life applications (such as sensors, aerospace, chemical industry, deepwell oil-field industry) can belong to several groups simultaneously.

In contrast to carbon (another type of hermetic coating), metal coatings do not need an additional protective polymer coating. Thus, metal coatings have no contender in applications for which the presence of a polymer coating is undesirable.

The known specific applications for metal-coated fibers are as follows:

• Radiation-resistant fiber optic systems intended for use in the nuclear industry (e.g., plasma diagnostic systems in thermonuclear reactors,...