Moisture Resistant Optical Fiber for Sensor Applications

Fiber Optic Gyroscopes (FOGs) are used extensively as rotation rate sensors in both inertial navigation systems, weapons guidance systems and gun mount stabilization platforms. A FOG consists of a long length of single mode, polarization maintaining (PM) optical fiber wound into a coil. Light from a semiconductor laser is launched into both ends of the coil to create two counter rotating light beams which are them recombined to form interference patterns, which change as the coil is rotated. The fiber used is typically coated with a double layer of sicone and UV curable acrylate polymer, to protect it from abrasion and moisture, which can cause stress corrosion. This coating material has a tendency to absorb small quantities of moisture from the atmosphere and swell, causing stresses which result in slight deformation of the fiber coil, which in turn result in degradation of the gyroscope performance. We propose to eliminate this problem by replacing the outer acrylate coating with an extruded jacket of a novel material. This material is extremely impervious to moisture, and has superior chemical, mechanical and thermal properties. It is thermoplastic, which means that it can be applied using standard extrusion techniques. It may also be possible, using an appropriate co-polymer mix, to fuse the coil together after winding simply by raising the temperature to melting point of the co-polymer. This will further enhance the stability and durability of the FOG.