Acrylates and Polyimides. Acrylates and polyimides were being designed into flight grade optical fiber in the 1980’s. Optical cable with acrylate coated fiber was qualified by GSFC during that period and is now considered the heritage design for flight projects. Like the array of materials used for cable jacketing, fiber coating materials each have advantages and disadvantages. The advantages of the acrylate material is that it can be selected for strippability (example: methylene chloride) and is fairly soft which makes the fiber more flexible. Its disadvantages are that acrylates tend to have low temperature ratings (usually around 85°C although some are available that are rated as high as 200°C) and it is a well known outgasser (e.g. TML greater than or equal to 5%, CVCM greater than or equal to 0.5% [3]). Though terminated cable does not allow enough venting to consider acrylate coated fiber in a cable assembly a problem, acrylate can be a contamination problem when the fiber is used in a non-hermetic coupler.

Polyimide has been used successfully for coating space grade optical fiber and it comes with the advantage of a 125°C temperature rating. This is critical to programs such as Space Station and some JPL applications. Polyimide will tend to make the fiber seem stiffer than an acrylate coated fiber. Silicone and Gore-Tex,® applied as a second coating, have been used to improve the flexibility of polyimide coated fiber. Silicone has the disadvantage of introducing a contamination concern. Silicone is also a known outgasser. Even in a cable configuration (where when using small amounts of outgassing material the outer jacket could protect the system), large quantities of silicone are currently used and could cause problems as a result of this outgassing. The Gore-Tex® construction will be evaluated.

Of even more significance are the difficulties associated with stripping polyimide. Polyimide coating can be stripped chemically, with hot sulfuric acid, or mechanically, using a hot tweezers. Users find hot sulfuric acid to be dangerous and non-portable. Mechanical stripping methods are not recommended for flight cable assemblies because they have the capacity to introduce surface flaws to the glass fiber, compromising long term reliability. Connector manufactures have tried to circumvent this problem by offering a ferrule hole size that will accommodate the unstripped polyimide coating’s outer diameter. This approach has not been accepted readily by the industry because they make the connectors considerable more expensive and force the user to commit to a non-standard configuration. It tends to be a more viable option when military specified removable ferrules (MIL-T-29504) are being used, such as in the MIL-C-38999 (mini-circular, multi-terminus) or MIL-C-28776 (hermaphroditic, circular, multi-terminus) connectors. When polyimide coating is used in single mode applications (where perfect alignments are crucial to limiting the insertion losses), it must be removed before termination due to the tighter tolerance contraints of the single mode connectors.

Fibers coated with polyimide have been found to outperform acrylate coated fiber, using identical glass, in ionizing radiation testing. This phenomenon is discussed in detail in the Radiation Effects section.

Hermetic Coating: It has been established in fiber optic industry literature that exposure to water and/or water vapor can hinder the long term reliability of fiber optics and will accelerate the aging process. Hermetic coatings are currently being fabricated on fiber optics that require longer duration reliability and are generally in the form of a 200 angstrom layer of amorphous carbon applied directly to the outside of the cladding. The process by which an hermetic coating is applied can be crucial to the survivability of the fiber itself. In cases where hermetic coatings have been achieved using a sputtered metal the fiber can become embrittled. Though polyimide coating over amorphous carbon is the more available hermetic arrangement at this time, acrylate can be applied over amorphous carbon as well lending its advantages of mechanical flexibility to the hermetic design.

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