Massive infrared telescopes, such as the James Webb Space Telescope under development by NASA, are designed to collect infrared radiation emitted from some of the furthest celestial objects, providing clues to the origins and composition of the universe. Infrared radiation detected by such telescopes is transmitted to infrared cameras, spectrographs, and other instrumentation located many miles away. To preserve thermal infrared radiation and form an image, the entire system must be cooled to cryogenic temperatures. Given such difficult operating conditions, great care must be taken to preserve the optical properties of the entire system.
Observing that optical fibers have been shown to retain their flexibility at cryogenic temperatures, researchers at the Anglo-Australian Observatory in Australia conducted an experiment to assess the viability of using optical fibers as “light guides” in infrared telescope systems.1 All optical fibers alter the angular distribution of light to some degree, a phenomenon known as focal ratio degradation (FRD). This study examined the degree of additional FRD that results when optical fiber mounting assemblies are subjected to cryogenic temperatures (77K).
Each assembly consisted of an optical fiber housed within a flexible strain relief tube and then placed in a rigid ferrule, as is typical of optical fiber connectors used in astronomical instrumentation. Both the fiber and the flexible tube were affixed in place with an adhesive. Such assemblies are designed to allow free manipulation for connection and polishing purposes without risking damage to the optical fiber. The study tested various combinations of fiber material, flexible tubing types, and adhesive compounds. Researchers cited three important selection criteria, in addition to cryogenic serviceability, for the adhesive:
- Low outgassing, for use in a vacuum
- Long cure time, to allow time for manipulating parts during assembly
- Low shrinkage, to minimize stress on the fiber during cure
Of the six adhesives tested, Master Bond EP29LPSP produced the best results. In fact, the top-performing combination of assembly materials and EP29LPSP adhesive resulted in no additional FRD under cryogenic conditions as compared to warm conditions. The study demonstrated that the combination of appropriate materials and Master Bond EP29LPSP adhesive produce an optical fiber assembly that is serviceable in cryogenically-cooled thermal infrared telescope systems.