Fiberoptic Feedthrough/Sealing Material at Cryogenic Temperatures

Abstract

Development and testing of a novel bulkhead fiberoptic feedthrough for pressure differential applications is presented. The feedthrough is specifically targeted toward application in cryogenic temperature environments. Upon the initiation of this program, there did not exist any commercially available technology for hermetic sealing of fiberoptic feedthroughs at cryogenic temperatures. The developed feedthrough provides sealing of optical fibers in a bulkhead penetrator and allows transmission of the optical signal and optical power pulses at cryogenic-to-elevated temperatures. State-of-the art information is presented on cryogenic sealing material and optical fibers in terms of compatibility and ability to withstand high levels of vibration, temperature extremes, and harsh radiation levels. Prototype feedthroughs were designed and tested using a polycrystalline ceramic material as a sealant.

Some test results of environmental, mechanical and leak rate testing are shown, with other testing still in progress. The results and continuing testing indicate that the innovative design is able to withstand differential pressure from ambient to vacuum, vibration levels up to 21 grms from 0 to 2000 hz random vibration, three day exposure to a liquid nitrogen bath, and retain a leak rate below 10^-11 cc/sec as shown in helium leak rate test results. Shock and temperature cycling test results are favorable. Radiation, Salt Spray and Humidity testing will be done in the near future.