Interference due to thermal radiation in loss measurements during fire tests of optical fiber cables

Abstract

Certain applications of fused silica optical fibers, in particular fire-resistant cables and sensors working in hot environments (e.g., nuclear reactors) require short- or long-term operation at temperatures exceeding 800 ◦C. Peak temperatures during fire tests of fire-resistant cables vary between 830 ◦C and 1040 ◦C depending on applicable standards. If the fiber contains OH− ions and hydrogen, it exhibits both increased loss and incandescence in the corresponding absorption bands. Additionally, deteriorated fiber with multiple cracks and other microscopic defects collects radiation from glowing surroundings. During loss measurements with a standard setup including a light source and a power meter, thermal radiation from both sources adds to radiation from the light source, causing false decrease of indicated fiber loss. Several methods to eliminate this interference are presented.