Fiber optic sensors for the monitoring of cryogenic spacecraft tank structures

As a part of the surveillance system for liquid hydrogen tanks which is developed for future space programs of the European Space Agency, we have investigated hydrogen sensors, temperature sensors. and strain sensors. all of them based on fiber optic Bragg gratings. We present a new type of hydrogen sensor in the form of a micro-bending beam consisting of a D-shaped elliptical core fiber with an inscribed Bragg grating and a 2-10mum thin palladium foil glued onto the flat side of the fiber. The strain sensors are embedded in the inner tank wall, i.e., they are designed to function properly down to minimum temperatures of 20K. Temperature sensors are required for the separation of hydrogen or strain effects, respectively, from temperature influences. They measure the thermal elongation of glass substrates with particular sensitivity at cryogenic temperatures. The reversible shift of the Bragg wavelength which is caused by the elongation of the Bragg gratings in the multi-sensor network is monitored by a polychromator based signal processing unit.