This SBIR Phase I project will address the feasibility of using a fiber Bragg grating array as a means of detecting liquid and slush hydrogen in gravity and zero gravity environments. Fiber optic Bragg grating sensors offer the advantages of a single fiber feed through into the cryogenic vessel for reliability and the ability to multiplex many sensors on a fiber in order to locate a liquid level or a floating mass. The detected parameter will be the differences in the thermal properties of the three hydrogen phases. Methods will be developed to extend the sensitivity of the Bragg grating sensors to at least ten Kelvins. In Phase I, the signal processing for each sensor in a short array will be performed using a robust tunable laser and a curve-fitting algorithm. The feasibility of the method will be demonstrated by using a four-sensor array to detect levels of liquid nitrogen and a single point sensor calibrated to 10K to demonstrate low temperature sensitivity. In Phase II, an array of 50 sensors will be demonstrated and advanced signal processing will be developed. The ability of the tunable laser to withstand launch stresses will be tested.