A new fiber optic sensor for use in flexible structures is reported. The sensor is based on an extension of Rogowski's design, in which the laser-driven optical beam in the fiber is modulated at radio frequency and strain is detected by a shift in the phase delay as the fiber dimension is strained with the structure. This sensor -- FORISS -- differs in that it consists of an embedded closed loop of fiber coupled to a laser/detector/fiber optic delay line circuit through a 2 X 2 coupler. The closed loop has the characteristics of a Fabry-Perot cavity operating at radio frequency wavelengths within the fiber. A parametric model of the sensor that enables both physical characterization of prototype sensors and insights which guide design optimization of the sensor is described. Experiments were performed on fiber- embedded composite specimens tension-stressed to failure at 26 kpsi and 7400 microstrains. The sensor survived to the point of coupon failure. The data indicates that the sensor possesses the properties predicted by the theory.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.