A method and system for determining the length of collocated waveguides in a high erosion environment, such as a solid rocket motor or a braking system. The system provides for mating optical waveguides having different attenuation coefficients within the combusting, eroding, or otherwise regressing material. Optical energy generated by the environment (e.g., from burning fuel), or which is introduced and scattered into the environment, travels through the waveguides to detector means coupled thereto. The intensities of the arriving optical energy are compared and the length of the collocated waveguides calculated therefrom. By calculating the length of the waveguides over time, a regression rate is determined.