The compatibility of optical fiber sensors with composite materials enables one to install internal monitoring devices prior to curing. Proper installation of the minute, but strong, glass optical fibers within the laminate can provide a void-free component with minimal disturbance in the layup of the reinforcement filaments. We believe that the utility of such an installation will greatly improve the quality, as well as help extend the lifetime, of the composite structure by providing quantitative, realtime measures of its stress state. Furthermore, such protected, subsurface sensors can be used to facilitate measurements for active structural control. Two specific devices will be discussed for potential application to curing process control, residual stress assessment and in-service health monitoring. Both sensors use dual mode fibers. That is, measurements are conducted on the differential phase shifts between two modes. In essence, the sensors perform as linearly distributed interferometers. The relative phase change in response to a measurand is manifest as a crosstalk change in the twin-core fiber case and a polarization change in the birefringent fiber case. Furthermore, we introduce multiple wavelength operation of these dual mode fibers in order to resolve two parameters. For instance, the twin-core fiber can measure temperature and strain simultaneously, while the birefringent fiber can provide a principal 2-D stress analysis.

© 1988 Optical Society of America

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