An in-situ fiber optic technique for determining the endpoint of cure of a thermoset polymer has been demonstrated and described previously . The technique requires no calibration and is readily used as a cure detector in epoxy-based composite materials . In brief, a short sensing fiber element is prepared from the thermoset resin of interest, and brought to a fully-cured state. In order to detect the endpoint of cure of this same resin when used as a matrix in a composite panel for example, the sensing fiber element is embedded within the panel during layup. Conventional multimode silica optical fibers are joined to each end of the sensing fiber with a small amount of epoxy. The silica fibers extend beyond the panel itself. Near infrared light from an LED is transmitted through the fiber arrangement, and the intensity of the transmitted light is measured with a photodetector. The fully-cured resin sensing fiber has a larger refractive index than uncured or partially-cured resin, and therefore acts as an efficient waveguide of varying numerical aperture when it is surrounded by the progressively curing matrix. When the surrounding resin becomes fully cured, the waveguiding capabilities of the sensor fiber are lost, and the transmitted light decays to zero, independent of the cure temperature. Thus, a null sensor output indicates that the resin in the composite panel is fully cured. This occurs at any temperature, without calibration, due to the self-referencial nature of the sensor.