Spectrally based fiber optic sensors depend on a light beam modulated in wavelength by an environmental effect. Examples of these types of fiber sensors include those based on blackbody radiation, absorption, fluorescence, etalons, and dispersive gratings.


One of the simplest of these sensor types is the backbody sensor. A blackbody cavity is placed at the end of an optical fiber. When the cavity rises in temperature, it starts to glow and act as a light source. Detectors in combination with narrow band filters are then used to determine the profile of the blackbody curve and, in turn, the temperature, as in Fig. 2. This type of sensor has been successfully commercialized and used to measure temperature to within a few degrees C under intenseRF fields. The performance and accuracy of this sensor are better at higher temperatures and fall off at temperatures on the order of 200 Degree C because of low signal-to-noise ratios. Care must be taken to ensure that the hottest spot is the blackbody cavity and not on the optical fiber lead itself, as this can corrupt the integrity of the signal.