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Wavelength-modulated sensors use changes in wavelength to detect the sensing function. Fluorescence and phosphorescence emit a characteristic wavelength of light if perturbed in the proper way. For instance, a dye in the presence of an analyte can give off a characteristic excitation spectrum. Chapter 15 describes fluorescent sensors for chemical sensing and Chapter 11 for temperature sensing. The emitting spectrum provides a qualitative sensing function, but the intensity of the spectrum (usually ratiometric) is required for qualitative sensing measurements. Bragg gratings are truly wavelength-modulated sensors. The parameter being monitored is a direct function of the wavelength shift associated with the Bragg resonance condition. Once again it is important to mention that the fundamental parameter measured is light intensity. However, the wavelength shift is a direct effect of the associated environmental perturbation and is independent of the light source intensity. Gauge lengths can be as small as 0.01 mm. A distinct advantage of Bragg grating devices is their ability to be used as quasi-distributive sensors. They can be used in wavelength-division multiplexing schemes without additional wavelength-encoding filters.
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