A chirp is a linear variation in the grating period, that can be added to the refractive index profile of the grating. The reflected wavelength fluctuates with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the ability to add dispersion—especially, different wavelengths reflected from the grating will be subject to different delays.
A non-uniform resonance wavelength along the length of the grating in a CFBG can be accomplished by varying the period or by varying the average effective refractive index. The average refractive index can be changed using different methods, for example, changing the amplitude of the reflective index modulation profile or variation the fiber in the region of the grating length. The chirped FBG was manufactured with the usage of a chirped phase mask to generate a variation in the period of the refractive index.
Chirped fiber Bragg gratings have been widely used for dispersion compensations in high-speed fiber optic communications systems because they are able to retard pulsed light depending on its wavelength. Experience has proven that ideas in one field find applications in another. Actually, this type of optical device has been attracting significant attention in the fiber optic sensing community, in high sensitivity sensors or wavelength discriminators in interrogation systems.
There are two prevailing fields of application of chirped FBG: measurement of curvature based on chirped fiber Bragg gratings and new interrogation system, written in an Erbium-doped fiber. The increasing demand for measurement of curvatures has stimulated the appearance of few sensing systems that depend on the intrinsic characteristics of fiber Bragg gratings. A curvature measurement technique using a smart composite consists of two chirped fiber Bragg gratings. The two gratings are embedded on the opposite sides of the composite laminate and serve as curvature sensors and as wavelength discriminators enabling a temperature-independent intensity-based scheme for the measurement of the radius of curvature.
FBG interrogation relies on the usage of the edge filtering concept applied to a chirped fiber Bragg grating written in an erbium-doped fiber as the processing element. Through the combination of the photon amplification of the erbium-doped fiber and of the distributed wavelength reflection characteristics of the chirped FBG, it becomes possible to reach different reading sensitivities and amplification of the remote sensing signal. The ability of chirped FBG has also been employed successfully in the development of interrogation techniques. One of these techniques uses the group-delay in a Sagnac loop interferometer and another the spectra response of broadband chirped gratings.