A fiber Bragg grating is a longitudinally periodic refractive index modulation in an optical fiber. Reflections from successive periods reinforce near the vacuum wavelength , where is the period and is the guided mode's effective refractive index.
It acts as a wavelength-selective reflector while much off-resonant light continues through the optical fiber. Stretching the fiber changes the period and usually the effective refractive index, moving the reflected peak. This supports strain sensing; temperature also shifts the peak, so a strain measurement needs temperature control or independent compensation.
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Fiber Bragg Grating (FBG) is a specific type of optical device that is used primarily in fiber optic communication and sensing applications. An FBG consists of a periodic variation in the refractive index of the optical fiber core, which is created through various techniques such as exposure to UV light. This periodic change in refractive index acts as a wavelength-selective reflector.