Does HJ represent fiber Bragg grating
A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. The term type in this context refers to the underlying mechanism by which grating fringes are produced in the fiber. The grating period can be uniform or graded, and either localised or distributed in a superstructure.
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