A long-period fiber grating (LPG) is a one dimension (1D) periodic structure, and is formed by introducing periodic modulation of the refractive index along an optical fiber. The coupling from the guided mode to cladding modes is wavelength dependent so we can obtain a spectrally selective loss. It is an optical fiber. In essence, a long period fibre grating (LPFG) is an all-fibre device with wavelength dependent loss. As a band rejection filter, all light in a spectral slice is discarded without affecting the amplitude and phase of neighbouring wavelengths, with the additional advantage of low insertion losses. Long period fiber gratings (LPFGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond (fs) laser direct writing. Firstly, the techniques of fabricating HLPGs by CO 2 laser, hydrogen–oxygen flame heating, and arc discharge are summarized.
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