Description:

　　long period fiber grating make by the period refractive index moudulation generated by optical fiber axis upward , whose period generally is more than 100um.

　　Its coupling principle: Fiber fundamental mode transmitting forward is coupled in several certain central wavelength cladding mode transmitting forward which lost fast. Thus LPFG basically do not have backward refletivity, but only several absorption peak with certain wavelength in spectrogram.