Principle and transmission characteristics of long-period photonic crystal fiber grating based on structural change

A kind of long-period photonic crystal fiber grating based on structural change is studied, whose transmission spectrum is composed of five loss peaks. When a photonic crystal fiber is heated by a CO₂ laser, the air-holes in photonic crystal fiber will shrink with the certain period, depth and envelope profile, this will lead to the change of the refractive index in cladding. This paper computes and compares the change of the effective refractive index brought by this kind of structural change in the collapse laws of sine and rectangle envelope, and gets the relationship between the effective refractive index and the shrinkage of air-holes, it is a new progress that the principle of forming grating is proved in the mathematic way. The model of photonic crystal fiber grating based on structural change is built, the effective refractive index varying with the wavelength is discussed. The mode field distribution is computed with finite difference beam propagation method, then transmission spectrum with multi peak is got. The results of this study indicate that the periodic air-holes shrinkage can form grating, not only the degree of air-holes shrinkage, but also the shrinkage envelope has effect on the magnitude and the distribution of the effective refractive index. There are many modes transmitting in photonic crystal fiber grating based on structural change, however, the coupling can only arise from a few modes.