Theoretical Study On Novel Cladding Index Modulated Fiber Grating

A new type of fiber grating, cladding index modulated fiber grating, is presented in this paper. The key feature of this new grating is that the index modulation is only in the cladding, while the core is kept unperturbed. The modes coupling mechanism of the new grating was first analyzed with the coupled-mode theory based on the model of a three-layer step-index fiber geometry. The reflection spectrum of the cladding index modulated fiber Bragg grating (FBG) and the transmission spectrum of the cladding index modulated long-period fiber grating (LPFG) were simulated numerically. The characteristics of the new grating were predicted. With the same parameters this new grating has the reliable filter functionality which is similar to the traditional fiber grating. In addition, the spectra of the new grating can be designed flexibly by changing the grating parameters. Lower attenuation is demonstrated in comparison with traditional grating through calculating the attenuation of both gratings. The new grating's attenuation is ten times less than fiber core index modulated grating under the same-doped condition.Through exposing the cladding photosensitive fiber to periodic ultraviolet, the new cladding index modulated fiber grating can be formed. Thus, the various fabrication methods for traditional fiber grating can be easily transplanted to the new grating. Meanwhile, the research results show that the new grating can be a solution to the high attenuation problem of the gratings with the high photosensitive fiber core.