The Research On The Feature Of Rectangle Refractive Index Modulation Model Cascade Long-period Fiber Grating

Tough the application of fiber optic sensor has been adopted in many fields widely, it is still one of important optical fiber passive components at present. As for the existing theories of long-period fiber grating, most LPFGs are modulated through the sinusoidal refractive index. Therefore, the paper introduces the grating which is made from amplitude mask method. From the method, it can be seen that the distribution of refractive index is presented in the form of matrix waveform. In the process of simulation from the effective refractive index of fiber core to the transmitted spectra of fiber grating, we can obtain the result that demonstrates the feasibility of this algorithm. The algorithm should be applied to the study of the cascade fiber grating, and then we can make research on the spectral characteristic of the cascade fiber grating. Actually, the accuracy of grating can be reached a higher degree through the amplitude mask method. Additionally, the grating obtained better consistency/conformity and the spectrum characteristic. For the transmitted/transmission spectrum of cascade long-period fiber grating, its bandwidth interference peak is smaller/less than the bandwidth of single LPFG (which is)presented from the loss peak of transmitted spectrum. As a result, the cascade long-period fiber grating can reach a rather high sensor resolution. Therefore, the cascade long-period FBG is applied more widely than the LPFG.The paper conducts a study on the theoretical analysis of rectangle refractive index modulated LPFG, as well as the features of the cascade long-period fiber grating. The main content in the paper are elaborated as follows:1. The paper introduces the developing background, classification, and the production methods of long period fiber grating in detail. And then the significance of studying fiber grating is proposed.2. The author makes a more detailed outline about the feature of the cascade long-period fiber grating in all aspects and its recent years’ development in the application fields, which elaborates the great significance of studying the cascade long-period fiber grating in a better way.3. The paper makes a detailed derivation about the basic/essential operating principles/working principles of long-period fiber grating modulated by the rectangle refractive index. And then the graphical method and dichotomy are employed by solving the effective refractive index and cladding refractive index of the fiber core. Besides, through the numerical simulation, we can get the transmitted spectrum of the long-period fiber grating modulated by the rectangle refractive index.4. The rectangle refractive index modulation should be applied to the cascade long-period fiber grating. The length of the intermediate cascade fiber (in the cascade long-period fiber grating), the relative position are under intensive study in this paper. In addition, it also probes into the influence of the spectral characteristics of the cascade long-period fiber grating under the circumstance of phase shift.