Spectral Shape Control Of Cfbg Based On Polynomial Function Apodization

The edge filter demodulation system based on triangular spectral chirped fiber grating has the advantages of low cost and its performance is not affected by mechanical elements and stability of light source.However,in order to obtain a triangular spectral grating that meets the requirements,its spectral shape needs to be controlled to make its bandwidth,reflectance and linearity meet the application requirements of the scene.Therefore,we propose that the existing spectrum control technology can be divided into two parts: grating spectrum reconstruction and grating production.Here "reconstruction" refers to the process of obtaining the refractive index modulation distribution from the target spectral shape,namely the theoretical design."Production" refers to the actual grating characterizes platform according to the results of the "reconstruction" conform to the requirements of the chirp grating process,namely the actual script.In the "production" stage of the grating,the researchers proposed that after a laser beam is homogenized,a triangular apodization board is used to control the exposure intensity of the optical fiber at different positions,and the production is expanded to further increase the length of the optical fiber that can be exposed at one time,thereby improving Production efficiency.However,in the stage of spectral "reconstruction",the linear apodization function corresponding to the triangular apodization board is not obtained by the spectral "reconstruction" algorithm.The study found that the distribution of the refractive index modulation corresponding to the triangular apodized board has a large gap with the true refractive index modulation distribution of the target spectrum,and the resulting grating spectrum has obvious ripple oscillations.Therefore,it is necessary to optimize the secondary exposure of Gauss toe board.To solve this problem,this paper tries to obtain the corresponding polynomial toe cutting function of the specific spectral shape through the grating "reconstruction" algorithm,and then processes the corresponding toe cutting mask template to realize the spectral shape control of the chirped grating.The specific contents of this paper are as follows:(1)Previous studies pointed out that the CEO algorithm,as a grating reconstruction algorithm with high design accuracy,can obtain a more accurate refractive index modulation distribution of the target spectrum.However,in terms of the apodization mask exposure method,the distribution of the refractive index modulation of the grating is controlled by the apodization function corresponding to the apodization plate.Therefore,this paper proposes that the "reconstruction" of the spectrum at this time should further include the conversion process of the continuous apodization function obtained by the CEO algorithm.Specifically: First,starting from the parameters and shape of the target spectrum,a discrete refractive index modulation distribution is obtained by the CEO algorithm;second,after the refractive index modulation distribution is smoothed,a polynomial function is fitted to obtain its corresponding apodization function.By this method,the apodization function of the triangular spectrum chirped grating with target spectrum bandwidth of 7nm and maximum reflectance of 97% is obtained.Finally,with the help of the transfer matrix method,the simulation is carried out,and the results of the design spectrum parameter bandwidth of 7 nm and the maximum reflectance of 93.4% are obtained.The experimental results except for the slight decrease in the maximum reflectance,the spectral shape is basically consistent with the target spectrum,and the relative error is 1.17%,which theoretically verifies the feasibility of the method.(2)In order to further verify the correctness of the apodization function,this study designed and processed the corresponding apodization mask.The apodization mask is used to control the light intensity at different positions of the optical fiber,and realize the "production" of the grating.The resulting optical fiber has a bandwidth of 7nm,a maximum reflectance of 92.7%,and has good linearity,which is in good agreement with the design results.The results show that the apodization function obtained by this method can realize the spectral shape control of triangulated chirped grating.Because the apodization function involved in this study is based on a strict spectral "reconstruction" algorithm,there is no need to use a Gaussian apodization plate to expand the secondary exposure to optimize the spectrum,which simplifies the grating production process.This research has certain practical significance in promoting the mass production of triangulated spectrum chirped fiber gratings,and then the engineering application progress of the edge filter demodulation system.