| With the development of information technology,the rate of information transmission is getting faster and faster.When the rate exceeds 40Gbit/s,all-optical communication must be used due to the existence of electronic bottleneck,which makes the clock extraction technology in all-optical communication become very important,this paper studies the all-optical clock extraction scheme based on FP cavity.Using the optimized fiber gratings to obtain a better single-wavelength clock signal.Firstly,the transfer equation of F-P cavity is deduced according to the structure of FP cavity and the T light transmission in the cavity.The important parameters of F-P cavity are given.The relationship between the fineness and the reflectance R of the mirror is studied.At about R=0.95,the fineness has a quick increase.The basic principle of fiber grating is studied and analyzed by using coupled-mode equation.The coupling model of uniform grating and chirped grating is solved by transfer matrix method,and the reflection spectra of various gratings are simulated by Matlab.Secondly,in order to better know the characteristics of uniform grating and chirped grating,the influence of the reflection spectrum and the delay curve is observed by changing the parameters.For uniform gratings,the grating length,the refractive index modulation depth,and the effective refractive index are changed to obtain the effect on the reflectance peak,bandwidth,and delay.For the chirped grating,the influence of the grating length,chirp coefficient and refractive index are changed to obtain the effect on the reflectance peak,bandwidth,and delay.Based on the study of grating characteristics,the fiber grating F-P cavity is simulated by transfer matrix method.The effects of grating length,refractive index modulation depth,cavity length and effective refractive index on the reflectance spectra of the two same gratings were studied.At the same time,the influence of different grating length and different refractive index modulation depth on the reflection spectrum of the two different gratings is studied.Then,because of the disadvantages of chirped grating,using the apodize method to optimize.Compared the difference of reflection characteristics and time delay characteristics among the unapodized chirped fiber Bragg gratings,the Gaussian-apodized chirped fiber Bragg gratings and the partial Gaussian-apodized chirped fiber Bragg gratings.The grating is divided into three parts,we focus on how to select an optimal apodization parameters ?a‘ of each part by comparing the changing of FWHM and the average delay between unapodized chirped fiber Bragg gratings and apodized chirped fiber Bragg gratings.And ?a‘s are a1=8,a2=4,a3=5.Finally,on the basis of time domain and frequency domain analysis of RZ code,40 GHz optical clock signal is extracted by ordinary F-P cavity.By changing the reflectivity R of the plane mirror(R=0.9?0.93?0.99?0.999),the frequency domain and the time domain of the extracted clock signal are obtained,and the conclusion is obtained that the larger the fineness is,the better the clock extraction effect is,but the great fineness will cause the transmission spectrum becoming narrower,and it will lead to difficult to align,and the latter part of the fineness increase in the effect of optimization is not obvious.So we choose R=0.99.As single wavelength optical clock recovery it just need 0.96 nm bandwidth.Firstly,we use the fiber Bragg grating F-P cavity to recover the single wavelength optical clock,which is not feasible due to the uncontrollable position of transmission window.Analyzing the wavelength selection advantage of using fiber grating to filter the signal,before using the F-P cavity to achieve all-optical clock recovery..Due to the side lobes of FBG and CFBG reflection spectrum,the effect of filter is not quite good,and this results the bad quality of all-optical clock recovery,and point out that it is very difficult to achieve the 0.96 nm bandwidth of apodized FBG.So,the CFBG should be apodized,and we change ?neff to ?neff=1*10-4 to get a good quality of all-optical clock recovery. |
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