% The following program is used to calculate the reflection spectrum of uniform FBG  
% First time:    2006-10-02   PS: The result is good.

close all
clear all
Neff=1.45900;         
delta_N=8e-4;      % 'dc' index chang
v=1;               %  fringe visibility                                                                            
lamda_D=1558e-9;   %  designed wavelength
L=1e-3;            %  grating length
Period=lamda_D/2/Neff;
lamda=[1535:0.1:1585]*1e-9;

for i=1:501
   k=pi*v*delta_N/lamda(i);
   Beita=2*pi*Neff/lamda(i);
   delta=2*pi*delta_N/lamda(i)+(Beita-pi/Period);
%  delta=2*pi*Neff*(1/lamda(i)-1/lamda_D)+2*pi/lamda(i)*delta_N;
   R(i)=(sinh((k^2-delta^2)^0.5*L))^2/((cosh((k^2-delta^2)^0.5*L))^2-delta^2/k^2);    % formula 22 from Erdogan Fiber Grating Spectra
end
initial_R=R;

plot(lamda,abs(R));
axis([1.535e-6 1.585e-6 0 1]);
title('Calculated reflection spectrum of uniform FBG');                                                                                              
xlabel('wavelength /m');                                                                                         
ylabel('Reflectivity');