calc_index.m

计算光纤模场分布和传播常数

function [Index] = calc_index(B,field,R,k0)
format long
clf
hold on
for nn=1:length(B)
    U=field(nn,:);
    j=0;sum=0;
    num=length(U);
    for i=2:num-1
        j=j+1;    
        Field(j)=U(i);
        dr(j)=R(i)-R(i-1);
        sum=sum+dr(j);
        r(j)=sum;
    end
    num1=length(r);
    for s=2:num1-1
        as(s)=(dr(s))^-2-1/(2*r(s)*dr(s));
        cs(s)=(dr(s))^-2+1/(2*r(s)*dr(s));
        bs(s)=((B(nn))^2*Field(s)-cs(s)*Field(s+1)-as(s)*Field(s-1))/Field(s);
    end
    as(1)=(dr(1))^-2-1/(2*r(1)*dr(1));             cs(1)=(dr(1))^-2+1/(2*r(1)*dr(1));
    as(num1)=(dr(num1))^-2-1/(2*r(num1)*dr(num1)); cs(num1)=(dr(num1))^-2+1/(2*r(num1)*dr(num1));
    
    bs(1)=(B(nn))^2*Field(1)-cs(1)*Field(2)-as(1)*U(1);
    bs(num1)=(B(nn))^2*Field(num1)-cs(num1)*U(num)-as(num1)*Field(num1-1);
    in=sqrt((bs+2./(dr.^2))./k0^2);
%     index(1)=in(1);index(num)=in(num1)
%     for ss=1:num1
%         index(ss+1)=in(ss);
%     end
    Index(nn,:)=in;
    plot(r(2:num1-1),in(2:num1-1));
end
hold off