Fraunhofer diffraction simulation in matlab

Hello,

I have simulated how light would diffract by going through a phase mask. Below is the code.

lambda=0.6*10^-6; % wavelength, unit:m
delta=80*lambda; % sampling period,unit:m
z=2; % propagation distance, unit:m
M=512; % space size
c=1:M;
r=1:M;
[C, R]=meshgrid(c, r);
THOR=(((R-M/2-1).^2+(C-M/2-1).^2).^0.5)*delta;
%OB=zeros(M); % Object
%OB = continuos_phaseshift;
FD=fftshift(fft2(fftshift(exp(1i*continuos_phaseshift))));
FD=abs(FD);
FD=FD/max(max(FD));
C=C*lambda*z/M/delta*1000;
R=R*lambda*z/M/delta*1000;
figure; mesh(R, C, FD);
figure; imshow(FD);

Now, I get some output of what the diffraction looks like at 1 meter from the diffracting surface. But now I want this new ouput to go through the same phase mask again.

Does anyone know how to achieve this?