use "Angular-Spectrum Propagation" to calculate intensity distribution at focal plane of a lens, but the outcome is not only a central spot, but accompanying with some "diffraction order" as shown below:

clear,clc

lamuda=550e-9;k=2*pi/lamuda; radius=2e-3;diameter=2*radius; foc_length=400e-3;

L=255; x=linspace(-radius,radius,L);delt=x(2)-x(1); [x,y]=meshgrid(x,x);

E=exp(-i*k*(x.^2+y.^2)./2/foc_length); intensity=angular_spectrum(E,delt,L,foc_length); figure(4),mesh(x,y,intensity)

%% This is a user-defined fuction:

function intensity=angular_spectrum(E,delta_x,L,z) % This function compute focal spot intensity distribution, using angular % spectrum methord.

% *E is amplitude of light field after lens. % *delta_x is sampling interval of pupil or lenslet. %% System parameters i=sqrt(-1); lamuda=551e-9;k=2*pi/lamuda; % wavelength of light.

%% Generation of Fourier transfrom of propogator fx=1/2/delta_x*linspace(-1,1,L);fy=fx; [fx,fy]=meshgrid(fx,fy); r2=fx.^2+fy.^2; Fh=1/i/lamuda/z.*exp(i*z*k.*sqrt(1-lamuda^2*r2)); % Fh is fourier transform of propogator.

%% Generation of Trasnform of diffract spot FU0=fft2(E); % fourier transform of optical amplitude.

Fh=ifftshift(Fh); FU=FU0.*Fh; % FU is transform of light amplitude at observatory plane. We use this equation because amplitude on observatory plane is convolution of pupil amplitude and propogator. U=ifft2(FU); intensity=abs(U).^2; end

[Image Analyst]

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http://www.mathworks.com/matlabcentral/answers/8626-how-do-i-get-help-on-homework-questions-on-matlab-answers

Answer 1

[Han St]

Hi. The function for angular spectrum propagation needs to apply the same wavelength as you use for creating the initial field...I suggest using lamuda as an additional input constant for the function: intensity = angular_spectrum(E,delta_x,L,z, lamuda).