initial commit

2026-09-18 00:09:10 +09:00 · 2026-08-07 15:56:42 +09:00
commit 91ad25aca9
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+% FUNCTION  [u_1, H, h, dH] = near_field_evolution(u_0, z, lambda, extent, use_ASM_only)
+% Description: nearfield evolution function, it automatically swithch
+% between ASM and Fraunhofer propagation 
+
+function [u_1, H, h, dH] = near_field_evolution(u_0, z, lambda, extent, use_ASM_only)
+
+
+    H = [];
+    h = [];
+    u_1 = [];
+    dH = [];
+        
+
+    if nargin < 5
+        use_ASM_only = false; 
+    end
+    
+    extent = extent(:)' .* ones(1,2);
+    if z == 0
+        H = 1;
+        u_1 = u_0;
+        return
+    end
+    if z == inf
+        return
+    end
+
+    Npix = size(u_0);
+
+    xgrid = (0.5+(-Npix(1)/2:Npix(1)/2-1))/Npix(1);
+    ygrid = (0.5+(-Npix(2)/2:Npix(2)/2-1))/Npix(2);
+
+    k = 2 * pi / lambda(1);
+    
+    % Undesamplling parameter
+    F = mean( extent.^2 ./   (lambda(1) .* z .* Npix ));
+        
+    if abs(F) < 1 && ~use_ASM_only
+        % farfield propagation 
+        warning('Farfield regime, F/Npix=%g', F )
+        Xrange = xgrid*extent(1);
+        Yrange = ygrid*extent(2);
+        [X,Y] = meshgrid(Xrange, Yrange);
+        h =  exp(1i*k*z +1i*k/(2*z) * (X'.^2 + Y'.^2));
+       
+        % this serves as low pass filter for the far nearfield 
+        H = ifftshift(fft2(fftshift(h))); 
+        H = H / abs(H(end/2+1, end/2+1)); % renormalize to conserve flux in image 
+    else
+        % standard ASM 
+        kx =  2 * pi .*xgrid / extent(1) * Npix(1);
+        ky =  2 * pi .*ygrid / extent(2) * Npix(2);
+        [Kx, Ky] = meshgrid(kx, ky);
+        
+        dH =  ( -1i*(Kx'.^2+Ky'.^2)/(2*k) );  
+        
+        H = exp( 1i*z*sqrt( k^2 - Kx'.^2-Ky'.^2));  % it make it a bit more sensitive to z distance
+        h = [];
+    end
+    
+    u_1 = ifft2( bsxfun(@times, ifftshift(H), fft2(u_0)));
+end
+