Hello, perhaps this answer is late, but it might help someone else.
I have tried your code and I don't know much about Fresnel diffraction, but I can only see one peak. You never provided a value for ii, but it doesn't seem to matter in my case.
Instead, to answer what I do know about, conv is producing a linear convolution. To convince yourself you can check the definition or use FFT with zero padding.
So,
n=5;
a = rand(1,n);
tmp1 = conv(a,a); %Matlab conv
tmp2 = ifft(fft([a, 0*a]).*fft([a, 0*a])); %FFT with zero padding = linear convolution
and the results are that tmp1(1:n) and tmp2(1:n) are identical. So, conv is thus convolving linearly, just like for a zero padded FFT.
