Does diffraction of a coherent laser beam affect its polarization state?

Does diffraction of a coherent laser beam affect its polarization state?
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nezivande0u
Yes, diffraction can lead to polarization effects. However, this requires moving out of the regime of scalar optics. Scalar optics is the regime when you can neglect the polarization of light and only treat its spatial complex amplitude.
Scalar optics can break down when the paraxial limit breaks down. This happens around when you introduce spatial features into your field which are close to as small as or smaller than the wavelength of light you are considering.
A good example of this is an extremely tightly focused Gaussian beam. Suppose the beam is propagating along the z direction. To use some language from Fourier optics. If the focus is so small it means the beam is composed of a very large range of wavevectors kx and ky. The plane waves corresponding to these large wavevectors can have a large angle with the optical axis. That means it is possible for those plane waves with large transverse wavevectors to have large components in the direction of propagation. This is not what is typically the case for paraxial Gaussian beams which are purely transversely polarized. At the focus the superposition of these polarization vectors can lead to surprising polarization patterns. See for example:
Polarization of tightly focused laser beams
You ask specifically about diffraction. Well just reverse the effect I have described above. Shine a transversely linearly polarized plane wave onto an opaque screen which has a hole on it with dimensions on the order of λ. What you will find is that in the vicinity of the hole the diffracted light will have non-transverse polarization.