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I am currently studying laser interferometry. I read that we can describe the spot size at the target as (1) a diffraction-limited value with a focusing objective or as (2) a free space propagation value with some divergence $\theta$. In this context of spot size, what is meant by a 'diffraction-limited value with a focusing objective' and a 'free space propagation value with some divergence $\theta$'? My particular interest is in laser diodes, so I would appreciate an answer that is also within that particular context (if that changes anything).

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  • $\begingroup$ For imaging, diffraction limited has to do with interference patterns (Airy discs) caused by the aperture (which is like a slit causing diffraction). No idea if that helps as you asked for another context... $\endgroup$ Jul 6, 2021 at 13:02
  • $\begingroup$ @CharlesTucker3 So I guess the aperture in the case of a laser diode would be the aperture of the laser diode chip? $\endgroup$ Jul 6, 2021 at 13:05
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    $\begingroup$ Yes, I would expect either the diode itself or some optical component (shade, aperture) limiting the beam, depending on the experiment. (But unfortunately without knowing it, just guessing...) $\endgroup$ Jul 6, 2021 at 13:13
  • $\begingroup$ @CharlesTucker3 Ok, thanks for the insight. $\endgroup$ Jul 6, 2021 at 13:14

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"Diffraction-limited" means the spot size you calculate given the wavelength, aperture, and focal length. This is the ideal minimum, assuming a perfectly flat wavefront and so on.

Then, once you get close to this limit, there's no point in designing a lens whose geometric raytrace indicates a smaller spot size at the point of minimum confusion (aka smallest encircling spot for all incoming rays from a point source at infinity on the optic axis).

Free-space divergence angle typically means the plane angle (not solid angle) over which the beam is spreading, either due to noncollimation or self-diffraction. In the case of laser diodes, the raw output is elliptical and you'd expect to measure two different angles at the two orthogonal axes. If the diode contains a correcting lens, you may have a "round" rather than elliptical beam profile.

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  • $\begingroup$ Hmm, what exactly is the "plane angle"? litron.co.uk/wp-content/uploads/2019/06/Beam-Divergence-Dia.jpg $\endgroup$ Jul 6, 2021 at 15:08
  • $\begingroup$ system (in)sensitivity may depend on the spot size of the geometric ray trace, so designing for smaller than diffraction limit is not necessarily useless $\endgroup$
    – hyportnex
    Jul 6, 2021 at 15:28
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    $\begingroup$ @ThePointer I mean the angle of divergence in a specified plane which contains the optic axis $\endgroup$ Jul 7, 2021 at 16:00
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Parallel rays of light (as from a very distant point source), after passing through a lens (or reflecting from a parabolic mirror) will ideally be brought to focus at a point. Actually, they form a circular aperture diffraction pattern ( a circular spot at low intensity which may show concentric rings if brighter). A larger mirror gives a smaller pattern. If you have two point sources very close together, their spots may overlap so much that you cannot tell that there are two different sources. Supporting structures in a telescope may also contribute to the diffraction pattern giving it "spikes".

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