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We are currently setting up an experiment in which we need to pipe laser light from one optical table to another. One way we were thinking of doing this was by coupling into an optical fiber, running it to the second table, and then recollimating the light using a lens. However, if we then wanted to focus that light down again onto a sample, would our spot size be limited by the core diameter of the fiber we used since it would be in effect acting as an extended point source? E.g. if we use a 100 micron fiber to source the light, would our spot size for focusing onto a sample also be limited to 100 microns?

Thanks for reading!

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    $\begingroup$ There's no reason in physics why this isn't possible in principle. In practice, getting good results will depend on having just the right lenses available. $\endgroup$
    – The Photon
    Commented Mar 4, 2020 at 17:41
  • $\begingroup$ To clarify, are you saying that 100 microns is the limit for our focused spot size (if we use a 100 micron fiber optic cable), and whether we can approach that limit depends on the quality of our lenses? Or are you saying that we should be able to focus smaller than 100 microns as long as we have good quality optics? $\endgroup$
    – bbsmitz
    Commented Mar 4, 2020 at 17:59
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    $\begingroup$ No, I mean you should be ale to de-magnify, but you'll need well-chosen optics to do it (particulary without aperture losses). If you were starting with 9 um fiber it would be more difficult to de-magnifiy (but still possible in principle). $\endgroup$
    – The Photon
    Commented Mar 4, 2020 at 18:15
  • $\begingroup$ Sorry again for not fully understanding. In your parlance, is 'de-magnify' equivalent to collimating the beam? $\endgroup$
    – bbsmitz
    Commented Mar 4, 2020 at 18:24
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    $\begingroup$ You don't want to collimate your fiber output, you want to image onto some target. The easiest way to do that is probably collimate the output from the fiber, then use a 2nd lens to focus it onto the target. But the diameter of the collimated beam is not particularly relevant to the size of the final focused spot. $\endgroup$
    – The Photon
    Commented Mar 4, 2020 at 18:30

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The spot size of your system will be defined by the diffraction limit which means numerical aperture (a flavor of beam divergence) and wavelength will be determining factors. Your NA is defined by the fiber, yes, but can be manipulated with lenses. It is very possible to increase your NA by inserting diverging lens systems. However the more lenses you include, the lower you beam quality becomes, you get uncertainty from back calculating through lens magnification, abberations, etc. So in principle yes, you can VERY easily go below 100um in spot size. But if you need it to have an M-squared of certain quality or a pointing/centration below a certain value, then it will require trade-offs.

TLDR: You can do it, but you won't be able to retain beam quality. It will be a balance of tolerances at the end of the day.

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You will have better luck if you can couple into a single-mode fiber. In that case, you will get a spot of minimal size, maximal intensity. A larger (100 micron) fiber will definitely detract from beam quality and reduce the spatial coherence of the light, so that the beam cannot be well collimated nor focused to a spot much smaller than 100microns without throwing light away.

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Keep in mind the conservation of luminance (or law of brightness) when you consider such problems. The answer of using a microscope objective to couple the laser light into a single-mode fiber is the best. If you couple light into a multimode fiber, the output will contain mode-mode interference and will not have nearly the beam quality as the single mode fiber. If you want you could one-to-one image the fiber end onto your sample on the other table.

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