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I understand the requisites to couple a laser beam into a single-mode fiber, including mode-field diameter and numerical aperture. However, when trying to couple diffuse light into a single-mode fiber, I guess a very low component of the field will fulfill the requirements for propagation when the fiber is put in "direct contact" with the source.

What role can a coupling lens or lenses play in getting more light into the fiber? Could I get more light than from butt-coupling? Would that be only because I could catch light from a larger area and focus it on the fiber?

I am working with light coming out of a diffusive medium. If I could get more light even from a bigger area, I would be interested in not catching more than one speckle.

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Butt coupling will give you the easiest practical result. But also beware of cladding modes, or modes that propagate a short distance.

If you don’t really need single mode at the output, you will collect significantly more light even with a few more modes. But if a coherent source those modes may interfere.

The single fiber core is quite small, 8-10 um, Multi mode fiber is often 50 or 62.5 um in diameter so that should be smaller than your speckle pattern in most situations I think.

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  • $\begingroup$ Yes, I am interested in a single mode at the output, so that there is no decorrelation going on in the fiber (that's also the reason not to get more speckles). The problem is I need more power coupled into the fiber than what I get with butt coupling. How can I do this? (This is what I thinking when I wrote "best way" in the question, i.e. more light) $\endgroup$ Commented Aug 3, 2022 at 8:55
  • $\begingroup$ Without knowing the details of the setup I think you are limited by the etendue. energetiq.com/etendue-and-optical-throughput-calculations . The problem is single mode really is single mode . Having a antireflection coating on the fiber tip or index matching the fiber tip helps a tiny bit. If you can modulate or chop your light source that can help by allowing you to use a better signal recovery technique. Same with choosing a light source that matches your detector response. Or chose a better detector and doing photon counting. $\endgroup$
    – UVphoton
    Commented Aug 3, 2022 at 11:53
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    $\begingroup$ To get an idea if lens would help you could set up a spatial filter with a pinhole and see how much light you get through a pinhole similar to the diameter of your fiber core. Also if you really want single mode at the output of your fiber you may want to use a long fiber and mode scramble the fiber to lose the higher modes. $\endgroup$
    – UVphoton
    Commented Aug 3, 2022 at 11:58

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