I am attaching a picture of the beam on a IR card as well as the tapered end.
This fiber, 50 um core 125 um cladding diameter, was tapered to 12-13 um cladding diameter, so core diameter became 5 um. The input beam is 1064 nm, randomly polarized, Gaussian with 1/e2 diameter of 5 mm, focused onto the fiber's untapered end by NA 0.22 10x objective.
I observed a similar doughnut shape with a 532 nm linearly polarized beam.
Maybe this is trivial and someone else who observed this already knows the answer.
Hi Elena,
this is due to the total reflection and refraction of the beam at the taper surface. The ring pattern pops up due to at least first (up to about third) order internal reflection within the taper before the beam is able to escape from the taper. So a lot of beams having a primary direction close to the fibre axis significantly are changed in direction and end up in the doughnut pattern.
In the case of the butt end fibre those rays would go straight forward and end up at the center area of the spot.
I observed a similar behavior on a home made PMMA light guiding device which converts a 5x5mm quadratic beam to a circular crosss-section of about 2,5mm in diameter via a tapered section. The doughnut pattern could be seen without any magnifying lense.
Thank you, this is a clear answer. So, now I need to make a taper with smaller angle, or a shorter one that has larger final cross-section.
Dear Elena
I guess you will taper your 50µm multimode fibre because you will couple the tapered end onto the front end of a single mode fibre.
The taper angle (cone-angle) should be as small as possible; so you have to have a very, very smooth transition of core cross-section.
But nevertheless you will have a large photon flux loss of about 100 [pi*50*50/(pi*5*5)] due to the ratios of the involved core areas.
When changing the cross-section the product of cross-section area A times the numerical aperture(NA) remains constant: A*NA=const.
The taper is used for mixing the modes of the multimode fiber in order to effectively excite the optical mono-mode in the small core fibre.
Sorry for this discouraging answer.
Please see the very long dicussion on this subject at the attached RG link.
The last answer (of M. Alsous) describes your taper approach, which seem to be successful at least for his case(s).
https://www.researchgate.net/post/How_can_i_achieve_efficient_light_coupling_from_multimode_to_single_mode_fibers
Thank you for these tips. I would like to say that I didn't observe large power loss from a tapered fiber compared to a blunt one. However, I do not know if all that power is emitted from the core, not from cladding.
I am not coupling light into an SMF. I just need a single mode beam, and I had a hard time coupling the laser energy from free space directly into an SMF.
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