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I am currently attempting to couple a 405nm laser diode into FC/APC fibre. The fibre is mounted on a flexure stage approximately 15.3mm from an f=15.29mm lens. I have managed to couple into the core but I am getting a large, asymmetric cladding mode that is not being suppressed by the fibre coating. Here is a beam profile of the output of the fibre after collimation:

enter image description here

I have attempted swapping the ends of the cable around and the effect stays. The cladding mode persists at the same relative intensity to the core mode no matter how I translate the fibre entrance with respect to the focusing lens.

Does anyone have any ideas why this may be occurring and any simple solutions on how to suppress it? Stripping back the coating and applying a cladding mode stripper is ideally a last resort option.

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    $\begingroup$ That is curious. What's the numerical aperture of the lens? $\endgroup$ – garyp Jan 8 '16 at 15:09
  • $\begingroup$ I'm using a 405 nm, f=4.02 mm, NA=0.60 FC/APC fiber collimation package. The lens used to launch into fibre has an NA = 0.16. Oddly enough, I've just measured the beam profile without the collimation package and it looks distinctly Gaussian... $\endgroup$ – user103271 Jan 8 '16 at 16:12
  • $\begingroup$ What kind of fiber is it - i.e. core diam, polarization maintaining, etc.? Do you have a fiber-coupled source you could test (with a fiber-fiber coupler) to make sure this isn't a problem with the fiber's radial structure itself? Also, do you know the modal structure of the free-space beam? $\endgroup$ – Carl Witthoft Jan 8 '16 at 19:11
  • $\begingroup$ ONe other random thought: APC means the fiber face is at an angle other than $90^{\circ}$ to the core, so you may be prisming light into the cladding right there. I don't remember whether that is a known effect or a known-not-to-happen effect. $\endgroup$ – Carl Witthoft Jan 8 '16 at 19:15
  • $\begingroup$ @CarlWitthoft It's just a single-mode fibre (not PM). It's a good idea to test another source. I have tested the free-space beam and it is distinctly Gaussian. This suggests it's a problem with the fibre collimator. $\endgroup$ – user103271 Jan 11 '16 at 11:54
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Quote is taken from: https://www.linkedin.com/pulse/whats-difference-between-upc-apc-connectors-cheer-chen

APC connectors feature a fiber end face that is polished at an 8-degree angle. With UPC connectors, any reflected light is reflected straight back towards the light source. However, the angled end face of the APC connector causes reflected light to reflect at an angle into the cladding versus straight back toward the source.

Can you dip the opposite end of the cable into index matching fluid to suppress reflections off of the opposite end of the cable? (Vegetable oil might work for this.) If the cladding mode disappears or is significantly suppressed, then using an APC connector is a likely culprit.

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  • $\begingroup$ Using multiple SE accounts is allowed in the SE network until there is no misuse, for example you don't vote for yourself. But in most cases you don't need it after a successful registration and it makes everything only more complex. For example, you have to wait the reviewers until they approve your change on your own post. Doing the same with your primary account, you could have edited your own post any time. $\endgroup$ – peterh says reinstate Monica Jan 30 '17 at 5:36

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