# Coupling into FC/APC fiber

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:

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.

• That is curious. What's the numerical aperture of the lens? – garyp Jan 8 '16 at 15:09
• 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... – user103271 Jan 8 '16 at 16:12
• 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? – Carl Witthoft Jan 8 '16 at 19:11
• 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. – Carl Witthoft Jan 8 '16 at 19:15
• @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. – user103271 Jan 11 '16 at 11:54