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.