I have coupled a Thorlabs HNL050L-EC – HeNe, 632.8 nm, 5 mW, Polarized Laser to a 2 meter long single mode fiber patch chord using a Thorlabs F230-FC-B aspheric lens. While I am certainly able to obtain a pure single mode Gaussian at the output, the total output intensity seems to be fluctuating over time scales of about a second. In some sense, the mode appears to be "breathing". The aspheric lens has been mounted on a stable mount, and the fiber is at the correct wavelength. I have also verified that the fluctuations are over and above the intrinsic fluctuations from the laser itself. Has anyone had this issue before? If so, what is the cause and what could be the best way to work around it to get a stable single mode Gaussian output?
P.S Please drop a comment if you require any further details to diagnose this issue.
Best Answer
My bet is that your fiber is very short (something like one meter or so) and that the fluctuations you see on the output mode are due to cladding modes, i.e. a part of the injected light propagating into the cladding of the fiber instead of the core. The resulting fluctuations are due to external perturbations of the fiber (thermal fluctuations or you touching the fiber and stuff). Usually these modes are attenuated over long distances but I've already had this problem when using short fibers.
Try to shake your fiber or heat it up with your hands to see if it's doing something on the "breathing" that you see. If so then it could be those cladding modes. They should disappear when using a longer fiber though (for instance 5 meters).