Polishing the optical fibers

Problem description

Imaging with one of the excitation arms had a much lower image intensity (lower peak of the histogram) even though the output power of both fibers was roughly the same at 561 nm (40 vs. 45 mW).

Diagnosis

After unplugging the fiber from the FC/APC adaptor on top of the shutters, the fiber output was directed against a white sheet of paper (This should only be done with proper laser safety equipment). Instead of a nice Gaussian beam profile, the beam is highly irregular. This is caused by dirt on the fiber tip. High laser intensities and static electricity can cause an accumulation of dirt on the output aperture of the fiber. In this case, the fiber should be cleaned and - if necessary - repolished.

Solution

The fiber tip was polished using Thorlabs LF03P 11" x 9" Calcined Alumina Lapping 0.3 µm Grit polishing sheets and a D50-FC/APC Polishing Disc. The polishing sheet was fixed on a CTG913 Glass Polishing Plate. If you have never polished a single-mode fiber before, please refer to the polishing manual available here

After polishing, the fiber output looked much more regular: