We want a measurement of the fiber modes at either end, with the collimators, because these will be the modes that we'll be trying to match in order to couple light into the fibers, for FOL and/or future projects.
In order to measure these modes, I used the beam profiler (Thorlabs BP 209-VIS) to take measurements of the beam diameter (cut off at 13.5% of the amplitude) along the optical axis, for each of the fiber ends.
The ends are arbitrarily labelled End 1 and End 2.
For each measurement, the fibers were coupled to roughly 30%, or 25mW at the output.
Regarding the issue of free rotation in the collimator stages: while End 1 was relatively stable, End 2 tended to move away from its optimal coupling position. In order to correct for this, I chose a position where coupling was good, and repositioned the stage to that coordinate (124 degrees) before taking each measurement.
The data were then entered into A La Mode, which gave waist measurements as follows:
End 1--- X Waist: 197um at Z = 4.8mm Y Waist: 190um at Z = 13.6mm
End 2--- X Waist: 192um at Z = 7.4mm Y Waist: 190um at Z = 6.0mm
A La Mode code is attached in .zip file
These are the types of profiles that we will hopefully be matching the PSL and AUX lasers to, for use in frequency offset locking.
More characterization of the fibers is to follow, including Polarization Extinction Ratio.
We also hope to be testing the overall setup soon.