I have a setup which includes a 637 nm toptica iBeamSmart laser which is reflected off two mirrors, passes through a line of dichroics (that should completely transmit it, they are used to couple other lasers) and then hits the entrance of a single mode optical fiber. When measuring the coupling ratio with a thorlabs photodiode, I came across some odd behavior: for some power settings of the laser, i measure more power behind the fiber than at the laser exit or in front of the fiber. You can see my measurements in this plot (all values in mW):
Now, since I took these measurement series at different times, at first I assumed the laser was just not accurrately setting its output power. However, what I tested afterwards was leaving the laser power untouched, simply moving the photodiode back and forth from the laser exit to the fiber exit and the measurements matched what I saw before. I can't come up with any explanation as to how the power could be higher behind the fiber.
I have done these same measurements with the other 4 lasers in my setup (405, 488, 532 and 568 nm) and reached more standard results (i.e) coupling ratios of 50-70% for all laser powers.
I actually took a video of my setup while doing the measurements without touching the laser controls so you can check out my setup if you would like. The video
Obviously laser power can not be created from thin air, but I really would like to know how this is possible or if this has been observed before.
EDIT: I tested the power range where this is happening more thorougly, as some comments suggested: (I also tried to observe the beam profile at different power levels but it just always looks round to me. I don't have a way to measure the spatial distribution of the beam behind the fiber)
Some notes on this: This is entirely new data, and I measured at the same laser settings as before as well as at some more power settings.
Lowering the power steps even more is basically impossible as there are some power steps where clearly the laser did not change output, as both the pre fiber measurement as well as the post fiber did not change at all. I still can't think of a convincing reason for these measurements.