I am trying to couple a 532nm laser with a 200 ns square pulse into an optical fiber (fused silica with damage threshold ~ 500 MW/cm^2). At lower intensities, < ~30 MW/cm^2, I see about what I would expect to see at the output. However, as I increase the energy, I see the development of a temporally-based attenuation in the pulse; this continues to get worse at increasing energy (pictures attached). The structure is very repeatable in time and seems the be insensitive to alignment into the fiber. I was curious if this is a behavior anyone is familiar with? And if there is a solution?
- There is no visible damage to the fiber upon inspection after running it at these energies.
- The transmission through the fiber is ~60%, regardless of the intensity (should be ~95%).
- The spot quality at the output of the fiber looks good. That is, we are not propagating much through the cladding, and the spot is fairly uniform in intensity.
- This is not a detector feature. If I move the detector closer to (or further away from) the fiber output I see the overall amplitude of the pulse change but the structure remains unaltered.
- If I switch to a shorter fiber (5 ft as opposed to 30ft) I am able to transmit more energy before this structure begins to appear