I repeated the same measurements with another spectrum analyzer, the Agilent35670a

This instrument spectra are much cleaner, I did only 32 averages instead of 256 with the Yokogawa sa2400

First plot: gain 1000 in-loop pd: PM100D

Second plot: gain 1000 in-loop pd: DET10N

Third plot: gain 5000 in-loop pd: DET10N