The IMC REFL error signal was measured to compare it with the other spectra (if we have).

The blue curve is the in-loop IMC error and the red is the dark noise. So they are not an apple-to-apple comparison. But the red noise is going to be suppressed by the loop, and still the red is below blue. This means that the blue curve is the measured noise rather than the readout noise.

We suspect that the current issue is the PC drive saturation (as usual). Does this indicate that the laser freq noise is actually increased?

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Another suspect was that the degradation of the LO level. We used to have the issue of slowly dying ERA-5 (ERA-5SM indeed). The RF levels on the demod board were measured using an active probe.

The LO input: 0dBm, ERA-5 input: -2.7dBm and -2.1dBm for I and Q. I found that the outputs of the ERA-5SM were +10.5dBm and +10.6dBm.
This lead me to replace the chips but the situation was not changed. Then I realized that the LO levels should have been measured with the load replaced from the mixers to a 50Ohm load. Somehow these mixers lower the apparent LO levels. So I decided to say this is OK.