Here is a new plot for the differential noise measurement. I plot a noise contribution from the intensity noise (brown curve).

If we believe this data, the differential noise is NOT dominated by the intensity noise of the PSL.

(intensity noise coupling measurement)

 Here is a plot for the transfer functions (TFs) from the intensity noise DCPD to the beat signal.

   In principle these TFs tell us how much intensity noise are contributed into the differential noise.

When I measured the spectra shown above, the frequency offset of the beatnote was at about 8 MHz from the zero cross point.

Keeping the same lock, I measured the transfer function (red curve) by using the swept sine technique on DTT. The setup for this measurement is depicted on the last entry (#4389).

Then I made the spectra above by multiplying the intensity spectrum by this TF.

  Later I measured another transfer function when the beatnote was at about 2 MHz from the zero cross point.

According to this measurement, our MFD gets insensitive to the intensity noise as the beat offset goes close to the zero cross point. This is consistent with what we expected.