I changed the attenuator to -1dBm. From a quick measurement, there's no improvement in the beat signal at high frequency. Any improvement in the band we might be able to see is dominated by acoustic and PLL. [more details later]
I increased the power to the 14.75MHz EOM from 0.5 dBm to 3.5 dBm. This was done by changing the attenuator from -6dBm to -3dBm.
As a quick test to see improvement in electronic noise level, I increased the detection gain. The modulation depth was changed from 0.14 to 0.2 rad (following the EOM's paramters in psl:855). I could not see any improvement yet. So I'll try to damp the mechanical peaks in the signal so that flat noise level can be measured better.
The slope of error signal for RCAV and ACAV are 44.25/57.62 kHz/V