We are limited by the intensity noise of the X arm transmitted green light.
Since the intensity noise from the PSL wasn't big enough to explain the differential noise (#4392), so this time I measured the noise contribution from the X arm transmitted light.
I performed the same intensity noise coupling measurement, but this time between the DC signal of the beatnote RFPD and the beatnote signal.
While measuring it, I excited the intensity of the PSL laser by using the same AOM like I did yesterday. This AM cause the observable intensity noise on the beatnote RFPD.
With the excited AM, we can pretend to have an excited AM on the green transmitted light from the X arm, of course assuming the intensity noise coupling from the PSL is less.