Sarah and I are trying to remeasure the coupling from RIN to frequency noise. Here we list the problems that might cause the error in the measurement.
Before measuring anything, we aligned the beam to ACAV and RCAV , adjusted the power so the power to ACAV is 2mW and RCAV has 0.2mW.
We found out that:
Since the TF remains the same at lower frequency, we thought that the gain of ACAV loop cannot keep up with the frequency noise and cause the deviation in the measure TF, so we checked the UGF of ACAV with different power level (*this is done roughly for a quick check, we will come up with more careful analysis later).
The measurements above were done with an extra ND filter that reduce the power on RFPD by a factor of 2. The shape of the TF is roughly 1/f. The UGF is significantly higher than the interested frequency (DC-1kHz), so we think it is not the insufficient gain in ACAV loop.
So we change the gain in RCAV loop, by increasing the power to 0.4 mW. With this level to RCAV, the TF does not change with power to ACAV that much, unless the power goes down to ~0.4mW. On the other hands, the TF(RIN coupling) also changes if we reduce RCAV gain to certain level. We will investigate more about this.
I tried to remeasure the coupling from RIN to frequency noise (same setup as in psl:1005). However I have not been able to reproduce the result yet.
The result we got in psl:1005 does not have the same TF shape as we expected from the calculation. The slope is off by 1/f^0.25. So There might be something wrong with the setup, for example bad alignment in EAOM that adds PM to the beam and effect the PDH signal. So in order to verify the problem I redo the measurement with the similar setup. This time, however, when I change the power into ACAV, the measured TF changes as well (this did not happen before), so I'm checking what's wrong with the setup.
Checking plan:Once I can remeasure the TF (ACAV AOM feedback/ ACAV TRANSPD). I want to check