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:

1. EAOM alignment does not change the measured TF. We misaligned the EAOM and measured the TF. There was no significant change in the measured TF.
2. The measured TF does not change with excitation level (from SR785). With output level of 1-5Vpkpk, the measured TF remains the same.
3. However, the TF changes if we change the power to ACAV. The measure TFs with different power level differ at high frequency and they converge at low frequency depend on the power level. For example, if we choose RFpower on Marconi to 12 dBm, the measured TF will be higher than the TF with 13dBm power ~ 2dB at 1kHz, and converge around 300Hz.

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.