I'm measuring the TF between ACAV (ref) and  feedback to VCO (response).

The TF looks ok, but I still need to verify by increasing the laser power and see if the TF goes up with the same factor.

We are determining the coupling coefficient from intensity to frequency change via thermo-optic effect.

Using a swept sine measurement with 4000 integration cycles, from 10Hz to 100kHz, to measure the TF 

between A) ACAV RCTRANSPD (ref) and B) feedback to VCO(response), we can see some reasonable results.

I tried 2000 integration cycles with feedback to PZT, but it seems the laser is too noisy. I might try longer integration time later.

To make sure that the measured TF is real, we must check that when the power to the cavity is increased, the magnitude of

the TF should increase by the same factor. 

Once we have the TF between ACAV_RCTRANSPD and VCO feedback, we can calculate how RIN would couple in to frequency noise by

Thermo optic noise (RIN) [Hz] = VCO calibration [Hz/V] x TF between VCO feedback and ACAV_RCTRANSPD [V/V] x

                                                    VCO TF [V/V]    x RIN x DC level of ACAV_RCTRANSPD

Note the TF of the VCO box (VCO TF) is measured and fit. One pole is at 1.25 Hz, and one zero is at 38 Hz. see fig 1.

 VCO's calibration for V to Hz can be found here, The temperature is quite stable and VCO_MON has been around -0.7 to -0.3 V, we can approximate to be 1.4 MHz/V for now.