I make a plan to measure the open loop gain transfer function of the TTFSS.
I have to make sure that the TTFSS has a high bandwidth gain. It should be higher than 0.5 MHz to have a good frequency stabilized laser.
To measure the OLG TF of the whole loop, I have to:
1) Lock the laser to the cavity, inject the excitation at TEST2 EXC, measure the reference signal at common OUT2, response signal at common OUT1. This will take care almost all of the TFs of the entire loop. Don't forget to turn on the switch for the excitation signal on s4 (on the flat box).
2) Then I need to know the TF between common OUT2 and common OUT1. I can do this without closing the loop. By injecting the signal at TEST IN, measure the reference signal at common OUT1, response signal at common OUT2.
3) Add two measured TF together to get the TF of the whole loop. This TF will include both contributions from FAST and PC actuators because the reference points and response points are in the common path.
I'll have to make sure that the signal is not saturated during the TF measurement.