We have measured the open-loop transfer function of the Y-end green PDH loop. From the measurement, the loop UGF is ~12kHz.
We have been trying to measure this transfer function for some time now, and playing around with various points of injecting the excitation and measuring the output. Koji helped arrive at one that actually worked, and the scheme used to make this measurement is shown in the sketch below. The SR785 signal analyzer was used to make the measurement, while an SR560 preamp was used to sum the output from the PDH box (PZT-OUT) and the excitation, with this sum being delivered to the auxiliary laser PZT via a pomona box that sums the servo output and the signal from the LO. The transfer function measurement made was a1/a2 w.r.t the sketch attached.
Set-up to measure Y-end Green PDH transfer function:
Measured Open Loop Transfer Function: