[Yuhang, Eleonora]

While waiting for the quibig EOM to be repaired, we have tried to estimate the ampituede of the PDH signal expected both in reflection and trasmission of the OPO.

We used the following paramenters:

R1:  99.975%  (crystal)

R2:  92%         (incoupling mirror)

INPUT POWER: 6 mW

MOD. DEPTH: 0.3 

PD GAIN tot: 16e2 [V/W]  (photosensitivity at 1064:  0.1 [A/W], amplification:16e3 [V/A])

BS before OPO:  R = 4% T= 87% (p-pol)

We did a simulation with Finesse (see attached plot) and confirm it with analytical computation. (We will upload the code on the wiki ) 

Conclusions:

1) The PDH signal is larger in transmission (due to the fact that in this configuration the cavity is undercoupled) 

2) The PDH signal is anyway very small (less than 10 mV pp) with the current values. 

It is for sure convenient to lock the cavity in transmission (as done in Virgo1500 and GEO) but we should also consider how to increse the signal. Some possibilities:

1) Increase the input power (change EOM telescope to increase allowed power? How much power is used in GEO and Virgo?)

2) Increase the PD gain (the current one has low photosensitivity for IR) 

3) Increase the modulation depth (is it feasable?) 

Note that the simulation has been done considering nominal reflectivity values for OPO but we observed that the transmission is lower than expected (0.18% instead of 1%), this may result in a discrepancy between the  simulation and the real signal.