[Yuhang, Eleonora, Matteo]
The goal is to lock the OPO using P-pol beam
Preliminar information: The p-pol beam is modulated at 88 MHz (with resonant Qubig EOM). Because of the lack of space, the beam had to be focuesed inside the EOM and thus the power sent to it has to be reduced. Currently we have 5.5 mW reach the OPO and this power cannot be increased without changing the EOM telescope configuration.
After some difficulties due to a wrog setting of the gain of the photodiode in trasmission, we managed to align also the p-pol beam. (see pic1. left)
S-pol is also reasonably well aligned. (see pic1. right)
The trasmitted power for p-pol is about 10 uW while the input is power is 5.5 mW. The transmissivity for p-pole is about 0.18%. For the s-pol we found 0.25% and the nominal is 1.2%.
[Note that since the cavity has not be locked, we made the measurent by manually driving the piezo in order to bring the cavity on the top of 00 resonance. It may be not very accurate.]
After that we installed the locking set up but we couldn't find any PDH signal from the PD in reflection.
Some tests we did:
1) We ruled out that it is a malfunctioning of the PD, as we checked that it was able to sense PDH signal from the green mode cleaner. (But note that responsivity for IR is more than a factor two smaller)
2) We double check the alignement of the PD.
3) We lowpassed the signal with a SR560 to get rid of some high frequency noise, but it didn't help
4) We check the signal coming from the PD(before demodulation) with a spectrum analyzer but we couldn't find any line at 88 MHz. So we know that the problem doesn't come from the demodulation, but it's present before.
5) We checked that the driving signal was reaching EOM.
6) We checked that the EOM orientation was the one required by the input beam polarization
7) We tried to reduce the modulation depth but we coudn't see any change in the amplitude of the TEM00 peaks in trasmission.
Some hypotesis:
1) The EOM may not work. How to check it?
2) The signal on the lock PD may be too weak? We send it 200uW ( which should be fine) but the responsivity at 1064nm is small (see attached PD datasheet)
