[Eleonora, Yuhang]

Last week, after removing the qubig EOM, we took the chance to test two resonant EOMs from TAMA, in order to see if we could see a error signal at least with them. Unfortunately even with these EOMs we could not get any PDH error signal from OPO.

Some details:

1) The EOMs from TAMA are Newport 4003(damage threshold 4W/mm2, aperture D = 3mm, length = 5.5cm) resonant at 40 MHz and 76.18 MHz resplactively.

2) We did small modifications on the p-pol path to get a reasonable matching into the OPO. (We mainly moved the lens after EOM). The reflected beam from OPO looks quite astigmatic but it was supposed to be just a quick test so we didn't spent much time to optimize matching and alignment.

3) On the newport EOM manual we read that the beam into the EOM ideally should be collimated. This in not the case for us, as we have it focused into the EOM, but it shouldn't prevent to see some modulation effect.

As a general remark, we are puzzled by the fact that even if the setup is not optimized, we cannot see any trace of error signal. Of course TAMA EOMs are ten years old and there is no guarantee they are still working but it makes us to suspect that there could be something else wrong in the set up.