[Matteo, 2*Eleonora]

We tried to check the effect of CC loop on the squeezing.

While bringing up the whole bench we had some trouble acquiring and keeping the lock of the CC PLL.

By modulating the green phase (10 Hz, 1.5 Vpp) we obtained a nice error signal for CC in reflection from OPO. Since the amplitude of such signal is maximum when the parametric gain is maximum we used it to find the optimal freqency offset for the p-pol PLL lock. It is 170 MHz and the correspondent CC error signal amplitude is ~ 65mV pp.

We centered the error signal around zero adding an offset and stopped the green phase modulation. See pic1.

We closed the loop (acting on the green phase shifter) using two stanford SR560 in series with the following parameters:

With only one SR560 we were not able to make it work.

We checked the CC error signal from the homodye: it shows a strange, not sinusoidal shape (even after checking and optimizing alignment). See pic2. We tried to close the loop on the LO phase shifter with one SR560 but we didn't succeed.

We measured squeezing at zero span at 200 kHz with the CC loop on the green phase closed and scanning the LO phase. Even if we saw the usual McDonald shape modualtion in the shot noise,  its value was much higher than the shot noise level reference without squeezing.  This is probably due to the CC beam reaching the homodyne. We will investigate the possibility to reduce its power.