This afternoon we continued the work of last night on SQZ BAB alignment. The issue with OPO locking was solved during the morning, and we had a stable bright beam from the squeezer.

We started by moving the folding mirror underneath the SDB1 Faraday isolator, in order to bring the beam heigth on the external squeezing bench to the nominal value (110 mm). We did -1850 steps in horizontal and +1450 steps in vertical with the corresponding picomotor

We then repeated the mirror steering to superpose the two beams (BAB and ITF) on two diaphragms on the sqz bench at our best, i.e. with the beam centres overlapped within about 0.3 mm over a distance of 2.5 m.

Then we started looking for a spot of the BAB on the B1p camera. On first we unlocked and misaligned the ITF arms, keeping only NI aligned, and then we made an angular scan of NI by  +-0.5 mrad in both Tx and Ty, but we did not observe the BAB spot on B1p camera.

So we switched off the ITF beam by unlocking and misaligning the IMC, and we found that the BAB  was perfectly overlapped with the ITF beam so that the spot on B1p camera was hidden beneath it.

Then we made a scan of the OMC with only the BAB present, from which we found no significant power in the TEM00 mode.

We finally made some beam tweaking with a walk-off of the last two steering mirrors on sqz bench, to improve the OMC matching. However we lost the alignment before being able to make another OMC scan. At 12:45 we decided to postpone the matching improvement to another shift in the next future.

We leave a beam dump in front of the sqeezer as usual.

In conclusion, the pre-alignment with two diaphragms on external sqz bench is a reliable and fast method to align the sqz beam on the ITF. Since we expect the ITF alignment to evolve during commissioning, we leave the two diaphragms installed on the bench, although one of them shall be moved to a different position when the new Faraday isolators will be installed.

N.B. we found that the camera looking at SDB1 from the East viewport does not respond. We tried to reset it by repeatedly unplugging and replugging the etherned cable without success.