[Yuhang, Eleonora]

We designed and implemented a new telescope for the EOM.  The goal was to increase the allowed power of the p-pol beam and find a reasonable PDH signal. 

We followed the recomandation of the seller from Qubig which suggests to use a beam of 1 mm diameter inside the EOM (the crystal side is 3mm). 

As input, we have a collimated beam ( 2 mm diameter) and we want to recover a beam with this same size after the EOM, before it recombines with the s-pol beam. In order to do so, we have  simply put two lenses with  f = 200 mm at a distance of 40 cm from each other and place the EOM  at a distance of about 10cm from the first lens, where the beam diamater is between 1 mm and 0.7 mm (see Fig1, 2  and the updated the optical layout in the wiki)

For our laser power, that is 170 mW, the power density (considering diameter of 0.7mm) is about 1W/mm^2  which is less than the treshold of the EOM (10W/mm^2).

We have installed all the optics (except for the EOM which is not yet back) and aligned and matched the beam into the OPO. In parallel we have also recovered the alignment of the s-pol beam.

Pic 3,4,5 show respectively the OPO optical spectum with p-pole beam, s-pole beam and both beams. The heighest HOM are LG modes wich are not easy to get rid of. Anway the final aligment will be done once we have also the EOM.

We repeted the transmissivity measurement.

Some comments:

1) Since we cannot lock the OPO, in order to make the mesurement, we manually adjust the piezo offset to bring the cavity on the top of the TEM00 resonance, this can affect the precison of the measurament

2) The nominal trasmissivity shoud be 1%, while we found a value that is 5 times smaller.  Note that the matching (in particular for the p-pol) is not optimal.

Conclusion: The new design of the EOM telescope seems fine and in this new configuration we should be able to increase the power on the p-pol and get a detectable PDH error signal for the OPO.