The frequency shift between cc and p-pol is used for the coherent control in the future. At the same time, we need to control this frequency at a certain point with a proper value. This is pretty similar with the double control of infrared and green resonanting inside the filter cavity. In FC case, we use AOM. Here we use PLL to control this fixed frequency shift.

Today I set up the PLL and it works well.(see attached picture) Note here the singal is 1/10 of the signal. However, since we use PBS while we are aligning OPO, we need to take it out to make p-pol and cc both go to OPO. The reason is we will use cube BS in the future. We use cube PBS to simulate the optical path change by the substrate. Taking out cube PBS change the phase of beam quite a lot. It makes beam not resonate inside OPO. So I stopped today's experiment. We will wait the arriving of cube BS.