Since last week, we tried to set up a working telescope to be able to lock the filter cavity with the AOM.

Our design is the following : one lens (L1) before the AOM, two lenses (L2 and L3) after the AOM and before the Faraday isolator.

With the configuration f1=100mm , f2 = 100mm and f3 = 150mm we were able to align the beam into the filter cavity but we saw Laguerre Modes.

By amplifying 5 times the Pound-Drever-Hall signal we were able to lock the cavity but this lock was really unstable.

Indeed, either the input beam in the PR chamber was at a good size but the reflected beam from the IM was too big, either the input beam was too small and the reflected beam was at the good size.

Despite many attents to slightly move the lenses, we couldn't manage to obtain a better beam size.

We also check the beam size on the bench which confirm that this telescop can't work.

We then tried to use a f2=100mm f3=175mm configuration. But this telescop couldn't work neither. We think it was because the beam wasn't collimated at all.

In order to be able to use a bigger value of f3, we decide to move the AOM before the sterring mirror on the bench which allows us to have a bigger range for our new telescop. Indeed we mesure a beam size of 1.3mm just after the beam-splitter close to L1. Because L1 shrinks the size of the beam, we can have a small enough beam size for the AOM between this beam-splitter and the sterring mirror.

Then we choose a f2=100mm and f3=250mm configuration.

With this telescop, we were able to obtain a good beam size on the 2'' mirror and send the beam trough the filter cavity.

We obtain transmission peaks around 1V (better than from the first telescop) and gaussian modes. By amplifying 5 times the Pound-Drever-Hall signal, we could obtain a quite stable lock of the filter cavity on the green with the AO

We check the power exiting the optical table and found out that the AOM first order efficiency is now 67%. We are now trying to improve this value as well as the beam shape ( a bit astimatic) which might come either from the AOM or because the beam size entering the Faraday might be a little big.

We put back a camera on the IR path on the end room optical table.

Because the green power is quite smaller than before, we are able to see the IR modes flashes.

When the filter cavity is locked on the green, it seems that the IR flashes disappear and we can see a lock on some higher modes. We need to check if it is reamaining green or really IR.