Found the possible reason for high frequency oscillation

The first check we did today is to install a PD on the after the first BS mirror to look at the infrared beam came out from the laser,so the first picture is the error signal when the output power of the laser about 500mW, and the second is the signal from the new-set PD which shows that the infrared signal from the laser somehow also has the high frequency oscillation. And the situation are same with or without the green.

Then we increased the power a little bit, do the alignment again, then the error signal and the infrared signal shows in the third picture. There is a flashing spot in the infrared signal.

But the changing part is when we lock the cavity, the 500mW situation has the oscillation also(picture 4), but the second seams not.(pic 5)

According to this circumstance, we at least got one conclusion is that the high frequency oscillation has nothing to do with the green.

Then we rethought the polarization again, found out no matter s or p polarization reflect back, it is should stopped by the Faraday Isolator, but now it seems not.

We decided to check the polarization again from the beginning. Firstly one thing for sure is that, the input infrared beam to the cavity is in p polarization, and the reflect infrared beam is also p polarization.

Then we try to put another FI and half wave plate after the BS mirror and before the telescope to cancel the reflect infrared beam from the cavity. We use the same FI as we used before, but found out the two polarization cube point to different direction of the original one. Then the new put FI did reflect out the reflection beam, and get rid of the oscillation.

Then we guess that the original FI did not do its job maybe because the two wave plate before it did not clean the polarization as they should do.So we put a PBS after the first quarter wave plate and second wave plate, found out exactly what we suspected, so we adjusted them firstly then the Faraday and the other two half wave plate, got pure p polarization inject into the cavity.

Now the oscillation seems not there but the efficiency of green beam is only about 10%, so we tried to increased the alignment, but it seems we should do it from the start. After we make a progress in the alignment, we can check again if the oscillation really disappear. Then we can be sure that it is caused by the polarization problem.