Before the holiday we tried to move the lens to improve the beam shape, but still have some limitation, so we thought it maybe because the beam size is too large at the input port of the Faraday Isolator. So we tried to move the Faraday further from the lens, to let the beam converge more, but even we move the furthest we can, the beam is still large compared to the Faraday. Then we thought maybe we can change the lens into another one with smaller focal length, now we are using 100mm one.
But if we change to one with 50mm focal length,there is a possibility that the before the beam goes out from the Faraday it will expand. So I did the simulation today. I used the initial beam waist is 52.7 micrometer and 82.2cm away from the output of EOM, this is the value we had from the infrared beam, I am not sure if the green beam has the same size and position as the infrared one, but if I did the simulation with 100mm lens in its position now, the simulation result is close to the real beam size on the bench, so I did the after simulation with this starting point. Then I tried to change the lens to 50mm one in the simulation, and got the result in the first picture. The length of the Faraday is about 5cm, so if I put the beam waist at the center of the Faraday, the beam size at the input and output port of the it will be 762 micrometer in diameter which I think it is small enough.
But if we changed like this, there may be another problem, the aberration on the first lens will be much serious, since the beam size on the first lens does not change, but the focal length reduce.
We got the PBS for green today, so we can check the polarization of the green beam. If I remember right, we got pure P polarization went into the cavity, so the green should be S polarization, so we adjust the waveplate to maximum the S polarization.