Participants : Eleonora, Yuefan, Yuhang
Since last friday we started designing the EOM telescope.
EOM parameters:
Following the EOM datasheet (attached to this entry) the beam conditions inside the EOM are the following :
Max beam size defined by EOM aperture (3x3mm) : max beam radius = 425 um
Min beam size defined by max optical intensity (20W/mm^2) : min beam radius = 75 um (as the input power is around 350mW)
This requirements can be meet if we use a f=175 lens and place the EOM 10cm after it. (actually we first used the wrong value of max optical density first meaning that the beam is now more than 100um inside the EOM)
Issues :
Because of the Faraday Isolators, the beams after the two 98:2 are quite astigmatics (the datas will be added tomorrow morning).
The beam were also vertically tilted (3.1 mrad for the beam going to the EOM).
By using only 1 lens after the 98:2 we couldn't achieve better than 86% of transmission.
Possible solution and future work :
We then installed 2 steerings mirrors before the lens in order to correct the beam tilt. This means that the EOM path is now shifted 5cm away from the laser with respect to the nominal position.
It seems that there is enough space to use this solution (and to recombine the 2 beams we could then use 1 steering mirror and rotate the PBS).
Tomorrow we will installed EOM and characterized the output beam.
It should then be possible to use this solution to recombine the 2 beams.
You can find attached the complete data sheet for the 88 MHz EOM which I get from Quibig. Specs may be a bit different from that reported in the entry.
As Eleonora pointed out we used a wrong datasheet for the EOM (and also did some wrong calculations for the max beam size inside the EOM...)
Here is the good size range : between 300 and 80 um
We designed a new telescope (Fig1) as the following : f=125mm lens and 10cm after f=-25mm lens.
This should allow to have a beam size around 200um inside the EOM.
Question : For the green EOM, the astigmatism depended a lot on the lens position.
Is the astigmatism also that problematic? We will have quite a short beam path until the OPO.
Anyway, we found 2 trails on which we can translate the lenses borrowed from Manuel's experiment.
Fig 2 : beam size before the EOM
The beam didn't seem to be too much astigmatic after the EOM (posted soon)
At the begining, we used the wrong beam dimmsion, the initial beam DIAMETER is 2000um.
BUT, all the telescopes we designed are using RADIUS as 2000um.
Today, we realized this problem. I designed the telescope again. The EOM doesn't make a large difference. This design can be a fine reference.
Lesson: actually we have many chances to realized this problem, we checked every time after putting the lens. But everytime, we checked only the beam waist position. We never checked the beam waist size. So we didn't realize this problem. So next time we should check both of them carefully.