Participaint: Chienming, Shurong and Yuhang
Situation before changing telescope: there is a higher order Laguerre-Gaussian mode appeared in the spectrum while we were scanning SHG. The conversion efficiency of SHG is 13%.
Motivation: Have more green production for the measurement of non-linear gain measurement of OPO. Decide to improve mode-matching situation. Certainly, more coupling of infrared into SHG will produce more green.
So we removed all the lens set by yuefan and implement the telescope we designed before. I am so sorry that I didn't ask Matteo to buy some new rails for telescope, so we put only one lens on the rail. Now the situation is we put 200mm at hole between (26, 7) and (26,8)(closer to 7) while put 125mm at hole (18,17). The lens on the rail is 125mm one. See attached figure 1 and 2.
Situation after changing telescope: The mode matching now is shown in the attached figure 3. We can see from the plot that mode-matching now is 94%. So there is still possibility to increase mode matching and further increase conversion efficiency. We need to note here that the movement of the second lens mainly change the position of waist while the movement of the first lens change mainly the waist size. And since we know the measurement result of yuefan, the waist size should be 54um. We can improve the mode matching further easily if we have another rail. Then we can easily move two lenses together and achieve a good beam waist size and position together. After did the mode matching we measure the power of infrared going inside SHG, which is 419.5mW(shown in attached figure 4). Then we locked SHG, while the invert is set as 'on'(shown in attached figure 5) and the gain is set as full gain(shown in attached figure 6). Then we measured the green production, now the green power is 101mW(shown in attached figure 7).
Situation after changing temerature: As suggested by Chienming, the mode-matching change will change phase matching situation. We increased temperature and measured the green power generation. The result is shown in attached figure 8. We found the best phase matching temperature is between 3.151 and 3.147kOm, which is smaller than before. This mode-matching difference of 20% bring optimal temperature difference of 0.2K. Now the best temperature is around 331.4K. Now the green power can reach 147mW(as shown in attached figure 9) Then we lock SHG again, we found now the alignment is quite sensitive. Even we touch a little bit mirror mount, we will degrade the green power by several mW. This means we may need a better mirror mount(so this can be somthing be improved in the future). And now the tranmission voltage is 1.46V(as shown in the attached figure 10). This is a little bit higher than the peak value of scanning as we expected. And also now we change temperature to 3.151kOm(as shown in attached figure 11). This means conversion effciency of 35% now. As pointed out by manufacture, it can reach 45%. So we still can improve it anyway. The good thing is we have enough green as we want.
Problem we found and solved:
1. One of the lenses is with a wrong coating. This can expalin the strange ratio of power we found before. But anyway we removed it.
2. The telescope for matching SHG's transmission into PD. The beam size was very large and collimated with using a lens of 100mm. We replaced it with a 75mm lens as shown in attached figure 12 . But now the pd saturates, we put a ND filter (ND = 1).
Problem found but not solved:
1. stray light: the stray light hit on the mount of mirrors or lenses. Maybe this is something we should consider in the future.
2. alignment after SHG is changed as shown in attached figure 13.
Additional check and work needs to be done:
1. check the beam shape before EOM(for filter cavity)
2. Buy a new rail and improve alignment further more.
3. Replace mirror mount for the two steering mirrors in front of SHG by two very stable mirrors.
4. Align the path after SHG.
