11/15 Participants Chien-Ming, Shu-Rong, and Yuhang

Today we insert the second Faraday Isolator IO-2-YAG (FI 2) into the main laser light path and modify the telescope for SHG.(see Fig. 1)
The transmission of this small aperture (<2mm) isolator is around 85% even if the beam radius inside the isolator is less than 500 um.

We achieve a good mode matching (see Fig. 2) for SHG by using the lens L1 f = 75 mm and lens L2 f= 100 mm.
The output power of 532nm is 148 mW when injecting 426mW 1064 nm light into SHG. This efficiency is consistent with the previous results on 11/9 and 11/12.

However, we did find that the laser power stability of both SHG and Main laser have improved a lot after adding the isolator FI 2.
The SHG 532 nm output power have the ratio of Max/Min is 1.007:1 (see Fig. 3) which is much better than the result 1.088:1 measured yesterday.
The ratios Max/Min of the main laser are both improved as 1.010:1 and 1.012:1 when operating in a higher power and low power. (see Fig. 4 and 5). The power meter is placed behind the BS1.

We believe that such stability should be enough even though it is still somewhat different from the situation of blocking the reflected light from SHG with the ratio of Max/Min is 1.002 :1.(see Fig. 6)

Since the output polarization of the FI 2 is 45 degrees, we use a half waveplate (HWP1) to ensure the s-polarization for BS2 with the split ratio of T/R is 20:80. Otherwise, the BS2 split ratio of T/R is 38:62 when using the p-pol light. The second half waveplate (HWP2) rotates the polarization to p-pol. for serving the SHG.

Fig. 1 A schematic of adding the second isolator FI 2 and modify the telescope for SHG
Fig. 2 The mode matching result of SHG
Fig. 3 SHG output power stability
Fig. 4 Main laser power stability at higher power scale. 
Fig. 5 Main laser power stability at lower power. 
Fig. 6 Main laser power stability when blocking the reflected beam from SHG