Applications
The FASOR in the image is used for laser guide star experiments. It is tuned to the D2A hyperfine component of the sodium D line and used to excite sodium atoms in the mesospheric upper atmosphere. The FASOR consists of two single-frequency injection-locked Nd:YAG lasers close to 1064 and 1320 nm that are both resonant in a cavity containing a lithium triborate (LBO) crystal, which sums the frequencies yielding 589.158 nm light.
Read more about this topic: Frequency Addition Source Of Optical Radiation