FASOR is an acronym for frequency addition source of optical radiation. The name is used for a certain type of guide star laser deployed at US Air Force Research Laboratory facilities SOR and AMOS. The laser light is produced in a sum-frequency generation process from two solid-state laser sources that operate at different wavelengths. The frequencies of the sources add directly to a summed frequency. Thus, if the source wavelengths are ${\displaystyle \lambda _{1}}$ and ${\displaystyle \lambda _{2}}$, the resulting wavelength is

${\displaystyle \lambda =\left({\frac {1}{\lambda _{1}}}+{\frac {1}{\lambda _{2}}}\right)^{-1}.}$