In physics, a FASOR is an acronym for Frequency Addition Source of Optical Radiation. It is a device similar to a laser where the emitted light is produced in a sum-frequency generation process from two laser sources that operate at different wavelengths. The frequencies of the sources add directly to a summed frequency. Thus, if the source wavelengths are $\lambda_1$ and $\lambda_2$, the resulting wavelength is
$\lambda = \left(\frac{1}{\lambda_1} + \frac{1}{\lambda_2} \right)^{-1}.$