Frequency Addition Source of Optical Radiation

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For other uses, see FASOR (disambiguation).
A 50W FASOR used at the Starfire Optical Range.

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}.


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.