Frequency-offset separated oscillatory fields: A demonstration of a new technique for a measurement of the helium n=2 triplet P fine structure

We perform a proof-of-principle demonstration of the frequency-offset separated oscillatory field (FOSOF) technique [Phys. Rev. A 92,052504 (2015)]. For the FOSOF technique, the two separated field have frequencies which are offset from each other, so that the relative phases of the fields varies linearly in time. This proof-of-principle demonstration measures the $2^3$P$_1$~$m$=1 to $2^3$P$_2$~$m$=1 transition in atomic helium and demonstrates the usefulness of the FOSOF technique for high-precision atomic measurements.