Figure 1
From: Direct frequency comb optical frequency standard based on two-photon transitions of thermal atoms
Three-level scheme used to describe the two-photon transitions excited by frequency comb.
(a) Schematic of the joint participation of comb lines for TPT excitations. Once the comb is resonant with the TPT resonance, comb line pairs can jointly excite TPTs. The 5P state enables the occurrence of stepwise TPTs, involving the 5S → 5P (780 nm) and 5P → 5D (776 nm) transitions. (b) Doppler-free TPT excitations by two counter-propagating laser beams. For the elimination of Doppler-broadening TPTs background, various coherent quantum techniques are employed in this scheme (see methods).
