Shift of a resonance frequency in spatially separated optical fields

An analysis is made of the problem of a resonance frequency shift in three spatially separated optical fields. The dependence of the resonance position on the optical field phases gives rise to an uncontrolled shift which in the case of a Maxwellian gas and a thermal atomic beam is of the order of the resonance width. The physical reference point for frequency measurements is a saturated absorption resonance (Lamb dip).