Doppler-free two-photon absorption (${\Delta }_{\mathrm{Doppler}}=({\mathbf{k}}_1+{\mathbf{k}}_2)\cdot \mathbf{v}$)

Absorbing one photon from each of two opposing equal-frequency beams cancels the first-order Doppler shift through their nearly zero summed wavevector. Absorbing both photons from the same beam does not share this cancellation and can create a broad background.