Optomechanical drift between the two pump arms results in phase shifts in the rephasing and nonrephasing signals. Because the signals are generated as a cross interaction between the two pump beams, any time shift, Δ , between the leading pulses, and , will cause a corresponding phase shift, Δ , of equal magnitude and opposite signs in and (Figure ). Thus, the polarization that generates the emitted electric field in this geometry becomes sensitive to these terms. The phase cycling scheme of = 0, is used to collect signal on every shot and remove the background. The final signal maintains the phase shifts;