A modified Michelson interferometer was used to probe the electron density in plasma plumes produced by pulsed XeCl laser ablation of Al2O3. With one of the mirrors of the interferometer translating steadily, the phase angle of the sinusoidal interference signal was tracked and the XeCl laser was fired whenever the detected phase angle matched a predefined value. The transient interference waveform produced by the plasma plume was then synchronously captured. This moving‐mirror interferometer features minimal vibration isolation, fast response time (∼10 ns), powerful noise rejection, and a detection limit of a thousandth of a fringe shift, or ∼1015electrons per cm3for mm size plumes.