A method of locking the relative phase to provide stable constructive or destructive interference between the phase-modulated sidebands from a pair of phase modulators is demonstrated. It is discussed theoretically for optimal fringe visibility related to the phase noise from faulty system. After phase locking using the phase modulating and lock-in technique, the drift of the relative phase is focalized around +-0.0016 rad and the fringe visibility is restricted to 2×10?4.