An apparatus using an optical signal for actively tuning an optical interferometer without introducing any dither in its optical path length. The apparatus comprises a dither generator, means applying a portion of the dithering signal to the optical signal so as to provide an optical signal having a varying wavelength, an optical interferometer having a path length that is tunable and that responds to the optical signal having a varying wavelength and a path length adjustment drive signal and develops a first interference pattern when said path length is a prescribed value and a second interference pattern when the optical path length is changed, photodetector means responsive to the optical interference pattern and operative to develop an electronic feedback signal when the first interference pattern is not present, and a synchronous detection lock-in amplifier that is responsive to a portion of the dithering signal and the electronic feedback signal and operative to produce the optical path length adjustment drive signal. The drive signal changes the optical path length until it reaches the prescribed value, thereby producing the first interference pattern and tuning the optical interferometer. also a method of using an optical signal for tuning an optical interferometer by adjusting its optical path length without introducing dither into the path length is invented.