Method and apparatus for synchronizing two different types of modulators in an optical transmission system includes a first modulator generating an optical pulse train, a second modulator encoding data onto the optical pulse train, an optical filter resolving upper and lower modulation sidebands of the optical data and an analyzer measuring the optical power of modulation sidebands and converting the received optical power of the sidebands into a control signal for synchronizing the two modulators. A wedged etalon is the filter element selecting the USB and LSB from the optical data spectrum. The analyzer contains photo-detectors measuring the optical power of the filtered USB and LSB and an electronic differential amplifier producing a control signal based upon photo-detector output. The phase shifter, in response to the control signal, adapts the temporal delay of the first modulator to reduce differences between the power levels of the upper and lower sidebands.