Analog Stabilization of an Electro-Optic I/Q Modulator with an Auxiliary Modulation Tone

Abstract: Proper operation of electro-optic I/Q modulators rely on precise adjustment and control of the relative phase biases between the modulator's internal interferometer arms. We present an all-analog phase bias locking scheme where error signals are obtained from the beat between the optical carrier and optical tones generated by an auxiliary 2 MHz RF-tone to lock the phases of all three involved interferometers for operation up to 10 GHz. With the developed method, we demonstrate an I/Q modulator in carrier-suppressed single-sideband mode, where the suppressed carrier and sideband are locked at optical power levels $< -27 dB$ relative to the transmitted sideband. We describe a simple analytical model for calculating the error signals, and detail the implementation of the electronic circuitry for the implementation of the method.