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Conventional interferometers split and recombine an optical signal after some limited optical processing in one or more of their interference paths. We study an interferometer structure that converts the signal to a different wavelength range for processing and converts it back to the original wavelength for interference. In particular, we consider intradyne conversion of the optical signal in one interferometer arm to digital electronic baseband, followed by digital electronic signal processing and subsequent electro-optic conversion, using the same laser for local oscillator and retransmitted signal. This allows for arbitrary time/frequency manipulations of an optical signal within the system's bandwidth capabilities while letting the other frequency portions of the signal pass through the system unchanged. We quantify the performance of such an opto-electronic interferometer as a subcarrier add/drop node in a digital optical communication system and study its tolerance to important practical hardware limitations.