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Balanced heterodyne architecture for improving the noise performance of electro-optic probing systems

We present an improved heterodyne EO probing setup to gain benefits from balanced detection. Compared to a conventional heterodyne EO probing setup, the addition of another optical modulator driven by a shifted LO allows us to generate a second IF signal in phase opposition with the first one. As a consequence, a balanced detector can be used, improving the noise performance of the overall probing scheme. The setup remains simple as we are not relying on optical heterodyning, which would involve accurate LO control, and some polarization manipulation. There is also no need for an accurate time delay between the two arms as we just need to control the phase shift between the two IF signals, which are at low frequencies (typically 1 MHz). An experimental comparison between the conventional and the balanced heterodyne systems is performed and confirms >6dB improvement of the noise performance and the sensitivity of the EO probing system.