Two-Photodetector In-Phase and Quadrature Demodulation Interferometer Scheme Using Temporally Shifted Differential Signals

Four photodetectors, two in the balanced homodyne detector for in-phase (I) channel measurements and two in that for quadrature (Q) channel measurements, are needed for a conventional homodyne I/Q interferometer. Here, we present a new homodyne I/Q interferometer scheme for which two photodetectors, one for I-channel and the other for Q-channel measurements, are used for I/Q demodulation of the interference signal. The DC bias terms on the output signal from each photodetector are rejected by taking the difference between the signal and the temporally shifted signal. Then the induced phase on the interference signal can be measured by using the algorithm for I/Q phase demodulation. It will be shown that the sensitivity is limited by the corresponding photon noise. We will also show that in the vicinity of the turning points of a periodic modulation, where the phase difference may become smaller than the corresponding photon noise, or in the region where there is no measurable phase change the phase can be calculated through temporal averaging, which not only removes high-frequency noises but also increases the time delay for obtaining the difference signal.