A differential-phase interferometric system includes a polarized heterodyne interferometer for generating reference and signal beam that travel along reference and signal channels, respectively. The signal beam is directed to a specimen and contains measured information of the specimen. The interferometer further generates a first electrical signal output corresponding to first linear polarized waves of the reference and signal beams, and a second electrical signal output corresponding to second linear polarized waves of the reference and signal beams. A differential amplifier receives the first and second electrical signal outputs, and generates a differential signal output therefrom. A data acquisition unit is used to measure amplitudes of the first and second electrical signal outputs and the differential signal output. A computing unit computes the amplitudes measured by the data acquisition unit to determine a phase difference between the electrical signal outputs, which corresponds to the measured information of the specimen.