Environment-noise-free optical heterodyne retardation measurement using a double-pass acousto-optic frequency shifter.

We demonstrate an environment-noise-free optical heterodyne interferometer (OHI) based on a double-pass acousto-optic frequency shifter (AOFS) for phase retardation measurements. The OHI generates intermediate-frequency (IF) heterodyne beat signals for each orthogonal linear polarization mode of a birefringence sample. The phase differences of the IF signals are compared by using a wide-bandwidth lock-in amplifier. This scheme provides 20 dB rejection of common-mode environmental disturbances. Measurements of the half-wave voltage of an electro-optics modulator and the electro-optic coefficient of an LiNbO3 plate are demonstrated as application examples.

Low-coherence heterodyne interferometry using an achromatic frequency shifter based on a frequency-domain optical delay line.

In this paper a high-sensitivity low-coherence heterodyne interferometric system consisting of two polarizing Michelson interferometers arranged in tandem is presented. A compact frequency-domain optical delay line was placed in the first interferometer to produce a 4.4 kHz frequency shift for broadband near-infrared light. The frequency shift was wavelength independent because the scanning delay line had a zero-group-delay configuration. The fringe amplitude and phase of a low-coherence interference signal were detected using a lock-in amplifier. The experimental results show that the signal-to-noise ratio in the proposed technique is more than 30-fold higher than that of conventional low-coherence homodyne interferometry.