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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.