In addition to a conventional phase α the interference signal of a sinusoidal-wavelength-scanning interferometer has a phase-modulation amplitude Z b that is proportional to the optical path difference L and amplitude b of the wavelength scan. L and b are controlled by a double feedback system so that the phase α and the amplitude Z b are kept at 3π/2 and π, respectively. The voltage applied to a device that displaces a reference mirror to change the optical path difference becomes a ruler with scales smaller than a wavelength. Voltage applied to a device that determines the amplitude of the wavelength scan becomes a ruler marking every wavelength. These two rulers enable one to measure an absolute distance longer than a wavelength in real time.
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