Sinusoidal-wavelength-scanning interferometer with double feedback control for real-time distance measurement

In addition to a conventional phase a the interference signal of a sinusoidal-wavelength-scanning interferometer has a phase-modulation amplitude Zb 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 alpha and the amplitude Zb are kept at 3pi/2 and pi, 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