A novel method is presented for of measuring absolute displacement with a synthesized wavelength interferometer. The optical phase of the interferometer is simultaneously modulated with a frequencymodulated laser diode and optical path-length difference. The error signal originating from the intensity modulation of the source is eliminated by a signal processing circuit. In addition, a lock-in technique is used to demodulate the envelope of the interferometric signal. The displacement signal is derived by the self-mixing technique.
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