Tunable laser diode for absolute distance measurement

We propose an interferometric method for absolute distance measurement. This method uses the fact that the wavelength of a laser diode can be tuned continuously, by addition of an external cavity. The laser system, which has a small linewidth and therefore a large coherence length, is used to feed two interferometers. One of them serves as a reference distance. The phase variation of the signal is extracted by the Fourier transform technique. The magnitude of phase variation is proportional to the rate of measuring and reference distances. If the reference distance is known, a measuring distance can be obtained from measurement of the phase variation. We have done some fundamental experiments and shown that the precision is essentially limited by calibration of reference distance. The application of this technique has already allowed to measure distance of 3m with relative accuracy of 1.8 10(-4).