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In the measurement using conventional heterodyne interferometer, `high-speed' and `high-resolution' were methodologically contradictory problems. Previously, authors have solved this contradiction by using two conjugate beat signals. In this work, we realized highly sensitive measurement without speed limit of the target using a three- mode laser. The synthetic wavelength of (lambda) /2 generated by three-mode laser enabled to increase the measurement sensitivity twice. The three-mode laser also has an advantage of higher output power.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new method of high-resolution two-wavelength interferometry was proposed for real-time and absolute measurement of micron-order distance or step-shapes in the depth. Simultaneous phase measurement of synthetic wavelength and one of two wavelengths was executed to measure the distance in the range of synthetic wavelength with interferometric resolution. A heterodyne light source constructed of two longitudinal modes was also proposed. The intermode beat of two modes was used as heterodyne career. Experiments were done with He-Ne lasers of 633 nm and 544/612 nm to measure 3 - 18 micrometers range.

In conventional heterodyne interferometer it Is necessary to provide a high-beat frequency laser when measurement for a high-speed target Is required1 . But use of the high-beat frequency laser makes the " nanometerdivisions" difficult. We have developed a novel interferometer system that has a sufficient response to high-speed movement of the target without Increase of the laser beat frequency. In this work a two frequency laser light passes through the same optical path of the interferometer so that two conjugate beat signals are obtained. By processing the multiple beat signals with a newly developed signal processor the above methodological contradiction is solved. l. OPTICS FOR LINEAR DISTANCE MEASUREMENT Optical system of the interferometer Is shown In Flg. l in which two frequency lights Fl and F2 (freq. f and f2) of orthogonally linear-polarized components of a two mode laser are used. A reference beat signal R (freq. r ) is generated from Fl and F2. Fl and F2 are passed through exactly the same path of the interferometer. A half power of Fl and F2 Is reflected and the rest Is transmitted In the beam splitter ( BS ). Polarization direction of the reflected beam is rotated for ir/2 after passing the X/4 plate twice. The transmitted beam Is reflected by a moving mirror (MM) mounted on the moving target so that frequency of the reflected beam Is shifted by Doppler