ABSTRACf For interferometric distance measurements on rough surfaces multiple wavelength interferometry is a powerful tool. It allows to reduce the sensitivity and to extend the range of unambiguity for interferometric measw-ement. A new kind of a double wavelength beterodyne interferometer is presented which uses only one laser source which generates two wavelengths A, and ~ simultaneously. The two different heterodyne frequency shifts of the double heterodyne interferometer (DHI) are achieved by combining an acousto-optica1 modulator (driven in suppressed carrier mode) with a fixed diffraction grating. In a first prototype a multi·wavelength HeNe laser was used as light source. The synthetic wavelength A in this system is 55.5 IJIll and the resolution about 0.15 1JIll· Measurement results obtained with the setup described arc presented . I , INTRODUCTION Applying interferometric technique to the analysis of optically rough surfaces two serious problems arise. When USing a reflection set·up the unambiguity range of the measurement is limited to the half laser wavelength. In addition the speckle effect makes measurements very difficult or even impossible. To overcome these problems interferometers with grazing incidence' or increased laser waveJengtbl were developed. Alternatively double heterodyne interferometry is used. By applying two wavelengths simultaneously to the object the sensitivity is reduced to an effective wavelengthl.4 given by A = A.l . A2/1Al + A21 . Fercher et al.' described a two wavelength heterodyne speckJe interferometer where the phase of the effective wavelength is evaluated from two independent detector signals. In this paper a scanning double heterodyne technique is descn'bed that generates a low frequency detection signal with a phase shift that corresponds to the effective wavelength. 2. PRINCIPLE OF SUPRESSED CARRIER MODE

Two wavelength heterodyne absolute ranging technique using suppressed carrier modulation

There are some techniques of absolute remote distance measurement. Some of it deserve the title of absolutness without restriction (triangulation e.g.) but are not useful measuring at distances of some ten metres because of practical reasons. The other techniques have an unambiguity range within the measurement is absolute. Among the incoherent techniques the phase measurement of modulated light seem to have reached some limitation with an resolution of less than 1 mm. This limitation is set by the bandwidth of the photodetector. The coherent techniques operate with two wavelengths to extend the unambiguity range of the classical interferometry. The unambiguity range is then half the synthetic wavelength and can be adjusted quite arbitrarily from some ten micrometers on. If heterodyne detection is involved a relative resolution of the electronic phase measurement of 10−4 is achievable. As is valid for the phase measurement of intensity modulated light cascading of 2 or more stages of different synthetic wavelength is necessary in order to improve the relative resolution of the whole system.

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Silke Kreuz

Papers

Two wavelength heterodyne absolute ranging technique using suppressed carrier modulation

Interferometry for high resolution absolute distance measuring by larger distances