In two-wavelength interferometry, synthetic wavelengths are generated in order to reduce the sensitivity or to extend the range of unambiguity for interferometric measurements. Here a novel optoelectronic technique, called superheterodyne detection, is presented, which permits measurement of the phase difference of two optical frequencies that cannot be resolved by direct optoelectronic heterodyne detection. This technique offers the possibility for operation of two-wavelength interferometry in real time with arbitrary synthetic wavelengths from micrometers to meters in length. Preliminary experimental results are reported. An optical arrangement for absolute range-finding applications using tunable-laser sources (e.g., semiconductor lasers) is proposed.

Two-wavelength laser interferometry using superheterodyne detection

In two-wavelength interferometry, synthetic wavelengths are generated in order to reduce the sensitivity or to extend the range of unambiguity for interferometric measurements Here a novel optoelectronic technique, called superheterodyne detection, is presented, which permits measurement of the phase difference of two optical frequencies that cannot be resolved by direct optoelectronic heterodyne detection This technique offers the possibility for operation of two-wavelength interferometry in real time with arbitrary synthetic wavelengths from micrometers to meters in length Preliminary experimental results are reported An optical arrangement for absolute range-finding applications using tunable-laser sources (eg, semiconductor lasers) is proposed

Optical Metrology of Surfaces

A two-wavelength interferometer is described that can be used to measure distances to rough surfaces with micrometer resolution. The measuring range can be made as large as several centimeters. Owing to the small numerical aperture of the object beam, the interferometer is particularly suitable in applications where a long working distance is required. The interferometer uses two single-frequency diode lasers to create a virtual two-wavelength laser. The equivalent wavelength of this laser can be easily tuned by changing the temperature of the laser diodes in opposite directions.

Two-wavelength scanning spot interferometer using single-frequency diode lasers.

It appears that only three different physical principles are required to classify all range sensors: (classical) interferometry on smooth surfaces (type III), broad-band interferometry on rough surfaces (type II), and triangulation (type I). We will show that signal formation in classical interferometry is completely different from signal formation in white-light interferometry on rough surfaces. The latter (type II) principle has some remarkable features: the distance uncertainty does not depend on the aperture of observation, and it displays some kind of “superresolution” by acquiring surface roughness, even if the lateral microtopology of the object is unresolved. We will demonstrate this by experiments and theory.

Limits of Optical Range Sensors and How to Exploit Them

This paper is a study of the principles of a new method of rough-surface interferometry (ROSI). We review some properties of the higher-order probability density function (pdf) of correlated speckle fields and discuss some assumptions that are usually made silently. This pdf is used to investigate statistical properties of intensities and phases in a dichromatic speckle field; the results are applied to ROSI. The experiments described confirm the theoretical results.

U. Vry

Papers

Higher-order statistical properties of speckle fields and their application to rough-surface interferometry