The unambiguous distance measurement range in two-color interferometry is generally understood to be limited to the equivalent or synthetic wavelength, which is inversely proportional to the wavelength separation of the two colors. Here it is shown that one may extend the unambiguous range well beyond this limit by using optical phase information to determine the synthetic-wavelength fringe order.

Extending the unambiguous range of two-color interferometers

Apparatus and methods particularly suitable for use in electro-optical metrology and other applications to measure and monitor the refractive index of a gas in a measurement path and/or the change in optical path length of the measurement path due to the gas while the refractive index of the gas may be fluctuating due to turbulence or the like and/or the physical length of the measuring path may be changing. More specifically, the invention employs electronic frequency processing to provide measurements of dispersion of the refractive index, the dispersion being substantially proportional to the density of the gas, and/or measurements of dispersion of the optical path length, the dispersion of the optical path length being related to the dispersion of the refractive index and the physical length of the measurement path. The refractive index of the gas and/or the optical path length effects of the gas are subsequently computed from the measured dispersion of the refractive index and/or the measured dispersion of the optical path length, respectively. The information generated by the inventive apparatus is particularly suitable for use in interferometric distance measuring instruments (DMI) to compensate for errors related to refractive index of gas in a measurement path brought about by environmental effects and turbulence induced by rapid stage slew rates. In preferred embodiments, differential plane mirror interferometer architectures are utilized, the operating wavelengths are approximately harmonically related and may be monitored and/or controlled to meet precision requirements, heterodyne and superheterodyne processing are beneficially used, and phase redundancy is resolved.

Interferometer and method for measuring the refractive index and optical path length effects of air

Contactless measuring techniques are becoming increasingly important for industrial applications. The use of a laser, solid-state detector arrays and powerful small computers leads to a very efficient fringe analysis in holography as well as in Moiré and speckle techniques. Due to the computer analysis, much more information can be extracted from interferograms, leading to higher sensitivities and accuracies. The application of different fringe analysis procedures is discussed, together with some potentials of the application of interferometry, holography, and speckle and Moiré techniques.

/pdf/optical-methods-for-precision-measurements-an-invited-paper-3izpg79zgk.pdf

Optical methods for precision measurements: An invited paper

Contactless measuring techniques are becoming increasingly important for industrial applications. The use of a laser, solid-state detector arrays and powerful small computers leads to a very efficient fringe analysis in holography as well as in Moire and speckle techniques. Due to the computer analysis, much more information can be extracted from interferograms, leading to higher sensitivities and accuracies. The application of different fringe analysis procedures is discussed, together with some potentials of the application of interferometry, holography, and speckle and Moire techniques.

Optical methods for precision measurements

https://dr.ntu.edu.sg/bitstream/10356/90734/1/Dual-wavelength%20image-plane%20digital%20holography%20for%20dynamic%20measurement.pdf

Dual-wavelength image-plane digital holography for dynamic measurement

Two-wavelength double heterodyne interferometry is applied for topographic measurements on optically rough target surfaces. A two-wavelength He-Ne laser and a matched grating technique are used to improve system stability and to simplify heterodyne frequency generation.

/pdf/two-wavelength-double-heterodyne-interferometry-using-a-56r07bzylo.pdf

Two-wavelength double heterodyne interferometry using a matched grating technique.

Photothermal interferometry for nondestructive subsurface defect detection

For nondestructive material evaluation (NDE) ultrasonic and X-ray methods have been well established for many years. About two decades ago an old effect, discovered by BELL /1/, was theoretically examined by WHITE /2/. A comprehensive theory for NDE utilizing a laser heat source was given by ROSENCWAIG et al./3/. It is called the thermoacoustic effect and describes the phenomenon of sound generation, when periodically heating the surface of a body with an audio frequency. Since heat generation is usually done optically, using a modulated laser beam or a laser pulse /4/, the process is called photoacoustic or photothermal depending on whether the main interest is in the acoustic or the thermal aspect respectively. One should however keep in mind that all effects happen at the same time. At first, light is converted into heat, which heats the surrounding air and generates sound. Second, heat causes thermal expansion, leading to sound transport in the material. We decided to call our experiment a photothermal process, although both effects are involved.

/pdf/nondestructive-evaluation-of-solids-by-photothermal-1ahepcaalh.pdf

Nondestructive evaluation of solids by photothermal interferometry on nonreflective surfaces

Over the past years, Synopta have successfully applied adaptive optics (AO) in optical ground stations to enhance the capabilities in satellite-to-ground laser communication links. In this paper, we present the outcome of an upgrade activity to the ESA Optical Ground Station (ESA OGS). The activities comprise the design, fabrication and site acceptance test results of the Cassegrain Adaptive Receiver Optics (“CARO”) add-on instrument to the 1 m telescope of the ESA OGS. Additionally, we present a baseline design concept for the future use of a bi-directional adaptive optics system in the Coudé path of the ESA OGS, expected performance characteristics for a selected uplink application.

/pdf/adaptive-optics-upgrades-for-laser-communications-to-the-esa-15cucy1lpy.pdf

Adaptive optics upgrades for laser communications to the ESA optical ground station

Fur die beruhrungslose Entfernungsmessung bieten sich verschiedene optische Verfahren an. Zu den nicht-koharenten Methoden zahlen die Triangulation /l,2,3,4/, Moire-, Laufzeit-und Phasenmestechniken. Geht man von einer Mesdistanz von 100 m und einer geforderten Auflosung von 0.1 mm aus, so erweisen sich die oben erwahnten Verfahren als nicht geeignet: Die Triangulation und die Moire-Technik benotigt fur die genannten Anforderungen eine zu grose Basis, was einen kompakten Aufbau ausschliest. Bei der Laufzeitmessung und der inkoharenten Phasenmestechnik liegen die derzeit erzielbaren Auflosungen im Millimeterbereich. Eine Verbesserung der Auflosung erfordert einen grosen elektronischen Aufwand.

/pdf/doppelheterodyn-interferometrie-fur-hochgenaue-vermessung-im-3vsgs0x6l1.pdf

Zoran Sodnik

Papers

Two-wavelength double heterodyne interferometry using a matched grating technique.

Photothermal interferometry for nondestructive subsurface defect detection

Nondestructive evaluation of solids by photothermal interferometry on nonreflective surfaces

Adaptive optics upgrades for laser communications to the ESA optical ground station

Doppelheterodyn-Interferometrie für hochgenaue Vermessung im Nahbereich