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The ''N''-slit interferometer is an extension of the double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of ''N''-slit arrays in optics was illustrated by Newton. In the first part of the twentieth century, Michelson described various cases of ''N''-slit
diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
. Feynman described
thought experiment A thought experiment is an imaginary scenario that is meant to elucidate or test an argument or theory. It is often an experiment that would be hard, impossible, or unethical to actually perform. It can also be an abstract hypothetical that is ...
s the explored two-slit quantum interference of electrons, using Dirac's notation. This approach was extended to ''N''-slit interferometers, by Duarte and colleagues in 1989, F. J. Duarte and D. J. Paine, Quantum mechanical description of ''N''-slit interference phenomena, in ''Proceedings of the International Conference on Lasers '88'', R. C. Sze and F. J. Duarte (Eds.) (STS, McLean, Va, 1989) pp. 42–47. using narrow-linewidth laser illumination, that is, illumination by indistinguishable photons. The first application of the ''N''-slit interferometer was the generation and measurement of complex interference patterns. These interferograms are accurately reproduced, or predicted, by the ''N''-slit interferometric equation for either even (''N'' = 2, 4, 6,...), or odd (''N'' = 3, 5, 7,...), numbers of slits.


''N''-slit laser interferometer

The ''N''-slit laser interferometer, introduced by Duarte,F. J. Duarte, Electro-optical interferometric microdensitometer system, ''US Patent'' 5255069 (1993)
.
uses prismatic beam expansion to illuminate a transmission grating, or ''N''-slit array, and a photoelectric detector array (such as a CCD or
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
) at the interference plane to register the interferometric signal.F. J. Duarte, in ''High Power Dye Lasers'' (Springer-Verlag, Berlin, 1991) Chapter 2. The expanded laser beam illuminating the ''N''-slit array is single-transverse-mode and narrow-linewidth. This beam can also take the shape, via the introduction of a convex lens prior to the prismatic expander, of a beam extremely elongated in the propagation plane and extremely thin in the orthogonal plane. This use of one-dimensional (or line) illumination eliminates the need of point-by-point scanning in
microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...
and
microdensitometry A microdensitometer is an optical instrument used to measure optical densities in the microscopic domain. J. C. Dainty and R. Shaw, ''Image Science'' (Academic, New york, 1974).T. H. James, ''The Theory of the Photographic Process'' (Eastman Koda ...
. Thus, these instruments can be used as straight forward ''N''-slit interferometers or as interferometric microscopes. The disclosure of this interferometric configuration introduced the use of digital detectors to ''N''-slit
interferometry Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...
.


Applications


Secure optical communications

These interferometers, originally introduced for applications in imaging, are also useful in optical metrology and have been proposed for secure optical communications in free space, between spacecraft. This is due to the fact that propagating ''N''-slit interferograms suffer catastrophic collapse from interception attempts using macroscopic optical methods such as beam splitting. Recent experimental developments include terrestrial intra-interferometric path lengths of 35 meters and 527 meters. These large, and very large, ''N''-slit interferometers are used to study various propagation effects including microscopic disturbances on propagating interferometric signals. This work has yielded the first observation of diffraction patterns superimposed over propagating interferograms. These diffraction patterns (as shown in the first photograph) are generated by inserting a
spider web A spider web, spiderweb, spider's web, or cobweb (from the archaic word ''Wikt:coppe, coppe'', meaning 'spider') is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey ...
fiber (or
spider silk Spider silk is a protein fibre or silk spun by spiders. Spiders use silk to make webs or other structures that function as adhesive traps to catch prey, to entangle and restrain prey before biting, to transmit tactile information, or as nest ...
thread) into the propagation path of the interferogram. The position of the spider web fiber is perpendicular to the propagation plane.


Clear air turbulence

''N''-slit interferometers, using large intra interferometric distances, are detectors of
clear air turbulence In meteorology, clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues such as clouds, and is caused when bodies of air moving at widely different speeds meet. The atmospheric region most susceptibl ...
. The distortions induced by clear air turbulence upon the interferometric signal are different, in both character and magnitude, from the catastrophic collapse resulting from attempted interception of optical signals using macroscopic optical elements.


Expanded beam interferometric microscopy

The original application of the ''N''-slit laser interferometer was interferometric imaging.F. J. Duarte, Interferometric imaging, in ''Tunable Laser Applications'' (Marcel-Dekker, New York, 1995) Chapter 5. In particular, the one dimensionally expanded laser beam (with a cross section 25-50 mm wide by 10-25 μm high) was used to illuminate imaging surfaces (such as silver-halide films) to measure the microscopic density of the illuminated surface. Hence the term interferometric
microdensitometer A microdensitometer is an optical instrument used to measure optical density, optical densities in the microscopic domain.Christopher Dainty, J. C. Dainty and R. Shaw, ''Image Science'' (Academic, New york, 1974).T. H. James, ''The Theory of the P ...
. Resolution down to the nano regime can be provided via the use of interinterferometric calculations. When used as a microdensitometer the ''N''-slit interferometer is also known as a laser microdensitometer. The multiple-prism expanded laser beam is also described as an extremely elongated laser beam. The elongated dimension of the beam (25-50 mm) is in the propagation plane while the very thin dimension (in the μm regime) of the beam is in the orthogonal plane. This was demonstrated, for imaging and microscopy applications, in 1993. Alternative descriptions of this type of extremely elongated illumination include the terms line illumination, linear illumination, thin light sheet illumination (in light sheet microscopy), and plane illumination (in selective plane illumination microscopy).


Other applications

''N''-slit interferometers are of interest to researchers working in
atom optics Atom optics (or atomic optics) "refers to techniques to manipulate the trajectories and exploit the wave properties of neutral atoms". Typical experiments employ beams of cold, slowly moving neutral atoms, as a special case of a particle beam. Li ...
, Fourier imaging,
optical computing Optical computing or photonic computing uses light waves produced by lasers or incoherent sources for data processing, data storage or data communication for computing. For decades, photons have shown promise to enable a higher bandwidth than the ...
, and quantum computing.


See also

*
Beam expander Beam expanders are optical devices that take a collimated beam of light and expand its width (or, used in reverse, reduce its width). In laser physics they are used either as intracavity or extracavity elements. They can be telescopic in nature ...
*
Clear air turbulence In meteorology, clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues such as clouds, and is caused when bodies of air moving at widely different speeds meet. The atmospheric region most susceptibl ...
*
Diffraction from slits Diffraction processes affecting waves are amenable to Quantity, quantitative description and analysis. Such treatments are applied to a wave passing through one or more slits whose width is specified as a proportion of the wavelength. Numerical a ...
*
Double-slit experiment In modern physics, the double-slit experiment demonstrates that light and matter can exhibit behavior of both classical particles and classical waves. This type of experiment was first performed by Thomas Young in 1801, as a demonstration of ...
*
Free-space optical communication Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking over long distances. "Free space" means air, oute ...
*
Laser communication in space Laser communication in space is the use of free-space optical communication in outer space. Communication may be fully in space (an inter-satellite laser link) or in a ground-to-satellite or satellite-to-ground application. The main advantage o ...
*
Microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...
*
Microdensitometer A microdensitometer is an optical instrument used to measure optical density, optical densities in the microscopic domain.Christopher Dainty, J. C. Dainty and R. Shaw, ''Image Science'' (Academic, New york, 1974).T. H. James, ''The Theory of the P ...
* ''N''-slit interferometric equation *
List of laser articles This is a list of laser topics. A * 3D printing, additive manufacturing * Abnormal reflection * Above-threshold ionization * Absorption spectroscopy * Accelerator physics * Acoustic microscopy * Acousto-optic deflector * Acousto-optic mo ...


References

{{reflist Interference Interferometry Interferometers Optical instruments American inventions