16 Holography

8/14/2019 16 Holography

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HOLOGRAPHYHOLOGRAPHYDr. Na. Venkat Nathan


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Holographic reconstructionrocess usin a oint source

An interference pattern is formed which in thiscase is in the form of curves of decreasingseparation with increasing distance from the

centre.When the plate is illuminated by the referencebeam alone, it is diffracted by the grating intodifferent angles.

Which depend on the local spacing of thepattern on the plate.


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The net effect of this it to reconstruct theobject beam.

It appears that light is coming from a point

source behind the plate, even when thesource has been removed.

The light emerging from the photographicplate is identical to the light that emerged

from the point source that used to bethere.


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Comparison to Concave

LensThis sort of hologram is effectively a

concave lens.

Since it "converts" a plane wave frontinto a divergent wave front.

It will also increase the divergence of

any wave which is incident on it inexactly the same way as a normallens does.


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Making a hologram of acom lex ob ect

The laser beam is split in two by the beamsplitter.

One beam illuminates the object which

then scatters light onto the recordingmedium.

The second (reference) beam illuminates

the recording medium directly.According to diffraction theory, each pointin the object acts as a point source oflight.


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Each of these point sources interferes

with the reference beam, giving rise toan interference pattern.

The resulting pattern = (sum of a large

number of point source + referencebeam) interference patterns.

The hologram is illuminated by the

reference beam for the reproductionof hologram.


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The diffraction grating will diffract part of the

reference beam to re-construct the wavefront coming from its point source.

These individual wave fronts add together toreconstruct the whole of the object beam.

The viewer perceives a wave front which isidentical to the wave front scattered by theobject,

This image is known as a "virtual" image as itis generated even though the object is nolonger there.


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The holographic recording is the random

variation in intensity which is an objectivespeckle pattern.When one looks at a object, each eye

captures a portion of the light scattered

from the object. The lens of the eye forms an image of theobject on the retina.

Where light from each angular position is

focused to a specific angular position in theimage plane.


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The hologram reconstructs the whole of thescattered light field that was incident on thehologram.

The viewer sees the same image whether itis derived from the light field scattered fromthe object, or the reconstructed light fieldproduced by the hologram.

It is difficult to tell whether it is real or virtualobject.


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A photograph records an image of the

recorded scene from a single viewpoint.Which is defined by the position of the

camera lens.

The hologram is not an image, but anencoding system which enables thescattered light field to be reconstructed.

Images can then be formed from any pointin the reconstructed beam either with acamera or by eye.


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Resolution of a

The spacing of the fringes depends on the

angle between object and reference beam. For example, if this angle is 45o, and the

wavelength of the light is 0.5m, the fringespacing is about 0.7m or 1300 lines/mm.

A working hologram can be obtained even ifall the fringes are not resolved.

But the resolution of the image is reduced as

the resolution of the recording mediumreduces.


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Mechanical stability

Very important when making a hologram.

Any relative phase change between theobject and reference beams due to vibrationor air movement will cause the fringes on therecording medium to move.

If the phase changes are greater than , thefringe pattern is averaged out, and noholographic recording is obtained.

Recording time can be several seconds ormore.

A phase change of is equivalent to amovement of /2 this is quite a stringentstability requirement.


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The coherence length of the light determinesthe maximum depth in the object of interest

that can be recorded holographically.A good holography laser will typically have a

coherence length of several meters, amplefor a deep hologram.

Certain laser pointers have been used tomake small holograms.

The size of these holograms is not restrictedby the coherence length of the laser pointers

(which can exceed several meters), but bytheir low power of below 5 mW.


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Condition for ReconstructionThe reference beam is not normally a

plane wave front. It is usually a divergent wave front that is

formed by placing a convex lens in the

path of the laser beam.From transmission hologram the

reference beam must have the same

wavelength and curvature.Must illuminate the hologram at the same

angle as the original reference beam.


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Any slight departure from any of theseconditions will give a distorted re-construction.

If the difference between the

reconstruction and original referencebeam is too great, no re-construction isobtained.

The reconstructed hologram would beenlarged if the light used to reconstructthe hologram had a higher wavelength.


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This initially generated some interestsince it seemed to be possible to use X-rays to make holograms of molecules.

View them using visible light. However X-ray holograms have not been created todate.

This effect can be demonstrated using alight source which emits several differentfrequencies.


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Holographic recording

It must also be sufficiently sensitive torecord the fringe pattern in a time periodshort enough for the system to remainoptically stable.

Any relative movement of the two beamsmust be significantly less than /2.

The recording medium has to convert the

interference pattern into an opticalelement which modifies either theamplitude or the phase of a light beam.


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ApplicationsHolographic data storage is a technique that

can store information at high density insidecrystals or photopolymers.

As current storage techniques such as Blu-ray reach the denser limit of possible datadensity.

Holographic storage has the potential tobecome the next generation of popularstorage media.

The advantage of this type of data storage isthat the volume of the recording media isused.


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In 2005, 120 mm disc that uses a

holographic layer to store data to apotential 3.9 TB (terabyte) aremanufactured under the nameHolographic Versatile Disc.

Security holograms are very difficult toforge because they are replicated froma master hologram which requiresexpensive, specialized andtechnologically advanced equipment.They are used widely in manycurrencies.


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Holographic art is often the result of

collaborations between scientistsand artists.Holographic interferometry is a

technique which enables static anddynamic displacements of objectswith optically rough surfaces.

Can be measured to opticalinterferometric precision (i.e tofractions of a wavelength of light).


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It can also be used to detectoptical path length variations intransparent media.

For example, fluid flow to bevisualized and analyzed.

It can also be used to generatecontours representing the formof the surface.

INTERFEROMETRIC


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INTERFEROMETRIC

Several holograms may keep information about

the same distribution of light, emitted to variousdirections.

The numerical analysis of such hologramsallows one to emulate large numerical aperture

which, in turn, enables enhancement of theresolution of optical microscopy.

The corresponding technique is calledinterferometric microscopy.

Recent achievements of interferometricmicroscopy allow one to approach the quarter-wavelength limit of resolution.


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Non-optical

Electron holography is the application ofholography techniques to electron wavesrather than light waves.

Today it is commonly used to study electricand magnetic fields in thin films, asmagnetic and electric fields can shift thephase of the interfering wave passing

through the sample. The principle of electron holography can

also be applied to interference lithography.


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Acoustic holography is a methodused to estimate the sound field neara source by measuring acoustic

parameters away from the sourcevia an array of pressure and/orparticle velocity transducers.

Atomic holography has evolved outof the development of the basicelements of atom optics.

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16 Holography