lecture 6 application holography

8/13/2019 Lecture 6 Application Holography

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PH 0101 UNIT 3 Lecture 6 1

Holography

Holography (Holo

whole; graphy

recording)

A technique of recording complete information of an object (ie.its amplitude and phase)

Invented by Dennis Gabor (Nobel prize 1971)

Holography (Holo

whole; graphy

recording)

A technique of recording complete information of an object (ie.its amplitude and phase)

Invented by Dennis Gabor (Nobel prize 1971)


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3D Rainbow hologram Space ship (Star wars)


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T-Rex skeleton

Toyata Car


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Requirements and properties of Holograms

1. Records Amplitude and phase

2. Need a coherent source of light

3. Need high resolution recording film

4. Need a vibration isolation table

5. Gives a 3 D effect

6. Each point on the hologram has the complete information about

the picture.

In an hologram the image is captured as a interference pattern recorded

on a film.


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Comparison between holography and photography

Property Photography Holography

1. Illumination Ordinary light Laser

2. Recording

parameter

Amplitude Both amplitude and phase

3. Imaging 2-Dimensional 3-Dimensional

4. Recording

medium

Ordinary Photographic

film

Very high resolution film

5. Specialrequirement

Not Applicable Vibration isolation table (itrequires long exposure)

6. Special property When cut into pieces,

information is lost

When cut into pieces each

bit carries full information


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Reference: How stuff works ?


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Reference: Optics by Hecht

a:The Hologram

b,c,d: Show the photo of theHologram taken from

Different angles.


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Holograms are true 3 dimensional images, the perspective of theimage changes when the same hologram is seen from different angles.


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Recording Technique : Construction of a Hologram

A monochromatic laser beam from the source is made to fallon beam splitter.

Beam splitter splits the incident beam into two.

One beam is made to fall on silver coated mirror M1 and after

reflection, it is directed towards the photographic plate

reference wave.

Another beam is made to scatter by the object object wave.

The reference wave and object wave interfere and the

interference pattern is recorded on a high resolutionphotographic plate

The developed photographic plate is known as hologram


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Recording process


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Reconstruction Process:

The hologram is illuminated by the reference wave

The holographic image is created by the reconstructedwavefront from the film.

A virtual image is seen by the observer from thereconstructed wavefront.


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Reconstruction process


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Hologram construction and reconstruction


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Ref: HyperPhysicswebsite

Hologram created using He-Ne laser Holographic image
http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/hph.html


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Applications of holography

1. Holographic interferometry

Double exposure holographic interferometry:The hologram of a undeformed object and the deformed object are

recorded on the same film.

The interference fringe pattern of the resulting image can be used

to measure small displacement and distortions of an object.

Real time holographic interferometry:

Reconstructed holographic image and the actual object overlap with eachother.

The changing fringe pattern can be used to measure strains on an object as they

actually deform.

Time-average holographic interferometry:

An oscillating object by exposed to the holographic film for a longtime.

The image obtained can be used for examination of spatial characteristics oflow amplitude vibrations of the object.


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Applications: Holographic interferometry


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2. Holographic computer memories

High Density Optical Storage

Next generation data storage use optical holography to store information in3-D.

Many holograms (2-D) are superimposed within the same volume of therecording medium

Information up to 1012bits (1Terabit) can be stored in 1 cc of the recordingmedium.

Data is transferred as 2-D images composed of pixels having single bit ofinformation.

The (Re)construction process is sensitive to the angle of incidence and thewavelength of the reference beam & is exploited to store and retrieveinformation.


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Holographic data storage


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Information is transferred as pages to and from the storagemedium so the data transfer rate is very high ( 1 Gbits/s)[DVD10Mbits/s]

Different holograms are recorded at different angles ofincidence of the reference beam in the medium

Data are imprinted onto the object beam by shining the lightthrough a pixelated device called a spatial light modulator (LCD

panels)

The reference beam overlaps with the object beam on thestorage material, where the interference pattern is stored as achange in absorption or refractive index.


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To maximize the storage density, the hologram is usuallyrecorded where the object beam is tightly focused

For readinga pair of lenses image the data through the storagematerial onto a pixelated detector array, such as a chargecoupled device (CCD).

A correct reference beam must first be directed to theappropriate spot within the storage media.

The hologram is then reconstructed by the reference beam, aweak copy of the original object beam continues along theimaging path to the camera, where the optical output isdetected and converted to digital data.


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The data transfer rate depends on

a. Latency:Time needed to access the data in a particularlocation (time delay between asking for and receivinga particular bit of data).

The time delay is dictated by mechanical movements

b. Readout rate: Time taken to read the particular data.

It is dictated by the charge integration time of the

CCD camera.If integration time is 1 ms, then 1000 pages can beretrieved in every second.


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Photopolymer materials:Polymerizes in the presenceof light illumination in an irreversible process.

The refractive index of the material changes becauseof polymerization .

Photochromic materials: Molecules that change theirconfiguration in the presence of light .

Change in configuration results in change in refractiveindex or absorption which can be used to storeinformation

Recording Medium (Permanent holograms)


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Inorganic photorefractive crystals doped with transition

metals or rare-earth ions are used as rewritable medium.

Eg. Lithium niobate, strontium barium niobate and bariumtitanate doped with iron, cerium, praseodymium or manganese(1 cm thick).

These materials trap and transport photo-ionized electrons,creating a change in the refractive index .

The photo-ionized electrons move from the bright regions to thedark regions creating a refractive index change depending onthe intensity of light falling in any region.

The trapped electrons can be rearranged by later illumination, to

erase recorded holograms and replace them with new ones.

Recording Medium (Rewritable)


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Books and resources

1. Lasers Ajoy Ghatak

2. Optics- Hecht

3. Fiber Optics- Ajoy Ghatak

4. Web: How Stuff Works

5. Web: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

6. Engineering Physics- Gupta & Gaur (Photometry,

Photoelasticity, fiber optics etc.)
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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