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Holography

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History of Holography

Invented in 1948 by Dennis Gabor for use in electron microscopy, before the invention of the laser

Leith and Upatnieks (1962) applied laser light to holography and introduced an important off-axis technique

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Conventional vs. Holographic photography Conventional:

2-d version of a 3-d scenePhotograph lacks depth perception or parallaxFilm sensitive only to radiant energyPhase relation (i.e. interference) are lost

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Conventional vs. Holographic photography Hologram:

Freezes the intricate wavefront of light that carries all the visual information of the scene

To view a hologram, the wavefront is reconstructed View what we would have seen if present at the

original scene through the window defined by the hologram

Provides depth perception and parallax

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Conventional vs. Holographic photography Hologram:

Converts phase information into amplitude information (in-phase - maximum amplitude, out-of-phase – minimum amplitude)

Interfere wavefront of light from a scene with a reference wave

The hologram is a complex interference pattern of microscopically spaced fringes

“holos” – Greek for whole message

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Hologram of a point sourceConstruction of the hologram of a point source

Any object can be represented as a collection of points

Reference wave - plane

Photosensitive plate1. Records

interference pattern (linear response)

2. Emulsion has small grain structure ()

Object wave - spherical

Photographic plate

x

z

y

Working Principle

Continues..

Creating Holograms

Pattern left on media

Reconstructing the image

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Point object hologram construction:

Intensity distribution on plate Reference wave

Object wave

Intensity distribution on plate

ROORRROOROyxI

zyxrwhere

oeezyxozyxO

reezyxrzyxR

ikrzyxi

ikzzyxi

****2

222

),,(

),,(

),(

),,(),,(

),,(),,(

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Hologram construction

)cos(2),(

0

)cos(2),,(

22

22

krororyxI

planefilmz

ororzyxI

Maxima for kr=2m or r=m

i.e. if the OPL difference OZ – OP is an integral number of wavelengths, the reference beam arrives at P in step with the scattered (i.e. object) beam.

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Hologram

When developed the photographic plate will have a transmittance which depends on the intensity distribution in the recorded plate

tb – backgrond transmittance due to |R|2 term B – parameter which is a function of the

recording an developing process

)( **2ORROOBtt b

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Hologram reconstruction

When illuminated by a coherent wave, A(x,y), known as the reconstruction wave, the optical field emerging from the transparency is,

i.e. a superposition of 4 waves

If A(x,y)=R(x,y), i.e. reconstruction and reference waves are identical,

ORBOBRRBOOttyxR

ABORRABOABOOAttyxA

bp

bp

2*2*

***

)(),(

),(

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Hologram reconstruction

Three terms in the reconstructed wave

ORBOBRRBOOttyxR bp

2*2*)(),(

Direct waveDirect wave – identical to

reference wave except for an

overall change in amplitude

Object wave – identical to object wave except for a change in intensity

Conjugate wave – complex

conjugate of object wave

displaced by a phase angle 2

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Hologram reconstruction Three terms in the reconstructed wave of

the point hologram

ikrikrkziikzbp erBeBeeoBttyxR

222)(),(

Direct waveDirect wave – identical to

reference wave (propagates

along z) except for an overall

change in amplitude

Object waveObject wave – Spherical wave

except for a change in intensity

B|r|2i.e. reconstructed wavefront

Conjugate waveConjugate wave – spherical wave

collapsing to a point at a distance z to the right of the hologram

-a real image- displaced by a phase angle 2kz

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Hologram: Wavelength

With a different color, the virtual image will appear at a different angle – (i.e. as a grating, the hologram disperses light of different wavelengths at different angles)

Volume hologram: emulsion thickness >> fringe spacing Can be used to reporduce images in their original

color when illuminated by white light. Use multiple exposures of scene in three primary

colors (R,G,B)

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Hologram: Some Applications

Microscopy M = r/s Increase magnification by viewing hologram with longer

wavelength Produce hologram with x-ray laser, when viewed with visible

light M ~ 106

3-d images of microscopic objects – DNA, viruses Interferometry

Small changes in OPL can be measured by viewing the direct image of the object and the holographic image (interference pattern produce finges Δl)

E.g. stress points, wings of fruit fly in motion, compression waves around a speeding bullet, convection currents around a hot filament

Capacity: 700 mb Capacity: 4.7 Gb Capacity: 30-50 Gb Capacity: 1-4 Tb

What is Holographic Memory ?

• It is a memory that can store information in form of holographic image.

• It is a technique that can store information at high density inside crystals or photopolymers.

• It provides data to be written beneath the surface of the disc.

• Holographic memory can store up to 1 Tb in a storage medium the size of a sugar cube crystal.

Why do we need this ?

• “For Internet applications alone, industry estimates are that storage needs are doubling every 100 days”

• By the year 2010, a storage system serving an average LAN will need … 100 TB and a WAN server will require 10TB to 1 petabyte …of storage” (Red Herring)

To spilt the laser beam

To direct the laser beams

spatial light modulator

photopolymer

Interprets the digital

informationBlue-green argon laser

Beam splitters

Mirrors

LCD panel

Lithium-niobate crystal

CCD camera

Basic Components

Properties of Hologram

• A block or sheet of photosensitive material which records the diffraction of two light sources.

• A laser beam is splatted into two beams: Source beam Reference beam

• The two beams diffracts to form the image on the recording medium

Spatial light modulator

• It is used for creating binary information out of laser light.

• It is a 2D plane, consisting of pixels which can be turned on and off to create binary 1.s and 0.s.

• It contains a two-dimensional array of windows, which are only microns wide.

Page Data Access

• As the data is stored in the form of holograms, data retrieval must be in the same form.

• So, a holographic system sends data in the form of pages.

• It provides fast access times.

Error Correction

• As the density of data is massive, it is prone to errors.

• Errors can be controlled by:o Recording errorso Page level parity bitso Interfacing

Interfacing

• While reading the data, they are sent to the computer as sheets.

• Such thing may exceed the processor throughput

• Remedy: Defining set of rules which can be used for correction and reading of data efficiently.

Applications

Data Mining1

Petaflop Computing2

Future Computer Systems3

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Future

• Built on technology that’s around for 40+ years

• Holographic Memory is the future of data storageo HUGE capacity, Very fast, Smallero Parallel processing

• Current storage methods nearing there fundamental limits of storage density

• Large market and little new competition