# Holography 1 History of Holography Invented in 1948

• Slides: 31

Holography 1

History of Holography Invented in 1948 by Dennis Gabor for use in electron microscopy, before the invention of the laser n Leith and Upatnieks (1962) applied laser light to holography and introduced an important off-axis technique n 2

Conventional vs. Holographic photography n Conventional: ¨ 2 -d version of a 3 -d scene ¨ Photograph lacks depth perception or parallax ¨ Film sensitive only to radiant energy ¨ Phase relation (i. e. interference) are lost 3

Conventional vs. Holographic photography n 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 4

Conventional vs. Holographic photography n 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 5

Hologram of a point source Construction of the hologram of a point source Any object can be represented as a collection of points Photographic plate Reference wave plane x z y Photosensitive plate 1. Records interference pattern (linear response) 2. Emulsion has small grain structure ( ) Object wave - spherical 6

Working Principle

Continues. .

Creating Holograms

Pattern left on media

Reconstructing the image

Point object hologram construction: Intensity distribution on plate n Reference wave n Object wave n Intensity distribution on plate 12

Hologram construction Maxima for kr=2 m 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. 13

Hologram n When developed the photographic plate will have a transmittance which depends on the intensity distribution in the recorded plate n tb n – backgrond transmittance due to |R|2 term B – parameter which is a function of the recording an developing process 14

Hologram reconstruction n When illuminated by a coherent wave, A(x, y), known as the reconstruction wave, the optical field emerging from the transparency is, n i. e. a superposition of 4 waves n If A(x, y)=R(x, y), i. e. reconstruction and reference waves are identical, 15

Hologram reconstruction n Three terms in the reconstructed wave Direct wave – identical to reference wave except for an overall change in amplitude Conjugate wave – complex conjugate of object wave displaced by a phase angle 2 Object wave – identical to object wave except for a change in intensity 16

Hologram reconstruction n Three terms in the reconstructed wave of the point hologram Direct wave – identical to reference wave (propagates along z) except for an overall change in amplitude Conjugate 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 2 kz Object wave – Spherical wave except for a change in intensity B|r|2 i. e. reconstructed wavefront 17

Hologram: Wavelength n n 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) 18

Hologram: Some Applications n 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 ¨ n 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 ¨ 19

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 10 TB to 1 petabyte …of storage” (Red Herring)

Basic Components spatial light modulator LCD panel To direct the laser beams photopolymer Lithiumniobate crystal Mirrors Beam splitters CCD camera To spilt the laser beam Blue-green argon laser Interprets the digital information

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 2 D 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 o o Recording errors Page level parity bits 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 1 Data Mining 2 Petaflop Computing 3 Future Computer Systems 4

Future • Built on technology that’s around for 40+ years • Holographic Memory is the future of data storage o HUGE capacity, Very fast, Smaller o Parallel processing • Current storage methods nearing there fundamental limits of storage density • Large market and little new competition