Holography. How is that different than photography? How is it accomplished? Amplitude & Phase

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2 Holography 1948: Dennis Gabor proposes lensless imaging: wavefront reconstruction. Calls it total recording or Holo gram Concept: record and recreate wavefront incident on film. Amplitude & Phase How is that different than photography? How is it accomplished? Inclusion of a reference wave, record the interference, capture the phase.

3 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

4 History Continued In 1962 Emmett Leith and Juris Upatnieks realized that holography could be used as a 3-D visual medium From their work, they used a laser to create the first hologram in history, that of a toy train and bird This type of hologram required laser light to be viewed, though.

5 White Light and High Speed Objects In 1962 Dr. Uri Denisyuk of the former U.S.S.R. developed a white light reflection hologram, which could be viewed in light from a normal incandescent bulb. In 1960, with the invention of the pulsedruby laser, holograms of high speed objects was made possible

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

8 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

9 Conventional vs. Holographic photography Hologram: Converts phase information into amplitude information (in-phase - maximum amplitude, out-ofphase 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

10 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 Photosensitive plate 1. Records interference pattern (linear response) 2. Emulsion has small grain structure () y z Object wave - spherical

11 Point object hologram construction: Intensity distribution on plate Reference wave Object wave Intensity distribution on plate R O OR RR OO R O y x I z y x r where oe e z y x o z y x O re e z y x r z y x R ikr z y x i ikz z y x i * * * * ),, ( ),, ( ), ( ),, ( ),, ( ),, ( ),, (

12 Hologram construction I( x, y, z) r 2 o 2 2or cos( ) Fresnel zone plate z 0 film plane I( x, y) r 2 o 2 2or cos( kr) 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.

13 Hologram When developed the photographic plate will have a transmittance which depends on the intensity distribution in the recorded plate * t tb B( O O R 2 OR * ) t b background transmittance due to R 2 term B parameter which is a function of the recording and developing process

14 Hologram reconstruction When illuminated by a coherent wave, A(x,y), known as the reconstruction wave, the optical field emerging from the transparency is, A( x, y) t p t b A BOO * A BO * RA BOR * A i.e. a superposition of 4 waves If A(x,y)=R(x,y), i.e. reconstruction and reference waves are identical, R( x, y) t p ( t b BOO * ) R BR 2 O * B R 2 O

15 Hologram reconstruction Three terms in the reconstructed wave 2 * R( x, y) t ( t BOO * ) R BR O p b B R 2 O 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 Three terms in the reconstructed wave of the point hologram R 2 ikz i2kz ikr ( x, y) t p ( tb B o ) e Be e B r 2 e ikr 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 2kz Object wave Spherical wave except for a change in intensity B r 2 i.e. reconstructed wavefront

17 Direct, object and conjugate waves Reference wave Object wave Real image Virtual image Conjugate wave Direct wave -z z z=0

18 Hologram : Direct, object and conjugate waves Direct wave: corresponds to zeroth order grating diffraction pattern Object wave: gives virtual image of the object (reconstructs object wavefront) first order diffraction Conjugate wave: conjugate point, real image (not useful since image is inside-out due to negative phase angle) first order diffraction In general, we wish to view only the object wave the other waves just confuse the issue

19 Off-axis- Direct, object and conjugate waves Use an off-axis system to record the hologram, ensuring separation of the three waves on reconstruction Reference wave Object wave Direct wave Virtual image Conjugate wave Real image

20 Holography

21 Hologram Playback Images are formed when light is projected through the hologram. The observer sees a virtual image formed behind the hologram.

22 Hologram and images

23 Sum of the Parts? Every part of a hologram contains the image of the whole object. You can cut off the corner of a hologram and see the entire image through it. For every viewing angle you see the image in a different perspective, as you would a real object. Each piece of a hologram contains a particular perspective of the image, but it includes the entire object. The top image is the view through the larger part of the hologram, while the bottom image is through a small corner cut off the hologram. The view through the small corner is from a particular point of view, but contains the whole object.

24 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 reproduce images in their original color when illuminated by white light. Use multiple exposures of scene in three primary colors (R,G,B)

25 Hologram Reflection vs. Transmission Transmission hologram: reference and object waves traverse the film from the same side Reflection hologram: reference and object waves traverse the emulsion from opposite sides View in Transmission View in reflection

26 Recording of Types of Holograms

27 White light reflection holography laser reference wave object wave lamp

28 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 ~ 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

29 Vibrating Surface

30 Holographic Storage