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Holography

Holography. Mon. Dec. 2, 2002. 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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Holography

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  1. Holography Mon. Dec. 2, 2002

  2. 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

  3. Conventional vs. Holographic photography • 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

  4. 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

  5. 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

  6. 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 • Photosensitive plate • Records interference pattern (linear response) • Emulsion has small grain structure () Reference wave - plane x z Object wave - spherical y

  7. Point object hologram construction: Intensity distribution on plate • Reference wave • Object wave • Intensity distribution on plate

  8. Hologram construction Gabor zone plate Maxima for kr=2m or r=m i.e. if the OPL difference OZ – OP is an integral number of wavelengths, the referencebeam arrives at P in step with the scattered (i.e. object) beam.

  9. 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

  10. 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,

  11. Hologram reconstruction • Three terms in the reconstructed wave Direct 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 

  12. Hologram reconstruction • 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 2kz Object wave – Spherical wave except for a change in intensity B|r|2 i.e. reconstructed wavefront

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

  14. 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

  15. 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 Conjugate wave Virtual image Real image

  16. Hologram – Reflection vs. Transmission • Transmissionhologram: 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

  17. 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)

  18. 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

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