2-Principle of holography

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20 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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Optical Holography

1
-
Introduction:


In (1948) Dennis Gabor outlind a two step lensless imaging process. It
is
radically a new technique of
photographing the obj
ects and is known as wave front
reconstruction. The technique is also called
holography. The word 'holography' is
formed by combining parts of two Greek words: 'holos', meaning "whole", and
'graphein' meaning "to write". Thus holography means writing the complete image.
Holography is actually are cording of interference pattern for
med between two
beams of coherent light coming from the same source. In this process both the
amplitude and phase components of light wave are recorded on a light sensitive
medium such as a photographic pla
te. The recording is known as a
hologram.
Holograp
hy required an intense coherent light source. Laser was not available
when Gabor formulated the idea of holography. Holographic technique became a
practical proposition only

after the invention of lasers.
Leith and Upatnicks
prepared laser holograms for th
e first time.






2
-
Principle of holography:


Holography is a two
-
step process. Fir
st step is the recording of hologram
where the object is transformed into a photographic record and the second step is
the reconstruction in which the hologram is transformed into the image. Unlike in
the conventional photography, lens is not required in e
ither of the steps. A
hologram is the result of interference occurring between two waves, an object
beam which is the light scattered off the object and a coherent background, the
reference beam, which is the light reaching the photographic plate directly.

In
Gabos's original experiments, the reference beam and object beams
were coaxial.
Further

advance was made by Leith and Upatnieks, who used the reference beam
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at an offset angle. That made possible the recording of holograms of three
-
dimensional objects.




2
-
1
-
Recording of the hologram:

In the off
-
axis arrangement a broad laser
beam is divided into two beams, namely a
reference beam and object beam by a beam splitter. The reference beam goes
directly to the photographic plate. The second beam of light is directed onto the
object to be photographed. Each point of the object scatte
rs the incident light and
acts as the source of spherical waves. Part of the light, scattered by the object,
travels towards the photographic plate. At the photographic plate the innumerable
spherical waves from the object combine with the plane light wave

from the
reference beam. The sets of light waves are coherent because they are from the
same laser. They interfere and from interference fringes on the plane of the
photographic plate. These interference fringes are a series of zone
-
plate like rings,
but
these rings are also superimposed, making a complex pattern of lines and
swirls. The developed negative of these interference fringe
-
patterns is a hologram.
Thus, the hologram does not contain a distinct image of the object but carries a
record of both the

intensity and the relative phase of the light waves at each point.

3









Figure

(1)
:

Hologram recording: the interference pattern produced by the reference
wave the object
wave is recorded



2
-
2
-
Reconstruction of the image:



Whenever required, the object can be
viewed.
For

reconstruction of the
image, the hologram is illuminated by a parallel beam of light from the laser. Most
of the light passes straight through, but the complex of line fringes acts as an
elaborate diffraction grating. Light is diffracted at a fairly wide a
ngle. The
diffracted rays from two images: a virtual image and a real image. The virtual
image appears at the location formerly occupied by the object and is sometimes
called as the true image. The real image is formed in front of the hologram. Since
the l
ight rays pass through the point where the real image is it can be photographed.
The virtual image of the hologram is only for viewing. Observer can move to
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different positions and look around the image to the same extent that he would be
able to, were he
looking directly at the real object. This type of hologram is known
as a transmission hologram since the image is seen by looking through it.






The ori
ginal configuration adopted by Gabor for recording hologram was a
coaxial arr
angement, as illustrated in figure
(2). In this arrangement the real image
is loca
ted in front
of the virtual image and is inconvenient for viewin
g or
photographing. The advantage

of the off
-
axis

configuration is that the two images
are separate.







The fundamental difference between a hologram and an

ordinary
photogra
ph is like this. In a photograph the information is stored in an orderly
fashion: each point in the object relates to a con
jugate point in the image. In a

hologram there is no such rela
tionship; light from every
object point goes to the
entire hologram. Th
is has two
main advantages


1
-
As the observer moves sideways
viewing the hologram, the image is
seen in
three dimensions


2
-
If the hologram were shattered or cut into small pieces, each fragment would still
reconstruct
the whole object, not
just part of the object.








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Figure:

(2)

Image reconstruction: light diffracted by the hologram
reconstructs the object wave.


3
-
Applications:

3
-
1
-
Holographic optical elements


A hologram can be used to transform an optical wave front in the same
manner as a lens. In addition, computer generated holograms can produce a wave
front having any arbitrary shape. As a result, holographic optical elements (HOEs)
can perform un
ique functions and have been used in several specialized
applications.



A major advantage of HOEs over conventional optical elements is tha
t their
function is independent of substrate geometry. In addition, since they can be
produced on thin substrates, they are quite light, even for large apertures. Another
advantage is that several holograms can be recorded in the same layer, so that
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spatia
lly overlapping elements are possible. Finally, HOEs provide the possibility
of correcting system aberrations in a single element, so that separate corrector
elements are not required.




The recording material for a HOE must have high resolution, good stability,
high diffraction efficiency and low scattering. Photoresists and dichromated gelatin
are, at present, the most widely used materials. Photopolymers are
an attractive
alternative.






3
-
2
-
holographic neural networks:


Holographic neural networks are attractive because they offer large storage
capacity as well as parallel access and processing capabilities during both the
learning and reading phases.





In a holographic neural network, neurons are represented by the pixels on a
spatial light modulator. The brightness of a pixel corresponds to the activation
level of the neuron. If a pair of pix
els are illuminated with a coherent beam, a
volume hologram can be formed in a suitable recording material. If, subsequently,
one of the original two beams is used to address the hologram, the other beam is
reconstructed with an efficiency that represents
the weight between these neurons.
With a photorefractive recording material, a process of learning can be
implemented by increasing, or decreasing, the weights selectively.






3
-
3
-
Acoustic holography


It is easy to produce coherent sound waves. Sound waves readily propagate
in solids. Therefore, a three dimensional acoustical hologram of an opaque object
can be made. By viewing such hologram in
visible light the internal structure of
the object can be observed. Such techniques will be highly useful in the fields of
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medicine and technology. In one of the techniques, two submerged coherent sound
generators emit the reference and the signal, scatter
ed by an object, respectively.
On a calm surface of water, these two contributions produce ripples. The ripple
pattern is the hologram. The pattern may be photographed and then reconstructed.
As sound waves can propagate through dense liquids and solids, a
coustical
holography has an advantage in locating underwater submarines etc and internal
body organs show in fig
ure
(3).







Fig
ure (3)