Holography

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Hologram artwork at the MIT Museum.

Holography is the science of producing holograms; it is an advanced form of photography that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store, retrieve, and process information.

Contents

Holography was first discovered by Dennis Gabor while working to improve the resolution of an electron microscope at the British Thomson-Houston Company in Rugby, England. Gabor coined the term "hologram" from the Greek words holos, meaning "whole," and gramma, meaning "message." Continued development was not possible until the development of the laser in 1960, which was able to supply a monochromatic (single-color) light source from a single point.

Overview

Identigram as a security element in a German identity card (Personalausweis).

The first holograms which recorded 3D objects were made by Emmett Leith and Juris Upatnieks in Michigan, in 1963, and by Yuri Denisyuk in the Soviet Union.

Several types of holograms can be made. The very first holograms were "transmission holograms," which were viewed by shining laser light through them. A later refinement, the "rainbow transmission" hologram, allowed viewing by white light and is commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission holograms are formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminum coating which provides the light from "behind" to reconstruct their imagery. Another kind of common hologram (a Denisyuk hologram) is the true "white-light reflection hologram" which is made in such a way that the image is reconstructed naturally using light on the same side of the hologram as the viewer.

One of the most promising recent advances in the short history of holography has been the mass production of low-cost, solid-state lasers-—typically used by the millions in DVD recorders and other applications, but sometimes also useful for holography. These cheap, compact, solid-state lasers can compete well with the large, expensive gas lasers previously required to make holograms, and are already helping to make holography much more accessible to low-budget researchers, artists, and dedicated hobbyists.

Technical description

The difference between holography and photography is best understood by considering what a black and white photograph actually is: A point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a color photograph, slightly more information is recorded (in effect the image is recorded three times, viewed through three different color filters), which allows a limited reconstruction of the wavelength of the light, and thus its color.

However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost, and with it the three-dimensional effect. In a hologram, information from both the intensity and the phase is recorded. When illuminating the hologram with the appropriate light, it diffracts part of it into exactly the same wave (up to a constant phase shift invisible to human eyes) which emanated from the original scene, thus retaining the three-dimensional appearance. Although color holograms are possible, in most cases the holograms are recorded monochromatically.

Holographic recording process

To produce a recording of the phase of the light wave at each point in an image, holography uses a "reference beam," which is combined with the light from the scene or object (the "object beam"). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram or the interference pattern.

Holographic recording

It is also important to note that these recorded fringes do not only directly represent their respective corresponding points in the space of a scene (the way each point on a photograph will only represent a single point in the scene being photographed). Rather, an individual section of even a very small size on a hologram's surface contains enough information to reconstruct the entire original scene (within limits) as viewed through that point's perspective. This is possible because during holographic recording, each point on the hologram's surface is affected by light waves reflected from all points in the scene, rather than from just one point. It can be thought of as if during recording, each point on the hologram's surface were an eye that could record everything it sees in any direction. After the hologram has been recorded, looking at a point in that hologram is like looking "through" one of those eyes.

To demonstrate this concept, one could cut out a small section of a recorded hologram, then view that cut-out section. One could still see most of the entire scene simply by shifting the viewpoint, the same way one would look outside from a small window in a house, for example.

Holographic reconstruction process

Holographic reconstruction.

Once the film is processed, if illuminated once again with the reference beam, diffraction from the fringe pattern on the film reconstructs the original object beam in both intensity and phase (except for rainbow holograms, where the depth information is encoded entirely in the zoneplate angle). Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move his or her viewpoint and see the image rotate exactly as the original object would.

Because of the need for interference between the reference and object beams, holography typically uses a laser in production. The light from the laser is split into two beams, one forming the reference beam, and one illuminating the object to form the object beam. A laser is used because the coherence of the beams allows interference to take place, although early holograms were made before the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.

In simple holograms the coherence length of the beam determines the maximum depth the image can have. A laser will typically have a coherence length of several meters, ample for a deep hologram. Also certain pen laser pointers have been used to make small holograms. The size of these holograms is not restricted by the coherence length of the laser pointers (which can exceed 1 m), but by their low power of below 5 mW.

Holograms as diffraction gratings

A diffraction grating receives incoming light from the left which is diffracted by the slits.

A diffraction grating is a transparent sheet with thin slits, the distance between them and their diameter being on the order of the wavelength of the light. Light rays traveling towards it are bent at an angle determined by the distance between the slits and the wavelength of the light.

When holograms are constructed the reference beam and the object beam interfere with one another, and the dark and light fringes of the interference pattern are recorded. When this photograph is developed, the light parts become clear and the dark parts opaque. The clear, light parts become like the slits of a diffraction grating, and the angle at which they bend incoming light (the reconstruction beam) is determined by the spacing between them, which in turn was determined originally by the object beam and reference beam, when the hologram's interference pattern was made. Thus the slits bend the reconstruction beam to be the exact angles at each point that the object beam was going at.

An incoming plane wave interferes with another, traveling at a certain angle to the other.

The distance (d) between the slits is determined by the wavelength of the waves (they are in phase both time-wise and space-wise, so this is the same for both) and the angle between them. If one records the interference pattern through a particular one dimensional slice of the overall two dimensional pattern, one gets the plate with the yellow stripes which represent where destructive interference occurs. If one develops this photographic plate so that the destructive interference stripes become slits, and take away the wave that was traveling at an angle (the object beam), leaving the one traveling perpendicular to the recorded pattern (the reference beam, which then becomes the reconstruction beam). The reconstruction beam will get bent by the slits left by the pattern.

The destructive interference leaves behind slits that become a diffraction grating. The incoming light is bent by them.

The diffraction grating created by the two waves interfering has recreated the "object beam:" the plane wave that was originally traveling at an angle. To further demonstrate the concept, consider a point source and a plane wave interfering:

A point source of light and a plane wave interfere.

The spacing (d) between the destructive interference fringes gets smaller and smaller the further from the plate the light from the point source is. With a smaller (d), the angle the reconstruction wave will get bent through will become sharper. If the photographic plate is developed, and the plane wave shone back through, the light will be bent at differing angles depending on the distance (d) between the slits.

The reference beam is shone through the developed photographic plate.

The diffraction grating reconstructs the point source. The light emerging from the photographic plate is identical to the light emerging when the point source used to be there. If you were standing on the other side of this simple hologram, your eyes would see the curved light rays (these are lines perpendicular to the wavefronts) and follow them perpendicularly back to where they meet, and tell the brain that there is a point there.

This is what human eyes do every day to see images. This is why people can see things that don't correspond directly to reality, like bent spoons in glasses, mirages, and reflections in mirrors, because eyes faithfully follow back the light to where it came from, whether the light actually started there or not: Every time there is a discrepancy between reality and what is seen, it is because light waves have been deviated or bent from their original course.

All objects that humans see, they see as a collection of point sources. Each point on the object radiates out light as a point source and the collection of points eyes see becomes a whole object. It is the same with holograms: Every single point on the object records its own interference pattern, which gets individually reconstructed, and someone's eyes see all these points reconstructed together to see the whole picture of the hologram all at once.

This explains why one's view of the object in the hologram changes with his position; each time he moves, he is seeing a different ray emitted from each point source (like moving around in front of a window, you see the ray from different sides of objects depending where you're standing). With normal photography, the camera records just one view, so when one moves, he is in effect seeing the same ray again and his view doesn't change. (One is seeing different rays from each droplet of ink, but each droplet of ink is one ray of the picture.) The hologram, in comparison, records every possible view there is to see, all at once.

Materials

The table below shows the principal materials for holographic recording. Note that these do not include the materials used in the mass replication of an existing hologram, which are described in the following section. The resolution limit given in the table indicates the maximal number of interference lines per mm of the gratings. The required exposure is for a long exposure. Short exposure times (less than 1/1000th of second) require a higher exposure due to reciprocity failure.

General properties of recording materials for holography.[1]
Material Reusable Processing Type of hologram Max. efficiency Required exposure [mJ/cm²] Resolution limit [mm-1]
Photographic emulsions No Wet Amplitude 6 percent 0.001–0.1 1,000–10,000
Phase (bleached) 60 percent
Dichromated gelatin No Wet Phase 100 percent 10 10,000
Photoresists No Wet Phase 33 percent 10 3,000
Photothermoplastics Yes Charge and heat Phase 33 percent 0.01 500–1,200
Photopolymers No Post exposure Phase 100 percent 1–1,000 2,000–5,000
Photochromics Yes None Amplitude 2 percent 10–100 >5,000
Photorefractives Yes None Phase 100 percent 0.1–50,000 2,000–10,000

Mass replication

An existing hologram can be replicated, either in an optical way similar to holographic recording, or in the case of surface relief holograms, by embossing. Surface relief holograms are recorded in photoresists or photothermoplastics, and allow cheap mass reproduction. Such embossed holograms are now widely used, for instance as security features on credit cards or quality merchandise. The Royal Canadian Mint even produces holographic gold and silver coinage through a complex stamping process.

The first step in the embossing process is to make a stamper by electrodeposition of nickel on the relief image recorded on the photoresist or photothermoplastic. When the nickel layer is thick enough, it is separated from the master hologram and mounted on a metal backing plate. The material used to make embossed copies consists of a polyester base film, a resin separation layer, and a thermoplastic film constituting the holographic layer.

The embossing process can be carried out with a simple heated press. The bottom layer of the duplicating film (the thermoplastic layer) is heated above its softening point and pressed against the stamper so that it takes up its shape. This shape is retained when the film is cooled and removed from the press. In order to permit the viewing of embossed holograms in reflection, an additional reflecting layer of aluminum is usually added on the hologram recording layer.

Dynamic holography

The discussion above describes static holography, in which recording, developing, and reconstructing occur sequentially and a permanent hologram is produced.

There exist also holographic materials that don't need the developing process and can record a hologram in a very short time. This allows one to use holography to perform some simple operations in an all-optical way. Examples of applications of such real-time holograms include phase-conjugate mirrors ("time-reversal" of light), optical cache memories, image processing, and optical computing.

The amount of processed information can be very high (in the Terabits range), since the operation is performed in parallel on a whole image. This compliments the fact that the recording time, which is in the order of a microsecond, is still very long compared to the processing time of an electronic computer. The optical processing performed by a dynamic hologram is also much less flexible than electronic processing. On one side, one has to perform the operation always on the whole image, and on the other side, the operation a hologram can perform is basically either a multiplication or a phase conjugation.

The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photorefractive crystals, but also in semiconductors or semiconductor heterostructures (such as quantum wells), atomic vapors and gases, plasmas, and even liquids it was possible to generate holograms.

A particularly promising application is optical phase conjugation. It allows one to remove the wavefront distortions a light beam receives when passing through an aberrating medium, by sending it back through the same aberrating medium with a conjugated phase. This is useful, for example, in free-space optical communications, to compensate the atmospheric turbulence.

Holographic data storage

Holography can be applied to a variety of uses other than recording images. Holographic data storage is a technique that can store information at high density inside crystals or photopolymers. As current storage techniques such as Blu-ray reach the denser limit of possible data density (due to the diffraction-limited size of the writing beams), holographic storage has the potential to become the next generation of popular storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.

Currently available Spatial light modulators (SLMs) can produce about 1000 different images a second at 1024 × 1024 bit resolution. With the right type of media, probably polymers rather than something like lithium niobate (LiNbO3), this would result in about 1 gigabit per second writing speed. Read speeds can surpass this and experts believe 1 terabit per second readout is possible.

In 2005, companies such as Optware and Maxell have produced a 120 mm disc that uses a holographic layer to store data to a potential 3.9 TB (terabyte), which they plan to market under the name Holographic Versatile Disc. Another company, InPhase Technologies, is developing a competitive format.

Digital holography

An alternate method to record holograms is to use a digital device like a CCD (charge-coupled device) camera instead of a conventional photographic film. This approach is often called digital holography. In this case, the reconstruction process can be carried out by digital processing of the recorded hologram by a standard computer. A 3D image of the object can later be visualized on the computer screen or TV set.

Holography in art

Salvador Dalí claimed to have been the first to employ holography artistically. He was certainly the first and most notorious surrealist to do so, but the 1972 New York exhibit of Dalí holograms had been preceded by the holographic art exhibition held at the Cranbrook Academy of Art in Michigan, in 1968, and by the one at the Finch College gallery in New York in 1970, which attracted national media attention.[2]

The Dalí Holograms were mastered in St. Louis, at the McDonnell Douglas Company, which had just invested in a Ruby Pulse Laser and decided to, aside from meteorological purposes, make industrially oriented projection Holograms for presentations and trade shows. In London, Dalí assembled his models by hanging objects with wires inside of wooden frames. This technique allowed for overlapping and differences in depth.

Since then the quality of the holograms has increased dramatically, mainly due to better holographic emulsions. As of 2005, there are many artists who use holograms in their creations.

Notes

  1. Lecture Holography and optical phase conjugation held at ETH Zürich by Prof. G. Montemezzani in 2002
  2. Holophile, Inc., History of Holography. Retrieved August 4, 2007.

References

  • Hariharan, P. 2002. Basics of Holography. Cambridge, UK: Cambridge University Press. ISBN 0521807417
  • Kasper, Joseph Emil, and Steven A. Feller. 2001. The Complete Book of Holograms: How They Work and How to Make Them. Mineola, N.Y.: Dover Publications. ISBN 0486415805
  • Saxby, Graham. 2004. Practical Holography. Bristol, UK: Institute of Physics Pub. ISBN 0750309121
  • Schnars, Ulf, and Werner Jueptner. 2005. Digital Holography: Digital Hologram Recording, Numerical Reconstruction, and Related Techniques. Berlin: Springer. ISBN 354021934X

External links

All links retrieved March 4, 2014.

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