Modern Communication: The Laser and Fiber-Optic Revolution. Basic Research Remains Vital
Amid those fast-paced developments, basic research continued to yield important improvements. In early fiber-optic 
systems, the amplifiers for regenerating a weak signal constituted a bottleneck. Although optical devices could be used to detect an incoming laser signal, some sort of electronic circuitry was needed to convert it to electric current, amplify the current, and then drive a new laser to recreate the optical signal. This limited the system to the capacity of the electronic amplifiers, which was considerably less than the potential capacity of the lasers and optical fibers. But in 1985, at England's University of Southampton, physicist S. B. Poole discovered a solution. Adding a small quantity of the element erbium to the glass used in optical fibers would make it possible to build an all-optic amplifier. A short strand of erbium- doped glass, spliced into the main fiber, would receive energy from an external source and act as a laser in its own right, amplifying a weak optical signal without using electronics.
Poole's colleagues at Southampton, David Payne and P. J. Mears, and Emmanuel Desurvire at Bell Laboratories, proceeded to turn the discovery into practical and effective fiber-optic amplifiers. In 1991, investigators at Bell Laboratories demonstrated that an all-optic system would have a carrying capacity about 100 times that then achievable with electronic amplifiers. In short order, both European and American communication firms installed all-optic cables across the Atlantic, and a Pacific cable entered service in 1996.
Clearly, progress has been remarkable and rapid. Impressive as these accomplishments are, even more dramatic advances are on the horizon. Although today's fiber-optic systems serve as trunk lines, carrying large numbers of voice and data channels between central telephone stations, industry specialists speak wistfully of the "last mile"--from the central station to your home. Today's telephone system spans that last mile with conventional copper-wire equipment, which provides good voice connections but is still inadequate for carrying large quantities of high-speed data. High-speed data lines for that last mile are available, and many businesses have them, but they are generally more expensive than is practical for home use today. Whatever new technology turns out to provide the last crucial link from individuals to the rest of the world, the research that gives rise to it will have come from scientists who probe beneath the immediate needs of any given industry, investigating seemingly unrelated processes to understand the fundamental nature of the world.
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| MODULE 9 HOLOGRAPHY Texts: A. Fundamentals of Holography B. Recording and Reconstruction Processes C. Y. Denisyuk's Discovery D. Holographic Information Storage |
Terminology A:
1) three-dimensional image – трехмерное изображение;
2) complete record – полная запись;
3) optical scene – оптический объект, картина;
4) conventional – обычный, традиционный;
5) converging lens – сходящаяся, собирающая, линза;
6) interference pattern – интерференционная картина;
7) reference beam – опорный пучок;
8) development – зд.: проявление, проявка;
9) obscure – неясный, смутный;
10) viable – действующий, жизнеспособный.
Vocabulary Practice
1. Underline the suffixes:
viability, emulsion, reference, researcher, diffusiveness, referable, radiant, exposed, dimensional, diffusive, commonly, uniformly, quietly, directly, obscurely, efficiently.
2. Complete the table using a dictionary if necessary:
| verb | noun(s) | adjective | adverb |
| research | |||
| complete | |||
| transparently | |||
| develop | |||
| variation | |||
| record | distinct | ||
| displacement | |||
| arrange | |||
| distinguishing | |||
| determine | observation | ||
| separately | |||
| compare | applicability | ||
| creative | |||
| suggest |
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4. Choose the correct form of each word to complete the following sentences:
a) creation, creator, creating, creative
A hologram can be considered to be an optical device capable of ________ three-dimensional images.
b) expose, exposure, exposed, exposer
In real-time holographic interferometry the photographic plate after development is placed back in the same position it occupied during ___________.
c). irradiate, irradiance, irradiator, irradiant
The wave front to be regenerated does not require the time coherence of the ________.
d). converge, converging, convergence, convergent
In conventional photography a ___________ lens is used for focusing the light reflected from a scene.
READING AND DISCUSSION
TEXT 9A FUNDAMENTALS OF HOLOGRAPHY
Dennis Gabor published the first papers on holography in 1948. From 1948 through 1960 progress in the use and development of holography was slow because researchers working in this discipline did not have a light source available that could produce intense light with a long coherence length. Emmett Leith and Juris Upatnieks working at the University of Michigan Institute of Science and Technology in the early 1960s produced the first holograms using a laser. In doing so Leith and Upatnieks transformed holography from an obscure concept to a viable scientific and engineering tool. In 1965 Powell and Stetson published the first paper on holographic interferometry. Dennis Gabor received the Nobel Prize in physics for his work in 1972.
For our purposes, a hologram can be thought of as an optical device produced by using photographic techniques and laser light that is capable of creating three-dimensional images. Theword hologram stems from the Greek root “holos”, which means whole, complete, or entire, and “gram”, which means message. Thus a hologram is a complete record of an optical scene. In conventional photography the light reflected from a scene is focused – using a converging lens system – onto photographic emulsion. Variation of the irradiance due to the image being focused on the emulsion is related only to the electric field intensity amplitude of the light. In holography photographic emulsion, usually on a glass plate, is exposed to an interference pattern produced by two coherent laser beams. One beam called the object beam is reflected from an object or scene to the photographic emulsion. The other beam called the reference beam is reflected directly from the laser to the photographic emulsion by using mirrors. Using this technique both the amplitude and the phase information about the electric field due to the light reflected from the scene can be recorded. After photographic development the resulting transparency is a hologram. By shining laser light – in some cases, white light – through a hologram, three-dimensional images can be produced.
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2200 п.зн.
5. Answer the following questions:
1) Who was the inventor of holography? 2) Why couldn't Dennis Gabor produce a hologram in 1948? 3) What source of light was used to produce the first holograms? 4) How was D. Gabor's work appreciated? 5) What does the word "hologram" mean? What is the origin of this word? 6) What is the difference in recording images by conventional photography and holography? 7) What are two coherent laser beams which produce interference pattern? How are three-dimensional images produced?
Review
6. Match the parts of sentences in column A with those in column B to produce true statements:
| A.1) Holography is a way of | B. a) complete or entire message. |
| 2) A hologram is a device | b) recording and then reconstructing waves. |
| 3) The word “hologram” means | c) both about its amplitude and its phase. |
| 4) The physical foundation of holography is | d) the reference beam |
| 5) A beam reflected from the laser to the photоemulsion is called | e) the science of waves, their interference and diffraction. |
| 6) Holography records complete diffraction information about a wave | f) capable of creating three-dimensional images. |
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7. Study the following sentences carefully and state whether they are true or not. Correct the false ones:
1) Holography was invented in 1948 by Dennis Gabor, professor, 1972 Nobel Prize winner.
2) Using a laser as a source of light he ran his experiments in the Research Laboratory in England.
3) Since then, holography began to develop at a very rapid rate. 4) A hologram just like conventional photography regenerates not a two-dimensional image of an object but the field of the wave which it scatters. 5) Emulsion in holography is exposed to an interference pattern produced by two coherent laser beams. 6) The beam reflected from an object or scene to the photoemulsion is called a reference beam.
8. Make sentences by matching the dates with the events listed in the text:
| 1) In 1948 | a) first holograms |
| 2) From 1948 through 1960 | b) photographic interferometry |
| 3) In the early 1960s | c) the first papers on holography |
| 4) In 1965 | d) the Nobel Prize |
| 5) In I972 | e) didn't have a light source |
9. Rewrite the sentences to make a logical paragraph:
In holography photographic emulsion is exposed to an interference pattern produced by two coherent laser beams. Thus it is a complete record of an optical scene. By shining laser light we see the object from different angles sensing its three-dimensional and realistic nature. The word “holography” originates from the Greek "holos" meaning the whole? These are the object beam and the reference one. Thus it is a complete record of an optical scene.
Terminology B:
1) to mount – устанавливать;
2) transparent phenomena – явления, происходящие в реальном масштабе времени;
3) split – расщеплять, splitter – расщепитель;
4) pinhole – пиксель (элемент разложения);
5) bias – смещать;
6) spatial filter – пространственный фильтр;
7) amplitude transmittance – коэффициент пропускания;
8) quiescent point – рабочая (неподвижная) точка;
9) exposure curve – характеристическая кривая;
10) fringe – полоса;
11) virtual image – мнимое изображение;
12) real image – действительное изображение.
Vocabulary Practice
1. Give as many derivatives of the following words as you can:
transit, space, finite, quiet, curve, unify, photo, image, consider, develop, reflect, observe.
2. Continue the examples for each prefix meaning:
not (in-, un-, im-, ir-): inappropriate, incalculable, unnecessary, irrelevant, improbable…
again (re-): reassure, recover, recoil, refresh…
without (dis-): disability, disbelieve, disorder…
3. Choose the correct form of each word to complete the sentence that follows:
a) split, splitter, splitting, splitted
By means of a beam _________the laser beam is divided into two different beam.
b) dust, duster, dusty, dustiness
The rings and stripes a hologram often contains are due to the diffraction of light on particles of ________ getting onto the mirrors.
c) scatter, scattering, scatted, scatterer
The colour of the sky is due to the small amount of _________the sunlight by molecules of the air.
d) virtual, virtualization, virtual, virtually
Apparently, images from which the light is diverging are termed _________images.
READING AND DISCUSSION
TEXT 9B RECORDING AND RECONSTRUCTION PROCESSES
Recoding process
All the optical elements used in the production of optical holograms are mounted on an essentially vibration-free surface. Usually a heavy table with a steel or granite top isolated from floor vibration by air tubes provides this surface. A continuous or pulsed laser operating in the TEM00 mode at a wavelength within the visible portion of the electromagnetic spectrum is used for holography. Pulsed ruby lasers are used to produce holograms of transient phenomena or when vibration is a problem.
To produce a hologram a laser beam is first split into two teams by abeam splitter. The reference beam is reflected by a mirror and then spread by a converging lens to illuminate uniformly a photographic emulsion on a glass plate. The object beam is spread by a converging lens and then reflected to the object by a mirror so that the object is uniformly illuminated as seen from the position of the photographic plate. Pinholes are used in the focal planes of the converging lenses as spatial filters to remove optical noise from the laser beams. The optical noise is primarily due to light scattered by dust and flaws on the optical element surfaces. Maximum coherence of the two beams at the photographic plate is obtained by making the object and reference beams of equal length.
It is desirable that objects used in holography have diffusive surfaces. Flat white paint is often used to give industrial parts uniform diffusive surfaces. When illuminated, each point on these surfaces will act as point sources scattering light in all possible directions. Thus optical information about every point on the object facing the photographic plate can be recorded on an infinite number of points on the photographic emulsion.
To obtain a linear recording, the emulsion is “biased” to establish a quiescent point in the linear portion of the transmittance versus exposure curve. By making a linear recording it can be assumed that the amplitude transmittance of the emulsion after development is a linear function of the irradiance incident on the emulsion during exposure. That is, t = to + β (Io + UgUo/2z + UgUo/2z), where to is the “bias” transmittance established by the reference beam and β is the product of the slope of the transmittance versus exposure curve at the quiescent point and the exposure time (Goodman, 1968).
Reconstruction
Holograms produced by using the optical system described above can be used to form virtual and real images. To “play back” a virtual image a reconstruction beam is used that is identical to the reference beam used in the production of the hologram. In most industrial applications a laser is used for reconstruction. When a reconstruction beam, which is identical to the reference beam, is used to illuminate the hologram, much of the light passes straight through the hologram. Some light is diffracted by the fringes recorded in the emulsion due to its exposure to the reference pattern produced by the object and reference beams.
An observer viewing the diffracted light will see a virtual image of the object in space behind the hologram. This image is three-dimensional and in the same location as the object was relative to the emulsion. The image will appear to be the same as if the observer were looking at the original object illuminated by the light used for reconstruction through a window the size of the hologram.
3500 п.зн
4. Answer the following questions:
1) Why is it necessary to mount all optical elements used in producing a hologram on a vibration - free surface? 2) What lasers are considered to be the mostly commonly used for holography?
3) Why is it desirable that objects used in holography have diffusive surface? Explain your answer. 4) What material is used to make such surfaces? 5) When is maximum coherence of the two beams obtained? 6) Compare an emulsion used in holography before and after development. Is it different? 7) Under what condition is a linear recording obtained?
Review
5. Match the parts of sentences in column A with those in column B to produce true statements:
| A. 1) For producing a hologram all optical elements are mounted | B. a) pulsed ruby lasers are used to produce holograms. |
| 2) In case of vibration | b) pinholes as spatial filters are used in the focal planes of lenses. |
| 3) To remove optical noise from the laser beams | c) its exposure to the interference pattern. |
| 4) Optical information about every point on the object | d) on a heavy table providing vibration-free surface. |
| 5) In reconstructing a hologram some light is diffracted by the fringes recorded in the emulsion due to | e) on a infinite number of points on the photographic emulsion. |
6. Study the following sentences carefully and state whether they are true or not. Correct the false ones:
1) In order to provide a vibration-free surface all elements used to produce holograms are mounted on a heavy table. 2) Pulsed ruby lasers are commonly used for holography. 3) As a result of splitting a laser beam both the reference beam and the object one are reflected by a mirror simultaneously. 4) Diffusive surfaces of objects are provided by thick layers of a silver halide. 5) Before and after development a light amplitude transmittance of an emulsion is identical. 6) To reconstruct a real image a reconstruction beam opposite the reference beam is used to illuminate the hologram. 7) In this case all the light passes in straight through the hologram. 8) To reconstruct a virtual image a collimated reconstruction beam is used.
7. Read the text below and answer the following questions:
1) What are Lippmann layers? 2) What contribution into holography was made by Yu. Denisyuk?
In 1962, just before the “holography explosion” the Russian physicist Yu N. Denisyuk published an important paper in which he combined holography with the ingenious method of photography in natural colours, for which Gabriel Lippmann received the Nobel Prize in l908. Lippmann produced a very fine-grain emulsion with colloidal silver bromide and backed the emulsion with mercury serving as a mirror, light falling on the emulsion was reflected at the mirror, and produced as a set of standing waves. Colloidal silver grains were precipitated in the maxima of the electric vector in layers spaced by very nearly half a wavelength. After development, when the complex of layers illuminated with white light reflected a narrow waveband around the original colour did the wavelets scattered at the Lippmann layers add up in phase.
Denisyuk’ s suggestion is as follows. The object wave and the reference wave fall in from opposite sides of the emulsion. Again standing waves are produced and Lippmann layers, but these are no longer parallel to the emulsion surface, they bisect the angle between the two wavefronts. If now, and this is Denisyuk’s principle, the developed emulsion is illuminated by the reference wave, the object will appear in the original position and (unless the emulsion has shrunk) in the original colour.
Though Denisyuk showed considerable experimental skill, lacking a laser in 1962 he could produce only an “existence proof”. A colour reflecting hologram which could be illuminated with white light was first produced in 1965 by G.V. Stroke and A. Labeyrie.
1600 п. зн.
8. Translate text D in writing:
TEXT 9D HOLOGRAPHIC INFORMATION STORAGE
The application of holography which is certain to have gained high importance is information storage. Holography allows storing 100-300 times more printed pages in a given emulsion than ordinary microphotography. Even without utilizing the depth dimension the factor is better than 50. The reason is that a diffused hologram represents almost ideal coding, with full utilization of the area and of the gradation of the emulsion, while printed matter uses only about 5-10% of the area, and the gradation not at all. A further factor arises from the utilization of the third dimension, the depth of the emulsion. This possibility was first pointed out in an ingenious paper by P.J. van Heerden in 1963.Theoretically it appears possible to store one bit of information in about one wavelength cube. This is far from being practical but the figure of 300 previously mentioned is entirely realistic.
However, even without this enormous factor, holographic storage offers important advantages. A binary store, in the form of a checkerboard pattern on microfilm can be spoiled by a single grain of dust, by a hair or by a scratch, while a diffused hologram is almost insensitive to such defects. The holographic store is according to its author L.K. Anderson (1968) only a modest beginning, yet it is capable of accessing, for instance, any one of 64 x 64 printed pages in about a microsecond. Each hologram, with a diameter of 1.2 mm can contain about 104 bits. Reading out this information sequentially in a microsecond would of course require an impossible waveband but powerful parallel reading means can be provided. One can confidently expect enormous extensions of these “modest beginnings” in the near future.
1700 п. зн.
10. Render the text given below in English. Use the following words:
to improve, resolving power, imagery to assume, considerable, undisturbed, to be neglected.
В 1947 г. Габор заинтересовался электронной микроскопией. Пытаясь усовершенствовать разрешающую способность электронного микроскопа, он предложил двухступенчатый метод получения оптического изображения. Сначала объект освещается когерентной волной. Предполагается, что объект находится в таком состоянии, что значительная часть волны проникает через него невозмущенной. Это означает, что, если волну выразить в виде суммы падающей и дифракционной волны, рассеяние последней незначительно.
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