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OPTICS TODAY- AN OVERVIEW
OPTICS TODAY- AN OVERVIEW
By- Dr. Ratnam Challa
"Optics, as a field of science, is well into it's second millennium of life; yet in spite of its age, it remains remarkably vigorous and youthful"
Optics is an enabling science and is an important component of modern, high technological , societies and economies. According to the Australian Optical Society Report , March, 1994, " Both in research and industry, optics is a major growth area of modern science and technology that shows no signs of abating in the foreseeable future". "Prediction is very difficult", said NIELS BOHR, "especially about the future". Two decades ago optical computing was a thriving area of research that offered the prospect of computers much faster than those available at the time. Of course, computers are indeed much faster today than they were 20 years ago, but they are still not optical. However, there have been more than enough developments in other areas of optics, both pure and applied, over this period to compensate for that particular disappointment. Some of these advances were planned or predicted, but many were completely unexpected (PETER RODGERS, Physics world' Editor, April 2005)
Optics is one of the growth areas of modern physics and engineering. As a result, optics finds applications in almost every walk of life in our society. Optical devices are crucial components in many sectors of industry. The development of our modern technological lifestyle is becoming increasingly linked to developments in the field of optics. This encompasses areas that are well known in traditionaloptics:
- optical materials (glasses, crystals, thin film technology)
- optical components (lenses, mirrors, gratings),
- photometry (light measurement, colour definition)
- solar collectors and concentrators
- Optical systems (cameras, telescopes, microscopes, photocopiers etc.)
- lighting and display systems
Conventional optical components are already present in many devices and instruments, which are in everyday use in our society.
The ability to measure, and to compute with speed and accuracy are the hallmark requirements of today's physical and exact sciences. Since its introduction in the late 1950's, the digital computer has furthered the computational abilities of man beyond the wildest dreams. Today computer is used in almost every field of research in optics, not to say less about any other field. The fields of optics that have particularly profited from the use of computers are 1) Lens design and Thin Film design 2) Image Processing and Evaluation 3) Atmospheric Optics 4) Space Optics 5) Remote Sensing and in more recent times 6) Medical Optics.
IMPORTANCE OF RESEARCH IN MODERN OPTICS
There is a considerable quality, breadth and depth in optics research, and in the industrial applications of research in optics. For any country Optics is an appropriate industry in terms of its contribution to wealth generation and employment and its low environmental impact (AUSTRALIAN OPTICAL SOCIETY REPORT, March, 1994).
The new fields of Modern Optics encompass growing areas which are at the forefront of current scientific research, such as Lasers, Fibre optics, Photonics, Optical Computing , Atom Optics, Fourier Optics etc. More importantly, the applications of modern optics increasingly underpin our social and economic well being, and are an important component of modern, high technology industry. Examples include:
- lasers in medicine (cancer surgery, laser angioplasty, eye surgery)
- optical communications (mass information transfer, optical switching)
- imaging and sensing (bar code readers, laser alignment tools)
- optical data storage (compact discs, digital cameras)
- precision measurement (laser rangers and gyroscopes, interferometers)
- holography (data storage, structural testing, security devices)
- materials processing (laser annealing, drilling and cutting)
History of Development of Modern Optics
Since the early 1960`s, it has gradually become accepted that a modern academic training in optics should include a heavy exposure to the concepts of Fourier Analysis and Linear systems theory. During the middle of the 20thcentury, various events and discoveries have given new life, energy and richness to the field of optics. The most important events have been
(1) The introduction of the concepts and tools of Fourier Analysis and communication theory into optics (1940-1960).
(2) The discovery and successful realization of the LASER in the late 1950 `s.
(3) The origin of the field of non –linear optics in 1960`s.
(4) The infusion of statistical concepts and methods of analysis into the field of optics.
(5) Computers have further accelerated the pace of development in the field of optics throughout the entire century.
A realization of the utility of Fourier methods in the analysis of optical systems arose rather spontaneously in the late 1930's when a number of research workers began to advocate the use of sinusoidal test patterns for system evaluation. Much of the initial stimulus was supplied by a French scientist P.M.DUFFIEUX, whose work culminated in the publication of a book, in 1946, on the use of Fourier methods in optics. Unfortunately this book has never been translated into English and is not widely available. In the United States, much of the interest in these topics was stimulated by an electrical engineer named OTTO SCHADE, who very successfully employed the methods of linear systems theory and communication theory in the analysis and improvement of television camera lenses. However the foundations of Fourier optics were in fact laid considerably earlier than 1940, particularly in the works of Ernst Abbe (1840-1905) and Lord Rayleigh (1842- 1919) (GOODMAN, 1968)
In the late 19th century LORD RAYLEIGH had solved many fundamental problems in acoustics and optics. The discovery of the quantized nature of light had dramatically increased the need for statistical interpretation of Quantum Mechanics in optics, which was first introduced by MAX BORN (GOODMAN 1984). In 1954 EMIL WOLF introduced an elegant and broad framework for considering the coherence properties of waves, which laid foundation for several important statistical problems in 0ptics that could be treated in a uniform way. An event of special mention is the classical theory of light detection pioneered by L.MANDEL which tied together in a comparatively simple way, the knowledge of the fluctuations of classical wave quantities (fields, intensities) and fluctuations associated with the interaction of light and matter.
A student level course in Physics, today, encounters optics in an entirely deterministic framework. Physical quantities are represented by mathematical functions that are either completely specified in advance or assumed to be precisely measurable. These physical quantities are subjected to well-defined transformations that modify their form in perfectly predictable manner. For example, the path of a monochromatic light wave with a known complex field distribution at a certain distance away from the screen can be calculated precisely by using the well-established diffraction formulae of wave optics.
Thirty years ago, the enormous impact of lasers on modern optics and technology could not have been predicted. Even now, the full range of possible applications afforded by lasers has yet to be realized. There are also signs that other fields in modern optics, for example atom optics (where atoms are controlled with light in the same way as "conventional" optics controls light with matter) offer similar potential for new technological applications. Optics is no longer an isolated branch of physics. There is a great deal of transformation in the study of Optics over the decades with the use of computers. Optics has developed to its present status by borrowing mathematical concepts from diverse fields such as communication engineering, electronics, nuclear physics, etc. It is the wave nature of light that makes this link possible.
CURRENT RESEARCH AREAS IN OPTICS:
University research in optics is spread across applied and fundamental areas. These activities have been coarsely classified into areas and are briefly described below.
(a) Optical Fibres and Photonics
One of the major advances in this technology is the development of facilities for the design, fabrication and testing of specific optical fibres. This activity includes the research and development of next generation components and devices including optical switches, fibre lasers and fibre amplifiers. These devices are of importance to the advancement of photonics. Photonic devices and circuits are being developed for application to very high bit rate optical communication systems and networks including local and metropolitan area networks for delivery of pay television etc. Other research is being undertaken into nonlinear optical materials and devices with applications to all optical switching. This work is at the leading edge of technology for the future.
(b) Lasers and Applications
Lasers ranging from semiconductor, solid state and CO2 systems operating in the infrared- to- metal vapour regions and rare gas lasers operating in the visible and ultra violet are being researched and developed. The application of laser technology is prevalent, particularly in non-destructive testing and diagnostics, high precision machining of polymers using ultraviolet lasers. The study of man-made plasmas and hypersonic gas flows using laser diagnostics, medical diagnostics employing lasers, and holography are just some of the research topics undertaken in universities in which laser technology is directly applied.
(c) Conventional Optics
The field of conventional optics plays a role of ever increasing importance. Diverse activities such as microscopy, imaging, optometry, solar collectors and astronomy are ongoing activities in various research institutes across the world. A wide variety of specialist microscopes are currently under development in a number of active research groups which are likely to be in departments of physiology and anatomy as well as in departments of physics. Many hospitals are also very active as there is promise, using advanced imaging techniques, for developing methods for the early detection of diseases such as glaucoma. Astronomy is also dependent on high quality optics as highlighted by the custom precision optics, lasers and quantum imaging devices used by the Sydney University Stellar Interferometer. The development of suitable optical components is essential to the advancement of solar energy technology that has the potential to deliver cheap and efficient electricity and also to become an export earner. (AUSTRALIAN OPTICAL SOCIETY REPORT, March, 1994).
The study of vision spans scientific disciplines from mathematics to physiology. Understanding the processes of pattern recognition and visual processing will have implications in problems associated with machine vision such as range sensing and measurement of optic flow. There is growing research interest and activity in this area, which is evolving as an important component in high technology industry across the world.
(e) Atomic and Molecular Physics and Quantum Optics
This predominately fundamental area of research is important to optics in two ways. Firstly, most modern methods of investigating processes in atomic and molecular physics involve the use of state of the art lasers together with high quality optics. Secondly, major advances in modern optics have emanated from this area. For example, the discovery of optical bistability in atomic sodium was the precursor to the field of photonics. Atomic physics gave the world the laser and more recently, squeezed states. The list can be continued. Research in atomic and molecular physics and quantum optics is an area of strength in institutes of advanced research.
(f) Particle and Atom Optics.
Over the last ten years there has been explosive growth of worldwide interest in the optics of massive particles such as atoms and neutrons.
(g) X-ray Optics.
This area is of considerable importance as it not only assists fundamental studies in materials science and astronomy but also underpins the areas of medical imaging. Many of the experimental techniques in x-ray optics, although common in principle with other areas of optics, present a unique set of challenges.
(h) FOURIER OPTICS
JEAN BAPTISTE JOSEPH FOURIER (March 21, 1768 – May 16, 1830) was a French mathematician and physicist who is best known for initiating the investigation of Fourier series and their applications to problems of heat flow. The Fourier Transform is named in his honor. The mathematical technique of Fourier transforms in association with correlation and convolution, when applied to optics resulted in the subject of Fourier transform optics or Fourier Optics, which is an important branch of Modern Optics. The foundation to Fourier Optics was laid in the works of Ernest Abbe and Lord Rayleigh (GOODMAN, 1988) and Michelson (STEWARD, 1983).
The importance and the art of the Fourier transform in optics was briefly but beautifully summarized by FRANCON (1974). According to him the wave aspect of light is the fundamental reason for the important role played by Fourier transform in the field of optics. In fact, the Fourier transform comes in as a natural tool for representation of all vibrational phenomena in physics. It translates the principle of Huygens and enables the study of diffraction phenomena. It translates the formation of images of extended objects and brings in the notion of transfer function. It describes the degree of spatial coherence in terms of source geometry and further more, the degree of temporal coherence in terms of source spectrum. It plays a fundamental role in spatial filtering, character recognition and image processing. Finally, in the domain of Interference spectroscopy, it relates the intensity distribution in the source to the spectral distribution of source energy. Thus, the use of Fourier analysis, justified by the wave aspects of light, presents for optics, the possibilities which are far from having being exhausted (LAKSHMANA RAO, 1994).
The statistical properties of light play an important role in determining the outcome of most optical experiments. A description in terms of certain second order averages known as coherence functions is entirely adequate for predicting experimental outcomes. The origins of the modern concept of coherence can be found in the scientific literature of the 19th and early 20thcenturies. Particularly noteworthy early contributions were made by E.VERDET (1865), M.VON LAUE (1906), (1907), (1907), (1909), (1910), (1915), (1915); M.BEREK, (1926), (1927); P.H.VANCITTERT ,(1934), (1939); F.ZERNIKE (1938), (1948) and others. The next developments of major importance are found in the work of H.H.HOPKINS (1951), A.BLANC-LAPIERRE and DUMONTET (1954), E.WOLF (1953), (1954), (1954). The historical evolution of concept of coherence is presented with an extensive bibliography by L.MANDEL and E.WOLF (1970)
"Emil Wolf is a living legend in the field of physical optics. An icon in the world of optics, Emil Wolf laid the foundations of contemporary physical optics by documenting the concept of spatial coherence before lasers were introduced. This powerful concept has influenced many areas of optical science and engineering", (Tribute to Emil Wolf: Science and Engineering Legacy of Physical Optics, Editor(s): TOMASZ P.JANNSON Publication Date: Dec 2004).
(i) MEDICAL IMAGING
Since its conception, the medical field has been searching for better ways to understand the human body. Today, the optical field is finding new ways to assist in this process. One of those ways is through electromagnetic radiation imaging. On the cutting edge of this field is the g- ray coded aperture imaging.
What is Coded Aperture Imaging?
The coded aperture imaging is an indirect method of imaging in which the conventional pinhole or multi-channel collimator is replaced by a plate containing a coded hole pattern or opening. A point gamma ray source will cast a shadow of this hole pattern on a detector and this shadow is the coded image of the point source , which may also be called the point-spread function of the coded aperture. A source distribution (two-dimensional or three-dimensional) is encoded as a superposition of many such shadows. This encoded pattern on the detector plane is called the shadowgram of the object distribution of intensity. The design of the coded aperture is chosen to facilitate the image reconstruction or decoding process in order to ensure a faithful reproduction of the object or the source distribution on the image plane. The decoding can be accomplished by a number of methods- both optically and electronically.
Coded Aperture Imaging is a technique originally developed for X-ray astronomy by MERTZ (1961) and YOUNG (1963) where typical imaging problems are characterized by far-field geometry and an object made of point sources distributed over a mainly dark background. These conditions provide, respectively, the basis of artifact-free and high Signal-to-Noise Ratio (SNR) imaging. In a report titled "Computer aided Zone Plate 3-D Imaging: Theory, Software and Implementation" sponsored by Laser Plasma Division, Centre for Advanced Technology, Department of Atomic Energy, Govt. of India, (June, 2003), the authors G.S.SOLANKI and H.C.PANT have reported that Coded Imaging (CI) Techniques are being much preferred to conventional Imaging Techniques especially for imaging sources of short wavelength radiations. The CI Techniques have made significant contribution in the following fields:
a) X-ray Astronomy: [ Diek (1968);Young(1963)]
b) Nuclear medicine: [Barrett (1972); Barrett et al(1973); Rogers et al (1973)]
c) Nuclear engineering [Rose et al (1975), Rose (1976)]
d) Inertial Confinement Fusion [Ceglio and Coleman (1977), Ceglio and Larson (1980); Bruno et al (1979)]
Prof. H.H.BARRETT, who is at present Regents Professor in Optical Sciences Centre in the University of Arizona, U.S.A, has contributed an enormous wealth to field of Coded Aperture Imaging Technique and has a large number of U.S. Patents to his credit. He has a huge team of researchers who have themselves published very useful work in this field of research.
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was when recorded. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the recorded image (hologram) appear Three-Dimensional. The technique of holography can also be used to optically store, retrieve, and process information.
Holographic data storage is a technique that can store information at high density inside crystals or photopolymers. The ability to store large amounts of information in some kind of media is of great importance, as many electronic products incorporate storage devices. As current storage techniques such as Blue- Ray Disc reach the 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. In 2005, companies such as Opt ware 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, In Phase Technologies, is developing a competing format.
Security holograms are very difficult to forge because they are replicated from a master hologram which requires expensive, specialized and technologically advanced equipment. They are used widely in many currencies such as the Brazilian Real 20 note, British Pound 5/10/20 notes, Estonian Croon 25/50/100/500 notes, Canadian Dollar 5/10/20/50/100 notes, Euro 5/10/20/50/100/200/500 notes, South Korean Won 5000/10000/50000 notes, Japanese Yen 5000/10000 notes, etc. They are also used in credit and bank cards as well as Passports, ID cards, books, DVDs and sports equipment.
This is only a brief exposition on the ever growing importance of optics in the various fields of life. The world of optics is far reaching and a thorough study is beyond the scope of this article. However, anyone interested in a career in Optics can definitely get an idea of this ever growing branch of Physics.
AUSTRALIAN OPTICAL SOCIETY REPORT, (March, 1994).
BARRETT, H. H., STONER,W.W.,WILSON,D.T ., DE MEESTER,G.D.,(1974),
Optical Engineering.,13, no.6, 539
BARRETT, H. H., WILSON,D.T.,and DE MEESTER,G.D.,(1972), Opt.commun., 5, 398
BARRETT, H.H and DE MEESTER,G.D.,(1974), Appl.Opt.,13, no.5, 1100
BARRETT, H.H and HORRIGAN,F.A.,(1973), Appl.Opt., 12,2686
BARRETT, H.H, (1972), Proc.IEEE 60,723
BORN and WOLF, (1959), "Principles of Optics", 1st edition, p.394,397.,
Pergamon Press, N.Y BORN and WOLF, (1965), "Principles of Optics", p.396,397,441,
Pergamon Press, Oxford
BORN, M. and WOLF,E., (1984), In "Principles of optics", (Pergamon Press, Oxford, SixthEdition), p.524, 441, 435. 440, 526, 522, 881.
BORN, M and WOLF,E.(1970) , "Principles of Optics"
(Pergamon Press,G.B. )4th Ed.,p.333,396,398,424,436,438,462,469&524.
CEGLIO, N.M., and SWEENEY,D.W., (1984), in "Zone plate coded imaging; theory and applications", in Progress in Optics- XXI, Ed. By E.WOLF, Pub. by Elsevier Science publishers, B.V., 1984
DICKE, R.H., (Aug 1968),Scatter-Hole cameras for X-ray and Gamma-rays, Astrophysical J.,vol.153, L101
FRANCON, M., (1974), J.Opt. Soc. Am., 64, p.519. (Abstract)
GHATAK, A.K., THYAGARAJAN,K.(1984), "Contemporary Optics", (MacMillan India,Ltd.)p.17.
GOODMAN, J. W., (1984), In "Statistical Optics". (John Wiley & Sons, New York). P.1
GOODMAN, J.W., (1988), In "Introduction of Fourier Optics", Re-Issue, (Mcgraw-Hill Publ.co., New York), p.1,101, 103, 19, 17, 131
GOODMAN, J.W.(1968) "Introduction to Fourier Optics"(McGraw- Hill,USA) P.1,17,83,101,103,139&148.
LAKSHMANA RAO, V.,(1994), Ph.D Thesis: "Studies on Resolution of two unequally
bright object points by apodised annular optical systems in partially coherent illumination"
RATNAM, C.,(2006), Ph.D Thesis: "FOURIER ANALYTICAL INVESTIGATIONS ON THE PERFORMANCE OFMULTIPLE - ANNULI CODED APERTURES IN MULTIPLEXED TOMOGRAPHY"
About the Author
Doctorate in Physics. Worked as a Physics Lecturer for 30 years. Presently concentrating on Web content writing.
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