fibra óptica – SBPMat – Sociedade Brasileira de Pesquisa em Materiais

And now we are back to the history of optical fibers. [See the first part of our story]

In the late 1950s, short optical fibers were already industrially produced and used in certain segments, especially in medicine to inspect the interior of the human body using endoscopes.

In this figure on the electromagnetic spectrum, we can compare the different types of electromagnetic radiation. Source: https://en.wikipedia.org/wiki/Electromagnetic_spectrum#/media/File:EM_Spectrum_Properties_edit.svg

In telecommunications, the transmission of information through copper wires and radio waves was established and continued to advance. The first transatlantic copper wire cable was installed in 1956, and the first telecommunications satellite, which used radio waves, was launched two years later. However, the increasing use of the telephone and television was creating an urgent demand to increase the capacity to transmit information.

Telecommunication companies in Europe and the United States began to seek solutions in their research labs. Most of the research focused primarily on the use of microwaves and short-wave radio waves, but did not consider the waves of the so-called “optical region,” which is mainly made up of visible light. Yet it was in the waves of visible light where the greatest potential for communications could be found. To give you an idea, for example, these waves can carry tens of thousands of times more information than radio waves.

The emergence of the laser somewhat changed the story of optical telecommunications. Invented in 1960 at a research center of an aerospace company of the United States, the laser began to gain new and better versions throughout the decade. With its ability to emit light in the form of very narrow beams that are preserved over large distances, the laser could be a great partner to fiber optics.

However, the optical fiber was left out because of its enormous attenuation – a reduction in the intensity of the light signal between two points, which is measured in decibels lost per kilometer (dB/km). In fact, using the available optical fibers at that time, only 1% of the light injected into the fiber remained within it 20 meters ahead. Faced with this very low efficiency, other ways of guiding light began to be proposed and tested by some groups, while other researchers continued to invest efforts and resources into radio or microwave waveguides.

The few groups that opted for fiber optics or similar optical waveguides (thin films, for example) in the early 1960s were located at STL (research center of the British telecommunications company STC); at CSF (a strong French business group active in areas such as telecommunications, defense, materials and electronics); at the Bell Labs (US industrial research laboratory then connected to the AT&T telecommunications company), and at the Japanese university of Tohuku.

Charles Kao, probably in 1966. Source https://www.youtube.com/watch?v=2-5sScP_fiw

In the STL group, there was a man called Charles Kao, who would go on to win the Nobel Prize for Physics in 2009 in recognition of his work with fiber optics. Born in Shanghai, China, Kao attended high school at a British college in Hong Kong and moved to England to pursue university studies in electronics and communications, which he loved. He graduated in Electrical Engineering from the University of London in 1957, and soon began working for STC, until he received and accepted a proposal to do a business doctorate in the company’s research arm, STL. There he helped the researcher Antoni E. Karbowiak in his studies on various waveguides until Karbowiak left STL to take up a professorship. At that time, Kao dedicated himself to a project at STL which he believed in, the development of fibers composed of core and coating to be used in telecommunications as guides of visible light waves.

Kao then relied on the help of his colleague, the young engineer George Hockham, to develop his studies on fiber optics. Together they set out to understand the causes of light loss in the fiber to assess whether they could be eliminated or diminished, or whether, on the contrary, trying to lower the attenuation meant facing a losing battle. While Hockham studied the imperfections in the shape or size of the fibers, Kao concentrated on the characteristics of the material, in particular its structure and the impurities and defects. The results of their studies were published in June 1966 in the IEEE Proceedings [K.C. Kao and G.A. Hockham, “Dielectric-Fibre Surface Waveguides for optical frequencies”. Proc. IEE, 113, 1151 (1996)].

This paper can be considered a milestone in the history of fiber optics, since it is the first to report the causes of light loss in fiber optics and it has shown the way forward and the goal to be reached in order to achieve suitable for use in telecommunications.

Based on the characteristics of existing light emitters (laser) and detectors, Kao and his co-author argued that in order to use the fibers in optical telecommunications, it was necessary to lower their attenuation to 20 dB/km. The goal was very challenging, because in the fibers then available the light attenuated 20 dB… every 20 meters! At its best. However, by showing that the main causes of light loss in optical fibers were related to the presence of impurities in the material, which absorbed or scattered light and diverted it from its path, the paper pointed out a way to reduce the attenuation: the use of purest glasses.

Representation of the frontal cut of an optical fiber (in which proportions were not considered) with the two main parts of the fiber: the core, with n1 refraction index, and the coating, with lower refractive index (n2). Source https://pt.wikipedia.org/wiki/Fibra_%C3%B3ptica#/media/File:Optical_fiber.svg.

The article concluded that cylindrical fibers composed of a core and a coating, both made of vitreous materials with slightly different refractive indices (higher in the nucleus), could be a much better means for transmitting information than those existing at the time, in addition to being inexpensive.

In these fibers, the information would travel encoded in light signals that would run through the core, while the coating would ensure that the light remained in the nucleus, even in the curves.

After that, Charles Kao continued to focus on fiber optics, investing his time not only in research but also in dissemination. In fact, he lectured on his studies and on the potential of fiber optics in several laboratories and companies around the world. In addition, STL released a press release highlighting the possibilities of fiber optics in the field of telecommunications, with little impact on the press.

In parallel, along with new collaborators, Kao performed a series of experiments with various glasses and other materials and showed, among other results, that in fused silica glass, the attenuation could reach only 5 dB/km. The result was encouraging, but turning that material made of pure silicon dioxide (SiO2) into an optical fiber was another story. Due to its purity, this glass could only be melted at very high temperatures, above 1,500 °C. In addition, after melting, its viscosity made it difficult to transform into any product. Finally, the refractive index of the fused silica was extremely low. Thus, using it to make the fiber core, if on the one hand it would be advantageous in terms of purity, on the other hand it would be very complicated, not only because of the difficulty of processing the material, but also because of the impossibility of finding a material with a lower refraction index for the coating.

At that time, some laboratories from companies in Germany, the United States, France, the United Kingdom and Japan decided to face the challenge of developing low-attenuation fiber optics. Faced with the difficulty of dealing with the fused silica, most of them gave up on this material and tried to make optical fibers with other glasses, removing the impurities. Other groups, on the other hand, gave up making low attenuation optical fibers upon hearing from glass experts who claimed that it would be impossible to remove the impurities that were so problematic.

Only one of these groups made different choices, the Corning company in the United States. Founded in 1851, the company always worked with glasses, but far from stagnating in the production of low value-added products, it led the development of many innovations, starting with the glass globe of Thomas Edison’s incandescent lamp. In the early 1930s, it was at Corning that the chemist Franklin Hyde created the flame hydrolysis method that enabled the manufacture and processing of fused silica. This method, instead of fusing silicon dioxide crystals, is based on a silicon-based liquid compound which when heated on top of a flame, generates a powder that can be deposited forming layers of silica.

Peter Schultz, Donald Keck and Robert Maurer, and optical fiber. Source http://ethw.org/File:Corning_Fiber-optic_Inventors_3.jpg

In 1966, Corning commissioned physicist Robert Maurer to research and develop fiber optics of less than 20 dB km attenuation for use in optical communications. In 1968, two more scientists had joined Maurer in this project: Peter Schultz, PhD in Glass Science, and Donald Keck, PhD in Physics.

The trio firmly worked on ideas that were opposite to those that the other groups in the world were following. When choosing the material, Corning’s group opted to use the purest glass and added impurities when necessary, instead of removing impurities from less noble glass until it reached the desired attenuation. The Corning scientists then used pure fused silica to coat the optical fiber, which required a material with a lower refractive index, and silica with very small amounts of titanium in the core, in order to increase the refractive index only as necessary and to reduce purity as little as possible.

For the fiber manufacturing method, the Corning group also followed its own path, based on the method Hyde had developed more than thirty years ago. The trio made a tube of pure silica and deposited the doped silica into it. With this fiber, about four years after the start of the low attenuation fiber optic development project, the Corning group obtained the first attenuation measure of less than 20 dB / km. The first low attenuation optical fiber was developed!

In May 1970, the team filed two patents disclosing, respectively, the composition and manufacturing method of this fiber and, thereafter, began to disclose the results.

In 1971, Corning decided that the project could move from the research phase to the development phase, in which engineers worked to make the manufacturing process adequate to make the fiber stronger (the first fiber was more fragile than desirable) and to finalize the development with companies that were interested in buying the fiber. In the mean time, the research team continued to explore, with good results, new possibilities for better optical fibers. Subsequently, Maurer, Schultz, and Keck were forced to devote much of their time to litigation related to the fiber optic patents granted to Corning in 1972 and 1973.

In the early 1970s, fiber optics was not yet commercially available. In fact, it took more than 10 years for insertion of this technology in the market to take place. That part of the story, also interesting, will not be addressed here, but we can cite some landmarks. In 1975, in the United Kingdom, the first non-experimental optical fibers were installed. In 1976, Corning inaugurated its first industrial fiber optic plant. In 1983, in the United States, the first national fiber-optic telephone network was installed. In 1988, the first transatlantic fiber optic cable was installed.

Today, with billions of kilometers of fiber optics installed, telecommunications on planet Earth, mainly via the Internet, relies heavily on these fine glass or plastic wires. With regard to other technologies, fiber optics maintains first place in speed of data transmission, with immense amounts of information that can be transmitted in 1 second between distant points in the planet. With respect to the radio waves that prevailed in optical communications 60 years ago, this capacity increased by no less than a million times. All the effort of everyone involved in the history was worth it, wasn´t it?

To learn more

Two revolutionary optical technologies. The Royal Swedish Academy of Sciences. https://www.nobelprize.org/nobel_prizes/physics/laureates/2009/advanced-physicsprize2009.pdf
Making the first low-loss optical fibers. Peter C. Schultz. http://www.peterschultzconsulting.com/wp-content/uploads/2015/03/SchultzOPN2010Makinglowlossfiber.pdf
Optical fiber spans 30 years. Dr Donald B. Keck. http://www.corning.com/media/worldwide/coc/documents/Fiber/Resource%20Center%20-%20General/r3461.pdf
City of light. The story of fiber optics. Jeff Hecht. Oxford University Press (1999).

Fiber optics bundle. https://commons.wikimedia.org/wiki/File:Fibreoptic.jpg

You’re probably reading this on some Internet-connected device, aren’t you? It doesn’t matter if you are using a smartphone that receives the information from an antenna, or if the data arrives to your computer, or to the electric post of your neighborhood, through copper wires or coaxial cables. At some point, your access to the web will depend on optical fibers, those transparent, glass or plastic wires, whose diameter is similar to that of human hair strands (from a few tens of microns to a few hundreds of microns).

Fiber-optic cables are large data highways that connect continents, countries, cities, and data centers to each other. Only in the sections closest to the user, the information travels through other types of routes, in slower traffic. According to data from Corning Incorporated there are more than 2 billion kilometers of optical fibers installed in the world, enough to circle the Earth by the equatorial line 50 thousand times!

If optical fibers are highways, light is the means of transport that transits through them, and the data are the passengers, which travel coded as optical signals. On these highways, the passengers can travel about 200,000 km (five laps over the Equatorial line) in 1 second.

The first fiber “usable” in communications was produced in 1970 in the United States, more precisely at Corning Glass Works (now Corning Inc.), a company specializing in vitreous and ceramic materials. But the history of the development of fiber optic begins much earlier. Let’s describe the moments we believe are the most important in this story, carried out by many scientists of different nationalities. It should be noted that there were many patent litigations and that several of the researchers involved did not recognize the work of the previous ones.

Nineteenth Century: guiding light by sinuous paths

In the nineteenth century, some renowned scientists experimentally demonstrated that light could be driven by certain means (in this case, a jet of water) forcing it to follow a certain path, including following curves. There are records of demonstrations and publications carried out in scientific societies in Europe in the 1840s and 1850s by the Swiss Jean-Daniel Colladon (1841), the Frenchman Jacques Babinet (1842) and the Irishman John Tyndall (1854). The experiment, which is illustrated in the image below, shows that light, guided by water, was diverted from its rectilinear path to make a curved path.

On the left: Colladon, Babinet and Tyndall. On the right, the drawing of the experiment they presented.

But was the light, following the water “path” really describing a curved trajectory? Of course not. What these scientists were showing was light reflecting again and again in the water stream, describing a kind of zigzag due to the phenomenon known as total internal reflection. This is more or less as follows. When the flow of water begins to curve, the light, which travels in a straight line, ends up reaching the interface between water and air. Then it is reflected by the inner “wall” of the water stream and hits the opposite “wall” where it is reflected back. And so it continues its zigzag path into the water. The phenomenon occurs due to differences between the way water and air interacts with light (refractive indexes). For the phenomenon to happen, it is critical that light describes a greater angle to the so-called “critical angle” when reaching the interface between water and air. Obviously, the phenomenon can happen in various media, not just in the water-air system.

The nineteenth-century experiment resembles the one that appears at the beginning of this video (except, of course, for some updates implemented in the video, such as the use of lasers). See how beautiful it is:

Flexible glass fibers to explore the digestive tract

Apparently Jaques Babinet went a little further in exploring full internal reflection and demonstrated that a curved glass rod could also guide the light.

The idea was in all probability taken up again in the 1920s, when attempts were made to use flexible glass rod bundles to conduct light and be able to see otherwise inaccessible places, such as the interior of the digestive tract (medicine). Not that there were no instruments for this, for instance endoscopes, but they were rigid and therefore feared by the patients (rightly so!).

But is glass flexible? Yes, when it’s thin, it’s very flexible.

However, these fiber ancestors were not efficient; the phenomenon of full reflection was not full at all there, the light escaped through the “walls” and the fibers did not fulfill their role of illuminating the human body.

Narinder Singh Kapany. http://www.sikhfoundation.org/people-events/jewels-of-punjab-dr-narinder-singh-kapany/

Fortunately, in the 1960s flexible glass fibers capable of guiding light were now ready for various applications, largely due to two scientific contributions. On the one hand, Indian Narinder Singh Kapany and his doctoral advisor Harold Hopkins, working at Imperial College London (England), overcame the technological challenge of making a glass fiber bundle that delivered a quality image. The bundle, which the authors called the fibroscope, had several hundred fibers of 75 cm in length. On the other hand, the Dutchman Abraham C.S. van Heel of the Technical University of Delft (The Netherlands) successfully demonstrated the idea of coating glass fibers with lower refractive index materials to achieve full (or close to) reflection. Both papers were published in the same issue of the journal Nature (volume 173, number 4392). Published January 2, 1954, the edition can be considered a milestone in the field of optical fibers. The articles made public important developments and demonstrations, and encouraged the development of products.

Detail of the patent on the manufacture of fiberglass coated with glass. https://patents.google.com/patent/US3589793A/en?inventor=Lawrence+E.+Curtiss

One such product was the flexible gastrointestinal endoscope (gastroscope), which began to materialize when Basil Hirschowitz, a South African surgeon and a gastroenterologist in London, read the cited papers from Nature. The surgeon was currently doing a research internship at the University of Michigan (USA) and recruited an under graduate Physics student, Lawrence Curtiss. Young Larry, apparently very capable, was heavily involved in the project, enough not to give up given the difficulties he encountered in trying to reproduce the processes reported in the articles and in trying to reduce the light losses of the fibers. In 1956, Curtiss made an important contribution to the development of fiber optics: he developed the first fiber with glass core and coating. In fact, he was able to carry out his idea of coating a fiber made of a certain glass with another glass of lower refractive index. To do this, he developed a simple manufacturing method: introduce a stick of the core glass into a tube of the other glass, heat it all together and pull out a fiber from the medium. The resulting fibers were extremely thin, about 5 microns. In 1957, the patent application for this optical fiber and the process was deposited, and in 1971 it was granted.

The same year the fiber patent was deposited, Hirschowitz began testing and disseminating the fiber optic endoscope with glass core and coating. The flexible endoscope subsequently made a number of significant advances in medicine, such as minimally invasive surgeries such as laparoscopy.

However, in the 1950s, no applications were found or any attempts to apply optical fiber in the communications of the time (telephony, radio, television). However, data transmission through copper wires was not meeting the demands. In 1956, the first transatlantic telephone cable was installed, which actually worked. Nevertheless, it allowed only 36 simultaneous calls…

On the other hand, the laser had just been invented, opening possibilities for transporting information (including voice) encoded in light. This, however, required a medium that would guide the light through the desired routes.

For us, in 2018, it is obvious that this medium is the optical fiber, but fifty years ago it was discarded by many researchers. Why? Because of the strong attenuation (loss of light along the way) the fibers presented at that time. The amount could be negligible for an endoscope of less than a meter, but it was extremely significant in a cable of thousands of kilometers.

New work needed to be developed. New researchers would emerge. Who, when, where and how? We will tell you soon.

Interested? Our reading tip: Hecht, “City of light. The story of fiber optics”, Oxford University Press (1999).

Tag: fibra óptica

From idea to innovation: The glass wire that connected the world (part 2).

From idea to innovation: The glass wire that connected the world.