Fiber optic communication has revolutionized people's lifestyles, but its development has not been unimpeded, and in the early days, people once thought that fiber optic communication was impossible. At the insistence of a few, and coinciding with the development of medical and lasers, seemingly unrelated factors have come together to give the optical fiber industry an opportunity for development. In terms of specific technologies, understanding the role of impurities and defects in glass is one of the keys to its success.
撰文 | 彼得·汤森（Peter Townsend）
Translation | Zhao Qian
Over the past 70 years, United Kingdom's largest earthworks project has not been the construction of the Channel Tunnel to France, but the laying of fibre on streets across the country. Fiber optic communication has revolutionized the way we live and relax, and it hasn't improved the quality of TV shows, but it has brought about a lifestyle revolution. The science behind these assumptions is well established, and while many leading communications industries once generally believed that long-distance communication with fiber optics was unrealistic, they are now a reality.
Overcoming the technical difficulties depends largely on how we understand the role of impurities and defects in glass, and how we design light sources and detectors so that light signals carry information. The reason why people think that fiber optics cannot be used in communication is mainly related to the historical background and science of fiber optics, but I must also emphasize that the success of fiber optic communication is due to the focus of a very small number of people, coupled with the timing of medical and laser development. The combination of these seemingly unrelated factors gives the fiber optic industry an opportunity to grow. I really appreciate Jeff · Hecht's brilliant commentary on scientific progress and the charisma of the characters in his 1999 book, City of Light.
Sending long-range light signals is not an innovative idea. This method has been used by many countries over the past millennia. The Romans who invaded United Kingdom would light beacons as they paved roads, and in the distance they could see where a straight road was to be built. Later United Kingdom seem to have neglected this particular skill in transmitting information, and it was not until a thousand years later that it was used again in the construction of railways and highways. In ancient China and Rome, as well as later civilizations, people would light beacons on high places to signal an attack by invaders.
Information can be sent by controlling the amount of smoke ejected to transmit the signal, as well as by reflecting the sunlight through the daylight reflecting signal. In the Vatican elections, colored smoke is still used to convey the progress of the elections. Optical signaling methods are capable of transmitting larger amounts of information, encoded by intermittent pulses, which is the modern binary digital signal transmission method, with only two digital signals, 0 and 1. It is ideal for modern fiber optic data transmission. While the concept may be the same, the pulse frequency is a few times per minute when using a daylight reflection annunciator, and it increases to more than 1 billion times per second when using optical fiber. Some of the improvements made to fiber optics are that signals can be sent in many different colors of light, and wavelength-selective coding means that there can be 100 different color-coded channels on a single fiber.
Reflecting sunlight through daylight reflective signalers can be seen by others for miles, but they have the disadvantage of relying on a straight line of vision (and only during the day when the weather is clear). In order to improve it and make optical transmission reach the convenience of transmitting electrical signals over wires, three basic components are needed. One is a directional strong pulse light source, the second is a system that directs light around corners, and the third is a sensitive detector. Signal processing equipment is required at both ends of the fiber to encode and decode the information. These are huge challenges, and only with all of the above conditions in place can the speed from signal encoding to decoding be improved.
Figure 1 Light can bounce inside a piece of glass or optical fiber. If the core of an optical fiber is larger compared to the wavelength of light, there are multiple feasible bounce angles, known as modulus, written as m=0, m=1, m=2, and so on. The angle is exaggerated here for the sake of effect. In the case of communication fibers, the high refractive index cores tend to be very small, so light can actually only travel in a straight line along the fiber in one pattern.
Problems such as dirt and scratches can occur on the surface of the bare fiber, so the solution is to wrap it in another glass with a lower refractive index. This is very important in fiber optic systems. It may sound like a mundane idea, but a science student in a school can come up with a solution without reading modern literature. But in fact, even top scientists have imperfections in not discovering the problem early and not thinking of a simple solution with protective glass cladding, which not only hinders the development of optical fiber, but also means that some major industrial laboratories have abandoned the research.
Medicine needs to promote fiberglass manufacturing
We now make fibers that are tens or hundreds of kilometers long, but just 50 years ago, we could only make fibers that were a few meters long. Technically, even medium-length fibers are difficult to manufacture. If short, thin, and flexible glass rods are useless, the enthusiasm for fiber optic systems will soon fade. Fortunately, there is great encouragement from the medical community in this regard, as doctors want to examine not only the condition of the patient's superficial layer, but also its internal organs. The first attempt by doctors to peek inside an organ, such as the stomach, is done by inserting a tube and light source through the mouth in a manner similar to a sword-swallowing. The use of a larger diameter tube to obtain images of the interior often has catastrophic consequences, with many patients being injured or killed in the process. As a result, more patients died than sword-swallowers. Surprisingly, doctors never gave up, because in those days, doctors couldn't find other surgical methods to help them diagnose, even though there were obvious medical problems with this method.
The need for a smaller, bendable endoscope has been addressed by many researchers, both with more flexible tubes and a series of lenses to transmit images, and by using fiberglass bundles. With the addition of these devices, the lens system becomes very cumbersome and has an astonishingly large diameter. Using a glass lens system half an inch (1 inch = 2.54 centimeters) in a tube to check for prostate problems sounds worse than a condition. Fiberglass bundles are more flexible and have a slightly smaller diameter. If the fibers can be optically separated, then each fiber sends a signal from only one observation point. For example, 50 fibers can provide a 50-point image, which is terrible compared to modern multi-megapixel cameras, but it does make improvements in terms of tubes and lenses.
The pioneers realized that not only did they need very transparent glass to transmit light, but they also needed to add cladding material to confine light to the fibers and avoid light leakage when the fiberglass/rods touched each other. If people have touched glass with their hands, the light loss can be very severe because the grease on the fingers causes scattering on the glass surface. Initially, it was found that coating the surface of the glass with a layer of metal seemed like a good way to limit the light, but as the light bounced multiple times, most of the light was lost even when it was reflected back into the glass rod from the stomach, and the metal mirror lost 15% of the signal on each reflection. For staple fibers with only 10 reflections, the intensity of light is reduced by more than 80% (only 20% remains).
Later, people began to experiment with plastic outer layers instead of metal mirrors. These plastic coatings have a lower absorption rate than metals, but their bond is less strong, and some waste can be trapped at the interface between glass and plastic, resulting in a large amount of signal scattering and loss. The first real success was with a very clean and sanded and fire-polished glass rod. Insert it into an equally clean and polished low refractive index glass tube. The device is heated, softened, and then pulled into a glass rod to produce a semi-flexible "fiber" that confines light to the glass core. Image information is transmitted by aligning one end of many fibers and fixing them into a bundle. These devices are so popular that they create a huge medical market that keeps the enthusiasm for fiber optics unabated.
The language spoken by the medical profession and the general public is not identical. Using fiber optics to examine the inside of the body does not require surgery, so the procedure is euphemistically described as "non-invasive." But friends who have experienced this kind of examination use completely different words (which are not convenient to quote here) to describe them. Non-invasive tests also often lead to infection, possibly because fibrous systems are difficult to disinfect thoroughly.
Clever manufacturing methods
The initial attempt to make controllable ultra-thin glass filaments in the form of fibers may have been due to the need to use very thin glass fibers to make twisted wires for electrical measuring instruments in the 19th century. One of these instruments is a very sensitive mirror galvanometer that measures tiny currents. Mirrors mounted on the system can deflect the beam, making the torsion caused by the coil energized more apparent. One type of "thread" that can be used to hang mirrors is fiberglass. Although skilled glass craftsmen can make fiberglass with short lengths and large diameters, the diameter varies with length, and this method is not easily repeatable. The real technological advancement came in 1887, when Charles Vernon ·· Vernon Boys designed a miniature bow crossbow with a heated glass rod placed on the crossbow. This glass bow is fired, creating an elongated strip of molten glass that cools into an even and strong fiberglass of glass. Mechanically, this fiberglass is stronger than steel of the same diameter. It is finer and more transparent than hand-drawn fiberglass, and its diameter remains virtually unchanged even when it is more than a few meters long. The range of this crossbow is only a few tens of meters, but it makes people aware of some of the properties of fiberglass and measures it. This makes it possible for optical fibers to become a reality in the laboratory.
Sensitive mirror galvanometers are an important part of the transatlantic submarine cable system and are used to detect electrical impulses of Morse code signals. Now we have seen that derivative technologies completely obscure and replace the original system.
Longer fibers
By the 60s of the 20th century, the main challenge of endoscopy was the cladding of optical fibers, thus preventing the loss of transmitted signals. It became clear that it was possible to create a precisely controlled fiber with a core with a higher refractive index to transmit light, while the outer cladding had a lower refractive index than the core. The cladding makes the fiber stronger, prevents the surface from reacting with water vapor, and more importantly, does not escape light when the fiber is bent.
The possibility of using fiber optics for long-distance signal transmission is still completely denied by most large United States companies. This is partly due to the poor performance of existing fibers, but in the meantime, these companies are working on microwave and radio links between signal towers. They thought it would be possible to build a microwave system inside a metal pipe laid underground. Due to wartime needs and the military's interest in radar, microwaves were well understood, and microwave sources and detectors were built. The problems with microwaves themselves are: (1) metal waveguides cause extremely high losses, so the signal strength will be attenuated; (2) The signal cannot be bent at a sharp turn. To solve these problems, one needs to detect in many stages, amplify the signal or otherwise employ ways to repeat and enhance the signal every few hundred meters. Microwave waveguides are also sensitive to distortion, thermal effects, and weather conditions, which affect the air (and water vapor) inside the pipes that transmit microwave signals. From today's perspective, we may wonder why people at the time were still enthusiastic, unwavering, and invested so much money in this approach.
Optical loss and scattering in early fibers
Initially, few scientists seriously studied the problems associated with optical fibers, let alone believed that optical fibers could transmit signals over long distances. The most obvious problem was that the fiberglass of that period would have reduced the signal strength very quickly, and it was not clear how to make long fibers and connect them together. It is believed that the making of joints is extremely difficult. Indeed, modern optical fibers are almost as diameter as a human hair, and the core that transmits signals is only one-tenth the diameter of a human hair. Telephone engineers sometimes have to perform repairs outdoors in bad weather, and it seems impossible to align these two glass cores with an error of less than 1%. Today, this task is still not easy, but there are more reliable and conventional methods.
The idea of directing light through optical fibers was simple, but in 1960, in the first type of optical fibers that could be manufactured, the absorption and scattering of light caused very significant losses. Even with the best quality glass materials, every meter of fiber results in a 50% reduction in signal strength. This is an advance for a lab demonstration, but even with 10 meters of fiber optics for communication in a room, the light coming out of the fiber is 1,000 times weaker than the input light. At that time, there was no technology capable of making thousands of meters of optical fiber, but it didn't matter, because the light was lost in the glass and could not transmit the signal at all. Overall, one needs a material that has a light-absorbing capacity of one-millionth the size of window glass.
Light loss can also be seen through window panes, and when viewed from the thinner direction of the glass (a few millimeters thick), we might think that the loss is only due to dust and surface reflections. The refractive index of glass is 1.5, and the reflection loss of visible light at the glass-air interface is about 4% (so that about 92% of visible light can pass through the glass in the absence of light absorption). As mentioned earlier, if we look at a window pane from the edges, we will see that it is significantly less transparent and has a slight green color (due to the presence of iron impurities in the glass). The reflection loss did not increase, but we could see the light absorption effect even if the length of the glass was only a dozen centimeters. For the kilometres of fiber required for signal transmission, this means that the signal is destroyed.
A material that is more transparent than window glass (a silicate composed of a variety of metal oxides) is pure silica (SiO2). But people initially refused to make any fibrous material out of it because, although it is the most transparent material available, it has a very low refractive index (about 1.46). People haven't thought about how to make it into a core, because it requires a glass cladding with a lower refractive index. In addition, one limitation that many experimentalists face is that very high temperatures are required to pull a piece of silicon rod that is close to melting into silicon fibers. The melting point of silica is around 1713 degrees Celsius. In 1960, furnace and crucible materials capable of achieving such temperatures were rare, and the relatively simple method of heating was the use of hydrogen-oxygen welding torches.
Unfortunately, the glass industry at the time did not yet understand what the factors that limited the transmission of glass, because there was no need to solve the problem. It is believed that glass absorbs a certain amount of light, and this is due to the presence of impurities such as iron or other metals in the sand in which glass is made. However, one advantage of materials such as silica is that it has only one simple constituent component, silica. This means that it is unlikely to change significantly in composition or density, i.e., there will be no scattering sites.
Scattering is related to the wavelength of light, which is very intuitive. Optically, the scattering intensity is inversely proportional to the fourth power of the wavelength, so blue light with a wavelength of 400 nanometers (i.e., 400×10-9 meters) scatters 9.4 times more intensely than red light (700 nanometers). The difference in scattering intensity between blue and red light is in line with what we are familiar with as an everyday phenomenon and explains why the sky is blue. Direct sunlight mixes all the radiation waves emitted by the sun, and when you look in the direction of the sun, you can see intense, unscattered yellow and red light. But the light seen from all other directions has been scattered many times. Because blue light with shorter wavelengths scatters more intensely than red light, the rest of the sky is blue.
Under long-wave light (wavelengths greater than those that can transmit silica), silica is transparent and scatters to a low degree, which also facilitates the study of heavy metal fluoride glasses. People put a lot of effort into making various materials called ZBLAN. ZBLAN glass is a complex mixture of fluorides such as zirconium, barium, lanthanum, aluminum, and sodium. While they do absorb long-wave light better than silica, they scatter light due to uneven density/composition, resulting in significant loss. Instead of operating at wavelengths close to 1.5 microns, this complex material was studied with the aim of moving to a longer wavelength, such as 10 microns, which would reduce the original scattering loss by a factor of about 250. In fact, the composition of ZBLAN is highly variable and brittle, so we have been using silica.
Harness impurities for progress
The reality at that time was very different from what we know now, and in the 60s of the 20th century, attention was paid to how to make and use the most transparent materials. In order to reduce the absorption of light, it is necessary to remove impurities such as metal and water. The best transparent material to choose is silica, but as mentioned earlier, it has an extremely high melting point and a lower refractive index than all glasses. The focus of solving this problem is to remove all impurities before considering the subsequent melting and cladding issues. This is a sensible approach because it allows us to see an increase in the transparency of the material. What's more, it has the potential to attract a certain level of research funding and support, even if the likelihood of success is limited. As always, impurities come in two forms, beneficial and harmful. These harmful impurities absorb light, but if you focus only on them, you will ignore the benefits of other impurities.
Glassmakers dope silicate glass with large amounts of other oxides (such as boron, sodium, calcium, etc., to lower the melting point or act as stabilizers to make non-brittle glass) or lead (to increase the refractive index). These are well-known facts, so a similar approach can be applied to silica fibers. Surprisingly, people didn't immediately accept this. Not all standard glass dopants are compatible with fiber optic applications. For example, boron absorbs light from the infrared (wavelength 1.54 microns) transmitted by modern optical fibers, but it is suitable for the red laser signal transmitted earlier (wavelength close to 800 nanometers, or 0.8 microns).
Due to the very large range of values, Figure 2 shows the pattern of attenuation loss in the fiber. The lowest loss value occurs at wavelengths close to 1.5 microns, at which point the lowest trough of the curve occurs. Early optical fibers only transmitted light at wavelengths close to 1.3 microns, which was limited by the choice of light sources and detectors at the time, and there was a trough in the optical loss in this band. Modern materials are much more transparent than the examples used here. The absorption crest between the two troughs is due to the action of residual water in the fibers. The curve on the graph changes significantly, but we need to keep in mind that the influencing factor is impurities (such as water), which may only make up a few parts per million of glass. Even at the wavelengths with the greatest losses, it's still a very transparent glass, but we're going to look at the loss per kilometer, not just the thickness of the window pane.
Figure 2 Pattern of attenuation loss in optical fiber
The silicon in silica can be replaced with other tetravalent elements, such as titanium or germanium. Heavier atoms carry more electrons, which interact more with light, which slows down the propagation of light and increases the refractive index. An early glass fiber produced by an United States company contained some titanium to increase the refractive index of glass. However, replacing part of the silicon ions with germanium ions is more suitable in size and the ionic bonds are more matched. Germanium also increases the refractive index. Therefore, with germanium silicate glass as the core and pure silicon dioxide as the cladding, such an optical fiber can meet the needs of high refractive index core and low refractive index cladding. We consider germanium to be a beneficial dopant rather than a harmful impurity.
Similarly, the melting point problem can be solved by adding a low melting point material. The chemistry of these materials may differ from that of tetravalent silicon, so not all chemical bonds may be fully satisfied. If trivalent aluminum is used, it is necessary to compensate for it by adding other materials, such as pentavalent materials such as phosphorus, to correct the electronic states in the material (i.e., the average of 5 and 3 is 4, which matches the covalent bonds of the silicon atoms). If there is an error in the bonding process, it can lead to discoloration of the glass and light absorption. The specifics may vary, but this example illustrates the principle of lowering the melting point of germanium silicate glass.
It is essential to remove water and metal impurities that cause light absorption. The key fact we need to recognize is that a large number of impurities can be added as long as they do not affect the properties of the glass required in the application. Today, the "harmful" impurities in fiber optics have been reduced to one in a billion, which is the number used in hype and marketing. There is no mention of large quantities of germanium added to increase the refractive index or sodium, aluminum, or fluorine added to adjust the melting temperature. If there are optical amplifiers and lasers in the fiber, other impurities (e.g. erbium) need to be added.
While cutting-edge science can describe the structure of long-range transparent fibers, the details and requirements in terms of limitations and difficulties are rapidly escalating. For example, the refractive index of the core and cladding are very similar, such as 1.48 (core) and 1.46 (cladding). Larger cores are more likely to result in laser coupling of signals, but larger diameters result in more optical patterns that produce different transmission speeds as the bounce of light increases the path length and decreases the signal speed. For pulse-coded signals, a larger core diameter limits the available pulse rate. Therefore, the goal of the solution is to shrink the core and make the refractive index change in step at the boundary.
Summary of Defects in Fiber Optic Science
Summarizing the role of defects, we can see that in the development of fiber optic materials, the transparency of glass needs to be increased by at least 1 million times. To a large extent, this means removing many of the metals and water vapors from the glass, which absorb the transmitted light and cause light attenuation. At the same time, the addition of other impurities (useful impurities, i.e., dopants) can control the refractive index, melting point and tensile temperature, and also help to form glass with high tensile strength.
Many prominent figures and industrialists fail to understand the potential of fiber-optic communications, have a serious bias against technology that is ahead of its time and are firmly established, underfunding, corporate competition, corporate bankruptcies, and devastating patent litigation (see Jeff ·'s book The City of Light), all of which have led to a completely different set of societal defects. Even though I'm an academic researcher, I've been inspired by a deep understanding of corporate competition, deliberate suppression of competitors, bias, and a clear lack of intelligence. Of course, these problems also exist in academia, but they are rarely mentioned in the usual literature or research teaching. The most positive conclusion I can draw from this is that if enough people have the vision, charisma, sales skills, and hard work, it will take a lot of effort, but they will eventually make progress.
To demonstrate how to benefit from the defects, I will cite a recent example of fiber optic transmission. When buried fiber optic cables are deformed, such as ground tremors caused by heavy vehicles, earthquakes, landslides, or ground impacts, tiny optical fibers can also bend, causing light to bend back in the direction of the light source, resulting in partial signal loss. Obviously, this defect can interfere with optical communications.
However, due to the increasing demand for communication capacity, fiber optics are often phased out and replaced. However, the obsolete fiber optic systems will still be retained. It was realized that these signal reflections could be used to locate crustal activity. For example, Celeste Labedz, a graduate student in geology, detected some glacial earthquakes as a result ·of his discovery of "noise" in Alaska's optical fiber. Rather than installing a single local seismic sensor in Alaska, Labez added multiple sensors using fiber optics. In addition, fiber optics are used to map seafloor fault zones and seismologies to gather predictive information about major earthquakes and volcanic eruptions.
About the Author
Peter · Townsend is Professor Emeritus of Physics at the University of Sussex. He has worked in 9 countries, spanning academia and industry, with expertise in solid state physical defects, ion implantation, luminescence, glass, optoelectronics, cancer detection, and more. He has published more than 550 research articles and 8 books and holds honorary doctorates from the Autonomous University of Madrid and the Bulgaria Academy of Sciences.
This article is excerpted with permission from Chapter 7 "Optical Fiber Communication" of "The Beauty of Defects: Nature, Technology and the Key to Survival" (China Science and Technology Press, May 2024 edition).
Special Reminder
1. Enter the "Boutique Column" at the bottom menu of the "Huipu" WeChat official account to view a series of popular science articles on different themes.
2. "Back to Park" provides the function of searching for articles by month. Follow the official account and reply to the four-digit year + month, such as "1903", to get the article index in March 2019, and so on.