Chapter 1: Introduction Free

Greek philosophers thought that sight was something emanating from the eyes, although Lucretius wrote in On the Nature of the Universe (55 b.c.e), “The light and heat of the Sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.” The ancient Greeks mused a lot on the nature of light but the understanding and manipulation of light, i.e., optics, did not happen until much later.

It has been postulated, but never completely proven, that the Vikings (7th to 10th century c.e.) used the mineral calcite (CaCO₃) to observe polarized light from the Sun to enable them to see where the Sun was even on overcast and cloudy days. This, along with a few other tricks, such as observing the length of a shadow at noon, would give them some idea of direction and latitude in the unfriendly waters of the North Sea and North Atlantic.

In 1669, three years before Newton first presented his particle theory of light, Danish philosopher Erasmus Bartholin had begun experimenting with transparent calcite crystals, which had been discovered in Iceland. He found that when an image is placed behind a crystal it is duplicated, with one copy appearing slightly higher than the other. He thus revealed the previously hidden world of polarization.

Isaac Newton showed how sunlight could be split into its constituent parts with the use of a prism in 1672. At the time many people thought that sunlight must be perfect and the result he was reporting was a defect in the glass prism. When he took the green light from his spectrum, put it through another prism, and revealed that monochromatic light going through a prism would only deliver the same monochromatic light from the other side, his case was made.

Much more thought was put into the subject from the 15th and 16th centuries onwards. Kepler and Galileo helped to develop optical telescopes, Snell and Descartes worked on the laws and theory of refraction, and in 1676 Danish astronomer Ole Christensen Romer used data from the eclipses of Jupiter's moons to get the first reasonable value for the speed of light. He observed that the time between eclipses of Jupiter's moons by the planet was shorter when the Earth was on the same side of the Sun as Jupiter, and became longer when the Earth and Jupiter moved towards opposite sides of the Sun. He suggested that this was due to the time it takes for light to cross the increased distance. He calculated the speed as 225 000 km/s, which is not too far from the accepted value today of just under 300 000 km/s.

The next few hundred years were taken up by arguments about the nature of light and whether it was a wave or a particle. This is a difficult question, as it has the properties of both, but is one that taxed the minds of the likes of Newton, Euler, and Fresnel.

In 1864, the brilliant Scottish mathematician James Clerk Maxwell wrote,

The four partial differential equations, now known as Maxwell's equations, that completely describe the classical electromagnetic theory appeared in fully developed form in Maxwell’s book A Treatise on Electricity and Magnetism (1873). Planck, who made one of the next major breakthroughs, said on the occasion of the centenary of Maxwell's birth in 1931, that this theory, “… remains for all time one of the greatest triumphs of human intellectual endeavour” (Raymer and Smith, 2005).

Maxwell was an inspiration for Albert Einstein, who said, “The special theory of relativity owes its origins to Maxwell's equations of the electromagnetic field” Einstein (1931). Einstein also said: “Since Maxwell's time, physical reality has been thought of as represented by continuous fields, and not capable of any mechanical interpretation. This change in the conception of reality is the most profound and the most fruitful that physics has experienced since the time of Newton.”

Before the discovery of electricity, the light that most humans experienced was astronomical in nature. The next time you look at the Moon, consider that the photons you are seeing were probably first created in the core of the Sun about the time modern humans first left Africa, then finally left the Sun's surface about eight minutes ago, then hit the Moon's surface about one and a half seconds ago. Then 88% of the photon's compatriots will have been absorbed by the Moon's surface. Of the 12% reflected, some of them were absorbed in the back of your eye, where the energy of the photon was converted into an electrical nerve impulse, which your brain then interpreted as an image of the Moon.

In this book, we will look at the physics and mathematics behind the generation and transmission of light; we will also look at the practical effects and benefits this understanding brings to the world, which leads us to the science and engineering of optical communications.

We will build on the works of Newton, Snell, Maxwell, and many others to understand the basics of the physics of manipulation of light. The first part of the book looks at the basics of electromagnetic transmission, refraction and reflection, and polarization. It is only from an understanding of these fundamentals that we can understand how we can manipulate light to our advantage in lenses and prisms. We can then progress to the creation of our own light sources with LEDs and lasers.

With this basic grounding in physical principles we can move the discussion to optical communications and methods of encoding information onto optical signals. Information has been encoded onto optical signals for centuries; the smoke signals of the native Americans of the plains and southwest were using optical communications but the data rate would have been somewhat basic.

A heliograph allows a coding and transmission system about as fast as somebody can manipulate a mirror, which would be about two bits per second. Despite the classical connotations of the Greek name (Sun + write) there is no evidence that communicating by flashing the Sun off a mirror was ever used in classical times. The heliograph appears to have been introduced by the British Army in India in about 1869 and by the US Army in 1878. The heliograph's range can span tens of kilometers but it is dependent upon the appearance of the Sun. In Scotland there is only 1200 hours a year of a visible (and highly unpredictable) Sun as opposed to Phoenix, Arizona, with 3782 hours of Sun. It is obvious that for reliable communications an artificial source of light is required.

People have signaled with lamps at night throughout history but it was only the arrival of Morse code in 1844 that allowed comprehensive encoding of information onto light or onto electrical signals on a telegraph wire. After the use of flags as a signaling method for many centuries, the British Royal Navy introduced signaling by flashing lamps in the middle of the 19th century. At first kerosene lamps were used but by the beginning of the 20th century electric lamps were used. The first manufacturer was by a British company called Aldis, named after the name of the inventor, Arthur C. W. Aldis (1878–1953). Now, an aldis lamp is a generic name for any kind of lamp that can rapidly be turned on and off to effect a method of communications. To save the life of the lamp, and to overcome slow start up and turn off times, the signaling is usually done by some form of mechanical shutter over the lens.

Before radio was widespread, the aldis lamp became an indispensable method of communication. After radio was invented, the aldis lamp became an indispensable method of communicating without being overheard, at least not by anybody out of the same line of sight.

Ships at sea still have to carry a signaling light. The SOLAS (Safety of Life at Sea) Convention is published by the IMO (International Maritime Organization), and it states that,

A modern aldis lamp would expect to be able to provide a daylight signaling range of 8 miles and a transmission rate of 12 words per minute.

In America, the FAA (Federal Aviation Administration) calls the signaling lamps signal light guns and mandates their use and availability in control towers as a last ditch communications methods to aircraft. In this case colors are used, along with steady or flashing signals, to indicate commands to an aircraft.

The U.S. Navy still uses Morse code ship-to-ship signaling lights when strict radio silence has to be observed, but Morse code proficiency is becoming a difficult skill to find. In 2017, the U.S. navy trialed a system that consists of either step motors to operate the lamp shutters or LEDs, while at the other end a camera captures Morse code flashes from the other ship. In between sits a proprietary converter in the form of a handheld device or a laptop that runs specialized software. This converts messages tapped out on the screen into Morse code that is flashed from the lamp, or converts received Morse code flashes back into text messages that are displayed on screen. This makes it possible for anyone to send and receive signals without knowing anything about Morse code, while allowing a more traditional signalman to communicate with them without difficulty (U.S. Navy tests signal, 2017).

The invention of LEDs and lasers in the 1960s allowed optical communications to really take off. These relatively cheap sources of light can be turned on and off very quickly, allowing a rapid speed of communications traffic. Optical communications can be in free space, which is either atmospheric or literally in space. Optical communications within the atmosphere are plagued by rain, fog, snow, and atmospheric turbulence, which limits the range. Optical communications can also take place through water, but there the absorption characteristics of the media are even more demanding. This has led to a lot of research over the decades to understand the optical attenuation by wavelength of the light used in different media and allowed signals to be optimized by picking the best wavelength. This is an area that has been known to astronomers for many centuries and who are well versed in the attenuation performance of the atmosphere.

The vast majority of optical communications now takes place through the medium of optical fiber. Some people claim that Anglo-Chinese physicist Dr. Charles Kao invented optical fiber in England in 1966; he was awarded the Nobel Prize in Physics in 2009 for this work. Others claim it was invented by Corning Glass in America in 1970. Corning must at least take some credit for the commercialization and mass production of optical fiber that allowed for its commercial take-off in the late 1970s.

In optical fiber, the minimum attenuation is in the infrared wavelengths and, just like any other transmission media, an intimate knowledge of absorption characteristics by wavelength is vital. After a review of LEDs and lasers, we will consider the methods of encoding information onto light from these sources. This ranges from simple on–off keying to more sophisticated methods. There are many different ways of modulating a signal, but we shall focus on the three types most often seen in optical links: non return to zero (NRZ), pulse amplitude modulation (PAM), and quadrature amplitude modulation (QAM). Using complex codes to squeeze more data into a limited bandwidth is always going to be limited by noise and, in the case of optical fiber, especially limited by dispersion of the signal.

In Chap. 8 we look at optical fiber in more detail and, in particular, at the differences between multimode and singlemode and the great problems of modal and chromatic dispersion. There are many books on optical physics that explore this subject in more mathematical detail, and in this work we hope we have supplied sufficient mathematical detail for the reader to follow the main lines of argument. Where this book does differ is in tying the theoretical performance of optical fiber to the expectations of the real world; for example, how multimode mostly relates to the world of short distance local area networks and singlemode to the world of long-haul telecommunications. The hard borders of these two worlds have become greyer and I suspect will one day merge—but not yet.

In all cases we have tried to tie together the world of international standards to the products on the market and the communications interfaces available. In particular, we will refer to the work of the International Telecommunications Union (the ITU), for their work on telecommunication singlemode fiber, the International Standards Organization and the International Electrotechnical Commission (ISO/IEC), for their work on multimode fiber and the other vital components of optical communications, and the Institute of Electronic and Electronic Engineers (the IEEE), for their work on optical communications interfaces and coding for the Ethernet communications standards. Without the works of these three organizations nothing would actually communicate with anything else.

In Chap. 9 we will consider some of the other optical components that are essential to a practical and working optical communication system. There are optical connectors and splices, patch panels and cables, and an essential device called an erbium-doped fiber amplifier (or EDFA). EDFAs, isolators/circulators, Bragg gratings, and MEMs (micro-electro mechanical systems) all contribute to the essential requirement of wavelength division multiplexing, but without the accompanying array of connectors, splices, and cables the optical communication system would not be complete or usable.

In the final chapter we will review how optical systems are tested to prove they work in the field. In testing optical systems, there are generally four parameters of interest: attenuation of the link, its return loss, the continuity and polarity of the optical links, and the length of the link. Other parameters of interest, such as bandwidth, cannot easily be tested outside of a laboratory and system bandwidth is normally inferred from the optical fiber specification. Attenuation of the link, at all the wavelengths of interest, is a key factor in determining if an optical link will actually work. Enough light must reach the receiver for the signal to be distinguishable from the background noise. In this chapter we will focus on the techniques of light source/power meter measurements and optical time domain reflectometry (OTDR). An OTDR is an essential tool for fault finding and characterizing an optical fiber installation, but such a sophisticated device demands a sophisticated understanding of how to use it; some of the pitfalls of using such devices outside of a laboratory environment are discussed.

Optical communications will only get faster, cheaper, and more imaginative as time goes by. A review of today's technical publications brings us “Sky-Fi” (https://eandt.theiet.org/content/articles/2020/10/telescope-facility-to-enable-high-speed-data-connections-from-space/), space-to-ground laser-based communications system to bring secure high-definition, real-time video from as far away as the Moon. Li-Fi will be office-based gigabit speed optical communications based on cheap LEDs already on the market, and we have Aqua-Fi (https://eandt.theiet.org/content/articles/2020/06/laser-system-provides-wireless-internet-underwater/), a 520-nanometre underwater communication system to allow divers to communicate with each other and the surface using Ethernet interfaces and transmission speeds.

We hope this book will provide a theoretical foundation and practical guidance for all engineers and scientists working in this exciting and expanding field.