Optical Networking: Fundamentals of Light
The idea of using light to represent data isn't particularly new. Over a hundred years ago, Alexander Graham Bell demonstrated that light could carry voice through the air. What is new, however, is the ongoing improvement of the fibers and other basic optical components. While scientists in the '60s predicted that fibers would be limited to 500 meters, today's optical pythons can reach 1000 times that distance.
Those improvements are made possible by an understanding of light's basic properties. Reflection and refraction play critical roles in enabling light to travel down a fiber, while scattering becomes important on extended lengths of fiber.
The irony is that, while scientists know how light behaves, they are less clear on what comprises light. Experiments show that light can be explained both as a particle and as an electromagnetic wave, a form of energy caused by the excitation of an atom's electrons. Both explanations will be important to our understanding of optical networking.
Atoms, Electrons, and the Like
Until the 19th century, light was conceived as a series of particles emitted by some object and in turn viewed by another object. The particle theory of light, as its called, largely stems from the Newtonian understanding of light that stretched back to the Greeks who dubbed these tiny particles corpuscies. Under the Newtonian understanding proposed in 1666, light was a series of particles emitted by a light source that stimulated sight in the eye. By conceiving light as a series of particles, Newton was able to explain reflection and refraction.
Even within Newton's lifetime, however, the particle theory was challenged. In 1678, Christian Huygens, a Dutch physicist, argued that reflection and refraction could also be explained by understanding light as a wave. Huygens' views were rejected at the time by opponents who argued that if the wave theory were true, light should bend around objects and we should be able to see around corners. As we'll see later, light in fact does bend around cornerswhat we call diffractionthough it isn't easily observable because light waves have short wavelengths.
The wave theory of light laid largely dormant for just over a century when in 1801, Thomas Young, a British physicist, physician and Egyptologist (who also helped decipher the Rosetta Stone that led to the understanding of Egyptian hieroglyphics), first demonstrated light's wave properties. Young's experiment proved that light rays interfere with one another, a phenomena unexplainable under the particle theory as no two particles could inhabit cancel each other out. Additional research during the 19th century, gradually swayed the scientific community towards viewing light as a wave that passed through an invisible substance, called ether, the same substance after which Metcalfe named the popular local area network, Ethernet (see Chapter 2).
This theory, that light is a wave, can be easy understood by using the atomic model proposed by Ernest Rutherford (18711937), the "layman's" view of the atom. With the Rutherford model, the electron's orbit around a nucleus, comprised of protons and electrons, like planets orbiting around the sun. The nucleus exerts a force, the electric force, on the electrons, holding them in their respective states (see Figure 3.1). The closer an electron is to the nucleus, the greater the attraction. The area of, shall we say, "reach" of the protons is called the force field (in physics, not Trekkian, terms). Positive charges placed in this field are repelled by the protons; negatively charged particles, like electrons, are attracted.
Figure 3.1 Classical view of the atom.
As energy is introduced, the electrons are excited and begin to vibrate in their place. The electrons' vibrations distort the electric field holding them, forming an electromagnetic wave (see Figure 3.2).
Figure 3.2 Under the classic view, electromagnetic waves are formed when an electron vibrates, causing a distortion in the electric force field exerted by a positively charged particle.
With their understanding of wave coupled with their understanding of electricity and magnetism, 19th century scientists were able to explain most known properties of light. Yet some phenomena, notably the photoelectric effect, could not be explained. A new model was to be developed, the quantum model of light, that combined elements of both particles and waves.
Under the photoelectric effect, electrons can be released when light strikes a semiconducting material. Using the wave theory of light, the kinetic energy of the released electron should increase with the intensity of the light. Experiments, however, showed that the amount of additional energy was independent of the light's intensity.
It was only until the start of the 20th century that this problem was solved. Albert Einstein proposed a theory based on Max Planck's theory of quantization, which assumes energy to be present in a light wave in packages, called photons. Einstein theorized that the energy of these photons is proportional to the frequency of the electromagnetic wave.
Using Plancks original quantum theory and Einstein's conception of light as a series of photons, Niels Bohr in 1913 introduced a new model of the atom to replace the Rutherford model. The problem with Rutherford's model is that if energy is produced though an electron's vibrations, then according to this model electrons should be emitting energy all the time. Do you see why? When you map an electron's orbit onto a two-dimension plane the electron appears to be constantly vibrating, because in fact it is! (see Figure 3.3).
Figure 3.3 An electron's orbit appears the same as an electron vibrating in place, which under the classic view should lead to an electromagnetic wave.
This means, then, that according to the conservation of energy, the electron would slow down each time energy was emitted. After enough time, the electron would be unable to hold its position, eventually crashing into the nucleus, destroying the atom. Matter would exist for a fraction of a second and this book would never have been written.
Bohr postulated that classical radiation theory doesn't hold for atomic-sized systems. He thought that that electrons were contained at certain energy levels around the nucleus. The term energy levels is used for many reasons, one of which is that although electrons might appear to move, they don't actually orbit around the nucleus (see Figure 3.4).
Figure 3.4 Under Bohr's model, electrons are shown as inhabiting different energy states; the farther they are from the nucleus, the more energy they contain.
Whereas classical physics allowed for nearly any orbit, the quantum view says that only "special" energy levels are possible. Electrons are pushed to higher energy levels through particles of light, called photons, sharing the same frequency. When the electron drops from a higher energy level to a lower one, it emits a photon equal to the energy difference between the two states. When enough photons are emitted of the right frequency, visible light is produced.
The quantum understanding of light view might sound much like the original particle view and, in fact, there is a strong similarity to the quantum model. What's important here though is that Einstein's theories contain aspects of both the wave and particle theories. The photoelectric effect then results from the energy transfer of a single photon to an electron in the metal. Yet this photon's energy is determined by the frequency of the electromagnetic wave.
So is light a particle or wave? It's both, or perhaps more accurately, light exhibits qualities of both particles and waves depending on the situation. Much of optical networking can be explained with the wave theory of light. We'll resort to the particle theory where necessary.