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An OXC can also be used as an optical add/drop multiplexer (OADM). That is, it can terminate the optical signal of a number of incoming wavelengths and insert new optical signals on the same wavelengths in an output port. The remaining wavelengths are switched as described above.
An OXC can switch wavelengths in a static or dynamic manner. In the static case, the OXC is configured to switch permanently the incoming wavelengths to the outgoing wavelength. In the dynamic case, the OXC will switch a particular incoming wavelength to an outgoing wavelength on demand. An OADM can also add/drop wavelengths either in a static manner or dynamically (i.e., on demand).
Figure 8.2 An example of an optical network.
HOW LIGHT IS TRANSMITTED THROUGH AN OPTICAL FIBER
A typical WDM optical network, as operated by a telecommunication company, consists of WDM metro (i.e., metropolitan) rings, interconnected by a mesh WDM optical network, i.e. a network of OXCs arbitrarily interconnected. An example of such a network is shown in Figure 8.2.
There are many different types of optical components used in a WDM optical network, and some of these components are described in Section 8.3. We now proceed to examine some of the basic principles of light transmission through an optical fiber.
8.2 HOW LIGHT IS TRANSMITTED THROUGH AN OPTICAL FIBER
Light radiated by a source can be seen as consisting of a series of propagating electromagnetic spherical waves (see Figure 8.3). Along each wave, one can measure the electric field, indicated in Figure 8.3 by a dotted line, which is vertical to the direction of the light. The magnetic field (not shown in Figure 8.3) is perpendicular to the electric field.
The intensity of the electrical field oscillates following a sinusoidal function. Let us mark a particular point, say the peak, on this sinusoidal function. The number of times that this particular point occurs per unit of time is called the frequency. The frequency is measured in Hertz. For example, if this point occurs 100 times, then the frequency is 100 Hertz. An electromagnetic wave has a frequency f, a speed v, and a wavelength X. In vacuum or in air, the speed v is approximately the speed of light which is 3 x 108 meters/sec. The frequency is related to the wavelength through the expression: v = fX.
An optical fiber consists of a transparent cylindrical inner core which is surrounded by a transparent cladding (see Figure 8.4). The fiber is covered with a plastic protective cover. Both the core and the cladding are typically made of silica (SiO2), but they are made so that to have different index of refraction. Silica occurs naturally in impure forms, such as quartz and sand.
The index of refraction, known as the refractive index, of a transparent medium is the ratio of the velocity of light in a vacuum c to the velocity of light in that medium v, that
Figure 8.3 Waves and electrical fields.
Figure 8.4 An optical fiber.
OPTICAL FIBERS AND COMPONENTS
(a) Step-index fiber (b) Graded-index fiber
Figure 8.5 Step-index and graded-index fibers.
is n = c/v. The value of the refractive index of the cladding is always less than that of the core.
There are two basic refractive index profiles for optical fibers: the step-index and the graded-index. In the step-index fiber, the refractive index of the core is constant across the diameter of the core. In Figure 8.5(a), we show the cross-section of an optical fiber and below the refractive index of the core and the cladding has been plotted. (For presentation purposes, the diameter of the core in Figure 1, Figure 5, and some of the subsequent figures is shown as much bigger than that of the cladding.) In the step-index fiber, the refractive index for the core (n1) remains constant from the center of the core to the interface between the core and the cladding. It then drops to n2, inside the cladding. In view of this step-wise change in the refractive index, this profile is referred to as step-index. In the graded-index fiber, the refractive index varies with the radius of the core (see Figure 8.5(b)). In the center of the core it is nb but it then drops off to n2 following a parabolic function as we move away from the center towards the interface between the core and the cladding. The refractive index is n2 inside the cladding.
Let us investigate how light propagates through an optical fiber. In Figure 8.6, we see a light ray is incident at an angle Ot at the interface between two media with refractive indices ni and n2, where ni > n2. Part of the ray is refracted - that is, transmitted through the second medium - and part of it is reflected back into the first medium. Let Ot be the angle between the incident ray and the dotted line, an imaginary vertical line to the interface between the two media. This angle is known as the incidence angle. The refracted angle Of is the angle between the refracted ray and the vertical dotted line. We have that Ot < Of. Finally, the reflected angle Or is the angle between the reflected ray and the vertical dotted line. We have that Or = Of.