Optical networking is some flashing light moving back and forth across a glass rod. This flashing light carries the information traveling between networking components, such as optical switches and routers.
Optical networking is enabled because of two improvements in the field of shining a flashlight in one end of a glass rod:
-> The glass used in fiber optics is specially designed so that it is low in impurities. Further, within the fiber itself, the light can bounce around and propagate to the far end.
-> Very powerful lights called lasers are capable of traveling much farther than regular light.
Optical networking, offers enhancements over conventional networking because it provides three important network performance improvements: speed, capacity, and distance.
How Optical Networking Works
The transmission of light in optical fiber is most commonly explained using the principle of total internal reflection (TIR). This means that 100 percent of light that strikes a surface is reflected. For means of comparison, a mirror reflects about 90 percent of the light that strikes it, so you can see that TIR is a high standard to meet.
When light is emitted be it from a powerful laser or from candlelight, the radiated light can bounce, assuming it strikes the right material. Light can be manipulated in two basically different ways:
-> Reflection means that the light bounces back.
-> Refraction means that the light’s angle is altered as it passes through a different medium (like a glass of water, a prism, or a fiber). The angle is determined by the angle of incidence. The angle of incidence is the angle at which light strikes the interface between an optically denser material and an optically thinner one.
For TIR to occur, the following conditions must be present:
-> Beams of light must pass from a dense material to a less dense material.
-> The incident angle must be less than the critical angle. The critical angle is the angle of incidence at which light stops being refracted and is insteadtotally reflected.
The core and cladding are constructed out of optically denser and optically thinner types of highly pure silica glass. These components are mixed with components called dopants (like erbium), which adjusts their refractive indices. The difference between the refractive indices of the two different kinds of glass causes most of the emitted light to bounce off the cladding and stay within the core, traveling to the endpoint. The critical angle requirement is met by controlling the angle at which light is beamedinto the core.
Optical Fibers
For optical networking to happen, it must use a fantastically pure kind of glass. In this case, silica glass is the blend of choice. Even though it is exceptionally pure, there is still a little loss of light as the light travels through. The loss, however, is much less pronounced than with ordinary glass.
Once you have a core of pure silica, an extra layer of glass (called cladding) is wrapped around the core. The cladding has a lower refractive index than the core. The difference in refractive indices guides the light into the core and prevents the light from escaping through the sides of the fiber.
An optical signal can generate many different light waves, which can travel through the fiber simultaneously. This is the method used in multimode fibers (which provides a medium over which a number of concurrent transmissions can be sent). Unfortunately, this can also cause problems as the waves arrive at the end of the fiber and are out of sync. Most optical networks use single-mode fiber, which has a rather small fiber core (about 9 micrometers—a micrometer is a millionth of a meter), thus ensuring that only a single light wave traverses the fiber, alleviating receiving problems
Fiber optic cables contain extremely thin strands (10 micrometers across) of silica glass, which are then encased in a thicker, denser layer of silica glass (about 125 micrometers across). Once a protective wrap is applied, the fiber optic cable is a quarter of a millimeter in diameter. The glass used within the fiber is highly pure, thus ensuring that the photons keep moving. Also, these strands of glass can be hundreds or thousands of miles long.
Design
Optical fiber is composed of three parts:
-> The core, which carries the light
-> Cladding, which traps the light in the core, causing total internal reflection
-> The Buffer, which is the insulating wrap protecting the fiber
1. First, the light source (in this case a laser) converts the network’s electrical signal into pulses of light.
2. The light is injected into the core of the fiber.
3. The photons bounce off of the border between the core and the cladding. Because the core and the cladding have different refractive indices, the photons are bounced back into the core.
4. The photons continue through the length of the fiber.
5. Ultimately, they exit the fiber and are converted back into electrical signals by the light detector.
6. Because the fiber doesn’t exist in a harm-free environment, the core and cladding are encased in a protective wrap called the buffer. The buffer makes the fiber more durable and easy to handle.
Types of Fiber
Fiber comes in two basic types, single-mode and multimode.
-> Single-mode fiber has a core size of only about six times the wavelength of the fiber. In turn, this causes all the light to travel across a single path (in optical networking parlance, a path is referred to a mode). Single-mode fiber is useful because modal dispersion disappears and the bandwidth of the fiber is at least 100 times greater than graded index fiber.
-> Multimode fiber allows light to travel across several different paths through the core of the fiber, which enter and leave the fiber at various angles
Multimode fiber can further be broken down into two different types of fiber.They differ based on the index.
-> Step index fiber has a core made from a single type of glass. Light within the fiber travels in straight lines and reflects off the cladding. Step index fiber has a numerical aperture that is determined by the differences in the indices of refraction of the core and cladding. Because each mode of light travels a different path, a pulse of light is dispersed while traveling through the fiber, thereby restricting the bandwidth of step index fiber.
-> Graded index fiber has a core that is composed of many different layers of glass. The layers differ because of their densities, thereby transmitting light along a parabolic path. In glass with a lower index of refraction, light travels faster as it approaches the outside of the core. Conversely, light traveling closest to the core will travel at the slowest pace. Since the fiber contains these different layers of glass, the bandwidth capacity of the fiber is 100 times greater than for step index fiber.
Size Matters
Fiber can also be categorized by its size. The most popular fiber in multimode today is 62.5/125.
The first multimode fiber in wide use was 50/125. Telephone companies needing more bandwidth for long-distance uses were the first to adopt this type of fiber. Because it has a small core and a low numeric aperture, it was difficult to connect to LED sources. Because of this difficulty, a move was made to 100/140 fiber. It worked well, but because the core was so large, it was expensive to manufacture. Further, it had a unique cladding that required connector manufacturers to develop connectors specifically for it. The next popular size of fiber was 85/125. It provided connectivity to LED sources but used the same connectors as other fibers. Finally, 62.5/125 became the de facto standard for when IBM turned to 62.5/125 for its fiber optic hardware. When this occurred, other types of fiber have dropped away, leaving 62.5/125 as the industry standard.
No comments:
Post a Comment