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Working with optical fiber is a specialty of its own. This is only an overview of fiber optics.
However, you can buy pre-made cables for short runs and use optical Ethernet without too much special knowledge or skill.
Fiber optic cabling uses ultra-pure glass made from silicon and germanium oxides. Light is confined inside a fiber of glass by different methods depending on the cable design. Fiber optic cabling for networking has two concentric layers of glass. The core carries the light signal and the cladding surrounds the core. The cladding protects the core from the outside environment, provides mechanical strength, provides uniform support for the core and controls the characteristics of the reflections in the core. The cladding is not required to keep the light confined in the core. Decorative fiber optics and plastic fiber optic cable used for digital sound have no cladding.
Multimode fiber has a large diameter core (compared to the diameter of the fiber itself). Typically, the core is 62.5 microns in diameter where the cladding is 125 microns in diameter. This would be specified as 62.5/125. It is called multimode because the core is wide-enough that light can bounce around in the core along an infinite number of paths (modes).
Multimode fiber is usually fed with LEDs, but lasers and laser-optimized multimode fiber are becoming popular. The following are the two main types of multimode fiber optic cable.
Step index multimode fiber has a core that has a uniform index of refraction (density). Therefore, the light travels at the same speed (about 200,000,000 meters per second) in all parts of the core. The cladding is always a lower density than the core. If rays of light enter the core at a shallow enough angle, 100% of the light is reflected back into the core when the light meets the core-cladding interface. This phenomenon is called total internal reflection and happens whenever light is passing from one medium to a less dense medium (such as glass to air) and only if the angle is shallow enough. After a short distance, all light that is not reflected back into the core (because it is striking the core-cladding interface at too-steep an angle) is lost but what remains will travel a long distance.
Graded index multimode fiber has a core that is more dense toward the center than the edge. This causes light that is traveling from the center of the core toward the edge to be gently bent back toward the center (rather than bouncing off the core-cladding interface as with step index fiber). Like step-index multimode fiber, the light has to enter the core at a relatively shallow angle. The difference is that in graded index fiber, the rays of light make a serpentine path through the central part of the core rather than bouncing off the core-cladding interface.
Multimode fiber optics are usually fed with light emitting diodes (LEDs). The characteristics of the light LEDs emit and the existence of multiple modes limits the data rate and the distance a usable signal can travel. The maximum practical distance between nodes is usually specified as 2 km. Multimode fiber optics are usually used for LANs where the distances are short.
Singlemode fiber optic cable has a core size that is around eight times the wavelength of the light used to carry signals. This causes the electromagnetic energy to travel along the core-cladding interface; the light is not free to bounce around inside the core. Therefore, there is only one mode (path) for the light to follow.
Singlemode fiber optic cable is usually fed with infrared lasers. The existence of only a single mode and the use of lasers mitigates certain problems that limit the data rate and the distance a usable signal can travel in the fiber. Therefore, a signal can travel many kilometers in a single mode fiber. It is used for long-distance telecommunications, cable television and FTTx (see below).
Light travels 50% faster in air than it does in glass so a hollow core will give significantly better performance at extremely high data rates. However, it can’t work. Hollow fiber has an air core and a glass cladding. This makes the cladding with a higher index of refraction than the core. For total internal reflection to work the cladding must be a lower index of refraction than the core. Recent developments have overcome this problem, making hollow fiber optic cable possible.
Wavelength is the distance a wave travels during one oscillation. Electromagnetic radiation travels at about 300,000,000 meters per second (in vacuum or air). If an electromagnetic wave is oscillating 1,000,000 times per second (1 MHz), it will have a wavelength of 300 meters. Light is electromagnetic radiation with wavelengths between 400 nm and 700 nm. (a nanometer is 1 billionth of a meter).
Fiber optics use infrared light. Technically, infrared light is any electromagnetic radiation with a wavelength longer than 700 nm but shorter than 1 mm (1,000,000 nm). Wavelengths used for fiber optics range from 850 nm (actually visible to humans) to about 1,600 nm. Shorter wavelengths tend to scatter more and therefore go shorter distances. As devices that work with longer wavelengths were developed, signals were able to be sent longer distances through fiber optics.
Two main factors cause rays of light to take different times to travel from one end of a fiber optic cable to the other. These are modal dispersion and chromatic dispersion. As the rays go farther along the cable, they spread out more. This is like an Indy car race where 33 cars start at the same time but by the end of the race cars are all over the track. If you ask, when did “the cars” finish the race, you would have to take an average or something. When it comes to sending bits down a cable, the bits will start to overlap after some distance and become undiscernable.
Modal dispersion happens in multimode fiber where there is an infinite number of paths (modes) that rays of light can take. Light rays that take longer paths arrive at the end of the cable later than those that take shorter paths. Graded index fiber has less modal dispersion than step index fiber because the light is confined to a smaller part of the core. Modal dispersion is eliminated in singlemode fiber.
Longer wavelengths of light travel faster than shorter wavelengths. An 850 nm LED actually has a wide range of wavelengths that are near 850 nm. This results in modal dispersion and limits the distance a usable signal can travel. Lasers produce a narrow range of wavelengths. Lasers combined with singlemode fiber results in the greatest distances usable signals can travel with fiber optics.
The greatest distances are achieved with longer-wavelength lasers and singlemode fiber.
Optical Ethernet transmits Ethernet signals over a pair of fiber optic cables, one for transmitting and one for receiving. You will probably read that the maximum distance for Optical Ethernet is 2 km. However, this is an old specification for Fast Ethernet using short-wavelength infrared light and multimode cables. This system is still common, but newer systems use longer-wavelength light and singlemode fiber. Both increase the distance a usable optical signal can travel in the fiber. 10-Gigabit Ethernet is specified with distances of up to 80 km.
1. Fiber optic cable has a mirror coating between the core and cladding to keep the light in the core.
There is no mirror-like coating between the core and the cladding. It is just two layers of glass fused into a single fiber.
2. The cladding is a special glass that confines the light in the core.
Both the core and the cladding are made of a mix of silicon oxide and germanium oxide. The cladding has a lower index of refraction (density) but is the same composition as the core.
3. The purpose of the cladding is to reflect light back into the core.
Early fiber optics had no cladding. However, the surface of the glass fiber was easily contaminated or damaged. Anything touching the fiber could change the reflective properties. The cladding was invented to protect the core, not to contain light in the core. So, even though the cladding does confine the light in the core, no cladding is required to do so.
4. All fiber optic cable works by total internal reflection.
Only step-index multimode fiber works by total internal reflection. In graded-index multimode fiber optic cable, light travels in a serpentine path and remains near the center of the core. In singlemode fiber optic cable, light energy travels along the core-cladding interface. It does not reflect off the cladding.
5. Wavelength Division Multiplexing (WDM) sends multiple data streams through a multimode fiber by sending different colors along different paths (different modes)
or
multimode fiber carries multiple signals but only for a short distance, but singlemode fiber carries one signal a long distance.
WDM is usually implemented with singlemode fiber, not multimode fiber. Second, the multiple paths in multimode fiber degrade the signal at all wavelengths. You cannot isolate different modes and use them for different data streams. Therefore, singlemode fiber often carries multiple data streams using different wavelengths of light. Multimode fiber is rarely if ever used for multiple data streams. Review the paragraphs on multimode and singlemode fiber optics above.
6. Fiber optic Ethernet can convey signals up to 2 km.
You can tell when an author repeats something he or she read in another book. This obsolete specification is nearly 25 years old and is still repeated in book after book. Optical Ethernet can go up to 80 km these days.
The following illustration shows a green laser beam reflecting inside an acrylic rod. When the light goes from a dense medium (glass, plastic, etc.) to a less dense medium (air, or less-dense glass)
Light contained in the core of the fiber
Wide
bandwidth due to high frequency (Terahertz range)
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| A green laser reflecting inside an acrylic rod. |
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