Wiring Tip 6: Fiber Optic Cable

Wiring Tip 6: Fiber Optic Cable

In the 1950s, more research and development on optical fiber video transmission was successful in the medical field in the field of remote lighting and observation instruments. In 1996, Charles Kao and George Hockham proposed the idea of ​​transmitting information through glass fiber, and realized that the cable loss must be greatly reduced to realize this idea.

This was the driving force behind the development of improving fiber losses in the fiber manufacturing industry, and today our fiber losses are well below the goals originally set by Charles Kao and George Hockham.

Advantages of using fiber optics

Due to its low loss and high bandwidth properties, optical fiber can be used over longer distances than copper cables. In data networks, this distance can reach up to 2 kilometers without the use of repeaters. At the same time, optical fiber's light weight and small size make it ideal for use in places where copper cables are not suitable, and through the use of multiplexers, one optical fiber can replace hundreds of copper cables. Such a small glass filament is really eye-opening, but the real benefit to the data industry is that it is completely immune to Electro Magnetic Interference (EMI) and the glass itself is a non-conductor.

Since fiber is a non-conductor, it can be used in places where electrical isolation is necessary. For example, copper cables need to be crossed between buildings to reduce differences in ground potential. At the same time, fiber poses no threat to hazardous environments, such as chemical plants where a spark could cause an explosion. Last but not least, there is the issue of security; it is very, very difficult to hack into fiber optics and read digital signals.

Fiber structure

There are many types of fiber optic cables, but for the purpose of this article we will discuss the most common type - 62.5/125 micron fiber protection tube. The numbers represent the diameter of the fiber core and the sheath, and it is measured in microns, which is one millionth of a meter.

Fiber optic cable can be used indoors or outdoors, or both. Outdoor cable tubes are usually filled with gel hose to protect against moisture. The number of wires in the cable can be anywhere from 4 to 144.

Over the years, there have been a variety of wire sizes, but the three most common sizes used in digital communications are: 50/125, 62.5/125, and 8.3/125. In data networks, 50/125 and 62.5/125 micron multimode cables are still widely used, but recently 62.5 has become more popular. This is quite unfortunate because 50/125 has been considered a better choice for Gigabit Ethernet applications.

8.3/125 micron is a single-mode cable, and until now it has not been widely used in data networks due to the high cost of single-mode hardware. However, this is beginning to change. The Gigabit Ethernet length limit over 62.5/125 fiber has been reduced to about 220 meters, and using 8.3/125 may now be the only option for some campus networks. Hopefully, this shift will drive down the cost of single-mode fiber.

The difference between single mode and multimode

Larger dimensions in copper cables mean less resistance and therefore more current, but the opposite is true for optical fibers. To explain this, we first need to understand how light travels through the fiber core.

Light Propagation

The process by which light travels through an optical fiber is called "Total Internal Reflection" (TIR); this is possible by using two glasses with different reflectivity. The inner core has a high reflectivity index and the outer cladding has a low reflectivity index. This reflection works the same way as if you were looking into a pond. The water in the pond has a higher reflectivity index than the air, so if you look into the pond from a small angle you will see a reflection of the surroundings, but if you look at it from a vertical angle you will probably be looking at the bottom of the pond.

At a certain angle between these two viewpoints, the light does not reflect off the water but travels straight through the air/water interface allowing you to see the bottom of the pond. Multimode fiber, as the name implies, means there are multiple ways to propagate the light. This ranges from low-mode to high-mode. Low-mode takes the most direct route to the center, while high-mode takes the longest route because they bounce from one side to the other.

Since the time for a light pulse to reach the far end is not the same, this causes the effect of signal scattering. This is called inter-modal dispersion (sometimes called differential mode delay, DMD). In order to solve this problem, gradient optical fiber is available. In the example above, there is a clear boundary between the inner core and the cladding, and here there is a high reflectivity index in the center, and it gradually decreases to a low reflectivity index toward the edge. This slows down the low-mode transmission speed, making the order in which the light waves arrive at the far end closer, thereby reducing inter-modal dispersion and improving the shape of the signal.

Single mode fiber

What is the best way to remove intermodal dispersion? Simple, it is to allow only one mode to propagate. Therefore a smaller fiber core means higher bandwidth and longer distance. It is that simple.