Last month, we took a look at the reasons for using optical fiber. The big advantage is optical fiber can handle transmission rates many times higher than copper wire. Plus, optical fiber is immune to electromagnetic interference (EMI) and radio frequency interference (RFI). We also discussed basics like field-installed components and the importance of testing your work as an installment progresses.
This month's article explores the principles behind how light travels through the optical fibers. It also defines the three main types of optical fibers and explains the manufacturing process.
The principles of light travel
Total Internal Reflection. Optical fiber function well for signal transmission because of the principle of total internal reflection. Here is how total internal reflection works:
When light goes from one material to another of a different density (in scientific terms, this difference in density is called index of refraction), the light's path will bend. This is why you can see the bottom of a clear pond at the edge you are standing near, but when you look across the pond, you can see only a reflection of the other side. At a certain angle, light will not pass through the surface; it bounces off. When the light bends at a certain angle (and this angle is different for different types and densities of materials), all of it is reflected, and none passes through the boundary between the two materials.
This phenomenon is used to bend the light at the core/cladding boundary of the fiber and trap the light in the core. By choosing the material differences between the core and cladding, one can select the angle of light at which total internal reflection occurs.
This angle defines a primary fiber specification, the numerical aperture (NA) of a fiber.
Numerical aperture (NA). The NA designates the angle called the angle of acceptance-the angle beyond which the light rays injected into an optical fiber are no longer guided. The light rays will pass through the core/clad boundary and be lost.
Fibers with higher NAs will accept a wider range of light paths (the technical term for paths is modes). Because there are so many modes, the signal will be distorted. So, a fiber with a higher NA will cause increased signal distortion, and will be able to carry less signal. Therefore, high-NA fibers have less bandwidth, and low-NA fibers have greater bandwidth.
Index of refraction. The index of refraction of a material is the ratio of the speed of light in vacuum to that in the material. In other words, the index of refraction is a measure of how much light slows down after it enters the material. Because light has its highest speed in vacuum (approximately 300,000 kilometers per second), and because light slows down whenever it enters any medium, (water, plastic, glass, crystal, oil, etc.), the index of refraction of any material is greater that one.
For example, the index of refraction for a vacuum is 1. For glass and plastic optical fibers, it is around 1.5. Water has an index of refraction of around 1.3. You can see that the light signals sent through an optical fiber travel at considerably less than the "speed of light" as most people think. The much-stated speed of light (again, 300,000 kilometers per second) is the speed of light in a vacuum-not the speed of light in all materials.
Pulse spreading. Signal distortion comes from two primary causes-the colors of light sent through the fiber (chromatic dispersion), and the paths light takes as it moves through the fiber (modal dispersion). Both of these causes for distortion have the same final effect-distorting the signal by pulse spreading.
Pulse spreading is illustrated in Fig. 1. Notice the digital signal sent into the fiber is square. As the signal travels down the fiber, it is distorted and begins to spread.
Pulse spreading is not a loss of light because as much light is leaving the fiber as entered it. The light signals, however, are distorted. If the pulses spread too much, they will be unintelligible to the receiver, and the communications will not go through.
One at a time, we will now explain why this pulse spreading is caused by both color and path.
Chromatic dispersion. Chromatic dispersion (pulse spreading due to the colors of light sent through the fiber) occurs because different colors of light (which we also call different wavelengths of light) travel at different speeds in an optical fiber.
For example, if two different wavelengths (colors) of light are sent into a long fiber at the same time, one of them will reach the far end before the other. The time difference between the two different wavelengths arriving at the end of the long fiber would tend to spread a data pulse. This causes the pulse spreading shown in Fig. 1.
Because of chromatic dispersion, it is important to use light sources that put out only one color of light. Many of the newer lasers can do this well. And even though these lasers are more expensive than LED light sources, they are often used because they cause far less chromatic dispersion. The LED sources are used only for shorter runs where higher chromatic dispersion is manageable.
Good laser sources are said to have a narrow spectral bandwidth, putting out light within a 1 nm range. So, the light output from a 1550 mn laser will be within a range of 1549.5 and 1550.5 nm.
LED sources, on the other hand have a broad spectral bandwidth. Many LEDs have a spectral bandwidth of 20 nm. So, the light output from an 850 nm LED would be between 840 and 860 nm.
Modal dispersion. Modal dispersion (pulse spreading due to the paths of light sent through the fiber), occurs because some paths through a fiber are more direct than others, as Fig. 2 illustrates.
In Fig. 2, one of the light rays goes right down the middle of the fiber, another enters the fiber at a severe angle and must bounce from side to side all the way through the fiber.
You can see how the light ray going down the middle of the fiber has a significantly shorter path, it will reach the far end considerably sooner than the other ray of light. Notice how different paths of light traveling through a fiber will reach the end at different times, and will cause a data pulse to spread.
Modal dispersion is a major factor in determining the design of optical networks, even in the design of fibers themselves.
The three main types of optical fibers commonly used today are single-mode fibers; multi-mode, graded-index fibers; and multi-mode, step-index fibers 1. Single-mode fibers. A single-mode fiber allows only one light wave ray to be transmitted down the core. The core is extremely small, usually between 8 and 9 microns. Because of quantum mechanical effects, the light traveling in the narrow core stays together in packets rather than bouncing around the core of the fiber. Therefore, single-mode fiber has an advantage over all other types because it can handle more signal over greater distances.
2. Multi-mode, graded-index fibers. Graded-index fibers contain many layers of glass, each with a lower index of refraction as it moves outward from the center. Because light travels faster in the glass with lower indexes of refraction, the light waves refracted to the outside of the fiber are speeded up to match those traveling in the center. This type of fiber allows for high-speed data to be transmitted over a reasonably long distance.
Multi-mode fibers are used with LED light sources, which are less expensive than the laser light sources used for single-mode. Graded index fibers come in core diameters of 50, 62.5, 85, and 100 microns.
3. Multi-mode, step-index fibers. Step index fibers are used far less than the other types, having a far lower capacity. They have a relatively wide core, like multi-mode, graded index fibers, but because they are not graded, the light put through them bounces wildly through the fiber and exhibits high levels of modal dispersion (pulse spreading due to path losses). All three types of fiber are shown in Fig. 3.
The size of an optical fiber is referred to by the outer diameter of its core and cladding. For example, a size given as 62.5/125 indicates a fiber with a core of 50 microns and a cladding of 125 microns. The coating is not typically mentioned in the size because it has no effect on the light-carrying characteristics of the fiber.
The core is the part of the fiber carrying the light pulses used for transferring data. This core may be made of either plastic or glass. The size of the core is important because core sizes of joined fibers must match. Larger cores have greater light-carrying capacity than smaller cores, but may cause greater signal distortion.
The cladding sets a boundary around the fiber, so light running into this boundary is reflected back into the cable. This keeps the light moving down the cable, keeping it from escaping. Claddings can be made of either glass of plastic, and they always have a different density than that of the core. (If they did not have a different density [index of refraction], they could not reflect escaping light back into the core.)
Coatings are typically multiple layers of ultraviolet curable acrylate plastic. This is necessary to add strength to the fiber, to protect it, and to absorb shock. These coatings come between 25 and 100 microns thick. (One micron is equal to one millionth of a meter. For comparison purposes, a sheet of paper has a thickness of approximately 25 microns.) Coatings can be stripped from the fiber (and must be for terminating) either mechanically or chemically, depending what type of plastic is used.
Manufacturing fiber Manufacturing optical fiber is a difficult and complex process. We won't go through all of the details associated with making optical fiber, but we will briefly explain the process.
In general, the process entails three parts: 1. The manufacture of a preform, which is a cylinder of glass that the optical fiber will be made from. They are generally about 3-ft long and 1-inch wide. The preform has a physical make up identical to the final fiber, including both core and cladding, except it is much wider and shorter.
2. Pulling the fiber. The preform is heated very precisely, and a thin strand of glass is pulled off of one end. The diameter of this strand is carefully controlled through variances in heating and pulling tension. This strand is the optical fiber, containing both the core and cladding.
3. Cooling, coating, winding. Once the fiber is pulled off the preform, it must be carefully and slowly cooled, covered with the final coating, and wound on to reels.
There are three methods used today to fabricate moderate- to low-loss waveguide fibers: 1. Modified Chemical Vapor Deposition (MCVD)
2. Outside Vapor Deposition (OVD)
3. Vapor Axial Deposition (VAD) Modified Chemical Vapor Deposition (MCVD) is one of the methods currently being used to manufacture fiber. A hollow glass preform, approximately 3-ft long and 1 inch in diameter, is placed in a horizontal or vertical lathe and spun rapidly. A computer-controlled mixture of gases is passed through the inside of the tube. On the outside of the tube, a heat source (oxygen/hydrogen torch) passes up and down as illustrated in Fig. 4.
Each pass of the heat sources fuses a small amount of the precipitated gas mixture to the surface of the tube. Most of the gas is vaporized silicon dioxide (glass), but there are carefully controlled remounts of impurities (dopants), which cause changes in the index of refraction of the glass. As the torch moves and the preform spins, a layer of glass is formed inside the hollow preform. The dopant (mixture of gases) can be changed for each layer so the index may be varied across the diameter.
After sufficient layers are built up, the tube is collapsed into a solid glass rod, which is the preform. It is now a scale model of the desired fiber-but much shorter and thicker.
The preform is then taken to the drawing tower where it is pulled into a length of fiber up to 10 kilometers long.
The Outside Vapor Deposition method utilizes a glass target rod placed in a chamber and spun rapidly on a lathe. A computer-controlled mixture of gases is then passed between the target rod and the heat source. On each pass of the heat source, a small amount of the gas reacts and fuses to the outer surface of the rod. After enough layers are built up, the target rod is removed and the remaining soot preform is collapsed into a solid rod. The preform is then taken to the tower and pulled into fiber.
The Vapor Axial Deposition process utilizes a very short glass target rod suspended by one end. A computer-controlled mixture of gases is applied between the end of the rod and the heat source. The heat source is slowly backed off as the preform lengthens due to tile soot build-up caused by gases reacting to the heat and fusing to the end of the rod. After sufficient length is formed, the target rod is removed from the end, leaving the soot preform. The preform is then taken to the drawing tower to be heated and pulled into the required fiber length.
Fiber coatings. After the fiber is pulled, a protective coating is applied very quickly after the formation of the hair-thin fiber. The coating is necessary to provide mechanical protection and prevent the ingress of water into any fiber surface cracks. The coating typically is made up of two parts, a soft inner coating and a harder outer costing. The coating overall thickness varies between 62.5 and 187.5 microns, depending on fiber applications. These coatings are typically strippable by mechanical means and must be removed before fibers can be spliced or fitted with connectors.