Fiber-optic cable has two propagation modes: multimode and single mode. They perform differently with respect to both attenuation and time dispersion. The single-mode fiber-optic cable provides much better performance with lower attenuation. To understand the difference between these types, you must understand what is meant by "mode of propagation."
Light has a dual nature and can be viewed as either a wave phenomenon or a particle phenomenon that includes photons and solitons. Solitons are special localized waves that exhibit particle-like behavior. For this discussion, let's consider the wave mechanics of light. When the light wave is guided down a fiber-optic cable, it exhibits certain modes. These are variations in the intensity of the light, both over the cable cross section and down the cable length. These modes are actually numbered from lowest to highest. In a very simple sense, each of these modes can be thought of as a ray of light. For a given fiber-optic cable, the number of modes that exist depends on the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section. The various modes include multimode step index, single-mode step index, single-mode dual-step index, and multimode graded index.
Multimode Step Index
Consider the illustration in Figure 3-8. This diagram corresponds to multimode propagation with a refractive index profile that is called step index. As you can see, the diameter of the core is fairly large relative to the cladding. There is also a sharp discontinuity in the index of refraction as you go from core to cladding. As a result, when light enters the fiber-optic cable on the left, it propagates down toward the right in multiple rays or multiple modes. This yields the designation multimode. As indicated, the lowest-order mode travels straight down the center. It travels along the cylindrical axis of the core. The higher modes, represented by rays, bounce back and forth, going down the cable to the left. The higher the mode, the more bounces per unit distance down to the right.
Figure 3-8 Multimode Step Index
The illustration also shows the input pulse and the resulting output pulse. Note that the output pulse is significantly attenuated relative to the input pulse. It also suffers significant time dispersion. The reasons for this are as follows. The higher-order modes, the bouncing rays, tend to leak into the cladding as they propagate down the fiber-optic cable. They lose some of their energy into heat. This results in an attenuated output signal. The input pulse is split among the different rays that travel down the fiber-optic cable. The bouncing rays and the lowest-order mode, traveling down the center axis, are all traversing paths of different lengths from input to output. Consequently, they do not all reach the right end of the fiber-optic cable at the same time. When the output pulse is constructed from these separate ray components, the result is chromatic dispersion.
Fiber-optic cable that exhibits multimode propagation with a step index profile is thereby characterized as having higher attenuation and more time dispersion than the other propagation candidates. However, it is also the least costly and is widely used in the premises environment. It is especially attractive for link lengths up to 5 kilometers. It can be fabricated either from glass, plastic, or PCS. Usually, MMF core diameters are 50 or 62.5 m. Typically, 50-m MMF propagates only 300 modes as compared to 1100 modes for 62.5-m fiber. The 50-m MMF supports 1 Gbps at 850-nm wavelengths for distances up to 1 kilometer versus 275 meters for 62.5-m MMF. Furthermore, 50-m MMF supports 10 Gbps at 850-nm wavelengths for distances up to 300 meters versus 33 meters for 62.5-m MMF. This makes 50-m MMF the fiber of choice for low-cost, high-bandwidth campus and multitenant unit (MTU) applications.
Single-Mode Step Index
Single-mode propagation is illustrated in Figure 3-9. This diagram corresponds to single-mode propagation with a refractive index profile that is called step index. As the figure shows, the diameter of the core is fairly small relative to the cladding. Because of this, when light enters the fiber-optic cable on the left, it propagates down toward the right in just a single ray, a single mode, which is the lowest-order mode. In extremely simple terms, this lowest-order mode is confined to a thin cylinder around the axis of the core. The higher-order modes are absent.
Figure 3-9 Single-Mode Step Index
Consequently, extremely little or no energy is lost to heat through the leakage of the higher modes into the cladding, because they are not present. All energy is confined to this single, lowest-order mode. Because the higher-order mode energy is not lost, attenuation is not significant. Also, because the input signal is confined to a single ray path, that of the lowest-order mode, very little chromatic dispersion occurs. Single-mode propagation exists only above a certain specific wavelength called the cutoff wavelength.
The cutoff wavelength is the smallest operating wavelength when SMFs propagate only the fundamental mode. At this wavelength, the second-order mode becomes lossy and radiates out of the fiber core. As the operating wavelength becomes longer than the cutoff wavelength, the fundamental mode becomes increasingly lossy. The higher the operating wavelength is above the cutoff wavelength, the more power is transmitted through the fiber cladding. As the fundamental mode extends into the cladding material, it becomes increasingly sensitive to bending loss. Comparing the output pulse and the input pulse, note that there is little attenuation and time dispersion. Lower chromatic dispersion results in higher bandwidth. However, single-mode fiber-optic cable is also the most costly in the premises environment. For this reason, it has been used more with metropolitan- and wide-area networks than with premises data communications. Single-mode fiber-optic cable has also been getting increased attention as local-area networks have been extended to greater distances over corporate campuses. The core diameter for this type of fiber-optic cable is exceedingly small, ranging from 8 microns to 10 microns. The standard cladding diameter is 125 microns.
SMF step index fibers are manufactured using the outside vapor deposition (OVD) process. OVD fibers are made of a core and cladding, each with slightly different compositions and refractive indices. The OVD process produces consistent, controlled fiber profiles and geometry. Fiber consistency is important, to produce seamless spliced interconnections using fiber-optic cable from different manufacturers. Single-mode fiber-optic cable is fabricated from silica glass. Because of the thickness of the core, plastic cannot be used to fabricate single-mode fiber-optic cable. Note that not all SMFs use a step index profile. Some SMF variants use a graded index method of construction to optimize performance at a particular wavelength or transmission band.
Single-Mode Dual-Step Index
These fibers are single-mode and have a dual cladding. Depressed-clad fiber is also known as doubly clad fiber. Figure 3-10 corresponds to single-mode propagation with a refractive index profile that is called dual-step index. A depressed-clad fiber has the advantage of very low macrobending losses. It also has two zero-dispersion points and low dispersion over a much wider wavelength range than a singly clad fiber. SMF depressed-clad fibers are manufactured using the inside vapor deposition (IVD) process. The IVD or modified chemical vapor deposition (MCVD) process produces what is called depressed-clad fiber because of the shape of its refractive index profile, with the index of the glass adjacent to the core depressed. Each cladding has a refractive index that is lower than that of the core. The inner cladding a the lower refractive index than the outer cladding.
Figure 3-10 Single-Mode Dual-Step Index
Multimode Graded Index
Multimode graded index fiber has a higher refractive index in the core that gradually reduces as it extends from the cylindrical axis outward. The core and cladding are essentially a single graded unit. Consider the illustration in Figure 3-11. This corresponds to multimode propagation with a refractive index profile that is called graded index. Here the variation of the index of refraction is gradual as it extends out from the axis of the core through the core to the cladding. There is no sharp discontinuity in the indices of refraction between core and cladding. The core here is much larger than in the single-mode step index case previously discussed. Multimode propagation exists with a graded index. As illustrated, however, the paths of the higher-order modes are somewhat confined. They appear to follow a series of ellipses. Because the higher-mode paths are confined, the attenuation through them due to leakage is more limited than with a step index. The time dispersion is more limited than with a step index; therefore, attenuation and time dispersion are present, but limited.
In Figure 3-11, the input pulse is shown on the left, and the resulting output pulse is shown on the right. When comparing the output pulse and the input pulse, note that there is some attenuation and time dispersion, but not nearly as much as with multimode step index fiber-optic cable.
Figure 3-11 Multimode Graded Index
Fiber-optic cable that exhibits multimode propagation with a graded index profile is characterized as having levels of attenuation and time-dispersion properties that fall between the other two candidates. Likewise, its cost is somewhere between the other two candidates. Popular graded index fiber-optic cables have core diameters of 50, 62.5, and 85 microns. They have a cladding diameter of 125 micronsthe same as single-mode fiber-optic cables. This type of fiber-optic cable is extremely popular in premise data communications applications. In particular, the 62.5/125 fiber-optic cable is the most popular and most widely used in these applications. Glass is generally used to fabricate multimode graded index fiber-optic cable.