Signal Degradation in Fiber
Attenuation and dispersion are the two critical characteristics that determine the maximum distance an optical signal can be transmitted before the receiver is unable to detect it.
Attenuation
Compared to conventional transmission media the most attractive feature of fiber optic cable is its extremely low signal attenuation. The attenuation profile of a fiber is wavelength dependant; standard non-dispersion compensated fiber (i.e. corning SMF28) has an attenuation profile shown below in figure 9.
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| Figure 9: Attenuation profile of Corning single mode SMF28 optical cable |
Typically systems are designed centered around 1310nm and 1550nm having attenuation values of 0.4dB/km and 0.3dB/km respectively. Comparatively standard Belden 1694 coaxial cable transmitting SDI video at 270MB/s has an attenuation of 100dB/km; the attractiveness of fiber becomes obvious.
Dispersion
To send information over a fiber bits of information are successively encoded and represented digitally by pulses of light. The more rapidly a laser pulses on and off the higher the bit rate and the closer encoded bits of information are spaced temporally. As these pulses of light propagate through a fiber, figure 10, they tend to lose their shape and spread out eventually over lapping each other causing inter-symbol interference. The higher the data rate of the channel the more sensitive the overall system is to the effects of dispersion. Hence dispersion limits the information carrying capacity of a fiber.
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| Figure 10: A square pulse of light spreads as it travels through an optical fiber eventually two adjacent pulses will overlap one another. This is known as dispersion |
Multimode fiber is much more sensitive to dispersion than single mode fiber because of its large core, as shown in figure 5 different rays making up a bit of data take different paths as they propagate. These rays arrive at the receiver at slightly different times resulting in the broadening of the optical pulse. To compensate for pulse broadening the multimode fiber core has a graded refractive index profile shown in figure 11. The refractive index of the fiber is greatest in the center of the core and decrease rapidly until it becomes constant near the core/cladding interface. The refractive index value effects the speed of light, the greater the refractive index the slower light propagates making the ray passing directly through the center of the core travel slower than rays near the outside.
Rays arrive at the receiver with less temporal distortion reducing the dispersion of the fiber. Figure 8 illustrates the spectral output of a laser is not monochromatic but has a finite spectral width made up of many different wavelengths. Each of these wavelengths propagates through a fiber at slightly different speeds leading to pulse broadening. Although this type of dispersion is far less apparent than that experienced in multimode fiber it still affects the transmission distance of high data rate single mode systems.
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| Figure 11: A standard core index profile of a multimode fiber. The graded index reduces the amount of dispersion introduced in the fiber |
Bending loss
As the title implies, bending loss is introduced in a fiber when there is a sharp bend in the optical cable. Bends introduce an interruption in the path of light causing some of the optical power to leak into the cladding where it is lost. We recommends a minimum-bending radius of 5cm on any fiber pigtail and when bundling fibers together with tie wraps or twist ties avoid tying these fibers too tight as to not introduce micro bending into the fiber, which will further introduce loss into your system
