Model dispersion can be reduced in three ways :
(I) Use a smaller core diameter, which allows fewer modes.
(II) Use a graded -index fiber so that light rays that allow longer paths also travel at a faster velocity and thereby arrive at the other end of the fiber at nearly the same time as rays that follow shorter paths.
(III) Use a single-mode fiber, which permits no modal dispersion.
Different wavelengths (colours) also travel at different velocities through a fiber, even in the same mode, as
n = c/v
where n is index of refraction, c is the speed of light in vacuum and v is the speed of the same wavelength in the material. The value of V in the equation changes for each wavelength, Thus Index of refraction changes according to the wavelength. Dispersion from this phenomenon is called material dispersion, since it arises from material properties of the fiber.
Each wave changes speed differently, each is refracted differently. White light entering the prism contains all colours. The prism refracts the light and its changes speed as it enters the prism. Red light deviates the least and travels the fastest. The violet light deviates the most and travels the slowest.
The amount of material dispersion depends on two factors :
(I) The range of light wavelengths injected into the fiber. A source does not normally emit a single wavelength, it emits several. This range of wavelengths, expressed in nanometer is the spectral width of the source. An LED has a much higher spectral width than a LASER – about 35 nm for a LED and 2 to 3 nm for a LASER.
(II) The centre operating wavelength of the sources
Around 850nm, longer (reddish) wavelengths travel faster than the shorter (Bluish) ones. At 1550nm however the situation is reversed. The shorter wavelengths travel faster than the longer ones. At some point, the cross over must occur where the bluish and reddish wavelengths travel at the same speed. This crossover occurs around 1300nm, the zero-dispersion wavelength. At wavelengths below 1300nm, dispersion is negative. So wavelengths travel or arrive later. Above 1300 nm, the wavelengths lead or arrive faster.
This dispersion is expressed in Pico seconds per kilometer per nanometer of source spectral width (ps/km/nm).
WAVEGUIDE DISPERSION :
Waveguide dispersion, most significant in a single- mode fiber, occurs because optical energy travels in both the core and cladding, which have slightly different refractive indices. The energy travels at slightly different velocities in the core and cladding because of the slightly different refractive indices of the materials. Altering the internal structures of the fiber, allows waveguide dispersion to be substantially changed, thus changing the specified overall dispersion of the fiber.
BANDWIDTH AND DISPERSION :
A bandwidth of 400 MHz -km means that a 400 MHz-signal can be transmitted for 1 km. It means that the product of frequency and the length must be 400 or less. We can send a lower frequency for a longer distance, i.e. 200 MHz for 2 km or 100 MHz for 4 km.
Multimode fibers are specified by the bandwidth-length product or simply bandwidth.
Single mode fibers on the other hand are specified by dispersion, expressed in ps/km/nm. In other words for any given single mode fiber dispersion is most affected by the source’s spectral width. The wider the source spectral width, the greater the dispersion.
Conversion of dispersion to bandwidth can be approximated roughly by the following equation.
BW = 0.187/ (Disp) (SW) (L)
Disp = Dispersion at the operating wavelength in seconds/ nm/ km.
SW = Spectral width of the source in nm.
L = Fiber length in km.
So the spectral width of the source has a significant effect on the performance of a single mode fiber.
OPTICAL WINDOWS :
Attenuation of fiber for optical power varies with the wavelengths of light. Windows are low-loss regions, where fiber carry light with little attenuation. The first generation of optical fiber operated in the first window around 820 to 850 nm. The second window is the zero-dispersion region of 1300 nm and the third window is the 1550 nm region.
High loss regions, where attenuation is very high occur at 730, 950, 1250 and 1380 nm. One wishes to avoid operating in these regions. Evaluation of losses in a fiber must be done with respect to the transmitted wavelength.
Figure shows a typical attenuation curve for a low loss multimode fiber.
Making the best use of the low loss properties of the fiber requires that the sources emit light in the low loss region of the fiber. Plastic fibers are best operated in the visible light area around 650 nm. One important feature of attenuation in an optical fiber is that the constant at all modulation frequencies within the bandwidth.
Attenuation in a fiber has two main causes.
We can obtain losses less than 2.5 dB/km in the first window at 850 nm. Graded index fibers in the second window with loss below 1 dB/km and in the thrid window below 0.5 dB/km are obtained. Even lower losses are regarded as feasible for monomode fibers in all the three windows. Typically minimum loss in the three windows for the multimode fiber is 2.5 dB/km, 0.44 dB, km and 0.22 dB/km respectively. The corresponding figures for a monomode fiber are 1.9 dB/km, 0.32 dB/km and 0.048 dB/km.
Cabling is an outer protective structure surrounding one or more fibers. Cabling protects fibers environmentally and mechanically from being damaged or degraded in performance. Important consideration in any cable are tensile strength, ruggedness, durability, flexibility, environmental resistance, temperature extremes and even appearance. Evaluation of these considerations depends on the application.
Fiber Optic Cables have the following parts in common ;
(I) Optical Fiber
(III) Strength member
|Buffer||Protect fiber From Outside||Nylon, Mylar, Plastic|
|Central Member||Facilitate Stranding
|Primary Strength Member||Tensile Strength||Aramid Yarn, Steel|
|Cable Jacket||Contain and Protect
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