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Optical Fiber Communication chapter3


CHATER3 SIGNAL DEGRADATION IN OPTICAL FIBERS
3.1 ATTENUATION 3.2 SIGNAL DISTORTION IN OPTICAL WAVEGUIDES 3.3 DESUGN OPTIMIZATION OF SIGNAL MODE FIBERS

CHATER3 SIGNAL DEFRADATION IN OPTICAL FIBERS
1. What are the loss or signal attenuation mechanisms in a fiber? 2. why and to what degree do optical signals get distorted as they propagate along a fiber ? Signal attenuation is one of the most important properties of an optical fiber, because it largely determines the maximum unamplified or repeaterless between a transmitter and a receiver. receiver. Of equal importance is signal distortion. The distortion distortion. mechanisms in a fiber cause optical signal pulses to broaden as they travel along a fiber. If these pulses travel sufficiently far, fiber. the will eventually overlap with neighboring pulses, thereby creating errors in the receiver output. The signal distortion output. mechanisms thus limit the information-carrying of a fiber. informationfiber.

3.1ATTENUATION
Attenuation of a light signal as it propagates along a fiber is an important consideration in the design of an optical communication system . The basic attenuation mechanisms in a fiber are absorption, scattering and radioactive losses of the optical energy. Absorption is related to the fiber material, whereas scattering is associated both with the fiber material and with structural imperfections in the optical wave guide. Attention owing to radioactive effects originates from perturbations of the fiber geometey.

3.11 Attenuation Units
As light travels along a fiber, its power decreases exponentially with distabce . For simplicity in calculating optical signal attenuation in a fiber, the common procedure is to express the attenuation coefficient in units of decibels per kilometer, denoted by dB/km. This parameter is generally referred to as the fiber loess, and it is a function of the wavelength .

3.12 Absorption
Absorption is caused by three different mechanisms Absorption by atomic defects in the glass composition. composition. xtrinsic absorption by impurity atoms in the glass material. material. Intrinsic absorption by the basic constituent atoms of the fiber material.

Atoms defects are imperfections in the atomic structure of the fiber material. material. Radiation damages a material by changing its internal structure. structure. The damage effects depend on the energy of the ionizing particles or rays, the radiation flux, and the influence. influence. The total dose a material receives expressed in units rad, which is a measure of radiation absorbed in bulk silicon. This unit is defined as silicon.

1rad=100erg/g=0.0j/kg
The basic response of a fiber to ionizing radiation is an increase in attenuation owing to the creation of atomic defects, or attenuation, centers, that absorb optical energy. energy. The higher the radiation level, the larger the attenuation. attenuation. The dominant absorption factor in fibers prepared by the directdirectmelt method is the presence of impurities in the fiber material. material. Impurity absorption results predominantly from transition metal ions, such as iron, chromium, cobalt, and copper, and from OH(water)ions. OH(water)ions. The peaks and valleys in the attenuation curve resukted in the designation of various “transmission windows” to optical fivers. fivers. By reducing the residual OH content of fibers to around 1 ppb, standard commercially available single-mode fibers have singlenominal attenuations of 0.5Db/km in the 1300-nm window and 13000.3Db/km in the 1550-nm window, as shown by the solid Fig. 1550Fig.

Intrinsic absorption is associated with the basic fiber material and is the principal physical factor that defines the .transparency windows of a material over a specified spectral region. region. Intrinsic absorption results from electronic absorption bands on the ultraviolet region and from atomic vibration bands in the near-infrared region. The nearregion. electronic absorption bands are associated with the band gas of the amorphous glass materials. materials. As showed in Fig.3-3,the ultraviolet loss in small Fig. compared with scattering loss in the near-infrared nearregion. region. These mechanisms results in wedge-shaped wedgespectralspectral-loss characteristic. Within this wedge, characteristic. losses as low as 0.154dB/km at 1.55um in a single154dB/km 55um singlemode fiber have been measured. measured.

3.13 Scattering Losses
Scattering losses in glass arise from microscopic variations in the material density, from compositional fluctuations, and from structural inhomogeneities or defects occurring during fiber manufacture. Glass is manufacture. composed of a randomly connected network of molecules. molecules. Such a structure naturally contains regions in which the molecular density is either higher or lower than the average density in the glass. Since glass is glass. made up of several oxides. These two effects give oxides. rise to refractive –index variation which occur within the glass over distances that are small compared with the wavelength. These index variations cause a wavelength. RayleighRayleigh-type scattering of the light .

Structural inhomogeneities and defects created during fiber fabrication can also cause scattering on light out of the fiber. These defects may be in the form of fiber. trapped gas bubbles, unreacted starting materials, and crystallized regions in the glass. In general ,the glass. perform manufacturing methods that evolved have minimized these extrinsic effects the to point where scattering that results from them is negligible compared with the intrinsic Rayleigh scattering. scattering. The losses of multimode fibers are generally higher than those of single-mode fibers. This is a result of singlefibers. higher dopant concentrations and the accompanying larger scattering loss due to greater compositional fluctuation in multimode fibers. In addition,multimode fibers. fibers are subjextto higher-mode kisses owing to higherperturbations at the core-cladding interface. coreinterface.

3.14 Bending losses
radiative losses occur whenever an optical fiver undergoes a bend of finite radius of curvature. Fiber curvature. can be subject to two types of bends: bends:(a)macroscopic bends having radii that are large compared with the fiber diameter.(b) random diameter. microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. cables. Let’s first examine larger-curvation losses, which largerare known as macrobending losses or simply vending losses. For slight beds the excess loss is extremely small and is essentially unobservable. As the radius of curvature decreases, the loss increase exponentially until at a certain critical radius the curvature loss becomes observable. If the bend radius id made a bit smaller once this threshold point has been reached , the losses suddenly

Anther form of radiation loss in optical waveguides results from mode coupling caused by random microbends of the optical fiber.

3.15 Core and Cadding Losses
Upon measuring the propagation in an actual fiber, all the dissipative and scattering losses will be manifested simultaneously. simultaneously. It has generally observed that the loss increases with increasing mode number.

3.2 SIGNAL FISTIORTION IN OPTICAL WAVEGUIDES
An optical signal becomes increasingly distorted as it traveled along a fiber. This distortion is a consequence fiber. of intramodal dispersion and intermodal delay effects. effects. These distortion effects can be explained by examing the behavior of the group velocities of the guided modes, where the group velocity is the speed at which energy in a particular mode travels along the fiber. fiber. Intramodal sispersion or chromatic dispersion is pulse spreading that occurs within a single mode. The mode. spreading arises from the finite spectral emission width of an optical source. This phenomenon is also source. known as group velocity dispersion ,since the dispersion is a results of the group velocity being a function of the wavelength. wavelength.

The two main causes of intramodal dispersion are as follows:

1.

Material dispersion, which arises from the variation of the refractive index of the core material as a function of wavelength . 2. Waveguide dispersion, which occurs because a single-model fiber confines only singleabout 80% of the optical optical power to the 80% core. core. The other factor giving rise to pulse spreading is intermodal delay, which is a result of each mode having a different value of the group velocity at a single frequency.

Of these three, waveguide dispersion usually can be ignored in multimode fibers. However ,this effect is fibers. significant in single –mode fibers. fibers.

3.2.1 INFORMATION CAPACITY DETERMINATION
A result of the dispersion-induced signal distortion is that dispersiona light pulse will broaden as it travels along the fiber. fiber. As shown in fig.3-10, this pulse broadening will fig. 10, eventually cause a pulse to overlap with neighboring pulse. pulse. After a certain amount of overlap has occurred, adjacent pulses can no longer be individually distinguished at the receiver and errors will occur. occur. Thus, the dispersive properties determine the limit of the information capacity of the fiber. fiber. A measure of the information capacity of an optical waveguide is usually specified by the bandwidth – distance product in MHz·km .

The information-carrying capacity can be determined by informationexamining the deformation of short light pulses propagating along the fiber. The following discussion fiber. on signal distortion is thus carried cut primarily from the standpoint of pulse broadening, which is representative of digital transmission. transmission.

3.2.2 GROUP DELAY
The group delay depends on the wavelength, each spectral component of any particular mode takes a different amount of time o travel a certain distance. As a result of this difference in distance. time delays, the optical signal pulse spreads out with tine as it is transmitted over the fiber. Thus, fiber. the quantity we are interested in is the amount of pulse spreading that arises from the group delay variation. variation. The factor is designated as the dispersion .it defines the pulse spread as a function of wavewavelenfth and is measured in picoseconds per kilometer. It is a result of material and

3.2.3MATERIAL 3.2.3MATERIAL DISPERSION
Material dispersion occurs because the index of refraction varies as a function of the optical wavelength. wavelength. The various spectral components of a given mode will travel at different speeds, depending on the wavelength. wavelength. Material dispersion is, therefore, an intramodal dispersion effect, and is of particular importance for single-mode wave-guides and for LED singlewavesystem (since am LED has a broader output spectrum than a laser diode). diode). Material dispersion can be reduced either by choosing sources with narrower spectral output widths ( reducing ) or by operating at longer wavelengths.

3.2.4 WACEGUIDE DISPERSION
The effect of waveguide dispersion on pulse spreading of approximated by assuming that the refractive index of the material is independent of wavelength. wavelength. For a fixed value of V the group delay is different foe every guided mode. These various modes arrive at the mode. fiber end at different times depends on their group delay, so that a pulse spreading results. For results. multimode fibers the waveguide dispersion is generally very small compared with material dispersion and can therefore be neglected. neglected.

3.2.5 SIGNAL DISTORTION IN SINGLE-MODE FIBER SINGLEFor single mode fibers, waveguide dispersion is of importance and can be of the same order of magnitude as material dispersion. dispersion. For a standard non-dispersion shifted fiber, nonwaveguide dispersion is important around 1320nm. At this point, the two dispersion factors cancel to give a zero total dispersion. However, material dispersion dominates waveguide dispersion at shorter and longer wavelengths.

3.2.6 Polarization-mode PolarizationDispersion
The effects of fiber birefringence on the polarization states of an optical signal are anther source of pulse broadening. This is particularly critical for high-rate highlonglong-haul transmission links that are designed to operate near the zero-dispersion wavelength of the zerofiber. Birefringence can result from intrinsic factors such as geometric irregularities of the fiver core or internal stresses in it. in addition, external factors, such as bending ,twisting, or pinching of the fiber ,can also lend to birefringence. There will be a varying birefringence along its length.

A fundamental property of an optical signal is its polarization state. Polarization refers to the electricstate. electricfield orientation of a light signal, which can vary significantly along the length of a fiber. Signal energy fiber. at a given wavelength occupies two orthogonal polarization modes. A varying birefringence along its modes. length will cause each polarization mode to travel at a slightly difference velocity and the propagation orientation will rotate with distance. The result distance. difference in propagation times △T between the two orthogonal polarization modes will results in pulse spreading. spreading. This is the polarization-mode dispersion. polarizationdispersion.

3.2.7 Intermodal Distortion
The final factor giving rise to signal degradation is intermodal distortion, which is a result of different values of the group delay fir each individual mode at a single frequency. This frequency. distortion mechanism is eliminated by singlesinglemode operation, but is important in multimode fibers. fibers.

3.3 DESUGN OPTIMIZATION OF SIGNAL MODE FIBERS
Since telecommunication compares use single mode fibers as the principal optical transmission medium in their network ,and because of the importance of singlesingle-mode fibers in microwave –speed localized applications, this sections addresses their basic design and operational properties. properties. Here, we shall examine design –optimization characteristics, cutoff wavelength ,dispersion, modemodefield diameter, and bending loss. loss.

3.3.1 Refractive-Index Profiles RefractiveIn the design of single-mode fibers, dispersion behavior singleis a major distinguishing feature, since this is what limits long-distance and very high speed transmission. longtransmission. Whereas the dispersion of a single mode silica fiber is lowest at 1300nm,its attenuation is a minimum at 1300nm,its 1550nm,where 1550nm,where the dispersion is higher. To achieve higher. this, one can adjust the basic fiber parameters to shift the zero-dispersion minimum to longer wavelengths. zerowavelengths. The most popular single mode fibers used in telecommunication network are near-step-index fibers, near-stepwhich are dispersion-optimized for operation at dispersion1300nm. 1300nm.

As we saw from Eqs (3-20) and(3-26),whereas material 20) and(3 26),whereas dispersion depends on composition of the material, waveguide dispersion is a function of the core radius ,the refractive index difference, and the shape of the refractive index profile. By creating a fiber with a profile. lager negative waveguide dispersion and assuming the same values for material dispersion can they shift the zerozero-dispersion point to longer wavelengths. The wavelengths. resulting optical fibers are known as dispersion-shifted dispersionfibers. fibers. The results total dispersion curve is showed in Fig.3Fig. 24bfor fibers with a zero-dispersion wave-length at 1550 24bfor zerowavenm. nm. An alternative is to reduce fiber dispersion by spreading the dispersion the minimum out over a wider range. The range. approach is known as dispersion flattening. flattening.

3.3.2 Cutoff Wavelength
The cutoff wavelength of the first higher-order highermode is an important transmission parameter for singer-mode fibers ,since it separates the singersinger mode from the multimode region. region. Since in the cutoff region the field of the Lp11 mode is widely spread across the fiber cross section, its attenuation is strongly affected by fiber bends, length, and cabling.

3.5.3 Dispersion calculation
The total dispersion in single-mode fibers consists mainly of singlematerial and waveguide dispersion. dispersion. The dispersion behavior varies with wavelength and also with fiber type. type. Thus, the EIA and the IUT-T have recommended different IUTformulas to calculate the chromatic dispersion for specific fiber types operation in a given wavelength region. region. Figure 3-27 illustrates the importance of controlling dispersion in single-mode fibers. As optical pulse travel down a fiber, singlefibers. temporal broadening occurs because material and wavelength dispersion cause different wavelengths in the optical pulse to propagate with different velocities. Thus, as Eq. (3-54)implies, velocities. Eq. 54)implies, the broader the spectral width of the source, the greater the pulse dispersion will be. This effect is clearly seen in Fig. 3-27. be. Fig. 27.

3.5.4 Mode-field Diameter ModeSection 2.5.1 gives the definition of the mode-field fibers. modefibers. One uses the mode-field diameter in describing the modefunctional properties of a single-mode fiber, since it singletakes into cladding. This is shown in Fig. 3-28 for cladding. Fig. 1300-nm\ 1300-nm\optimized, dispersion-shifted, dispersionand dispersiondispersion-flattened single-mode fibers. singlefibers.

3.5.5 Bending Loss
macrobending and microbending losses are important in the design of single-mode fibers. These losses are singleprincipally evident in the 1550-nm region, and show 1550up as a rapid increase in attenuation when the fiber is bent smaller than a certain bend radius. The lower the cutoff wavelength relative to the operating wavelength, the more susceptible single-mode fibers are to singlebending. For example, in a fiber which is optimized for operation at 1300nm, both the microbending and macrobending losses are greater at 1300 nm by a factor of 3to 5 , as Fig. 3-29 illustrates. A fiber thus 3might be transmitting at 1300nm but have a significant loss at 1550nm.

The bending losses are primarily a function of the modemodefield diameter. Generally, the smaller the mode-field diameter. modediameter, the smaller the bending loss. This is true for loss. both matched-clad and depressedclad fibers. matchedfibers.


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