1、CHATER3 SIGNAL DEFRADATION IN OPTICAL FIBERS,3.1 ATTENUATION 3.2 SIGNAL FISTIORTION 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 s
2、ignals 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. Of equal importance is signal distortion. The distortion m
3、echanisms in a fiber cause optical signal pulses to broaden as they travel along a fiber. If these pulses travel sufficiently far, the will eventually overlap with neighboring pulses, thereby creating errors in the receiver output. The signal distortion mechanisms thus limit the information-carrying
4、 of a fiber.,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
5、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,
6、 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
7、it is a function of the wavelength .,3.12 Absorption,Absorption is caused by three different mechanismsAbsorption by atomic defects in the glass composition.xtrinsic absorption by impurity atoms in the glass material. Intrinsic absorption by the basic constituent atoms of the fiber material. Atoms d
8、efects are imperfections in the atomic structure of the fiber material.Radiation damages a material by changing its internal structure. The damage effects depend on the energy of the ionizing particles or rays, the radiation flux, and the influence. The total dose a material receives is expressed in
9、 units rad, which is a measure of radiation absorbed in bulk silicon. This unit is defined as,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. The high
10、er the radiation level, the larger the attenuation.The dominant absorption factor in fibers prepared by the direct-melt method is the presence of impurities in the fiber material. Impurity absorption results predominantly from transition metal ions, such as iron, chromium, cobalt, and copper, and fr
11、om OH(water)ions.The peaks and valleys in the attenuation curve resukted in the designation of various “transmission windows” to optical fivers. By reducing the residual OH content of fibers to around 1 ppb, standard commercially available single-mode fibers have nominal attenuations of 0.5Db/km in
12、the 1300-nm window and 0.3Db/km in the 1550-nm window, as shown by the solid Fig.,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.,Intrinsic absorption results
13、from electronic absorption bands on the ultraviolet region and from atomic vibration bands in the near-infrared region. The electronic absorption bands are associated with the band gas of the amorphous glass materials.As showed in Fig.3-3,the ultraviolet loss in small compared with scattering loss i
14、n the near-infrared region.These mechanisms results in wedge-shaped spectral-loss characteristic. Within this wedge, losses as low as 0.154dB/km at 1.55um in a single-mode fiber have been measured.,3.13 Scattering Losses,Scattering losses in glass arise from microscopic variations in the material de
15、nsity, from compositional fluctuations, and from structural inhomogeneities or defects occurring during fiber manufacture. Glass is composed of a randomly connected network of molecules. Such a structure naturally contains regions in which the molecular density is either higher or lower than the ave
16、rage density in the glass. Since glass is made up of several oxides. These two effects give rise to refractive index variation which occur within the glass over distances that are small compared with the wavelength. These index variations cause a Rayleigh-type scattering of the light .,Structural in
17、homogeneities and defects created during fiber fabrication can also cause scattering on light out of the fiber. These defects may be in the form of trapped gas bubbles, unreacted starting materials, and crystallized regions in the glass. In general ,the perform manufacturing methods that evolved hav
18、e minimized these extrinsic effects the to point where scattering that results from them is negligible compared with the intrinsic Rayleigh scattering.,The losses of multimode fibers are generally higher than those of single-mode fibers. This is a result of higher dopant concentrations and the accom
19、panying larger scattering loss due to greater compositional fluctuation in multimode fibers. In addition,multimode fibers are subjextto higher-mode kisses owing to perturbations at the core-cladding interface.,3.14 Bending losses,radiative losses occur whenever an optical fiver undergoes a bend of f
20、inite radius of curvature. Fiber can be subject to two types of bends:(a)macroscopic bends having radii that are large compared with the fiber diameter.(b) random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables.Lets first examine larger-curvation losse
21、s, which are 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 t
22、he bend radius id made a bit smaller once this threshold point has been reached , the losses suddenly become extremely large.,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 t
23、he propagation in an actual fiber, all the dissipative and scattering losses will be manifested 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 a
24、long a fiber. This distortion is a consequence of intramodal dispersion and intermodal delay 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 alon
25、g the fiber. Intramodal sispersion or chromatic dispersion is pulse spreading that occurs within a single mode. The spreading arises from the finite spectral emission width of an optical source. This phenomenon is also known as group velocity dispersion ,since the dispersion is a results of the grou
26、p velocity being a function of the 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
27、fiber confines only about 80% of the optical optical power to the 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
28、 in multimode fibers. However ,this effect is significant in single mode fibers.,3.2.1 INFORMATION CAPACITY DETERMINATION,A result of the dispersion-induced signal distortion is that a light pulse will broaden as it travels along the fiber. As shown in fig.3-10, this pulse broadening will eventually
29、 cause a pulse to overlap with neighboring pulse. After a certain amount of overlap has occurred, adjacent pulses can no longer be individually distinguished at the receiver and errors will occur. Thus, the dispersive properties determine the limit of the information capacity of the fiber.A measure
30、of the information capacity of an optical waveguide is usually specified by the bandwidth distance product in MHzkm .,The information-carrying capacity can be determined by examining the deformation of short light pulses propagating along the fiber. The following discussion on signal distortion is t
31、hus carried cut primarily from the standpoint of pulse broadening, which is representative of digital 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
32、 this difference in time delays, the optical signal pulse spreads out with tine as it is transmitted over the fiber. Thus, the quantity we are interested in is the amount of pulse spreading that arises from the group delay variation.The factor is designated as the dispersion .it defines the pulse sp
33、read as a function of wave-lenfth and is measured in picoseconds per kilometer. It is a result of material and waveguide dispersion.,3.2.3MATERIAL DISPERSION,Material dispersion occurs because the index of refraction varies as a function of the optical wavelength. The various spectral components of
34、a given mode will travel at different speeds, depending on the wavelength. Material dispersion is, therefore, an intramodal dispersion effect, and is of particular importance for single-mode wave-guides and for LED system (since am LED has a broader output spectrum than a laser diode).Material dispe
35、rsion 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 independe
36、nt of wavelength. For a fixed value of V the group delay is different foe every guided mode. These various modes arrive at the fiber end at different times depends on their group delay, so that a pulse spreading results. For multimode fibers the waveguide dispersion is generally very small compared
37、with material dispersion and can therefore be neglected.,3.2.5 SIGNAL DISTORTION IN SINGLE-MODE FIBER,For single mode fibers, waveguide dispersion is of importance and can be of the same order of magnitude as material dispersion.For a standard non-dispersion shifted fiber, waveguide dispersion is im
38、portant 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 Dispersion,The effects of fiber birefringence on the polarization states of
39、an optical signal are anther source of pulse broadening. This is particularly critical for high-rate long-haul transmission links that are designed to operate near the zero-dispersion wavelength of the fiber. Birefringence can result from intrinsic factors such as geometric irregularities of the fiv
40、er 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 t
41、o the electric-field orientation of a light signal, which can vary significantly along the length of a fiber. Signal energy at a given wavelength occupies two orthogonal polarization modes. A varying birefringence along its length will cause each polarization mode to travel at a slightly difference
42、velocity and the propagation orientation will rotate with distance. The result difference in propagation times T between the two orthogonal polarization modes will results in pulse spreading. This is the polarization-mode dispersion.,3.2.7 Intermodal Distortion,The final factor giving rise to signal
43、 degradation is intermodal distortion, which is a result of different values of the group delay fir each individual mode at a single frequency. This distortion mechanism is eliminated by single-mode operation, but is important in multimode fibers.,3.3 DESUGN OPTIMIZATION OF SIGNAL MODE FIBERS,Since
44、telecommunication compares use single mode fibers as the principal optical transmission medium in their network ,and because of the importance of single-mode fibers in microwave speed localized applications, this sections addresses their basic design and operational properties. Here, we shall examin
45、e design optimization characteristics, cutoff wavelength ,dispersion, mode-field diameter, and bending loss.,3.3.1 Refractive-Index Profiles,In the design of single-mode fibers, dispersion behavior is a major distinguishing feature, since this is what limits long-distance and very high speed transmi
46、ssion. Whereas the dispersion of a single mode silica fiber is lowest at 1300nm,its attenuation is a minimum at 1550nm,where the dispersion is higher. To achieve this, one can adjust the basic fiber parameters to shift the zero-dispersion minimum to longer wavelengths. The most popular single mode f
47、ibers used in telecommunication network are near-step-index fibers, which are dispersion-optimized for operation at 1300nm.,As we saw from Eqs (3-20) and(3-26),whereas material dispersion depends on composition of the material, waveguide dispersion is a function of the core radius ,the refractive in
48、dex difference, and the shape of the refractive index profile. By creating a fiber with a lager negative waveguide dispersion and assuming the same values for material dispersion can they shift the zero-dispersion point to longer wavelengths. The resulting optical fibers are known as dispersion-shif
49、ted fibers. The results total dispersion curve is showed in Fig.3-24bfor fibers with a zero-dispersion wave-length at 1550 nm.,An alternative is to reduce fiber dispersion by spreading the dispersion the minimum out over a wider range. The approach is known as dispersion flattening.,3.3.2 Cutoff Wav
50、elength,The cutoff wavelength of the first higher-order mode is an important transmission parameter for singer-mode fibers ,since it separates the singer mode from the multimode 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.,
