Optical Fiber Communications

Optical Fiber Communications Week 4 Bahria University

Dispersion • Dispersion of the transmitted optical signal causes distortion for signal transmission along optical fibers. • Dispersion mechanisms within the fiber cause broadening of the transmitted light pulses as they travel along the channel. • The phenomenon is illustrated in Figure (next slide), where it may be observed that each pulse broadens and overlaps with its neighbors, eventually becoming indistinguishable at the receiver input. Bahria University

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Bandwidth of Fiber • Signal dispersion alone limits the maximum possible bandwidth attainable with a particular optical fiber to the point where individual symbols can no longer be distinguished. • For no overlapping of light pulses down on an optical fiber link the digital bit rate ‘‘Bt’’ must be less than the reciprocal of the broadened (through dispersion) pulse duration (2τ). Bahria University

Alternative method • Another more accurate estimate of the maximum bit rate for an optical channel with some dispersion may be obtained by considering the light pulses at the output to have a Gaussian shape with an rms width of σ. Bahria University

Signal Dispersion in Various types Fibers • It may be observed that the multimode step index fiber exhibits the greatest dispersion of a transmitted light pulse and the multimode graded index fiber gives a considerably improved performance. • Finally, the single-mode fiber gives the minimum pulse broadening and thus is capable of the greatest transmission bandwidths which are currently in the gigahertz range, whereas transmission via multimode step index fiber is usually limited to bandwidths of a few tens of megahertz. Bahria University

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Conti… • Hence, the number of optical signal pulses which may be transmitted in a given period, and therefore the information-carrying capacity of the fiber, is restricted by the amount of pulse dispersion per unit length. • The typical best bandwidth–length products for the three fibers are 20 MHz km, 1 GHz km and 100 GHz km for multimode step index, multimode graded index and single-mode step index fibers respectively. Bahria University

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Dispersion Types • Dispersion in optical fibers is of various types these are as follows: • Material dispersion, • Waveguide dispersion • Modal Dispersion Material& waveguide dispersions are main causes of Intramodal Dispersion. Bahria University

Modal Dispersion • Multimode fibers can guide many different light modes since they have much larger core size. • Each mode enters the fiber at a different angle and thus travels at different paths in the fiber. • Since each mode ray travels a different distance as it propagates, the ray arrive at different times at the fiber output. So the light pulse spreads out in time which can cause signal overlapping so seriously that you cannot distinguish them any more. Bahria University

Conti… Model dispersion is not a problem in single mode fibers since there is only one mode that can travel in the fiber Bahria University

Chromatic Dispersion • Chromatic dispersion is the pulse spreading that arises because the velocity of light through a fiber depends on its wavelength. • So, the refractive index of the silica is different for different frequency of waves. • Optical sources do not emit just a single frequency but a band of frequencies then there may be propagation delay differences between the different spectral components of the transmitted signal. Bahria University

Material Dispersion • Pulse broadening due to material dispersion results from the different group velocities of the various spectral components launched into the fiber from the optical source. Bahria University

Conti…. • The pulse spread due to material dispersion may be obtained by considering the group delay τg in the optical fiber which is the reciprocal of the group velocity vg. • The pulse delay τm due to material dispersion in a fiber of length L is therefore: Bahria University

Conti…. • For a source with rms spectral width σλand a mean wavelength λ, the rms pulse broadening due to material dispersion σm may be obtained from the expression: Bahria University

Conti… • Material dispersion parameter M which is defined as: Example: Bahria University

Material Dispersion Versus Wavelength Bahria University

Waveguide Dispersion • The waveguiding of the fiber may also create chromatic dispersion. • Waveguide dispersion is only important in single mode fibers. It is caused by the fact that some light travels in the fiber cladding compared to most light travels in the fiber core. Bahria University

Conti…. • Since fiber cladding has lower refractive index than fiber core, light ray that travels in the cladding travels faster than that in the core. • While the difference in refractive indices of single mode fiber core and cladding are minuscule, they can still become a factor over greater distances Bahria University

Polarization • Orientation of the electric and magnetic fields w.r.t earth or axis. • Cylindrical optical fibers do not generally maintain the polarization state of the light input for more than a few meters. • The optical signal is thus detected by a photodiode which is insensitive to optical polarization or phase of the lightwave within the fiber. Bahria University

Fiber birefringence • Single-mode fibers in fact support two modes simultaneously, which are orthogonally polarized. • In an ideal circular-core fiber, these two modes will propagate with the same phase velocity; However, practical fibers are not perfectly circularly symmetric. • As a result, the two modes propagate with slightly different phase and group velocities. Bahria University

Conti… • Hence in an optical fiber with an ideal optically circularly symmetric core both polarization modes propagate with identical velocities. • Manufactured optical fibers, however, exhibit some birefringence resulting from differences in the core geometry (i.e. ellipticity) resulting from variations in the internal and external stresses, and fiber bending. • As rays travelling at different speeds so, modes therefore have different propagation constants βx and βy ( Slow and fast mode respectively) Bahria University

Conti…. • where λ is the optical wavelength, is modal Birefringence Constant • Light polarized along one of the principal axes will retain its polarization for all L. • The difference in phase velocities causes the fiber to exhibit a linear retardation Φ(z) which depends on the fiber length L in the z direction and is given by assuming that the phase coherence of the two mode components is maintained. Bahria University

Normal Scenario of Birefringence • Polarization mode dispersion (PMD) is a source of pulse broadening which results from fiber birefringence and it can become a limiting factor for optical fiber communications at high transmission rates. Bahria University

Birefringent Coherence • Polarization-maintaining fibers can be broadly divided into the following two categories: (i) high-birefringence (Hi-Bi) fibers (ii) single-polarization single-mode (SPSM) fibers. • We Will discuss Hi-Bi fibers, as working phenomenon is same for both. Bahria University

Conti…. • Birefringent coherence is maintained over a length of fiber, • Incident linear polarization which is at 45° with respect to the x axis becomes circular polarization at Φ =π/2 and linear again at Φ =π. The process continues through another circular polarization at Φ = 3π/2 before returning to the initial linear polarization atΦ = 2π. Bahria University

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Beat Length • The characteristic length LB for this process corresponding to the propagation distance for which a 2π phase difference accumulates between the two modes is known as the beat length. It is given by: • Typical single-mode fibers are found to have beat lengths of a few centimeters. Bahria University

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Maximum Bit rate • Maximum bit rate for the digital transmission in relation to the polarization mode dispersion is: Bahria University

Optical Fiber Communications