Subject Name Optical Fiber Communication Subject Code 10

Subject Name: Optical Fiber Communication Subject Code: 10 EC 72 Prepared By: Sreepriya Kurup, Pallavi Adke, Sowmya L(TE) Department: Electronics and Communication Engineering Date: 10/11/2014 8/12/2014

UNIT-2 TRANSMISSION CHARACTERISTIC OF OPTICAL FIBERS 6/4/2021

Contents • • • Introduction Attenuation Absorption Scattering losses Bending loss Dispersion – Intramodal dispersion – Intermodal dispersion 6/4/2021

Introduction Most optical fibers are used for transmitting information over long distances. Two major advantages of fiber: (1) wide bandwidth and (2) low loss. Attenuation cause mainly by absorption and scattering. Bandwidth is limited by an effect called dispersion. 6/4/2021

Attenuation There a number of major causes of attenuation in fiber ü Attenuation mainly due to material absorption, material scattering. ü Others include bending losses, mode coupling losses and losses due to leaky modes üThere also losses at connectors and splices Effect of Attenuation A receiver in an optical system requires a minimum optical input power to operate with a specific error probability. Attenuation reduces the optical power available, degrading the error probability. Most system specifications allow a maximum error probability of 1 X 10 -9 6/4/2021

Attenuation • Logarithmic relationship between the optical output power and the optical input power • Measure of the decay of signal strength or light power • Po= Pi e(-α’L) where: P(o) = Optical Power at distance L from the input Pi= Input optical power - ’ = Fiber attenuation coefficient, [d. B/km] 6/4/2021 6

Attenuation • Usually, attenuation is expressed in terms of decibels • Attenuation Conversion: = 4. 343 ’ Po= Pi 10(-αL/10) • where: P(L) = Optical Power at distance L from the input Po = Input optical power = Fiber attenuation coefficient, [d. B/km] (Signal attenuation per unit length in decibels) = scattering + absorption + bending 6/4/2021

A 30 km long fiber at 1300 nm has an attenuation of 0. 8 d. B/km. if 200µW power is launched into fiber; find the output power in d. Bm and in watts? 6/4/2021

Types of Attenuation 1 - Material Absorption losses Types of Absorption 2 - Intrinsic Absorption 3 - Extrinsic Absorption 4 - Scattering loss (Linear and nonlinear) 5 - Bending loss 6/4/2021

Material Absorption Losses ♣ Material absorption is a loss mechanism related to the material composition and the fabrication process for the fiber, which results in the dissipation of some of the transmitted optical power as heat in the waveguide. ♣ The absorption of the light may be intrinsic or extrinsic 6/4/2021

• Intrinsic Absorption: Caused by interaction with one or more of the components of the glass. ℓ Intrinsic absorption is a natural property of glass. It is strong in the ultraviolet (UV) region and in infrared (IR) region of the electromagnetic spectrum. ℓ However both these considered insignificant since optical communication systems are normally operated outside this region ℓ Non oxide glasses like fluorides and chlorides, in which longer wavelengths give less attenuation compared to oxide glasses 6/4/2021

Pure silica-based glass has two major intrinsic absorption mechanisms at optical wavelengths: (1) a fundamental UV absorption edge, the peaks are centered in the ultraviolet wavelength region. This is due to the electron transitions within the glass molecules. The tail of this peak may extend into the shorter wavelengths of the fiber transmission spectral window. (2) A fundamental infrared and far-infrared absorption edge, due to molecular vibrations (such as Si-O). The tail of these absorption peaks may extend into the longer wavelengths of the fiber transmission spectral window. 6/4/2021

• Extrinsic Absorption: Caused by impurities within the glass • A- Extrinsic Absorption (OH ions): Caused by dissolved water in the glass, as the Hydroxy or (OH) ion. In this case absorption due the same fundamental processes between (2700 nm, and 4200 nm) gives rise to so called absorption overtones at 1380, 950, 720 nm. Typically a 1 part per million impurity level causes 1 d. B/ km of attenuation at 950 nm. 6/4/2021

B- Extrinsic Absorption (metallic ions): For some of the more common metallic impurities in silica fiber, the table shows the peak attenuation wavelength caused by impurity concentration of 1 in 109. Modern fabrication techniques can reduce impurity levels below 1 part in 1010 **Extrinsic absorption is much more significant than intrinsic 6/4/2021

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Linear Scattering Losses • Scattering - Linear Scattering Losses Scattering is a process whereby all or some of the optical power in a mode id transferred into another mode. Frequently causes attenuation, since the transfer is often to a mode that does not propagate well. (also called a leaky or radiation mode). o 6/4/2021 Two major type: 1. Rayleigh 2. Mie scattering

– Raleigh Scattering - most common form of scattering • caused by microscopic non-uniformities making light rays partially scatter • The non-uniformities (in homogeneities) manifest themselves as refractive index fluctuations and arise from density and compositional variations which are frozen into the glass lattice on cooling • Compositional variations can avoided by improved fabrication but index fluctuations cannot be avoided. • nearly 90% of total attenuation is attributed to Raleigh Scattering • becomes important when wavelengths are short - comparable to size of the structures in the glass: long wavelengths are less affected than short wavelengths 6/4/2021 • The subsequent scattering due to the density fluctuations, which is in almost all directions, produces an attenuation proportional to 1/ λ 4

• the Rayleigh scattering formula , For a single-component glass this is given by: • Where, γR is the Rayleigh scattering coefficient, λ is the optical wavelength, n is the refractive index of the medium, p is the average photoelastic coefficient, βc is the isothermal compressibility at a fictive temperature TF, and K is Boltzmann’s constant. • the Rayleigh scattering coefficient is related to the transmission loss factor (transmissivity) of the fiber following the relation • where L is the length of the fiber, Rayleigh scattering is strongly reduced by operating at the longest possible wavelength. 6/4/2021

Mie Scattering ▪ caused in inhomogeneities which are comparable in size to the guided wavelength. ▪ These result from the non-perfect cylindrical structure of the waveguide and may be caused by fiber imperfections such as irregularities in the core-cladding interface, core-cladding refractive index differences along the fiber length, diameter fluctuations, strains and bubbles. ▪ The inhomogeneities may be reduced by: (a) removing imperfections due to the glass manufacturing process; (b) carefully controlled extrusion and coating of the fiber; (c) increasing the fiber guidance by increasing the relative refractive index difference. 6/4/2021

Nonlinear Scattering Losses • Non linear scattering causes the power from one mode to be transferred in either the forward or backward direction to the same or other modes, at the different frequency. • The most important types are; 1. Stimulated Brillouin scattering 2. Stimulated Raman scattering • Both are usually only observed at high optical power density in long single mode fibers 6/4/2021

– Stimulated Brillouin Scattering (SBS) • Stimulated Brillouin scattering (SBS) may be regarded as the modulation of light through thermal molecular vibrations within the fiber. • The scattered light appears as upper and lower sidebands which are separated from the incident light by the modulation frequency. • The incident photon in this scattering process produces a phonon* of acoustic frequency as well as a scattered photon • Brillouin scattering is only significant above a threshold power density, the threshold power Pb is given by: • where d and λ are the fiber core diameter and the operating wavelength, micrometers, αd. B is the fiber attenuation in decibels per kilometer and ν is the source bandwidth (i. e. injection laser) in gigahertz. 6/4/2021

– Stimulated Raman Scattering (SRS) • Stimulated Raman scattering (SRS) is similar to SBS except that a highfrequency optical phonon rather than an acoustic phonon is generated in the scattering process. • Also, SRS can occur in both the forward and backward directions in an optical fiber, and may have an optical power threshold of up to three orders of magnitude higher than the Brillouin threshold in a particular fiber. • the threshold optical power for SRS PR in a long single-mode fiber is given by: • SBS and SRS are not usually observed in multimode fibers because their relatively large core diameters make threshold optical power levels extremely high. 6/4/2021

• Fiber Bend Loss – Optical fibers suffer radiation losses at bends or curves on their paths. – This is due to the energy in the evanescent field at the bend exceeding the velocity of light in the cladding and hence the guidance mechanism is inhibited, which causes light energy to be radiated from the fiber 6/4/2021

• • microbending - result of microscopic imperfections in the geometry of the fiber macrobending - fiber bending with diameters on the order of centimeters (usually unnoticeable if the radius of the bend is larger than 10 cm) 6/4/2021

• The loss can generally be represented by a radiation attenuation coefficient • • where R is the radius of curvature of the fiber bend and c 1, c 2 are constants large bending losses tend to occur in multimode fibers at a critical radius of curvature Rc • Macrobending losses may be reduced by: (a) designing fibers with large relative refractive index differences; (b) operating at the shortest wavelength possible. • the critical radius of curvature for a single-mode fiber Rcs can be estimated as: 6/4/2021

Dispersion • Different modes take a different amount of time to arrive at the receiver. Result is a spread-out signal • each pulse broadens and overlaps with its neighbors, eventually becoming indistinguishable at the receiver input. The effect is known as intersymbol interference (ISI) • 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). • For NRZ BT=2 B, RZ BT=B 6/4/2021

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The typical best bandwidth–length products for the three fibers shown in Figure 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. 6/4/2021

• 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 pulse broadening increases linearly with fiber length and thus the bandwidth is inversely proportional to distance. the bandwidth–length product (i. e. Bopt × L) 6/4/2021

Dispersion • Dispersion - spreading of light pulses in a fiber – limits bandwidth – most important types • Intramodal or chromatic dispersion – material dispersion – waveguide dispersion • Intermodal/multimode dispersion 6/4/2021

Material Dispersion • The refractive index of the material varies as a function of wavelength, • Material-induced dispersion for a plane wave propagation in homogeneous medium of refractive index n: • The pulse spread due to material dispersion is therefore: is material dispersion(M) 6/4/2021

Waveguide Dispersion • Waveguide dispersion is due to the dependency of the group velocity with wavelength for a particular mode. • It is equivalent to change of incident angle w. r. t which subsequently leads to variation of transmission times for the rays and hence dispersion • For SMF d 2β/dλ 2 ≠ 0 • For MMF the majority modes propagate far from cut-off are almost free of waveguide dispersion (0. 1 to 0. 2 ns/km) 6/4/2021

Polarization Mode dispersion • The effects of fiber-birefringence on the polarization states of an optical are another source of pulse broadening. Polarization mode dispersion (PMD) is due to slightly different velocity for each polarization mode because of the lack of perfectly symmetric & anisotropicity of the fiber. If the group velocities of two orthogonal polarization modes are then the differential time delay between these two polarization over a distance L is • The rms value of the differential group delay can be approximated as: 6/4/2021

Dispersion Intermodal dispersion Because the different modes follow different paths through the fiber, a light pulse is broadened in proportion to the length of the fiber. 6/4/2021

• Intermodal or modal dispersion causes the input light pulse to spread. • The input light pulse is made up of a group of modes. As the modes propagate along the fiber, light energy distributed among the modes is delayed by different amounts. • The pulse spreads because each mode propagates along the fiber at different speeds. Since modes travel in different directions, some modes travel longer distances. • Modal dispersion occurs because each mode travels a different distance over the same time span, as shown in figure. • The modes of a light pulse that enter the fiber at one time exit the fiber a different times. This condition causes the light pulse to spread. • As the length of the fiber increases, modal dispersion increases. 6/4/2021

Estimating Modal Dispersion (Step Index Fiber) Assume: • Step index fiber • An impulse-like fiber input pulse • Energy is equally distributed between rays with paths lying between the axial and the extreme meridional ray What is the difference in delay for the two extremes over a linear path length L? 6/4/2021

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The rms pulse broadening at the fiber output due to intermodal dispersion for the multimode step index fiber 6/4/2021

Quantifying Dispersionin a GI Fibre (I) As in the step index case one determines maximum time difference between the two most extreme modes Most common expression is: By comparison the equivalent value for a step index fiber has been shown to be: Because of the dependence for graded index the dispersion is much lower since Δ is << 1. 6/4/2021

A 6 km optical link consists of multimode step index fiber with a core refractive index of 1. 5 and a relative index difference of 1%. Estimate : (a) the delay difference between the slowest and fastest modes at the fiber output (b) the rms pulse broadening due to intermodal dispersion on the link; (c) the maximum bit rate that may be obtained without substantial errors on the link assuming only intermodal dispersion; (d) the bandwidth-length product corresponding to (c). 6/4/2021