2b - signal degradation

8/3/2019 2B - Signal Degradation

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2Optical Fiber

Signal Degradation

2 Effects Pulse Spreading Dispersion (Distortion)

Causes the optical pulses to broaden as they travel

along a fiber

Overlap between neighboring pulses creates errors

Resulting in the limitation of information-carrying

capacity of a fiber

2

Determines the maximum repeaterless separationbetween optical transmitter & receiver


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Pulse Spreading

Successive pulses

overla as the s readInitial instantaneous pulses

Spreading increases

with distance

Degree of dispersion

depends on fiber type

3

Unintelligible

Multipath/Modal Dispersion Modes are oscillation/propagation paths

Mode velocities differ in step-index multimodefiber

Visualize as difference in ray pathsRed ray goes shorter distance thanblue

n2

n1

4


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Time Delay & Bandwidth Length product

Time delay between the two rays takinglongest and shortest paths is a measure of

=

=

2

2

11

n

n

c

LL

Sin

L

c

ndT

c

Time delay can be related to the information

carrying capacity of the fiber through bit rate Bcn

Derive ?

5

1.


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Group Velocity, Vg

The actual velocity at which the signal information &energy is traveling down the fiber. It is always lessthan the speed of light

The observable delay experienced by the opticalsignal waveform & energy, is commonly referredto as group delay

The group velocity depends on frequency and isgiven by:

d=

7

dg

Basics about Plane Waves

Wave Front

8


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Phase Velocity

vp

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Group of Waves

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Carrier and Envelope

vp vg

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Group Velocity

vg

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Dispersion can be described as:

Any phenomenon in which the velocity ofpropagation of any electromagnetic wave is

wave eng epen en .

Any process by which any electromagnetic signal

propagating in a physical medium is degraded

because the various wavelength signals have different

propagation velocities within the physical medium

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Types of Dispersion Intermodal/Modal Dispersion

(already discussed)

Intramodal Dispersion

1- Material Dispersion

2- Waveguide Dispersion

Polarization-Mode Dispersion

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Intramodal Dispersion/GVD

The propagation constant, (), is frequencydependent over band width , with the center

frequency ,

Each frequency component has a specific delay time

As the output signal is collectively represented by

group velocity & group delay this phenomenon is

called intramodal dispersion or Group Velocity

Dis ersion (GVD)

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In the case of optical pulse propagation down the

fiber, GVD causes pulse broadening, leading to InterSymbol Interference (ISI)

How to characterize dispersion? If the spectral width of the optical source is not too wide,

For spectral components which are apart,

symmetrical around center wavelength, the total delay

difference over a distance L is:

=

=

== L

d

dL

V

L

d

d

d

dT

g

g

22

2

dL==

1

16

2 is called GVD parameter, and shows how much alight pulse broadens as it travels along an optical fiber

dVV ggg, ,


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The more common parameter is called Dispersion,and can be defined as the delay difference per unit

length per unit wavelength as follows:

21 cdD =

=

In the case of optical pulse, if the spectral width of theoptical source is characterized by its rms value of theGaussian pulse , the pulse spreading T, over thelength of L, can be well approximated by:

=

Vd g

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D has a typical unit of [ps/(nmkm)]

Material DispersionCladding

CoreEmitter

Very short

vg(

2)

vg(

1)

Input

Output

t

Spread,

t0

Spectrum,

12o

Intensity Intensity Intensity

light pulse

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- within a spectrum, , of wavelengths.

Waves in the guide with different free space wavelengths travel at

different group velocities due to the wavelength dependence of n1.

The waves arrive at the end of the fiber at different times and hence

result in a broadened output pulse.


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Material Dispersion

The refractive index of the material varies as a function of

wavelength, n ()

Material-induced dispersion for a plane wave propagation

in homogeneous medium of refractive index n:

d

dn

cd

dnD

gg

M

22

2

12==

=

c

n

d

d

Vd

d 2

2

1

19

e pu se sprea ue o ma er a spers on s ere ore:

)( matg DLT =

material dispersion

Wavelength dependence of Dispersion

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Waveguide Dispersion

Waveguide dispersion is due to the dependency of thegroup velocity of the fundamental mode as well as other

modes on the Vnumber Normalized Fre uenc .

In order to calculate waveguide dispersion, we consider

that n is independent of wavelength.

Waveguide dispersion is given by:

+

=

dV

Vbd

d

dn

dV

VbdV

n

nD

gg

W

)()(2 22

2

2

2

2

2

21

where, group delay is expressed in terms of the

normalized propagation constant, b, also calledwaveguide parameter

Total Dispersion, Zero Dispersion

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Minimum loss here


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Dispersion

ShiftedFiber Profiles

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Polarization Mode Dispersion Effect of fiberbirefringence on polarization (electric

field orientation) states of an optical signal

Due to geometric irregularities, internal stresses,

deviations in circularity, external factors like bending,

twisting & pinching

Resulting difference in propagation times between the

two orthogonal modes causes pulse spreading, hence

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ea ng o PMD varies randomly along the fiber, ps/km


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Polarization Mode Dispersion

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Core

z

n1y

// Ex

Output light pulse

t

Intensity

n1x

//x Ey

Ex

Ey

E

= Pulse spread

Input light pulse

t

26

Suppose that the core refractive index has different values along two orthogonaldirections corresponding to electric field oscillation direction (polarizations). We cantakexand axes along these directions. An input light will travel along the fiber with ExandEpolarizations having different group velocities and hence arrive at the output at

different times


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Polarization mode dispersion (PMD) is due to slightlydifferent velocity for each polarization mode because ofthe lack of perfectly symmetric & anisotropicity of the

fiber

If the group velocities of two orthogonal polarization

modes are vgxand vgy, then the differential time delay

Tpol between these two polarization over a distance L is

pol

LLT =

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The rms value of the differential group delay can beapproximated as:

gygx

LDT PMDpol

Losses in Optical FibersCoaxial cable Vs. Optical Fiber Attenuation

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Attenuation (fiber loss)

Power loss along a fiber:

The parameter is called fiber attenuation coefficient

Z=0

P(0) mWZ= l

lpePlP

= )0()( mw

zpePzP

= )0()(

p

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having the unit of for example [1/km] or [nepers/km].

A more common unit is [dB/km] that is defined by:

]km/1[343.4)(

)0(log

10]dB/km[ p

lP

P

l =

=

Fiber loss in dB/km

dBmmW

mWmWP 10

1

10log1010

10 =

==

Z = 0 Z = l

]dBm)[0(P

]km[]dB/km[]dBm)[0(]dBm)[( lPlP =

30

mWmWdBmP 50110127 1027

=

==


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Fiber loss

Example: 10mW of power is launched into an optical fiberthat has an attenuation of a=0.6 dB/km. What is thereceived power after traveling a distance of 100 km?

n a power s: in = m

Received power is: Pout= Pin L = 10 dBm (0.6)(100)= - 50 dBm

Example: 8mW of power is launched into an optical fiberthat has an attenuation of a=0.6 dB/km. The received powerneeds to be -22dBm. What is the maximum transmissiondistance?

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in 10

Received power is: Pout = 1mW 10-2.2 = 6.3 mW

Pout - Pin = 9dBm - (-22dBm) = 31dB = 0.6 L L = 51.7 km

Optical fiber attenuation vs. wavelength

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Attenuation in Optical Fibers

Three causes of Losses

Absorption

Scattering

Radiative Losses

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Absorption-I (Extrinsic) Absorption is caused by three different mechanisms:

1- (a) Impurities in fiber material: from transition metal [iron,

,

1~10 ppb)

(b) OH ions with absorption peaks at wavelengths 1400

nm, 950 nm & 725nm (overtones of the fundamental

absorption peak of water around 2700nm).

Reduction of residual OH contents to around 1 ppb,

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0.3 dB/km in 1550 nm window.

An effectively complete elimination of water molecule

results in the Allwave fiber


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850 nm

window

1.3 m

window

1.55 m

window

absorption

Infrared

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Scattering

minimum

Absorption

Of silica

Absorption-II2- Intrinsic absorption (fundamental lower limit): electronic

absorption band (UV region) & atomic bond vibration

band (IR region) in basic SiO2.

3- Radiation defects

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2 Scattering losses

Linear scattering losses: transfer of opticalpower from one mode (proportionally to the

Rayleigh scattering

Mie scattering

Nonlinear scattering losses: Disproportionateattenuation, usually at high optical power levelsin long SM fibers

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Stimulated Brillouin scattering

Stimulated Raman scattering

Linear Scattering Rayleigh scattering: - Refractive index

variations oversmall distances compared with

wavelength due to:

Microscopic variations in the material density

Defects during fiber manufacture

Compositional fluctuations due to oxides

kmdBc /1

=

38

where cR is the Rayleigh scattering coefficient and

is the range from 0.8 to 1.0 (dB/km)(mm)4


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2

Mie scattering: ---- inhomogeneities comparable insize to the guided wavelength due to non perfect

cylindrical structure: e.g

Imperfections at core-cladding interface

Diameter fluctuations

Core-cladding refractive index differences along

the fiber length

Mie scatterin is t icall ver small in o tical

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fibers

Nonlinear ScatteringInsignificant unless the power is greater than 100 mW

Stimulated Brillouin scattering:modulation of light through thermal molecular

produces a phonon ofacoustic frequency aswell as a scattered photon

Stimulated Raman scattering: highfrequency optical phonon is generated

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Phonon is a quantum of elastic wave in lattice structure


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2

Absorption & Scattering Losses in

Fibers

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Radiative Losses (Bending Loss)Macrobending Loss:

Lightwave suffers sever lossdue to radiation of theevanescen e n ecladding region. As the radiusof the curvature decreases, theloss increases exponentiallyuntil it reaches at a certaincritical radius.

For any radius a bit smaller

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,suddenly becomes extremelylarge.

Higher order modes radiateaway faster than lower ordermodes.


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Microbending Loss:

Microscopic bends of thefiber axis that can arise

when the fibers are

incorporated into cables.

The power is dissipated

through the microbended

fiber, because of the

repetitive coupling of

energy between guided

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modes & the leaky or

radiation modes in the

fiber

Losses in standard SM fiber

.

850 nm 1.8 dB/km 2.72 dB/km

1300 nm 0.35 dB/km 0.52 dB/km

1380 nm 0.50 dB/km 0.92 dB/km

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nm . m . m

2b - signal degradation

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