fibre plus dispersion

8/13/2019 Fibre Plus Dispersion

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The Basics of Fiber

Optics

Fiber Optics



Introduction

An optical fiber is essentially a waveguide for lightIt consists of a core and cladding that surrounds core

The index of refraction of the cladding is less than that of the

core, causing rays of light leaving the core to be refracted back intothe core

A light-emitting diode (LED) or laser diode (LD) can be used forthe source

Advantages of optical fiber include:• Greater bandwidth than copper• Lower loss• Immunity to crosstalk • No electrical hazard



Advantages of Fiber

• Fiber has these advantages compared with metal wires

• Bandwidth – more data per second

• Longer distance

• Faster

• Special applications like medical imaging and quantum

key distribution are only possible with fiber because they

use light directly



Elements of a Fiber Data Link

• Transmitter emits light pulses (LED or Laser)

• Connectors and Cables passively carry the pulses

• Receiver detects the light pulses

Transmitter ReceiverCable



Repeaters

• For long links, repeaters are needed to compensate for signal loss

FiberRepeaterRepeater Repeater

Fiber FiberFiber

• Optical fiber transmits light pulses

• Can be used for analog or digital transmission

• Voice, computer data, video, etc.

• Copper wires (or other metals) can carry the same types of signals with

electrical pulses

Fiber v. Copper (comparison)



Optical Fiber

• Optical fiber is made from thin strands of either glass or

plastic• It has little mechanical strength, so it must be enclosed in a

protective jacket

• Often, two or more fibers are enclosed in the same cablefor increased bandwidth and redundancy in case one of thefibers breaks

• It is also easier to build a full-duplex system using twofibers, one for transmission in each direction



Optical Fiber

• Core• Glass or plastic with a higher index of

refraction than the cladding

• Carries the signal

• Cladding• Glass or plastic with a lower index of

refraction than the core

• Buffer

• Protects the fiber from damage andmoisture

• Jacket

• Holds one or more fibers in a cable



Optical Fiber & Communications System



Optical fibers work on the principle of total

internal reflection. With light, the refractiveindex is listed The angle of refraction at theinterface between two media is governed bySnell’s law: n1sin 1 n2 sin 2

Total Internal Reflection



Index of Refraction

• When light enters a dense medium like glass orwater, it slows down

• The index of refraction (n) is the ratio of the speed

of light in vacuum to the speed of light in themedium

• Water has n = 1.3• Light takes 30% longer to travel through it

• Fiber optic glass has n = 1.5• Light takes 50% longer to travel through it



The numerical aperture of the fiber is

closely related to the critical angle andis often used in the specification for

optical fiber and the components thatwork with it

The numerical aperture is given by theformula:

The angle of acceptance is twice thatgiven by the numerical aperture

2

2

2

1.. nn A N

Numerical Aperture



Numerical Aperture

• If the core and cladding have almost the same index

of refraction, the numerical aperture will be small

• This means that light must be shooting right down

the center of the fiber to stay in the core



Multimode Fiber

• Can transmit more than a single mode

• Relatively inexpensive

• Easy to couple with LEDs and detectors

• Large bandwidth

• NA ~ 0.20

Types of FiberSingle-Mode Fiber

– Allows only a single mode to propagate

– Difficult to handle and couple – More expensive

– Requires a laser source

– Large bandwidth

– High speed/large bandwidth systems – NA ~ 0.12



Types of Fiber



Types of FiberBoth types of fiber described earlier are known as step-index fibersbecause the index of refraction changes radically between the coreand the claddingGraded-index fiber is a compromise multimode fiber, but the index

of refraction gradually decreases away from the center of the coreGraded-index fiber has less dispersion than a multimode step-index

fiberGraded-Index Fiber

• Refractive index variesbetween the central core andthe cladding

• More expensive

• Dispersion and bandwidthimproved

• Works best for multimode

• Rays refract continuously

• Step-Index Fiber

– Most common

– Two distinct refractiveindices

– Core refractive indexconstant



Singlemode Fiber

• Singlemode fiber has a core diameter of 8 to 9 microns, which only

allows one light path or mode

• Images from arcelect.com

Index ofrefraction



Multimode Step-Index Fiber

• Multimode fiber has a core diameter of 50 or 62.5 microns (sometimes

even larger)

• Allows several light paths or modes

• This causes modal dispersion – some modes take longer to

pass through the fiber than others because they travel a

longer distance

• See animation at link Ch 2f

Index ofrefraction



Multimode Graded-Index Fiber

• The index of refraction gradually changes across the core

• Modes that travel further also move faster

• This reduces modal dispersion so the bandwidth is greatly

increased

Index ofrefraction



Step-index and Graded-index

• Step index multimode was developed first, but rare today because it

has a low bandwidth (50 MHz-km)

• It has been replaced by graded-index multimode with a bandwidth

up to 2 GHz-km.

• Large core (1 mm) step-index multimode fiber

• Easy to cut and work with, but high attenuation (1 dB /

meter) makes it useless for long distances

Plastic Optical Fiber



Special Fiber Types

Plastic Fiber• High attenuation

• Less expensive than glass• Easy to work with

• Step-index fibers

• Used in automobiles, consumer products, industrial control, andsmall LANs

Dispersion-Shifted Fiber

– Adjusts for pulse spreading caused by material and

waveguide dispersion

Photonic Crystal (Holey) Fibers

– Dispersion can be controlled – Nonlinear properties

– Single-mode

– Wide wavelength

– Cladding region consists of air holes

– Two categories: High-index and low-index guiding fibers



V-number in Optical Fiberswhere

• N is the number of modes• a is the radius of the fiber

• λ is the wavelength of light

For mode fiber if , V < 2.405

For mode fiber if, V > 2.405

2

2

2V N

aNAV

4

2V

N



Examples of Dispersion



INTRODUCTION

• Dispersion Fundamentals

• Light is spread out (in time) as it travels through the fiber

• Parameter: Pulse Widening (measured at 50%), Δt = TRX-TTX

• Dispersion causes INTERSYMBOL INTERFERENCE



Fiber Dispersion

Dispersion is the spreading of a light pulse as itpropagates down the fiber. Dispersion may beeither modal or chromatic.

Modal Dispersion• The temporal spreading of a pulse in an optical

waveguide caused by modal effects

• Intermodal, or modal, dispersion occurs only inmultimode fibers.

• Contributes to pulse broadening



Modes and Materials

• Since optical fiber is a waveguide, light can propagate in a number ofmodes

• If a fiber is of large diameter, light entering at different angles willexcite different modes while narrow fiber may only excite one mode

• Multimode propagation will cause dispersion, which results in thespreading of pulses and limits the usable bandwidth

• Single-mode fiber has much less dispersion but is more expensive toproduce. Its small size, together with the fact that its numericalaperture is smaller than that of multimode fiber, makes it moredifficult to couple to light sources



Dispersion

• Dispersion in fiber optics results from the fact that in multimodepropagation, the signal travels faster in some modes than it would inothers

• Single-mode fibers are relatively free from dispersion except forintramodal dispersion

• Graded-index fibers reduce dispersion by taking advantage of higher-order modes

• One form of intramodal dispersion is called material dispersion because it depends upon the material of the core

• Another form of dispersion is called waveguide dispersion

• Dispersion increases with the bandwidth of the light source



• Types of Dispersion

• Modal Dispersion, Δtmod

• Intermodal: between different modes

• Intramodal: inside each mode

• Chromatic Dispersion, Δtcro

• Mechanisms

• Material

• Waveguide

• PMD, Δtpmd

• Polarization Mode Dispersion

• Between orthogonal polarizations



Material Dispersion• Material dispersion occurs because the spreading of a light pulse is

dependent on the wavelengths' interaction with the refractive indexof the fiber core.

• Material dispersion is a function of the source spectral width, whichspecifies the range of wavelengths that can propagate in the fiber.

• Material dispersion is less at longer wavelengths.

Waveguide dispersion

– Waveguide dispersion occurs because the modepropagation constant is a function of the size of the fiber'score relative to the wavelength of operation.

– Waveguide dispersion also occurs because light propagatesdifferently in the core than in the cladding.

Polarization Mode Dispersion

– Polarization mode dispersion (PMD) occurs when different

planes of light inside a fiber travel at slightly different speeds,

making it impossible to transmit data reliably at high speeds.



CHROMATIC DISPERSION (I)

• Fundamentals (I)

• Colors arrive at different times

• Types

• Due to material, Δtmat

• Due to waveguide effect, Δtwg

• Total chromatic dispersion, Δtcro: both contributions are added

• Chrom. disp. increases linearly with fiber length

IN

t

OUT

t



-20

-10

0

10

20

Frequency (GHz)

0

0.2

0.4

0.6

0.8

Power

0

50

100

150

200

250

300

Time/Distance

Frequency components

of modulated signaltravel at different

velocities in fiber

Chromatic Dispersion



t

Spread, ²

t 0

Spectrum, ²

1 2 o

Intensity Intensity Intensity

Cladding

CoreEmitter

Very short

light pulse

v g (

2)

v g ( 1)

Input

Output

All excitation sources are inherently non-monochromatic and emit within aspectrum, ² , of wavelengths. Waves in the guide with different free spacewavelengths t ravel at different group velocities due to the wavelength dep endenceof n1. The waves arrive at the end of the fiber at different t imes and hence result in

a broadened output p ulse.

© 1999 S.O. Kasap,Optoelectronics (Prentice Hall)

Material Dispersion



Material Dispersion

• The refractive index of the material varies as a function of wavelength,

• Material-induced dispersion for a plane wave propagation in homogeneousmedium of refractive index n:

• The pulse spread due to material dispersion is therefore:

)( n

d

dnn

c

L

nd

d L

cd

d L

cd

d Lmat

)(2

22ω

22

[3-19]

)(2

2

mat

mat g D L

d nd

c L

d d [3-20]

)( mat D is material dispersion



Dispersion in multimode fibre

31

4

8cn

NA Lt GI

1

2

2cn

NA Lt SI



Fiber Dispersion

Total Dispersion

• Total dispersion is due to all types of dispersion

pol chromtot t t t t 22mod

2

Fiber Losses

– Absorption loss occurs at wavelengths greater than

1.55µm due to infrared vibration.

– Scattering can be significant at shorter wavelengths.

– Attenuation describes the total loss of a optical fiber

system

– Bending loss occurs when total internal reflection

deteriorates because of installation procedures.



Group Velocity

• Wave Velocities:

• 1- Plane wave velocity: For a plane wave propagating along z -axis in anunbounded homogeneous region of refractive index , which isrepresented by , the velocity of constant phase plane is:

• 2- Modal wave phase velocity: For a modal wave propagating along z -axisrepresented by , the velocity of constant phase plane is:

3- For transmission system operation the most important & useful type ofvelocity is the group velocity, . This is the actual velocity which thesignal information & energy is traveling down the fiber. It is always less thanthe speed of light in the medium. The observable delay experiences by theoptical signal waveform & energy, when traveling a length of l along thefiber is commonly referred to as group delay.

1n

)ωexp( 1 z jk t j

11 n

c

k v

)ωexp( z jt j

ω

pv

g V



Group Velocity & Group Delay• The group velocity is given by:

• The group delay is given by:

• It is important to note that all above quantities depend both on frequency &the propagation mode. In order to see the effect of these parameters ongroup velocity and delay, the following analysis would be helpful.

d

d V g

ω

[3-6]

ωd d l

V l

g

g [3-7]

Inp t/O tp t signals in Fiber Transmission



Input/Output signals in Fiber TransmissionSystem

• The optical signal (complex) waveform at the input of fiber of length l is f(t).

The propagation constant of a particular modal wave carrying the signal is. Let us find the output signal waveform g(t).

z-=0 Z=l

c

c

d e f t f t j

)(~

)( [3-8]

c

c

d e f t g l jt j )()(~

)( [3-9]

bandwidth.signalopticaltheis

If



...)(2

1)()()(

If

2

2

2

ccc

c

ccd

d

d

d

)()(

)(~

)(~

)(~

)(

)()(

)(2/

2/

)(

)]()([2/

2/

)(

2/

2/

g

l jl j

d

d l t j

l j

l d

d jt j

l jt j

t f ed

d

l t f e

d e f e

d e f d e f t g

c

c

c

c

c

c

c

c

c

cc

c

c

c

g

g V

l

d

d l

c

[3-10]

[3-11]

[3-14]



Intramodal Dispersion• As we have seen from Input/output signal relationship in optical fiber, the

output is proportional to the delayed version of the input signal, and the delay

is inversely proportional to the group velocity of the wave. Since the propagation constant, , is frequency dependent over band widthsitting at the center frequency , at each frequency, we have one

propagation constant resulting in a specific delay time. As the output signal iscollectively represented by group velocity & group delay this phenomenon iscalled intramodal dispersion or Group Velocity Dispersion (GVD). This

phenomenon arises due to a finite bandwidth of the optical source,dependency of refractive index on the wavelength and the modaldependency of the group velocity.

• In the case of optical pulse propagation down the fiber, GVD causes pulse

broadening, leading to Inter Symbol Interference (ISI).

)ω( ωcω



Dispersion & ISI

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

A measure of information

capacity of an optical fiber for

digital transmission is usually

specified by the bandwidthdistance product

in GHz.km.

For multi-mode step index fiber

this quantity is about 20

MHz.km, for graded index fiber

is about 2.5 GHz.km & for singlemode fibers are higher than 10

GHz.km.

L BW



How to characterize dispersion?

• Group delay per unit length can be defined as:

• If the spectral width of the optical source is not too wide, then the delaydifference per unit wavelength along the propagation path is approximately

For spectral components which are apart, symmetrical around centerwavelength, the total delay difference over a distance L is:

d

d

cdk

d

cd

d

L

g

2

1

ω

2

d

d g

2

2

2

22

22

d

d L

V

L

d

d

d

d

d

d

d

d

c

L

d

d

g

g

2

d



• is called GVD parameter, and shows how much a light pulse broadens as it travels along an optical fiber. The more common parameter iscalled Dispersion, and can be defined as the delay difference per unit length

per unit wavelength as follows:

• In the case of optical pulse, if the spectral width of the optical source ischaracterized by its rms value of the Gaussian pulse , the pulsespreading over the length of L, can be well approximated by:

• D has a typical unit of [ps/(nm.km)].

22

d

d

22

211

c

V d

d

d

d

L D

g

g

g

DLd

d g g

M i l Di i



t

Spread, ²

t 0

Spectrum, ²

1 2 o

Intensity Intensity Intensity

Cladding

CoreEmitter

Very short

light pulse

v g ( 2)

v g (

1)

Input

Output

All excitation sources are inherently non-monochromatic and emit within a

spectrum, ² , of wavelengths. Waves in the guide with different free spacewavelengths travel at different group velocities due to the wavelength dependenceof n1. The waves arrive at the end of the fiber at different times and hence result in

a broadened output pulse.

© 1999 S.O. Kasap,Optoelectronics (Prentice Hall)

Material Dispersion



Material Dispersion

• The refractive index of the material varies as a function of wavelength,

• Material-induced dispersion for a plane wave propagation in homogeneousmedium of refractive index n:

• The pulse spread due to material dispersion is therefore:

)( n

d

dnn

c

L

nd

d L

cd

d L

cd

d Lmat

)(2

22ω

22

)(2

2

mat

mat g D L

d nd

c L

d d

)( mat D is material dispersion



Material Dispersion Diagrams


W id Di i



Waveguide Dispersion

• Waveguide dispersion is due to the dependency of the group velocity of thefundamental mode as well as other modes on the V number, (see Fig 2-18 of

the textbook). In order to calculate waveguide dispersion, we consider that n is not dependent on wavelength. Defining the normalized propagationconstant b as:

• solving for propagation constant:

• Using V number:

21

22

2

2

1

2

2

22//

nn

nk

nn

nk b

[3-21]

)1(2 bk n [3-22]

2)( 2

2/12

2

2

1 kannnkaV [3-23]



Waveguide Dispersion

• Delay time due to waveguide dispersion can then be expressed as:

dV

Vbd nn

c

Lwg

)(22 [3-24]




Waveguide dispersion in single mode fibers

• For single mode fibers, waveguide dispersion is in the same order of material

dispersion. The pulse spread can be well approximated as:

2

2

2 )()(

dV

Vbd V

c

Ln D L

d

d wg

wg

wg

[3-25]


)( wg D



Losses

• Losses in optical fiber result from attenuation in the materialitself and from scattering, which causes some light to strike thecladding at less than the critical angle

• Bending the optical fiber too sharply can also cause losses bycausing some of the light to meet the cladding at less than thecritical angle

• Losses vary greatly depending upon the type of fiber

• Plastic fiber may have losses of several hundred dB per kilometer• Graded-index multimode glass fiber has a loss of about 2 –4 dBper kilometer

• Single-mode fiber has a loss of 0.4 dB/km or less



Fiber Loss

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

Distance (km)

F r a c t i o n o

f P o w e r

R e m a

i n i n g

80 km 100x loss

A i (fib l )



Attenuation (fiber loss)

• Power loss along a fiber:

• The parameter is called fiber attenuation coefficient in a units of for

example [1/km] or [nepers/km]. A more common unit is [dB/km] that isdefined by:

Z=0

P(0) mW

Z= ll pe P l P

)0()( mw

z pe P z P

)0()( [3-1]

p

]km/1[343.4)(

)0(log

10]dB/km[

pl P

P

l

[3-2]

T f L



Types of Losses

1. Macro bending

2 Mi b di L



2. Microbending Loss

• Microbending Loss:microscopic bends of the fiberaxis that can arise when thefibers are incorporated intocables. The power is dissipated

through the microbended fiber, because of the repetitivecoupling of energy betweenguided modes & the leaky orradiation modes in the fiber.


3 Ab i



3. Absorption

• Absorption is caused by three different mechanisms:1- Impurities in fiber material: from transition metal ions (must be in order of ppb) & particularly from OH ions with absorption peaks at wavelengths 2700 nm, 400 nm, 950 nm & 725nm.

2- Intrinsic absorption (fundamental lower limit): electronicabsorption band (UV region) & atomic bond vibration band (IRregion) in basic SiO2.

3- Radiation defects

4 S i L



4. Scattering Loss

• Small (compared to wavelength) variation in material density, chemicalcomposition, and structural inhomogeneity scatter light in other directionsand absorb energy from guided optical wave.

• The essential mechanism is the Rayleigh scattering. Since the black bodyradiation classically is proportional to (this is true for wavelengthtypically greater than 5 micrometer), the attenuation coefficient due toRayleigh scattering is approximately proportional to . This seems to menot precise, where the attenuation of fibers at 1.3 & 1.55 micrometer can beexactly predicted with Planck’s formula & can not be described withRayleigh-Jeans law. Therefore I believe that the more accurate formula for

scattering loss is

4

4

1

5)exp(

T k

hc

B

scat

eTemperatur : ,JK 103806.1 Js, 10626.6-12334 T k h B



Sources and Wavelengths

• Multimode fiber is used with

• LED sources at wavelengths of 850 and 1300 nm for slower local area

networks

• Lasers at 850 and 1310 nm for networks running at gigabits per second or

more

• Singlemode fiber is used with

• Laser sources at 1300 and 1550 nm

• Bandwidth is extremely high, around 100 THz-km



Fiber Optic Specifications

• Attenuation

• Loss of signal, measured in dB

• Dispersion

• Blurring of a signal, affects bandwidth

• Bandwidth• The number of bits per second that can be sent through a data link

• Numerical Aperture

• Measures the largest angle of light that can be accepted into the core



Attenuation and Dispersion

• See animation at link

Ch 2e



Measuring Bandwidth

•The bandwidth-distance product in units of

MHz × km shows how fast data can be sent

through a cable•A common multimode fiber with bandwidth-

distance product of 500 MHz × km could carry• A 500 MHz signal for 1 km, or

• A 1000 MHz signal for 0.5 km

• From Wikipedia



Fiber Types and Specifications

• From Lennie Lightwave (www.jimhayes.com/lennielw/fiber.html)

http://www.jimhayes.com/lennielw/fiber.html

http://www.jimhayes.com/lennielw/fiber.html



Popular Fiber Types

• At first there were only

two common types of fiber

• 62.5 micron multimode, intended for LEDs and 100 Mbps networks

• There is a large installed base of 62.5 micron fiber

• 8 micron single-mode for long distances or high bandwidths, requiring laser

sources

• Corning’s SMF-28 fiber is the largest base of installed fiber in the world (links Ch 2j, 2k)



Gigabit Ethernet

• 62.5 micron multimode fiber did not have enough bandwidth for

Gigabit Ethernet (1000 Mbps)

• LEDs cannot be used as sources for Gigabit Ethernet – they are too

slow

• So Gigabit Ethernet used a new, inexpensive source:

• Vertical Cavity Surface Emitting Laser (VCSEL)



Multimode Fiber Designed for VCSELs

• First came laser-rated 50 micron multimode• Bandwidth 500 MHz-km at 850 nm

• Then came laser-optimized 50 micron multimode

• Bandwidth 2000 MHz-km at 850 nm

• Distinctive aqua-colored jacket

• See links Ch 2g, 2h, 2i



Don’t Mix Fiber Types

• You can’t mix singlemode and multimode fiber – you lose 20 dB at

the junction (99% of the light!)

• Mixing 50 micron and 62.5 micron multimode is not as bad, but you

lose 3 dB (half the power) which is usually unacceptable



Drawing

• The fiber is drawn from the preform and thencoated with a protective coating



Fiber Applications



Step-index Multimode

• Large core size, so source power can be efficiently coupled to thefiber

• High attenuation (4-6 dB / km)

• Low bandwidth (50 MHz-km)

• Used in short, low-speed datalinks

• Also useful in high-radiation environments, because it can be madewith pure silica core



Graded-index Multimode

• Useful for “premises networks” like LANs, security systems, etc.

• 62.5/125 micron has been most widely used

• Works well with LEDs, but cannot be used for Gigabit Ethernet

• 50/125 micron fiber and VSELS are used for faster networks



Singlemode FIber

• Best for high speeds and long distances

• Used by telephone companies and CATV



Fiber Performance

Att ti



Attenuation

• Modern fiber material is very pure, but there is still someattenuation

• The wavelengths used are chosen to avoid absorption bands• 850 nm, 1300 nm, and 1550 nm

• Plastic fiber uses 660 nm LEDs

• Image from iec.org (Link Ch 2n)



Three Types of Dispersion

• Dispersion is the spreading out of a light pulse as it travels throughthe fiber

• Three types:

• Modal Dispersion

• Chromatic Dispersion• Polarization Mode Dispersion (PMD)



Modal Dispersion

• Modal Dispersion

• Spreading of a pulse because different modes (paths) through the fiber takedifferent times

• Only happens in multimode fiber

• Reduced, but not eliminated, with graded-index fiber



Chromatic Dispersion

• Different wavelengths travel at different speeds through the fiber

• This spreads a pulse in an effect named chromatic dispersion

• Chromatic dispersion occurs in both singlemode and multimode fiber

• Larger effect with LEDs than with lasers

• A far smaller effect than modal dispersion



Polarization Mode Dispersion

• Light with different polarization can travel at different speeds, if the fiberis not perfectly symmetric at the atomic level

• This could come from imperfect circular geometry or stress on the cable,and there is no easy way to correct it

• It can affect both singlemode and multimode fiber.



Modal Distribution

• In graded-index fiber, the off-axis modes go a longer distance thanthe axial mode, but they travel faster, compensating for dispersion

• But because the off-axis modes travel further, they suffer more attenuation



Equilibrium Modal Distribution

• A long fiber that has lost the high-order modes is said to have anequilibrium modal distribution

• For testing fibers, devices can be used to condition the modaldistribution so measurements will be accurate



Mode Stripper

• An index-matching substance is put on the outside of the fiber toremove light travelling through the cladding

• Figure from fiber-optics.info (Link Ch 2o)



Mode Scrambler

• Mode scramblers mix light to excite every possible mode oftransmission within the fiber

• Used for accurate measurements of attenuation

• Figure fromfiber-optics.info(Link Ch 2o)



Mode Filter

• Wrapping the fiber arounda 12.5 mm mandrel

• Exceeds the critical angle for total internal reflection for very oblique modes

• The high-order modes leak into the cladding and are lost

• That creates an equilibrium modal distribution

• Allows an accurate test with a short test cable• Figure from fiber-optics.info (Link Ch 2o)



Decibel Units



Optical Loss in dB (decibels)

• If the data link is perfect, and loses no power

• The loss is 0 dB

• If the data link loses 50% of the power• The loss is 3 dB, or a change of – 3 dB

• If the data link loses 90% of the power

• The loss is 10 dB, or a change of – 10 dB

• If the data link loses 99% of the power

• The loss is 20 dB, or a change of – 20 dB

dB = 10 log (Power Out / Power In)

Data LinkPower In Power Out



Absolute Power in dBm

• The power of a light is measured in milliwatts

• For convenience, we use the dBm units, where-20 dBm = 0.01 milliwatt

-10 dBm = 0.1 milliwatt0 dBm = 1 milliwatt

10 dBm = 10 milliwatts

20 dBm = 100 milliwatts

fibre plus dispersion

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