chapter 01 - introduction optical measurement

7/25/2019 Chapter 01 - Introduction optical measurement

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Chapter One Introduction

Contents

1. Measurement of Optical Fiber and Optical Components

2. Radiometry and Photometry


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Optical Measurements

Introduction

Early fiber optic systems need only modest test.Now the industryis eolin!" thus optical fibre systems and measurement technolo!y need

to be improed.

Narrow wavelength spacing#

$%M systems with 1&& '()

E.!. power" si!nal*to*noise ratio" waelen!th

(i!h data rates#

+ 1& 'b,s re-uires compatible components characteristic

E.!. spectrum width" dispersion" bandwidth response

Optical amplifier#

Enablin! $%M systems

E.!. !ain" noise fi!ure

uestion

$hy need accurate and reliable optical test / measurement techni-ues0


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Introduction

Expansionof optical communication systemsReplacin! copper cables eerywhere" towards access area

Complexfibre optic systems

ll optical networks3 passie and actie

Self-reviewof the basic features of a fiber*optic communication lin4 are necessary.

Fibre optic lin4 measurements determine if the system meets its end desi!n goals.

ll of the components contained within the lin4 must be characteried and specified to

!uarantee system performance.

uestion

$hat are the thin!s to 4now before proceedin! with fiber optic test / measurement0


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Introduction

Optical fibres#Singlemode fibres 3 6tandard fibre" %ispersion*shifted fibre" Non*)ero %ispersion*

shifted fibre" Polari)ation Maintainin! fibre" Erbium*doped fibre

Multimodefibres 3 6tep inde7" 'raded*8nde7

Optical components#

!wo-port optical components# hae optical input and optical output" E.!. $%Mcoupler" 9andpass filter" 8solator

Single-portcomponents. E.!. :ransmitter" Receier

:his chapter will briefly introducethe types of measurements that can be made to the fibre

optic and optical components.

:he detailsof each measurement will be discussed in the dedicated chapters.

uestion

$hat are the parameters to measure0


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Measurement of Optical #i$re and !wo-port Components

Insertion %oss

9oth a sourceand receiverare necessary6ource 3 a waelen!th tuna$le laseror a $road$and source

Receier 3 an optical power meter


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Insertion %oss

Optical power meterCali$ratedoptical to electrical conerter

&owaelen!th information

Optical spectrum analy)er

:unable $andpass filter? power meter

uestions

%oes an optical spectrum analy)er proide waelen!th information and why0

(ow to use an OPM but still !ettin! the waelen!th information0


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Insertion %oss

:A6 ? OPMAar!e measurement ran!e" but B 2&&nm

#inewaelen!th resolution

Maor limitation 3 broadband noisefrom :A6

uestions

$hat is the noise referrin! to0

(ow to improe the measurement usin! the :A60


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Insertion %oss

:A6 ? O6'ighestperformance solution

:A6 proides narrowspectral width

O6 proides additional filteringof the broadband noise emission

uestions$hat is the direct effect on the measured spectrum by usin! the aboe confi!uration0


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Insertion %oss

9roadband emission source ? O6(idewaelen!th ran!e coera!e

Moderatemeasurement ran!e

#astmeasurement speed

:un!sten lamp emitters 3 entirefibre*optic communication waelen!th ran!e

Optical amplifiers 3 narrower waelen!th ran!es" but with much higherpower

uestion

$hat is the disadanta!e of a tun!sten lamp source0


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)mplifier *ain and &oise #igure

'ain measurementsOften done in large signalconditions 3 !ain saturation

Re-uires a high-powere7citation source

Characteri)ation of noise

Optical domain3 measure the leel of 6E comin! from the amplifier

Electrical domain 3 use a photodetector and an electrical spectrum analyser tocharacteri)e the total amount of detected noise produced by the system

uestion

$hat is the potential error in the measurement of the amplifier noise0


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)mplifier *ain and &oise #igure

:he fi!ure below shows a test configuration used to measure !ain and noise fi!ure ofoptical amplifier

For $%M systems 3 characteri)ation needs the same si!nal*loadin! conditions as in the

actualapplication

uestion

$hy is there a difference in the optical amplifier characteri)ation between sin!le* and

multi*channel systems0


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Chromatic +ispersion

Measurement is accomplished by analy)in! the group delaythrou!h the fiber,componentsas function of wavelength

Procedure

waelen!th tunable optical source is intensity modulated

:he phase of the detected modulation si!nal is compared to that of the transmitted

modulation

:he waelen!th of the tunable source is then incrementedand the phase comparison

is made a!ain

:he phase delay is conerted into the !roup delay

uestion

$hat is the waeform shape of the modulation si!nal0


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:he fi!ure shows the resultfor the measurement of the !roup delay with waelen!th

uestion

(ow can the !roup delay be calculated from the phase delay0


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:he fi!ure shows the chromatic dispersion measurement set-upfor two*port optical deices

ccurate characteri)ation of the minimum fibre dispersion waelen!th is important in the

desi!n of high-speed:%M and $%M communication systems

%ispersion compensationcomponents also re-uire accurate measurement of dispersion

uestion

$hy is it important to characteri)e chromatic dispersion of fibre0


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,olariation

Polari)ation of the li!htwae si!nal refers to the orientation of the electric fieldin spaceE.!. insertion loss and !roup delay of a two*port optical component varyas a function of the

input polari)ation

Polari)ation transfer function characteri)ation

,olariation analy)er measures the polariation state

:he polari)ation state is represented by a ones polariation-state vector

ones state ector contains two comple7 numbers that -uantify the amplitude and

phaseof the ertical and hori)ontal components of the optical field

uestion

(ow does the polari)ation state of a linearly polari)ed li!ht eole in a fibre0


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,olariation

:he ones matri7 measurementpply threewell*4nown polari)ation states at the input

Characteri)e the resultin! output polari)ation state in the polariation analyer

:he ones matri7 of the polari)ation transfer function will predict the output polari)ation state

for anyinput polari)ation state

:he fi!ure below illustrates a measurement techni-ue to characteri)e the polari)ationtransfer function of optical fibre and components.


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.eflection

Optical time-domain reflectometry


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.eflection

:he fi!ure shows an e7ample O:%R display

:he locations and ma!nitudes of faults

%etermined by measurin! the arrival timeof the returnin! li!ht

Reduction in .aleigh scatteringand occurrence of #resnelreflection

uestion

(ow to determine the locations and ma!nitudes of faults0


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Measurement of !ransmitter and .eceiver

,ower

:he fi!ure illustrates a basic power*meter instrument dia!ram

Process

6ource 3 optical fibre 3 photodetector 3 electrical current

.esponsivity

:he conversion efficiencybetween the input power and the output current

Gnits of mps,$att

function of wavelengthfor all photodetectors

Must be cali$ratedin order to ma4e optical power measurements


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,ower

:hermal*detector headsMeasure the temperaturerise caused by optical si!nal absorption

Hery accurate and are wavelength-independent

6uffer from poor sensitivity

:hermal detectors are used to cali$ratephotodetectors

Gpper power limit

%etermined by saturationeffects

Responsiity decreasesbeyond this point

Aower power limit

Aimited by the averagingtime of the measurement and the dark current

%esi!n considerations

Power meters hae to be independent of the input polariation

:he reflectivityof the optical head has to be eliminated


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,olariation

Ai!ht sourcesAaser sources are predominantly linear polariedsources

AE%s hae no preferred direction of polari)ation and are predominantly unpolaried

Polari)ation effects

Polari)ation*dependent loss" !ain" or elocity

:hese are influenced by the am$ient conditions" e.!. stress" temperature

:hus" a polari)ed input will perform unpredicta$ly

Polari)ation measurement

:o determine the fraction of the total li!ht power that is polaried

:o determine the orientationof the polari)ed component

uestion

'ies the names for the polari)ation effects0


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,olariation

:he fi!ure illustrates a polariation analyerinstrument

Polari)ation analy)er

Four power meterswith polariation characteriingoptical components

8t measures the Stokes parameters# S&" S1" S2" S

S&3 total powerof the si!nal

S13 power difference between verticaland horiontalpolari)ation components

S23 power difference between /01and -01de!rees linear polari)ation

S3 power difference between right-handand left-handcircular polari)ation

S1and S2are measured with polariersin front of detectors

Sis measured with a waveplatein front of a detector


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,olariation

:he polari)ation state of a source is coneniently isuali)ed usin! a ,oincar2 spherePoincarI sphere

:he a7es are the 6to4es parameters normaliedto S&3 alues are between & and 1

Polari)ation state is represented by the three-dimensional coordinates


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,olariation

:he de!ree of polari)ation


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Optical Spectrum )nalysis

n optical spectrum analy)er


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O6Consists of a tuna$le $andpass filterand an optical power meter

:he li!ht from the input fibre is collimatedand applied to the diffraction !ratin!

:he diffraction !ratin! separates the input li!ht into different angles dependin! on

waelen!th

:he li!ht from the !ratin! is then focused onto an output slit:he !ratin! is rotatedto select the waelen!th that reaches the optical detector

uestion

$hat are the components in the O6 that constitute to the tunable bandpass filter0


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:he filter bandwidth is determined bythe diameterof the optical beam that is incident on the diffraction !ratin!

the aperturesi)e at the input and output of the optical system

Fabry*Perot


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:he fi!ure below shows a spectral plot for a +#3 laserthat is modulated with 2.; 'b,s di!italdata

ccurate spectral measurement

:he O6 must hae a ery narrowpassband and steeps4irts

filter stopband should be K 14 d3down to measure the smaller sidelobes.

O6s do not hae sufficient resolution to loo4 at the detailed structure of a laser

lon!itudinal mode

uestion

$hat determines the alue of the stopband0


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)ccurate (avelength Measurement

:he fi!ure below illustrates a method by which ery accuratewaelen!th measurementscan be made

Michelson interferometer confi!uration

:he li!ht from the un4nown source is splitinto two paths

9oth are then recom$inedat a photodetector

One of the path len!ths is varia$leand the other is fi7ed in len!th


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s the ariable arm is moed" the photodetector current aries

:o accurately measure the waelen!th of the un4nown si!nal" a reference laser with a

4nown waelen!th is introduced into the interferometer

uestion

$hy does the photodetector current ary0


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:he waelen!th meter comparesthe interference pattern from both lasers to determine thewaelen!th

:his procedure ma4es the measurement method less sensitiveto enironmental chan!es

Reference lasers

(elium*neon


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%inewidth and Chirp Measurement

'eterodyneand homodyneanalysis tools are used to e7amine the fine structure of optical

si!nals

:hese analysis methods allow the measurement of modulatedand unmodulatedspectral

shapes of the lon!itudinal modes in laser transmitter

'eterodyne

:he fi!ure illustrates a heterodyne measurement setup

:he un4nown si!nal is combined with a stable" narrow*linewidth local oscillator


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'eterodyne

:he AO must hae the same polariationfor best conersion efficiency

:he two si!nals mi7 in the photodetector to produce a difference fre-uency


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'omodyne

%imitedinformation on the optical spectrum

Much easierto perform

AO is a time-delayed ersion of itself


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:he fi!ure shows a homodyne measurement of an unmodulated %F9 laser

uestion

$hat is the measured linewidth of the %F9 laser0


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Modulation )nalysis: #re;uency +omain

:his characteri)ation methods display information as a function of the modulation

fre;uency

:he fi!ure shows a dia!ram of a lightwave signal analyer

8t consists of a photodetector followed by a preamplifier and an electrical spectrum

analyer

:he modulation fre-uency response of these components must be accurately cali$rated as

a unit

:his modulation domain si!nal analy)er measures the followin! modulationcharacteristics#

%epth of optical modulation

8ntensity noise

%istortion


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Modulation )nalysis: #re;uency +omain

:he fi!ure shows the power of the modulation si!nal as a function of the modulation

fre;uency3 a %F9 laser modulated at > '()

:he relative intensity noise


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Modulation )nalysis: Stimulus-.esponse Measurement

:he fi!ure shows the instrument for measurin! the modulation response of optical

receiers" transmitters and optical lin4s

Electrical ector analy)er

8ts electrical sourceis connected to the optical transmitter

n optical receier is connected to the input

Compares both the ma!nitude and phase of the electrical si!nals enteringand leavingthe analy)er


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Modulation )nalysis: Stimulus-.esponse Measurement

:he fi!ure shows measurements of a %F9 laser transmitter and an optical receier

Maor challen!es 3 cali$rationof the O,E and E,O conerters in both ma!nitude and phase

response


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Modulation )nalysis: !ime +omain

:he shape of the modulation waveformas it pro!ress throu!h a lin4 is of !reat interest

n oscilloscopedisplays the optical power ersus time" as shown in the fi!ure below

'igh speed sampling oscilloscope

Often used in both telecommunication and data communication systems

%ue to the giga$itper second data rates inoled


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:he fi!ures below illustrate eye diagrammeasurement

Eye dia!ram

:he cloc4 waeform is applied to the triggerof the oscilloscope

:he laser output is applied to the inputof the oscilloscope throu!h a calibrated optical

receier

:he display shows all of the di!ital transitions overlaidin time

8t can be used to troubleshoot lin4s that hae poor bit*error ratio performance


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8nternational standards such as 6ONE:


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Optical .eflection Measurements

:he fi!ure shows the apparatusto measure the total optical return*loss

Optical return*loss measurement

n optical source is applied to a deice under test throu!h a directional coupler

:he reflected si!nal is separatedfrom the incident si!nal in the directional coupler

9y comparing the forward and reerse si!nal leels" the total optical return*loss is

measured

uestion

$here are the possible reflections0


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:he fi!ure shows the return*loss ersus waelen!th for a pac4a!ed laser usin! a tunable

laser source for e7citation

Aar!e total return*loss

:he locationsof the reflectin! surfaces become important

Re-uires optical time*domain reflectometry


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Optical component characteri)ation re-uires very finedistance resolution in the milimeter to

micron ran!e

:he fi!ure illustrates a hi!h resolution O!+. measurement based on broadband source

interferometry


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(i!h resolution O:%R

Gses a Michelson interferometerand a $road$and light sourceto locate reflections

with 2&Lm accuracy

Constructie interference occurs only when the moable mirror to the directional

coupler distance e;uals the distance from the deice under test reflection to the

directional coupler

:he resolution of the measurement is determined by the spectral width of thebroadband li!ht source


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.adiometry and ,hotometry

Radiometry

:he science of measuringli!ht in any portion of the electroma!netic spectrum" in terms

of a$solutepower

8n practice" the term is usually limited to the measurement of infrared" isible" andultraiolet li!ht usin! optical instruments


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Photometry

:he science of measurin! isible li!ht in units that are wei!hted accordin! to the

sensitiity of the human eye

8t is a -uantitatie science based on a statistical model of the human visual responseto li!ht * that is" our perception of li!ht * under carefully controlled conditions.

:he standardi)ed model of the eyes response to li!ht as a function of wavelength is!ien by the luminosityfunction.

:he eye has different responses as a function of waelen!th when it is adapted to li!htconditions


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%ifference

Radiometry includes the entireoptical radiation spectrum" while photometry is limited to

the visi$lespectrum as defined by the response of the eye.

uantities

:here are two parallel systems of -uantities 4nown as photometric and radiometric-uantities.

Eery -uantity in one system has an analogous-uantity in the other system.

:his table !ies the radiometric and photometric -uantities" their usual sym$olsand theirmetric unitdefinitions.

oule" $ watt" lm lumen" m meter" s second" sr steradian


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Proected area is defined as the rectilinear proection of a surface of any shape onto a planenormalto the unit ector

where is the an!le between the local surface normaland the line of sight

:he radian is the plane anglebetween two radii of a circle that cuts off on the circumferencean arc e-ual in len!th to the radius

uestion

%erie the proected area for the shapes of flat rectan!ular" circular disc and sphere0

. di t d ,h t t


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One steradian


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>uantities and ?nits ?sed in .adiometry

Radiometric units can be diided into twoconceptual areas#

:hose hain! to do with poweror energy" and

:hose that are geometricin nature.

Ener!y

8t is an 8nternational 6ystem of Gnits


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Now" incorporatin! power with the !eometric -uantities areaand solid angle.

8rradiance


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Radiant intensity

8t is another 68 deried unit and is measured in (@sr.

8ntensity is power per unit solid an!le" d/d. :he symbol is I.

Radiance

8t is the last 68 deried unit we need and is measured in (@m7sr.

8t is power per unit proected area per unit solid an!le" d/ddAcos


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>uantities and ?nits ?sed in ,hotometry

:hey are basically the same as the radiometric units e7cept that they are wei!hted for the

spectral responseof the human eye

:he symbols used are identical to those radiometric units" e7cept that a subscript isadded to denote visual.

Candela

8t is the luminous intensity" in a !ien direction" of a source that emits monochromaticradiation of fre-uency 104AB4B7hertand that has a radiant intensity in that direction ofB@56 watt per steradian.

:he candela is abbreiated as cd and its symbol is Iv.

. di t d ,h t t


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Aumen

:he lumen is an 68 deried unit for luminous flux. :he abbreiation is lm and the

symbol is v.

:he lumen is deried from the candela and is the luminous flu7 emitted into unit solidan!le


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8lluminance

8t is another 68 deried unit which denotes luminous flux density.

:he unit has a special name" the lux" which is lumens per s-uare metre" or lm,m2.

:he symbol is Ev

Auminance

8t is notincluded on the official list of deried 68 units.

8t is analo!ous to radiance" differentiatin! the lumen with respect to both area and

direction.

:his unit also has a special name" the nit" which is cd,m2or lm,m2sr if you prefer.

:he symbol is Lv.

8t is most often used to characteri)e the $rightness of flat emittin! or reflectin!surfaces.



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Properties Of :he Eye

:he eye has two !eneral classes of photosensors" conesand rods.

Cones

:he cones are responsible for light-adapted vision they respond to colorand haehigh resolutionin the central fovealre!ion

:he li!ht*adapted relatie spectral response of the eye is called the spectral luminousefficiency functionfor photopic ision" V


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Rods

:he rods are responsible for dark-adapted vision" with no color information and

poor resolutionwhen compared to the foeal cones.:he dar4*adapted relatie spectral response of the eye is called the spectral luminousefficiency functionfor scotopic ision" V


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Photopic


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Conersion 9etween Radiometric and Photometric Gnits

$e 4now from the definition of the candela that there are >D lumens per watt at a fre-uency

of ;5&:()" which is ;;; nm


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8n order to conert a source with non-monochromatic spectral distribution to a luminous-uantity" the spectral natureof the source is re-uired.

:he e-uation used is in a form of#

where Xv is a luminous term" Xis the correspondin! spectral radiant term" and V

chapter 01 - introduction optical measurement

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