HP AN1550 6_High Speed Lightwave Component Analysis

8/14/2019 HP AN1550 6_High Speed Lightwave Component Analysis

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High-spe ed lightwavecomponent analysis

Application Note 1550-6

Characterizing

sys tem components

Laser and LED transmit ters

Ph otodiode receivers

External modulators

Optical components


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As lightwa ve tra nsm ission systems become m ore advan ced, component designers an d

ma nu factu rers m ust ma ximize the per form an ce of th eir devices. For example, one

par am eter often used t o specify digita l system performan ce is bit err or rat e. However,

it is difficult to specify individual components in such terms. Rather, fundamentalmeasu rement s such as gain, bandwidth, frequency response an d retu rn loss can be

appr opriat e. The Lightwave Component Analyzer (LCA) is used t o measur e th e linear

tr an smission an d r eflection char acteristics of a component as a function of modulation

frequency. Measuremen ts ar e calibrated a nd can be performed a t modulat ion ra tes up

to 20 GHz.

Table of Conte nts

Introduct ion

Genera l measurement t echniques and considera t ions 3The Lightwave Component Analyzer (LCA) family 4

Lightwave transmitter measu remen ts (E/O)

Modu la tion ba ndwidth , frequ en cy r espon se, a nd con version efficien cy 5

The effect s of bias on laser per formance 7

Laser pulse measurements 7

Laser reflect ion sensit ivity 8

Modula t ion phase response 8

Laser input impedance 9

Electro-opt ic e xternal m odulator me asurem en ts (E/O)

Modula tor bandwidth and responsivity 11

Lighwave receive r measurem ents (O/E)

Modu la tion ba ndwidt h, fr equ en cy r espon se, a nd con ver sion efficien cy 12

Photodiode pulse measurements 13

Photodiode modula t ion phase measurements 15

Photodiode output impedance 15

Optical compo nen t meas ureme nts (O/O)

Transmiss ion measurements 17

Fiber length and propagat ion delay 17

Fiber modulation pha se sta bility 18

Reflect ion me asurements 18

Methods for measur ing lightwave reflect ions vs. distance 18

Achieving both high resolut ion an d long ra nge 20

Electrical compo ne nt measu remen ts (E/E) 21

Appen dix 1: Signal relat ionships in op to-electric de vices 21

Appen dix 2: Operat ion in the t ime dom ain

Basic considera t ions 22

Range and resolu t ion 23

Improving measurement accu racy through ga t ing 23


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Introduct ion

General Measurem ent

Techn i ques a ndConsiderations

The concept of lightwa ve

component a nalysis is str aight-

forward. Measurements ar e made

of th e small-signal linear tra ns-

mission and reflection character-

istics of a va riety of lightwa ve

componen ts. A pr ecise electrical

(signal genera tor) or optical (laser)

source is used to stimulate th e

component under test an d a very

accurate optical or electrical

receiver measures t he t ransmit-

ted (or r eflected) signal. Sin ce

characterization over a range

of modulation frequencies is

required, the frequency of mod-

ulat ion is norma lly swept over

the bandwidth of interest.

Measurem ents a re typically

comprised of the appr opriate ra tio

of microwave modulat ion cur ren t

(or power) and lightwave modu-

lation power (see Figure 2).

While Figure 1 demonstra tes

th e basic concepts of lightwa ve

component analysis, the specific

measur ement processes are illus-

tr ated later. An a na lysis of how

various signa ls are used in the

measu remen t pr ocess is found

in Append ix 1, "Signa l Relation-

ships in O pto-electric Devices."

LWSource

LWReceiver

Display

Amp

O/O

E/E

RF Source

E/O

O/E

O/O, E/O, O/E, or E/E

Deviceunder test

Modulated Lightwave

modOpticalModulationPower

modElectricalModulationCurrent

InputDevice

Under Test Output

P : I :

E/O measurement =

O/O measurement =

P Out

I In

P Out

P In

mod

mod

O/Emeasurement =

I Out

P In

mod

mod

mod

mod

Laser

Photodiode

Fiber

Figu re 1. LCABlock diagram

Figure 2.Measurementsignals


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O/ O Measurem ents

Characteristics of purely optical

devices can a lso be measur ed. Inthis case, both the stimulus and

response are m odulated light. The

rat io measur ement is simply one

of gain or loss versu s modula -

tion frequen cy.

Measurement Process

To simplify th e pr ocess of mak ing

measur ements , LCAs ha ve a

built in Guided setu p featu re.

This will lead th e user th rough

the basic measurement setup

an d calibration featu res.

Measurement Calibration

The key to making accurate E/O,

O/O, or O/E m easur ements is

calibrated instrument ation. Th e

instru ment lightwave source and

receiver a re individua lly cha ra c-

terized. The systematic responses

of the components ma king up t heLCA can th en be rem oved, yield-

ing the response of the device

under test (DUT). (See Appendix

1, Signal relationships in opto-

electric devices for m ore det ail.)

The LCA Family

There are several instrum ents

in the LCA family. Their charac-

teristics are sum mar ized below:

Please refer to the Hewlett-Packard

Lightwave Test an d Measur e-

ment Catalog for a complete

listing of Lightwave Component

Analyzers a s well as other light-wave test equipment.

An LCA measu res input m odulat-

ing curr ent and output modulation

power a nd displays the rat io of the

two in Wat ts/Amp, either linearly

or in decibels.

O/ E Measurements (Photodiodes)

The m easur ement process for O/E

devices is s imilar to E /O devices.

The measu rement consists of th e

ratio of output electrical modu-

lation cur rent to inpu t optical

modula tion power. Slope respon-

sivity for O/E devices describes

how a change in optical power

produces a cha nge in electrical

current . Graph ically this is

shown in Figure 4.

The LCA measures th e input

optical m odulation power a nd

output modulation current and

displays the rat io of th e two in

Amps/Watt.

E/ O Measurem ents

(Lasers, LED's)

The measurement of an E/O

tr an sducer is a combinat ion ofinput modulating current and

outpu t optical modulat ion power.

Slope r esponsivity is used to

describe how a chan ge in input

current produces a cha nge in

optical power. Graph ically th is

is shown in Figure 3.

Responsivity Rs (W/A)= Pout/ IinRs (dB)= 20 log10 (Rs(W/A))/(1(W/A))

PoutmW

Iin mA

Figure 3. E/O sloperesponsivi ty

Responsivity Rr (A/W)= Iout/ PinRr (dB)= 20 log10 (Rr(A/W))/(1(A/W))

IoutmA

Pin mW

Figu re 4. O/Eslope responsivi ty

Modulation

LCA (nm) Frequency Range

HP 8702 850, 1300 or 300 KHz3/6 GHz1550

HP 8703 1300 or 1550, 130 MHz20 GHzFP or DFB

HP 8510/834201 1300 or 1550, 45 MHz20 GHzor FP or DFB

HP 8720/83420

1 See HP Product Note 8510-15.


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Figu re 5. E/Omodulation band-width measurement

Lightwave TransmitterMeasure me nts (E/O)

The LCA is used t o cha ra cterizeth e tra nsmission and r eflection

para meters of laser and LED

sources with respect to modula-

tion frequency. The transmission

measu remen ts to be discussed

include:

modulation bandwidth and

frequency response

conver sion efficiency

th e effects of bias

pulse measurements

reflection sensitivity modulation ph ase response

laser input impedance

Other laser m easurements includ-

ing linewidth, chirp, and RIN

ar e discussed in H P Applicat ion

Note 1550-5 (or 371).

Modulat ion Bandw idth,Frequency Response , andConversion Efficiency

Modulat ion ban dwidth r efers

to how fast a laser can be inten -

sity modu lated, wh ile conversionefficiency (responsivity) refers

to how efficient ly an electr ical

signal driving a laser is converted

to modulated light . Alth ough

responsivity is often used to

describe a sta tic or DC par am e-

ter, the conversion efficiency of

a device for modulation signals

is a dynam ic chara cteristic and

can be r eferred t o as slope

responsivity.

It is not un usu al for slope

responsivity to vary according

to how fast the electr ical signal

is varied. As th e frequency of

modulation increases, eventua lly

th e conversion efficiency will

degrade or roll off. The fre-

quency wher e th e conversion

efficiency drops to one-half of

th e maximum is the 3 dB

point (when dat a is displayed

logarith mically) an d det ermines

a laser s modulat ion ba ndwidth .

Distortion of modulation signalswill occur if the frequency response

is not flat an d th ere a re fre-

quency componen ts wh ich exceed

a laser s ban dwidth.

The measu remen t of modulation

bandwidth consists of stimulat-

ing a laser with a n electr ical

(microwave or RF) signal a nd

measu ring its response (modu-

lated light) with a l ightwave

receiver. Normally th e frequency

of an electrical signal into alaser is swept t o allow cha racter-

ization of the laser over a wide

range of modulation frequencies.

Measurement Results

and Interpretation

Figure 5 shows the m easure-

men t of the conver sion efficiency

of th e laser as a function of mod-

ulation frequency. The display

uni ts a re Watt s per Amp (the

vertical axis). In t his case, the

display is in a logarithmic format

where 0 dB represents 1 wat t

per a mp. The h orizontal axis is

modulation frequency, indicating

that the measurement is being

made over a wide range of fre-

quencies, in th is case from

300 kHz to 3 GHz.

As stated, this measurement

indicates h ow fast th e laser can

be modulated. This part icular

laser has a modulation ban dwidthof about 1.5 GHz. Beyond th is

frequen cy, t he conver sion effi-

ciency is gradually degraded.

There are two significant compo-

nents tha t limit t he modulation

bandwidth. One is the actual

const ruction of a laser including

the physical dimensions and

fabrication process. The other is

how efficient ly an electr ical signa l

is delivered to th e laser. (See

Laser input impedance.)Measurement Procedu re

An accura te measur ement

requires a user calibration. A

user calibration will allow the

LCA to remove the response of

the t est system, including the

electrical cables, optical fiber,

and the instr ument i tself. Prior

to the a ctu al calibrat ion st ep,

th e LCA needs to be configured.

This in cludes:

start and st op frequencies sweep type (linear or logarithmic)

number of measurement points

measurement sweep t ime

sour ce power level

Note: LCAs have a Guided

Setup feature t hat leads th e user

thr ough al l the s teps that a re

described here. Guided setu p is

accessed by press ing SYSTEM

key an d th e [Guided setup] soft-

key. The following t ext d iscusses

th e processes that th e guidedsetup executes.


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Figu re 7. E/Omeasurement

To perform a simple frequency

response calibration, th e con-

nections sh own in F igur e 6 must

be made. The analyzer mea suresthe a ppropriate path s so the fre-

quency and phase response of the

un known path (s) is/are then

char acterized. The a nalyzer then

uses t his inform at ion in conjunc-

tion with t he int ernal calibrat ion

data to generate an error matrix.

(The lightwave source and receiver

characteristics a re pre-determined

dur ing a factory calibration an d

stored in m emory. The st orage

meth od depends on th e type of

LCA used). The end r esult is th edisplayed response of the laser

under test alone.

After the calibration is complete,

one might expect t o see a flat

response at 0 dB indicating th e

test system r esponse ha s been

removed. When using an HP 8702,

th e display seen u pon completion

of th e response calibra tion pr o-

cess will not n ecessarily be a flat

line. The laser used in th e cali-

bra tion is sti l l conn ected a ndha s become the DUT. Thus, its

response is displayed unt il it is

replaced with th e actual tes t

device. When the HP 8703 cali-

bration is completed, no response

(other th an noise) is displayed

until an E/O test device is con-

nected between the electrical and

optical measurement planes.

In a ddition to the simple response

calibration, there are also the

resp onse plus isolation andthe response plus match cali-

brat ions. Th e isolation calibra-

tion is used for high insert ion

loss (low conver sion efficiency)

devices wher e an y signal leak-

age within the instrument m ay

be significan t r elative to the

actual signals measured. The

ma tch calibrat ion is used to

remove the effects of reflections

between the instrument electri-

cal test port a nd th e laser undertest. If the laser being tested ha s

a poor electr ical input ma tch, the

response and m at ch calibration

can pr ovide a significan t improve-

ment in measurement accuracy.

(The response an d ma tch cali-

bra tion is only available with the

HP 8703 LCA.) An exam ple of

the r esponse plus ma tch calibra-

tion is found in t he section on O/E

receiver measurements.

Once the setup a nd calibrat ionha ve been completed, th e laser

under test is connected an d accur-

ate measur ements can be made.

Accuracy Considerations

There a re several items to con-

sider with respect to measur e-

ment accuracy. These include:

Keeping all electrical an doptical connectors clean and in

good cond ition

Operating the test device in

l inear regions (unsat ura ted

conditions)

Avoid overdriving th e instr u-

ment receiver

Minimizing cable movement

Allowing the instru ment t o

warm-up

Keeping both optical an d elec-

tr ical reflections at a m inimum

(for tra nsmission measur ements)

Figu re 6. E/Ocalibrationconfiguration

HP 8702 HP 8703

HP 8340XSource

HP 8341XReceiver

HP 8702 HP 8703

E/O DUT HP 8341XReceiverE/O DUT


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The Effects of Biason Laser Performance

The frequency response of

a laser is also dependent onbiasing conditions. As th e DC

bias of th e laser is increased,

the bandwidth will generally

increase. This is typically due

to th e rela xat ion oscillation

characteristics that vary with

bias. The relaxation oscillation

phenomenon creates a r esonan ce

in th e frequency response, noise,

an d distortion of th e laser.

Figur e 8 is a composite of a

bandwidth measurement made

at th ree different bias levels.

(The horizont al a xis is log fre-

quen cy.) As bias is increa sed,

both responsivity and band-

width increase. For th is laser,

as bias rea ches a certa in point,

the high-end r esponse begins to

degrade.

Note in th e two lower tr aces

that the response tends to peakbefore rolling off. This is t he

region of relaxat ion oscillation.

Care must be tak en when mod-

ulat ing a laser in th is region,

becau se this is where n oise and

distortion properties ar e often

at th eir worst. (See HP Appli-

cation N ote 1550-5 (or 371),

Measur ing Modulated Light.)

Laser Pulse Measurements

Fr equency doma in informa tion

(modulation bandwidth) is related

to time domain perform an ceusing th e an alyzer s time domain

featu re. An LCA uses the mea-

sur ed frequency domain (band-

width) data and math ematically

man ipulates it thr ough a form of

an inverse Fourier tra nsform to

pred ict t he effective step an d/or

impulse r esponse of a laser. (See

Appendix 2, "Operat ion in t he

time domain;" Basic considera-

tions.)

Measurement Resultsand Interpretation

Figure 9 shows th e predicted

impulse response of a high-speed

laser. The da ta is displayed in

a linear m agnitude format (as

opposed t o logar ithmically in dB).

Several items of informa tion ar e

available from this measurement.

One is basic impulse width, which

is a measure of device speed. Two

time values a re shown. The PW

value is the time between mark-

ers at the half-maximum points.However, part of the response is

due to th e finite bandwidth of

the inst ru ment itself. The Net

PW value is impu lse response

with th e instru ment s response

removed.

Figure 8. Compositeplot of bandwidth at3 bias levels

Mark er 1, at 10.337 ns, is the

effective delay or propa gat ion

time through the laser device

from t he electr ical inpu t t o theoptical outpu t. The device ha s a

long len gth of fiber pigtail wh ich

is the main cont ributor to the

tota l delay.

Note also th at t here is a

secondary impu lse. This typi-

cally indicates the presence of a

reflection and re-reflection.

Figure 10 shows the pr edicted

step r esponse of th e sam e laser.

From this measur ement we can

determine r isetime, ringing, andovershoot performance. In gen-

eral, these parameters a re directly

related to the frequency response

of th e device. (For a compa rison

of time-doma in measu remen ts

generat ed by an LCA versus a n

oscilloscope, see Figur e 24, page

14 under Ph otodiode Pu lse

Measurements).

Figu re 9. E/Oimpulse response

Figu re 10. E/Ostep response


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Figure 11.Reflectionsensi t ivi tyse tup

Measurement Procedu re

Pulse measurements ar e gener-

ated by manipulating measured

frequency response da ta. Conse-quently, the measurement pro-

cedure is almost identical to that

used for th e modulation ban d-

width. (Potent ial differences

exist du e to requirements of th e

math ematical t ransform. See

Appendix 2, "Operat ion in th e

time domain.")

Laser Reflection Se nsit ivity

The frequ ency response of a

laser ma y be modified if light is

reflected back int o th e laser scavity. The r eflection sen sitivity

of a laser can be mea sur ed as

shown in F igure 11.


The measurement setup is simi-

lar to the measu remen t of mod-

ulation bandwidth. In addition,

a directiona l coupler is inserted

in the optical path (prior to cali-

brat ion) in order t o monitor th e

tra nsmitted light and minimize

th e instru ment s response to the

reflected light. The controlled

reflection is conn ected to the other

arm of the coupler. For an accu-

rat e measurement , it is essential

th at all optical reflections, except-

ing th e cont rolled reflection, be

kept at a minimum.

Typically, a las er s frequ ency

response with back-reflected

light is compared t o the response

when n o reflections are pr esent.

The r esponse calibration for the

reflection sen sitivity measu re-ment (un der th e Guided setup

menu ) norma lizes the frequency

response to a flat line when no

reflections a re pr esent. As the

back-reflection is increased, and

the polarization of the reflectedlight is adjusted for worst case

results, th e modulation response

will deviate from th is norma lized

tra ce and sh ow the r eflection

sensitivity.

Measurement Interpretation

In t his case, the r esponses for

severa l levels of reflections ar e

shown in F igure 12, a composite

diagram (th rough offsett ing

subsequent measu rements by

changing th e display reference

level). The magnitude and polar-

ization of the reflected light are

adjusted wh ile th e laser s outpu t

is monitored by the LCA. Depend-

ing on how well the las er is iso-

lated, and its inherent sensitivity,

the frequen cy response of the

laser can be significan tly impacted

by reflected light. In the worst

case, (a reflection of approximately

4 dB retu rn loss) the modulation

response shows a 3 dB pea k-to-

peak variation.

When a laser is used in an actualsystem, th e amoun t of back-

reflected light m ay be u nkn own.

Thus, it is desirable to develop

a r obust laser wh ose chara cteris-

tics will be consist ent over a diver-

sity of opera ting en vironm ents.

Modulat ion Phase Response

Idea lly, a laser s m odulat ion

envelope will exhibit a linea r

phase response versus modula-

tion frequen cy. If th e relat ive

pha se relationships of the mod-ulation frequencies do not remain

const an t, a form of distortion will

occur. The pha se response of the

laser can be displayed in two

ways. One way is to display the

pha se r esponse directly. The

second is to display th e pha se

resp onse in a delay form at .

HP 8702 (or 8703)

E/O DUT HP 8157/8Attenuator

v

HP 81000BRReflector

HP 11894Polarization

Adjuster

HP 11890/1Directional

Coupler

HP 8341XReceiver

Figure 12. Reflectionsensi t ivi ty for severallevels of reflection


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and Interpretation

Ph ase dat a is displayed by

simply choosing the dat a formatto be ph ase as opposed to th e

defau lt log mag. If th e DUT

ha s any significan t length in

either th e optical or electrical

pat h, some compensat ion in

length (thr ough the electr ical

delay function un der the Scale

Ref key) will be r equir ed for

viewing the ph ase r esponse of

the laser. In th is measurement,

10.315 ns of electr ical dela y is

added, because t he fiber pigtail

is about 2 m long.

The p ha se res ponse often fol-

lows th e frequ ency response.

The frequency response of this

laser rolls off at the same fre-

quency range where the phase

begins to deviate from a linear

response.

Figu re 14. E/Odelay measurement

Figu re 13. E/Ophase response

Sometimes the phase response

is easier to interpret an d use

when viewed in the delay dataform at . The plot of delay is us ed

to indicate t he effective time it

ta kes for a m odulat ing signal at

the input of the E /O DUT t o exit

the device as m odulated light.

Ideally, th is tr an sition time will

be th e sam e for all modulationfrequencies of interest.

Figure 14 sh ows the delay for a

3 GHz laser. The average pr opa-

gation t ime over the 3 GH z band-

width is near 6.3 ns.

Laser Inpu t Imped ance

The convers ion efficiency of a

laser is dependen t n ot only on

the inh erent pr opert ies of the

laser, bu t a lso on h ow efficientlythe electr ical m odulation signa l

is delivered to th e laser. High-

speed modulation signals are

general ly t ran smit ted to the

laser over tr an smission lines

with a 50 or 75 ohm char acteris-

tic impedance. Maximum power

tra nsfer will occur if th e input

impedan ce of th e laser is th e

same as the t ran smission line.

Unfortun at ely, the input imped-

an ce of an active laser is much

lower tha n th e tran smission sys-

tem used to drive it. Two problemsoccur when such an impedan ce

misma tch exists. First, a signifi-

cant amoun t of energy will be

reflected at the transmission

line/laser inter face. This r eflected

energy may eventu ally be re-

reflected a nd distort the desired

data signal. The second problem

is tha t t he reflected ener gy is

wast ed since it is never effec-

tively used t o modulat e the las er.

Thu s, th e overall conversion

efficiency of the laser is degra ded.


Figure 15 shows th e retur n loss

of a laser with a simple resistive

matching circuit as measured

on the component an alyzer. The

measurement is made by send-

ing a swept RF signa l to the laser

under test and measuring the

energy tha t r eflects back. The

setup a nd calibrat ion pr ocedur e

will depend on the model of LCA

used. In all cases, a calibrat ionkit containing k nown electr ical

reflection stan dar ds is required

to improve the accuracy of the

reflection measur ement s.

Figure 15. E/O returnloss measurement


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Measurement In terpretation

The retu rn loss over a 6 GHz

ra nge varies from a best case of

near ly 34 dB to a worst case of17 dB. It is not unu sua l for th e

reflection level to get worse as

th e modulation frequency is

increased.

Retur n loss is th e rat io of

reflected to incident energy

(10 Log (P refl/ P inc)). The larger

the retu rn loss magnitude, the

smaller th e reflected signal an d

the better t he impedance match.

Figure 16 uses the same mea-

sured dat a as th e return loss plot,except in th is case th e data is

displayed in a Sm ith Cha rt for-

ma t. A Smith Ch art is a form of

an impeda nce map. The display

shows the laser input impedan ce

as a fun ction of frequen cy. For

th is laser, the impedan ce is close

to 50 ohms over th e 6 GHz ran ge,

as t he response does not deviate

much from t he center of the char t.

The Smith Char t data presenta-

tion is selected under the Format key menu.

Figure 16. Retu rnloss in Smith chartformat

The impedance data from the

Smith Cha rt can be used to model

the inpu t str uctur e of th e laser.

The laser s effective inpu t imped-

an ce can be improved with a

mat ching network. Simple meth -

ods ar e usua lly resistive, while

more efficient bu t complex meth-

ods use reactive elements.

Im plications of

Impedance Mism atch

on Measurement Accuracy

When th e inpu t impedan ce ofth e E/O device under t est is far

from 50 ohms, a significant por-

tion of the electrical energy sent

to th e device will be reflected.

This r eflected energy can degrade

measu remen t a ccur acy. This is

typically seen a s r ipple in fre-

quency r esponse measurements.

Two techn iques ar e available to

overcome t his pr oblem including

th e response/match calibra tion

(discussed in O/E measurements)

an d gating (discussed in Appen-dix 2, "Operat ion in th e time

domain").


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Electro-opticExternal ModulatorMeasurements

External intensity modulators

can be cha ra cterized in mu ch

th e same way as laser sources.

This is an other class of E/O

measurements where the stimu-

lus is a swept frequency electri-

cal signal an d th e response out

of th e modulator is inten sity

modulated light . In par ticular,

modulation bandwidth, phase,

an d electrical impedance mea-

surements are made with the

component analyzer in t he sam e

configur at ion th at is used for

laser measurements.

However, a significant difference

exists due to the modulator being

a thr ee-port device. While the

frequency response of a m odula-

tor is often independent of th e

input optical power, th e respon-

sivity is not. Th e conver sionefficiency of th e modu lat or is

not only a function of th e elec-

tr ical inpu t, but also the level of

th e optical inpu t.

The LCA measu rement compar es

the output modulation power to

the input modulat ion current .

A responsivity in Wat ts per Ampis then computed an d displayed.

If the inpu t optical power is

increased, th e output modulation

will typically also increase. Th us,

the a ppar ent r esponsivity will

increase. This means th at t he

modulator responsivity mea-

sur ement is valid only for the

specific optical inpu t power t ha t

existed when the measu rement

was performed. Th e frequency

resp onse is t ypically valid over

a wide ra nge of input powers.

Figure 18 is a measu rement of a

wide bandwidth external modu-

lator. The unu sua l response at

the low frequency ra nge is due

to the efficiency of the electrical

impedance ma tching circuitry.

Similar t o the process used for

laser measurements, the phase

response an d electrical input

impedan ce can a lso be cha ra c-terized. The frequency domain

informa tion can also be used to

predict the step and impulse

responses.

Lasers are typically described

by an input current versus output

power relationship. The preferred

description for a modulat or is

often a n inpu t voltage versus out -

put power relat ionsh ip. Becau se

LCA measur ements assum e a

50 ohm m easurement environ-ment , the LCA modulator mea-

sur ement in Watts per Amp can

be convert ed to Watts per volt by

scaling (dividing) the measure-

ment by 50. With th e HP 8703,

th is can be achieved by setting

th e nu mer at or K (gain ) term of

th e coefficient model t o 50, load-

ing the model into memory, and

dividing th e dat a by memory.

These functions are under the

Display key.

HP 8703

CW LWSource

DUT

RF in

LW out

Figu re 17. E/Omodulator mea-surement setup

Figure 18.Modulatorbandwidth


12/24

electr ical r eceiver. Normally th e

frequency of the modulation is

swept to allow examination of

the photodiode over a wide ran geof modula tion frequen cies.

Measurement Results

and Interpretation

The instru ment display of Figure

19 shows the conversion efficiency

of th e ph otodiode as a fun ction

of modu lat ion frequ ency. The

vertical axis display un its ar e

Amps per Watt an d th e horizon-

tal axis is modulation frequency.

In t his case, the vertical axis is

in a logarith mic format where0 dB (the center line of the dis-

play) represent s 1 Amp per Wat t.

12

Lightwave ReceiverMeasurements(O/E)

The measurements that t he LCAmakes on lightwave receivers

are in m any ways similar to those

ma de on lightwa ve sources. In

th is case, th e stimulus will be

modulated light an d the response

will be demodulat ed electrical

signals. Measurements include:

photodiode responsivity an d

modulation bandwidth

step and impulse response

char acterization and improve-

ment of the electrical outpu timpedance

As with the laser source, band-

width m easurements a re relevant

to pulse rise and fall times, while

impedance measurements ar e

important to minimize signal

reflections and maximize elec-

tr ical power tr an sfer. Optical

power reflections are discussed

in Optical components: Reflec-

tion measurement s.

Photodiode ModulationBandwidth, FrequencyResponse , andConversion Efficiency

As discussed ea rlier, photodiode

conversion efficiency refers to

how a change in optical power is

convert ed to a change in output

electr ical curr ent. As th e fre-

quen cy of modula tion increases,

eventually the receiver conver-

sion efficiency will rolloff. Thus,

th e device ha s a limited modu-

lation bandwidth.

The m easurement of modulation

band width consists of stimulat -

ing the photodiode with a source

of modulated light a nd mea sur-

ing the outpu t r esponse (RF or

microwave) curren t with a n

Figu re 19. O/Ebandwidth andresponsivi tymeasurement

The photodiode under test h as a

modulation bandwidth of appr oxi-

ma tely 1.5 to 2 GHz. The fre-

quency response a lso shows some

distinct resonances that will

impact th e time-doma in (step

or impulse) performa nce, as

shown in Figures 22 and 23.Measurement Procedu re

The measu remen t process is

virtua lly identical t o the laser

measur ement . An accura te

measurement r equires a user

calibration. This will allow the

LCA to remove the response of

the test system including the

electrical cables, optical fiber,

and the instr ument i tself. Prior

to the actua l calibration step,th e LCA needs to be configured.

This in cludes:

start and st op frequencies

sweep type (linear or

logarithmic)

number of measurement

points

measurement sweep t ime

sour ce power level

Note: LCAs have a Guided

Setup featur e tha t leads the

user th rough all the steps tha tar e described here. This is the

recommended measurement pro-

cedur e. Guided setup is a ccessed

by pressing the SYSTEM key

an d th e Guided Set up softkey.

The following text discusses th e

processes that the guided setup

executes.

To perform a s imple frequency

response calibration, th e con-

nections in Figur e 20 must be

made. The analyzer then mea-sures the appr opriate path s. The

frequency an d pha se responses

of th e un kn own pat h(s) is/are

then characterized. The analyzer/

system u ses this inform ation in

conjunction with the internal

calibration data to generate an

error ma tr ix. (The light wave

source and receiver chara cteris-

tics are predetermined and stored

in m emory. The st orage meth od

depends on t he t ype of LCA used.)

The end resul t i s that the f re-quency and pha se responses of

the ent ire test system are removed

from the measurement so that

th e displayed response is only

that of the ph otodiode under t est.


13/24

13

After complet ion of the calibra -

tion, one might expect to see a

flat response at 0 dB indicat ing

the test system response has

been removed. When u sing anHP 8702, th e display seen u pon

completion of the response cal-

ibra tion pr ocess will not neces-

sar ily be a flat l ine. The O/E

receiver used in th e calibrat ion,

which is sti l l in th e measu re-

ment path , has become the DUT.

Thu s its response is now dis-

played. When th e HP 8703 cali-

bra tion is completed, no response

other th an n oise is displayed

until an O/E test device is con-

nected between t he electr icaland optical measur ement planes.

In a ddition to the simple response

calibrat ion, ther e are also the

response plus isolation and t he

response plus ma tch calibrations.

The isolation calibrat ion is used

for high-inser tion loss (low con-

version efficiency) devices, where

an y signal leakage within t he

instr u-ment m ay be significant

relat ive to the actua l signa ls

measu red. The ma tch calibration

is used to rem ove th e effects of

reflections between the instrument

electrical test port a nd t he ph o-

todiode under test. (The response

an d ma tch calibrat ion is only

available with the HP 8703 LCA.)

Once th e setup an d calibrations

have been completed, the instr u-

ment is now ready to make accur-

ate m easur ements. The receiver

to be tested is placed in the m ea-surement path and its response

can be seen, as in F igure 19

O/E bandwidth and responsivity

measurement, previously

shown.

Response an d

Match Calibration

The response and ma tch calibra-

tion is used to impr ove measu re-

ment u ncertainty when t he O/E

test device ha s a poor outpu t

mat ch. Impedan ce mismat ch

leads to s tan ding waves tha t

degrade th e measurement of

device responsivity. Typically,

th is problem is m ore pronounced

at higher modulation frequencies.

The response and match calibra-

tion uses network an alysis error

correction techniques to minimize

th e effects of misma tch. The cali-

brat ion requires a 1-port electr ical

reflection calibration in addition

to the thru tra nsm ission calibra-

tion for t he optical and electr icalpaths.

Figure 21 is a composite m easur e-

ment of a high-speed photodiode.

The lower tra ce is a measur ement

with only th e normal responsecalibrat ion. The upper trace, which

ha s lower ripple, is ma de using

the response and match calibra-

tion. The tr aces are intent iona lly

offset for cla rit y.

HP 8702 HP 8703

HP 8340XSource

HP 8341XReceiver

Figu re 20. O/Ecal ibration con-figuration

Figure 21. Responseand match cal ibration

The r esponse m atch calibration

can be executed by following

the steps in the Guided Setup

procedure.

Photodiode PulseMeasurements

To see what implications th e

device bandwidth and frequency

response have on the t ime domain

performa nce, the time domain

tra nsform can be used. This trans-

form uses t he measured frequency

response data t o predict the small

signal st ep and impulse responses

of the photodiode. (See Appendix 2,

"Operat ion in th e time domain;"

Basic considerations. )


14/24

14

Figu re 22. O/Estep response

gat ing removes t he effects of

reflections and is discussed in

deta il in Appendix 2, "Opera tion

in th e time domain ;"Improvingmeasurement accuracy through

gating.

It is interestin g to compare th e

predicted time-domain response

with a tr ue time domain mea-

sur ement . Figur e 24 shows a

composite of the step response

generat ed by an LCA in com-

parison with the step response

when measured using a sharp

optical pu lse and a h igh-speed

HP 54120 oscilloscope.

The two measur ements agree very

well. It is importan t t o remember

that the oscilloscope measurement

displays the combined response

of the optical pulse, the oscillo-

scope, an d th e photodiode. The

LCA measurement can calibrat e

out the response of the test system

in order t o isolat e th e response

of th e DUT. The tr ace magnitu de

differences are du e to unequa l

instr um ent vertical scales.

Figure 22 shows the predicted step

response of th e sam e photodiode

whose bandwidth was measured

in Figure 19. There are severalpoints of interest. Th e tr an sition

from off to on or risetime (on

the order of 180 ps) is dependent

upon th e device ban dwidth

(roughly 2 GHz). There is some

ringing in the step response.

The frequen cy of th e ringing

corr elates d irectly to th e fre-

quency response resona nce at

3.2 GHz. Anoth er inter esting

chara cteristic is the seconda ry

step th at occur s roughly 600 ps

after th e initial step. This is dueto reflections with in th e device,

and is easier to understand by

viewing the impulse response.

Figure 23 shows the pr edicted

impulse response of the photodi-

ode using t he low-pass impulse

data tran sform. This measure-

ment provides several pieces of

informa tion. First, we see the

impulse width. The t ime between

the m ark ers is 123 ps at th e full-width ha lf-maximum points.

(This is due not only to the ph o-

todiode bandwidth , but a lso th e

finite bandwidth of th e instru -

men t its elf. The net pulsewidt his th e effective pulsewidth of th e

photodiode alone after r emoving

th e effect of th e inst ru men ts

bandwidth.) Another importan t

data point is noted by mar ker 1

at the pea k of th e response. This

value is 621 ps a nd is t he effec-

tive delay of th e ph otodiode or

in other words, the a verage

propagation t ime experienced

by the modulation signal from

the optical input to the electrical

outpu t. A second impulse is notedby mark er 2. This response is

due to an intern al reflection and

re-reflection. Th e r e-reflected

signal tra vels a longer distance

than the primary impulse, and

ther efore shows up with a rela-

tive delay.

Figu re 23. O/Eimpulse response

Figure 24. Compos ite

t ime domain mea-surements

The reflection in the photo-

diode has an adverse affect on

the frequency response of the

device. If this reflection could beremoved, the r esponse would be

improved. A technique called


15/24


16/24

16

The dat a can a lso be displayed

simply as Retu rn Loss, th e ratio

of reflected to incident power

(10 Log (P re fl/ P inc).

Figu re 28. O/Ereturn lossmagnitude

Figure 29. Timedomain display of electrical reflec-t ions

Figu re 27. O/Ereturn loss inSmith chartformat


and Interpretation

The setup and measur ement

of photodiode ret ur n loss ar eidentical to th e procedure u sed

in characterizing laser r eturn loss.

See Laser inpu t impedan ce

on page 9. Figure 27 sh ows the

ret ur n loss of an optical receiver

measured with the component

an alyzer, displayed on a Smith

Char t. A Smith Ch ar t is a form

of an impedance map . The dis-

play shows the outpu t impedan ce

as a fun ction of frequen cy. For

this receiver, an electrical amp-

lifier follows the photodiode, sothe measur ed impedan ce is essen-

tially that of the amplifier. Over

the 6 GHz measurement ran ge,

the impedance stays reasonably

close to 50 Ohms (th e center of

the Sm ith Cha rt). The ideal case

would be for the impeda nce to

be a consta nt 50 (or 75) Ohms.

The Smith Chart data presenta-

tion is selected u nder the Format

key menu.

Using the time domain feature

of th e LCA can help to determine

the locations of any discontinu-

ities in the electr ical pat h of theph otodiode as sembly. Figur e 29

is a t ime/distance represent ation

looking back int o a ph otodiode

assembly. (This is the same pho-

todiode measu red on pa ges 12

to 14, where it wa s shown in a

tra nsmission measurement th at

there were significant reflections.)

SMAConnector

Joint50 Ohm

Transmission Line

PhotodiodeAssembly


17/24

17

Optical Componen ts(O/O): Tran sm iss ionand Re f lect ion

Measurements

Consistent, accurate measure-

ments of multi-mode fiber band-

width ar e difficult to achieve using

an LCA, principally due to th e

fibers inherent instability in m ode

stru ctur e and distribution. How-

ever, frequency response data can

be used in a time domain forma t

to yield pr ecision length an d

propagation delay tra nsm ission

measurements and high-resolu-

tion reflection measu remen ts.

TransmissionMeasurements

Fiber Leng th andPropagat ion Delay

In the following example we want

to determine the length of a sec-

tion of singlemode fiber. The mea-

surement will be made by using

a modulated optical signa l with

a swept modulation frequency.

The ran ge and resolut ion a redirectly dependent upon the m od-

ulation frequency bandwidth and

the nu mber of measur ement

point s. A useful tool built into the

HP 8702 and HP 8703 that assists

in making time domain measure-

ments is th e tra nsform param -

eter s function. See Appendix 2,

Operat ion in th e time domain;"

Transform Param eters.

Measurement Results

and InterpretationFigure 30 shows the result

of th e fiber tr an smission m ea-

surem ent displayed in th e time-

domain. The frequency-domain

data has been t ran sformed to

predict t he impu lse response of

th e fiber.

Figure 30. Impulseresponse of a lengthof fiber

Placing a marker a t the peak

of the pulse indicat es th e propa-gation time th rough th e fiber,

24.429 ns. If we know the index

of refraction, we can calculat e th e

physical length of th e fiber. Con-

versely, if we kn ow the p hysical

length , we can calculat e th e fibers

index of refraction. The impulse

width is due t o the finite ban d-

width of the LCA and not th e

fiber itself.


The measurement setup is

stra ightforwar d. The swept m od-

ula ted optical source is connected

th rough a short piece of fiber to

the inst ru ment s light wave

receiver. A mea sur ement cali-

bration is required to remove the

tra nsmission path length an d

frequency response er rors of theLCA sour ce and receiver.

Care must be tak en in setting

the instrum ent sweeptime and

IF ba ndwidth , part icular ly for

long devices. This is because

th e LCA tun ed receiver cont in-

ues to sweep while the stimu lus

signal is delayed through th e

fiber. The m inimum sweeptime

for a given device delay is det er-

mined by the combination of IF

bandwidth, number of measure-ment point s, and th e frequen cy

span . There are no simple rules

to follow in set tin g th e critical

parameters. The best procedure

is to set th e sweeptime to a large

value, such a s 10 seconds, with

th e DUT conn ected, prior to per-

form ing a calibra tion. (If th ere is

no response, the sweeptime may

need to be increased furt her ). The

sweeptime is sequent ially reduced

un til the response cha nges. The

sweeptime is then increased backto a level giving a st able mea-

surement .

Figu re 31. O/Ocal ibration s etup

HP 8702 HP 8703

HP 8340XSource

HP 8341XReceiver


18/24

18

Once th e measur ement calibra-

tion h as been performed, th e test

fiber can be conn ected between

th e short fiber and th e test sys-tem. The initial measurement is

ma de in the frequency domain.

Actual length measur ements are

determined through t he t ime-

domain t ran sform. Measurement

accuracy is discussed in Appen-

dix 2, Operat ion in th e time-

domain.

Fiber ModulationPhase Stabil i ty

In cert ain fiber optic microwave

link applications, it is importan t

for t he m icrowave signal to ha ve

a very stable phase response

relative to other signals propaga-

ting on different fibers or through

different media. If the index of

refraction varies with temper a-

tur e, or some other environmen tal

parameter, the carrier (l ight)

velocity an d th us t he m odulat ion

envelope will experience a rela -

tive phase sh ift.

Because we ar e attempt ing to

measure a change in the fiber

char acter is t ics , the setup a nd

calibra tion procedur es ar e dif-

ferent t han for m ost measu re-

ment s. In th is case, we calibrate

the instru ment with the fiber

un der test conn ected to the

instrument .

HP 8703

Figure 32. Pha sestability calibration

With th e fiber under t est in

place during th e calibrat ion, we

effectively rem ove an y respons e

present in an ambient environ-ment. Care must be taken th at

effects other tha n th e param eter

of interest (for example t emper-

ature) do not impact the measure-

ment. For instance, any bending

of th e cable after calibrat ion can

cause a change in the pha se

response.

For this measurement, the

device un der test is a 10 km

spool of fiber. The mea sur emen t

is made with a CW modulat ionfrequen cy of 10 GHz. In stea d of

sweeping frequency, th e m easur e-

ment is ma de over a 16 minute

time span. It can be seen t hat

the modulation phase response

does vary significantly with time.

In this measurement , the rela-

tive phase response begins a t

roughly 60 degrees (some phase

change ha s already occurred

between the time the calibration

was completed and t he measu re-

ment began). The pha se contin-ues t o cha nge to 180 degrees,

where th e an alyzer rolls over

to +180 degrees. For the given

time span , the total variation is

appr oximately 150 degrees.

As shorter lengths of fiber ar e

examined, the phase response

variance versus time will become

sma ller. However, other pa ra m-eters such as temperatu re or

physical stress can cause ph ase

variat ion, even over short ru ns

of cab le.

Reflection Measurements

In a high-speed fiber optic

system, reflected light can cause

a variety of problems and come

from several different sources.

Both distributed feedback (DFB)

and Fabry-Perot lasers are sen-sitive to light reflecting back int o

their resonant structures. Both

noise an d modulation chara cter-

istics can be degraded. In a com-

munication system, re-reflected

light can arr ive at th e receiver

an d potentia lly cause bit errors.

To minim ize th ese effects, it is

importa nt to char acter ize the

am ount of light th at is reflected

off of optical componen ts an d

determ ine where t he r eflections

occur.

Methods for Measu ringLightwave Reflectionsvs. Distance

In component development it is

often n ecessary to determine t he

physical location of the r eflection.

If ther e ar e mu ltiple reflections,

we mu st determine which reflec-

tions contribute significantly to

the total a mount of reflected light.

There are a variety of methods for

measu ring reflected light versu s

distance or position. Among t hesemethods are optical time-domain

reflectomet ers (OTDR), optical

coher ence-domain reflectometer s

(such as the HP 8504A precision

reflectomet er), and optical fre-

quency domain reflectometers

(OFDR). The LCA uses the OFDR

Figu re 33. O/Ophase measure-ment vs. t ime


19/24

19

HP 8702

Test Port

HP 8341XReceiver

HP 8340XSource

HP 8703

TestPort

HP 11890/1 Figure 34.OFDR setup

technique. Each technique has

advanta ges an d disadvant ages.

(When m easur ements of total

retur n loss are required, withoutspatial informa tion, a power m eter

solut ion su ch as the H P 8153A

is used.)

Determining both th e ma gnitude

an d locat ion of reflections in light-

wave components require t ech-

niques beyond th e capabilities

of a multimet er or conventional

OTDR.

The LCA is well suited for

ma king h igh resolution reflec-

tion measu remen ts of light wavecomponen ts. Th e LCA does not

use a pu lse techn ique and con-

sequently does not suffer from

deadzone problems typical of

OTDRs. Instead, a wide band-

width swept frequency technique

is used, which leads to precision

location a nd resolut ion of each

reflection.

The set up for a r eflection

measurement requires that

th e light wave source be rout edto th e input of a dir ectional cou-

pler. The DUT is conn ected to

the coupler output a rm. The

coupled ar m is connected t o the

LCA receiver.

The r esolution of th e LCA in

OFDR mode is dependen t u pon

the modulation frequency range.

The wider the bandwidth, thehigher is the two-event r esolution.

The closest t ha t t wo reflections

can be and st ill be resolved is refer-

red to as response resolution. (See

Appendix 2, Operation in the

time domain;Basic considera-

tions.) A 20 GHz instr umen t band-

width can pr ovide 5 mm of two-

event resolution wh ile a 3 GH z

bandwidth can provide 33 mm

(in fiber). If higher resolut ion is

required, th e HP 8504 precision

reflectometer offers better tha n25 micron 2-event resolution. Mea-

surement sensitivity is enhan ced

thr ough trace averaging and set-

ting the LCA IF bandwidth to a

low value, su ch as 30 H z. This

usually slows the measurement

rate, but will reduce the effects

of noise. Sm aller reflections can

then be seen.

Once the frequency range ha s

been set, a calibration must be

perform ed. The simplest cali-brat ion is achieved by using th e

open-ended test port a s a Fresn el

reflection stan dard. This assu mes

tha t t he port is polished, clean,

an d in good condit ion. With t his

calibrat ion sta nda rd in pla ce,

the a nalyzer measur es the light

reflected off the test port as th e

frequency of modulation is sweptover th e selected bandwidth.

Thu s, the frequency response

imper fections of th e LCA are

ma th emat ically removed from

the measurement .

Figure 35 shows the reflections

from a light wave cable consisting

of th ree pa tchcords with simple

PC conn ectors. The ma gnitude

of th e r eflection for each conn ec-

tor is easily seen. Settin g the

index of refra ction t o 1.46, an dusing the marker functions, the

length of each pa tchcord can be

determ ined, at 1.514m, 1.761m,

an d 1.756m r espectively.

Figure 35. Multipleref lect ion measu rement

Measur ement accur acy is dis-

cussed in Appendix 2, Opera tion

in the time domain.


20/24

(The sweeptime considerations

discussed in O/O tr an smission

measurements are even more

critical here, since the signal istr aversing the length of the fiber

twice before being detected.)

98.513 s of electr ical dela y is

added, and while in t he t ime

mode, the spa n is reduced to

show two reflections very close

together. These are th e Fr esnel

reflections at the two coupler

outpu ts, which ar e measured to

ha ve a different ial path length

of 18 mm . The t wo-pass mea -

surement technique has pr o-

vided m illimeter resolut ion atth e end of a 10 km cable.

Note: This technique is sus-

ceptible to alias r esponses. The

reflection a t t he inst ru men ts

test port, or where t he coupler

is conn ected to the fiber spool, can

potentially show up in a nother

region du e to th e limitations of

th e tr an sform. To determine if a

response is real or an alias, the

num ber of measurement pointsshould be chan ged and th e mea-

surement repeated. True events

will ma intain their locat ion, wh ile

alias events will move.

20

There are l imitations in the

OFDR technique. The higher

th e two-event resolution, the

smaller th e overal l measur e-ment range. For instance, the

20 GHz configuration with 201

measu remen t point s offers th e

best two-event resolution (5 mm

in fiber), but t he one-way ran ge

is only 1 meter. The mea sure-

ment ran ge can be increased by

increasing the nu mber of mea-

sur ement p oint s, or decreasing

the instruments frequency range,

which will in tur n degrade t he

two-event resolution. (See

Appendix 2, Operat ing in t hetime domain.)

Achieving Both HighResolut ion an d Long Range

Some measurement scenarios

require both high r esolution and

long range. This can be achieved

using t he LCA in a 2-pass

measurement technique. The

ana lyzer is first set up in a nar -

row bandwidth mode that pro-

vides a long enough ra nge to

locate th e region of inter est. Thepropagation time to the ar ea of

interest is deter mined. The LCAs

frequency range is then widened

to provide the two-event resolu-

tion required t o isolat e th e indi-

vidua l reflections. The electrical

delay equal to th e propagation

time t o the r eflections is added

to the measurement (using the

electrical delay function under

th e Scale Referen ce key). This

effectively pu lls th e r eflections

of inter est into the instr um ent sreduced range.

A High-resolution Measurem ent

of Differential L ength

To demonst ra te th is procedure,

a long s pool of fiber with a 1X2coupler at t he end was m easured.

The different ial length of the two

arm s of the coupler is t he desired

measurement.

The first ta sk is to locate the

coupler a t t he end of the fiber. The

spool is estimated to be 10 km

in length. The measurement span

is configured t o provide 12 k m of

ran ge. This requires a frequency

spa n of only 2.5 MHz. After a

response calibrat ion similar totha t described above, th e spool

an d coupler a re th en conn ected

to the test port, and the measur e-

ment of Figure 36 is generat ed.

Two reflections a re seen . One

at time 0, corr esponding to th e

fiber connection to the instrument,

and another over 98.513 s (two-

way) or 10.114 km (one way), at

the cable end.

The t ask is now to zoom in on

the cable end an d examine the

reflections in high resolution

mode. The an alyzer ban dwidth

is increased to 20 GHz an d placed

in set freq low pass mode. The

ana lyzer is then recal ibrat ed

with t he DUT disconnected.

Figure 36. OFDRmeasurement inwide span

Figure 37. Zoomin gin on the cable end


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21

Electrical Compone ntMeasurements (E/E)

Lightwave component analyzershave t he capability to operate a s

RF an d microwave network ana -

lyzers. They can th en be used t o

chara cterize the electr ical compo-

nent s used in lightwa ve systems

including amplifiers, filters, coup-

lers etc. For tu torial informa tion

on RF an d microwave network

an alysis please refer to the HP

"Vector Semina r" booklet (HP lit-

erature number 5954-8355.)

Appen dix 1:Signal Relat ionships inOpto-electric Device s

Signal Relationsh ipsUsed in ComponentMeasurements

The LCA measurement technique

is built upon concepts u sed in

cha racterizing RF and m icrowave

devices. S-para met er or s cat-

tering matr ix techniques have

proven to be convenient ways to

chara cterize device per form an ce.

The following section will discuss

how similar techniques ar e usedin char acterizing devices in t he

lightwave domain. This is intended

to show the basis on which E/O

an d O/E responsivity measu re-

ments are defined.

Figure 38 is a general represen-

ta tion of a l ightwa ve system,

showing input and output signals

in term s of term inal voltages,

input and output currents, and

optical modulat ion power.

S-para meters are used todescribe the tran smitted and

reflected s ignal flow with in a

device or network . For the model,

the following S-par am eters a re

defined:

S11=b1 (a2= 0)a1

S22

=b2 (a

1

= 0)a2

where:

a1 =V1 incident on E/O deviceZ0

= I1 Z0

b1 =V1 reflected from E/O deviceZ0

a2 =V2 incident on O/E deviceZ0

b2 = V2 transmitted from O/E device

Z0

= I2 Z0

It is interesting to note tha t

delta volta ges and curren ts

ar e used a s opposed to RMS

values. Th is is done because we

deal with m odulation signals in

describing lightwa ve tra nsdu c-

ers, where a cha nge in optical

power is proportional to a change

in electrical current or voltage.The overall system forwar d gain

is defined as:

S21 =b2

(a2= 0)a1

S12 = 0 (no reverse transmission

is assumed)

5050

PI

50

PO P

2

I2

II

PO

E/O O/E

PO

f

Rr(A)

WR

s(W)

A

50

Figure 38. Signaldefini t ions


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Data Subject to ChangeCopyright 1992Hewlett-Packard CompanyPrinted in U.S.A. 4/975091 6478E

With the t ime-domain tra nsform

turned off, the gate function may

rema in a ctive. The frequency

response is now shown, butwith th e effect of th e reflection

removed. It is apparent th at t he

reflection has a significant effect

on the frequency response. Thus,

gating p rovides a useful tool to

simulat e the resu lts of actu ally

removing unwanted responses.

The t ime-doma in gat ing function

acts a s a time ban dpass filter

tha t passes the primary response

an d removes the responses dueto r eflections. Once t he reflec-

tions ha ve been gated out , th e

measurement can be returned

to the frequency doma in. The

frequency response displayed is

as if th e reflected signals wer e

no longer presen t.

Figure 39 shows a photodiode

response th at is degraded due

to int erna l reflections.

Analyzing the r esponse in t he

time domain, the secondary

impulse is determined t o be

du e to a reflection.

Using th e gating function (part

of the tra nsform m enu), the time

gate or filter is centered an dthe spa n a djusted to reject a ll

but th e primary response. The

gate center is noted by the T

an d width by the two flag ma rk -

ers. The gate is turned on, and

the reflection response is

removed.

Figure 39.Degraded fre-quency response

Figure 40. Timedomain response(with ref lect ions)

Figure 42.Frequency responsewi th and wi thoutgating act ive (gatedtrace is offset)

Figu re 41. "Gated "time domainresponse

For more information aboutHewlett-Packard test and measure-ment products, and for a cu rrentsales office l isting, visit our website, http:www.hp.com/go/tmdir.

HP AN1550 6_High Speed Lightwave Component Analysis

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