hp-pn71452-4_pulsed or time-dependent optical spectra measurements

8/14/2019 HP-PN71452-4_Pulsed or Time-Dependent Optical Spectra Measurements

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HP 71452BOptical Spec trum Ana lyzer

Product Note 71452-4

Pu lsed or Time-Depe nde nt OpticalSpectra Measurem ents


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Table of Conten ts Introduction 3

OSA Block Diag rams 4

Free Run Mode 5Zero Span Mode 7

OSA Trigge r Modes 8Tr iggered Sweep 8ADC Trigger Mode 8ADC AC Mode 10Gated Sweep Mode 10

Application Examp les 12Pulsed Light Signals 12High bit ra te signals 12Solitons 12

Test / cont rol signals 12Pulsing a Laser 14Synchronizing The OSA With a Tun able Laser 15Time Doma in Extinction 16Re-circula t ing Loop 18

Appendix 22Impor tant Keyst rokes 22

Notice

The informa tion contained in th is document is subject t o cha ngewithout notice.

Hewlett-Packard makes no warranty of any kind with regard to

this ma terial, including but not limited to, th e implied warra nt iesof merchan tability and fitness for a par ticular pur pose. Hewlett-Pa ckard sh all not be liable for errors conta ined her ein or forincidenta l or consequen tial dam ages in conn ection with th efur nishing, performa nce, or u se of this m ater ial.

Copyright 1996 Hewlett-Packard Compa ny

All right s reserved Reproduction, ada pta tion, or t ran slationwithout pr ior written permission is prohibited, except as allowedun der th e copyright laws.


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Introduct ion The r apid pr ogress of light wave technology constan tly leads t omeasu remen ts of more complex optical signals. In a va riety ofapplications, the p ower or th e spectru m of the light changes within

a sh ort period of time. For exam ple, a pu lsed laser ha s to be measu reddifferent ly tha n a cont inuous wa ve signal. In a re-circulat ing loopincorporat ing optical amplifiers, th e spectru m cha nges significantlywith in a fraction of a second. HP s family of optical spectr uman alyzers (OSAs) support a var iety of measur ement featu res whichallow a ccur at e char acterization of time-dependent optical spectra .

This product n ote first d iscusses h ow th e HP OSA synchronizes itsintern al activities with extern al signals. Then it shows how majorapplications can ta ke advan tage of it.

Product informat ion:

HP 71450B / 71451B / 71452B (or 70950B / 70951B / 70952B):

All informat ion in t his document is applicable with out exception.

HP 71450A / 71451A (or 70950A / 70951A):

Triggered sweep, gated sweep an d ADC Sync Outpu t ar e stan dard .Many older instr umen ts ha ve a limitat ions of the a na log bandwidthand the analog slew rate. The analog-to-digital conversion triggermodes and th e TRNZLCK fun ction r equire additiona l har dware.

To improve the an alog an d timing ha rdwar e, and t o get th e firmwar efunctions offered in t he B version, cont act your local H P salesoffice and order th e HP 70953A Time Domain Extinction upgra de.

References:[1] Optical Spectrum Ana lysis Basics, HP Applicat ion N ote 1550-4(HP literature number 5963-7145E)

[2] EDFA Testing with the Time Doma in Extinction TechniqueHP Pr oduct Note 71452-2 (HP litera tu re nu mber 5963-7147E)

[3] EDFA Noise Gain Pr ofile an d Noise Gain Peak Measu remen tsHP Pr oduct Note 71452-3 (HP litera tu re nu mber 5963-7148E)

[4] HP 71450B/1B/2B Optical Spectrum Ana lyzers User s Guide(HP par t nu mber 70950-90049)

[5] HP 71450B/1B/2B Optical Spectrum Ana lyzers Reference(HP par t nu mber 70950-90051)


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OSA Block D iagrams Measurements of optical spectra require a complex combination ofoptics, electronics, and firmware. Figure 1 shows a simplified modelof an OSA. HP s a ctu al implement ation is mu ch more complex tha n

the m odels shown in th is document t o illust ra te th e timing andsynchronization issues.

If th e light a t th e OSA input cha nges with time, then t he signal tobe measu red mu st be described as a fun ction of wavelength an d time.However, the operation of th e instr um ent depends on time t oo theoptics inside the OSA filter th e spectrum. In order to measu re a llspectr al components, th e grating rotat es so th at d ifferent wavelengthsare brought t o the slit. The OSA fun ction sweep time (ST) and t hewavelength ra nge (CENTER, SPAN) control the sp eed of th e grat ing.

Figure 1. OSA Princip le of Operation

The grat ing together with th e slit behaves like an optical bandpa ss.Such a combination is called a monochromator. Its spectral filterchara cteristic (ma inly center wavelength ) cha nges with th e gratingmotion. Th e ph otodetector receives a power tha t can be describedas a convolution of the input signal and filter function of the mono-chromator. Independent of th e wavelength, th e photodetectorconverts all the light coming t o the slit int o an electr ical signal t ha tis amplified and then sampled. The analog-to-digital conversion(ADC) occur s every 37.5 s. In best case, it ta kes t ra ce length timesADC conver sion time (800 x 37.5 s = 30 m s) to scan a givenwavelength range.

After th e ADC, a digital signa l processor (DSP) furth er pr ocesses th edata. For example, the video bandwidth (VBW) function is implement-ed in t he digita l processor. Finally, th e dat a is log-conver ted (dBm)and tran sferred to a display unit.

Controller& Display

Light input

Photodiode

Slit

A/DConverter

DigitalProcessing

Grating

v

v

v

v


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In ma ny cases th e spectr um a t a given point with in the modulationperiod is more mean ingful th an th e average spectru m. A TTL signa lat th e trigger input of th e OSA can synchronize a var iety of the

electr onic blocks (Figur e 2). For examp le, th e signa l can (mut ua llyexclusive):

sta rt t he grat ing motion (Triggered Sweep)

sam ple and A/D convert a dat a p oint (ADC Trigger Mode)

tell the DSP when t he optical spectru m is valid (Gated Sweep)

Becau se th ere is only one t rigger inpu t conn ector, th e timing un itblock has t o distribut e the t rigger inpu t t o either t he motor contr ol,or to t he an alog-to-digita l conversion, or t o the digital signa l processor.If the grat ing is triggered, then t he a na log-to-digita l conversion r un s

freely. If the ADC is tr iggered, th en t he gra ting m oves independen tlyof an y externa l signal. The DSP cant be t riggered directly (it h as t osynchronize its a ctivities both with th e ADC and t he gra ting).However, th e trigger input t ells the DSP h ow to han dle the dat afrom th e ADC.

Figure 2. Trigger Signa l Distribution

Before the next section discusses t he var ious trigger modes in

detail, let u s first look a t h ow th e OSA operat es independent of an yexterna l signal.

Fr ee Run ModeThis is the basic mode. The grat ing sweeps thr ough the selectedwavelength ra nge (Figur e 3). When th e sweep has been completed,the grat ing moves back into the star t position. This cycle repeatsitself as long as continuous sweep is active.

+

A

D

Timing Unit

Digital

SignalProcessor

ADC Sync Out

Ext. Trigger In

MotorControl

Unit

Grating

Analog-to-Digital Conversion

Transimpedances

v

v

v v

v

v

v

v

v

v


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Figure 4. Video Ban dw idth Effects

1 The OSA can also increase its sensitivity by changing a different amplitude range (i.e., byselecting a different amplifier gain in the a nalog path). Because t his can cause a dditionalproblems with m odulated signals, we assume t hat ranging does not occur du ring the m easure-ment. Several t rigger and gated sweep modes tur n off AUTO RANGE aut omatically.

Figure 3. Free Run Mode

If th e input power and spectru m ar e constan t over time, then onlythe gra t ing motion a nd t he digital f il ters in th e DSP mu st besynchronized to genera te an accur ate t ra ce on the screen. In thiscase, th e grating speed mainly depends on the wavelength ra nge tobe covered an d th e required sen sitivity1: th e slower the gra tingrotat es, the more sam ples from th e ADC can be averaged by thevideo bandwidth (VBW) function into one trace point on the screen.Sometimes such a trace point is called a trace bucket because itactually combines several ADC values.

If th e signa l is modulated t hen t he OSA still can measu re th eaverage spectrum without an y external synchronization. The VBWmust be significantly smaller th an the lowest modulation frequencycomponent, otherwise the signal can look very distorted (Figure 4).

Active Sweep

Grating Motion

Data Acquisition

Low VideoBandwidth

Trace Buckets

v v v v v v v v v vvvvvvvvvvvvvvvv

v v v vv

VBW = 700 kHz VBW = 100 Hz


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Zero Span ModeIf the spa n is zero (i.e., star t wa velength = stop wavelength ), thenthe grat ing remains at the a ngular position representing the center

wavelength . Therefore, th e rest of th e OSA beha ves similar to adigital oscilloscope. The x-axis on the screen represents the sweeptime. The sa mpling period is 37.5 s. If the sweep time is longerthan trace length t imes sample rate, then a tr ace bucket representsthe average of several ADC values.

Besides looking a t a low frequen cy (< 10 kHz) modula tion, th ismode has a major speed benefit for an accurate power measurementat one wavelength: Instea d of placing a ma rker a t a desired wave-length an d then r eading its power level, use zero span a t th at wave-length a nd t hen read the a verage power of the wh ole tra ce (remotely:MEAN TRA?;). Becau se a tr ace consist s of ma ny point s, th e mea ntr ace power ha s avera ged the noise or m odulation, even if the

sweep time is very short. To achieve the same noise or modulationsuppr ession for a sweep with span > 0, the video bandwidth (VBW)must be very low, and therefore the sweep time becomes very long.


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There is also a relation between sweep time and t he longest t imeperiod between tr igger events: in order t o get a t least one validsamp le for each t race point, th e maximum time between tr igger

events mu st be less tha n sweep time divided by tra ce length. If thegrat ing passes from one tr ace point (which represent s a specificwavelength ) to th e next one with out a t rigger, then t he OSA reportsthe error Sweep time too fast . In t his case, adjust t he sweep timeman ua lly to 1.2 .. 2 x tra ce length x ma ximum t rigger period (th efactor 1.2 .. 2 allows some time for p rocessing overh ead).

Figure 6. ADC Trigger Mode

Two additional aspects should be kept in mind when u sing the ADCtr igger mode (or ADC AC Mode discussed below): th e impa ct on th eeffective video ban dwidth, an d th e amp litu de ra nge settin g.

Becau se th e VBW filter is implemented a s par t of th e DSP firm ware,the OSA displays a VBW value based on th e assumpt ion th at t hesampling period is 37.5 s. If the trigger period in the ADC triggermode is longer, th en th e effective VBW is a ccordingly lower. Theeffective VBW can be calculat ed as th e displayed VBW times 37.5 sdivided by the trigger period.

ADC trigger is available in a ll ranges, including t he m ore sensitiveones. If the input signals chara cterized with th e ADC tr igger m odeshave higher modulation frequency components, then the trans-impedan ce am plifier between th e photodiode and th e ADC musthave enough bandwidth. Because t he sensi t ive gain ra nges areslower3, the OSA mu st sta y in th e two fast gain sta ges. The easiestway is to tu rn on th e function TRNZLCK (tra nsimpeda nce lock)and to turn off AUTORANGE.

3 Bandwidth of the OSA ra nges: 700 kHz, 600 kHz, 60 kHz, 20 kHz, 2 kHz an d 200 Hz bandwidth .

Active Sweep

Trigger Input

Grating Motion

Data Acquisition

Trigger Events: NEG EDGE

Trigger Delay

Sync Output

v v v v v v v

vv


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ADC AC Mode

Similar t o the ADC tr igger mode, ADC AC sam ples the da tadelayed after a t rigger event. While the first one tr iggers on either

a p ositive or a n egative edge, the ADC AC mode altern at es betweenpositive an d nega tive edges (Figure 7). In a ddition, t he DSPprocesses the data differently: it calculates the absolute differencebetween th e samples acquired after t he positive trigger edge andthe ones acquired after t he n egative edge. The resulting tr ace pointrepresent s only the modulation amplitude, so th at a ny constan tlight or light modulated a t a different frequency can cels out .

In t his mode, the DSP ru ns t wo VBW filters on t he r aw dat a fromth e ADC (one for t he positive and one for t he n egative sam ples).Therefore, i t reduces ra ndom noise with out affecting th e tru eamp litu de of th e signal.

Figu re 7. ADC AC Trigge r

The ADC AC mode is similar to lock-in t echn iques. It m easur es th emodulation portion of the light only, and it suppresses light whichis not modulat ed. The impa ct on th e effective video bandwidth an dthe a mplitude ra nge setting considerat ions discussed above applyto th e ADC AC mode a s well.

Applications for this mode include tests for systems incorporatingEDFAs (see Figure 19), or open beam setups which u se a 270 H zmodulation in order t o suppress ambient light.

Gated Sw eep Mode

The gated sweep mode tells th e DSP when to retain or ignore thedat a coming from th e ADC. Both the gra ting an d th e ADC runwithout synchronizat ion to any externa l signal. If th e trigger inputis high, then t he DSP ta kes the ADC value as a valid data point.Otherwise, it replaces the sa mple by the value -200 dBm. In bothcases it continues processing according to the functions selected(e.g., video ban dwidth , ma x hold, etc.).

Trigger Delay

Trigger Input

Active Sweep

Grating Motion

Data Acquisition

Input Signal

+

-

+

+

+

v v v v v v v v

vv


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Figure 8. Gated Sw eep Mode

If the time of the low level is longer t ha n t he t ime needed for t hegrat ing to move from one tra ce point to the n ext one, th en th e tra cewill ha ve gaps (Figur e 8). There a re t wo alternat ives to close the gaps:either increase th e sweep time to at least 1.2 .. 2 x tra ce length xlongest low level period, or activate t he MAX HOLD function a ndlet th e OSA sweep severa l times. In t he first case, the DSP willha ve at least one data samp le marked valid (high level) per tr acepoint . In t he second case, mu ltiple sweeps fill the gaps becau se th ehigh and low levels of the gating signal occur independently of thegrat ing position.

Gated sweep has no time limit for the high or low level. Therefore,

it can be u sed to chara cterize pulses a s na rrow as a few microseconds,or obtain a spectr um whose timing exceeds the m aximum 6.5 msdelay of th e ADC tr igger mode. This can occur in a pplications asshown in Figure 20.

Active Sweep

Trigger Input

Grating Motion

Data Acquisition

Trace On Screen

v v v v v v v v v v v v v v v v v v v v v v v v v v


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Pulse d Light Signals

There is a great variety of pulsed light sources. Many applicationsfall into one of these categories (or at least have similar

requirements):

1. High bit r at e signals (34 Mb/s to 10 Gb/s)

2. Solitons (pulse width 1 GHz)

3. Test/contr ol signa ls (pulse width 1 s to 1 ms, du ty cycle 10 to 90%)

4. Active component test s (laser s on a chip, etc.)

High Bit Rate SignalsThe signals of this category have little modulation frequency com-ponents below 10 MHz. The best case ana log band width of the OSA

is about 700 kHz. Therefore, th e OSA measures only the a veragespectru m of typical t elecom tr affic signa ls.

Figure 9. OSA Frequency Ran ges

SolitonsSignals using solitons and other picosecond pulses also have afrequency spectru m ma inly in t he gigah ertz ra nge. Similar to thecase above, the a na log bandwidth of the tr an simpedance am plifieravera ges the pu lses so th at t he signal looks like a cont inuouswaveform to th e OSA.

Test/Contr ol Signa ls

HP s family of OSAs offers t he m ost var iety to char acter ize testsignals from component s, sensors, subsystem s or other light sour cesmodulated in t he frequency ra nge approxima tely between 10 Hzand 250 kHz. Again, i t is possible to measur e only the averagespectru m by choosing a low video ban dwidth (Figure 10). Even ifthe a na log bandwidth is higher tha n th e modulat ion of th e signal,th e VBW function will low-pass filter th e sam ples.

Applicat ion Examp les

1 kHz 1 MHz 1 GHz 1 THz

Electrical Range(DC600 kHz)

Optical Range(RBW 0.0810 nm)

Frequency

Traffic Signals(34 Mb/s10Gb/s)

Rel.Amplitude


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Puls ing a Laser

Testing a source component , such as a laser or LED on a chip, is acomm on OSA application which often a llows th e OSA to contr ol the

modu lat ion of th e light source. HP s family of OSA offers personalit iesto cha ra cterize the spectru m of an LED, FP or DFB laser. Optiona lly,a curr ent source can drive the component up t o 200 mA (Figure 12).

Figure 12. Pulsing a Laser With The Bu ilt-In Current Sou rce

The OSA provides current pulses according to the selected pulsewidth. Becau se th e OSA synchronizes th e current source pulseswith its dat a acquisition t iming requirement s, the actua l duty cycleof th e curr ent m ay be slightly lower but n ever higher th an the valueenter ed by the u ser.

If th e device un der test n eeds more cur rent th an a vailable from th eOSA, or if th e OSA does not h ave th e curren t sour ce option, then itis still possible to contr ol th e light pu lses by using t he ADC syncoutput (Figure 13): this outpu t conn ector pr ovides the timing infor-mat ion which otherwise modulates th e optional curr ent sour ce. Thewidth of tha t TTL-compatible pulse and its ma ximum du ty cyclecan be chosen in wide ra nges. The OSA will always samp le a dat apoint immediately before th e falling edge of th e sync output pulse.

Figure 13. Synchroniz ing an External Laser Driver

Optical Spectrum AnalyzerHP 7145XB with #001

Optical Spectrum AnalyzerHP 7145XB with #001

TTL Signal

Laser Driver

Sync Out


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When th e OSA contr ols the t iming, measurem ent speed is at itsoptimum because t he OSA allows just enough t ime to acquire thedat a before it cont inues. Ther e is no time lost while the OSA waits

for a trigger signal, nor does the OSA miss or s kip a trigger.

Synch ronizing th e OSA With a Tunable Laser

Many advanced measurement setups use a tuna ble laser source(TLS) an d an OSA. For exam ple, passive optical component s suchas deep, na rrow filters r equire a swept source and a wavelengthselective an alyzer. The EDFA test option 051 of th e OSA dependson a TLS to cha racterize the gain a nd t he noise figure.

The TLS in Figure 14 mak es a wavelength sweep step-by-step, i.e.,i t tu rns the laser off, moves th e intern al mechanics to the nextwavelength, an d tu rns the laser on again. The TTL-compa tiblemodulation output goes low before the laser is t ur ned off, an d h igh

after th e laser is on a gain. Each rising edge of that signal triggersan OSA sweep. It is essential th at th e TLS dwell time (i.e., howlong it sta ys at t he wavelength before it ma kes th e next wavelengthstep) is long enough so that th e OSA can complete th e sweep andretu rn ba ck to th e star t point . The more calculations t he OSA ha sto mak e, the longer the dwell time. As a ru le of thum b, choose th eTLS dwell time greater th an 1.5 x ST + 0.3 s (ST = displayed sweeptime) for passive component tests, a nd 2 x ST + 5 s for th e EDFAtest personality (option 051).

Figure 14. Synch ronizing the OSA with a Tunable Laser

The TLS and OSA combination is often used in conjunction withthe MAX HOLD function of the OSA. For each wavelength step, theTLS provides only one na rr ow laser signal (plus some sourcespontaneous emission). Therefore, each OSA sweep results in onlyone needle. MAX HOLD builds th e wavelengt h dep enden ce of theDUT by collecting the ma ximum of all sweeps. For il lustr at ionpur poses, th e step size of the OSA in F igure 15 is much grea tertha n th e resolution ba ndwidth (RBW) of the OSA. If th e step size issmaller th an t he RBW, then th e peaks combine and t he obta inedtra ce continuously shows th e chara cteristic wavelength sha pe ofthe device under t est.

Ext. Trig

Tunable LaserHP 8168X #003

Optical Spectrum AnalyzerHP 71452B

DUT

runs the

l-Sweepapplication

uses the functions

EXT. TRIGandMAX HOLD

Mod Out

v


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In order t o watch t he effects of the m in an d ma x function on t heOSAs color display, choose CLEAR/ WRT A, MAX HOLD B, andMIN HOLD C. Trace A displays each sweep at th e top, and t races B

and C hold th e maxima an d minima r eached over time.

Figure 15. MAX HOLD Function

Time Domain Extinc tion

Time domain extinction is a technique to char acterize Erbium-dopedamplifiers (EDFA). It takes advantage of the relatively slow relaxa-tion time of Er bium. One point of interest is the qu estion h ow anEDFA amplifies a sm all signal such a s n oise in t he pr esence of alarge one. In Figure 16, th e TLS drives the a mplifier into its oper-ating point , and a n E dge Emitting LED (EELED) provides the probesignal. Becau se th e OSA has to measu re t he a mplified EELED onlybut n ot th e TLS signal, the TLS is modulated an d the measu rementoccurs only in t he brea k of th e modulation. The EE LED is modu-lated a s well, so that its a verage power has only a negligible impacton th e operat ing cond ition of th e OSA.

Figure 16. Noise Gain Profile Setu p

Actual Sweep

Maximum ofprevious sweeps

Optical Amplifier

Mod Out

Ext. Trig



25 kHz

Edge Emitting LEDHP 83424A #H37

3dBCouplerHP 11890A

Ext. Mod.

Sync Out

v

v


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In t he examp le shown, th e OSA uses the ADC trigger m ode. Whenthe OSA recognizes the negat ive trigger edge, then it switches t heADC Sync Outpu t h igh, which t urn s th e EE LED on (Figur e 17).

Ten m icroseconds later, the OSA samples a data point and thenimmediately resets th e ADC sync output .

At t he sa mpling time, th e TLS is off. With a locked tra nsimpeda nce,th e OSA ha s enough time t o recover from the st rong laser pulse.Therefore, th e OSA measur es only the spectrum of th e am plifiedspontaneous emission and the amplified EELED. (For a furtherdiscussion of th is measu remen t, see pr oduct n ote 71452-3).

Figure 17. Noise Gain P rofile Timing

The time-doma in extinction t echn ique is also a useful method tolook at syst ems, even if th e ends of th e link ar e at d ifferent locat ions

(Figur e 18). While the signal from th e tra nsm itter side is squar e-wave modulated, th e amplified spontan eous emission from th eam plifier is not. The t rigger receiver shown cont ains an opticalcoupler (10:1) and a receiver that re-creates the modulation clock.

Tunable Laser(Internal Mod.)

ASE Power

OSA Sample10 s

Edge Emitting LED

120 s vv

vv


Optical Amplifier System

25 kHz

Tap and Receiver


Ext.

Trigger

v

Figure 18. AC Mode Se tup For Installed Sy stems


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Depending on the ADC trigger mode and delay, it is possible to lookat th e combined or individua l spectr al components. Without a nysynchronization an d a low VBW, the OSA measu res t he t otal

average spectrum consisting of th e am plified signal an d th e ASE.When th e ADC triggers on th e negative edge with 10 s delay time,th en t he t ra ce shows only the noise (ASE) spectrum . However, whenchoosing th e ADC AC modes, th en th e un modulated ASE cancelsan d only the modulated signal appear s. Note tha t th e signa l ampli-tud e on the OSA screen is 3 dB higher in this m ode. The ADC ACmode measur es the full amplitude of the squar e wave while th e lowVBW measur es only its a verage.

Figure 19. EDFA Output Se paration

Re-Circulating Loop

In order to observe how an optical spectrum can change significantlywhen tr aveling along thousa nds of kilometer s and t hr ough manyoptical am plifiers without actu ally building such a system , the setu pin F igure 20 u ses a loop techniqu e. Typically, th e loop consists ofonly a few EDFAs with 30 to 70 km long fibers between th em, an done circulation lasts about 0.2 to 2 ms. Several circulations of asignal then simulat e how that signal would behave on a very longlink.

Amplified Signal

ASE Spectrum

Amplified Signal+ ASE Spectrum


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Figure 20. Re-Circulating Loop Se tup

A pulse genera tor cont rols the t iming. First, th e acoust o-opticmodula tor (AOM) 1 is open a nd AOM 2 is closed (Figur e 21). At t histime, th e TLS in conjunction with an externa l modulator fills theloop with a pseudo-ran dom bit pat tern . Second, switch 1 closes an dswitch 2 opens a llowing the pa tter n t o circulat e adequa tely (about5 ms per 1000 simu lated kilometer s). Third, the OSA measur es thespectru m a t a variable delay (i.e., at a var iable simulat ed distan ce).The thr ee steps are repeat ed unt il th e OSA ha s built a completetrace.

Figure 21. Re-Circulating Loop Timing

To mea sur e the spectru m at a given dista nce, two tr igger techniquesmay be used: ADC trigger or gated sweep. When using ADC trigger,the OSA samples only one data point per tr igger but th e time of th esamp ling is very well known. The OSA sweep time m ust exceed th etotal tr avel time times t he n umber of tr ace points (e.g., 50 ms x 800= 40 s. To allow pr ocessing overh ead, choose 50 to 80 s).


Coupler

Data PatternModulator

AOM Switch 1

AOM Switch 2

Optical SpectrumAnalyzerHP 71452BPulse Generator

HP 8110A

v

v

v

AOM 1

AOM 2

OSA

Trigger Input

5..50 ms(Total Travel Time)

VariableDelay

0.2 .. 2 ms(Loop Delay)

vv

vv

vv


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Time-Domain ScanWhen th e OSA span is set to zero, then the gra ting behaves as afilter wit h fixed cent er wa velength . If th e signal going to AOM 1

triggers a sweep, then subsequen t ADC values represent th e powerversus t ime of th e cent er wavelength of th e OSA. With 50 m s sweep-time a nd 800 point s / trace, the dista nce resolut ion is about 7.5 km5.This measu remen t h as t o be repeat ed for N wavelengths (typically50 to 200) in order t o creat e th e 3-D plot (Figure 23). N a nd th espan to be covered determ ine th e wavelength a ccur acy. Assum ingtha t a t race tra nsfer to a computer last s only few seconds, the dataacquisition for t his 3-D plot ta kes several m inut es.

Figure 23. 3-D Scan U sing th e Zero Span Method

5 Dista nce resolution = (sweep time * speed of light) / (tr ace length * refractive index)

Amplitude

Wavelength

Time(D

ista

nce)

12 3 N. . .


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Appendix This section lists importa nt keystroke sequences required for t heexamples shown. For a detailed description of these functions, seeth e HP 7145X User s Guide.

The n otat ion below uses a bold font for front pane l keys and aregular font for softkeys.

Always after power on and warm -up:

Pr ovide light to the monochr omator input an d hit AUTO ALIGN .

To reset the instrum ent and start a new m easurement:

INSTR PRESET, AUTO MEAS.

To define measurement ranges:

Use START, STOP or CENTER, SPAN to select the desired wave-length r an ge. If necessary, adjust RBW, SENS , or REV LEVEL.

To use t races effectively:

MENU , Traces, CLEAR WRT A, MAX HOLD A, MIN HOLD A,STORE A / VIEW A, tr ace B, .. , MORE, MORE, TRACE LEN GTH.

To control the sweep and video bandwidth:

MENU , BW Swp, CONT SWEE P, SINGLE SWEE P, VIDEO BW,SWPTIME. In most cases SWPTIME = AUTO works fine. UseSWPTIME MAN only to make it longer tha n t he value calculat edautomatically.

To select vari ous trigger m odes:

MENU, BW Swp, MORE, TRIGGER E XT, TRIGGER FREE, .. , MORE,GATESWP ON/OFF, adc trigger, ADCTRIG POSEDGE, ADCTRIGDELAY. Lock the tr an simpedance when using a ny ADC trigger mode.

To lock th e tran sim pedan ce am plifier:

MENU , Amptd , MORE, AUTORNG OFF (to keep the gain in t hepat h det erm ined by th e referen ce level), TRNSZLCK ON (to lock italways into the high speed path ).

To turn th e current source on (OS A w ith option #001):

MENU , State, curren t source, PULSE WIDTH (set th e pulse widthbefore t he d ut y cycle), DUTY CYCLE % (th e ADC Sync Out putbecomes a ctive if th e dut y cycle is less th an 100 %), IGEN x mA

(enter a positive or negat ive current ), IGEN ON.

To turn on th e ADC Sync Outpu t (OS A w ithout #001):

MENU , State, sync out, PULSE WIDTH, DUTY CYCLE %.

To watch th e VBW as a function of theREV LEVEL :

MENU , Amptd, LOG dB/DIV, MORE, AUTORNG OFF, BW Swp,VID BW Auto.

To analyze the spectrum of a LE D, Fabry-Perot or Distribut ed

Feedba ck L aser:

USER , LED or F P or DFB


23/23

For more information on Hewlett-Packard Test and Measurementproducts, applications, or services,please cal l your local Hewlett-

Pack ard sales office. A currentlisting is a vailable via th e World-wide Web through AccessHP athttp://ww w.hp.com. If you do nothave access to the internet pleasecontact one of the HP cen ters l i stedbelow and th ey wi l l direct you toyour nearest HP representative .

United States:Hewlett-Packard Compan yTest and Measu rement Organization5301 Stevens Creek Blvd.Bldg. 51L-SCSanta Clara, CA 95052-80591 800 452 4844

Canada:

Hewlett-Packard Canada Ltd.5150 Spectru m WayMississauga, Onta rioL4W 5G1(905) 206 4725

Europe:Hewlett-PackardEuropean Marketing CentreP.O. Box 9991180 AZ AmstelveenThe Netherlands

Japan:Hewlett-Packard Japan Ltd.Measurement Assistan ce Center9-1, Taka kur a-Cho, Hachioji-Shi,Tokyo 192, Ja pan

(81) 426 48 3860Latin America:Hewlett-PackardLatin American Region Headquar ters5200 Blue Lagoon Drive, 9th FloorMiami, Florida 33126, U.S.A.(305) 267 4245/4220

Australia/New Zealand:Hewlett-Packard Australia Ltd.31-41 J oseph StreetBlackburn , Victoria 3130, Austr alia131 347 Ext. 2902

Asia Pacific:Hewlett-Packard Asia Pacific Ltd.17-21/F S hell Tower, Time Squa re,

1 Math eson St reet, Causeway Bay,Hong Kong(852) 2599 7070

Data Subject to ChangeCopyright 1996Hewlett-Packard CompanyPrinte d in U.S.A. 2/965964-6416E

hp-pn71452-4_pulsed or time-dependent optical spectra measurements

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