HP-AN1550-4_Optical Spectrum Analysis Basics

8/14/2019 HP-AN1550-4_Optical Spectrum Analysis Basics

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Opt ica l Spe c t rum Analys i s

Applica t ion Note 1550-4

Opt ica l Spec t rumAnalys i s Ba s ics


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Table of Conten ts PageIntroduct ion 1

Chapter 1Types of opt ica l spect rum an alyzers 2In ter ferometer-Based Opt ica l Spect rum Ana lyzers 3Diffr act ion -Gr at in g-Ba sed Op tica l Sp ect ru m An alyzer s 4

Chapter 2

Diffract ion-gra t ing-based opt ica l spe ct rum an alyzers 10Wavelength Tun ing and Repea tability 10Wavelength Resolu t ion Bandwidth 10Dynamic Range 11Sensit ivity 12Tun ing Speed 13

Pola r iza t ion Insensit ivity 15Inpu t Coupling 17

Chapter 3

L igh t -emi t t ing d iodes and semiconduc to r d iode l a se r s 18Ligh t Emit t ing Diodes (LE Ds) 18Fabry-Perot Lasers 21Dist r ibu ted F eedback (DFB) Lasers 25

Refe rences 28

Appendix

Opti ca l and mic rowave spec t rum ana lyze r s compared 29


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In t roduc t ion This application note is intended t o provide th e reader with a basicun dersta nding of optical spectr um an alyzers, th eir technologies,specifications, and applications. Chapter 1 describes interfero-

meter-based a nd diffraction-grating-based optical spectru m an alyz-ers. Chapt er 2 defines man y of th e specified performan ce para m-eters of diffraction-g rat ing-based optical spectru m an alyzers an ddiscusses th e rela tive mer its of th e single monochroma tor, doublemonochromator, and double-pass-monochromator-based opticalspectru m a na lyzers. For rea ders familiar with electr ical spectr uman alyzers, some of the sam e term s ar e used, but with differentdefinitions. The final chapter of this application note describeslight emitting diodes (LEDs) an d semicondu ctor diode lasers, a ndtheir para meters th at are measu red by optical spectrum ana lyzers.

Opt ica l spect rum a nalys isOptical spectrum an alysis is th e measu remen t of optical power a sa function of wavelength . Applications include test ing laser an dLED light sources for spectra l purity a nd power distribut ion, aswell as testing transmission characteristics of optical devices.

The spectr al width of a light source is an importan t pa ram eter infiber-optic communication systems due to chromatic dispersion,which occurs in t he fiber a nd limits t he m odulat ion ba ndwidth of the system. The effect of chromatic dispersion can be seen in thetime domain as p ulse broadening of a digita l waveform. Sincechromatic dispersion is a function of the spectral width of the lightsource, na rrow spectra l widths ar e desirable for high-speed com-munication systems.

Figure 1 shows th e spectr um of a Fa bry-Perot laser. The laser isnot pu rely monochroma tic; it consist s of a s eries of evenly spa cedcoheren t spectral lines with a n a mplitude pr ofile determ ined by thechara cteristics of the gain media.

Optical spectrum an alyzers can be divided into th ree categories:diffra ction-grating-based an d two int erferometer-based ar chitec-tur es, the F abry-Perot an d Michelson int erferometer-based opticalspectru m an alyzers. Diffraction-grating-based optical spectru man alyzers a re capable of measuring spectra of lasers an d LEDs.The resolution of these instr umen ts is var iable, typically ran gingfrom 0.1 nm to 5 or 10 nm . Fa bry-Perot-int erferometer-based

optical spectr um a na lyzers have a fixed, nar row resolution, typi-cally specified in frequen cy, between 100 MHz a nd 10 GHz. Th isna rrow resolution allows th em to be used for m easur ing laserchirp, but can l imit their measurement spans mu ch more tha n th ediffraction-grating-based optical spectrum analyzers. Michelson-interferometer-based optical spectr um an alyzers, u sed for directcoheren ce-length measu remen ts, display the spectrum by calculat-ing the Fourier transform of a measured interference pattern.

Figure 1. Optical

spec t rum ana lyzermeasurement of aFabry-Perot lase r.


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Chapter ITypes of op t ica ls p ec t rum an a lyze r s

Basic b lock diagramA simplified optical spectrum analyzer block diagram is shown infigure 2. The incoming light passes t hr ough a wa velength-tu na ble

optical filter (monochromator or interferometer) which resolves theindividua l spectr al components. The ph otodetector then convertsthe optical signal t o an electrical curr ent proport iona l to the inci-dent optical power. An exception to this description is theMichelson interferometer, which is not actually an optical filter.

The curr ent from th e photodetector is converted t o a voltage by thetra nsimpedan ce amplifier and t hen digitized. Any rema iningsignal processing, such as applying correction factors, is performeddigita lly. The signal is t hen applied to the display as t he vert ical,or amplitude, data. A ramp generator determines the horizontallocation of the tra ce as it sweeps from left t o right . The r am p a lsotun es the optical filter so tha t its r esona nt wavelength is propor-tiona l to the h orizont al position. A tr ace of optical power ver suswavelength resu lts. The displayed width of each mode of the laseris a fun ction of the spectra l resolut ion of the wa velength -tu na bleoptical filter.

Figure 2.S impl i fied op t ica lspec t rum ana lyzerb lock d iagram.


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In te r fe rome ter-based o p t ica l spec t rum a na lyzersFabry-Perot in te r ferometersThe Fabry-Perot interferometer, shown in figure 3, consists of twohighly reflective, par allel mirrors t ha t a ct as a r esona nt cavitywhich filters the incoming light. The resolution of Fabry-Perot-interferometer-based optical spectr um an alyzers, dependent on thereflection coefficient of th e mir rors a nd th eir spa cing, is typicallyfixed, and th e wavelength is varied by changing th e spacingbetween the mirr ors by a very small amount.

The advan ta ge of the F abry-Perot interferometer is its very nar rowspectra l resolution, which a llows it to mea sur e laser chirp. Themajor disadvanta ge is th at a t an y one position mu ltiplewavelengths will be passed by the filter. (The spacing betweenthese responses is called th e free spectr al ra nge.) This problem can

be solved by placing a m onochr omator in cascade with th e Fa bry-Perot int erferometer to filter out all power outside th e interfer-ometer's free spectral ra nge about t he wavelength of interest .

Figure 3. Fabry-Perot-interferomete r-based op tical spectrum a nalyzer.

Michelson in ter ferometersThe Michelson interferometer, shown in figure 4, is based oncreating an interference pattern between the signal and a delayedversion of itself. The power of th is interference patt ern is measu redfor a ra nge of delay values. The r esulting waveform is theau tocorrelation function of the inpu t signal. This ena bles theMichelson-interferometer-based spectru m an alyzer to ma ke direct

measu remen ts of coherence length, a s well as very accura tewavelength mea sur ements . Other t ypes of optical spectru man alyzers cannot ma ke direct coheren ce-length m easur ements .

To determine t he power spectra of the input signal, a Fouriertra nsform is perform ed on th e au tocorrelation waveform. Becau seno rea l filterin g occur s, Michelson-inter ferometer-based opt icalspectru m an alyzers cannot be put in a spa n of zero na nometers,which would be useful for viewing the power at a given wave-length a s a fun ction of time. This type of an alyzer a lso tends toha ve less dynamic ran ge tha n diffraction-grat ing-based opticalspectrum analyzers.


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Figure 4.Michelson-interferometer-basedoptical spectrum analyzer.

Diffract ion-grat ing-based op t ica l spec t rumanalyzers

Figure 5.Concept of prism-based optical spectrum analyzer.Di ffrac t ion gra t ings a re used ins tead of p r i smsbecause d i ffrac t ion gra t ings provide grea te r

separa t ion among wavelengths of ligh t .

Diffra ction gra tings ar e used inst ead of prisms becau se th ey pro-vide a great er separ at ion of wavelength s, with less at tenu at ion.This a llows for better wavelength r esolution.

A diffraction grating is a mirror with grooves on its surface, asshown in figure 6. The spacing between grooves is extremelyna rrow, approximately equal to the wa velength s of interest . Whena pa rallel light beam st rikes th e diffraction gra ting, the light isreflected in a number of directions.

The most common optical spectrum an alyzers u se monochroma torsas the tunable optical filter. In the monochromator, a diffractiongrat ing (a m irror with finely spaced corr ugat ed lines on t he su rface)separa tes th e different wavelengths of light. The r esult is similar totha t achieved with a prism. Figure 5 shows wha t a pr ism-basedoptical spectr um an alyzer might look like. The prism separ at es thedifferent wavelengths of light, an d only th e wavelength tha t pa ssesthr ough the aper tu re reaches th e photodetector. The an gle of th eprism deter mines th e wavelength t o which the optical spectru manalyzer is tuned, and the size of the aperture determines thewavelength r esolution.


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The first r eflection is called th e zero-order bea m (m=O), an d itreflects in the same direction as it would if th e diffraction gr atin gwere replaced by a plane mirr or. This beam is not separ ated int o

different wavelengths a nd is n ot used by th e optical spectrumanalyzer.

The first-order beam (m=l) is creat ed by th e const ructive interfer-ence of reflections off each gr oove. For cons tr uctive int erferen ceto occur, the pa th -lengt h differen ce between r eflections fromadjacent grooves, must equ al one wavelength . If the input lightcont ains m ore tha n one wavelength component, t he beam willha ve some a ngular dispersion; th at is, the r eflection an gle for eachwavelength mu st be different in order to satisfy th e requiremen ttha t the pat h-length difference off adjacent grooves is equal to onewavelength. Thus, the optical spectru m an alyzer separ at esdifferent wavelengths of light.

Figure 6.The d i ff rac t ion gra t ing sep ara tes theinput beam in to a number of ou tputbeams. Wi th in each output beam ,except the zero order beam, d i ffe ren twavelengths a re separa ted .

For the second-order beam (m=2), the path-length difference fromadjacent grooves equals two wavelengths. A th ree wavelengthdifference defines t he t hird-order beam, a nd so on.

Optical spectru m an alyzers ut ilize multiple-order beam s t o covertheir full wavelength ra nge with na rr ow resolut ion.

Figure 7 shows the operation of a diffraction-grating-based opticalspectru m a na lyzer. As with t he pr ism-based a na lyzer, the diffra ctedlight pa sses thr ough an a pertu re to the ph otodetector. As thediffra ction grat ing rotates, the inst ru ment sweeps a ra nge of wavelengths, allowing th e diffra cted light -- th e par ticular wa ve-length depends on the position of the diffraction grating -- to passthr ough to the a pertu re. This technique a llows th e coverage of awide wavelength ra nge.


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Figure 7.Diffraction-grating-basedoptical spectrum analyzer.

Sing le Monochromato rDiffra ction-grating-based optical spectru m an alyzers cont ain eithera single monochromator, a double monochromator, or a double-passmonochromator. Figure 8 shows a single-monochromator-basedinstrument. In these instruments, a diffraction grating is used toseparate the different wavelengths of light. The second concavemirror focuses the desired wavelength of light a t t he ap ertu re.The apert ur e width is variable and is used to determ ine the wave-length resolut ion of the inst rum ent.

Figure 8.S ingle -monochromator-based opt ica l spec t rumanalyzer.


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Doub le Monochromato rDouble monochr omators, such a s sh own in figure 9, a re sometimesused to improve on th e dyna mic ran ge of single monochromator

systems. Double monochr omators a re equivalent to a pair of sweep-ing filters. While this technique impr oves dynam ic range, doublemonochr oma tors typically have reduced span width s due t o thelimita tions of monochr oma tor-to-monochr omator tu ning ma tch;double monochr omators a lso have degraded sen sitivity du e tolosses in t he m onochr omators.

Figure 9.Double-monochromator-based opt ica l spec t rumanalyzer.

Aperture


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Double-Pass MonochromatorHewlett-Packar d's H P 71450B/1B/2B optical spectru man alyzers u se a un ique wavelength-selection scheme -- th e double-

pass monochromator. The double-pass monochromator providesthe dyn amic-ran ge advant age of the double monochr omator an dthe sensitivity a nd size advan ta ges of the single monochr omator.Figure 10 shows th e double-pass monochr omator.

Figure 10.Block d iagram of double-pass -monochromator op t ica lspec t rum ana lyzer.

Wavelen gth Sele ct ive Fi l ter ingThe first pass t hrough t he double-pass m onochromator is similar toconvent iona l single monochroma tor systems. In figure 10, th e inputbeam (1) is collimated by the optical element an d dispersed by t hediffra ction gra ting. This results in a spat ial distribution of th e

light, based on wavelength. The diffraction grating is positionedsuch tha t t he desired wavelength (2) passes through th e apertu re.The width of the a pertu re determines t he bandwidth of wavelengthsallowed to pass t o the detector. Various aper tu res a re a vailable toprovide resolution ba ndwidth s of 0.08 nm a nd 0.1 nm to 10 nm in a1, 2, 5 sequence. In a single-monochr omator instr umen t, a largephotodetector behind th e apert ur e would detect the filtered signal.


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The Second PassThis system shown in figur e 10 is unique in th at th e filtered light(3) is sent th rough th e collimat ing element an d diffra ction grat ing

for a second t ime. During th is second pass t hr ough th e monochr o-mat or, th e dispersion p rocess is reversed. This creates an exactreplica of th e input signal, filtered by th e apert ur e. The smallresu ltan t ima ge (4) allows th e light to be focus ed onto a fiber whichcarries th e signal to th e detector. This fiber acts a s a secondapert ur e in the system. The implementa tion of this second pa ssresults in th e high sen sitivity of a single monochroma tor, th e highdynamic range of a double monochromator, as well as polarizationinsensitivity (due to the half-wave plate). This process is discussedmore completely in Chap ter 2.


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Chapter 2Diffract ion-Grat ing-Based Opt ica lSpec t rum Analyzers

Operation and Key Specifications

Wavelen gth Tunin g a nd Repea tab i l ityTuningThe wavelength t un ing of the optical spectrum an alyzer is con-trolled by th e rotat ion of the diffra ction grat ing. Each a ngle of thediffraction grating causes a corresponding wavelength of light to befocused directly at t he center of the apert ur e. In order t o sweepacross a given span of wavelengths, th e diffraction gra ting isrotat ed, with the initial a nd final wavelength s of th e sweep deter-mined by th e initial and fina l angles. To provide accurat e tu ning,the diffra ction-grating a ngle mu st be precisely controlled an d veryrepeat able over time.

Tunin g Tech nique sConventional optical spectrum an alyzers u se gear r eduction sys-tems t o obtain the required a ngular resolution of the diffra ctiongrating.

To overcome problems associated with gear driven systems,Hewlett-Packar d optical spectrum an alyzers h ave a direct-drivemotor system which provides very good wavelength accuracy(1 n m), wavelength reproducibility a nd repeat ability (0.005 n m),and fast t uning speed.

Wavelen gth Re peatabi l i ty vs . Wavelen gth Re producibi l i tyWavelengt h r eprodu cibility, as defined for m ost optical spectr uman alyzers, specifies wavelength tun ing drift in a one-minu te per iod.

This is specified with t he optical spectrum an alyzer in a continuoussweep mode and with n o cha nges made to the tu ning.

In addition t o wavelength reproducibility, H ewlett-Pa ckard speci-fies an additional par amet er: wavelength repeat ability. Wave-length r epeata bility is the a ccura cy to which t he optical spectruman alyzer can be retu ned to a given wavelength after a change intuning.

Wavelen gth Reso lu t ion Ban dw id thFull Width at Half Maximu mThe a bility of an optical spectrum an alyzer t o display t wo signalsclosely spaced in wavelength as two distinct responses is d eter-mined by t he wa velength resolution. Wavelength r esolution is, intur n, deter mined by th e bandwidth of th e optical filter, whose keycomponent s ar e th e monochr omator a pertu re, photodetector fiber,input image size, and qu ality of the optical components.


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The wavelength r esolution is specified as t he filter bandwidth atthe ha lf-power level, referr ed to as full width at ha lf maximu m.This is a good indication of the optical spectrum analyzer's ability

to resolve equal amplitude signals. The HP 71450B/1B/2B opticalspectru m a na lyzers h ave selecta ble filters of 0.08 nm a nd 0.1 nm to10 nm in a 1, 2, 5 sequ ence, which ma ke it possible to selectsut ticient resolution for most mea sur ement s.

Figure 11 shows thr ee spectra l component s of a F abry-Perot lasermeasu red with t hr ee different r esolution bandwidths. In ea ch case,the a ctual spectra l width is mu ch less tha n th e resolution band-width. As a resu lt, each response shows the filter sha pe of theoptical spectru m an alyzer's r esolution-bandwidth filter. The ma incomponent of th e filter is th e apert ur e. The physical width of thelight beam at t he aper tur e is a fun ction of th e input ima ge size.If th e physical width of th e light beam a t th e apert ur e is narr owcompa red to the a pertu re itself, the response will have a flat top,as sh own in figure 11 for the 0.5 nm resolut ion ba ndwidth . Thisoccurs as t he na rrow light beam is swept a cross the aper tur e. Thena rrower resolut ion-bandwidth filters r esult in a r oun ded responsebecau se the ima ge size at th e apert ur e is similar in size to th eapert ur e. Each r esponse onscreen is th e convolut ion of the aper-tur e with th e optical image.

Figure 11.Three Fabry-Perotlaser spec t ra lcomponents , eachmeasured wi th adifferent resolutionbandwidth .

Dynam ic R a ngeBased on F i l te r Shape Fac to rFor man y measur ement s, the various spectral component s to bemeasu red ar e not equal amplitude. One such examp le is the

measu remen t of side-mode suppr ession of a distributed feedback (DFB) laser, as shown in figure 12. For th is measur ement , thewidth of th e filter is n ot th e only concern. Filt er sh ape (specified interm s of dynam ic ra nge) is also importan t. The adva nta ge of double monochr omators over single monochr omators is t ha t double-monochr oma tor filter skirt s ar e much st eeper, and t hey allowgreater dyn amic range for th e measur ement of a sma ll spectra lcomponent located very close to a large spectral component. Thedouble-pass m onochr omator ha s th e same dyna mic-rangeadvan tages as th e double monochromator.

Dynamic range is commonly specified at 0.5 nm and 1.0 nm offsetsfrom t he m ain response. Specifying dynamic ran ge at th ese offsets

is driven by th e mode spacings of typical DFB la sers . A 60 dBdynam ic-range specificat ion a t 1.0 nm a nd grea ter indicates th atthe optical spectrum an alyzer's response to a pu rely monochromaticsigna l will be 60 dBc or less at offsets of 1.0 nm an d grea ter. Inaddition to the filter shape factor, this specification is also anindicat ion of th e str ay light level and th e level of spuriousresponses within th e ana lyzer.

Figure 12.DFB Laser s ide modesuppress ionmeasurement .


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Single monochromators t ypically have sen sitivity a bout 10 to 15 dBbetter t ha n th at of double monochromators due to the a dditiona lloss of th e second d iffraction gra tin g in double monochr omat ors.

The double-pass m onochr omator ha s th e sam e high sensitivity of single monochr omators even t hough t he light st rikes th e diffra ctiongrat ing twice. The high sensitivity is ma de possible by the h alf-wave plate an d the u se of a sma ller photodetector th at h as a lowernoise equivalent power (NE P). The sen sitivity improvement fromthe half-wave plate is discussed in the section, PolarizationInsensitivity," later in t his chapt er.

Sensitivity can be set directly on Hewlett-Packard optical spectruman alyzers, which then au tomatically adjust t o optimize the sweeptime, while ma inta ining th e desired sen sitivity. Sensitivity iscoupled directly to video bandwidth, as shown in figure 15. As thesens itivity level is lowered, t he video band width is decreased (orthe t ra nsimpedan ce amplifier gain is increased), which r esults in alonger sweep time, since th e sweep t ime is inversely proport iona lto the video bandwidth. Th e sweep time can be optimized becau sethe video bandwidth is contin uously variable and just enough videofiltering can be performed. This avoids the problem of smallincreases in sensitivity cau sing large increases in sweep time,which can occur wh en only a few video bandwidths are available infairly large st eps.

Figure 15. Video ban dwid th directly affects sens it ivi ty.

Tu n i n g SpeedSwe ep-Time LimitsFor fast sweeps, sweep time is limited by th e maximum tu ning ra teof th e monochroma tor. The direct-drive-motor system allows forfast er sweep rat es when compa red with optical spectr um a na lyzerstha t u se gear-reduction systems to rotate t he diffraction gra ting.

For high-sensitivity sweeps tha t t end t o be slower, the sma ll photo-detector an d cont inuously variable digital video bandwidth s a llowfor faster sweep t imes. The sma ll photodetector r educes the sweeptime becau se it has a lower NEP th an t he large photodetectors used


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in oth er optical spectru m an alyzers. Lower NEP mean s th at for agiven sensitivity level, a wider video bandwidth can be used, whichresults in a faster sweep. (Sweep time is inversely proportiona l to

the video bandwidth for a given spa n a nd r esolution bandwidth .)

The cont inuously variable digital video bandwidth s improve thesweep time for h igh-sensitivity sweeps in two ways. First, t heimplementa tion of digita l video filtering is fast er t ha n the responsetime required by na rrow an alog filters dur ing aut oranging. Second,since the video bandwidth can be selected with great resolut ion,

just enough video filtering can be employed, resulting in noun necessary sweep-time pena lty due to using a nar rower videobandwidth th an is required. Figure 16 shows a 20 second filter-response measu remen t. This filter, for a n E rbium am plifier, wasstimula ted by a white-light source, and figur e 16 shows the norma l-ized response. The purpose of this filter is t o attenu at e light a t t hepum p wavelength, while passing th e am plified laser output of 1550nm . Due to the low power level of whit e-light sources, this mea -sur ement r equires great sensitivity, which tr aditionally ha sresulted in long sweep times.

Figure 16.Improved sweep t imes ,even for h igh sens i t iv i ty

measurements tha ttradit ionally result ins low sw eeps . This p lo tshow s the normal izedoutput o f an Erb iumamplifier f i l ter that w ass t imula ted by a whi te -l igh t source . Autoranging Mode

Autoranging m ode is a ctivat ed au tomat ically for sweeps withamplitude ranges greater th an about 50 dB. The am plitude ra nge isdeterm ined by the t op of the screen an d th e sensitivity level set bythe u ser. With t he au toran ging mode activated, when th e signalamp litu de crosses a th reshold level, the sweep pau ses, thetra nsimpedan ce amplifier's gain is cha nged to reposition the signa lin the mea sur ement r an ge of the a na lyzer's int erna l circuitry, andthe sweep cont inues. This repositioning explains th e paus e tha tcan occasionally be seen in a sweep with a wide measur ementrange.

Chopper ModeThe m ain pur pose of the chopper mode is to provide sta ble sensitiv-ity levels for long sweep times , which could oth erwise be a ffected bydrift of the electronic circuitr y. The des ired st ability is achieved byautomatically chopping the light to stabilize electronic drift insweeps of 40 seconds or great er. The effect is to samp le th e noisean d stra y light before each tra ce point a nd subt ra ct them from th etra ce point reading. In all modes of operat ion, H ewlett-Pa ckard

optical spectrum analyzers zero the detector circuitry before eachsweep.

Improved dynamic range is an oth er benefit of samp ling th e stra ylight before each tr ace point. For measu remen ts requ iring thegreatest dynam ic ran ge possible, some improvement can beobtain ed with th e use of the chopper mode. While th is mode doesimprove dynam ic ra nge, it is not required for th e ana lyzers to meettheir dynam ic range specificat ions.

Figure 17 shows the impr oved dynam ic ra nge obtained byactivating th e chopper m ode.


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Figure 17.Dynamic range improvementf rom choppe r mode .

Polar iza t ion Insens i t iv i tyPolar iza t ionAccordin g to electr omagn etic th eory, electric- and ma gnet ic-fieldvectors mu st be in the plane perp endicular to the d irection of wavepropagation in free space. Within this plane, the field vectors canbe evenly distr ibuted in all directions an d pr oduce un polar izedlight. A surface emitting LED provides a good illustration of thephen omena . The electric field, however, can be oriented in onlyone direction, as with a las er. This is called linear polar ization an dis shown in figure 18. Altern at ively, th e electr ic field can r otat e by360 degrees within one wavelength, such as with th e vector sum of

two orthogonal linearly polarized waves. Circular polarization isthe t erm t ha t describes two ort hogona l waves that ar e of equalamplitude.

Figure 18.Linear and circular polarization


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Cause of Polar iza t ion Sen si t iv i tyPolarization sensitivity results from the reflection loss of thediffraction grating being a function of the polarization angle of the

light t ha t st rikes it. As the polarization a ngle of the light varies,so does the loss in the monochromator. Polarized light can bedivided int o two component s. The component par allel to thedirection of th e lines on th e diffraction gra ting is often labeled Ppolar ization a nd t he component perpendicular t o the direction of the lines on the diffraction grating is often labeled S polarization.The loss at the diffraction grating differs for the two differentpolar izations, a nd each loss var ies with wa velength. At ea chwavelength, the loss of P polarized light and the loss of S polarizedlight r epresent the m inimum an d ma ximum losses possible forlinearly polarized light. At some wavelengths, the loss experiencedby P polarized light is greater tha n t ha t of S polarized light, whileat other wavelengths, th e situa tion is reversed. This polar izat ionsensitivity results in an amplitude uncertainty for measurementsof polarized light and is specified as polarization dependence.

Solut ion to Polar iza t ion Sensi t iv i ty ProblemTo reduce polarization sensitivity, a half-wave plate has been placedin th e path of th e optical signal between th e first a nd second pa ssin t he double-pass monochr omator, as sh own in figure 19. This ha lf-wave plate rotates th e component s of polar ization by 90 degrees.The resu lt is tha t t he component of polarization tha t r eceived themaximu m at tenu ation on th e first pass will receive the minimumatt enua tion on th e second pass, and vice versa .

Figure 19.Half-wa ve plate in

the double-passmonochromatorreduces po la r iza t ionsens i t iv i ty andimproves ampl i tudesensit ivi ty.


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The r esult is r educed polarization sensitivity, as t he t otal loss is th eproduct of the minimum an d ma ximu m losses, regardless of polarization. Also, because the monochromator is polarization

insensitive, the monochroma tor output of th e HP 71451B is alsopolarization insensitive. Other polarization-sensitivity-compensa-tion techn iques are curren tly in u se, but none have a m onochr oma-tor output tha t is polarization insen sitive. This monochr oma toroutput allows th e monochr omator portion of the optical spectruman alyzer to be us ed as a preselector filter for other s igna l-process-ing applications.

Improved am plitude sen sitivity over double monochroma tors isan other ben efit of the h alf-wave plat e. This improved sensitivity isbecau se the signa l polarization can never hit t he ma ximum lossangle twice, as can occur with a double monochromator. Thisbenefit, along with t he low NEP of th e photodetector, givesHewlett-Packar d optical spectrum an alyzers t he h igh sen sitivity of single-monochr omator-based a na lyzers while ma intain ing th e highdynamic range of double-monochromator-based analyzers.

Input Coupl ingVariety o f Inp ut Conn ect ors Availa bleAt th e inpu t of Hewlett-Packard optical spectru m ana lyzers is ashort, straight piece of 62.5 m core-diameter graded-index fiber.Connection to this fiber is made using one of the interfaces listedbelow. The inpu t en d of th is fiber is flat . The other end of this fiber,in the monochromator, is angled to help minimize reflections.

Hewlett-Packar d optical spectrum an alyzers u se user-exchangeableconnector interfaces, which allow easy cleaning of the analyzer'sinput connector as well as th e u se of different conn ector t ypes withthe same analyzer. Available connector interfaces include FC/PC,D4, SC, Diam ond H MS-10, DIN 47256, Biconic, an d ST.


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Chapter 3Light Emi t ting Diode sand Sem i c o n duc to rDiode Lasers

This chapt er describes th e operation of light emitt ing diodes(LEDs) an d semicondu ctor diode lasers a nd describes th eirpar amet ers which are commonly measur ed with optical spectr um

analyzers.

Hewlett-Packar d optical spectru m an alyzers ha ve built-in measu re-ment r outines tha t a re designed to measure a utomatically manypar amet ers of LEDs, Fabr y-Perot lasers, an d distributed feedback (DFB) lasers. These au tomatic measur ement r outines are discussedin this chapter.

Light Emi t t ing Diode s (LEDs)Light emitting diodes produce light with a wide spectral width , andwhen used in fiber optic commun ication systems, t hey can be modu-lated at frequencies up to about 200 MHz. LEDs ha ve the advan -tages of low tem perat ure sensitivity a nd n o sensitivity to back reflections. Additionally, the incoherent emitted light is not sensi-tive to optical inter ference from reflections.

A light emitting diode genera tes light by sponta neous emission.This occurs when an electr on in a h igh energy conduction ba ndchanges to a low energy valence band, a s sh own in figure 20.The ener gy lost by th e electr on is released as a photon. For a givenmat erial, discrete en ergy levels repr esent t he different orbitalsta tes of the electron. The energy of th e released photon is equal tothe en ergy lost by the electron, an d th e wavelength of the emittedphoton is a function of its energy. As a result, the wavelength of thephoton is determined by the ma terial used to make th e LED.

Figure 20.Spontaneous emiss ion .

Movement of e lec t ronsf rom con duct ionband to va lence banddur ing recombina t ion .


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The difference in energy between t he condu ction ba nd a nd t hevalence band is called the ba ndgap en ergy (E.) and is expressed inun its of eith er joules or electr on volts (eV). The wavelengt h of the

emitted photon is determ ined by th e bandgap energy as shown:

h . C 1.24 m = = E g E g (eV)

Where h (Planck's constant) is equal to 6.62 * 10 34 Ws 2, c (Spe ed of light) is 2.998 * 10 8 m/s, and E g (bandga p ener gy of the m at erial) isexpressed in u nit s of joules. Altern at ively, E g can be expressed inunits of electron volts. (1 electron volt is equal to 1.6022*10 19 joule.)

These condu ction-band electrons ar e generat ed by a forwar d biasplaced on th e p-n jun ction of the diode. A forwar d-biased p-n

jun ction is shown in figure 21. The mat erial on th e n-layer side of the junction ha s immobile positive charges evenly distributedthr oughout the layer, with m obile negat ive charges, or electr ons,resp onsible for electr ical cur ren t flow. Conversely, th e ma ter ial onthe player side of the junction ha s immobile negat ive cha rgesevenly distribut ed t hroughout th e layer, with mobile positivelychar ged holes, actua lly locations of missin g electr ons, respons iblefor electr ical curr ent flow.

At the jun ction, th e mobile electr ons from th e n-layer a nd t hemobile holes from th e player r ecombin e, producing photons. Wh ileLEDs in use today actually consist of multiple layers of semi-condu ctor mat erial, rath er tha n just th e two shown in figure 21,the light-genera tion process is the sa me.

Figure 21.Diagram of forwardbiased p-n junctionshow ing the loca t ionof immobi le chargesand mobi le cur ren tcamers .

Figure 22 sh ows the spectrum of a light emitting diode. As can beseen, this pr ocess results in a broad distribut ion of wavelengthscent ered about t he wavelength calculat ed by the above equa tion.The spectra l width is often specified by the full width a t h alf ma ximum (half-power points of the spectr um ). Typical values forfull width at ha lf maximum ra nge from 20 nm to 80 nm for LEDs.

Figure 22.Spec t rum of l igh t emi t t ingdiode .


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Including those parameters mentioned above, there are manypar amet ers of light-emitting diodes tha t a re commonly measur ed.These parameters can be automatically measured as shown in

figure 23. Some param eters (such as mea n wavelength an d spectra lwidth) have two methods by which t hey can be measu red. Onemeth od ta kes into account t he ent ire spectru m, while the othertakes into account only a few points of the spectrum. The definitionof each pa ra met er is described below.

Total Power - The su mma tion of th e power at each tra ce point ,norma lized by th e ra tio of the tr ace point spacing/resolut ionbandwidth . This norm alization is required becau se the spectrum of the LED is cont inuous, rat her th an containing discrete spectralcomponents (as a laser does).

N Trace point spacingTotal Power = P i ( ) = P oi=1 Resolution bandwidth

Mean (FWHM ) - This wavelength repr esents t he center of ma ss of the t ra ce points. Th e power an d wavelength of each tra ce point areused to calculat e the m ean (FWHM) wavelength.

N Trace point spacingMean (FWHM) = = P i ( ) i /P o

i=1 Resolution ban dwidth

Sigma - An r ms calculation of th e spectral width of the LED ba sedon a Gau ssian distr ibution. The power and wavelength of eachtra ce point are u sed to calculate sigma.

N Trace point spacingSigma = = P i ( ) ( i - )2/P o

i=1 Resolu t ion bandwidth

FWHM (Full Width at Half Maximum) - Describes th e spectralwidth of the h alf-power points of the LED, a ssum ing a continuous,Gaussia n power distribut ion. The h alf-power points a re th osewhere t he power-spectr al density is one-ha lf th at of the peak amplitude.

FWH M = 2.355 * Sigma

Figure 23.Resul t s o f au tomat icLED measurement

rout ine .


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3 dB Width - Used to describe the sp ectr al width of the LED ba sedon the separ ation of the t wo wavelengths th at each have a power-spectr al dens ity equal t o one-half the peak power-spectral den sity.The 3 dB width is determ ined by finding the pea k of th e LEDspectr um, a nd dr opping down 3 dB on each side.

Mean (3 dB ) - The wavelength th at is th e average of th e twowavelengths determined in the 3 dB width m easurement.

Peak Wavelength - The wavelength a t wh ich t he peak of th e LED'sspectr um occur s.

Density (1 nm) - The power-spectral density (normalized to a 1 nmband width) of th e LED at th e peak wavelength .

Distribution Trace - A tr ace can be displayed th at is based on thetotal power, power distribut ion, an d mea n wa velength of th e LED.This trace has a Gaussian spectral distribution a nd r epresents aGaussian a pproximation t o the measur ed spectrum .

Fabry-Pe ro t LasersLasers are capa ble of producing high out put powers a nd directiona lbeams. When used in fiber-optic communication systems, semi-condu ctor lasers can be modulated a t ra tes up t o about 10 GH z.However, lasers ar e sensitive to temperat ure an d back reflections.Additionally, the coherent emitted light is sensitive to opticalinterference from reflections.

Of the two laser t ypes discussed in t his chapter, the F abry-Perot isthe simpler. It is, however, more susceptible to chromatic dispersionwhen used in fiber-optic systems because it h as a wider spectra lbandwidth.

A Fabr y-Perot laser differs from a light -emittin g diode in t ha t itgenerat es light mainly by stimulat ed emission. Some of th e photonsar e generat ed by spont an eous emission, as described for the LE D.But t he ma jority of th e photons a re generat ed by stimulatedemission, where photons trigger additional electron-holerecombinat ions, resu lting in additional photons as shown infigure 24. A stimu lated ph oton tr avels in t he sa me direction a ndhas t he same wavelength and phase as t he photon th at tr iggeredits generation.


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Figure 24.S t imula ted emiss ionis the re lease of aphoton , due to anelectron-holerecombina t ion ,t r iggered by anoth er

photon .

Stimu lated emission can be t hought of as the amp lification of light(laser is an acronym for light amplification by stimulated emissionof rad iation). As one ph oton p asses t hr ough th e region of holes an dcondu ction band electrons, ad ditiona l photons a re genera ted. If themat erial were long enough, enough ph otons might be genera ted toproduce a significan t a mount of power a t a single wavelength.

An easier wa y to build up power is t o place a reflective mirror a teach end of th e region just d escribed so th at th e photons tra velback and forth between the m irrors, building up th e num ber of photons with ea ch t rip. These mirrors form a r esona tor, which isa requ irement for laser operat ion.

Laser operation has two additional requirements. One requirementis tha t for st imulated em ission t o occur, a great er nu mber of condu ction-band electr ons th an valence-ban d electr ons mu st bepresent . This is called a population inversion. It is a chieved byforcing a h igh curren t den sity in t he a ctive layer of th e diodestru cture. The second requirement is th at t he gain exceeds thelosses due to absorption and r adiat ion. Pa rt of th e radiat ion lossesis the amount of light released at t he laser output . As the curren tincreases, the gain increases. The curren t for wh ich st imulat edemissions occur is the thr eshold curr ent of th e laser.

The r esonat or is often just highly reflective, cleaved su rfaces onthe edges of the diode. As th e light reflects between th e mirr ors,the ph otons of a given wavelength must be in phase t o add con-stru ctively. The resona tor acts as a Fa bry-Perot interferometer,as sh own in figur e 25, in t ha t only that light for wh ich t he resona-tor spacing is an int egral nu mber of half wavelengths will addconst ru ctively. As a r esult, th e spectrum of a F abry-Perot lasercont ains multiple discrete-wavelength component s, as shown infigur e 26.


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Figure 25. Reflectivesur faces a t the edges of l aser d iode ac t as aFabry-Perot typeresonator.

The possible wavelengths pr oduced by the resonator a re given by:

m cf r es =

2 l n

Wh er e m = in teger

c = speed of light

l = length of cavity

n = refactive index of cavity

The actua l outpu t power at each of these wavelengths is determ inedby the laser gain a nd m irror reflectivity at tha t wavelength. As withthe LE D, the center wavelength can be determined from th e bandgapenergy. The separ at ion between th e different wavelength s, modespacing, can be determ ined from th e separa tion of the mirrors a sfollows:

c 2Mode Spa cing = (Hz) = (m)

2 1 n 2 1 n

Many of the commonly measu red pa ram eters of Fa bry-Perot lasersar e discussed a bove. As with t he LE D, Hewlett-Packard opticalspectru m a nalyzers ha ve an a utomatic measurement r outine forFa bry-Perot lasers. The resu lts from the F abry-Perot lasermeasu remen t r out ine ar e shown in figure 27. The followingparam eters are often of interest and are m easured by theautomatic routine.

Figure 26.Spec t rum of Fabry-Perotlaser.

Figure 27.Resul t s o f au tomat icFabry-Perot lasermeasurement rou t ine .


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peak m ust r ise, an d then fall, by at least t he peak excursion valueabout a given spectral component. Set ting th e value too high willresult in failure to include the sma ller responses nea r th e noise

floor. Settin g th e value t oo low will cau se all spectra l component sto be accepted, but un want ed responses, including noise spikes an dthe second peak of a response with a slight dip, could beerroneously included.

Peaks Fun ction - The peaks function displays a vertical line fromthe bottom of the grid t o each counted spectra l component of th esignal. This fun ction is u seful to determ ine if an adjustm ent of th epeak excursion value is required.

Distribution Trace - A tra ce is displayed th at is based on the t otalpower, individual wavelengths, mean wavelength, an d modespacing of the laser. This tr ace has a Gaus sian spectra l distributionan d represent s a continuous appr oximat ion to the actua l, discretespectrum.

Dis t r ibu ted F ee dback (DFB) LasersDistributed feedback lasers a re similar to Fa bry-Perot lasers,except that all but one of their spectral components are significantlyreduced as sh own in figure 28. Becau se its spectrum ha s only oneline, th e spectra l width of a distr ibuted feedback laser is mu ch lesstha n t ha t of a Fabr y-Perot laser. This greatly redu ces th e effect of chroma tic dispersion in fiber-optic system s, allowing for grea tertran smission bandwidths.

Figure 28.Spec t rum of ad is t r ibu tedfeedback laser.


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Figure 29.

Dis t r ibu ted feedbacklasers use a se r ies of reflecting ridges toreduce the ampl i tudeof al l but one of thespec t ra l componentsof the lase r.

The distribut ed feedback laser u tilizes a gra ting, a series of corrugated ridges, along the active layer of the semiconductor, asshown in figure 29. Rath er th an using just th e two reflectingsur faces at t he ends of the diode, as a F abry-Perot laser does, the

distributed feedback laser uses ea ch ridge of the corr ugat ion a s areflective surface. At the resonant wavelength, all reflections fromthe different ridges add in pha se. By having much sma ller spa cingsbetween th e resona tor elements, compared to the F abry-Perotlaser, th e possible resona nt wa velengths a re much farth er apa rt inwavelength, an d only one r esona nt wavelength is in th e region of laser gain. This results in th e single laser wavelength .

The ends of th e diode still act as a resonator, however, an d pr oducethe lower a mplitude side m odes seen in figur e 28. Ideally, th edimensions a re selected so tha t t he end r eflections add in ph asewith t he gra ting r eflections. In th is case, th e ma in mode will occurat a wa velength ha lfway between th e two adjacent side modes; an y

deviation is called a mode offset. Mode offset is mea sur ed as t hedifference between the m ain-mode wavelength a nd t he a veragewavelength of the two adjacent s ide modes.

The am plitude of the largest side mode is typically between 30 a nd50 dB lower th an th e main sp ectra l out put of th e laser. Becau seside modes ar e so close to th e ma in mode (typically between 0.5 nman d 1 nm ) the dynam ic ra nge of an optical spectrum an alyzerdeterm ines its ability to measur e th em. Dynam ic ran ge is specifiedat offsets of 0.5 nm a nd 1.0 nm from a large response. Hewlett-Pa ckard optical spectr um an alyzers, for exam ple, specify a dynam icrange of 55 dBc at offsets of 0.5 nm and greater, and 60 dBc atoffsets of 1.0 nm and greater. This indicates the amplitude level of side modes th at can be detected at t he given offsets.

As with th e LED an d Fabr y-Perot laser, Hewlett-Packar d opticalspectrum ana lyzers h ave an aut omat ic measurement routine fordistributed feedback lasers. The r esults from the DFB lasermeasu remen t r outine ar e shown in figure 30. The following para m-eters ar e often of interest a nd ar e measur ed by the au tomat ic routine.

Figure 30.Resul t s o f au tomat ic DFBlaser measurement rou t ine .


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Peak Wavelength - The wavelength a t which the main spectra lcomponent of the DFB laser occurs.

Side Mode Suppression Ratio (SMSR) - The amplitude differencebetween th e main spectral component a nd t he largest side mode.

Mode Offset - Wavelength separ ation (in n an ometers) between th emain spectra l component an d th e SMSR mode.

Peak Am plitude - The power level of the main spectral componentof the DFB laser.

S top Band - Wavelength spa cing between t he u pper a nd lower sidemodes adjacent t o the ma in mode.

Center Offset - Indicat es how well the m ain m ode is centered in t hestop band. This value equals the wavelength of the m ain spectralcomponent m inus th e mean of th e upper an d lower stopbandcomponent wavelengths.

Bandwidth - Measur es the displayed bandwidth of th e mainspectra l component of th e DFB La ser. The am plitu de level, relativeto the peak, that is used to measure th e bandwidth can be set bythe user. In figure 30, th e amp litu de level used is 20 dBc. Due tothe n ar row line width of lasers, the r esult of this m easur ement foran u nm odulated laser is strictly dependen t upon th e resolution-bandwidth filter of the optical spectru m an alyzer. With modulationapplied, the r esultan t wa veform is a convolut ion of the a na lyzersfilter and the modulated laser's spectrum, causing the measuredbandwidth to increase. The combinat ion of th e modulated rea dingand unmodulated reading can be used to determine the bandwidthof the modulated laser an d th e presence of chirp.

Peak E xcursion - The peak excur sion value (in d B) can be set by t he

user a nd is used to determine which thr ee onscreen responses willbe accepted a s discrete spectral responses. To be counted, t he t racemust rise, and th en fall, by at least the pea k excur sion value abouta given spectral component. Set ting th e value t oo high will result infailure to count small responses near the noise floor.

Peaks Fun ction - The peaks function displays a vertical line fromthe bottom of the grid t o each counted spectra l component of th esignal. This fun ction is u seful to determ ine if an adjustm ent of th epeak excursion value is required.


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Re f e r e n ces

1. C. Hentschel, Hewlett -Pa ckard F iber Optics Ha ndbook,

January 1988.

2. C. B. Hitz, Understanding Laser Technology, Second Edition,PennWell Publishing Company, Tulsa OK 1991.


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AppendixOpt ica l and Microw ave Sp ec t rum Analyzers Compared

Key Funct ional BlocksThe key signal pr ocessing blocks of the H ewlett-Pa ckar d optical spectrum an alyzers are shown infigure 31. The apert ure is th e primar y resolution-ban dwidth filter, and it det ermines t he full-width-ha lf-maximum band width of the an alyzer. Secondary filtering is perform ed by th e coupling of theoptical signal onto the fiber. This filter ha s a wider bandwidth th an th e prima ry filter, but it is veryeffective at in creasing th e filter sha pe at offsets greater th an 0.3 nm from the full-width a t h alf-ma ximumpoints on th e resolution ba ndwidth filter. While the seconda ry filter ha s very little impact on t he full-width a t h alf-maximum band width, it does provide th e rejection at close offsets r equired to give thedouble-pass monochr omator t he high dyna mic ran ge of double monochr omators.

Following th e filters is t he ph otodetector, which acts as a power det ector on t he light signa l. Thephotodetector converts th e optical power t o an electr ical curr ent. This electr ical curr ent is converted to

a volta ge by th e tra nsimpeda nce amplifier. For the pu rpose of determ ining the int erna l noise level andsensitivity of th e optical spectrum an alyzer, the tr an simpedan ce am plifier is the m ain n oise source.The electrical signal is digitized after the transimpedance amplifier. The video bandwidth filter, whichhelps to deter mine t he sen sitivity, is implemened digita lly, an d th en t he conversion to logarith micam plitu de values is performed.

Figure 31 . Key s igna l p rocess ing b locks of the Hewle t t -Packard double-pass monochromator based opt ica lspec t rum ana lyzers .

Block Diagram Differenc esThe operat ion of optical spectru m an alyzers is very similar t o microwave spectr um an alyzers; howeverth ere a re some differences, especially in relationship t o the sensitivity of the a na lyzer. Figure 32 sh owsth e key signal-processing blocks of the H ewlett-Pa ckard optical spectr um an alyzers a nd th e equivalentblocks of a t ypical microwave spectr um an alyzer.

The order of the key signal processing elements is different, and this difference is most noticed in thesensitivity level of the analyzers. As can be seen in figure 32, the most significant source of internalnoise for the m icrowave spectru m a na lyzer is at th e front-end of th e instru ment , from th e inputatt enuator a nd m ixer to th e EF amplifiers. The resolution ban dwidth then determines the rm s value of the broadband intemal noise.


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Reducing th e resolut ion ba ndwidth reduces the inst ru ment noise level. The signa l is then converted t oa logarith mic scale by th e log am plifier an d t he en velope of tha t signa l is detected by the det ector. Thenoise signal seen onscreen is th is envelope of the original intern al n oise. As a result, t he r esolution

band width, which h ad chan ged the rm s value of th e original noise, cha nges th e average value of thedisplayed noise. The video band width filter th en det ermines th e peak-to-peak width of the displayednoise, without cha nging th e avera ge level.

Figure 32 . Key s igna l -process ing b locks of Hewle t t -Packard op t ica l spec t rum ana lyzers and a typ ica l microwavespec t rum ana lyzer.

The m ost significan t sour ce of int erna l noise for th e optical spectrum an alyzer comes after t heresolut ion ba ndwidth filters an d th e detector. The resolution ban dwidth h as n o direct effect on t heinternal noise level. Following digitization, the video bandwidth filter is applied to the internal noise.Since th is noise has not been affected by t he det ector, th e avera ge noise level is still 0 V. The videofilter in th e optical spectrum an alyzer affects t he rm s value of the int erna l noise but t he avera gerema ins 0 V. This is the sam e effect th at the r esolution bandwidth filter ha d on the int erna l noise atth at point in th e microwave spectru m a na lyzer. The filtered signal is then convert ed to a logarith micscale for display. The average value of the displayed internal noise is 0 W (because the noise sourcefollows the detector), which is equal to minus infinity dBm. As a result, the optical analyzer's noisefloor differs because, due to the envelope detector, the microwave spectrum analyzer has a non-zeroaverage n oise level. It is the pea ks of the noise floor tha t det ermine t he optical spectrum an alyzer'ssensitivity. The sensitivity is defined as 6 times the rms noise level. In order to keep the display frombeing too cluttered, the internal noise is clipped 10 dB below the sensitivity point.

In su mma ry, microwave spectr um an alyzers h ave a n on-zero average noise level th at is determined byth e resolut ion ban dwidth, an d th e displayed width of th e noise is determined by th e video ban dwidth.The sen sitivity of th e microwave spectrum an alyzer is defined as th e avera ge noise level. Opticalspectr um an alyzers h ave a zero avera ge (minu s infinity dBm) noise level th at is not affected by th eresolut ion ban dwidth, but th e rms level of the noise is determ ined by the video bandwidth . Thesensitivity of th e optical spectrum an alyzer is defined a s 6 times t he r ms of the noise.

For convenience, opera tors of Hewlett-Packard optical spectrum an alyzers can enter th e desiredsensitivity, and a s a resu lt, the appr opriate instru ment settings, including video ban dwidth an d sweeptime, are automatically determined and set.

No Effect on Noise


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Data Subjec t to ChangeCopyright 1996

HP-AN1550-4_Optical Spectrum Analysis Basics

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