Semiconductor opiicai anplifiers Beyond their role as transmitters in fiber-optic systems, la^r diodes can be made to amplify light, /^rfmps promising simple all-optical repeaters

Part of the original vision of integrated optics is the uttimaie creation of a fiberoptic system that would be 3 coiTsplete opiicai analog to ί radii iona! elec­tronic systems. In such a system the light, once gen­erated, would not only be transmitted optically, but would also be amplified optically and received by dtrea optical detection.

Today, research and development on semicon­ductor optical amplifiers is bringing that original vi-sion closer to reality. In several laboratories in the United States, Eufupc, and Japan, scmiconuuciur opiicai amplifiers offering gain of more than 30 decibels and bandwidth exceeding 10 gigahertz are being siudied as possible alternatives to existing repeaters in conventionaJ amnliiude-modulated sys­iems. Sefriicuiiduclor opiicai amplifiers are also promising drvices for the poiential cohereri? opiicai iransmlssion system of the future, because of their capability to preserve the coherence of light in frequency-modulated and phase-modulated signals. Furthermore, owing to their miniature size, semiconductor opti­ca] amplifiers can be integrated monolithically with other optica! circuits, opening the way to the eventual development of optical data processing.

By contrast, today's long-haul oi^raiional systems are optical-electronic hybrids, containing regenerative repeaters that deteci thc light photoclectrically, amplify the resulting current electroni­cally, and use the amplified current to pump a diode laser that reconverts the electric signal into an intense light signal that is sent on to the next repeater in the system. Direct optical amplifi­cation, circumventing the electronic steps, would allow sysiems to be built with more compact repeaters of a simpler design.

Two decades of research It has long been a desire of optical-transmission engineers to

build a praaical optical amplifier that could make the direct amplification of optical signals possible. Research on direct opti­ca! amplificaiicn can be divided inio three areas. The firsi was roughly from 1963 to 1968. Just after semiconductor lasers were invented independently in l%2 at International Business Machines Corp., C^nerai Electric Co.. and thc Lincoln Labora-tori^ of the Massachusetts Institute of Technology, the principle of optical amplification began to be studied by groups at each of I h ^ plaoes in ihe Uniied States. At that time gallium arsenide (GaAs) semiconductor amplifiers, cooled to -2(K)°C, were employed. These studies were not developed for practical appli­cation because the devices did not operate at room temperature.

The second era of research extended from 1973 to 1975. After semiconductor lasers were made to operate at room temperature by groups at Bell LalK ratories and at the loffe Physico-Technical institute in the USSR in 1970, the study of direct optica! amplification was revived, especially by a group at Siemens AG

Soichi Kobayashi and Tatsuya Kinmra Nippon Telegraph and Telephone Public Corp,

in West Germany. The German group's intention was to use these devices as repeaters in optical-fiber communicaiion systems. However, since stable sin­gle-frequency semiconductor lasers and other high-performance optical devices were not available at that time, the group was not able to adequately fine-tune wave lengths or efficiently couple light into op­tical amplifiers; thus it was difficult to determine the propeaies of optical amplifiers and to find a way to suppress optical noise.

The third era o f research siarted in 1980 and is continuing in the present. In 1980 a research group at the Musashino Electrical Communications Laboratory of the Nippon Telegraph and Tele­phone Public Corp. (NTT) in Tokyo started to study semicon^ ductor opiicai amplifiers with the hope of finding that they could be used in optical-fiber transmission systems, Iht researchers, who included the authors, have examined optical amplification mechanisms, beginning with basic principles, and have both theoretically and experimentally elucidated such characteristics as gain, bandwidth, maximum output pK^wer, and noise. In so doing they were able to use well-controlled single-frequency laser diodes and thus have been able to overcome some of the obsta­cles that hampered the earlier researchers: fine-tuning the optical wavelength between the laser transmitter and the optical ampli­fier, coupling the light into the amplifier with minimum loss, and achieving stable amplification with gains as high as 300 to 1000 times (25 to 30 decibels). They also have found that optical noise can be suppressed by applying an antirenective coating to the in­put crystal facet of the am.plifier diode. These findings have led to several successful experiments in systems with optical repeat­ers. Most of these experiments have been conducted with discrete semiconductor optica! elements. Monolithic integration o f the elements had been tried as early as 1980 by a number of re­searchers, including M.B. Chang and Elsa Garmire at the Uni­versity o f Southern California. These studies showed, however, that the feasibihty o f fabncating integrated opucal devices anu their satisfactory practical operation as optical amplifiers seem premature.

Optical amplifiers are also being studied at the Centre National d'Etudes de TSl communications (CNET) of France's postal telephone and telegraph utility, the British Telecom Research LalK)ratories in England, and USC in the United States, all with the aim of applying them to optical-fiber communications. Semi­conductor optical amplifiers have been studied not only for direct amplification of an optical signal, but also for use as op­tical switches, in which the optical signal is turned on and off by switching the injection current. They have also been studied for use as optical bistable devices at NTT and at the University of Tokyo in 1983.

LEDs, optical amplifiers, and lasers A semiconductor opiicai amplifier can be identical in structure

to a light-emitting diode (LED) and a semiconductor laser; in

26 O018-9235/84/O5OO^26$L00^ 1984 IEEE lEEESPECTRUM MAY 1984

[Ι] Λ cross section of an indium gallium arsenide piiospiiide (InGaAsP) Fafiry-Perot semiconductor optical amplifier, shown in this scanning electron micrograph, reveals something of its in­ternal siructure (left). That siructure is more clearly elucidated in the diagram (right). Each iayer of the diode has a slightly dif fprent cofr,po5ii!Gn from trie neighboring lasers. The n-^ype semiconductors have an excess of negative charge carriers (elec­

trons) carrying the current while the ^-type semiconductors have an excess of positive charge carriers (holes). This particular amplifier is designed to work at a wavelength of 1.5 micrometers. The active layer (red) is 2.8 micrometers wide, 190 micrometers long, and 0.09 micrometer thick—thinner tlian that of a conven­tional iastrr Uiude, which results in a hinher saturoiion-oiitpiJt-power levei.

fact, in many cases tiie same diode can behave sequentially as an LED, an amplifier, ar.d a laser, depciiJing on ihe magnitude of the injection current witn which it is biased. In these cases, the diode consists of a sandwich of two or more types of semicon­ductors doped with impurities to have different electricai proper­ties [see Fig. 1]. At each end there is a cleaved crystal facet; together the two end facets act as plane-parallel, partially reflec­tive mirrors.

When an electric current is applied across the diode, valence electrons in the crystal lattice leave their places and move ihrough the material, leaving behind ''holes" in their places that other electrons can fill. When electrons and holes recombine in an ac­tive region bordered by junctions of the semiconductor

Defining terms Baseband: the band of frequftnries occupied by the signal before it modulates the carrier frequency (in a fiber-optic system the frequency of the infrared light emitted by the source laser) to form the transmitted signal. Beat noise: noise generated in a FabryPerot amplifier by mix­ing uciween me mam amplified signal and the central longi­tudinal mode of amplified spontaneous emission (signal-spontaneous beat noise), or v ithin components of the ampli­fied spontaneous emission (spontaneous-spontaneous beat noise). Dark current: a leakage current that flows in a detector even in the absence of incident light. dBm: a unit for expressing power levels in dB with reference to a power ievel of 1 milliwatt. For example, -2UdBmis20dB below 1 milliwatt, or a power of 0.01 milliwatt. 3-dB gain bandwidth: the bandwidth at which the gain of an amplifier drops to half its peak value. Gain saturation: the natural limit of any amplifying device beyond which a further change in input power no longer results in an appreciable change in output. The power at which gain saturates in a device is the saturation output power. Internal gain coefficient: the gain of an optical amplifier per unit length. Total gain of the device for each pass of the light through the active region is exponentially related to the inter

materials, they spontaneously emil radiation at a wavelength determined by the material's band gap—essentially the dif­ference m energy between the electrons and the holes.

If the current applied is relatively low, the active region of the device begins lo glow with light spontaneously emitted over a broad specirui ^ of wavelengths; ai this point is an LED. Ary light inserted into one end of the diode from an external source will be absorbed by the crystal.

As the current is gradually increased, there comes a point at which the diode will turn transparent to the incoming radiation: light inserted into il from an external source wiii no longer be ab­sorbed, but will pass ihrough the diode and be emitted out the other end. That transparency point marks the threshold of

na! gain coefficient mijltiniled by the total length, less ail losses. Longitudinal modes: discrete wavelengths of light reinforced at resonant frequencies along the longitudinal dimension of any FabryPerot cavity. Partition noise: noise arising from the tendency, under modu­lation, of a semiconductor laser to hop from one longitudinal mode to another. Shot noise: unavoidable random noise, an inherent conse­quence of the quantum nature of light, generated both by the amplified signal and by the amplified spontaneous emission. Spontaneous emission: light emitted by the spontaneous recombination of electrons and holes in ihe active region of a semiconductor diode. Tne light is emitted over a broad range of wavelengths. Ampiiiied spunianeous emission (ASE) is spontaneous emission that has been amplified within the cavity of the optical amplifier. ASE is the principal source of noise In optical amplifiers. Square-root gain-bandwidth product: the constant that ex­presses the tradeoff between gain and bandwidth for a semi­conductor amplifier. Gain increases with increasing injection current while bandwidth decreases with increasing injection current. The numerical value of the square-root gain-band­width product, expressed in gigahertz, is equal to the square root of the gain in decibels times the bandwidth in gigahertz.

Kobayashi—Semiconductor optical amplifiers 27

/ !

/ I

• 3 0 A i te^fjf

[2/ A single diode can behave sequentialiy as a hght-emitting diode (LED), a semiconductor opticai amplifier (SOA), and a laser, depending on ihe magnitudp of the injection current with which It iv biased. When ihe cum-ni is first applied to the diode, electrons and holes spontaneously recombine, emitting radia­tion, and the diode hegm to gio^: m mis fmm it is an LED. Any light coupled into it from an external source wiii he absorbed. As the current Ls gradually increased, there comes a point at which the diode H 7 / / turn transparent to the incoming radiation; at even higher currents, the diode has gain and incoming radiation wiii be amplified. The transparency point marks the threshold of ampli­fication—the current at which the diode becomes a semi­conductor optical amplifier. As the current Ls further increased, one of two things can happen. If the diode has internal feedback such as paniuiiy reflective end facets, the gain exceeds ail interna! losses in the material and the diode becomes a laser (solid line), if hnwei^r the facets are coated to be nonreflective, (he diode continues to amplify incoming light until—at some point in­trinsic to the maierial—the gain saturates (dashed line).

ampiification: the current at which the diode begins to act as an ainplifler (see Fsg 2J. At current levels above that amplification thr^hold, thedlcKle has net gain, intensifying incident light; that gain will increase as the current is increa^. In addition, the dicKle is still spontaneously emitting its own light and amplifying that sponianeous mission; this ampufied sf^ntaneous emission (ASE) has a somf^ha? narrower st^runi than the LED emis­sion and is the amplifier's principal ^urce of noise.

As the current to the diode ampiifier is further increased, at some point intrinsic to the maierial the gain begins to saturate: a furiher change in input current no longer results in an ap­preciable change in ouipui, Howrver, if the diode has some feed­back mechanism, such as partially refiective end mirrors, thc gain will not saturate; instead, at a second threshold current the net gain exceeds the losses to transmission through the end facets, absorption of free carriers wiihin the crystal, scattering, and oihef caviiy iosscs. Ai ihis lading ihrcshold current, iniernally generated light will resonate back and forth between the two crystal-facet mirrors, stimulating funher emission with each pass. A certain amouni of the radiation will be transmitted through the end mirrors, ^'aping the crystal, to be emitted over a fairly narrow spectrum of wavelengths as laser light. Thus the dicHJe has changed from an amplifier to a laser.

Semiconductor optical amplifiers. like LEDs and laser diodes, are typically made out of compounds of elements from Groups ill and V of the p^odic table, i^vices of different compounds r^pond to different wavelengths. As laser diodes, aluminum

gallium ars^tde (AlGaAs) devices are widely used as opticai sources for audio- and video-disk playback sy^ms and optical-disk recording systems, especially at the short wavelengths of 0.8 to 0.9 micTGirmer. Lasar dicKies of indium gallium arsenide phos­phide (InGaAsP) have b^n develop^ as optical sources for use in fiber-optic communications systems, operadng at the long wavelengths of low fiber dis^^ion (1.3 micTommers) and low fiber loss (1.55 micrometers). In spite of the fact that, when operated as lasers, diod^ of these two different materials emit different wavelengths of light, when they are operated as ampli­fiers little essential diff&mce has been obsn^ed in their gain and noise characteristics.

Two types of optical amplifiers Tncfc are iwo basic types of semiconductor opdcal amplifiers:

Fabry-Perot and traveung-wave [see Fig. 3]. in the Fabry-Pero! optical ampiificf ihe iwo eieaved-vrysia!

facets aa as paniaily refiective end mirrors that form what is known as a Fabry-Perot cavity. The natural reflectivity of the facet mirrors is approximaieiy 30 percent; their reflectivity can be modified over a wi' * range by the addition of dielectric coatings. Incident light is coa led into ihe cavity, where ii is airipiincd dur­ing successive passes between mirrors and emitted again at a higher intensity.

The structure of the traveling-wave optical ampiifier is identi­cai tc that cf thc Fabry Perci ampuHer, ith t.ne exception thai ihe facets ai each end cf ihc cavity are coated \v:ih a niiihuayer antiretieaion coating to prevent inimiai fecuback. TnuS the light to be amplified travels through the device only once, emerg­ing intensified at the other end.

(It should be noted that an injection-locked laser can also behave like an opticai ampiifier. When a weak narrow-band­width optical beam is coupled into the laser, the laser becomes a locked oscillator, forced to oscillate at the frequency of the inci­dent light. The injection-locked laser therefore emits its intense laser light at the same frequency as that of the incident signal. Its nonlinear gain characisristics make it most sukabie for amplify­ing frequency-modulated or phase-modulated optical signals, rather than for the iniensiiy-rnuduiaied signals of most existing digital fiber-optic systems. Because it operates above the lasing threshold, however, it is in a different category from ihc Fabry­Perot and traveling-wave amplifiers and will not be discussed.)

One important measure of any amplifier's performance is its signal gain—the ratio of the increase in the output power over the input power. In a semiconductor optical amplifier, the

Device

Spectral OUipiJi

L A j L

f3J Tlie optical paths of light in the two basic types of semi­conductor optical amplifiers are shown in cross section (left) along with the spectral output obtained from boih types (ngm). The Fabry-Perot amplifier (A) has facet reflectivities of about 32 percent, so that the light is partially reflected within the diode from the cleaved<rystal facets forming the ends of the device; amplified spontaneous emission (pink) ocairs in discrete longi­tudinal modes around the amplified signal. The traveling-wave amplifier {B) has end facets coated to have zero reflectivity so that the light travels through the device only once; amplified spontaneous emission occurs over a broad range of wavelengths. The output light that travels out the input facet of the Fabry­Perot amplifier must be removed with an optical isolator.

28 lEEESPECTRUM MAY 1984

gain is a function of the device's internal structure and mirror rcficctivities, the injection current, and the device's physical length.

The intemal gain coefficient—that is, the gain per unit length within the crystal—increases linearly in direct proportion to the magnitude of the injection current. The single-pass gain of the Fabry-Perot and traveling-wave amplifiers therefore increases ex-ponentiaily with increases in the device iengili or injeciion current.

Since the ideal traveling-wave amplifier refiects no energy from its facets, its signal gain is equal to the single-pass gain in the amplifier cavity. In the Fabry-Perot amplifier, however, the in­jected light is refiected back and forih between thc facet mirrors, increasing the effective length of the device and therefore en­hancing its signal gain. Thus the total power output from a Fabry-Perot amplifier is larger than the output from a traveling-u/a.ve amplifier when both the sinftle-pass gain and the length of ihe two anipuiiers arc tne sarne.

Tne amplifier gain is limiieu by gain saiuraticr. that is, for sufficiently large optical powers, further increases in the input signal power no longer resuk in an appreciable change in output. The gain saturation is partly a function of the ampiitier's length and partly rciaicu io the way ihc light is optically confined to the active region. Although gain saturation limits the maximum out­put power of both Fabry-Perot and traveling-wave amplifiers, it imooses a more serious limitation on the operation of Fabry-Pcrct devices.

.\nciher limit to the available gain is the onset of laser oscilla­tion, which cuiivciia uii dcvicc ίΓΟπ; ar. a.'T.piifier into ? •g' er Oscillation is a major limit to the Fabry-Perot anipiiuer bccdusc-of its reflective end facets. Tlie limit to the net gain below the laser oscillation threshold in both AlGaAs and InGaAsP Fabry-Perot amplifiers has been ineasurcu at 30 dB. Λ iraveljp.g-wave amplifier has virtually no internal optical feedback because of the low reflectivity of its end facets. Therefore it can be biased up to a higher injection current without setting up internal oscillation. Thus a higher single-pass gain is obtained through a traveling-wave ampiiner than inronpn Λ rauiy-Feioi ampliricr. Thc highest net gain of AlGaAs traveling-wave amplifiers has been measured at 22 dB—three times as large as ihe uncoated diode's lasing threshold current.

Noise limits effective gain Another limit to the effective gain is the fact that semi-

conduaor optical amplifiers have noise, and the noise is ampli­fied along with the signal. There are two types of noise: shot noise and beat noise, the major portion of which arises from the presence of amplified spontaneous emission (ASE) in the optical amplifier cavity.

Shot noise is unavoidable random noise that is an inhcrcni conseauencc of the quantum, nature of light. It exists even m so-called coherent light, because the particle behavior of light in­troduces intrinsic statistical randomness in the detected photo-current. Shot noise is generated both by the amplified signal and by the ASF and is directly proportional to the strengths of the signal and of the ASE.

As mentioned above, ASE is the amplification of spontaneous emission within the amplifier cavity. When electrons and holes spontaneously recombine, the frequency of the light they emit depends on the difference in energy between the electrons and holes. Since there is a range of energies, the maieriai in ihe acuve region spontaneously emits light over a wavelength range of greater than 50 nanometers, and this light is amplified along with the main signal.

In a Fabry-Perot amplifier, the cavity resonances will reinforce and amplify only those frequencie<i of light at which an integral number of half-wavelengths will fit between the two end mirrors of the cavity. Because these various wavelengths originate from light resonating along the longitudinal dimension of the diode, they are also cailed longitudinal modes. The emission from the amplifier therefore consists of a central longitudinal mode at a

-

Spontfineous- 1 X apontaneoLiS «« boat noise

! Ik. Signal-spontaneoua 1 " beat noise

* ^ Signal j shot noise

1 1 1

Amplified output power, dbm

/ V / A ' c r n f po^tfi μνι itiUt u^^r.civ.'icith, expressed :r. c.nipen's squared per hertz, from an AiUaAs Fabry-Peroi umplifier is sliOnn relative to its αίπρίΐββ٨ signal out nut nnwor, tn illustrate how different forms of noise become dominant at different power ieveis. The unsaiuraied signal gain of this device is 20 dB. The upper curve shows the heat-noise power, ui power levels below -20 dBm the spontaneous-spontaneous beat noise is dominant, while above - 20 dBm signal-spontaneous beat noise is dominant. The lower curve shows the shot-noise power; at power /?v'p/c fnwpr than - /i7 dnrn ihv ihoi nohe arising from thc amplified spnrijnnpnu^ emission is dominant, while above - iO dBm the amplified signal shot noise is dominam.

certain wavelength—the amplified signal—surrounded by a dozen or so weaker components of ASE 0 .5 to 2 .0 nm apart creaicd by ihe other longitudinal mode resonances within the cavity.

When the total output from a fabry-Peroi amplifier—signal plus ASE—is detected by a photodiode, the various modes com­bine to generate what is known as beat noise. Signal-spontaneous beat noise is generated by mixing he twcen the amplified signal and the ASE cuiiiponenis in the centra! Icngiiudina! mode; sponiancOus-sponiaiicOus beat noise is generated by mixing within the ASE components themselves [see Fig. 4) .

Aniplineu spontanccus emission is also generated in traveling-wave amplifiers. Because there is no internal resonant optica! feedback within a iraveiing-wave ampliuer, however, the spontaneous-spontaneous beat noise is spread out over a wide continuum instead of being concentrated in discrete modes.

To design a high-quality system with semiconductor optical amplifiers, mechanisms must be devised to suppress both shot lioisc and beat noise. !n a Fabry-Pcrot amplifier, if the cain is 20 dB and the amplified output power is greater than - 20 dBm (that is, greater than 20 dB below a reference power level of I milliwatt), the signal-spontaneous beat noise is dominant; if the output power is less than - 2 0 dBm, the spontaneous-spontaneous beat noise becomes dominant. The total power level of the shot noise is about two orders of magnitude below that of the total beat noise. The best way to suppress both the shot noise and spontaneous-spontaneous beat noise is to install a separate narrow-bandpass filter at the output of a Fabry-Perot or traveling-wave amplifier.

29

Opfk^t pmmpl i f tm: prmntskig only at high data rates

ςΛ5ί::ίΐ5ίί٥5> ίτοί^Φ 'ΑΐΛίΑ ImplemenlirHi ٦ as pan of a

uarm of t?^ nr^etical GOI>«k!erfi!lons involved In incor-porsTrfiQ donicoiKhiCtor optical amplliiers into an actual T jer-q iic tf«MWNIII»wr» sy^c^n. for thc csKs cf brevity, the diactidelon it conflnsd to oiw application—semiconductor optfGdl «mp«flem &a sw«emplin§fi&. However, a NURTIYW oi ihe «r»9lneeriffO th«t arise—such as frequency stabiliza­tion. poiarteatJcft $«A9ltlvlty. and coupling losses—must also

$%e$»hr«ri| in many current fiber optit; svctems are ava-lanc^ph^^odfooes (APOs). v^icl) conve l a very weak opticai sl^f^ η κ Ο a sUOiW <»i6Ctrlcal one by ciestlt^ ^ ν ^ ί Λ Μ ί Μ of etemjwe and botes Ui tlw cisvtee. roi kmy-haui fssiT'^^w # j r^r iw, m 5vaie?icns pnoi^ikw rs-

PAII Lsfaoriiiodes in Crawford HHI. Ν J . . Is al^nfncamty aenaHlve than earlier Qiermmlxm APoa.

a standard A T O , thia ncNv <^/lo$ has senate ^ fuB ^^^Cfpilvn ^ mustisilcation of eiactroi-vhole p f i ^ a e p a n ^ by m UnOaAsP iayer that enhances the Qevica'a a p s ^ ϋ ί» ucr^mamOd an SAi^M APD.

Soichi Kot>dyashi Tatsuya Kimura of Nippon Telegraph ar>d Telephone Public Οοφ. are proposli«ci that simple P I N

feceivars ia nonavalanche p-l^ photodiode deiector plus S f!e*{!-*?*f«*-« ifftftsisiot a m u i m m i m v.It 2^—: condtictor optical preamplifiers will have oetier rece iver sen-^ithrii;^ ;r^n t^05^ ob*eir.ab*e ^i?h APC^. II is therefore use-fuJ >o cofnpare potential o? ttte new S A G M A P D with that of it«i optical preamp- PINFET receiver for enhancing the per-fo^msAce Of prac tcai ορ٥ί^٢٥ commuviic ticr.s syetefnt.

in cort^inn the two. the Important component and sys­tem !S8U99 ir w^ rnust be considerod inciude excess notss fac tors, exterriai nrcer requirvnioxuS. poisrirstior. effccie, irar.s-mlttino laser asabltity. and Ιηρυ! coupUng tosses. The per­formances of iwo F^^SARAIETIVO opticas prcamp siruct'jrss are consklered: a traveUng wave preamp with t>oth facets cci st^ ic: fis?iect*^*y and a Fabry-Perot an^plifier with

facet iimrsaied for ^ p e r o w i i reiitjtiiviiy. it is a^usurnuu ihdi the cavity length of bcAh preamps is iuO micron^eters. απ٦ that e«cft device has a signal gain or 30 decibols.

Any eamiconductor optical amplifier is itseif a source o> n<^8« becatise of smplifiod i^wianecu! ; emissior. (ASB Tr e lo**i inpM*f»cet reflectivity of the Fabry-Perot preamp lowe.^ its i i%C^ ??0!se factor by reducing the fraction of the ASE that escapes from tfie output f ecet. Even so. a consideration of phOlcm stelUitics reveals that the k>wer Internal-gain coef f i-c.'sisi of the FtriHy-Pserot airpllfier causes It to have a higher e x c ^ noise fa^or in«i«irav^ing A S V C smplifief of equiva­lent eigf>»l sain. Realistic excesa noise factors for the travel­ing-wave ana FaiHV-woi ampiiiiwa consr::*-^.' r.svc rc iuiUMfM) receiver siana}*to^lse-ratio penalties of 2 dB and 4 dS, respectively, rotative to an loeai traveiing-wev^ pieamp operate kn the signai-spontaneous beat-noise limit. The KHiQSQCloms βρνΓηίΓτθΟ;;^ wpC;:i^neOii£ 0 O i ? « afi-tina frem ttve A^H prodded by a travelingwave preamp can aavanHy clegrad^ tha raceme tignaMo^oise ratio unieas a m m l d l ^ ^ issss^lhns optica fiitS? b p i ^ C ^ |h« ρτΜίιψ am) the PtN-FET receiver. The Impfovement in the rec^tw's sensitivity increases markβd^y with a narrowing of the optical filter's bandwidih [iippet iifustrationj; for a fixed

filler t>andwidth, sensitivity improven>ent Increases linearly Wi l l i isiui uu tu 7^"^::; ~ —

Data rate, in fact, is ihe tmporiarti quantity that must be cc-'^?-i i^^ ™ ~ -- '^s- ' ig ii"-r rvrsciicaiiiy ύί opiicai pre­amps: since practical dielectric muiiilayer or grating filters for sysferrt appiications have bandwidths equal to or gnbater than 1.6 nanometers (200 gigahertz) at a wavelongth of 1.55 mi­crometers, the curves show that a traveling-wave preamp doss not significantly improve the performance of a PIN-FFT receiver for data rates lower than about 200 Mb/s. Even at 1 Gbis. the 7-io-&dB imorovement in receiver sensitivity provid­ed by SAGM-APD is equivalent to that obtained from a travel ir*g wave optical PROENIP r^llu α 1.6 .nm bandwidth opucal filter. Traveling-wave optical preamps will therefore be most useful in systems operating at data rates exceeding 2 Gb/s. where the filtering requirements are less stringent and where

barvd'- id!h product of ?^AGMAPD devices may not be large enough to provide comparable improvements in re­ceiver sensiiiviiy.

The noisofiltering problem is more tractable for the Fabry­Perot preamp. where the narrov/ gain bandwidths of the am­plifier's longitudinal modes amplify only a small portion of the spontaneous emission in the cavity. The narrow (0.03 nm) 3-dB gain bandwidth of the Fabry-Perot preamp does, however, pre­sent a fundamental obstacle to its application: the necessity io maintain an exUeiTi Jy C!O2G match bcii".':?cn ?ho wnvc i«*»^F Κ rvi it^A INURR-fi in?if»r NNRI IHA Dre?rr:?'* ΓΓίΞίπ !cr,5iiwdir;a! niCrJe. A iypicai v;ava!ori5th-ierr?per«iu^e r.nf»uirAer\\ or a laser diode emitting light a] 1.55 micrometers is 0.08 nanome­ter per degree Geisius (lO \ J H Z / X ) , Fur a 1-Gb/s trar.sn^issicr: system, this strong dependence of emission wavelength on tcimnerature reauires ihat the temperature diiference l>e-tween the source laser and the preamp be controiiea to less ihan 0.*1. FrsqusPfCy stabilization of the laser and the preamp is fijrthftf complicated by the fact that ":ε the devices aga, their leakage currents and opticai losses chango and iheir operating wavelengths will drift. Precisely controlling the temperature and the injection current of both the source laser and Fabry-Perot preamp to the tolerances required would be j)IFFIR^»ju In AN actual operating environment.

IN OUUIIRO'i, bcth ihc ffoqu'CPcy sr?d the QR ln P'^'^ry-P'=»O? ariipJifiers—Linlikc iravcung --'a -3 emniifiArcs—ara sensitive to changes in tho poiarization of the'lncoming signal. In a Fat>ry-Perot preamp, bolt) the facet reflectivity and the index of refraction for each longitudinal mode in the cavity are d fpendent on polarization. Drifts in tne poiarization oi tha in-pui eignaS caused by ,~.st'jr£!iy^cc'jrr!n« fvqrturtiations In the optical fiber (from temperature variations, vibrations, or mechanical stress) would therefore have to be conected by placing an optical polarization compensaior in front of the Fabry-Perot preamp. A iravelingwave preamp would not re­quire such a compensator, making it easier to install in an ac-

From tfie abo\^ considerations of optical filtering and gam bsnd^idths, it is clear that semiconductor optical amplifiers could be usod only in transmission systemci with stabilized sinqie-frequency diode laser sources. If the frequency of the source laser is not stabilized, an opncai preamp uuuid cAacer bate the effects of the laser's nrKKie-partition noise (that is, the tendency under n>odulalk)n for an unstabllized laser to hop from one longitudinal mode to another)—completely extinguishing the received signal when the laser has hopped to a frequency to which the preamp will not respond. \r, addi-

The bandwidth of any communications device is intrinsically related to its information-carrying capacity: the greater the band­width, the faster the device can be modulated and the more infor­mation it can cany per unit of lime. An elemeniar>' result from information theory siates that the minimum bandwidih (in megahenz) necessary to carry a digital signal is equal to half the bit rate in gigabits per second.

The bandwidih of a Fabry-Perot semiconductor optical ampli­

fier decreases dramatically with injection current. It is often ex­pressed in terms of thc 3-dB bandv^dth—that is, the bandwidth at which the amplifier gain drops to half its peak value. Since gain increa^ with the injection current, there is a tradeoff be­tween bandwidih and gain.

In a Fabry-Perot amplifier, the tradeoff is explicitly quanti­fied: the pr(Kluct of the bandwidth and the square root of the gain is a constant called the square-root gain-bandwidth product.

30 lEEESPECTRUM MAY 1984

S 10 100 20C 10C0 2CCC o.otc aoe 0.3 Β,Ο ιβ.ο Fllt^ bandwKllh, GHz δτκΐ nm 10,000 GH2

Blocked Febry-Ρβτο. Muitliayer kiterferenoe Mon

The improvement that fabr/^Pemt and traveling-wave preamps make in PINFET receiver sensitivity is compared for different dat9. retes fund Wmr tandAfidths. The imorovement in sensitivi ly is pfotied for data rates of 140 Mb/s (dashed cun/es) ana i Gb/s (solid curves) as a function of tho bandwidth On nsnome-

ana gtganenzi or the finer usea with the device. At both data ^ate^ ite trsveiing^svo smplifief (red tines) v^iih a vury narrow-bandwidih filter yields a higher gain than the fabry Perot amplifier (brown lines). Moreover, the filter requirement bscOwTTC^ tscs st'ni^ni St higher data ίΰίϋ». Aiso plotted is the pwti/nrmnvv Or irm ησύντύίγ announced SAGavalanche pho todiode (horizontalblue fine)measured at a bit rate of 1 Gb/s. As «Λη h» ΛΛΛτ». Us ttM^ifhiity LM/^M fUtcr Z{ c!i is CQ'jal to or tyei ter than that of either pr*^mp. A 5^8 coupling loss has been subtracted from the preamp sensitivities.

i»r»n, ihfi fiber-optic svstem miflht fall altoqether if aoing or therma! effects cause the source laser s emission lo drift out­side the bandwidth of the preamp or its optical filter. By con­trast, an APD is not a frequency-sensitive device; drifts In laser frequency and poiarizaticn do not pose a problem for an .APD f^wClvef.

Lastly, adding any device to a f Iber-optic system Introduces optical coupling loss, which takes its toil on the ultimate slgnal-tofiolse ratio. With an APD ihme la ΐίίΐίβ ο*- no coupling loss bec*»"?i^ th»ro fife no sxirs devices (pieaiTip, filtei, polarization compensator, and so on) In front of the receiver. Thus the gain of a preamp would have to more than compen­sate for these lcss€?s in order tc improve the system's overall SignaHo^noisd ratio over that obtained with an APD. Experi­ence with coupling diode lasers to single-mode fibers indi­cates that coupurig a semiconductor optica! amplifier to β siagle-n>ode fiber will Introduce a coupling loss on the order of 5 dS. Thsrsfore, as noiwd in iho iiiustrations, a b-ab input coupling loss has been assumed m calculating the practical sensitivity of the optical preamps. It is worth noting, however, thst inserting the optical filter necessary to reduce the preamp's spontaneous-spontaneous beat noise does not par­ticularly degrade the receiver's baseband sIgnaMo-noise

ratio. The reason is that gain of the preamp i^soierg© thiat the overall signal-to r.oise ratio i i!rr«iied b y hA^t ^nd by receiver thermal noise, A I a aata rate or i uws.a iraveimflr wave preamp with a gain of 30 dB coupled tc a PiNFET re* ceiver through a l.e-nm optical filter can tolerate filter losses as n i n h as 10 uG bsforo the bSSSba.'K! «ΐΛηΛΗο-ΛΛίΛ ri tiii Is measurably reduced by receiver thermal noise.

In summary, from a system.s viewpoirxt the e)?treme|y nar­row 3-c!B gain bartdwidth, required temperatureiatftbittefetion, end polarization sensitivity of Fabry-Perot devices maKethem less attractive than traveling-wave amplifiers for u$e as prac­tical preamps In optical communication systems, likewise. ;r.c r,zTTZ\\' c p * J c ? - l m q u l r e d for suDDrossihQ epontane-ous-spontaneous beat noise ficm irav^lr^-wave preamps makes their application difficult for data r^te$ lower than about 1 Qb/s. At data rates below 1 or 2 Qb/s, the recently developed high-speed SAGMAPD is a more practice! device for increasing receiver sensitivity than is an optical preamp coupled to a PIN-FET receiver. Since the filter bandwidth necessary for suppressing spontaneous-spontaneous beat noise broadens linearly with the system's data rate, the most promising applicaiion of optical pieamps will be as traveling-wave amplifiers to Increase the sensitivity of receivers in

fIber-opiiii Ryslftrps opf lrat inn at nnijlttQinahtt rtata retO?.

--^Donald M. Fye Senior Member, Technical Staff υ Ib Laooraiones

80100 200 400 6001000 2000 4000 Oflta rat«. Mt>/e

ThQ perfGrmancQ of tho troveh.ng wave preamp improves-dramatically wiih decreasing filter bandwidth and increasing data rate. Here the improvement in ihe sensitivity cf a PIN­FET receiver using a traveling wave preamp is plotted with resect to data rstc. The thrco cur^'cz, from left to right. <5/TC»V the performance of the preamp used with a fiiw' haying a bandwidth of 20 GHz (solid red), 200 GHz (dashed red), and 1250 GHz (pink). The cunres reveal that ttie traveling wave op tical preamps do not significantly improve the perfarmance of a PiN'Ftr receiver tor aata nates buior^ 200 !^t/s. At toth 400 Mb/s and at 1 Ob/s, the Tto-S-dB Improvement of th9 proamp with a 200ΌΗΖ filter Is roughly aqulvelent to the performance of the SAQMAPO (dots). The preamp shows Its best perfor­mance at multlglgabit data rates. Again, a S<iS couptlng hss is assumed for the preamps.

For example, in a conventional AlGaAs amplifier having a Fabry-Perot cavity 300 micrometers long and facets that are 32-percent refleriive, the square-root gain-bandwidth product is 25 GHz. If the reflectivity of both facets is reduced to 10 percent, the square-root gain-bandwidth productivity increases to 50 GHz. If the refiectivity is further decreased to 1 percent, the square-root gain-bandwidth product becomes 200 GHz. The lower the facet reflectivity is made, the larger the square-root

gain-bandwidth product becomes—until thc refieciivities are so low that the Fabry-Perot amplifier essentially becomes a traveling-wave amplifier.

Direct optical repeaters appear feasible The semiconductor optical amplifier is suitable for optical

communication systems because of its direct amplification of op­tical signals, its compactness, and its power efficiency. In conven-

Kobayashi—Simiconductor optical amplifiers 31

mml ioiig-haui nber-omk: transmission syst@ns, the light at-tenua!^ after a^vorai tens of kiiometers is ampiined by digitai reg^erative repeaters. Such r^eneraiive repeatm incofporate complex drcuits to perform all the tasks of o^nverting the optical sipials into el^ric signals—miipUf)ins t hm , r^haping the pute , and driving the semiconductor l a ^ that re-emits light. Semiconduaor optical amplifiers, by coiitrasi, can directly amplify incident light, n ^ i n g only a small power supply so pro­vide the necessary injmion current.

It may be feasible to insert miniaturized optical signal repeaters into the iiba transmission line to reduce the number of complex elmronic regenerative repeaters needed in the system. In such a hybrid transmission system, the spacing be tw^ the regoierative re^^ters can 1^ expand^; if the performance of semiconductor optical amplifiers can be made good enough, they may cveii replace regenerative rq^ters eKcep! at terminals where ii is desired to convert the optical signals into electric signals.

The Fabry-Perot amplifier is suitable for relatively low-gain and broad-bandwidth os^ation because its maximum gain is limited by the onset of laser oscillation. The frequency of the in­coming signal must precisely matched to the frequency of the main Fabry-Perot mode for the signal to be properly amplified with a maximum si^al-to-noise ratio. The frequency can be ef-fmively matched with an accuracy of 12 gigahertz per degree Celsius and 3 gigahertz per milliampere by controlling the temi^ature of the AlGaAs optical amplifier and the magnitude of the injection current, r^pectively. An optical isolator would be n ^ ^ to prevent the amplified signals from being emitted out the input faca.

The traveling-wave amplifier would probably make a simpler optical-signal r q ^ e r . Its saturation output ^wer is high—that is, it cars be pumped to deuver a large gain before reaching satura­tion. Moreover, there is no ne^ to control its frequency because il will amplify any signal over a bandwidth of 4300 GHz (10 nm) wiih the AlGaAs optical amplifier. And no optical isolator would be needed because it is a single-pass device.

in 1982 the NTT researchers in Tokyo experimented with send­ing intensity-modulated signals through AlGaAs Fabry-Perot amplifiers. In this exj^ment, the Fabry-Perot amplifiers had

fct'r Of fepe.itf rs

/ 5 / Theoretical baseband signal-tonoise ratio of a fiber-optic system is shown with respect to an increasing number of amplifier repeaters for two types of AlGaAs amplifiers. The red curve is for a traveling- wave amplifier with an optical narro w-bandpass filter Tlie brown curve is for a Fabry-Perot amplifier with an input facet reflectivity of 6 f^rcent and an output facet reflectivity of 32 per­cent, used without an optical filter. The output power of both types of amplifier is -5 dBm, and the data rate is Mb/s, The curves show that, all things being equal, approximately 10 times more traveling-wave amplifiers can be used in a system than Fabry-Perot amplify iMfom the stgrtsi-to-noise ratio drops below the minimum accepmbk bit-error rate oform in 10^ (das^ line}.

facets with different input and output leO^vities; the input fac^ was coated so ii was aboui 6-pefc^i reueaive, while the output facet was left at the crystal's natural 32-percent reflec­tivity. Reducing the reflectivity of the facet on the side of the inci­dent light reduces the fraction of the ASE emitted in the direction of the deteaor, therdjy improving the signal-to-noise ratio.

In the exp^nmeni-, res^er gain of 33 dB was m^ured uang two optical amplifiers in tand^ on an optical bmch dlfectly coupled to each other with no intervening fiber. Tne researchers estimated how many direct optical repeaters could be insoted be­tween electronic regenerative repeaters to obtain a signal-to-noi^ ratio of 21.6 dB, which would correspond to a bit-error rate of one part in 10*. From the experiment, the r^archers extrapo­lated the behavior o f a 100-megabit-per-second transmission system, assuming low-loss fibers having an attenuation of 0.2 dB/km and 3-dB coupling loss per facet. In it each optical repeater would have a gain of 20 dB and an output power of - 5 dBm. The experiment predicted that 62 optical repeaters o f the Fabry-Perot type could be inserted at intervals o f about 70 km, thus expanding the distance between regenerative repeaters to seme 4300 km.

If the optical repeaters were to use traveling-wave amplifiers with a 2(X>-megahertz bandpass filter, and the spacing between optical repeaters could be about 70 km, the regenerative repeater spacing could expanded up to 16 times farther—to 69 000 km. Although these distances may be idealized values, they do seem to indicate that optical amplifiers could be desirable components for very long-distance systems (see Fig. 5j.

Optical amplifiers m prmmps The two tyjKss of semiconductor optical amplifiers are suited

to acmewhat differoit purposes iri an cptical transmission system. For optical transmission, the Fabry-Perot and the traveling-wave devices can amplify AM, FM, or phase-modu­late signals.

In addition to optical repeaters, ampliOers in a transmission system serve several other basic functions, Stemiconductor op­tical amplifers ^ow more promi^ for some applications than for others; one of the most promising is as preamplifiers.

Preamps are placed front of a receiver to boost a very weak signal to a power high enough to be detcaed. As optical pre^ps, saniconductor optical amplifiers might be instaUed in from of a PIN-FET detector or an avalanche photodiode of a conventional regoierative rq)eater, thus lowering the minimum power level for detection and aiiowing the repeaters to be spaced farther apart. From the NTT experimcsitiyi r^uits at 1(H) Mb/s, an AlGaAs opticaipreamp improves the minimum detectable power by 7.4 dB. The researcliers found that the Fabry-Perot device witii asymmetric facet reflec­tivities also significantly suppressed the amplified spon= taneous emission emitted from die exit facet. The optical signal-to-noisc ratio can be further improved by inserting a narrow-b^dwidth filter after the amplifier—a filter whose passband is mmched to tiie spectrum of the optical signais.

The researchers also calculated the minimum detectabk power at 1 gigabit per s ^ n d cn Fabry-Perot and traveling-wave preamp systems and comjmrvd the figures with those of oonvoitional avalandie photodiode direct-detection systems [see Fig. 6]. Tlie calculations showed tiiat a Fabry-Perot preamp with clmed facets improves the minimum detectable power at an avalanche photodiode by 5 dB, with an error mte of one part in 10 . An i d ^ traveling-wave amplifier improves the minimum detectable powa- requirement even further, to 10 dB. The improvement is more pronounced at wavelengths longer than 2 micrometCTs—the wavelength r ^on at which the attenuation in opticd fibers could be incredibly low with the new fluorick glasses now under investigation. At those long wavdengths the performance of avalanche photodiode is degraded by a large dark current (one that fiows even in the absence of an optical signal).

32 lEEESPECTRUM MAY 1984

13 15

Optical wavelength, ,.ητι

16J Minimum signal power necessary for detection ai a daia rate of 1 Gb/s is shown for an uvulunche-photodiode (blue) direct-detection scheme versus a PiN-FET deiector used with a Fabry­Perot preamp (brown) or a traveling-wave preamp (red). The bands indicate a range of minimum detectable power for each device, depending on certain critical factors within each one. The multiplication of shot noise induced from avalanche multiplica­tion is the critical factor in the APD; the range is shown for a noise coefficient ranging between 0.6 (bottom limit) and LO (top iimii). The criiicai factor for ihe Fabry-Perot preamp is the number of longitudinal modes of light allowed through ihe filietj the range is shown for three modes (bottom limit) to 30 modes (top limit). The bandwidih of ihe opiicai filter is the critical fac­tor for the traveling-wave amplifier; the range is shown from a hand width of 2 GHz (bottom limii) to 200 GHz (iop limii).

Several significant engineering problems remain to be solved before semiconductor optical amplifiers can l)ecome practical in conmiercial fiber-optic systems. More studies arc needed on the materials for the amplifiers to obtain the highest gain amplifiers with low noise. One solution might be to control the doping of thc diodes to possible luw-noise opucai amplitiers that can operate at reduced bias current. Increasing the doping level reduces the stimulated absoφtion, which would increase the gain and decrease the signal-spontaneous beat noise.

The structural design of semiconductor optical amplifiers is also an important area that needs more study The structure of an amplifier affects its capability to attain a high output power before saturation with a minimum of noise. A key necessity is to opiimize the optical mode-confinement factor, which represents the distribution of the optical field in the active layer, to thus ob­tain a high saturation output power. Moreover, amplifier struc­tures have io be designed to couple their ligui into the adjuiiiing optical fibers with a minimum of loss.

Finally, before semiconductor optical amplifiers can be put in­to commercia! use, coating techniques for depositing ultrathin fiims must be refined, i nesc coatings are mandator>' for obtain­ing large amplifier bandwidths, reducing coupling losses, and suppressing noise.

The fabrication of traveling-wave optical amplifiers calls for coating thin antireflective films on the facets of resonant Fabry­Perot ajnplifiers. The traveling-wave type has the greatest poten­tial for amplifying optical signals with large bandwidth, large gain, and litUe sensitivity to environmental temperature changes. However, this amplifier must be used with optical narrow-bandwidth fiiters to suppress spontaneous-spontaneous beat noise. Bandpass filters that are narrow enough to transmit only a single longitudinal mode are needed to reduce the spontaneous-spontaneous beat noise in traveling wave amplifiers.

Although the ouUook for the future has its practical obstacles,

semiconduaor optical amplifiers do show some promise in realizing the ultimate vision of a complete integrated optical communications system.

To probe further For a general introduction to optical amplifiers, ihe reader is

referred iu die paper. "Use of laser ampiifiers in a glass-fibcf communiciitions system;*' a paper by G. Zeidler and D. Shicke-tanz in the Siemens Forch, -M. EntwickL, -Ber. Bd. 2, nr. 4 (1973), pp. 227-34. See also the analysis by M.B. Chang and Elsa Garmire in the paper **Amplification in cleaved-substrate lasers,** IEEE Journal of Quantum Electronics, September 1980, Vol. QE-i6, no. 9, pp. 997-1001.

The mechflnic«*s of ampuuwiiiion in semiconductor lasers arc detailed in Heterostructure Lasers, Part A, by H.C. Casey Jr. and M.B. Panish, published by Academic Press, New York, in 1978.

A more specific introduction to semiconductor optical ampli­fiers is found in "Characteristics of AlGaAs Fabry-Perot cavity-type laser ampiifiers,'* a paper by Y. Yamamoto, in ihe iEEE Journal of Quantum Electronics, October 1980, Vol. QE-16, no. 10. pp. 1047-52.

For material on praaical design of semiconductor optical am­plifier systems, see **S/N and error-rate performance in AlGaAs semiconduaor laser preamplifier and linear repeater systems,** a paper by T. Mukai, Y. Yamamoto, and T. Kimura, in the IEEE Journal of Quantum Electronics, October 1982, Vol. QE-18, no. 10. pp. 1560-68.

Fabry-Perot GaAs semiconductor optical amplifiers are ob­tained with optica! isolators i; presented in "Gain and saturation power of resonant AlGaAs laser amplifier,** a paper by S. Kobayashi and T. Kimura, Electronics Letters, Vol. 16, no. 6, 1980, pp. 230-32.

A study on traveling-wave amplifiers is described in "Polariza­tion characteristics of traveling-wave-type semiconductor laser amphfiers," by J.C. Simon, Electronics Letters, Vol. 18, no. 11, 1982, pp. 438-39.

Noise characteristics of the various semiconductor optical amplifiers are discussed in "Comparison of noise characteristics of Fabry-Pcrct-type and traveling-wave-type semiconduaor laser ampiifiers," by J.C. Simon, J.L. Favennec, and J. Charil. Electronics Letters, Vol. 19, no. 8, April 14,1983, pp. 288-89.

Some practical issues are addressed in "Semiconductor laser ainpiificr for single mode opiicai fiber communications,** by J.C. Simon, Journal of Optical Communications, Vol. 4, no. 2., June 1983, pp. 51-62.

About the authors Soichi Kobayashi (M) is assistant chief of the electronic-equip­

ment development division at the Musashino Electrical Com­munications Laboratory^ Nippon Telegraph and Telephone Public Corp., Tokyo. He has been engaged in researc* ^ a optical devices and applied technology—holography, opi iw iiber fabri­cation, semiconduaor optica! amplifiers, and injeaion locking in semiconductor lasers. He received ihe B.3., M.S., and Fh.D. degrees in elearical engineering from Keio University, Kana-gawa, Japan, in 1969, 1971, and 1983, respeaively.

Tatsuya Kimura (SM) is associate director o f the physical science division in the Musashino Elearical Communications Laboratory at NTT. He has been engaged in research on optical transmission systems for public telecommunication naworks and has also studied optical equipment such as modulators, detectors, lasers, and fibers. He received a B.S. in electrical engineering, M.S. in eiearonics engineering, and Ph.D. in quan­tum electronics, all from the University of Tokyo, in 1%2,1964, and 1967, respeaively. ^

The authors wouid like to thank Τ Mukal, Τ Saitoh, and Y. Yamamoto for their stimulating discussions.

Kobayashi—Semiconductor optical ampUriers 33