Amplifiers for the Masses

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004 63

Amplifiers for the Masses: EDFA, EDWA, and SOAAmplets for Metro and Access Applications

Donald R. Zimmerman and Leo H. Spiekman, Member, IEEE

Tutorial Paper

Abstract—Small erbium-doped amplets and semiconductoroptical amplifiers will be used in current and future metro andenterprise networks in various configurations. Many new systemarchitectures will be enabled as these low-cost technologies areused to compensate for transmission and impairment-compen-sating component losses. This paper discusses the definition, use,and technologies associated with these new classes of optical am-plifiers which, though little, will impact next-generation networksa great deal.

Index Terms—Metropolitan area networks, optical commu-nication, optical fiber amplifiers, optical planar waveguidecomponents, semiconductor optical amplifiers.

I. INTRODUCTION

S INCE the early days of optical amplifier usage in networks,the variety of applications served and the value provided

has increased dramatically. Early applications used single-pumperbium-doped fiber amplifiers (EDFAs) in booster configura-tions to extend the range of 1.5 m transmission links. Shortlythereafter, coarse wavelength-division multiplexing (CWDM)was being employed with booster amps to double the capacityof installed fiber routes. Cascades of EDFA were under inves-tigation for undersea use; first in single wavelength configura-tions. It soon became apparent that multiple wavelengths couldbe supported with cascaded optical amplifier (OA) systems andan entire industry was born based on dense wavelength-divisionmultiplexing (DWDM) and EDFA technology.

A. Leveraging DWDM

The success of optically amplified DWDM systems inlong-haul applications was primarily driven by the costefficiency of sharing these relatively new and expensivegain-flattened EDFAs across many wavelengths. InitialDWDM deployments were used in back-bone networks wherenetwork reconfigurations were few and far between. The eco-nomics of DWDM were next applied to metropolitan networkswhere reconfigurations were much more frequent. Wavelengthrouting and optical protection switching have become com-monplace in these networks. This forces additional complexity

Manuscript received June 26, 2003; revised October 7, 2003.D. R. Zimmerman is with the Light Systems Associates, Farmingdale, NJ

07727 USA (e-mail: [email protected]).L. H. Spiekman is Vrijkensven 17, 5646HP Eindhoven, The Netherlands

(e-mail: [email protected]).Digital Object Identifier 10.1109/JLT.2003.822144

onto the OA designer to suppress transient crosstalk betweenwavelengths. As OAs are pushed further toward the edges ofthe network, cost and complexity become key concerns.

As with many maturing technologies, the bifurcation of theamplifier space into higher and lower performance solutions hasoccurred. Costs for providing basic amplification functionalityhave plummeted, driven by improvements in technology andmanufacturing efficiency developed during the golden age ofDWDM systems. The everchanging landscape of technologicaldevelopment and the economics afforded by each solution forceus to ponder the question: Are there better and more cost-ef-fective ways to deploy the available technologies than we havedone in the past? In the early days, it was all about deploying themaximum bandwidth from point A to B with the lowest overallcapital cost. Now carriers are much more concerned with firstdeployment cost, capacity growth profile, and operational ex-penditure. Architectural decisions from the past may not be thebest solutions for the future.

B. The Amplet Arrives

New architectures that take advantage of low-cost, modestperformance amplifiers are being developed today for many dif-ferent applications. These amplifiers have been called “amplets”by the early adopters of the technology to distinguish them fromtheir larger and more complex predecessors. Although it is dif-ficult to give one overarching definition for this class of opticalamplifier, the common thread is reduced performance and lowercost as compared to a traditional broadband DWDM amplifier.As an example, subbanded DWDM system application ampletsare being used with only 4 to 8 ITU channels (compared to 32 to40 channels of typical broadband DWDM OA) therefore, totalpower requirements are reduced by as much as 9 to 10 dB. Foran EDFA, this allows for less pump power and fewer gain stagesresulting in a much lower cost product.

Each application dictates the required performance andoperational characteristics of an amplet but a few criticalrequirements are shared by all. Amplets are expected to below cost. Price is dictated by performance and function butmust be sufficiently low to supplant other architectural choices.Amplets must be small in size. Functional packing densities ofsystems will increase over time to lower both first installationand operational expenses. Amplets must have low powerconsumption. As systems get smaller cooling becomes moreproblematic. Amplets must have at least the reliability of theirbig brothers in the telco central office environment. As they

0733-8724/04$20.00 © 2004 IEEE

64 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004

Fig. 1. Typical specified performance of three amplet types: EDFA, EDWA,and SOA.

make their way out to the network edge, they may be requiredto withstand the harsher conditions of underground vaults andcustomer premises. Lastly, amplets must be easy to use. If anamplet has a limited operational range or must be controlledvery accurately it may limit the usefulness of the solution.

As can be seen in Fig. 1, the performance of the variousamplet technologies under consideration has improved dra-matically to the point where they are all quite comparable.Throughout the remaining few sections the authors will en-deavor to describe various amplet applications, technologies,and design considerations, to shed light on the benefits andpitfalls with each available solution.

II. APPLICATIONS OF AMPLETS

Current and future architectural solutions will certainly takeadvantage of lower cost, modest performance amplets to providebetter overall network performance with a modularity that scaleswith deployed capacity.

A. DWDM Subbanded Line Systems

Single wavelength and subbanded architectures can takeadvantage of amplets to provide gain on an incremental growthbasis. For example, a 32-channel DWDM system could bedesigned using transient controlled DWDM amplifiers and32-channel wavelength-division multiplexers (WDMs) at theterminus points. This is a very cost-effective architecture forfully loaded systems when no wavelength add/drop occurs.Next, consider the situation where two nodes require wave-lengths to be dropped using fixed WDM and back-to-backDWDM amplifiers. Finally, consider using up to 16 amplets pernode with subbanded multiplexing as shown in Fig. 2. Whenthe number of add/drop nodes is high and the channel count islow, this architecture can be most cost effective. There may bea small price premium for a fully loaded system but the controlis simpler and it offers the added flexibility of simpler networkreconfigurations. Its first installed cost may be substantiallylower than a DWDM amplifier solution. Fig. 3 shows how in afully reconfigurable wavelength add/drop node amplets wouldbe used at both drop and add ports to overcome device losses

and boost signal levels for transmission. This application isideal for an erbium-doped waveguide amplifier (EDWA) arraywhere pump sharing and VOA control could be utilized.

B. High-Speed Systems Improvement

As transmission speed increases to 40 Gb/s and beyond, itbecomes harder to maintain OSNR through the labyrinth of dis-persion compensation devices in a system. Use of amplets inboth the transmitter and receiver circuit modules should be con-sidered to increase system margin through OSNR improvement(see Fig. 4). Wavelength tunable transmitters typically do nothave as much power as their DFB counterparts and high bit-ratemodulators are considerably more lossy than lower bit-rate de-vices. In addition, dispersion precompensation is required inmany links. An amplet located after modulating and before anydispersion precompensation allows greater launch power intoeither a DWDM mux or a fiber link preserving OSNR. Receiversensitivity is enhanced through the use of a preamplifier am-plet before a PIN receiver. In either single channel or DWDMsystems, the link performance can be improved. Improvementsof greater than 2 dB have been seen at 10 Gb/s with an EDWApreamplifier [1].

C. SOAs in Dynamic Channel Count Applications

Since semiconductor optical amplifiers (SOAs) amplifyingWDM channels will nearly always be run as a linear system,such a system will automatically be suited to operate at a non-constant total average power. Both changes in the number ofWDM channels, e.g., due to reconfiguration of the network, anduse of bursty data, e.g., in a packet network, do not change thegain of the amplifiers in such a configuration. This is in con-trast with networks using erbium which, operated in heavy sat-uration, reacts strongly to such slow changes in average powerwith its millisecond gain dynamics [2].

Demonstrations of SOA-amplified systems with a varyingnumber of channels use 8 or 16 10-Gb/s DWDM channels, halfof which are switched on and off at a slow (kHz) rate [3], [4].Fig. 5 shows received spectra from one of these experiments: 16channels on; 8 channels on and 8 channels off; and 8 channelson and 8 channels switched at 100 kHz, respectively; all afterfour 40-km spans and four SOAs. It is clear that the amplifiergains, and therefore the received channel powers, do not vary ap-preciably with the number of channels. Good eye diagrams andlow error rates were observed in both experiments. Repetitionof one of the experiments with nontransient-controlled EDFAs[4] shows the clear advantage of running a linear system underthese conditions; see Fig. 6.

D. Coarse WDM Systems

Another application area where SOAs can offer an advan-tage is in the amplification of coarse WDM (CWDM) data. TheCWDM standard defines a coarse wavelength grid of 20-nmspaced channels with 13-nm passbands, to allow use of cheapfilter technology and uncooled lasers. Eighteen channels are de-fined from 1270 to 1610 nm. CWDM can add capacity to simple,e.g., Gigabit-Ethernet, point-to-point links, and can add OADMflexibility to more complex datacom networks. Introduction of

ZIMMERMAN AND SPIEKMAN: AMPLIFIERS FOR THE MASSES 65

Fig. 2. Example of incremental growth in 32 channel subbanded DWDM system with fixed add/drop.

Fig. 3. Use of amplets at drop and add sides of flexible wavelength router.

Fig. 4. High-speed applications with wavelength agile transmitter andpreamplified PIN receiver.

the WDM multiplexers adds loss, however, which can be recov-ered using amplets. Varying the composition of the active layer,the gain peak of a SOA can reach any CWDM channel, and thewide gain bandwidth of the SOA (80 nm 3-dB width typical)allows it to amplify a decent number of CWDM channels at atime. A single SOA has even been shown to amplify up to eightCWDM channels over a bandwidth of 140 nm [5]. The outputspectrum of the amplifier is shown in Fig. 7. A margin improve-ment varying from 17 dB in the center of the bandwidth to 5 dB

Fig. 5. Received spectra in a SOA-amplified system with dynamic numberof channels. Top to bottom: all channels on; even channels off; even channelsswitched on and off at 100 kHz.

Fig. 6. Eye diagrams of one of the surviving channels of Fig. 5 (20 ps/div).Left: quasi-linear system using SOAs; right: system with saturated EDFAs.

at the edges was obtained, which allowed extension of the reachof this CWDM system by 30 km.

III. OPTICAL AMPLIFICATION BASICS

Optical gain is the most important property of an amplet.The two families of amplet discussed in this paper, erbium-doped devices and semiconductor-based devices, provide op-tical gain based on different but comparable interactions of lightwith matter. In erbium-based devices, light from a pump sourceelevates ions of the rare-Earth element erbium to an excited state(see Fig. 8). Optical signals with wavelengths that fall withinthe gain spectrum of the erbium induce stimulated emissionand are thereby amplified. In semiconductor devices, the energylevels of the erbium ion are replaced with the energy bands of


Fig. 7. Output spectrum of a SOA amplifying 7 CWDM and 8 DWDMchannels.

Fig. 8. Energy level scheme of the Er ion.

Fig. 9. Carrier recombination in the active layer of a semiconductor amplifier.

the semiconductor crystal, but other than that the gain mecha-nism is similar. The semiconductor is brought into an excitedstate by pumping it electrically, populating the bands with elec-trons and holes. An optical signal propagating through the de-vice gives rise to carrier recombination, and the associated stim-ulated emission amplifies the signal (see Fig. 9).

Note that the device properties that are described in this para-graph apply to traditional optical amplifiers as well as amplets.After all, the characteristics distinguishing amplets from theirlarger cousins are not qualitative but rather quantitative.

A. Device Structure

In order to be amplified efficiently, the signal must propagatethrough the amplifier in a well-confined manner. Therefore, am-plifiers are usually waveguides with gain. The EDFA is the mostwell-known example: a waveguide (the optical fiber) is heavilydoped with erbium ions, which provide gain when optically ex-cited by injection of pump light (Fig. 10). Erbium can also beimplanted into a planar waveguide structure, forming an EDWA.

Similarly, a SOA is formed by enclosing an amplifying activelayer, usually indium gallium arsenide phosphide (InGaAsP) ofan appropriate band gap, between cladding layers of lower re-fractive index, creating a waveguide structure. Light is usuallycoupled into and out of it by means of lenses (see Fig. 11). Thecladding layers of the SOA waveguide are p- and n-doped, re-spectively, allowing electrical pumping by current injection.

Fig. 10. Basic EDFA configuration. A design with counter-propagating pump.

Fig. 11. Typical packaged SOA chip. Lenses are often used to make the twofiber-chip couplings.

Fig. 12. Typical gain versus output power curve of an optical amplifier. The3-dB gain compression point is indicated, usually denoted by P .

B. Gain

The gain spectrum of the optical amplifier is determined bythe energy levels of the erbium ion, or by the bandgap of thesemiconductor. The gain bandwidth of erbium extends fromabout 1525 to 1565 nm, covering a considerable part of thelow-loss window of standard single-mode fiber. The spectralproperties of a SOA are determined by the composition of theInGaAsP active layer, which can be varied to provide gain from1200 to 1650 nm. For a given composition, the gain bandwidthis about 80 nm. The gain spectrum is not the only difference be-tween erbium and semiconductor devices. The lifetime of theexcited state is another distinguishing characteristic. The ex-cited state of erbium has an extremely long lifetime ( 10 ms),leading to slow gain dynamics. As a result, high-data rate sig-nals do not cause any significant gain modulation even in deeplysaturated amplifiers.

In contrast, the carrier lifetime in a SOA typically is 100 ps,i.e., of the order of the bit period in a 10-Gb/s modulated signal.Therefore, amplifying such a signal using a saturated SOA willnormally lead to intersymbol interference (ISI). A third differ-ence is the polarization dependence of the device. An erbium-doped fiber has circular symmetry, and, therefore, the gain of anEDFA will exhibit negligible polarization dependence. EDWAsand SOAs based on asymmetric planar waveguides on the otherhand may exhibit polarization-dependent gain. This is reducedto acceptable levels by proper waveguide design (EDWA) or byintroducing crystal strain (SOA).

C. Output Power

An optical amplifier driven with lots of input power will satu-rate, i.e., its gain will drop from its small-signal gain value. The


reason is that the power source of the amplifier, the number ofexcited erbium atoms or the number of available electron-holepairs, is depleted. The saturation of an optical amplifier is usu-ally referenced to the output power at which the gain has beencompressed by 3 dB, as indicated in Fig. 12. An EDFA can beoperated deeply in saturation (when the input power does notslowly vary, i.e., when the number of optical channels remainsconstant).

A saturated SOA, on the other hand, may give rise to ISI and,in WDM systems, to interchannel crosstalk due to the fast gaindynamics. Therefore, operation of the SOA is usually restrictedto the quasi-linear regime, and consequently it is more difficultto get high output power out of a SOA.

D. Noise Figure

Besides the stimulated emission that creates gain, the gainmedium also produces spontaneous emission, which gives riseto the amplified spontaneous emission (ASE) spectrum of theamplifier. This ASE noise limits the optical signal-to-noise ratio(SNR) of a cascade of amplifiers and is quantified in the ampli-fier’s noise figure (NF). This can be denoted as ,in which is the inversion parameter ofthe amplifier (i.e., the degree of population inversion, withand the fractional number of erbium atoms or carriers in theground and excited states, respectively), and is the input cou-pling loss. Both well-designed EDFAs and SOAs have inversionfactors close to unity, but the fiber-chip coupling loss of the SOAputs it at a disadvantage. EDFA noise figures typically are 4–6dB, while SOA noise figures are usually 6–8 dB.

E. Gain Ripple

Different phenomena are denoted by the term gain ripple inEDFAs and in SOAs. Gain ripple in an EDFA refers to theshape of the gain spectrum which is determined by the wave-length-dependent emission and absorption coefficients of the er-bium-doped fiber, weighed by the fractional populations of theexcited and ground states of the erbium. Gain flattening filtersare sometimes used to reduce this gain ripple. If channel loadingor input levels are changed from their design center, inversionvariation and spectral hole burning will affect the gain flatnessof an EDFA. In-line attenuators are often used in DWDM lineamplifiers to control the inversion and fix the erbium gain, thus,controlling the spectral tilt [6]. This degree of control is seldomused in amplet applications due to its added cost and complexity.

The overall gain spectrum of a SOA is determined by thesemiconductor bands, and has a smooth parabolic shape withoutthe excursions seen in an EDFA gain shape. However, SOAsare extremely short devices ( 1 mm, compared to many me-ters for an EDFA), so that reflections at the end facets can giverise to round-trip resonances that lead to a ripple with a periodof a few tenths of nanometers in the wavelength domain. Withcountermeasures like antireflection coatings and angled facets,the magnitude of this gain ripple can be reduced to 0.1 dB.

IV. ERBIUM AMPLET DESIGN AND TECHNOLOGY

There are two fundamentally different amplet technologiesthat utilize erbium-doped glass as the gain medium: one is

fiber-based (EDFA) and the other is planar waveguide-based(EDWA). Although the guiding structures and design approachare significantly different, one can expect them to have similartemporal, spectral, and saturation performance.

A. EDFA

Erbium-doped fiber has been in use since the late 1980s as thegain medium of choice for optical amplifiers [7]–[9]. In the early1990s, great improvements in efficiency, spectral performance,splicability, and numerical modeling were made [10]–[12] re-sulting in a robust gain medium that was ideal to exploit forlong-haul and metro-area systems. Recent improvements in thecontrol of concentration quenching and reduction in cladding di-ameters to 80 m have yielded fibers that are much better suitedfor amplet use. Shorter EDF lengths and tighter bend radii allowthe developer greater flexibility in the design of smaller pack-ages.

A typical EDFA amplet consists of a pump laser, pump WDMcoupler, EDF spool, input and output isolators, and input andoutput tap/detectors. Each component takes on new characteris-tics for use in an optimal EDFA amplet. Small size and low costdrive the design decisions toward new component choices.

The most costly component for the EDFA amplet is the pump.Design choices favor 980 nm devices for their improved noiseperformance and reduced power consumption. New coolerlessmini-DIL pumps are currently being offered at up to 200 mWoperating power. These devices must operate over large temper-ature and drive current ranges. It is not uncommon to see thepump chip gain peak shift by greater than 20 nm as the temper-ature changes from 0 to 70 degrees C and the output power isvaried from 20 to 200 mW. So that the pump energy remainscentered in the peak erbium absorption region, the pumps arewavelength locked with a fiber grating, often with polarizationmaintaining fiber for improved lock-range over all polarizationstates. Although the mini-DIL pumps are intended to be lowerin cost than the larger butterfly packages, their small size andreduced power consumption are the primary drivers for use inan EDFA amplet [13], [14].

New advances in erbium-doped fiber have created an oppor-tunity to shrink the package size while still maintaining theperformance of larger single pump amplifiers. Newly availablefibers have peak absorptions greater than 30 dB/m while main-taining pumping efficiency and satisfactory noise performance[15]. These fibers allow shortening of the fiber by as much as 3xas compared to standard EDF optimized for DWDM. Vendorsare beginning to offer 80 m cladded versions of these fibers sothat EDF spools can be wound tighter without incurring unduefailure risk.

Optical components are chosen for their small form factorand ease of use. Many optical components are now being of-fered with 80 m cladded fiber. When high NA fiber designs arespecified, bend losses are reduced allowing very tight packagedesigns. The fused fiber components are shorter due to a reducedtaper length that the smaller cladding diameter affords. Photode-tectors with integrated taps are also worthy of consideration dueto their dual-use status while incurring a minimal size penalty.For multiple amplifier array applications, one might even con-sider hybrid architectures that use passive waveguide devices


coupled with EDF to leverage the higher pump efficiency of thefiber and the multicomponent cost and size savings of passivewaveguides.

EDFA technology is clearly maturing. In the future, EDFAamplets will leverage component improvements and design ex-perience in new ways to push the boundaries of size, cost, andperformance.

B. EDWA

The great promise of erbium-doped planar waveguidetechnology is the integration of many functions onto an easilymass-produced photonic IC. Great strides have been madein this area with recent results yielding amplifiers of verygood performance with high degrees of integration. Twobasic technologies have been used to achieve these recentresults: plasma-enhanced chemical vapor deposition (PECVD)and metal ion exchange (IE). With PECVD the passive anderbium-doped waveguides are deposited directly onto thesame silicon substrate in an integrated fashion [16]. With IEtechnology, metal ions are imbedded into a glass substrateto selectively raise the refractive index in the waveguide.Erbium-doped glass is used for the active waveguides and clearglass is used for the passive waveguides. The active and passivesections are then joined together for the final integration [17].

One of the main performance differences between EDFA andEDWA can be seen in pumping efficiency. The concentration oferbium in a EDWA is approximately 10–20 times higher thanthat of an EDFA. Due to the high concentrations of erbium in thewaveguide system, concentration quenching occurs at the higherpumping levels. Additionally, waveguide losses are much higherin planar waveguides than in fiber. With large input signals asmuch as twice the pump power may be required for an EDWA toreach output powers on parity to an EDFA. For some amplet ap-plications this could be a concern. But as available pump powercontinues to go up and pump failure rates diminish, this shouldbecome less of a concern [18].

A key benefit of waveguide technology is the ability to in-tegrate many functions in a cost effective manner, automatingmany of the tasks now currently required to assemble an EDFA.For applications where multiple amplets are required, array am-plifier technology has proven effective at reducing size and ex-pense (see Fig. 13). Pump sharing architectures have been de-veloped to utilize either a one or two high-powered pump(s) anddistribute them to either four or eight individual EDWAs [19].Mach-Zender VOAs are optionally written into the pump pathsto control each EDWA individually as required. Photodetectionof both input and output signals for control purposes has beendemonstrated using numerous schemes. Stray light managementis a key concern that all waveguide designers must consider toachieve accurate monitoring.

The integration of all necessary amplifier components ishampered by the availability of Faraday-effect materials forintegrated isolation. Most suppliers are currently experimentingwith methods to attach bulk isolators to their waveguides withsufficient performance and stability to eliminate fiber coupleddevices. The focus of this activity is cost and size reduction.

As with the EDFA, the cost of the pump is a major concern.Maybe even more so since the pumping efficiency is lower in

Fig. 13. Array EDFA with pump sharing and independent pump control.

Fig. 14. SOA device structure. Mesa, blocking layers, and cladding are oftengrown in three separate MOCVD runs.

EDWA. One method under consideration for controlling costand reducing size is to couple a pump directly to the waveguidewithout an intermediary fiber. The difficulties of achieving astable and robust pump package are well known and as suchwe might expect this activity to take some time to achieve com-mercial acceptance.

Further integrations with additional network functions arejust around the corner. As market demand picks up and a drivetoward the next generation platform that is smaller and lesscostly commences, designers will have greater tools and muchmore flexibility to incorporate amplification into their networkrouting components.

V. SOA DESIGN, TECHNOLOGY, AND DEVICE PHYSICS

SOA device design is similar to semiconductor laser design.The typical SOA is an MOCVD-grown layer structure con-sisting of an active layer sandwiched between p- and n-dopedcladding layers which allow current injection. Lateral opticalconfinement is accomplished by etching a mesa, which isovergrown with a current blocking structure, which can besemi-insulating InP or a diode structure in reverse direction(see Fig. 14).

As aforementioned, a SOA is supposed to deliver gain in atraveling-wave fashion. Unlike a laser structure, that dependson facet reflections, in a SOA reflections must be avoided asmuch as possible, which usually leads to an implementationwith an angled gain stripe [20] and facet antireflection coat-ings [21]. An other important difference is that a laser emitsin one (usually TE) polarization, while a SOA should amplifyincoming signals independent of their polarization. This is ac-complished by tuning the geometry and composition of the ac-tive layer. In particular, the type and amount of crystal strainhas a large influence: Compressive strain leads to TE amplifi-cation, while a tensile strained layer mainly amplifies TM-po-larized light. Careful tuning of the strain in alternating tensileand compressive quantum wells [22], or control of the amount


of tensile strain in quantum wells [23] or in a bulk active layer[24], can deliver small ( 0.2 dB) polarization dependence.

A. Output Power and Gain Dynamics

The output power of a SOA is reported in terms of its ,the power at which the gain is compressed by 3 dB. The highestpower SOAs that have been reported to date possess valuesof 17 dBm [25], [26]. For a single-polarization device, a valueof 20 dBm has been reported [27]. It must be noted that in am-plification applications, the SOA can not be operated at its ,since the fast gain dynamics of the device (carrier lifetime 100ps) would cause its gain to be modulated by the bit pattern onthe input signal. Likewise, cross-gain modulation (XGM) willcause crosstalk in amplified WDM signals. When the device isoperated in its (nearly) linear regime (see Fig. 12), the gain mod-ulation is negligible and WDM operation is feasible, as will bediscussed later.

B. Four-Wave Mixing

The phenomenon of four-wave mixing (FWM) occurs in theSOA as a result of intraband processes such as spectral holeburning and carrier heating [28]. Compared to FWM in fiber,the interaction length in a SOA is so short that no walkoff oc-curs between different wavelength signals, so the strength of themixing products is solely determined by the power of the inter-acting signals and by the FWM-efficiency,which strongly varies with the frequency spacing of the in-teracting signals. The signals must be copolarized for FWM tooccur.

FWM mixing products appear one above and below theinteracting signals. In a WDM system, this usually means theyinterfere with an other channel. Therefore, the power levels ina SOA-based WDM system must be controlled to minimize theoccurrence of FWM. Since the output power of the SOA mustbe confined to the (quasi) linear regime anyway to avoid XGM,this poses no additional limitation in WDM operation for cur-rent generation devices. However, in future higher power SOAs,FWM and not XGM may be the limiting phenomenon when de-signing the system power map.

C. SOA-Based WDM Amplification

Design of systems based on SOAs is different from designingan erbium-based system, in that SOAs are essentially constantgain devices, that should not be saturated in order to avoidXGM, while EDFAs are typically used in constant outputpower mode under heavy saturation. Consequently, the SOAgain has to be matched to the (span or passive component) lossit is meant to compensate. Between the minimum per-channelinput power required to maintain good optical signal-to-noiseratio (OSNR) and the maximum total output power limited byXGM, this leads to moderate span lengths and channel counts.

As an example, a 32-channel (10-Gb/s) system is shown inFig. 15. Here, four SOAs with a gain of 13 dB are used to com-pensate the loss of 40-km spans of standard single mode fiberplus appropriate amounts of dispersion compensating fiber. TheSOAs are operated at an average output power of 7 dBm,which puts the peak power about 2 dB below the of these

Fig. 15. Transmission of 32 WDM channels modulated at 10 Gb/s across four40-km spans of standard fiber using SOAs as line amplifiers.

Fig. 16. Q-factors measured at the end of the system as shown inFig. 15. Varying the launched optical power reveals the limits of SNR andnonlinearities. The left curve (squares) reflects a quasi-linear system; the rightcurve (diamonds) shows the effect of adding a reservoir or ballast channel.

devices, which is 12 dBm. This way, the maximum gain com-pression remains below 1 dB [29]. The (per-channel) SOA inputpower of 21 dBm is sufficient for these devicesto yield reasonable OSNR after four spans.

Fig. 16 shows Q-factors measured at the receiver versuslaunched power. In the optimum, with an OSNR 20 dB, anaverage Q-factor of 16.8 dB is observed ( for allchannels). Based on the OSNR alone, a Q of 18 dB would beexpected (left dashed line). The XGM distortion due to gaincompression in the SOAs (right dashed line) deteriorates the Qwith 1.2 dB. Still, the BER is for all channels.

The method of adding a ballast or reservoir channel has beensuggested to reduce XGM distortion. The always-on reservoirchannel reduces the power swing at the output of the SOA andtherefore partly suppresses the gain modulation. The effect ofthis method depends on the system in which it is used. In an early32 2.5 Gb/s experiment the reservoir channel made a lot ofdifference [30]. On the other hand, in the experiment discussedhere, it allows use of larger output powers, but does not improvethe Q-factor (see Fig. 16).

The output powers delivered by the SOAs in this example aresufficiently moderate to stay out of the regime of fiber nonlin-earities. In such an, essentially linear, system, improvement ofeither the noise figure or the of the devices directly leadsto an equal performance improvement in terms of channel count


or span length. Therefore, with recent anddevices, large improvements over the results as dis-

cussed here are to be expected.

VI. SUMMARY

In this paper, several technologies have been discussed to con-struct optical amplifiers that are suitable for the low-cost, mod-erate performance application space. These amplifiers must besmall in size and easy to control to allow their use in many placesin the network. The different technologies, EDFA, EDWA, andSOA, have different properties making them suitable for a va-riety of applications. Gain, noise figure, and output power ofamplets currently made in these technologies seem to be suit-able for single- and multichannel metro and access operation.The best choice among them is highly application and archi-tecture dependent. Therefore, it pays to be aware of the com-monalities and differences between the members of this class ofdevices.

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Donald R. Zimmerman, photograph and biography not available at the time ofpublication.

Leo H. Spiekman (M’97), photograph and biography not available at the timeof publication.

Amplifiers for the Masses

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