Amplifiers for the Masses: EDFA, EDWA, and SOA Amplets for Metro and Access Applications
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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004 63
Amplifiers for the Masses: EDFA, EDWA, and SOA
Amplets for Metro and Access Applications
Donald R. Zimmerman and Leo H. Spiekman, Member, IEEE
Tutorial Paper
Abstract—Small erbium-doped amplets and semiconductor onto the OA designer to suppress transient crosstalk between
optical amplifiers will be used in current and future metro and wavelengths. As OAs are pushed further toward the edges of
enterprise networks in various configurations. Many new system the network, cost and complexity become key concerns.
architectures will be enabled as these low-cost technologies are
used to compensate for transmission and impairment-compen- As with many maturing technologies, the bifurcation of the
sating component losses. This paper discusses the definition, use, amplifier space into higher and lower performance solutions has
and technologies associated with these new classes of optical am- occurred. Costs for providing basic amplification functionality
plifiers which, though little, will impact next-generation networks have plummeted, driven by improvements in technology and
a great deal. manufacturing efficiency developed during the golden age of
Index Terms—Metropolitan area networks, optical commu- DWDM systems. The everchanging landscape of technological
nication, optical fiber amplifiers, optical planar waveguide development and the economics afforded by each solution force
components, semiconductor optical amplifiers. us to ponder the question: Are there better and more cost-ef-
fective ways to deploy the available technologies than we have
I. INTRODUCTION done in the past? In the early days, it was all about deploying the
maximum bandwidth from point A to B with the lowest overall
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-pump
capital cost. Now carriers are much more concerned with first
deployment cost, capacity growth profile, and operational ex-
penditure. Architectural decisions from the past may not be the
erbium-doped fiber amplifiers (EDFAs) in booster configura-
best solutions for the future.
tions to extend the range of 1.5 m transmission links. Shortly
thereafter, coarse wavelength-division multiplexing (CWDM) B. The Amplet Arrives
was being employed with booster amps to double the capacity
New architectures that take advantage of low-cost, modest
of installed fiber routes. Cascades of EDFA were under inves-
performance amplifiers are being developed today for many dif-
tigation for undersea use; first in single wavelength configura-
ferent applications. These amplifiers have been called “amplets”
tions. It soon became apparent that multiple wavelengths could
by the early adopters of the technology to distinguish them from
be supported with cascaded optical amplifier (OA) systems and
their larger and more complex predecessors. Although it is dif-
an entire industry was born based on dense wavelength-division
ficult to give one overarching definition for this class of optical
multiplexing (DWDM) and EDFA technology.
amplifier, the common thread is reduced performance and lower
cost as compared to a traditional broadband DWDM amplifier.
A. Leveraging DWDM As an example, subbanded DWDM system application amplets
The success of optically amplified DWDM systems in are being used with only 4 to 8 ITU channels (compared to 32 to
long-haul applications was primarily driven by the cost 40 channels of typical broadband DWDM OA) therefore, total
efficiency of sharing these relatively new and expensive power requirements are reduced by as much as 9 to 10 dB. For
gain-flattened EDFAs across many wavelengths. Initial an EDFA, this allows for less pump power and fewer gain stages
DWDM deployments were used in back-bone networks where resulting in a much lower cost product.
network reconfigurations were few and far between. The eco- Each application dictates the required performance and
nomics of DWDM were next applied to metropolitan networks operational characteristics of an amplet but a few critical
where reconfigurations were much more frequent. Wavelength requirements are shared by all. Amplets are expected to be
routing and optical protection switching have become com- low cost. Price is dictated by performance and function but
monplace in these networks. This forces additional complexity must be sufficiently low to supplant other architectural choices.
Amplets must be small in size. Functional packing densities of
Manuscript received June 26, 2003; revised October 7, 2003. systems will increase over time to lower both first installation
D. R. Zimmerman is with the Light Systems Associates, Farmingdale, NJ and operational expenses. Amplets must have low power
07727 USA (e-mail: drzim@optonline.net). consumption. As systems get smaller cooling becomes more
L. H. Spiekman is Vrijkensven 17, 5646HP Eindhoven, The Netherlands
(e-mail: lspiekman@ieee.org). problematic. Amplets must have at least the reliability of their
Digital Object Identifier 10.1109/JLT.2003.822144 big brothers in the telco central office environment. As they
0733-8724/04$20.00 © 2004 IEEE64 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004
and boost signal levels for transmission. This application is
ideal for an erbium-doped waveguide amplifier (EDWA) array
where pump sharing and VOA control could be utilized.
B. High-Speed Systems Improvement
As transmission speed increases to 40 Gb/s and beyond, it
becomes harder to maintain OSNR through the labyrinth of dis-
persion compensation devices in a system. Use of amplets in
both 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 not
have as much power as their DFB counterparts and high bit-rate
modulators are considerably more lossy than lower bit-rate de-
vices. In addition, dispersion precompensation is required in
Fig. 1. Typical specified performance of three amplet types: EDFA, EDWA,
and SOA. many links. An amplet located after modulating and before any
dispersion precompensation allows greater launch power into
either a DWDM mux or a fiber link preserving OSNR. Receiver
make their way out to the network edge, they may be required sensitivity is enhanced through the use of a preamplifier am-
to withstand the harsher conditions of underground vaults and plet before a PIN receiver. In either single channel or DWDM
customer premises. Lastly, amplets must be easy to use. If an systems, the link performance can be improved. Improvements
amplet has a limited operational range or must be controlled of greater than 2 dB have been seen at 10 Gb/s with an EDWA
very accurately it may limit the usefulness of the solution. preamplifier [1].
As can be seen in Fig. 1, the performance of the various
amplet technologies under consideration has improved dra- C. SOAs in Dynamic Channel Count Applications
matically to the point where they are all quite comparable.
Since semiconductor optical amplifiers (SOAs) amplifying
Throughout the remaining few sections the authors will en-
WDM channels will nearly always be run as a linear system,
deavor to describe various amplet applications, technologies,
such a system will automatically be suited to operate at a non-
and design considerations, to shed light on the benefits and
constant total average power. Both changes in the number of
pitfalls with each available solution.
WDM channels, e.g., due to reconfiguration of the network, and
use of bursty data, e.g., in a packet network, do not change the
II. APPLICATIONS OF AMPLETS gain of the amplifiers in such a configuration. This is in con-
trast with networks using erbium which, operated in heavy sat-
Current and future architectural solutions will certainly take uration, reacts strongly to such slow changes in average power
advantage of lower cost, modest performance amplets to provide with its millisecond gain dynamics [2].
better overall network performance with a modularity that scales Demonstrations of SOA-amplified systems with a varying
with deployed capacity. number of channels use 8 or 16 10-Gb/s DWDM channels, half
of which are switched on and off at a slow (kHz) rate [3], [4].
A. DWDM Subbanded Line Systems Fig. 5 shows received spectra from one of these experiments: 16
Single wavelength and subbanded architectures can take channels on; 8 channels on and 8 channels off; and 8 channels
advantage of amplets to provide gain on an incremental growth on and 8 channels switched at 100 kHz, respectively; all after
basis. For example, a 32-channel DWDM system could be four 40-km spans and four SOAs. It is clear that the amplifier
designed using transient controlled DWDM amplifiers and gains, and therefore the received channel powers, do not vary ap-
32-channel wavelength-division multiplexers (WDMs) at the preciably with the number of channels. Good eye diagrams and
terminus points. This is a very cost-effective architecture for low error rates were observed in both experiments. Repetition
fully loaded systems when no wavelength add/drop occurs. of one of the experiments with nontransient-controlled EDFAs
Next, consider the situation where two nodes require wave- [4] shows the clear advantage of running a linear system under
lengths to be dropped using fixed WDM and back-to-back these conditions; see Fig. 6.
DWDM amplifiers. Finally, consider using up to 16 amplets per
node with subbanded multiplexing as shown in Fig. 2. When D. Coarse WDM Systems
the number of add/drop nodes is high and the channel count is Another application area where SOAs can offer an advan-
low, this architecture can be most cost effective. There may be tage is in the amplification of coarse WDM (CWDM) data. The
a small price premium for a fully loaded system but the control CWDM standard defines a coarse wavelength grid of 20-nm
is simpler and it offers the added flexibility of simpler network spaced channels with 13-nm passbands, to allow use of cheap
reconfigurations. Its first installed cost may be substantially filter technology and uncooled lasers. Eighteen channels are de-
lower than a DWDM amplifier solution. Fig. 3 shows how in a fined from 1270 to 1610 nm. CWDM can add capacity to simple,
fully reconfigurable wavelength add/drop node amplets would e.g., Gigabit-Ethernet, point-to-point links, and can add OADM
be used at both drop and add ports to overcome device losses flexibility to more complex datacom networks. Introduction ofZIMMERMAN 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. 5. Received spectra in a SOA-amplified system with dynamic number
of channels. Top to bottom: all channels on; even channels off; even channels
switched 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 reach
of this CWDM system by 30 km.
Fig. 4. High-speed applications with wavelength agile transmitter and III. OPTICAL AMPLIFICATION BASICS
preamplified PIN receiver.
Optical gain is the most important property of an amplet.
the WDM multiplexers adds loss, however, which can be recov- The two families of amplet discussed in this paper, erbium-
ered using amplets. Varying the composition of the active layer, doped devices and semiconductor-based devices, provide op-
the gain peak of a SOA can reach any CWDM channel, and the tical gain based on different but comparable interactions of light
wide gain bandwidth of the SOA (80 nm 3-dB width typical) with matter. In erbium-based devices, light from a pump source
allows it to amplify a decent number of CWDM channels at a elevates ions of the rare-Earth element erbium to an excited state
time. A single SOA has even been shown to amplify up to eight (see Fig. 8). Optical signals with wavelengths that fall within
CWDM channels over a bandwidth of 140 nm [5]. The output the gain spectrum of the erbium induce stimulated emission
spectrum of the amplifier is shown in Fig. 7. A margin improve- and are thereby amplified. In semiconductor devices, the energy
ment varying from 17 dB in the center of the bandwidth to 5 dB levels of the erbium ion are replaced with the energy bands of66 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004
Fig. 10. Basic EDFA configuration. A design with counter-propagating pump.
Fig. 7. Output spectrum of a SOA amplifying 7 CWDM and 8 DWDM
channels.
Fig. 11. Typical packaged SOA chip. Lenses are often used to make the two
fiber-chip couplings.
Fig. 8. Energy level scheme of the Er ion.
Fig. 12. Typical gain versus output power curve of an optical amplifier. The
3-dB gain compression point is indicated, usually denoted by P .
B. Gain
The gain spectrum of the optical amplifier is determined by
the energy levels of the erbium ion, or by the bandgap of the
Fig. 9. Carrier recombination in the active layer of a semiconductor amplifier. semiconductor. The gain bandwidth of erbium extends from
about 1525 to 1565 nm, covering a considerable part of the
the semiconductor crystal, but other than that the gain mecha- low-loss window of standard single-mode fiber. The spectral
nism is similar. The semiconductor is brought into an excited properties of a SOA are determined by the composition of the
state by pumping it electrically, populating the bands with elec- InGaAsP active layer, which can be varied to provide gain from
trons and holes. An optical signal propagating through the de- 1200 to 1650 nm. For a given composition, the gain bandwidth
vice gives rise to carrier recombination, and the associated stim- is about 80 nm. The gain spectrum is not the only difference be-
ulated emission amplifies the signal (see Fig. 9). tween erbium and semiconductor devices. The lifetime of the
Note that the device properties that are described in this para- excited state is another distinguishing characteristic. The ex-
graph apply to traditional optical amplifiers as well as amplets. cited state of erbium has an extremely long lifetime ( 10 ms),
After all, the characteristics distinguishing amplets from their leading to slow gain dynamics. As a result, high-data rate sig-
larger cousins are not qualitative but rather quantitative. nals do not cause any significant gain modulation even in deeply
saturated amplifiers.
A. Device Structure In contrast, the carrier lifetime in a SOA typically is 100 ps,
In order to be amplified efficiently, the signal must propagate i.e., of the order of the bit period in a 10-Gb/s modulated signal.
through the amplifier in a well-confined manner. Therefore, am- Therefore, amplifying such a signal using a saturated SOA will
plifiers are usually waveguides with gain. The EDFA is the most normally lead to intersymbol interference (ISI). A third differ-
well-known example: a waveguide (the optical fiber) is heavily ence is the polarization dependence of the device. An erbium-
doped with erbium ions, which provide gain when optically ex- doped fiber has circular symmetry, and, therefore, the gain of an
cited by injection of pump light (Fig. 10). Erbium can also be EDFA will exhibit negligible polarization dependence. EDWAs
implanted into a planar waveguide structure, forming an EDWA. and SOAs based on asymmetric planar waveguides on the other
Similarly, a SOA is formed by enclosing an amplifying active hand may exhibit polarization-dependent gain. This is reduced
layer, usually indium gallium arsenide phosphide (InGaAsP) of to acceptable levels by proper waveguide design (EDWA) or by
an appropriate band gap, between cladding layers of lower re- introducing crystal strain (SOA).
fractive index, creating a waveguide structure. Light is usually
coupled into and out of it by means of lenses (see Fig. 11). The C. Output Power
cladding layers of the SOA waveguide are p- and n-doped, re- An optical amplifier driven with lots of input power will satu-
spectively, allowing electrical pumping by current injection. rate, i.e., its gain will drop from its small-signal gain value. TheZIMMERMAN AND SPIEKMAN: AMPLIFIERS FOR THE MASSES 67
reason is that the power source of the amplifier, the number of fiber-based (EDFA) and the other is planar waveguide-based
excited erbium atoms or the number of available electron-hole (EDWA). Although the guiding structures and design approach
pairs, is depleted. The saturation of an optical amplifier is usu- are significantly different, one can expect them to have similar
ally referenced to the output power at which the gain has been temporal, spectral, and saturation performance.
compressed by 3 dB, as indicated in Fig. 12. An EDFA can be
operated deeply in saturation (when the input power does not A. EDFA
slowly vary, i.e., when the number of optical channels remains Erbium-doped fiber has been in use since the late 1980s as the
constant). gain medium of choice for optical amplifiers [7]–[9]. In the early
A saturated SOA, on the other hand, may give rise to ISI and, 1990s, great improvements in efficiency, spectral performance,
in WDM systems, to interchannel crosstalk due to the fast gain splicability, and numerical modeling were made [10]–[12] re-
dynamics. Therefore, operation of the SOA is usually restricted sulting in a robust gain medium that was ideal to exploit for
to the quasi-linear regime, and consequently it is more difficult long-haul and metro-area systems. Recent improvements in the
to get high output power out of a SOA. control of concentration quenching and reduction in cladding di-
ameters to 80 m have yielded fibers that are much better suited
D. Noise Figure for amplet use. Shorter EDF lengths and tighter bend radii allow
Besides the stimulated emission that creates gain, the gain the developer greater flexibility in the design of smaller pack-
medium also produces spontaneous emission, which gives rise ages.
to the amplified spontaneous emission (ASE) spectrum of the A typical EDFA amplet consists of a pump laser, pump WDM
amplifier. This ASE noise limits the optical signal-to-noise ratio coupler, EDF spool, input and output isolators, and input and
(SNR) of a cascade of amplifiers and is quantified in the ampli- output tap/detectors. Each component takes on new characteris-
fier’s noise figure (NF). This can be denoted as , tics for use in an optimal EDFA amplet. Small size and low cost
in which is the inversion parameter of drive the design decisions toward new component choices.
the amplifier (i.e., the degree of population inversion, with The most costly component for the EDFA amplet is the pump.
and the fractional number of erbium atoms or carriers in the Design choices favor 980 nm devices for their improved noise
ground and excited states, respectively), and is the input cou- performance and reduced power consumption. New coolerless
pling loss. Both well-designed EDFAs and SOAs have inversion mini-DIL pumps are currently being offered at up to 200 mW
factors close to unity, but the fiber-chip coupling loss of the SOA operating power. These devices must operate over large temper-
puts it at a disadvantage. EDFA noise figures typically are 4–6 ature and drive current ranges. It is not uncommon to see the
dB, while SOA noise figures are usually 6–8 dB. pump chip gain peak shift by greater than 20 nm as the temper-
ature changes from 0 to 70 degrees C and the output power is
E. Gain Ripple varied from 20 to 200 mW. So that the pump energy remains
centered in the peak erbium absorption region, the pumps are
Different phenomena are denoted by the term gain ripple in
wavelength locked with a fiber grating, often with polarization
EDFAs and in SOAs. Gain ripple in an EDFA refers to the
maintaining fiber for improved lock-range over all polarization
shape of the gain spectrum which is determined by the wave-
states. Although the mini-DIL pumps are intended to be lower
length-dependent emission and absorption coefficients of the er-
in cost than the larger butterfly packages, their small size and
bium-doped fiber, weighed by the fractional populations of the
reduced power consumption are the primary drivers for use in
excited and ground states of the erbium. Gain flattening filters
an EDFA amplet [13], [14].
are sometimes used to reduce this gain ripple. If channel loading
New advances in erbium-doped fiber have created an oppor-
or input levels are changed from their design center, inversion
tunity to shrink the package size while still maintaining the
variation and spectral hole burning will affect the gain flatness
performance of larger single pump amplifiers. Newly available
of an EDFA. In-line attenuators are often used in DWDM line
fibers have peak absorptions greater than 30 dB/m while main-
amplifiers to control the inversion and fix the erbium gain, thus,
taining pumping efficiency and satisfactory noise performance
controlling the spectral tilt [6]. This degree of control is seldom
[15]. These fibers allow shortening of the fiber by as much as 3x
used in amplet applications due to its added cost and complexity.
as compared to standard EDF optimized for DWDM. Vendors
The overall gain spectrum of a SOA is determined by the
are beginning to offer 80 m cladded versions of these fibers so
semiconductor bands, and has a smooth parabolic shape without
that EDF spools can be wound tighter without incurring undue
the excursions seen in an EDFA gain shape. However, SOAs
failure risk.
are extremely short devices ( 1 mm, compared to many me-
Optical components are chosen for their small form factor
ters for an EDFA), so that reflections at the end facets can give
and ease of use. Many optical components are now being of-
rise to round-trip resonances that lead to a ripple with a period
fered with 80 m cladded fiber. When high NA fiber designs are
of a few tenths of nanometers in the wavelength domain. With
specified, bend losses are reduced allowing very tight package
countermeasures like antireflection coatings and angled facets,
designs. The fused fiber components are shorter due to a reduced
the magnitude of this gain ripple can be reduced to 0.1 dB.
taper length that the smaller cladding diameter affords. Photode-
tectors with integrated taps are also worthy of consideration due
IV. ERBIUM AMPLET DESIGN AND TECHNOLOGY to their dual-use status while incurring a minimal size penalty.
There are two fundamentally different amplet technologies For multiple amplifier array applications, one might even con-
that utilize erbium-doped glass as the gain medium: one is sider hybrid architectures that use passive waveguide devices68 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 1, JANUARY 2004
coupled with EDF to leverage the higher pump efficiency of the
fiber and the multicomponent cost and size savings of passive
waveguides.
EDFA technology is clearly maturing. In the future, EDFA
amplets will leverage component improvements and design ex-
perience in new ways to push the boundaries of size, cost, and
performance.
B. EDWA
The great promise of erbium-doped planar waveguide
technology is the integration of many functions onto an easily
mass-produced photonic IC. Great strides have been made
in this area with recent results yielding amplifiers of very Fig. 13. Array EDFA with pump sharing and independent pump control.
good performance with high degrees of integration. Two
basic technologies have been used to achieve these recent
results: plasma-enhanced chemical vapor deposition (PECVD)
and metal ion exchange (IE). With PECVD the passive and
erbium-doped waveguides are deposited directly onto the
same silicon substrate in an integrated fashion [16]. With IE
technology, metal ions are imbedded into a glass substrate Fig. 14. SOA device structure. Mesa, blocking layers, and cladding are often
grown in three separate MOCVD runs.
to selectively raise the refractive index in the waveguide.
Erbium-doped glass is used for the active waveguides and clear
EDWA. One method under consideration for controlling cost
glass is used for the passive waveguides. The active and passive
and reducing size is to couple a pump directly to the waveguide
sections are then joined together for the final integration [17].
without an intermediary fiber. The difficulties of achieving a
One of the main performance differences between EDFA and
stable and robust pump package are well known and as such
EDWA can be seen in pumping efficiency. The concentration of
we might expect this activity to take some time to achieve com-
erbium in a EDWA is approximately 10–20 times higher than
mercial acceptance.
that of an EDFA. Due to the high concentrations of erbium in the
Further integrations with additional network functions are
waveguide system, concentration quenching occurs at the higher
just around the corner. As market demand picks up and a drive
pumping levels. Additionally, waveguide losses are much higher
toward the next generation platform that is smaller and less
in planar waveguides than in fiber. With large input signals as
costly commences, designers will have greater tools and much
much as twice the pump power may be required for an EDWA to
more flexibility to incorporate amplification into their network
reach output powers on parity to an EDFA. For some amplet ap-
routing components.
plications this could be a concern. But as available pump power
continues to go up and pump failure rates diminish, this should
V. SOA DESIGN, TECHNOLOGY, AND DEVICE PHYSICS
become less of a concern [18].
A key benefit of waveguide technology is the ability to in- SOA device design is similar to semiconductor laser design.
tegrate many functions in a cost effective manner, automating The typical SOA is an MOCVD-grown layer structure con-
many of the tasks now currently required to assemble an EDFA. sisting of an active layer sandwiched between p- and n-doped
For applications where multiple amplets are required, array am- cladding layers which allow current injection. Lateral optical
plifier technology has proven effective at reducing size and ex- confinement is accomplished by etching a mesa, which is
pense (see Fig. 13). Pump sharing architectures have been de- overgrown with a current blocking structure, which can be
veloped to utilize either a one or two high-powered pump(s) and semi-insulating InP or a diode structure in reverse direction
distribute them to either four or eight individual EDWAs [19]. (see Fig. 14).
Mach-Zender VOAs are optionally written into the pump paths As aforementioned, a SOA is supposed to deliver gain in a
to control each EDWA individually as required. Photodetection traveling-wave fashion. Unlike a laser structure, that depends
of both input and output signals for control purposes has been on facet reflections, in a SOA reflections must be avoided as
demonstrated using numerous schemes. Stray light management much as possible, which usually leads to an implementation
is a key concern that all waveguide designers must consider to with an angled gain stripe [20] and facet antireflection coat-
achieve accurate monitoring. ings [21]. An other important difference is that a laser emits
The integration of all necessary amplifier components is in one (usually TE) polarization, while a SOA should amplify
hampered by the availability of Faraday-effect materials for incoming signals independent of their polarization. This is ac-
integrated isolation. Most suppliers are currently experimenting complished by tuning the geometry and composition of the ac-
with methods to attach bulk isolators to their waveguides with tive layer. In particular, the type and amount of crystal strain
sufficient performance and stability to eliminate fiber coupled has a large influence: Compressive strain leads to TE amplifi-
devices. The focus of this activity is cost and size reduction. cation, while a tensile strained layer mainly amplifies TM-po-
As with the EDFA, the cost of the pump is a major concern. larized light. Careful tuning of the strain in alternating tensile
Maybe even more so since the pumping efficiency is lower in and compressive quantum wells [22], or control of the amountZIMMERMAN AND SPIEKMAN: AMPLIFIERS FOR THE MASSES 69
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 highest
power SOAs that have been reported to date possess values
of 17 dBm [25], [26]. For a single-polarization device, a value
of 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 100
ps) would cause its gain to be modulated by the bit pattern on
the input signal. Likewise, cross-gain modulation (XGM) will
cause crosstalk in amplified WDM signals. When the device is Fig. 15. Transmission of 32 WDM channels modulated at 10 Gb/s across four
operated in its (nearly) linear regime (see Fig. 12), the gain mod- 40-km spans of standard fiber using SOAs as line amplifiers.
ulation is negligible and WDM operation is feasible, as will be
discussed later.
B. Four-Wave Mixing
The phenomenon of four-wave mixing (FWM) occurs in the
SOA as a result of intraband processes such as spectral hole
burning 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 the
mixing 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 to
occur. Fig. 16. Q-factors measured at the end of the system as shown in
FWM mixing products appear one above and below the Fig. 15. Varying the launched optical power reveals the limits of SNR and
nonlinearities. The left curve (squares) reflects a quasi-linear system; the right
interacting signals. In a WDM system, this usually means they curve (diamonds) shows the effect of adding a reservoir or ballast channel.
interfere with an other channel. Therefore, the power levels in
a SOA-based WDM system must be controlled to minimize the devices, which is 12 dBm. This way, the maximum gain com-
occurrence of FWM. Since the output power of the SOA must pression remains below 1 dB [29]. The (per-channel) SOA input
be confined to the (quasi) linear regime anyway to avoid XGM, power of 21 dBm is sufficient for these devices
this poses no additional limitation in WDM operation for cur- to yield reasonable OSNR after four spans.
rent generation devices. However, in future higher power SOAs, Fig. 16 shows Q-factors measured at the receiver versus
FWM and not XGM may be the limiting phenomenon when de- launched power. In the optimum, with an OSNR 20 dB, an
signing the system power map. average Q-factor of 16.8 dB is observed ( for all
channels). Based on the OSNR alone, a Q of 18 dB would be
C. SOA-Based WDM Amplification expected (left dashed line). The XGM distortion due to gain
Design of systems based on SOAs is different from designing compression in the SOAs (right dashed line) deteriorates the Q
an erbium-based system, in that SOAs are essentially constant with 1.2 dB. Still, the BER is for all channels.
gain devices, that should not be saturated in order to avoid The method of adding a ballast or reservoir channel has been
XGM, while EDFAs are typically used in constant output suggested to reduce XGM distortion. The always-on reservoir
power mode under heavy saturation. Consequently, the SOA channel reduces the power swing at the output of the SOA and
gain has to be matched to the (span or passive component) loss therefore partly suppresses the gain modulation. The effect of
it is meant to compensate. Between the minimum per-channel this method depends on the system in which it is used. In an early
input power required to maintain good optical signal-to-noise 32 2.5 Gb/s experiment the reservoir channel made a lot of
ratio (OSNR) and the maximum total output power limited by difference [30]. On the other hand, in the experiment discussed
XGM, this leads to moderate span lengths and channel counts. here, it allows use of larger output powers, but does not improve
As an example, a 32-channel (10-Gb/s) system is shown in the Q-factor (see Fig. 16).
Fig. 15. Here, four SOAs with a gain of 13 dB are used to com- The output powers delivered by the SOAs in this example are
pensate the loss of 40-km spans of standard single mode fiber sufficiently moderate to stay out of the regime of fiber nonlin-
plus appropriate amounts of dispersion compensating fiber. The earities. In such an, essentially linear, system, improvement of
SOAs are operated at an average output power of 7 dBm, either the noise figure or the of the devices directly leads
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