1Advance in Optical Fiber Amplifier

Chun Jiang, Qingji Zeng, Hua Liu, Xiaodong Tang, Xudong Yang

Center for Broadband Optical Networking Technology, College of Electronics & Information, Shanghai

Jiaotong University, Shanghai 200030, P.R.China

ABSTRACT

Advances in optical fiber amplifier for optical network are reviewed in this paper. Considerable progress hasbeen made in optical amplifier technology in recent years. The bandwidth of amplifiers has increased severaltimes and flat gain amplifiers with more than 80 nm of bandwidth have been demonstrated. With the advent ofRaman fiber amplifiers, more wider bandwidth is obtained. Progress has also been made in the understanding ofamplifier gain dynamics. Several control schemes have been successfully demonstrated to mitigate the signalimpairments due to fast power transients in a chain of amplifiers and will be implemented in optical networkdesign. Terrestrial optical systems have been increasing in transmission capacity. In this review, we focus on the

recent progress in some important aspects of several optical fiber amplifier technology.

Keywords: Erbium doped fiber amplifier, Raman fiber amplifier, Optical network

1.INTRODUCTION

In the past decades years, tremendous progress has been made in the development of optical amplifier

components and technology, including erbium-doped fiber amplifier, Raman fiber amplifier and waveguideamplifier, semiconductor pump lasers, passive components, and splicing and assembly technology. In theresearch area, Optical amplifier with a bandwidth of 80 nm was achieved for the first time. In the meantime, anenormous effort has been under way to incorporate optical amplifier into commercial optical communication

systems. After intensive laboratory research and development, optical amplifiers technology offer anunprecedented cost-effective means to meet the ever-increasing demand for transport capacity, networkingfunctionality, and operational flexibility. In this paper, we focus on the recent progress in some important aspects of

several optical fiber amplifier technology.

2.ERBIUM DOPED FIBER AMPLIFIER

2.1 Optical Network Demand

An standard EDFA is equipped with the features of gain, noise figure, output power, dynamic range andreliability. In practice, however, different network functions require only some of these features. Amplifierfeatures can generally be divided into static parameters and dynamic parameters. To obtain good static

1: Correspondence Author: Emai1:cjiangonhine.sh.cn; Telephone:0086-021-62932166;Fax:0086-021-62820892.

In Rare-Earth-Doped Materials and Devices IV, Shibin Jiang, Editor,318 Proceedings of SPIE Vol. 3942 (2000) • 0277-786X/O0/$1 5.00

parameters, EDFAs with two or more stages are generally used. Zyskind et al.' discuss the basics of two-stageamplifier and some of the related design issues. In this subsection, we concentrate on the recent progress inseveral important aspects of EDFAs technology. In the next subsections, we represent wideband amplifier forhigh capacity applications and conclude with a discussion ofthe current status ofthe EDFA.

2.2 Wideband EDFA

The recent exponential growth in data communications places urgent demands on high capacity communicationnetworks. To increase total capacity, research and development teams can works on one or more of the following

parameters: High speed, which is currently limited by high speed electronics, fiber dispersion, and nonlineareffects; Channel spacing, which is limited by filtering technology and non-linear effects. Amplifier bandwidth,which has paid much attention in recent years. For WDM application, uniform gain is desired for all signalschannels. Generally, the gain is flat somewhere between 1540nm and 1 560nm for an inversion level of 40% to60%.Actually, it is this generic gain that was used in initial WDM systems. Since the ASE power around the1530nm region can be high enough to cause saturation, an ASE filter can be added in the middle stage to blockthe ASE in this band 2 This type of optical amplifier has been successfully used in CATV and early WDMoptical networks3. To fully utilize the gain band between 1530nm and 1565nm, gain equalization filters(GEFs)can be used to flatten the gain spectrum. Several technology have been studied to fabricate GEFs, including thin

film filters, long period gratings ", short-period gratings5,silica wavelength structure6, fused fiber, and acousticfilters7.A GEF is inserted between two erbium doped fibers in order to form a wideband optical amplifier.

Figure 1 Design of wideband optical amplifiers with a C-band and L-band structure

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region ( the C—band) 8,9• Additionally there is variable attenuation in the middle stage whose function will be

discussed in the optical amplifier for practical WDM networking systems" section. this kind of amplifier with35nm of flat bandwidth was used in the long distance transmission of 32nm and 64 channels at 10 Gb/s 8,10

2.3. C-band and L-band

Since the gain drops sharply on both sides of the C-band at 40 to 60 % inversion, it is impractical to furtherincrease the bandwidth with a GEF. However, a flat gain region between 1565nm and 1615nm (the L-band) can

be obtained at a much lower inversion level (20 to 40%)h112. By combining the C-band and the L-band, a much

wider bandwidth can be realized, with the principle shown in figure 1 .Since the initial demonstration ofprinciple12"3"4 much progress has been made in the understanding and design of ultrawideband optical amplifiers

with a split -band structure.

2.4. Automatic gain control

EDFAs are employed in present-day multi-wavelength optical networks to compensate for the loss of fiber spans

and network elements. In these applications, the amplifiers are normally operated in a saturated mode. In theevent of either a network reconfigration or a failure, the number of WDM signals traversing the amplifiers wouldchange and the power of surviving channels would increase or decrease due to a cross-saturation effect in theamplifiers. Dropping channels can give rise to surviving channel errors, since the power of these channels maysurpass the threshold for nonlinear effects such as Brilluin scattering. Adding channels can cause errors bydepressing the power of surviving channels below the receiver sensitivity. To overcome such enor bursts insurviving channels in the network, the signal power transients must be controlled. The response speed requiredfor surviving channel protection is governed by the EDFA transient response, the size of the WDM system, and

the power margins build into the transmission system.

2.4.1 dynamic gain controlBecause of the saturation effect, the speed of gain dynamics in a single EDFA is generally much faster than thespontaneous lifetime of about lOms.The time constant of gain recovery in single-stage amplifier was reported tobe between 110 and 340 s15 The time constant of gain dynamics is a function of the saturation caused by thepump power and the signal power. With the development of high-power EDFAs for WDM communicationsystems, the saturation factor becomes higher and the transient time constant shorter. In a recent report,characteristic transient times have been reported to be tens ms in a two-stage EDFA.16. Figure 2 shows thetransient behavior of surviving channel power for the cases of one, four, and seven dropped channels in an eight-

channel system. In the case of seven dropped channels, the transient time constant is nearly 52 i- s. As shown in

the figure, the transient becomes faster as the number of dropped channels decreases. The time constantdecreases to 29 i- s when only one out of eight channels is dropped. The rate equations 17 for the photons and

the populations of the upper (I3/2) and lower (I5/2) states can be used to derive the following approximateformula for the power transient behavior18:

P(t) c) (1)where P(O) and P( ') are the optical powers at time t =0 and t = , respectively. The characteristic time T

the effective decay time of the upper level averaged over the fiber length. It is used as a fitting parameter to

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obtain the best fit with the experimental data. The model has been used to calculate the fractional powerexcursions in decibels of the surviving channels for the cases of one, four, and seven dropped channels. Thetimes required to limit the power excursion to 1 dB are 18 i s and 8 1-' s when four and seven channels aredropped, respectively. As EDFA technology advances further to support larger numbers of WDMchannels inoptical networks, the transient times will fall below 10 is. Dynamic gain control of EDFAs with fasterresponse times will be necessary to control the signal power transients.

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Figure 2 the transient behavior of surviving channel power for the cases of one, four, and seven droppedchannels in an eight-channel system

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2.4.2 Power transients

In recent studies, the phenomenon of fast power transients in cascaded EDFA was reported.19'20. The effect ofdropped channels on surviving powers in an amplifier chain is illustrated in Figure 3 .When 4 out of 8 WDMchannels are suddenly lost, the output power of each EDFA in the chain drops by 3 dB, and the power in eachsurviving channel then increases toward double the original channel power to conserve the saturated amplifieroutput power. Although the gain dynamics of an individual EDFA are unchanged, the increase in channel powerat the end of the system becomes faster for longer amplifier chains. The fast power transients result from theeffects of the collective behavior in chains of amplifiers. The output of the first EDFA attenuated by the fiberspan loss acts as the input to the second EDFA. Since both the output of the first EDFA and the gain of thesecond EDFA increase with time, the output power of the second amplifier increases at a faster rate. Thiscascading effect results in faster and faster transients as the number of amplifiers increases in the chain. To

prevent performance penalties in a large-scale WDM optical network, surviving channel power excursions mustbe limited to certain values depending on the system margin. For the Multi-channel Optical Networking(MONET), the power swing should be within 0.5 dB when channels are added and within 2 dB when channels

are dropped., In a chain consisting of 10 amplifiers, the response times required to limit the power excursions to0.5 dB and 2 dB would be 0.85 i,i s and 3. 5 i s, respectively. The response times are inversely proportional tothe number of EDFAs in the transmission system.21 .The time response of EDFAs can be divided into threeregions: the initial perturbation region, the inter- mediate oscillation region, and the final steady-state region. Inthe initial perturbation region, the gain of the EDFA increases linearly with time, and the system gain and output

power increase at a rate proportional to the number of EDFAs. Assuming that the amplifiers work under sameconditions, the rate of change of gain at each EDFA is the same and is proportional to the total lost signal power.

The slope increases linearly with the number of cascaded EDFAs. These experimental results have beenconfirmed by modeling and numerical simulation from a dynamic model.22. From the results of bothexperimental measurements and numerical simulation on a system with N EDFAs, the time to reach the peak isfound to be inversely proportional to N, and the slope to the peak is found to be proportional to N 23 These

properties in the perturbation and oscillation regions can be used to predict power excursions in large opticalnetworks.

2.5 Channel protection

Channels in optical networks can suffer from error bursts caused by signal power transients resulting from a linefailure or a network reconfiguration. Such error bursts in surviving channels represent a service impairment,which is absent in electronically switched networks and unacceptable to service providers. The speed of powertransients results from channel loading and there are the speed required to protect against such enor burstsproportional to the number of amplifiers in the network and can be extremely last for large networks. Severalschemes to protect against fast power transients in amplified networks--pump control, link control, and lasercontrol-have been demonstrated in recent years.

2.5.1 Pump controlThe gain of an EDFA can be controlled by adjusting the pump current. Early reported work addressed pumpcontrol on time scales of the spontaneous lifetime in EDFAs24. One of the studies demonstrated low-frequency

feed-forward compensation with low-frequency control. 16.After the discovery of fast power transients, pumpcontrol on short time scales 25 was demonstrated to limit the power excursion of surviving channels. In the

322

absence of gain control, the change in surviving channel signal power exceeds 6 dB. When the pump control on

both stages is active, the power excursion is less than 0.5 dB for both drop and add conditions. The controlcircuit acts to correct the pump power within to 8 i s, which effectively limits the surviving channel powerexcursion.

2.5.2 Link control

The pump control scheme described above requires protection at every amplifier in the network. Anothertechnique uses a control channel in the transmission band to control the gain of amplifiers. Earlier workdemonstrated gain compensation in an EDFA at low frequencies using an idle communication network element.The control channel is stripped at the next network element to prevent improper loading of downstream links.The power of the control channel is adjusted to hold constant the total power of the signal channels and thecontrol channel at the input of the first amplifier. This maintains constant loading of all EDFAs in the link. Theexperimental demonstration of link controlled surviving channel protection is set up with several signal channelsand 1 control channel. A fast feedback circuit with a 4 ii s response time is used to adjust the line controlchannel's power to maintain constant total power. The signal channels and the control channels are transmittedthrough amplified spans of fiber, and the bit error rate (BER) performance of one of the signal channels ismonitored. When most out of several signal channels are added/dropped, the surviving channel suffers from apower penalty over 2 dB and a severe BER floor. With fast link control in operation, power excursions aremitigated, BER penalties are reduced to a few tenths of a dB, and error floors disappear.

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2.5.3 Laser control

Automatic gain control by Laser has been extensively studied since it was experimentally demonstrated26. A newscheme for link control based on laser gain control has recently been reported 271n this work, a compensating

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signal in the first amplifier is generated using an all-optical feedback laser loop; the signal then propagates downthe link. Stabilization is reached within a few tens ms, and output power excursion after 6 EDFAs is reducedfrom more than an order of magnitude to a few tenths of a dB. For laser gain control, the speed is limited bylaser relaxation oscillations28 which are generally on the order of tens of ms or slower. Inhomogeneousbroadening of EDFAs and the resulting spectral hole burning can cause gain variations at the signal wavelength,which limit the extent of control from this technique. The same is true for link control schemes.

2.6.EDFA for WDMSystems

The advent of practical EDFAs has revolutionized almost all aspects of optical communication systems. Thissection focuses on the practical issues that must be addressed in order to design and engineer the optically

amplified communication systems.

2.6.1 SNR

In an optically amplified system, the channel power into the receiver is usually well above the receiversensitivity. The optical signal is optically degraded by the accumulated ASE noise from the optical amplifiers in

the chain. At the photodetector, ASE noise is converted to electrical noise primarily through signal-ASE beating,

which leads to BER flooring. System performance therefore places a stringent requirement on the optical signal-to-noise ratio (OSNR) of each of the optical channels. OSNR is thus the most important design parameter for an

optically amplified system. Other optical parameters in system design consideration are channel powerdivergence and maximum channel power relative to the threshold levels of optical nonlinearities--for example,

self-phase modulation, cross-phase modulation, and four-wave mixing.29

2.6.2 Noise Figure and Output Power

EDFAs are conventionally classified as power amplifiers, in-line amplifiers, and preamplifiers, state-of-the-artWDM systems require all three types of amplifiers to have low noise figure, high output power, and uniformgain spectrum. But here we do not distinguish these three types of amplifiers. The nominal OSNR for a 1 .55mWDM system with N optical transmission spans can be given by the following :

OSNR = 58 +P0- lOloglO(Nh)-L-NF-lO 1og10(N+l) (2)where OSNR is normalized to 0.1 nm bandwidth, P01 is the optical amplifier output power in dBm, Nh is thenumber of WDM channels, L, is the fiber span loss in dB, and NF is the amplifier noise figure in dB. Forsimplicity, we assume that both optical gain and noise figure are uniform for all channels.Equation (2) showshow various system parameters contribute to the OSNR. This equation indicates that we can make tradeoffsbetween the number of channels and the number of spans in designing a system. Note that the tradeoffs may not

be as straightforward in a practical system because of the mutual dependence of some of the parameters. Other

system requirements impose additional constraints, for instance, optical nonlinearities place an upper limit onchannel power, which depends on the number of spans, the fiber type, and the data rate. This simple formula

highlights the importance of two key amplifier parameters--noise figure and output power. While it providesvaluable guidelines for amplifier and system design, it is always necessary to simulate the OSNR evolution in achain of amplifiers when designing a practical WDM system. The amplifier simulation is usually based on anaccurate mathematical model of amplifier performance. Amplifier modeling is also a critical part of the end-to-end system transmission performance simulation that incorporates various linear and nonlinear transmission

penalties.

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2.6.3 Gain FlatnessGain flatness is another key parameter for WDM system design. The worst WDM channel that experiences the

lowest amplifier gain will have an OSNR value lower than the nominal value given in Equation (2). This deficit,

which can be viewed as a type of penalty resulting from amplifier gain non-uniformity, is a complicated functionof the individual amplifier gain shapes and the correlation of the shapes of the amplifiers in the chain. It isassumed the same gain shape for all amplifiers in the chain and calculate the OSNR penalty due to gain non-uniformity, while the absolute penalty may have a value in practical cases, the experimental result show that gainflatness is a parameter that can have a significant impact on the bottom-line OSNR, the penalty is especiallysevere for a long amplifier chain, as in the case of long haul applications, the impact of gain non-uniformity,however, is not limited to the OSNR penalty; it also causes power divergence of WDM channels in a long chain,while the weak channels see an OSNR penalty that limits the system performance, the strong channels continue

to grow in power that may reach the nonlinear threshold, also limiting system performance. Additionally, largepower divergence increases the total crosstalk from other WDMchannels at the optical DMUX output. State-of-

the-art optical amplifiers usually incorporate a gain equalization filter to flatten the gain spectrum. To minimize

the residual gain nonuniformity requires careful design, modeling ,and engineering of the amplifier componentsin particular, the gain equalization filters.2.6.4 Amplifier ControlAn optical amplifier may not always operate at the gain value at which its performance, especially gain flatness,is optimized. Many factors contribute to this non-optimal operating condition, including the fact that the spanloss can be adjusted at system installation and maintained in the system's lifetime only to a finite range withrespect to the value demanded by the amplifiers for optimal performance. As a result, amplifier gain will betilted, and such tilt can have significant impact on system performance in ways similar to gain non-uniformity. Ifnot corrected, gain tilt can result in an OSNR penalty and increased power divergence. Control of opticalamplifier tilt is often necessary to extend the operational range of the amplifiers and compensate for loss tilt inthe system due to, for example, fiber loss variation in the signal band. Control of amplifier gain tilt can beachieved by varying an internal optical aUenuator°. Implementation of such a tilt control function requires afeedback signal that is derived from, for example, measured amplifier gain or channel power spectrum, and analgorithm that coordinates the measurement and adjustment functions. By changing the loss of the attenuator, theaverage inversion level of the erbium doped fiber can be adjusted, which affects the gain tilt in the EDFA gain

spectrum. Another important amplifier control function is amplifier power adjustment. In a WDM system, thereis a need to adjust the total amplifier output as a function of the number of equipped channels. The total output

power must be adjusted so that while the per-channel power is high enough to ensure sufficient OSNR at the endof the chain, it is low enough not to exceed the nonlinear threshold. Additionally, per-channel power must bemaintained within the receiver dynamic range as the system channel loading is changed. Such power adjustmenthas been conventionally achieved by a means of a combination of channel monitoring and software-based pump

power adjustment.The recent advances in WDM optical networking have required a power control fast enough to minimize

channel power excursion when a large number off channels are changed due to, for example, catastrophic partial

system failure. Various techniques have been demonstrated to stabilize amplifier gain, thereby achieving the goal

of maintaining per-channel power. In addition to amplifier dynamics control, practical implementation in asystem also requires a receiver design that can accommodate power change on a very short time scale.

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2.6.5 Performance Monitoring and Fault LocationTo maintain an optically amplified WDM system, it is essential to be able to monitor system parameters andlocate and isolate faults quickly. To achieve this goal, the system must be continually monitored, and thegathered information must be transmitted in a timely manner to the endpoints. A telemetry channel in atraditional regenerated system uses the overhead bits on one signal channel to transmit the maintenance and faultlocation data to the end terminal and provide communication to remote repeater sites. In an optically amplifiedsystem, there is no access to these bits; Instead, other methods have been developed to transport telemetryinformation. The use of a separate optical channel just for telemetry has been widely adopted in practicaloptically amplified systems. Telemetry is added and dropped at each repeater amplifier site to serve the function

of collecting, processing, and transporting maintenance information. It is usually operated at 15 10 urn with acapacity 1 .5 Ivfb/s- a slow data rate for optical networking technology.

2. RAMAN FIBER AMPLIFIER

Recent advances in high power diode lasers and Bragg gratings enable the use of stimulated Raman scattering to

frequency-shift a pump wavelength to amplify optical signals at a long wavelength31, in a 1 .3 1-' m intracavitycascaded Raman amplifier, Braggings were made at both ends of a fiber loop, to selectively confine the pumpand stokes line within the fiber loop. This configuration permits an efficient transfer over several stokes shiftsfrom the 1 .064 ii m pump wavelength to 1 .3 i' m for signals amplification. Most fibers employed in Ramanamplifier are standard germanium doped fibers having a high delta and small core diameter for powerconfinement. Since a relatively long (0.5-2km) fiber loop is used, a reasonably low attenuation loss is requiredbetween the pump wavelength (around 1 ii m) and the signals wavelength (1 .3 U m). Laser at differentwavelengths can be fabricated by employing configurations such as that illustrated in figure 4. This is composedof high-NA germanosilicate fiber. Typically 0. 1 to 1.0 km length, nested within cascaded Bragg gratings andusing a high-power pump sources. The gratings selectively confine and re-circulate the pump and Ramanfrequencies up to the (n-1)-th order while permitting lasing from the n-th order2. Such lasmg can be contrived at

almost any wavelength using different pump and accessing different order Raman peaks.

Figure 5 Schematic diagram of a Raman fiber laser

Amplification of signals in the frequency ranges of the n-th order Raman peak can also be achieved usinggratings that confine Raman frequencies up to the (n-1)-th order. Gain is obtained through n-th order stimulated

326

A1 A2 AA0

A A2 A

Raman emission as the signal propagates through the same fiber. Using this method, amplification of 13 lOnmsignals was demonstrated in germanosilicate fibers using Nd3 cladding pumped laser at 1064nm and cascadedgratings for 11 1 ,1 1 5 and 1240nm Gain up to 25dB has been achieved with 350mW pump power4.Ultrahigh output power of 8.5 W at 14 2nm and having 4 % slope efficiency has been demonstrated in acascade Raman fiber laser. Broadband operation is feasible since Raman peaks are relatively broad and the pumpwavelength of cladding-pumped fiber lasers can be varied by as much as lOOnm.It has been observed that thefrequency shift (l320cm1) of the first-order Raman scattering peak in P-doped silica is about three times largerthan that for Ge-doped and un-doped silica Recently, It has been demonstrated that strong Bragg gratings canbe written using ArF excimer (193nm) in P-doped fibers sensitized by deuterium loading 338.Thus ,P-dopedfibers can permit the use of a fewer number of Stokes transitions between the pump wavelength and 1.3 itm forsignal amplification. Since the Raman gain coefficient at 1320 cm1 in P-doped difficulties in incorporatting ahigh concentration of P-dopant, the delta is usually less than 1 .5-2% in fibers doped exclusively with phosphorus.A significant increase in attenuation loss is also observed in fibers containing a high phosphorus dopantconcentration. Thus, further improvement in the fiber processing will be required to fully utilized the advantagesof P-doped fibers for Raman amplification applications.

3.CONCLUSIONS

The availability of high-performance optical amplifiers and other advanced optical technologies, as well as themarket demand of more bandwidth at lower costs, have made optical networking an attractive solution foradvanced networks. Optical network uses the WDM wavelengths not only to transport large capacity but also toroute and switch different channels. Compared to point-to-point systems, optical networking applications need

higher optical amplifier requirements such as gain flatness, wide bandwidth, and dynamic gain control.Flatness affects system performance in many ways. Flat gain amplifiers are essential for achieving the

system OSNR margin for routed channels and minimizing power divergence to allow practical implementationof networking on the optical layer. Wide bandwidth can either enable large channel spacing as a countermeasureof the filter bandwidth narrowing effect or allow more optical channels for more flexibile routing of traffic.Dynamic gain control is critical to maintaining system performance under varied channel loading conditionscaused by either a network reconfiguration or a partial failure. In addition to the traditional optical amplifier

attributes--output power and noise figure—future amplifiers are not only expected to deliver more (wide signalband) bandwidth and higher-quality (flat gain spectrum) bandwidth, but managed bandwidth with well-controlled gain shape and amplifier dynamics.

Considerable progress has been made in optical amplifier technology in recent years. The bandwidth ofamplifiers has increased several times and flat gain amplifiers with 84 urn ofbandwidth have been demonstrated,made possible by addition of the L-band branch. With the advent of Raman fiber amplifiers, more widerbandwidth is obtained. Progress has also been made in the understanding of amplifier gain dynamics. Severalcontrol schemes have been successfully demonstrated to mitigate the signal impairments due to fast powertransients in a chain of amplifiers and will be implemented in optical network design. Terrestrial optical systems

have been increasing in transmission capacity. To meet the enormous capacity demand, the presently availablehundreds of Gb/s capacity system with more than 50 channels on a single optical fiber will soon be followed by

systems having terabit capacity.

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ACKNOWLEDGEMENTS

Present work is funded by China Government "863-3 1 ".

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