This new amplifier architecture design, while useful in the local, regional, and long haul networks, is even more important in the ULH network, where it can significantly reduce network costs, simplify line architectures, and minimize system maintenance requirements. The ability to utilize EDFA/Raman rather than traditional EDFA amplifiers to eliminate traditional short span (about 80 km) line architecture design was due to three characteristics; 1) a much higher total amplifier effective gain, 2) a much lower amplifier effective noise figure, and 3) a much higher amplifier OSNR
White PaperIn-Line Amplifiers and Hut-Skipping NetworksThis paper summarizes a new application technology in advanced optical transmission amplifier development and a series of successful field trials for hut-skipping line architecture using hybrid Raman/Erbium-Doped Fiber Amplifiers (EDFA). The trials were conducted to show that the application for hut-skipped Ultra Long-Haul (ULH) at 10 to 40 GB/s mixing over 1600 km linear field fibers with an average span loss of 38 dB (160 km/span) is possible. The goal was to develop a new optical amplifier which can handle 10 G/s and 40 G/s copropagating on the same fiber and achieve excellent performance in metro, regional, and ULH networks. This new amplifier architecture design, while useful in the local, regional, and long haul networks, is even more important in the ULH network, where it can significantly reduce network costs, simplify line architectures, and minimize system maintenance requirements. The ability to utilize EDFA/Raman rather than traditional EDFA amplifiers to eliminate traditional short span (about 80 km) line architecture design was due to three characteristics; 1) a much higher total amplifier effective gain, 2) a much lower amplifier effective noise figure, and 3) a much higher amplifier OSNR. IntroductionVerizon has spent over two years on new optical amplifier development in order to improve the current photonic backbone line architectures and save future ULH deployment and maintenance costs. Optovia, a startup company with lot of business, research, and development momentum, has brought several proof of concept amplifiers to Verizon s 400 International Lab and has participated in several technology trials over the last two years.The primary focus of the technology trials was to determine the feasibility of long span (160+ km, up to 40 dB) transmission while maintaining Verizon s required Ultra Long Haul transmission distance of 1500+ km. Optovia s proof-of-concept amplifiers were tested with a commercial Siemens 10 GB/s terminal, Movaz metro 10 GB/s WDM system as well as Mintera s commercial 40 GB/s terminal and Cisco s OC-768c router. Tests were carried out on two fiber types, NDSF (using Verizon s Dallas Fiber Loop) and MDF (Verizon s newly selected transmission fiber). Using the Dallas Fiber Loop, several 160 km NDSF spans were created with an average loss of 38 dB. Excellent transmission performance was shown with the Siemens 10 GB/s system (with pre-FEC error rates less than 1.0E-12 in 2003), Movaz 10 GB/s metro applications, Mintera s 40 GB/s ULH system and Cisco OC-768c router in 2004. The expected transmission length at 10 GB/s is definitely in excess of 1500 km for the normal NRZ modulation on NDSF. Verizon conducted a record breaking field trial in November 2004, achieving a transmission of 1600 km at 40 GB/s with CS-RZ modulation. We predict that 40 GB/s, DPSK modulation, hybrid EDFA/Raman amplifier, and hut-skipping line architecture may exceed 2000 km in the field.The impact of this technology on Verizon s network transmission costs would be significant. The new hybrid amplifier low-cost pump technology lets Verizon use one long span amplifier which is less costly than installing two EDFAs. Add to this the reduced number of OSCs and cheaper DCMs, and the total capital savings could be up to 30 percent of Day-1 costs. From an operational perspective, the reduced number of sites has many advantages including reduced site preparation, less equipment installation, and fewer equipment provisioning requirements. Savings are also expected in sparing and maintenance costs. This paper gives a detailed By David Z. Chen and T. J. XiaNext-Generation ArchitecturesPage 1 of 10Untitled DocumentWhite Paperintroduction of the hybrid EDFA/Raman hut-skipping amplifier technology. At the end, we are going to show the entire lab and field trial results, which demonstrate that the new technology is ready for future deployment.Overview of Hut-Skipping SystemsCurrent optically amplified systems typically deploy line amplifiers every 70 km to 90 km along the transmission route. This distance is determined by the pure fiber loss and the ability of optical amplifier boost power to balance the loss with some margin. In the early 1980s, this spacing generally stemmed from the regenerator spacing used before optical amplifiers were available. Early regenerators were placed every 30 km to 35 km (7 dB to 9 dB span loss) and, as lasers, receivers, and fiber loss improved in performance, the sites were spaced every 40km to 45 km (10 dB to 12 dB) apart. When optical amplifiers became available in the early 1990s, it was a natural choice, given their optical gain capability (up to 25 dB), to place the amplifiers at every other line site resulting in the 70 km to 90 km spacing used today. Let s use the first regeneration-amplifier transition as an example and call these early amplifiers the first hut-skipping solution with respect to the old pure OEO line architectures.In the context of this paper, hut-skipping is now applied to the deployed amplifier systems, resulting in span lengths typically from 140 km to 180 km. A historical perspective of transmission system design, along with the expected new hut-skipping design is shown in Figure 0-1. The figure does not represent the total reach capability of a hut-skipped system (expected to achieve ULH distances) but is used to illustrate the site reduction possibility with these next-generation amplifiers. Figure 0-1: Typical Site Spacing in Transmission System DesignIn the 320 km example of Figure 0-1, the three current line amplifier sites are reduced to a single line amplifier site. In order to understand how this improvement is possible it s important to analyze a chain of optical amplifiers using the systems optical signal-to-noise ratio (OSNR). The following section provides an overview and some basic mathematical analysis of the noise properties of a chain of amplifiers and the impact of extending the spacing between sites. The most important key point for developing the new hybrid EDFA/Raman amplifier is the OSNR improvement.Noise Treatment of Optically Amplified SystemsIn optically amplified transmission systems, the dominant noise term originates from the amplified spontaneous emission (ASE) noise from an amplifier. ASE power spectral density is used in the definition of OSNR; therefore, this ratio must be specified within a given bandwidth.Equation 1Page 2 of 10OSNR =SignalPowerASEPowerUntitled DocumentWhite PaperThe Signal and ASE powers are measured at the output of the amplifier. Figure 1-2 shows the optical transmission line architecture, where G represents gain and L represents fiber loss. Figure 1-2: Schematic of a Uniform Span Loss Link Having a Unity Link GainMoving along this chain, the ASE power from the first amplifier will experience loss (L) as it reaches the input of the second amplifier. The ASE from the first amplifier will experience gain (G) in the second amplifier. Since gain is equal to loss, the ASE power from the first amplifier will return to its original level at the output of the second amplifier. The second amplifier also introduces ASE power, so the total ASE power is now twice the value of ASE generated by a single amplifier. This effect is shown schematically in Figure 1-2. In the case where span loss is equal to gain, and every span loss is identical, the OSNR of a chain of amplifiers is given by:Equation 2Where Nspans is the number of uniform amplifier spans, G and F are the gain and noise figure of a single amplifier in decibels, respectively. PSIG is the per-channel signal launch power in dBm. Most transmission systems designs employ the principal of unity gain; however, the real network is not comprised of uniform span losses. In this case, the above equation does not apply easily. However, approximating the network as uniform spans enables Equation 2 to provide a powerful tool for OSNR estimation. Optimization of optical amplifier deployment in a network requires a balance between amplifier spacing and OSNR performance. The trade-off between these two parameters can easily be understood using Equation 2. Consider a baseline network example of 16 spans having a uniform loss of 18 dB. Table 1 details the trade-off between amplifier spacing and OSNR for traditional Long-Haul EDFAs and Hybrid Raman-EDFAs. As an approximation, an EDFA designed for Long-Haul applications (multiple stages, high output power, mid-stage DCM) can have a noise figure in the vicinity of 6 dB. However, using hybrid Raman-EDFA amplifiers, the same Long Haul amplifiers can have a noise figure in the vicinity of zero dB. In all examples, the signal launch power will be 2 dBm.Table 1: OSNR AnalysisPage 3 of 10OSNR[dB] = 58 + PSIG G F 10 " log10 (NSPANS)OSNR AnalysisGain/Loss [DB]F[DB]Number of SpansOSNR [DB]TypeComments1861624EDFANo hut-skipping. Excess OSNR margin. 36689EDFAHut-skipping with EDFA only. Not enough OSNR margin.360815HybridHut-skipping with Hybrid amp. Enough OSNR margin.Untitled DocumentWhite PaperThe emergence of forward-error correction (FEC) has enabled contemporary 10 GB/s DWDM transmission systems to reduce the beginning-of-life (BOL) OSNR below 15 dB (0.1 nm bandwidth). A system design with BOL OSNR well in excess of this limit is over-designed and consequently not optimized for cost. As Table 1 illustrates, the 16 span x 18 dB route using EDFAs on every span results in an OSNR well in excess of the BOL limits. This result justifies the need to increase the average span at the expense of OSNR. However, an attempt to skip huts using an EDFA-only system falls short of the BOL requirements. Since doubling the span loss leads to 18 dB degradation in OSNR and halving the number of spans leads to a 3 dB increase in OSNR, a reduction of 15 dB OSNR must be accounted for. In this example, there is only 9 dB of excess OSNR so hut-skipping with the EDFA only approach does not work. However, if the noise figure of an amplifier is reduced by 5 to 6 dB hut-skipping can be achieved. This is precisely the value that a hybrid Raman-EDFAs system can deliver.Why Do Hybrid Raman-EDFA Amplifiers Have Lower Noise Figures?Raman amplifiers are generally used to improve the OSNR of a transmission line. This is accomplished by distributing the gain within the transmission fiber. Figure 1-3 demonstrates the impact of the distributed gain on signal power at the end of the transmission span. The red curve is the signal power with the Raman amplifier turned off. The signal is linearly attenuated by the fiber loss. Figure 1-3: Signal Power vs. Distance Within a Fiber Span (With and Without a Raman Amplifier)The blue curve shows the signal power with the Raman amplifier turned on. It s evident that the signal experiences gain within the transmission span. The signal power difference at the end of the span is often referred to as the Raman on/off gain. It should be noted that this expression of gain differs from that used with discrete amplifiers such as EDFAs. With discrete amplifiers the gain is expressed as a net change in signal power from device input to device output. With the gain occurring in the transmission span the effective loss of the span is reduced. This, in turn, can be represented in Equation 2 in two ways, either as a reduced span loss or as a reduced noise figure. In general it is typical to use the reduced noise figure approach for the amplifier site. The effective reduction in span loss is proportional to the Raman gain present in the transmission span. Therefore, the amplifier site-noise figure is also proportional to the distributed Raman gain. Major Field Trial and World Record-Breaking ResultsBeginning in November 2003, Optovia brought three proof-of-concept, hut-skipping amplifiers to the 400 International Lab as part of the 2003 Technology Showcase. To determine the potential capability of Optovia s hut-skipping solution, 160 km spans were created using the Dallas Fiber Loop as a field test bed. This consists of an 80 km fiber ring around the Dallas metro area. Each amplifier span therefore consisted of two loops around the ring. The trial environment is shown in Figure 1-4. Page 4 of 10Untitled DocumentWhite Paper Figure 1-4: Dallas Metro Fiber Loop Trial Environment2003 Technology Showcase ConfigurationFor the Technology Showcase 2003 the Optovia amplifiers were tested using Siemens 10 GB/s terminal equipment. The raw (uncorrected) BER of the Siemens receiver was accessible through the OC-192 craft interface. The system configuration is shown in Figure 1-5. Seven additional wavelengths were multiplexed with the Siemens transmitter to provide additional optical loading on the transmission line. Figure 1-5: 2003 Technology Showcase Configuration Included in Figure1-5 is a photograph of the prototype amplifiers, with color indicators for each location along the transmission line. Measurements of OSNR along the amplifier chain, received BER, transmission line Polarization Dependent Gain, and receiver eye diagrams were carried out on the system. The long-term BER of the transmission system also was measured. The major field trial results are presented in the following section.Page 5 of 10Untitled DocumentWhite Paper2003 Technology Showcase ResultsThe eye diagram at the end of the transmission system is shown in Figure 1-6. Other than the expected noise in the trail, the eye diagram shows no signs of distortion. The symmetrical shape of the eye indicates the transmission system is operating in the linear regime with no non-linear distortion. This is expected given the reduced number of amplifiers in the transmission line compared to a traditional EDFA system with twice as many amplifiers. The final system test monitored the BER of the Siemens OC-192 system over the life of the 2003 Technology Showcase. The first eight hours of data is shown in Figure 1-7 and the BER remained at or below 1.0E-12 for the entire month. This equates to a Q value of 17 dB giving a start-of-life system margin of 8 dB which would be 9.5 dB using the optimized ( final ) amplifier configuration in the following trials. Figure 1-6: Eye Diagram of OC-192 Transmitter After 3 x 160 km NDSF Spans Figure 1-7: Long-Term BER after 3 x 160 km NDSFConclusions for 2003 ShowcaseThis test was the first time a signal from a commercial 10 GB/s terminal was transmitted across a hut-skipped amplifier system operating over field-deployed fiber. The test successfully demonstrated the strong system performance of the transmission line with a BER of the proof of concept Optovia amplifiers it s expected that a full system reach (with sufficient margin) of 1500 km is feasible on NDSF using production amplifiers.2004 Technical Showcase and Verizon Press ReleaseField Trial With Mintera 40 GB/s ApplicationOptovia and Mintera carried out a series of tests to determine the feasibility of 40 GB/s transmission over a hut-skipped transmission system. Tests were carried out on MDF fiber in Verizon s laboratory as well as field-deployed NDSF fiber in the Dallas Fiber Loop. Here, we are only going to show you the field trial results and the trial results with Verizon s recirculating loop which is covered in Verizon s white paper Next Generation Optical Transmission Fibers for Metro, Regional, and Ultra Long-Haul Network Applications and Field Deployment. Page 6 of 10Untitled DocumentWhite PaperMintera 40 GB/s Application OverviewMintera s 40 GB/s MI 40000 unit is a commercially available 40 GB/s transport platform capable of multiplexing a variety of signals into a 40 GB/s stream (OC-48, OC-192, OC-768, and 10GE). The tests carried out in the lab use the MI 4010S card, a transceiver capable of transparently multiplexing four OC-192s into one 40 GB/s stream. The modulation format is a Carrier-Suppressed Return to Zero (CS-RZ) which supports Ultra Long Haul transmission distances. The transmitter contains a built-in mini optical amplifier enabling high launch powers up to +15 dBm. The receiver contains a built-in automatically controlled tunable dispersion compensator (TDC) with a nominal 400 ps/nm tuning range as well as a mini optical pre-amplifier to achieve -15 dBm receiver signal sensitivity. Multiplexed Signal Configuration for 3 Span SystemAll configurations in the following test cases used the same multiplexed set of signals. A combination of 11 CW signals, one 10 GB/s signal and the 40 GB/s signal were prepared using the set-up shown in Figure 1-8. The 3 span receiver eye-diagram is shown in Figure 1-9. The launch spectrum is shown in Figure 1-8. The 40 GB/s wavelength is at 1554.13 nm. Figure 1-8: Multiplexed Transmit Spectrum Figure 1-9: MI 40000 Transmitter Eye DiagramThe nominal output power of the CW and 10 GB/s signals was set to 5 dBm per channel which corresponds to a total output power of 21 dBm for a 40-channel system. Hence, with only 13 channels present, the total output power of the line amplifiers was set to 16 dBm. The channels were selected to cover both edges and the center of a typical C-band EDFA system to simulate the Dense WDM ULH system. The launch power of the 40 GB/s system was scaled individually (up to 13 dBm) to test the non-linear launch power limit on both MDF and NDSF.Page 7 of 10Untitled DocumentWhite Paper4 x 160 km NDSF System on Dallas Loop FiberUsing the MI 40000 s Raman amplifier (incorporated on the LEM card) an additional 160 km span was added to the transmission system, now totaling 640 km of NDSF. The configuration is shown in Figure 1-10. The dispersion compensation required to compensate for the final span was provided by the MI40000 unit. In this configuration, the launch power was scanned from +6 dBm to +13 dBm per channel. Additionally the dispersion tolerance of the receiver was measured and compared to the dispersion tolerance of the back-to-back transceiver.Figure 1-10: 160 km NDSF System Configuration Using Dallas Loop FiberIt was not possible to monitor the received optical spectrum after the MI 40000 Raman amplifier; therefore, the optical spectrum was measured at the output monitor port of the last Optovia amplifier in the system. Overnight results were collected and its stability matched industry standard. Based on the OSNR measurements after each of the initial 3 spans, it s possible to extrapolate the expected system transmission distance at 40 GB/s over 160 km NDSF spans. The extrapolated performance, scaled for reduced launch power with increasing number of spans results in at least a six span system design point, or 960 km. The received eye diagram was measured at +6 dBm, +9 dBm, +10 dBm, and +12 dBm launch power.Figure 1-11: Eye Diagrams for a 4 x 160 km NDSF System (at +6 dBm and +9 dBm Launch Power) Figure 1-12: Eye Diagrams for 4 x 160 km System at +10 dBm and +12 dBm Launch PowerPage 8 of 10Untitled DocumentWhite PaperFinally the dispersion tolerance of the MI 40000 receiver was measured after 4 spans and compared with the back-to-back dispersion tolerance. The results are shown in Figure 1-13 with the 40 GB/s channel launch power set to +10 dBm.Figure 1-13: Receiver Dispersion Tolerance ComparisonThe eye diagram and dispersion tolerance results indicate that the transmission line incurred little or no dispersion penalty on the 40 GB/s system.Finally, Verizon had a record-breaking hut-skipping ULH trial in the middle of November 2004. The trial setup is shown in Figure 1-14. The final BER at the receiver before the FEC was at 2x10^-4 with approximated OSNR of 15 to 16 dB.Figure 1-14: 1600 km Hut-Skipping 10 GB/s and 40 GB/s Mix ULH Trial DiagramPage 9 of 10Untitled DocumentPage 10 of 10White Paper 2006 Verizon. All Rights Reserved. WP10601 01/06The Verizon and Verizon Business names and logos and all other names, logos, and slogans identifying Verizon s products and services are trademarks and service marks or registered trademarks and service marks of Verizon Trademark Services LLC or its affiliates in the United States and/or other countries. All other trademarks and service marks are the property of their respective owners.We never stop working for you.ConclusionsThe Optovia amplifier chain supports 40 GB/s transmission on both NDSF and MDF. Long-span spacing up to 200 km at 40 GB/s is achievable with adequate lifetime system margin. Depending on the total system reach requirement, span lengths of 160 km to 200 km are possible. The system performance indicates that a mixed 10 to 40 GB/s transmission reach of 1600 km on NDSF with 160 km amplifier spacing.The Mintera MI 40000 product utilizes a CSRZ modulation format, suitable for ULH Transmission. The combination of advanced modulation format, powerful Forward Error Correction, and built-in Tunable Dispersion Compensation enables easy deployment over hut-skipped transmission systems.Network Cost Analysis and AdvantagesThe ability to double the amplifier spacing has many advantages from a network point of view. From a CapEx perspective, the Optovia product amplifier using low-cost Raman technology is equivalent to two traditional EDFAs. In addition to the reduced amplifier cost, the other transmission line-item costs also will be reduced. The number of OSCs will be reduced in line with the reduced number of amplifier sites. It s also more cost effective to purchase large DCMs (160+ km) compared to two smaller DCMs (80+ km) due to reduced packaging costs. From an OpEx perspective, the reduced number of amplifier sites has many advantages:" Fewer sites to prepare for install" Fewer sites to install equipment" Fewer sites to provision" Fewer sites to maintain" Reduced sparing costs" Reduced network footprint (floor space, AC space, etc.)" Increased network availability Although it s hard to capture the exact OpEx savings, it s clear that there are multiple benefits that work to reduce the ROI timeframe compared to today s solution. If the route is a new build (i.e., it is not an over-build of an existing route) the installation savings are enormous. The cost of creating a site for amplifier deployment is very large compared to the initial equipment cost. For routes being overbuilt with new fiber it could be possible to save on the fiber install costs due to the reduced number of fiber termination points. This may be something to consider when rolling out the MDF fiber into the network.ReferencesChen, D., and Wellbrock, G. (2005). Long haul hut-skipped transmission of mixed 10 and 40 GB/s signal over deployed SSMF fiber in Verizon s Dallas fiber loop using hybrid Raman erbium-doped fiber amplifers. Optical Fiber Communications Conference.