Design and Manufacturing Process Management for Tera-bit/FP Class Submersible Plant Primary author s name: Hiroshi Sakuyama All secondary authors names: Akira Hagisawa, Tomoyuki Harada, Shohei Yamaguchi, Hisatomi Saitoh Primary author s e-mail address: h-sakuyama@ax.jp.nec.com Primary author s company: NEC Corporation Primary author s business address: Shin-Tamachi Building 34-6, Shiba 5-chome, Minato-Ku, Tokyo 108-0014, Japan Abstract: The transmission capacity of submarine cable system has been dramatically increased by enhancing the transmission speed up to 10Gb/s or more and adopting the Dense-WDM technologies ever since the first 5Gb/s optical amplified submarine cable system was constructed in 1990 s. Technologies for maximizing the transmission capacity, such as high-performance 10Gb/s modulation format, high-gain forward-error correction (FEC), narrow-channel spacing multiplexer and demultiplexer, wide-band optical repeater and Dispersion Managed Fibre (DMF), have been developed and improved over the years. Nowadays, submarine cable systems with tera-bit class capacity per fibre pair, which carries more than hundred wavelengths of 10Gb/s optical signals, are being constructed around the world. The wide-band and long-distance submersible plant, especially those with several thousands kilometres of submarine cable and more than hundreds of cascaded repeaters, require extremely flat gain characteristics of less than 0.1dB per repeater section over 30nm bandwidth. On the other hand, in order to achieve high quality DWDM transmission, utilization of DMF is also necessary in order to precisely compensates the chromatic dispersion in each repeater section within several ps/nm/km by combining a plus dispersion fibre and a minus dispersion fibre. Although these requirements seem to be theoretically possible to archive, special considerations are necessary to construct an actual submersible plant meeting them which are most likely more stringent than the manufacturing deviations. This paper introduces NEC s system design approach through our project implementation, and describes how the characteristics of submersible plant mentioned above can be designed and managed in actual manufacturing stages. It will also discuss how the submersible plant can be re-adjusted when some unexpected design changes are required during the project implementation stage. 1. Introduction The demands of communication traffic have been growing every year. In order to support the traffic demands for international communications, the innovative technologies aimed at increasing the capacity of long-distance optical signal transmission systems are being researched and developed continuously. The optical submarine cable system applying these latest technologies are also being planned and constructed one after the other. submarine equipment by connecting submarine cables and submarine repeaters, construction of the submarine plant using cable ships, installation of the terminating equipment, construction of land cables and system testing. The period required from design to completion of a submarine cable system is about a year for a small-scale system or a few years in the case of a large-scale system. Month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Design Repeater Manufacturing Cable Manufacturing System, Equipment 2. Processes of Submarine Cable System Construction Figure 1 shows an example of works for constructing a submarine cable system. It includes system design, equipment design / manufacturing, assembly / testing of the System Assembly Marine Installation Land Plant Terminal Equipment System Test Figure 1 Copyright 2010 SubOptic Page 1 of 6 Permit for Survey Survey Manufacturing Permit for Installation Manufacturing Installation Installation Test Installation Works of submarine cable system construction
3. System Design 3.1 Submarine Plant Design As a first step, a Straight Line Diagram (SLD) showing the configuration of the submarine plant is drawn up based on the cable route and marine chart information. The type of each submarine cable to be utilized is decided based on the sea depth and the geological context of its location, and these cable types are reflected in the SLD. Figure 2 shows the types of submarine cables and the depths where they may be applied. The SLD also outlines the positions of submarine repeaters/joint Boxes (JBs) and the land cable lengths. The interval of the submarine repeaters (repeater span) is decided at the optical signal performance design stage because this affects the transmission performance. The SLD is updated sequentially reflecting the marine survey results and the following sequence of works. Cable Station Figure 2 Double Armour Single Armor Light Weight Screened DA SA LWS Water Depth < 500m WD < 500m Burial or Surface lay WD < 1,500m Surface lay WD < 3,000m Surface lay Light Weight LW Submarine Cable Types and Applicable Depths WD > 3,000m Surface lay 3.2 Transmission Performance Design The Transmission performance is designed based on the system length specified in the SLD. Currently, 10Gb/s wavelength-division multiplexing technology is commonly used. The optical signal performance is designed by considering; the cable length, the loss of optical fibres, the optical output power and noise figure (NF) of submarine repeaters, the number of multiplexed wavelengths, the performance of the 10Gb/s optical transceiver, the transmission penalty, the manufacturing margins of the equipment/systems and the repair margin for the service life. In this step, the intervals of submarine repeaters are adjusted to preserve the required transmission quality and the manufacturing /repair margins. Decreasing the repeater intervals improves the signal to noise ratio (SNR) of the optical signals and makes it possible to preserve more margins, but it also increases the number of repeaters and the system cost. It is therefore required to select the longest possible repeater intervals, so far as they can preserve the desirable system margins. Table 1 shows an example of optical signal performance design (power budget) for a 10Gb/s x 96-wave x 9,000-km system. The optical signal design uses the Q-value, which is the signal quality index generally used in the submarine systems industry. Table 1 Example of the Main Signal Performance Design (Power Budget) Item Value System length 9,000 km 10Gb/s signal multiplexing 96 waves 1 Theoretical transmission 16.0 db performance (Q-value) 2 Transmission 3.2 db penalty/stability 3 Equipment/system 1.6 db manufacturing margin 4 Repair margin 1.4 db 5 System margin 1.0 db 6 System performance limit (1-2-3-4-5) 8.8 db 3.3 Power Feeding Design Power Feeding design is also required as well as the transmission design. The power feeding equipment (PFE) is installed at cable landing stations to supply power to the submarine repeaters via the power feeding conductor in submarine cables. Each submarine plant has multiple submarine repeaters connected in series. They are powered by approximately 1-ampere of DC current from the PFE. The resistance of Copyright 2010 SubOptic Page 2 of 6
the submarine cable is typically less than 1-Ω/km. The voltage drops are caused by; the submarine repeaters, cables, the earth potential difference between landing points (variable between 0 and 0.1 V/km approximately depending on regions) and the insertion of spare repeaters and/or spare cables for repairs. A power feeding voltage of about 11 kv is thus required for a 9,000 km system with four fibre pair arrangement. The power feeding is normally performed from PFEs installed at both landing stations in double-end power feeding system with single-end power feeding capability. Under this configuration, one of the PFEs feeds positive voltage while the other feeds negative voltage, so that each PFE feeds half the required voltage. By adapting this configuration, the system operation can be maintained even when either PFE needs to be turned down for maintenance because another PFE can feed all sufficient voltage in the single-end power feeding mode. 3.4 Monitoring Network Design Each equipment at landing station is monitored for maintenance activities. The Element Management System (EMS) at each landing station monitors both the system and its peripheral equipment. The EMS in the two stations are interconnected via an order-wire channel that utilizes the overhead of the 10Gb/s optical signal of the submarine line terminating equipment (SLTE) so that the EMS at one station can also monitor the equipment status of the other station. Figure. 3 shows an example of a monitoring network configuration for a submarine cable system. The server and client terminal of the EMS are connected to the router at the station, and the router is connected to the order wire channel of the 10Gb/s SLTE, which is subsequently connected to the router at the other station as well as to its EMS server and client. Connecting the landing stations to a remote monitoring centre at a distant location via exclusive lines and networks allows the client terminal at the remote monitoring station to monitor the status of the landing stations and of the submarine plant. Figure 3 Example of Monitoring Network Design 4. Detailed Submarine Plant Design and System Adjustment Technologies The submarine plant of a large-capacity ultra-long distance submarine cable system requires a high-accuracy gain equalization technology in order to achieve the uniform transmission of wavelength-division multiplexed signals without any decreases in their levels. This function is needed even when the dispersion management technology for ultra-long distance transmission is applied. 4.1 Gain Equalization Technology To construct a Trans-Pacific DWDM system such as 10Gb/s x 96-wave, it is necessary to connect about 130pieces of repeaters which have 28-nm optical amplification band. As the requirements of gain flatness within the amplification bandwidth for the overall system is very severe, the gain flatness of submarine repeaters is required to be very accurate. However, it is not practical to manufacture repeaters with such precise characteristics of gain flatness per repeater. Accordingly, a block equalization technique is used. For this technique, characteristics of each repeater and Copyright 2010 SubOptic Page 3 of 6
cable are recognised from manufacturing data in advance, and the end-to-end characteristics are controlled by the combination of amplifiers, cable and block equalizers. The loss of optical fibres in submarine cables has a slight wavelength dependency and its effect is not ignorable. Therefore, the shape of the amplification bandwidth of submarine repeaters should be designed in consideration of the wavelength dependency of the optical fibre loss so that the combined amplification bandwidth in each repeated section will be flat. Although the accuracy of gain flatness of the submarine repeaters and submarine cables is controlled during the manufacturing process, the manufacturing deviations are accumulated in the actual system, and it is hard to achieve the optimum gain flatness required for a 9,000km system. As mentioned above, the block equalization technique solves this issue by compensating the accumulated gain flatness by the insertion of gain equalizers during the submarine equipment assembly, a process connecting submarine repeaters with submarine cables. Figure 4-A) shows the concept of gain equalization. The gain equalizers include the tilt equalizer for correcting tilts inside the amplification bandwidth and the shape equalizer for correcting accumulated amplification bandwidth fluctuations. The tilt equalizers are inserted after every optimum number of repeaters with optimum tilts according to the characteristics under actual conditions. The shape equalizers are designed and fabricated by complementing the gain shapes of the amplification bandwidth of submarine repeaters, and they are inserted at the same position of selected tilt equalizers. As these gain equalization characteristics shall be managed during manufacturing process of submersible plant, summary characteristics data of submersible plant shall be updated and reflected to optimize the gain flatness every day. 4.2 Dispersion Management Technology Figure 4-B) shows an example of a dispersion map, specifically of the dispersion management fibre (DMF) system. Optical fibres both with positive and negative wavelength dispersions are combined in the repeatered sections in such a way that the aggregate dispersion value of each repeatered section will be slightly negative. The negative dispersion accumulated due to multiple repeaters spans is corrected in the dispersion compensation sections that are inserted after each optimum number of repeaters. Again the dispersion value of the optical fibres in each repeater s span shall be strictly controlled during the manufacturing process. The accumulated manufacturing errors caused by the multiple repeaters spans are compensated at the dispersion adjustment sections that are inserted every some repeaters span. A) Concept for Gain Equalization λ1 Repeater λ96 B) Dispersion Map for DMF Dispersion ps/nm λ1 Shape Equalizer λ96 Tilt Equalizer λ1 λ96 Distance (km Figure 4 Gain Equalization and Dispersion Management of Submarine Plant 5. System Assembly Test (SAT) As the last stage of the manufacturing process, submarine repeaters and equalizers are connected with cables. At this stage, System Assembly Test is performed to confirm that the performance of the submarine plant meets the technical requirements which include the followings; Optical spectrum and optical SNR (Signal to Noise Ratio) Gain flatness Trace measurement of optical fibres (C-OTDR) Copyright 2010 SubOptic Page 4 of 6
Wavelength dispersion of optical fibres Insulation of the power feeding line and voltage drop The insertion of gain equalizers and adjustments for dispersion compensation, both described in section 5, are also performed as a part of this process. Since the gain flatness of submarine repeaters varies slightly depending on the repeater temperatures, the system test of an ultra-long distance system with a high accuracy specification is performed at the same temperature conditions as that of the ocean bed. Figure 5 shows examples of measurements for the gain flatness and dispersion values in a 6,000km section of 9,000km system. The measured data indicate that the submarine equipment has been manufactured with extremely high accuracy in accordance with its design specification. A) System Gain Flatness (An example) Gain Flatness (db) 2 1 0-1 -2 1533 1538 1543 1548 1553 1558 1563 1568 Wavelength (nm) B) System Dispersion Map (An example) Accumulated Dispersion (ps) 4000 2000 0-2000 0 1000 2000 3000 4000 5000 6000 Distance (km) Figure 5 Examples of the Characteristics of a Manufactured 6,000km Submarine Plant Section If design parameter change is required to accommodate cable route change for example, re-adjustments of cable length in dispersion compensation section and change of gain equalizers may be required. This adjustment is also performed during SAT. Therefore, it is important to secure the methods and arrangements of re-adjustments for the gain equalization and the dispersion management at the SAT stage, such as preparation of the various type devices for gain equalization and the extra positive/negative fibre cables for dispersion management. 6. System Construction and Commissioning Test As the final stage for system supply, commissioning test is performed after installation of submersible plant, land plant and terminal equipment. During the laying of the submarine plant, the insulation resistance, voltage drop, optical fibre trace and optical SNR are measured periodically to make sure that the laying is performed correctly. Land cable and PFE are installed firstly at the landing station in order to feed the power to submersible plant for monitoring the marine installation works. Then, installation of other terminal equipment in the landing stations is performed in parallel with the laying of the submarine plant. Upon the completion of the submarine plant installation, commissioning test takes place to confirm the performance of the system. The test includes the following items; Electrical performance of submarine plant, such as insulation resistance and voltage drop Optical performance such as SNR and Q-value Network management test (alarm, monitor, etc.) Long-term stability (Q-value, alarm) After the successful completion of commissioning test, the constructed optical submarine cable system is delivered to the customers. 7. Conclusion The construction process of submarine cable systems is performed sequentially from designs for several aspects, the submarine plants and equipment manufacturing, the system Copyright 2010 SubOptic Page 5 of 6
assembly/testing, installation of land equipment, the loading onto the cable ship, the submersible plant laying and finally commissioning. Any interruption in the process will cause significant impacts on the delivery term and costs. It is therefore essential to precisely define the specifications for the equipment and cables from the system design stage, as well as the correction techniques to be applied during the submarine plant assembly, if necessary. These activities should be completed before proceeding to the installation stage. It is also essential to process each stage of design, assembly, construction and testing described above with precise caution in order to provide high-quality submarine cable systems in a timely manner. NEC is committed to introducing the latest technologies while maintaining its high quality standards. NEC will thus contribute to the construction of international and domestic submarine networks with global reputation as a competent player in the submarine cable industry. - End of Paper - Copyright 2010 SubOptic Page 6 of 6