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development progress of 40 gbs (stm 256) sdh optical system in

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									Feature Articles: Optical Fiber Communications

FEATURE ARTICLES

Development Progress of 40 Gb/s (STM-256) SDH Optical System in China
Mao Qian Professor, Wuhan Research Institute of P&T. Wuhan, Hubei, China

ABSTRACT
According to the requirement of large information transport, 40Gb/s (STM-256) SDH optical communication system is the very useful technology for national information infrastructure. In this paper, the development progress has been introduced in terms of the key technologies of 40Gb/s equipment and system, and some test results have been showed. Key words: 40Gb/s, STM-256, SDH, optical communication, dispersion, PMD

(STM-256) SDH optical communication system in China is described based on the "10th five year plan of National Key Technologies R&D Programme (NKTRDP)".

II. 40GB/S (STM-256) SDH OPTICAL COMMUNICATION SYSTEM
TDM is the basic technology to increase the system's capacity or bit-rate. The highest level of SDH hierarchy is STM-256, i.e. 40Gb/s. In the world, there are only a few companies can provide such equipment. In 2002, Wuhan Research Institute of P&T, China (WRI) initialized the STM-256 optical communication system R&D project, or the NKTRDP project. The final target of this project is to establish a 40Gb/s optical transmission field trial and provide commercial use in the future. The trial system consists of two terminal equipment and five optical repeaters (i.e. optical amplifiers) for a linear link system or three ADM nodes for ring systems, which are shown in Fig 1. This project was implemented at the end of 2004, and it was checked and accepted by MII of China in June, 2005.

I. INTRODUCTION
The total length of installed optical cable is more than 3.38M km in China, including 646 K km of core network and 2.73M km of local network. But the microwave line length is only 190 K km. So we can see almost 95% of the total information is transported through optical transport network in China. On the other hand, there are a total of 700M telephone subscribers (340M POTS subscribers and 360M mobile phone users) in China. This is the largest telecommunication network in the world. Thus, more information transportation is needed. In this paper the development status of 40Gb/s

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Feature Articles: Optical Fiber Communications

Fig

(a) STM

Linear link

(1) Where, L is the maximum transported distance, C is the velocity of light, D is the dispersion coefficient of used optical fiber, is the operating wavelength, B is the bit rate of transported signal. Table 1 shows the calculation results for transported signal from 2.5Gb/s to 40Gb/s. Table.1 Limited distances for transported signal from 2.5Gb/s to 40Gb/s.
Bit rate 1550nm(G.652) 1550nm(G.655) 1310nm(G.652) 2.5Gb/s 10Gb/s 40Gb/s 928km 58km 3.6km 4528km 283km 18km 6400km 400km 25km

(b) STM

ring

III. KEY TECHNOLOGIES IN 40GB/S SYSTEM
There are several key technologies in the 40Gb/ s system, such as chromatic dispersion compensation, Polarization Mode Dispersion (PMD) compensation, nonlinear control, optical amplifier technology, OSNR control and so on. All of such problems have been well resolved in this system by theoretic studies, computer simulations and practical experiments. For example, the chromatic dispersion in optical fiber could cause a pulse to pread out progressively as it travels along the fiber. This spreading leads to interference between adjacent pulses (called inter-symbol interference), which limits the distance of system signal. The limited distance calculation is as following formula:

So, to send the 40Gb/s signals over hundreds of kilometers in G.652 and G.655 fiber operating at 1550nm, we must use dispersion compensation technologies to reduce the dispersion effects. Our methods combine the use of Dispersion Compensation Fiber (DCF), tunable dispersion compensator and chirp fiber grating. Thus system dispersion could be compensated exactly. Another example is PMD effect. PMD results from the fact that light signal energy at the operating wavelength in the fiber actually occupies two orthogonal polarization states or modes. The resulting difference in propagation time between the two

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orthogonal polarization states will result in pulse spreading. So, high bit rate signal could not be transported to long distance due to the PMD effect. The limited distance could be calculated using formula (2)

(2)

ensure that the OSNR could be more than 25dB at system sink end. All of those technologies mentioned above have been applied in our 40Gb/s optical communication system, and the experiment results could show the coherence between theoretic calculation and practical system test.

IV. TEST RESULTS OF Where: L is the maximum transported distance, 40GB/S TRIAL SYSTEM B is the bit rate of transported signal, D PMD is the PMD coefficient of used optical fiber. Fig.2 shows the calculation results for different bit rate signals and PMD coefficients. We can see only 25km could be transported for 40Gb/s signal in G.652A and G.655A fiber from Fig.2. But right now, almost all of the fiber's PMD coefficients are very small, typically less than 0. 05ps/km 1/2. So, the Fig The limited transport distances for different bit rate signals and PMD limited distance of coefficients 40Gb/s signal could We have implemented the 40Gb/s STM-256 optical reach to 2 500km, even there is no PMD compensatransmission system trial using NRZ line code, withtion work needed in the general project. out FEC, without PMD compensation and without For nonlinear effect, we could reduce its influence electrical regenerator over 560km (7 80km by incidence optical power control. According to based on Fig. 1a) in G.652 and G.655 fiber our theoretic calculation and computer simulation, respectively. Table 2 shows the test results of 1~+3dBm (for G.655 fiber) and 0~+2 dBm (for G. major objectives of STM-256 trial system. 652 fiber) is the optimum incidence optical power in Fig. 4 shows the optical signal eye pattern at our system. transmitter end and receiver end after 560km Due to the fact that the noise figure of Raman transmission respectively. We could see a very optical amplifier is less than 0 dB, so we use EDFA plus Raman optical amplifier in the line system to clear eye open and there are more margins in the

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Table.2 Major objectives of STM-256 trial system
Parameter Application code Operating wavelength range Max. -20dB spectral wide Side mode suppression ratio Mean launched power Min. return loss at S point Min. extinction ratio Eye pattern mask Receiver Min. sensitivity Receiver Min. overload Max. Reflectance at R point S-R path penalty Max. non-congestion cross Cap. Tributary signal level dB dB Gb/s dBm nm nm dB dBm dB dB Unit Objective I.256-2, L.256-2, L.256-3 1530~1565 0.52 45.2 +1.5 36 11.9 Compliance with Fig.3 and Table 3 - 20.1 0 - 42 1.8 320 STM16, 64, GE

receiver end eye pattern, so it is possible to extend more transmission distance. We have done the trial test for 40Gb/s system transmission pass through 480km in G.655 fiber over 30 days. The error performance is as follows: B1 error: 0 B2 error: 0 B3 error: better than 2 10-18

V. CONCLUSION
Fig Eye pattern mask

This 40Gb/s STM-256 SDH optical transmission system is the first one in China. A transmission Table.3 Parameters of eye pattern mask distance of 560km without electrical regeneraSTM-256 tors is the longest record for the single channel X1/X4 40Gb/s STM-256 system in China, and even in X2/X3 the world. X3-X2 0.2 Another important project program is the " 863 Y1/Y2 0.30/0.70 Hi-tech Research and Development Program of Y3/Y4 0.25/0.25 China " ( 863 in short). There is a "80 40Gb/s DWDM transmission system" in 863 programs, which has been completed by Wuhan Research Institute of P&T in June 2005. The longest transmission distance of 80 40Gb/s DWDM signal is more than 300km both in G652 and G655 fibers.

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Feature Articles: Optical Fiber Communications

(a)
Fig Optical signal eye pattern: (a) Transmitter end (b) Receiver end after

(b)
km transmission

Both the single channel and the DWDM system based on 40Gb/s will be applied in commercial telecommunication network in the near future in China.

REFERENCES
[1] ITU-T Rec. G.707/Y.1322 (10/00) " N e t work node interface for the synchronous digital hierarchy (SDH)" [2] ITU-T Rec. G.783 (10/00) "Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks" [3] ITU-T Rec. G.959.1 (12/03) "Optical transport network physical layer interface"

BIOGRAPHY
Mr. Mao Qian, Graduated from Wuhan Posts and Telecommunications Institute in 1964. Received Master degree from Wuhan Research Institute of posts and telecommunications in 1982. He is mainly engaged in R & D on optical fiber communication equipment, systems, optical transport networks, digital communications and data communications and science & technologies managements. He received national and ministerial or province level Science & Technology Progress Award several times. At present he works as Professor, vice president and chief engineer of Wuhan Research Institute of posts and telecommunications president of

FiberHome Technologies Institute, director of Quality Supervision & Inspect Center of optical communication products of MII. He is the professor of Huazhong University of Sciences and Technologies and Dalian University of Technologies. He is national level expert and enjoyed special government subsidy. He is ITU-T Study Group 15 member council member and vice chairman of transport network & access network technical committee of China communication standards association. Also, he is the director of optical communication committee, vice director of science working committee of China Institute of Communications, council vice chairman and director of optical communication committee of Communication Institute of Hubei province, council vice chairman and director of communication transmission committee of Electronics Institute of Hubei province. He has published three books and more than one hundred papers in national & international conferences and magazines.

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