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Figuer 1. DWDM System Block Diagram

Point-to-Point DWDM System Design and Simulation

Gao Yan1, Zhang Ruixia1, Du Weifeng2*, and Cui Xiaorong1 1 College of Computer Science and Technology, Henan Polytechnic University, Jiaozuo, Henan, China

Email: gaoyan@hpu.edu.cn 2 School of Mathematics and Information Engineering, Jiaxing University, Jiaxing, Zhejiang, China

Email: woodmud@tom.com

Abstract—As for the problems of complexity and long cycle in the DWDM design, part of problems needed to be considered in DWDM system design were analyzed. An optimized point-to-point DWDM transmission system was designed. Optical fiber communication system with 40Gbit / s and 32 channel was designed and simulated by Optisystem. Based on the study above, the model witch can inhibit dispersion and fiber linear loss has been successfully manufactured. Index Terms—DWDM, Dispersion Compensation, Linear loss

I. INTRODUCTION

DWDM technology is known as a kind of technology coupling and transmitting optical signals of different frequency (wavelength) to an optical fiber by using the tremendous bandwidth of SMF’s low-loss area in DWDM system, which is not only conducive to the realization of switching and recovery in optical networks but also convenient to the expansion and upgrade, and thus the further realization of transparent and high survivability optical networks[1].

DWDM technology is now in a mature development period. With the development of the society, the requirement of people to communication quality and speed is higher and higher. How to use the optical bandwidth huge resources and to upgrade the capacity of fiber-optic communications systems is an important theoretical and technical subject[1]. Simulation software takes all kinds of parameters into account for DWDM system, through which the measurement results of various instruments can be get, and it can simplify design process and save a lot of time and funding for theoretical research.

Based on the simulation software Optisystem, a 32-channel DWDM point-to-point transmission system simulation model was designed in this paper.

II. THE BASIC PRINCIPLES OF DWDM SYSTEM

DWDM technology is to make full use of huge bandwidth resources from low-loss single-mode fiber District. According to the difference of each channel

wave frequency (or wavelength), the low-loss optical window can be divided into several channels. The light waves as a signal carrier, in the sending end the use of Wavelength Division Multiplexer (MUX) to different provisions of the signal wavelength carrier merged into a single fiber for transmission. At the receiving end these different wavelengths of light signals carrying different carrier is separated by the wave of sub-use device (ODU). As different wavelengths of light carrier signal can be seen as independent of each other (without regard to fiber nonlinear time), optical fiber can be achieved in multi-channel optical signal the use of transmission[2]. The use of different submultiplexer can be used several different wavelengths, from two to several hundred range. Their system is as shown in Figure 1 [1].

III. THE 32-CHANNEL AT 40GBIT/S DWDM SYSTEM

An optical communication system consists of transmitter, communication channel and receiver. The role of the optical transmitter is to convert the electrical signal into optical form and launch the resulting optical signal into the optical fiber. Optical signals were transmitted through optical fiber to the optical receiver, and then the optical receiver converted the distorted and attenuated weak optical signal output from the fiber-optic lines to electrical signals, enlarged and processed into the pre-launch signal, thus the entire transfer process was completed.

A. Transmitter Optical transmitter is the core equipment of fiber optic

transmission system[3], and consists of optical source, electrical pulse generator and optical modulator. DWDM system requires very high accuracy and stability of wavelength, and high performance requirements in dispersion. For 40Gbit / s DWDM, the adoption of the direct modulation makes optical pulse waveform

*Corresponding Author. Du Weifeng, School of Mathematics andInformation Engineering, Jiaxing University, Jiaxing, Zhejiang, China,Email:woodmud@tom.com

ISBN 978-952-5726-02-2 (Print), 978-952-5726-03-9 (CD-ROM)Proceedings of the 2009 International Symposium on Information Processing (ISIP’09)

Huangshan, P. R. China, August 21-23, 2009, pp. 090-092

© 2009 ACADEMY PUBLISHER AP-PROC-CS-09CN002

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Figuer 3. The Structure of Receiver

Figuer 2. The Structure of M-Z-Externally Modulated Transmitter

Figuer 4. Simulation Model of 32-Channel DWDM System

deterioration in long-distance transmission due to frequency chirp added to fiber dispersion[2],therefore, M-Z modulator external modulation model was used in system design, for its advantages of high-speed, high extinction ratio, low insertion loss as well as easy production[4]. Optical transmitter structure was shown in Figure 2.

B. Transmission line SMF is applicable for high-capacity, long-distance

optical fiber communications due to its tremendous bandwidth. In optical fibers, after a long haul, the signal’s intensity was greatly attenuated, therefore, need to be enlarged by a relay optical in order to be received or continue transmission. At present, the EDFA was currently used. Designing a channel, an EDFA was added after a period of 50 meters SMF to compensate for the linear loss.

The most fundamental reason that restrict the transmission of high-speed signals on the 1550nm optical fiber is the linear dispersion[5], the dispersion of SMF in the 1550nm window is 17ps/(nm·km), therefore the DFC should be used for compensating their dispersion performance. DFC’s chromatic dispersion is negative (dispersion coefficient is - 90ps/(nm·km)), its dispersion characteristics is coincides contrary with the SMF’s, if the length of DFC is the SMF’s 1/5, then the total transmission line dispersion value close to zero[1]. However, the DFC attenuation is larger, to solve this problem, EDFA was added to compensate linear loss after the DFC and near to the receiver.

From above, in the design of communication channel the parameters of SMF and DCF will be corresponding to Table I.

C. Receiver Optical receiver is composed of the photoelectric

detectors and filters and demodulator. Its performance impacts on the optical fiber communication system transmission (relay) distance, which is one of the most important part of the entire fiber-optic communication systems and its design to a large extent depends on the transmitter used modulation[5];[6]. APD detector has high sensitivity and costs than PIN detectors. Due to the loss of smaller lines, in the design of a shorter distance transmission system the sensitivity of such indicators can be relaxed for optical receiver and the cost of PIN

detector is lower. Receiver noise is directly proportional to the receiver bandwidth, but can be reduced through a bandwidth of less than bit-rate of low-pass filter. Pulse propagation will affect the bit-o-detection beyond of the time period. Decision-making circuit will contrast linear channel output with the threshold value, which maks decision that the signal is 1 or 0 with realtime clock recovery circuit[5];[6]. Therefore, in the design of the receiver, PIN detectors and Bessel low-pass filter whose bandwidth is 4 × Bit rate and cut-off frequency is 0.75 × Bit rate are selected with built-clock recovery and decision making circuit BER analyzer as a receiver. The structure is shown in Figure 3.

IV. SIMULATION AND RESULTS

DWDM system simulation model is shown in Figure 4. Optical transmitter output power indicates the largest optical fiber loss which can be accepted. It is an important design parameters, which is set to 4dBm in system. In order to reduce inter-symbol crosstalk, 200GHz channel spacing wavelength is selected in design.

Transmitter and receiver is selected in design of section III-A, but only SMF channel instead of taking the linear dispersion and loss of compensation measures, the running model is shown in Figure 5.

When compensation measures have not been taken, it can be seen from Figure 5 that the results of serious

TABLE I. PAREMETERS OF SMF AND DCF

DispersionCoefficient

DispersionSlope

Nonlinear Coefficient

Linear Loss Length

Units ps/(nm·km) ps/(nm2·km) 1/(km·W) dB/km km

SMF 17 0.075 0.31 0.2 50

DCF -90 -0.3 5.24 0.5 10

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Figuer 5. The Eye Diagram of DWDM System without Compensation Measures

Figure 6. The Eye Diagram of DWDM System with SMF+EDFA+DCFas Channel

Figure 7. The Eye Diagram of DWDM System in Witch Linear Loss and Dispersion has been Compensated

distortion of the signal and the receiver can not determine the received signal and communication lines are not available coming from the cumulative effect of dispersion, which causes that optical signals have been spreading and inter-symbol interference and noise model.

According to the theoretical analysis of III-B, channel is improved. A again of 10dB and noise figure of the EDFA for 6dB to compensate for the linear wear and tear are added after the 50km of SMF, and then for some 10km of DCF chromatic dispersion compensation by Figure 6.

Table I in section III-A shows that DCF has a lot of linear loss which should be taken into account in the design of channel. Therefore, EDFA of a gain of 5dB and the noise index for 6dB should be added after DCF to compensate SMF and DCF linear loss, and Figure 7 is get.

It can be seen from Figure 6 and Figure 7 that the eye diagram height and the lowest error rate and the quality factor have declined when DCF has not EDFA linear loss compensation. It can not be ignored in high-quality communication system.

V. CONCLUSION

To solve the problems in process of the DWDM optical transmitter design, Optimized DWDM point-to-point transmission system is designed. 32-channel optical

fiber communication system, whose transmission speed is 40Gbit/s, is designed and researched through Optisystem. From the above, externally modulated transmitter should be used in design of long-distance transmission DWDM system. DCF Dispersion Compensation and Loss linear EDFA compensation should be used in SMF channel to ensure the communication quality. Though DWDM system, which is a complex system, has achieved very good results, it has many problems, such as various nonlinear optical crosstalk and linear crosstalk, ASE noise of EDFA and Wavelength channel instability, etc. Such of these should be studied further.

ACKNOWLEDGMENT

This work is supported by National Natural Science Foundation of China (Grant No.60875034), the Special Research Funding to Doctoral Subject of Higher Education Institutes in P.R. China (Grant No. 20060613007), Henan Province Key Attack Project (082102210079), Henan Province Science and Technology Tackle Key Project (0424460013), Zhejiang Province Fatal Project (priority subjects) Key Industrial Project (2008C11011).

REFERENCES

[1] Dong Tianlin. “Fiber-optic communications and fiberoptic information network” .Beijing: Tsinghua University Press, 2005.9.

[2] Zhuang Jianzhong. “DWDM optical transmitter design” CATV Technology, No.12, 2006: 77-81.

[3] Feng Weizhu. “To select and use optical transmitter.” China Digital Cable TV, No.05, 2006: 596-599.

[4] Hen Yan; Zhang Shilin; Zhang Bin; Mao Luhong; Guo Weilian. “Review on Electro-Optic Modulator”, Semiconductor Technology, No.4, 2008: 286-288.

[5] C.M. Weinert, R. Ludvig, W. Papier, H.G. Weber, D. Breuer, K. Petermann, and F. Kuppers, “40 Gbit/s Comparison and 4 x 40 Gbit/s TDM/WDM Standard Fiber Transmission”, Journal of Lightwave Technology, Vol. 17, pp. 2276-2284, 1999.

[6] M.I. Hayee and A.E. Willner, “NRZ Versus RZ in 10-40 Gb/s Dispersion- Managed WDM Transmission Systems” IEEE Photonics Technology Letters, Vol. 11, pp. 991-993, 1999.