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This paper describes a theoretical study of the impact of dispersion map design upon the long-haul RZ-DPSK system using the dispersion flattened fiber. Numerical simulations are conducted to clarify the transmission performance of different dispersion map designs. As a result, number of dispersion blocks has a significant impact upon the transmission performance, and reduction of the number improves the nonlinear tolerance of the system.
©2010 Optical Society of America
1. Introduction
Return-to-zero differential phase shift keying (RZ-DPSK) is a promising modulation format for the long-haul large capacity optical fiber transmission system, and it demonstrated a superior performance compared to the conventional intensity-modulation direct-detection (IM-DD) system experimentally [1–3]. For the IM-DD based long-haul system, a dispersion map is commonly used to improve the transmission performance, and so-called “block type” dispersion map is the most popular style used for the long-haul IM-DD system [4]. For this block type dispersion map, both positive and negative dispersion fibers are combined to compose one dispersion block, and an entire system is composed of several dispersion blocks. Even though the block type dispersion map is effective to improve the performance of the conventional IM-DD based system, it was reported that the performance of long-haul 10Gbit/s RZ-DPSK system with the block type dispersion map and the non-zero dispersion shifted fiber (NZDSF) demonstrated a performance degradation near the system zero dispersion wavelength [5,6]. It was also reported that it was possible to improve the transmission performance by modifying the dispersion map of the system. By reducing number of dispersion blocks, the system performance could be improved without changing any system parameters [7].
For the IM-DD based long-haul system, the dispersion flattened fiber (DFF) is another important technology to improve the transmission performance, and it was already installed in the Pacific Ocean [8]. The DFF is also effective to improve the transmission performance of the RZ-DPSK based system, and a comparison of the long-haul system using the NZDSF and the DFF with the blockless type dispersion map was reported [9].
For the NZDSF based system, block type dispersion map is not optimum for the RZ-DPSK format and number of dispersion blocks changes the transmission performance significantly. This implies that the transmission performance of the DFF based system is also affected significantly by the number of dispersion blocks. Therefore, this paper focuses on this issue whether the block type dispersion map causes the performance degradation of the DFF based RZ-DPSK system. The impact of the dispersion map design upon the transmission performance of the long-haul RZ-DPSK system using the DFF is characterized through numerical simulations.
2. Simulation model and dispersion map design
Figure 1 shows a schematic diagram of the simulation model. There were 96 optical transmitters (TX) to generate the RZ-DPSK signals. The wavelength was ranged between 1540.5nm and 1559.5nm with 0.2nm channel separation. The PSK signal was assumed to be generated by a Mach-Zehnder modulator (MZM), and the waveform applied for the two arms of the MZM was a raised cosine with the non return-to-zero (NRZ) waveform. The RZ waveform was applied after the PSK modulation, and the waveform was also raised cosine. The duty ratio of the pulse was 50%. The bit rate and the pattern for the modulation were 10Gbit/s and 29 De Brujin sequence, respectively. The multiplexer (MUX) did not have any wavelength selective function, and the modulated pattern of each transmitter was randomized at the output of the MUX. Three different sets of the initial pattern at the output of the MUX were simulated, and the obtained results were averaged.
The transmission line comprised the DFF and EDFA repeaters. The DFF was composed of the super large area fiber (SLA) and the inverse dispersion fiber (IDF) [10]. Each DFF span had negative chromatic dispersion of −240ps/nm, and the cumulative negative dispersion was compensated by the SLA. Table 1 shows the parameters of these fibers. The span length of the DFF was 100km, while that of the SLA only span was 108km. The total transmission distance was 6048km. The output power of the EDFA repeater was varied between + 13dBm and + 18dBm by 1dB step, and the noise figure of the repeater was set to 4.5dB. The wavelength dependent gain of the repeater was ignored in this simulation.
Table 1. Parameters of the transmission fiber
Six different dispersion maps were used for the simulation. Number of dispersion blocks was changed for each map. Map 1 had one dispersion block, Map 2 had two dispersion blocks, and so on. Map 1, 2, 3, and 6 had uniform dispersion blocks while Map 4 and 5 had two different block lengths within the system. Figure 2 shows the dispersion maps used for this study. Note that the difference was only the position of SLA only span, and the physical parameters of the fibers were identical.
The optical demultiplexer (DEMUX) had the third order Gaussian shape. The bandwidth of the DEMUX was 0.1nm. As the system was using the DFF, there was no cumulative chromatic dispersion after the transmission for all the channels. Therefore, no pre or post dispersion compensation was adopted for the simulation. For the signal demodulation, difference of the optical phase was directly calculated from the optical field, and the performance was evaluated by the Q-factor obtained from the rails of 0 phase and π phase [11].
3. Results and discussion
Figure 3 shows the performance of 96 channels after 6048km transmission as a function of the repeater output power. As seen in the figure, for small repeater output power of below + 14dBm, there is not any significant difference between the maps, but the performance of map 6 becomes inferior than the others when the repeater output power is increased above + 16dBm. These results clearly indicate that the nonlinear penalty of the system strongly depends on the dispersion map design.
Figure 4 shows the average Q-factor of 96 channels as a function of the repeater output power and the dispersion map. It is obvious that increasing the number of dispersion blocks leads to performance degradation in higher repeater output power (i.e., higher nonlinear regime). Regarding dispersion map design, the tendency is the same as the NZDSF based system [7], and it is favorable for the DFF system to reduce number of dispersion blocks to improve the performance.
One might suggest that the major reason of the performance difference is not the number of dispersion blocks but the maximum cumulative dispersion, because the maximum was around 7000ps/nm for Map 1 and 2200ps/nm for Map 6 as shown in Fig. 2. Then, the effect of the maximum cumulative dispersion was studied. As it was impossible to increase the maximum cumulative dispersion of Map 6 while maintaining fiber parameters, span configuration, and number of dispersion blocks, the chromatic dispersion parameter was adjusted to increase the maximum cumulative dispersion. Figure 5 shows modified Map 6. The chromatic dispersion parameters of the SLA and IDF were tripled to be + 60ps/nm/km and −132ps/nm/km, respectively. The other parameters were maintained. Figure 6 shows the simulated results of modified Map 6. As seen in Fig. 6, the modified map showed comparable performance as Map 1. In addition, there was no significant channel dependency, and the performance of individual channels was similar to those shown in Fig. 3. This result implies that larger maximum cumulative dispersion improves the performance. On the other hand, comparing Map 4 and Map 5, even though the maximum cumulative dispersion is the same, Map4 showed better performance than Map 5 as seen in Fig. 4. This shows that number of dispersion blocks tends to have more significant impact than the maximum cumulative dispersion. As the fiber parameters used for Figs. 5 and 6 were not realistic, further studies are required to clarify the impact of the maximum cumulative dispersion.
4. Conclusions
The transmission performances of the long-haul RZ-DPSK system using the DFF was characterized theoretically. It was revealed that the dispersion map design was crucial for the system especially higher repeater output power regime. It was effective to reduce number of dispersion blocks to improve the system performance, and this tendency was the same as the case of the NZDSF based system [7].
Acknowledgments
This work is supported partially by National Science Council 96-2221-E-110-049-MY3, partially by key module technologies for ultra-broad bandwidth optical fiber communication project of Ministry of Economy, Taiwan, R.O.C., and partially by Aim for the Top University Plan of the National Sun Yat-Sen University and Ministry of Education, Taiwan, R.O.C.
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