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Transmission performances of 6,000km RZ-DPSK system using dispersion flattened fiber (DFF) and non-zero dispersion shifted fiber (NZDSF) were theoretically investigated and compared. Both fibers showed similar performance when the repeater output power was small, but the DFF showed superior performance in the higher repeater output power regime. The result shows that the merit of the DFF is fully utilized when the repeater output power is high enough.
©2009 Optical Society of America
1. Introduction
The dispersion flattened fiber (DFF) is a well-known solution to improve the performance of the long-haul intensity-modulation direct-detection (IM-DD) system compared to the non-zero dispersion shifted fiber (NZDSF) [1]. Currently, both the DFF and the NZDSF are used for the long-haul undersea system [2]. Return-to-zero differential phase shift keying (RZ-DPSK) modulation is attractive, because it can improve the transmission performance [3–5]. It is possible to improve the transmission performance by a combination of the RZ-DPSK and the DFF, but a quantitative comparison between the DFF and the NZDSF using the RZ-DPSK format has not been conducted yet to the best of my knowledge.
In this paper, a numerical study of the long-haul RZ-DPSK system performance using the DFF and the NZDSF is conducted. Ninety-six 10Gbit/s RZ-DPSK channels with 0.2nm channel spacing are transmitted over 6,000km. The result shows a merit of the DFF in the higher optical power regime.
2. Simulation method and model
The numerical simulator solved coupled nonlinear Schrödinger equations using the split-step Fourier method [6]. The equation used for the simulation is:
where β2j is the second order group velocity dispersion (GVD) coefficient, β3j is the third order GVD coefficient, αj is the fiber loss coefficient, βj is the nonlinear parameter of the fiber, and the suffix j and k are the channel number. Four wave mixing (FWM) was ignored in the simulation, because the generated power through the FWM [7] was negligibly small owing to the fiber parameters. The fiber step length for the calculation was set to non-uniform, and it was expanded exponentially from the initial length of 100 meter [8].
Figure 1 shows a schematic diagram of the simulation model. There were 96 optical transmitters (TX), and the signal wavelengths were ranged from 1540.5 nm to 1559.5 nm with 0.2 nm channel separation. The bit rate and the pattern were 10Gbit/s and 29 De Brujin sequence, respectively. In the simulation, 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) format. The RZ waveform was applied after the PSK modulation, and the waveform was also raised cosine. 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 used for the simulation to reduce the pattern dependent XPM impact [9], and the obtained results were averaged over these three sets.
The transmission line comprised Erbium-doped fiber amplifier (EDFA) repeaters and fibers. The noise figure of the EDFA repeater was set to 4.5dB. The amplifier spontaneous emission (ASE) noise generated by the repeater had a random complex electrical field, and it was added to the complex electrical field of the optical signal. The repeater span length was 100km. The wavelength dependent gain of the repeater was ignored in the simulation.
The fiber span comprised the DFF or the NZDSF. The DFF was composed from the super large area fiber (SLA) and the inverse dispersion fiber (IDF) [10], and there were two types of the NZDSF. Figure 2 shows the dispersion map of the DFF system and the NZDSF system. As the conventional periodical dispersion map degraded the performance of the NZDSF system [11], dispersion compensation of the system was accomplished at the center of the system. Both maps had pure positive fiber spans in the center of the system, and the span length of this section was 96km for the DFF system and 100km for the NZDSF system. Thus, the total transmission distance was 6,272km and 6,300km for the DFF system and the NZDSF system, respectively. The positive dispersion fiber used for the DFF system was the SLA, and that for the NZDSF system was the single-mode fiber (SMF). The parameters of these fibers are summarized in Table 1. The DFF span loss was 21.1dB and the NZDSF span loss was 21.0dB.
The optical demultiplexer (DEMUX) had the second order Gaussian shape. The 3dB bandwidth of the demultiplexer was set to 0.1nm. As the relative dispersion slope of the SLA and the IDF was chosen to be the same, the cumulative dispersion of all signal channels was equal to zero for the DFF system. On the other hand, the cumulative dispersion of each channel was equalized to be 100ps/nm for the NZDSF system after the DEMUX [12]. For the signal demodulation, difference of the optical phase was directly calculated from the optical field [13]. The difference of the phase was defined as the phase difference between two sampling points separated by one bit period, and an eye-like diagram of the phase can be obtained within the phase range between -π/2 to 3 π /2. The performance was evaluated by the Q-factor obtained from the rails of 0 phase and π phase explained in reference [13].
Table 1. Parameters of the transmission fiber
3. Results and discussions
At first, the system performance was evaluated as a function of the repeater output power. Figure 3 shows the results. The horizontal axis shows the repeater output power, and the vertical axis shows the averaged Q-factor of 96 channels. As seen in the figure, when the repeater output power was smaller than +14dBm, the performance of both systems was similar. This result shows that the difference of the fiber parameters did not have so significant impact on the transmission performance when the optical fiber nonlinearity was not dominant. The performance was improved as the repeater output power was increased up to +18dBm for the DFF system and up to +16dBm for the NZDSF system. This result clearly shows that the DFF had smaller transmission impairment caused by the optical fiber nonlinearity than the NZDSF. When the repeater output power was +16dBm, averaged Q-factors for the DFF system and the NZDSF system were 13.9dB and 12.6dB, respectively. There was 1.3dB performance advantage for the DFF system when the repeater output power was set to optimum for the NZDSF system. Furthermore, when the repeater output power was +18dBm, averaged Q-factor for the DFF system was improved to 14.9dB. Therefore, the DFF system had 2.3dB performance advantage against the NZDSF system when the repeater output power was set to optimum for each case.
Figures 4 and 5 shows channel dependence of the Q-factor for the DFF system and the NZDSF system. Figure 4 shows +14dBm repeater output power, and Fig. 5 shows +16dBm repeater output power. As seen in Fig. 4, the DFF system showed slightly improved performance than the NZDSF system. On the other hand, as seen in Fig. 5, the performance of the DFF system was clearly better than that of the NZDSF system. In addition, Fig. 6 shows the comparison at the optimum repeater output power. The DFF system at +18dBm repeater output power showed superior performance than the NZDSF system at +16dBm. These results show that the DFF is effective to improve the transmission performance of the long-haul RZ-DPSK system, but the repeater output power should be high enough to fully utilize the advantage of the DFF.
As already mentioned, the DFF system had 2.3dB performance advantage against the NZDSF system when the repeater output power was set to optimum for each case. The reason of this 2.3dB advantage could be explained in two steps. Firstly, there is 2dB difference of the optimum repeater output power. This difference could be justified by the effective area difference between the SLA and NZDSF1. As the effective areas of the SLA and NZDSF1 are 107μm2 and 70μm2, respectively, the difference in dB scale is 1.8dB, and this could cause 2dB difference of the optimum power level. Roughly speaking, 2dB improvement of the repeater output power improves the optical signal to noise ratio of 2dB, and it can improve the Q-factor of 2dB. Secondly, remaining 0.3dB discrepancy could be attributed to channel dependent degradation of the NZDSF system at +16dBm output power. As shown in Fig. 5, channel performance of NZDSF system above the system zero dispersion wavelength clearly exhibits gradual degradation as channel wavelength becomes longer. Actually, if the average Q-factor of the NZDSF system is calculated using only 48channels existing in the shorter wavelength region, it is improved to 12.8dB.
The results shown above are based on the dispersion map shown in Fig. 2. The previous study showed that the performance of the RZ-DPSK system using the NZDSF depended significantly on the dispersion map design especially number of zero crossing points [10]. Therefore, the dispersion map design might cause some impact on the performance of the RZ-DPSK system using the DFF. Further study is required for this issue.
4. Conclusions
A theoretical investigation of the transmission performance of the long-haul RZ-DPSK system using the DFF and the NZDSF was conducted. There was no significant performance difference between the DFF system and the NZDSF system in the smaller repeater output power regime, but the DFF system showed superior performance in the higher repeater output power regime. As a result, the DFF system had 1.3dB advantage when the repeater output power was set to +16dBm, and 2.3dB improvement when both systems were compared at the optimum repeater output power. The results proved the benefit of the DFF for the long-haul RZ-DPSK system. Further study might be required to clarify the impact of the dispersion map design using the DFF.
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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