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A numerical study focusing on the feasible upgrade scenario of long-haul undersea optical fiber cable system is discussed. At first, appropriateness of the simulated performance is confirmed through conventional IM-DD modulation format. Then, capacity upgrade of the system is attempted by adopting the RZ-DPSK modulation format. As the RZ-DPSK format alone does not make capacity upgrade possible, additional methods are considered, and the modification of the dispersion map is revealed to be effective. The results show that it is possible to double the capacity of the long-haul undersea system using the RZ-DPSK modulation combined with the dispersion map modification.
©2008 Optical Society of America
1. Introduction
Long-haul undersea optical fiber cable system is one of the most important infrastructure in the world to enable broad bandwidth telecommunication. In general, the undersea system is designed to operate for 25 years to recover the total cost of the system. Even though, some of the undersea systems terminate their service operation before they reach their life time, because the progress of the new technologies increase the system capacity so rapidly, old generation systems become cost consuming to maintain them until their life time. Such an early-retired system is utilized for scientific investigations in the world [1], but it might be more attractive if an old generation system is re-vitalized by some advanced technologies those become available recently.
Wavelength division multiplexing (WDM) technology and optical amplifier technology are commonly used for the latest undersea cable systems. As they are transparent for the bit-rate and the modulation format, it might be possible to increase the transmission capacity of the system beyond the original design by adopting newly developed technologies. Such capacity upgrade should be effective to operate the old generation systems until their life time. The differential phase shift keying (DPSK) is one example of new technologies to enable such capacity upgrade, and the feasibility of it has already been reported [2].
One technical issue of the capacity upgrade using the DPSK technology is the dispersion map of the system. It has been reported that the conventional dispersion map used for the undersea system caused the performance degradation of the return to zero DPSK (RZ-DPSK) signal near the system zero dispersion wavelength [3, 4]. Then, some methods to overcome this issue are required to realize feasible upgrade technology. In this paper, a scenario to realize capacity upgrade of long-haul undersea system is discussed. As this scenario can spare the cost to install a new undersea cable system, it would be quite attractive for the undersea system operators.
2. Simulation method and model
The numerical simulator solved coupled nonlinear Schrödinger equations using the split-step Fourier method [5]. 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, γj is the nonlinear parameter of the fiber, and the suffix j and k are the channel number. The fiber step length for the calculation was set to non-uniform, and it was expanded exponentially from the initial length of 100 meter [6].
At first, long-haul WDM undersea cable system with intensity modulation direct detection (IM-DD) chirped return-to-zero (CRZ) format was simulated. Figure 1 shows a schematic diagram of the simulated system. Parameters used in the simulation were based on previously published papers like [7, 8].
There were 16 optical transmitters whose wavelengths were ranged from 1545.5nm to 1554.5nm with 0.6nm channel separation. The bit rate and the pattern of each transmitter were 10Gbit/s and 29 a De Brujin sequence, respectively. The output waveform of the transmitter was raised cosine, and the pulse duty ratio was 50%. 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 in order to reduce the pattern dependent XPM impact [9], and the obtained results were averaged over these three sets.
The transmission line comprised amplifier repeaters and optical fibers. The amplifier repeater had +9dBm total repeater output power and 4.5dB noise figure. The wavelength dependent gain of the repeater amplifier was ignored in the simulation. The repeater span length was 50km. The fibers used for the span were non-zero dispersion shifted fiber (NZDSF) and conventional single mode fiber (SMF). There were two types of NZDSF, and one has large effective area (NZDSF1) while the other has small dispersion slope (NZDSF2). The loss and the chromatic dispersion at 1550nm of two different NZDSFs were identical, and they were 0.21dB/km and -2ps/nm/km, respectively. The effective area and the dispersion slope of NZDSF1 were 75μm2 and 0.1ps/nm2/km, respectively, and those of NZDSF2 were 50μm2 and 0.06ps/nm2/km, respectively. The loss, chromatic dispersion, effective area, and dispersion slope of SMF were 0.18dB/km, 18ps/nm/km, 75μm2 and 0.06ps/nm2/km, respectively. Each fiber span comprised only NZDSF or only SMF, and NZDSF span comprised both NZDSF1 and NZDSF2. The length of NZDSF1 and NZDSF2 was 25km each. The dispersion map was formed by twelve dispersion blocks. Each dispersion block comprised nine NZDSF spans and one SMF span, and the SMF span was placed at the sixth span in the block. The system zero dispersion wavelength was 1550nm.
The optical demultiplexer (DEMUX) had the second order Gaussian shape with 0.4nm bandwidth. The cumulative chromatic dispersion for each channel was compensated to have residual dispersion of 100ps/nm to include a compensation error of actual system. 50% of the dispersion compensation was done at the transmitter side, and the other 50% was conducted at the receiving end. The optical receiver had 7.5GHz electrical bandwidth. A Q-factor [10] was used to evaluate the transmission performance. The Q-factor was calculated from the distributed sampled points of the electrical eye of the optical receiver.
For the upgraded system, the RZ-DPSK modulation format with the raised cosine waveform was adopted. Number of wavelengths was increased to 32, and the signal wavelengths were ranged between 1545.5nm to 1554.8nm with 0.3nm channel separation. The bandwidth of the DEMUX was reduced to be 0.2nm, and the dispersion compensation was conducted at the receiving end only. For the signal demodulation, difference of the optical phase was directly calculated from the optical field [11]. 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 3π/2. The performance was evaluated by the Q-factor obtained from the rails of 0 phase and π phase [11].
3. Simulation results of original and upgraded system
The performances of the original system and the upgraded system were compared. Figure 2 shows the simulated performance of the original system and the upgraded system. The performance of the upgraded system shows degradation near the system zero dispersion wavelength, and this result qualitatively agrees the experimental result [3]. In addition, the performance of the original system also agrees the experimental result qualitatively [7], therefore, the performance shown in Fig. 2 should be reasonable as the comparison between before and after upgrade using the RZ-DPSK format.
The upgraded system with the RZ-DPSK format is not attractive, because from Fig. 2, the center channels might not be able to satisfy the required transmission performance of the undersea cable system. Then, possible scenarios to solve this issue are discussed in the next section.
4. Possible scenario to improve performance of RZ-DPSK based upgraded system
As the issue of the upgraded system is the performance dip near the zero dispersion wavelength of the system, hybrid combination of the CRZ and the RZ-DPSK was simulated to improve the overall transmission performance. As the performances of the CRZ channels near the center wavelength were better than those of the RZ-DPSK channels, the CRZ format was used for the center channels, while the RZ-DPSK format was used for the edge channels. Actual wavelength allocations were twelve RZ-DPSK signals from 1545.2nm to 1548.5nm, five CRZ signals from 1549.1nm to 1551.5nm, and ten RZ-DPSK signals from 1552.1nm to 1554.8nm, because five CRZ channels showed superior performance than corresponding RZ-DPSK channels in Fig. 2. Wavelength separations were 0.6nm and 0.3nm for the CRZ and the RZ-DPSK, respectively. The CRZ signal power was maintained the same as the original system, and the power of the RZ-DPSK signal was set 3dB smaller than that of the CRZ signal.
Figure 3 shows the results. Even though we expected that the worst performance of the system would be limited by the CRZ channels, the RZ-DPSK channels at the interface of the CRZ channels exhibited the worst performance. From this result, it is concluded that the hybrid configuration is not attractive for the choice of upgrade scenario of the long-haul undersea system using the RZ-DPSK format.
It was confirmed that modification of the dispersion map while maintaining the fiber parameters improved the performance of the RZ-DPSK system [12]. Then, as a next possible scenario, the dispersion map of the system was modified. Figure 4 shows the original map and three different modified maps. Modified map A unifies the SMF spans in the center of the system, modified map B unifies the SMF spans at the beginning and the end of the system [13], and modified map C replaces the SMF spans by the NZDSF spans.
Using these modified maps, 32 channels RZ-DPSK transmission was simulated. Figure 5 shows performance comparison of the upgraded system with the original map and the modified maps. As seen in the figure, the performance dip of the original map near the center channels is perfectly eliminated, and modified maps greatly improved the performance of the upgraded system. In addition, there is not any significant performance deference between modified maps A and B, while a slight degradation is observed for modified map C. Therefore, it seems that the best scenario of the capacity upgrade of the long-haul undersea system is to combine the RZ-DPSK format and the dispersion map modification. Even though, from a practical point of view, one significant issue is the dispersion map modification itself. As the optical fiber cable is installed under the ocean, it would be difficult to change the position of the SMF. Still, it is possible to change the position of the SMF by adopting the conventional cable repair method. It will require some cost, but it should be much smaller than that of installing a new system.
4. Conclusions
Transmission performance of long-haul undersea system has been simulated, and the scenario to upgrade the capacity of the system is proposed. RZ-DPSK format combined with modified dispersion map could enable to double the capacity of the original CRZ system, and this upgrade should be cost effective compared to installing a new system. As the capacity upgrade of the system would be able to extend the actual life-time of the system, this proposed scenario should be quite useful.
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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