Experimental Set-up
B. Transmission Link, dispersion map and amplification
Figure 2 shows the optical link used in this transmission demonstration. 20 odd channels from one CSRZ-DPSK transmitter are combined with 20 even channels from the other CSRZ-DPSK transmitter in an interleaver. The resulting 40 channel WDM signal is then sent through -1100 ps/nm of precompensation before it is launched in the loop which is composed of four spans of 100 km Raman amplified UltraWaveT dispersion-managed fiber [6,15]. Each span consists of two identical sections of large effective area fiber with an effective area of 107 
and a dispersion of 20 ps/nm/km separated by a section of fiber with an effective area of 31 
and a dispersion of -45 ps/nm/km. The lengths of the three sections are adjusted to give the desired net dispersion per span. The first three spans in the loop have a net dispersion of +122 ps/nm and the fourth span has a dispersion of -276 ps/nm giving a loop round-trip dispersion of +90 ps/nm; all values refer to the center of the C-band. Extensive numerical simulations were carried out to find this dispersion map that optimizes CSRZ-DPSK transmission over distances in the 10,000 km region. It was found that the transmission penalty generally decreases with increasing absolute value of the net dispersion per loop round-trip. A high loop round-trip dispersion means, however, that the dispersion accumulated in many loop round-trips becomes very high, and it was found that a positive loop round-trip dispersion of about 100 ps/nm is a good compromise between transmission performance and the practical issues related to compensation of the accumulated dispersion. For a given net dispersion per loop roundtrip, it was furthermore found that the transmission performance generally improves slightly if the four spans in the loop are not identical leading to the employed "3+1" dispersion map. Figure 3 shows an example of the results of the transmission penalty simulations. The figure shows the simulated eye opening penalty after 25 loop round-trips (=10,000 km) as a function of the precompensation and the dispersion of the first three 100 km spans; the dispersion of the fourth span is always adjusted to give a loop round-trip dispersion of 90 ps/nm. The dot in the figure shows the dispersion map used in this transmission demonstration. The optimum precompensation of about -1100 ps/nm is approximately the value that gives a transform limited pulse (corresponding to an accumulated dispersion of 0) in the middle of the considered nonlinear link, i.e. after 12.5 loop round-trips:
ps/nm in agreement with [16]. Since the transmission penalty increases sharply if the precompensation becomes too negative as shown in figure 3, we decided to operate near the left border of the low penalty valley shown in the figure.
Fig. 3. Simulated eye opening penalty in dB after 10,000 km transmission as a function of the amount of precompensation and the per span dispersion of the first three 100 km spans in the loop.
The dot shows the dispersion map used in this transmission demonstration.
The loss of the transmission link is compensated by forward and backward distributed Raman amplification. Each span has a fiber loss of 21 dB and an additional total loss of 1.2 dB in the two Raman pump couplers. Moreover, and not shown in figure 2, is a wavelength dependent loss of a fixed gain flattening filter inserted between the second and third fiber span. Semiconductor lasers with low RIN provide depolarized co-propagating pump waves at 1418, 1438 and 1465 nm giving approximately 6 dB of forward gain. The rest of the required Raman gain is provided by counter-propagating pump waves at 1427 and 1455 nm from Raman lasers. WDM couplers are used to couple the co-propagating Raman pump waves to the transmission fiber, and circulators are used for the counter-propagating Raman pump waves. These circulators furthermore prevent the backward traveling spontaneous emission from one 100 km fiber span to propagate into the preceding 100 km fiber span.
The described distribution of forward and backward Raman gain was determined on the basis of simulations of the influence of the forward Raman gain on the optical signal to noise ratio (OSNR, referred to 0.1 nm) and the double Rayleigh backscatter (DRBS) relative to the signal. The result of these simulations is shown in figure 4 which displays the OSNR and the DRBS per 100 km span as a function of the forward Raman gain keeping the total Raman gain constant. To get a fair picture of the transmission improvement brought about by the forward Raman gain, the nonlinear distortion is kept approximately constant by adjusting the launch power for each value of the forward gain so that the time average nonlinear phase shift per span is 0.007 rad. This is the nonlinear phase shift experienced by the signal in our 10,000 km transmission demonstration where -11 dBm per channel was launched into spans with 6 dB forward Raman gain. It was found experimentally that this launch power gives the lowest BER after transmission, i.e. the best tradeoff between OSNR (the higher the channel power, the better) and nonlinear transmission penalty (the lower the channel power, the better). Figure 4 shows that for fixed nonlinearity, the optimum OSNR is achieved with 6 dB of forward gain. The OSNR per span is 34.6 dB in this case corresponding to 14.6 dB after 100 spans (10,000 km). This is a 0.5 dB improvement compared to a purely backward pumped Raman amplified system with the same nonlinear distortion. Equally important, however, is the fact that the crosstalk from DRBS is 5 dB smaller in a system with 6 dB forward Raman gain than in a system that relies completely on backward Raman amplification. This can be seen from the DRBS curve in figure 4 that shows a reduction of the DRBS level from about -40 dB ( -20 dB after 100 spans) in the system with no forward Raman amplification to about -45 dB ( -25 dB after 100 spans) in the system with 6 dB forward gain.
The results shown in figure 4 correspond as mentioned to a nonlinear phase shift per span of 0.007 rad. However, it should be noted that the main conclusion from the figure - that 6 dB forward gain maximizes the OSNR and gives at least 5 dB DRBS reduction compared to no forward gain - is the same for signal power levels up to a level roughly 10 times higher than what was used in figure 4, i.e. up to about 0.07 rad nonlinear phase shift per span.
Fig. 4. OSNR and DRBS per span as a function of the forward Raman gain in the transmission fiber.
Finally in the discussion of the transmission link, it should be mentioned that the loop switch in figure 2 includes a dynamic gain flattening filter as well as an EDFA that compensates the loop switch related loss.
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