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15 Tb/s unrepeatered transmission is achieved over 409.6 km (68.2 dB) of large effective area fiber using forward and backward distributed Raman amplification and a remotely-pumped erbium-doped optical amplifier. This result provides a record capacity-reach product of 6.14 Pb/s-km over a single fiber type. We also demonstrate channel growth from 10 to 150 waves within 61 nm amplification bandwidth.
© 2014 Optical Society of America
1. Introduction
Larger network bandwidth to meet ever increasing traffic demand requires higher transport capacity. For unrepeatered applications, already several reports have been published on both high capacity and long reach unrepeatered transmission. Figure 1 depicts the trend in the capacity-reach product (Pb/s-km) for unrepeatered transmission in recent years [1–10]. These results often require either a mix of fiber types in the span [2, 5–9] or very strong (> 5W) high-order Raman pumping [1, 5], which are less practical in real deployments, or 200 Gb/s PM-16QAM channels [3, 9].
In this paper, we report 15 Tb/s (150 x 120 Gb/s) unrepeatered transmission over 409.6 km of large effective area (Aeff) G.654B fiber (Corning Vascade EX2000). The transmission is achieved using mature 100G technology over a single fiber type span with pump power of less than 2.5 W (per direction), resulting in a practical solution for deployments. To our knowledge, this result also represents the highest capacity-reach product (6.14 Pb/s-km) for unrepeatered transmission demonstrated to date.
2. Experimental Setup
A diagram of the unrepeatered transmission experiment is shown in Fig. 2. Two modified 100G channel cards (hereafter referred to as comb modulators) are used to generate multiple modulated 100G loading channels. A comb modulator receives at its input a CW comb generated by a bank of external signal sources and outputs a 120 Gb/s NRZ PM-QPSK modulated comb with signals spaced 100 GHz apart. A total of 150 external distributed feedback (DFB) lasers in the C-band (1531.51 nm ~1567.13 nm) and L-band (1567.54 nm ~1592.10 nm) are separated into two groups (odd and even). The channels in each group (spaced at 100 GHz) are multiplexed through a polarization multiplexed (PM) Arrayed Waveguide Grating (AWG) and a PM 3-dB coupler. The channels in each group are then modulated by a comb modulator and combined with a 3-dB coupler to generate a comb of 150 modulated channels spaced at 50 GHz. The channels are amplified by a lumped Raman amplifier (LRA), LRA-1. In addition, three tunable commercial 100G channel cards (with real-time processing; one in the L-band and two in the C-band) are combined with the modulated comb channels using a wide-band Wavelength Selective Switch (WSS), WSS-1. LRA-2 is used to boost the combined signals on the transmit side. The dispersion compensating fiber used as the Raman gain medium in LRA-2 also provides approximately −670 ps/nm of dispersion pre-compensation, improving the transmission performance. In this experiment, all the channels are modulated at 120 Gb/s, which accounts for the 15% overhead of the Soft-Decision Forward Error Correction (SD-FEC). This SD-FEC can correct a BER of 1.9 x 10−2 to less than 10−15. At the receive end, LRA-3 is used to amplify the receiving signals and WSS-2 is used to select channels for Q measurement using the 100G channel cards.
Forward and backward Raman pumping is provided by commercial Raman pump modules which consist of seven pump wavelengths distributed in the range between 1400 nm and 1500 nm.
The unrepeatered span was assembled with only one fiber type - Corning Vascade EX2000. This fiber is G.654B-compliant (cutoff-shifted single mode fiber with cutoff wavelength ≤ 1530 nm) with an average chromatic dispersion of ≤ 20.2 ps/nm-km and has been widely deployed in submarine systems. It has a large Aeff of 112 μm2, enabling higher launch powers into the fiber, therefore making it attractive for unrepeatered transmission systems. The span consists of 409.6 km of fiber with a loss of 68.2 dB (mean fiber attenuation of 0.166 dB/km including splice and connector losses) and a Remote Optically-Pumped Amplifier (ROPA). The ROPA consists of 16 m of erbium doped fiber and is placed at 120.9 km from the receive end.
3. Transmission results
Figure 3(a) and 3(b) show the measured spectra of the transmitted (at the output of LRA-2 in Fig. 2) and received (at the output of LRA-3 in Fig. 2) channels, respectively. The transmit channels are pre-emphasized to achieve an approximately uniform Q across all channels at the receiving end. The average channel launch power is −2.8 dBm/ch. The large peak-to-peak signal power ripple (~10 dB) at the receive side comes from unequal ROPA gain, but is flattened by the WSS before the receiver.
Figure 4 (a) shows the simulated gain profile in the section between the span input and the ROPA with an average gain of −34.0 dB, including the 48 dB span section loss. The estimated on/off gain from the forward pump is 14.2 dB. Figure 4(b) depicts the simulated ROPA gain using an estimated 14.2 mW of residual pump power (shown in Fig. 4(e)) from the backward Raman pumping. The ROPA provides 21.5 dB average net gain for all 150 channels but – as expected – shows much lower gain in the L-band. The distributed backward Raman gain profile which is adjusted to compensate the gain tilt introduced by the ROPA is shown in Fig. 4(c). The average backward Raman gain (out/in) is 7.7 dB, including the 20.2 dB span section loss between the ROPA and the span output.
Fig. 4 Simulated gain (a) between the input of the span and the ROPA, (b) of the ROPA, (c) between the ROPA and the output of the span, Simulated (d) signal profile and (e) pump profiles over the span distance.
Figure 4(d) and Fig. (e) provide the simulated signal power profiles and pump power profiles over the transmission distance, respectively. The maximum signal power reaches + 11.2 dBm at 18.7 km from the transmit side. In this experiment, the longest pump wavelength (with less “walk-off” between pump and signal in a dispersive fiber) of the forward pump module is turned off to reduce the RIN transfer penalty [11], therefore, 6 (of the 7 available) pump wavelengths (in the range between 1400 and 1480 nm) are used. The backward pump module uses all 7 pump wavelengths. The forward and backward distributed pump powers are 2230 mW and 2450 mW, respectively.
The measured and simulated OSNR as well as the calculated Q values from the measured pre-FEC BER are plotted in Fig. 5. Since the noise level cannot be assessed due to the dense 50 GHz spacing, the OSNR is measured by switching off adjacent channels. The signals at shorter wavelengths experience higher nonlinear transmission penalty due to the signal pre-emphasis (shown in Fig. 3(a)) and stronger forward Raman gain and therefore, require higher OSNR. The average OSNR is 14.2 dB and the simulated OSNR show a good match with the measured values. The Q for all 150 channels is measured with a real-time processing ASIC 100G channel card. For the measurement, each individual comb channel is replaced by the channel of a 100G channel card tuned to the same wavelength and power level. The Q values of all 150 channels, with an average of 7.0 dB, are greater than the (pre-SD-FEC) Q threshold of 6.4 dB (BER 1.9 x 10−2). All the channels show error-free operation after FEC over a period of more than 16 hours. The result of a 16-hour BER stability test is plotted in Fig. 6.
4. Transmission as a function of channel growth
Generally, unrepeatered transmission systems operate with a lower capacity (lower channel count) at the beginning of the life. Capacity is increased by adding channels over time. The commercial system used in this experiment can adaptively operate based on the channel count and therefore does not require loading channels for low capacity transmission.
To support different channel counts, the LRA-2 in Fig. 2 is automatically controlled to meet signal output set points and the forward Raman pump power is controlled to provide optimal performance based on the number of channels. Since the backward distributed Raman gain is unsaturated due to the low signal power, the pump power in the backward pump module does not need to be adjusted with the addition of channels. LRA-3 and WSS-2 operate together to provide optimal power at the receiver.
To simulate a real deployment situation, the system is first equipped with 10 x 100G channels in the C-band and then 30, 50, 100, and 150 channels. Figure 7 shows the changes in (a) transmitted (at the output of LRA-2 in Fig. 2) and (b) received, at the end of the span (at the input of LRA-3 in Fig. 2) optical spectra for different channel counts. LRA-2 output signal set point uses a constant output signal profile for the full capacity (shown as dotted line in Fig. 7 (a)). The change in forward pump power (per wavelength) versus channel count is provided from the simulation.
Fig. 7 Spectra as a function of channel growth (10, 30, 50, 100, 150 CHs) at (a) transmit side (output of LRA-2 out in Fig. 2), and (b) span output (input to LRA-3 in Fig. 2).
Figure 8 shows the transmission performance changes in OSNR and Q for different channel counts. As the number of channels increases from 10 to 30, 50, 100, and 150, the average OSNR and Q change from 23.2 dB to 20.1 dB, 18.6 dB, 15.7 dB, 14.2 dB and from 11.6 dB to 10.2 dB, 9.8 dB, 8.3 dB, 7.0 dB, respectively. The system provides more margins at lower channel count without using loading channels.
5. Conclusion
A record capacity-reach product of 6.14 Pb/s-km (15 Tb/s over 409.6 km / 68.2 dB) unrepeatered transmission has been demonstrated. This result is achieved by using a single G.654-compliant fiber type, commercial Raman pump modules and real-time processed 100G channel cards, providing a practical solution for high-capacity and long-reach unrepeatered systems.
Acknowledgments
The authors would like to thank Hector de Pedro, Jeff Stone, and Kim Nguyen for helpful assistance in the experiment.
References and links
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