4Gb/s Coherent DP-PSK for Submarine Applications Jamie Gaudette, Elizabeth Rivera Hartling, Mark Hinds, John Sitch, Robert Hadaway Email: <jamiega1@nortel.com> Nortel, 3 Carling Ave., Ottawa, ON, Canada Abstract: Performance of two variants of 46Gb/s coherent DP-PSK transceivers over repeatered links of 6 and 1 in length and unrepeatered links with span losses up to 75 db is presented. All transceivers in this work employ electronics for polarization state recovery as well as correction of CD, PMD, and all other deterministic linear impairments, thereby eliminating the need for optical compensation. 1. INTRODUCTION Dual Polarization Phase Shift Keying (DP- PSK) with coherent detection is an interesting alternative to traditional OOK/DPSK for submarine applications. Commercially available 46Gb/s DP-PSK transceivers offer up to eight times the raw spectral efficiency of existing state of the art RZ technology [1, 2]. Furthermore, when DP-PSK with coherent detection is paired with DSP, all deterministic linear impairments, such as CD and PMD, can be tracked and compensated. Thus, all optical compensation is removed from the physical layer. With favorable WDM properties and excellent noise tolerance, 46Gb/s DP-PSK can be positioned to overlay with 1Gb/s technology on existing submarine cables, or to build new capacity on dark fibers offering comparable reach to 1Gb/s technology, without costly re-engineering of the submarine cable plant. In this work, two implementations of DP- PSK are presented. In the first implementation, Dual Polarization Quaternary Phase Shift Keying (DP- QPSK) is transmitted. As shown in Figure 1-1, DP-QPSK offers 46Gb/s with an 11.5GHz NRZ line rate. In the second implementation, Dual Polarization Binary Phase Shift Keying is transmitted. To achieve an aggregate data rate of 46Gb/s while maintaining an 11.5GHz baud rate, two carriers are modulated and frequency division multiplexed (FDM) in one GHz optical channel. Frequency selectivity of coherent detection is used at the receiver to de-multiplex the FDM carriers, and avoid ultra narrow optical filters. This process is termed coherent frequency division multiplexing (CoFDM). In this implementation, 31.5GHz of optical bandwidth is required to offer a 46Gb/s line-rate on two CoFDM carriers modulated at 11.5Gbaud. Signal [dbm] -15-2 -25-3 -35 4G DP-BPSK CoFDM and 4G DP-QPSK 31.53 GHz 11.53 GHz 11.53 GHz 23 Gb/s DP-BPSK CoFDM 1555.9 1556 1556.1 1556.2 Figure 1-1 Spectral Occupancy of DP-BPSK CoFDM and DP-QPSK 2. REPEATERED APPLICATIONS DP-QPSK Experimental results were obtained during a field trial on a 6 trans-atlantic cable [3]. Nortel submarine line terminal equipment (SLTE) was placed on the ends of a dark fiber pair in the sub-sea link. No alterations were made to the line system. The SLTE consisted of Nortel s Optical Multi-service Edge 6 (OME6), Copyright 21 SubOptic Page 1 of 5
Dispersion [ps/nm] which housed all coherent transponders, and Nortel Common Photonics Layer (CPL), which provided optical multiplexing and amplification. Figure 2-1 illustrates the SLTE configuration. Field trial, measurements were taken on the 6 point to point trans-atlantic link, and the 12, trans-atlantic loopback. Figure 2-1 Nortel Repeatered SLTE Design The optical fiber in the submarine cable plant was a hybrid mixture of large core fiber (LCF, G.655) and reduced slope fiber (RSF, G.655), with periodic use of dispersion compensating fiber (DCF, G.654). Before performance was characterized, in-service CD and PMD measurements were taken with the coherent receiver. The in-service measured CD for both the 6 point-to-point and the 12, loopback is given in Figure 2-2. The measured mean PMD was 6ps for the 6 point-to-point link, and 13ps mean for the 12, trans-atlantic loopback. Trans-Atlantic Dispersion Profile x 14 1.2 6 Route.9 1 Loopback.6.3 -.3 -.6 -.9-1.2 154 1545 15 1555 156 1565 Figure 2-2. In-Service CD Measurement To estimate performance of a full-fill of 46Gb/s DP-PSK channels on a GHz optical grid, a grouping of 46Gb/s channels was moved across the operating bandwidth and the optical line performance was measured in three locations: mid-band, red-band edge and blue-band edge. Optical performance was measured as pre- FEC BER converted to dbq [2logQ]. The pre-fec BER is available from the coherent receiver, and can be accessed via the network management software. The effect of up to 1 WDM interferers was measured. Five idle channels were employed to consume unused power from the sub-sea amplifiers. The optical spectrum of the received signal in the midband test location is given in Figure 2-3. The optimal launch power was measured at -5 dbm per channel, per repeater for 4G DP-QPSK, and -6dBm per channel, per repeater for 46Gb/s DP-BPSK CoFDM. [dbm] -15-2 -25-3 -35 6 Atlantic Crossing Rx 1545 15 1555 156 Figure 2-3. Field Trial Rx Optical Spectrum The results of the performance measurements at 6 are given in Figure 2-4. Results clearly indicate that industrial margins are expected with a fullfill of 42x46Gb/s channels on a GHz ITU grid when 46Gb/s DP-BPSK CoFDM is employed. Results also indicate that 46Gb/s DP-QPSK is not well suited for trans-atlantic distances as low operating margins were observed, and large channel spacing were required. From other field trial measurements it was concluded that 46Gb/s DP-QPSK is better suited for links up to 4 in reach. The margin gained using DP-BPSK is due to increased phasenoise tolerance of each BPSK subcarrier. Copyright 21 SubOptic Page 2 of 5
Margin [dbq] 4 3.5 3 2.5 2 1.5 1.5 Estimated Trans-Atlantic Margin 1545 15 1555 156 Est. SOL 42x46Gb/s DP-BPSK GHz ITU Grid Est. SOL 5x46Gb/s DP-QPSK 4GHz Spacing Est. SOL 42x46Gb/s DP-BPSK Simulated Worst-Case Margin Figure 2-4. Trans-Atlantic Measured Operating Margins [2logQ] performance measurements of a single channel at 12, are given in Figure 2-5. To our knowledge, this is the longest 4G demonstration with real-time processing. It is important to note that this system was optimized for 6. Higher operating margins are expected on a properly optimized 12, system. An optical recirculation loop was used to further investigate the performance limitations of 46Gb/s DP-PSK. The recirculation loop was designed to closely resemble the physical properties of the trans-atlantic cable presented in this work. Figure 2-6 shows the experimental setup of the optical recirculation loop. 1.4 12, Atlantic Loopback Measured dbq Margin 1.2 Margin [dbq] 1.8.6.4 Figure 2-6 Optical Recirculation Experimental Configuration.2 Est. SOL 46Gb/s DP-BPSK Simulated Single Channel 1545 15 1555 156 Figure 2-5. 46Gb/s DP-BPSK Measured Opearting Margins over 1 [2logQ] The operating margins for 42x46Gb/s channels on the 6 Atlantic crossing were also simulated using our in-house split-step Fourier propagator. The worst case simulated channel is presented in Figure 2-4. In this case, a.25 dbq overestimation in worst case SOL performance was simulated when compared to measured operating margins. In addition to 6 point-to-point testing, the cable plant was looped back to create a 1 propagation distance. For this measurement, a single channel was swept across the available bandwidth as performance was measured in terms of pre- FEC BER converted to dbq. The Figure 2-7 Single Channel Recirculation of 46Gb/s DP-QPSK and 46Gb/s DP-BPSK In Figure 2-7, single channel performance of both 46Gb/s DP-QPSK and 46Gb/s DP- BPSK CoFDM is shown at varying distances. This experiment illustrates the additional reach achieved with DP-BPSK CoFDM in place of DP-QPSK as performance above the FEC limit was measured out to 14. 3. UNREPEATERED APPLICATIONS For traditional un-repeatered applications, the goal is to launch the highest possible per wavelength power into the stretched Copyright 21 SubOptic Page 3 of 5
conference & convention single span at the transmit end to maximize system. The maximum launch power is often limited by fiber nonlinearities, such as self-phase modulation (SPM), cross phase modulation (XPM), and stimulated Bouillon scattering (SBS). In the case where launch powers are limited such that sufficient cannot be provided for system operation, improvements can be realized by counter-pumped and co-pumped amplifiers. amplification increases signal powers further into the fiber, thus raising the average signal power in the fiber while reducing the peak power of the signals. When feasible, remote optically pumped amplifiers (ROPA) can be built into a cable plant to provide further improvements in system. Performance of 46Gb/s DP-QPSK over unrepeatered systems has been demonstrated in laboratory setups on stretched single spans with submarine quality low loss G.654 fiber, with effective span losses up to 75dB. A 75 db span loss corresponds to 415 of ultra low loss NDSF (.18dB/ loss). To allow effective link analysis, power taps were placed strategically to measure power evolution in the single stretched span. In this work we consider three primary unrepeatered solutions: (1) Counter- pumping only, (2) Co- + Counter, and (3) Co- + Counter with an in-line counter-pumped ROPA. The three experimental setups are shown in Figure 3-1, Figure 3-2 and Figure 3-3 respectively. Counter Pump Figure 3-1. Counter-Pump Configuration Copyright 21 SubOptic Co-Pump Counter Pump Figure 3-2. Co-Pump + CounterPump Configuration Co-Pump Counter& ROPA Pump ROPA Figure 3-3. Co-Pump + CounterPump + Counter Pumped ROPA The Co-Pump examined was a 7mW multi-laser diode (MLD) co-pump. The Counter-pump examined was a 3W Super Counter Pump (SCRP) as described in [4]. The ROPA pump was a 4mW MLD 148nm counter-pump. The ROPA was a single section of Erbium Doped Fiber (EDF) with input and output isolators. 3 was used for fiber propagation. An in-line variable optical attenuator () was used after of fiber propagation to adjust the unrepeatered span loss. A maximum number of 11 channels with 1GHz spacing were supported with this vintage of SCRP without ROPA. Current Counter- technology supports up to 44 channels with GHz channel spacing. A maximum of 16 channels with 1GHz channel spacing was supported with the ROPA solution. No optical compensation was required, as the coherent receiver eliminates effects of CD and PMD. Figure 3-4 outlines the results of the laboratory analysis. The single channel case provided the best performance. However, performance and maximum span loss decreased as additional channels were Page 4 of 5
added. To achieve optimum performance with multiple wavelengths, the launch power of all 46Gb/s transceivers was dropped to reduce the penalty incurred from XPM. Figure 3-5 illustrates the effect XPM has on the optimal launch power into the un-repeatered span. Performance [2logQ] 12.5 12 11.5 11 1.5 1 9.5 9 72 7 68 66 Measured Unrepeatered 11x4G - Counter 11x4G Co+Counter 16x4 - Co+Counter + Ropa 1x4G - Co+Counter + ROPA 8.5 6 62.5 65 67.5 7 72.5 75 Los s [db] Figure 3-4. Unrepeatered Performance vs Maximum for Un-Repeatered 76 16-Channels 74 Single Channel Max [db] Furthermore, industrial margins for a fullfull of 42x46b/s DP-BPSK CoFDM channels was presented for an aggregate data rate of over 1.9Tb/s per fiber pair across the Atlantic Ocean. In-service CD and PMD measurements at 6 and 1 using the coherent receiver are also presented. 5. REFERENCES [1] Roberts, K. et al., Performance of Dual- Polarization QPSK for Optical Transport Systems, JLT Vol 27, No 16, pp. 3546-59, 9 [2] L. Nelson, Performance of a 46Gbps Dual-Polarization QPSK Transceiver in High PMD Fiber Transmission Experiment, OFC8, PDP9 [3] Nortel Networks and Reliance GlobalCom, Nortel Achieves 1 4G Milestone on Submarine Cable, http://www2.nortel.com/go/news, September 1, 9 [4] Papernyi, S.B. et al., Third-Order Cascaded Amplification, OFC 2 Post-deadline Paper FB4-1. 64-1 -7.5-5 -2.5 2.5 5 Fiber (dbm Per Channel) Figure 3-5. Fiber vs Performance with Co-Pump Experimental results also indicate that, as expected, Co-Pumping and the addition of a ROPA significantly improved 46Gb/s DP-QPSK performance over the un-repeatered span when compared with Counter-pumping only. 4. CONCLUSIONS DP-PSK optical modulation for modems with coherent detection and DSP has been demonstrated at reaches as far as 12, on repeatered submarine systems, and on links with up to 75dB span losses on unrepeatered submarine systems. Copyright 21 SubOptic Page 5 of 5