This paper reports record unrepeatered transmission results using forward and backward distributed Raman amplifications, large effective area ultra-low loss fiber and enhanced ROPAs with additional pumping fibers. Coherent 100 Gb/s with PM-QPSK modulation format and OOK 10G transmission are demonstrated over 607 km (97.2 dB) and 632 km (101.0 dB), respectively.
© 2015 Optical Society of America
Ultra-long, “thin” (low-capacity) unrepeatered transmission systems can provide a cost-effective solution compared to repeatered solutions for many applications. Such applications include terrestrial routes in remote and hostile areas (i.e., tropical forest, desert…) for which the use of unrepeatered transmission alleviates the need for intermediate amplification sites (and associated capital and operational expenses) and subsea links to connect sparsely populated islands for which an unrepeatered system – without submerged repeaters and associated power feed equipment – yields a more economically viable solution. Another application is communication links to offshore oil and gas platforms, and over power utility grids in remote areas .
There have been a number of publications reporting increased unrepeatered distances [2–12]. Figure 1 shows record distances of unrepeatered transmission distances with several generations of channel bit rate. At 662 Mb/s and 2.5 Gb/s, forward and backward Remote Optically Pumped Amplifiers (ROPA) and dedicated pumping fibers were used to achieve transmission over 530 km (88.8 dB) and 570km (99.7 dB), respectively [2,3]. The longest unrepeatered distance reported to date is 601 km (97.3 dB) at 10 Gb/s using RZ DPSK modulation format, third order Raman pumping and a ROPA . At 40 Gb/s, the record distance of 525 km (84 dB) was achieved using PDM-RZ-BPSK modulation combined with a coherent receiver, third order Raman, ROPA and large effective area fiber .
At 100 Gb/s, unrepeatered transmission over 520.6 km of G.652 fiber and 556.7 km of G.654 fiber using PM-QPSK with a coherent received, Soft-Decision FEC (SD-FEC), forward and backward ROPAs were recently reported .
In this paper, we introduce a novel enhanced ROPA configuration which utilizes multiple additional pumping fibers and demonstrates 100G unrepeatered transmission over 607.3 km (97.2 dB) and 10G transmission over 632.3 km (101.0 dB), which are (to our knowledge) the longest unrepeatered distances reported to date. These results are obtained through the use of a commercially available DWDM Line Terminal Equipment (LTE) with distributed Raman pump modules, large effective area ultra-low loss fiber.
2. Experimental Setup
The experimental setup is shown in Fig. 2. The setup is configured to transmit 100G at 1563.86 nm or 10G at 1563.05 nm. The 100G signal is NRZ-PM-QPSK modulated at 120 Gb/s which accounts for the 15% overhead of the Soft-Decision Forward Error Correction (SD-FEC) code. The SD-FEC can correct a BER of 1.9 x 10−2 to less than 10−15 (NCG of 11.1 dB). The 10G channel operates at 12.5 Gb/s which includes the 25% overhead of the Ultra-FEC (FEC threshold is 9.5 x 10−3). The signal is amplified through a double-stage Erbium-doped Fiber Amplifier (EDFA) with a mid-stage Dispersion Compensation Unit (DCU) followed by a Wavelength Selective Switch (WSS) used to filter out the ASE from the transmit EDFA. At the receive end, an EDFA amplifies the received signal and another WSS is used to de-multiplex the channels. For 10G operation, two EDFAs and multiple DCUs are used to provide optical dispersion compensation (shown in insert of Fig. 2). At the transmit side, approximately −1,600 ps/nm of dispersion pre-compensation is placed in the mid-stage of the EDFA to improve transmission performance for both 100G and 10G transmission. Approximately −11,000 ps/nm dispersion post-compensation at the receiver side is used only for 10G transmission.
The span is assembled with Corning® Vascade® EX2000 optical fiber. Vascade EX2000 fiber is an ITU-T G.654B (cutoff-shifted single mode fiber with cutoff wavelength ≤ 1530 nm) compliant fiber with an average chromatic dispersion of 20.2 ps/nm-km and has a large Aeff of 112 μm2, enabling high optical launch powers into the fiber. The picture of the spools and ROPAs is shown in Fig. 3.
In the signal-path, the forward and backward ROPAs are located at 128.1 km and 151.6 km from the terminals, respectively. For the 100G transmission, the distance between the ROPAs is adjusted to 327.6 km for a total span length of 607.3 km and a span loss of 97.2 dB (loss of the ROPAs not included), resulting in an average fiber (including splices and connectors) loss of 0.160 dB/km. In the case of 10G transmission, the total distance is increased to 632.3 km (352.6 km between ROPAs) for a span loss of 101.0 dB. As for the dedicated pump paths, fiber length of 128.3 km and 154.3 km are used in pump-path1 for forward and backward pumping, respectively. For pump-path2, 128.8 km and 151.8 km long fibers are used.
The span distance and the loss are carefully verified by OTDR measurement and direct loss measurement with an Optical Power Meter (OPM). Figure 4 shows details of the OTDR measurements and analyzed results.
Figure 4 (a) shows measured OTDR traces of the signal path for 100G transmission. For the length analysis, 1.4623 at 1550nm is used as refractive index of EX2000 fiber. Due to the limit in dynamic range of the OTDR, the entire span for signal path is divided into four sections and each section was measured separately. Figure 4 (b) and (c) show measured OTDR traces for dedicated pumping fibers. The losses of the entire 607.3 km (for 100G) and 632.3 km (for 10G) spans are also measured using an OPM after splicing the four sections together without ROPAs. The total losses of 97.2dB and 101.0 dB are measured for 607.3 km and 623.3 km, respectively.
All distributed Raman pumps use the same commercial Raman pump module (Nu-Wave Optima SE24) that consists of five pump wavelengths distributed in the range between 1420 nm and 1500 nm. However, the Raman pump modules in the signal-path do not use the pump at the longest wavelength, thus operating with 4 pump wavelengths in the range between 1420 and 1480 nm. Turning off the longest pump wavelength (with less “walk-off” between pump and signal in a dispersive fiber) helps to reduce the RIN transfer penalty in the forward direction and also provides more efficient Raman gain to the signal wavelength around 1563 nm.
The pump modules in the pump-paths use all 5 pump wavelengths. Due to the Raman interaction between the pump wavelengths along the fiber, the longest wavelength in both the forward and backward pump modules has the highest power at the ROPA and is primarily used to excite the erbium fiber. The blue and green arrows in Fig. 2 represent residual pumps from the signal path and pump paths, respectively.
Figure 5 shows the details of the enhanced ROPA configurations which utilize two additional fibers for pumping. In the forward ROPA (Fig. 5 (a)), the residual pump power which comes from the signal-path is separated from the signal at a hybrid filter (a in the figure) and combined with residual pump power from pump-path1 using pump λ1/λ2 Mux (c in the figure). The combined powers then pump 20 m of erbium-doped fiber in the backward direction via sig/pump MUX filter (b in the figure). The residual pump from pump-path2 is combined with the signal by the hybrid filter (a) and then pumps the erbium-doped fiber in the forward direction. The backward ROPA (Fig. 5 (b)) uses similar schemes to combine residual pump powers from each fiber paths, with the difference that in the signal-path, the signal and residual pump travel in opposite directions. The backward ROPA uses an additional hybrid filter (a’), to split erbium gain into two sections while allowing the pumps to excite erbium fiber sections from both directions, which improves the Noise Figure (NF) of the backward ROPA.
3. Transmission Results
Figure 6 (a) shows the simulated power profiles of a single 100G channel over 607 km and of a single 10G channel over 632 km. Measured input signal powers, forward and backward pump powers and the characteristics of the Vascade EX2000 fiber  are used in the simulations. The signal first experiences the forward distributed Raman amplification. Then the signal is amplified by the forward ROPA, attenuated by the fiber, and amplified again by the backward ROPA. Finally, the signal experiences the backward distributed Raman amplification. The signal power launched to the span is −9.4 dBm for 100G and −3.6 dBm for 10G transmission. The same distributed Raman pump powers are used for both 100G and 10G transmission. The launched pump powers are 1860 mW in the signal-path and 2060 mW in the pump-paths (same pump powers for both forward and backward pumping). The residual pump power reaching the EDF in the forward ROPA is measured to be 5.2 mW from the signal-path and 8.7 mW, 8.2 mW from the pump-path1 and pump-path2, respectively. The forward ROPA gain is 18.8 dB for 100G, and 13.2 dB for 10G. The maximum power of the signal right after the forward ROPA at 128 km is + 11.9 dBm for 100G and + 12.2 dBm for 10G.
At the backward ROPA, the residual pump powers to the EDF sections are measured to be 1.7 mW, 2.4 mW and 3.3 mW from the signal-path, pump-path1, and pump-path2, respectively. The backward ROPA provides 25.4 dB gain at 100G and 26.2 dB at 10G. The measured spectra after the end of the span are shown in Fig. 6 (b) and (c). The measurement is done with 0.067 nm resolution using an EXFO Optical Spectrum Analyzer (OSA, FTB-5240S). The measured OSNR at the receiver is 13.7 dB (0.1nm) for 100G and 10.0 dB (0.1 nm) for 10G, in very good agreement with the simulations (13.6 dB, 10.1 dB).
The result of a 45-hour BER stability test at 100G transmission is plotted in Fig. 7 (a). The average pre-FEC BER over the duration of the test is 1.18 x 10−2 (corresponding to a Q of 7.1 dB) with less than 0.3 dB Q fluctuation and no uncorrected errors were observed after SD-FEC. The total signal propagation penalty which includes nonlinear, RIN and MPI penalties is estimated to be 0.6 dB in Q compared to the back-to-back performance (Q = 7.7 dB at 13.7 dB OSNR). Figure 7 (b) shows the result of a 30-hour stability test at 10G unrepeatered transmission. The average pre-FEC BER is 7.80 x 10−3 (corresponding to a Q of 7.8 dB) with less than 0.2 dB Q fluctuation and no uncorrected errors were observed after UFEC.
To the best of our knowledge, we have demonstrated for the first time 100G unrepeatered transmission over 600 km distance. The transmission of 10G over 632 km also represents the longest unrepeatered transmission ever demonstrated and the first time over a 100 dB span loss. Such record results are achieved by using a single fiber type, commercial Raman pump modules and 100G/10G channel cards, providing a practical solution for real field deployments.
The authors would like to thank John McLaughlin, Edwin Zak, Jeff Stone, Tung Nguyen and Kim Nguyen for helpful assistance in the experiment.
References and links
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