We demonstrate unrepeatered transmission of 8x128Gb/s PDM-QPSK signals over a 515k-m fiber link. This ultra-long distance of 800 Gb/s unrepeatered transmission in a single fiber configuration is achieved by employing enabling techniques such as large-effective-area ultra-low-attenuation fibers, co-propagating and counter-propagating 2nd-order-pumped distributed Raman amplification, and remote optically pumped amplifier (ROPA). The ROPA itself is also counter-propagating 2nd-order Raman pumped. The designs and characteristics of the ROPA and 2nd-order pumped distributed Raman amplification are described, and optimization of the transmission performance of this ultra-long reach 800Gb/s unrepeatered transmission fiber link is discussed in this paper.
© 2016 Optical Society of America
There is a growing interest in unrepeatered transmission technologies for wide range applications including coastal festoons, island hopping, lake crossings for submarine optical fiber links, and even bridging ultra-long spans in terrestrial systems [1–5]. The technology development for such unrepeatered systems is targeted to achieve transmission distances as long as possible without any in-line active elements, so it provides a cost effective solution for point-to-point connections over several hundred kilometers of fiber links without the need for any in-line repeaters.
It is very difficult to increase the transmission distance in unrepeatered links, as the optical power is launched only from the terminals stations. Large effective area (Aeff) ultra-low attenuation fibers are essential to extend the distance of unrepeatered transmission system. Large Aeff enables a high optimum launch power to be delivered into the fiber while ultra-low fiber attenuation extends the physical distance achievable for any specific span loss budget of a system. The unrepeatered submarine cable systems also employ a high power booster  at the transmitter station, and/or co-propagating and counter-propagating distributed Raman amplification [1–3, 5] to provide high system gains. The receiver characteristics can be improved by means of remote optically pumped amplifier (ROPA) which is remotely usually backward pumped from the receiver station. High-order (e.g. 2nd, or 3rd-order) Raman pumping [1–5] has also been employed for distributed Raman amplification and ROPA to improve system performance.
Recently, a number of unrepeatered transmission experimental demonstrations on large capacity systems have been reported, for example, a 6.3 Tb/s unrepeatered transmission over 402km fiber was achieved using high power booster and 2nd-order Raman pumped ROPA in L-band transmission . An 8 Tb/s unrepeatered transmission over 363km fiber was reported using 200Gb/s PDM-16-QAM signals and a strong 3rd-order Raman pumping scheme . Other work has been focused on achieving ultra-long reach but with smaller capacity, such as 8x120Gb/s over 480.4km large-Aeff ultra-low-loss fiber with mixed fiber types . Four 100G channels and one 100G channel unrepeatered transmission over 523.2km and 556.7km of G.654 fiber using both forward and backward ROPA have been demonstrated respectively . Two 50Gb/s PDM-RZ-BPSK signals unrepeatered transmission over 610km has also been reported using 3rd -order Raman pumping .
In this work, we demonstrate 800Gb/s (8x128Gb/s) unrepeatered transmission over a 515km fiber by using new large Aeff ultra-low-loss fiber and co-propagating and counter-propagating 2nd-order pumped distributed Raman amplifications. A backward-pumped ROPA, which is counter-propagating 2nd-order Raman pumped, is also used to achieve this result. To the best of our knowledge, this is the longest reach of 800Gb/s unrepeatered transmission in a single fiber configuration. In this paper, we describe the designs and characteristics of the ROPA and 2nd-order pumped distributed Raman amplifications, and discuss the system performance of this ultra-long reach 800Gb/s unrepeatered transmission fiber link for submarine applications
2. Experimental set-up
The schematic diagram for the unrepeatered transmission experiment is shown in Fig. 1. The transmitters consisted of 8 distributed feedback (DFB) lasers and one tunable external cavity laser (ECL) at wavelengths ranging from 1556.15nm to 1561.83nm on 100-GHz-spacing. Two sets of four 200-GHz spaced channels, corresponding to odd and even channels, were combined by the polarization-maintaining (PM) coupler. The odd and even channels were modulated independently by two separate nested Mach-Zehnder modulators whose in-phase/quadrature (I/Q) modulators were driven by 32-Gb/s 215-1 pseudo-random bit sequences (PRBS) with suitable inversion and delay to assure decorrelation of the I and Q data. The outputs from two modulators were combined together by another PM coupler, and then amplified by a polarization-maintaining EDFA. The signals output from PM-EDFA were split into two paths with a relative delay of 386 symbols between the two polarization tributaries before being polarization-multiplexed by a polarization beam combiner (PBC) to form polarization-division-multiplexed (PDM) quadrature-phase-shift keying (QPSK) channels at 128-Gb/s. This bit-rate accounts for a 20% overhead of soft-decision forward-error-correction (SD-FEC), which corrects a bit-error ratio (BER) of 2.4x10−2 (Q = 5.92dB) to better than 10−15 using quasi-cyclic low-density-parity-check code . A tunable ECL with a linewidth of ~100 kHz was used for BER measurement. Each channel under measurement was switched from the DFB source to the tunable ECL source. The eight 128Gb/s PDM-QPSK signals were passed through a dispersion compensating fiber (DCF) with a dispersion of −940ps/nm at 1550nm in order to improve transmission performance. The eight PDM-QPSK signals were then amplified again by an EDFA before being launched into the link.
The link consisted of two sections using OFS TeraWaveTM ULL fiber. The first section fiber span from transmitters to ROPA was 366km length, while the second section from ROPA to receiver was 149km long. The TeraWaveTM ULL fiber is ITU-T G.654.D  compliant and it has a typical Aeff of 125μm2, and ultra-low attenuation of 0.155dB/km at 1550nm wavelength. The macro- and micro-bending performances of this large Aeff fiber are improved by optimized waveguide design . It exhibits excellent bending performance and meets all macro-bending requirements in G.654.D. The total link loss including splices and connectors at 1550-nm was 81.2dB.
A low relative-intensity-noise (RIN) (~-130 dB/Hz for frequencies from approximately 1 to 100-MHz) semiconductor diode at 1455nm combined with a fiber laser at 1363nm (as the 2nd-order pump) using CWDM coupler were employed to provide co-pumped distributed Raman amplification in the 1st section. A high gain efficiency Er-doped fiber (EDF) was used in the ROPA which was remotely pumped by 1489nm semiconductor lasers from receiver station. A 2nd-order pump at 1393-nm was also employed in the counter-propagating direction to provide Raman gain to the 1489nm pump light. Additionally, a 1447nm laser, which was combined with the 1489nm and 1393nm pump lasers, was used to provide the distributed Raman gain of the signals in counter-propagating direction from the 2nd section of fiber link.
The receiver was a typical digital coherent receiver as described in ; it consisted of a 90° polarization-diversity optical hybrid, an optical local oscillator (OLO) using a tunable ECL, and four balanced photo-detectors. The electrical waveforms were digitized by four 50-GSamples/s analog-to-digital converters (ADCs) using Tektronix real-time sampling scope (DPO72004B) with 20-GHz bandwidth. The digitized waveforms of 1-million samples each were processed offline to perform electronic CD compensation, polarization de-multiplexing, and frequency/phase recovery. Finally the BERs were calculated using direct error-counting and averaged over 1-million samples, and then averaged over the two polarizations, from which Q2 factors were calculated. The back to back performance of the PDM-QPSK was characterized, and the required OSNR for SD-FEC threshold is 11.3dB at 0.1nm resolution bandwidth (RBW).
3. ROPA design and link amplifications characteristics
The ROPAs, employing a high efficiency EDF, were designed first by simulation, and then further optimized using a set of measurement data such as input signals powers and available pump powers after transmission system optimization. The EDF used for ROPA has the peakabsorption of 5.9dB/m, NA of 0.33 and MFD of 4.4-μm at 1550nm, Fig. 2 shows one example of the ROPA gain vs the length of EDF from simulation using an input signal power of −35.2dBm/channel and 1489nm pump of 5 and 6mW respectively. It can be seen that a 20m EDF provides the optimized gain at this operation condition. In the experiment, a 20 m EDF was used for the ROPA. At the system optimized operation (see below), the residual power of 1489nm pumps reaching ROPA from receiver station was estimated to be 5.7 mW by simulation. It should be noted that the location of EDF was also carefully chosen by considering the balance between the 1489nm pump power arriving at the EDF from the receiver station and the power of the 128Gb/s PDM-QPSK WDM signals arriving from the transmitter station in order to maximize the system gain and improve noise performance of the ROPA.
Figure 3 shows the simulated results of evolution of optical powers for the eight 128Gb/s signal channels and Raman pumps along the 515km fiber line under optimized operation conditions, illustrating the benefits of the 2nd-order Raman amplification and the ROPA for extending the reach of unrepeatered system. The input signal power per channel, and co- and counter-pump powers, which were measured at optimized transmissions (see below), were used in the simulation. It can be seen from Fig. 3 that the signals first experience the co-pumping distributed Raman gain; then are attenuated by fiber loss. The signals are amplified again by backward-pumped ROPA, finally, they experience the backward distributed Raman amplification before reaching the receiver station. The signals’ peak power per channel reaches ~ + 12.7 dBm at ~45km from the transmitter station. It can be seen that the 2nd -order Raman pump at 1363nm allowed the co-pumping 1455nm pump lights to penetrate deep into the large-Aeff ultra-low loss fiber spans, thus sufficient input powers of the 128Gb/s PDM-QPSK signals can be delivered into the EDF of the ROPA. The 2nd-order Raman pump at 1393nm provided majority of Raman gain for 1489nm pump light, so that a sufficient 1489nm pump power can be delivered to ROPA. The semiconductor laser at 1447nm was relatively far from the Raman gain peak of 1393nm pumps and experienced less Raman gain from the 2nd-order Raman pump 1393nm. This design helped to keep enough residual 1489nm pump power for the ROPA, while providing enough Raman gain for the signals in 2nd section of fiber link.
Figure 4 plots the measured gain from the co-propagating distributed Raman gain in the first section of fiber link under the optimized operation conditions for the transmission (see below), an averaged 24.5 dB Raman gain was obtained with a slight negative tilt of the gain spectrum. The total gain from ROPA and counter-propagating distributed Raman gain from second section of fiber spans is shown in the Fig. 4, and 47.8dB average gain was achieved with a positive tilt of the total gain spectrum in second section of the fiber link.
4. Transmission results
The best transmission performance required collective optimization of the signal launch power, co- and counter-pump powers. The powers of the 1455 and 1447/1489nm from semiconductor lasers were fixed to be 250, and 160, 320 mW respectively, and the powers of 2nd-order pump fiber lasers (1363nm and 1393nm) were varied. First, a rough optimum for each power was found; then one of the powers was scanned while other two were fixed. Figure 5(a) shows the Q-factor of 1560.20nm (near center) channel vs. 2nd -order co- and counter-pump (1363/1393nm) power when the total signal launched power was fixed at around 3.5dBm, and optimum pump powers of 2nd-order 1363nm and 1393nm pumps were found about 1.85W and 2.0W respectively. Figure 5(b) shows the performance of 1560.20nm channel as a function of total launched power when 2nd-order co- (1363nm) and counter (1393nm) pump powers were fixed at 1.85W and 2.0 W respectively, and the total optimal signal launch power was found to be ~3-dBm (−6dBm per channel). From the measured Q and OSNR in Fig. 5(b) at low input power regime and back-to-back BER performance, the transmission Q penalty of this unrepeatered transmission experiment was estimated to be about 0.7dB which was mainly caused by fiber laser pumps that had relative high RIN noises.
As shown in Fig. 4, the total gain of the ROPA and counter-pumped Raman gain from second section fiber links was positive tilted due to Raman gain from 1489nm, hence the channel pre-emphasis were adjusted at transmitters in order to obtain relative flat spectrum of the received signal channels. Figure 6(a) shows the measured optical spectra at transmitter and the receiver, showing slightly negative tilt in the transmitters’ spectrum and relative flat spectrum from receiver. The measured OSNR is shown in Fig. 6(b), and the average received OSNR over eight channels in 0.1nm RBW was 13.7dB. The BERs of all channels were measured and Fig. 6(b) shows all Q values. The averaged Q-factor is 6.21dB and the worst channel has Q-factor of 6.0dB; all channels are therefore above the FEC limit of 5.92 dB and would yield a BER below 10−15 after correction by SD-FEC.
We have demonstrated unrepeatered transmission of 8x128Gb/s PDM-QPSK signals over a 515km fiber link. This 800 Gb/s unrepeatered transmission was achieved by employing enabling techniques including large-effective-area ultra-low-attenuation fibers, co-propagating and counter-propagating 2nd-order pumped distributed Raman amplifications, and ROPA, which was also 2nd-order Raman pumped in a counter-propagating configuration. The design and characteristics of the ROPA and co- and counter-propagating distributed Raman amplification are described, and the collective optimization of these amplifiers to achieve unrepeatered transmission of 800 Gb/s over a record distance of 515km is discussed.
References and links
1. H. Bissessur, P. Bousselet, D. Mongardien, G. Boissy, and J. Lestrade, “4x100Gb/s unrepeatered transmission over 462km using coherent PDM-QPSK format and real-time processing,” in ECOC 2011 (2011), paper Tu.3.B.3.
2. D. Chang, W. Pelouch, and J. McLaughlin, “8 x 120 Gb/s unrepeatered transmission over 444 km (76.6 dB) using distributed raman amplification and ROPA without discrete amplification,” in ECOC 2011 (2011), paper Tu.3.B.2.
3. B. Zhu, P. Borel, K. Carlson, X. Jiang, D. W. Peckham, and R. Lingle Jr., “Unrepeatered transmission of 3.2-Tb/s (32x120-Gb/s) over 445-km fiber link with Aeff managed span,” in Proc. OFC/NFOEC 2013 (2013), paper OTu2B.2. [CrossRef]
4. B. Zhu, P. Borel, K. Carlson, X. Jiang, D. W. Peckham, R. Lingle, Jr., M. Law, J. Rooney, and M. F. Yan, “6.3-Tb/s unrepeatered transmission over 402-km fiber using high power Yb-free clad-pumped L-band EDFA,” in Proc. OFC 2014 (2014), paper W1A.2.
5. H. Bissessur, C. Bastide, D. Dubost, S. Etienne, and D. Mongardien, “8Tb/s unrepeatered transmission of real-time processed 200 Gb/s PDM 16-QAM over 363 km,” in ECOC 2014 (2014), paper Tu.1.5.3.
6. D. Chang, P. Patki, S. Burtsev, and W. Pelouch, “8 x 120 Gb/s transmission over 80.8 dB/480.4km unrepeatered span,” in Proc. 2013 (2013), paper JTh2A.42.
7. T. J. Xia, D. L. Peterson, G. A. Wellbrock, D. Chang, P. Perrier, H. Fevrier, S. Ten, C. Tower, and G. Mills, “557 km Unrepeatered 100G transmission with commercial Raman DWDM system, enhanced ROPA, and cabled large Aeff ultra low loss fiber in OSP environment,” in Proc.OFC 2014 (2014), paper Th5.A7.
8. S. Etienne, H. Bissessur, C. Bastide, and D. Mongardien, “Ultra-Long 610 km unrepeatered transmission of 100 Gb/s using single fiber configuration,” in ECOC 2015 (2015), paper Th2.2.5.
9. D. Chang, F. Yu, Z. Xiao, Y. Li, N. Stojanovic, C. Xie, X. Shi, X. Xu, and Q. Xiong, “FPGA verification of a single QC-LDPC code for 100 Gb/s optical systems without error floor down to BER of 10−15,” in Proc. OFC 2011 (2011), paper OTuN2. [CrossRef]
10. ITUT, “Characteristics of a cut-off shifted single-mode optical fiber and cable”, www.itu.int/rec/T-REC-G.654-201210-I/ (2012).
11. D. Peckham, A. Klein, P. Borel, R. Jensen, O. Levring, K. Carlson, M. Yan, P. Wisk, D. Trevor, R. Lingle Jr, A. McCurdy, B. Zhu, Y. Zou, R. Norris, B. Palsdottir, and D. Vaidya, “Optimization of large area, low loss fiber designs for C+L band transmission,” in Proc. OFC 2016 (2016), paper Tu3G.1. [CrossRef]
12. B. Zhu, C. Xie, L. E. Nelson, X. Jiang, D. W. Peckham, R. Lingle, Jr., M. F. Yan, P. W. Wisk, and D. J. DiGiovanni, “70 nm seamless band transmission of 17.3 Tb/s over 40x100km of fiber using complementary Raman/EDFA,” in Proc. OFC 2015 (2015), paper W3G4.