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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 2 — Jan. 16, 2012
  • pp: 706–711
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WDM/SDM transmission of 10 x 128-Gb/s PDM-QPSK over 2688-km 7-core fiber with a per-fiber net aggregate spectral-efficiency distance product of 40,320 km⋅b/s/Hz

S. Chandrasekhar, A. H. Gnauck, Xiang Liu, P. J. Winzer, Y. Pan, E. C. Burrows, T.F. Taunay, B. Zhu, M. Fishteyn, M. F. Yan, J. M. Fini, E.M. Monberg, and F.V. Dimarcello  »View Author Affiliations


Optics Express, Vol. 20, Issue 2, pp. 706-711 (2012)
http://dx.doi.org/10.1364/OE.20.000706


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Abstract

We demonstrate 2688-km multi-span transmission using wavelength-division multiplexing (WDM) of ten 50-GHz spaced 128-Gb/s PDM-QPSK signals, space-division multiplexed (SDM) in a low-crosstalk 76.8-km seven-core fiber, achieving a record net aggregate per-fiber-spectral-efficiency-distance product of 40,320 km⋅b/s/Hz. The demonstration was enabled by a novel core-to-core signal rotation scheme implemented in a 7-fold, synchronized recirculating loop apparatus.

© 2012 OSA

1. Introduction

2. Seven-core MCF and tapered multicore fiber connector characteristics

The MCF [3

3. B. Zhu, T. F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112-Tb/s space-division multiplexed DWDM transmission with 14-b/sHz aggregate spectral efficiency over a 76.8-km multicore Fiber,” Opt. Express 19, 16665–16671 (2011). [CrossRef] [PubMed]

,5

5. B. Zhu, T. F. Taunay, M. F. Yan, J. M. Fini, M. Fishteyn, E. M. Monberg, and F. V. Dimarcello, “Seven-core multicore fiber transmission for optical data links,” Opt. Express 18(11), 11117–11122 (2010). [CrossRef]

] is designed for operation in C and L bands for applications in high SE and high capacity optical networks, and consists of seven cores arranged in a hexagonal array with 9-μm core diameter and 46.8-μm core pitch (shown as a photograph in the inset of Fig. 1
Fig. 1 MCF with the TMC and photograph of cross-section of the MCF. Also shown is the span loss table. The center core loss was increased with a 3-dB attenuator to match the loss of the other cores.
.). The cladding diameter is 186.5 μm and the polymer coating diameter is 315-μm. The cutoff wavelength of each core is ~1.44 μm, and the mode-field diameter (MFD) at 1.55 μm is 9.6 μm. The dispersion and dispersion slope of each core in the MCF at 1.55 μm is about 16.5 ps/km-nm and 0.06 ps/km-nm2, respectively. The measured attenuation for the center core is about 0.23 dB/km and 0.37 dB/km at 1.55 μm and 1.3 μm respectively. The measured attenuation for each of the six outer cores is about 0.26 dB/km and 0.40 dB/km at 1.55 μm and 1.3 μm respectively. The attenuation characteristics of all cores are low across the entire transmission window [3

3. B. Zhu, T. F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112-Tb/s space-division multiplexed DWDM transmission with 14-b/sHz aggregate spectral efficiency over a 76.8-km multicore Fiber,” Opt. Express 19, 16665–16671 (2011). [CrossRef] [PubMed]

] and the values are similar to that of a single-core standard single-mode fiber.

It is crucial for multi-span MCF transmission to have an efficient coupling scheme between each core of the MCF and external single-core components such as optical amplifiers. A fiber-based tapered multicore coupler (TMC) was fabricated to match the core spacing and modefield properties of the MCF, as illustrated in Fig. 1. The TMC was designed to minimize power coupling between cores, eliminating the coupler as an additional source of crosstalk. Two TMCs were used, one at the input end and the other at the output end of the MCF. The measured insertion loss of each core of the two TMCs ranges between 0.5 dB and 2.8 dB, and the crosstalk between cores is less than −45 dB [5

5. B. Zhu, T. F. Taunay, M. F. Yan, J. M. Fini, M. Fishteyn, E. M. Monberg, and F. V. Dimarcello, “Seven-core multicore fiber transmission for optical data links,” Opt. Express 18(11), 11117–11122 (2010). [CrossRef]

]. The high insertion loss for some of the connections is attributed to small core misalignment, modefield mismatch and core asymmetry.

3. Core-to-core signal rotation concept

The transmission performance of SDM systems using MCF must be maximized to reap the benefits associated with such a multiplexing scheme. In particular, performance variations among the signals traveling through the multiple cores of a multi-span MCF system need to be minimized. (As an example, the signals propagating in the center core of a hexagonally symmetric MCF, as shown in Fig. 1, experience crosstalk from signals propagating in all six outer cores, which is 3-dB higher than the crosstalk from the three neighboring cores experienced by signals propagating in the outer cores.) A potentially viable solution is to increase the uniformity of the transmission characteristics of the cores in the MCF, such as loss, dispersion, and crosstalk, through better fiber design and manufacturing. However, this approach has its limitations and also makes fabrication issues challenging.

4. 2688-km SDM-DWDM transmission experiment

Figure 3
Fig. 3 Schematic of the experimental setup: (a) transmitter; (b) multiple recirculating loops with core-to-core rotation every round trip and (c) coherent receiver. Inset shows transmitter eye diagram.
shows the schematic of the experimental setup.

Ten C-band external cavity lasers (ECLs), each with a nominal linewidth of 100 kHz, were operated on a 50-GHz frequency grid from 193.200 THz to 193.650 THz (1548.15 nm to 1551.72 nm). The odd and even channels were separately combined using passive power combiners, and each group of five channels was separately modulated using integrated double-nested Mach-Zehnder modulators with 35-GHz 3-dB bandwidth and Vπ of 2.5 V. The in-phase (I) and quadrature (Q) branches of the modulators were driven by 32-Gb/s binary electrical signals from a pulse pattern generator with a pseudo-random bit sequence (PRBS) of length 215-1, with several tens of bits decorrelation between the I- and Q- waveforms. Following modulation, polarization multiplexing was achieved by 3-dB splitting the WDM signals, delaying one copy by several hundred symbols, and recombining them in a polarization beam splitter (PBS) using manual polarization controllers (PCs). The odd and even channels were then combined with a 100-GHz to 50-GHz interleaver (IL).

A 1x7 optical switch (SW) was used to direct the received signal from the outputs of each of the seven re-circulating loops to the eighth WB, where the WDM channel to be detected was selected. This selected channel was then sent to a coherent receiver consisting of a polarization-diversity 90-degree hybrid, followed by four balanced detectors. The signal was combined with another ECL serving as the local oscillator (LO), tuned to within ± 100 MHz of the signal carrier. The four signal components (Ix, Qx, Iy, Qy) were digitized asynchronously using two synchronized 2-channel 80-GS/s real-time oscilloscopes. Digitized waveforms of 1-million samples each were processed offline in a computer to perform electronic dispersion compensation, polarization de-multiplexing, frequency/phase recovery, and bit error ratio (BER) measurement using efficient PDM-QPSK algorithms [7

7. S. J. Savory, “Digital coherent optical receivers: Algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]

] without any crosstalk-compensating multiple-input multiple-output (MIMO) processing.

5. Measurement results

Figure 4(a)
Fig. 4 (a) Measured back-to-back BER performance of the 128-Gb/s PDM-QPSK channel at 193.400 THz. Inset: typical recovered signal constellation at an OSNR of 40 dB. (b) Spectra of the 10x128-Gb/s DWDM channels before and after transmission.
shows the measured back-to-back BER performance of the 128-Gb/s PDM-QPSK channel at 193.400 THz, while Fig. 4(b) shows the DWDM spectra before and after transmission. The required optical signal-to-noise ratio (OSNR), defined with a 0.1-nm noise bandwidth, is 15.5 dB at BER = 1 × 10−3, and 12.8 dB at BER = 1.5x10−2, the threshold for 19.5% overhead hard-decision forward error correction (FEC) that is assumed in this investigation following [8

8. F. Chang, “Application aspects of enhanced HD-FEC for 40/100G systems,” ECOC’10, workshop talk WS11–6. See also http://www.vitesse.com/products/download.php?fid=4424&number=VSC9804.

], where a BER of 1.5x10−2 (a Q2 of 6.7 dB) was shown to be correctable to a BER better than 1x10−15. Compared to the theoretical performance (red curve in Fig. 4(a)), the implementation penalty was 1.6 dB at BER = 1 × 10−3. With the overhead assumed in this investigation, the net data rate of each wavelength channel is 107.3-Gb/s, yielding a net DWDM SE of 2.15-b/s/Hz in each core, and a net aggregate per-fiber SE of 15-b/s/Hz.

Figure 5(a)
Fig. 5 (a) BER as a function of launch power per core at a distance of 2688 km of the channel at 193.400 THz, measured at the output of each of the seven loops. Inset: typical recovered signal constellation at 2688 km from the output of loop 1. (b) BER performances of the channel at 193.400 THz as a function of distance, measured at the output of each of the sevens loops, with 0-dBm launch power into each core.
shows the received BER as a function of launch power per core at a distance of 2688 km (35 round trips) for the channel at 193.400 THz, measured at each of the seven loop outputs. At this distance, the variations in BER from core to core are uncorrelated and mostly determined by the uncertainty in equalizing the DWDM channel powers. The optimum launch power for this reach is around 0 ± 1 dBm, where the BER from all the cores is below the FEC threshold of 1.5x10−2. It is instructive to mention here that the spread in the BER for a given power from the seven loop outputs is small, as expected for the CCR scheme implemented here. In the absence of such a “scrambling”, the spread is generally too large from signal power variations originating in the finite granularity of the WB attenuator setting used for channel equalization. In our experiment, we found the BER varied between 3x10−3 to 5x10−2 at a given “nominal” launch power of 0 dBm.

In an independent experiment, a single span of 80-km single-core standard single mode fiber (SSMF) was incorporated in a recirculating loop with one EDFA at the launch, one EDFA following the span transmission, and one WB for channel equalization. The span loss of this setup was adjusted to match that of the center core of the MCF with attenuators placed both at the input (to emulate the input TMC loss) and at the output (to emulate the output TMC loss). The optimum launch power for transmission over 35 spans was found to be between −2 dBm and 0 dBm, limited by the accuracy of setting the channel powers. The received BER was between 5x10−3 and 2.0x10−2 for the channel at 193.400 THz, similar to that measured with the MCF. These results confirm that SDM transmission performance with the MCF matches that of single-core SSMF spans, providing further evidence of the low crosstalk characteristics of the MCF as well as the benefits of CCR for averaging out the impairments.

In all subsequent experiments, the power of each channel was set to a nominal value of 0 dBm in each core. Figure 5(b) shows the measured BER as a function of distance traversed in the MCF for the channel at 193.400 THz. Measurements were made on this channel at all seven loop outputs. The spread in the BER from the different cores is low, demonstrating the benefit of using CCR at every span to equalize the performance of all the cores.

Finally, the BER performance of each of the ten DWDM channels is shown in Fig. 6
Fig. 6 BER performance of all the ten DWDM channels after transmission over 2688 km (35 spans), measured at the outputs of the seven loops, with 0-dBm launch power into each core.
, at the transmission distance of 2688 km. Measurements were made at the output of each loop and WB8 was set to select each of the ten channels. The measurements indicate that the worst and best BER values are 1.3x10−2 (Q2 value of 7 dB) and 2.7x10−3 (Q2 value of 9 dB), respectively, below the FEC threshold of 1.5x10−2. The delivered OSNR at the nominal launch power of 0 dBm was between 14 and 16 dB among the ten channels at the seven loop outputs, indicating a transmission penalty of about 1dB.

6. Summary

We have successfully transmitted ten 50-GHz spaced 128-Gb/s PDM-QPSK channels over 35 spans of a low-crosstalk MCF, achieving a net aggregate per-fiber spectral efficiency of 15 b/s/Hz and a net aggregate per-fiber spectral efficiency times distance-product of 40,320 km⋅b/s/Hz. A novel core-to-core rotation scheme was implemented that allowed for statistical averaging of the transmission performance over multi-span MCF. The demonstration highlights the promise of using the newly available low-crosstalk MCF to enable transmission over long distances with mature and well-established coherent reception techniques.

Acknowledgments

The authors wish to thank A. R. Chraplyvy and D. J. DiGiovanni for support.

References and links

1.

J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, and M. Watanabe, “109-Tb/s (7x97x172-Gb/s) SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multicore fiber,” OFC’11, PDPB6.

2.

B. Zhu, T.F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E.M. Monberg, F.V. Dimarcello, K. Abedin, P.W. Wisk D.W. Peckham, and P. Dziedzic, “Space-, wavelength-, polarization-division multiplexed transmission of 56-Tb/s over a 76.8-km seven-core fiber,” OFC’11, PDPB7.

3.

B. Zhu, T. F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112-Tb/s space-division multiplexed DWDM transmission with 14-b/sHz aggregate spectral efficiency over a 76.8-km multicore Fiber,” Opt. Express 19, 16665–16671 (2011). [CrossRef] [PubMed]

4.

R. W. Tkach, “Scaling optical communications for the next decade and beyond,” Bell Labs. Tech. J. 14(4), 3–9 (2010). [CrossRef]

5.

B. Zhu, T. F. Taunay, M. F. Yan, J. M. Fini, M. Fishteyn, E. M. Monberg, and F. V. Dimarcello, “Seven-core multicore fiber transmission for optical data links,” Opt. Express 18(11), 11117–11122 (2010). [CrossRef]

6.

S. Chandrasekhar, A. H. Gnauck, X. Liu, P. J. Winzer, Y. Pan, E. C. Burrows, B. Zhu, T.F. Taunay, M. Fishteyn, M. F. Yan, J. M. Fini, E.M. Monberg, and F.V. Dimarcello, “WDM/SDM transmission of 10 x 128-Gb/s PDM-QPSK over 2688-km 7-core fiber with a per-fiber net aggregate spectral-efficiency distance product of 40,320 km×b/s/Hz,” ECOC'11, paper Th.13.C4.

7.

S. J. Savory, “Digital coherent optical receivers: Algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]

8.

F. Chang, “Application aspects of enhanced HD-FEC for 40/100G systems,” ECOC’10, workshop talk WS11–6. See also http://www.vitesse.com/products/download.php?fid=4424&number=VSC9804.

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems

ToC Category:
Fibers, Fiber Devices, and Amplifiers

History
Original Manuscript: November 14, 2011
Manuscript Accepted: December 12, 2011
Published: January 3, 2012

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
S. Chandrasekhar, A. H. Gnauck, Xiang Liu, P. J. Winzer, Y. Pan, E. C. Burrows, T.F. Taunay, B. Zhu, M. Fishteyn, M. F. Yan, J. M. Fini, E.M. Monberg, and F.V. Dimarcello, "WDM/SDM transmission of 10 x 128-Gb/s PDM-QPSK over 2688-km 7-core fiber with a per-fiber net aggregate spectral-efficiency distance product of 40,320 km⋅b/s/Hz," Opt. Express 20, 706-711 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-2-706


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References

  1. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, and M. Watanabe, “109-Tb/s (7x97x172-Gb/s) SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multicore fiber,” OFC’11, PDPB6.
  2. B. Zhu, T.F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E.M. Monberg, F.V. Dimarcello, K. Abedin, P.W. Wisk D.W. Peckham, and P. Dziedzic, “Space-, wavelength-, polarization-division multiplexed transmission of 56-Tb/s over a 76.8-km seven-core fiber,” OFC’11, PDPB7.
  3. B. Zhu, T. F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112-Tb/s space-division multiplexed DWDM transmission with 14-b/sHz aggregate spectral efficiency over a 76.8-km multicore Fiber,” Opt. Express19, 16665–16671 (2011). [CrossRef] [PubMed]
  4. R. W. Tkach, “Scaling optical communications for the next decade and beyond,” Bell Labs. Tech. J.14(4), 3–9 (2010). [CrossRef]
  5. B. Zhu, T. F. Taunay, M. F. Yan, J. M. Fini, M. Fishteyn, E. M. Monberg, and F. V. Dimarcello, “Seven-core multicore fiber transmission for optical data links,” Opt. Express18(11), 11117–11122 (2010). [CrossRef]
  6. S. Chandrasekhar, A. H. Gnauck, X. Liu, P. J. Winzer, Y. Pan, E. C. Burrows, B. Zhu, T.F. Taunay, M. Fishteyn, M. F. Yan, J. M. Fini, E.M. Monberg, and F.V. Dimarcello, “WDM/SDM transmission of 10 x 128-Gb/s PDM-QPSK over 2688-km 7-core fiber with a per-fiber net aggregate spectral-efficiency distance product of 40,320 km×b/s/Hz,” ECOC'11, paper Th.13.C4.
  7. S. J. Savory, “Digital coherent optical receivers: Algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron.16(5), 1164–1179 (2010). [CrossRef]
  8. F. Chang, “Application aspects of enhanced HD-FEC for 40/100G systems,” ECOC’10, workshop talk WS11–6. See also http://www.vitesse.com/products/download.php?fid=4424&number=VSC9804 .

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