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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 17 — Aug. 15, 2011
  • pp: 16665–16671
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112-Tb/s Space-division multiplexed DWDM transmission with 14-b/s/Hz aggregate spectral efficiency over a 76.8-km seven-core fiber

B. Zhu, T.F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello  »View Author Affiliations


Optics Express, Vol. 19, Issue 17, pp. 16665-16671 (2011)
http://dx.doi.org/10.1364/OE.19.016665


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Abstract

We describe a new multicore fiber (MCF) having seven single-mode cores arranged in a hexagonal array, exhibiting low crosstalk among the cores and low loss across the C and L bands. We experimentally demonstrate a record transmission capacity of 112 Tb/s over a 76.8-km MCF using space-division multiplexing and dense wavelength-division multiplexing (DWDM). Each core carries 160 107-Gb/s polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) channels on a 50-GHz grid in the C and L bands, resulting in an aggregate spectral efficiency of 14 b/s/Hz. We further investigate the impact of the inter-core crosstalk on a 107-Gb/s PDM-QPSK signal after transmitting through the center core of the MCF when all the 6 outer cores carry same-wavelength 107-Gb/s signals with equal powers, and discuss the system implications of core-to-core crosstalk on ultra-long-haul transmission.

© 2011 OSA

1. Introduction

Demand for large transmission capacity has been increasing exponentially due to the rapid spread of Internet services [1

1. A. R. Chraplyvy, “The coming capacity crunch,” ECOC2009, Vienna, Austria, plenary talk.

]. Today’s optical fiber communication systems employ time-, wavelength-, and polarization-division multiplexing to carry as much traffic as possible through conventional single-mode fibers. As these dimensions become exhausted, one must consider multiplexing techniques in other dimensions, for example, space-division multiplexing (SDM) [2

2. P. J. Winzer and R.-J. Essiambre, “Advanced Optical Modulation Formats”, in Optical Fiber Telecommunications V, I. Kaminow, T. Li, and A. E. Willner (eds.), Elsevier (2008)

,3

3. T. Morioka, “New generation optical infrastructure technologies: ‘EXAT initiative’ towards 2020 and beyond”, OECC2009, paper FT4.

] using multicore fiber (MCF) [4

4. S. Inao, T. Sato, S. Sentsui, T. Kuroha, and Y. Nishimura, “Multicore optical fiber,” in Optical Fiber Communication, 1979 OSA Technical Digest Series (Optical Society of America, 1979), paper WB1.

6

6. T. Hayashi, T. Toshiki, S. Osamu, S. Takashi, and S. Eisuke, “Ultra-low-crosstalk multicore fiber feasible to ultra-long haul transmission”, OFC’11, PDPC2 (2011).

]. This approach is now being researched worldwide as a potential solution to meet the ever-increasing demand of capacity in optical networks. However, there are a great deal of technical challenges in the design and manufacture of high-core-count, low-loss, and low-crosstalk MCF. It is also increasingly challenging to couple signals into and out of each core in an MCF because the cores are closely spaced. Recently, impressive transmission results based on SDM using MCF have been reported [7

7. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, M. Watanabe, “109-Tb/s (7x97x172-Gb/s) SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multicore fiber”, OFC’11, PDPB6 (2011).

9

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

]. A transmission capacity of 109 Tb/s was demonstrated in the C and L bands over a 16.8-km seven-core MCF [7

7. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, M. Watanabe, “109-Tb/s (7x97x172-Gb/s) SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multicore fiber”, OFC’11, PDPB6 (2011).

], and a capacity of 56 Tb/s transmission was demonstrated in the C-band alone over a 76.8-km seven-core MCF [8

8. 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 (2011).

].

In this paper, we present in more depth the fiber optical properties of the seven-core MCF reported in Ref [8

8. 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 (2011).

], which exhibits low attenuation and low core-to-core crosstalk across the entire C and L transmission bands. We further experimentally demonstrate a record transmission capacity of 112 Tb/s (7 × 160 × 107 Gb/s) through SDM and DWDM over the 76.8-km seven-core MCF with an aggregate spectral-efficiency (SE) of 14 b/s/Hz. The SDM is achieved using a newly designed tapered multicore connector (TMC) fusion spliced to the MCF with a commercially available polarization maintaining (PM) fiber splicer. Each core in the MCF carries 160 107-Gb/s DWDM channels using polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) on a 50-GHz grid in the C and L bands. In addition, we study the impact of the inter-core crosstalk on the signal passing through the center core, and discuss the system implications of core-to-core crosstalk for long-haul transmission.

2. Seven-core MCF and tapered multicore fiber connector

The MCF 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 in the inset of Fig. 1(a)
Fig. 1 (a) Measured attenuation spectra of seven-core MCF, Inset: cross-section photograph of the MCF, (b) measured optical core-to-core crosstalk spectra of the MCF.
). The cladding diameter was increased to 186.5 μm to reduce attenuation of the outer cores due to coupling to the polymer coating, which has 315-μm diameter. The cutoff wavelength of each core is ~1.44 μm, and the mode-field diameter (MFD) at 1550 nm is 9.6 μm. The dispersion and dispersion slope of each core in the MCF at 1550 nm are about 16.5 ps/km-nm and 0.06 ps/km-nm2, respectively. The measured attenuation spectra for all cores are shown in Fig. 1(a). The center core has 0.23 dB/km and 0.37 dB/km at 1550nm and 1300nm respectively. The average losses for 6 outer cores are 0.26dB/km and 0.40dB/km at 1550nm and 1300nm respectively. It can be seen that the loss of all cores are low across the entire transmission window and the values of losses are similar to that of a single-core standard single-mode fiber.

Connectivity to individual cores becomes very difficult due to the dense core spacing in the MCF. A fiber-based tapered multicore coupler was fabricated to match the core spacing and modefield properties of the MCF, as illustrated in Fig. 2
Fig. 2 Schematic diagram of tapered multicore fiber connectors.
. The TMC was designed to minimize power coupling between cores, eliminating the coupler as an additional source of crosstalk. The measured insertion loss of each core of two TMCs ranges between 0.45 dB and 2.77 dB, and the crosstalk between cores is less than −45 dB (Table-1). High insertion loss is attributed to slight core misalignment, modefield mismatch and slight core asymmetry.

3. 112-Tb/s (7 x 160 x 107-Gb/s) SDM-DWDM transmission experiment

The schematic diagram for the 100-Gb/s SDM-DWDM transmission experimental setup is shown in Fig. 3
Fig. 3 Schematic diagram of experimental setup for 107-Gb/s SDM-DWDM seven-core fiber transmission.
. The WDM transmitters consisted of 80 DFB lasers at wavelengths ranging from 1530.33 to 1561.83 nm in the C-band and 80 DFB lasers at wavelengths ranging from 1570.42 to 1603.60 nm in the L-band, with both groups of channels on the 50-GHz-spaced ITU frequency grid. In each wavelength band, two sets of 100-GHz spaced channels, corresponding to odd and even channels, were multiplexed separately by two 1x40 arrayed waveguide grating (AWG) routers. The C-band and L-band (either all odd or all even) channels were combined together using polarization maintaining (PM) couplers. The odd and even channels were modulated independently by two separate QPSK modulators, each fed with 26.75-Gb/s pseudo-random bit sequences (PRBS) with a length of 215-1. The output from each modulator was split into two paths with a relative delay of 84 symbols (or 3.14 ns) before being polarization-multiplexed by a polarization beam combiner (PBC) to form a PDM-QPSK channel at 107 Gb/s, which supports a net data rate of 100 Gb/s after excluding a typical 7% overhead for forward error correction (FEC). The odd and even channels were then spectrally interleaved through a 50-GHz interleaver (IL) to form 160 DWDM channels. Tunable external cavity lasers (ECLs), one each for C-band and L-band, with a linewidth of ~100 kHz were used for bit-error ratio (BER) measurement on the respective channel under test. Each channel under test was switched from the DFB source to the tunable ECL source. The 160 DWDM channels were separated by a C/L band splitter into the C-band channels and the L-band channels, which were separately amplified by a C-band EDFA and an L-band EDFA, respectively. For each band, the 80 DWDM channels were split by a 1:8 power splitter, whose seven outputs were amplified by seven EDFAs. The amplified C-band and L-band channels were combined together using C/L band couplers, then launched into each of the seven cores of the MCF.

At the receiver side, the 160 wavelength channels from each core were amplified separately by EDFAs and separated by another interleaver (IL) and AWG combination, before being measured individually. The receiver was a typical digital coherent receiver consisting of a polarization-diversity optical hybrid, an optical local oscillator (OLO) using a tunable ECL, and four single-ended photo-detectors. The electrical waveforms were digitized by four 50-GS/s analog-to-digital converters (ADCs) in a real-time sampling scope. The 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 BER measurement using typical PDM-QPSK algorithms [12

12. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008). [CrossRef] [PubMed]

].

4. Measurement results

We first assessed the impact of crosstalk on a 107-Gb/s PDM-QPSK signal after passing it through the center core when all the 6 outer cores carried same-wavelength 107-Gb/s signals with equal powers. We fixed the signal launch power for each of the 6 outer cores (Pouter) at 16 dBm, while varying the signal launched power for the center core for different measurements. This allows us to effectively vary the crosstalk in the center core, defined as the ratio between the power of the crosstalk from the outer cores and that of the signal in the center core. Figure 5
Fig. 5 In-band crosstalk Q-factor penalty of center-core as a function of the ratio between the outer-core signal power (Pouter) and the center-core signal power (Pcenter).
shows the in-band crosstalk penalty as a function of the ratio between the outer-core signal power (Pouter) and the center-core signal power (Pcenter). The penalty was obtained by finding the difference in the measured Q2 values obtained with and without the presence of the outer-core signals. For a 1-dB penalty, Pouter can be 15 dB and 10 dB higher than Pcenter, at 1561-nm and 1600-nm wavelengths, respectively. If the transmission distance of system is not limited by OSNR and/or other non-linear impairments, the above in-band crosstalk penalty measurement results indicate that the 107-Gb/s PDM-QPSK signal could pass ~30 spans in the C-band, or ~10 spans in the L-band, with a small crosstalk penalty of ~1 dB, when the signal powers in all the seven cores are the same. The difference in the crosstalk penalty is also reasonable considering the crosstalk difference shown in Fig. 4. The above results also suggest that the inter-core crosstalk of the current MCF fiber design is sufficiently low in the C-band to support ultra-long-haul transmission over 2000 km for PDM-QPSK signals, but further crosstalk reduction is desirable for supporting ultra-long-haul transmission in the L-band, and/or for supporting higher-level modulation formats such as PDM-16QAM.

For DWDM transmission, the average launch power into each core was 18.3 dBm for the C-band and 18.0 dBm for the L-band. The span losses including TMC1, two spools of MCF, TMC2, and SW ranged from 21 to 25 dB. A typical received optical spectrum of all the 160 channels is shown in Fig. 6
Fig. 6 Typical optical spectrum and OSNR measured at one output of the seven-core fiber
. The non-uniformity of the optical spectrum is due to the wavelength dependence of the optical components, including the EDFAs, used in this experiment. The measured received OSNR (0.1-nm resolution bandwidth) ranged from 25.8 dB to 29.8 dB in the C-band, and 22.6 dB to 27.3 dB in the L-band. The lower OSNR in the L-band is mostly due to the higher loss of the optical components used.

Figure 7
Fig. 7 Signal Q2-factor (derived from the measured BER) after 76.8-km transmission as a function of signal wavelength for all the seven cores. Insets: typical recovered signal constellations after transmission in the C-band (a) and in the L-band (b).
shows the measured Q2 factors for all channels and for all cores. The Q2 factor is derived from the measured BER through error counting at the receiver. For those channels that yielded no errors over the sampled data records, Q2 values were estimated from the variances of the constellation points. The mean BER values in the C and L bands are 8 × 10−6 and 1.6 × 10−4, respectively, which are much lower than the typical 7%-overhead FEC thresholds (~3.8 × 10−3). Relatively lower Q2 factors in the short wavelength region of the L-band (near 1570 nm) are due to the lower OSNR, and lower Q2 factors at the long wavelength region in the L-band (near 1600 nm) are attributed to the fact that the polarization-diversity optical hybrid used in this experiment was not optimized for operation in the L-band. Nevertheless, the worst BER value for the 1120 (7 × 160) measurements was 6 × 10−4, lower than the FEC threshold. Negligible crosstalk penalty was found in this SDM-DWM transmission experiment.

5. Summary

We have presented a new seven-core fiber exhibiting low crosstalk among all cores and low loss across the entire C and L bands. We have experimentally demonstrated space-division multiplexed DWDM transmission of 1120 107-Gb/s PDM-QPSK signals over a 76.8-km seven-core fiber having low crosstalk in both C and L bands, achieving a record total capacity of 112 Tb/s (7 × 160 × 107 Gb/s) with an aggregate spectral efficiency of 14 b/s/Hz on a single fiber. We further have investigated the impact of crosstalk on a 107-Gb/s PDM-QPSK signal after passing it through the center core of the MCF when all the 6 outer cores carried same-wavelength 107-Gb/s signals with equal powers, and discussed the system implications of crosstalk properties in MCF for ultra-long-haul transmission.

Acknowledgements

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

References and links

1.

A. R. Chraplyvy, “The coming capacity crunch,” ECOC2009, Vienna, Austria, plenary talk.

2.

P. J. Winzer and R.-J. Essiambre, “Advanced Optical Modulation Formats”, in Optical Fiber Telecommunications V, I. Kaminow, T. Li, and A. E. Willner (eds.), Elsevier (2008)

3.

T. Morioka, “New generation optical infrastructure technologies: ‘EXAT initiative’ towards 2020 and beyond”, OECC2009, paper FT4.

4.

S. Inao, T. Sato, S. Sentsui, T. Kuroha, and Y. Nishimura, “Multicore optical fiber,” in Optical Fiber Communication, 1979 OSA Technical Digest Series (Optical Society of America, 1979), paper WB1.

5.

K. Imamura, K. Mukasa, T. Yagi., “Investigation on multicore fibers with large Aeff and low bending loss”, OFC2010, San Diego, CA, paper OWK6.

6.

T. Hayashi, T. Toshiki, S. Osamu, S. Takashi, and S. Eisuke, “Ultra-low-crosstalk multicore fiber feasible to ultra-long haul transmission”, OFC’11, PDPC2 (2011).

7.

J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, M. Watanabe, “109-Tb/s (7x97x172-Gb/s) SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multicore fiber”, OFC’11, PDPB6 (2011).

8.

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 (2011).

9.

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

10.

J. M. Fini, T.F. Taunay, B. Zhu, and M. F. Yan, “Low cross-talk design of multicore fibers”, in CLEO2010, OSA Technical Digest, DC, paper CTuAA3.

11.

K.-P. Ho, “Effects of homodyne crosstalk on dual-polarization QPSK signals,” J. Lightwave Technol 29, 124 (2011).

12.

S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008). [CrossRef] [PubMed]

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:
Space-Division Multiplexing

History
Original Manuscript: May 24, 2011
Revised Manuscript: June 16, 2011
Manuscript Accepted: July 15, 2011
Published: August 15, 2011

Virtual Issues
Space Multiplexed Optical Transmission (2011) Optics Express

Citation
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/s/Hz aggregate spectral efficiency over a 76.8-km seven-core fiber," Opt. Express 19, 16665-16671 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-17-16665


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References

  1. A. R. Chraplyvy, “The coming capacity crunch,” ECOC2009, Vienna, Austria, plenary talk.
  2. P. J. Winzer and R.-J. Essiambre, “Advanced Optical Modulation Formats”, in Optical Fiber Telecommunications V, I. Kaminow, T. Li, and A. E. Willner (eds.), Elsevier (2008)
  3. T. Morioka, “New generation optical infrastructure technologies: ‘EXAT initiative’ towards 2020 and beyond”, OECC2009, paper FT4.
  4. S. Inao, T. Sato, S. Sentsui, T. Kuroha, and Y. Nishimura, “Multicore optical fiber,” in Optical Fiber Communication, 1979 OSA Technical Digest Series (Optical Society of America, 1979), paper WB1.
  5. K. Imamura, K. Mukasa, T. Yagi., “Investigation on multicore fibers with large Aeff and low bending loss”, OFC2010, San Diego, CA, paper OWK6.
  6. T. Hayashi, T. Toshiki, S. Osamu, S. Takashi, and S. Eisuke, “Ultra-low-crosstalk multicore fiber feasible to ultra-long haul transmission”, OFC’11, PDPC2 (2011).
  7. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, M. Watanabe, “109-Tb/s (7x97x172-Gb/s) SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multicore fiber”, OFC’11, PDPB6 (2011).
  8. 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 (2011).
  9. B. Zhu, T. F. Taunay, M. F. Yan, J. M. Fini, M. Fishteyn, E. M. Monberg, and F. V. Dimarcello, “Seven-core multicore fiber transmissions for passive optical network,” Opt. Express 18(11), 11117–11122 (2010). [CrossRef] [PubMed]
  10. J. M. Fini, T.F. Taunay, B. Zhu, and M. F. Yan, “Low cross-talk design of multicore fibers”, in CLEO2010, OSA Technical Digest, DC, paper CTuAA3.
  11. K.-P. Ho, “Effects of homodyne crosstalk on dual-polarization QPSK signals,” J. Lightwave Technol 29, 124 (2011).
  12. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008). [CrossRef] [PubMed]

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