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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B567–B573
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2.56 Tbit/s/ch polarization-multiplexed DQPSK transmission over 300 km using time-domain optical Fourier transformation

Pengyu Guan, Toshiyuki Hirano, Koudai Harako, Yutaro Tomiyama, Toshihiko Hirooka, and Masataka Nakazawa  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B567-B573 (2011)
http://dx.doi.org/10.1364/OE.19.00B567


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Abstract

We demonstrate a 2.56 Tbit/s/ch polarization-multiplexed single-carrier transmission over 300 km using subpicosecond DQPSK signals. We adopted an ultrafast time-domain optical Fourier transformation technique to reduce waveform distortions. For such an ultrashort optical pulse, depolarization components resulting from second-order polarization-mode dispersion (PMD) become a dominant factor as regards signal distortion because of the broad signal bandwidth. The influence of inter-polarization crosstalk induced by second-order PMD, is presented in detail.

© 2011 OSA

1. Introduction

Ultrahigh-speed single-carrier transmission technology will make it possible to achieve ultrahigh capacity optical networks with a simple configuration, large flexibility, and low power consumption at the switching nodes through the use of fewer wavelength channels. Optical time division multiplexing (OTDM) is expected to be a driving force for realizing a serial transport system with a Tbit/s channel capacity [1

1. H. G. Weber and M. Nakazawa, Ultrahigh-Speed Optical Transmission Technology (Springer, 2007).

]. The first Tbit/s/ch transmission was demonstrated in 2000 by using a 640 Gbaud polarization-multiplexed on-off keying (OOK) signal [2

2. M. Nakazawa, T. Yamamoto, and K. R. Tamura, “1.28 Tbit/s-70 km OTDM transmission using third- and fourth-order simultaneous dispersion compensation with a phase modulator,” Electron. Lett. 36(24), 2027–2029 (2000). [CrossRef]

]. Since then, the bit rate has been increased to 2.56 Tbit/s by employing differential quadrature phase-shift keying (DQPSK) [3

3. H. G. Weber, S. Ferber, M. Kroh, C. Schmidt-Langhorst, R. Ludwig, V. Marembert, C. Boerner, F. Futami, S. Watanabe, and C. Schubert, “Single channel 1.28 Tbit/s and 2.56 Tbit/s DQPSK transmission,” Electron. Lett. 42(3), 178–179 (2006). [CrossRef]

], and to 5.12 Tbit/s with 16-quadrature amplitude modulation (QAM) [4

4. C. Schmidt-Langhorst, R. Ludwig, D.-D. Gross, L. Molle, M. Seimetz, R. Freund, and C. Schubert, “Generation and coherent time-division demultiplexing of up to 5.1 Tb/s single-channel 8-PSK and 16-QAM signals,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), PDPC6.

]. Efforts have also recently been made to increase the symbol rate to 1.28 Tbaud [5

5. H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett. 45(5), 280–281 (2009). [CrossRef]

], and the fastest single-carrier bit rate yet achieved is 10.2 Tbit/s [6

6. T. Richter, E. Palushani, C. Schmidt-Langhorst, M. Nölle, R. Ludwig, and C. Schubert, “Single wavelength channel 10.2 Tb/s TDM-data capacity using 16-QAM and coherent detection,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), PDPA9.

]. However, a long-haul transmission at such a high symbol rate over several hundreds of kilometers such as in a metro or backbone network is still a major challenge. A higher symbol rate increases susceptibility to chromatic dispersion (CD) and polarization-mode dispersion (PMD), whereas a higher bit rate realized by multi-level modulation leads to reduced OSNR tolerance. Taking this trade-off into account, the adoption of polarization-multiplexed DQPSK is expected to provide optimum performance at a single-channel bit rate in the Tbit/s regime, as demonstrated by 1.07 Tbit/s-480 km [7

7. C. Schmidt-Langhorst, R. Ludwig, H. Hu, and C. Schubert, “Single-channel 1-Tb/s transmission over 480 km DMF for future terabit ethernet systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), OTuN5.

] and 2.56 Tbit/s-160 km [3

3. H. G. Weber, S. Ferber, M. Kroh, C. Schmidt-Langhorst, R. Ludwig, V. Marembert, C. Boerner, F. Futami, S. Watanabe, and C. Schubert, “Single channel 1.28 Tbit/s and 2.56 Tbit/s DQPSK transmission,” Electron. Lett. 42(3), 178–179 (2006). [CrossRef]

] transmissions.

2. Experimental setup

Figure 1
Fig. 1 Experimental setup for 2.56 Tbit/s polarization-multiplexed DQPSK transmission over 300 km. Abbreviations are defined in the text.
shows the experimental setup for a 2.56 Tbit/s/ch-300 km polarization-multiplexed DQPSK transmission. A 40 GHz mode-locked fiber laser (MLFL) was used as an optical pulse source, which emits a 1.6 ps pulse train at 1540 nm. The pulse was amplified to 18 dBm and launched into a 2 km-long highly nonlinear dispersion-flattened fiber (HNL-DFF), with a dispersion of – 0.2 ps/nm/km, a dispersion slope of 0.002 ps/nm2/km and a nonlinear coefficient γ = 5 W−1km−1, in which the pulse was compressed to 600 fs after the self-phase modulation (SPM)-induced chirp had been compensated with a single-mode fiber (SMF). The time-bandwidth product of the compressed pulse was 0.45, indicating that it was almost a transform-limited Gaussian pulse (0.44). The 600 fs pulse train was then DQPSK modulated with a 40 Gbaud (80 Gbit/s), 27 − 1 PRBS using an IQ modulator. The DQPSK signals were optically time-division multiplexed to 1.28 Tbit/s with a single polarization using an optical delay-line multiplexer. Figure 2(a)
Fig. 2 (a) 1.28 Tbit/s DQPSK signal waveform, (b) switching gate of NOLM switch.
shows the 1.28 Tbit/s DQPSK signal waveform, which was measured with an optical sampling oscilloscope with 800 fs time-resolution. The pulse train was well aligned with a slight power variation between different tributaries. The 1.28 Tbit/s DQPSK signal was then polarization multiplexed to 2.56 Tbit/s.

At the receiver, the 2.56 Tbit/s DQPSK signal was divided into two orthogonal polarization channels with a polarization beam splitter (PBS) and demultiplexed from 1.28 Tbit/s to 80 Gbit/s using a nonlinear optical loop mirror (NOLM) switch. The NOLM was composed of a 100 m HNLF with a nonlinear coefficient γ = 17 W−1km−1, a dispersion slope of 0.03 ps/nm2/km, and a zero-dispersion wavelength of 1548 nm. The insertion loss was 9 dB. As a control pulse source, we used an MLFL directly emitting a 40 GHz 720 fs pulse train at 1563 nm, which was PLL-operated with a 40 GHz clock extracted from the polarization-demultiplexed 640 Gbaud data using an electro-optical PLL clock recovery unit [9

9. C. Boerner, V. Marembert, S. Ferber, C. Schubert, C. Schmidt-Langhorst, R. Ludwig, and H. G. Weber, “320 Gbit/s clock recovery with electro-optical PLL using a bidirectionally operated electroabsorption modulator as phase comparator,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2005), OTuO3.

]. The walk-off between the signal and control pulse was 230 fs. The optical power at the HNLF input was set at 18 dBm for the data signal and 16 dBm for the control pulse, respectively. The switching gate window of the NOLM is shown in Fig. 2(b), which was measured by launching CW light instead of the data pulse. The switching gate width was 1.0 ps with an extinction ratio of > 17 dB.

After removing the control pulses with 15 and 5 nm optical filters, we coupled the demultiplexed DQPSK signal to an ultrafast time-domain optical Fourier transform circuit (OFTC) including an LN phase modulator in a round-trip configuration and an SMF [10

10. T. Hirano, P. Guan, T. Hirooka, and M. Nakazawa, “640-Gb/s/channel single-polarization DPSK transmission over 525 km with ultrafast time-domain optical Fourier transformation,” IEEE Photon. Technol. Lett. 22(14), 1042–1044 (2010). [CrossRef]

]. With a combination of linear chirp K and dispersion D, it is possible to transform an optical waveform between the time and frequency domains under the condition K = 1/D. Since the spectral envelope profile remains unchanged even if its time-domain waveform is distorted by linear perturbations including higher-order PMD and time-varying perturbations, OFT enables us to obtain a distortion-free pulse waveform in the time domain and thus eliminate linear transmission impairments [11

11. M. Nakazawa, T. Hirooka, F. Futami, and S. Watanabe, “Ideal distortion-free transmission using optical Fourier transformation and Fourier transform-limited optical pulses,” IEEE Photon. Technol. Lett. 16(4), 1059–1061 (2004). [CrossRef]

]. We successfully obtained the chirp rate K = 0.71 ps−2 by adopting the round-trip configuration, that is a sufficiently large amount of chirp required for the OFT of ultrashort pulses. As a dispersion medium, we used a 2 m-long SMF, which was chosen for minimum pulse width at the output of OFTC. The Fourier-transformed DQPSK signal was then preamplified and demodulated with a one-bit delay interferometer, which was biased at + π/4 or – π/4 depending on the phase component being measured. Finally, the bit error rate (BER) was measured after detection with a balanced photo-detector (PD).

3. Experimental results

We first measured the PMD characteristics of the 300 km transmission line that we used in this work. Figure 3(a)
Fig. 3 PMD characteristics of the 300 km transmission link. (a) DGD, (b) PSP vector on a Poincaré sphere, (c) depolarization.
and 3(b), respectively, show the differential group delay (DGD) versus wavelength characteristics, Δτ(λ), and the evolution of the PSP vector on a Poincaré sphere with respect to the wavelength, n(λ), which were measured with the Jones matrix eigenanalysis [12

12. B. L. Heffner, “Accurate, automated measurement of differential group delay dispersion and principal state variation using Jones matrix eigenanalysis,” IEEE Photon. Technol. Lett. 5(7), 814–817 (1993). [CrossRef]

]. The DGD value was 0.33 ps at 1540 nm, and the PSP axis rotated more than 180 deg. between 1530 and 1550 nm. The second-order PMD characteristics can be obtained by taking the derivative of the DGD and the PSP vector with respect to wavelength. The magnitude of the depolarization |(n/ω)Δτ(ω0)| is plotted as a function of wavelength in Fig. 3(c). As the depolarization value at 1540 nm was 0.13 ps2, the PSP direction changes on a Poincaré sphere at a rate |n/f| = 0.79 π rad/THz with respect to frequency, or 0.09 π rad/nm with respect to wavelength. This result indicates that the PSP vector is rotated by 180 deg. within a wavelength of 11 nm.

The BER characteristics after a 300 km transmission are shown in Fig. 5
Fig. 5 2.56 Tbit/s/ch-300 km transmission results. Squares and triangles show different polarization-component, and I and Q components are plotted with closed and open symbols, respectively.
. The result with single polarization (1.28 Tbit/s) [8

8. P. Guan, H. C. Hansen Mulvad, Y. Tomiyama, T. Hirano, T. Hirooka, and M. Nakazawa, “Single-channel 1.28 Tbit/s-525 km DQPSK transmission using ultrafast time-domain optical Fourier transformation and nonlinear optical loop mirror,” IEICE Trans. Comm E 94-B, 430–436 (2011).

] is also shown for comparison. Because of higher OSNR requirement for DQPSK signals, an error floor already exists under the back-to-back condition. The BER performance was mainly limited by OSNR in single polarization transmission. A large error floor exists at a BER of 10−5, which is more than two orders of magnitude worse than that for the single polarization. The BER improvement with OFT was less significant in the polarization-multiplexed transmission. The pulse waveform obtained after employing the OFT is shown in Fig. 6(b)
Fig. 6 Waveforms of demultiplexed DQPSK signals after a 300 km (a) Without OFT, (b) with OFT.
. Compared with the waveform without OFT shown in Fig. 6(a), the pulse width was shortened and the timing jitter was reduced by OFT, but the impairment around the peak was not completely eliminated. This is because an OFT is only applicable to the distortion of its own signal, but cannot be applied to distortions induced by crosstalk from different signals. Comparing the BER curves with that of single-polarization transmission in Fig. 5, the performance improvement was almost the same. This indicates that the polarization-multiplexed transmission is limited not only by the crosstalk but also by the distortions within the single-polarization, and the OFT is beneficial for the latter distortions.

4. Conclusion

We have demonstrated a 2.56 Tbit/s/ch polarization-multiplexed DQPSK transmission over 300 km. Ultrafast OFT was used to reduce waveform distortions. We found that the performance was mainly limited by the polarization crosstalk induced by second-order PMD. PMD control and management techniques that include such higher-order effects are indispensable for exploring the feasibility of Tbit/s/ch transmission systems.

References and links

1.

H. G. Weber and M. Nakazawa, Ultrahigh-Speed Optical Transmission Technology (Springer, 2007).

2.

M. Nakazawa, T. Yamamoto, and K. R. Tamura, “1.28 Tbit/s-70 km OTDM transmission using third- and fourth-order simultaneous dispersion compensation with a phase modulator,” Electron. Lett. 36(24), 2027–2029 (2000). [CrossRef]

3.

H. G. Weber, S. Ferber, M. Kroh, C. Schmidt-Langhorst, R. Ludwig, V. Marembert, C. Boerner, F. Futami, S. Watanabe, and C. Schubert, “Single channel 1.28 Tbit/s and 2.56 Tbit/s DQPSK transmission,” Electron. Lett. 42(3), 178–179 (2006). [CrossRef]

4.

C. Schmidt-Langhorst, R. Ludwig, D.-D. Gross, L. Molle, M. Seimetz, R. Freund, and C. Schubert, “Generation and coherent time-division demultiplexing of up to 5.1 Tb/s single-channel 8-PSK and 16-QAM signals,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), PDPC6.

5.

H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett. 45(5), 280–281 (2009). [CrossRef]

6.

T. Richter, E. Palushani, C. Schmidt-Langhorst, M. Nölle, R. Ludwig, and C. Schubert, “Single wavelength channel 10.2 Tb/s TDM-data capacity using 16-QAM and coherent detection,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), PDPA9.

7.

C. Schmidt-Langhorst, R. Ludwig, H. Hu, and C. Schubert, “Single-channel 1-Tb/s transmission over 480 km DMF for future terabit ethernet systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), OTuN5.

8.

P. Guan, H. C. Hansen Mulvad, Y. Tomiyama, T. Hirano, T. Hirooka, and M. Nakazawa, “Single-channel 1.28 Tbit/s-525 km DQPSK transmission using ultrafast time-domain optical Fourier transformation and nonlinear optical loop mirror,” IEICE Trans. Comm E 94-B, 430–436 (2011).

9.

C. Boerner, V. Marembert, S. Ferber, C. Schubert, C. Schmidt-Langhorst, R. Ludwig, and H. G. Weber, “320 Gbit/s clock recovery with electro-optical PLL using a bidirectionally operated electroabsorption modulator as phase comparator,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2005), OTuO3.

10.

T. Hirano, P. Guan, T. Hirooka, and M. Nakazawa, “640-Gb/s/channel single-polarization DPSK transmission over 525 km with ultrafast time-domain optical Fourier transformation,” IEEE Photon. Technol. Lett. 22(14), 1042–1044 (2010). [CrossRef]

11.

M. Nakazawa, T. Hirooka, F. Futami, and S. Watanabe, “Ideal distortion-free transmission using optical Fourier transformation and Fourier transform-limited optical pulses,” IEEE Photon. Technol. Lett. 16(4), 1059–1061 (2004). [CrossRef]

12.

B. L. Heffner, “Accurate, automated measurement of differential group delay dispersion and principal state variation using Jones matrix eigenanalysis,” IEEE Photon. Technol. Lett. 5(7), 814–817 (1993). [CrossRef]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4230) Fiber optics and optical communications : Multiplexing

ToC Category:
Transmission Systems and Network Elements

History
Original Manuscript: September 29, 2011
Revised Manuscript: November 7, 2011
Manuscript Accepted: November 18, 2011
Published: November 30, 2011

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

Citation
Pengyu Guan, Toshiyuki Hirano, Koudai Harako, Yutaro Tomiyama, Toshihiko Hirooka, and Masataka Nakazawa, "2.56 Tbit/s/ch polarization-multiplexed DQPSK transmission over 300 km using time-domain optical Fourier transformation," Opt. Express 19, B567-B573 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B567


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References

  1. H. G. Weber and M. Nakazawa, Ultrahigh-Speed Optical Transmission Technology (Springer, 2007).
  2. M. Nakazawa, T. Yamamoto, and K. R. Tamura, “1.28 Tbit/s-70 km OTDM transmission using third- and fourth-order simultaneous dispersion compensation with a phase modulator,” Electron. Lett.36(24), 2027–2029 (2000). [CrossRef]
  3. H. G. Weber, S. Ferber, M. Kroh, C. Schmidt-Langhorst, R. Ludwig, V. Marembert, C. Boerner, F. Futami, S. Watanabe, and C. Schubert, “Single channel 1.28 Tbit/s and 2.56 Tbit/s DQPSK transmission,” Electron. Lett.42(3), 178–179 (2006). [CrossRef]
  4. C. Schmidt-Langhorst, R. Ludwig, D.-D. Gross, L. Molle, M. Seimetz, R. Freund, and C. Schubert, “Generation and coherent time-division demultiplexing of up to 5.1 Tb/s single-channel 8-PSK and 16-QAM signals,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), PDPC6.
  5. H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett.45(5), 280–281 (2009). [CrossRef]
  6. T. Richter, E. Palushani, C. Schmidt-Langhorst, M. Nölle, R. Ludwig, and C. Schubert, “Single wavelength channel 10.2 Tb/s TDM-data capacity using 16-QAM and coherent detection,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), PDPA9.
  7. C. Schmidt-Langhorst, R. Ludwig, H. Hu, and C. Schubert, “Single-channel 1-Tb/s transmission over 480 km DMF for future terabit ethernet systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), OTuN5.
  8. P. Guan, H. C. Hansen Mulvad, Y. Tomiyama, T. Hirano, T. Hirooka, and M. Nakazawa, “Single-channel 1.28 Tbit/s-525 km DQPSK transmission using ultrafast time-domain optical Fourier transformation and nonlinear optical loop mirror,” IEICE Trans. Comm E94-B, 430–436 (2011).
  9. C. Boerner, V. Marembert, S. Ferber, C. Schubert, C. Schmidt-Langhorst, R. Ludwig, and H. G. Weber, “320 Gbit/s clock recovery with electro-optical PLL using a bidirectionally operated electroabsorption modulator as phase comparator,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2005), OTuO3.
  10. T. Hirano, P. Guan, T. Hirooka, and M. Nakazawa, “640-Gb/s/channel single-polarization DPSK transmission over 525 km with ultrafast time-domain optical Fourier transformation,” IEEE Photon. Technol. Lett.22(14), 1042–1044 (2010). [CrossRef]
  11. M. Nakazawa, T. Hirooka, F. Futami, and S. Watanabe, “Ideal distortion-free transmission using optical Fourier transformation and Fourier transform-limited optical pulses,” IEEE Photon. Technol. Lett.16(4), 1059–1061 (2004). [CrossRef]
  12. B. L. Heffner, “Accurate, automated measurement of differential group delay dispersion and principal state variation using Jones matrix eigenanalysis,” IEEE Photon. Technol. Lett.5(7), 814–817 (1993). [CrossRef]

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