Abstract

Optical quadrature amplitude modulation (QAM) attracts considerable attention in the enhancement of communications capacity, due to its high spectral efficiency. However, this modulation format is more sensitive to signal distortion than conventional formats, making it more difficult to create, transmit, and detect signals. In this paper, we propose an adaptive scheme for demodulation of the distorted optical signals with paying attention to use of DSP-technology. The scheme is based on the adjustment of the thresholds against the signal distortion. Successful demodulation of optical QAM signals is demonstrated with a sufficiently low bit-error rate.

© 2008 Optical Society of America

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References

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  1. M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).
  2. N. Kikuchi, K. Mandai, K. Sekine, and S. Sasaki, "First experimental demonstration of single-polarization 50-Gbit/s 32-level (QASK and 8-DPSK) incoherent optical multilevel transmission," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP21.
    [PubMed]
  3. M. Nakazawa, J. Hongou, K. Kasai, and M. Yoshida, "Polarization-multiplexed 1 Gsymbols/s, 64 QAM (12Gbit/s) coherent optical transmission over 150km with an optical bandwidth of 2 GHz," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP26.
    [PubMed]
  4. Y. Kamio, M. Nakamura, and T. Miyazaki, "Pre-equalization for 10 Gsymbol/s 16-QAM in a vector modulator," in Proceedings of IEEE/LEOS summer topical meetings (IEEE, 2008), paper MC1.2.
  5. M. Nakamura, Y. Kamio, and T. Miyazaki, "Real-time 40-Gbit/s 16-QAM self-homodyne using a polarizing-multiplexed pilot-carrier," in Proceedings of IEEE/LEOS summer topical meetings (IEEE, 2008), paper WC2.3.
  6. M. Seimetz, "Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2008) paper OTuM2.
  7. K. Kasai, M. Yoshida and M. Nakazawa, "Optical Phase-locked Loop for Coherent Transmission over 500 km," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper JWA49.
    [PubMed]
  8. A. P. T. Lau and J. M. Kahn, "Signal design and detection in presence of nonlinear phase noise," J. Lightwave Technol. 25, 3008-3016 (2007).
    [CrossRef]
  9. M. S. Alfiad, D. van den Borne, F. N. Hauske, A. Napoli, B. Lankl, A. M. J. Koonen, and H. de Waardt, "Dispersion tolerant 21.4-Gb/s DQPSK using simplified Gaussian Joint-Symbol MLSE," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2008), paper OthO3.
  10. M. G. Taylor, "Coherent detection for optical communications using digital signal processing," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper OMP1.
    [PubMed]
  11. D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).
    [CrossRef]
  12. H. Noguchi, N. Yoshida, H. Uchida, M. Ozaki, S. Kanemitsu, and S. Wada, "A 40Gb/s CDR with adaptive decision-point control using eye-opening monitor feedback," in Proceedings of IEEE International Conference on Solid-State Circuits, ISSCC2008 (Institute of Electrical and Electronics Engineers, San Francisco, 2008), pp. 228-229.
  13. A. Chiba, T. Sakamoto, and T. Kawanishi, "Adaptive symbol discrimination method for distorted multilevel optical signal and its application to decoding of high-speed optical quadrature amplitude modulation," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2008) paper JWA63.
    [PubMed]
  14. A. Chiba, T. Sakamoto, and T. Kawanishi, "Simple, low fluctuation voltage signal generation for 8-Gb/s quadrature amplitude modulation using dual-parallel Mach-Zehnder modulator," IEICE Elect. Express 5, 497-502 (2008).
    [CrossRef]
  15. T. Sakamoto, A. Chiba and T. Kawanishi, "50Gb/s 16 QAM by a quad-parallel Mach-Zehnder modulator," in Proceedings of ECOC2007 (Berlin, German, 2007), paper PD2.8.

2008 (1)

A. Chiba, T. Sakamoto, and T. Kawanishi, "Simple, low fluctuation voltage signal generation for 8-Gb/s quadrature amplitude modulation using dual-parallel Mach-Zehnder modulator," IEICE Elect. Express 5, 497-502 (2008).
[CrossRef]

2006 (1)

M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).

2005 (1)

D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).
[CrossRef]

Chiba, A.

A. Chiba, T. Sakamoto, and T. Kawanishi, "Simple, low fluctuation voltage signal generation for 8-Gb/s quadrature amplitude modulation using dual-parallel Mach-Zehnder modulator," IEICE Elect. Express 5, 497-502 (2008).
[CrossRef]

Hongou, J.

M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).

Kasai, K.

M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).

Katoh, K.

D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).
[CrossRef]

Kawanishi, T.

A. Chiba, T. Sakamoto, and T. Kawanishi, "Simple, low fluctuation voltage signal generation for 8-Gb/s quadrature amplitude modulation using dual-parallel Mach-Zehnder modulator," IEICE Elect. Express 5, 497-502 (2008).
[CrossRef]

Kikuchi, K.

D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).
[CrossRef]

Ly-Gagnon, D. -S.

D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).
[CrossRef]

Nakazawa, M.

M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).

Sakamoto, T.

A. Chiba, T. Sakamoto, and T. Kawanishi, "Simple, low fluctuation voltage signal generation for 8-Gb/s quadrature amplitude modulation using dual-parallel Mach-Zehnder modulator," IEICE Elect. Express 5, 497-502 (2008).
[CrossRef]

Yoshida, M.

M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).

Elecron. Lett. (1)

M. Nakazawa, M. Yoshida, K. Kasai, and J. Hongou, "20 Msymbol/s, 64 and 128 QAM coherent optical transmission over 525km using heterodyne detection with frequency-stabilized laser," Electron. Lett. 42, 57-58 (2006).

Electron. Lett. (1)

D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s quadrature phase-shift keying signals over 210km using homodyne phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).
[CrossRef]

Express (1)

A. Chiba, T. Sakamoto, and T. Kawanishi, "Simple, low fluctuation voltage signal generation for 8-Gb/s quadrature amplitude modulation using dual-parallel Mach-Zehnder modulator," IEICE Elect. Express 5, 497-502 (2008).
[CrossRef]

Other (12)

T. Sakamoto, A. Chiba and T. Kawanishi, "50Gb/s 16 QAM by a quad-parallel Mach-Zehnder modulator," in Proceedings of ECOC2007 (Berlin, German, 2007), paper PD2.8.

H. Noguchi, N. Yoshida, H. Uchida, M. Ozaki, S. Kanemitsu, and S. Wada, "A 40Gb/s CDR with adaptive decision-point control using eye-opening monitor feedback," in Proceedings of IEEE International Conference on Solid-State Circuits, ISSCC2008 (Institute of Electrical and Electronics Engineers, San Francisco, 2008), pp. 228-229.

A. Chiba, T. Sakamoto, and T. Kawanishi, "Adaptive symbol discrimination method for distorted multilevel optical signal and its application to decoding of high-speed optical quadrature amplitude modulation," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2008) paper JWA63.
[PubMed]

N. Kikuchi, K. Mandai, K. Sekine, and S. Sasaki, "First experimental demonstration of single-polarization 50-Gbit/s 32-level (QASK and 8-DPSK) incoherent optical multilevel transmission," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP21.
[PubMed]

M. Nakazawa, J. Hongou, K. Kasai, and M. Yoshida, "Polarization-multiplexed 1 Gsymbols/s, 64 QAM (12Gbit/s) coherent optical transmission over 150km with an optical bandwidth of 2 GHz," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP26.
[PubMed]

Y. Kamio, M. Nakamura, and T. Miyazaki, "Pre-equalization for 10 Gsymbol/s 16-QAM in a vector modulator," in Proceedings of IEEE/LEOS summer topical meetings (IEEE, 2008), paper MC1.2.

M. Nakamura, Y. Kamio, and T. Miyazaki, "Real-time 40-Gbit/s 16-QAM self-homodyne using a polarizing-multiplexed pilot-carrier," in Proceedings of IEEE/LEOS summer topical meetings (IEEE, 2008), paper WC2.3.

M. Seimetz, "Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2008) paper OTuM2.

K. Kasai, M. Yoshida and M. Nakazawa, "Optical Phase-locked Loop for Coherent Transmission over 500 km," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper JWA49.
[PubMed]

A. P. T. Lau and J. M. Kahn, "Signal design and detection in presence of nonlinear phase noise," J. Lightwave Technol. 25, 3008-3016 (2007).
[CrossRef]

M. S. Alfiad, D. van den Borne, F. N. Hauske, A. Napoli, B. Lankl, A. M. J. Koonen, and H. de Waardt, "Dispersion tolerant 21.4-Gb/s DQPSK using simplified Gaussian Joint-Symbol MLSE," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2008), paper OthO3.

M. G. Taylor, "Coherent detection for optical communications using digital signal processing," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper OMP1.
[PubMed]

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Figures (5)

Fig. 1.
Fig. 1.

Schematic for deciding thresholds after evaluation of the optical signal histograms in the constellation map. (a) Calculation of intersection of lines (solid squares (1a)–(1i)) composed of the symbol positions corresponding to the peaks in the histogram (solid circles (A)–(P)). (b) Calculation of midpoint (open squares (2a)–(2l)) between symbols located at the edges of whole symbols. (c) Thresholds determined by connecting the joints of (1a)–(1i) and (2a)–(2l).

Fig. 2.
Fig. 2.

Obtained BER for the numerically-generated QAM data with several OSNR by the conventional method (BERconv, horizontal axis) and the proposed method (BERadap, vertical axis). Circles, triangles inverted triangles and squares are for orthogonality θ of 90, 85, 80 and 75 deg., respectively. Solid symbols are results for OSNR of 12 dB. Thin solid line indicates that obtained BERadap is the same as BERconv, and the dashed line corresponds to the case where BERadap is less than BERconv by a ratio of 0.1. Inset: characteristic of BERconv (open squares) and BERadap (solid squares) against OSNR of the emulated QAM signal with an orthogonality of θ of 75 deg.

Fig. 3.
Fig. 3.

(a). Obtained BER for optical 16-QAM signals by the conventional method (BERconv, horizontal axis) and the proposed method (BERadap, vertical axis). Dotted line indicates the case where the obtained BERs are the equal and the dashed line corresponds to the case where BERadap is less than BERconv by a ratio of 0.1. (b)–(d) constellation maps of obtained optical QAM signals. BER of each signal corresponds to the black circle enclosed by dotted-dash line labeled by the same character as in Fig. 3(a).

Fig. 4.
Fig. 4.

(a). Constellation map of an optical QAM signal generated by a QPMZM and thresholds (white-dashed line) and normalized histograms of the optical signal (b) shown in Fig. 4(a) and (c) generated by a optical I-Q modulator driven by two four-level signals.

Fig. 5.
Fig. 5.

Example block diagram for proposed demodulation method.

Equations (7)

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s = α i A i + β i ( B i A i )
1 s < T
T s < 0
0 s < T
T s < 1
[ I n ( θ ) Q n ( θ ) ] = 1 A 0 [ 1 cos θ 0 sin θ ] [ I n ( θ = 90 ° ) Q n ( θ = 90 ° ) ] + [ Δ I Δ Q ] ,
A 0 = 1 4 n = 0 3 m = 0 3 I m ( θ ) + j Q n ( θ ) 2 .

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