Abstract

We experimentally investigate the performance of a spectrally efficient multi-carrier channel consisting of two or more optical carriers spaced around the baud rate, with each carrier modulated with polarization division multiplexed (PDM) quadrature phase shift keyed (QPSK) format. We first study the performance of a 100-Gb/s 2-carrier PDM-QPSK channel with each carrier modulated at 12.5 Gbaud as a function of various design parameters such as the time alignment between the modulated carriers, the frequency separation between the carriers, the oversampling factor at the receiver, and the bandwidth of the digital pre-filter used for carrier separation. While the measurements confirm the previously reported observations, they also reveal some interesting additional features. The coherent crosstalk between the modulated carriers is found to be minimized when these carriers are symbol aligned. Spacing the carriers at the baud rate, corresponding to the orthogonal frequency-division multiplexing (OFDM) condition, leads to a local maximum in performance only for some specific cases where large oversampling (>2 × ) is applied. It is found that 4 × oversampling, together with a constant modulus algorithm (CMA) based digital equalizer having multiple quarter-symbol (T/4) spaced taps, gives much better overall performance than 2 × oversampling with a CMA-based equalizer having T/2 spaced taps. In addition, using a T/4-delay-and-add filter (DAF) as a pre-filter for assist carrier separation is found to give better performance than the commonly used T/2-DAF. In addition, it is possible to set the carrier spacing to be as small as 80% of the baud rate while incurring negligible penalty at BER≈10−3. 3-carrier and 5-carrier PDM-QPSK channels at 12.5-Gbaud with frequency-locked carriers spaced at 12.5 GHz and 4 × oversampling are also studied, and shown to perform reasonably well with small relative penalties. Finally, increasing the baud rate of the 2-carrier PDM-QPSK to 25 Gbaud and 28 Gbaud is investigated. It is found that with a fixed sampling speed of 50 Gsamples/s, scaling from 12.5 Gbaud to 25 and 28 Gbaud causes excess crosstalk penalties of about 2.8 dB and 4.8 dB, respectively, indicating the need to increase the sampling speed and transmitter bandwidth in order to support these high-data-rate channels without excessive coherent crosstalk.

© 2009 OSA

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References

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  4. K. Yonenaga, A. Sano, E. Yamazaki, F. inuzuka, Y. Miyamoto, A. Takada, and T. Yamada, “100 Gbit/s All-Optical OFDM Transmission Using 4 x 25 Gbit/s Optical Duobinary Signals with Phase-Controlled Optical Sub-Carriers”, in Proc. Optical Fiber Commun. Conf. (OFC) 2008, San Diego, CA, February 2008, Paper JThA48 (2008).
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    [CrossRef]
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    [CrossRef]
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    [CrossRef] [PubMed]
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  12. A. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. 17(2), 504–506 (2005).
    [CrossRef]
  13. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s per channel coherent optical OFDM transmission with subwavelength bandwidth access,” OFC’09, post-deadline paper PDP1.
  14. R. Dischler and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 but/s/Hz over 400 km of SSMF,” OFC’09, post-deadline paper PDP2.
  15. S. Chandrasekhar, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” ECOC’09, post-deadline paper PD2.6.

2008

2007

G. Goldfarb, G. Li, and M. G. Taylor, “Orthogonal wavelength-division multiplexing using coherent detection,” IEEE Photon. Technol. Lett. 19(24), 2015–2017 (2007).
[CrossRef]

2005

A. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. 17(2), 504–506 (2005).
[CrossRef]

1992

F. Derr, “Coherent optical QPSK intradyne system concept and digital receiver realization,” J. Lightwave Technol. 10(9), 1290–1296 (1992).
[CrossRef]

Derr, F.

F. Derr, “Coherent optical QPSK intradyne system concept and digital receiver realization,” J. Lightwave Technol. 10(9), 1290–1296 (1992).
[CrossRef]

Ellis, A.

A. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. 17(2), 504–506 (2005).
[CrossRef]

Goldfarb, G.

G. Goldfarb, G. Li, and M. G. Taylor, “Orthogonal wavelength-division multiplexing using coherent detection,” IEEE Photon. Technol. Lett. 19(24), 2015–2017 (2007).
[CrossRef]

Gunning, F. C. G.

A. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. 17(2), 504–506 (2005).
[CrossRef]

Li, G.

G. Goldfarb, G. Li, and M. G. Taylor, “Orthogonal wavelength-division multiplexing using coherent detection,” IEEE Photon. Technol. Lett. 19(24), 2015–2017 (2007).
[CrossRef]

Savory, S. J.

Taylor, M. G.

G. Goldfarb, G. Li, and M. G. Taylor, “Orthogonal wavelength-division multiplexing using coherent detection,” IEEE Photon. Technol. Lett. 19(24), 2015–2017 (2007).
[CrossRef]

IEEE Photon. Technol. Lett.

G. Goldfarb, G. Li, and M. G. Taylor, “Orthogonal wavelength-division multiplexing using coherent detection,” IEEE Photon. Technol. Lett. 19(24), 2015–2017 (2007).
[CrossRef]

A. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. 17(2), 504–506 (2005).
[CrossRef]

J. Lightwave Technol.

F. Derr, “Coherent optical QPSK intradyne system concept and digital receiver realization,” J. Lightwave Technol. 10(9), 1290–1296 (1992).
[CrossRef]

Opt. Express

Other

Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s per channel coherent optical OFDM transmission with subwavelength bandwidth access,” OFC’09, post-deadline paper PDP1.

R. Dischler and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 but/s/Hz over 400 km of SSMF,” OFC’09, post-deadline paper PDP2.

S. Chandrasekhar, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” ECOC’09, post-deadline paper PD2.6.

X. Liu, S. Chandrasekhar, and A. Leven, “Self-coherent optical transport systems” (Chap. 4) in Optical Fiber Telecommunications V B, edited by I. P. Kaminow, T. Li, and A. E. Willner (Academic Press, 2008).

H. Yamazaki, T. Yamada, K. Suzuki, T. Goh, A. Kaneko, A. Sano, E. Yamada, and Y. Miyamoto, “Integrated 100-Gb/s PDM-QPSK modulator using a hybrid assembly technique with silica-based PLCs and LiNbO3 phase modulators”, in Proc. European Conf. Optical Commun. 2008., Brussels, Belgium, paper Mo3C1 (2008).

M. O’Sullivan, “Expanding network applications with coherent detection”, in Proc. Optical Fiber Commun. Conf. (OFC) 2008, San Diego, CA, February 2008, tutorial paper NWC3 (2008).

K. Roberts, “Digital Compensation of the Optical Line: Pre-distortion Tx & Coherent Rx”, in Proc. IEEE LEOS Summer Topicals, Mexico, July 2008, paper WD2.2 (2008).

E. Yamada, A. Sano, H. Masuda, T. Kobayashi, E. Yoshida, Y. Miyamoto, Y. Hibino, K. Ishihara, K. Takatori, K. Okada, K. Hagimoto, T. Yamada, and H. Yamazaki, “Novel no-guard-interval PDM CO-OFDM transmission in 4.1 Tb/s (50 x 88.8-Gb/s) DWDM link over 800 km SMF including 50-GHz spaced ROADM nodes”, in Proc. Optical Fiber Commun. Conf. (OFC) 2008, San Diego, CA, February 2008, post-deadline paper PDP8 (2008).

K. Yonenaga, A. Sano, E. Yamazaki, F. inuzuka, Y. Miyamoto, A. Takada, and T. Yamada, “100 Gbit/s All-Optical OFDM Transmission Using 4 x 25 Gbit/s Optical Duobinary Signals with Phase-Controlled Optical Sub-Carriers”, in Proc. Optical Fiber Commun. Conf. (OFC) 2008, San Diego, CA, February 2008, Paper JThA48 (2008).

A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, S. Matsuoka, R. Kudo, K. Ishihara, Y. Takatori, M, Mizoguchi, K. Okada, K. Hagimoto, H. Yamazaki, S. Kamei, and H. Ishii, “13.4-Tb/s (134 x 111-Gb/s/ch) No-Guard-Interval Coherent OFDM Transmission over 3,600 km of SMF with 19-ps average PMD”, in Proc. European Conf. Optical Commun. 2008., Brussels, Belgium, post-deadline paper Th3E1 (2008).

H. Masuda, E. Yamazaki, A. Sano, T. Yoshimatsu, T. Kobayashi, E. Yoshida, Y. Miyamoto, S. Matsuoka, Y. Takatori, M. Mizoguchi, K. Okada, K. Hagimoto, T. Yamada, S. Kamei, “13.5-Tb/s (135 × 111-Gb/s/ch) no-guard-interval coherent OFDM transmission over 6,248 km using SNR maximized second-order DRA in the extended L-band,” OFC’09, post-deadline paper PDPB5 (2009).

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Figures (8)

Fig. 1
Fig. 1

Schematic of the experimental setup. (a) Two 2-carrier generation methods; (b) generation of QPSK modulation and polarization multiplexing of the two data streams; and (c) coherent reception of the 2-carrier signal. Clock frequency fc = |f2- f1|/2; PC: polarization controller; MZM: Mach-Zehnder modulator; CSF: carrier-separation filter; PBS(C): polarization beam splitter (combiner); PM; power monitor; VOA: variable optical amplifier; BPF: band-pass filter; OLO: optical local oscillator.

Fig. 2
Fig. 2

Measured Q at OSNR = 35 dB as a function of the relative symbol alignment of one carrier with respect to the second carrier for the two carriers spaced at 12.5 GHz and at 12.5 Gbaud. Insets show recovered constellations for one polarization and directly detected eye diagrams of the 2-carrier system before polarization multiplexing for the two extreme alignment cases.

Fig. 3
Fig. 3

(a) Measured Q at OSNR = 35 dB as a function of the frequency separation between the two carriers under 2 × oversampling and 4 × oversampling. (b) Recovered constellations for one polarization at three characteristic frequency separations, (1) 18 GHz, (2) 12.5 GHz, and (3) 10 GHz (3). (c) Modulated spectra of the two-carrier PDM-QPSK signal corresponding to the three characteristic frequency separations.

Fig. 4
Fig. 4

(a) Measured Q at OSNR = 35 dB as a function of the frequency separation of the two carriers, with 4 × oversampling and two different pre-filter for carrier separation: T/4-DAF and T/2-DAF. The inset shows recovered constellation for one polarization at carrier frequency separation of 12.5 GHz using T/4-DAF. (b) The measured BER as a function of OSNR for the case of two carriers spaced 10-GHz apart. Inset shows recovered constellation at OSNR = 18 dB for one polarization at carrier frequency separation of 10 GHz.

Fig. 5
Fig. 5

Measured Q (normalized to the Q of its single carrier) as a function of the frequency separation between the two carriers for the case with frequency-locked carriers and for the case with independent carriers. 4 × oversampling and T/4-DAF are used.

Fig. 6
Fig. 6

Experimental setup to generate 5-carriers PDM-QPSK at 12.5-Gbaud per carrier. Insets show optical spectra of the carriers before and after modulation. 3-carrier PDM-QPSK can be similarly generated with the comb-generator producing 3 (instead of 5) carriers.

Fig. 7
Fig. 7

(a) Measured BER as a function of OSNR for 1-carrier, 2-carrier, 3-carrier and 5-carrier systems, with measurement made on the central carrier; (b) BER as a function of the normalized OSNR per 50-Gb/s carrier.

Fig. 8
Fig. 8

(a) Measured BER as a function of OSNR for the 2-carrier system at 12.5-Gbaud, 25-Gbaud and 28-Gbaud, respectively. (c) Normalized modulation spectra of a single carrier for each of the three baud rates.

Tables (2)

Tables Icon

Table 2 Required OSNR, excess OSNR penalty, and Q at OSNR = 35 dB for three different baud rates.

Tables Icon

Table 1 Required OSNR, relative OSNR penalty and Q at OSNR = 35 dB.

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