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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 23 — Nov. 18, 2013
  • pp: 28290–28296
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120 Gbit/s, polarization-multiplexed 10 Gsymbol/s, 64 QAM coherent transmission over 150 km using an optical voltage controlled oscillator

Yixin Wang, Keisuke Kasai, Tatsunori Omiya, and Masataka Nakazawa  »View Author Affiliations


Optics Express, Vol. 21, Issue 23, pp. 28290-28296 (2013)
http://dx.doi.org/10.1364/OE.21.028290


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Abstract

We report a polarization-multiplexed, 10 Gsymbol/s 64 QAM coherent transmission over 150 km using an optical voltage controlled oscillator (OVCO). The OVCO enables us to realize a low phase noise optical phase-locked loop (OPLL) due to its wideband operation independent of the frequency modulation (FM) bandwidth of an LD. As a result, 120 Gbit/s, 64 QAM data were successfully transmitted over 150 km with a power penalty as low as 1 dB.

© 2013 Optical Society of America

1. Introduction

A spectrally efficient digital coherent transmission technology with a multi-level modulation is making rapid progress with the aim of expanding the transmission capacity within a limited optical amplification bandwidth [1

1. M. Nakazawa, “Giant leaps in optical communication technologies towards 2030 and beyond,” Plenary talk in Euro. Conf. on Optical Communication (ECOC), Torino, 2010.

]. Of many modulation formats, multi-level quadrature amplitude modulation (QAM) is advantageous as regards expanding the spectral efficiency toward the Shannon limit. QAM has been utilized for a number of coherent optical transmission experiments, and 1024 QAM transmission has recently been achieved with a spectral efficiency of 13.8 bit/s/Hz [2

2. Y. Koizumi, K. Toyoda, M. Yoshida, and M. Nakazawa, “1024 QAM (60 Gbit/s) single-carrier coherent optical transmission over 150 km,” Opt. Express 20(11), 12508–12514 (2012). [CrossRef] [PubMed]

]. To realize coherent transmission with a high multiplicity, carrier-phase synchronization between transmitted data and a local oscillator (LO) is indispensable. A conventional scheme involves carrier-phase estimation based on digital signal processing where LO feedback control is not needed [3

3. K. Kikuchi, “Coherent detection of phase-shift keying signals using digital carrier-phase estimation,” in Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, 2006, OTuI4. [CrossRef]

, 4

4. R. Noe, “PLL-free synchronous QPSK polarization multiplex/diversity receiver concept with digital I&Q baseband processing,” IEEE Photon. Technol. Lett. 17(4), 887–889 (2005). [CrossRef]

]. However, as the multiplicity and the symbol rate increase, a high-speed and large-scale electronic circuit is required, resulting in greater computational complexity. Moreover, the accuracy of carrier-phase estimation becomes insufficient for higher-order QAM.

2. Experimental setup for 120 Gbit/s, 64 QAM coherent optical transmission employing OVCO

Figure 1
Fig. 1 Experimental setup for Pol-Mux, 10 Gsymbol/s, 64 QAM-150km coherent transmission.
shows our experimental setup for a 120 Gbit/s, polarization-multiplexed (Pol-Mux), 64 QAM coherent transmission with an OVCO. The coherent CW laser we used was a 1538.8 nm C2H2 frequency-stabilized ECLD with a linewidth of 4 kHz [11

11. K. Kasai and M. Nakazawa, “FM-eliminated C2H2 frequency-stabilized laser diode with an RIN of -135 dB/Hz and a linewidth of 4 kHz,” Opt. Lett. 34(14), 2225–2227 (2009). [CrossRef] [PubMed]

]. The output of the laser was split into two arms. In one arm, it was modulated by an IQ modulator, which was driven with a 10 Gsymbol/s, 64 QAM signal generated by an arbitrary waveform generator (AWG). Here, a pre-equalization process was adopted to compensate for the distortions, which were caused by individual components such as the AWG and the IQ modulator. Furthermore, the nonlinear phase rotation caused by self-phase modulation (SPM) during transmission was also pre-compensated. We introduced SPM compensation using software at the AWG by adding a phase shift,
Δϕc=γLeffN[PI(t)+PQ(t)]
(1)
Leff=1α0Leαzdz
(2)
to the QAM signal before transmission, where γ is a nonlinear coefficient of the transmission fiber, Leff is the effective span length that takes account of the fiber loss α defined as Eq. (2), N is the number of spans and PI and PQ are the optical powers of the I and Q data. Pol-Mux was performed with a polarization beam combiner (PBC).

Our receiver is composed of two parts. One is a polarization-diversity coherent receiver, and the other is an OPLL circuit. In the coherent receiver, the transmitted QAM data signal was amplified and then homodyne detected with an OVCO signal whose phase was locked to the data signal via the pilot tone by using a polarization-diverse 90-degree optical hybrid and four balanced photo-detectors (B-PDs). Finally, the detected data signals were A/D-converted using a high-speed digital oscilloscope (40 Gsample/s, 16 GHz bandwidth) and demodulated with software in an offline condition. We calculated the BER from 123 kbit demodulated signals.

The OPLL circuit consists of an OVCO, a photo-diode (PD), an RF amplifier, a double balanced mixer (DBM), a synthesizer, and a loop filter with a bandwidth of approximately 10 MHz. The OVCO is composed of an ECLD with a linewidth of 4 kHz as an LO, a LiNbO3 (LN) optical phase modulator, a tunable optical filter with a bandwidth of 6 GHz, and a 10 GHz RF-VCO. In the OVCO, the output signal of the ECLD (fLO) is phase modulated by the LN phase modulator driven by the RF-VCO (fmod = 10 GHz) with a modulation depth of 0.46π, and then the first longer-wavelength sideband of the modulated signal is extracted with the tunable optical filter. The frequency and phase of the OVCO signal (fOVCO = fLO-fmod) was changed by applying a voltage to the RF-VCO. In the OPLL circuit, the phase of the beat signal between the extracted pilot tone and the OVCO signals (IF signal: fIF = |fpilot-fOVCO|) was compared with that of a reference signal generated from the synthesizer (fsyn) at 10 GHz by the DBM. The voltage phase error signal from the DBM was fed back to the RF-VCO through a lag-lead loop filter.

In Fig. 3(a)
Fig. 3 Output characteristics of OVCO, (a) optical spectrum, (b) delayed self-heterodyne spectrum, (c) frequency tuning characteristics, and (d) FM response and phase characteristics.
the gray and red curves show the optical spectra of the OVCO before and after filtering, respectively. After filtering, extra sidebands are sufficiently eliminated with a side-mode suppression ratio of more than 35 dB. Figure 3(b) shows the delayed self-heterodyne spectrum of the OVCO with a measurement resolution of 2 kHz [12

12. T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett. 16(16), 630–631 (1980). [CrossRef]

]. The linewidth was approximately 4 kHz, which is the same as that of the LO. Figure 3(c) shows the frequency tuning characteristics of the OVCO. The frequency was tuned with a tuning ratio of 1.67 MHz/V. Figure 3(d) shows the FM response and phase characteristics of the OVCO. The 3 dB bandwidth was approximately 4 MHz, which was the same as that of the RF-VCO.

3. Experimental results

Figure 4
Fig. 4 Optimization of launch power in 120 Gbit/s, 64 QAM-150 km coherent transmission.
shows the BER of the demodulated 10 Gsymbol/s, 64 QAM signal after a 150 km transmission for various powers launched into each fiber span. From these results, the launchpower was optimized to −1 dBm, where the optical power of the QAM data and the pilot tone signal were −4 dBm/pol and −14 dBm, respectively. The optical spectra of the data signal before and after 150 km transmission are shown in Fig. 5
Fig. 5 Optical spectra of 120 Gbit/s, 64 QAM data signal before and after 150 km transmission (0.1 nm resolution bandwidth).
. The OSNR was degraded from 43 to 27 dB during the 150 km transmission.

Figures 8(a)
Fig. 8 Constellation maps for the 10 Gsymbol/s, 64 QAM signal for (a) back-to-back, and (b) after 150 km transmission.
and 8(b) show constellation maps for the 10 Gsymbol/s, 64 QAM signal under back-to-back and after a 150 km transmission at OSNRs of 43 and 27 dB, respectively. After the transmission, the constellation points were broadened due to the OSNR degradation.

4. Conclusion

We successfully transmitted a polarization-multiplexed, 10 Gsymbol/s, 64 QAM (120 Gbit/s) signal over 150 km with a low power penalty of 1 dB. This result was achieved by the use of OVCO with a low phase noise OPLL operation. The present coherent transmission scheme is expected to be a candidate for future multi-level coherent transmission systems, especially those with higher-order multiplicity.

References and links

1.

M. Nakazawa, “Giant leaps in optical communication technologies towards 2030 and beyond,” Plenary talk in Euro. Conf. on Optical Communication (ECOC), Torino, 2010.

2.

Y. Koizumi, K. Toyoda, M. Yoshida, and M. Nakazawa, “1024 QAM (60 Gbit/s) single-carrier coherent optical transmission over 150 km,” Opt. Express 20(11), 12508–12514 (2012). [CrossRef] [PubMed]

3.

K. Kikuchi, “Coherent detection of phase-shift keying signals using digital carrier-phase estimation,” in Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, 2006, OTuI4. [CrossRef]

4.

R. Noe, “PLL-free synchronous QPSK polarization multiplex/diversity receiver concept with digital I&Q baseband processing,” IEEE Photon. Technol. Lett. 17(4), 887–889 (2005). [CrossRef]

5.

A. Mizutori, M. Sugamoto, and M. Koga, “12.5 Gbit/s BPSK stable optical homodyne detection using 3-kHz spectral linewidth external-cavity laser diode,” in Proceedings of the Euro.Conf. on Optical Communication (ECOC), Amsterdam, 2010, P3.13.

6.

S. Norimatsu, K. Iwashita, and K. Noguchi, “An 8 Gb/s QPSK optical homodyne detection experiment using external-cavity laser diodes,” IEEE Photon. Technol. Lett. 4(7), 765–767 (1992). [CrossRef]

7.

Y. Wang, K. Kasai, and M. Nakazawa, “Polarization-multiplexed, 10 Gsymbol/s, 64 QAM coherent transmission over 150 km with OPLL-based homodyne detection employing narrow linewidth LDs,” IEICE Electron. Express 8(17), 1444–1449 (2011). [CrossRef]

8.

A. L. Scholtz, W. R. Leeb, H. K. Philipp, and E. Bonek, “Infra-red homodyne receiver with acousto-optically controlled local oscillator,” Electron. Lett. 19(6), 234–235 (1983). [CrossRef]

9.

S. Camatel, V. Ferrero, R. Gaudino, and P. Poggiolini, “Optical phase-locked loop for coherent detection optical receiver,” Electron. Lett. 40(6), 384–385 (2004). [CrossRef]

10.

K. Kasai, Y. Wang, and M. Nakazawa, “An LD-based ultra-low phase noise OPLL circuit using optical voltage controlled oscillator,” in Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, 2013, OW3D.2. [CrossRef]

11.

K. Kasai and M. Nakazawa, “FM-eliminated C2H2 frequency-stabilized laser diode with an RIN of -135 dB/Hz and a linewidth of 4 kHz,” Opt. Lett. 34(14), 2225–2227 (2009). [CrossRef] [PubMed]

12.

T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett. 16(16), 630–631 (1980). [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.4080) Fiber optics and optical communications : Modulation
(140.5960) Lasers and laser optics : Semiconductor lasers
(140.3425) Lasers and laser optics : Laser stabilization

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 22, 2013
Revised Manuscript: October 28, 2013
Manuscript Accepted: October 29, 2013
Published: November 11, 2013

Citation
Yixin Wang, Keisuke Kasai, Tatsunori Omiya, and Masataka Nakazawa, "120 Gbit/s, polarization-multiplexed 10 Gsymbol/s, 64 QAM coherent transmission over 150 km using an optical voltage controlled oscillator," Opt. Express 21, 28290-28296 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-23-28290


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References

  1. M. Nakazawa, “Giant leaps in optical communication technologies towards 2030 and beyond,” Plenary talk in Euro. Conf. on Optical Communication (ECOC), Torino, 2010.
  2. Y. Koizumi, K. Toyoda, M. Yoshida, and M. Nakazawa, “1024 QAM (60 Gbit/s) single-carrier coherent optical transmission over 150 km,” Opt. Express20(11), 12508–12514 (2012). [CrossRef] [PubMed]
  3. K. Kikuchi, “Coherent detection of phase-shift keying signals using digital carrier-phase estimation,” in Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, 2006, OTuI4. [CrossRef]
  4. R. Noe, “PLL-free synchronous QPSK polarization multiplex/diversity receiver concept with digital I&Q baseband processing,” IEEE Photon. Technol. Lett.17(4), 887–889 (2005). [CrossRef]
  5. A. Mizutori, M. Sugamoto, and M. Koga, “12.5 Gbit/s BPSK stable optical homodyne detection using 3-kHz spectral linewidth external-cavity laser diode,” in Proceedings of the Euro.Conf. on Optical Communication (ECOC), Amsterdam, 2010, P3.13.
  6. S. Norimatsu, K. Iwashita, and K. Noguchi, “An 8 Gb/s QPSK optical homodyne detection experiment using external-cavity laser diodes,” IEEE Photon. Technol. Lett.4(7), 765–767 (1992). [CrossRef]
  7. Y. Wang, K. Kasai, and M. Nakazawa, “Polarization-multiplexed, 10 Gsymbol/s, 64 QAM coherent transmission over 150 km with OPLL-based homodyne detection employing narrow linewidth LDs,” IEICE Electron. Express8(17), 1444–1449 (2011). [CrossRef]
  8. A. L. Scholtz, W. R. Leeb, H. K. Philipp, and E. Bonek, “Infra-red homodyne receiver with acousto-optically controlled local oscillator,” Electron. Lett.19(6), 234–235 (1983). [CrossRef]
  9. S. Camatel, V. Ferrero, R. Gaudino, and P. Poggiolini, “Optical phase-locked loop for coherent detection optical receiver,” Electron. Lett.40(6), 384–385 (2004). [CrossRef]
  10. K. Kasai, Y. Wang, and M. Nakazawa, “An LD-based ultra-low phase noise OPLL circuit using optical voltage controlled oscillator,” in Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, 2013, OW3D.2. [CrossRef]
  11. K. Kasai and M. Nakazawa, “FM-eliminated C2H2 frequency-stabilized laser diode with an RIN of -135 dB/Hz and a linewidth of 4 kHz,” Opt. Lett.34(14), 2225–2227 (2009). [CrossRef] [PubMed]
  12. T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett.16(16), 630–631 (1980). [CrossRef]

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