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512 QAM (54 Gbit/s) Coherent Optical Transmission over 150 km with an Optical Bandwidth of 4.1 GHz Seiji Okamoto, Kazushi Toyoda, Tatsunori Omiya, Keisuke Kasai, Masato Yoshida and Masataka Nakazawa Research Institute of Electrical Communication, Tohoku University 2-1-1, Katahira, Aoba-ku, Sendai-shi 980-8577, Japan E-mail: [email protected] Abstract We report the first demonstration of 512 QAM coherent optical transmission by using an optical PLL. A polarisation-multiplexed 54 Gbit/s data signal was successfully transmitted at 3 Gsymbol/s with an optical bandwidth of 4.1 GHz including a tone signal. SE exceeding 10 bit/s/Hz by using a frequencystabilised fibre laser and an optical phase locked 7,8 loop (OPLL) . In this paper, we report the first 512 QAM coherent transmission experiment, in which we successfully transmitted a polarisation-multiplexed 54 Gbit/s signal over 150 km within an optical bandwidth of 4.1 GHz. Introduction Recently, to cope with the rapid growth of information capacity, coherent optical transmission with a high spectral efficiency (SE) using a multi-level format has been intensively studied1,2. Of the many multi-level formats, quadrature amplitude modulation (QAM), in which information is encoded on both amplitude and phase, has attracted a lot of attention because of its high SE compared with other formats3,4. QAM has been applied to WDM transmission with a record-breaking capacity, in which 69.1 and 64 Tbit/s transmissions were achieved using 16 and 36 QAM with SEs as high as 6.4 and 8 bit/s/Hz, respectively. To achieve further increases in the transmission capacity, QAM with a higher multiplicity could play an important role by taking full advantage of the finite bandwidth over the Cand L-bands. Previously, we demonstrated 128 and 256 QAM transmissions that achieved an Pilot Intensity Arbitrary Waveform Generator Experimental set-up for polarisation multiplexed 512 QAM coherent optical transmission Figure 1 shows the experimental set-up for polarisation-multiplexed 512 QAM transmission. As a coherent light source, we used a 1.5 µm acetylene frequency-stabilised fibre ring laser 9 with a linewidth of 4 kHz . The output of the laser was divided into two paths, and one was coupled to an IQ modulator. A 3 Gsymbol/s 512 QAM baseband signal was produced by an arbitrary waveform generator (AWG) operating at 12 Gsample/s and fed into the IQ modulator. Amplifier 12 GSample/s QAM Signal 2.03 GHz Optical Frequency IQ Mod. C2H2 FrequencyStabilised Fibre Laser PC Att EDFA ⊥ ~ Optical Filter ( 3 nm) Att OFS PBC (fOFS=2.03 GHz) -4 dBm 75 km SLA FBG (6 GHz) 75 km SLA PBS Raman Pump (1.44 µm) Att Prec Baseband Signal Polariser ˚ 90 Optical Optical Hybrid Filter PD OFS: Optical Frequency Shifter PBC: Polarisation Beam Combiner FBG: Fibre Bragg Grating SLA: Super Large Area Fibre B-PD: Balanced Photo-Detector DBM: Double Balanced Mixer B-PD A/D B-PD A/D DBM Digital Signal Processor Feedback Circuit Synthesiser fsyn=2.03 GHz Optical Filter ( ~2.5 nm) Local Oscillator (Tuneable Fibre Laser) Fig.1: Experimental set-up for polarisation-multiplexed 512 QAM coherent optical transmission. Transmission results Figure 2 (a) and (b) show the optical spectra of the QAM signal before and after 150 km transmission, respectively. By using Raman amplifiers, the OSNR degradation after Resolution: 0.1 nm 0 -10 P ow er [dBm ] -20 -30 Resolution: 0.1 nm 0 -10 40.5 dB -40 -50 -60 -20 -30 36 dB -40 -50 -60 -70 1538.2 1538.4 1538.6 1538.8 1539 -70 1539.2 1538.2 1538.4 1538.6 1538.8 Wavelength [nm] 1539 1539.2 Wavelength [nm] (a) (b) Fig. 2: Optical spectra of the QAM signal before and after 150 km transmission (a), (b). Demodulation bandwidth =4.05 GHz 20 10 0 Power [dB] Pow er [dBm ] We employed a root raised-cosine Nyquist 10 filter with a roll-off factor of 0.35 at the AWG as well as in the digital signal processor (DSP) at the receiver using software, so that the bandwidth of the QAM signal was reduced to 4.05 GHz. In addition, a pre-equalisation process was adopted to compensate for the distortions caused by individual components such as AWG and the IQ modulator by using a finite impulse response (FIR) filter with 99 complex-valued taps. We also pre-compensated individually for the nonlinear phase rotation caused by self phase modulation (SPM) and for the waveform distortion caused by chromatic dispersion (CD) during transmission. The optical QAM signal generated by the IQ modulator was then orthogonally polarisation-multiplexed with a polarisation beam combiner (PBC). The other path from the laser output was led to an optical frequency shifter (OFS), which fed a frequency downshift of 2.03 GHz against the carrier frequency as shown in the inset of Fig. 1. This signal was used as the pilot tone signal required for optical phase tracking of the local oscillator (LO) under OPLL operation. The polarisation of the pilot signal was aligned with one of the two polarisation axes of the QAM signal. These signals were combined and launched into a 150 km fibre link with a transmission power of -4 dBm. The lowest BER was obtained with this transmission power, which is the optimum value for minimizing impairments caused by S/N and nonlinearity. The fibre link was composed of two 75 km spans of super large area (SLA) fibre, a backward Raman amplifier, and an EDFA. The SLA fibre had an average fibre loss of less than 2 0.19 dB/km, an effective area of 106 µm , a dispersion of 20 ps/nm/km, and a dispersion 2 slope of 0.06 ps/nm /km. The Raman amplifier provided 10 dB gain to the signal at each span. The power of the pilot tone signal was set -25 dB lower than the QAM signal power. At the receiver, the QAM signal was homodyne detected at a 90-degree optical hybrid after passing through a fibre Bragg grating (FBG) optical filter with a 6 GHz bandwidth and a polariser for ASE reduction. As an LO, we used a frequency-tuneable fibre laser whose phase was locked to the transmitted pilot tone signal via the OPLL. After detection with balanced photodiodes, the data were A/D converted at 40 Gsample/s. Then, the QAM data were demodulated with a DSP in an off-line condition. Pilot tone signal -10 -20 -30 -40 -50 -60 -3 -2 -1 0 1 2 3 RF frequency [GHz] Fig. 3: Electrical spectrum of the demodulated signal at the DSP. transmission was only 4.5 dB, and a value as high as 36 dB was obtained. Figure 3 shows the electrical spectrum of the demodulated signal at the DSP. The demodulation bandwidth was set at 4.05 GHz due to the adoption of a Nyquist filter. The 54 Gbit/s data could be transmitted within an optical bandwidth of 4.06 GHz including the tone signal. To realise a QAM transmission with a higher multiplicity, it is essential to reduce the phase noise of the IF signal. The phase noise of the signal was estimated by integrating the single side band (SSB) noise power spectrum of a beat signal between the tone signal (fc - 2.03 GHz) and an LO (fc) under OPLL operation. Under a back-toback condition, the phase noise was 0.3 degree, whereas it increased to 0.75 degree after a 150 km transmission because of the OSNR degradation. These values are less than half of the tolerable phase noise of 1.8 degree for 512 QAM, which is determined by the phase difference between the nearest symbols. Figure 4 shows the BER characteristics of a polarisation-multiplexed, 3 Gsymbol/s, 512 QAM (54 Gbit/s) transmission. Figure 5 (a) and (b) show the respective constellations of the 512 QAM signal before and after transmission at the maximum received power. The power penalty at Pol-MUX bb(⊥) -1 10 Pol-MUX bb(//) Pol-MUX 150 km (-4 dBm) (⊥) Pol-MUX 150 km (-4 dBm) (//) Theoretical bb curve (phase noise 0.3 degree) -2 10 BER FEC threshold -3 10 -4 10 -5 10 -35 -30 -25 -20 -15 Received Power [dBm] (a) Fig. 4: BER characteristis of poralisation-multiplexed, 3 Gsymbol/s, 512 QAM transmission over 150 km. the BER of the forward error correction (FEC) threshold (2 x 10-3) was about 8 dB for both polarisations, but it was possible to achieve a BER lower than the FEC threshold for both polarisations. Without Raman amplifiers, the maximum received power was limited to -23 -3 dBm, where the BER was as large as 3.9 x 10 . The solid curve shows the theoretical BER under a back-to-back condition including the distortions caused by imperfect implementation of the hardware and the phase noise of the IF signal of 0.3 degree. The magnitude of the distortion produced by the hardware was calculated from the distributions of the constellations shown in Fig. 5 (a). The theoretical curve fits the experimental result well. The power penalty after transmission is attributed to cross phase modulation (XPM) between the two polarisations, and imperfect compensation for the interaction between SPM and CD. The IF phase noise is also responsible for the power penalty after a 150 km transmission. Conclusion We described the first demonstration of a 512 QAM coherent optical transmission, where 54 Gbit/s data were successfully transmitted over 150 km with an optical bandwidth of 4.06 GHz including a tone signal. The adoption of Raman amplifiers contributed greatly to the realisation of this high QAM multiplicity. The result indicates the possibility of realising an ultrahigh SE of 12.4 bit/s/Hz in a multichannel transmission even when taking account of the 7 % FEC overhead. (b) Fig. 5: Constellations (32768 symbols) before (a) and after 150 km transmission (b). References 1 P. J. Winzer, IEEE LEOS Newsletter, vol. 23, pp. 4-10, 2009. 2 A. D. Ellis, OFC 2009, OMM4. 3 E. Ip et al., J. Lightwave Technol., vol. 23, pp. 4110-4124, 2005. 4 M. Nakazawa et al., Electron. Lett., vol. 42, pp. 710-712, 2006. 5 A. Sano et al., OFC 2010, PDPB7. 6 X. Zhou et al., OFC 2010, PDPB9. 7 H. Goto et al., IEICE Electron. Express, vol.5, pp. 776-781, 2008. 8 M. Nakazawa et al., IEEE Photon. Technol. Lett,. vol. 22, pp. 185-187, 2010. 9 K. Kasai et al., IEICE Electron. Express, vol. 3, pp. 487-492, 2006. 10 H. Nyquist, AIEE. Trans., vol. 47, pp. 617644, 1928.