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We realized a single-carrier, polarization-multiplexed 32 Gbaud 128 QAM coherent transmission. Digital frequency-domain equalization enabled us to achieve waveform distortion compensation of a wideband data signal with high frequency resolution. Thus, we successfully increased the QAM multiplicity to 128 at 32 Gbaud, and transmitted 448 Gbit/s data over 150 km with a potential spectral efficiency of 10.7 bit/s/Hz. This is the highest multiplicity and spectral efficiency yet achieved in a coherent QAM transmission at a baud rate of as high as 32 Gbaud.
© 2015 Optical Society of America
1. Introduction
The commercialization of 100 Gbit/s digital coherent transmission systems has made it important to achieve a channel rate of 400 Gbit/s and beyond. To realize such a high channel rate, superchannel transmission with a bit rate exceeding 1 Tbit/s has been investigated by employing an optical multi-carrier transmitter [1, 2]. In contrast, high baud rate single-carrier transmission has recently attracted attention since it enables us to reduce the number of optical carriers while maintaining the same transmission capacity, and to simplify the transmission system. Several high baud rate coherent transmission experiments have already been demonstrated with a polarization-multiplexed (pol-mux) 16 quadrature amplitude modulation (QAM) transmission system operating at 75 Gbaud [3] and 107 Gbaud [4] by employing an electrical time-division multiplexing (ETDM) scheme. By using a 72 Gsample/s digital-to-analog converter (DAC) with an analog bandwidth of 23 GHz and a field programmable gate array (FPGA), a 72 Gbaud 64 QAM transmission has also been reported [6]. In such a high baud rate transmission, the demodulation performance inevitably degrades due to waveform distortions caused by the insufficient bandwidth of individual optical and electrical components. In these experiments, a programmable optical filter [5] was employed to equalize such waveform distortions caused by the insufficient frequency response characteristics of the hardware. However, the frequency resolution of such an optical equalizer is inherently limited to several GHz. Therefore, it has been difficult to increase the multiplicity of QAM beyond 64 and the spectral efficiency (SE) has been limited to 6.4 bit/s/Hz [6].
In this study, by incorporating digital frequency-domain equalization (FDE) both at a transmitter and a receiver as a high-resolution waveform distortion compensation scheme [7], we were able to increase the QAM multiplicity up to 128 at 32 Gbaud. Here, we generated a pre-equalized 32 Gbaud 128 QAM baseband signal by using a high-speed arbitrary waveform generator (AWG) with a 65 Gsample/s DAC, and demonstrated a 448 Gbit/s single-carrier pol-mux 128 QAM-150 km coherent transmission. In this experiment, 448 Gbit/s data were successfully transmitted within an optical bandwidth of 39.2 GHz, resulting in a potential SE of 10.7 bit/s/Hz. This is the highest multiplicity and SE yet achieved in a 32 Gbaud coherent transmission.
2. Experimental setup for 448 Gbit/s, 32 Gbaud 128 QAM-150 km coherent optical transmission
Figure 1 shows the experimental setup for a pol-mux 32 Gbaud 128 QAM coherent optical transmission. We used a 1.5 μm CW, C2H2 frequency-stabilized fiber laser with a 4 kHz linewidth as a transmitter [8]. The coherent light from the laser passed through an erbium-doped fiber amplifier (EDFA) and was coupled to an IQ modulator consisting of a LiNbO3 nested Mach-Zehnder modulator with an E/O bandwidth of 30 GHz. The optical carrier was modulated with a 32 Gbaud 128 QAM baseband signal and a pilot tone signal was generated by an AWG. Here, we used a fixed pattern data signal with a length of 4096 symbols. The AWG was operated at 64 Gsample/s with an 8-bit resolution. At the AWG, we adopted a Nyquist filter with a roll-off factor of 0.2 to reduce the bandwidth to 38.4 GHz. In addition, we employed frequency domain pre-equalization (pre-FDE) by converting the data into the frequency domain by using a fast Fourier transform (FFT) with an FFT size of 8192 ( = 4096 symbols × 2 oversampling). The transfer function of the equalizer was determined that would compensate for the non-ideal frequency responses of individual components such as the AWG, IQ modulator, balanced photo-detectors (B-PDs) and A/D converter. A pilot tone signal, whose frequency was up-shifted by 20 GHz against the carrier frequency, was used for the optical phase tracking of the local oscillator (LO) under optical phase-locked loop (OPLL) operation, which enabled stable phase locking [9]. Figure 2 shows the optical spectra of the QAM data and pilot tone signals measured in front of the pol-mux circuit with a 0.01 nm resolution bandwidth. The M-shaped spectral profile of the data signal reflects the result of pre-equalization for the non-ideal frequency responses of the electrical and optical components at the receiver. After polarization-multiplexing by using a conventional delay-and-add circuit with 625-ps delay (20 symbols), these signals were coupled into a dispersion-managed fiber link consisting of two 75 km spans composed of a 50 km super large area (SLA) fiber with a dispersion of 19.5 ps/nm/km and a 25 km inverse dispersion fiber (IDF) with a −40 ps/nm/km dispersion. The fiber loss was compensated for by using an EDFA and Raman amplifiers. The Raman amplifiers were backward pumped and contributed a 10 dB gain to the total gain of 17.5 dB in each span.
Fig. 2 Optical spectra of 32 Gbaud 128 QAM data and pilot tone signals (0.01 nm resolution bandwidth).
At the receiver, the transmitted QAM data signal was homodyne-detected with an LO by a polarization-diverse 90-degree optical hybrid and four B-PDs with a bandwidth of 43 GHz. The LO was a 4 kHz linewidth frequency-tunable fiber laser whose phase was locked to the transmitted pilot tone via an OPLL with a phase noise variance of as low as 0.17 degrees. The baud rate is independent of the laser linewidth, but the modulation multiplicity depends on the linewidth. For high multiplicity QAM transmission, precise carrier-phase synchronization between the data signal and the LO is indispensable if we are to reduce the phase noise of the homodyne-detected data signal. The phase noise variance of the homodyne-detected signal obtained by using an OPLL circuit is given by σϕ2 = (δfT + δfL)/2fc, where δfT and δfL are the linewidths of the transmitter laser and LO, respectively, and fc is the bandwidth of the feedback circuit. In our OPLL circuit, fc is mainly limited by the bandwidth of the electrical devices in the feedback circuit. Thus, to achieve a high multiplicity QAM transmission while reducing the phase noise, the use of a narrow linewidth laser is very important. A linewidth of 4 kHz is very effective for realizing 1024-2048 QAM [9].
The detected QAM data were A/D converted at 80 Gsample/s (bandwidth: 33 GHz) and processed with an offline DSP. In the DSP, the pol-mux QAM data were polarization-demultiplexed with an algorithm based on the Stokes vectors [9, 10]. We then compensated for nonlinearities with a digital back-propagation (DBP) method [11] where we employed a split-step Fourier analysis of the Manakov equation with a 9.375 km step size. We also compensated for residual distortions caused by hardware imperfections by adopting FDE with an FFT size of 8192 ( = 4096 symbols × 2 oversampling). The frequency resolution of FDE was 9.77 MHz. In addition, we used an adaptive finite impulse response (FIR) filter to compensate for time-variant waveform distortions and polarization cross-talk while an FDE filter was employed as an equalizer for static waveform distortions. The frequency resolution of the FIR filter can be expressed as ΔfFIR = baud rate/NFIR, where NFIR is the number of FIR filter taps. To realize the maximum precise compensation for such distortions with a high frequency resolution, we used 99 taps, which was the maximum number of FIR filter taps in our digital signal processing circuit. Finally, the compensated QAM signal was demodulated into binary data, and the bit error rate (BER) was evaluated from 115 kbit data. Here, a 19.2 GHz low-pass digital filter was used to eliminate the pilot tone signal.
3. Experimental results
Figure 3(a) shows the BER of the demodulated 32 Gbaud 128 QAM signal after a 150 km transmission obtained for various powers launched into each fiber span. From these results, the launch power was optimized to 0 dBm, where the optical power of the QAM data and the pilot tone signal were −3 dBm/pol and −14 dBm, respectively. Figure 3(b) shows the optical spectra of the data signals obtained before and after a 150 km transmission measured with a 0.1 nm resolution bandwidth. There was an optical signal-to-noise ratio (OSNR) degradation of 9.5 dB during the 150 km transmission.
Fig. 3 (a) Optimization of launch power in 448 Gbit/s, 32 Gbaud 128 QAM-150 km coherent transmission, (b) Optical spectra of 448 Gbit/s-data signal before and after 150 km transmission (0.1 nm resolution bandwidth).
Figure 4(a) shows the beat spectrum between the LO and the pilot tone as an intermediate frequency (IF) signal under OPLL operation with 2 MHz spans. The linewidth of the spectrum was less than 10 Hz, which was below the measurement resolution. The phase noise was suppressed over a wide range, which resulted in a low noise IF signal with an SNR of approximately 70 dB. Figure 4(b) shows its single sideband (SSB) phase noise spectrum. The phase noise variance (RMS) of the IF signal estimated by integrating the SSB noise power spectrum from 10 Hz to 1 MHz was 0.17 degrees. This is sufficiently small for 128 QAM demodulation.
Fig. 4 (a) IF spectrum at 20 GHz with 2 MHz span and (b) its SSB phase noise spectrum (10 Hz~1 MHz) under OPLL condition.
Figure 5 shows the electrical spectrum of the demodulated 32 Gbaud 128 QAM signal for a back-to-back condition with a maximum OSNR of 42 dB. The signal bandwidth including the pilot tone was 39.2 GHz due to the adoption of a Nyquist filter. Its SNR was approximately 27 dB.
Figures 6(a) and 6(b) show constellations for a 32 Gbaud 128 QAM signal obtained for back-to-back and 150 km transmissions with OSNRs of 42 and 32.5 dB, respectively. The error vector magnitudes (EVM) for back-to-back and 150 km transmissions were 2.45 and 3.28%, respectively. When the FDE was not employed, the data clock could not be recovered by DSP due to excessive waveform distortions. This indicates that precise waveform distortion compensation is indispensable for a high baud rate, high multiplicity QAM transmission.
Fig. 6 Constellations for 32 Gbaud 128 QAM signal for (a) back-to-back and (b) 150 km transmissions.
The BER performance for a 448 Gbit/s pol-mux 32 Gbaud 128 QAM transmission over 150 km is shown in Fig. 7. As for the back-to-back condition, the OSNR penalty at a BER of 2 × 10−3 compared with the theoretical BER curve was 1 dB. An error floor was observed in the back-to-back condition due to the insufficient SNR of the 32 Gbaud 128 QAM baseband signal. A 128 QAM signal requires a theoretical Eb/N0 as high as 20 dB to achieve a BER below 1x10−5. The value of Eb/N0 corresponds to SNR = (Eb/N0) × b = 28.5 dB, where b ( = 7) is bit per symbol. This indicates that the present SNR estimated from Fig. 5 is slightly insufficient for achieving a BER below 1 × 10−5. The insufficiency is mainly attributed to the insufficient SNR of the 32 Gbaud 128 QAM baseband signal generated by the AWG. The OSNR penalty at a BER of 2 × 10−3 was 2.5 dB for both polarizations. The BERs for both polarizations were below the 7% FEC threshold at an OSNR of 32.5 dB. Thus, 448 Gbit/s data were successfully transmitted within an optical bandwidth of 39.2 GHz including the pilot tone, resulting in a potential SE of 10.7 bit/s/Hz taking into account the 7% FEC overhead.
4. Conclusion
We successfully demonstrated a single-carrier, pol-mux 32 Gbaud 128 QAM coherent transmission for the first time. By adopting digital FDE and digital back-propagation schemes, a 448 Gbit/s data signal was transmitted over 150 km within an optical bandwidth of 39.2 GHz. In this transmission, we were able to achieve a spectral efficiency of 10.7 bit/s/Hz in a multi-channel system by taking account of the 7% FEC overhead.
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