We demonstrate a record QAM multiplicity of 1024 levels in a single-carrier coherent transmission. A frequency-domain equalization technique and a back-propagation method are adopted to compensate for distortions caused by hardware imperfections and fiber impairments, respectively. As a result, 60 Gbit/s polarization-multiplexed transmission over 150-km has been achieved at 3 Gsymbol/s within an optical bandwidth of only 4.05 GHz.
©2012 Optical Society of America
Increasing the spectral efficiency toward the Shannon limit has been the subject of intensive research with a view to meeting the growing demand for higher transmission capacity in optical backbone networks . Higher-order quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM) have been adopted to achieve a spectral efficiency exceeding 10 bit/s/Hz [2–4], which makes it possible to realize an ultra-large wavelength-division multiplexing (WDM) capacity of > 100 Tbit/s by fully utilizing the finite bandwidth over the C- and L-bands in a single core . The highest QAM multiplicity yet reported in a single-carrier coherent transmission is 512 , , and 1024 QAM subcarrier modulation at 1 Msymbol/s has recently been achieved with the OFDM technique . Such an extremely high-order QAM modulation on a single optical carrier is a challenging target in terms of the increased susceptibility to both phase noise and optical signal-to-noise ratio (OSNR).
In this paper, we demonstrate the first 1024 QAM single-carrier coherent optical transmission, in which a 60 Gbit/s polarization-multiplexed signal was transmitted at 3 Gsymbol/s over 150 km within an optical bandwidth of 4.05 GHz. In addition to the frequency-stabilized coherent light source, optical phase-locked loop (OPLL), and Raman amplifiers that we employed in our previous 512 QAM experiment, we newly adopted a frequency-domain equalization (FDE) technique to compensate for distortions caused by hardware imperfections in the transmitter  and a digital nonlinear compensation scheme with a back-propagation method .
2. Experimental setup
Figure 1 shows the experimental setup. At the transmitter, coherent CW light emitted from an acetylene frequency-stabilized fiber laser at 1538.8 nm with a linewidth of 4 kHz  was modulated by an IQ modulator driven with a 3 Gsymbol/s, 1024 QAM baseband signal generated by an arbitrary waveform generator (AWG). The AWG was operated at 12 Gsample/s with a 10-bit resolution. The bandwidth of the 1024 QAM baseband signal was reduced to 4.05 GHz with the adoption of a digital Nyquist raised-cosine filter with a roll-off factor of 0.35 [9–11]. When the roll-off factor is too small, the eye pattern is closed and the clock cannot be extracted at the receiver. We considered the margin of the system and set the factor at 0.35, which is often used in the wireless communication. The signal bandwidth maybe reduced further by optimizing the roll-off factor. In addition, digital pre-equalization was employed by converting the data into the frequency domain with a fast Fourier transform (FFT). The transfer function of the equalizer was determined in order to compensate for the non-ideal frequency response of individual components, mainly at the IQ modulator, whose E/O response is shown in Fig. 2 . In the IQ modulator, surface acoustic waves are generated by the piezoelectric effect in the LiNbO3 crystal , as a result the E/O response has fluctuations with a period of several MHz. To compensate for the frequency response, a frequency resolution of less than 1 MHz is needed in the pre-equalization. Here the FFT size for FDE was set at 16384. This enabled us to improve the frequency resolution to 0.73 MHz, compared with the 30 MHz resolution obtained with a conventional 99-tap FIR filter. The optical QAM was polarization multiplexed with a polarization beam combiner to generate 60 Gbit/s data. In parallel with these processes, part of the laser output was divided in front of the IQ modulator, and its frequency was down-shifted by 2.03 GHz against the carrier frequency. This signal was combined with the QAM data, and used as a pilot tone in the receiver for OPLL . The transmission link was composed of two 75 km spans of super large area (SLA) fiber with an effective area of 106 μm2, whose loss (0.20 dB/km) was compensated for by using EDFAs and Raman amplifiers. The power launched into each span was set at −1 dBm, which was optimally chosen to maximize the OSNR and minimize the nonlinear impairments. The Raman amplifiers were backward pumped and provided 9.5 dB of the total gain of 16.0 dB at each span.
At the receiver, which was followed by a 0.7 nm optical filter and an EDFA preamplifier, the transmitted QAM data were combined with a local oscillator (LO) and received by a polarization-diverse 90-degree optical hybrid and four balanced photodiodes. Here, we controlled the polarization of the QAM signal by using a polarization controller (PC) located before the 90-degree optical hybrid to avoid any digital polarization separation. The LO was a frequency-tunable fiber laser whose phase was locked to the transmitted pilot tone via an OPLL. The detected signals were A/D converted at 40 Gsample/s and processed with an offline digital signal processor (DSP). Figure 3 shows the RF 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. In the DSP, we first compensated for nonlinearities and dispersion simultaneously with a digital back-propagation method. We employed a split-step Fourier analysis of the Manakov equation, which describes the pulse propagation in a fiber with dispersion, SPM, and XPM between two orthogonal polarizations under a randomly varying birefringence . Here the step size in terms of distance was set at 9.375 km. Finally, the compensated QAM signal was demodulated into binary data, and the bit error rate was evaluated.
3. QAM coherent optical transmission results
We first measured the OSNR and phase noise of the transmitted data, and compared them with the requirement for 1024 QAM. Figure 4(a) and 4(b), respectively, show the optical spectra of the 1024 QAM signal before and after a 150 km transmission. It can be seen that the optical bandwidth was accommodated within a bandwidth of 4.05 GHz including the pilot tone (not visible in Fig. 3 because the tone level was 20 dB below the data). The OSNR, which we measured with a 0.1 nm resolution before transmission, was 40 dB, and it had degraded slightly to 36.5 dB after a 150 km transmission. It should be noted that 1024 QAM requires a theoretical Eb/N0 value as high as 24 dB to achieve an FEC threshold of BER = 2x10−3 . This corresponds to OSNR = (R/Δν)Eb/N0 = 27.8 dB, where R ( = 30 Gbit/s) is the bit rate per single polarization and Δν ( = 12.5 GHz) is the bandwidth used for optical signal detection. This indicates that the present OSNR is sufficient for the FEC threshold after a 150 km transmission.
Figure 5 shows the single side-band (SSB) noise power spectrum of a heterodyne beat note between the LO and the pilot tone after a 150 km transmission. By integrating this spectrum, the phase noise was estimated to be 0.46 degrees, whereas it was 0.25 degrees before transmission. This slight increase was mainly caused by OSNR degradation. On the other hand, the phase tolerance for 1024 QAM, determined by the phase difference between the two nearest symbols, is ± 0.95 degrees. This implies that the OPLL successfully achieved a residual phase noise in the IF signal that was within the tolerance for 1024 QAM even after a 150 km transmission.
Figure 6(a) and 6(b) show the constellation diagrams obtained under a back-to-back condition when we adopted an FIR filter and FDE for distortion compensation, respectively. The error vector magnitude (EVM) of the back-to-back constellation was greatly improved from 1.1 to 0.79% as a result of the adoption of FDE in the transmitter. Figure 7 shows the measured BER after a 150 km transmission as a parameter of the fiber launched power without and with digital nonlinear compensation using a back-propagation method. The optimum launched power was increased from −6 to −1 dBm by using the back-propagation method. Figure 8(a) and 8(b) show the constellation diagrams at an optimum launched power without and with nonlinear compensation. The EVM of the constellation was improved from 1.30 to 1.12% by employing nonlinear compensation.
Of the various linear and nonlinear transmission impairments, XPM between the two polarizations was a major cause of performance degradation. It should be noted that the individual equalization of dispersion, SPM, and XPM are insufficient for 1024 QAM, and simultaneous equalization with digital back-propagation plays an important role.
The BER characteristics are shown in Fig. 9 . The power penalty at a BER of 2x10−3 was approximately 7 dB for both polarizations, but both sets of polarization data achieved a BER lower than the FEC limit of 2x10−3. The power penalty was mainly caused by the residual phase noise that remained after nonlinear compensation.
We achieved a record QAM multiplicity of 1024 levels in a single optical carrier and successfully demonstrated ultrahigh density transmission over a 150 km SLA fiber. 60 Gbit/s data were generated at a symbol rate as low as 3 Gsymbol/s, and transmitted within an optical bandwidth of only 4.05 GHz by virtue of the extremely high QAM multiplicity. The present result is scalable to a net spectral efficiency as high as 13.8 bit/s/Hz in a multi-channel transmission even when taking account of the 7% FEC overhead.
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