We describe an injection-locked 64 QAM homodyne coherent transmission, which is the highest QAM multiplicity realized with an injection locking technique. The frequency locking range of the local oscillator (LO) was as wide as 1 GHz. The phase noise was only 0.2 deg, which is 1/3 of that obtained with our previous OVCO-based OPLL (0.6 deg.). As a result, a 120 Gbit/s polarization-multiplexed 64 QAM signal was successfully transmitted over 150 km with a simple receiver configuration and low DSP complexity.
© 2014 Optical Society of America
For the past ten years, internet traffic has been growing at an annual rate of 40%. Motivated by this rapidly growing capacity demand in optical fiber backbone networks, intensive studies have been undertaken on digital coherent optical transmission with a multi-level modulation format. In coherent transmission, the phase and frequency recovery of transmitted data is very important for carrier synchronization, especially for quadrature amplitude modulation (QAM) transmission with higher multiplicity. Phase synchronization has been realized with digital phase estimation schemes [1,2]. However, as the modulation multiplicity increases, computational complexity becomes too high to realize with digital signal processors. On the other hand, analog phase synchronization techniques, such as optical phase-locked loop (OPLL)  and injection locking [4–6] schemes, are potentially applicable to high multiplicity. Among various methods, an injection-locking scheme is expected to be the simplest way to realize carrier synchronization with low phase noise .
Recently, several demonstrations of coherent transmission using injection locking have been reported [8,9]. One example was an 8-QAM orthogonal frequency-division multiplexed (OFDM) 400 km coherent transmission at 15.3 Gbit/s, where a local oscillator (LO) was injection-locked by a transmitted data signal via a pilot carrier . A QPSK-OFDM 155 km coherent transmission was also reported with carrier recovery based on injection locking with a low injection power .
However, these experiments all used a pilot carrier among the OFDM subcarriers as an injection seed, and this requires a large residual carrier with sufficient separation from the OFDM data subcarriers. This means that a high ratio is needed between the optical intensity of the residual carrier and that of the OFDM signal. Since this leads to degradation in the S/N ratio of the data, it is difficult to apply to higher-order QAM transmission. The highest multiplicity reported thus far has been only 8 QAM in an OFDM injection-locked coherent transmission . In addition, the bit rate remains below 15 Gbit/s with a spectral efficiency (SE) of 2.3 bit/s/Hz.
In this paper, we demonstrate a single-carrier 120 Gbit/s, 64 QAM transmission using injection locking. A pilot tone, which is added to the input data at 10 GHz lower than the center frequency, is used as the injection seed to realize injection locking. The frequency locking range of an LO is as wide as 1 GHz, which is larger than that of an OPLL by three orders of magnitude. The present result constitutes the highest multiplicity and bit rate yet realized with injection locking and an LD-based optical source.
2. Experimental setup for 120 Gbit/s, 64 QAM injection-locked coherent optical transmission
Figure 1 shows an experimental setup for a 120 Gbit/s, polarization-multiplexed (Pol-Mux), 64 QAM coherent transmission with an injection-locking scheme. A coherent CW light emitted from a 1538.8 nm external cavity LD (ECLD) with a linewidth of 6 kHz was first split into two arms. In one arm, it was IQ-modulated with a 10 Gsymbol/s, 64 QAM signal, which was generated by an arbitrary waveform generator (AWG). In the AWG, a pre-equalization process was adopted to compensate for the distortions caused by individual components such as the IQ modulator by using a 99-tap finite impulse response (FIR) digital filter. Furthermore, the nonlinear phase rotation caused by self-phase modulation (SPM) during transmission was pre-compensated. In the other arm, the frequency of the ECLD output was down-shifted by 10 GHz against the carrier frequency. This was used as a pilot tone, namely as an injection-locking seed at the receiver. Here, the polarization of the pilot tone was the same as that of the Y-polarization data signal. The QAM data was polarization-multiplexed with a polarization beam combiner (PBC). The RF spectrum of the generated signal is shown in Fig. 2.
The pilot-embedded 64 QAM signal was transmitted over a 150 km dispersion-managed fiber link. Each span of the fiber link consisted of a 50 km super large area (SLA) fiber with adispersion of 19.5 ps/nm/km and a 25 km inverse dispersion fiber (IDF) with a dispersion of −40 ps/nm/km. The average span loss of the transmission fiber was 17 dB.
At the receiver, the transmitted signal was split into two paths after an EDFA. In one path, the pilot tone signal was extracted by an etalon filter with a 50 MHz bandwidth, and then injected into the LO with an injection power of 0 dBm. The polarization state of the pilot tone, which has the same polarization direction as the LO, was manually controlled by using a polarization controller so that the power of the seed signal was kept constant after it had passed the polarization sensitive circulator. Better polarization adjustment can be realized with certainty by employing an automatic polarization tracking controller. Here, the electrical spectrum of the self-heterodyne beat note between the injection seed and the ECLD is shown in the inset of Fig. 1. After the etalon filter, there were still several data components around the pilot tone at 60 dB down from the peak. We used a frequency-tunable ECLD with a linewidth of 6 kHz as an LO whose configuration was the same as that of the transmitter except that an isolator was removed. The output signal of the ECLD (fLO) was frequency down-shifted by 10 GHz with a single-sideband (SSB) modulator, which was used as an LO signal in homodyne detection. The lengths of these two optical paths were adjusted with an accuracy of 1 m. The QAM and LO were homodyne-detected 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 digital oscilloscope (40 Gsample/s, 16 GHz bandwidth) and demodulated with software in an offline condition. In the software, a 10 GHz low-pass digital filter was used to eliminate the pilot tone signal. We calculated the bit error rate (BER) from 123 kbit demodulated signals.
3. Experimental results
First, we optimized the injection-locking condition in terms of pilot tone level and injection power. The injection power was optimized with back-to-back demodulation experiments. Figure 3(a) shows the optimization of the power ratio between the pilot tone and the data. We set Ppilot/Pdata at −10 dB because it is the lowest power ratio at which we can maintain the lowest error vector magnitude (EVM) value (2.1%) of the demodulated 64 QAM signal. Figure 3(b) shows the EVM of the 64 QAM signal as a function of the injection power. The EVM remained 2.1% when the injection power was varied from −30 dBm to 0 dBm. The locking becomes unstable below −30 dBm or above 0 dBm. Figure 4(a) shows the injection-locking range characteristics of the LO as a function of the injection power. Here, we used a free-running erbium fiber laser (EFL) with a linewidth of 6 kHz as a master laser, and evaluated the injection-locking performance by changing its oscillation frequency. Figure 4(b) shows the relationship between EVM and the detuning frequency under an injection power of Pinj = + 5, 0 and −5 dBm. The EVM remained the same as long as the frequency offset between the data and the LO was within the locking range. As shown in Figs. 4(a) and 4(b), the maximum locking range was 1 GHz with an injection power of 0 dBm. Compared with a conventional optical phase-locked loop (OPLL) , whose detuning range is typically several MHz, the injection-locking scheme allows a larger frequency offset with three orders of magnitude. Hereafter, we chose an injection power of 0 dBm for our experiment.
Figure 5 shows the BER of a demodulated 10 Gsymbol/s, 64 QAM signal after a 150 km transmission for various powers launched into each fiber span, obtained with and without SPM compensation. From these results, the launch power was optimally set at 2 dBm, where the optical power of the QAM data and the pilot tone were −1 dBm/pol and −11 dBm, respectively. Figure 6 shows the optical spectra of the data signal before and after 150 km transmission measured with a 0.1 nm resolution. The optical signal-to-noise ratio (OSNR) was degraded from 43 to 36 dB during the 150 km transmission.
Figure 7(a) shows the beat spectrum between the injection-locked LO and the pilot tone as an intermediate frequency (IF) signal, measured after a 150 km transmission with a 200 MHz span. A low noise IF signal was obtained with an SNR of approximately 60 dB. The residual spectral components below −60 dB originate from the data components in the injection seeds as seen in the inset of Fig. 1. Figure 7(b) shows an IF signal with a 2 MHz span, where the linewidth was below 10 Hz. Figure 7(c) shows the single-side band (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 only 0.2 degrees, which is 1/3 of that obtained with our previous OPLL scheme using an OVCO (0.6 deg.) . The SSB phase noise after 150 km transmission was the same as that before transmission.
Figure 8(a) and 8(b) show constellation maps for the 10 Gsymbol/s, 64 QAM signal for back-to-back and 150 km transmissions, measured at OSNRs of 43 and 34 dB, respectively. After the transmission, the constellation points had broadened due to the OSNR degradation. Figure 9 shows the BER characteristics as a function of the OSNR. After transmission, error-free demodulation was realized when the OSNR exceeded 30 dB. The transmission penalty (1 dB) was mainly caused by cross-phase modulation (XPM) between the two polarizations.
We successfully transmitted a polarization-multiplexed, 10 Gsymbol/s, 64 QAM (120 Gbit/s) signal over 150 km with an injection locking circuit and an ECLD. The injection locking circuit had a low phase noise of 0.2 deg.(10 Hz~1 MHz). Due to the large locking range of 1 GHz, the allowable frequency offset between the data and the LO can be made three orders of magnitude wider than that with an OPLL. The present coherent transmission scheme is expected to be a candidate for a multi-level coherent transmission system with higher-order multiplicity.
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