We demonstrate an 80 Gbit/s, 5 Gsymbol/s 256 QAM coherent optical transmission by employing an injection-locked homodyne detection circuit based on both fiber lasers and LDs. With either circuit, low phase noise carrier-phase synchronization between the transmitted data signal and an LO were achieved with a phase noise variance of only 0.2 degrees. As a result, we have successfully transmitted an 80 Gbit/s data signal over 150 km with a simple receiver configuration. This is the highest QAM multiplicity yet realized with injection-locked homodyne detection.
© 2015 Optical Society of America
Digital coherent quadrature amplitude modulation (QAM) transmission has been intensively investigated to satisfy the rapidly growing demand for capacity in optical fiber backbone networks. In such transmissions, precise carrier-phase synchronization between transmitted data and a local oscillator (LO) is indispensable. Many multi-level QAM coherent transmissions have already been demonstrated by employing carrier-phase estimation (CPE) with digital signal processing (DSP)  or an optical phase-locked loop (OPLL) . A 2048 QAM-3 km multi-core fiber transmission has been realized with the CPE method . By using an optical phase-locked loop (OPLL), we have successfully transmitted a polarization-multiplexed (pol-mux), 3 Gsymbol/s 2048 QAM signal over 150 km with an optical bandwidth of 3.6 GHz corresponding to a potential spectral efficiency (SE) of 15.3 bit/s/Hz . On the other hand, an injection-locking scheme is very attractive for realizing low phase noise carrier-phase synchronization with a very simple receiver configuration .
Several coherent transmission experiments employing an injection-locking technique have recently been demonstrated with single-channel orthogonal frequency-division multiplexing (OFDM) quadrature phase shift keying (QPSK) signals , 8 QAM signals  and 16 QAM signals . The potential spectral efficiency (SE) was approximately 3.5 bit/s/Hz with 16 QAM. In addition, single-channel 12 Gsymbol/s  and 50 Gsymbol/s  16 QAM demodulation experiments with injection locking have also been demonstrated. In these experiments, the LO was injection-locked to the data signal by using a residual carrier in the middle of an OFDM signal or a QAM spectrum as a seed signal. Here, a residual carrier was generated from an IQ modulator by shifting its bias voltage away from the optimum value, resulting in an increase in the effect of the nonlinear response characteristics of the IQ modulator. This inevitably causes waveform distortions in the data signal. In addition, the generation of a residual carrier inevitably reduces the data signal power since the total signal power including a residual carrier is kept to be constant for optimum fiber transmission. This results in the degradation of the optical signal-to-noise ratio (OSNR) of a data signal itself. Furthermore, a QAM signal generally has data components very close to the carrier frequency. Therefore, it is difficult to extract only the residual carrier with an optical filter. The extracted residual carrier inevitably contains the data signal components resulting in the degradation of the injection locking performance. Because of these drawbacks, it is difficult to apply the injection locking scheme with a residual carrier to higher-order QAM transmission.
To overcome this bottleneck, we have developed a new injection-locked homodyne detection circuit, where a high OSNR pilot tone is generated and added very close to the QAM data spectrum without degradation of the data signal characteristics . By employing an LD-based injection-locked homodyne detection circuit that employed an InP-based external cavity laser diode (ECLD)  as both a transmitter and an LO, we achieved a single-channel, pol-mux 10 Gsymbol/s, 128 QAM (140 Gbit/s) transmission with a potential SE of 4 bit/s/Hz . Recently, we also applied this detection scheme to a pol-mux 5 Gsymbol/s, 256 QAM (80 Gbit/s) transmission employing fiber lasers as both a transmitter and an LO, and reported preliminary transmission results .
In this paper, we describe in detail an 80 Gbit/s, 256 QAM-150 km coherent transmission that we realized by employing a fiber laser-based injection-locked homodyne receiver. Furthermore, we adopt an LD-based injection-locked homodyne detection circuit for an 80 Gbit/s, 256 QAM-150 km coherent transmission, and we compare the two sets of transmission characteristics. In both experiments, 80 Gbit/s data were transmitted within an optical bandwidth of 8 GHz, resulting in a potential SE of 9.3 bit/s/Hz. These are the highest QAM multiplicity and SE yet achieved in a coherent optical transmission with an injection-locked homodyne detection scheme.
2. Experimental setup for 80 Gbit/s, 256 QAM-150 km injection-locked coherent optical transmission
Figure 1 shows the experimental setup we used for an 80 Gbit/s, pol-mux, 5 Gsymbol/s 256 QAM coherent transmission with an injection-locking technique. In a fiber laser-based transmission system, we used a 4 kHz linewidth, C2H2 frequency-stabilized erbium fiber laser emitting at 1538.8 nm  as a coherent transmitter. On the other hand, for the LD-based transmission, we used a 1538.8 nm 4 kHz linewidth, InP-based ECLD with an external Bragg grating on a silica planar lightwave circuit  as the transmitter. Figure 2 shows the delayed self-heterodyne spectra (delay line: 50 km) of these transmitters. The linewidths (full width at half maximum: FWHM) of these lasers are the same, but the tail of the spectral profile of the ECLD is much broader than that of the fiber laser. Here, two peaks are observed in the spectrum of the fiber laser at around ± 143 kHz. These are due to relaxation oscillation of the Er ions. The output from the coherent transmitter was IQ-modulated with a 5 Gsymbol/s, 256 QAM signal from an arbitrary waveform generator (AWG). The AWG was driven at 10 Gsample/s with a 12-bit resolution and directly generated 1 Vpp (peak-to-peak) baseband 256 QAM data signal. Here, we adopted a Nyquist filter with a roll-off factor of 0.2 at the AWG that enabled us to reduce a bandwidth of the QAM signal to 6 GHz. We also employed pre-equalization to compensate for the distortions caused by individual components such as the IQ modulator, AWG, balanced photo-detectors (B-PDs) and A/D converter by using a 99-tap finite impulse response (FIR) digital filter. At the same time, we pre-compensated for the nonlinear phase rotation caused by self-phase modulation (SPM) during transmission . The pol-mux was achieved with a polarization beam combiner (PBC). In parallel, a part of the transmitter output was coupled to an optical frequency shifter (OFS-1) consisting of a LiNbO3 (LN) single-sideband (SSB) modulator driven by a synthesizer at 5 GHz. Here, the first lower frequency sideband was generated as the pilot tone signal whose frequency was downshifted by 5 GHz against the carrier frequency of the transmitter laser. This signal was used as an injection seed signal at the receiver. These signals were transmitted over two 75-km spans of super large area (SLA) fiber with an average loss of 17.5 dB/span and a dispersion of 19.5 ps/nm/km. The launch power was optimized at 1 dBm (QAM data: −2 dBm/pol. and pilot tone: −10 dBm).
At the receiver, the received signal was split into two arms. On one arm, the pilot tone signal was injected into the LO after extraction by an etalon filter with a 50 MHz bandwidth. Here, we used a tunable erbium fiber laser as the LO, and its configuration is shown in Fig. 3 . The local fiber laser consists of a 1.48 μm LD, a polarization-maintained (PM) erbium-doped fiber (EDF), a 4-port PM optical circulator, a 1.2 GHz narrowband PM-fiber Bragg grating (PM-FBG) filter, and a PM optical isolator. The total cavity length is 1.9 m. This configuration provides a single frequency output with an output power of 10 mW (160 mW pump power) and a 5 kHz linewidth. The laser frequency is continuously tuned by simultaneously applying a voltage to the piezoelectric transducer (PZT) around which the EDF is wound, and to the multi-layer PZT on which the FBG is laid . The pilot tone signal was injected as a seed into the LO through a PM coupler with a coupling loss of 5 dB. In the LD-based system, we used a 4 kHz linewidth, frequency-tunable ECLD with an output power of 10 mW as the LO, and its configuration was the same as that of the transmitter LD except that an isolator was removed for injection locking. These LOs have the same spectral profiles as transmitter lasers.
The output signal of the injection-locked LO was frequency-upshifted by 5 GHz with an OFS-2 consisting of an LN intensity modulator driven by a synthesizer at 5 GHz and a narrowband optical filter. Here, the first higher frequency sideband of the intensity modulated LO signal with an OSNR of 55 dB was extracted with a narrowband filter. This was used for homodyne detection. The 256 QAM signal was homodyne-detected with the LO by using a polarization-diversity 90-degree optical hybrid and four balanced photo-detectors (B-PDs). Here, polarization demultiplexing was carried out by placing a polarization controller in front of the 90-degree optical hybrid. We adjusted the path length difference between the two arms with an accuracy of several tens of cm to remove an interference fluctuation from the intermediate frequency (IF) signal which was caused by fiber dispersion. The detected data signals were then A/D-converted using a digital oscilloscope (40 Gsample/s, 16 GHz bandwidth, 8-bit resolution) and demodulated with a digital signal processor (DSP) in an offline condition. In the DSP, a clock recovery circuit was used to compensate for the desynchronization of the clock between the A/D and D/A converters. We also used an adaptive 99-tap FIR filter to compensate for time-variant waveform distortions by minimizing the error vector magnitude (EVM) with a decision directed least mean square (DD-LMS) algorithm. Finally, the QAM data signal was demodulated into binary data, and the bit error rate (BER) was measured. Here, we used a 3 GHz low-pass digital filter to eliminate the pilot tone signal. The BER was measured from 131 kbit data.
3. Experimental results
3.1 80 Gbit/s, 256 QAM-150 km coherent transmission with fiber laser-based injection-locked homodyne receiver
We evaluated the injection-locking performance with a 5 GHz IF signal between the pilot tone signal and the injection-locked LO. Figures 4(a) and 4(b) show the locking range characteristics and SSB phase noise of the IF signal as a function of the injection power of the pilot tone signal (Pinj), respectively. Here, the locking range is defined as the maximum frequency detuning between the pilot tone frequency (ftone) and the original LO frequency (fLO), where the original LO frequency can be pulled toward the tone frequency. In the high injection power region over 5 dBm, the injection-locking operation became unstable, where the injection-locked LO began to be intensity modulated. Therefore, the injection power was set at 5 dBm to obtain a wideband locking range while maintaining a stable locking condition. Here, the locking range was 18 MHz. Figures 5(a) and 5(b) show the IF spectrum within a 2 MHz span and its SSB phase noise spectrum measured after a 150 km transmission, respectively. Here, the relaxation oscillation components of the transmitter laser were observed at around ± 143 kHz. 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 the same performance as that obtained with our OPLL circuit .
Figure 6(a) shows the optical spectra of the QAM data and pilot tone signals measured after the PBC with a 0.01 nm resolution bandwidth. These signals were coupled into a dispersion-managed fiber link with a launch power of 1 dBm. Figure 6(b) shows the optical spectra of data signals obtained before and after a 150 km transmission measured with a 0.1 nm resolution bandwidth before the pre-amplifier at the receiver. There was an OSNR degradation of 5 dB during the 150 km transmission.
Figures 7(a) and 7(b) show constellations for the 5 Gsymbol/s, 256 QAM signal obtained for back-to-back and 150 km transmissions, measured at OSNRs of 42 and 35 dB, respectively. The EVM for back-to-back and 150 km transmissions were 1.62% and 2.0%, respectively.
Figures 8(a) and 8(b) show the BER characteristics as a function of the OSNR for a single-polarization and a pol-mux transmission, respectively. The OSNR penalty was 1 dB during a single-polarization transmission at a BER of 2x10−3, which indicates the forward error correction (FEC) threshold with a 7% FEC overhead. This may be attributed to the degradation of the injection locking performance. Because the phase and amplitude noise of the injection-locked LO increased due to degradation of the pilot tone signal OSNR after transmission. During pol-mux transmission, the OSNR degradation was 1.5 dB. This was mainly due to the cross-phase modulation (XPM) between two polarizations. The BERs for both sets of polarization data were below the FEC threshold. In the present experiment, 80 Gbit/s data were transmitted within an optical bandwidth of 8 GHz including the pilot tone, which corresponds to a potential SE as high as 9.3 bit/s/Hz taking account of the 7% FEC overhead.
3.2 80 Gbit/s, 256 QAM-150 km coherent transmission with LD-based injection-locked homodyne receiver
We employed an ECLD as both a transmitter and an LO for an 80 Gbit/s, pol-mux, 5 Gsymbol/s 256 QAM injection-locked coherent transmission. Figures 9(a) and 9(b) show the locking range characteristics and SSB phase noise of the IF signal as a function of the injection seed power, respectively. The locking range increased with the increase in injection power as shown in Fig. 9(a). The phase noise increased under a strong injection power as shown in Fig. 9(b). From these results, the optimized injection power was 0 dBm, where the locking range was 1 GHz. This is 55 times wider than the result obtained with a fiber laser LO as shown in Fig. 4(a). This wide locking range enabled us to realize more stable phase tracking operation. The frequency drift of the ECLD was about 200 MHz peak-to-peak, where the frequency fluctuation with time was 15 MHz/hr. From this result, frequency stabilization was unnecessary at the transmitter laser in our LD-based injection-locked coherent detection system. Figures 10(a) and 10(b), respectively, show the IF spectrum within a 2 MHz span and its SSB phase noise spectrum measured after a 150 km transmission. The phase noise variance (RMS) of the IF signal was only 0.2 degrees. Although the LD has a broad spectrum at the bottom as shown in Fig. 2, the phase noise was widely suppressed and the same low phase noise characteristic was achieved for the IF signal as that obtained with a fiber laser. This is due to the wide range of the LD-based injection locking.
The locking range of the fiber laser was 18 MHz and that of the ECLD was 1 GHz. Although there was a large difference between the locking ranges of the two circuits, the phase noise variance in both injection locking circuits was the same (0.2 deg.). This may be attributed to the fact that the tail of the spectrum of the ECLD was much broader than that of the fiber laser as shown in Fig. 2.
Figures 11(a) and 11(b) show the constellations obtained for the 5 Gsymbol/s, 256 QAM signal with back-to-back and 150 km transmissions, measured at OSNRs of 42 and 35 dB, respectively. The EVM for back-to-back and 150 km transmissions were 1.70 and 2.15%, respectively. Figures 12(a) and 12(b) show the BER characteristics as a function of the OSNR for a single-polarization and a pol-mux transmission, respectively. The OSNR penalties during transmission at a BER of 2x10−3 were 1.2 and 3 dB, respectively. The BERs for both sets of polarization data were below the FEC threshold. These transmission results are almost the same as those obtained with the fiber laser-based injection-locked homodyne receiver shown in Figs. 8(a) and 8(b). This may be due to the fact that the LD-based injection locking circuit realized a precise phase locking operation with a phase noise variance of only 0.2 degrees, which is the same as that obtained with fiber laser-based injection locking.
We demonstrated an 80 Gbit/s, 256 QAM digital coherent transmission by using a fiber laser-based circuit and an LD-based injection-locked homodyne detection circuits. These circuits enabled us to realize low phase noise carrier-phase synchronization with a phase noise variance of 0.2 degrees. As a result, we successfully transmitted an 80 Gbit/s, 256 QAM data signal over 150 km with a potential SE of 9.3 bit/s/Hz in both case. These are the highest multiplicity and SE yet achieved in an injection-locked coherent transmission. The present coherent transmission scheme is expected to be a strong candidate for a multi-level coherent transmission system with higher-order multiplicities since a precise homodyne detection can be realized with a very simple receiver configuration.
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