Open Access
Add to BibSonomy
Abstract
We demonstrate a 235-channel wavelength division multiplexing (WDM), polarization-multiplexed (pol-mux) 18-Gbaud 64 QAM coherent transmission of 160 km over the full C-band. By applying an injection-locked homodyne detection circuit to WDM coherent transmission, we have achieved low noise optical carrier-phase locking between transmitted data and a local oscillator over the full C-band range. As a result, a potential capacity of 42.3 Tbit/s data with a spectral efficiency of 9 bit/s/Hz was transmitted.
© 2017 Optical Society of America
1. Introduction
Spectrally efficient wavelength division multiplexing (WDM) digital coherent transmission with multi-level modulation such as quadrature amplitude modulation (QAM) has been intensively investigated with the aim of increasing transmission capacity. Several WDM transmissions with capacities exceeding 1 Pbit/s have been demonstrated using multi-core and multi-mode fiber [1–5]. On the other hand, increasing the transmission capacity of single-core, single-mode fiber is also important as it achieves the aim with a simple, conventional transmission system and without multiple-input and multiple-output (MIMO) signal processing.
Several high capacity transmissions using single-core, single-mode fiber have already been reported. By using a polarization-multiplexed (pol-mux) 128 QAM, 1200-subcarrier OFDM signal, a 101.7 Tbit/s transmission through a 165 km SMF was demonstrated over the full C + L band with a spectral efficiency (SE) of 11 bit/s/Hz. In this transmission, the baud rate per subcarrier was ~1 Mbaud, and the transmission capacities in the C- (bandwidth: 4.8 THz) and L-bands (bandwidth: 4.4 THz) were 52.8 and 48.9 Tbit/s, respectively [6]. A 102.3 Tbit/s, pol-mux 5.71 Gbaud 64 QAM transmission was demonstrated in the C- and extended L-band, where 1792-channel (224-grid × 8-subcarrier) WDM signals were transmitted over 240 km of pure silica core fiber with an SE of 9.1 bit/s/Hz. The transmission capacities in the C- (bandwidth: 4.9 THz) and extended L-bands (bandwidth: 6.3 THz) were 44.77 and 57.53 Tbit/s, respectively [7].
These transmissions employed an intradyne detection system with digital carrier-phase estimation for carrier-phase recovery. One attractive feature of this scheme is that there is no need for a hardware optical carrier-phase synchronization circuit. However, as the modulation multiplicity increases, the computational complexity becomes high, thus degrading the precise phase estimation. Therefore, the demodulation performance is degraded in a higher order QAM transmission. By contrast, an analogue optical carrier-phase synchronization scheme including an optical phase-locked loop (OPLL) and injection locking can realize precise carrier-phase locking independent of baud rate and modulation multiplicity [8, 9]. In particular, injection locking has attracted attention because this scheme enables precise optical carrier-phase locking with a very simple receiver configuration [9]. We have applied a homodyne detection system with injection locking to a single carrier coherent transmission, and demonstrated 256~512 QAM transmissions [9,10]. Then, we recently applied this scheme to a WDM coherent transmission, and reported preliminary results obtained with a 235-channel WDM, pol-mux 18 Gbaud 64 QAM signal. In this transmission, a potential capacity of 42.3 tbit/s data was transmitted over 160 km [11].
In this paper, we present the detail of a 42.3 Tbit/s, WDM 64 QAM-160 km transmission in the C-band including the dependence of the injection locking performance on wavelength. In our previous report [11], we roughly evaluated the demodulation performance of several channels including those obtained under maximum and minimum optical signal-to-noise ratio (OSNR) conditions. This time, we newly report the demodulation characteristics of the channels with low OSNR in detail.
2. Experimental setup for 42.3 Tbit/s, WDM 64 QAM-160 km transmission in C-band with injection locking technique
Figure 1 shows the experimental setup for a 42.3 Tbit/s, WDM 64 QAM-160 km transmission with an injection locking technique. The transmitter consists of a channel under measurement and loading channels. For the main channel, the outputs from a full C-band wavelength-tunable external cavity laser diode (ECLD) with an 8-kHz linewidth (Koshin Kogaku) [12] and four LDs with a 100-kHz linewidth were modulated with 18 Gbaud 64 QAM data, and a pilot tone (PT) signal was generated for injection locking with an arbitrary waveform generator (AWG). The AWG was driven at 54 Gsamples/s with an 8-bit resolution and a 25 GHz analogue bandwidth. The data pattern length was 16384 symbols. The PT to signal power ratio (PSPR), PPT/Pdata, was optimized at −10 dB to obtain the best bit error rate (BER) performance. Here, we adopted a Nyquist filter with a roll-off factor of 0.05. The waveform distortions caused by individual components such as an IQ modulator, an AWG, a balanced photo-detector (B-PD) and an A/D converter were pre-equalized by using a 99-tap finite impulse response (FIR) digital filter. Figure 2 shows a schematic diagram of the signal layout of the measurement channel. Here, five 18 Gbaud, 64 QAM data signals were set with a 20 GHz spacing, which corresponded to a guard band of 1.1 GHz. The PT was attached to each channel and its frequency was set 10.125 GHz higher than the carrier frequency. We located these signals on one ITU grid with a 100-GHz spacing. For the loading channels, the outputs from 46 LDs with a 100-kHz linewidth were phase modulated to generate 230 carriers. These carriers were IQ modulated in the same way as the main channel. The main and loading channels were combined with a wavelength selective switch (WSS), thus generating a total of 235-ch WDM signals located on 47 ITU grids. These signals were then transmitted over two 80-km ultra-large-area (ULA) fibers with an Aeff of 153 μm2. Here, we used ULA fibers to reduce fiber nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM) during transmission. We compensated for the fiber loss by using EDFAs. Here, the launch power into each span was optimally set at 15.5 dBm (−8 dBm/channel).
Fig. 1 Experimental setup for 42.3 Tbit/s, WDM 64 QAM-160 km transmission with an injection locking technique.
At the receiver, we used a tunable distributed feedback (DFB) LD array [13] as a local oscillator (LO). This consisted of 12 DFB-LDs with a linewidth of several MHz in a free running condition. We operated one of the 12 LDs, which oscillated at a different wavelength in the full C-band. The optical isolator at the output port of this laser was removed to allow injection locking. The LO was injection-locked to the PT signal, which was extracted by using a 50 MHz-bandwidth etalon filter (finesse ~1000, FSR = 50 GHz). The polarization state of PT was set manually to the polarization axis of the LO so that the power of the seed signal after passing the polarization sensitive circulator was kept constant. Better polarization adjustment can be realized by employing an automatic polarization controller. To realize low noise injection locking, it is only necessary to extract a PT signal in such a way that no data components remain around it. As for the filter bandwidth that was used to extract the PT, we optimized the guard band bandwidth and the PT frequency so that the best injection locking performance could be obtained. The output of the injection-locked LO was frequency downshifted with an optical frequency shifter consisting of a LiNbO3 (LN) intensity modulator and an optical filter to coincide with the carrier frequency for homodyne detection. In our experiments, the clock signal of the AWG and that of the signal source that generated a 10.125 GHz sinusoidal signal for the frequency shift were not synchronized. Although there was a static phase difference and slow phase fluctuation between the AWG clock and the signal source clock, we could compensate for them with digital signal processing (DSP) at the receiver. The measured channel (ECLD channel) was demultiplexed by using an optical tunable filter, and homodyne detected with the LO by employing a polarization-diversity 90°-optical hybrid and B-PDs. The detected signals were then A/D-converted with a 40-Gsample/s, 16-GHz bandwidth and demodulated with DSP in an offline condition. With the DSP, we carried out polarization demultiplexing with an algorithm based on the Stokes vectors [14]. We employed a digital back-propagation (DBP) method [15] to compensate for nonlinear phase rotation caused by SPM and XPM between two polarizations during transmission, and the chromatic dispersion of the transmission fiber. We also compensated for the waveform distortions caused by hardware imperfections by using an adaptive 99-tap FIR filter. Finally, the compensated QAM signal was demodulated into binary data, and the BER was evaluated from 98-kbit data. There was a slow phase fluctuation between the injection-locked LO and the transmitted data signal due to their different optical paths. However, we could easily compensate for such slow phase fluctuation by using an adaptive FIR filter in the receiver DSP.
3. Experimental results
First, we evaluated the injection locking performance under a back-to-back condition via the 10.125 GHz intermediate frequency (IF) signal of the beat between the PT signal and the injection-locked LO. Figures 3(a) and 3(b) show the locking range characteristics and single-sideband (SSB) phase noise of the IF signal as a function of the injection power of the PT signal, respectively. The injection power was defined as the optical power of the extracted PT signal. The OSNR of the PT signal was 40 dB. Here, the locking range is defined as the maximum frequency detuning between the PT frequency and the original LO frequency, where the LO frequency can be pulled toward the PT 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 −10 dBm to obtain a wideband locking range while maintaining a stable locking condition. Here, the locking range was 5.8 GHz. To realize precise carrier phase locking, a wide feedback bandwidth that corresponds to the locking range is an important factor [16]. Injection locking has a tracking range that is more than one order of magnitude wider than a conventional OPLL whose feedback bandwidth is typically limited to less than 100 MHz [17]. This wide locking range enabled us to realize a more stable and precise phase tracking operation. We also evaluated the relationship between the injection locking performance and the OSNR of the PT. Figure 3(c) shows the IF signal SSB phase noise dependence on the PT OSNR. The SSB phase noise characteristics did not degrade even when the PT OSNR fell from 40 dB to 25 dB. This indicates that amplified spontaneous emission (ASE) noise at such a low power level did not affect the injection locking performance. Figure 4(a) shows the IF spectrum within a 2 MHz span. It was obtained with heterodyne detection between the PT signal and injection-locked LOs with OSNRs of 40 dB and over 55 dB, respectively. Figure 4(b) shows its SSB phase noise spectrum with injection locking. A low noise IF signal was obtained with a signal-to-noise ratio (SNR) of approximately 65 dB. The residual spectral components below 65 dB originate from the electrical noise from a ground line. 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 0.24 degrees. This value is sufficiently small for the demodulation of a 64 QAM signal. Figure 5 shows the wavelength dependence of the SSB phase noise, where a phase noise variance of less than 0.24 degrees was obtained over the full C-band range.
Fig. 3 (a) Locking range characteristics of LO. (b) Relationship between SSB phase noise of IF signal and injection seed power. (c) PT OSNR dependence of SSB phase noise.
Fig. 4 (a) IF spectrum of beat between PT and injection-locked LO in 2 MHz span, and (b) SSB phase noise spectrum (10 Hz ~1 MHz).
We show the demodulation characteristics for the back-to-back condition. Figure 6(a) shows the optical spectrum of WDM signals measured after the WSS with a 0.1 nm resolution bandwidth. Figure 6(b) shows an enlarged view of the test channel block measured with a 0.02 nm resolution bandwidth. Here, an ECLD channel used for a BER measurement is highlighted with a black curve. The M-shaped spectral profile around the peak of this signal reflects the result of pre-equalization at the AWG.
Fig. 6 (a) Optical spectra of 235-channel WDM signals under back-to-back condition (0.1 nm resolution). (b) Enlarged view of test channel block (0.02 nm resolution).
Figure 7(a) shows the BER characteristics of channel 118 at 1546.12 nm after WDM demultiplexing as a function of the OSNR. The BER characteristics of single-channel 18 Gbaud 64 QAM and the theoretical BER curve are also shown. The OSNR penalty at a BER of 1 × 10−3 between the theoretical BER and the BER for single-channel 64 QAM was 0.9 dB. This OSNR penalty may be increased when employing conventional intradyne detection with a free running LO because of the large phase noise. In the WDM case, the additional penalty of 1.2 dB may be caused by the linear crosstalk between the adjacent channels. This additional penalty may also increase when using a free running LO. In our experiment, we realized WDM demultiplexing by using an optical tunable filter (OTF-1) as shown Fig. 1. We set the filter bandwidth at an optimal 18 GHz to obtain the best BER characteristics. When the filter bandwidth was reduced, the BER was degraded due to the waveform distortions caused by excessive spectral narrowing. On the other hand, when the filter bandwidth exceeded 18 GHz, adjacent channels were simultaneously input into the coherent detection circuit with the channel under test. This induced the SNR degradation of the homodyne detected signal for the BER measurement channel due to the input power limitation of the B-PDs. This resulted in the degradation of the BER characteristics. Figure 7(b) shows the constellations of an 18 Gbaud 64 QAM signal (channel 118) at an OSNR of 32 dB for which the error vector magnitude (EVM) was 3.7%. This value was well below the forward error correction (FEC) limit of 2 × 10−3 for a 7% overhead.
Fig. 7 Demodulation characteristics under back-to-back condition. (a) BER characteristics of channel 118 at 1546.12 nm as a function of the OSNR, (b) constellations of an 18 Gbaud 64 QAM signal measured at an OSNR of 32 dB.
Figure 8 shows the optical spectrum of 235-channel WDM signals measured at a 0.1 nm resolution bandwidth and the wavelength dependence of the OSNRs after a 160 km transmission. The OSNRs degraded at shorter wavelength channels due to the gain characteristics of the EDFA. We separately measured the BER characteristics for 13 channels including those obtained under maximum and minimum OSNR conditions. This was accomplished by shifting the wavelength of the measured channel. We roughly evaluated the BER characteristics of 8 channels (1532.68 ~1564.679 nm) with relatively high OSNRs by arbitrarily selecting a measurement channel. In a shorter wavelength region where the OSNRs of the data signals were relatively low (1527.68 ~1528.31 nm) compared with those at longer wavelengths, we carefully measured the BERs of 5 channels (channels 1 ~5) as shown Fig. 8(b).
Fig. 8 (a) Optical spectra of 235-channel WDM signals after 160 km transmission and wavelength dependence of the OSNRs. (b) Enlarged view of shorter wavelength region.
The BER characteristics are shown in Fig. 9. At an OSNR of 25.2 dB (1528.31 nm, channel 5), the BER was 6.2 × 10−3 (worst), while a BER of 1.7 × 10−3 (best) was obtained with an OSNR of 26.6 dB at 1559.79 nm (channel 203). In conventional WDM experiments, there is a threshold OSNR that can realize an error-free transmission with a certain FEC overhead. Below this threshold, WDM transmission is not possible. In our case, the BERs of the lowest order OSNR channels are still all below the FEC threshold. This indicates that all other channels with higher OSNRs definitely realize error-free transmission. It is interesting to see that the BER at channel 5 was worse than that at channel 1. This may be attributed to the fact that channel 5 was adversely affected by WDM signals at shorter and longer wavelengths while channel 1 experienced fewer WDM effects as it was an edge channel. The BERs at channel 118 (1546.12 nm) before and after transmission were 1.4 × 10−3 and 2.5 × 10−3, respectively, at an OSNR of 25.6 dB. The transmission penalty was very small. This indicates that the XPMs induced by adjacent WDM channels were still negligibly small in this experiment.
Fig. 9 (a) BER characteristics of 13 channels after 160 km transmission. (b) Enlarged view of shorter wavelength region.
Figure 10 shows the constellations for channels 203 (best) and 5 (worst) measured at OSNRs of 26.6 dB and 25.2 dB, respectively. The EVMs were 5.0 ~5.1%, and 5.7 ~5.8%, respectively. The constellation points of the channel 5 data shown in Fig. 10 are broader than those of channel 203. This is because the OSNR of channel 5 is lower than that of channel 203.
Fig. 10 Constellations of 18 Gbaud 64 QAM signal after 160 km transmission. (a) Channel 203 (best) at 1559.79 nm measured at an OSNR of 26.6 dB. (b) Channel 5 at 1528.31 nm measured at an OSNR of 25.2 dB.
In this transmission, a potential capacity of 50.76 Tbit/s data was transmitted within an optical bandwidth of 4.7 THz over the C-band, resulting in an SE as high as 9 bit/s/Hz with a 20% FEC overhead, where the potential net data capacity was 42.3 Tbit/s.
4. Conclusion
We demonstrated, for the first time, a 235-channel WDM pol-mux 18-Gbaud 64 QAM signal over 160 km in the C-band with the use of an injection locking technique. Our injection-locked homodyne detection system enabled us to realize low noise optical carrier-phase locking between the transmitted data and the LO that resulted in precise demodulation of the 64 QAM data. It can be said that a potential capacity of 42.3 Tbit/s data was transmitted with an SE of 9 bit/s/Hz. For use as a practical injection-locked homodyne receiver, we believe that in the near future all the parts including the DFB LD, the optical modulator, and the optical filter should be optically integrated and applied to an ultra-multilevel QAM coherent transmission system as a compact coherent receiver with high performance.
Funding
National Institute of Information and Communications Technology (NICT), Japan.
Acknowledgments
The research was supported by the National Institute of Information and Communications Technology (NICT), Japan, as a part of the “Research and development for practical application of innovative optical fiber.”
References and links
1. H. Takara, A. Sano, T. Kobayashi, H. Kubota, H. Kawakami, A. Matsuura, Y. Miyamoto, Y. Abe, H. Ono, K. Shikama, Y. Goto, K. Tsujikawa, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba, and T. Morioka, “1.01-Pb/s (12 SDM/222 WDM/456 Gb/s) crosstalk-managed transmission with 91.4-b/s/Hz aggregate spectral efficiency,” in European Conference on Optical Communication (Optical Society of America, 2012), paper Th.3.C.1. [CrossRef]
2. D. Qian, E. Ip, M. Huang, M. Li, A. Dogariu, S. Zhang, Y. Shao, Y. Huang, Y. Zhang, X. Cheng, Y. Tian, P. Ji, A. Collier, Y. Geng, J. Linares, C. Montero, V. Moreno, X. Prieto, and T. Wang, “1.05Pb/s transmission with 109b/s/Hz spectral efficiency using hybrid single- and few-mode cores,” in Frontiers in Optics (Optical Society of America, 2012), paper FW6C.3.
3. B. J. Puttnam, R. S. Luís, W. Klaus, J. Sakaguchi, J.-M. Delgado Mendinueta, Y. Awaji, N. Wada, Y. Tamura, T. Hayashi, M. Hirano, and J. Marciante, “2.15 Pb/s transmission using a 22 core homogeneous single-mode multi-core fiber and wideband optical comb,” in European Conference on Optical Communication (Optical Society of America, 2015), paper PDP3.1. [CrossRef]
4. D. Soma, K. Igarashi, Y. Wakayama, K. Takeshima, Y. Kawaguchi, N. Yoshikane, T. Tsuritani, I. Morita, and M. Suzuki, “2.05 Peta-bit/s super-nyquist-WDM SDM transmission using 9.8-km 6-mode 19-core fiber in full C band,” in European Conference on Optical Communication (Optical Society of America, 2015), paper PDP3.2. [CrossRef]
5. T. Kobayashi, M. Nakamura, F. Hamaoka, K. Shibahara, T. Mizuno, A. Sano, H. Kawakami, A. Isoda, M. Nagatani, H. Yamazaki, Y. Miyamoto, Y. Amma, Y. Sasaki, K. Takenaga, K. Aikawa, K. Saitoh, Y. Jung, D. J. Richardson, K. Pulverer, M. Bohn, M. Nooruzzaman, and T. Morioka, “1-Pb/s (32 SDM/46 WDM/768 Gb/s) C-band dense SDM transmission over 205.6-km of single-mode heterogeneous multi-core fiber using 96-Gbaud PDM-16QAM channels,” in Optical Fiber Communication Conference (Optical Society of America, 2017), paper Th5B.1. [CrossRef]
6. D. Qian, M. Huang, E. Ip, Y. Huang, Y. Shao, J. Hu, and T. Wang, “101.7-Tb/s 370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in Optical Fiber Communication Conference (Optical Society of America, 2011), paper PDPB5.
7. A. Sano, T. Kobayashi, S. Yamanaka, A. Matsuura, H. Kawakami, Y. Miyamoto, K. Ishihara, and H. Masuda, “102.3-Tb/s (224 × 548-Gb/s) C- and extended L-band all-Raman transmission over 240 km using PDM-64QAM single carrier FDM with digital pilot tone,” in Optical Fiber Communication Conference (Optical Society of America, 2012), paper PDP5C.3. [CrossRef]
8. S. Beppu, K. Kasai, M. Yoshida, and M. Nakazawa, “2048 QAM (66 Gbit/s) single-carrier coherent optical transmission over 150 km with a potential SE of 15.3 bit/s/Hz,” Opt. Express 23(4), 4960–4969 (2015). [CrossRef] [PubMed]
9. Y. Wang, K. Kasai, M. Yoshida, and M. Nakazawa, “320 Gbit/s, 20 Gsymbol/s 256 QAM coherent transmission over 160 km by using injection-locked local oscillator,” Opt. Express 24(19), 22088–22096 (2016). [CrossRef] [PubMed]
10. Y. Wang, K. Kasai, M. Yoshida, and M. Nakazawa, “Single-carrier 216 Gbit/s, 12 Gsymbol/s 512 QAM coherent transmission over 160 km with injection-locked homodyne detection,” in Optical Fiber Communication Conference (Optical Society of America, 2017), paper Tu2E.1. [CrossRef]
11. T. Kan, K. Kasai, M. Yoshida, and M. Nakazawa, “42.3-Tbit/s, 18-Gbaud 64QAM WDM coherent transmission of 160 km over full C-band using an injection locking technique with a spectral efficiency of 9 bit/s/Hz,” in Optical Fiber Communication Conference, (Optical Society of America, 2017), paper Th3F.5. [CrossRef]
12. K. Kasai, M. Nakazawa, Y. Tomomatsu, and T. Endo, “Full C-band, mode-hop-free wavelength-tunable laser diode with a linewidth of 8 kHz and a RIN of −130 dB/Hz,” in Optical Fiber Communication Conference (Optical Society of America, 2017), paper W1E.2. [CrossRef]
13. H. Ishii, K. Kasaya, and H. Oohashi, “Spectral linewidth reduction in widely wavelength tunable DFB laser array,” IEEE J. Sel. Top. Quantum Electron. 15(3), 514–520 (2009). [CrossRef]
14. B. Szafraniec, B. Nebendahl, and T. Marshall, “Polarization demultiplexing in Stokes space,” Opt. Express 18(17), 17928–17939 (2010). [CrossRef] [PubMed]
15. C. Paré, A. Villeneuve, P. A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett. 21(7), 459–461 (1996). [CrossRef] [PubMed]
16. A. C. Bordonalli, C. Walton, and A. J. Seeds, “High-performance phase locking of wide linewidth semiconductor lasers by combined use of optical injection locking and optical phase-lock loop,” J. Lightwave Technol. 17(2), 328–342 (1999). [CrossRef]
17. Y. Wang, K. Kasai, T. Omiya, and M. Nakazawa, “120 Gbit/s, polarization-multiplexed 10 Gsymbol/s, 64 QAM coherent transmission over 150 km using an optical voltage controlled oscillator,” Opt. Express 21(23), 28290–28296 (2013). [CrossRef] [PubMed]
