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Abstract
We demonstrate real-time 24-Tb/s dense wavelength division multiplexing (DWDM) transmission over a 1910-km field-deployed G.654.E fiber link using 24 in-line wide-bandwidth Erbium-doped fiber amplifiers with a widened bandwidth of 6 THz in the cost-effective C-band. The DWDM system consists of 60 100-GHz-spaced 400-Gb/s wavelength channels, modulated with probabilistic constellation shaped polarization-division-multiplexed 16-point quadrature-amplitude modulation and fast-than-Nyquist shaping. This field trial shows the feasibility of achieving a record per-fiber capacity of 24 Tb/s in field-deployed 2000-km-class terrestrial fiber links by using the widened C-band.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical networks are facing an explosive increase in data traffic as the backbone of the Internet. The advancement of 5G, cloud computing, and high-definition video services are demanding unprecedented higher capacity in all levels of the optical networks. Coherent technologies such as high baud rate and advanced modulation format, has driven single-carrier transmission rate up to 100 Gb/s and beyond. Under the same baud rate, it usually needs higher-level quadrature amplitude modulation (QAM) format to achieve higher channel rates. These high-order QAMs require higher optical signal-to-noise ratio (OSNR), and they are more sensitive to laser phase noise and fiber nonlinear effects, leading to reduced transmission reach. To extend the transmission reach, low-loss and large-effective-area fibers and Raman amplifiers can be used. As an example, real-time single-carrier 800 Gb/s transmission has been reported recently, in which the transmission link consists of 1600-km ITU-T G.654.E link with 16 equal-length spans, hybrid distributed Raman amplifiers (DRA) and erbium doped fiber amplifiers (EDFA) installed in the laboratory [1]. Due to frequent changes in physical environment and complex terrestrial network deployment, a huge performance gap was observed between laboratory demonstrations and real field trials [2]. Furthermore, it is required to reserve certain amount of power and OSNR margin for static and dynamic optical impairment in field-installed links. Therefore, lower-order modulation format is preferred for long haul field transmission, which is more robust to optical impairments and system noise. Recently, several field trials based on G.654.E fiber have been reported [3,4], in which the transmission distance of single-carrier 400 Gb/s is extended to several hundred kilometers [4]. Real-time 10 × 400-Gb/s-per-wavelength transmission over 2019-km G.654.E fiber with combined EDFA and backward DRA has been demonstrated [5]. The transmission link consists of 18 equal-length loops between two buildings.
In this work, we report a field trial in a deployed telecom carrier’s optical network using transponders that support a net data rate of 400 Gb/s per wavelength. A 60 × 400-Gb/s dense wavelength division multiplexing (DWDM) system with 6-THz (or 48 nm) working bandwidth at the widened C-band (1524 nm∼1572 nm) is demonstrated, which can achieve a net system capacity of 24 Tb/s only using EDFAs as optical amplifiers. The 400-Gb/s signals are generated with probabilistic constellation shaping (PCS) [6–8], polarization-division-multiplexed 16-point quadrature-amplitude modulation (PDM-16QAM), and faster-than-Nyquist (FTN) shaping [9]. The 400-Gb/s wavelength channels with 95-GBd baud rate are placed in a 100-GHz frequency grid. Moreover, with deployed G.654.E fiber, a point-to-point transmission distance of 1910 km has been realized, which, to the best of our knowledge, is the longest deployed terrestrial point-to-point G.654.E link and the longest reach of single-carrier 400-Gb/s field transmission at present.
2. System setup
Figure 1 shows the schematic diagram of the 1910-km point-to-point field trial system connecting two major cities in China: Shanghai, and Guangzhou. A pair of G.654.E fibers is used to realize the bidirectional point-to-point transmission. A total of 24 optical inline amplifier sites and 1 Reconfigurable Optical Add-Drop Multiplexer (ROADM) site are deployed along this fiber link, dividing the whole link into 26 spans that range between 32 km and 98 km, which differs from a laboratory link with equal-length spans. The G.654.E fiber has an attenuation coefficient of 0.17 dB/km and an effective area of 130 um2. During field installation, optical fiber cables with length of 2-3 km are joined together through fusion splicing. There are several concatenate fusion splices along the whole link. The fusion splices will introduce non-negligible losses to each fiber span. Figure 2 shows the measured average attenuation coefficient of different spans, which can exceed 0.2 dB/km, after field deployment. It is also worth noting that each of the spans has been added with power attenuation of more than 3 dB by a variable optical attenuator (VOA) for engineering link budget reservation. All the above factors add impacts to the performance of the field trial system, making it tougher to achieve similar transmission performance as in a laboratory demonstration.
Fig. 1. Schematic diagram of field trial system with 1910km G.654.E point to point terrestrial link (upper) and the geographic location of the field-deployed fiber link (lower).
Since point-to-point transmission is investigated in this work, a wavelength selective switching (WSS) deployed in the ROADM site functions as an optical gain equalizer to compensate for the cascaded uneven gain of EDFA. At the two terminal sites, two identical DWDM transponders are used in pair to realize bidirectional transmission. Each side of the terminal sites contains four real-time testing channels and fifty-six dummy channels. These dummy channels are generated by using a flat noise source with a WSS, which are shaped like the same as the spectrum of testing channels. The real-time channels are configured into the format of probabilistic shaping PDM-16QAM with a 400-Gb/s net rate. All the 60 channels are multiplexed by a WSS with 100-GHz channel spacing at the transmitter side, while the receiver side uses another WSS to drop the testing channels into their corresponding transponders. Thus, a total of four WSSs are used at the two ends of this link.
In the Guangzhou site, a 100GE bit error rate test stream is connected to a client-side processing card, which is subsequently packed into optical transport network (OTN) frame in the OTN processing card. The OTN stream is transmitted to the Shanghai site, dropped to transponder 1 and looped back at the same transponder. The loopback of OTN frame allows the 100GE bit error stream to be received and tested at the Guangzhou site in convenience. The real-time transponder is comprised of continuous-wave laser, high baud-rate commercial linear driver, optical modulator, integrated coherent receiver, and real-time application specific integrated circuit (ASIC) chip. The transmitter digital signal processing (DSP) contains several modules, including, forward error correction (FEC) coding, symbol mapping, pulse shaping, bandwidth pre-compensation, skew compensation, which work consequently. Inside the receiver side DSP, resampling, chromatic dispersion (CD) compensation, frequency-offset compensation, polarization de-multiplexing, phase estimation, and FEC decoding are employed to recover transmitted information. The modular architecture of the 400 Gb/s transceiver based on PDM-16QAM is illustrated in Fig. 1.
3. Results and discussion
The transmission performance of the 400 Gb/s transceiver and system is first studied in a laboratory environment prior to the field trial. The back-to-back received constellation map of one polarization of the PDM-16QAM signal is shown in Fig. 3. The result clearly indicates the PCS technology used in the transceiver and the non-uniform distribution it has imposed on the equidistant constellation points of the 16QAM signal. Additionally, the square shape and equal level distance shows that the transceiver DSP module has already compensated the optical and electrical impairments in transceiver except ground noise.
A comparative study of the optimum launch power of the 60-channel DWDM system is also carried out for different wavelength ranges and different fiber types in the laboratory setup with equal-length spans, and the results are shown in Fig. 4. The transmission performances in G.652.D and G.654.E fiber are investigated. Since the fundamental losses of these two fibers are different, we choose 1040-km G.652.D fiber link and 1600-km G.654.E fiber link to obtain the similar received signal quality for comparison. The span lengths of these two links are 80 km and 100 km for G.652.D and G.654.E fiber respectively, while they have the same 22-dB span loss (including additional optical attenuation introduced by VOA for engineering link budget reservation). Three testing channels are placed in short-, middle-, and long-wavelength ranges, respectively. The pre-FEC bit error rates (BER) of the three channels are measured with different launch powers. It can be seen that different wavelengths have the similar performance in the same system. However, the optimum launch power is about 4.5 dBm and 6 dBm for G.652 and G.654.E fiber link, respectively. This indicates that G.654.E fiber system can tolerate a higher launching power, thanks to its large effective area, resulting in better transmission performance and longer transmission distance. Moreover, the average lower loss of G.654.E fiber can extend transmission distance at the same time. In the field trial link, the optimal launch power of 400 Gb/s system is also around 6 dBm via G.654.E fiber, which is similar as the laboratory setup.
The 100GE BER testing result is shown in Fig. 5, which confirmed error free transmission over this 1910-km DWDM transmission system. Figure 6 shows the optical spectrum of the whole 60 channels with 100-GHz DWDM channel spacing at the receiver side, which is measured by an optical spectrum analyzer. It can be found that the system achieves a 48-nm working bandwidth by using a widened C-band. The inserter in Fig. 6 shows the optical spectrum of the 400-Gb/s signal. It can be observed that the baud rate of the signal is very high, resulting in the signal spectral width that is much close to the 100-GHz channel spacing used by the system. Therefore, FTN shaping technology is used to avoid excessive filtering costs and crosstalk that could lead to deterioration in transmission performance.
A flat received DWDM signal is achieved by fine-tuning the in-line WSS. The flatness of DWDM signal is further reflected by the BER of each channel at the receiver side, as shown in Fig. 7. The BER of all channels are measured by sweeping the testing channels along the 48-nm working bandwidth range. Optical power is kept the same as each sweeping group during the whole testing period. It can be concluded from Fig. 7 that all measured 60 channels from the short-wavelength to the long-wavelength range have nearly the same performance. The OSNR at the receiver end of all the channels is also measured and shown in Fig. 7. The back-to-back OSNR tolerance is about 18.9 dB, so there is still certain amount of OSNR margin for dynamic impairment penalty. The Q-factor threshold is 5dB, so there is more than 2-dB Q-factor margin.
Figure 8 shows the long-term pre-FEC BER tracing curve, which is captured from the optical module every five seconds from channel 1 (the longest wavelength channel). This curve shows a stable BER monitoring result over more than two hours. It is verified that the 60-channel 400G system can withstand the dynamic optical impairment from real field circumstance.
4. Conclusions
In this work, we have conducted a successful real-time field trial of 400-Gb/s-per-wavelength transmission in a deployed optical network over a 1910-km G.654.E terrestrial fiber link, which is the longest G.654.E terrestrial link to the best of our knowledge. Moreover, a record DWDM system capacity of 24 Tb/s for 2000-km-class terrestrial fiber links is achieved in the cost-effective C-band by broadening the amplification window to 6 THz. Even with more than 3-dB power budget reserved for each fiber span, the system performance shows sufficient OSNR margin to accommodate dynamic channel impairment penalties. This real-time demonstration shows that the combined use of widened C-band, low-loss and low-nonlinearity G.654.E fiber, and advanced DSP techniques such as PCS and FTN shaping provides a competitive solution for 2000-km-class terrestrial optical networks to achieve 24 Tb/s per-fiber transmission capacity. With the additional use of the L-band with over 5 THz of bandwidth [10], over 11 THz of total amplification bandwidth could be achieved, enabling the per-fiber transmission capacity to be improved further.
Funding
National Key Research and Development Program of China (2018YFB1801200).
Acknowledgments
This work is supported by the National Key R&D Program of China.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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