key: cord-0262133-28ggtwla authors: Pittala, Fabio; Braun, Ralf-Peter; Boecherer, Georg; Schulte, Patrick; Schaedler, Maximilian; Bettelli, Stefano; Calabro, Stefano; Kuschnerov, Maxim; Gladisch, Andreas; Westphal, Fritz-Joachim; Xie, Changsong; Chen, Rongfu; Wang, Qibing; Zheng, Bofang title: 1.71 Tb/s Single-Channel and 56.51 Tb/s DWDM Transmission over 96.5 km Field-Deployed SSMF date: 2021-08-04 journal: nan DOI: 10.1109/lpt.2022.3142538 sha: d7604d85eec0d75207d03b6e43ebb3d1dc1e6490 doc_id: 262133 cord_uid: 28ggtwla We report an industry leading optical dense wavelength division multiplexing (DWDM) field trial with line rates per channel exceeding 1.66 Tb/s using 130 GBaud dual-polarization probabilistic constellation shaping 256-ary quadrature amplitude modulation (DP-PCS256QAM) in a high capacity data center interconnect (DCI) scenario. This research trial was performed on 96.5 km of field-deployed standard single mode G.652 fiber infrastructure of Deutsche Telekom in Germany employing Erbium-doped fiber amplifier (EDFA)-only amplification. A total of 34 channels were transmitted with 150 GHz spacing for a total fiber capacity of 56.51 Tb/s and a spectral efficiency higher than 11bit/s/Hz. In the single-channel transmission scenario 1.71 Tb/s was achieved over the same link. In addition, we successfully demonstrate record net bitrates of 1.88 Tb/s in back-to-back (B2B) using 130 GBaud DP-PCS400QAM. XPONENTIAL internet traffic growth continues with emerging bandwidth-hungry applications such as 5G, video, artificial intelligence and virtual/augmented reality. In addition, the Covid health crisis has led to a rapid expansion of our digital activity, which will continue to grow beyond the pandemic. In order to ensure sufficient connectivity, constant This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. evolution in research and standardization of high-capacity optical transmission systems is required. Recent records for single-carrier experiments with net bitrates exceeding 1 Tb/s are summarized in Fig. 1 . Filled markers show experiments using one digital-to-analog converter (DAC) per dimension, while non-filled markers show data for transmitters using multiple DACs per dimension. Back-to-back (B2B) experiments are shown in grey. The high-speed DAC is the key component to generate high-bitrate and high-symbol rate signals. The sampling rates of DACs have reached 128 GSa/s for technologies based on Silicon-Germanium (SiGe) [1, 2, 5, 6] and 120 GSa/s for complementary metal-oxide semiconductor (CMOS) [3] . Using a single DAC per dimension, net bitrates up to 1.61 Tb/s have been reported using 128 GSa/s SiGe DACs [6] , while the highest reported net bitrate with 120 GSa/s CMOS DACs is 1.15 Tb/s [3] . Using multiple DACs per dimension allows to achieve higher symbol rates, however bandwidth multiplexing techniques affect the signal integrity limiting the achievable information rate (AIR) [4, [7] [8] [9] . Recent dense wavelength division multiplexing (DWDM) aggregate capacity records are shown in Fig. 2 . In these experiments, the DWDM grid ranges from 100 GHz to 150 GHz and the symbol rates from 96 GBaud up to 130 GBaud. Triangle markers in Fig. 1 and in Fig. 2 show the net bitrates achieved in this work. We report record symbol rates up to 134 GBaud using off-theshelf hardware components and in particular using only a single DAC per dimension. We achieved net bitrates up to 1.88 Tb/s for single-carrier optical B2B, 1.78 Tb/s and 1.71 Tb/s, respectively, after transmission over 61.3 km and 96.5 km of field-deployed ITU-T G.652 standard single-mode fiber (SSMF) belonging to the infrastructure of Deutsche Telekom in Germany employing EDFA-only amplification at the Tx/Rx of location ① in Fig. 3 . In contrast to all previously reported records, it should be noted that the transmitter used in this paper does not use polarization division multiplexing emulation, but generation of the two polarizations is obtained using four independent lines of components (DAC, driver, modulator) reflecting the actual implementation of a product. In addition, we report the first DWDM transmission experiment over 96.5 km field-deployed SSMF link with a per-carrier net bitrate exceeding 1.6 Tb/s on each of the 34 DWDM channels and yielding to a total capacity of 56.51 Tb/s in C-band with a spectral efficiency of 11.08 bits/s/Hz. To the best of our knowledge, this is the highest single-channel and C-band aggregate net bitrate ever trans-mitted on a field-deployed fiber network of a service provider. The experimental setup is described in section II, while the main findings are reported in section III. Our conclusions are reported in section IV. The system setup is shown in Fig. 4 . The data signal consisting of four real components is generated by four SiGe DAC application-specific integrated circuits (DAC-ASICs) having a typical bandwidth of 65 GHz (with sin(x)/x roll-off mathematically compensated), effective number of bits (ENOB) of 5 bit for the same bandwidth, and a typical sampling rate of up to 128 GSa/s [14] . With respect to the DAC employed in [5, 6] , this device uses a novel type of package for the DAC-ASIC and a built-in amplifier delivering high speed and quality output signals with a smooth frequency roll-off. In this experiment we have increased the sampling rate of the DACs up to 134 GSa/s (over the limit indicated in the data sheet of this device) by supplying a reference clock up to 67 GHz. To keep the system stable (synchronized over time with the input reference clock) and to reduce thermal noise in the DAC-ASICs a careful temperature control was performed by using an external air cooling system. Based on the prototype version of this AWG and time interleaving two 128 GSa/s outputs a record symbol rate demonstrating 220 GBaud signal generation has been recently published [15] . The electrical outputs of the DACs are connected to four single-ended SiGe RF amplifiers with 72 GHz 3 dB-bandwidth and 11 dB gain driving two electro-optic GaAs IQ modulators having 6 dB-bandwidth exceeding 50 GHz. Fig. 5 shows the frequency transfer function (without compensating the sin(x)/x roll-off) for the four DAC-ASICs, when the peaking of the built-in amplifier is set to its maximum value, without and with the 72 GHz driver amplifiers. The output optical signal of a tunable external cavity laser (ECL) with <100 kHz linewidth is split in a polarization maintaining (PM) splitter and amplified by two PM Erbiumdoped fiber amplifiers (EDFAs) before feeding the two IQ modulators with 18 dBm optical power. The dual-polarization (DP) signal is obtained by recombining the output of the IQ modulators using a polarization beam combiner (PBC). The DP signal is amplified by an EDFA before applying linear preemphasis by a waveshaper that, with 8 dB pre-emphasis, flattens the power spectral density of the modulated signal. The transceiver can be tuned over 5.36 THz from 191.1 THz to 196.46 THz. The optical power of the modulated signal at the output of the waveshaper can reach up to 12 dBm. The optical links are part of the Deutsche Telekom fiber infrastructure in Germany, as shown in Fig. 3 . A set of singlecarrier experiments is based on multiple spans of 61.3 km SSMF each having ~19.5 dB attenuation, while the DWDM experiment is based on a single span of 96.5 km of SSMF exhibiting 23 dB attenuation. Amplification is performed using EDFAs only. In B2B and single-channel configurations no booster EDFA is required and after fiber transmission, the signal is pre-amplified before a tunable bandpass filter set to 150 GHz to reduce amplified spontaneous emission noise at the receiver. To emulate a DWDM system, an ASE bandwidth loading method [16] is applied assuming a 150 GHz-spaced 34channel DWDM from 191.225 THz to 196.325 THz. In the DWDM scenario, optical equalization is done in the MUX followed by a booster amplifier and after selecting the channel under test with the DEMUX, an additional EDFA is used to keep the optical power at the input of the receiver at 7 dBm. The receiver consists of a coherent mixer and four 100 GHz balanced photodetectors (BPDs), with 0.45A/W responsitivity, connected to a 256 GSa/s 80 GHz oscilloscope. Another ECL with linewidth <100 kHz is used as local oscillator. The receiver digital signal processing (DSP) makes use of advanced and fully adaptive nonlinear component equalizers, targeting imperfections such as bandwidth limitations, frequency dependent I/Q imbalance and skew, phase ripple, I/Q crosstalk and high-order nonlinearities at transmitter and receiver. At the receiver, a first digital Volterra equalizer (Rx-NLE in Fig. 4 ) addresses the imperfections of the receiver components, i.e. optical-electronic frontend and analog-to-digital converter (ADC). After channel equalization and demodulation (including carrier phase recovery), another Volterra equalizer (Tx-NLE in Fig. 4) compensates for the residual imperfections of the transmitter. Since the transponder imperfections arise mostly in the electrical domain, where the four tributaries are independently processed, also the equalizers operate on the real tributaries rather than on the complex baseband signal. Finally, partial-response equalization (PREQ) with impulse response 1+αD is implemented to whiten the noise, followed by a complex-valued BCJR algorithm with one memory tap used for sequence detection, similarly to [17, 18] . Measurements are performed using a family of probabilistic constellation shaping (PCS) formats of variable entropy (H) obtained from high-order square quadrature-amplitude modulation (QAM) formats: 256QAM, 324QAM, 400QAM, 484QAM and 576QAM, compatible with probabilistic amplitude shaping (PAS) [19] . In B2B, for each base constellation and symbol rate (128-134 GBaud) we varied the alphabet entropy using Maxwell-Boltzmann distributions. Then we selected the configuration resulting in the maximum AIR: 256QAM with H=7.9 bit/symbol, 324QAM with H=8.14 191 191.5 192 192.5 193 193.5 194 194.5 195 195.5 196 196 [20] . For experiments over the 96.5 km link, DP-PCS256QAM and DP-PCS400QAM have been chosen. In Fig. 7 -left (singlechannel) and Fig. 7 -right (DWDM), we observe that DP-PCS400QAM has a higher AIR than DP-PCS256QAM. The number of bitlevels per complex symbol is 8 and 10 for 256QAM and 400QAM, respectively, since 400QAM is based on 1024QAM. The net bitrate backoff is − _ and the bitwise FEC margin is ( − _ )/ . Thus, considering the impact of practical FEC, following [21] , the net bitrate backoff translates into a larger bitwise FEC margin for lower . As we can observe in Fig. 7 -center (single-channel), if we target a fixed net bitrate of 1.6 Tb/s, DP-PCS256QAM has a larger bitwise FEC margin (translated into SNR [dB]) than DP-PCS400QAM. Similarly, for a given FEC margin, a lower results in a smaller net bitrate backoff. For this reason, we observe in Fig. 7 -left and Fig. 7 -right, that when selecting from a set of practical codes with similar FEC margins [20] , DP-PCS256QAM achieves a higher net bitrate than DP-PCS400QAM. In single-channel transmission, the highest net bitrate is 1.71 Tb/s and for all 34 channels of the DWDM system the net bitrate is ≥1.66 Tb/s for DP-PCS256QAM and above 1.62 Tb/s for DP-PCS400QAM. Fig. 8 shows the stability of the transmission system monitoring the AIR for the worst DWDM channel over the course of 14 hours, with measurements taken every 45 seconds. We have reported record symbol rates up to 134 GBaud using one DAC per signal dimension, record net bitrates up to 1.88 Tb/s employing high-order modulation formats up to DP-PCS576QAM in optical B2B and record single-channel transmission achieving net bitrates ≥1.6 Tb/s up to 183.9 km of field-deployed fiber using 130 GBaud DP-PCS400QAM. In addition, we reported a capacity record of 56.51 Tb/s in a 34channel DWDM configuration over a 96.5 km link with 23 dB attenuation. All 34 carriers achieve error-free net bitrates ≥1.66 Tb/s using DP-PCS256QAM. Transmission experiments were performed on a field-deployed fiber network infrastructure of Deutsche Telekom in Berlin, Germany, using EDFA-only amplification of the C-band in ITU-T G.652 SSMF. Single Carrier 1.2 Tbit/s Transmission over 300 km with PM-64 QAM at 100 GBaud 1.3-Tb/s single-channel and 50.8-Tb/s WDM transmission over field-deployed fiber 1.1 Tb/s/λ at 9.8 bit/s/Hz DWDM transmission over DCI distances supported by CMOS DACs 1.04 Tbps/Carrier Probabilistically Shaped PDM-64QAM WDM Transmission Over 240 km Based on Electrical Spectrum Synthesis 1.52 Tb/s single carrier transmission supported by a 128 GSa/s SiGe DAC Single-channel 1.61 Tb/s Optical Coherent Transmission Enabled by Neural Network-Based Digital Pre-Distortion 1.3-Tbps/carrier net-rate signal transmission with 168-GBaud PDM PS-64QAM using analogue-multiplexer-integrated optical frontend module Generation and Intradyne Detection of Single-Wavelength 1.61-Tb/s Using an All-Electronic Digital Band Interleaved Transmitter Transmission of 200-GBaud PDM Probabilistically Shaped 64-QAM Signals Modulated via a 100-GHz Thin-film LiNbO3 I/Q Modulator 41-Tbps C-band transmission with 10-bps/Hz SE using 1-Tbps 96-GBd PS-256QAM for DCI 49.2-Tbit/sWDM Transmission Over 2x93-km Field-Deployed Fiber 52.1 Tb/s C-band DCI transmission over DCI distances at 1.49 Tb/s/λ 54.5 Tb/s WDM Transmission over Field Deployed Fiber Enabled by Neural Network-Based Digital Pre-Distortion 220 GBaud Signal Generation Enabled by a Twochannel 256 GSa/s Arbitrary Waveform Generator and Advanced DSP Investigation of bandwidth loading in optical fibre transmission using amplified spontaneous emission noise 800ZR+ DWDM Demonstration over 600km G.654D Fiber Enabled by Adaptive Nonlinear TripleX Equalization Recurrent Neural Network Soft-Demapping for Nonlinear ISI in 800Gbit/s DWDM Coherent Optical Transmissions Bandwidth Efficient and Rate-Matched Low-Density Parity-Check Coded Modulation Transoceanic Transmission Systems Using Adaptive Multirate FECs Information Rate of Probabilistically Shaped QAM with Non-Ideal FEC