1. Introduction

400-Gb/s per channel WDM transmissions on ITU-T grid have attracted a lot of research interest since 400 Gb Ethernet (400GbE) has been envisioned as the foundation of the next-generation transport standard to cope the exponential growth of IP traffic [1–10]. Along that line, 400G transmission on a single optical carrier emerged as an attractive solution since, although it would push the boundaries of opto-electronic components and modules, it would offer benefits in terms of system complexity [5–10]. Namely, the employment of the highest feasible electronic multiplexing rates with a minimum number of channel subcarriers reduces the number of deployed optical components, which typically determinate the transponder cost [5–10]. There are two viable options to how to modulate a single optical carrier. On one side, by employing higher level modulation formats, such as 16QAM and beyond, will require higher optical-signal to noise ratio (OSNR) and limit the transmission distance while increasing complexity and power consumption of DSP modules in coherent receivers [5, 10]. On the other side, to increase baud in combination with more robust modulation formats, such as PMD-QPSK, would lead to longer transmission distances and less complex DSP implementation [7–9]. We believe that increasing the baud to realize single-carrier 400-Gb/s WDM channels is a promising and cost effective solution for future large-capacity optical transmission networks having in mind that it can also serve a solid foundation for construction of multicarrier channels with multi terabit capacity.

So far, a single-carrier 400G submarine system transmission over 7,200-km fiber link is reported as the longest distance one [10]. Transmission was done by using 64-GBaud 16QAM format with net spectral efficiency (SE) of 5.33b/s/Hz over cascaded 50-km fiber spans. As for terrestrial systems with 80 to 100-km fiber spans, a single-carrier 400G transmission over 4800-km is reported in [8]. It was realized with net SE of 3.64 bit/s/Hz based on 107GBaud QPSK format. Also, 3600-km transmission with net SE of 4 bit/s/Hz based on 110-GBaud QPSK, which has been the highest ETDM symbol rate so far, is reported in [9].

In our work reported here, we successfully increased the limit of ETDM-based opto-electronic schemes by employing the symbol rate of 128.8-GBaud over record breaking distances. Namely, we successfully transmitted single-carrier 515.2-Gb/s PDM-QPSK signals over 10,130 km link consisted of 100km-spans of TeraWave SLA + fiber. To the best of our knowledge, it is the highest symbol rate reported so far, and the longest WDM transmission distance with single-carrier 400G signals. We have also demonstrated the transmission of single carrier 128.8-GBaud filtered QPSK signals in 100-GHz-grid over 6,078-km, which has the line SE of 5.152 (b/s/Hz).

2. Experimental setup

Figure 1 shows the experimental setup of the system based on 515.2-Gb/s single-carrier generation on 128.8-GBaud PDM-QPSK format. The same setup was used to generate 9-QAM-like filtered QPSK signal that effectively doubles the spectral efficiency. At the transmitter side, 8- (for PDM-QPSK) or 16 (for 9-QAM-like filtered QPSK) external cavity lasers (ECLs) with linewidth less than 100-kHz, have been used. They are spaced apart in carrier frequency by 200GHz or 100GHz producing the output power of 14.5dBm. As such they are divided into two groups as the odd and even channels to form the WDM channel setup in 200- or 100GHz-grid. The odd/ even channels are multiplexed by two arrayed waveguide gratings (AWGs). After AWG, the signals are boosted in power to 23dBm by PM-EDFAs. The two-pairs of 128.8-GBaud in-phase (I) and quadrature (Q) data signals are generated by three-stage all-electronic time-division multiplexing (ETDM) blocks with 2:1, 4:1, and 2:1 electrical multiplexing ratios, thus producing 128.8-GBaud rate from 8.05-GBaud binary pseudo-random binary sequence (PRBS) signals with a word length of 2¹⁵-1. The odd and even channels are modulated independently by using two I/Q modulators directly driven by the generated 128.8-GBaud PRBS signals. The quite clear electrical eye diagram of 128.8-GBaud binary signal is shown as inset (a) in Fig. 1. In our case the employed 4:1 MUX is a 56 Gb/s 4:1 broadband multiplexer module, while the 2:1 MUX is a 120Gb/s 2:1 broadband multiplexer module. As we confirmed, the 4:1 MUX performs quite well with 64.4-Gb/s output and the 2:1 MUX also works well with the 128.8-Gb/s output. The obtained output of the 4:1 MUX has peak-to-peak value V_pp = 500mV and the output of 2:1 MUX has a V_pp = 400mV. For QPSK modulation format, the I/Q modulator is biased at the null point. The 3-dB bandwidth of the two-arm LiNbO3 I/Q modulator is ~37 GHz. After modulation, the polarization multiplexing of each path is realized via the polarization-multiplexer, which consists of a polarization-maintaining optical coupler (PM-OC), used to split the signal, optical delay lines to provide delay of over 100 symbols, and a polarization beam combiner (PBC) to recombine the signal. The odd and even channels are then combined by a programmable wavelength selective switch (WSS).

 

Fig. 1 Experimental setup. (ECL: external cavity lasers; Mux: time-division Multiplexer; IQ Mod.: IQ modulator; Pol. MUX: polarization multiplexer; BW: bandwidth; WSS: wavelength selective switch; OC: optical coupler; SW: switch; ATT: attenuator; TOF: tunable optical filter; LO: local oscillator; ADC: analog to digital convertor). Insets: (a) the electrical eye diagram of 128.8-GBaud binary signal; (b) the frequency response of the WSS with different filter alpha factors; (c) the QPSK signal spectrum after passing WSS with different alpha factors.

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Two types of WDM channel arrangements are tested, including: (i) eight WDM channels spaced apart by 200 GHz (line SE = 2.64 b/s/Hz) with transmitter-side optical pre-equalization; and (b) sixteen 100-GHz- super-Nyquist WDM (SN-WDM) channels spaced apart by 100 GHz (line SE = 5.152 b/s/Hz) without pre-equalization. We first tested the long distance transmission performances for the single-carrier 400G transmission with transmitter-side optical pre-equalization using 200GHz-grid channels. For eight 200-GHz grid channels, the transmitter-side pre-equalization by enhancing the power of high-frequency components while attenuating the low-frequency components, was used [8, 11, 12]. In this case, the transfer function of WSS is designed via the obtained channel response. The selection of taps number and the frequency value of WSS impulse response are based on the similar scheme we proposed for digital pre-equalization using receiver-side channel estimation under back-to-back self-homodyne coherent detection [12]. Inset (b) in Fig. 1 shows the frequency response of the WSS with different filter alpha factors (we use the alpha factor to adjust the pre-emphases strength; no pre-equalization when alpha is 0, and full pre-equalization when alpha is 1), while inset (c) in Fig. 1 shows the QPSK signal spectrum after passing WSS. After that, sixteen 100-GHz-grid super-Nyquist WDM (SN-WDM) channels of 9-QAM-liked QPSK signals without any pre-equalization were tested. For the 100-GHz-grid SN-WDM channels, the odd and even channels were spectrally filtered to achieve the 9-QAM-like constellation signals and combined by the WSS at 100-GHz fixed grid and 94.8-GHz 3-dB bandwidth (BW).

The fiber transmission was conducted over a 405.2-km recirculating loop, which consisted of four 101.3-km spans of TeraWave-SLA + fiber with an average effective area A_eff of 122 μm², an attenuation coefficient of 0.185 dB/km (20-dB span loss including connectors) and a chromatic dispersion coefficient of 20.0 ps/(nm·km) at 1550 nm. One backward-pumped Raman amplifier with about 20-dB ON-OFF gain was used for each span to compensate for the signal loss. The average power of the Raman pumps was about 950 mW. One attenuator was used to control the launch power per channel. Also, one WSS was employed to flatten the gain slope band-pass filter. At the coherent receiver side, tunable optical filter (TOF) with 3-dB bandwidth of 0.9 nm was employed to select the desired sub-channel. An ECL with a linewidth less than 100 kHz was utilized as a local oscillator (LO) with the input to a polarization-diverse 90° hybrid. The bandwidth of the balanced detector that we used was 50GHz. Next, the sampling and digitization (A/D) was realized by the real-time digital oscilloscopes with 160-GSa/s sampling rate and 65-GHz electrical bandwidth, which was followed by off-line DSP applied to four channel sampled data sequence. The data was first re-sampled to 257.6-GSa/s with CD compensation applied, and then processed by either regular constant-modulus equalization (CMEQ) algorithm for QPSK [8] or the multi-modulus equalization (MMEQ) algorithm with maximum likelihood sequence estimation (MLSE) for 9-QAM-like signal [9, 13]. Here, MLSE is used to equalize the inter-symbol-interference (ISI) caused by the filtering effect [13–15]. The total errors were counted over 12 million bits. The line bit rate of 515.2 Gb/s also implies that some overhead was added to 400 G data. In reality, it would be ≥20% soft-decision forward-error-correction (SD-FEC) enabling error free transmission at BER≤ 2.4 × 10⁻².

3. Experiment results and discussions

We have initially tested the performance of WSS-based pre-equalization scheme. The back-to-back (BTB) BER of signals with different filtering alpha factors is shown in Fig. 2(a) . We can see that for single-channel case (or in 200-GHz-grid) the optimal alpha factor for QPSK recovery is ~0.75. The BER performance gets worse when alpha is less than 0.75 due to the ISI penalty and it slightly degrades when alpha is larger than 0.75. We believe there are two reasons. First, since the bandwidth setting accuracy of the WSS for optical pre-equalization is larger than 5-GHz, it is not possible to exactly obtain the channel response. An optical compensation factor should be found by the scanning method. Second, by scanning the alpha factors, it is possible to have the optimal filters for maximizing the SNR in the AWGN channels, which makes the WSS and other band-limited devices as matched filters. However, for the 9-QAM recovery scheme with MMEQ, the alpha value has less impact on the system performance due to the equalization to the ISI based on MLSE. On the other hand, for SN-WDM case with 100-GHz-grid, 9-QAM recovery without optical pre-equalization shows better performance by minimize the inter-channel-interference (ICI) and equalizing the ISI, as demonstrated in [9, 13–15]. Finally, Fig. 2(b) shows the BTB BER results of 128.8-GBaud signals as a function of OSNR (0.1 nm resolution) under different transmission cases for a single channel. The required OSNR at the BER of 2 × 10⁻² for the 200 GHz-grid channels with pre-equalization is about 19 dB, and it increases to 20.5 dB for the 100 GHz-grid SN-WDM channels. In addition, we also verified that all other channels exhibit similar performance except that the side channel has ~0.5 dB better OSNR tolerance as shown in Fig. 4 (b). The corresponding error-free constellation diagrams of 9-QAM and QPSK signals are shown by insets (i) and (ii) in Fig. 2 (b).

 

Fig. 2 (a) The back-to-back BER performances by using QPSK and 9-QAM recovery schemes under different alpha values; (b) The back-to-back BER results versus the OSNR for the 128.8-GBaud PDM-QPSK signals. Insets: the corresponding error-free constellation diagrams of 9-QAM and QPSK signals are shown by insets (i) and (ii).

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The spectrum of the 16 SN-WDM 100 GHz-grid channels before and after 6,078-km transmission is shown in Fig. 3(a) , while Fig. 3(b) shows spectrum of the 8 channels in 200-GHz-grid before and after the 10,130-km transmission. We also measured the BER performance of 100-GHz-grid channel #7 after 6,078-km TeraWave SLA + fiber transmission as function of the launch power per channel, which is shown in Fig. 3(c). The optimal input power per channel is ~0.75-dBm. The BER degrades at the launched powers higher than 0.75-dBm due to nonlinear impairments.

 

Fig. 3 (a) The spectrums of the 16 SN-WDM channels in 100-GHz-grid before and after 6,078-km transmission, (b) the 8 channels in 200-GHz-grid before and after the 10,130-km transmissions, (c) The BER performance after 6,078-km transmission of channel 7 versus the launch power per channel.

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As an illustration, Fig. 4(a) shows the measured BER of channel #7 inside the SN-WDM channels as well as the measured BER of channel #3 inside 200GHz-grid channels as function of transmission distance under the optimal launch power. The measured BERs of 100GHz-grid channel #7 after 6,078-km transmission and 200GHz-grid channel #3 after 10,130 km are 1.6x10⁻² and 2.15 x10⁻², respectively, which are all below the SD-FEC limit. The constellation diagram of the channel #3 after 10,130-km transmission is also shown as inset (i) in Fig. 4(a). We have also confirmed that the BER of all WDM channels in 100 GHz- or 200 GHz grid after 6,078-km or 10,130-km transmission are below the BER threshold of the 20% SD-FEC as shown in Fig. 4(b), respectively. There are several factors which impact the final performances after transmission, such as power fluctuations over different sub-carriers, the gain and noise figure differences of optical amplifier over different wavelength, the fiber nonlinear impairments and the final OSNR. In our experiment, one WSS was employed to flatten the gain slope band-pass filter and equalize the power of different channels in order to have similar performances. Since the transmission distance of 200- and 100-GHz-grid signals are different with different channel spacing and channel number, the performance distribution shows slight differences over the sub-channels.

 

Fig. 4 (a) The BER performance versus the transmission distance. Inset: (i) the constellation diagram of the channel #3 after 10,130-km transmission. (b) The BER of all 16 channels after 6,078-km transmission and 8 channels after 10,130-km transmission.

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4. Conclusion

We successfully transmitted single-carrier 515.2-Gb/s PDM-QPSK/9-QAM signals over 10,130km/6,078-km, respectively, over 100-km spans of TeraWave SLA + fiber. As far as we know, this is the first time to realize single-carrier 400G signal transmission over distances above 10,000km in terrestrial transmission environment. We have also demonstrated the coherent transmission of single carrier 128.8-GBaud 9-QAM-like filtered QPSK signals in 100-GHz-grid over 6,078-km.