1. Introduction

Dramatic improvements in system capacity are underway in submarine transmissions systems thanks to the capability of coherent systems [1] to digitally compensate large amounts of optical dispersion [2]. The digital dispersion compensation reduces the nonlinear penalties which are particularly significant in dispersion managed systems. Using lower baud rate subcarriers for the channels further increases tolerance to the interplay of dispersion, inter-channel and intra-channel nonlinear effects [3]. In concert with tight frequency control of lasers [4] sharp digital filters further improve spectral efficiency (SE) which ultimately drives higher fiber capacity.

The recent reported achievements of real-time transmissions on ultra long-haul submarine links are shown in Fig. 1. Focusing on the links exceeding 10,000 km in length Ait Sab et al. [5] and Kamalov et al. [6] achieved transmissions at an SE of 3 b/s/Hz using the 8-ary quadrature amplitude modulation (8QAM) format with 50 GHz channel spacing. Just recently a real-time transmission at SE of 4 b/s/Hz using the same modulation format with 37.5 GHz spacing has been reported [7]. By employing advanced coherent and digital technologies of Infinera’s fourth-generation Infinite Capacity Engine (ICE4) we report, to the best of our knowledge, the most spectral efficient, 4.66 b/s/Hz, real-time error-free superchannel transmission over a submarine link longer than 10,500 km using 8QAM format with 22.1 GHz channel spacing.

 

Fig. 1 Published values of SE vs. distance for real-time transmission on straight line systems [5–9].

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2. Link description

SEABRAS-1 is a new 6-fiber pair, 72 Tb/s submarine dispersion-unmanaged cable system between São Paulo (Brazil) and New York (US). The wet plant (Fig. 2) uses a fiber with 130 µm² effective area and the link dispersion varies from 204 s/m at the blue end of the spectrum to 221.5 s/m at the red end of the spectrum.

 

Fig. 2 Schematic diagram of the link and received spectrum. ROADM, Reconfigurable optical add-drop multiplexer; LMAF, Large mode area fiber; EDFA, Erbium-doped fiber amplifier.

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Optical signals are inserted into a multiplexer card based on wavelength selective switch (WSS) and the received spectrum is shown in Fig. 2. The central wavelength of the live traffic is 1564.8 nm and the trial superchannel is tuned to 1535.8 nm. In order to emulate the effect of full loading of the fiber, an amplified-spontaneous emission (ASE) source is added to one of the WSS inputs providing consistent coverage over the C-band at a spectral density 1.75 dB lower than the superchannel. The ASE is carved such that there is a guard band of 50 GHz for the superchannel under test. Unmodulated laser sources are added as idler channels to maintain optimal modulated launch power.

3. System description

The XTS-3300 units are 6-channel Infinera line cards with data rate capabilities of up to 200 Gb/s per channel (with 16QAM modulation format and baud rate of ~33 GBd) that contain a transceiver photonic integrated circuit (PIC). The transmitter comprises six widely-tunable lasers monolithically integrated with nested Mach-Zehnder modulators and with integrated radio frequency (RF) drivers and electronics. The receiver comprises six local oscillators with optical 90^◦ hybrids and transimpedance amplifiers. The packaged part includes the polarization multiplexing optics, thermoelectric coolers, and control electronics. Both the transmit and receive sides of the module are described in detail in [10–12].

The digital coherent application-specific integrated circuit (ASIC) architecture relies on digitally synthesized subcarriers [13] to achieve tolerance to transmission fiber nonlinearity and dispersion [14]. Previous work has shown that subcarrier baud rate should be optimized to balance interference from subcarrier nonlinearities with adjacent carriers [15] therefore we configure the subcarrier baud rate to 5.4 GBd. Our internal calculation using our own DSP comparing 5.4 GBd subcarrier versus for instance 21.6 GBd showed similar high-order format gains of 9 to 15 % extra reach (over fiber plants of this type) as in [16]. On top of that, it is well known that balancing the dispersion compensation between the transmitter and receiver improves the tolerance to nonlinearities [17]. We used the ICE4 ASIC capability to scan the transmitter pre-compensation and found the optimum to be 76.5 s/m (37.5 %) as shown in Fig. 3.

 

Fig. 3 Transmitter dispersion pre-compensation scan of a superchannel pair.

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Each data channel is split into four subcarriers which are digitally shaped using a root-raised cosine filter. Two shaping roll-off factors α = 6 % and α = 2 % are selected for the real-time transmission tests. Internal pattern generators are used to generate a client payload of 103.125 Gb/s prior to the soft decision feed-forward correction (SD-FEC) overhead insertion. Figure 4 shows the case of α = 2 % where all six wavelengths are multiplexed using an optical power combiner on the PIC to form an 8QAM dual-polarization superchannel carrying 618.75 Gb/s of payload and occupying 132.7 GHz of optical spectrum giving a spectral efficiency of 618.75/132.7 = 4.66 b/s/Hz. Further capacity gain is achieved with SD-FEC gain sharing where the FEC codeword is striped across two wavelengths so the performance of each individual channel can be balanced across the spectrum. The line cards also use a novel wavelocking architecture which ensures that adjacent lasers maintain their spacing to <100 MHz [4] providing the superchannel with extremely high precision and stability.

 

Fig. 4 Received high-resolution optical spectrum of 6-channel superchannel with 22.1 GHz spacing.

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4. Performance

The real-time performance measurements were carried out for different channel spacings within the superchannel and the two aforementioned root-raised cosine shaping filters in order to quantify the system penalty. Figure 5 shows the Q values inferred from pre-FEC bit-error rates as a function of spectral efficiency. First we applied the 6 % roll-off digital shaping filter to all subcarriers and varied the channel spacing from 27 to 22.4 GHz. The dashed curves in Fig. 5 depict the performance of all three pairs of gain shared channels. The performance slowly and linearly deteriorated with increasing spectral efficiency reaching the maximum at 4.52 b/s/Hz with 22.8 GHz of spacing. Beyond this value, i.e., with tighter spacing, the degradation rapidly increased and transmission was no longer error-free. However, we changed the filter roll-off factor to 2 % for each subcarrier and pushed the channel spacing even further to 22 GHz. The solid curves show how the error-free performance steadily edged toward the SE boundary with the maximum at 4.66 b/s/Hz and 22.1 GHz channel spacing. The receiver average optical signal to noise ratio (OSNR) was ∼16.6 dB implying the dispersion/nonlinearity penalty was ~1.7 dB. We launched at −2.7 dBm per channel which agrees well with our simulations for optimum power given the nonlinearity and noise accumulation in our model. It is worth noting that this is 0.7 dB more wet plant amplifier power than full C-band loading at this superchannel spectral density could support. We also ran a 9-hour performance soak for this case proving the exceptional stability and error-free performance over time, see Fig. 6. (The longest performance soak we had time to execute was almost 120 hours post-FEC error free at SE of 4.65 b/s/Hz, i.e., 22.2 GHz channel spacing with 2 % shaping). Figure 7 portrays real-time captures of received symbols of one of the channels (α = 6 %, 27 GHz spacing) shown as constellation diagrams. It is worth noting that the measurements were done at the red and blue edges of the spectrum and the performance difference was observed to be small.

 

Fig. 5 Q as a function of SE for 6 % and 2 % roll-off factors.

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Fig. 6 Stability of Q values over time with a superchannel width of 132.7 GHz (22.1 GHz channel spacing) and 2 % roll-off factor.

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Fig. 7 Channel constellation diagrams per subcarrier (columns) per polarization (rows) for 6 % roll-off factor and 27 GHz spacing.

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We would like to emphasize that similar to other published works [7] the validity of the achieved superchannel SE is mainly based on the performance of channels 3/4 where channels 1/2 and 5/6 act as the closest and most significant interferers. This value might be reduced, for instance, when the C-band is fully loaded to additional nonlinear interference and Raman gain tilt.

5. Conclusion

We show a high spectral efficiency real-time system by using a combination of advanced coherent and digital techniques such as Nyquist subcarriers, high precision wavelocking, SD-FEC gain sharing, and superchannel PIC optics. Owing to these technologies we demonstrated a record real-time error-free transmission of 4.66 b/s/Hz through more than 10,500 km SEABRAS-1 link.