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We demonstrated the first 30Tb/s (350 × 85.74Gb/s) optical transmission over 10,181km using bidirectional C/L-bands, 121.2km hybrid fiber spans, DP-QPSK modulation and EDFAs. Achieved 306Petabit/s•km capacity × distance product is the highest reported to date for WDM transmission.
©2013 Optical Society of America
1. Introduction
Fiber communications research is truly driven by two fundamental challenges, namely: capacity and reach. As a consequence, multiple terabit transmission experiments over ultra-long distance were initially reported using 10G RZ [1,2], 10G RZ-DPSK [3,4] and 40G DPSK [5]. Recently, as the development of digital coherent receiver, transmission of 15.5Tb/s over 7,200km was reported using 100G DP-QPSK, both C- and L-bands and hybrid Raman/EDFA amplification [6]. For 10,610km distance, transmission of 9.6Tb/s was also demonstrated using 100G DP-RZ-QPSK, maximum a posteriori probability (MAP) algorithm, C-band and EDFA amplification [7]. Such record has been rapidly updated to 20-Tb/s over 6,860km using below Nyquist rate 100G DP-QPSK, maximum likelihood sequence estimation algorithm, C-band and EDFA amplification [8]. In order to further increase the capacity, higher order modulation formats have been introduced to ultra-long haul transmission. DP-8QAM-OFDM [9] and DP-16QAM-OFDM [10] wavelength-division multiplexed (WDM) signals have been transmitted over 10,181km using advanced soft-decision FEC, ultra-low-loss/ultra-large-core fiber, fiber nonlinearity compensation and digital transmitter. Recently, transmission of 30Tb/s over 6,630km has been reported using DP-16QAM, joint MAP and LDPC decoder and 5THz bandwidth C-band EDFA [11].
Bidirectional transmission has been reported to reduce system cost and the complexity of operation, administration and maintenance [12–18]. The bidirectional WDM transmission systems are classified as being “interleaved” [14] or “band-separated” [15–18] wavelength arrangements. Interleaved schemes are combining two counter-propagating interleaved channel grids which show four-wave mixing (FWM) and cross talk suppression due to the increase of adjacent co-propagating channels spacing. Band-separated schemes are transmitting two groups of wavelengths separated from each other in counter-directions. It is useful for suppression of inter-channel crosstalk [15] and mitigation of Raman gain depletion [17] using simple optical components. Bidirectional C/L-band WDM transmission [17,18] has also been demonstrated as an effective method to achieve high aggregate capacity transmission.
In this paper, an overall capacity of 30Tb/s is transmitted in a single fiber over 10,181km using the longest span length reported so far in a ultra-large capacity (>20Tb/s)/ultra-long distance transmission (>9,000km) demonstration. Total 350 Nyquist shape DP-QPSK channels at channel net data rate of 85.74Gb/s together with a bidirectional C/L-bands EDFA configuration are employed to obtain what is, to the best of our knowledge, the first report of a capacity × distance product (C × D) in excess of 300Pb/s·km. In order to take advantage of the latest developments low loss/large effective area of newly developed fibers, this C × D record is obtained using a 121.2km span length. This is the largest capacity × span-length product of 3636 Tb/s·km for transpacific-range transmission (>9,000km). A summary of C × D reported of recent progress in WDM transmission experiments is illustrated in Fig. 1.
2. Experimental Setup
The experiment setup is shown in Fig. 2. At the transmitter, 350 CW DFB lasers at a channel spacing of 25 GHz spanning the C- and L-bands (1529.94-1565.5nm and 1569.4-1604.67nm) are used. The lasers are distributed into odd and even channels both in C- and L-bands with four independent modulation paths at the transmitter. Four DACs with differential output are used to drive four single-drive I/Q modulators with pre-loaded Nyquist shape QPSK signal. Positive (p) outputs are used for C-band signals and negative (n) outputs are used for L-band signals. In order to generate the Nyquist shape QPSK signal, 239400 random information bits are encoded by FEC encoder with rate 0.875 to generate 10 codewords. The FEC code employed is quasi-cyclic (QC) non-binary LDPC (13680, 11970) code of girth 8 [19]. The encoded non-binary symbols are mapped with QPSK modulation at 24.5GHz symbol rate. For the baseband QPSK signal, two times up-sampling and a digital low pass filter are used to reduce the signal bandwidth within 24.7GHz. At the end, the two times sampling signal is re-sampled to 64GS/s to match the DAC output sampling rate. Within C- or L-bands, the odd and even channels are amplified with polarization maintaining (PM) EDFA and combined with a PM coupler. The inset in Fig. 2 shows the optical spectrum. Polarization multiplexing is performed for both C- and L-bands separately by splitting the signal, delaying one copy by 100ns and rotating it to the orthogonal polarization, followed by polarization beam combining. At the end, the WDM signal of C- and L-bands is amplified with EDFAs, combined using a C/L-bands coupler and sent to the transmission line.
The 484.8km circulating loop testbed consists of four 121.2km spans using hybrid large-core/ultra low-loss fiber. Each span consists of 60.6km of ultra low loss (0.16dB/km) very large effective area (146µm2) fiber (Type I) spliced to another 60.6km ultra low loss (0.16dB/km) large effective area (112µm2) fiber (Type II). The mid-band chromatic dispersion is around 21ps/nm/km. Each span is equipped with a C/L-bands EDFA set. In each set, one C-band EDFA and one L-band EDFA are connected in bidirectional mode using two C/L-bands splitters. The output power of both C- and L-band EDFAs are 20dBm. The loop contained two specific spans. The loop control span contains a loop synchronous polarization controller (LSPC). And the gain equalization span has one C- and one L-band waveshaper and the automatic feedback control loop to compensate residual loop gain un-flatness error where the optical spectrum analyzer (OSA) keeps monitoring the spectral flatness in the gated mode.
At the receiver, the 25GHz channel of interest is selected by a C/L-bands splitter followed by a 0.4nm optical tunable filter (OTF). The local oscillator (LO) is a separated tunable external cavity lasers (ECL) with a linewidth of 100-kHz. For each measurement at the wavelength of interest, the corresponding DFB laser and seven neighboring DFB lasers are replaced by eight tunable ECLs. The signal and LO are combined using a polarization-diversity 90° hybrid followed by four balanced photo-detectors. The electrical signals are sampled and digitized using a 4-channel real-time sampling scope with a sampling rate of 50GSa/s and 20GHz analog bandwidth. The captured data is then processed offline.
The DSP algorithm used to recover the transmitted signal consists of four steps: first, chromatic dispersion compensation is performed using overlapping frequency domain equalization (FDE) method. Next, CMA is used to perform the polarization de-multiplexing and channel estimation/equalization before carrier phase recovery. Then, a 16-state/128-depth-sliding-window reduced-complexity Bahl–Cocke–Jelinek–Raviv (BCJR) equalizer [20,21] is used for each polarization to boost the Q factor performance before QC-LDPC decoder. Finally, the demodulated QPSK symbols are demapped into QC-LDPC symbols that are fed into the QC-LDPC decoder to compute the coded bit error rate (BER). All BER were calculated from 10 million bits sequence obtained from two polarizations. The raw data rate before FEC is thus 2 × 2 × 24.5GHz = 98-Gb/s per 25-GHz channel. After accounting for the 14.3% FEC overhead, the achieved net data rate per 25-GHz channel is 85.74-Gb/s. Therefore, the capacity × distance product can be calculated as 98-Gb/s × 0.875 × 350 × 10181km = 306Pb/s·km in this experiment. Figure 3 shows the simulation performance of the QC-LDPC decoder as a function of line Q-factor. FEC threshold is found at Q = 6.2dB which is the required input BER to BCJR equalizer so that the decoder output BER is equal or less than 10−15.
3. Experiment results
The back-to-back Q-factor versus OSNR performance is shown in Fig. 4, where OSNR is defined as the power of a 25GHz channel divided by the noise measured at 0.1-nm bandwidth. The Q-factors are calculated from the measured BER for the 98-Gb/s DP-QPSK channel at 1549.3nm. The OSNR required to achieve the soft-decision FEC threshold of 6.2dB is 10.6dB, which is about 1.6dB away from the theoretical limit. The penalty between single channel and WDM case is negligible. The 350 × 98-Gb/s WDM signals are launched into the test-bed with a very flat spectrum. After 10,181km transmission (21 loops), the optical spectrum is shown in Fig. 5, and the average received OSNR (in 0.1nm RBW) of the WDM channels is 11.5dB.
The Q-factors calculated from the BER measured before and after BCJR equalizer for all 25GHz channels are shown in Fig. 6. The average Q-factor improvement from the sliding-window BCJR equalizer is about 0.5dB. After BCJR equalizer, Q-factor of the worst channel was 6.28dB at 1569.8nm. The received bit errors after the BCJR equalizer are corrected using QC-LDPC decoder for all 350 channels. No any error was found after QC-LDPC decoder which processed in total 3.5 billion bits received from all channels. Both C- and L-bands have similar overall transmission performance which was jointly decided by inter-band Raman gain, fiber non-linearity penalty, EDFA noise figure and gain profile, fiber loss and effective area, etc. Figure 7 shows the similar achievable transmission distance for channels at 1549.3nm and 1588.3nm.
4. Conclusion
We report the first time beyond 300Pb/s•km transmission over transpacific distance. 30Tb/s (350 × 85.74Gb/s) transmission over 10,181km has been achieved using DP-QPSK modulation, 121.1km hybrid large-core/ultra low-loss fiber span, bidirectional C/L-bands EDFA amplification, Nyquist shape spectra, reduced-complexity sliding-window BCJR equalizer and QC-LDPC coding.
Acknowledgments
The authors would like to thank H. B. Matthews, S. K. Mishra, S. R. Bickham and R. R. Khrapko from Corning Incorporated for their great work to develop the new fiber.
References and links
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