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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 2 — Jan. 28, 2013
  • pp: 1555–1560
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25 Tb/s transmission over 5,530 km using 16QAM at 5.2 b/s/Hz spectral efficiency

J.-X. Cai, H. G. Batshon, H. Zhang, C. R. Davidson, Y. Sun, M. Mazurczyk, D. G. Foursa, O. Sinkin, A. Pilipetskii, G. Mohs, and Neal S. Bergano  »View Author Affiliations


Optics Express, Vol. 21, Issue 2, pp. 1555-1560 (2013)
http://dx.doi.org/10.1364/OE.21.001555


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Abstract

We transmit 250x100G PDM RZ-16QAM channels with 5.2 b/s/Hz spectral efficiency over 5,530 km using single-stage C-band EDFAs equalized to 40 nm. We use single parity check coded modulation and all channels are decoded with no errors after iterative decoding between a MAP decoder and an LDPC based FEC algorithm. We also observe that the optimum power spectral density is nearly independent of SE, signal baud rate or modulation format in a dispersion uncompensated system.

© 2013 OSA

1. Introduction

2. Experiment

A schematic of the transmitter is shown in Fig. 1
Fig. 1 Schematic of de-correlated 4 rail PDM RZ-16QAM transmitter.
. We combine 250 lasers onto a 20 GHz frequency grid using two separate rails for odd and even channels. We add 4 additional external cavity lasers (ECL) for each rail which are tuned to 8 contiguous channels that replace coinciding lasers during the bit error ratio (BER) measurements. The bit pattern for the drive signal of the modulators is generated offline using digital signal processing (DSP) where the input information bit-stream (a truncated 218-1 PRBS) is demultiplexed (serial to parallel) into seven data streams that are independently encoded by seven identical LDPC encoders with rate 0.93 [6

6. H. Zhang, J.-X. Cai, H. G. Batshon, C. R. Davidson, Y. Sun, M. Mazurczyk, D. G. Foursa, A. Pilipetskii, G. Mohs, and N. S. Bergano, “16QAM transmission with 5.2 b/s/Hz spectral efficiency over transoceanic distance,” Opt. Express 20(11), 11688–11693 (2012). [CrossRef]

]. The LDPC code used in this setup is of codeword length 32,000, girth 8 and column weight 4. The encoded bit streams are then multiplexed, interleaved and forwarded to a 7/8 rate SPC encoder and the resulting data is mapped onto the 16QAM constellation four bits at a time using Gray mapping. We use the encoded data to program a 4 channel pulse pattern generator (PPG) that drives our optical I/Q modulators at 16 GBd. To create the 4 level in phase (I) and quadrature (Q) drive signals, we combine the four outputs of the PPG two at a time with 6 dB power difference between the most significant bit and least significant bit using passive power combiners [7

7. P. J. Winzer, A. H. Gnauck, S. Chandrasekhar, S. Draving, J. Evangelista, and B. Zhu, “Generation and 1,200-km transmission of 448-Gb/s ETDM 56-Gbaud PDM 16QAM using a single I/Q modulator,” in Proceedings of ECOC 2010, ECOC 2010, (19–23 Sept. 2010), PD2.2.

]. No DAC or digital spectral shaping is used in our experiment. The drive signal for the second rail is generated in a similar fashion using the four inverted outputs of the PPG.

Each rail further comprises a pulse carving stage (RZ) and a polarization division multiplexing (PDM) stage where we split the signal into two equal parts, delay one part with respect to the other by ~100 symbols and recombine the two parts with orthogonal polarization using a polarization beam combiner (PBC) to create 128 Gb/s channels with 23% overhead and a net data rate of 104.16 Gb/s. To emulate four independent rails, we also de-correlate the nearest neighbors on both the odd and even channels using back-to-back 40 GHz optical interleaving filters (OIF) with a fiber delay. A 20 GHz OIF is added to each rail for pre-filtering and both rails are then combined with a third 20 GHz OIF. We transmit 250 PDM RZ-16QAM channels at all times during our transmission experiments.

The 16QAM receiver structure is shown in Fig. 2(a)
Fig. 2 (a) Circulating loop test bed, 128 Gbit/s PDM RZ-16QAM receiver, (b) receiver DSP schematic.
as part of the loop setup schematic. The channels are first demultiplexed by a tunable optical band pass filter and by double passing through a 20 GHz OIF before the selected channel is mixed with a local oscillator in a polarization diversity 90° optical hybrid. The signals from the four balanced photo detectors connected to the optical hybrid are sampled at 50 GHz using a digital oscilloscope with 16 GHz analog BW. There are ~8 million bits used for the BER calculation from each data acquisition.

Figure 2 also shows a schematic of our circulating loop testbed. The transmission path consists of ten low loss 50-km spans (8.5 dB) with large effective area fiber (~132 μm2) and single stage C band EDFAs. The EDFAs are equalized to 40 nm BW and operate at 18 dBm output power which corresponds to an average power per channel of −6 dBm launched into the transmission fiber. We configure the 10 spans into a 503 km transmission loop that includes a gain equalization filter to correct residual gain error and a loop synchronous polarization controller (LSPC) to properly account for polarization dependent loss (PDL) and polarization mode dispersion (PMD) in the loop. The average fiber dispersion is 20.7 ps/nm/km at 1550 nm and the average differential group delay of the loop is ~1.5 ps.

3. Transmission results

Figure 3
Fig. 3 Performance vs. transmitter pre emphasis after 5,530 km along with noise loaded back to back of our 16QAM setup at 5.2 b/s/Hz SE.
shows performance vs. transmitter pre-emphasis curves at 1529.6 nm, 1547.4 nm and 1560.1 nm after 5,530 km transmission along with the noise loaded back to back performance of our 16QAM setup at 5.2 b/s/Hz SE. We achieve a minimum required OSNR of 14.6 dB at the FEC threshold which corresponds to an implementation penalty of 1.4 dB compared to the single channel theoretical limit. We change the pre-emphasis by varying the power of a group of 8 contiguous channels and plot the performance of the channel at the center of the group vs. received OSNR. Zero dB pre-emphasis (flat launch) corresponds to the nominal operating point of the loop with 17.3 dB average OSNR across all channels.

Similar to what is commonly observed in transmission using EDFAs with >30 nm BW, the short wavelength region is more noisy [11

11. C. R. Davidson, C. Chen, M. Nissov, A. Pilipetskii, N. Ramanujam, H. Kidorf, B. Pedersen, M. Mills, C. Lin, I. Hayee, J.-X. Cai, A. Puc, P. Corbett, R. Menges, H. Li, A. Elyamani, C. Rivers, and N. Bergano, “1800 Gb/s transmission of one hundred and eighty 10 Gb/s WDM channels over 7,000 km using the full EDFA C–band,” in Proceedings of OFC 2000, PD25.

] for several reasons. First, the EDFA noise figure is wavelength dependent and higher in the short wavelength region for single stage EDFA. Second, the fiber loss exhibits a parabolic shape with minimum loss near 1575 nm resulting in higher loss in shorter wavelength region. Third, the Raman effect becomes significant at wide BW increasing the apparent loss in the short wavelength region even further. Therefore, to achieve a uniform OSNR over the wavelength range, more signal power is required in the short wavelength region and the short wavelength channels reach their nonlinear limit at lower OSNR. Therefore, the OSNR at the nonlinear limit in the short wavelength region is typically 1-2 dB lower as shown in Fig. 3. Furthermore, we observe a residual offset from the back to back curve at low transmitter pre emphasis. We believe that this offset is caused by broadband nonlinear interaction of the measurement channel with the rest of the 40 nm amplifier BW [12

12. P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical Modeling of Nonlinear Propagation in Uncompensated Optical Transmission Links,” IEEE Photon. Technol. Lett. 23(11), 742–744 (2011). [CrossRef]

]. The magnitude of this impairment depends on the amplifier BW, amplifier output power, and fiber characteristics. All above makes it difficult to extrapolate high SE transmission results obtained with narrow amplifier BW [3

3. S. Zhang, M. Huang, F. Yaman, E. Mateo, D. Qian, Y. Zhang, L. Xu, Y. Shao, I. Djordjevic, T. Wang, Y. Inada, T. Inoue, T. Ogata, and Y. Aoki, “40×117.6 Gb/s PDM-16QAM OFDM transmission over 10,181 km with soft-decision LDPC coding and nonlinearity compensation,” in Proceedings of OFC/NFOEC2012, (4–8 March 2012), PDP5C.4.

] to the full C band.

Figure 4
Fig. 4 Transmission performance for three channels at nominal power. The inset shows the recovered constellation for Ch125 after 5,530 km.
shows the mean performance for the same three channels as a function of distance at nominal power. The FEC limit is reached at the longest distance (7,500 km) for the longest wavelength channel measured (1560.1 nm). At this point the operating OSNR for this channel is ~1.5 dB lower than optimum (see Fig. 3). For full capacity measurement we choose a distance of 5,530 km to allocate some margin for Q fluctuations due to PDL. The recovered 16QAM constellation for Ch125 after 5,530 km is shown in the inset of Fig. 4.

Figure 5
Fig. 5 Received OSNR (in 0.1nm RBW) and optical spectrum after 5,530 km.
shows the received OSNR (1 dB/div) and optical spectrum (5 dB/div) for 250 channels after 5,530 km transmission with flat launch (no transmitter pre-emphasis). In our experiment we purposefully tilt the gain of the test bed to achieve nearly equalized OSNR and Q-factor across the band. The received OSNR is constant within ± 0.5 dB at an average of 17.3 dB. As seen in Fig. 5, the optical power of short wavelength region is higher in order to achieve similar OSNR as in long wavelength region.

The result of the full loading experiment (again with no transmitter pre-emphasis) is shown in Fig. 6
Fig. 6 Performance at 5.2 b/s/Hz after 5,530 km.
. For each channel we report the polarization averaged BER converted to Q factor obtained from ten data acquisitions. This corresponds to ~80 million bits processed for each channel and more than 20 billion bits processed in total. An additional advantage of our SPC code is making our 16QAM receiver algorithms tolerant to cycle slips and no cycle slips were detected in all of the data. The average Q-factor of all 250 channels is ~6.5 dB with individual channels ranging from 6.2 dB to 6.8 dB. The whiskers show the best and worst recorded Q-factor out of the ten data sets for each channel and polarization to give an indication of performance variations with PDL. The lowest performance data set was measured with a Q-factor of ~5.9 dB.

All data sets are further processed with our FEC decoder and decoded with no errors within 3 outer iterations as shown in Fig. 7
Fig. 7 BER after different iterations showing no errors for all channels within 3 iterations.
. The total number of bits processed experimentally (20 billion) supports our expectation of no error flaring to at least BER = 5x10−11.

4. Optimum power spectral density

5. Conclusions

References and links

1.

A. Sano, T. Kobayashi, S. Yamanaka, A. Matsuura, H. Kawakami, Y. Miyamoto, K. Ishihara, and H. Masuda, “102.3-Tb/s (224 x 548-Gb/s) C- and extended L-band all-Raman transmission over 240 km using PDM-64QAM single carrier FDM with digital pilot tone,” in Proceedings of OFC/NFOEC2012, (4–8 March 2012), PDP5C.3.

2.

J.-X. Cai, Y. Cai, C. R. Davidson, A. Lucero, H. Zhang, D. G. Foursa, O. V. Sinkin, W. W. Patterson, A. Pilipetskii, G. Mohs, and N. S. Bergano, “20 Tbit/s capacity transmission over 6,860 km,” in Proceedings of OFC/NFOEC2011, (6–10 March 2011), PDPB4.

3.

S. Zhang, M. Huang, F. Yaman, E. Mateo, D. Qian, Y. Zhang, L. Xu, Y. Shao, I. Djordjevic, T. Wang, Y. Inada, T. Inoue, T. Ogata, and Y. Aoki, “40×117.6 Gb/s PDM-16QAM OFDM transmission over 10,181 km with soft-decision LDPC coding and nonlinearity compensation,” in Proceedings of OFC/NFOEC2012, (4–8 March 2012), PDP5C.4.

4.

M. Mazurczyk, D. G. Foursa, H. G. Batshon, H. Zhang, C. R. Davidson, J.-X. Cai, A. Pilipetskii, G. Mohs, and N. S. Bergano, “30 Tb/s transmission over 6,630 km using 16QAM signals at 6.1 b/s/Hz spectral efficiency,” in Proceedings of ECOC 2012, ECOC 2012, (16–20 Sept. 2012), Th.3.C.2.

5.

O. Sinkin, J.-X. Cai, D. Foursa, H. Zhang, A. Pilipetskii, G. Mohs, and N. S. Bergano, “Scaling of nonlinear impairments in dispersion uncompensated long–haul transmission,” in Proceedings of OFC/NFOEC2012, OTu1A.2.

6.

H. Zhang, J.-X. Cai, H. G. Batshon, C. R. Davidson, Y. Sun, M. Mazurczyk, D. G. Foursa, A. Pilipetskii, G. Mohs, and N. S. Bergano, “16QAM transmission with 5.2 b/s/Hz spectral efficiency over transoceanic distance,” Opt. Express 20(11), 11688–11693 (2012). [CrossRef]

7.

P. J. Winzer, A. H. Gnauck, S. Chandrasekhar, S. Draving, J. Evangelista, and B. Zhu, “Generation and 1,200-km transmission of 448-Gb/s ETDM 56-Gbaud PDM 16QAM using a single I/Q modulator,” in Proceedings of ECOC 2010, ECOC 2010, (19–23 Sept. 2010), PD2.2.

8.

I. B. Djordjevic, M. Cvijetic, L. Xu, and T. Wang, “Proposal for beyond 100 Gb/s optical transmission based on bit-interleaved LDPC-coded modulation,” IEEE Photon. Technol. Lett. 19(12), 874–876 (2007). [CrossRef]

9.

H. G. Batshon, I. B. Djordjevic, L. Xu, and T. Wang, “Multi-dimensional LDPC-coded modulation for high-speed optical communication systems,” in Proceedings of IEEE Photonics Society Summer Topicals2009, (20–22 July 2009), WC1.3.

10.

I. B. Djordjevic, L. Xu, and T. Wang, “On the reverse concatenated coded-modulation for ultra-high-speed optical transport,” in Proceedings of OFC/NFOEC2011, (6–10 March 2011), OWF3.

11.

C. R. Davidson, C. Chen, M. Nissov, A. Pilipetskii, N. Ramanujam, H. Kidorf, B. Pedersen, M. Mills, C. Lin, I. Hayee, J.-X. Cai, A. Puc, P. Corbett, R. Menges, H. Li, A. Elyamani, C. Rivers, and N. Bergano, “1800 Gb/s transmission of one hundred and eighty 10 Gb/s WDM channels over 7,000 km using the full EDFA C–band,” in Proceedings of OFC 2000, PD25.

12.

P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical Modeling of Nonlinear Propagation in Uncompensated Optical Transmission Links,” IEEE Photon. Technol. Lett. 23(11), 742–744 (2011). [CrossRef]

13.

G. Bosco, P. Poggiolini, A. Carena, V. Curri, and F. Forghieri, “Analytical results on channel capacity in uncompensated optical links with coherent detection,” in Proceedings of ECOC 2011, ECOC 2011, (18–22 Sept. 2012), We.7.B.3.

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Transmission Systems and Network Elements

History
Original Manuscript: October 4, 2012
Manuscript Accepted: October 30, 2012
Published: January 15, 2013

Virtual Issues
European Conference on Optical Communication 2012 (2012) Optics Express

Citation
J.-X. Cai, H. G. Batshon, H. Zhang, C. R. Davidson, Y. Sun, M. Mazurczyk, D. G. Foursa, O. Sinkin, A. Pilipetskii, G. Mohs, and Neal S. Bergano, "25 Tb/s transmission over 5,530 km using 16QAM at 5.2 b/s/Hz spectral efficiency," Opt. Express 21, 1555-1560 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-2-1555


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References

  1. A. Sano, T. Kobayashi, S. Yamanaka, A. Matsuura, H. Kawakami, Y. Miyamoto, K. Ishihara, and H. Masuda, “102.3-Tb/s (224 x 548-Gb/s) C- and extended L-band all-Raman transmission over 240 km using PDM-64QAM single carrier FDM with digital pilot tone,” in Proceedings of OFC/NFOEC2012, (4–8 March 2012), PDP5C.3.
  2. J.-X. Cai, Y. Cai, C. R. Davidson, A. Lucero, H. Zhang, D. G. Foursa, O. V. Sinkin, W. W. Patterson, A. Pilipetskii, G. Mohs, and N. S. Bergano, “20 Tbit/s capacity transmission over 6,860 km,” in Proceedings of OFC/NFOEC2011, (6–10 March 2011), PDPB4.
  3. S. Zhang, M. Huang, F. Yaman, E. Mateo, D. Qian, Y. Zhang, L. Xu, Y. Shao, I. Djordjevic, T. Wang, Y. Inada, T. Inoue, T. Ogata, and Y. Aoki, “40×117.6 Gb/s PDM-16QAM OFDM transmission over 10,181 km with soft-decision LDPC coding and nonlinearity compensation,” in Proceedings of OFC/NFOEC2012, (4–8 March 2012), PDP5C.4.
  4. M. Mazurczyk, D. G. Foursa, H. G. Batshon, H. Zhang, C. R. Davidson, J.-X. Cai, A. Pilipetskii, G. Mohs, and N. S. Bergano, “30 Tb/s transmission over 6,630 km using 16QAM signals at 6.1 b/s/Hz spectral efficiency,” in Proceedings of ECOC 2012, ECOC 2012, (16–20 Sept. 2012), Th.3.C.2.
  5. O. Sinkin, J.-X. Cai, D. Foursa, H. Zhang, A. Pilipetskii, G. Mohs, and N. S. Bergano, “Scaling of nonlinear impairments in dispersion uncompensated long–haul transmission,” in Proceedings of OFC/NFOEC2012, OTu1A.2.
  6. H. Zhang, J.-X. Cai, H. G. Batshon, C. R. Davidson, Y. Sun, M. Mazurczyk, D. G. Foursa, A. Pilipetskii, G. Mohs, and N. S. Bergano, “16QAM transmission with 5.2 b/s/Hz spectral efficiency over transoceanic distance,” Opt. Express20(11), 11688–11693 (2012). [CrossRef]
  7. P. J. Winzer, A. H. Gnauck, S. Chandrasekhar, S. Draving, J. Evangelista, and B. Zhu, “Generation and 1,200-km transmission of 448-Gb/s ETDM 56-Gbaud PDM 16QAM using a single I/Q modulator,” in Proceedings of ECOC 2010, ECOC 2010, (19–23 Sept. 2010), PD2.2.
  8. I. B. Djordjevic, M. Cvijetic, L. Xu, and T. Wang, “Proposal for beyond 100 Gb/s optical transmission based on bit-interleaved LDPC-coded modulation,” IEEE Photon. Technol. Lett.19(12), 874–876 (2007). [CrossRef]
  9. H. G. Batshon, I. B. Djordjevic, L. Xu, and T. Wang, “Multi-dimensional LDPC-coded modulation for high-speed optical communication systems,” in Proceedings of IEEE Photonics Society Summer Topicals2009, (20–22 July 2009), WC1.3.
  10. I. B. Djordjevic, L. Xu, and T. Wang, “On the reverse concatenated coded-modulation for ultra-high-speed optical transport,” in Proceedings of OFC/NFOEC2011, (6–10 March 2011), OWF3.
  11. C. R. Davidson, C. Chen, M. Nissov, A. Pilipetskii, N. Ramanujam, H. Kidorf, B. Pedersen, M. Mills, C. Lin, I. Hayee, J.-X. Cai, A. Puc, P. Corbett, R. Menges, H. Li, A. Elyamani, C. Rivers, and N. Bergano, “1800 Gb/s transmission of one hundred and eighty 10 Gb/s WDM channels over 7,000 km using the full EDFA C–band,” in Proceedings of OFC 2000, PD25.
  12. P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical Modeling of Nonlinear Propagation in Uncompensated Optical Transmission Links,” IEEE Photon. Technol. Lett.23(11), 742–744 (2011). [CrossRef]
  13. G. Bosco, P. Poggiolini, A. Carena, V. Curri, and F. Forghieri, “Analytical results on channel capacity in uncompensated optical links with coherent detection,” in Proceedings of ECOC 2011, ECOC 2011, (18–22 Sept. 2012), We.7.B.3.

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