We compare the transmission performance of three different optical fibers in separate 256 Gb/s PM-16QAM systems amplified with erbium doped fiber amplifiers (EDFAs) and distributed Raman amplification. The span length in each system is 100 km. The fibers studied include standard single-mode fiber, single-mode fiber with ultra-low loss, and ultra-low loss fiber with large effective area. We find that the single-mode fiber with ultra-low loss and the large effective area fiber with ultra-low loss afford reach advantages of up to about 31% and 80%, respectively, over standard fiber measured at distances with 3 dB margin over the forward error correction (FEC) threshold. The Raman amplified systems provide about 50% reach length enhancement over the EDFA systems for all three fibers in the experimental set-up. For the best performing fiber with large effective area and ultra-low loss, the absolute reach lengths with 3 dB margin are greater than 1140 km and 1700 km for the for EDFA and Raman systems, respectively.
© 2013 OSA
As worldwide data traffic continues to grow, similar growth requirements are placed on the capacity of long-haul fiber-optic communication. This imperative has led researchers to investigate signal bit rates and spectral efficiencies beyond 100 Gb/s, 2.0 b/s/Hz in coherent optical systems. The next step up in capacity without increasing the system symbol rate may be 200 Gb/s per wavelength channel using 16-ary quadrature amplitude modulation (16QAM), which provides 4 bits/symbol per polarization. Consequently, there has been a significant level of effort recently directed towards demonstrating 200 Gb/s per channel with polarization multiplexed 16QAM (PM-16QAM) signals [1–6].
However, a primary challenge of this modulation format is its significantly reduced reach in comparison to 100 Gb/s PM-QPSK signals. The reduced reach is mainly attributable to the higher optical-signal-to-noise (OSNR) required for 16QAM signals than QPSK signals with the same symbol rate to achieve a comparable bit error ratio (BER). For ideal signals, 16QAM signals require approximately 7 dB higher OSNR than QPSK . This would lead to a reach reduction for 200 Gb/s PM-16QAM by a factor of about 5x in comparison to 100 Gb/s PM-QPSK. In practice, however, the reduction in reach may be more severe  due to larger implementation penalties associated with 16QAM transmitters. Because of the higher OSNR requirements for 200 Gb/s PM-16QAM systems, it is very important to increase system OSNR through every means available. A primary route to enhancing OSNR and thus increasing system reach is by the use of optical fiber with lower attenuation and/or larger effective area. Both attributes can serve to increase OSNR in comparison to standard fiber.
In this work, we directly compare experimentally measured transmission reach for 256 Gb/s PM-16QAM systems over three different optical fibers with both EDFA and Raman amplification. The three fibers include standard G.652-compliant single-mode fiber, G.652-compliant fiber with ultra-low loss, and a G.654-compliant fiber originally developed for submarine applications with both ultra-low loss and a larger effective area. We find that the large effective area, ultra-low loss fiber demonstrates a reach advantage over the standard single-mode fiber of up to 80%. The reach advantage of the Raman systems was about 50% over the EDFA systems for all fibers in the experimental configuration.
2. Experimental set-up
A schematic diagram of the general experimental set-up is shown in Fig. 1. 20 optical wavelengths were modulated together with a 32 Gbaud 16QAM transmitter. The channel under test was encoded on an external cavity laser (ECL) with 100 kHz linewidth, and the other 19 channels were encoded on DFB lasers. The channel spacing was 50 GHz. The channels were polarization multiplexed and then passed through a short piece of fiber (about 8 km, 160 ps/nm dispersion) to de-correlate adjacent channels by > 2 symbols before being launched into a re-circulating loop built with the given fiber under test (FUT). The loop transmission medium was comprised of three 100 km spans of fiber with either a single-stage EDFA or backward-pumped Raman amplifier at the end of each span. In the case of the Raman amplifiers, the pump currents were adjusted to provide transparency, i.e. Raman gain sufficient to compensate for the preceding span loss. Three pump wavelengths at 1427 nm, 1443 nm, and 1462 nm were used. The remainder of the loop included a loop synchronous polarization scrambler, a gain equalizing filter, and another EDFA to compensate for the loop element loss.
Channels were selected for detection in a polarization- and phase-diverse digital coherent receiver with a free-running local oscillator with 100 kHz linewidth. The four signals from the balanced photodetectors were digitized by analog-to-digital converters operating at 50 Gsamples/s using a real-time sampling oscilloscope with 20 GHz electrical bandwidth. BER values were calculated via direct error counting over more than 3 million bits with offline digital signal processing steps including quadrature imbalance compensation, up-sampling to 64 Gsamples/s, chromatic dispersion compensation using a frequency-domain equalizer, digital square-and-filter clock recovery, polarization demultiplexing and equalization using a 21-tap adaptive butterfly structure with filter coefficients determined by a radius-directed constant modulus algorithm (CMA)  following pre-convergence using a standard CMA, carrier frequency offset using a spectral domain algorithm, and phase recovery using a feed forward algorithm . All chromatic dispersion was compensated digitally in the offline processing.
The 16QAM optical transmitter is shown schematically in Fig. 2. Four binary 32 Gbaud signals output from a pulse pattern generator were combined in pairs with 6 dB relative attenuation difference between the signals in each pair. Relative delays of several tens of symbols served to de-correlate the individual 215-1 PRBS signals from each other, as well as the resulting pair of 4-level pulse amplitude modulation (PAM) electrical signals driving an I-Q optical modulator. The 32 Gbaud symbol rate produces an overall bit rate of 256 Gb/s for the PM-16QAM signals, with a 28% total overhead above the nominal 200 Gb/s data rate. The assumed soft-decision FEC (SD-FEC) has a raw BER threshold of 2x10−2 for a post-FEC error rate of < 10−15 .
The three fibers tested were Corning® SMF-28e + ®, SMF-28® ULL, and Vascade® EX2000 optical fibers. The attenuation values for these 3 fibers were 0.191 dB/km, 0.166 dB/km, and 0.161 dB/km, respectively. The fiber effective areas were 82 μm2, 82 μm2, and 112 μm2, respectively. The nominal chromatic dispersion at 1550 nm is about 16.5 ps/nm/km for SMF-28e + ® fiber, 16 ps/nm/km for SMF-28® ULL fiber, and about 20 ps/nm/km for Vascade® EX2000 fiber.
3. Experimental transmission results
A single-channel back-to-back OSNR sensitivity characterization of the PM-16QAM transmitter and receiver was first performed before transmission tests over fiber. The results are shown in Fig. 3. As compared to an ideal transmitter and receiver pair, the experimental implementation penalty was about 3.2 dB at a BER value of 1 x 10−3.
Transmission experiments over the three different optical fibers were first performed for the EDFA systems. The EDFA at the end of each of the first two spans was a single-stage EDFA. The EDFA at the end of the third span was a 2-stage pre-amp and booster with the loop synchronous polarization scrambler and tunable gain equalization filter placed in the mid-stage. The same EDFAs were used for all 3 fiber systems. For each fiber tested, we first determined the optimal launch power into the spans by measuring BER as a function of channel power at the distance of 600 km (2 loop circulations). The results for the channel power optimization for all three fibers are shown in Fig. 4(a). The optimal channel power was about 0 dBm for both SMF-28e + ® and Vascade® EX2000 fibers, while it was about −1 dBm for SMF-28® ULL fiber. The optimal launch power is determined by both the fiber effective area as well as the fiber attenuation.
With the optimal launch power set for each fiber system, we then measured the BER of the 1550.92 nm channel in the center of the 20 channel system as a function of transmission distance. The results of these measurements are shown in Fig. 4(b) in terms of 20log(Q) in dB as a function of distance. Q is calculated by Q = √2erfc−1(2∙BER). Figure 4(b) also shows two dashed lines corresponding to Q values of 6.25 dB (the SD-FEC threshold), and 9.25 dB (3 dB margin above the threshold). The longest transmission reach is clearly obtained with the ultra-low loss and large effective area Vascade EX2000 fiber. All 20 channels were measured with Q values above the FEC threshold at 2700 km with this fiber. The spread of the Q values over all 20 channels is represented by the error bars in Q at this distance. Similarly, all 20 channels were also measured for the other two fibers at the maximum loop distances supported and the Q spread in each case is represented by error bars. For all three fibers, the OSNR of the measurement channel for the distance with Q closest to the SD-FEC threshold was about 22 dB.
Experimental transmission set-ups often include a separate modulator for odd and even channels as a means of ensuring that adjacent channels have de-correlated data streams. Our set-up used one modulator for all channels due to hardware limitations but included a piece of optical fiber on the transmitter side which served to de-correlate adjacent channels by a few symbols prior to launch into the re-circulating loop via chromatic dispersion. To verify this arrangement’s effectiveness, we also measured the BER of the central 1550.92 nm measurement channel with the two adjacent channels turned off as a function of distance to compare to the data with the adjacent channels on. An example of the results of this comparison for the SMF-28 ULL fiber are shown in Fig. 5 in which Q is plotted vs. OSNR, with each data point representing a different distance. The nearly negligible difference between the cases with adjacent channels on and off suggests that the single modulator used in the transmitter does not significantly affect the nonlinear behavior of the system with all dispersion compensation performed digitally in the receiver.
The transmission results of the EDFA systems are summarized in Figs. 6(a) and 6(b) as the reach length for each fiber for varying levels of Q margin above the SD-FEC threshold. The reach was determined for the 1550.92 nm channel by interpolation of the measured data points taken for various loop circulations. The reach lengths in km are given in Fig. 6(a) while Fig. 6(b) shows the reach values normalized to that of the standard single-mode fiber. At the important 3 dB margin level, the Vascade EX2000 fiber and the SMF-28 ULL fiber have reach advantages over the standard single-mode fiber of 80% and 31%, respectively. These relative results for SMF-28 ULL fiber compared to standard single-mode fiber are consistent with earlier results obtained for 40 Gb/s non-coherent transmission systems .
Systems using these same three optical fibers amplified by backward Raman pumping were investigated next. For these systems, the Raman pump powers were adjusted in each case to fully compensate for the loss of the preceding span and the discrete loss of the Raman module. The estimated total Raman pump power per span needed was about 850, 750, and 935 mW for the SMF-28e + , SMF-28 ULL, and Vascade EX2000 fibers, respectively. The booster EDFA was used at the end of the re-circulating loop to compensate for the loss of the loop elements. As before, optimal channel launch powers were first determined, this time at 900 km distance, and then the BER values of the center 1550.92 nm channel within the 20 channel system were measured at the optimal channel power as a function of transmission distance for all fiber systems. The results from these two sets of measurements are shown in Figs. 7(a) and 7(b), respectively. The optimal channel powers were 2.5-3 dB lower for each fiber compared to the EDFA systems. The error bars in Fig. 7(b) again represent the spread in Q values measured over all 20 channels at the distances at which all channels were above the SD-FEC threshold. For the Raman systems, these distances were 2100 km, 2700 km, and 3600 km for SMF-28e + , SMF-28 ULL, and Vascade EX2000, respectively. At the SD-FEC threshold, the OSNR values of the measurement channel were about 0.3-0.7 dB higher than in the EDFA systems, perhaps reflecting small penalties from double Rayleigh back-scattering (DRBS) for the Raman systems. The system constructed with Vascade EX2000 had the smallest such penalty due to the fiber’s larger effective area and smaller required gain.
The Raman amplified systems data are summarized in Figs. 8(a) and 8(b). In Fig. 8(a), the absolute reach lengths of the fibers at different Q margins are given, while in Fig. 8(b), the reach lengths are normalized to that of the standard single-mode fiber. With 3 dB Q margin, the reach lengths of the Vascade EX2000, SMF-28 ULL, and SMF-28e + fiber systems were 1700 km, 1240 km, and 970 km, respectively. These results correspond to reach advantages of the two ultra-low loss fibers with 3 dB margin of 76% and 28%.
For these re-circulating loop experimental systems, the reach advantage of the Raman systems compared to the EDFA systems are about 50% for all three optical fibers at the 3 dB margin level. It is expected that this reach advantage would be greater in straight-line systems without additional loop element losses.
4. Summary and conclusions
We have studied 256 Gb/s PM-16QAM transmission over three optical fibers with two different amplification schemes in 100 km span systems. The reach lengths of each fiber with each type of amplification were compared. For EDFA systems, the reach of the standard single-mode fiber was about 630 km with 3 dB Q margin above the SD-FEC threshold, while an ultra-low loss G.652-compliant fiber reach was about 830 km, and it was about 1140 km for an ultra-low loss, large effective area G.654-compliant fiber. For Raman amplified systems, those reach values increased to about 970 km, 1240 km, and 1700 km, respectively. Raman amplification provided about a 50% reach increase for all three fibers tested. The experiments highlight the benefits of using advanced optical fibers with ultra-low attenuation and/or larger effective area, as well as Raman amplification, to achieve reach lengths with sufficient margin that may be practical and cost-effective in commercial deployments.
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