ECOC 2014, Cannes - France PD.1.4 40-Gb/s TDM-PON over 42 km with 64-way Power Split using a Binary Direct Detection Receiver (1) (1) (2) (1) Doutje van Veen , Vincent Houtsma , Alan Gnauck , Patrick Iannone (1) (2) Bell-Labs/Alcatel Lucent, Murray Hill, New Jersey, USA, firstname.lastname@example.org Bell-Labs/Alcatel-Lucent, Holmdel, New Jersey, USA Abstract We demonstrate a 40-Gbps TDM-PON over a 42-km, 64-split fiber plant using optical duobinary modulation. Experimental results show that our architecture supports 31 dB of power budget for a differential reach of 26 km at 1550 nm without DSP. Introduction Until now, all generations of commercially deployed passive optical networks (PONs), whether standardized by ITU-T or IEEE, have been bi-directional, power-splitter-based lowcost TDM-TDMA systems with downstream data sharing one wavelength and the upstream data sharing another (aside from an optional downstream video enhancement wavelength). The main challenges associated with increasing the serial data-rate are achieving the needed optical power budget, the decreased dispersion tolerance and the possible increased cost of implementation at a higher rate. Therefore for the next ITU-T standard beyond the 10-Gbps TDM-PON standard, the (Full Service Access Network) FSAN group has proposed NGPON2, a time- and wavelengthdivision multiplexed (TWDM) PON that multiplies the bit rate by stacking 4 or 8 1 wavelengths with 10-Gbps each . Still, the development of a practical low-cost single-wavelength solution beyond 10-Gbps, has the potential to either supplant NGPON2 as the next-gen solution, or serve as a perwavelength upgrade path for NGPON2. We previously demonstrated a 26-Gbps TDM PON over 40 km using the limited bandwidth of a low cost commercial 10-Gbps receiver to convert 2 NRZ into 3-level duobinary , with more options for upgrading a 10-Gbps PON discussed in ref. . In this paper we explore the use of optical duobinary transmission instead of 3-level duobinary detection as an upgrade path to 40Gbps TDM PON. ODB moves some of the 40-Gb/s 215-1 PRBS 10-Gb/s Generator 4:1 40 Gb/s Mux ODB Tx 1550-nm DFB laser Transmitter 64 40-Gbps TDM-PON experimental setup The experimental setup, shown in Fig. 1, consists of an optical line terminal (OLT) connected via up to 42 km of feeder fiber to a 1:64 power splitter serving user optical network units (ONUs). Our focus here is demonstrating 40-Gb/s downstream performance, since a lower upstream rate is acceptable for most PON deployments. One viable upstream option is 10Gbps time division multiple access, as is already 5 available for 10G-EPON . The modulation scheme we applied in this 7 paper is optical duobinary modulation (ODB) which has the following advantages: A binary direct detection receiver can still be used, helping to keep the cost down in the ONU; Higher tolerance to chromatic dispersion can be achieved compared to NRZ, thus increasing the reach. Since ODB requires modulating both TIA APD-ROSA RF combiner RF splitter 10 dB Real-time two-tap FFE 40-Gb/s Amp Clock recovery Recovered clock 1:4 40 Gb/s DeMux MZM Dual drive MZM ~ 35 GHz 3-dB BW Biased at the null ONU complexity from the ONU (3-level detection) to the OLT (phase and intensity modulation needed) compared to 3-level duobinary. We successfully achieved a 40-Gbps downstream bitrate using a 25-Gbps APD-based 4 receiver with 31 dB of optical power budget, and 26 km differential reach at 1550 nm over an outside plant consisting of 42 km of SSMF and a 64-way power split. Data Data Outside Plant APD/ TIA Rcvr Fig. 1: 40-Gbps PON experimental setup LPF Data ATT OLT Data (4) Quarter-pattern shifted 215-1 PRBS ATT 1-bit delay APD 2 Various fiber lengths EDFA 15-GHz low-pass filters LPF 1 FBG-based dispersion precompensation (4) 10-Gb/s 215-1 PRBS Receiver Fig. 2: Setup details on the optical duobinary transmitter (left) and the APD-based receiver setup (right). 10-Gb/s Analyzer ECOC 2014, Cannes - France PD.1.4 -2 -17 NRZ w/o EQ ODB w/o EQ NRZ w/ EQ -3 ODB w/ EQ ODB w/ EQ -3 Log (BER) Received Power for BER= 1×10 ODB w/o EQ -4 -5 -6 -7 -8 -9 -10 -26 -24 -22 -20 -18 -16 -14 -18 NRZ w/ EQ -19 -20 -21 -22 -23 -300 -12 -200 -100 Received Power (dBm) 200 300 NRZ 10 ODB 0 NRZ ODB 0 -10 Relative Power (dB) optical amplitude and phase, a Mach Zehnder Modulator (MZM) biased at the null is used at the OLT side to modulate light from a 1550-nm DFB laser. Use of a MZM can be justified because it is a shared component so it is much less cost sensitive compared to components at the ONU side. Fig. 2 shows more detail on the transmitter and receiver setup. The MZM electrical drive signals are derived from four 10-Gbps PRBSs of 15 length 2 -1, multiplexed together to form a 4015 Gbps 2 -1 PRBS signal. The differential duobinary drive signals are generated by lowpass filtering the 40-Gbps data and inverteddata with 15-GHz low-pass filters (instead a 15GHz MZM could also be used). To avoid error propagation and to simplify the receiver circuitry when using real data, a precoder is needed at the transmitter when using duobinary modulation. In the case of a pseudorandom binary sequence (PRBS) pattern, like in our experiments it is not needed. Simultaneously satisfying the stringent link budget and reach requirements for the latest PON standards creates a tension in the choice of wavelength band between C-band (low loss and high chromatic dispersion) and O-band (higher loss and low chromatic dispersion). For our experiment we have selected the C-band, however the results can be easily translated to O-band as well, where the increased fiber loss will limit the split to 1:32 or less for the maximum-reach case. To compensate for the accumulated chromatic dispersion as a function of fiber length, we propose using Fiber Bragg Grating (FBG)-based dispersion compensation, thus enabling 42 km reach (or more) in the Cband. The FBG-based dispersion compensator is followed by an erbium doped fiber amplifier (EDFA), which is used to boost the launched optical power to +12 dBm. Naturally, an O-band implementation must utilize an SOA, which are also available with these output powers. 100 Fig. 4: Dispersion tolerance for ODB with and without the 2-tap equalization scheme, and for NRZ with 2-tap equalization, with 0 dBm launched power. Reflected Power (dBm) Fig. 3: APD/TIA Back-to-Back performance for NRZ and ODB, with and without 2-tap equalizer (EQ). 0 Dispersion (ps/nm) -10 NRZ ODB -20 -30 -40 -50 -100 -50 0 50 100 Frequency (GHz) -20 -30 0 5 10 15 20 Launch Power (dBm) Fig. 5: SBS Measurements for NRZ and ODB in 42 km of SSMF In conjunction with the transmitter’s high 4 launch power, a 25-Gbps APD-based ONU receiver was used to achieve the large optical power budget typically required in standardized PONs, at a lower cost and lower power consumption relative to an optically preamplified receiver. This 25-Gbps APD-TIA receiver optical subassembly (ROSA) was 6 developed for the 100G Ethernet standard , a technology that is expected to mature and become low cost in the near future. The receiver’s measured -3 dB bandwidth of 15 GHz is smaller than the required ~28 GHz, resulting in a receiver power penalty. To ameliorate the bandwidth limitation of the receiver a simple 2-tap equalizer was implemented, combining the differential outputs of the receiver with one output attenuated and delayed by one bit. After the equalizer, the clock was recovered and the 40-Gbps data stream was de-multiplexed to four 10-Gbps streams, each with virtually identical performance, for real-time bit-error-rate (BER) measurements. As is the ONU in the 40-Gbps TDM-PON needs to be able to process a data-rate of 40Gbps. We therefore propose to use Bit Interleaving (BI) to relax the requirements ECOC 2014, Cannes - France PD.1.4 -5 Experiments The back-to-back receiver performance is depicted in Fig. 3, where red squares and blue circles represent BER data for NRZ and ODB modulation, respectively. In both cases, the addition of the equalizer (solid symbols) improves performance by 1.3 dB or greater at a -3 BER of 1.0x10 , the raw BER corresponding to -12 a post-FEC BER of 1.0x10 . Fig. 4 uses the same symbols to plot the -3 variation in received power at a BER of 10 as a function of dispersion. Positive dispersion points were measured with various lengths of standard single-mode fiber (SSMF). Negative dispersion points were measured with a -330-ps/nm FBG concatenated with SSMF. For received powers of approximately –19 dBm or less, as required for our system, the range of dispersion (and therefore maximum reach) for equalized optical duobinary exceeds that for NRZ modulation by over a factor of 2. Fig. 5 plots backscattered power as a function of launch power for both NRZ and duobinary modulation in a 42-km spool of SSMF. The linear shape of the duobinary curve (blue circles) indicates that Rayleigh backscattering is the dominant process, with the total backscattered power approximately 30 dB below the launch power for launch powers up to 20 dBm. The red squares show that the onset of stimulated Brillion scattering (SBS) limits the maximum launch power for NRZ modulation to approximately 10 dBm. This launch power advantage of ODB relative to NRZ is due to the fact that the ODB spectrum does not have a carrier, as seen in the inset in Fig. 5. Fig. 6 shows the 1:64 PON transmission results with +12 dBm of optical launch power, which was found to be the optimum power for maximum margin at 42.4 km (fiber nonlinearity limited). From this it can be seen that we have a -3 31-dB power budget at a BER of 1.0x10 . We demonstrate two different reaches (0-26.9 km or 15.7-42.4 km) by adjusting the dispersion precompensation: The -330-ps/nm FBG was concatenated with either a length of SSMF or dispersion-compensating fiber to obtain the required precompensation values, -224 ps/nm and -488 ps/nm. Obviously, an FBG could be Open Symbols: -224 ps/nm Precompensation Solid Symbols: -488 ps/nm Precompensation Maximum Received Power RX Sensitivity BER=1.0×10-3 -10 Power (dBm) needed to process the user data. The bit 8 interleaving PON scheme can be used to reduce the bitrate that an ONU needs to handle down to a lower rate, for example to 10-Gbps which enables the use of commercially available 10G PON electronic ONU parts, thus minimizing cost and also enabling a lower power implementation. -15 Margin -20 -25 0 10 20 30 40 50 SSMF Length (km) Fig. 6: Transmission results at 1550 nm over the 1:64 PON for two values of dispersion precompensation. designed for a certain required dispersion value. Conclusions We successfully demonstrated a 40-Gbps TDMPON over a 42-km, 64-split fiber plant. Experimental results show that our proposed system supports 31 dB of power budget, for a -3 BER of 1.0x10 with a differential reach of 26 km at 1550 nm without any form of digital signal processing (DSP) using a low cost 25-Gbps APD/TIA receiver. We used a booster EDFA to launch up to +12 dBm into the fiber and a fiber Bragg grating to offset dispersion in the outside fiber plant for operation in the C-band. The demonstrated dispersion tolerance of more than 200 ps/nm also enables operation over a large window in the O-band (about 1275 1355 nm) over at least 0 - 40 km SSMF without needing to offset dispersion with the FBG. Acknowledgements The authors thank Peter Vetter and Ed Harstead for enlightening discussions and Atul Srivastava and the NTT Electronics Corporate team for their technical contribution. 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