We present a colorless network-embedded self-tuning transmitter assisted by a remotely pumped erbium-doped double-pass amplifier located at the remote node for a conventional hybrid stacked-WDM/TDM-PON. The scheme provides up to 256-split PON with 80-Gb/s aggregate upstream capacity obtained with RSOA direct modulation at 2.5 Gb/s.
© 2013 OSA
One of the main concerns for next generation access networks (NGANs) is migration and coexistence with pre-existing network infrastructure . Wavelength division multiplexed passive optical networks (WDM PON) present some issues in this regard . On the contrary the hybrid approach between time division multiplexed (TDM) and WDM PON, which uses a combination of arrayed waveguide grating (AWG) and power splitters, accommodates several conventional PON systems and satisfies different users needs, as shown in the topology of Fig. 1(a) . Moreover as the hybrid approach exploits the efficient TDM PON resources utilization, it allows channel sharing among multiple ONUs, and provides a solution to the asymmetry of channels usage due to users behaviour non-uniformity , while WDM PON , under non-uniform load condition, saturates channels that serve heavy users and underutilizes channels dedicated to light users.
Recently it has been experimentally demonstrated an unconventional hybrid TDM/WDM PON using self-seeded reflective semiconductor optical amplifiers (RSOA), providing a 512-split PON with 20-Gb/s aggregated upstream capacity  achieved by overlaying multiple WDM PONs on a legacy optical distribution network deployed for TDM PONs, as in Fig. 1(b). This unconventional scheme takes advantage of the properties of the RSOA-based self-seeded transmitter scheme, which is self-tuning, and thus colorless, and immune to the distributed backscatterings , which limit the ultimate bridgeable distance when a single feeder fibre is exploited in remotely-seeded solutions . The unconventional hybrid topology is motivated by the fact that the self-seeded transmitter heavily suffer from cavity insertion losses , thus power splitting for TDM overlay is performed after wavelength demultiplexing (in upstream direction).
In this paper we present for the first time to the best of our knowledge the experimental analysis of a colorless network-embedded self-tuning transmitter for a conventional hybrid stacked-WDM/TDM-PON . To overcome the additional cavity losses due to power splitters the optical network unit (ONU) transmitter is assisted by a remotely-pumped optical amplifier, placed in the completely passive remote node (RN) and shared between all the users . The pump is placed at the Central Office (CO), thus preserving the passive nature of the RN and in this sense differs greatly from solutions proposing Erbium doped fibre amplifiers (EDFA) at the remote node . Though remote amplification has been already proposed for extending the reach of PON, the remotely pumped EDFA is here employed in a double pass configuration due to the properties of the self-seeded source. The discussed scheme sustains a 256-split PON with 80-Gb/s aggregate upstream (US) capacity.
In Section I the transmitter topology adopted to allow for the exploitation of the remotely-pumped EDF is described and the choice of the doped fibre length is motivated trough a numerical analysis. In Section II the complete experimental set up is presented with particular attention to cavity active devices. The experimental measurements relative to the demonstration of operation at an aggregate capacity up to 80 Gb/s are presented in Section III with a feeder fibre link of 25 km of standard single mode fibre (SSMF). Conclusions are finally drawn evidencing limits and possible improvements of the proposed stacked-WDM/TDM-PON topology.
2. Double pass EDFA
As already stated the self-tuning cavity (STC) performance depend on the cavity losses, the exploitation of a power splitter inside the ONU transmitter to implement Fig. 1(a) conventional hybrid stacked-WDM/TDM-PON drastically raises the cavity losses of an amount equivalent to the doubled splitter rate. It is thus mandatory to overcome them with assisted amplification. In order to maintain the passive nature of the RN, a remotely pumped EDFA has been chosen. The STC topology  must be modified to integrate the remotely pumped EDFA in the RN, the final scheme is presented in Fig. 2. The pump source, which is located at the central office (CO), after propagation in the feeder fibre, is coupled with the EDF by two 1550-1480 nm WDM couplers: the first one, WDM-1, also routes the DS signal to the RN cyclic AWG via the STC coupler output branch. The second one (WDM-2) common port is conversely connected to the EDF, while its 1550-nm port allows the US cavity signal to be coupled to the remotely pumped EDFA. Both the cavity signal and the pump are reflected by the RN cavity mirror, which can be for instance a Faraday Rotator Mirror (FRM) to overcome the polarization issue in a cavity exploiting a low polarization dependent gain (PDG) RSOA . The realized amplifier is thus a double-pass EDFA.
In order to define the proper length of EDF the remotely pumped amplifier was simulated exploiting the classical rate-equation model, as presented for instance in . The EDF parameter were obtained through measurements and Erbium doping concentration estimated in 5·1024 ions/m3. In order to determine the appropriate length, N1 and N2 population densities respectively of the 4I15/2 and 4I13/2 levels have been plotted in Fig. 3(a) as a function of the doped fibre length, for a pump power of 13 dBm. This value of pump power has been estimated as the reasonably available after the feeder fibre losses. The two populations intersect at nearly 20 m, which represents the length after which the EDF does not provide further gain, thus it has been chosen as the proper length for the double pass EDFA .
For the 20-m length the amplifier gain has been numerically evaluated as a function of the effective pump power; in Fig. 3(b) the simulations results are plotted (continuous blue line) together with the experimental results (red squares) obtained in double pass EDFA characterization. The good agreement found proves the correctness of both experimentally extracted parameters and of the exploited model for the RN remotely pumped EDFA.
3. Experimental set up
Figure 4 shows the experimented upstream transmitter based on the RSOA self-tuning cavity assisted by a remotely pumped EDF amplifier together with the whole experimental setup. The cavity is delimited by two mirrors: one belonging to the RSOA located at the ONU and one FRM located at the RN. The transmitter remaining passive part is embedded in the network itself: the WDM C/L coupler for US and downstream (DS) separation, the distribution fibre, here 420 m of SSMF, the 32-channel 100-GHz spacing cyclic athermal AWG and the 80/20 output coupler. A power splitter between the AWG and the ONU, here emulated by a variable optical attenuator (VOA), allows for hybrid TDM/WDM PON transmitter validation. The RN topology follows the description of Section 2 in which the exploited EDF length has been 20 m. The employed pump is a Raman-fibre laser at 1486 nm. The pump source is inserted into the feeder fibre through a 1550-1480 WDM coupler.
At the ONU the active element is constituted by a low PDG RSOA, with more than 2-GHz E/O bandwidth, which can be directly modulated at 2.5 Gb/s. The RSOA Electro/Optical (E/O) response is measured for bias currents from 100 mA to 175 mA, which covers the exploited bias current range; the measurements are presented in Fig. 5(a), showing for all the bias-current range, a 3-dB bandwidth larger than 2 GHz. The RSOA spectrum at the bias current of 100 mA is presented in Fig. 5(b), as can be seen it is centered at 1580 nm, thus AWG channels lower than 20 are not within the RSOA full width half maximum (FWHM) gain bandwidth.
The exploited pump power is 25 dBm, which allows coupling an effective power in the EDF of nearly 16.6 dBm, due to fibre and coupler losses. As expected , pump propagation in the feeder fibre determines also Raman amplification in the L band, where the DS signal is spectrally located. In our set up, exploiting 25-km SSMF feeder fibre, the average net Raman gain in L band has been measured in 5 dB. The measured double-pass gain was 26 dB at 1543 nm at −10 dBm input power.
The cavity losses, including the 1550-1480 nm WDM coupler, but excluding the doped fibre, with no VOA extra losses, have been estimated in 17.4 dB. Of course the emulation of 1:8, 1:16 or 1:32 splitters adds to the cavity losses of 18 dB, 24 dB and 30 dB respectively, which need to be recovered by the remotely pumped EDFA gain.
4. Experimental results
Figure 6(a)-6(c) presents the measured back to back eye diagrams, i.e., taken at point A of Fig. 4, for channels 1, 16 and 32. They correspond to different bias points, that is respectively 155 mA, 130 mA and 102 mA; the increasing bias currents for lower number channels are consistent with the RSOA gain bandwidth: to get the necessary gain for lower channels higher currents are needed. The applied data are 4 Vpp and the relative extinction ratios (ERs) are 4.7 dB, 6.7 dB and 8 dB. The operating points have been chosen to minimize the BER and represent a trade off between desirable high ER and cavity recirculating signal cancellation . Figure 7(d)-7(f) shows the output spectra for the same channels in CW regime (red narrower spectrum) and when applying 2.5-Gb/s modulation, modulated spectra (blue line).
Table 1 shows received powers to achieve the enhanced FEC limit of 4·10−3 BER and the Reed-Solomon limit of 10−4 BER  at 2.5 Gb/s. The measurements have been performed after 25-km SSMF propagation and after OLT AWG demodulation with a 2.5-GHz APD followed by a clock and data recovery module. With VOA set to emulate 1:8 split, i.e. 9 dB attenuation, all the 32 channels reach a BER lower than the enhanced FEC limit for received power lower than 29 dBm, as it is also demonstrated by Fig. 7, where BER values for each of the 32 channels at −29 dBm received power are presented. This allows an aggregate upstream capacity of 80 Gb/s shared by 256 end-users. Table 1 first column also shows that, with reference to 10−4 BER, for channel numbers lower than 24 the necessary received power rapidly increases, consistently with the RSOA gain allocation. The same trend is confirmed by the performance with VOA emulating 1:16 and 1:32 split losses: only for the channel range 24-32 the performance reaches the Reed-Solomon or enhanced FEC limits . With these splitting ratios, which imply heavier losses, the aggregate upstream capacity is thus limited to 20-Gb/s, differently from what has been demonstrated for the 256 users (1:8 split). The results seem to indicate that with the proper RSOA gain bandwidth allocation, up to 80 Gb/s capacity could be shared by up to 1024 users.
4. Discussion and conclusion
We have demonstrated a colorless network embedded self-tuning transmitter assisted by remotely-pumped EDF amplification for a conventional stacked-WDM/TDM PON propagating over a single 25-km feeder fiber, which is bidirectionally exploited for both up- and down-stream. The proposed topology takes advantage of the characteristics of the self-seeded transmitter, while maintaining the passive nature of the network and allowing also L-band amplification for the DS. The scheme allows up to 80 Gb/s aggregate upstream capacity shared by 256 users. The number of end-users is severely limited by the available RSOA gain bandwidth and consequently could be improved. As previously evidenced , the full exploitation of the scheme requires long burst durations (100 ms to 1ms) due to the turn-on time of the network embedded transmitter.
The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement ERMES no. 288542
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