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This paper describe a truly-passive coexistence of 10G-PON and GPON compatible reach extension system with a novel optical configuration, by using laser pumps to provide reverse-pumped distributed Raman gain for both 1270nm 10G-PON and 1310nm GPON upstream (US) signals, and using semiconductor optical amplifiers (SOA) as boosters to improve the loss budgets for both 1577nm 10G-PON and 1490nm GPON downstream (DS) signals. The Raman interaction between laser pumps and the two US signals is investigated, and the system transmission penalties of US signals due to Raman ASE noises is measured. The transmission impairments of 1490nm DS signals due to pattern-dependent distortion caused by gain dynamics of the SOA is discussed in this paper. Finally, we present experimental demonstration of coexisting 10G-PON and GPON bi-directional transmission over 50-km of AllWaveTM fiber with entirely passive fiber plant and a total 1:96 split, accommodating link loss budget more than 39-dB for both 10G-PON and GPON US signals.
©2012 Optical Society of America
1. Introduction
The advantages of deploying cost effective passive optical networks (PONs) have been largely recognized worldwide. PON are now being deployed in large numbers and will play an increasingly important role in future access networks. The maximum reach of a typical deployed gigabit PON (GPON) today is 20-km or less with 1:32 split, limited by the 28-dB loss budget for Class B + system (G.984.2 Amd2) [1]. Over the last several years, network operators have shown a great deal of interest in GPON reach extension to longer than 20-km and split ratios larger than 32 [2–4]. GPON reach extension has also been standardized by the ITU-T (G.984.6) [5]. However, the reach extension approaches considered in G.984.6 require the use of electrically powered units in the field containing optical amplifiers or optical-electrical-optical repeaters; this poses disadvantages in a PON system and may not be cost effective, particularly in the environments where there is no electrical powering (for example, in rural areas). Truly passive reach extension techniques would be much more attractive for operators. It has recently been proposed that distributed Raman amplification as a technique could improve the GPON loss budgets and extend the reach without the need of electrical powering in the field [6–9]. An economic study by network operators has shown that this technique offers great savings in operational and capital costs [10].
Furthermore, the future access network will require increased bit rates up to 10Gbit/s in order to satisfy the ever-increasing traffic demands, and 10-Gigabit PON (XG-PON) has recently been considered in an ITU-T standard [11]. To ensure the smooth upgrade from GPON to XG-PON for network operators, coexistence of both systems is mandatory. A coexisting XG-PON and GPON long-reach system with entirely passive fiber plant can be very cost effective: it allows the economic benefits of sharing common infrastructure and reducing the number of central offices (CO). In this paper, we present in more depth our recent demonstration [12] of coexisting XG-PON and GPON bi-directional transmission over 50-km of AllWave® fiber with entirely passive fiber plant using distributed Raman amplifications. We further discuss and report new results including system impacts of the Raman interaction between laser pumps and upstream (US) signals, and the transmission impairments of downstream (DS) signals due to pattern-dependent distortion caused by gain dynamics of the SOA. The optimization of negative chirp to mitigate the impact of chromatic dispersion on XG-PON DS from the feeder fiber is also briefly described.
2. System configuration for coexistence of XG-PON and GPON reach extender
Figure 1 shows the schematic of the system configuration for coexisting XG-PON and GPON reach extension, illustrating a CO with optical-line-terminal (OLT) which is connected to the remote node (RN) by 50-km of AllWave fiber as the feeder fiber and optical network users (ONU) at the subscriber premises. The OLT consists of a DFB laser diode (LD) at 1490 nm as GPON DS transmitter, an electroabsorption modulated laser (EML) at 1577nm as XG-PON DS transmitter, two APDs as US receivers and two pump lasers. The two pumps at wavelengths 1206nm and 1240nm are selected to be coupled into the feeder fiber to provide reverse-pumped distributed Raman gains for the XG-PON US signal band (1260-1280nm) and GPON US signal band (1300-1320nm, with narrow wavelength option as defined in ITU G.984.5 [13]) respectively. Two compact SOAs are used to boost both 1577nm and 1490nm signal powers before launching into the feeder fiber to accommodate high loss budget for long-reach. A WDM combiner is employed tocombine the XG-PON/GPON DS signals and Raman pump lasers, and separate XG-PON/GPON US signals. The passband of the WDM combiner is designed to ensure compatibility with the wavelength band specifications for XG-PON and GPON signals as defined in ITU-T standards [11,13]. The WDM combiner also acts as an optical bandpass filter to remove undesired Raman ASE noise outside of the US signal bands to improve the transmission performance. In order to reduce the total link losses and component costs in ONUs, the RN architecture is simplified by a new cyclic 1:2 WDM multiplexer/demultiplexer (mux/demux) [14] in combination with 1:64 passive splitters for GPON and a 1:32 splitter for XG-PON. The ONU consists of a wavelength pair of 1270/1577nm or 1310/1490nm DFB LD and APD receivers. Note only passive cyclic WDM mux/demux and splitter are used in RN, the outside fiber plant is entirely passive, therefore no electrical power is required in RN.
3. Experiment
The experimental set-up is similar as shown in schematic of Fig. 1, except that the losses for optical power splitters with ratios of 1:32 and 1:64 were simulated by using two pairs of variable optical attenuators (VOA), and a loss of 18 dB and 21dB were assumed for the 1:32 and 1:64 way splitters respectively. The 1206nm and 1240nm pump lights, that provided reverse-pumped distributed Raman gain for both 1270nm and 1310nm US, were generated from cascaded Raman resonator fiber lasers [15]. The fiber lasers were spliced to the input ports of WDM combiner due to high pump powers used. It should note that compact semiconductor quantum-dot lasers [9] at 1206nm and 1240nm may be used as the pumps; however, the high power consumption and low output power of semiconductor lasers at such short wavelength (e.g.1206nm) can possibly pose limitations. The cyclic WDM mux/demux used in this experiment was made by a thin-film technique, and it is shown in the schematic diagram Fig. 2(a) . The measured transmissivity of this device is plotted in Fig. 2(b). The measured fiber losses at 1270, 1310, 1490 and 1577nm are about 0.37, 0.32, 0.21 0.19 dB/km respectively and measured insertion losses for WDM combiner and cyclic WDM mux/demux as well as the total link loss budget are shown in Table 1 .
Fig. 2 (a) Schematic diagram of cyclic WDM mux/demux, (b) The measured transmissivity of cyclic WDM mux/demux
Table 1. Link loss budget
The booster SOAs for 1490nm and 1577nm DS have about 15 and 16dBm saturated power respectively. The noise figure (NF) and polarization dependent gain (PDG) are less than 6dB and 0.7 dB respectively for both devices. An optical isolator was used at the input of each SOA, and no polarization controller was used for SOA. A VOA was used to adjust the input power into each SOA, ensuring that the SOA was operated at near linear regime in order to reduce the pattern-dependent distortion caused by gain dynamics of the SOA. The commercially available EML at 1577nm was packaged in a transmit-optical-sub-assembly (TOSA) and mounted on an EML driver board. This EML was designed for 10-Gb/s transmission over only 400ps/nm dispersion, therefore it was operated by negative bias to generate strong negative chirp of pulses [16] in order to mitigate the dispersion impairment of a 10G signal over 50-km of AllWave fiber. The EML was modulated at 10.7-Gb/s (223-1) PRBS using a pattern generator (PG) with output power 0dBm. The un-cooled DFB LDs operating at 1270nm, 1310nm and 1490nm as the XG-PON US, GPON US and DS transmitters respectively were directly modulated at 2.5-Gbit/s (231-1) PRBS using another PG. A commercially available 10G APD with a broadband pre-amplifier and clock recovery circuit was used as XG-PON DS receiver.
4. Results and discussion
First, we investigated the Raman interaction between two pumps (1206nm and 1240nm) and the two US signals by measuring the Raman on-off gain, optical signal to noises ratio (OSNR) of 1270nm and 1310nm US signals with and without other pump and signals operated. Figure 3 shows the Raman on-off gain and OSNR (0.1 nm resolution) of (a) the 1270nm and (b) 1310nm US signals as a function of input signal power into the 50-km feeder fiber with (“all P&S”) and without (“only”) other pump and signals operated. The various input signal power levels represent the cases of different losses from RN with various split ratios. In the measurements, the pump powers for 1206 and 1240nm were fixed at 850 and 520mW respectively, which were optimized to ensure XG-PON and GPON error-free operation, with each operating alone or both at the same time, for 50-km reach with 1:96 way split system. The Raman on-off gain is virtually constant, and OSNR is reduced when the input power into the feeder fiber is decreased. Due to the pump interaction, the OSNR of 1270nm signal drops by about 0.8dB and Raman on-off gain is slightly changed when 1240nm pump/1310nm US is turned on (Fig. 3a). On the other hand, the OSNR and Raman on-off gain of 1310nm US are increased by 0.9 and 2.2 dB respectively when 1206 pump/1270nm US are turned on (Fig. 3b), this is because the pump 1206nm also provides Raman gain for 1240nm pump and 1310nm signal.
The received spectra for US and DS signals are shown in Fig. 4 , and it can be seen the OSNR of US signals is degraded due to ASE noise in the reverse-pumped distributed Raman amplifiers. The WDM combiner at the OLT has passband bandwidth of about 20nm, which permits the use of low cost un-cooled DBF LD as transmitters in the ONU, meanwhile improves the system performance by filtering out the ASE noise outside of the US signal bands for 1270nm and 1310nm signals.
The system transmission impairments of both US signals due to Raman ASE noises was investigated by adjust the VOAs which were used to simulate the losses in the power splitters. The bit-error-ratio (BER) performance as a function of receiver power is measured at various loss conditions. The receiver power penalties for both XG-PON 1270nm and GPON 1310nm US signals as a function of total link losses is shown in Fig. 5 . Higher losses within larger split ratio cause lower input power into the feeder fiber which acting as amplifier medium and lower OSNR (see Fig. 3), resulting in higher receiver power penalty. Eventually lower OSNR due Raman ASE noises at larger split ratio limits the transmission performance.
Fig. 5 Receiver power penalty as a function of total link losses for XGPON 1270nm (a) and GPON 1310nm (b)
Secondly, we studied the transmission impairments of 1490nm DS signals due to pattern-dependent distortion caused by gain dynamics of the SOA by adjusting the input power into SOA from 1490nm transmitter. The measured BER of 1490nm DS through 50-km fiber as a function of receiver power under various SOA operation conditions are shown in Fig. 6(a) , and the recorded output power /gain of SOA, and the receiver power penalties as a function of SOA input power are also plotted in Fig. 6(b). It can be seen that the receiver power penalties are increased when the input power into SOA is high due to the nonlinear distortions if the SOA operates close to saturated regime. The receiver power penalty can be less than 1dB by operating the SOA at near linear regime, meanwhile output power of SOA about 10dBm is still large enough to accommodate the total link loss budget for DS signals.
Fig. 6 (a), BER of 1490nm DS through 50-km fiber as a function of receiver power under various SOA operation conditions, (b) the recorded output power /gain of SOA, and the receiver power penalties v.s SOA input power
Finally, we measured the BER performance for all US and DS channels bi-directional transmitted through the 50-km link with total 1:96 split. In the measurements, the losses of 1:32 way and 1:64 way were adjusted to be 18dB and 21dB respectively, the pump powers for 1206 and 1240nm were fixed at 850 and 520mW respectively. These power values were optimized to ensure to have enough gain and OSNR for both XG-PON and GPON error-free operation for a 50-km reach with total 1:96 way split system. The OSNR would be reduced if further increasing the pump power, i.e. the amount of useful Raman gain will be limited by the double Rayleigh backscattering as discussed in [8]. Figure 7 shows the BER of US and DS signals as a function of receiver power for the GPON and XG-PON each operating alone (“only”) and for both systems operating simultaneously (“all”). Due to added ASE noise in the distributed Raman amplification, the 1310nm BER performance of the GPON-only system was degraded by as much as 2.4dB (@BER 10−10) relative to back-to-back (B2B) baseline (Fig. 7a). Its penalty was reduced to less than 1.8dB when both GPON and XG-PON were operated simultaneously. This is because the 1206nm pump also improved the OSNR of the 1310nm signal (Fig. 3b). The receiver power penalty for the 1270nm signal was about 1.7dB (@BER 10−4 FEC threshold) when operating the XG-PON only, and its penalty was degraded about 2.2dB when all channels operated simultaneously, mainly due to depletion of the 1206nm pump by the 1240nm pump, and the resulting degradation in OSNR of the 1270nm signal (Fig. 3a). For 1490nm DS, there was about 1.1 dB power penalty (@BER 10−10) (Fig. 7b) after 50 km transmission with total link loss of 37.7dB, and this penalty is attributed mainly to pattern-related nonlinear effects in the SOA. The BER performance for 1577nm 10.7-Gb/s DS was optimized with negative biasing of EML at Vb about −0.47V to generate strong negative chirp of the 10G pulses in order to minimize the chromatic dispersion impairment through 50-km of AllWave fiber link, however about 1.0 dB power penalty (@BER 10−3 FEC threshold) remained for a 10.7Gb/s 1577nm DS signal. Nevertheless, the receiver sensitivity is better than −29dBm at BER of 1x10−3 (a pre-FEC BER corresponding to a post-FEC BER < 1x10−12). All 4 channels have more than 1 dB receiver power margin for a system with 50-km reach and 1:96 split.
Fig. 7 BER performance of the extended PON systems (a) 1270nm and 1310nm US, (b) 1490nm and 1577nm DS
Although we have demonstrated only continuous US BER performance, the system performance with burst-mode US signals shall not be impacted by the transient effect due to the reversed-pumped distributed Raman amplification. This is because the Raman amplifier has fast response time (~50 fs) [17], the transient effects shall be negligible in GPON burst-mode operation. Furthermore the performance with burst-mode US signal using reversed-pumped distributed Raman amplifier was recently confirmed by experimental demonstration [7, 9]. It should also be pointed out that the Raman amplified XG-PON and GPON system should provide optical safety features for automatic shut-down of the Raman pump lasers when a fiber break or damages occur in feeder fiber because high power of Raman pumps are involved.
5. Summary
In this paper, we have described a truly-passive coexisting XG-PON and GPON compatible reach extension system with a novel optical configuration, using two lasers to provide Raman gain for both US signals and low cost SOA as power boosters to improve the loss budgets for DS signals. We have presented the first experimental demonstration of a coexisting XG-PON and GPON bi-directional transmission over 50-km of AllWave fiber with entirely passive fiber plant and a total 1:96 split, accommodating link loss budget more than 39-dB for both GPON and XG-PON US signals
Acknowledgments
We thank David DiGiovanni at OFS Labs for his support, and we thank Dave Au, Farooq Khan, and Yaowen Li at OFS SPD for making the 1206nm fiber laser cavity used in this work.
References and links
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