In previous investigation of extended GPON system, we employed 1240nm and 1427nm dual pumps within optical line terminal (OLT) equipments at central office (CO) to provide distributed Raman gains of upstream 1310nm and downstream 1490nm signals. These pump wavelengths were selected to ensure compatibility with the standard GPON wavelengths and reduce the unwanted pump-to-signal interactions. In this paper, we propose a new system scheme for an entirely-passive extended reach GPON to further enhance the system performance by eliminating the pump-to-signal interactions. In this scheme, a 1240 nm laser is employed to provide counter-pumping distributed Raman amplification of the upstream 1310nm signal, and a discrete Raman amplifier is integrated with the 1490nm transmitter to booster the downstream signal power and to improve the link loss budget. An operation over 60-km of zero-water-peak Allwave® fiber with a 1:128 way splitter is experimentally demonstrated at 2.5 Gbit/s. The system performance of such purely passive GPON extender is investigated in the paper. The system transmission limitation of upstream signal due to Raman ASE noises is discussed, and the non-linear impairment on downstream signal due to high launch power into feeder fiber is also examined.
© 2010 OSA
The advantages of deploying cost effective passive optical networks (PONs) have been largely recognized worldwide. The PONs are now being deployed in many countries and will play an increasingly important role in future broadband access networks [1,2]. The gigabit PON (GPON) standard  permits a logical reach of 60 km and 128 addressable optical network units (ONU). However, the 28 dB loss budget for Class B + systems [G.984.2 Amd1] limits typical GPON deployments to 1:32 split and < 20 km reach. There have been several reports on techniques to extend the reach of PON systems using semiconductor optical amplifier (SOA) and thulium or praseodymium doped fiber amplifiers [4–6]. GPON reach extenders have also recently been standardized by the ITU-T (G.984.6). The reach extension techniques considered in G.984.6 require the use of electrically powered units in the field containing optical amplifiers or optical-electrical-optical (OEO) repeaters, but these techniques negate some of the advantages of PON systems and may not always be practical or cost effective for operators, particularly in certain environments where there is no electrical powering.
Techniques that enable PON reach extension while maintaining a totally passive outside plant could be very attractive for network operators. Raman amplification in the transmission fiber is one such technique that could improve the PON loss budget by coupling suitable pump lasers to the fiber at the central office. Recently, we have reported a purely-passive GPON compatible reach extender using distributed Raman amplification . Operation over 60-km of AllWave® fiber with split ratio of 1:64 has been demonstrated. A fiber laser pump at 1240 nm was chosen to provide gain for the GPON upstream 1310nm signal in order to ensure compatibility with standard GPON wavelengths with the narrow upstream wavelength band (1300-1320nm) option as defined in ITU G.984.5. This wavelength tolerance allows the use of low cost, un-cooled DFB diode transmitters. A pump at wavelength around 1400-nm would provide the maximum Raman gain efficiency for co-pumping of downstream signal band (1480-1500nm); however the upstream 1310 nm signal would be significantly depleted if 1400-nm pump were used. Hence, the 1427-nm pump was used in that experiment as a trade-off between a minimal upstream 1310nm signal depletion and an adequate downstream 1490nm signal gain. While these pump wavelength designs partially reduced most unwanted pump-to-signal interactions, detectable depletion of the upstream 1310nm signal was observed in that experiment. In addition, the de-polarized semiconductor lasers with relative intensity noise (RIN) as low as –150dB/Hz are required for the co-pumping scheme in order to reduce the pump noises coupled into the downstream signal.
In this paper, we describe the purely passive GPON compatible reach extender using both discrete and distributed Raman amplifiers. In order to eliminate the depletion for 1310nm upstream signal as mentioned above, we propose a new system scheme for entirely-passive reach-extended GPON. In this scheme, an S-band optical amplifier is integrated with the 1490nm transmitter at OLT to amplify the downstream 1490nm signal power and to improve the link loss budget, and a 1240 nm fiber laser is employed to offer counter-pumping distributed Raman amplification of upstream signal. For the first time, an operation over 60-km of zero-water-peak AllWave® fiber with a 1:128 way splitter is demonstrated at 2.5 Gbit/s using such system scheme. The transmission performance of this purely passive GPON extender is investigated based on C + optics specifications defined in ITU G.984.2 (amd2) . The system limitation of upstream signal due to Raman ASE noises is discussed, and the possibilities of non-linear impairment on downstream signal due to high launch power into feeder fiber are also examined in the paper.
We propose a new system configuration for purely passive GPON reach extender to improve the system performance. Figure 1 is a schematic diagram of experiment set-up, illustrating a CO with OLT which is connected to the remote node by 60 km of AllWave® fiber as the feeder fiber and ONUs at the subscriber premises. The OLT consists of a DFB laser diode (LD) at 1490 nm as the transmitter for the downstream signal, 1310 nm APD receiver, a CWDM combiner. A Raman pump laser at 1240nm is selected to provide optical pumping for the distributed Raman amplification of the 1310nm upstream signal. A discrete Raman amplifier is used to booster the downstream signal power after the 1490nm transmitter before launching into the feeder fiber. Since the remote node uses only passive optical splitter, the outside fiber plant is entirely passive. Each ONU uses a DFB LD operating 1310 nm to transmit upstream signal, a 1490nm receiver and a CWDM splitter to split the 1310nm upstream signal and 1490nm downstream signal. It should be pointed that a SOA at 1490nm with adequate gain can also be used as a booster amplifier for downstream signal. Other types of optical booster amplifier can be integrated with 1490nm transmitter, or high power DFB LD (without optical booster amplifier) can be used to improve the total link loss for downstream signal. Compared with the approach in , this scheme eliminates the 1310nm upstream signal depletion due to 1427 nm pumping.
The 1490 nm and 1310 nm LDs located at the OLT and ONU are commercially available un-cooled DFBs with 3 dBm output power. The measured fiber losses at 1310 nm and 1490 nm are about 0.32dB/km and 0.21dB/km respectively. The 128-way splitter loss is set to be 24 dB in the experiment, and the total loss of CWDM coupler and connectors are about 1.7 dB. Hence the total link losses between OLT and ONU are 44.9dB and 38.3 dB for upstream 1310 nm and downstream 1490nm signals, respectively. Assuming that C + optics devices as defined in  can be used in future GPON extender, the receiver sensitivities would be −30 dBm and −32 dBm for upstream and downstream signals respectively. The total link loss budget is shown in Table 1 .
The 1240 nm pump light, that provides counter-propagating Raman gain for the 1310 nm upstream signal, is generated in Raman fiber laser . Low cost multimode semiconductor diodes at 915 nm are used to pump an Yb-doped cladding-pumped fiber laser (CPFL). The output of CPFL at 1117 nm is input to a cascaded Raman resonator (CRR), which consists of 400m spool of high Raman gain efficiency fiber and a cascaded grating set to shift the output wavelength up to 1240 nm. An output power of 1.3 W is readily obtained using this approach. The residual Raman shifted wavelengths at 1117nm and 1172nm are more than 20 dB below the 1240nm and do not contribute significantly to the Raman gain. The discrete Raman amplifier at 1490nm wavelength band to booster the downstream signal power consists of a Raman gain medium and 1400nm semiconductor diode pumps. The Raman gain medium is a 1.6-km length of high Raman gain efficiency fiber, and it is configured as backward-pumping. A net Raman gain of 12 dB can be ready obtained in the experiment.
Figure 2(a) shows the Raman on-off gain and OSNR (0.1 nm resolution) of the 1310 nm signal as a function of input power into the feeder fiber when the 1240nm pump laser is counter-propagating into 60-km of AllWave® fiber at 950mW pump power. The different input signal powers represent the cases of different losses from optical splitter (i.e. different split ratio) at remote node and distributed fibers. The Raman on-off gain is virtually constant, however OSNR is reduced when the input power into feeder fiber is decreased. The OSNR of 1310nm signal at transmitter is more than 45dB and an OSNR of 18 dB (at 0.1nm resolution) is obtained when the 1310nm upstream signal power input feeder fiber is about −21 dBm. This corresponds to the case with the split ratio of 1:128 ( + 3 dBm output from 1310nm transmitter, and 24 dB loss is assumed for 1:128 way splitter). Figure 2(b) shows the Raman on-off gain and OSNR of upstream signal as a function of pump power when the 1310 signal input power is kept at −21dBm. The Raman on-off gains are increased monotonically with the pump power, while the maximum OSNR is obtained at 950 mW pump power, the OSNR is rolled off upon a further increase in the 1240nm pump power; and this indicates that double Rayleigh backscattering  limits the amount of useful Raman gain. For the deployed fiber (e.g. SSMF), the losses at 1310nm and 1240nm may be higher than that of AllWave® fiber in the Labs, and large losses at connectors and splices  may occur in some GPON links, which will impact the Raman gain, noise figure, and required Raman pump power. In addition, the large reflections in connectors and splice, and double-Rayleigh backscattering in some deployed old fibers may occur, which cause multi-path interference (MPI) , hence degrade overall system performance in terms of reach and splitter ratio. Therefore a careful estimation and reduction of localized losses in fiber links will be required in order to deploy this purely passive Raman amplified GPON reach extension system.
The transmitted and received optical spectra of upstream and downstream signal are shown in Fig. 3 . It can be seen that a minimal OSNR degradation for the 1490 nm signal [see Fig. 3(d)] is due to the relatively high input power into the discrete Raman amplifier from the transmitter, and the received OSNR for downstream signal is more than 38dB/0.1nm. Conversely, the OSNR of 1310 nm signal is degraded [Fig. 3(c)] due to the low input signal power in the counter-propagating distributed Raman amplifier. In this experiment, CWDM couplers with a bandwidth more than 16nm are used, which permits the use of low cost, un-cooled DBF diodes as the transmitters as defined in ITU G/984.5, meanwhile improve the system performance by filtering out the ASE noises.
3. System results and discussion
The un-cooled DFB LDs at 1310 nm and 1490 nm are directly modulated at 2.5 Gbit/s (231-1) PRBS, and bi-directionally transmitted through the 60-km AllWave® fiber to APD receivers. The output power from modulated 1310nm and 1490nm diodes are 3 dBm, and the extinction ratio of the transmitters are 11.3dB. The back-to-back APD receiver sensitivities for 1310nm and 1490nm signals are −32.4 and −32.2 dBm (at BER = 10−10) respectively, which are slightly better than the specification of class C + transceivers . The loss of optical splitter is set to be 24 dB. The 1240nm fiber laser at 950 mW pump power provides counter-propagating distributed Raman gain of upstream 1310nm signal, while the launched optical power is set at 10 dBm for downstream signal from the discrete Raman amplifier. Figure 4 shows the bit-error-ratio (BER) performance for upstream and down stream signals with both channels operating through the system simultaneously. It can be seen that less than 0.5 dB power penalty is observed for the 1490 nm signal, after 60 km transmission with total link loss of 38.3 dB. The power penalty may be caused by chromatic dispersion at 1490 nm. The upstream BER performance is degraded by as much as 5.3 dB relative to the baseline (at BER = 10−10) due to added noises in the distributed Raman amplifier. However, there is no indication of error floors, and error-free bi-directional 1310 nm and 1490 nm signal transmission is achieved at 2.5 Gbit/s over 60 km fiber with 1:128 split ratio.
The BER performance for upstream and downstream signals is studied under various operation conditions when both channels operate through the system simultaneously. First, the BER performance of 1310nm signal due to Raman ASE noises in counter-pumping scheme is investigated by varying input signal power into feeder fiber (60-km AllWave® fiber) at a fixed pump power (950mW). This represents the cases of different losses from optical splitter and distributed fiber. The BER performance of 1310nm signal at various input signal power into feeder fiber is shown in Fig. 5(a) , where it can be seen the BER floors occur when input powers into the feeder are below −22 dBm. The higher losses from splitter and distributed fibers or a lower input signal power into the feeder fiber result in higher Raman ASE noises built-up along the feeder fiber when Raman pump power is fixed (no pump depletion/saturation occurs), which result in lower OSNR at the 1310nm receiver [see Fig. 2(a)] and therefore poor BER performance. However even at −24dBm input power, the BER of 1310 signal is still below 10−4, leading to an error-free operation if FEC with 10−4 pre-FEC BER threshold is used (based on C + optics specification in ). This means an additional 3 dB loss margin can be obtained if C + optics devices are used in the GPON extender. It should note that the performance of upstream signal is limited by OSNR, but not by the received optical power in this case. Second, since the performance of the downstream 1490nm transmission can be affected by non-linear impairments  such as stimulated Brillouin scattering (SBS) and self-phase modulation (SPM) due to high launched power into the feeder fiber, the BER performance is examined at different launch power after the discrete Raman amplifier. Figure 5(b) shows the 1490nm BER performance with launch power at 3, 10 and 14 dBm. It can be seen that the BER performance at launch powers of 3, 10 and 14dBm are virtually the same for this 60-km AllWave® fiber link. At launch power of 14 dBm, SBS and SPM effects are not observed in the experiment. It is expected that SBS threshold can be as high as 17 dBm for a 60-km link using AllWave® fiber or standard single-mode fiber (SSMF) and 2.5Gb/s directed modulated DFB LD transmitter. For 14 dBm launch power, a link for total loss of 46 dB can be realized if C + optics device  is used. This can ready accommodate 60-km reach with 128-way splitter with an adequate loss margin for the realistic GPON systems. Although we have demonstrated only continuous upstream BER performance, the performance with burst-mode upstream signals should not suffer any transient effects due to the Raman amplification .
In this paper, we propose a new system scheme to enhance system performance of entirely-passive reach-extended GPON. In this scheme, a 1240 nm laser is employed to provide counter-pumping distributed Raman amplification of the upstream 1310nm signal, and a discrete Raman amplifier, that can be integrated with 1490 nm transmitter at OLT, is used to booster the downstream signal power and improve the link loss budget. An operation over 60-km of zero-water-peak AllWave® fiber with a 1:128 way splitter is experimentally demonstrated at 2.5 Gbit/s. The system performance of this purely passive GPON extender is investigated based on C + optics specifications . The system limitation of upstream signal due to Raman ASE noises is discussed, and the possibilities of non-linear impairment on downstream signal due to high launch power into feeder fiber is also examined in the paper.
We thank David DiGiovanni at OFS Labs for his encouragement and support and we also thank Dr. Derek Nesset at BT for his interesting discussion on the work.
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