We analyze the performance of bidirectional WDM PON architecture which utilizes distributed Raman amplification and pump recycling technique. The maximum reach at data rates of 622 Mb/s and 1.25 Gb/s in the proposed WDM PON architecture is calculated by taking into account the effects of power budget, chromatic dispersion of transmission fiber, and Raman amplification-induced noises with a given amount of Raman pump power. From the result, the maximum reach for 622 Mb/s and 1.25 Gb/s signal transmission is calculated to be 65 km and 60 km with a Raman pump power of 700 mW, respectively. We also find that the calculated results agree well with the experimental results which were reported previously.
©2007 Optical Society of America
Long-reach passive optical network (PON) has been considered as a promising way to realize a cost-effective subscriber network [1–4]. This is because the PON architectures with an extended reach could reduce the number of equipment interfaces, network elements and central offices (CO). Previously, long-reach PONs based on time-division multiplexed (TDM) technology have been demonstrated by using various optical amplifier modules, such as erbium-doped fiber amplifiers (EDFAs), semiconductor optical amplifiers (SOAs) and Raman amplifiers [1–3]. Long-reach wavelength-division multiplexed (WDM) PON has been also demonstrated with a channel data rate of 125 Mb/s . However, we do not believe that WDM PON with a low data rate might be an attractive solution for cost-effective subscriber networks, as compared to TDM PON. Therefore, in order to make WDM PON competitive with TDM PON in the deployment of cost-effective subscriber networks, the feasibility of high-speed (beyond 1.25 Gb/s) signal transmission based on low-cost WDM light sources should be demonstrated with an extended reach and a high channel-count. Recently, we have proposed and demonstrated a novel bidirectional WDM PON architecture that utilized distributed Raman amplification and pump recycling technique . A key feature of the proposed WDM PON was that spectrum-sliced amplified spontaneous emission (ASE) light generated by pumping EDF with a remnant Raman pump power was used for the upstream light source of each subscriber. Therefore, each optical network unit (ONU) needs not to have an active light source. Using the proposed architecture, we have achieved a 622 Mb/s signal transmission over a 50-km long conventional single-mode fiber (SMF) link. In this paper, we theoretically analyze the performance of the proposed WDM PON for 622 Mb/s and 1.25 Gb/s signal transmission. The maximum reach and required Raman pump power in the proposed WDM PON architecture are calculated by taking into account the effects of power budget, chromatic dispersion of transmission fiber, and Raman amplification-induced noises. From the results, we find that the calculated results agree well with our previously reported experimental results in Ref. .
2. Proposed WDM PON architecture
Figure 1 shows our proposed bidirectional WDM PON architecture based on the distributed Raman amplification and pump recycling technique . An ASE source is employed as a downstream light source at a CO and is spectrally sliced by using a conventional arrayed-waveguide grating (AWG) to generate downstream signals. The spectrum-sliced downstream lights are modulated by use of electro-absorption modulators (EAMs) and then multiplexed with another AWG. A cyclic AWG that exhibits a periodic wavelength passband is used for downstream signal multiplexing and upstream signal demultiplexing. The multiplexed downstream signals are transmitted through a SMF link.
One of the key features in our proposed scheme is to use distributed Raman amplification over a transmission fiber to obtain lossless, bidirectional signal transmission. Raman pump module is located at the CO for the purpose of fully-centralized control of all active light sources. Using the fact that non-negligible pump power remains unused after distributed Raman amplification, residual Raman pump power is recycled as a pump for an upstream ASE source after the transmission fiber. The residual Raman pump is separated from the downstream signals using a pump/signal WDM at a remote node (RN) and coupled into a length of EDF via a circulator. The generated ASE light is spectrally sliced through a conventional AWG and distributed to ONUs for upstream data transmission. On the other hand, the downstream signals are demultiplexed at the RN using a cyclic AWG and distributed to each subscriber.
Each of the ONUs consists of a EAM, a receiver, and a circulator. The demultiplexed downstream signals from the cyclic AWG share a common path between the RN and the ONUs with the modulated upstream signals owing to periodic wavelength response of the cyclic AWG. The multiplexed upstream signals at the RN are transmitted over the transmission fiber and demultiplexed through the cyclic AWG at CO.
3. Results and discussion
Previously, we demonstrated the feasibility of proposed bidirectional WDM PON at a data rate of 622 Mb/s . In that experimental demonstration, the achieved transmission length (i.e. reach) was 25 km and 50 km with a Raman pump power of 350 mW and 650 mW, respectively. Here, we theoretically calculated the maximum reachable length of proposed WDM PON for 622 Mb/s and 1.25 Gb/s signal transmission with a given amount of Raman pump power. The system parameters used in our calculation were summarized in Table 1. In the calculation, we assumed that the maximum reach of our proposed WDM PON was mainly determined by the output power of an upstream ASE source which was generated with a residual Raman pump at RN. That is to say, the output power of downstream ASE source could be easily increased to be larger than one of upstream ASE source just by increasing the pump power.
We first calculated the maximum reach of the proposed WDM POM with a Raman pump power of 500 mW which was chosen by considering the laser hazard classification. For the first step of performance evaluation, the obtainable Raman gains were calculated with a Raman gain efficiency coefficient of 0.64 (W-1km-1). Figure 2(a) shows the Raman on/off gain and Raman net gain as a function of transmission fiber length with a Raman pump power of 500 mW. As it can be seen in Fig. 2(a), the obtained Raman net gain was calculated to be larger than 0 dB even after 100 km of SMF link. Previously, it was reported that a significant amount of Raman pump power was unused after Raman amplification in a transmission fiber unless the transmission fiber was long enough to attenuate all of the pump power . Therefore, we could also calculate the residual Raman pump power after Raman amplification as shown in Fig. 2(b). As explained before, this residual Raman pump power was launched into EDF located at RN to generate an upstream ASE source. Total ASE power generated with this amount of residual pump power was also calculated as shown in Fig. 2(c). The modeling of EDF based on the standard Giles model  was used to calculate the power of backward ASE light with Giles parameters even only peak absorption and gain parameters of EDF were represented in Table 1. From the results in Fig. 2(c), we found that the generated ASE power decreased to be less than 5 mW after a 50-km long SMF link.
Next, the spectrum-sliced channel ASE power was calculated from total generated ASE power with an AWG which had a channel spacing of 100 GHz and a 3-dB bandwidth of 0.4 nm. In this calculation, we assumed that the ASE power was uniformly distributed in the wavelength range of 1540 nm to 1560 nm (which, we assumed, was used for upstream signal transmission) even though there would exist a gain peak around 1530 nm. From the channel ASE power level, the input power level of upstream signal into the receiver located at CO was calculated by taking into account the transmission loss of SMF and the obtained Raman net gain. The insertion losses of AWG and EAM were assumed to be 5 dB and 6 dB, respectively. Finally, the minimum required ASE power levels for 622 Mb/s and 1.25 Gb/s signal transmission were calculated with the receiver sensitivity of -27 dBm for bit-error rate of 10-9 at 622 Mb/s. In addition to the power budget consideration, the effect of chromatic dispersion of transmission fiber was taken into account for the minimum required channel ASE power level. It has been reported that the chromatic dispersion of transmission fiber could induce a power penalty for high-speed signal transmission due to the wide bandwidth of spectrum-sliced ASE source . Therefore, we took into account both power budget and chromatic dispersion-induced power penalty for the calculation of required channel ASE power level in Fig. 3. From the results, we found that the maximum reach for 622 Mb/s and 1.25 Gb/s signal transmission in our proposed WDM-PON with Raman pump power of 500 mW were 55 km and 50 km, respectively. In this calculation, we did not take into account the effects of pump to signal crosstalk and double Rayleigh backscattering-induced inband crosstalk for Raman amplification. This is because the effects of these crosstalks would be negligible for Raman on/off gain of <22 dB with Bragg-grating-stabilized semiconductor laser diode pump even in co-pumping configuration [9–11].
The maximum reach of proposed WDM PON with a different level of Raman pump power was also calculated for 622 Mb/s and 1.25 Gb/s signal transmission as shown in Fig. 4. In this calculation, we assumed that the Raman on/off gain was limited to be less than 32 dB in order to consider the effect of double Rayleigh backscattering-induced inband crosstalk. Generally, the Raman on/off gain of 25 dB could induce a power penalty of 1-dB due to double Rayleigh backscattering when a laser diode source was used for signal transmission in distributed Raman amplified system . It has been also known that the spectrum-sliced ASE source was more tolerant to inband crosstalk than laser source . Considering the equivalent optical and electrical filter bandwidths for the spectrum-sliced ASE source , we assumed that the spectrum-sliced ASE source was 14-dB more tolerant to inband crosstalk than laser source. Since the amount of double Rayleigh backscattering-induced inband crosstalk is proportional to the square of Raman gain, the allowable Raman on/off gain for spectrum-sliced ASE source could be increased by 7 dB with a negligible inband crosstalk-induced power penalty, as compared to laser source. Moreover, the pump to signal crosstalk in a co-pumped Raman amplifier could not be negligible with a Raman on/off gain of larger than 32 dB even for using a Bragg-grating-stabilized semiconductor laser diode pump . From the results, we found that the maximum reach of proposed WDM PON was mainly determined by the Raman gain limitation due to Raman amplification-induced noises when the Raman pump power was higher than 700 mW. Then, as it can be seen in Fig. 4, the maximum reach of proposed WDM PON for 622 Mb/s and 1.25 Gb/s signal transmission was 65 km and 60 km with a Raman pump power of 700 mW, respectively. The calculation results agree well with the experimental results reported in  even though there would exist a small discrepancy. We believe that this discrepancy between the calculation and experimental results was mainly due to the fact that we used 25-km spools of SMF in an experimental demonstration and did not try to maximize the transmission length with a given amount of Raman pump power.
We have theoretically analyzed the maximum coverage of our proposed WDM PON architecture that utilized distributed Raman amplification and residual pump recycling technique. From the calculation results, the maximum reach at data rates of 622 Mb/s and 1.25 Gb/s were 65 km and 60 km with a Raman pump power of 700 mW, respectively. This result agreed well with the previously reported experimental result .
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-331-D00386).
References and links
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