We propose and demonstrate an OCDMA-PON scheme with optical network unit (ONU) internetworking capability, which utilizes low-cost gain-switched Fabry–Pérot (GS-FP) lasers with external dual-wavelength injection as the pulse sources on the ONU side. The injection-generated optical pulses in two wavelengths from the same GS-FP laser are used separately for the PON uplink transmission and ONU internetworking. Experimental results based on a two-user OCDMA system confirm the feasibility of the proposed scheme. With OCDMA technologies, separate ONU-internetworking groups can be established using different optical codes. We also give experiment results to analyze the performance of the ONU-ONU transmission at different power of interference signals when two ONU-internetworking groups are present in the OCDMA-PON.
© 2010 OSA
Optical-code-division multiple access (OCDMA) technique  has attracted a lot of interests due to its various advantages including asynchronous operation, high network flexibility, protocol transparency, simplified network control and potentially enhanced security [2–4], and is often considered as one of the effective multiple-access ways for passive optical networks (PONs) applications [5–7]. To satisfy the need of PON users that share information among optical network units (ONUs), it is advantageous to establish peer-to-peer internetworking within OCDMA PONs. ONU internetworking independent of the optical line terminal (OLT) is considered as an attractive feature for PONs, because it can reduce packet traffic load on the OLT . However, few effective schemes have been given and demonstrated to realize the ONU internetworking independent of OLT on optical layer in OCDMA PONs, although the study on related topics has been popular in TDM-PONs and WDM-PONs [8–10]. For ONU internetworking in an OCDMA-PON, multiple ONU-ONU transmissions can be simultaneously performed by using different optical codes. And the simple architecture proposed for TDM-PON in , in which a fiber Bragg grating (FBG) is located before the optical splitter at the remote node (RN), can also be utilized in OCDMA-PON, avoiding using the complex RN configurations, e.g. 2N*2N AWG used in .
Recently, we have proposed and demonstrated a low-cost scheme for OCDMA PONs, using optical injected gain-switched Fabry-Pérot (GS-FP) lasers as short-pulse sources on the ONU side . Compared with the OCDMA system using the conventional pulse source, e.g. mode-locked lasers (MLLs), the scheme with GS-FP lasers gives an effective solution to decrease the ONU cost and shows reasonable system performance. In this paper, we propose an OCDMA PON scheme with optical ONU internetworking capability, which utilizes the low-cost GS-FP lasers with external dual-wavelength injection on the user side. Both the uplink and ONU-ONU transmission signals can be carried on the two injection-generated wavelengths from the same GS-FP laser to decrease the cost of ONUs. The fiber Bragg grating (FBG) located before the optical couplers at RN reflects the ONU-ONU traffic data towards all users without passing through the OLT. ONUs belonging to the same ONU-internetworking group can communicate with each other by sharing the same optical code and multiple groups can be established using different optical codes. A two-user OCDMA system with symmetrical 1.25-Gb/s downlink and uplink transmission, as well as 1.25-Gb/s ONU-ONU data transmission is set up to experimentally demonstrate the proposed scheme. In addition, the performance of the desired ONU-ONU transmission data as the power of interference signal varies is also analyzed when two simultaneous ONU-internetworking groups are present in the OCDMA-PON.
2. Proposed network architecture
Figure 1 shows an OCDMA PON architecture providing ONU internetworking capability. The system is based on low-cost GS-FP lasers with external dual-wavelength injection at the ONUs. In this architecture, data for up/downlink transmission and ONU-ONU traffic can be simultaneously transmitted since they are at different wavelengths. Two continuous wave (CW) lasers used as the injection seeding sources are located at OLT and shared by all ONUs to reduce the cost and relax the management at ONUs. The injecting lights are first combined with the encoded downlink signals by a wavelength division multiplexer (WDM1) and then transmitted to the ONUs. At each ONU, the injecting light is separated from encoded downlink signals by a WDM demultiplexer (the same type as WDM1) and then injected into the FP laser. At the same time, a matched decoder at the receiving end is used to decode the desired channel of the encoded downlink signals. Short pulses in dual wavelengths generated from the injection-locked GS-FP laser are separated by a WDM demultiplexer (WDM2). One branch is modulated and coded for the uplink and the other for the ONU-ONU traffic.
At the RN, a FBG is located before the uplink power splitter to pass uplink OCDMA signals and reflect the ONU-ONU signals towards ONUs. To realize multiple ONU-internetworking groups within the OCDMA PON, ONUs in different groups use different optical en/decoders. Only ONUs in the same internetworking group communicate with each other by sharing the same optical code. Reconfigurable en/decoders  need to be used if an ONU needs to switch among internetworking groups.
To resolve the possible collision during the intra-group ONU-ONU transmission, media access control (MAC) protocol, such as carrier sense multiple access with collision avoidance protocol for optical networks , is needed. Despite of this, several intra-group ONU-ONU transmissions, which belong to different ONU-internetworking groups, can be performed simultaneously and asynchronously, because they are optical-code divided.
3. Experiment setup
Figure 2 presents the experimental setup of the proposed system. For the downlink transmission, an optical pulse train with 1.8-ps pulse width and 10-GHz repetition rate is generated by a commercial MLL, followed by an optical filter with 3-dB passing bandwidth of 0.6 nm and central wavelength at 1552.3 nm, in an effort to match the pulse width of the GS-FP lasers on the ONU side. The pulse train is gated down to 1.25 GHz and modulated with 215-1 pseudo-random bit sequence (PRBS), and then encoded by two different 127-chip, 320-Gchip/s super-structured fiber Bragg gratings (SSFBGs). The encoded signals are combined with two CW lights generated from the distributed-feedback (DFB) lasers at wavelength of 1549.1 nm and 1550.7 nm, respectively, by a WDM (WDM 1; passing band: 1551.7-1553.3 nm; reflection band: others) and launched into a 20-km single mode fiber (SMF) followed by a dispersion compensation fiber (DCF).
On the ONU side, the downlink encoded signals are separated from the CW lights by a WDM demultiplexer (the same type as WDM 1), decoded by a matched SSFBG and directly detected for bit error rate (BER) analysis. The two CW lights are injected into the FP laser and then the generated 10-GHz pulse train from the injection locking GS-FP laser is modulated with 27-1 PRBS at 1.25 Gb/s. A polarization controller (PC) is put before the GS-FP to obtain the matched polarization state. In a practical application, a depolarizer at OLT can be used to eliminate the polarization dependence of injection locking, as proposed in . The generated signal is splitted into two branches to emulate the users in two different ONU-internetworking groups. For each branch, the 1.25-Gb/s signal is separated by another type of WDM (WDM 2; pass band: 1550.1-1551.7 nm; reflection band: others) and then encoded by other two different 127-chip, 320-Gchip/s SSFBGs for the uplink and ONU-ONU traffic. Optical delay lines (DLs) are used to decorrelate these encoded signals. A WDM demultiplexer (the same type as WDM 2) is employed in the experiment to replace the FBG. Hence, the ONU-ONU transmitting signal is redirected downlink to another ONU through a 6-km SMF, while the uplink signal is transmitted back to the OLT through another 20-km SMF followed by the DCF. The generated pulse train from the GS-FP laser is characterized by using an optical spectrum analyzer with a resolution of 0.1 nm and an Agilent 20-GHz sampling oscilloscope with 17.5-ps time resolution in conjunction with a 42-GHz pin photodetector. Due to the acceptable dispersion-induced penalty, no DCF is used for the ONU-ONU transmission.
4. Results and discussion
At bias current of 28 mA, RF signal power of 28 dBm and the injection power at each wavelength of around −4.5 dBm, dual-wavelength 10 GHz pulses with a pulse width of ~20 ps (see Fig. 3(d) ) and side-mode suppression ratio (SMSR) exceeds 24 dB (comparing Fig. 3(a) and (b)) can be generated from the commercial GS-FP laser. The measured optical spectra for the downlink, uplink and ONU-ONU transmission are all illustrated in Fig. 3(c). One can see that the three wavelength channels are 1.6 nm spacing and correspond to the 200 GHz ITU grid channels. The reason of using 200 GHz channel spacing is that the mode spacing of the GS-FP laser used in this experiment is 200 GHz (seeing Fig. 3(a)) and the effective spectral range of SSFBGs used is around 1549-1553 nm.
The measured eye diagrams of the decoded signals are illustrated in Fig. 4 . Since different wavelengths are used in OLT downlink, ONU uplink and internetworking, there is no significant multiple user interference (MUI) cross the three wavelength channels (seeing one-user cases in Fig. 4(a), (b) and (c)). However, in each wavelength channel, a two-user OCDMA system is carried and thus the MUI exists, which can be shown by comparing one-user cases in Fig. 4(a), (b) and (c) with two-user cases in Fig. 4(d), (e) and (f), respectively. From the BER curves shown in Fig. 5 , one can see that the 1.25-Gb/s uplink and ONU-to-ONU interconnecting signals show very similar BER performance (in the back-to-back cases, their power penalty difference at BER = 10−9 is ~0.3dB and ~0.4dB for the cases of one user and two users, respectively), which means the two wavelengths from the dual wavelength injection GS-FP laser have similar system performance. Here we note that the pulse width of the generated pulses from GS-FP laser is larger than the chip duration (~3 ps) of the SSFBG en/decoders used in the experiment, which may cause the BER performance degradation. However, from the BER curves in Fig. 5, one can see the low-cost GS-FP laser can still work reasonably well with the 320 Gchip/s SSFBGs in our experiments. SSFBGs with chip duration of ~20 ps may be a choice to compensate this degradation. However, bit rate or security of the OCDMA system will be decreased in this case. In addition, although the experiments we conducted here are at 1.25Gb/s due to the relatively broad pulse width from GS-FP laser (~20 ps), it is possible to realize 10-Gb/s OCDMA systems by using shorter pulses, which can be generated by GS-FP lasers with higher bandwidths plus optional fiber compressors, such as in .
Separate ONU-internetworking groups can be supported in the PON, and signals from each group may have the different power loss induced by different transmitting fiber lengths. Therefore, the ONU will receive different ONU-ONU transmitting signals with different optical power, which will result in additional power penalty. Figure 6 shows the measured optical power penalty at BER = 10−9 for the desired ONU-ONU transmitting signal versus the received power difference, when two ONU-internetworking groups are supported in the OCDMA-PON. Here we keep the optical power of the desired signal constant while adjusting the optical power of the interfering signal. Considering that the distribution-fiber-length difference is often less than 10 km in the PON and the typical power loss at 1550 nm for the SMF is 0.2 dB/ km, the received power difference between these two signals is kept within 2 dB. When the optical power of the interfering signal is 2 dB more than the desired signal, ~3.4-dB power penalty is observed. And the BER performance of the desired signal is with ~1.1 dB improved, when the optical power of the interfering signal is 2 dB less than the desired signal. These power penalties should be considered during system design.
We have proposed and demonstrated an OCDMA PON scheme supporting ONU-ONU transmission independent of the OLT, which employs low-cost GS-FP lasers wit h external dual-wavelength injection on the user side. The data for the uplink transmission and the traffic among ONUs can be carried on two injection-generated wavelengths from the same GS-FP laser. Several ONU-internetworking groups can be simultaneously performed using different optical codes. Experiment results based on a two-user OCDMA system have proved the feasibility of the scheme.
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