We present the first experimental demonstration of a bi-directional cascaded arrayed-waveguide grating (AWG) access network combining one N×N AWG in the central office with multiple 1×N AWG’s at the distribution points, such as to individually address N 2 users with only N wavelengths. Downstream and upstream data share the same optical path. BER curves were measured using 2.5Gb/s data stream in each direction, and error free transmission achieved for downstream and upstream, with only 0.3dB power penalty for simultaneous transmission. The addition of two orthogonal polarization-multiplexed channels per wavelength doubled the number of possible end users. Error free transmission was achieved with simultaneous upstream and downstream transmission of a composite signal featuring eight 2.5Gb/s channels (2 polarizations ×4 wavelengths).
© 2004 Optical Society of America
The increase in end-user bandwidth demand, along with the decrease in WDM component cost, implies that WDM-based devices are likely to offer performance enhancements in multiple-access networks. Arrayed-waveguide gratings (AWG’s), with their potential low cost, high wavelength selectivity, low insertion loss, small size and cyclic wavelength routing characteristics (Latin routing) are probably one of the best candidates for such an application. In a cascaded format, they offer high-level wavelength routing as well as flexible and efficient bandwidth allocation . In addition, they can be tuned by varying the characteristics of their waveguides and perform space-wavelength switching functions .
So far, cascaded architectures have mainly been investigated theoretically [1,3], whilst experiments have been carried out using star architectures . In this paper we present the first experimental investigation of a scalable access network architecture that can serve large numbers (thousands) of concurrent clients, using only a small number of wavelengths and sharing the same optical path for both downstream and upstream transmission. This is achieved by cascading AWG’s and exploiting their Latin routing characteristics. In Section 2, we discuss the proposed network architecture and comment on its performance by presenting typical eye diagrams and BER measurements. Simultaneous bi-directional error free transmission of 2.5Gb/s downstream and 2.5Gb/s upstream per end-user data was achieved, with negligible crosstalk between the downstream and upstream channels.
In Section 3 we apply the concept of polarization division multiplexing (PDM) to further double the potential number of end users of the original architecture. The different aspects of the new network architecture are described and the quality of transmission is investigated. We simultaneously and bi-directionally transmitted four wavelengths, each carrying two orthogonal PDM 2.5Gb/s data streams. Measurements showed that the degree of orthogonality between the two multiplexed polarizations is maintained within acceptable limits , resulting in negligible crosstalk between the channels.
2. Access network based on Latin routing characteristics of AWG’s
2.1 Network architecture
A cascaded AWG access network architecture was proposed in , where the central office housed an N×N voltage controlled AWG. The advantage of such a device, in comparison with commercially available passive AWG’s, is that spatial-wavelength routing can be easily achieved by tuning the applied voltage. Thus, just one input port of the central office AWG can be used for downstream transmission, whilst the remaining ports are used for upstream detection. Due to the fact that such active space-wavelength switching AWG’s are not yet commercially available, the architecture was modified in order to demonstrate the desired wavelength routing using a passive N×N AWG, circulators, and an opto-mechanical switch instead. Figure 1 shows a block diagram of the network architecture as built in the laboratory.
A passive N×N (16×16) AWG was used in the central office, whilst passive 1×N (1×32 or 1×40) AWG’s were used in the distribution points. The use of an opto-mechanical switch, along with the AWG in the central office, resulted in a similar function to that of the voltage controlled AWG. Thus, we could address any end-user by allocating any wavelength to any output port, by selecting the appropriate input port of the N×N AWG. The central office transmitter was based on an array of DFB lasers. The lasers were multiplexed by a WDM-coupler, modulated, and launched through the N×N AWG to the desired distribution point. An Erbium-doped fiber amplifier (EDFA) was used to boost the signal powers before the N×N AWG. The optical network units (ONU) were designed so that detection of the downstream data and transmission of upstream data could be simultaneously performed. This was achieved by connecting a DFB laser at the assigned wavelength and a photodiode to a circulator. For the upstream detection, circulators were inserted at the input ports of the N×N central office AWG, thus directing the signals from the ONU’s to a photodiode array. For the experimental network, standard single-mode fiber (SMF) lengths of 4m were used between the CO and DP, and DP and ONU’s respectively, although in practice lengths of several kilometers will be required. All AWG’s had 100GHz channel spacing and channel isolation greater than 25dB. The N×N device had 9dB insertion loss, whilst the 1×N AWG’s had up to 4.5dB insertion loss. The lasers at the central office were modulated at 2.5Gb/s, using Mach-Zehnder (MZ) modulators, whilst the ONU lasers were directly modulated at the same bit rate.
2.2 Experimental results
Simultaneous downstream and upstream transmissions were achieved using up to four different lasers. All the wavelengths matched ITU-standard frequencies and were modulated with 27-1 PRBS data. The PIN diodes had a 2.5GHz bandwidth. Typical eye diagrams for three different wavelengths are shown in Fig. 2, where top and bottom eyes correspond to downstream and upstream transmission respectively, whilst data were simultaneously sent in both directions.
The lasers were set at 1542.1nm, 1550.1nm and 1553.3nm wavelengths, corresponding to Figs. 2(a), (b) and (c) respectively, and had output power in the 1.5dBm range. Error free transmission was achieved. The bit rates were limited by the available equipment but could be further increased to at least 10Gb/s without any degradation. The eye diagrams are wide open and prove that the system is stable and that the crosstalk between channels is negligible. BER measurements were performed for several channels and found to be similar. Typical BER curves are given in Fig. 3 for the wavelength 1542.1nm. Measurements were performed for both one-way and simultaneous bi-directional transmissions. The comparison of the back-to-back with the downstream or upstream measurements shows a 1.1dB loss introduced by the network. This was caused by the addition of two AWG’s and a circulator into the optical path, increasing both power loss and sources of crosstalk, thus degrading the signal-to-noise ratio. An additional loss of only 0.3dB was introduced due to crosstalk between the downstream and upstream channels in the simultaneous bi-directional transmission case. The 1.1dB shift (green arrow, Fig. 3) between the downstream and upstream measurements is mainly due to the distortion introduced by the directly modulated lasers employed for the upstream sources.
3. Polarization multiplexing technique
3.1 Polarization multiplexing source and network architecture
We then applied the PDM technique to further double the number of concurrent users that can be served. Although it is currently not economically viable in the access, we believe PDM may still be a potential upgrade possibility in the future. PDM is based on the transmission of two orthogonal polarizations at the same wavelength and demultiplexing them at the ONU, so that two different users can be addressed by using just a single wavelength . Previous work has demonstrated PDM over 45km of SMF at 4Gbit/s, suitable for access distances . A block diagram of the modified network is shown in Fig. 4. A polarization and wavelength division multiplexed source (PWMD)  featuring two polarizations times four wavelengths (8 channels in total), each carrying a 2.5Gb/s 27-1 PRBS data stream, was used for both upstream and downstream transmission after splitting its output through a 50% coupler. Two EDFA’s made possible an independent power adjustment in both directions. The simple use of the PWDM source instead of directly modulated lasers for the upstream transmission required a slight modification of the ONU, such that one out of the possible four wavelengths was fed to the upstream through a circulator. In the downstream direction, a polarized beam splitter (PBS) placed after the circulator was used to demultiplex the two orthogonal polarizations (Fig. 4 ONU (a)). When based on stand-alone emitters at the ONU, polarizations would first be demultiplexed with a PBS, with each of its two outputs connected to a circulator with an emitter-detector assembly, as shown in Fig. 4 ONU (b). In order to demultiplex upstream data in the central office, a circulator was inserted on the input side of the N×N AWG. Following wavelength demultiplexing, the two orthogonally polarized signals were again demultiplexed. The remaining part of the cascaded AWG architecture was similar to that of Section 2.
3.2 Experimental results
The PWDM source was optimized to supply two orthogonal polarizations carrying 2.5Gb/s 27-1 PRBS data at 1544.5nm, 1545.3nm, 1546.1nm and 1546.9nm, with signal output power in the 1.5dBm range. The composite signal was then simultaneously transmitted, both in the downstream and upstream directions. The upper part of Fig. 5 ((a) first polarization A and (b) second polarization B) shows typical eye diagrams for downstream transmission at 1546.1nm, whilst the bottom part shows the corresponding case for the upstream path. These measurements were taken whilst the eight available channels were simultaneously transmitted in both directions. Although there is slight crosstalk between the two PDM channels, error free transmission was achieved and the system found to be stable.
BER measurements were carried out at both ONU and exchange receivers for all four wavelengths, using simultaneous bi-directional transmissions together with PDM. Typical BER curves for the 1546.1nm wavelength are shown in Fig. 6. The blue and green plots represent the two downstream channels, whilst the red and black are for the two upstream channels. Comparison of the downstream channels shows a 0.2dB difference. This is due to the slightly different characteristics of the MZ-modulators used for the two channels. Again the network introduces an additional 1dB power loss, with respect to the back-to-back transmission. The same power loss can be noticed for the upstream transmission channels, since the same source was used for both transmissions and they followed the same path.
PDM requires realignment of the orthogonally polarization-multiplexed states of polarization (SOP’s) at the ONU. Fluctuations of the SOP’s are mainly due to thermal and vibrational effects on the transmission fibers, and as these are intrinsically slow, they can be actively compensated using a feedback loop in the polarization controllers of the ONU. The preservation of orthogonality during the transmission is also of crucial importance, as any degradation is directly converted into crosstalk. However, we have recently shown that the effect of a single AWG on orthogonality is negligible. Moreover, we found that the effect of two cascaded AWG’s is mainly additive, and thus does not significantly further degrade the quality of the transmission. Temperature effects on the AWG’s characteristics have also been found to be negligible . With respect to polarization, AWG’s are linear devices, and thus should not degrade the degree of polarization. However, they suffer from polarization dependent losses (PDL), which is the main cause for the loss of orthogonality. It is also important to note that the SOP at the ONU can be recovered as long as the polarization mode dispersion is smaller that the coherence length of the laser. Thus, the laser linewidth is the ultimate limitation of this transmission scheme.
To our knowledge, this is the first demonstration of a bi-directional cascaded-AWG-based access network using 4 wavelengths, one 16×16 AWG router in the central office, and 1×32 and 1×40 AWG’s at the distribution points, with common optical path for downstream and upstream transmission, offering identical 2.5Gb/s data rate in both direction. Error free transmission was achieved, with only a 0.3dB power penalty due to simultaneous transmission in the common optical path. Moreover, this is also the first demonstration of PDM for doubling the number of users in such access network architecture. The experiment features the use of up to four wavelengths, each carrying two orthogonally multiplexed polarizations, modulated at 2.5Gb/s. Measurements of BER curves showed error free transmission, whilst simultaneously transmitting the eight available channels in both the downstream and the upstream direction. Moreover, we found that the reduction of orthogonality between the two polarization channels, due to the AWG’s, is negligible, and results in minimal crosstalk. Thus, cascaded AWG’s and polarization both offer a viable solution to the upgrade of existing passive optical networks towards active wavelength switched high capacity access networks.
References and links
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