1. Introduction

Bandwidth demand to support new broadband applications requires an upgrade on the existing copper access infrastructure comprising the local loop. Perceived wisdom dictates that passive optical network (PON) architectures will provide the final wired solution for residential users and small businesses, offering bandwidths ranging from 30Mbit/s to more than 50Mbit/s, according to incumbent initiatives [1].

To utilize the optical fiber bandwidth effectively and therefore increase subscriber volume, wavelength division multiplexing (WDM) is expected to provide the next step in PON deployment. WDM-PONs have been reported with up to 128 users at 1.25 Gbit/s to 2.5 Gbit/s per user/wavelength, offering both great security and protocol transparency [2].

The focus of the work presented in this paper is to capture the merits of time and wavelength multiplexing in a new access network architecture encapsulating scalability and interoperability of TDM and WDM-PONs. This can be achieved by utilizing coarse WDM (CWDM) [3, 4] to route multiple physical PON locations to a common optical line terminal (OLT). Hence the network will exhibit dynamic wavelength assignment to manage network resources across multiple physical PONs according to traffic penetration and requirement in bandwidth.

2. Network architecture

The new network architecture, shown in Fig. 1, utilizes a single routing device based on a coarse array waveguide grating (AWG) which is placed in the OLT. The AWG is designed to serve concurrently multiple TDM and WDM physical PONs using a single tunable laser (TL) at the OLT. The 4×4 AWG channels utilize 20 nm-spaced standard coarse passband windows, centered on four ITU-T channels λ₁-λ₄, each of which can incorporate up to 16 dense wavelengths to address all ONUs on a PON. This is illustrated in Table I where each row represents a coarse channel, populated by 16, 0.4 nm spaced dense wavelengths. Each PON could potentially support up to 32 ONUs resulting to a total of 128 subscribers over 4 physical PONs.

 

Fig. 1. Multi-PON network architecture

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In downstream, TL1 will utilize the dense wavelength positioned at the centre of the AWG coarse channel λ₁ to address for example all ONUs connected to TDM-PON1 in a broadcasting manner, although it could potentially use any wavelength within the same passband.

To service a WDM-PON, TL1 will have to employ 16 dense wavelengths, centered ±3.2 nm around another AWG coarse channel e.g. λ₂, to address all 16 ONUs in WDM-PON4. This is possible since the AWG exhibits standard 7 nm passband channels [4] where network ONUs are mapped and routed in the form of single coarse channels.

The interoperability of TDM and WDM-PON is a key feature of the architecture since it allows smooth migration to multi-wavelength optical access by substituting the splitter in the distribution point with a multiplexing unit, justifying parallel operation of TDM and WDM PONs to address bandwidth requirements from subscribers with different service levels more effectively.

Table 1. Coarse and Dense channels allocation

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In the ONU reflective semiconductor optical amplifiers (RSOAs) are intended to be integrated to implement colorless transceivers, thus no modification in customer premises equipment is required in the migration from TDM to WDM. In addition, the use of a TL in the OLT allows for cross-operation integration management of network load by dynamically assigning downstream-transmission wavelengths to each PON in tandem with continuous waves to be used by the RSOAs [5] for upstream transmission. The continuous waves will be modulated by the RSOAs in the ONUs and subsequently routed as coarse channels upstream through the same path used for downstream transmission due to the AWG’s reciprocal nature.

The network is also highly scalable since extra tunable lasers can be easily incorporated at unused AWG ports in the OLT to increase bandwidth provision at high traffic conditions. As shown in Fig. 1, TL1 uses 16 dense wavelengths λ₂ ^1-16, positioned within coarse channel λ₂ to address all ONUs of WDM-PON4. In case the total bandwidth demand in the network cannot be met by TL1 alone, an extra tunable laser, TL2, will be connected to the second AWG port to share the load using the Latin-routing property of the AWG [6]. As shown in Fig. 1, TL2 uses 16 dense wavelengths λ₁ ^1-16, this time positioned within coarse channel λ₁, to address all ONUs of WDM-PON4. This is possible since the 1×16 AWG in the distribution point utilizes multiple free spectral ranges (FSRs) to cover all the coarse channels used by the 4×4 AWG. In that case a suitable algorithm should be adapted to prevent both TLs from addressing the same ONU simultaneously.

Finally, the ratio of subscriber number to OLT transmitters and receivers in the network is fairly high, allowing for low inventory count in the OLT. As shown in Fig. 1, all ONUs served by TL1 downstream, will be terminated upstream to a single receiver, RX1.

3. Network modeling and results

A physical layer simulation was initially devised in Virtual Photonics Inc. (VPI) to model the routing performance of a Gaussian coarse passband AWG. Previous simulation work has demonstrated the capability of a flat-response device to multiplex 16 ONUs comprising two physical WDM-PONs over its 7 nm coarse channels to a common destination [7].

To investigate in particular the influence of the Gaussian response, in the power distribution and routing of wavelengths within the AWG coarse bandpass, WDM-PON4 was utilized to demonstrate bidirectional transmission of a total of 16 ONUs, as shown in Fig 1. A Mach-Zehnder (MZ) modulator was utilized to externally modulate a fixed laser source at 2 dBm upstream to allow for a safety margin of 3 dB. A 2^7-1 long pseudo-random bit sequence (PRBS) at 2.5 Gbit/s was applied at the modulator’s RF input to reduce the simulation time. Externally modulated lasers were used at this stage of experimentation instead of RSOAs for the purpose of demonstrating the AWG Gaussian channel routing.

In the subscriber premises each of the modulated wavelengths, λ₂ ^1-16, corresponding to 16 ONUs, ranging from λ₂ ¹=1553.33 nm to λ₂ ¹⁶=1547.32 nm, was applied to a circulator to allow bidirectional transmission, multiplexed and transmitted over 20 km of standard single-mode fiber (SMF) and subsequently applied to the AWG router. In the OLT another circulator was used to separate upstream and downstream transmission followed by a PIN photodetector in the RX1 module where all upstream data was terminated.

In downstream a random sequence of similar specifications to upstream was employed to externally modulate a fixed laser source. Wavelengths centered around coarse channel λ₂=1550 nm were utilized to address the ONUs of the WDM-PON, all of which were terminated to a single PIN detector.

3.1 Network routing performance

The potential of each Gaussian coarse channel to route all 16, 0.4 nm-spaced dense wavelengths of each PON over the same passband at acceptable error rates, was investigated by consolidating the polarization properties of the device. Polarization-dependent wavelength (PDW) shift of wide passband AWGs in particular is known to impose an obstacle in multi-wavelength transmission and therefore limit the routing performance of the network. To perform simulations for the worst case scenario, PDW shifts of up to 1.8 nm at room temperature [4] were considered in the model. Device thermal stability was not taken into consideration since in the proposed topology the AWG is placed in the OLT and consequently it is assumed to operate at steady temperature.

Figure 2 displays the measured PDL versus operating wavelength at polarization shifts of 0, 1.5 and 1.8 nm, using the Mueller Method [8]. The vertical lines represent the ±3.2 nm spectral width required for transmission of all wavelengths in a PON. As becomes evident from Fig. 2, the PDL varies from 0 dB at centre wavelength to more than 5 dB outside the transmission band. As expected, the PDL for 0 nm curve exhibits negligible PDL across the entire transmission passband. On the contrary, the PDL for the 1.5 nm and 1.8 nm curves demonstrate asymmetric, linearly increasing loss with regard to the centre wavelength since, due to shifting, the longer wavelengths suffer extended PDL. It can be further observed that the longest wavelength in the passband, at λ₂ ¹=1553.33 nm, experiences the highest PDL with approximately 2.5 dB more loss than the central wavelength at λ² ₉=1550.12 nm. However, for the worst case scenario 1.8 nm PDL curve, the loss measured at the most affected wavelength of λ₂ ¹=1553.33 nm, does not exceed 3 dB. This is acceptable since it falls within the allowed 3 dB margin.

 

Fig. 2. PDL wavelength dependent

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Phase errors in the AWG can also contribute towards the rise of PDL. Although typical phase errors of approximately 108º have been recorded in literature [9], assuming 30 mm waveguide lengths [4] a worst case phase error of 270° was considered in the simulation model to abide by vendor’s communications [10].

To demonstrate the effect of wavelength shift, phase errors and loss superimposed at the transmitted wavelengths, Fig. 3(a) displays the PDW shift of the central channel λ₂=1550 nm, for which the transverse magnetic (TM) response is -1.8 nm shifted with respect to the transverse electric (TE). Figure 3(b) shows the resulting passband, where all 16 wavelengths are linearly vertically polarized and consequently routed using the TM response solely. It can be observed that the longest wavelength at λ₂ ¹=1553.33 nm suffers additional 5.5 dB loss, compared to the central wavelength at λ₂ ⁹=1550.12 nm, due to the increase of PDL and the Gaussian response. Similar behavior was monitored for all other coarse channels.

 

Fig. 3. Polarization-dependent wavelength shift of central channel

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Consequently to evaluate the network bidirectional performance, BER responses were drawn for the longest wavelengths at λ₂ ¹=1553.33 nm and λ₄ ¹=1592.52 nm of the coarse channels λ₂=1550 nm and λ₄=1590 nm respectively as shown in Fig. 4. In that sense the best and worst case scenario coarse channels in the middle and boundaries of the device FSR and the highly attenuated wavelengths within are investigated. Results confirm error free transmission with measured BERs of 10^-9 at -29.5 dBm receiver sensitivity for both upstream and downstream with no more than 0.5 dB penalty between the longest wavelengths due to the AWG routing across the device FSR.

 

Fig. 4. Measured BER for upstream and downstream transmission

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4. Conclusion

The novel access network architecture presented in this paper demonstrates high scalability and interoperability among TDM and WDM PONs by means of a single OLT based on a coarse AWG router with dynamic bandwidth allocation features. Simulation results have confirmed successful routing of 16 ONUs per physical PON using the 7 nm-wide Gaussian passband windows of a devised AWG with no more than 3 dB PDL incurred on transmitted wavelengths due to a worst case polarization-dependent wavelength shift of 1.8 nm and worst case device phase errors of 270°. BER analysis demonstrated error free performance with achieved rates of 10^-9 at -29.5 dBm receiver sensitivity and 0.5 dB maximum power variation between the coarse channels. The ability of the OLT to reach each interconnected PON in more than one path allows the proposed architecture to allocate bandwidth on demand according to subscriber service levels.