WDM-PON Technologies

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White Paper

WDM-PON Technologies

Contents1- Introduction 2- WDM-PON Architectures -Type 1 : Tuneable Laser -Type 2 : Sliced Broadband Source -Type 3 : Reflective Architectures a) Injection locked Fabry-Perot Laser b) Single polarisation RSOA c) Polarisation independent RSOA d) Reflective EAM 3- Experimental evaluation of a Single Polarisation RSOA in WDM-PON 4-Conclusion 5- References 6- Appendix ( RF and bias settings)

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1. IntroductionPassive optical networks (PONs) were originally developed in the 1980s [1] as a cost effective method of sharing fibre infrastructure for narrowband telephony (TPON) to business premises. Since those early days the application of PONs has moved on to interactive broadband networks implemented as either BPON (Broadband PON) or Ethernet PON (EPON) and now GPON (Gigabit PON) [2].Optical Network Unit #1 Central Office (CO) ONU #2 ONU #3

Passive Optical Power Splitter

ONU #4

ONU #N

Figure 1: Basic PON diagram

All these systems are based on the same idea of time sharing the optical medium by TDMA (time division multiplexed access). However, it has long been realised that using wavelength division multiplexing (WDM) offers an alternative method of sharing the capacity of a PON between multiple users and would offer advantages in terms of capacity, low latency and service transparency. Whilst the concept of using WDM within a PON has been widely demonstrated within research projects it is only recently that the enabling technology has become sufficiently mature for commercial consideration. Recently the interest in WDM -PONs has grown significantly, especially in parts of Asia, and it is widely believed to be the route towards the next generation of PONs. The technology challenge for WDM-PON has been to avoid the need for expensive wavelength selective optical components in each end-users optical network unit(ONU). In practice this means that it is cost prohibitive to use the type of lasers currently available for long haul dense WDM (DWDM) transmission within a WDM-PON. Moreover, it would be impractical for each customers terminal to be built with a fixed single wavelength laser because managing the inventory of lasers would be complex and costly for the network operator. For the customers ONU to be colourless either a tuneable laser is required or an alternative WDM-PON network design based on a reflective architecture must be used. In the2

longer term the tuneable laser approach probably offers the highest performance WDM-PON with the greatest potential number of wavelength channels but at present the cost of tuneable lasers is still far too high. Moreover, a tuneable laser solution may require additional network control and management to set and maintain wavelengths. The reflective architecture takes a different approach since all of the individual wavelengths are provided by a shared network resource, such as shown in fig 2. In this scheme the upstream transmitter within the ONU only requires a reflective optical modulator.Optical Line Terminal (OLT)

Upstream Shared Network Source (e.g. ASE source) Downstream Signal

AWG

ONU #1 ONU X

1, 2, ,N

Downlink

RX R-MOD

Uplink

ONU #N RX X

Figure 2: Reflective PON diagram

Over the past 5 years there have been a wide range of reflective WDM-PON architectures reported in the research literature and more recently extensive network trials have been announced. This white paper first reviews the alternative approaches to the reflective WDM-PON architecture and then goes onto to discuss the attributes required from the key optical component within the ONU .

2. WDM-PON ArchitecturesWithin this paper only the upstream path is considered because this is the most demanding as far as the cost critical customers ONU is concerned. In principle the same transmission architecture can be used for both upstream and downstream as might be the case when a symmetric service in terms of bit rate is required. However, this may not always be the case as higher data rates are often required for the downstream. A further variation of the overall architecture is when it is necessary to both wavelength-share and time-share the PON [3]. In this situation a wavelength multiplex is used to3

increase the capacity of the primary feeders in the access network and the distribution PON capacity is then further shared using optical power splitters in a classical TDMA approach. Where combinations of both time sharing and WDM are used a greater power budget is required which can be stretching for some types of reflective architecture PON. In the schemes outlined below only wavelength sharing of the PON is considered for simplicity. Type 1 Tuneable Laser Scheme This uses a tuneable laser within the ONU and is a scheme that offers the ultimate in terms of optical performance and flexibility. If the splitters in the network are solely WDM devices, such as planar array waveguide grating (AWG) devices, the number of ONUs supported will be determined by the channel spacing of the AWGs and the tuning range of the laser. Use of broadband splitter/combiners at the distribution point and more wavelength selective filters at the central office would allow more channels to be accessed, although available power budget might then be a consideration. To design a system which used the available spectrum most efficiently and make good use of the available power budget a combination of both WDM splitter/combiners and broadband splitter/combiners would be used in the distribution network.

Central OfficeData Out

Upstream Spectrum A

A

1530nm-1565nm C band

Optical Receiver

AWG Splitter Splitters (or AWG) A

Customers ONUData In

Tx Spectrum Modulator 0.1nm

Tuneable Laser

Figure 3: Type 1 architecture: WDM-PON with tunable laser at the ONU.

The problem with the tuneable approach is that a much more sophisticated laser is required within the customers ONU compared to conventional EPON and GPON systems. Tuneable lasers also usually require internal wavelength lockers or an external network wavelength reference to ensure they operate at the correct wavelength channel. Furthermore, to maintain a stable laser operating regime, external modulation rather than simple direct laser modulation is the norm. When the PON uses WDM devices for combining signals, the constraints on wavelength control can be slightly eased because4

only when the laser is operating at the correct wavelength can the signal pass through the WDM, so the demands of wavelength control and blanking during laser start up are eased.

Type 2 Sliced Broadband Source In this case each ONU contains a broad optical spectrum source within the transmitter, such as a superliminescent light emitting diode (SLED). The broad spectral output of the customers ONU is connected to one port on a WDM device, which could be thin film filter or AWG based. Only the optical spectral components from the LED which can pass through the WDM channel are transmitted through to the central office and the remaining power is wasted. Although all the customers ONUs have identical SLEDs, because each is connected to a different port on the WDM combiner, it is possible to slice a different part of the available optical spectrum for each ONU. SLEDs would normally be used as the transmitter device in the customers ONU although, alternatively, the self-amplified spontaneous emission from a reflective semiconductor optical amplifier (SOA) can be used instead. The latter has the advantage of producing a greater optical output power for a lower drive current but has the disadvantage that it can be sensitive to optical back reflections resulting in an amplitude ripple in the output spectra and in the worst case laser action. When a simple LED is used as the transmitter it is only practical to have a few upstream data channels of 155Mbit/s from this scheme. With a RSOA as the slicing source up to 32 X 155Mbit/s has been reported [4]. The number of channels and the data rate for each channel is determined by the excess optical intensity noise produced by the slicing process which is itself a function of the ratio of the bit rate to the optical bandwidth of the sliced source.Central OfficeData Out C BA Upstream Spectrum 10nm

Optical Receiver

WDM Splitter WDM (Slicer)

C B A

Customers ONU

Data In

Spectrum from SLD Super Luminescent Diode (SLD)

1530nm-1565nm C band

Figure 4: Type 2 architecture: WDM-PON with broad spectrum optical source for the uplink at the ONU.

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Type 3 Reflective Schemes In this class of schemes a separate sliced source is used to seed the return path modulators within the customers ONU. This has the advantage over the previous non-seeded scheme because the optical power produced by the ONU transmitter is now all within the required spectral channel and so none is wasted. A second advantage with some schemes is that the excess intensity noise produced by the slicing process can be reduced by gain saturation effects. Four types of reflective WDM-PON are outlined below. Type 3A Reflective : Spectral Slicing with Injection locked Fabry-Perot LaserSLD or EDFA Broadband SourceSeed Spectra

Central OfficeData Out1530nm-1565nm C band

Seed

AWG Combiner

Optical Receiver

AWG SplitterUpstream Spectrum

Customers ONUA Data In

Return Spectrum (with FP modes)

1530nm-1565nm C band

Laser with Asymmetric facet reflectivity 1nm

Figure 5: Type 3A architecture: WDM-PON with Injection locked FP as ONU transmitter

In this scheme [5] a central broadband seeding source is used, this is typically the amplified spontaneous emission of an erbium doped fibre