WO2010000307A1

WO2010000307A1 - Apparatus and modules for an optical network

Info

Publication number: WO2010000307A1
Application number: PCT/EP2008/058407
Authority: WO
Inventors: Fabio Cavaliere; Pierpaolo Ghiggino
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-06-30
Filing date: 2008-06-30
Publication date: 2010-01-07
Also published as: EP2301171A1

Abstract

A set of modules for use in constructing an optical network, the set comprising at least two different types of module, wherein the different module types comprise: (a) a multi-wavelength distribution module comprising: at least one optical output port; at least two wavelength generators each capable of generating an optical signal comprising at least N different wavelengths, where N is an integer greater than 1; and a protection switching mechanism arranged to selectively connect only one of said wavelength generators at anyone time to said at least one optical output port; and (b) an optical modem module comprising: N electrical input ports;N electrical output ports;an optical input port for receiving modulated optical signals;an optical output port for outputting modulated optical signals;a further optical input for receiving an optical output signal comprising N different wavelengths from a multi-wavelength distribution module;a modulator arranged to receive up to N electrical input signals from the N electrical input ports and an optical output signal comprising N different wavelengths from the further optical input, and modulate up to N of the different wavelengths in dependence upon a respective one of the electrical signals to generate an output modulated optical signal comprising up to N channels for output at said optical output port; and a demodulator coupled to the optical input port for demodulating an optical signal comprising up to N channels into a set of N electrical signals, each electrical signal corresponding to one of the N channels, the modulator being coupled to the N electrical output ports for supplying a respective one of the signals to each port.

Description

APPARATUS AND MODULES FOR AN OPTICAL NETWORK

TECHNICAL FIELD

This invention relates to improvements in optical networks and in particular to a set of modules for use in constructing an optical network such as a broadband access network or an optical transport network, and to apparatus including such modules.

BACKGROUND

Optical networks have developed rapidly and development has generally been split into two types of network or transmission system. These types are sometimes known as the Broadband access network and the Transport network. They have evolved separately because of the different and sometimes conflicting requirements of each network in terms of distance reach, carried traffic and cost requirements. Broadband access networks have a shorter distance reach than Transport networks, the former enabling an end user to access the longer distance transport network.

Equipment for Optical Transport Networks (OTN) is generally more costly than access network equipment as it is more complex, with the last generation now able to carry up to 80 WDM (Wavelength Division Multiplexed) channels at 40Gb/s each over more than 1000km of links. Installation in nodes of Reconfigurable Optical Add Drop Multiplexers (ROADM) and Optical cross connects (OXC) based on wavelength selective switching (WSS) allows complex mesh topologies to be implemented without needing costly regeneration sites. ONT are currently often built to comply with a set of standards set by ITU-T (International Telecommunication Union - Telecommunication Standardization Section). ITU-T standards which define an OTN as being composed of a set of Optical Network Elements connected by optical fibre links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals.

Whilst Optical transport networks can cope with large distances, for the final relatively short hop from the network to a home a different optical network infrastructure has evolved known as Broadband fibre access. These networks are mainly based on passive optical networks (PON's). Classically these use one wavelength downstream from the central office to end users and another wavelength in the upstream direction from the end users to the central office. Conventional Broadband access systems are limited in how many end users can be supported and in reach distance. Typically the limits are 32 end users and a distance limited to around 1- 20km, due to splitting loses at the passive nodes used to distribute the signals.

Current progress in Broadband access is towards exploiting techniques used in transport networks in future broadband access systems in order to make more efficient use of the fibre capacity. However costs are an issue as the systems developed for transport networks are relatively complex.

On the other hand, the ever increasing demand for increased capacity of ONT and its transformation from a series of point-to-point links to a meshed topology increases the likelihood of the uncontrolled growth of the equipment required at each node. In conventional WDM systems, each transponder typically requires a separate card, as does each amplifier, dispersion compensating module, multiplexer, demultiplexer, ROADM, WSS OXC etc. Thus, deploying a fully equipped node often means in practise filling one or more rooms with racks full of power consuming equipment. SUMMARY

According to a first aspect the invention A set of modules for use in constructing an optical network, the set comprising at least two different types of module, wherein the different module types comprise: (a) a multi-wavelength distribution module comprising: at least one optical output port; at least two wavelength generators each capable of generating an optical signal comprising at least N different wavelengths, where N is an integer greater than 1 ; and a protection switching mechanism arranged to selectively connect only one of said wavelength generators at any one time to said at least one optical output port; and

(b) an optical modem module comprising: N electrical input ports; N electrical output ports; an optical input port for receiving modulated optical signals; an optical output port for outputting modulated optical signals; a further optical input for receiving an optical output signal comprising N different wavelengths from a multi-wavelength distribution module; a modulator arranged to receive up to N electrical input signals from the N electrical input ports and an optical output signal comprising N different wavelengths from the further optical input, and modulate up to N of the different wavelengths in dependence upon a respective one of the electrical signals to generate an output modulated optical signal comprising up to N channels for output at said optical output port; and a demodulator coupled to the optical input port for demodulating an optical signal comprising up to N channels into a set of N electrical signals, each electrical signal corresponding to one of the N channels, the modulator being coupled to the N electrical output ports for supplying a respective one of the signals to each port.

The present inventors have realised that this set of modules forms a basic building block that can be used to construct a number of different types of nodes for different types of network, with such modules optimally allowing the exploitation of the advantages of integrated technology whilst maintaining the degree of modularity needed to form different types of node.

The set of modules may further comprise at least one of the following module types:

(c) an FEC module comprising: a FEC encoder; a FEC decoder; a first group of ports comprising N electrical input ports and N electrical output ports, each input port being connected to one of the output ports through the FEC decoder; and a second group of ports comprising N electrical input ports and N electrical output ports, each input port being connected to one of the output ports through the FEC encoder; and

(d) a cross connect module comprising: K sets of ports where K is an integer, each set comprising N electrical input ports and N electrical output ports; and an array of switchable interconnections which are configurable to enable any input port of a set of N inputs to be connected to any output port of any of the other sets of N output ports.

Using the above building modules it is possible to configure all of the nodes in a transport or access optical network, allowing for volume, stocks & manufacturing optimization (and thus reducing the overall cost of forming the nodes). Further, use of such modules allows for the network design rules to be relatively simple (due to the limited number of modules) and thus optimizes the network design process, reducing the design time and increasing the efficiency of the network be it a process.

Each module type may be provided as a single card or rack mountable board, or perhaps two cards or boards. By this we may mean that each module type may occupy only a single, or perhaps two or maybe three slots of a rack mounting unit. For example, ETSI defines standard dimensions for a rack (e.g. 600 cm x 300 cm x 2 m). A typical example of board dimensions that can be rack mounted is 367 mm x 220 mm x 41 mm. It has been realised by the inventors that a set of the described four types of module provide all the key modules needed to implement almost any type of optical network node or terminal. The manner in which functionality is split between the modules gives a good compromise between increased integration on the one hand (putting more functions on each module) with reduced sizes and increased flexibility on the other hand (by making each module reasonably specific in its functions).

It has been appreciated that over integration can increase the cost of each module as they become more complex and this can also lead to many parts of a module being under used for a particular role in a network. Unused parts increases costs as it can lead to unwanted duplication between modules when they are combined.

In the past, the functionality of any one of the modules of the invention has been provided by coupling together many different sub-units which each have a dedicated role, e.g. individual modulators, amplifiers, single wavelength generators etc. Network nodes and terminals have essentially been bespoke arrangements of individual building blocks which each perform only one specific role.

Dividing the components in the manner of the described module types has been found to offer several other advantages. The module types (c) and (d) are all electrical and lend themselves well to integration on a single chipset or processor board. Standard CMOS processing techniques can be used which are relatively well established and inexpensive.

Module (a) includes mostly optical components and the others mostly electronic components. In particular modules (b) (c) and (d) may be free of any optical sources. By splitting the modules in this way all of the components of modules (b) to (d) can be fabricated using CMOS technologies if required. More specifically, providing the FEC module separate from the modem module is advantageous because its specific implementation strongly depends on signal format and bit-rate and it is power consuming, while the same generic modem could be used in principle for various bit-rates, e.g. 1.25 Gb/s, 2.5 Gb/s and 10 Gb/s signals. Moreover, it could be necessary to split each FEC module over two boards, due to power consumption issues, while keeping a single multi-modulator board.

There is a further advantage to separating the multi-wavelength source from the modem. It is generally not possible today implement a laser using CMOS techniques. Keeping a separate multi-wavelength source, it is possible, at least in principle, to use CMOS techniques for all the other modules.

Finally, there is an advantage in keeping the cross-connect matrix module separate from the others because it can be skipped in some configurations and may be bit-rate independent, at least partially.

In the case of module (a) the optical sources may comprise laser diodes or solid state lasers or light emitting diodes. These may be more expensive and time consuming to produce than the all electrical modules and so the functionality is best kept in relatively dedicated modules.

Said at least one optical output port of said multi-wavelength distribution module may comprise M output ports where M is an integer greater than 1. Said multi-wavelength distribution module may further comprise an optical splitter coupled between the protection switching mechanism and said M output ports for splitting the optical signal into M output optical signals, and for supplying each of said M output optical signals to a respective output port. The M output optical signals may each comprise said at least N different wavelengths. The modem module type (b) may be arranged to receive at least one optical signal from an output port of the multi-wavelength distribution module and to modulate that signal using its modulating means. Where the optical signal from the multi-wavelength distribution module has up to N discrete wavelengths or frequencies the modulator may modulate each of the frequencies with a different one of the up to N electrical signals received at its N electrical input nodes.

In a preferred arrangement of module (b) the electro-optic modulator may include N semi-conductor optical amplifiers (SOA's) in an array together with N electroabsorption modulators (EAM's) and an arrayed waveguide grating (AWG). The N optical signals, each at one of the N wavelengths from the module (a) may each be applied to a respective one of the SOA's which then modulates the optical signal with the electrical signal applied to a respective one of the N input port, the output of each SOA being passed through the EAM and finally all the outputs from the EAMs being combined by the AWG to be passed to the optical output port. The module (b) therefore does not need to include an optical source in its componentry.

The use of SOA's to modulate optical signals is well known. Of course, other electro-optic modulators could be used if preferred in some arrangements.

In a preferred arrangement the module (d) may be configured to function as a K way cross-connect or K way splitter with a capacity of N channels for each of the K ways in which each of the N channels of a set is connected to a corresponding channel in each of the remaining sets. K is typically small, say 1 , 2, 3, 4 or 5 or so. The cross connections may be user configurable although they may be hard wired in the cross connect and non-configurable. In a preferred implementation, the same module (b) may be compatible with signals having various bit-rates, such as 1.25 Gb/s, 2.5 Gb/s, 10 Gb/s, 40 Gb/s.

In the module (a) the same signal is preferably present at each port: it is composed by optical waves of at least N different wavelengths (typically N=40 or 80, according to the frequency grid defined by the ITU-T G.694 recommendation family). There are various methods reported in the specialized literature to implement the source of multi-wavelength generator: wavelength locked laser arrays, frequency multiplication effects in non-linear optical media, optical loops including semiconductor optical amplifiers and periodic optical filters, etc.

The multi-wavelength distribution module type may include an optical splitter which splits the M output ports into two or more groups of output ports, each port of a set carrying signals which replicate the signals carried by a corresponding port of the other set or sets. A set may comprise 1 or more output ports, and most preferably 2, 3, 4 or more sets.

The splitter of the multi-wavelength distribution module may supply all of the N generated wavelengths to each of its M output ports. Alternatively it may only supply selected ones of the wavelengths. The splitter may be replaced by an M port filter which enables the module type to only pass selected ones of the wavelengths generated by the generator to the output ports. For example, an optical deinterleaver (a widespread components in optical transport systems) could be used to send N/2 frequencies ("odd channels") to one of two ports and other N/2 frequencies ("even channels") to a second port. In this example, M=2.

The multi-wavelength distribution module may include a generator which comprises a comb generator that supplies N different frequencies onto a single port which is passed to the (optional) splitter. For resilience it may comprise two generators, each replicating the other, and a protection switching mechanism which selectively connects one of the generators but not the other at any one time to the output ports. If one fails, the protection switching mechanism may switch over to the other generator.

The value of K is preferably a small integer typically less than or equal to 4. The value of N may be chosen to be equal to or greater than 40 which provides compatibility with current standard such as the ITU-T grid standard. N may be equal to 40 or 80 for example. Where N is 40 the multiwavelength module may generate N wavelengths at 100 GHz spacing, and where N is 80 they may have a 50GHz spacing. The module type (a) may include a generator which generates 40 or 80 wavelengths, and the module types (b) and (c) may include electrical input and output ports that are arranged in sets of 40 or 80 to match the generator.

According to a second aspect the present invention provides an optical network node for use in an optical communications system having a modular architecture which is constructed from a combination of at least two different module types, the module types comprising: (a) a multi-wavelength distribution module comprising: at least one optical output port; at least two wavelength generators each capable of generating an optical signal comprising at least N different wavelengths, where N is an integer greater than 1 ; and a protection switching mechanism arranged to selectively connect only one of said wavelength generators at any one time to said at least one optical output port; and

The network node may further comprise at least one of the following module types:

The node may be constructed from modules of a module set in accordance with the first aspect of the invention, and as such any features described in relation to that first aspect may be incorporated into the second aspect of the invention.

The modules may be configured such that the optical modem modules provide the connection between the node or terminal and an input and output optical fibre path of the network which connects the node or terminal to and from at least one other terminal or node of the network.

The modules may comprise discrete units perhaps provided on a single card or two cards. They may embody any of the features of the modules of the first aspect of the invention.

In one arrangement the optical network terminal or node may comprise a general node or terminal in which the modules are arranged as: at least one cross connect module, an FEC module whose first set of N FEC decoded electrical output ports pass signals are connected to one of the sets of N electrical input ports of the cross connect module and whose second set of N FEC encoded electrical input ports receive signals from a corresponding one of the same set of N electrical output ports of the cross connect module; an Optical modem module whose N electrical output ports pass signals to the first set of N electrical input ports of the FEC module and whose N electrical input ports receive signals from the second set of N electrical output ports of the FEC module; and a multi-wavelength distribution module of which at least optical output port is connected to the Optical modem module to provide an optical signal that is to be modulated.

The cross connect module may be omitted in some general node or terminal arrangements.

The arrangement may be extended to include the same arrangement of FEC, modem and distribution module for each of the K sets of N electrical input/output ports of the cross connect module. In this general node, multiplexed optical signals with N channels can be sent and received by the modem, and converted into electrical demodulated signals for passage through the FECs and cross connect.

In another notable arrangement of modules, the optical network node or terminal may comprise a modular optical line terminal (OLT) architecture for a WDM PON (broadband access) or Transport line terminal (Transport network).

The OLT terminal may comprise: an optical modem module which receives optical signals from a network at its optical input node and transmits modulated optical signals to the network at its optical output node; an FEC module whose first set of electrical inputs receive signals from the electrical outputs of the modem and whose second set of electrical outputs pass signals to the electrical inputs of the optical modem; and a multi-wavelength distribution module of which at least optical output port is connected to the Optical modem module to provide an optical signal that is to be modulated.

In a still further alternative the node may comprise a 3R multichannel regeneration node for a transport network comprising: two FEC modules which are cross connected so that the first set of electrical output ports of a first FEC module are connected to the second set of electrical input ports of the other FEC module and vice versa, two modems, each connected having its electrical output ports connected to the first set of electrical input ports of a respective FEC module and its electrical input ports connected to the second set of electrical output ports of the respective FEC module, and a multi-wavelength distribution module of which a first optical output port passes and optical signal to one of the modem modules and a second optical output port passes an optical signal to a second one of the modem modules. The two output ports of the multi-wavelength distribution module may each pass the same optical signal, which may comprise N discrete wavelengths.

The regeneration node does not require a cross -connect module.

In a still further arrangement the optical network node may comprise an K way N channel cross connect for a transport network comprising K modem modules, K FEC modules, one cross connect module and one multi- wavelength distribution module.

For example, in a four way N channel cross connect there will be provided a cross connect module having 2 x k x N output ports where k=4. Four FEC and four modem modules will be needed. A single modulator module having 4 output ports can be connected to each of the modems, or two modulators could be used or four. The former is preferred as it minimises the number of modules and hence cards or boards needed and saves space.

In a third aspect, the present invention provides a method of configuring an optical network comprising: determining the required functionality of a node for use in the network, and selecting any of the above modules required to form the node so as to provide the desired functionality. The method may further comprise connecting the modules to form the node.

BRIEF DESCRIPTION OF THE DRAWINGS

There will now be described by way of example only several embodiments of the invention, with reference to the accompanying drawings in which:

Figure IA is a schematic illustration of an embodiment of a Multi- wavelength distribution module type (a); Figure IB is a schematic illustration of an embodiment of an optical modem module type (b;

Figure 1C is a schematic illustration of an embodiment of an FEC module type (c);

Figure ID is a schematic illustration of an embodiment of a cross connect module type (d);

Figure 2 is a schematic overview of a general node/terminal architecture illustrating the possible inter-relationships between the different module types of Figures IA to ID;

Figure 3 is a schematic overview of a modular optical line terminal (OLT) architecture for a WDM PON (broadband access) or Transport line terminal (Transport network);

Figure 4 is a schematic overview of a 3R multichannel regeneration node for a transport network;

Figure 5 is a schematic overview of a 4 way 40 channel cross connect for a transport network; and

Figure 6 is a flow chart of a method in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Figure l(a) to (d) illustrates four different optical integrated modules which together form an embodiment of a module set according to a first aspect of the invention. The modules represent the building blocks from which a wide range of terminals or nodes of different networks can be constructed, ranging from local broadband access terminals to transport networks. Each module is a single, stand alone, preassembled, integrated unit. It may be available off the shelf having been preassembled and tested by a supplier. A module may be provided on a single rack mountable card but could, if required due to limited space on a card, be provided on two cards which are to be interconnected. It is preferred that each module does not occupy more than one slot in a rack as this would reduce the advantage that the invention can provide, although in some cases it may occupy two slots. All the components of a module may otherwise be pre-assembled, tested and ready to be used by a network designer or installer. It is also notable that only module (a) includes an optical source and that all of the other modules may be constructed entirely using CMOS technology.

The first module 10 shown in Figure l(a) is a multi-wavelength distribution module and the second module shown in Figure l(b) is an optical modem module. These modules are essential modules in so far as at least one of each is required for each terminal or node of an optical network to be constructed. The other module types are an FEC (Forward Error

Correction) module (c) and a cross connect module (d) which are optional in that they need not be present in all nodes or terminal arrangements that can be constructed.

As shown in Figure l(a) a multi- wavelength distribution module type 10 has M optical output ports 110 where M is an integer, a wavelength generator for generating an optical signal comprising optical waves of N different wavelengths, and an optional optical splitter 4 for selectively connecting one or more (and typically all) of the N wavelengths generated by the generator to each of the M output ports. The generator may be a comb generator including a laser light source which produces N different discrete optical signals, each at a different wavelength. In fact, as shown two generators 1 , 2 are provided so as to provide a level of inbuilt redundancy should one fail. The generators may be identical. Such comb generators in themselves are well known and can be implemented without the need for N individual transmitters, e.g. ASE splicing, closed optical loops and non-linear fibre based generators. Typically a generator will generate 40 channels at 100GHz spacing or 80 channels at 50GHz spacing for use on an ITU-T grid.

A protection switching mechanism 3 ensures that only the light from one generator is used at a time. In a normal working condition the protection switching mechanism 3 selects the first generator but monitors the output of both generators. If the output of the first generator 1 falls too low, or is of poor quality, the mechanism 3 will switch to the second generator 2. The output of the protection switching mechanism 3 is a single optical fibre carrying a multi wavelength signal which is passed through a filter and splitter. The splitter 4 splits the signal onto multiple output ports. In the example shown the splitter is a 1 :2 splitter that splits the signal onto two output ports (M=2). It could split the signal more ways, e.g. M=3, 4, 5 or more. Each of the ports will be provided with N different wavelengths.

As shown in Figure l(b) the second module 20 is an optical modem module type having N electrical input ports 122, N electrical output ports 128, 1 optical input port 126 and 1 optical output port 124. A modulator 8

(electro optic modulator) is arranged to receive up to N electrical input signals at the N electrical input ports 122 and modulate each of them onto a separate wavelength of an optical signal to generate an optical signal having up to N modulated channels which is supplied to the 1 optical output port 124. A demodulator 9 (opto-electronic converter) is arranged to demodulate an optical signal having up to N channels which is received at the 1 optical input 126 into a set of up to N electrical signals, each corresponding to one of the N channels. The demodulator 9 connects each of the up to N electrical signals to a respective one of the N electrical output ports 128. This module 20 includes one or more chipsets (a reduced number less than N) to perform the demodulation and modulation. In the past this has been performed by N stand alone cards. The module may also include N photodiodes and N electrical receiver front ends (liner amplifier, limiting amplifier and clock and data recovery circuitry). The module 20 further comprises a further optical input 120 for coupling to an optical output of the multi-wavelength distribution module 10. The further optical input 120 is coupled to the modulator 8. The input 120 receives the optical signal comprising N discrete wavelengths, which is modulated by the modulator 8 in dependence upon the N electrical input signals to form the optical output signal comprising up to N channels.

Figure l(c) shows an FEC module 30 having a first group of ports comprising N electrical input ports 134 and N electrical output ports 136, each input port being connected to one of the output ports through an FEC decoder 6. A second group of ports comprises N electrical input ports 130 and N electrical output ports 132, each input port being connected to one of the output ports through an FEC encoder 7. In essence this module is a simple array, on one or two cards, of two sets of N independent codecs which all work in parallel to process N channels simultaneously.

The last module type 40, shown in Figure l(d) is a cross connect module having K sets of ports where K is an integer number e.g. K=3,4,5 or more. Each set comprises N electrical input ports and N electrical output ports, the cross connect module comprising an array 5 of interconnections which are configurable to enable any input port of a set of N inputs 142, 143 to be connected to any output port of any of the other sets of N output ports 144, 145.

Together these modules 10,20,30,40 provide the key building blocks from which a range of different types of network can be constructed. All that needs to be done, in a simple arrangement, is connect the ports on the modules to either electrical cable or optical cable (or leave unconnected in some cases) and connect to a power supply and the network node is ready to use. With rack mountable modules connection to a suitable power supply may require little more than slotting the modules into bays in a mounting rack.

Figures 2 through 5 illustrate some of the many and varied nodes and terminals that can be constructed from the modules of Figure 1.

General node architecture

Figure 2 is an example of a general node architecture showing the order in which the modules can be added together. The node presents the modular architecture, each module being represented by a dotted box in the picture. For ease of reference the numerals used in Figures IA to ID have been repeated to show common parts.

All four different types of module 10,20,30,40 are used in this general architecture i.e. a multi-wavelength distribution module, an optical modem module, a FEC module and a cross-connect module. The multi-wavelength distribution and the optical modem are present in all the configurations while the presence of the other modules depends on the node functionality. For example, the cross connect module is not present in a terminal node while is necessary in OXC and OADM nodes. Examples will be provided in the next section.

It will be seen that one set of N output ports from the cross connect module 40 is coupled to the FEC encoder 7 of the FEC module 30 for supplying electrical signals (the signals being received by the module 40 from another source, to which the cross connect module 40 is connected) to the FEC encoder 7 for encoding by the FEC encoder. Similarly, N ports from the FEC decoder 6 of the FEC module 30 are connected to the cross connector module 40, to allow the cross connect module 40 to switch the decoded electrical signals from the FEC module 30 to a desired destination. N electrical outputs from the FEC encoder 7 of the FEC module 30 are coupled to the multi-channel optical modulator 8 of the optical modem module 20. A further optical input of the optical modulator 8 is coupled to the multi-wavelength distribution module 10, for receiving an optical signal comprising N discrete wavelengths. The modulator 8 is arranged to modulate that optical signal with the data of the N electrical signals received at its input, and output the resulting modulated optical signal comprising up to N channels on the optical output. An optical input of the optical modem module 20 is connected to the demodulator 9 (i.e. an optoelectronic converter). The converter 9 is arranged to receive an optical signal comprising up to N channels and to demodulate the N channels into respective electrical output signals, with each of the respective electrical output signals being supplied via output ports to respective input ports of the FEC decoder 6 of the FEC module 30, for decoding.

A network node with this architecture can be provided which is much more compact than in the current generation of systems, thanks to the integrated technology used inside each module. With a limited number of building modules is possible to configure all the node in a transport or access optical network, allowing for volume, stocks and manufacturing optimization (and, as consequence, reducing the costs). Also the network design rules are much simpler than for current systems, thanks to the limited number of modules, and this allow to optimize the time and the efficiency of any bid process. Just one multi-wavelength distribution module can be used to distribute the wavelengths wherever they are needed in the node, saving the costs related to separated optical sources. Also thanks to the generation of all the wavelengths at the same time with one module, the system is always full loaded, with the consequent simplification of the optical amplifier design (no power setting tables vs. the channel count, no mechanism to compensate for power transients, no compensation of channel power depletion induced by the amplification noise). Modular OLT architecture

Figure 3 is an example of modular OLT architecture constructed using the set of modules. Specifically in a WDM Transport Terminal, the block 4 in the distribution module need not be present so that block is shown in dotted outline. In a WDM PON, as shown the block 4 could be a splitter or de- interleaver, that is a wavelength selective 1 :2 splitter that divides 80 input channels, 50 GHz spaced, generated by the WDM comb source, into two 100 GHz spaced WDM combs. The two combs are 50 GHz shifted each other. Then, one output ("odd frequencies") is sent to the multi-channel modulator (block 8) while the frequencies of the other output ("even frequencies") could be separated in the PON WDM splitter and sent to the ONTs, to modulate the upstream channel.

The FEC block can be present, if required to guarantee an acceptable performance.

Note that a 40 channels node requires from 3 to 6 cards (assuming from 1 to 2 cards for each module) while a conventional WDM terminal would require at least 40 transponders.

3R Multi-channel regeneration node

Figure 4 is an example of a 3R multi-channel regeneration node for a transport network. A 3R regeneration node regenerates optical signals in three domains, including power, shape and time. This node does not require the cross-connection matrix. For each propagation direction though the node the optical channels are detected and converted from optical-to- electrical -blocks 9 and 9'-, FEC decoded -blocks 6 and 6'-, and modulated - blocks 8 and 8' . The same multi-wavelength source is used for both the directions, using a simple optical splitter (block 4). A 40 channel system requires only 5 modules (from 5 to 10 cards) instead of 40 3R regenerator cards as is known. This gives a considerable reduction in complexity.

4ways x 40 channels cross connect

In the embodiment of a cross connect node for a transport network as shown in Figure 5 there are four bidirectional ways, each way supporting N channels: one channel coming from one way is redirected on whatever other way by the cross-connect module (block 5). For each way, the transmitted channels are modulated and, optionally, FEC encoded. The received channels are photodetected and, optionally, FEC decoded.

The advantage with respect to conventional systems is impressive: from 9 to 18 cards (9 modules) are required instead of 160 transponder cards, for a 40 channel system.

From the above, it will be appreciated that the different types of module may be used to form a number of different types of network node. Figure 6 shows a method of configuring an optical network using the set of nodes described herein. Firstly, the type of node is determined (102) i.e. the required functionality of the node is determined. Then, the appropriate combination of modules is selected (104) to form the node so as to provide the desired functionality. Subsequently, the node can be provisioned (106) i.e. the modules connected together, and coupled to the network.

From the above examples, it will be appreciated that the invention may provide many advantages over existing network arrangements using conventional components.

Claims

1. A set of modules for use in constructing an optical network, the set comprising at least two different types of module, wherein the different module types comprise:

(a) a multi-wavelength distribution module comprising: at least one optical output port; at least two wavelength generators each capable of generating an optical signal comprising at least N different wavelengths, where N is an integer greater than 1 ; and a protection switching mechanism arranged to selectively connect only one of said wavelength generators at any one time to said at least one optical output port; and

2. A set of modules according to claim 1 , further comprising at least one of the following module types:

(c) an FEC module comprising: a FEC encoder; a FEC decoder; a first group of ports comprising N electrical input ports and N electrical output ports, each input port being connected to one of the output ports through the FEC decoder; and a second group of ports comprising N electrical input ports and N electrical output ports, each input port being connected to one of the output ports through the FEC encoder; and (d) a cross connect module comprising: K sets of ports where K is an integer, each set comprising N electrical input ports and N electrical output ports; and an array of switchable interconnections which are configurable to enable any input port of a set of N inputs to be connected to any output port of any of the other sets of N output ports.

3. A set of modules according to any preceding claim in which each module type is provided as either a single rack mountable board or across two rack mountable boards.

4. A set of modules according to claim 3 in which each board is compatible with an industry standard racking system which enables the components of the board to be connected to a suitable remote power supply.

5. A set of modules according to claim 1 , 2, 3 or 4 in which the cross connect module is configured to function as a K way cross-connect or K way splitter with a capacity of N channels for each of the K ways in which each of the N channels of a set is connected to a corresponding channel in each of the remaining sets.

6. A set of modules according to any preceding claim in which the modem module does not include any optical sources.

7. A set of modules according to any preceding claim wherein said at least one optical output port of said multi-wavelength distribution module comprises M output ports where M is an integer greater than 1 , and said multi-wavelength distribution module further comprises an optical splitter coupled between the protection switching mechanism and said M output ports for splitting the optical signal into M output optical signals, and for supplying each of said M output optical signals to a respective output port.

8. A set of modules according to claim 7 wherein the M output optical signals each comprise said at least N different wavelengths.

9. A set of modules according to any preceding claim in which the multi-wavelength distribution module includes a generator which comprises a comb generator that supplies N different frequencies onto a single port which is passed to the (optional) splitter.

10. A set of modules according to any preceding claim in which N is equal to or greater than 40.

11. An optical network node for use in an optical communications system having a modular architecture which is constructed from a combination of at least two different module types, the module types comprising: (a) a multi-wavelength distribution module comprising: at least one optical output port; at least two wavelength generators each capable of generating an optical signal comprising at least N different wavelengths, where N is an integer greater than 1 ; and a protection switching mechanism arranged to selectively connect only one of said wavelength generators at any one time to said at least one optical output port; and

12. An optical network node, further comprising at least one of the following module types:

13. An optical network node as claimed in claim 12 or claim 13, wherein said node is an optical network terminal

14. An optical network node according to any one of claims 11 to 13, in which the modules are configured such that the optical modem module provides a connection between the node and an input and output optical fibre path of the network which connects the node to and from at least one other terminal of the network.

15. An optical network node or optical network terminal according to any one of claims 11 to 14 wherein the modules comprise discrete units provided on one of: a single card and two cards.

16. An optical network node according to any one of claims 11 to 15 which comprises a general node or terminal in which the modules are arranged as: at least one cross connect module, an FEC module whose first set of N FEC decoded electrical output ports pass signals are connected to one of the sets of N electrical input ports of the cross connect module and whose second set of N FEC encoded electrical input ports receive signals from a corresponding one of the same set of N electrical output ports of the cross connect module; an optical modem module whose N electrical output ports pass signals to the first set of N electrical input ports of the FEC module and whose N electrical input ports receive signals from the second set of N electrical output ports of the FEC module; and a multi-wavelength distribution module of which at least optical output port is connected to the Optical modem module to provide an optical signal that is to be modulated.

17. An optical network node according to any one of claims 11 to 16 which comprises a modular optical line terminal architecture for a broadband access WDM PON or Transport network line terminal.

18. An optical network node according to claim 17 which comprises: an optical modem module arranged to receive optical signals from a network at its optical input node and transmit modulated optical signals to the network at its optical output node; an FEC module whose first set of electrical inputs receive signals from the electrical outputs of the modem and whose second set of electrical outputs pass signals to the electrical inputs of the optical modem; and a multi-wavelength distribution module of which at least optical output port is connected to the optical modem module to provide an optical signal that is to be modulated.

19. An optical network node according to any one of claims 11 to 17 which comprises a 3R multichannel regeneration node for a transport network comprising: two FEC modules which are cross connected so that the first set of electrical output ports of a first FEC module are connected to the second set of electrical input ports of the other FEC module and vice versa, two modems, each connected having its electrical output ports connected to the first set of electrical input ports of a respective FEC module and its electrical input ports connected to the second set of electrical output ports of the respective FEC module, and a multi-wavelength distribution module of which a first optical output port passes and optical signal to one of the modem modules and a second optical output port passes an optical signal to a second one of the modem modules.

20. An optical network node according to any one of claims 11 to 17 which comprises a K way N channel cross connect for a transport network comprising K modem modules, K FEC modules, one cross connect module and one multi-wavelength distribution module.

21. A method of configuring an optical network comprising:

(i) determining the required functionality of a node for use in the network; and (ii) selecting the modules in accordance with any one of claims 1 to 10 required to form the node so as to provide the desired functionality.

22. A method according to claim 21 further comprising connecting the modules to form the node.