WO2003081825A1

WO2003081825A1 - Optical access method and optical add/crop apparatus

Info

Publication number: WO2003081825A1
Application number: PCT/GB2003/001194
Authority: WO
Inventors: Wei Huang
Original assignee: Colonsay Communications Limited
Priority date: 2002-03-20
Filing date: 2003-03-19
Publication date: 2003-10-02
Also published as: AU2003224239A1; GB0206564D0

Abstract

Apparatus and method capable of adding an optical information signal at a particular transmission wavelength to a Wave Division Multiplexing (WDM) optical medium that already contains an optical information signal at the transmission wavelength is described. The addition of the optical information signal is achieved without any requirement to OEO (Optical/Electrical/Optical) convert the optical information signal already present within the WDM optical medium. The WDM optical medium may comprise an optical network wherein one or more optical nodes provide the means for adding the optical information signal. Such apparatus and described method provide an ideal way for developing flexible, scaleable and a cost effective communication networks.

Description

OPTICAL ACCESS METHOD AND OPTICAL ADD/CROP APPARATUS

Field of the Invention

The present invention relates to optical networking and access systems in general and, more particularly, to optical access methods and optical node routing and add/drop systems in wavelength division multiplexing (WDM) communications systems.

Background of the Invention

Wavelength division multiplexing (WDM) is a technology that divides the fibre-optic bandwidth into many segments and each channel uses an individual bandwidth segment to transmit a signal. In WDM systems, several light beams at different wavelengths can simultaneously propagate over a single optical fibre medium without interference.

PCT Application No. WO 02/11336 in the name Marconi Communications Limited teaches of a WDM optical network comprising a ring configuration of optical fibre links connecting a plurality of nodes. At each node are located add and drop filters which are themselves connected in series. One or more of these filters are employed to add or drop at least two selected adjacent wavelength channels of the WDM optical signal while allowing the remainder of the channels within the WDM signal to pass substantially unattenuated. The wavelength channels of each node are selected so as to maximise the number of adjacent wavelength channels at each node.

PCT Application No. WO 02/03857 in the name The Government of the USA teaches of a tuneable wavelength add/drop device that utilises a multiwavelength input (which serves as a data input port) , a low loss optical circulator or an optical coupler, a wavelength division de-multiplexer which splits the input multi-wavelength data stream into its individual components, a modified multi-channel DOS, a telecommunications grade optical fiber, and a wavelength multiplexer for adding optical data channels. The input multi-wavelength data stream from a network is sent to a wavelength de-multiplexer where it is demultiplexed into individual wavelengths which are applied to an array of Y-branch digital .optical switching devices controlled by a computer. If a specific wavelength is to be dropped, it is diverted towards a branch of a given switch that has a fibre pig tail attached. If a specific wavelength is to be sent through (neither dropped or added) then the signal is diverted towards a mirrored end of the Y-branch, where it is reflected back towards the wavelength de-multiplexers to the circulator and goes out the output end of the device. If a wavelength 'slot' has been vacated by a dropping channel then a new data stream may be added in that slot. US Patent No. US 6,333,798 in the name Allan et al. teaches of a bi-directional wavelength division multiplexed optical communication network useful in numerous network configurations. The network includes a bi-directional optical waveguide carrying counter- propagating wavelength division multiplexed optical signals each including plural channels. Several optical nodes are positioned along the bi-directional optical waveguide, each of which includes an optically-amplified bi-directional optical add-drop multiplexer. Each bidirectional optical add-drop multiplexer includes channel selectors for selecting at least one optical channel from each of the counter-propagating WDM optical signals. Each optical node further includes at least one optical transmitter for supplying an optical channel to be added by the bi-directional optical add-drop multiplexer to the bi-directional optical waveguide and at least one optical receiver for receiving an optical channel dropped from the bi-directional optical waveguide by the bidirectional add-drop multiplexer.

PCT Application WO 01/67650 in the name Telstra R&D Management Ltd teaches of a communications network that includes a number of optical fibre loops that have respective access nodes, an optical wavelength group for traffic within the loop, and at least one other optical wavelength group for traffic to the other loop. The network has an optical cross-connect for routing traffic between the loops by selecting the wavelength groups. The optical cross-connect is passive, and the network may be a metropolitan area network with traffic being carried by WDM signals. US Patent Application No. US 2001/0017958 in the name Solheim et al teaches of a unidirectional or bidirectional node for use in an optical communications network, a network consisting of such nodes and a method of maintaining a target loss around a ring. The node comprises one or more optical couplers as well as either or both of drop circuitry connected to an output port and add circuitry connected to an input port. This allows for changes to the wavelength plan without interruption of the ring traffic. If add circuitry is used, the wavelengths in the filtered add signal should be distinct from those of the incoming signal on the main optical path which is merged with the add signal. When separate fibres are used for transmitting and receiving data between a hub and nodes in a ring, the through loss of the couplers is reduced for upstream couplers, which increases the available loss to be assigned to the fibre. The method of maintaining a target loss around a ring relies on the known through loss of the coupler at each node to set the gain of an amplifier connected to the node .

PCT Application WO 00/57665 in the name Chorumn Technologies Inc. teaches of an optical wavelength add/drop multiplexer employed to provide communications between two optical links that support wavelength division multiplexing (WDM) . A wavelength slicer spatially separates the input signal into two sets of channels. An optical filter, such as an interference filter, spatially separates the subset of the input channels into an array of separated channels. A programmable optical add/drop switch array selectively routes channels from an array of input ports to an array of drop ports, substitutes channels from an array of add ports in place of the dropped channels, and routes the remaining input channels and added channels to an array of output ports. The channels from the output ports of the said add/drop switch array are then combined and transmitted into the second optical link. A network of wavelength slicers can be used to spatially separate the input signal into a larger number of sets of channels that can either be accessed by a number of add/drop switch arrays, or passed unchanged as "express lanes" to the second optical link.

US Patent No. US 6,185,023 in the name Mizrahi teaches of add-drop multiplexers which are compatible with dense wavelength division multiplexed (WDM) systems having large numbers of optical channels. The add-drop multiplexers employ sets of Bragg gratings separated by an optical isolator to reliably add or drop optical channels without crosstalk. The Bragg grating sets and the optical isolator are interposed between first and second optical couplers. Optical channels to be dropped from a WDM optical signal are reflected by the first set of Bragg gratings and exit the add-drop multiplexer through the first coupler. Optical channels to be added to a WDM optical signal enter the add-drop multiplexer through the second optical coupler.

PCT Application WO 99/07097 in the name Ciena Corporation teaches of an optical add/drop multiplexer, that comprises an optical switch coupled to one or more outputs from a demultiplexer. Accordingly, a dropped optical channel can be switched between a receiver optical path for directing the dropped optical signal to an optical receiver and between a combiner optical path for routing the dropped optical channel back towards the optical transmission path, optionally passing the dropped optical channel through a remodulator. Alternatively, the optical switch routes the dropped optical channel towards an optical receiver while permitting a new optical channel to be routed to add port of the static add-drop multiplexer.

US Patent No. US 5,323,255 in the name Alcatel N.V. teaches of an optical transceiver arrangement to transmit information signals from a transmitter to a receiver over an optical waveguide. This is achieved by modulating electrical subcarrier waveforms with information signals using modulation, converting them to optical signals using electrical/optical converters, and by transmitting these optical signals, by means of time division multiplexing means, in different time slots if the subcarrier waveforms have the same freguency. The modulation and time division multiplexing means are controlled by corresponding individual control modules under control of a central control module (CC) included in the receiver arrangement.

US Patent No. 5,864,414 in the names British Telecom pic and Hitachi Limited teaches of a wavelength division multiplex communication system that comprises a head station and a plurality of terminal stations interconnected by an optical fibre cable. The head station transmits continuous wave modulated wavelengths, and a signalling wavelength which is used to indicate, in each time slot, which wavelengths in the following time slot are available for transmission. Each terminal station is arranged to receive the signalling wavelength, to determine therefrom whether the next time slot contains any data packets for that terminal station and, if so, to receive the packets. The terminal station is arranged, if it has a data packet to transmit, to determine from the signalling wavelength whether the next time slot already contains data packets for the destination station and, if so, to avoid data collision by .not transmitting its own data packet. The terminal station then determines a free wavelength from the signalling wavelength, modulates the free wavelength with the data packet it is to transmit, and modifies the signalling wavelength.

PCT Patent Application No. WO 00/74278 in the name Advanced Fibre Communications teaches of a tree structure passive optical network (PON) . The described PON supports an interactive communication by broadcasting the downstream information on the PON at a first wavelength, and carrying the upstream information on the PON at a second wavelength while broadcasting a further signal (such as CATV) on the PON at a third wavelength.

The problem with all of these approaches is that none of the systems are capable of optically adding information to a wavelength of a WDM system, when the wavelength already has optical data present, without requiring either the existing data to be dropped or the employment of optical / electrical / optical (OEO) conversion techniques. Although US patent No. 5,864,414 teaches that an electrical signal can be added at a terminal station onto a light source from a head station through a modulator, the signal source at the different location must be connected to the terminal station through an electrical cable or optical path with an OEO conversion before the signal is added onto the light source from a head station. Meanwhile, as with other PON technologies, the embodiments described in PCT Application No. WO 00/74278 are only suitable for use in tree structure passive optical fibre networks where no optical passive add/drop multiplexers are used or required.

In an optical TDM system, the TDM add/drop components usually include receivers and transmitters because these are required to convert optical signals into electrical signals, multiplex and demultiplex these electrical signals and then to convert the processed signals back into optical form. Active optical switches may be employed to switch the time slots to different directions, but the switch rates cannot usually be as high as the data rate.

Within WDM systems that employ passive add/drop devices (without OEO or optical wavelength conversion) , a wavelength can only be added into a single optical fibre medium while this wavelength channel does not yet contain information signals in the fibre. Since the number of wavelengths that can be used is limited then the number of optical channels that can be supported is also limited. Thus, conventionally, to increase the flexibility and scalability, OEO or wavelength conversion is required in the system, however this significantly increases the system cost and complexity. Summary of the Invention

It is an object of an aspect of the present invention to provide a method and apparatus capable of adding an optical information signal at a particular transmission wavelength to an optical medium that already contains an optical information signal at the transmission wavelength.

It is a further aspect of the present invention to employ the aforesaid method and apparatus so as to provide a flexible, scaleable and a cost effective communication network

According to a first aspect of the present invention, there is provided a method of transmitting an optical signal between two or more optical nodes located within an optical network comprising the steps of:

1) Passively determining whether or not one or more portions of one or more wavelength dependent optical signals, present within an optical medium, are in a state where an optical information signal is present;

2) Generating and adding one or more wavelength dependent optical signal to one or more of the portions determined to be in a state where an optical information signal is not present.

Most preferably the step of passively determining whether or not one or more portions are in a state where an optical information signal is present comprises directing at least one synchronisation wavelength dependent optical signal from the optical medium to an optical signal generator.

Preferably the synchronisation signal is transmitted within at least one portion of at least one wavelength channel .

Preferably the directed synchronisation signal is extracted and processed by the optical signal generator.

Preferably a digital signal processor of the optical signal generator is employed to extract the synchronisation signal.

Preferably the processing of the synchronisation signal comprises directing the signal to a receiving module of the optical signal generator and thereafter forwarding the signal to a corresponding transmitting module.

Optionally the digital signal processor extracts the wavelength dependent optical signal by employing an electrical time demultiplexing and demodulator process.

Alternatively the digital signal processor extracts the wavelength dependent optical signal by employing an electrical frequency demultiplexing and demodulator process .

Most preferably the synchronisation wavelength dependent optical signal is updated with information relating to the generation and addition of a wavelength dependent optical signal. Preferably the synchronisation signal further comprises information regarding whether a wavelength dependent optical signal is to be extracted from the optical network by an optical node.

Most preferably the synchronisation signal contains information that a wavelength dependent optical signal is to be extracted the appropriate wavelength dependent optical signal is directed to the appropriate receiver of the optical node.

Preferably on the appropriate wavelength dependent optical signal being directed to the appropriate receiver the required portion of one wavelength channel is extracted.

Preferably the step of generating and adding an optical signal comprises the conversion of an electrical signal to an optical signal as controlled by the digital signal processor of the optical node.

Most preferably the conversion of an electrical signal to an optical signal comprises the step of modulating a laser source.

According to a second aspect of the present invention there is provided an optical node, suitable for use with an optical fibre medium that is capable of carrying one or more optical signals within a plurality of wavelength dependent channels, each optical signal comprising a plurality of portions suitable for carrying optical information, comprising a signal dropping device and one or more optical signal generators, wherein the signal dropping device provides a means for passively determining whether or not one or more of the portions is carrying optical information while the optical signal generator provides a means for injecting optical information into a portion determined not to be carrying optical information.

Most preferably signal dropping device comprises an input channel demultiplexer, one or more wavelength selectors and a receiver module .

Most preferably the one or more optical signal generators comprise an electrical source, a digital signal processor an a laser device.

Preferably the optical node further comprises an output channel multiplexer and a plurality of optical signal carrying means .

Most preferably the input channel demultiplexer resolves an input signal from the optical fibre medium into one or more wavelength dependent optical signals .

Most preferably the output channel multiplexer converts the one or more wavelength dependent optical signals into an output signal for transmission into the optical fibre medium.

Preferably the optical signal carrying means are employed to optically connect the input channel demultiplexer, the one or more optical signal generators, the output channel multiplexer and the one or more wavelength selectors. Preferably the wavelength selectors comprise one or more circulators, an optical wavelength selecting element and at least one wavelength filter.

Preferably optical wavelength selecting element comprises an element selected from the group comprising a fibre Bragg grating, a Mach-Zehnder interfermometer or any other suitable optical device.

According to a third aspect of the present invention there is provided an optical network comprising:

1) a network control module;

2) an optical fibre medium capable of carrying one or more optical signals within a plurality of wavelength dependent channels; and

3) at least one optical node in accordance with the second aspect of the present invention.

Preferably the network control module comprises a plurality of transmitters wherein a transmitter provides a means for generating an optical signal of a predetermined wavelength.

Most preferably the optical signal comprises a plurality of portions wherein each portion is suitable for carrying optical information.

Optionally the portions comprise a time domain subchannel. Alternatively the portions comprise a frequency domain sub-channel.

Preferably the network control module further comprises an output channel multiplexer wherein the output channel multiplexer converts the optical signals of predetermined wavelength into an output signal for transmission into the optical fibre medium.

Preferably the network control module further comprises an input channel demultiplexer wherein the input channel demultiplexer resolves the output signal, having being transmitted through the optical fibre medium, into one or more optical input signals of predetermined wavelengths.

Preferably the network control module comprises a plurality of receivers wherein a receiver provides a means for detecting an optical input signal of a predetermined wavelength.

Optionally the output channel multiplexer, the input channel demultiplexer and the plurality of receivers are located such that the optical network comprises a ring structure.

Brief Description of the Drawings

Other aspects and advantages of the present invention may become apparent upon reading the following detailed description and upon reference to the following drawings in which:

Figure 1 shows a block diagram of a first embodiment of the WDM communications system according to an aspect of the present invention in which optical add/drop devices add signals with particular wavelengths into a single optical fibre medium without requiring signals employing the same wavelength to be initially dropped;

Figure 2 shows a detail block diagram of the optical add/drop devices and related access equipment;

Figure 3 shows an example of the implementation of the optical add/drop device;

Figure 4 shows a method that divides a single wavelength channel into different time slot sub-channels, and a method of access to the optical medium;

Figure 5 shows a method that divides a single wavelength channel into different frequency sub-channels, and a method of access to the optical medium;

Figure 6 shows a method of dropping a particular wavelength channel and for de-multiplexing to a sub-channel;

Figure 7 depicts a general optical WDM communication system with optical add/drop nodes;

Figure 8 depicts a general optical TDM communication system with OEO add/drop node or active optical packet switch add/drop node; and

Figure 9 depicts a general optical WDM ring network communication system with optical add/drop nodes; Detailed Description of the Invention

In WDM networks, with all-optical routing and add/drop nodes, it is generally conceived by those skilled in the art that a signal at a particular wavelength cannot be added into an optical fibre medium when that wavelength is already being used by another signal source. In addition if two or more channels employ the same fibre then they are required to employ different wavelength sub-channels so as to avoid interfering with one another.

Described below is a system that integrates WDM and TDM or FDM technologies in a WDM network such that optical signals can be added into a single optical fibre through optical add/drop nodes. The result is that two or more signals employing the same wavelength, originating from different sources can be optically added into an optical fibre medium through any add/drop node and coupler assembly, thereafter the same wavelength can be dropped and broadcast to the different destinations. This is implemented by dividing a single wavelength channel into several time (or frequency) sub-channels based on TDM (or FDM) technology, and employing passive add/drop devices to add/drop the wavelengths, as required.

Figure 1 presents an embodiment of an optical communications system that comprises an optical add/drop device 518-1 in accordance with an aspect of the present invention. Optical signals from different sources are sent by the use of transmitters Tx 18-1 to Tx 18-K. These signals are multiplexed into an optical fibre medium 66-1 through a multiplexer 16-1, such as a coupler. As a result, the optical medium 66-1 may contain a signal that comprises a plurality of wavelength λi, ••-, λ_κ.

The add/drop node 518-1 is representative of one of a number of nodes that may form a complete communications systems. On entering the node, the optical signal may be amplified by an optical pre-a plifier 14-1. The optical add/drop node 518-1 further comprises a demultiplexer 15- 1, a multiplexer 16-2, and optical connect lines such as 166-1, and 166-2.

It is known to those skilled in the art that wavelength selection devices, such as wavelength grating routers

(WGR) , can interact with the optical connect lines so as to divide optical signals by wavelength, as appropriate. Thus, within the node 518-1 several of the wavelengths signals may be simultaneously directed to the multiplexer 16-2. Alternatively, the add/drop node is capable of directing one or more of the wavelength to local transceivers (181-la, 191-la) , (181-lb, 191-lb) , and

(181-2 191-2) . For example, the signal with wavelength λi is dropped to receivers 181-la and 181-lb. Meanwhile, the signal with wavelength λj generated by the transmitter 191-la is added into the optical fibre 66-3 through the optical node even although a signal with wavelength λ_j has not been dropped from the input signal, and such a signal may still be present in the optical fibre 66-3. Transmitter 191-lb can also be used to generate signals at wavelength λj for transmission into the optical fibre 66-3. A similar adding and dropping process is used for signals at wavelengths λ_p and λ_q. The multiplexed signal produced by the node 518-1 is then sent, via optical fibre 66-3, to the demultiplexer 15-2 and thereafter demultiplexed to the appropriate receivers, 19-1 to 19-K. It should be noted that there is no requirement for the transmitters 18-1 to 18-K, multiplexer 16-1, demultiplexer 15-2, and the receivers 19-1 to 10-K to be located in the same physical location, as is the case presented in Figure 1.

Figure 2 shows a particular embodiment that implements the optical communications system of Figure 1. Through a synchronisation process of time or wavelength that controls all of the transmitters from 28-1 to 28-K, the signals with wavelength λi, • • • , λ_κ are sent into the optical fibre medium 66a-l through a multiplexer 26-1. An optical amplifier 24-1 is employed to enhance the optical power before the signals are sent into a demultiplexer 25-1. The optical add/drop node 268a is then employed to add and drop some of the signals as described above.

In this particular example the signals with wavelengths λi and λj are dropped, and the other signals pass through the optical node to the optical fibre medium 66a-2. In addition signals are added at the signal wavelengths λ_k and λi. This is achieved by sending the signal with the wavelength λi to a wavelength filter (WF) 20-1, such as a WDM dichroϊc filter, via path 201-3. This signal may then be split so as to arrive at more than one node 201- 2. Within a particular node 201-2 a second WF 201-la or 201-b is employed to direct the signal to the appropriate reciever 291-la or 291-lb. Within this embodiment transmitter 201-2a can be seen to generate a signal at wavelength λk. WF 20-2a is further employed to route the generated λ_k signal to path 266-3 while blocking it from the path 201-la. The signal is then sent to the path 201-4 through WF 20-1, and through the multiplexer 26-2 where it is added into the optical fibre medium 66a-2. Again it should be noted that in this embodiment the wavelength λ has also been used by other source. This earlier signal with wavelength λ_k, entering the node 268a directly from optical fibre medium 66a-l, has passed directly through the optical node 268a.

It is the synchronisation of the time or wavelength of the signals, employed between the transmitter and receiver, that permits this process to take place. The synchronisation signal is extracted from the receivers and sent to the transmitters to get the time or wavelength synchronisation required between the optical signals .

The same adding and dropping process described above is also used to drop a signal with wavelength λ_j and to add a new signal that re-uses the wavelength λi as shown in Figure 2. All the signals are finally sent to the receivers 29-1 to 29-K via the optical amplifier 24-2 and the demultiplexer 25-2.

Figure 3 gives a more detailed example of the configuration of the optical add/drop node 268a. WDM signals, consisting of wavelengths λi, ^••■, λ_κ, are sent into the optical add/drop node 268a. The circulator 36-1 drops the wavelength λi to the path 351-1 while passing the other wavelengths to the fibre Bragg grating 37-1. The wavelength λi is initially directed to its destinations through WF 318-1. Further WFs 318-2 and 318-3 are then used to direct the λi signal to the required destinations, which may be in different physical locations . On arrival at the required destinations the λi signals are broadcast to the associated end users.

In the opposite direction, optical signal using wavelength λ₂ are added from the source through the WFs to the path 135-2. Circulator 36-2 is then employed to combine these signals with the other passed signals, all of which are then transferred into the optical medium 35- 2. It is again pointed out that more than one of the signals from the various sources employ the same wavelength λ₂.

Alternative adding or dropping devices such as a fibre Bragg gratings or Mach-Zehnder interferometers can also be used in the optical add/drop process. Two integrated multiplexing and access technologies have been described in order to add a particular wavelength into the optical channel without initially requiring the dropping of a particular wavelength signal from the optical fibre, see Figures 1 through to Figure 3. Figure 4 describes the application of this method based on TDM and WDM technologies. The time slot channel of each single wavelength band is divided into several sub time slot channels. Each data source uses one time slot channel (or possibly more than one time slot channel) to transmit signals that employ the same wavelength. Through the information obtained from the received signals it is known which time slot channels are empty so that new signals can be added into the vacant channel. The synchronisation is achieved by sending the synchronisation signal in a data channel or sending the synchronisation signal through an independent wavelength control channel .

As shown in Figure 4, a signal from a source employs a time slot channel 46-1 with a wavelength λ in the optical medium 41-1. Meanwhile, a signal from the source 466-3 employs different time slot channels 46-2 and 46-3 to transmit data. The channels are assigned by a digital signal processor (DSP) 466-2 so that they are synchronised with time slot channel 41-1. Laser source

466-1 then generates the synchronised wavelength λi signals for transmission in time slot channels 46-2 and 46-3. The signal streams are then added into the same optical medium 41-3 through the multiplexer (or the coupler) 45-1.

Figure 5 explains the method that is based on FDM and WDM technologies. Each single wavelength band is divided into several sub-frequency bands. Each data source uses one frequency band channel to transmit the signals. Different signals from different sources using a different sub- frequency bands can transmit signals by employing different light sources that operate at same wavelength. Through the information from the received signals it is known which frequency bands are empty so that the signals can be added into the vacant channels. As shown in Figure 5, an optical signal from a source employs frequency band Freql 56-1 in the optical medium 51-1. Another signal from another source is added by the use of the frequency band Freq 2 46-1. The signal is generated by the use of the source 566-3, DSP 566-2 and laser source 566-1. A synchronisation process is used to realise the wavelength synchronisation between the sources. The signals from the different sources, having different frequency bands and the same wavelength, are then added by multiplexer 55-1 into the same optical medium 51-3.

Figure 6 shows the receiver of the access system for both the TDM/WDM and the FDM/WDM systems. A signal with a particular wavelength is dropped to one or more destination receivers through optical paths, such as 366c and 366d, by a demultiplexer or a splitter. The signal is converted into an electrical signal by the use of a photo-detector PD 566a. A Digital Signal Processor (DSP) 566b is then employed to extract the sub-channel signals. In a TDM/WDM system technology, this DSP is an electrical time demultiplexing and demodulating process. In a FDM/WDM system this DSP is an electrical frequency demultiplexing and demodulating process.

Illustrative embodiments of a communication network that employ the above described optical nodes and access methods are now described below. A WDM system is shown schematically in Figure 7. A transmitter, Tx 71-1, generates a data stream at wavelength λi, a transmitter, Tx 71-2 uses λ₂, and so forth. All of these signals are combined by a multiplexer (Mux) 74-1 (such as a coupler) and are transmitted simultaneously over a single optical fibre medium 76-1. Hence, the optical fibre medium contains optical signals with wavelengths λi, ^{• • •} , λ_κ.

Before the signals are input into the add/drop node 778a, they may or may not amplified by an optical amplifier 73a according to the particular application. The add/drop node 778a includes a wavelength demultiplexer (De-Mux) 75-1 and a wavelength multiplexer 74-2. These elements are connected to one another via optical paths.

The demultiplexer 75-1 separates signals with different wavelengths and directs them to the appropriate destinations through the employment of wavelength grating routers (WGRs), which route the incoming optical signals as a function of wavelength, to particular output ports of the WGR. Each port may be connected to a receiver that can convert the optical signals to electrical signal.

In this particular embodiment the incoming optical signal is demultiplexed by the demultiplexer 75-1 with the majority of the signals being routed directly to the multiplexer 74-2. However, the signal with wavelength λi is dropped to the receiver 711b and this wavelength is thereafter re-used by the transmitter 711a to add another signal into the optical medium 76-2 through multiplexer 74-2.

Similarly, in the node 778b, the signal with wavelength λ_j is dropped, and another signal from the location is added into the optical fibre medium by the re-use of the wavelength λj . All of the signals are then sent to the location of demultiplexer 75-2 via the amplifier 73-b. Thereafter, the signals are demultiplexed so as to be received by the K receivers 71a-l to 71a-K, as appropriate.

A more detailed description of a communication system that employs time-division multiplexing (TDM) is described in Figure 8. Signals from transmitters Txl 821 to Tx4 824 enter the multiplexer 861. The multiplexer takes a sample of each signal time slot 85a to 85d, assigns a specific time slot to each sample, combines (multiplexes) these samples, and transmits them over a same optical fibre medium by a time sequence 891.

An add/drop node 886 is then employed to add and drop signals. In this example the add/drop node drops the signal of channel 2 that was employing time slot 81a, and simultaneously adds a signal 82a to the vacant time slot 5, resulting in the new time sequence channels 892. The signals are sent to the location of demultiplexer 862. The demulitplexer 862 then divides and routes the channels to the separate intended receivers 821a to 824a.

A ring WDM optical network is presented in Figure 9 for further illustrative purposes. The ring network comprises a central office 900-1 that is connected to the other communication nodes 901-1, 901-2 to 901-k.

The central office comprises a number of transmitters (90-1 to 90-K) and receivers (99-1 to 99-K) which are capable of generating signals with different wavelengths. Each of the transmitters sends a signal that comprises time or frequency modulated optical sub-channels. The signals are multiplexed by the multiplexer 96-1 into to a single fibre.

The nodes 901-1 to 901-k comprise optical add/drop multiplexer that are again passive devices based on thin- film filters, arrayed-waveguide gratings, Bragg gratings, circulators, or some other wavelength-selective technology, as described above. In this example node 901- 1 drops wavelengths λi and λ_p while the other wavelengths effectively bypass the node. The dropped signals λi and λ_p, are then broadcast to several destinations as required.

Each destination is equipped with a transmitter and a receiver. The receiver receives any broadcast signals while the transmitter provides a means for generating a modulated signal suitable for transmission within an optical sub-channel of a particular wavelength. Again it should be noted that the wavelength of the generated signal may be the same as one already being employed by other users of the network.

An optical amplifier 922-1 may also be employed within the network in order to provide a means for optical amplification or optical repeating.

Once the various signals have been transmitted around the network they return to the central office 900-1. A demultiplexer 96-2 is then employed to de-multiplex the signals, depending on their particular wavelength, such that arrive at the required receivers 99-1 to 99-K. The apparatus and methods detailed above provide several distinct advantages of the teachings of the Prior Art. Through the combination of WDM and TDM or FDM the described integrated multiplexing methods can provide more optical channels in a WDM communications systems. A plurality of signals from different sources can be optically multiplexed across a single wavelength. Combining these features with optical wavelength add/drop technologies into an optical network, results in improved networking scalability, flexibility, and a significant reduction in network deployment costs.

In addition the described nodes may be implemented to reduce costs and increase the flexibility, scalability and to enhance the performance associated with current optical add/drop nodes and optical network access equipment architectures that employing integrated multiplexing technologies.

In accordance with certain embodiments, the optical add/drop system may include wavelength demultiplexers, optical filters, and wavelength multiplexers. The optical add/drop node receives optical signals with a plurality of wavelength, outputs optical signals having a subset of those wavelengths, and is capable of routing at least one optical signal, at a particular wavelength (or wavelength band), from the optical medium to an optical transceiver. Simultaneously, optical signals can be added into the optical medium at any required wavelength through the employment of the same optical node.

Although an exemplary embodiment of the present invention has been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications or alterations to the invention described herein may be made, none of which depart from the invention as defined in the appended claims. All such changes, modifications, and alterations should therefore be seen as being within the scope of the present invention.

Claims

1) A method of transmitting an optical signal between two or more optical nodes located within an optical network comprising the steps of:

2) A method of transmitting an optical signal as claimed in Claim 1 wherein the step of passively determining whether or not one or more portions are in a state where an optical information signal is present comprises directing at least one synchronisation wavelength dependent optical signal from the optical medium to an optical signal generator.

3) A method of transmitting an optical signal as claimed in Claim 2 wherein the synchronisation signal is transmitted within at least one portion of at least one wavelength channel.

4) A method of transmitting an optical signal as claimed in Claim 2 or Claim 3 wherein the directed synchronisation signal is extracted and processed by the optical signal generator. 5) A method of transmitting an optical signal as claimed in Claim 4 wherein a digital signal processor of the optical signal generator is employed to extract the synchronisation signal.

6) A method of transmitting an optical signal as claimed in Claim 4 or Claim 5 wherein the processing of the synchronisation signal comprises directing the signal to a receiving module of the optical signal generator and thereafter forwarding the signal to a corresponding transmitting module.

7) A method of transmitting an optical signal as claimed in Claim 5 or Claim 6 wherein the digital signal processor extracts the wavelength dependent optical signal by employing an electrical time demultiplexing and demodulator process .

8) A method of transmitting an optical signal as claimed in Claim 5 or Claim 6 wherein the digital signal processor extracts the wavelength dependent optical signal by employing an electrical frequency demultiplexing and demodulator process.

9) A method of transmitting an optical signal as claimed in Claim 2 to Claim 8 wherein the synchronisation wavelength dependent optical signal is updated with information relating to the generation and addition of a wavelength dependent optical signal.

10) A method of transmitting an optical signal as claimed in Claim 2 to Claim 9 wherein the synchronisation signal further comprises information regarding whether a wavelength dependent optical signal is to be extracted from the optical network by an optical node.

11) A method of transmitting an optical signal^' as claimed in Claim 10 wherein when the synchronisation signal contains information that a wavelength dependent optical signal is to be extracted the appropriate wavelength dependent optical signal is directed to the appropriate receiver of the optical node.

12) A method of transmitting an optical signal as claimed in Claim 11 wherein on the appropriate wavelength dependent optical signal being directed to the appropriate receiver the required portion of one wavelength channel is extracted.

13) A method of transmitting an optical signal as claimed in Claim 1 to Claim 12 wherein the step of generating and adding an optical signal comprises the conversion of an electrical signal to an optical signal as controlled by the digital signal processor of the optical node.

14) A method of transmitting an optical signal as claimed in Claim 13 wherein the conversion of an electrical signal to an optical signal comprises the step of modulating a laser source.

15) An optical node, suitable for use with an optical fibre medium that is capable of carrying one or more optical signals within a plurality of wavelength dependent channels, each optical signal comprising a plurality of portions suitable for carrying optical information, comprising a signal dropping device and one or more optical signal generators, wherein the signal dropping device provides a means for passively determining whether or not one or more of the portions is carrying optical information while the optical signal generator provides a means for injecting optical information into a portion determined not to be carrying optical information.

16) An optical node as claimed in Claim 15 wherein the signal dropping device comprises an input channel demultiplexer, one or more wavelength selectors and a receiver module.

17) An optical node as claimed in Claim 15 or Claim 16 wherein the one or more optical signal generators comprise an electrical source, a digital signal processor an a laser device.

18) An optical node as claimed in Claim 15 to Claim 17 wherein the optical node further comprises an output channel multiplexer and a plurality of optical signal carrying means.

19) An optical node as claimed in Claim 16 to Claim 18 wherein the input channel demultiplexer resolves an input signal from the optical fibre medium into one or more wavelength dependent optical signals.

20) An optical node as claimed in Claim 18 or Claim 19 wherein the output channel multiplexer converts the one or more wavelength dependent optical signals into an output signal for transmission into the optical fibre medium.

21) An optical node as claimed in Claim 18 to Claim 20 wherein the optical signal carrying means are employed to optically connect the input channel demultiplexer, the one or more optical signal generators, the output channel multiplexer and the one or more wavelength selectors.

22) An optical node as claimed in Claim 16 to Claim 21 wherein the wavelength selectors comprise one or more circulators, an optical wavelength selecting element and at least one wavelength filter.

23) An optical node as claimed in Claim 22 wherein the optical wavelength selecting element comprises an element selected from the group comprising a fibre Bragg grating, a Mach-Zehnder interfermo eter or any other suitable optical device.

24) An optical network comprising:

1) a network control module;

3) at least one optical node in accordance with Claim 15 to Claim 23.

25) An optical network as claimed in Claim 24 wherein the network control module comprises a plurality of transmitters wherein a transmitter provides a means for generating an optical signal of a predetermined wavelength.

26) An optical network as claimed in Claim 25 wherein the optical signal comprises a plurality of portions wherein each portion is suitable for carrying optical information.

27) An optical network as claimed in Claim 26 wherein the portions comprise a time domain sub-channel.

28) An optical network as claimed in Claim 26 wherein the portions comprise a frequency domain sub-channel.

29) An optical network as claimed in Claim 24 to Claim 28 wherein the network control module further comprises an output channel multiplexer wherein the output channel multiplexer converts the optical signals of predetermined wavelength into an output signal for transmission into the optical fibre medium.

30) An optical network as claimed in Claim 24 to Claim 28 wherein the network control module further comprises an input channel demultiplexer wherein the input channel demultiplexer resolves the output signal, having being transmitted through the optical fibre medium, into one or more optical input signals of predetermined wavelengths.

31) An optical network as claimed in Claim 24 to Claim 30 wherein the network control module comprises a plurality of receivers wherein a receiver provides a means for detecting an optical input signal of a predetermined wavelength. 2) An optical network as claimed in Claim 31 wherein the plurality of transmitters, the output channel multiplexer, the input channel demultiplexer and the plurality of receivers are located such that the optical network comprises a ring structure.