An optical network

Abstract

The invention is an optical add / drop multiplexor for SDM and wavelength division multiplexed (WDM) signals. SDM signals is an MCF 12a’ are fed through fan out 50a to splitters 24a which sends all wavelengths to a drop unit 30 (e.g. multicast switch (MCS)) and thence transducers 34. Splitter 24a also sends all wavelengths to switch 22 which connects all wavelengths from an input core to output cores. The output cores include coupler 24b where wavelengths from add transducers 42 and unit 40 can be inserted amongst wavelengths passed though the switch. The passed through and added wavelengths then go through fan in 50b to multicore fibre 12e. The invention is intended to avoid the use of filters.

Description

FIELD The present invention relates to an optical network. In particular, to a spatial division multiplexing optical network and an optical switch therefor. BACKGROUND Optical networks are used in multiple applications including telecommunications, data transmission networks, and data centres to transport optical signals typically modulated to encode information such as audio, video and I or data bits. Optical networks as generally deployed use single mode optical fibres to transport optical signals. Information may be encoded in different ways into an optical signal with wavelength division multiplexing (WDM) being most generally adopted. Network nodes are points in a network connected thereto for adding, receiving and / or creating information for across the network. Optical networks include optical add / drop multiplexers positioned at network nodes to permit “add” optical signals, e.g. specific optical wavelengths / channels, to be added to the network from a device by multiplexing the add optical signals to the optical signals travelling through the network for transmission to another point on the network and for “drop” optical signals, e.g. specific optical wavelengths I channels, to be dropped from the network by demultiplexing them from the optical signals to a device connected to the network intended to receive them. Such multiplexers are often reconfigurable, so-called RODAM, and include filters, e.g. Wavelength Selective Switches (WSS), which process optical signals as separate wavelengths with each wavelength forming a data or information channel to be transported through the network. RODAMs are deployed at different points along the network and can be configured to direct optical signals in different directions to respective nodes along the network as required for a particular data or information channel to reach the desired destination. This functionality is performed at wavelength level by the filters within each RODAM and the RODAM has separate ports for each of the wavelengths being filtered. The numbers of ports required per wavelength increases by the number of data / information channels, as well as the number of desired directions in which to send an optical signal. Proposals have been made to use multi-core fibres as opposed to single mode fibres, so-called "spatial division multiplexing" to increase network capacities. However, the ability to use such fibres in networks introduces issues, particularly because RODAMs for use in such networks would require filters, e.g. WSSs, with high numbers of ports which is impractical and increases cost significantly. BRIEF DESCRIPTION OF THE INVENTION According to an aspect of the present disclosure we provide a spatial division multiplexing optical network including: one or more network nodes connected by multi-core optical fibres, wherein each multi-core optical fibre includes optical fibre cores including respective optical signals formed by wavelength division multiplexing; and an optical switch node located at one of the network node, wherein the optical switch node is connected to the network nodes by first and second multi-core optical fibres arranged in respective first and second directions with respect to the optical switch node, wherein the optical switch node includes: an optical switch including: two or more sets of ports including a first set of ports connected to optical fibre cores of the first multi-core optical fibre and a second set of ports connected to optical fibre cores of the second multi-core optical fibre, wherein the optical switch is selectively configurable to route the optical signals between different ports of the first set and different ports of the second set; one or more splitter devices connected to respective ones of the optical fibre cores, wherein each splitter device connected to said one of the optical fibre cores may split the power of the optical signal travelling therethrough so that part of the optical signal travels along a drop optical path whilst retaining all the optical wavelengths of the optical signal and part of the optical signal travels to the port connected to the said one of the optical fibre cores whilst retaining all the optical wavelengths of the optical signal, and / or one or more coupler devices connected to respective ones of the optical fibre cores, wherein each coupler connected to said one of the optical fibre cores may receive an add optical signal having a respective add wavelength travelling along an add optical path and the coupler device couples the add optical signal to the optical signal travelling through the said optical fibre. According to an aspect of the present disclosure, we provide a spatial division multiplexing optical switch node for an optical system, wherein the optical switch node is for locating at a network node of a spatial division multiplexing optical network and connected by multicore optical fibres, wherein the optical switch node for connecting to the network node by first and second multi-fibres arranged in respective first and second directions with respect to the optical switch, wherein the switch includes: an optical switch including: two or more sets of ports including a first set of ports for connecting to optical fibre cores of the first multi-core optical fibre and a second set of ports for connecting to optical fibre cores of the second multi-core optical fibre, wherein the optical switch is selectively configurable to route the optical signals between different ports of the first set and different ports of the second set; one or more splitter devices for connecting to respective ones of the optical fibre cores, wherein each splitter device connecting to said one of the optical fibre cores may split the power of the optical signal travelling therethrough so that part of the optical signal travels along a drop optical path whilst retaining all the optical wavelengths of the optical signal and part of the optical signal travels to the port for connection to said one of the optical fibre cores whilst retaining all the optical wavelengths of the optical signal, and / or one or more coupler devices for connecting to respective ones of the optical fibre cores, wherein each coupler for connection to said one of the optical fibre cores may receive an add optical signal having a respective add wavelength travelling along an add optical path and the coupler device couples the add optical signal to the optical signal travelling through the said optical fibre. Optionally or preferably a network or switch according to any preceding aspect wherein the sets of ports including further sets of ports which are connected, or are for connecting, to optical fibre cores of further multi-core optical fibres arranged in other directions direction with respect to the optical switch node and the optical switch is selectively configurable to route optical signals between different ports of the first set, second set and further sets of ports. Optionally or preferably a network or switch according to any preceding aspect wherein: the one or more splitter devices include splitter devices connected, or for connection to optical fibre cores of the further multi-core optical fibres; and / or the one or more coupler devices include coupler devices connected, or for connection to optical fibre cores of the further multi-core optical fibres. Optionally or preferably a network or switch according to any preceding aspect wherein the optical switch, in use, may route optical signals between any of the ports of one set and any of the ports of another set of ports. Optionally or preferably a network or switch according to any preceding aspect wherein the optical switch includes: fibre core drop ports, wherein the optical switch, in use, may selectively connect one of the ports from the two or more sets of ports to one of the fibre core drop ports to route all the optical wavelengths in the optical fibre core connected to the said one of the ports to a drop point; and / or fibre core add ports, wherein the optical switch, in use, may selectively connect one of the ports from the two or more sets of ports to one of the fibre core add ports to route all the optical wavelengths from the fibre core add port to the said one of the ports. Optionally or preferably a network or switch according to any preceding aspect wherein: one or more fibre core drop ports are connected at positions on optical paths which are after any splitter or coupler devices; and / or one or more fibre core add ports are connected at positions on optical paths which are before any splitter or coupler. Optionally or preferably a network or switch according to any preceding aspect wherein the splitter device is adjustable to permit use with optical signals having different ranges of optical power ratio. Optionally or preferably a network or switch according to any preceding aspect wherein the coupler device is adjustable to permit use with optical signals having different optical power ratio. Optionally or preferably a network or switch according to any preceding aspect wherein the add optical signal may have a plurality of add wavelengths and the coupler device adds all of the plurality of add wavelengths to the optical signal. Optionally or preferably a network or switch according to any preceding aspect wherein the drop optical paths terminate at drop ports for connection to a drop device connectable to a filter for selectively filtering the optical wavelengths of the optical signal, optionally or preferably the drop device is a multicast switch and optionally or preferably the multicast switch is connected to a transponder. Optionally or preferably a network or switch according to any preceding aspect wherein the add optical paths start at add ports for connection to an add device which provides the add optical signals, optionally or preferably the add device is a multicast switch ad optionally or preferably the multicast switch is connected to a transponder. Optionally or preferably a network or switch according to any preceding aspect wherein the optical switch is formed on an integrated chip. Optionally or preferably a network or switch according to any preceding aspect wherein the optical switch and the one or more splitter devices and / or one or more coupler devices are formed on an integrated chip. Optionally or preferably a network or switch according to any preceding aspect including multi-core breakout devices connected to the multi-core fibres to fan out the said respective optical fibres from the multi-core fibres, optionally or preferably the multi-core breakout devices are integrally formed as part of the optical fibre switch. Optionally or preferably a network or switch according to any preceding aspect wherein the optical drop path is connected to a drop device, optionally or preferably the drop device is a demultiplexer connected to one or more transponders. Optionally or preferably a network or switch according to any preceding aspect wherein one or more transponders are connected to a respective optical add path connected to an add device, optionally or preferably the add device is a multiplexer connected to one or more transponders to provide the add optical signal. Optionally or preferably a network or switch according to any preceding aspect wherein one or more of the ports of one or more of the sets of ports are connected to, or are for connection to, optical fibre(s) which provide add optical signal(s) having respective add wavelength(s) to the said one or more ports. Optionally or preferably a network or switch according to any preceding aspect wherein one or more of the ports of one or more of the sets of ports are connected to, or are for connection to, optical fibre(s) connected to drop device(s) which filter one or more of the optical wavelengths of travelling through the optical fibre(s). Optionally or preferably a network or switch according to any preceding aspect including a plurality of said one or more nodes and said optical switch nodes connected together, optionally or preferably to form a metro optical transport network, to form a network of data centres or telecommunications network. Optionally or preferably a network according to any preceding aspect according to any preceding claim, including a central controller for issuing network commands to local controllers which control the nodes and / or optical switch, wherein the central controller is a software defined network controller. Optionally or preferably a network according to any preceding aspect wherein the network commands include one or more of the network configuration, operating the optical switch node, network monitoring and / or telemetry. Optionally or preferably a network or switch according to any preceding aspect wherein the controller includes algorithms for optimising the topology of the network; and / or routing, modulation, spectrum and core allocation (RMSCA). Optionally or preferably a network according to any preceding aspect wherein a network controller for controlling the one or more nodes carries out the following steps when receiving a network request to add or drop I assign an optical signal containing one or more optical add I drop wavelengths or sending to another node: a) for each request to send I assign the optical signal at network node s for sending to network node d; b) obtaining for each of the optical fibre cores the number of available channels and determining which of the optical fibre cores has the maximum number of available channels which can be routed to network node d', and c) based on the determination at step b), return the first available channel from the core having the maximum number of available channels and send to the node d. BRIEF DESCRIPTION OF THE FIGURES In order that the present disclosure may be more readily understood, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic drawing of a network node embodying aspects of the present disclosure; FIGURE 2 is a schematic drawing showing the architecture of one of the network nodes shown in figure 1; FIGURE 3 is a schematic drawing of a control system for a network embodying aspects of the present disclosure; FIGURE 4a shows various network performance graphs based on simulated blocking probabilities for different network configurations including networks embodying aspects of the present disclosure and networks according to the prior art; FIGURE 4b is a table showing component requirements for given network requirements based on a prior art RODAM based network architecture and a network architecture embodying aspects of the present disclosure; and FIGURE 4c is an algorithm for use in controlling a network architecture embodying aspects of the present disclosure. FIGURE 5 is a schematic drawing showing another example architecture for use in one of the network nodes shown in figure 1 embodying aspects of the present disclosure; FIGURE 6 is a schematic drawing showing another example architecture for use in one of the network nodes shown in figure 1 embodying aspects of the present disclosure; and FIGURE 7 is a schematic drawing showing an optical switch circuit for use in one of the network nodes shown in figure 1 embodying aspects of the present disclosure. DETAILED DESCRIPTION OF THE DISCLOSURE Referring to the figures, examples embodying aspects of the present disclosure are shown. In general terms, aspects of the present disclosure relate to a spatial division multiplexing optical network including one or more network nodes connected by multi-core optical fibres. Each multi-core optical fibre includes optical fibre cores including respective optical signals, e.g. formed by wavelength division multiplexing. The network includes an optical switch node located at one of the network nodes. The optical switch node is connected to the network node by first and second multi-core optical fibres arranged in respective first and second directions with respect to the optical switch. The optical switch node includes an optical switch, one or more splitter devices and / or one or more coupler devices. The optical switch includes two or more sets of ports including a first set of ports connected to optical fibre cores of the first multi-core optical fibre and a second set of ports connected to optical fibre cores of the second multi-core optical fibre. The optical switch is selectively configurable to route the optical signals between different ports of the first set and different ports of the second set. The splitter devices are connected to respective ones of the optical fibre cores. Each splitter device connected to said one of the optical fibre cores may split the power of the optical signal travelling therethrough so that part of the optical signal travels along a drop optical path whilst retaining all the optical wavelengths of the optical signal and part of the optical signal travels to the port connected to the said one of the optical fibre cores whilst retaining all the optical wavelengths of the optical signal. The coupler devices are connected to respective ones of the optical fibre cores. Each coupler connected to said one of the optical fibre cores may receive an add optical signal having a respective add wavelength travelling along an add optical path and the coupler device couples the add optical signal to the optical signal travelling through the said optical fibre core. Referring to figure 1, this shows an optical network 10 according to an example which embodies the above aspects of the present disclosure. The optical network 10 is a spatial division multiplexing optical network configured for a metro network. In other examples, it may be employed as part of a network connecting data centres and / or a telecommunications network. The optical network 10 includes network nodes N1, N2, N3, N4 and N5. Multi-core fibres 12a connect nodes N1 and N2. Multi-core fibres 12b connect nodes N2 and N3. Multi-core fibres 12c connect nodes N3 and N4. Multi-core fibres 12d connect nodes N4 and N5. Multi-core fibres 12e connect nodes N5 and N1. The present disclosure is not limited to the node configuration shown in the figure. It should be understood that nodes formed in other configurations are possible and that these do not depart from the scope of the present disclosure. Each of the nodes N1 to N5 include a respective optical switch node 20. Referring to figure 2, this is a schematic drawing showing an example architecture at node N1. It will be understood that the same or similar architecture may be employed at or more of the other nodes N2 to N5. Node N1 includes respective ones 12a’, 12e’ of the multi-core fibres 12a, 12e connected to an optical switch node 20. Each multi-core fibre 12a’, 12e’ includes a plurality of single mode optical fibre cores 12a”, 12e”, i.e. fibre cores, arranged within a single fibre. The optical fibre cores 12a”, 12e” include optical signals formed by wavelength division multiplexing in this example. The multi-core fibres 12a’, 12e’ are arranged in respective first and second directions D1, D2 with respect to the optical switch node 20. The optical switch node 20 is connected to multi-core fibres 12a’, 12e’ as will be described. The optical switch node 20 includes an optical switch 22 which permits switching between one fibre core to another fibre core. In the art, this may be referred to as “core switching” at full waveband level. The optical switch 22 includes a first set of ports 22a including ports connected to the optical fibre cores 12a” of the multi-core fibre 12a and a second set of ports 22b including ports connected to the optical fibre cores 12e”. In this example, the ports 22a, 22b are connected to the optical fibre cores 12a", 12e" by intermediate fibres 12a'", 12e"' and ports 22a', 22b'. In other examples, there may be no intermediate fibres and, instead, the fibre cores 12a” are connected directly to the ports 22a. The optical switch 22 is selectively configurable to route the optical signals between different ports of the first set of ports 22a and different ports of the second set of ports 22b. In examples, the optical switch 22, in use, may route optical signals between any of the ports of one set 22a and any of the ports of another set of ports 22b. Node N1 includes a drop point D1 to which optical signals from the optical fibre cores 12a” can be sent along drop optical paths d. Node N1 includes an add point A1 from which optical signals can be sent to the optical fibre cores 12e” along add optical paths a. The optical switch node 20 includes splitter devices 24a connected to respective ones of the optical fibre cores 12a”. In this example, splitter devices 24a are connected to the fibre cores 12a" via intermediate fibres 12a'". Each splitter device 24a may split the power of the optical signal travelling therethrough, e.g. the optical signal received from one of the optical fibre cores 12a”, so that part of the optical signal travels along a drop optical path d whilst retaining all the optical wavelengths of the optical signal and part of the optical signal travels to the port 22a connected to the said one of the optical fibre cores 12a” whilst retaining all the optical wavelengths of the optical signal. In examples, the splitter devices 24a may be adjustable to permit different optical power level of the drop signal. The optical switch node 20 may include coupler devices 24b connected to respective ones of the optical fibre cores 12e”. In this example, coupler devices 24b are connected to the fibre cores 12e" via intermediate fibres 12a'". Each coupler 24b connected to said one of the optical fibre cores 12e” may receive an add optical signal having a respective add wavelength travelling along an add optical path a and the coupler device 24b couples the add optical signal to the optical signal travelling through the said optical fibre cores 12e”. In examples, the coupler devices 24b may be adjustable to permit different optical power level of the add signal. In examples, the add optical signal may have a plurality of add wavelengths and the coupler devices 24b add all of the plurality of add wavelengths to the optical signal. It should be understood that, although figure 1 shows operation for which optical signals travel in one direction, e.g. from node 2 to node 1 to node 5, other modes of operation during use involve optical signals travelling in the opposite direction, or optical signals travelling in different directions across sub-sets of the optical fibres 12a’, 12e’. For example, optical signals travelling from node 2 to node 1 to node 5 for two of the optical fibres 12e’, and travelling from node 5 to node 2 to node 1 for the remaining optical fibre 12e’. The optical switch 22 may include fibre core drop ports 22b" and the optical switch 22, in use, may selectively connect one of the ports from sets of ports 22a to one of the fibre core drop ports 22b" to route all the optical wavelengths in the optical fibre connected to the said one of the ports 22a to a drop point D2. The optical switch 22 may include fibre core add ports 22a" and the optical switch 22, in use, may selectively connect one of the ports from the sets of ports 22b to one of the fibre core add ports 22a” to route all the optical wavelengths from the fibre core add port 22a” to the said one of the ports 22b. In the example shown in figure 1, the fibre core drop ports 22a” are connected at positions on optical paths which are after the splitter and coupler devices 24a, 24b. However, they need not be in other examples, e.g. they may be placed only after a splitter device with none after a coupler device, or only after a coupler device with none after a splitter device. Alternatively, they may be placed before any splitter and / or coupler. In examples, the optical switch 22 may be formed on a photonics integrated chip. In examples, the optical switch 22 may be an optical matrix switch as are known in the art. For example, such as the optical fibre switches manufactured by Polatis. Splitter and coupler devices for splitting I coupling optical light are known in the art, e.g. multi mode interferometer or the like would be suitable for use with optical switch node 20. In examples, such as that shown in figure 1, the drop optical paths d may terminate at drop ports, e.g. on an integrated chip on which the optical switch 22 is formed, and which are connected to a drop device 30, e.g. transponders positioned at, for example drop point D1. The drop device 30 may be connected to a filter 32 (e.g. a tunable filter) for selectively filtering the optical wavelengths of the optical signal. The drop device 30 may be a multicast switch. The multicast switch may be connected to a transponder 34 through filter 32. In examples, the drop device 30 may be a wavelength selective switch for filtering the required optical wavelengths of the optical signal. Multicast switches known in the art are suitable for use with optical switch node 20, e.g. MCS modules manufactured by the company Optico. Similarly, the add optical paths a may start at add ports, e.g. on an integrated chip on which the optical switch 22 is formed, for connection to an add device 40 which provides the add optical signals. The add device 40 may be a multicast switch similar to the multicast switch at drop device 30 above. The multicast switch may be connected to a transponder 42. In examples, the add device 40 may be a wavelength selective switch which filters the required optical wavelength(s) from an incoming optical signal. The optical switch 22 and splitter devices 24a, and coupler devices 24b may be formed on the same integrated chip. In examples, where provided, intermediate fibres 12a'", 12e"' and ports 22a', 22b' may be formed as part of the integrated chip. In this example, multi-core breakout devices 50a, 50b may be connected to the multi-core fibres 12a, 12b to fan out the said respective optical fibre cores 12a", 12e" from the multicore fibres 12a, 12b. The multi-core breakout devices 50a, 50b may be integrally formed as part of the optical switch 22 in examples, e.g. part of the same integrated chip. In examples in which they are not integrally formed, suitable devices are known in the art for use with examples embodying aspects of the present disclosure, e.g. fan-in and fan-out devices manufactured by Optoscribe such as laser scribed 3D waveguides utilised in the 3D OptoFan. The optical switch node 20 as described permits switching of optical signals through the optical switch 22 without any filtering of the optical signals at wavelength level, i.e. the switching is performed at full waveband level whilst still retaining the ability to add I drop specific wavelengths of optical signals. The network 10 may be controlled by a network controller (not shown) which issues control commands to the individual nodes N1 - N5 and can configure operation (e.g. through local controllers at each node) of the optical switch nodes 20 at each of the nodes N1 - N5 to transmit optical signals along different routes according to the desired destination and network requirements. In a general sense, the network controller may operate the optical switch nodes 20, by appropriate control of the optical switches 22, to perform core switching so that, referring to figure 2 as an example, all of the wavelengths of optical signals in one of the optical fibre cores 12a” may be sent to one of the other optical fibre cores 12e”. The network controller may also permit optical signals to added or dropped at fibre core level I single mode fibre level at the optical switch nodes 20 so that all of the wavelengths of optical signals in one of the optical fibre cores 12a”, 12e" are dropped at a particular node, e.g. not transmitted from multi-core fibre 12a to multi-core fibre 12b, or all the optical signals from an add fibre core (single mode fibre) could be introduced at a node for transmitting to one of the multi-core fibres 12a, 12b. For example, all of the wavelengths of optical signals from one of the optical fibre cores 12a”, 12e” may be sent to fibre core drop ports 22b", and / or all of the wavelengths of optical signals from an add fibre core could be introduced at one of the fibre core add ports 22a” and sent to another of the ports , 22b fortransporting via one of the multi-core fibre cores 12a”, 12b”. It can be seen that no filtering of the wavelengths forming the optical signals travelling in optical fibres 12a’, 12e' occurs when switching the optical signals to different directions relative to the optical switches 20. Any such switching is done in a filter-less manner compared to prior art solutions. Referring to figure 3, this shows a schematic figure of a similar network 100 as that of figure 1 to illustrate aspects of network control for four nodes N that may be employed in a similar way for the network 10 of figure 1. The network 100 includes a central controller 110 for issuing network commands to local controllers which control the nodes N and optical switches at the nodes N. The central controller 110 may be a software defined network controller. The network commands may include one or more of the network configuration, operating the optical switch, network monitoring and / or telemetry to obtain the necessary network performance I functionality. This could include algorithms for optimising the topology of the network; and / or routing, modulation, spectrum and core allocation (RMSCA) such as those that will be described hereinafter. Operation of the network 10 will now be described with reference to figures 1 and 2. Operation of the network 10 for which an optical signal provided by one of the optical fibre cores 12a” needs to be dropped to drop point D1 will be as follows. The optical signal is formed of multiple channels, e.g. as wavelength channels of a wavelength division multiplexed signal, from which one of the channels is to be dropped at drop point D1. The optical signal travels along the optical fibre core 12a” until it reaches a splitter device 24a. At this point, the splitter device 24a will divide the optical signal on a power basis so that the optical signal power is divided by, for example, two, so that a half power optical signal travels to the respective port 22a by the splitter device 24a and a half power optical signal travels along optical drop path d to the drop point D1. At this stage, no wavelength filtering of the optical signal has occurred, and all of the channels I wavelengths (shown as five encoded channels out of an available eight channels) are transmitted along the respective optical path d to the optical switch device 22 and drop point D1. The drop device 30 receives the optical signal and, together with the filter 32, extracts the required wavelength channel as necessary. By contrast to the prior art, the optical signal has not been filtered as part of the switching of the optical signal whilst retaining the flexibility to be able to drop channels from the optical signal at the desired points along the network. In a similar way, channels with encoded information may be introduced into an optical signal passing through optical fibre cores 12e” through the use of the add device 40 and the respective coupler device 24a so that the resultant coupled optical signal leaving the coupler device 24a includes all of the wavelengths of original optical signal together with the additional add wavelengths of the optical signal introduced by the add device 40 for transmission to the optical fibre core 12e” (via intermediate optical fibre 12e”’). Although the network and node shown in figures 1 and 2, has splitter devices 24a arranged on all of the optical fibres 12a’ (via intermediate optical fibres 12a’”) and coupler devices 24b (via intermediate optical fibre 12e”’) arranged on all of the optical fibres 12b’, in other examples, there may be a mixture of splitter devices and coupler devices connected to ones of the optical fibres 12a’ and 12b’. The use of optical splitter devices and coupler devices connected to the optical fibres 12a’, 12e’ as described permits flexibility to add extra channels, e.g. wavelength channels, as well as reconfigure the network at a fibre core level. Referring to figure 1, additional direct fibre core connections between nodes are shown as 60a, 60b, 60c. These additional node-to-node connections can be formed by appropriate configuration of the respective nodes N1 - N5. Without these additional node-to-node connections, the nodes N1 - N5 are only connected directly by respective pairs of multicore fibres 12a - 12e. Therefore, an optical signal can only reach a desired node by following a certain route along the optical fibres 12a - 12e. For example, a particular channel, e.g. wavelength channel, can only travel along one of the optical fibre cores 12a” of one of the multi-core fibres 12a’ from node N1 to node N2. So, consider a request to add a wavelength channel at node N1 to one of the optical fibre cores 12a” to route the channel to node N2. If there is no spectral capacity in the optical fibre core 12a” to add such a channel, then any requests to add the channel to the fibre cores 12a” would be denied by the network controller. However, due to the presence of additional node-to-node connections 60a, 60b, 60c, the network controller can use an alternative route to send the wavelength channel to node N2. For example, the network controller may operate an algorithm such as that shown in figure 4c for governing how such requests should be dealt with. In this specific example, the algorithm may cause the respective coupler device 24b at node 1 to add the wavelength channel to the optical fibre core 60b by sending the wavelength channel via one of the optical fibre cores 12e’ of multi-core fibre 12e to node N5 from where it is sent by filter-less dropping via fibre core 60b to node N2. Alternatively, the algorithm may cause the respective coupler device 24b at node 1 to add the wavelength channel to optical fibre core 60a from where it reaches node N3 and from where it is sent via multi-core fibre 12b to node N2. Simulations to evaluate network performance and cost of networks deploying aspects of the present disclosure will now be described with reference to figures 4a and 4b. The simulations are based on a five node network formed as a ring, e.g. similar to figure 1, with 7, 12 and 19 optical fibre cores within the multi-core fibres 12a-e. The links between nodes are bi-directional and each of the optical fibre cores permits 96 optical channels with 50Ghz bandwidth. Each service having capacity requirements 100G, 200G, and 400G require respective 50GHz, 100GHz, and 200GHz of bandwidth. Pairs of source and destination nodes are grouped as neighbour and non-neighbour nodes set out as in figure 1. Three service request ratios between neighbouring and non-neighbouring node pairs were 1:1,2:1 and 1:2 with corresponding virtual topology structures. In particular, for such request ratios in a 12 core network, the cores between node 1 and node 2 are allocated for node pairs node 1 to node 2, node 1 to node 3 and note 5 to node 1 as ratios 4:4:4, 6:3:3, and 2:5:5 respectively. The spectrum assignment of channels is based on a first fit scheme. Service blocking probabilities (BPs) were simulated and evaluation for the following regimes: RODAM-based SDM (i.e. using WSSs with SDM), pure-SDM (fibre core level switching only) and the filter-less network architecture of the present disclosure to compare them against each other for different numbers of cores (12 and 19), traffic ratio and traffic load. Figure 4a shows the results (a) to (e) of BSs against traffic load for the three configurations above. This shows that the filter-less network architecture reduces BPs and improves spectral utilisation compared to pure SDM switching for all scenarios, and compared to RODAM-based SDM when traffic load is low because it mitigates against spectral fragmentation. Figure 4b shows network hardware requirements for a RODAM-based SDM at each node compared to the presently disclosed filter-less network requirements at each node based on 7, 12, and 19 fibre core mesh metro networks with different network node structures. It can be seen that a substantial amount of wavelength selective switches (WSSs) are required for RODAM based network nodes which would incur high costs, and greatly complicates wavelength allocation for the network controllers whereas the filter-less network proposed herein has an equal number of splitter and coupler devices compared to WSSs which are less costly (respectively 96%, 95% and 94% cost reductions compared to RODAM based network nodes) and facilitate quicker optical switching by comparison. Figure 5 is a schematic drawing showing another example architecture for use at one of the nodes of the network 10 shown in figure 1 according to aspects of the present invention. The architecture is generally the same as that shown in figure 2 and common features are denoted by the same reference numerals. The main differences are also follows. The optical switch 22 does not have any fibre core add or fibre core drop ports. The ports of the set of ports 22a include ports 62a to which optical signals containing add wavelength(s) may be introduced for switching by the optical switch 22 to one of the ports of the set of ports 22b. There are multiple drop devices 30 to which optical signals from the splitter devices 24a are provided, and these drop devices 30 may be demultiplexers for obtaining the individual wavelengths from the optical signals. The drop devices 30 may be connected to, for example, transponder. Turning to the set of ports 22b, these may similarly include ports 62b which may receive optical signals from one of the ports 22a which are directly connected to the splitter devices 24a and provide the optical signals received therefrom to drop devices 30’. The drop devices 30’ may be demultiplexers which obtain the individual optical drop wavelengths therefrom. The coupler devices 24b may receive add optical signals from add devices 40’ directly connected thereto. The add devices 40’ may be multiplexers which form add optical signals based on inputs received. Operation of the described architecture will be readily understood based on the previously described examples and so will not be described in detail hereinafter. It should be understood that, in certain examples in accordance with the present disclosure, the different features shown in the examples of figure 5 may be combined with the examples of figure 4, e.g. drop devices 30 in the form of demultiplexers, ports 62a which receive optical add wavelengths for being provided to ports 22b, and ports 62b for receiving optical signals from the ports 22a directly connected to splitter devices 24a. Figure 6 is a schematic drawing showing another example architecture for use at a node similar to nodes of network 10 shown in figure 1 according to aspects of the present invention. The architecture is generally the same as that shown in figure 2 and common features are denoted by the same reference numerals. The main difference is that the node has respective pairs of multi-core fibres 12a, 12b, 12c, 12d arranged at four respective directions, e.g. west, east, north and south, of the optical switch node 20 to permit switching of optical signals through the node across the four directions. This permits for switching optical signals across more than two directions for use in more complicated network arrangements. There are splitter and coupler devices 24a, 24b connected to respective optical fibres of the multi-core fibres 12a, 12b, 12c, 12d similar to the corresponding devices of the optical switch node 20 shown in figure 2 to split and combine optical signals for dropping optical signals to drop points D1 and adding optical add wavelengths from add points A1. Their configuration is not described in detail as their operation will readily be understood based on the previously described examples. Similar to the previous examples, optical switch 22 includes optical fibre core add and drop ports to permit optical signals to be dropped and added at core level from core add and drop sources A’, D’ respectively. The arrangement shown permits for a complex number of different optical add and drop paths to be formed and switching to be performed at fibre core level, dropping and adding optical add wavelengths, and adding and dropping optical signals at fibre core level. Referring to figure 7, this is a schematic figure showing an optical switch node 20 with multicore fibres 12a, 12b connected thereto formed as an integrated chip as an example of an optical switch for use with the previously described examples. The optical switch node 20 includes the break out devices 50a, 50b, splitter and coupler devices 24a, 24b, as well as fibre core adding and fibre core dropping ports 22a”, 22b”. The optical switch node 20 is an integrated photonic chip having optical waveguides to which light can be coupled by grating couplers, e.g. to couple light from the individual optical fibres of each of the multi-core fibres 12a, 12b, and single mode fibre add / drop cores for adding I dropping optical signals into the optical switch node 20. In examples, the optical chip may be formed from variously known silicon photonics platforms, e.g. silicon on isolator wafer, and silicon nitride on silicon. In examples, heterogeneous structures of polymer and metal materials on a silicon permits various types of optical grating couplers and waveguide designs to be adopted for use with the present disclosure as will be understood by the skilled person. The photonic chip may include an integrated electrical control circuit and thermal electric control packaged therein to permit programmable control of the optical switch node 20 as desired. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The invention may also broadly consist in the parts, elements, steps, examples and / or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and / or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein. Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure. Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

Claims

1. A spatial division multiplexing optical network including:one or more network nodes connected by multi-core optical fibres, wherein each multi-core optical fibre includes optical fibre cores including respective optical signals formed by wavelength division multiplexing; andan optical switch node located at one of the network node, wherein the optical switch node is connected to the network nodes by first and second multi-core optical fibres arranged in respective first and second directions with respect to the optical switch node, wherein the optical switch node includes:an optical switch including:two or more sets of ports including a first set of ports connected to optical fibre cores of the first multi-core optical fibre and a second set of ports connected to optical fibre cores of the second multi-core optical fibre,wherein the optical switch is selectively configurable to route the optical signals between different ports of the first set and different ports of the second set;one or more splitter devices connected to respective ones ofthe optical fibre cores, wherein each splitter device connected to said one of the optical fibre cores may split the power of the optical signal travelling therethrough so that part of the optical signal travels along a drop optical path whilst retaining all the optical wavelengths of the optical signal and part of the optical signal travels to the port connected to the said one of the optical fibre cores whilst retaining all the optical wavelengths of the optical signal, and / orone or more coupler devices connected to respective ones of the optical fibre cores, wherein each coupler connected to said one of the optical fibre cores may receive an add optical signal having a respective add wavelength travelling along an add optical path and the coupler device couples the add optical signal to the optical signal travelling through the said optical fibre.

2. A spatial division multiplexing optical switch node for an optical system, wherein the optical switch node is for locating at a network node of a spatial division multiplexing optical network and connected by multi-core optical fibres, wherein the optical switch node for connecting to the network node by first and second multi-fibres arranged in respective first and second directions with respect to the optical switch, wherein the switch includes:an optical switch including:two or more sets of ports including a first set of ports for connecting to optical fibre cores of the first multi-core optical fibre and a second set of ports for connecting to optical fibre cores of the second multi-core optical fibre,wherein the optical switch is selectively configurable to route the optical signals between different ports of the first set and different ports of the second set;one or more splitter devices for connecting to respective ones ofthe optical fibre cores, wherein each splitter device connecting to said one of the optical fibre cores may split the power of the optical signal travelling therethrough so that part of the optical signal travels along a drop optical path whilst retaining all the optical wavelengths of the optical signal and part of the optical signal travels to the port for connection to said one of the optical fibre cores whilst retaining all the optical wavelengths of the optical signal, and / orone or more coupler devices for connecting to respective ones of the optical fibre cores, wherein each coupler for connection to said one of the optical fibre cores may receive an add optical signal having a respective add wavelength travelling along an add optical path and the coupler device couples the add optical signal to the optical signal travelling through the said optical fibre.

3. A network or switch according to claim 1 or 2 wherein the sets of ports including further sets of ports which are connected, or are for connecting, to optical fibre cores of further multi-core optical fibres arranged in other directions direction with respect to the optical switch node and the optical switch is selectively configurable to route optical signals between different ports of the first set, second set and further sets of ports.

4. A network or switch according to claim 3 wherein:the one or more splitter devices include splitter devices connected, or for connection to optical fibre cores of the further multi-core optical fibres; and / orthe one or more coupler devices include coupler devices connected, or for connection to optical fibre cores of the further multi-core optical fibres.

3. A network or switch according to any preceding claim wherein the optical switch, in use, may route optical signals between any of the ports of one set and any of the ports of another set of ports.

4. A network or switch according to any preceding claim wherein the optical switch includes:fibre core drop ports, wherein the optical switch, in use, may selectively connect one of the ports from the two or more sets of ports to one of the fibre core drop ports to route all the optical wavelengths in the optical fibre core connected to the said one of the ports to a drop point; and / orfibre core add ports, wherein the optical switch, in use, may selectively connect one of the ports from the two or more sets of ports to one of the fibre core add ports to route all the optical wavelengths from the fibre core add port to the said one of the ports.

5. A network or switch according to claim 4 wherein:one or more fibre core drop ports are connected at positions on opticalpaths which are after any splitter or coupler devices; and / orone or more fibre core add ports are connected at positions on optical paths which are before any splitter or coupler.

6. A network or switch according to any preceding claim wherein the splitter device is adjustable to permit use with optical signals having different ranges of optical power ratio.

7. A network or switch according to any preceding claim wherein the coupler device is adjustable to permit use with optical signals having different optical power ratio.

8. A network or switch according to any preceding claim wherein the add optical signal may have a plurality of add wavelengths and the coupler device adds all of the plurality of add wavelengths to the optical signal.

9. A network or switch according to any preceding claim wherein the drop optical paths terminate at drop ports for connection to a drop device connectable to a filter for selectively filtering the optical wavelengths of the optical signal, optionally or preferably the drop device is a multicast switch and optionally or preferably the multicast switch is connected to a transponder.

10. A network or switch according to any preceding claim wherein the add optical paths start at add ports for connection to an add device which provides the add optical signals, optionally or preferably the add device is a multicast switch and optionally or preferably the multicast switch is connected to a transponder.

11. A network or switch according to any preceding claim wherein the optical switch is formed on an integrated chip.

12. A network or switch according to any preceding claim wherein the optical switch and the one or more splitter devices and / or one or more coupler devices are formed on an integrated chip.

13. A network or switch according to any preceding claim including multi-core breakout devices connected to the multi-core fibres to fan out the said respective optical fibres from the multi-core fibres, optionally or preferably the multi-core breakout devices are integrally formed as part of the optical fibre switch.

14. A network or switch according to any preceding claim wherein the optical drop path is connected to a drop device, optionally or preferably the drop device is a demultiplexer connected to one or more transponders.

15. A network or switch according to any preceding claim wherein one or more transponders are connected to a respective optical add path connected to an add device, optionally or preferably the add device is a multiplexer connected to one or more transponders to provide the add optical signal.

16. A network or switch according to any preceding claim wherein one or more of the ports of one or more of the sets of ports are connected to, or are for connection to, opticalfibre(s) which provide add optical signal(s) having respective add wavelength(s) to the said one or more ports.

17. A network or switch according to any preceding claim wherein one or more of the ports of one or more of the sets of ports are connected to, or are for connection to, optical fibre(s) connected to drop device(s) which filter one or more of the optical wavelengths of travelling through the optical fibre(s).

18. A network according to any preceding claim including a plurality of said one or more nodes and said optical switche nodes connected together, optionally or preferably to form a metro optical transport network, to form a network of data centres or telecommunications network.

19. A network according to any preceding claim, including a central controller for issuing network commands to local controllers which control the nodes and / or optical switch, wherein the central controller is a software defined network controller.

20. A network according to claim 19 wherein the network commands include one or more of the network configuration, operating the optical switch node, network monitoring and / or telemetry.

21. A network according to claim 19 or 20 wherein the controller includes algorithms for optimising the topology of the network; and / or routing, modulation, spectrum and core allocation (RMSCA).

22. A network according to any preceding claim, wherein a network controller for controlling the one or more nodes carries out the following steps when receiving a network request to add or drop I assign an optical signal containing one or more optical add / drop wavelengths or sending to another node:a) for each request to send I assign the optical signal at network node s for sending to network node d;b) obtaining for each of the optical fibre cores the number of available channelsand determining which of the optical fibre cores has the maximum number of available channels which can be routed to network node d’, andc) based on the determination at step b), return the first available channel from the core having the maximum number of available channels and send to the node d.

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