Optical switch

An optical switch comprises a hub and a plurality of nodes with the hub being connected to each node by an optical communication link dedicated for clock signals and by an optical communication link dedicated or data signals. In use the hub transmits a clock signal to all of the nodes; each node re-transmits a copy of the clock signal to the hub and transmits a data signal to the hub. The hub returns each re-transmitted clock signal to its respective node and forwards a copy of each data signal to all of the nodes so that each node can receive a selected data signal by processing the re-transmitted clock signal.

This application is the US national phase of international application PCT/GB02/00155 filed 15 Jan. 2002 which designated the U.S.

BACKGROUND

1. Technical Field

This invention relates to the field of optical switches for communications networks and specifically optical switch fabrics.

2. Description of Related Art

Optical communications technology has advanced rapidly in recent years, with transmission systems capable of terabits per second now being deployed. However, advances in switching and routing technology have not been as dramatic, leading to system bottlenecks as signals are converted from optical format to electronic format for processing, before being re-converted to an optical format for onwards transmission.

BRIEF SUMMARY OF EXEMPLARY NON-LIMITING EMBODIMENTS

According to a first aspect of the present invention there is provided an optical switch, comprising a hub and a plurality of nodes, each node being connected to the hub by first optical communication link for clock signals and by second optical communication link for data signals; such that, in use: the hub transmits a clock signal to all of the nodes; in response to receiving said clock signal, each node re-transmits the clock signal to the hub and transmits a data signal to the hub; the hub transmitting each data signal to all of the nodes and returning each re-transmitted clock signal to its respective node, each node processing the re-transmitted clock signal to receive a selected data signal. Preferably, each node generates data signals by modulating the received clock signal. The clock signal may comprise a plurality of wavelength division multiplexed pulses and each data signal may comprise a plurality of wavelength division multiplexed data pulses.

Preferably, the data signal transmitted by each node has a temporal offset relative to the clock signal which is unique to the respective node. Additionally, the hub may transmit each data signal to all of the nodes and return each re-transmitted clock signal to its respective node, each node determining the respective temporal offset from the re-transmitted clock signal to receive a selected data signal.

According to a second aspect of the present invention there is provided a method of switching optical signals, the method comprising the steps of:(a) transmitting a clock signal from a hub to a plurality of nodes;(b) re-transmitting the clock signal back to the hub from each node;(c) transmitting a data signal from each node to the hub;(d) returning the re-transmitted clock signal to each respective node(e) transmitting all of the received data signals from the hub to all of the nodes;(f) at one or more of the nodes, processing the re-transmitted clock signal to select a data signal.

It is preferred that in step (c) the data signal transmitted by each node has a temporal offset relative to the clock signal which is unique to the respective node and that in steps (e) and (f), each node determines the respective temporal offset from the re-transmitted clock signal to receive a selected data signal.

DETAILED DESCRIPTION OF EXEMPLARY NON-LIMITING EMBODIMENTS

FIG. 1shows a schematic depiction of an optical switch according to the present invention. The switch comprises a hub10which is in communication with a plurality of nodes20, each of which is connected to the hub10by optical communication links30.

FIG. 2shows a schematic depiction of the connection of a hub10to one of the nodes20(only one node is shown inFIG. 2for the sake of clarity) via optical communications links30. Hub10comprises a 1×N optical coupler11, optical pulse source12, N×N optical coupler13and optical circulator14. Hub10is connected to node20by optical communications links31,32,33&34. Node20comprises write module200and read module250. Write module200comprises phase locked loop (PLL)201, optical circulator202, electro-absorption modulator (EAM)203, variable optical delay204, optical 1×2 couplers205&206, fibre stretcher207, optical receivers208&209and band pass filters210&211. Read module250comprises optical receiver251, electro-absorption modulator252, impulse generator253, variable micro phase shifter254, band pass filter255, optical receiver256and optical 1×2 coupler257.

Optical pulse generator12generates a stream of short optical pulses, for example picosecond duration pulses, that is transmitted to 1×N optical coupler11. Optical coupler11has an output leg for each node20that the hub10is in communication with and may have additional, unused output legs for connection to additional nodes20if the switch is to be extended.FIG. 2shows one particular node20; in this case the respective output leg of the optical coupler11is connected to optical circulator14, which forwards the pulse stream to node20via optical communications link31. For the other nodes (not shown inFIG. 2) the pulse stream is transmitted over optical communication links35to the optical circulators (also not shown inFIG. 2) associated with those nodes. Thus, the pulse stream generated by the optical pulse generator11is distributed to all of the nodes20that are connected to the hub10.

Hub10also comprises N×N optical coupler13, the inputs of which are connected to each of the circulators14in the hub10. The outputs of the N×N optical coupler13are connected to the node shown inFIG. 2by optical communication link34. Optical communication links36connect the outputs of N×N optical coupler13to respective hubs20(not shown inFIG. 2). Thus, each of the hubs20is inter-connected so that any signals transmitted from hub20to the node along optical communication link31will pass through N×N optical coupler13and then be sent to all of the other nodes20.

Write module200of node20receives the optical clock pulse from optical communication link31at circulator202. The clock pulse is sent through fibre stretcher207, which controlled by the phase locked loop (PLL)201. Two copies of the clock pulse are then made by optical 1×2 coupler206.

One of the output legs of optical 1×2 coupler206is connected to variable optical delay204and then EAM203. Data to be switched through the optical switch (i.e. to one of the other hubs connected to the node) is modulated onto each pulse of the pulse comb. Modulated data pulses are fed into optical circulator202and transmitted over optical communication link31to optical circulator14, which directs the data pulses to the N×N optical coupler13, which distributes the data pulses to all of the nodes connected to the hub. Optical communication link34carries the output of the N×N optical coupler13to the read module250of the node20and this output will be the combination of the data pulses from the write modules200of each of the nodes connected to the hub. In order to prevent the various data pulses from interfering with each other it is necessary to provide a separation mechanism. The preferred mechanism is time division multiplexing the different data pulses; the variable optical delay204adds a time delay with respect to the received clock pulse before the data is modulated onto the clock pulses. The delay added in each different node20is chosen such that data pulses from all of the nodes can successfully coexist within the same communication link.

The other output leg of optical 1×2 coupler206is connected to the input leg of optical 1×2 coupler205. Clock pulses are transmitted to the PLL201via one of the output legs of optical 1×2 coupler205, the optical receiver208and band pass filter210. The second output leg of optical 1×2 coupler205is transmitted to read module250of the node20via optical communication link32, hub10and optical communication link33. The clock pulse stream is returned to the write module200by optical 1×2 coupler257and is fed to the PLL201via optical receiver209and band pass filter211.

In order for the data pulses from the different nodes to retain the desired time separation it may be necessary to change the path length that the data pulses propagate over; this allows for the compensation of changes in temperature, especially when nodes are not co-located and thus may be subject to different environmental conditions. The PLL controls the fibre shifter to decrease the optical path length to enable the data pulse from the respective node to ‘speed up’ and to increase the optical path length to enable the data pulse from the respective node to ‘slow down’.

Read module250receives clock pulses from optical communication link33and data pulse combs(comprising a data pulse from each of the nodes) from optical communication link34. Clock pulse are returned to write module250via one of the outputs of optical 1×2 coupler257(see discussion above). The second output of optical 1×2 coupler257is connected to optical receiver256. The electrical signal generated by optical receiver256is passed through band pass filter255and variable microwave phase shifter254. As the data pulses are time division multiplexed to provide a relative time gap between each data pulse, it is possible to determine the arrival time of any of the data pulses at a given node from the arrival of the clock pulse from the write module of the node. This relative time delay is used to determine the shift applied by microwave phase shifter254such that the impulse generator253can drive the EAM252to receive the required data pulse from the data comb, by gating the EAM.

It will be readily understood that the present invention relates to a fabric for an optical switch or router. The method by which a suitable path through the switch (or router) is selected (i.e. the pairing if an input port and an output port), or the method of by which port connection is avoided is immaterial and does not effect the working of the present invention. Although the above description has specifically described a number of components it should be noted that it is the function of the described device that is critical, rather than it's structure. For example, EAM252could be replaced by any other optical device that could be used to provide a gate function to ‘drop’ the selected data pulse from the data comb.

Optical communication links31,32,33&34that connect each node to the hub should have the same optical characteristics so as to minimise the differences in optical path length and other propagation phenomena. The inventor has realised that this result can preferably be achieved by the use of ‘blown fibre’ optical cables, in which 4 optical fibres are tightly bound in a jacket (see EP-A-0 186 753 and EP-B-0 521 710).

The capacity of the switch fabric will be limited by the temporal width of the clock and data pulses used in the network and the width of the guardbands which will be necessary to prevent adjacent pulses from interfering with each other. The device limitations which will limit the switch capacity will be the capability of the optical pulse generator12and the capability of the gating devices to ‘drop’ a desired pulse whilst maintaining a sufficient extinction ratio such that the gating devices do not add noise by inadvertently ‘dropping’ a fraction of the pulses that are adjacent the desired pulse.

A second embodiment of the present invention is shown inFIG. 3to provide a multiple wavelength optical switch. Optical pulse generator12generates a clock data comb comprising a number of pulses each having a different optical wavelength rather than a single clock pulse as described above. In a similar manner as described above in relation to the first embodiment, the clock data comb is sent from the hub to each of the nodes; each of the nodes re-transmits the clock data comb back to the hub along with a modulated data comb (which has had the correct amount of temporal delay added); the hub transmits all of the data combs to each of the nodes such that a desired data pulse can be dropped from one of the data combs.

In order to achieve this performance, it is necessary to modify the structure of the write module and the read module of the nodes (seeFIG. 3). Before the data can be modulated over the different wavelength pulses which comprise the clock comb it is necessary to divide the clock comb into its constituent pulses. Arrayed waveguide212is connected to the output of variable optical delay204so that each of the different clock pulses can be separated (althoughFIG. 3shows only 4 outputs from the arrayed waveguide (AWG) it will be understood that this is an arbitrary value and that the number of wavelengths used will vary with the switch capacity that is desired). Each of the AWG outputs is connected to an electro-absorption modulator so that the desired data can be modulated over the clock pulse to generate a data pulse (for the sake of clarity only one of these EAMs is shown inFIG. 3). The outputs of these EAMs are connected to AWG213which re-combines the different data pulses to form a multiple wavelength data comb which is transmitted to the hub.

In order for the read module250to drop a data pulse from the multiple wavelength data comb it is necessary to add 1×N splitter258and arrayed waveguide259to the read module (seeFIG. 3); additionally the one set of the devices required to drop a data pulse (optical receiver251, electro-absorption modulator252, impulse generator253, variable micro phase shifter254, band pass filter255and optical receiver256) must be provided for each of the different wavelengths being used in the switch. One of the outputs of coupler257is connected to 1×N splitter258(where N is the number of wavelengths being used in the switch), which creates a copy of the clock pulse for each of the sets of devices needed to drop the data pulses. For the sake of clarityFIG. 3shows both 1×N splitter258and AWG259as having only four outputs and only one set of receiving devices is shown. AWG259splits the data comb into its constituent data pulses, each of which has an associated set of receiving devices such that the read node can simultaneously drop all N pulses from a data comb.

The use of the multiple wavelengths turns the switch into a wavelength-and time-division multiplexed switch, further increasing the capacity of the switch. However, the use of the different wavelengths causes an additional problem as the different wavelengths will propagate at different speeds in the optical fibres30. In order to prevent wavelength-dependent temporal skew (i.e. some of the data pulses getting out of step with other pulses from the same data comb) it is preferred to arrange the waveguides of AWGs212and259such that the wavelengths that incur the greatest delay in the fibres have the shortest optical path through the AWG and the wavelengths that incur the smallest delay will have a greater path length through the AWG. Suitable selection of the AWG path lengths enables any temporal skew to be removed at the output of the AWG such that all of the data pulses are temporally aligned before being either modulated by EAM203or dropped by EAM252.