WO2013013695A1

WO2013013695A1 - An optical switch and a method of switching an optical signal

Info

Publication number: WO2013013695A1
Application number: PCT/EP2011/062613
Authority: WO
Inventors: Claudio Porzi; Giampiero Contestabile; Antonella Bogoni
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2011-07-22
Filing date: 2011-07-22
Publication date: 2013-01-31
Also published as: GB201402753D0; GB2507445A

Abstract

An optical switch (10) comprising: a first signal input (12) to receive an OOK modulated optical traffic probe signal having a first wavelength and carrying communications traffic and an optical gate probe signal having a second wavelength; a first output (16); a second output (18); a first pump input (30) coupled to the first arm to deliver an optical gate pump signal having the second wavelength and an optical traffic pump signal into the first arm in the second direction of propagation; an optical interferometer (20) having a first state in which the traffic probe and traffic pump signals are synchronously present within the nonlinear medium, to produce a first phase difference between the first and second arms whereby the traffic probe signal is output at one of a first signal output and a second signal output and having a second state in which the traffic probe and pump signals, and gate probe and pump signals are synchronously present within the nonlinear medium, such that a second phase difference is produced between the first and second arms whereby an OOK modulated output optical signal at the second wavelength, carrying the communications traffic, is output at said output; anda controller (34) arranged to selectively cause the optical gate pump signal to be delivered to the first pump input, to selectively set the interferometer to its second state.

Description

AN OPTICAL SWITCH AND A METHOD OF SWITCHING AN OPTICAL

SIGNAL

Technical Field

The invention relates to an optical switch. The invention further relates to an optical communications network node comprising the optical switch. The method further relates to a method of switching an optical signal carrying communications traffic in a

communications network. Background

Wavelength conversion is an essential function in wavelength division multiplexed, WDM, optical communications systems in which a traffic stream at a specific wavelength is transferred to another wavelength in order to be routed on a different wavelength path and/or to release the original wavelength resource to another traffic stream (commonly referred to as an add/drop operation). For next-generation optical networks, in which dynamic wavelength switching/routing nodes are expected to increase network capacity and flexibility, this is typically performed by deflecting an incoming signal toward a wavelength conversion stage. Typically this is proposed to be done at an electrical control- plane level using an electrical control signal arranged to control a space-switch to deflect the incoming signal, and to turn on or off a continuous wave (CW) laser which generates a carrier signal for the wavelength conversion stage. Two separate devices are thus normally required to release the output fiber from the original input wavelength (space deflection operation) and transfer the signal information at a new output wavelength (wavelength conversion operation). All-optical signal processing has been attracting much research in recent years since it can potentially provide solutions for the ever expanding demand of bandwidth in telecommunications and ultrafast computing. To this end integrated nonlinear optical devices are of great interest because of compactness and reduced power

consumption. In particular, integrated semiconductor optical amplifier Mach-Zehnder interferometers (SOA-MZIs) have been used in a number of signal processing

demonstrations, including wavelength conversion, as reported in T. Durhuus et al, "All optical wavelength conversion by SOA's in a Mach-Zehnder configuration", IEEE Photon. Technol. Lett., vol. 6, no. 1, pp. 53-55, Jan. 1994, and switching functions suitable for optical packet-switched networks, as reported in J.-Y. Kim et al, "All-optical multiple logic gates using parallel SOA-MZI structures", Proc. LEOS 2005, paper MM1, 2005. Summary

It is an object to provide an improved optical switch. It is a further object to provide an improved optical communications network node comprising the optical switch. It is a further object to provide an improved method of switching an optical signal carrying communications traffic in a communications network.

A first aspect of the invention provides an optical switch comprising a first signal input, a first signal output, a second signal output, an optical interferometer, a first pump input, and a controller. The first signal input is arranged to receive an on-off key modulated optical traffic probe signal having a first wavelength and carrying

communications traffic. The first signal input is further arranged to receive an optical gate probe signal having a second wavelength different to the first wavelength. The optical interferometer comprises an input coupled to the first signal input and an output coupled to the first and second signal outputs. The interferometer further comprises a first arm having a first optical path and a second arm having a second optical path, each optical path extending between the input and the output. The interferometer input is arranged to receive said optical traffic probe signal and said optical gate probe signal propagating in a first direction and is further arranged to couple each said received probe signal into each of the first arm and the second arm. The first arm further comprises a nonlinear medium arranged to apply an intensity dependent phase change to an optical signal propagating through it. The first pump input is coupled to the first arm. The first pump input is arranged to deliver an optical gate pump signal having the second wavelength into the first arm in a second direction of propagation, opposite to said first direction. The first pump input is further arranged to deliver an optical traffic pump signal into the first arm in the second direction of propagation. The optical traffic pump signal is a copy of the optical traffic probe signal. The interferometer has a first state in which the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at one of the first signal output and the second signal output. The interferometer has a second state in which the optical traffic probe signal, the optical traffic pump signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium, such that a second phase difference, different to the first phase difference is produced between the first and second arms. In the second state an on-off key modulated output optical signal at said second wavelength and carrying said communications traffic is output at said one of the first signal output and the second signal output. The controller is arranged to selectively cause the optical gate pump signal to be delivered to said first pump input, whereby the interferometer is selectively set to the second state.

The optical switch is able to carry out all-optical simultaneous wavelength conversion and traffic erasing. The optical switch is able to convert bursts of traffic of the optical traffic probe signal into an output optical signal at the second wavelength, and to simultaneously erase the converted traffic from the optical traffic probe signal, using nonlinear interaction of optical signals within a single counter-propagated optical interferometer. The optical switch may be used to perform wavelength routing operation in WDM systems, where an input signal is wavelength-converted in order to release the original wavelength resource in the network and the wavelength converted signal can then be routed on a different channel. The performance penalty suffered by the prior art schemes comprising two separate switching and wavelength conversion elements may be avoided by the optical switch. The optical switch may therefore have a reduced number of active elements as compared to the prior art. In addition, the optical switch carries out both switching and wavelength conversion entirely in the optical domain, therefore operation speed may be faster than conventional electro-optic devices used to perform the same operations.

In an embodiment, the optical switch further comprises a second signal input. The first signal input is arranged to receive the optical traffic probe signal. The second signal input is arranged to receive the optical gate probe signal. In the first state the interferometer is arranged to output the optical traffic probe signal at the first signal output. In the second state the interferometer is arranged to output the output optical signal at the second signal output.

In an embodiment, the optical interferometer further comprises a phase shifter provided within the second interferometer arm. The phase shifter is arranged to provide an initial phase difference, different to the first phase difference between the first and second arms such that in the absence of the optical traffic pump signal and the optical gate pump signal the optical traffic probe signal undergoes destructive interference at the first signal output.

In an embodiment, the initial phase difference is 2ηπ, the first phase difference is

(2n + 1)π, and the second phase difference is the first phase difference plus (2n + 1)π, where n is an integer.

In an embodiment, the second interferometer arm further comprises a nonlinear medium. In the first state, the optical traffic probe signal propagates through the nonlinear medium in the first arm and the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium in the second interferometer arm, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at the first signal output. In the second state, the optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are synchronously present within the nonlinear medium in the second arm and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm, whereby the output optical signal is output at the second signal output. The controller is arranged to selectively cause the optical gate pump signal to be delivered to the first pump input, whereby the interferometer is selectively set to the second state.

In an embodiment, the controller is further arranged to selectively cause the optical gate probe signal to be delivered to the second signal input synchronously with the optical gate pump signal. This may ensure that an optical signal at the second wavelength is not output from the optical switch when the optical gate pump signal is not present.

In an embodiment, each interferometer arm comprises a phase shifter arranged to set the interferometer to the initial state in which, in the absence of the optical traffic pump signal and the optical gate pump signal, the optical traffic probe signal undergoes destructive interference at the first signal output.

In an embodiment, the optical interferometer comprises a Mach-Zehnder waveguide interferometer.

In an embodiment, the or each nonlinear medium comprises one of a

semiconductor optical amplifier, a nonlinear passive optical waveguide, and a nonlinear photonic crystal waveguide. Using one of these nonlinear media may provide switching times, which may be fast enough to enable the optical switch to perform wavelength conversion and traffic erasing on a continuous traffic stream without loss of traffic bits.

In an embodiment, the optical switch further comprises a first optical filter and a second optical filter. The first optical filter is provided after the first signal output and is arranged to transmit optical signals at the first wavelength. The second optical filter is provided after the second signal output and is arranged to transmit optical signals at the second wavelength. Transmitted optical traffic probe signals may therefore be selected at the first optical signal output and wavelength converted on-off key modulated output optical signals may be selected at the second optical signal output.

In an embodiment, the switch further comprises a first optical splitter and a second optical splitter. The first optical splitter is arranged to receive an on-off key modulated input optical signal and to split the input optical signal to form the optical traffic pump signal and the optical traffic probe signal. The second optical splitter is arranged to receive an optical gate signal and to split the optical gate signal to form the optical gate pump signal and the optical gate probe signal. This may ensure that each pump optical signal is a copy of the respective probe optical signal.

In an embodiment, each optical splitter is arranged to split the respective optical signal such that the respective pump signal has a higher optical power than the respective probe signal. In an embodiment, each optical splitter is arranged to split the respective optical signal substantially equally to form the respective pump signal and probe signal, and the switch further comprises optical power control apparatus arranged to modify the optical power of one of the respective pump signal and the respective probe signal such that the respective pump signal has a higher optical power than the respective probe signal. Where the respective nonlinear medium is a semiconductor optical amplifier the gain may therefore be saturated by the respective pump signal. Where the respective nonlinear medium is one of a nonlinear passive optical waveguide and a nonlinear photonic crystal waveguide this may ensure that the respective pump signal is arranged to induce a nonlinear phase variation within the nonlinear medium.

In an embodiment, the optical switch further comprises an input optical signal converter and an output optical signal converter. The input optical signal converter is arranged to receive an input optical signal having a modulation format different to on-off key and convert the input optical signal into an on-off key modulated input optical signal. The output optical signal converter is arranged to receive an on-off key modulated optical signal from one of the first signal output and the second signal output and to convert said signal into a further output optical signal having a different modulation format.

In an embodiment, the different modulation format is a higher modulation format. In an embodiment, the different modulation format comprises one of differential phase- shift keying, differential quadrature phase-shift keying, and 16 quadrature amplitude modulation. The optical switch may therefore by used with multi-level optical signals.

A second aspect of the invention provides an optical communications network node comprising an optical switch a first signal input, a first signal output, a second signal output, an optical interferometer, a first pump input, and a controller. The first signal input is arranged to receive an on-off key modulated optical traffic probe signal having a first wavelength and carrying communications traffic. The first signal input is further arranged to receive an optical gate probe signal having a second wavelength different to the first wavelength. The optical interferometer comprises an input coupled to the first signal input and an output coupled to the first and second signal outputs. The interferometer further comprises a first arm having a first optical path and a second arm having a second optical path, each optical path extending between the input and the output. The interferometer input is arranged to receive said optical traffic probe signal and said optical gate probe signal propagating in a first direction and is further arranged to couple each said received probe signal into each of the first arm and the second arm. The first arm further comprises a nonlinear medium arranged to apply an intensity dependent phase change to an optical signal propagating through it. The first pump input is coupled to the first arm. The first pump input is arranged to deliver an optical gate pump signal having the second wavelength into the first arm in a second direction of propagation, opposite to the first direction. The first pump input is further arranged to deliver an optical traffic pump signal into the first arm in the second direction of propagation. The optical traffic pump signal is a copy of the optical traffic probe signal. The interferometer has a first state in which the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at one of the first signal output and the second signal output. The interferometer has a second state in which the optical traffic probe signal, the optical traffic pump signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium, such that a second phase difference, different to the first phase difference is produced between the first and second arms. In the second state an on-off key modulated output optical signal at said second wavelength and carrying said communications traffic is output at said one of the first signal output and the second signal output. The controller is arranged to selectively cause said optical gate pump signal to be delivered to said first pump input, whereby the interferometer is selectively set to the second state.

The node is able to carry out all-optical simultaneous wavelength conversion and traffic erasing. The node is able to convert bursts of traffic of the optical traffic probe signal into an output optical signal at the second wavelength, and to simultaneously erase the converted traffic from the optical traffic probe signal, using nonlinear interaction of optical signals within a single counter-propagated optical interferometer. The node may be used to perform wavelength routing operation in WDM systems, where an input signal is wavelength-converted in order to release the original wavelength resource in the network and the wavelength converted signal can then be routed on a different channel. The performance penalty suffered by the prior art schemes comprising two separate switching and wavelength conversion elements may be avoided by the node. The node may therefore have a reduced number of active elements as compared to the prior art. In addition, the node carries out both switching and wavelength conversion entirely in the optical domain, therefore operation speed may be faster than conventional electro-optic devices used to perform the same operations.

In an embodiment, the optical interferometer further comprises a phase shifter provided within the second interferometer arm. The phase shifter is arranged to provide an initial phase difference, different to the first phase difference between the first and second arms such that in the absence of the optical traffic pump signal and the optical gate pump signal the optical traffic probe signal undergoes destructive interference at the first signal output. In an embodiment, the initial phase difference is 2ηπ, the first phase difference is (2n + 1)π, and the second phase difference is the first phase difference plus (2n + 1)π, where n is an integer.

In an embodiment, the second interferometer arm further comprises a nonlinear medium. In the first state, the optical traffic probe signal propagates through the nonlinear medium in the first arm and the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium in the second interferometer arm, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at the first signal output. In the second state, the optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are synchronously present within the nonlinear medium in the second arm and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm, such that the output optical signal is output at the second signal output. The controller is arranged to selectively cause the optical gate pump signal to be delivered to the first pump input, whereby the interferometer is selectively set to the second state.

In an embodiment, the controller is further arranged to selectively cause the optical gate probe signal to be delivered to the second signal input when the optical gate pump signal is provided. This may ensure that an optical signal at the second wavelength is not output from the optical switch when the optical gate pump signal is not present.

In an embodiment, the or each nonlinear medium comprises one of a

In an embodiment, the optical switch further comprises a first optical splitter and a second optical splitter. The first optical splitter is arranged to receive an on-off key modulated input optical signal and to split the input optical signal to form the optical traffic pump signal and the optical traffic probe signal. The second optical splitter is arranged to receive an optical gate signal and to split the optical gate signal to form the optical gate pump signal and the optical gate probe signal. This may ensure that each pump optical signal is a copy of the respective probe optical signal.

In an embodiment, the optical switch further comprises an input optical signal converter and an output optical signal converter. The input optical signal converter is arranged to receive an input optical signal having a modulation format different to on-off key and convert the input optical signal into an on-off key modulated input optical signal. The output optical signal converter is arranged to receive an on-off key modulated optical signal from one of the first signal output and the second signal output and to convert said signal into a further output optical signal having a different modulation format. In an embodiment, the different modulation format is a higher modulation format. In an embodiment, the different modulation format comprises one of differential phase- shift keying, differential quadrature phase-shift keying, and 16 quadrature amplitude modulation. The node may therefore by used with multi-level optical signals.

In an embodiment, the optical communications network node further comprises an optical gate signal source arranged to generate the optical gate signal.

A third aspect of the invention provides a method of switching an optical signal carrying communications traffic in a communications network. The method comprises: receiving an on-off key modulated optical traffic probe signal having a first wavelength and carrying communications traffic;

receiving an optical gate probe signal having a second wavelength different to the first wavelength;

causing the optical traffic probe signal and the optical gate probe signal to propagate through an optical interferometer in a first direction, the interferometer comprising a first arm having a first optical path and a second arm having a second optical path, each optical path extending between an input and an output of the interferometer, the first arm comprising a nonlinear medium arranged to apply an intensity dependent phase change to an optical signal propagating through it and the input being arranged to couple each said received probe signal into each of the first arm and the second arm;

providing an optical traffic pump signal into the first interferometer arm in a second direction of propagation, opposite to the first direction, the optical traffic pump signal being a copy of the optical probe signal;

causing the optical traffic probe signal and the optical traffic pump signal to be synchronously present within the nonlinear medium, such that the interferometer is caused to operate in a first state in which a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at one of a first signal output and a second signal output; and

selectively additionally causing the optical gate probe signal and the optical gate pump signal to be synchronously present within the nonlinear medium such that the interferometer is caused to operate in a second state in which a second phase difference, different to the first phase difference, is produced between the first and second arms whereby an on-off key modulated output optical signal is produced at said second wavelength and carrying said communications traffic and is output at said one of the first signal output and the second signal output.

The method may enable out all-optical simultaneous wavelength conversion and traffic erasing. The method may enable conversion of bursts of traffic of the optical traffic probe signal into an output optical signal at the second wavelength, and simultaneously erasing of the converted traffic from the optical traffic probe signal, using self- switching of optical signals within a single counter-propagated optical interferometer. The method may be used to perform wavelength routing operation in WDM systems, where an input signal is wavelength-converted in order to release the original wavelength resource in the network and the wavelength converted signal can then be routed on a different channel. The performance penalty suffered by the prior art schemes comprising two separate switching and wavelength conversion elements may be avoided using the method. The method may enable both switching and wavelength conversion entirely in the optical domain, therefore operation speed may be faster than methods utilising conventional electro-optic devices to perform the same operations.

In an embodiment, in the first state the optical traffic probe signal is output at the first signal output and in the second state the optical traffic probe signal is output at the second signal output.

In an embodiment, the second interferometer arm further comprises a nonlinear medium. In the first state, the optical traffic probe signal and the optical traffic pump signal are caused to be synchronously present within the nonlinear medium, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at the first signal output. In the second state, the optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are synchronously present within the nonlinear medium in the second arm and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm, such that the output optical signal is output at the second signal output.

In an embodiment, the optical gate probe signal is selectively caused to be provided when the optical gate pump signal is provided. This may ensure that an optical signal at the second wavelength is not output from the optical switch when the optical gate pump signal is not present.

In an embodiment, each interferometer arm further comprises a phase shifter arranged to set the interferometer to an initial state in which, in the absence of the optical traffic pump signal and the optical gate pump signal, the optical traffic probe signal undergoes destructive interference at the first signal output.

In an embodiment, the or each nonlinear medium comprises one of a

semiconductor optical amplifier, a nonlinear passive optical waveguide, a nonlinear photonic crystal waveguide. Using one of these nonlinear media may provide switching times which may be fast enough to enable the optical switch to perform wavelength conversion and traffic erasing on a continuous traffic stream without loss of traffic bits.

In an embodiment, the method further comprises filtering the optical signal output at the first signal output to transmit only optical signals at the first wavelength. The method further comprises filtering the optical signal output at the second signal output to transmit only optical signals at the second wavelength. Transmitted optical traffic probe signals may therefore be selected at the first optical signal output and wavelength converted on-off key modulated output optical signals may be selected at the second optical signal output.

In an embodiment, the method further comprises the initial steps of receiving an on-off key modulated input optical signal and splitting the input optical signal to form the optical traffic pump signal and the optical traffic probe signal. The method further comprises receiving an optical gate signal and splitting the optical gate signal to form the optical gate pump signal and the optical gate probe signal. This may ensure that each pump optical signal is a copy of the respective probe optical signal. In an embodiment, the method comprises splitting each respective optical signal such that the respective pump signal has a higher optical power than the respective probe signal. In an embodiment, the method comprises splitting each respective optical signal substantially equally to form the respective pump signal and probe signal, and the method further comprises modifying the optical power of one of the respective pump signal and the respective probe signal such that the respective pump signal has a higher optical power than the respective probe signal. Where the respective nonlinear medium is a

semiconductor optical amplifier the gain may therefore be saturated by the respective pump signal. Where the respective nonlinear medium is one of a nonlinear passive optical waveguide and a nonlinear photonic crystal waveguide this may ensure that the respective pump signal is arranged to induce a nonlinear phase variation within the nonlinear medium.

In an embodiment, the method comprises receiving an input optical signal having a modulation format different to on-off key and converting the input optical signal into the on-off key modulated input optical signal. The method further comprises converting an on- off key modulated optical signal received from one of the first signal output and the second signal output into a further output optical signal having a different modulation format.

In an embodiment, the different modulation format is a higher modulation format. In an embodiment, the different modulation format comprises one of differential phase- shift keying, differential quadrature phase-shift keying, and 16 quadrature amplitude modulation. The method may therefore by used with multi-level optical signals.

A fourth aspect of the invention provides a data carrier having computer readable instructions embodied therein. The said computer readable instructions are for providing access to resources available on a processor. The computer readable instructions comprise instructions to cause the processor to perform any of the above steps of the method of switching an optical signal carrying communications traffic in a communications network

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. Brief description of the drawings

Figure 1 is a schematic representation of an optical switch according to a first embodiment of the invention;

Figure 2 is a schematic representation of an optical switch according to a second embodiment of the invention;

Figure 3 is a graphic representation of the operation principle of the optical switch of Figure 2;

Figure 4 shows oscilloscope traces showing: a) Input data; b) Gate signal; c) Pass- through output data; d) Wavelength- shifted output data; and e) and f) Rising and falling edges of the output signals, of the optical switch of Figure 2;

Figure 5 shows BER measurements (top) and input/output eye diagrams (bottom) of the optical switch of Figure 2;

Figure 6 is a schematic representation of an optical switch according to a third embodiment of the invention;

Figure 7 is a schematic representation of an optical switch according to a fourth embodiment of the invention;

Figure 8 is a schematic representation of an optical switch according to a fifth embodiment of the invention;

Figure 9 is a schematic representation of an optical communications network node according to a sixth embodiment of the invention;

Figure 10 shows the steps of a method of switching an optical signal carrying communications traffic in a communications network according to a seventh embodiment of the invention;

Figure 11 shows the steps of a method of switching an optical signal carrying communications traffic in a communications network according to an eighth embodiment of the invention;

Figure 12 shows the steps of a method of switching an optical signal carrying communications traffic in a communications network according to a ninth embodiment of the invention; and Figure 13 shows the steps of a method of switching an optical signal carrying communications traffic in a communications network according to a tenth embodiment of the invention. Detailed description

A first embodiment of the invention provides an optical switch 10 as shown in Figure 1. The optical switch 10 comprises a first signal input 12, a second signal input 14, a first signal output 16, a second signal output 18, an optical interferometer 20, a first pump input 30, a second pump input 32 and a controller 34.

The first signal input 12 is arranged to receive an on-off key (OOK) modulated optical traffic probe signal having a first wavelength and carrying communications traffic. The communications traffic comprises T bits (logical Is) and '0' bits (logical 0s). The second signal input 14 is arranged to receive an optical gate probe signal having a second wavelength that is different to the first wavelength. The first signal output 16 is arranged to selectively output the optical traffic probe signal. The second signal output 18 is arrange to selectively output an OOK modulated output optical signal at the second wavelength and carrying the communications traffic.

The optical interferometer 20 comprises an input 22 and an output 24. The input 22 is coupled to the first signal input 12 and to the second signal input 14. The output 24 is coupled to the first signal output 16 and to the second signal output 18. The input 22 is arranged to receive the optical traffic probe signal and the optical gate probe signal, both propagating in a first direction. The interferometer 20 further comprises a first arm 26 having a first optical path and a second arm 28 having a second optical path. Each optical path extends between the interferometer input 22 and the interferometer output 24. In operation, each optical traffic probe signal and each optical gate probe signal received at the interferometer input 22 is split in two, with a copy of each signal being routed into each interferometer arm 26, 28. Each signal received at the interferometer output 24, from a respective arm 26, 28, is split in two and delivered to each signal output 16, 18. The signals propagating through the interferometer arms 26, 28 are therefore recombined at each signal output 16, 18. Each interferometer arm 26, 28 further comprises a non-linear medium arranged to apply an intensity dependant phase change to an optical signal propagating through the nonlinear medium.

The first pump input 30 is coupled to the first arm 26 and is arranged to deliver the optical gate pump signal having the second wavelength into the first interferometer arm 26 in a second direction of propagation, opposite to the first direction of propagation. The second pump input 32 is coupled to the second interferometer arm 28 and is arranged to couple the optical traffic pump signal into the second interferometer arm 28 in the second direction of propagation. The optical traffic pump signal is a copy of the optical traffic probe signal.

The interferometer 20 has a first state in which the optical traffic probe signal propagates through the nonlinear medium in the first arm 26 and the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium in the second interferometer arm 28, such that a first phase difference is produced between the first and second arms. As a result the optical traffic probe signal is output at the first signal output 16. The interferometer 20 has a second state in which the optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are

synchronously present within the nonlinear medium in the second arm 28 and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm 26. As a result the output optical signal is output at the second signal output 18.

The controller 34 is arranged to selectively cause the optical gate pump signal to be delivered to the first pump input 32 and the optical gate probe signal to be delivered to the second signal input 14.

In operation, the interferometer is arranged to have an initial state in which any signal entering from input 12, with sufficiently low power to not produce nonlinear effects in the nonlinear medium, suffers almost complete destructive interference at the first output 16, and any signal entering from input 14, with sufficiently low power to not produce nonlinear effects in the nonlinear medium, suffers almost complete destructive interference at the first output 18.

Once the interferometer has been set to the initial state, the optical traffic probe signal and the optical traffic pump signal are launched synchronously into the optical switch 10 so that they travel simultaneously, in counter-propagating directions, through the interferometer 20 and overlap and interact within the non-linear medium in the second interferometer arm 28. Therefore, when no optical gate pump signal is delivered to the interferometer, a phase shift in the second interferometer arm 28 is produced by the ones in the optical traffic pump signal as a result of the intensity dependent phase shift produced within the non-linear medium. This causes the optical traffic probe signal to be delivered to the first signal output 16.

When the controller 34 causes the optical gate pump signal to be delivered to the first pump input 30, an optical gate pump signal and the optical gate probe signal are delivered simultaneously to the interferometer 20 and propagate simultaneously through the first interferometer arm 26. They therefore overlap and interact with each other within the non-linear gain medium in the first interferometer arm 26. The optical gate pump signal produces a phase shift in the first interferometer arm 26 which matches the phase shift produced in the second interferometer arm 28 by the bits representing a logical one in the optical traffic pump signal, as described above. The gain/phase balance between the two interferometer arms 26, 28 is thereby restored, which causes the optical traffic probe signal to be switched to the second optical signal output 18.

At the same time, the optical gate probe signal that travels into the second interferometer arm 28 experiences a first phase shift due to nonlinear interaction with the counter-propagating bits of the optical data pump representing a logical one in the nonlinear medium and the gate probe signal travelling in the first interferometer arm 16 experiences a second phase shift (equal to the first phase shift) due to the nonlinear interaction with the counter-propagating gate pump signal in the nonlinear medium. As a consequence, the gate probe signal travelling in the first and second interferometer arms experiences a net phase difference only when the pump bits are a logical zero. When the two replica of the probe signal travelling in the first and second interferometer arms recombine at the interferometer output, this causes constructive interference at output port 18 in correspondence of the logical zeros in the data pump, resulting in the communications traffic carried by the optical traffic pump signal being transferred on to the optical gate probe signal with inverted logic. The resulting OOK modulated output optical signal carrying the communications traffic is output at the second signal output 18. A state of destructive interference can be obtained either with phase balancing (where there is a phase difference of 2ηπ, where n is an integer) or with a phase difference of (2η+1)π between the interferometer arms. In the second state, the phase difference between the interferometers arms 26, 28 is changed by π with respect to the phase difference between the arms in the first state, such that the OOK modulated output optical signal is output at the second signal output 18 and the traffic probe signal is simultaneously erased due to destructive interference at the first output 16.

The phase difference between the interferometer arms 26, 28 in the first state is dynamically set by the traffic pump signal such that, in correspondence of a bit 1 in the traffic pump signal, the copy of that bit 1 in the probe traffic signal is output at the first signal output 16 (because of the phase difference state set by the traffic pump signal bit 1 which is synchronous with its bit 1 copy in the traffic probe signal ). A traffic pump signal bit 0 does not induce a phase difference and, as a consequence, the corresponding bit 0 copy in the probe traffic is not switched to the first signal output. However, since there is no power in the traffic probe signal bit 0 this does not affect the data sequence of the traffic probe signal at the first signal output. The traffic pump signal bits therefore cause switching of the corresponding traffic probe signal bits. The second state of the interferometer changes dynamically so that the state of phase-balancing is attained only in the presence of logical 1 s in the traffic pump signal, which counteract the effect of the phase change induced by the gate pump signal in the other interferometer arm.

An optical switch 40 according to a second embodiment of the invention is shown in Figure 2. The optical switch 40 of this embodiment is similar to the optical switch 10 shown in Figure 1, with the following modifications. The same reference numbers are retained for corresponding features.

In this embodiment the optical interferometer comprises a Mach-Zehnder interferometer (MZI) 42, which may be provided as a planar waveguide device. Each nonlinear gain medium comprises a semiconductor optical amplifier (SOA) 44, 46. Each interferometer arm 26, 28 further comprises a phase shifter (PS) 48, 50.

A first optical filter 52 arranged to transmit at the first wavelength is provided after the first optical signal output 16. A second optical filter 54 arranged to transmit at the second wavelength is provided after the second optical signal output 18. In this embodiment the outputs of the optical filters 52, 54 are combined via an optical signal combiner 56 into a single optical output.

The optical traffic pump signal comprises a copy of the optical traffic probe signal and the optical gate pump signal comprises a copy of the optical gate probe signal. Each of the pump signals has a higher optical signal power than its corresponding probe signal in order to ensure that the respective SOA 44, 46 is gain saturated when the respective pump signal is present within it.

The controller (not shown) is arranged to selectively cause the optical gate pump signal to be delivered to the first pump input 30 and the optical gate probe signal to be delivered to the second signal input 14 whereby the interferometer 20 is selectively set to its second state.

The phase shifters 52, 54 are arranged to apply a phase shift to their respective interferometer arm 26, 28 such that that phase shift combined with the phase shift caused by the respective SOA, in its initial gain state, sets the interferometer 42 to an initial state in which the first and second arms 26, 28 are phase balanced. This means that in the absence of the optical traffic pump signal and the optical gate pump signal, the optical traffic probe signal undergoes destructive interference at the first signal output 16.

In operation, an optical traffic signal, at the first wavelength, data, is

simultaneously delivered to two inputs of the optical switch 40; one copy of the optical traffic signal, the optical traffic probe signal, Data Probe, is applied to the first optical signal input 12, EST 1, and the other copy of the optical traffic signal, the optical traffic pump signal, Data Pump, is applied to the second pump signal input 32, P 2. The optical traffic pump signal acts as a pump signal in the SOA 46 in the second interferometer arm 28. The optical traffic pump and probe signals are launched synchronously in the optical switch 40 so that optical traffic pump signal travels simultaneously, in a counter- propagating direction, with the optical traffic probe signal within the SOA 46. Similarly, an optical gate signal at the second wavelength, Xgate, is applied simultaneously to the second signal input 14, EST 2, and the first pump signal input 30, PI, acting as optical gate probe and a pump signal, respectively. The optical gate pump and probe signals are also synchronized at the respective inputs, so that they cross within the SOA 44 in the first interferometer arm 26 at the same time, in counter-propagating directions. As shown in Figure 2, each probe and pump signal couple is counter-propagating within the SOAs, which have similar characteristics. The operation principle can be explained as follows. By acting on the phase shifters 48, 50 and on the initial gain of each SOA 44, 46, the interferometer 42 is initially biased so that if no pump signals are present, the optical traffic probe signal would experience complete destructive interference at the first signal output 12, OUT 1. In normal operation, however, the optical traffic probe and pump signals are always applied at the same time, and they are synchronous within the SOA-MZI 42. Thus, when the optical gate pump signal is not present, or is low, a phase shift in the second arm 28 is produced by the Is in the optical traffic pump signal as an effect of carrier depletion in the SOA 46, causing the optical traffic probe signal to be self- switched from the second optical signal output 18 to the first optical signal output 16.

When the controller (not shown) causes the optical gate pump signal and the optical gate probe signal to be delivered to the interferometer 42, optical gate probe and pump signals synchronously enter the SOA-MZI 42. The optical power of the optical gate pump signal is arranged such that the carrier density is reduced in the SOA 44 in the first arm 26 as a result of the optical gate pump signal propagating through it. This restores the original gain/phase balance between the two interferometer arms 26, 28 set by the initial bias state. This switches the optical traffic probe signal back to the second optical signal output 18, OUT 2, when the optical gate pump signal is present, resulting in selective cancellation of the communications traffic on the optical traffic probe signal at the first optical signal output 16. Due to the symmetry of the interferometer 42, the initial bias settings of the phase shifters 48, 50 and the SOAs 44, 46 are such that the optical gate probe signal, entering from the second signal input 14, is output from the first signal output 16 when no optical gate pump signal is applied and is self-switched to the second signal output 18 when the optical gate pump signal is applied. However, the initial bias state can be restored in correspondence to the logical Is in the optical traffic pump signal propagating through the SOA 46 in the second arm 28. This results in the input communications traffic pattern being transferred onto the optical gate probe signal, to form an OOK modulated output signal with inverted logic, which is delivered to the second signal output 18.

In this example, the optical filters 52, 54 comprise optical bandpass filters arranged to transmit optical signals at the first wavelength and the second wavelength output from the first and second optical signal outputs 16, 18 respectively. The optical switch 40 is thereby arranged to select the optical traffic probe signal at the first wavelength (the pass- through data stream) delivered to the first signal output 16, and to select the inverted copy of the gated burst of communications traffic at the second wavelength (the wavelength- shifted signal) at the second optical signal output. The OOK modulated output signal, i.e. the wavelength-shifted signal, is present at the second signal output 18 only when the optical traffic probe signal at the first wavelength is suppressed at the first optical signal output 16.

The effect of the pump signals on the probe signals at the two SOA-MZI optical signal outputs is illustrated in Figure 3, which shows the typical periodic output/input power transfer functions relative to the different outputs/inputs of the SOA-MZI 42, as a function of the phase difference ΔΦ between the interferometer arms 26, 28.

The initial bias setting (in the absence of pump signals) is such that the operating point for the optical gate probe signal and optical traffic probe signal are indicated in the figure by A and A' respectively. The effect of the optical traffic pump signal on the optical traffic probe signal is to push the operating point of the optical traffic probe signal from A to B (high transmission), whereas the optical gate pump signal pulls back the optical traffic probe signal to the initial point A (low transmission). Similarly, the effect of the optical gate pump signal on the optical gate probe signal is to push the operating point of the optical gate probe signal from A' to B' (high transmission), whereas the optical gate pump signal pulls back the optical traffic probe signal to the initial point A' (low transmission).

To demonstrate the operation of the optical switch 42 and evaluate its performance, a 10 Gb/s Non Return to Zero ( RZ) continuous Pseudo-Random-Bit-Sequence (PRBS) optical traffic signal was generated and a squared optical gate signal with variable length was generated using a 10 GHz-bandwidth waveform generator. The rising and falling edges of the optical gate signal were measured to be around 35 ps and 40 ps, respectively. The signal wavelengths were: first wavelength, data, =1554 nm and second wavelength, Xgate, =1557 nm. Both signals were split to generate the probe and pump copies, as described above. Delivery of each probe and pump signal was synchronized at the respective SOA-MZI 42 inputs using respective optical delay lines. The oscilloscope traces of the optical signals for a 500 ns-long optical gate signal with 50% duty-cycle are shown in Figure 4. The corresponding average input power levels at the SOA-MZI were -9.5 dBm, 2 dBm, -9.2 dBm and 0.5 dBm for optical traffic probe signal, optical traffic pump signal, optical gate probe signal and optical gate pump signal respectively. The two SOAs 44, 46 were equally biased with a current of 300 raA. Figure 4a) and b) show the optical traffic signal and the optical gate signal respectively. The optical traffic probe signal (pass-through signal) output from the first optical signal output 16, OUT 1, is shown in Figure 4c). The OOK modulated output optical signal (i.e. the wavelength converted signal) output at the second optical signal output 18, OUT 2, is shown in Figure 4d). Details of the rising and falling edges of both output signals in switching operation are shown in Figure 4e) and f) respectively. Rising and falling times of the wavelength shifted signal and the falling time of the pass-through optical traffic probe signal are set by the transient times of the optical gate signal, which are close to the transients of the communications traffic on the optical traffic signal (about 35 ps, as measured on a 40 GHz sampling oscilloscope). The rising time of the communications traffic on the pass-through optical traffic probe signal are related to the carrier recovery time of the SOA 44 in the first arm 26. As can be seen in Figures 4e) and f), these dynamics do not affect noticeably the eye opening of the communications traffic bits at the leading and trailing edges. It is worth noting that a good extinction ratio of the

communications traffic in the pass-through optical traffic probe signal and OOK modulated wavelength shifted output signal, as well as high suppression of the optical traffic probe signal (which was estimated to be more than 15 dB from the oscilloscope traces) when the optical gate pump signal is present can be observed.

Bit error rate (BER) measurements of the optical traffic probe signal (both as input and as pass-through signals) and the wavelength shifted OOK modulated output optical signal are shown in Figure 5, relative to a 2^U-1 long PRBS (limited by the gate duration of 500 ns that we used in this experiment)

Eye diagrams of the input optical traffic probe signal, wavelength sifted OOK modulated output signal and pass-through optical traffic probe signals are also shown in Figure 5. It can be seen that the eyes of the output optical signal are clearly open. In particular, the pass-through optical traffic probe signal looks almost immune to pattern effects, due to the self- switching mechanism, which results in a negligible power penalty of about 0.2 dB (at a BER of 10^"9). On the other hand, the eye diagram of the wavelength shifted signal shows some slower transient time in its leading edge, corresponding to the falling edges of the optical traffic pump signal, because of carrier dynamics in the partially saturated SOA 46. For this signal, a power penalty of about 1.8 dB was measured at BER=10^"9.

Faster operation could be in principle possible by using a differential push/pull configuration in which an attenuated and delayed copy of optical traffic pump signal is also injected into the SOA 44 in the first arm 26, in order to speed up phase restoration between the interferometer arms.

The optical switch 40 described above enables simultaneous wavelength conversion and traffic erasing of a burst of traffic under optical gating, without traffic bit loss. The optical switch 40 is based on low-power self- switching operation of traffic and gate signals in a single SOA-MZI. Error free operation has been demonstrated at 10 Gb/s, with a power penalty of 1.8 dB for the wavelength shifted output signal and a negligible power penalty for the pass-through optical traffic probe signal. Fast switching time of about 40 ps, makes the optical switch 40 suitable for fast operation with very low guard time for WDM systems and optical packet switched networks.

The optical switch 40 enables simultaneous performance of two different operations, namely traffic erasing and wavelength conversion, in a single device; the optical traffic erasure/conversion is selectively controlled by an optical gate signal. The device can be suitably exploited to perform wavelength routing operation in WDM systems, where an input signal is wavelength-converted in order to release the original wavelength resource in the network; the wavelength converted signal can then be routed on a different channel.

An optical switch 60 according to a third embodiment of the invention is shown in Figure 6. The optical switch 60 of this embodiment is similar to the optical switch 10 of Figure 1 and the optical switch 40 of Figure 2. The same reference numbers are retained for corresponding features.

In this embodiment the optical switch 60 further comprises a first optical splitter 62 and a second optical splitter 64. The first optical splitter 62 is arranged to receive an OOK modulated input optical signal and to split that signal to form the optical traffic pump signal and the optical traffic probe signal. The second optical splitter 64 is arranged to receive an optical gate signal and to split the optical gate signal to form the optical gate pump signal and the optical gate probe signal.

The optical switch 60 further comprises an input optical signal converter 66 and an output optical signal converter 68. The input optical signal converter 66 is arranged to receive an input optical signal having a modulation format different to OOK and to convert the input optical signal into an OOK modulated input optical signal, for delivery to the first optical splitter 62. The received input optical signal will typically comprise a multi-level signal having higher order modulation such as QPSK, DQPSK or 16QAM. The output optical signal converter 68 is arranged to receive an OOK modulated optical signal from either the first signal output, filtered by the first optical filter 52, or from the second signal output 18, filtered by the second optical filter 54. That is to say, the output optical signal converter will either receive a transmitted optical traffic probe signal (i.e. a pass-through optical traffic probe signal) or a wavelength shifted output optical signal (i.e. an OOK modulated output signal carrying an inverted copy of the communications traffic from the optical traffic probe signal and having the second wavelength). The optical signal converter is arranged to convert the received OOK modulated signal into a higher order modulation signal, such as those set out above.

A fourth embodiment of the invention provides an optical switch 70 as shown in

Figure 7. The optical switch 70 of this embodiment is similar to the optical switch 10 of Figure 1, with the following modifications. The same reference numbers are retained for corresponding features.

In this embodiment, the optical switch 70 comprises a single pump input 30 and a single optical signal input 12. The pump input 30 is arranged to receive both the traffic pump signal and the gate pump signal and to deliver the pump signals into the first interferometer arm 26. The optical signal input 12 is arranged to receive both the traffic probe signal and the gate probe signal.

The interferometer 72 comprises a nonlinear medium 74 in the first arm 26. The controller 34 is arranged to selectively control delivery of the gate pump signal into the first pump input 30. The interferometer 72 has a first state in which the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium 74, such that a first phase difference is produced between the first and second arms 26, 28. As a result the optical traffic probe signal is output at one of the first signal output and the second signal output, in this example the second output 18. The interferometer 72 has a second state in which the optical traffic probe signal, the optical traffic pump signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium 74, such that a second phase difference, different to the first phase difference is produced between the first and second arms26, 28. As a result an OOK modulated output optical signal at the second wavelength and carrying the communications traffic is output at the second signal output 18. The interferometer 72 is selectively set to the second state by the delivery of the optical gate pump signal controller by the controller 34.

The optical switch 70 further comprises an optical splitter 75 at the second signal output 18, which is arranged to split each output optical signal and to deliver the split signals to a first optical filter 76 and a second optical filter 78. The first optical filter 76 is arranged to transmit at the first wavelength and the second optical filter 78 is arranged to transmit at the second wavelength.

It will be appreciated that either the first signal output 16 or the second signal output 18 may be selected as the output for both the signals. Depending on which signal output is to be selected the interferometer 72 will be set to have an initial bias state of a phase difference of 0 or π between the first and second arms 26, 28. Starting from an initial bias of destructive interference, applying the traffic pump signal will result in the traffic probe signal to be output at the corresponding filter output, whereas applying the gate pump signal will result in selective erasing of the traffic probe signal (due to the restoration of the initial destructive interference state) and the wavelength converted OOK modulated output signal will be output from the second optical filter.

A fifth embodiment of the invention provides an optical switch 80 as shown in Figure 8. The optical switch 80 of this embodiment is similar to the optical switch 40 of Figure 2, with the following modifications. The same reference numbers are retained for corresponding features. The interferometer 82 comprises a nonlinear medium 84 in its first arm 26 and a phase shifter 86 in its second arm 28. The optical switch 80 comprises a single pump input 30 which is arranged to receive both the traffic pump signal and the gate pump signal and to deliver the pump signals into the first interferometer arm 26.

The interferometer 82 of this embodiment is initially biased using the phase shifter 86 so that, in absence of both the traffic pump signal and gate pump signal, any signal entering from the first signal input 12 experiences destructive interference at the first signal output 16 (and by symmetry, any signal entering from the second signal input 14 would experience destructive interference at the second signal output 18.

The optical switch 80 further comprises a first optical filter 88 provided after the first signal output 16 and a second optical filter 90 provided after the second signal output 18. The first optical filter 88 is arranged to transmit at the first wavelength, data. The second optical filter 90 is arranged to transmit at the second wavelength, Xgate.

In the interferometer's first state, the traffic probe signal is delivered to the first signal input 12 (with sufficiently low power not to trigger nonlinear effects in the nonlinear medium 74) and the traffic pump signal is delivered to the pump input 30, having a power level in its logical 1 s that is high enough to induce an extra phase shift of π (or in general an extra phase shift) in the nonlinear medium 74. As a result the traffic probe signal (or more precisely the logical 1 s in the traffic probe signal) is output at the first signal output 16, and is transmitted by the first optical filter 88.

It will be appreciated by the person skilled in the art that any extra phase shift (different from 2ηπ) induced by the logical 1 s in the traffic pump signal will result in optical power at the first wavelength to be delivered to the first signal output 16. However, for maximized efficiency of the optical switch 80, a π phase shift is desirable.

Under the action of the controller (not shown), the gate probe signal is delivered to the second signal input 14, with a sufficiently low power not to trigger nonlinear effects in the nonlinear medium74, and the gate pump signal is delivered to the pump input 30 with a power level high enough to induce an extra phase shift in the nonlinear medium 74 such that the traffic probe signal entering from the first signal input 12 experiences destructive interference at the first signal output 16, thereby causing selective erasure of the traffic probe signal at the first signal output.

If the phase shift induced by the logical 1 s in the traffic pump signal is π (or (2η+1)π) an extra phase shift of π (or (2η+1)π) is required to be created by the gate pump signal. In the second state, the destructive interference state at OUT 2 of the initial bias for signals input at the second signal input 14 is no longer satisfied when logical 0s are present in the traffic pump signal. This causes optical power to appear at the second signal output 18 in correspondence with the 0s of the traffic pump signal, resulting in an OOK modulated output signal with inverted logic at the second wavelength being output at the second signal output 18. The output signal is then transmitted by the second optical filter 90.

A sixth embodiment of the invention provides an optical communications network node 100, as shown in Figure 9. The node 100 comprises an optical switch 10 as shown in Figure 1. It will be appreciated that the node 100 may alternatively comprise one of the other optical switches 40, 60, 70, 80 described above.

The steps of a method 110 of switching an optical signal carrying communications traffic in a communications network according to a seventh embodiment of the invention are shown in Figure 10.

The method 110 comprises receiving an OOK modulated optical traffic probe signal having a first wavelength and carrying communications traffic 112, and receiving an optical gate probe signal having a second wavelength different to the first wavelength 114.

The method 110 further comprises causing the optical traffic probe signal and the optical gate probe signal to propagate through an optical waveguide interferometer in a first direction 116. The interferometer comprises a first arm having a first optical path and a second arm having a second optical path. Each optical path extends between an input and an output of the interferometer. The first arm further comprises a nonlinear medium arranged to apply an intensity dependent phase change to an optical signal propagating through it. The input is arranged to couple each said received probe signal into each of the first arm and the second arm. The method 110 further comprises providing an optical traffic pump signal into the first interferometer arm in a second direction of propagation, opposite to the first direction 118. The optical traffic pump signal is a copy of the optical probe signal.

The method 110 further comprises selectively causing the optical traffic probe signal and the optical traffic pump signal to be synchronously present within the nonlinear medium, such that the interferometer is caused to operate in a first state in which a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at one of a first signal output and a second signal 120.

The method 110 further comprises selectively additionally causing the optical gate probe signal and the optical gate pump signal to be synchronously present within the nonlinear medium such that the interferometer is caused to operate in a second state 122. In the second state a second phase difference, different to the first phase difference, is produced between the first and second arms. As a result, an OOK modulated output optical signal is produced at the second wavelength and carrying the communications traffic and is output at said one of the first signal output and the second signal output.

The steps of a method 130 of switching an optical signal carrying communications traffic in a communications network according to an eighth embodiment of the invention are shown in Figure 11. The method 130 of this embodiment is similar to the method 110 of Figure 10, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, each arm of the interferometer comprises a nonlinear medium

132.

The method comprises causing the optical traffic probe signal and the optical traffic pump signal to be synchronously present within the nonlinear medium in the first arm, such that the interferometer is caused to operate in a first state 134. In the first state a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at the first signal output.

The method further comprises selectively additionally delivering the optical gate probe signal and the optical gate pump signal such that the interferometer is caused to operate in a second state 136. In the second state optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are synchronously present within the nonlinear medium in the second arm and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm. As a result the output optical signal is output at the second signal output.

The steps of a method 140 of switching an optical signal carrying communications traffic in a communications network according to an eighth embodiment of the invention are shown in Figure 12. The method 140 of this embodiment is similar to the method 110 of Figure 10, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, the method comprises two initial steps. The first initial step comprises receiving an OOK modulated input optical signal and splitting the input optical signal to form the optical traffic pump signal and the optical traffic probe signal 142. The second initial step comprises receiving an optical gate signal and splitting the optical gate signal to form the optical gate pump signal and the optical gate probe signal 144.

In this example, each respective optical signal is split such that the respective pump signal has a higher optical power than the respective probe signal.

The steps of a method 150 of switching an optical signal carrying communications traffic in a communications network according to a ninth embodiment of the invention are shown in Figure 13. The method 150 of this embodiment is similar to the method 110 of Figure 10, with the following modifications. The same reference numbers are retained for corresponding steps.

In this embodiment, the method comprises a first step of receiving an input optical signal having a modulation format different to OOK and converting the input optical signal into the OOK modulated input optical signal 152. The method further comprises a final step of converting an OOK modulated optical signal received from one of the first signal output and the second signal output into a further output optical signal having a different modulation format 154.

The different modulation formats are higher order modulation formats, such as QPSK, DQPSK and 16QAM.

Claims

1. An optical switch comprising:

a first signal input arranged to receive an on-off key modulated optical traffic probe signal having a first wavelength and carrying communications traffic and further arranged to receive an optical gate probe signal having a second wavelength different to the first wavelength;

a first signal output;

a second signal output;an optical interferometer comprising an input coupled to the first signal input and an output coupled to the first and second signal outputs, the interferometer further comprising a first arm having a first optical path and a second arm having a second optical path, each optical path extending between the input and the output, the interferometer input being arranged to receive the optical traffic probe signal and the optical gate probe signal propagating in a first direction and being further arranged to couple each said received probe signal into each of the first arm and the second arm, and the first arm further comprises a nonlinear medium arranged to apply an intensity dependent phase change to an optical signal propagating through it; a first pump input coupled to the first arm and being arranged to deliver an optical gate pump signal having the second wavelength into the first arm in a second direction of propagation, opposite to the first direction, and further arranged to couple an optical traffic pump signal into the first arm in the second direction of propagation, the optical traffic pump signal being a copy of the optical traffic probe signal;

and

a controller arranged to selectively cause the optical gate pump signal to be delivered to the first pump input,

wherein the interferometer has a first state in which the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at one of the first signal output and the second signal output and wherein the interferometer has a second state in which the optical traffic probe signal, the optical traffic pump signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium, such that a second phase difference, different to the first phase difference is produced between the first and second arms whereby an on-off key modulated output optical signal at said second wavelength and carrying said communications traffic is output at said one of the first signal output and the second signal output, whereby the interferometer is selectively set to the second state by said delivery of the optical gate pump signal.

2. An optical switch as claimed in claim 1, wherein the optical switch further

comprises a second signal input arranged to receive the optical gate probe signal and the first signal input is arranged to receive the optical traffic probe signal, and wherein in the first state the interferometer is arranged to output the optical traffic probe signal at the first signal output and in the second state the interferometer is arranged to output the output optical signal at the second signal output.

3. An optical switch as claimed in claim 1 or claim 2, wherein the second

interferometer arm further comprises a nonlinear medium and wherein in the first state the optical traffic probe signal propagates through the nonlinear medium in the first arm and the optical traffic probe signal and the optical traffic pump signal are synchronously present within the nonlinear medium in the second interferometer arm, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at the first signal output and wherein in the second state the optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are synchronously present within the nonlinear medium in the second arm and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm, whereby the output optical signal is output at the second signal output, and wherein the controller is arranged to selectively cause the optical gate pump signal to be delivered to the first pump input, whereby the interferometer is selectively set to the second state.

4. An optical switch as claimed in any preceding claim, wherein the optical

interferometer comprises a Mach-Zehnder waveguide interferometer.

5. An optical switch as claimed in any preceding claim, wherein the or each nonlinear medium comprises one of a semiconductor optical amplifier, a nonlinear passive optical waveguide and a nonlinear photonic crystal optical waveguide.

6. An optical switch as claimed in any preceding claim, wherein the switch further comprises a first optical splitter and a second optical splitter, the first optical splitter being arranged to receive an on-off key modulated input optical signal and to split the input optical signal to form the optical traffic pump signal and the optical traffic probe signal and the second optical splitter being arranged to receive an optical gate signal and to split the optical gate signal to form the optical gate pump signal and the optical gate probe signal, and wherein each optical splitter is arranged to split the respective optical signal such that the respective pump signal has a higher optical power than the respective probe signal.

7. An optical switch as claimed in any preceding claim wherein the optical switch further comprises an input optical signal converter and an output optical signal converter, the input optical signal converter being arranged to receive an input optical signal having a modulation format different to on-off key and convert the input optical signal into an on-off key modulated input optical signal and the output optical signal converter being arranged to receive an on-off key modulated optical signal from one of the first signal output and the second signal output and to convert said signal into a further output optical signal having a different modulation format.

8. An optical communications network node comprising an optical switch as

claimed in any of claims 1 - 7.

9. A method of switching an optical signal carrying communications traffic in a communications network, the method comprising:

receiving an on-off key modulated optical traffic probe signal having a first wavelength and carrying communications traffic;

providing an optical traffic pump signal into the first interferometer arm in a second direction of propagation, opposite to the first direction, the optical traffic pump signal being a copy of the optical traffic probe signal;

causing the optical traffic probe signal and the optical traffic pump signal to be synchronously present within the nonlinear medium, such that the

interferometer is caused to operate in a first state in which a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at one of a first signal output and a second signal output; and

10. A method as claimed in claim 9, wherein the second interferometer arm further comprises a nonlinear medium and wherein in the first state the optical traffic probe signal and the optical traffic pump signal are caused to be synchronously present within the nonlinear medium, such that a first phase difference is produced between the first and second arms whereby the optical traffic probe signal is output at the first signal output and wherein in the second state the optical traffic probe signal, the optical traffic pump signal and the optical gate probe signal are synchronously present within the nonlinear medium in the second arm and the optical traffic probe signal, the optical gate probe signal and the optical gate pump signal are synchronously present within the nonlinear medium in the first arm, such that the output optical signal is output at the second signal output.

11. A method as claimed in claim 9 or claim 10, wherein the method further

comprises the initial steps of receiving an on-off key modulated input optical signal and splitting the input optical signal to form the optical traffic pump signal and the optical traffic probe signal and receiving an optical gate signal and splitting the optical gate signal to form the optical gate pump signal and the optical gate probe signal, and wherein each respective optical signal is split such that the respective pump signal has a higher optical power than the respective probe signal.

12. A method as claimed in any of claims 9 to 11, wherein the method comprises receiving an input optical signal having a modulation format different to on-off key and converting the input optical signal into the on-off key modulated input optical signal and further comprises converting an on-off key modulated optical signal received from one of the first signal output and the second signal output into a further output optical signal having a different modulation format.