WO2024099550A1

WO2024099550A1 - Optical switching apparatus and communications network node

Info

Publication number: WO2024099550A1
Application number: PCT/EP2022/081268
Authority: WO
Inventors: Francesco Testa; Vito SORIANELLO
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2022-11-09
Filing date: 2022-11-09
Publication date: 2024-05-16

Abstract

Optical switching apparatus (100) for adding optical channel signals comprising: add ports (102); an optical bus (104); wavelength selective optical switching elements, WSS, (106), coupled to the optical bus, operable to add optical channel signals from respective add ports to the optical bus; optical coupling apparatus (108); polarisation splitter converters (110, 210) to split optical channels into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components, for coupling into the optical bus; and a polarization combiner converter (116) configured to receive first TE polarization components from one end of the optical bus and second TE polarization components from the other end of the optical bus, and to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for output. Corresponding apparatus for dropping channels and a communications network node are also provided.

Description

OPTICAL SWITCHING APPARATUS AND COMMUNICATIONS NETWORK NODE

Technical Field

The invention relates to optical switching apparatus for adding optical channels. The invention further relates to optical switching apparatus for dropping optical channels. The invention further relates to a communications network node.

Background

A wavelength selective optical switch, also known as a reconfigurable optical add-drop multiplexer, ROADM, is as a key device for the implementation of optical transport functions in Radio front-haul applications of 5G networks. US9806841 B2 describes a silicon photonics wavelength selective optical switch based on micro-ring resonators, MRR, as wavelength selective switching elements. The device architecture comprises two different network elements, one for adding wavelength channels to a ring optical network and one for dropping wavelength channels from the network. Auxiliary circuits are also integrated in the device including optical attenuators to keep crosstalk low, and a 1x2 optical switch to change the path direction in case of a fibre break.

A polarization insensitive ROADM is reported in P. lovanna et al., “Optical Components for Transport Network Enabling The Path to 6G”, Journal of Lightwave Technology, Vol. 40, No. 2, Jan. 152022. To achieve polarization independence the ROADM has a polarization diversity structure consisting of two equal buses each with a sequence of MRR switching elements. At the input port, a double polarization grating coupler, DPGC, or, alternatively, a edge coupler, EC, followed by a polarization splitter and rotator, PSR, separates received input light into its two orthogonal signal polarization components and couples the two polarization components into the two buses, each handling a different polarization component. The two polarization components can be dropped from/added to the respective bus or can pass through to the line output port. In both cases the two polarization components are recombined at the output by a DPGC/PSR before output fibre coupling. An array of 1x2 switch elements is used to implement direction switching in each of the two buses to protect the network connection in case of a fibre break.

P. lovanna et al achieves polarization insensitivity by duplicating all the photonic circuits, including optical buses, MRRs, 1x2 switches and attenuators. The consequence is increased chip complexity, resulting lower yield, and increased chip area, with a related increased cost and higher power consumption due to the need of controlling (by micro-heater) twice the number of MRRs and all the duplicated circuits.

Summary

It is an object to provide an improved optical switching apparatus for adding optical channel signals. It is a further object to provide an improved optical switching apparatus for dropping optical channel signals. It is a further object to provide an improved communications network node.

An aspect provides optical switching apparatus for adding optical channel signals. The apparatus comprises a plurality of optical add ports, a bus optical waveguide having a first end and a second end, a plurality of wavelength selective optical switching elements, optical coupling apparatus, a plurality of polarisation splitter converters, and a polarization combiner converter. The wavelength selective optical switching elements are coupled to the bus optical waveguide. The wavelength selective optical switching elements are operable to add optical channel signals at different preselected channel wavelengths received from respective optical add ports to the bus optical waveguide. The optical coupling apparatus has an input port, an input/output port and an output port. The optical coupling apparatus is configured to route optical channel signals input at the input port to the input/output port, and is configured to route optical channel signals input at the input/output port to the output port. The polarisation splitter converters are configured to split optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components, for coupling into the bus optical waveguide. The polarization combiner converter is provided between the optical coupling apparatus input/output port and the first end and second end of the bus optical waveguide. The polarization combiner converter is configured to receive first TE polarization components from one end of the bus optical waveguide and second TE polarization components from the other end of the bus optical waveguide. The polarization combiner converter is configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the input/output port of the optical coupling apparatus.

Converting optical channel signals to be added into two separate polarization components both having TE polarization advantageously overcomes polarization dependency of wavelength selective optical switching elements, enabling the wavelength selective switching elements to operate correctly, and in an effectively polarization agnostic manner. Recombining the two TE polarization components in the polarization combiner converter enables substantially the entire optical channel signal (bar any component losses) to be added to an optical network. The optical switching apparatus may thereby enable optical channels adding to an optical network with low polarization sensitivity, avoiding a duplication of optical components with benefit in terms of cost, size and power consumption, that are crucial for application in centralized radio access network, C-RAN, wireless networks.

In an embodiment, the polarization splitter converters comprise dual-polarization grating couplers or polarization splitter rotators.

In an embodiment, the polarization combiner converter comprises a dual-polarization grating coupler or a polarization splitter rotator. In an embodiment, the bus optical waveguide is a folded optical waveguide. This may minimize the length of the bus optical waveguide and enable a compact apparatus.

In an embodiment, the polarization splitter converters are provided between optical add ports and respective first and second optical add paths coupled to respective wavelength selective optical switching elements. The polarization splitter converters are configured to couple first TE polarization components into the respective first optical add path and to couple second TE polarization components into the respective second optical add path. The wavelength selective optical switching elements are operable to add first TE polarization components into the bus optical waveguide to travel in one direction and to add second TE polarization components into the bus optical waveguide to travel in an opposite direction.

This may minimize interference between the first and second TE polarization components of an optical channel signal within the bus optical waveguide.

In an embodiment, the optical channel signals carry information bits having a bit time. The respective optical path difference of the bus optical waveguide from each wavelength selective optical switching element to the first end of the bus optical waveguide and to the second end of the bus optical waveguide results in a delay between the respective first TE polarization component and second TE polarization component of a fraction of the bit time.

This advantageously means that the effect of the optical path difference experienced by the first and second TE polarization components on the eye diagram and bit error rate, BER, of the recombined optical channel signals output from the polarization combiner converter is negligible.

In an embodiment, the delay is up to 10% of the bit time. This advantageously means that the effect of the optical path difference experienced by the first and second TE polarization components on the eye diagram and bit error rate, BER, of the recombined optical channel signals output from the polarization combiner converter is negligible.

In an embodiment, apparatus for adding optical channel signals further comprises delay elements in optical add paths of optical add ports. The delay elements are configured to add different compensating delays to one of the first TE polarization component and second TE polarization component of channels being added. This may enable an increase in the bit rate that the channels can carry and/or an increase in the length of the optical bus waveguide and the number of wavelength selective switches that may be incorporated, and thus the number of different channels that can be handled.

In an embodiment, the wavelength selective optical switching elements are micro-ring resonators, MRRs. Converting optical channel signals to be added into two separate components having TE polarization advantageously overcomes the strong polarization dependency of MRRs, enabling the MRRs to operate correctly, and in an effectively polarization agnostic manner.

In an embodiment, the apparatus is fabricated as a silicon photonic integrated circuit. The optical switching apparatus advantageously enables optical channels adding to an optical network with low polarization sensitivity, avoiding a duplication of the silicon photonics processing circuits with benefit in terms of cost, chip real estate and power consumption, that are crucial in case of circuits with high scale of integration and for application in C-RAN wireless networks.

Corresponding embodiments and advantages also apply to the communications network node described below.

An aspect provides optical switching apparatus for dropping optical channel signals. The apparatus comprises a bus optical waveguide having a first end and a second end, a plurality of wavelength selective optical switching elements, a plurality of optical drop ports, optical coupling apparatus, a polarization splitter converter and a plurality of polarization combiner converters. The wavelength selective optical switching elements are coupled to the bus optical waveguide. The wavelength selective optical switching elements are operable to drop optical channel signals at different preselected channel wavelengths from the bus optical waveguide to respective first and second optical drop paths. The optical coupling apparatus has an input port, an input/output port and an output port. The optical coupling apparatus is configured to route optical channel signals input at the input port to the input/output port and configured to route optical channel signals input at the input/output port to the output port. The polarization splitter converter is configured to split optical channel signals into first TE polarization components and TM polarization components. The polarization splitter converter is configured to convert the TM polarization components into second TE polarization components. The polarization splitter converter is configured to couple first TE polarization components into the bus optical waveguide to travel in one direction and to couple second TE polarization components into the bus optical waveguide to travel in an opposite direction. The polarization combiner converters are provided between respective optical drop ports and first and second optical drop paths coupled to respective wavelength selective optical switching elements. The polarization combiner converters are configured to receive first TE polarization components and second TE polarization components from respective first and second optical drop paths. The polarization combiner converters are configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the respective optical drop port.

Converting optical channel signals to be dropped into two separate polarization components both having TE polarization advantageously overcomes polarization dependency of wavelength selective optical switching elements, enabling the wavelength selective switching elements to operate correctly, and in an effectively polarization agnostic manner. Recombining the two TE polarization components in the polarization combiner converters enables substantially the entire optical channel signal (bar any component losses) to be dropped from an optical network. The optical switching apparatus may thereby enable optical channels dropping from an optical network with low polarization sensitivity, avoiding a duplication of optical components with benefit in terms of cost, size and power consumption, that are crucial for application in centralized radio access network, C-RAN, wireless networks.

In an embodiment, the polarization splitter converter is provided between the optical coupling apparatus input/output port and the first end and second end of the bus optical waveguide to couple first TE polarization components into one end of the bus optical waveguide and to couple second TE polarization components into the other end of the bus optical waveguide. This may minimize interference between the first and second TE polarization components of an optical channel signal within the bus optical waveguide.

In an embodiment, the polarization splitter converter comprises a dual-polarization grating coupler or a polarization splitter rotator.

In an embodiment, the polarization combiner converters comprise dual-polarization grating couplers or polarization splitter rotators.

In an embodiment, the bus optical waveguide is a folded optical waveguide. This may minimize the length of the bus optical waveguide and enable a compact apparatus.

In an embodiment, the optical channel signals carry information bits having a bit time. The respective optical path difference of the bus optical waveguide to each wavelength selective optical switching element from the first end of the bus optical waveguide and from the second end of the bus optical waveguide results in a delay between the respective first TE polarization component and second TE polarization component of a fraction of the bit time.

This advantageously means that the effect of the optical path difference experienced by the first and second TE polarization components on the eye diagram and bit error rate, BER, of the recombined optical channel signals output from the polarization combiner converters is negligible.

In an embodiment, the delay is up to 10% of the bit time. This advantageously means that the effect of the optical path difference experienced by the first and second TE polarization components on the eye diagram and bit error rate, BER, of the recombined optical channel signals output from the polarization combiner converters is negligible.

In an embodiment, apparatus for dropping optical channel signals further comprises delay elements in optical drop paths of optical drop ports. The delay elements are configured to add different compensating delays to one of the first TE polarization component and second TE polarization component of channels being dropped. This may enable an increase in the bit rate that the channels can carry and/or an increase in the length of the optical bus waveguide and the number of wavelength selective switches that may be incorporated, and thus the number of different channels that can be handled.

In an embodiment, the wavelength selective optical switching elements are micro-ring resonators. Converting optical channel signals to be dropped into two separate components having TE polarization advantageously overcomes the strong polarization dependency of MRRs, enabling the MRRs to operate correctly, and in an effectively polarization agnostic manner. Corresponding embodiments and advantages also apply to the communications network node described below.

A communications network node comprising optical switching apparatus for adding optical channels and optical switching apparatus for dropping optical channels. The optical switching apparatus for adding optical channel signals comprises a plurality of optical add ports, a first bus optical waveguide having a first end and a second end, a plurality of first wavelength selective optical switching elements, first optical coupling apparatus, a plurality of first polarisation splitter converters, and a first polarization combiner converter. The first wavelength selective optical switching elements are coupled to the first bus optical waveguide. The first wavelength selective optical switching elements are operable to add optical channel signals at different preselected channel wavelengths received from respective optical add ports to the first bus optical waveguide. The first optical coupling apparatus has a first input port, a first input/output port and a first output port. The first optical coupling apparatus is configured to route optical channel signals input at the first input port to the first input/output port, and is configured to route optical channel signals input at the first input/output port to the first output port. The first polarisation splitter converters are configured to split optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components, for coupling into the bus optical waveguide. The first polarization combiner converter is provided between the first optical coupling apparatus first input/output port and the first end and second end of the first bus optical waveguide. The first polarization combiner converter is configured to receive first TE polarization components from one end of the first bus optical waveguide and second TE polarization components from the other end of the first bus optical waveguide. The first polarization combiner converter is configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the first input/output port of the first optical coupling apparatus. The optical switching apparatus for dropping optical channel signals comprises a second bus optical waveguide having a third end and a fourth end, a plurality of second wavelength selective optical switching elements, a plurality of optical drop ports, second optical coupling apparatus, a second polarization splitter converter and a plurality of second polarization combiner converters. The second wavelength selective optical switching elements are coupled to the second bus optical waveguide. The second wavelength selective optical switching elements are operable to drop optical channel signals at different preselected channel wavelengths from the second bus optical waveguide to respective first and second optical drop paths. The second optical coupling apparatus has a second input port, a second input/output port and a second output port. The second optical coupling apparatus is configured to route optical channel signals input at the second input port to the second input/output port, and configured to route optical channel signals input at the second input/output port to the second output port. The second polarization splitter converter is configured to split optical channel signals into first TE polarization components and TM polarization components. The second polarization splitter converter is configured to convert the TM polarization components into second TE polarization components. The second polarization splitter converter is configured to couple first TE polarization components into the second bus optical waveguide to travel in one direction and to couple second TE polarization components into the second bus optical waveguide to travel in an opposite direction. The second polarization combiner converters are provided between respective optical drop ports and first and second optical drop paths coupled to respective second wavelength selective optical switching elements. The second polarization combiner converters are configured to receive first TE polarization components and second TE polarization components from respective first and second optical drop paths. The second polarization combiner converters are configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the respective optical drop port.

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

Brief Description of the drawings

Figures 1 to 4 are block diagrams illustrating embodiments of optical switching apparatus for adding optical channels;

Figures 5 to 8 are block diagrams illustrating embodiments of optical switching apparatus for dropping optical channels; and

Figure 9 is a block diagram illustrating an embodiment of a communications network node.

Detailed description

The same reference numbers are used for corresponding features in different embodiments.

Referring to Figure 1 , an embodiment provides optical switching apparatus 100 for adding optical channel signals. The apparatus 100 comprises a plurality, N, of optical add ports 102, a bus optical waveguide 104 having a first end and a second end, a plurality, N, of wavelength selective optical switching elements 106, optical coupling apparatus 108, a plurality, N, of polarisation splitter converters 110 and a polarization combiner converter 116.

The wavelength selective optical switching elements 106 are coupled to the bus optical waveguide 104. The wavelength selective optical switching elements 106 are operable to add optical channel signals at different preselected channel wavelengths, i to N, received from respective optical add ports, to the bus optical waveguide.

The optical coupling apparatus 108 has an input port 1 , an input/output port 2 and an output port 3. The optical coupling apparatus is configured to route optical channel signals input at the input port to the input/output port and is configured to route optical channel signals input at the input/output port to the output port.

The polarisation splitter converters 110 are configured to split optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components, for coupling into the bus optical waveguide 104.

The polarization combiner converter 116 is provided between the input/output port 2 of the optical coupling apparatus 108 and the first end and second end of the bus optical waveguide 104. The polarization combiner converter is configured to receive first TE polarization components from one end of the bus optical waveguide and second TE polarization components from the other end of the bus optical waveguide. The polarization combiner converter is configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the input/output port of the optical coupling apparatus.

An embodiment provides optical switching apparatus 150 for adding optical channel signals, as illustrated in Figure 2. In this embodiment, the apparatus 150 additionally comprises respective first optical add paths 112 and second 114 optical add paths coupled to the wavelength selective optical switching elements 106. The bus optical waveguide 104 is a folded optical waveguide.

The polarization splitter converters 110 are provided between optical add ports 102 and respective first 112 and second 114 optical add paths. The polarization splitter converters are configured to couple first TE polarization components into the respective first optical add path and to couple second TE polarization components into the respective second optical add path, as illustrated with the different dashed arrows in Figure 2. The wavelength selective optical switching elements are operable to add first TE polarization components into the bus optical waveguide 104 to travel in one direction and to add second TE polarization components into the bus optical waveguide to travel in the opposite direction.

The coupling apparatus 108 is configured to receive transit optical channel signals (also known as pass-through optical channel signals) from an optical network. Transit optical channel signals are received at the input 1 and routed to the input/output port 2.

The polarization combiner converter 156 is configured to receive the transit optical channel signals from the optical coupling apparatus input/output port 2 and to split the transit optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components. The polarization combiner converter 156 couples first TE polarization components into the bus optical waveguide 104 to travel in one direction and to couple second TE polarization components into the bus optical waveguide to travel in an opposite direction, as indicated by the different dashed arrows. The polarization combiner converter 156 is additionally configured to receive first TE polarization components from one end of the bus optical waveguide and second TE polarization components from the other end of the bus optical waveguide. The polarization combiner converter is additionally configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the input/output port of the optical coupling apparatus. The polarization combiner converter thus also recombines transit optical channel signals following transmission along the bus optical waveguide and recombines optical channel signals being added, the transit channels and the added channels then being delivered to the input/output port of the optical coupling apparatus, where they are routed to the output port 3, for coupling into the optical network.

The optical coupling apparatus may, for example, be an optical circulator or a 3dB optical coupler with an optical isolator provided between the optical network and the input port 1.

The polarization combiner converter 156 may be a dual polarization grating coupler, DPGC, a polarisation splitter rotator, PSR, or a PSR preceded by an edge coupler, EC-PSR. Use of a DPGC or EC-PSR enables the polarization combiner converter 156 to accomplish coupling to an optical fibre, as well as the polarization splitting/combining and rotation.

The polarization splitter converters 110 may be DPGCs, PSRs or EC-PSRs.

In an embodiment, the optical channel signals carry information bits having a bit time. As can be seen in Figures 1 and 2, for each wavelength selective optical switching element 106 (1-N) there is a respective first optical path length from the wavelength selective optical switching element to the first end of the bus optical waveguide and a respective second optical path length from the wavelength selective optical switching element to the second end of the bus optical waveguide. The respective optical path difference between the first optical path length and the second optical path length for each wavelength selective optical switching element results in a delay between the respective first TE polarization component and second TE polarization component of channels being added of a fraction of the bit time.

In an embodiment, the delay is up to 10% of the bit time.

An embodiment provides optical switching apparatus 200 for adding optical channel signals, as illustrated in Figure 3. In this embodiment, the apparatus 200 is fabricated as a silicon photonic integrated circuit.

The wavelength selective optical switching elements are micro-ring resonators, MRR, 206. The coupling apparatus is an optical circulator 208; it may alternatively be a 3dB optical coupler with an optical isolator provided between the optical network and the input port 1 . The polarization combiner converter 216 is a dual polarization grating coupler, DPGC; it may alternatively be an edge-coupler, EC, followed by a polarisation splitter rotator, PSR. The polarization splitter converters 210 are similarly DPGCs or EC-PSRs. Use of a DPGC or EC- PSR enables the polarization combiner converters 216 and polarization splitter converters 210 to accomplish coupling to an optical fibre, as well as the polarization combining/splitting and rotation.

In operation, the apparatus 200 receives, at the input port 1 , optical channel signals (transit channels) at respective channel wavelengths of a wavelengths comb, to which local channel signals are to be added for transmission to the network. The transit channels are separated in polarization by the DPGC 216, as described above, and the first and second TE polarization components travel undisturbed through the folded optical bus waveguide 104 toward the line output port 3, as described above. Local optical channel signals are added from the add ports 102. The local channel signals are separated in polarization by the respective DPGCs 210, as described above, and the first and second TE polarization components are coupled to the respective 206 MRR one clockwise and the other counter-clockwise, as indicated by the differently dashed arrows in Figure 3. If the MRR corresponding to the wavelength to be added is set in resonance mode, the first and second TE polarization components of that wavelength are added to the folded optical bus waveguide and transmit along it to the DPGC 216 where they are recombined into optical channel signals (both transit and added channels) and output to the circulator, routes them to the line output port 3.

In an embodiment, the distance between two adjacent MRRs 206 is 10 p.m, to keep thermal crosstalk between MRRs low, and there are 12 MRR, therefore the length of the optical bus waveguide is about 100 p.m. The speed of the light in silicon is 85000 Km/s, therefore the maximum delay between the first TE polarization component and second TE polarization component of the channels (the worst case delays, i.e. for the channels corresponding to the first MRR and the last MRR) is about 1 ps. For a bit rate of up to 100Gbps, i.e. a bit time of 10 ps, the maximum/worst case delay of 1 ps is well inside the above stated requirement of up to 10% of the bit time.

An embodiment provides optical switching apparatus 250 for adding optical channel signals, as illustrated in Figure 4. In this embodiment, the apparatus 250 additionally comprises a delay element 252 in one of the optical add paths 114 of each optical add port 102. Each respective delay element, 252(1) to 252(N), is configured to add a different compensating delay to one of the first TE polarization component and second TE polarization component of the respective channel being added. Each compensating delay is a well-defined and fixed value time delay to compensate for the respective delay between the first TE polarization component and second TE polarization component caused by the respective optical path difference between the first optical path length and the second optical path length for each wavelength selective optical switching element.

In an embodiment, the delay elements 252 comprise phase shifters or optical waveguide delay lines. Since the delay between the first TE polarization component and the second TE polarization component is fixed and different for each channel, each phase shifter or optical waveguide delay line is configured to apply a different delay. Referring to Figure 5, an embodiment provides optical switching apparatus 300 for dropping optical channel signals. The apparatus 300 comprises a bus optical waveguide 304 having a first end and a second end, a plurality, N, of wavelength selective optical switching elements 306, a plurality of optical drop ports 302, optical coupling apparatus 308, a polarization splitter converter 310 and a plurality of polarization combiner converters 316.

The wavelength selective optical switching elements 306 are coupled to the bus optical waveguide. The wavelength selective optical switching elements 306 are operable to drop optical channel signals at different preselected channel wavelengths, i to N, received from the bus optical waveguide to respective first optical drop paths 312 and second 314 optical drop paths.

The optical coupling apparatus 308 has an input port 1 , an input/output port 2 and an output port 3. The optical coupling apparatus 308 is configured to route optical channel signals input at the input port to the input/output port, and is configured to route optical channel signals input at the input/output port to the output port.

The polarization splitter converter 310 is configured to split optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components. The polarization splitter converter 310 is configured to couple first TE polarization components into the bus optical waveguide 304 to travel in one direction and to couple second TE polarization components into the bus optical waveguide to travel in an opposite direction.

The first optical drop paths 312 and second 314 optical drop paths are coupled to respective wavelength selective optical switching elements 306. The polarization combiner converters 316 are provided between respective optical drop ports 302 and first 312 and second 314 optical drop paths. The polarization combiner converters 316 are configured to receive first TE polarization components and second TE polarization components from respective first and second optical drop paths. The polarization combiner converters 316 are configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the respective optical drop port 302.

An embodiment provides optical switching apparatus 350 for adding optical channel signals, as illustrated in Figure 6. In this embodiment, the polarization splitter converter 360 is provided between the optical coupling apparatus input/output port 2 and the first end of the bus optical waveguide 354 and second end of the bus optical waveguide. The bus optical waveguide 304 is a folded optical waveguide.

The polarization splitter converter 360 is configured to receive the transit optical channel signals from the optical coupling apparatus input/output port 2 and to split the transit optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components. The polarization splitter converter 360 couples first TE polarization components into the bus optical waveguide 304 to travel in one direction and couples second TE polarization components into the bus optical waveguide to travel in an opposite direction, as indicated by the different dashed arrows.

The polarization splitter converter 360 is additionally configured to receive first TE polarization components from one end of the bus optical waveguide and second TE polarization components from the other end of the bus optical waveguide. The polarization splitter converter is configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the input/output port of the optical coupling apparatus. The polarization splitter converter thus recombines transit optical channel signals following transmission along the bus optical waveguide, the transit channels then being delivered to the input/output port of the optical coupling apparatus, where they are routed to the output port 3, for coupling back into the optical network.

The polarization splitter converter may be a dual polarization grating coupler, DPGC, a polarisation splitter rotator, PSR, or a PSR preceded by an edge coupler, EC-PSR. Use of a DPGC or EC-PSR enables the polarization splitter converter 156 to accomplish coupling to an optical fibre, as well as the polarization splitting/combining and rotation.

The polarization combiners converters 316 may be DPGCs, PSRs or EC-PSRs. Use of a DPGC or EC-PSR enables the polarization splitter/combiner converters to accomplish coupling to an optical fibre, as well as the polarization splitting/combining and rotation.

In an embodiment, the optical channel signals carry information bits having a bit time. As can be seen in Figures 5 and 6, for each wavelength selective optical switching element 306 (1-N) there is a respective first optical path length from the first end of the bus optical to the waveguide to the wavelength selective optical switching element and a respective second optical path length from the second end of the bus optical waveguide to the wavelength selective optical switching element. The respective optical path difference between the first optical path length and the second optical path length for each wavelength selective optical switching element results in a delay between the respective first TE polarization component and second TE polarization component of channels being dropped of a fraction of the bit time.

In an embodiment, the delay is up to 10% of the bit time

An embodiment provides optical switching apparatus 400 for dropping optical channel signals, as illustrated in Figure 7. In this embodiment, the apparatus 400 is fabricated as a silicon photonic integrated circuit. The wavelength selective optical switching elements are micro-ring resonators, MRR, 406. The coupling apparatus is an optical circulator 408; it may alternatively be a 3dB optical coupler with an optical isolator provided between the optical network and the input port 1 . The polarization combiner converters 416 are dual polarization grating couplers, DPGC; they may alternatively be an edge-coupler, EC, followed by a polarisation splitter rotator, PSR. The polarization splitter converter 410 is similarly a DPGC or an EC-PSR. Use of a DPGC or EC- PSR enables the polarization combiner converters 416 and polarization splitter converters 410 to accomplish coupling to an optical fibre, as well as the polarization combining/splitting and rotation.

In operation, the apparatus 400 receives, at the input port 1 , optical channel signals at respective channel wavelengths of a wavelengths comb. There are two possible paths for the input optical channel signals: either transiting from the optical circulator input port 1 to the output port 3, via the bus optical waveguide 354, with low perturbations or being dropped at respective drop ports 302, for optical to electrical conversion at local transceivers.

Optical channel signals are dropped when the resonance of an MRR corresponds to the wavelength of the channel. The optical channels signals are split by the DPGC 410 into first and second TE polarization components, as described above, and transit the bus optical waveguide in two directions simultaneously. The first TE polarization components travel in one direction, clockwise in Figure 7, and the second TE polarization components travel in the other direction, counter-clockwise in Figure 7.

When an optical channel signal is to be dropped, the MRR corresponding to its wavelength is activated: the MRR is put in resonance mode and both TE polarization components, the one traveling clockwise and the othertraveling counter-clockwise, are coupled by the MRR, propagating in opposite directions around the MRR, and transferred into the first and second drop paths. The first and second TE polarization components are then sent to the respective DPGC 416 for where they are recombined into the optical channel and coupled to an output drop fibre.

The rest of the received wavelengths that need to transit through the apparatus 400 without being dropped (the transit optical channel signals) are also decomposed by the DPGC 410 into first and second TE polarization components. The two TE polarization components travel, as for the channels to be dropped, along the optical bus waveguide in the two opposite directions. The transit optical channel signals traverse the MRRs 406 without disturbance and without being dropped, since these MRRs are not at resonance. Therefore, the transit optical channel signals propagate through the optical bus waveguide until they arrive at the opposite port of the DPCG 410, where the first and second TE polarization components are recombined into the respective optical channel signals and output to the input/output 3 of the circulator 408 for coupling back into the line fibre. The optical circulator is used to separate the incoming optical channels signals and the outgoing optical channel signals, for use of the apparatus 400 with a unidirectional ring network. In an embodiment, the distance between two adjacent MRRs 406 is 10 p.m, to keep thermal crosstalk between MRRs low, and there are 12 MRRs, therefore the length of the optical bus waveguide is about 100 p.m. The speed of the light in silicon is 85000 Km/s, therefore the maximum delay between the first TE polarization component and second TE polarization component of the channels (the worst case delays, i.e. for the channels corresponding to the first MRR and the last MRR) is about 1 ps. For a bit rate of up to 100Gbps, i.e. a bit time of 10 ps, the maximum/worst case delay of 1 ps is well inside the above stated requirement of up to 10% of the bit time.

An embodiment provides optical switching apparatus 450 for dropping optical channel signals, as illustrated in Figure 8. In this embodiment, the apparatus 450 additionally comprises a delay element 452 in one of the optical drop paths 314 of each optical drop port 302. Each respective delay element, 452(1) to 452(N), is configured to add a different compensating delay to one of the first TE polarization component and second TE polarization component of the respective channel being added. Each compensating delay is a well-defined and fixed value time delay to compensate for the respective delay between the first TE polarization component and second TE polarization component caused by the respective optical path difference between the first optical path length and the second optical path length for each wavelength selective optical switching element.

In an embodiment, the delay elements 452 comprise phase shifters or optical waveguide delay lines. Since the delay between the first TE polarization component and the second TE polarization component is fixed and different for each channel, each phase shifter or optical waveguide delay line is configured to apply a different delay. Referring to Figure 9, an embodiment provides a communications network node 500 comprising optical switching apparatus 100 for adding optical channels and optical switching apparatus 300 for dropping optical channels.

The input 1 and the output 3 of the coupling apparatus 108 of the apparatus 100 for adding optical channels are coupled to a first ring optical network 502.

The input 1 and the output 3 of the coupling apparatus 308 of the apparatus 300 for dropping optical channels are coupled to a second ring optical network 504.

In an embodiment, the communications network node 500 comprises optical switching apparatus 200 for adding optical channels and optical switching apparatus 400 for dropping optical channels.

The apparatus 200 is fabricated as a first silicon photonic integrated circuit and the apparatus 400 is fabricated as a second silicon photonic integrated circuit. The first and second silicon photonic integrated circuits may be implemented in separate chips or may be implemented in a single chip.

Claims

1. Optical switching apparatus for adding optical channel signals, the apparatus comprising: a plurality of optical add ports; a bus optical waveguide having a first end and a second end; a plurality of wavelength selective optical switching elements, coupled to the bus optical waveguide, operable to add optical channel signals at different preselected channel wavelengths received from respective optical add ports to the bus optical waveguide; optical coupling apparatus having an input port, an input/output port and an output port, and configured to route optical channel signals input at the input port to the input/output port, and configured to route optical channel signals input at the input/output port to the output port; a plurality of polarisation splitter converters configured to split optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components, for coupling into the bus optical waveguide; and a polarization combiner converter provided between the optical coupling apparatus input/output port and the first end and second end of the bus optical waveguide, and configured to receive first TE polarization components from one end of the bus optical waveguide and second TE polarization components from the other end of the bus optical waveguide, and to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the input/output port of the optical coupling apparatus.

2. The optical switching apparatus of claim 1 , wherein the polarization splitter converters comprise dual-polarization grating couplers or polarization splitter rotators.

3. The optical switching apparatus of claim 1 or claim 2, wherein the polarization combiner converter comprises a dual-polarization grating coupler or a polarization splitter rotator.

4. The optical switching apparatus of any one of claims 1 to 3, wherein the bus optical waveguide is a folded optical waveguide.

5. The optical switching apparatus of any one of claims 1 to 4, wherein the polarization splitter converters are provided between optical add ports and respective first and second optical add paths coupled to respective wavelength selective optical switching elements, wherein the polarization splitter converters are configured to couple first TE polarization components into the respective first optical add path and to couple second TE polarization components into the respective second optical add path, and wherein the wavelength selective optical switching elements are operable to add first TE polarization components into the bus optical waveguide to travel in one direction and to add second TE polarization components into the bus optical waveguide to travel in an opposite direction. The optical switching apparatus of claim 5, wherein the optical channel signals carry information bits having a bit time and wherein the respective optical path difference of the bus optical waveguide from each wavelength selective optical switching element to the first end of the bus optical waveguide and to the second end of the bus optical waveguide results in a delay between the respective first TE polarization component and second TE polarization component of a fraction of the bit time. The optical switching apparatus of claim 6, wherein the delay is up to 10% of the bit time. The optical switching apparatus of claim 5, further comprising delay elements in optical add paths of optical add ports, the delay elements configured to add different compensating delays to one of the first TE polarization component and second TE polarization component of channels being added. The optical switching apparatus of any one of the preceding claims, wherein the wavelength selective optical switching elements are micro-ring resonators. Optical switching apparatus for dropping optical channel signals, the apparatus comprising: a bus optical waveguide having a first end and a second end; a plurality of wavelength selective optical switching elements, coupled to the bus optical waveguide, operable to drop optical channel signals at different preselected channel wavelengths from the bus optical waveguide to respective first and second optical drop paths; a plurality of optical drop ports; optical coupling apparatus having an input port, an input/output port and an output port, and configured to route optical channel signals input at the input port to the input/output port, and configured to route optical channel signals input at the input/output port to the output port; a polarization splitter converter configured to split optical channel signals into first TE polarization components and TM polarization components, and to convert the TM polarization components into second TE polarization components, and to couple first TE polarization components into the bus optical waveguide to travel in one direction and to couple second TE polarization components into the bus optical waveguide to travel in an opposite direction; and a plurality of polarization combiner converters provided between respective optical drop ports and first and second optical drop paths coupled to respective wavelength selective optical switching elements, the polarization combiner converters configured to receive first TE polarization components and second TE polarization components from respective first and second optical drop paths, and configured to combine first TE polarization components and second TE polarization components of respective optical channel wavelengths to form optical channel signals for delivery to the respective optical drop port. The optical switching apparatus of claim 10, wherein the polarization splitter converter is provided between the optical coupling apparatus input/output port and the first end and second end of the bus optical waveguide to couple first TE polarization components into one end of the bus optical waveguide and to couple second TE polarization components into the other end of the bus optical waveguide. The optical switching apparatus of claim 11 , wherein the polarization splitter converter comprises a dual-polarization grating coupler or a polarization splitter rotator. The optical switching apparatus of any one of claims 10 to 12, wherein the polarization combiner converters comprise dual-polarization grating couplers or polarization splitter rotators. The optical switching apparatus of any one of claims 10 to 13, wherein the bus optical waveguide is a folded optical waveguide. The optical switching apparatus of any one of claims 11 to 14, wherein the optical channel signals carry information bits having a bit time and wherein the respective optical path difference of the bus optical waveguide to each wavelength selective optical switching element from the first end of the bus optical waveguide and from the second end of the bus optical waveguide results in a delay between the respective first TE polarization component and second TE polarization component of a fraction of the bit time. The optical switching apparatus of claim 15, wherein the delay is up to 10% of the bit time. The optical switching apparatus of any one of claims 11 to 14, further comprising delay elements in optical drop paths of optical drop ports, the delay elements configured to add different compensating delays to one of the first TE polarization component and second TE polarization component of channels being dropped. The optical switching apparatus of any one of claims 10 to 17, wherein the wavelength selective optical switching elements are micro-ring resonators. A communications network node comprising: optical switching apparatus for adding optical channels, as claimed in any one of claims 1 to 9; and optical switching apparatus for dropping optical channels, as claimed in any one of claims 10 to 18.