GB2043240A - Improvements in or relating to the switching of signals - Google Patents

Abstract

An optical switch comprises a plurality of input ports 10 which are coupled via waveguide means 16 to a plurality of output ports 11. Each output port 11 has an associated wavelength selector 22 which can be a tunable filter. Each input port 10 is arranged to receive optical signals of a wavelength which is different from the wavelengths of signals received at the other inputs; and the particular wavelength required at a particular output is obtained by tuning the filter 22 associated with that output. Alternatively each wavelength selector 22 is a filter for passing a particular wavelength; and the wavelength required at a particular output 11 is provided by tuning a selected one or combination of the input wavelengths. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the switching of signals This invention relates generally to the switching of signals and in particular relates to a switch which is designed to switch signals having wavelengths in the optical range.

In this specification the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.

In recent years it has become common for switch designs, for use in telephone exchanges for example, to use digital switching techniques which are based upon time division multiplexing (TDM). Such systems have found favour since they result in the elimination of large amounts of equipment particularly space switches. However, a disadvantage of using time division multiplex switching is that the bit-rate per channel becomes severely limited. For example, a TDM switch capable of operating at 2 MBit/sec is quite capable of handling all the information required for thirty speech channels but it could not possibly handle the information for say thirty digitisedviewphone channels. In fact it could handle only one such channel. The fundamental limitation is the use of time-shared switching.

An object of the present invention is to provide a switch which is designed to operate with optical signals and which is capable of switching wide bandwidth signals.

According to the present invention there is provided an optical switch comprising a plurality of optical input ports, each input port being arranged to receive optical signals of a wavelength which is either fixed and different from the wavelengths of signals received at the other inputs, or has a value selectable independently from within a range of wavelengths, waveguide means coupling the input ports to a plurality of output ports such that each output port can receive optical signals from at least some of the input ports, and a wavelength selector associated with each output port which is either tunable independently within a range of wavelengths, or arranged to transmit a particular wavelength and control means for controlling the selection of said input wavelengths or operation of said wavelength selectors such that a given input port can be coupled to a desired output port.

The wavelength selector may for example comprise a tunable grating filter or a tunable Fabry-Perot device. Where tunable light sources are employed they may comprise separate lasers of different wavelength, any one of which is chosen to achieve a particular connection, alternatively the choice could be made between the longitudinal modes of a single laser, or even between different external grating regions to provide feedback into the lasing region at the wavelength desired.

In this type of switch any output port can be linked to any particular input port simply by tuning either or both the source and wavelength selector to a common wavelength. Connections between different input and output ports have to use different wavelengths in order to avoid crosstalk.

In one embodiment of the invention all the input ports are optically coupled to a single waveguide which is linked by a coupling to a plurality of output waveguides such that all wavelengths are transmitted along each output waveguide. A wavelength selector such as a tunable filter is associated with each outputwaveguide.

In another arrangement all the input ports are optically coupled to a single waveguide. A plurality of tunable filters are arranged in series in the single waveguide, each filter being linked by a further waveguide to a particular output port.

In a further arrangement N input ports and N output ports are connected by waveguides to a plurality of tunable filters arranged in an N x N matrix, each input port being connected to a row of filters and each output port being connected to a column of filters such that any input port can be coupled to any output port.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 is a schematic diagram showing one example of an optical switch which operates by wavelength division multiplex; Figure 2 is a schematic diagramof another example of an optical switch; Figure 3 shows in more detail part of the switch of Figure 2; Figures 4, 5 and 6 illustrate further examples of optical switches; Figure 7 is a schematic illustration of a four port optical switch implemented using micro-optic components; Figures 8 and 9 are schematic illustrations of other examples of four port optical switches implemented using micro-optic components, and Figure 10 is a schematic illustration of an exchange constituted using optical switches in accordance with the present invention.

The switch illustrated in Figure lisa 100 channel switch having a hundred input ports indicated generally at 10 and a hundred output ports indicated generally at 11. Each input port has a laser 12 which is optically coupled to a dielectric optical waveguide indicated at 14. The laser associated with a particular input port is arranged to emit light of a particular wavelength and that wavelength is different from the wavelength of the light emitted from all the other lasers. Thus it will be seen that there are one hundred input channels each having a particular optical wavelength associated therewith.

Each optical wave guide 14 is coupled to a single waveguide 16 such that all input wavelengths can be multiplexed onto the waveguide 16. The waveguide 16 is arranged to linkthe input ports to an output stage indicated generally at 18. The lasers 12 may be distributed Bragg reflector (DBR) lasers whose emission wavelengths are accurately defined by means of an external grating region.

The output stage 18 includes a coupling region indicated at 19 which is arranged to distribute light of all wavelengths transmitted along the waveguide 16 equally to each of one hundred output stage waveguides 20. Each output waveguide 20 has an associated wavelength selector indicated at 22 which can for example comprise a tunable grating filter. Each filter can be adjusted to accept any one of the wavelengths associated with the lasers 12. Each output port has an associated photodiode 24 which can convert the optical signals into corresponding signals for transmission to appropriate terminals.

The operation of the wavelength selectors 22 is controlled by a central processor not shown. The central processor decides which wavelength is to be transmitted to which output channel and adjusts the filters 22 accordingly. The signalling information on the basis of which the processor makes this decision appears in electrical signals which are applied to the input of the switch. Time division multiplexed techniques can be used for the central processor to keep to a minimum the amount of circuitry required. It is not necessary in the arrangement shown in Figure 1 for the lasers or the photodiode detectors to be located in the switch itself.

It will be appreciated that loss will be incurred due to the common supply of all wavelengths to all output channels. in a one hundred channel switch this will be approximately 20dB optical. This would appear to present no problem for the switching of G bit per sec signals. A possible problem would be crosstalk from unwanted wavelengths but the required level of approximately 20dB optical for digital signals could almost certainly be achieved.

It will be appreciated that there are several possible devices which can be used as the wavelength selection means. For example it is possible to use liquid crystal filters or attenuators in micro-optic form with expanded parallel beams. Another alternative is a Fabry Perot device which is tunable by means of an electro-optic effect. Other examples are interference filters, prisms, gratings, or simple on-off attenuators.

In the arrangement of Figure 1 all input wavelengths are distributed to every output waveguide prior to selection of particular wavelength by a selector in each output waveguide. This arrangement results in some waste of power since much of the energy in each wavelength is not usefully employed.

An arrangement which does not suffer from this disadvantage is shown in Figure 2. In this arrangement a plurality of input ports 40 are each optically coupled by an associated waveguide 44 to a single waveguide 45. A plurality of tunable filters 46 arranged in series in the waveguide 45. Each filter 46 is connected to a particular one of the output ports 50 and can be tuned to select any one of the wavelengths associated with the input ports. In this arrangement only the required wavelength is coupled out of the waveguide 45 for transmission to the desired output. The switch of Figure 2 allows only one wavelength to couple to each output and this can be used only as a 1:1 switch.Since all wavelengths pass through each filter, each filter must be highly selective in order to avoid crosstaik. It will be appreciated that the number of control connections in an N x N switch is only N and thus the propagation losses are likely to be moderately low since no wavelength passes through more than N filters.

An example of a tunable filter for use in the switch of Figure 2 is shown in Figure 3. A monomode waveguide 55 which couples to a particular output port which terminates within a grating region 56.

The grating region includes an electro-optic medium whose refractive index can be varied by varying an electrical potential applied thereto.

Within the grating region 56 light from the waveguide 45 couples to the waveguide 55. The wavelength which couples depends upon the refractive index within the region 56 and thus can be selected by adjusting the potential applied to the electro-optic medium.

Figure 4 shows a more complex version of the switch of Figure 2 in which a first plurality of tunable filters 58 are associated with the first output port, a second plurality of tunable filters 59 are associated with the second output and so on. This arrangement allows N input wavelengths to be distributed to each of N outputs.

Figure 5 illustrates a switch of the type described with reference to Figure 1 in which each output waveguide has a plurality of tunable filters connected in series.

In the switches described with reference to Figures 1, 2, 4 and 5 the light wavelengths from the input ports are multiplexed priorto distribution to the outputs. An arrangement which employs distribution prior to multiplexing is shown in Figure 6. In this arrangement N input ports 60 are coupled to N output ports 61 in an N x N matrix 62 of tunable filters 63. Each input port is connected by a respec tivewaveguideto a row of the filters and each output waveguide is connected by a respective waveguide to a column of filters. It will be noted that any filter receives only one wavelength. The outputs from a particular column of filters are multiplexed onto a single waveguide so that each output port can receive any input wavelength.Although there are N2 control connections or crosspoints no wavelength need pass through more than N filters. Furthermore because the filters are essentially on-off devices which either let through or couple out the single wavelength which passes through they can be less selective than those of the Figure 2 switch. Thus the filter characteristics can be less stringent and shorter grating lengths can be used provided there is sufficient high refractive index modulation available to shift the centre wavelength of the filter much further than the linewidth of the filter itself. Any other known form of waveguide switch could be used because wavelength selective properties are not specifically required in this case.

The description above has related to the switching of optical signals. It will be appreciated that the switches described can also act as distributors to distribute, for example, one wavelength to several outputs simultaneously. This is required in T.V.

distribution. In order to function as distributors minor adjustments to the switches may be necessary. In the arrangement of Figure 2 only one input can be connected to any given output at any time.

However, provided the coupling efficiency of each filter is reduced appropriately to allow sufficient power to be coupled to the outputs, it could be possible to distribute one wavelength to several outputs. In a similar manner the switches of Figures 4 to 6 could be made to operate as N : N distributors.

In fact in the Figure 6 arrangement any combination of inputs can be connected to any combination of outputs.

An embodiment of a 4 x 4 wavelength division switch and distributor is shown in Figure 7. The switch comprises four input ports 60 and four output ports 61. Each input port 60 is coupled optically by a respective optical fibre 63 to a multiplexer 65. The multiplexer 65 is a five port device which employs a cube beamsplitter type of arrangement in which three interference filters 68 take the place of conventional beamsplitters, transmitting and reflecting the four wavelengths in order to couple them into a single optical fibre 69. The optical fibre 69 is coupled to a beam divider 70 comprising three 50 : 50 cube beamsplitters 72. The beam divider 70 distributes the four wavelengths equally into four optical fibres 73,74,75,76.

Each fibre 73 to 76 (only shown for fibre 73) is coupled to a demultiplexer 78 whose function is to separate the wavelengths present in the fibre. The demultiplexer 78 has four outputs, one for each wavelength, each output having a controllable liquid crystal attenuator 79. The four outputs of the demultiplexer 78 are connected by optical fibres to a multiplexer 80 which is in turn connected to an output line 81.

By controlling the attenuators 79 from an appropriate central control unit unwanted wavelengths can be blocked and the required wavelength or wavelengths allowed to pass to connect any of the four inputs to a particular output. The liquid crystal attenuators can be switched from low loss to high loss thereby allowing switching to be achieved by altering the wavelengths that are selected.

The arrangement of Figure 7 performs multiplexing before distribution. This is costly in terms of the amount of equipment required when on-off attenuators are used as the selection devices, since demultiplexing and multiplexing stages must be included in every outgoing line. An arrangement which employs similar components and performs distribution before multiplexing is shown in Figure 8.

The Figure 8 arrangement requires fewer components. Of course the type of arrangement shown in Figure 8 will work equally well if the wavelengths B to 4 are not distinguishable by using conventional beamsplitters throughout. This is because the use of attenuators, which are not strictly tunable, naturally results in this kind of space switch arrangement. The advantage of wavelength multiplexing in these examples is the minimisation of the interconnection losses, rather than any saving in components.

It is possible to reduce the number of devices in the switches of Figures 7 and 8 since subscribers do not usually wish to communicate with themselves.

Thus one third of the beam dividers and multiplexers could be eliminated in this example.

A schematic view of a 4 x 4 wideband switch employing duplex transmission is shown in Figure 9.

This switch employs two channel devices. Each port 90 has associated beam dividers 91 which couple with multiplexers 92 and switchable liquid crystal attenuators 93 associated with the other three ports.

Coupling between devices is by optical fibres.

The switches shown in Figures 1 and 2 and 4 to 6 can be used as building blocks to construct an exchange. Figure 10 shows schematically a ten thousand line exchange. The exchange comprises one hundred input blocks indicated at 140, each input block comprising a switch of the form shown in one of Figures 1,2,4,5 or 6. Similarly the exchange has a hundred output blocks indicated at 142, each output block comprising a similar switch. The input blocks are connected to the output blocks by optical waveguides indicated at 144. The arrangement is such that, taking the upper input block which has one hundred output ports, the first output port is connected to the first output block 142, the second output port is connected to the second output block and soon down to the 100th output port which is connected to the 100th output block. The other input blocks are connected in a similar manner.Thus each input block has one connection to each output block.

By connecting up the blocks as shown in Figure 10 it is possible to design a ten thousand line exchange using only two hundred optical switches, with each call passing through only two optical switches.

A problem with this arrangement however is that when switches are used together, their operation necessarily becomes more complex. There are two reasons for this. Firstly it may occur that all one hundred inputs to a particular input block 140 are required to go out on the same output line to a single second output block 142. Therefore the input switches must have one hundred filters all capable of working simultaneously in every output waveguide so that anything from one to a hundred channels can be connected to a single output switch block. The input switch must therefore have ten thousand accessible control points on the output side.

A second complication is that if one of the output blocks is receiving a certain wavelength from a given input block, it must not receive the same wavelength from any other input block, otherwise it will not be able to distinguish between them. This means that the wavelengths emitted in the first input switch block must be variable, and the central processor must select the required wavelengths of every source and filter using selection algorithms which obey this criterion. It is reasonable to assume that situations will arise where the addition of one call will require the reallocation of source and filter wavelengths of many, if not all switches. However, this problem could be completely avoided by making the switches in the output block 142 of the form shown in Figure 6.By performing the distribution of each input 144 to the appropriate outputs 146 before multiplexing the beams together, there would be no chance of the two identical wavelengths occupying the same path in the switch, since they would have different destinations. The most useful design for both switches would be that of Figure 6, since this would allow either the source wavelengths to be fixed and the filters to be tuned, or the source wavelengths to be tuned and the filters to be fixed.

There would have to be a slight difference though, because in the switches of the output blocks each input line could be carrying all possible wavelengths simultaneously, and each filter must therefore be able to couple any of these wavelengths into each output line. These must therefore be narrow-band filters. In contrast, the switches in the input block are essentially just space switches, since they do not have to distinguish between wavelengths, and can therefore have very broad wavelength response, if any.

Since the number of cross-points contained in each switch in this example is N, where N is the number of exchange lines, and there are 2.N1/2 blocks altogether, the total number of cross-points throughout the entire exchange is 2.N3'2, which equals 2,000,000 for a 10,000 line exchange. In comparison a ten thousand line exchange implemented on a single space switch matrix would require 100,000,000 cross-points. It is evident that the use of wavelength division multiplexed switching described herein produces a significant saving in required connections (50-fold in this case). Of course if 10,000 separate wavelengths were available, there would only be 10,000 cross-points, which would represent a 10,000-fold reduction.

Since no attempt has been made in these examples to incorporate methods of concentration as used in electro-mechanical and electronic exchanges, the true savings in equipment that coukd be made by the use of wavelength division multiplexed switching obviously cannot be judged so simply as this. But large savings are certainly to be expected.

The various types of switch described are all of use in particular applications. The optimum design for an N x N switch for which connections are required only on a one to one (1:1) basis is that of Figure 2 (provided the loss through each selective filter is sufficiently low). For an N: 1 switch operating as an input block 140 the method of distribution before multiplexing shown in Figure 6 would probably be the best design. However, for general purpose N: N distribution the arrangement shown in Figure 5 is the most convenient, despite its intrinsic loss of 10.1Og,oN dB, since careful control of filter coupling efficiencies is not required. If the loss were unacceptable the arrangement of Figure 6 could be used with filters of variable coupling efficiency. For 1 N distribution the arrangement of Figure 2 can be employed, also using filters of variable coupling efficiency.

Claims (9)

1. An optical switch comprising a plurality of optical input ports, each input port being arranged to receive optical signals of a wavelength which is either fixed and different from the wavelengths of signals received at the other inputs, or has a value selectable independently from within a range of wavelengths, waveguide means coupling the input ports to a plurality of output ports such that each output port can receive optical signals from at least some of the input ports, and a wavelength selector associated with each output port which is either tunable independently within a range of wavelengths, or arranged to transmit a particular wavelength and control means for controlling the selection of said input wavelengths or operation of said wavelength selectors such that a given input port can be coupled to a desired output port.

2. An optical switch as claimed in claim 1 wherein each wavelength selector is a tunable grating filter.

3. An optical switch as claimed in claim 1 wherein each wavelength selector is a tunable Fabry-Perot device.

4. An optical switch as claimed in claim 1 including tunabie light sources which comprise separate lasers of differentwavelengths, any one of which is selectable to achieve a particular connection.

5. An optical switch as claimed in claim 1 wherein all input ports are optically coupled to a single waveguide which is linked by a coupling to a plurality of output waveguides such that all input wavelengths are transmitted along each output waveguide.

6. An optical switch as claimed in claim 5 wherein a wavelength selector is associated with each output waveguide.

7. An optical switch as claimed in claim 1 wherein all input ports are optically coupled to a single waveguide, a plurality oftunable filters are arranged in series in the single waveguide, and each filter is linked by a further waveguide to a particular output port.

8. An optical switch as claimed in claim 1 wherein said switch comprises N input ports and N output ports which are connected by waveguides to a plurality of tunable filters arranged in an N x N matrix, each input port being connected to a row of filters and each output port being connected to a column of filters such that any input port can be coupled to any output port.

9. An optical switch substantially as hereinbefore described with reference to and as shown in the accompanying drawings.