Photonics system

A photonics system comprising an optical circulator, comprising a plurality of ports; a first port arranged to receive a plurality of spatially separate optical signals each comprising a plurality of wavelengths; the optical circulator arranged to pass the plurality of optical signals to a second port so as to produce them as spatially separate signals; a plurality of optical wavelength filters arranged at the second port, in which each of the plurality of optical wavelength filters is arranged to align with a different one of the plurality of optical signals at the second port, each of the plurality of optical wavelength filters for selectively reflecting a selection of the plurality of wavelengths comprised in the respective optical signal and selectively passing others of the plurality of wavelengths.

BACKGROUND OF THE INVENTION
 The present invention relates to the field of photonics and in particular
 to arrangements for controlling, routing or switching optical signals by
 wavelength selection.
 An optical circulator as illustrated by way of example in FIG. 1 is a known
 photonic device for the unidirectional transmission of an optical signal
 or beam from one port to the next sequential port (i.e. from port 1 to
 port 2, 2 to 3, 3 to 4, etc), without allowing transmission in the reverse
 directions (i.e. 2 to 1, 3 to 2, 4 to 3, etc). Thus light input at port 1
 is output at port 2, light input at port 2 is output at port 3, etc. Here
 the term "light" is used to include both visible and non-visible radiation
 e.g. optical signals suitable for the purposes of photonics. To operate on
 several beams, as is necessary when handling signals from multiple optical
 fibres, several optical circulators are conventionally required. To
 perform an all-optical add-drop function that separates channels carried
 on different wavelengths in an optical signal (i.e. wavelength division
 multiplexing (WDM) wavelength channels), would also require a multiplicity
 of optical circulators.
 With the spread of WDM wherein a plurality of wavelength channels is
 carried by a single optical fibre, the need for a compact and cheap means
 for independently operating on large numbers of wavelength channels is
 becoming more acute.
 A requirement in wavelength multiplexed multi-channel optical networks is
 to have fully flexible optical add-drop and cross connect functionality at
 nodes of the network. This means that any wavelength channel carrying data
 to that node can be dropped into any one of a number of receivers at that
 node and that any of the node's transmitters may reuse that wavelength to
 send data on from that node.
 To achieve these functions requires switching and demultiplexing of the WDM
 wavelength channels. Tuneable reflective optical gratings can be used with
 optical circulators to sort a signal comprising WDM wavelength channels
 into two sets of wavelength channels. To carry out the add-drop function
 with 8, 16, 32 or more channels requires the sorting function to be
 repeated many times which requires the use of large numbers of filters and
 optical circulators.
 SUMMARY OF THE INVENTION
 The present invention provides a photonics system comprising an optical
 circulator, comprising a plurality of ports; a first port arranged to
 receive a plurality of spatially separate optical signals each comprising
 a plurality of wavelengths, the plurality of optical signals arranged to
 produce an array of spatially separate sources of light at the first port;
 the optical circulator arranged to pass the plurality of optical signals
 to a second port; a focussing means provided at the first port to produce
 at the second port an array of spatially separate images of the sources of
 light; a plurality of optical wavelength filters arranged at the second
 port, in which each of the plurality of optical wavelength filters is
 arranged to align with a different one of the plurality of optical signals
 at the second port, each of the plurality of optical wavelength filters
 for selectively reflecting a selection of the plurality of wavelengths
 comprised in the respective optical signal and selectively passing others
 of the plurality of wavelengths.
 The present invention further provides a photonics system comprising a
 second optical circulator, and a plurality of further filters provided at
 certain ports of the second optical circulator for performing a similar
 function to the first optical circulator in which the first optical
 circulator and the second optical circulator are arranged in communication
 with each other so that optical signals issuing from a selected port of
 the first optical circulator enter the second optical circulator at an
 input port thereof.
 The present invention further provides an optical network comprising the
 photonics system of the invention.
 The present invention further provides a telecommunications network
 comprising the photonics system of the invention.
 The present invention further provides a photonics system comprising an
 optical circulator, comprising a plurality of ports, a first port arranged
 to receive a plurality of spatially separate optical signals each
 comprising a plurality of wavelengths the optical circulator arranged to
 pass the plurality of optical signals to a second port, to produce a
 plurality of spatially separate optical signals at the second port; a
 plurality of optical wavelength filters arranged at the second port, in
 which each of the plurality of optical wavelength filters is arranged to
 align with a different one of the plurality of optical signals at the
 second port, each of the plurality of optical wavelength filters for
 selectively reflecting a selection of the plurality of wavelengths
 comprised in the respective optical signal and selectively passing others
 of the plurality of wavelengths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, there is shown an implementation, by way of example,
 of an optical circulator of the prior art comprising four ports 1, 2, 3,
 and 4. It will be noted that operation of the optical circulator is not
 reciprocal in that light input at port 1 is transmitted to port 2 but not
 vice versa. The optical circulator will now be described in more detail
 with reference to the figure. In this embodiment, lenses 6 are positioned
 at the entry to the optical circulator at each of ports 1 to 4. The lenses
 function to focus light input at one port at the corresponding output
 port. Lenses are not essential as this function could be achieved by means
 of other focussing means such as suitably curved mirrors.
 Light entering the optical circulator at port 1 will encounter polarising
 mirror 8. The polarising mirror 8 functions to split the light into two
 orthogonally polarised beams which travel along different paths through
 the optical circulator. For example, light input at port 1 will be split
 such that light with a first polarisation travels straight through the
 polarising mirror 8 whereas light with an orthogonal polarisation will be
 reflected through ninety degrees causing it to hit mirror 10 where it is
 reflected a second time. At the centre of the optical circulator are two
 polarisation rotators each of which acts to rotate the polarisation of
 incident light through an angle of forty-five degrees. The first
 polarisation rotator 12 acts reciprocally in that light passing in one
 direction (e.g. from left to right in the figure) will have its
 polarisation rotated clockwise and light passing in the opposite direction
 (e.g. from right to left in the figure) will have its polarisation rotated
 anti-clockwise. In contrast the second polarisation rotator 14 is a
 so-called Faraday rotator which acts in an non-reciprocal way. The
 directional properties of the Faraday Rotator material are influenced by
 magnetic fields and, in practice, a saturating magnetic field will be
 applied to it. Light passing through the Faraday rotator in a first
 direction (e.g. from left to right in the figure) will have its
 polarisation rotated in a first direction, say clockwise. Light passing
 through the Faraday rotator 14 in the opposite direction (e.g. from right
 to left in the figure) will have its polarisation also rotated clockwise.
 The effect of these two different types of polarisation rotator arranged
 together in the centre of the optical isolator so that all light input at
 any port must pass through both elements is as follows. Light passing
 through the rotators in a first direction (in our above example from left
 to right) will undergo a polarisation rotation in a clockwise direction of
 forty-five degrees in element 12 and a further rotation in a clockwise
 direction of forty-five degrees in element 14 resulting in an overall
 rotation of ninety degrees. However, light travelling through the rotating
 elements in the opposite direction (in our example above from right to
 left) will undergo a polarisation rotation in a clockwise direction of
 forty-five degrees in polarising element 14 and a contrary polarisation
 rotation of forty-five degrees in an anti-clockwise direction in element
 12 resulting in an overall rotation of zero degrees, i.e. the light passes
 straight through with no overall change in its orientation of
 polarisation.
 Operation of the optical circulator of FIG. 1 will now be illustrated by
 describing the passage of light through the optical circulator from port 1
 to port 2. As mentioned above, light input at port 1 first passes through
 lens 6 and encounters polarising mirror 8 where that portion of the input
 light with a first polarisation passes straight through the polarising
 mirror, whilst light with the orthogonal polarisation is reflected by the
 polarising mirror through ninety degrees. The light passing straight
 through the polarising mirror will experience an overall rotation of
 polarisation of ninety degrees as explained above as a result of passing
 through polarisation rotation elements 12 and 14. The light will then
 encounter the second polarising mirror 16. This light now has an
 orthogonal polarisation compared with the light transmitted by the first
 polarising mirror 8 and will therefore be reflected through ninety degrees
 and directed out at port 2.
 The portion of the input light with orthogonal polarisation which was
 reflected at the first polarising mirror 8 undergoes a second reflection
 through ninety degrees at mirror 10 and subsequently passes through
 polarisation rotation elements 12 and 14, undergoing an overall
 polarisation rotation of ninety degrees in exactly the same way as the
 light transmitted through the first polarising mirror 8, as described
 above. After leaving the polarisation rotation elements 12 and 14 the
 light is reflected again through ninety degrees by the second mirror 18 so
 as to enter the second polarising mirror 16 from the top, as shown in the
 figure. However the light entering the top of the polarising mirror 16 has
 a different polarisation due to the ninety degree rotation undergone in
 the rotational elements 12 and 14 and therefore passes straight through
 the second polarising mirror 16 to issue at port 2.
 In a similar way light entering the optical circulator at any of the ports
 will be split according to polarisation and routed separately so as to
 issue from the appropriate port. The passage of light input at port 2 will
 now briefly be described to demonstrate the non-reciprocal operation of
 the optical circulator. Light input at port 2 with a first polarisation
 will be reflected in second polarising mirror 16 so that it passes through
 polarisation rotation element 14 and element 12 from right to left. As
 explained above this introduces no overall rotation in polarisation so
 that the light entering first polarising mirror 8 from rotational elements
 12 and 14 has the same polarisation as the light reflected by second
 polarising mirror 16 with the result that this light is also reflected
 through ninety degrees by first polarising mirror 8 and hence it issues
 from port 3 rather than port 1. In a similar way, light input at port 2
 and transmitted by second polarising mirror 16 will similarly be passed by
 first polarising mirror 8 (after reflection at mirrors 10 and 18 and
 passing through rotational elements 12 and 14) such that this light also
 issues from port 3.
 The fibre Bragg grating (FBG) filter of the prior art will now be briefly
 described with reference to FIG. 2. The FBG finds applications in the
 field of photonics by providing a tuneable reflective optical filter. The
 conventional FBG filter consists an optical fibre or an array of optical
 wave-guides in which a series of Bragg defraction gratings have been
 arranged (e.g. by exposure to ultraviolet holographic radiation). The
 gratings are arranged such that they reflect incident light over a narrow
 band of wavelengths. In addition the wavelengths reflected may be adjusted
 by e.g. heating or stretching the fibre locally to a particular grating.
 An FBG filter arranged for use with wavelength channels would typically be
 fabricated such that the wavelength band at which the gratings reflected
 light just missed the wavelengths of respective WDM wavelength channels
 but, when a particular grating was activated, the reflected wavelength
 band would shift so as to include that of a desired wavelength channel
 which would then be reflected. Alternatively the FBG filter could be
 fabricated so that each grating reflects a WDM wavelength channel but,
 when activated, is detuned to pass that channel. The FBG filter thus
 provides a convenient way of extracting a particular WDM wavelength
 channel from a plurality of such channels being carried in a single fibre.
 FIG. 2 shows in the upper line a typical WDM signal comprising four
 wavelength channels. In the lower line the reflectivity of three FBGs is
 shown: the lower level of the line indicating transmission the upper level
 of the line indicating reflection. As can be seen from the figure, one FBG
 has had its reflection wavelength band shifted to coincide with the
 wavelength of one of the wavelength channels: which wavelength channel
 will thereby be separated from the other wavelength channels by
 reflection.
 An arrangement for adding and dropping single channels using an optical
 circulator together with FBGs will be now be described with reference to
 FIG. 3.
 FIG. 3 shows a photonics system according to a first embodiment of the
 invention in which a plurality of optical signals represented
 diagrammatically by the series of peaks 5 input on separate optical beams
 are circulated simultaneously in a single optical isolator and isolated by
 use of a plurality of FBG filters. Elements common to FIG. 1 are shown
 with the same reference numerals and will not be described further here.
 The arrangement of the invention provides a multi-channel optical
 circulator (MCOC), by which is meant an optical circulator capable of
 independently routing a plurality of signals carried as wavelength
 channels: i.e. as different optical wavelengths. This embodiment differs
 from the prior art in that the input signal is power split into a
 plurality of separate beams each carrying the same information, i.e. all
 of the wavelength channels comprised in the original input signal. A
 conventional optical splitter 22 with a sufficiently broad transmission
 band may be used for this purpose.
 With reference to FIG. 3 the splitter 22 is arranged to produce an array of
 light sources adjacent port 1 of the optical circulator. A lens or other
 focusing means 26 is interposed between the array of light sources and the
 optical circulator input at port 1 and causes an image of the array of
 light sources to be produced at the corresponding output of the optical
 circulator (in this example, port 2). At port 2 a plurality of optical
 wavelength (eg. FBG) filters 24 is arranged, one filter arranged to align
 with each image of a light source produced at port 2 by the lens at the
 input to the optical circulator at port 1 so that light from each of the
 outputs of the optical splitter 22 enters a different one of the filters
 24. Optical wavelength filter 24 comprises a series or "chain" of FBG
 elements, each element designed to "switch" (i.e. reflect or pass) a
 particular channel wavelength.
 The optical wavelength filters 24 have the ability to selectively reflect
 one, or more, of the optical wavelengths (or "colours") corresponding to
 the wavelength channels arriving at the input to the optical circulator at
 port 1 depending on which of the FBG elements are activated. Light
 reflected in this way will create an array of sources of light adjacent to
 the second port of the optical circulator (port 2).
 Hence each of the optical wavelength filters at port 2 receives all of the
 wavelength channels from the input at port 1. In operation each filter may
 be set to reflect a different wavelength (corresponding to a different
 wavelength channel) or a group of wavelengths back into the optical
 circulator. Light reflected back into the optical circulator at port 2
 will exit at port 3. A focussing means at port 2, interposed between the
 optical circulator and the filters, causes an image of the array of
 sources of light at port 2 to be produced at port 3. Hence at port 3 an
 array of light sources may be created, each light source corresponding to
 a different one or several ones of the input wavelength channels. Any
 wavelength channels not reflected by a particular optical wavelength
 filter at port 2 will pass through the filter and may be transmitted out
 from the end of the filter remote from the port.
 Alternatively each optical wavelength filter at port 2 may be set to
 reflect all except one of the wavelength channel input at port 1. If each
 filter is set not to reflect (i.e. is set to pass) a different wavelength
 channel, an array of light images may be created at the ends of the
 optical wavelength filters remote from port 2, each light source
 corresponding to a different one of the input wavelength channels.
 At each of these remote ends of the optical wavelength filters an optical
 guide (e.g. fibre or waveguide, not shown) may be connected to guide
 onward transmission of wavelength channels passed by the filter. Similarly
 an array of optical guides 42 may be arranged at port 3 so as to guide
 onward transmission of any wavelength channels issuing from that port.
 Each port 3 optical guide would need to be accurately aligned with a
 corresponding image produced at port 3 from a source of light produced by
 one of the filters reflecting light back into the optical circulator at
 port 2.
 Hence by controlling the reflectivity of the plurality of optical
 wavelength filters it is possible to selectively switch a plurality of
 wavelength channels using a single optical circulator so that any desired
 wavelength channel or desired combination of wavelength channels may be
 routed to any one of the output guides at ports 2 or 3.
 Thus a flexible and compact multi-channel optical de-multiplexer and
 combiner for WDM is created using a single optical circulator.
 Cross talk between channels may be kept to acceptable limits by achieving
 sufficiently small aberrations in the optics. The cost per channel will
 reduce as the number of channels increases. Waveguide array components
 (either one or two dimensional arrays) may be used to arrange the several
 input optical beams so that they may be imaged through the optical
 circulator system.
 FIG. 4 shows a second embodiment of the present invention. With reference
 to FIG. 4, the input signal 5 is taken to port 2 rather than port 1,
 however the actual port used for input is irrelevant as the invention will
 work in the same way independent of which port is selected due to the way
 optical circulators operate.
 No splitter is used on the input signal which is fed to the input port
 (here port 2) on the optical circulator by means of a single optical guide
 30 (i.e. fibre or wave guide). The input optical guide 30 shares the input
 port 2 with a plurality of other guides 32 looping around from the next
 port on the optical circulator (here port 3). In the arrangement of FIG.
 4, port 3 effectively takes over the function performed by port 2 in FIG.
 3. A focussing arrangement 26 at port 2 forms an image at port 3 of each
 light source, corresponding to each optical guide (i.e. both input and
 looped) terminating at port 2. At port 3, aligned with each of the images
 so formed, is an optical wavelength filter 34 (such as a fibre Bragg
 grating) that may be controlled to reflect or transmit a selected
 wavelength channel or channels differentiated on the basis of the
 wavelength of light used to propagate it.
 A first one of the filters 34 at port 3 will therefore be aligned with the
 image of the input signal incident at port 2. This input signal will
 comprise all the wavelength channels carried by the input optical guide.
 This first filter may be controlled to selectively reflect one of the
 wavelength channels received from the input at port 2 back into the
 optical circulator at port 3. Light input at port 3 of the circulator will
 issue at port 4. The other wavelength channels, i.e. those not selected by
 this first filter for reflection, will issue from the end of the first one
 of filters 34 remote from port 3. This remote end of the filter is
 connected to one of the optical guides 32 looped round to terminate at
 port 2 at a different point from the input optical guide. The focussing
 arrangement 26 at port 2 creates a second image at port 3 of this looped
 around signal (which now contains all of the wavelength channels except
 that one selected by the first filter). This second image is aligned with
 a second one of the filters 34 at port 3 which is controlled to
 selectively reflect a second wavelength channel for issue at port 4. As
 this second filter is at a different position at port 3 from the first
 filter, the second wavelength channel selected will issue at port 4 in a
 different position to the first wavelength channel selected. Again the
 non-selected wavelength channels will propagate through the relevant
 filter and will again be looped around to port 2 in another one of the
 optical guides 32 that terminates at a different position to the first two
 guides. This looping-round is repeated in a similar manner with a
 different wavelength channel being selectively reflected by a filter at
 the start of each loop until each one of the wavelength channels has been
 selected to issue at a unique position from port 4. So for n input
 wavelength channels, n filters 34 and n-1 optical guides 32 looping from
 port 3 to port 2 will be required. Advantageously for a large number of
 wavelength channels (and therefore loops 32) an optical amplifier (not
 shown) may be integrated into one of the loops 32 to compensate for
 attenuation introduced. An array of optical guides 44 may be arranged at
 port 4 so as to guide onward transmission of any wavelength channels
 issuing from that port.
 Hence the same function is achieved as the arrangement of the first
 embodiment, described above, but without the need for an optical splitter.
 A further embodiment will now be described with reference to FIG. 5. Given
 the arrangement described in the second embodiment, above, if some of the
 input wavelength channels are not selected for output at port 4, it may be
 desirable to insert wavelength channels from another source.
 As illustrated in FIG. 5, in this embodiment additional signals 36 are
 presented at port 1 via additional input optical guides 38 to produce an
 array of light sources adjacent to the input of the optical circulator at
 port 1. Focusing means 26 interposed between the optical circulator and
 the array of light sources at port 1 produces an image at port 2 of the
 light sources. The additional input optical guides 38 at port 1 are
 positioned so that the images generated at port 2 do not coincide with the
 input guide 30 but do coincide with some of the other guides 32 looping
 around from port 3. By virtue of the optical circulator operation, signals
 input at port 3 will propagate through the optical circulator and issue
 from port 4. With each image at port 2 of port 1 light sources aligned
 with one of the guides 32 as described above, a signal input at port 1 can
 be made to travel through a selected one of the guides 32 to one of the
 optical wavelength filters 34 at port 3. If this optical wavelength filter
 is set to pass the wavelength corresponding to a wavelength channel
 carried by that signal, then that wavelength channel will re-enter the
 optical circulator at port 3 and issue from port 4.
 If the filter through which this additional wavelength channel passes at
 port 3 is set to not reflect any of the wavelength channels then (i) no
 radiation from the input signal at port 2 will be reflected back into port
 3 at this position to issue from port 4 and (ii) all of the radiation from
 port 1 travelling via port 2 and the appropriate one of the optical guides
 32 will pass through the corresponding one of the filters 34 to enter port
 3 at the identical position as the radiation input from port 2 would enter
 had it been reflected back from the same one of the filters 34. As a
 result, the wavelength channel input at port 1 and passed by the
 corresponding filter at port 3, will issue from port 4 along the same one
 of the output optical guides 40 as the corresponding wavelength channel
 input at port 2 would do were it reflected by that filter at port 3.
 If it were to be decided to include the wavelength channel input from port
 2 in place of the corresponding wavelength channel input at port 1 all
 that would be needed is to activate the relevant one of the filters 34 at
 port 3 to reflect the wavelength corresponding to that channel.
 A further embodiment will now be described with reference to FIG. 6. In
 this embodiment the number of passes made by the input signal through the
 multi-channel optical circulator (MCOC) required for multi-channel
 de-multiplexing and/or combining is reduced. In the arrangement of the
 previous embodiment (see FIG. 5), for n wavelength channels, n+1 passes
 through the MCOC are required together with n-1 loops between ports 2 and
 3. As n, the number of wavelength channels, increases the signal
 attenuation becomes significant and the cost and complexity increase.
 In the arrangement of FIG. 6 each of the wavelength filters 46, 50 and 54
 is arranged to pass half of the wavelength channels received by that
 filter and to reflect the other half. The filters aligned at successive
 ports of the optical circulator are controlled to reflect a different
 selection of wavelength channels from the preceding filters. Hence for n
 wavelength channels, any one channel will pass through the circulator a
 maximum of (log.sub.2 n)+1 times and through log.sub.2 n filters.
 FIG. 6 shows an input signal consisting of eight wavelength channels
 (represented diagrammatically by the signal peaks 5) input at port 1,
 although the present invention may be used with more or fewer input
 wavelength channels. The filter 46 at port 2 reflects four of these
 wavelength channels so that the wavelength channels are split into two
 groups of four, one group being reflected back to port 2 so that they pass
 on to port 3 at a first position, the second group being transmitted
 through the filter and looped back to a second position at port 2 via an
 optical guide 48 to be propagated through the optical circulator to port 3
 at a second position. This process is repeated at port 3 where each of the
 two groups of four wavelength channels is divided into two further groups
 (in this case of two wavelength channels each) by filters 50 and
 propagated back (following either reflection by filters 50 or looping back
 through optical guides 52) through the optical circulator to port 4 at
 four different positions. Again at port 4 the groups (now two wavelength
 channels each) are again split in two by filters 54 resulting in eight
 separate wavelength channels travelling back (following either reflection
 by filters 54 or looping back through optical guides 56) into the optical
 circulator at port 4 to issue from port 1, each at a different position.
 For eight channels, port 1 will need to be supplied with nine guides i.e.
 one for the input signal and eight guides for the outputs.
 As before, by selecting the reflective characteristics of the optical
 wavelength filters 46, 50, 54, any of the input wavelength channels may be
 output to an arbitrary one of the output guides. This arrangement uses all
 four ports of the device to demultiplex eight channels.
 Hence for eight channels, four passes will be required, the output issuing
 (assuming the design of optical circulator shown in the figure) from the
 same port as the input signal is applied to. If a signal is reflected by
 the filters at each of the three ports it would return to the input port
 at the same location as the input signal. To avoid one of the selected
 output beams being imaged from port 4 back onto the input channel at port
 1 rather than a designated output channel, an extra optical guide loop 58
 is used to take the appropriate signal issuing at port 4 to a different
 location on port 3, and from there (via a filter at port 4) to the output
 at port 1 at a different location to the input signal. For de-multiplexing
 more than eight channels, more passes are required than there are ports on
 the optical circulator. This means that some of the ports must be used
 more than once. This generates a problem in that it may result in the same
 location at a port being used more than once. There is a need for some
 means of shifting the position of an optical signal from one position at a
 port to a different position. This will happen automatically where a
 signal is transmitted through the relevant filter and looped back to the
 same port but for reflected signals extra measures will be required. One
 way of achieving this is to dedicate one port to a series of looped
 optical guides taking signals received at the port back into the optical
 circulator at different positions on the same port, thereby providing the
 opportunity for another "round-trip" via the ports of the optical
 circulator.
 Another factor is that practical fibre Bragg grating filters are not
 capable of reflecting 100% of the selected frequencies so that, although a
 reflected signal can be restricted purely to a single channel, any
 transmitted signal will also have elements of any channels which have been
 selected for reflection and will therefore not be pure. As a result of
 this, ideally, every signal output from the MCOC de-multiplexer will be a
 reflected, rather than a transmitted, signal. Careful selection of the
 splitting algorithm may be necessary to achieve this. Alternatively, an
 additional reflection stage can be incorporated in the device to improve
 the wavelength purity of the selected wavelength channel.
 In a preferred implementation of any of the above embodiments, the
 plurality of optical signals pass through the optical circulator as
 parallel beams.
 The invention is not restricted to any particular form of optical
 circulator and the skilled worker would realise that alternative
 embodiments using other forms of optical circulator, e.g. with different
 numbers of ports, fall within the scope of the invention.
 The skilled worker would also realise that if, for example, the number of
 passes through the optical circulator became excessively large due, for
 example, to a large number of different wavelength channels, the present
 invention could advantageously be implemented by a plurality of connected
 optical circulators without falling outside the scope of the present
 invention. In one embodiment of such an arrangement (not shown), the last
 port on a first optical circulator is arranged in communication with the
 first port of a second circulator. The optical circulators may be
 interconnected by way of optical guides or directly, in effect imaging
 from a penultimate port of a first optical circulator through to a second
 port of a second optical circulator, with the last port of the first and
 the first port of the second optical circulator either touching or closely
 adjacent to each other. Additional optical circulators could be connected
 in a similar way to form a "chain" of optical circulators. With optical
 circulators of limited port numbers, such an arrangement would allow any
 number of ports to be added subject to the appropriate boosting of
 signals, for example by optical amplifiers, as required.