Abstract:
An optical branching unit is formed of a plurality of interconnected optical interleavers for separating input optical signals into separate components for output on separate branches and for combining separated component optical signals with other input optical signals for output on common branches.

Description:
The present invention relates to an optical branching unit for optical transmission systems. 
   BACKGROUND OF THE INVENTION 
   Modern optical communications systems typically use many separate but interconnected optical transmission lines. These optical transmission lines are typically interconnected using branching units which allow selected optical signals to be removed from any one optical transmission line and inserted in to another connected optical transmission line. 
   Current optical branching units typically employ optical components which permit selected portions of optical signals to be removed from a transmission line and inserted in to another transmission line on the basis of wavelength. This form of branching unit is particularly suited to use with wavelength division multiplex (WDM) signals comprising a plurality of distinct but closely spaced wavelength channels. The branching unit may be configured to drop and add specific WDM wavelength channels from a particular transmission line. 
     FIG. 1  illustrates an example of such an arrangement. The branching unit generally denoted  1  comprises a first circulator  2  a second circulator  3  and an intermediate reflective grating  4  placed in between the two circulators. The first circulator  2  has one optical input  5  and two optical outputs  6  and  7 , while the second circulator  3  has two optical inputs  8  and  9 , and one optical output  10 . One optical output  7  of the first circulator  2  is in optical connection with one optical input  8  of the second circulator  3 . 
   In use, when a WDM optical signal  11  is input to optical input  5  of the branching unit, the entire signal is output at optical output  7  of the first circulator  2 . Selected wavelength channels, such as channel  12 , are subsequently reflected by the reflective grating  4  and redirected in to the circulator  2  for output at optical output  6 . Thus one selected wavelength channel may be dropped from the input signal  11  by appropriately choosing the wavelength reflectivity of the reflective grating  4 . 
   The remaining components  13  of the optical signal  11  then pass to an optical input  8  of the second circulator  3 , and are combined with an optical channel  12  which is input at the other optical input  9  of the second circulator  3 . The channel  12  is reflected by the reflective grating  4 , in a manner similar to the reflection of the corresponding channel  12  output from the optical output  7  of the first circulator  2 . The combined optical signal  15  is then output at the optical output  10  of the second circulator  3 . Thus, a single optical channel  12  may be dropped from, and added to, optical signal  11 . 
   Additional optical channels may be dropped from optical input signal  11  by employing additional reflective grating elements placed in line with grating element  4  which each have a pre-selected reflectivity appropriate to the channel to be reflected thereby. Clearly, a disadvantage of such an arrangement lies in the fact that in order to be reflected by a given one reflective grating, each pre-selected channel must traverse at least one such grating two times. 
   The first traversal occurring as the optical channel approaches the grating, the second traversal occurring after reflection thereby. Each such traversal incurs loss in the respective channel, and therefore the greater number of traversals required, the greater the loss incurred. If a larger number of wavelength channels are required to be dropped, a correspondingly large number of gratings will be required. In such circumstances the losses may be high and often produce a large “loss tilt” whereby the channel reflected in the last grating suffers a much higher loss than the channel reflected in the first grating. Channels reflected by intermediate gratings suffer a loss intermediate these two extremes and the result is a strong drop-off, or “tilt”, in the intensities of successive channels. 
   It will be noted that in the branching unit illustrated in  FIG. 1 , the wavelength channel  12  that is dropped from the incoming signal  11  is the same wavelength channel that is subsequently added by the second circulator of the branching unit to provide output signal  15 . This is necessary in order to avoid optical hole burning in the optical signal output by the branching unit as would occur where the hole  16  in the intermediate signal  13  of the branching unit not filled by the optical channel  12  added thereto by the second circulator  3  of the branching unit. Clearly, this seriously reduces the network routing capacity of any network employing branching units of this type since any channels removed from an optical transmission line by the branching unit must be subsequently added to the transmission line by the same branching unit. 
   Additionally, wavelength dependant add and drop branching units based on reflective gratings are generally difficult to manufacture, and are limited in the number of channels they may add and drop. Indeed, due to problems associated with chromatic dispersion in the reflective grating implied in such branching units, data transmission rates are limited to 2.5 gigabits per second through such branching units. 
   SUMMARY OF THE INVENTION 
   Thus, there is a recognised need for optical branching units which do not suffer from the limitations inherent in branching units based on reflection gratings. The present invention aims to provide a branching unit which overcomes at least some of these deficiencies in the prior art. 
   At its most general the present invention proposes to employ a plurality of interconnected optical interleavers in a branching unit for the purposes of separating at least some optical signals input thereto into separate component signals for output on separate branches, and for combining separated component optical signals with other input optical signals for output on common branches of the branching unit. 
   An interleaver is an optical device having three optical transmission ports in which, the transmission characteristic between a first and a second of the optical ports has the form of a first series of spaced wavelength channels, and the transmission characteristic between the first and a third of the optical ports has the form of a second series of spaced wavelength channels, wherein the wavelength positions of the first series interleaves the wavelength positions of the second series and all the transmission characteristics impart substantially equal (or at least to within a small margin of difference e.g. about 1 dB or less over the channels employed) insertion loss upon all the channels transmitted through the interleaver within its transmission characteristics. 
   The use of interleavers obviates the need to use wavelength selective fibre gratings in an optical transmission line in order to redirect components of optical signals (such as DWDM optical channels) input to the branching unit, and thereby avoids the aforementioned disadvantages associated with their use, particularly the high loss tilt due to rapidly increasing loss across channels. Thus, interleavers substantially mitigate insertion loss tilt across channels. 
   Furthermore, it has been found that optical interleavers provide a thermally stable branching apparatus having low insertion loss. This typically makes the design and manufacture of network nodes employing such branching apparatus easier than when branching units employing reflective gratings are used. 
   The present invention may provide a branching unit having a first optical input for receiving optical signals, a second optical input for receiving optical signals, branching apparatus comprising a plurality of interconnected interleavers for separating component portions of optical signals input at the first optical input and for combining some of those separated optical signal component portions with other optical signals input at the second optical input so as to provide combined optical signals, a first optical output for outputting the combined optical signals, and a second optical output for outputting other of the separated optical signal component portions. 
   Thus, a branching unit may be provided in which the adding and dropping of signal components (e.g. DWDM channels) is performed by interconnected interleavers. 
   The plurality of interconnected interleavers of the branching apparatus may be configured to separate component portions of optical signals input at the second optical input of the branching unit, and to combine some of those separated component portions with separated component portions of optical signals input at the first optical input so as to provide the aforementioned combined optical signals. The branching unit preferably has a third optical output for outputting other of the separated component portions of optical signals input at the second optical input of the branching unit. 
   Thus, some of the separated portions of optical signals input at the first optical input of the branching unit may be combined with some of the separated portions of optical signals input at the second optical input, then output at the first optical output. The other separated portions of either of the input signals are then output at other outputs of the branching unit. 
   The branching unit may have a third optical input for receiving optical signals, and the branching apparatus may be configured to combine the other of the separated component portions of optical signals input at the second optical input with optical signals input at the third optical input so as to provide combined optical signals for output at the third optical output of the branching unit. Thus, a combined optical signal may be provided at several optical outputs (i.e. branches) of the branching unit simultaneously, the optical signal at each being derived in part from the same one input signal. It will be appreciated that this feature of the present invention enhances the interconnectivity and versatility of the branches of the present branching unit. 
   The branching apparatus is preferably configured to separate component portions of the optical signals input at the third optical input of the branching unit, to combine some of those separated component portions with the other of the separated component portions of optical signals input at the second optical input so as to provide the aforementioned combined optical signals for output at the third optical output of the branching unit, and to output at the second optical output of the branching unit other of the separated component portions of optical signals input at the third optical input of the branching unit. 
   Thus, the branching unit may provide at a given branch a combined optical output comprising parts of optical signals input at the first and second optical inputs, and at another branch a combined optical output comprising parts of optical signals input at the second and third optical inputs. 
   The branching apparatus of the branching unit may be configured to combine the other of the separated component portions of optical signals input at the third optical input of the branching unit with the aforementioned other of the separated component portions of optical signals input at the first optical input of the branching unit so as to provide combined optical signals for output at the second optical output of the branching unit. Thus, the branching unit may provide at a first branch a combined optical output comprising parts of optical signals input at the first and second optical inputs, at a second branch a combined optical output comprising parts of optical signals input at the second and third optical inputs, and at a third branch a combined optical output comprising parts of optical signals input at the first and third optical inputs. 
   Preferably, the branching apparatus comprises at least one concatenated group of interleavers comprising, an initial drop interleaver having one optical input for receiving optical signals and two separate optical outputs each for outputting respective component portions of optical signals received thereby, and a terminal add interleaver having two separate optical inputs each for receiving optical signals and one optical output for outputting a combination of optical signals concurrently received at the two inputs thereof, wherein one optical output of the initial drop interleaver is in optical connection with one optical input of the terminal add interleaver of the concatenated group. 
   Depending upon the function of the branching apparatus, there may be one or more intermediate concatenated interleavers connected between the initial drop interleaver and the terminal add interleaver of each concatenated group. Each such intermediate interleaver preferably has one optical input port connected to a neighbouring interleaver of the group, one optical output port connected to another neighbouring interleaver of the group, and a third optical port (which may be an input port or an output port) connected to interleavers not in the group (e.g. interleavers of other concatenated groups). 
   If the function of a concatenated group is to separate optical signals input at the initial drop interleaver into a multiplicity of component portions and to add one such component portion to other optical signals input to the terminal add interleaver of the group, then the intermediate interleavers are preferably all drop interleavers. Thus, a concatenated group of this configuration has the overall function of dropping component signals input thereto. 
   However, if the function of a concatenated group of interleavers is to combine a multiplicity of separate component portions of optical signals separately input at the initial drop interleaver and each intermediate interleaver, and to combine those component portions with other signals input at the add interleaver, then the intermediate interleavers are preferably all add interleavers. Thus, a concatenated group of this configuration has the overall function of adding (combining) component signals input thereto. 
   The branching apparatus preferably comprises a number of concatenated groups of interleavers, one or more of which groups have an add function and one or more have a drop function. Preferably, in the branching apparatus, the number of concatenated interleaver groups having an add function is equal to the number of concatenated interleaver groups having a drop function. This enables full connectivity between each optical input and each optical output of the branching unit. 
   The group of interleavers may comprise only one drop interleaver such that an optical output of that interleaver is in direct optical connection to the terminal add interleaver. However, a plurality of drop interleavers may be employed within the group, an optical output of all but the penultimate drop interleaver being connected indirectly via one or more intermediate drop interleavers each forming part of the group of interleavers. 
   Thus, each of the first to the penultimate drop interleavers of the group of concatenated interleavers may serve to “drop” one of two component portions of optical signals input thereto, and the terminal add interleaver may serve to add to the un-dropped optical signal portion input thereto by the penultimate interleaver optical signals input at the other optical input of the terminal add interleaver. 
   It will be appreciated that an add-drop capability is provided by just one concatenated pair of interleavers comprising one drop interleaver and one add interleaver, and the branching apparatus of the branching unit may comprise a single such pair of concatenated interleavers. Preferably, the branching apparatus comprises a plurality of such concatenated pairs. 
   For example, the branching apparatus may comprise six interconnected interleavers defining six distinct concatenated pairs of interleavers wherein the one optical input of the first interleaver of each pair is arranged to receive optical signals input at one of the optical inputs of the branching unit and the one optical output of the second interleaver of each pair is arranged to output optical signals to one of the optical outputs of the branching unit. The second of the two optical outputs of the first interleaver of each concatenated pair is connected to the second of the two optical inputs of the second interleaver of another concatenated pair. In this way, full interconnectivity is achieved between the three optical inputs and the three optical outputs of the branching unit. 
   The present invention may provide a network routing unit comprising one or more aforementioned branching units for use in an optical communications network, and may provide an optical telecommunications network comprising one or more such branching units or network routing units. The network routing unit may be a submerged or submarine network routing unit, and the optical telecommunications unit may be a submerged or submarine optical telecommunications network. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There now follows a non-limiting example of the present invention, reference being made to the following drawings: 
       FIG. 1  illustrates a prior art wavelength dependant branching unit based on reflection gratings; 
       FIG. 2  illustrates the functionality of an interleaver; 
       FIG. 3  illustrates a concatenated pair of interleavers; 
       FIG. 4  illustrates a branching unit comprising six distinct pairs of concatenated interleavers; 
       FIG. 5  illustrates four distinct groups of three concatenated interleavers configured to add one input optical signal to four separate optical transmission lines; 
       FIG. 6  illustrates an optical network employing two optical branching units. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2  there is illustrated an interleaver  17  possessing a first optical port  18 , a second optical port  19 , and a third optical port  20 . Dense wavelength division multiplexed (DWDM) optical channels are illustrated as being input at the first and second optical input ports,  18  and  19 , of the interleaver. Each of these optical inputs consists of a string of spaced optical wavelength channels at distinct optical wavelengths. The action of the interleaver is to interleave the string of optical channels input at the first optical port  18  of the interleaver with the second string of optical channels input at the second optical input  19  of the interleaver and to output the result as a combined optical signal at the third optical port  20  of the interleaver. The combined optical signal  23  therefore consists of the first string  21  of optical signals interleaved with the second string  22  of optical signals. 
   The operation of the interleaver  17  is also reversible, in that were the combined signal  23  to be input to the third optical port  20  of the interleaver, the outputs of the interleaver at ports  18  and  19  would consist of the separated strings  21  and  22 , respectively, as illustrated in FIG.  2 . Thus, any two separate strings of optical channels concurrently input at the first two optical ports of the interleaver will be combined thereby in to a composite signal which is output on the third of the optical ports of the interleaver. The interleaver therefore acts as an add interleaver. The interleaver may function in reverse whereby one optical signal input at the third optical port of the interleaver is separated thereby in to two component optical signals for separate output at the first and second optical ports thereof. 
   The interleaver therefore acts as a drop interleaver in this circumstance. 
     FIG. 3  illustrates a pair of concatenated interleavers providing a basic add and drop capability. The pair consists of first drop interleaver D connected to a second add interleaver A. The first interleaver D comprises a first input port  24 , a first output port  26  and a second output port  25 . The first output port of drop interleaver D is connected to a first input port  27  of the add interleaver A which itself has a second optical input port  28  and the first optical output port  29 . Thus, composite optical signals  30  input at the optical input port  24  of drop interleaver D are separated thereby in to component portions, a first of which is output on the first optical output port  26  and directed to the input port  27  of add interleaver A. The second component portion  32  of the optical signal  30  input to the drop interleaver D, is dropped from the pair of concatenated interleavers via the optical output port  25 . 
   Optical signals merely concurrently input to the second input port  28  of the add interleaver A for combination with optical signals  31  input at the first input port  27  thereof which emanate from the drop interleaver D. The add interleaver A serves to interleave these two optical signals and produce a combined optical signal  34  for output at its optical output port  29 . 
   In use, the optical signals input at the second input port  28  of the add interleaver A consist of a string of DWDM optical channels positioned at the same wavelengths as those of the channels  32  dropped by the drop interleaver D. This enables signal channels input to the add interleaver A to replace those dropped by the drop interleaver D. Thus, a basic add and drop capability may be provided by connecting one drop interleaver to one add interleaver as illustrated in FIG.  3 . 
     FIG. 4  illustrates a branching apparatus for a branching unit having three branches A, B and C. The branching apparatus consists of six interconnected interleavers three of which are add interleavers and three of which are drop interleavers. The six interleavers are interconnected so as to form six distinct pairs of concatenated interleavers configured in the manner illustrated in FIG.  3 . Each pair of concatenated interleavers comprises a first interleaver being a drop interleaver, and a second interleaver being an add interleaver. Each of the branches A, B and C of the branching unit have one optical input and one optical output each of which is connected to a respective optical input or output of a separate interleaver pair. 
   The six distinct pairs of interleavers are the drop interleaver D 1  with the add interleaver A 1 , the drop interleaver D 2  with the add interleaver A 2 , the drop interleaver D 1  with the add interleaver A 3 , the drop interleaver D 3  with the add interleaver A 1 , the drop interleaver D 3  with the add interleaver A 2 , and the drop interleaver D 2  with the add interleaver A 3 . 
   This pairing permits optical signals input at any one branch of the three branches A, B and C of the branching unit, to be separated in to two component portions which are then each separately combined with separated component portions of optical signals input at the other two optical inputs of the branching unit for subsequent output on a respective one output of the other two branches. 
     FIG. 4  illustrates this process in operation in terms of a first optical signal  59  input at the optical input of branch A concurrently with a similar optical signal  64  input at branch B and a third similar optical signal  69  input at the optical input of branch C of the branching unit. 
   Each one of these three signals consists of a string of DWDM optical channels, the string comprising even-numbered channels interleaved with odd-numbered channels. The optical signal  59  input at branch A of the branching unit is directed to the optical input  41  of drop interleaver D 1  of the first and third interleaver pair. The drop interleaver D 1  separates the input optical signal  59  in to the first portion comprising all of the even-numbered DWDM channels of the optical signal, and a second portion comprising all of the odd-numbered channels thereof. The first portion is output at the first optical output of the interleaver D 1  for input to the first optical input port  44  of the add interleaver A 1  of the first interleaver pair. 
   Concurrently, the second portion  61  of the input optical signal  59  is output at the output port  43  of drop interleaver D 1  for input to the first optical input port  53  of the add interleaver A 3  of the third interleaver pair. 
   In a similar manner, the optical signal  64  input at the optical input of branch B of the branching unit is directed to the optical input port  47  of the drop interleaver D 2  of the second and sixth interleaver pair. 
   This drop interleaver similarly separates the input optical signal  64  in to its component even-numbered and odd-numbered DWDM signals portions. 
   The even-numbered channels  65  being output at the first optical output port  48  of this drop interleaver for input at the second optical input port of the add interleaver A 3  of the sixth and third pairs of concatenated interleavers so as to be combined thereby with the separate group of even channels  61  input at the first input port  53  of add interleaver A 3  so as to provide a combined optical signal  70  the output port  55  thereof. This combined signal is subsequently output on the output port of branch C of the branching unit. 
   An optical signal  69  comprising interleaved even-numbered and odd-numbered DWDM channels input at branch C is directed to the input port  56  of drop interleaver D 3  of the fourth and fifth interleaver pairs, the odd-numbered channels  62  being output at the first output port  57  thereof to the second input port  45  of add interleaver Al of the fourth interleaver pair. Simultaneously, the even numbered channels  67  of the optical signal are output at the second output port  58  of drop interleaver D 3  and are directed to the second optical input port  51  of add interleaver A 2  of the second and the fifth interleaver pairs. Thus, odd numbered channels  62  input to interleaver A 1  are combined thereby with even numbered channels  60  concurrently input thereto so as to provide a combined optical output signal at output port  46  thereof comprising interleaved even and odd such signals. This combined signal is then directed to the optical output port of branch B of the branching unit. 
   Similarly, even numbered channels  67  input to add interleaver A 2  concurrently with odd numbered channels  66  input thereto from drop interleaver D 2  are interleaved by add interleaver A 2  so as to provide the combined optical signal  68  at the optical output  52  thereof. This combined optical signal is subsequently directed to the optical output of branch A of the branching unit. 
   Thus, it will be appreciated that any DWDM optical signal input at a given one of the three branches of the branching unit will be separated in to its even-numbered and odd-numbered DWDM channel portions which portions will be subsequently output on different ones of the other two output ports of the branching unit in combination with odd-numbered and even-numbered channel portions of DWDM optical signals respectively input at the other two optical input ports of the branching unit. Thus, full DWDM fibre inconnectivity is achieved in this way. 
     FIG. 5  illustrates branching apparatus of a branching unit comprising four distict groups of three concatenated interleavers. Each one of these interleaver triplets connects the optical input E of the branching unit with one of four optical output ports A, B, C, and D of the branching unit. Each interleaver triplet comprises an initial drop interleaver connected to an intermediate drop interleaver and terminating with a terminal add interleaver. 
   The four distinct interleavers triplets illustrated in  FIG. 5  include: a first interleaver triplet comprising initial drop interleaver D 4  in combination with intermediate drop interleaver D 5  and terminating with terminal add interleaver A 4 ; a second triplet comprising an initial drop interleaver D 4  combined with an intermediate drop interleaver D 6  and terminating with terminal add interleaver A 6 ; a third triplet comprising initial drop interleaver D 4  in combination with intermediate drop interleaver D 6  and terminating with terminal add interleaver A 5 ; and finally, a fourth triplet comprising initial drop interleaver D 4  in combination with intermediate drop interleaver D 5  and terminating with terminal add interleaver A 7 . 
   It will be appreciated that in any one such interleaver triplet, one optical output of the initial interleaver thereof is connected to the optical input port of the intermediate drop interleaver, and one optical output port of the intermediate drop interleaver is subsequently connected to one optical input port of the terminal add interleaver of the triplet. 
   First, a DWDM optical signal  100  input at the optical input port  71  of the initial drop interleaver D 4  of each of the four interleaver triplets is separated thereby in to component even-numbered and odd-numbered DWDM optical channels. The odd-numbered channels  101  are output at a first optical output port  73  of the drop interleaver D 4  while the even-numbered channels  102  are output at the second optical output port  72  thereof. The odd-numbered optical channels  101  are subsequently directed to the optical input port of intermediate drop interleaver D 5  of both the first and the fourth interleaver triplet, while the even-numbered channels  102  are directed to the optical input port  80  of the intermediate drop interleaver D 6  of the second and the third interleaver triplets. 
   The intermediate drop interleaver D 5  separates the optical channels input thereto in to even-numbered and odd-numbered channels outputting the odd-numbered channels on the first optical output port  76  thereof whilst simultaneously outputting the even-numbered channels on the second optical output port  75  thereof. Similarly, the intermediate drop interleaver D 6  separates the optical signal  102  input thereto in to odd-numbered channels for output at a first optical output port  82  thereof, and even-numbered channels for output at optical output port  81  thereof. Even-numbered channels output by the intermediate drop interleaver D 5  of the first interleaver triplet are directed to one optical input port  77  of the terminal add interleaver A 4  of that triplet for combination with optical signals  107  concurrently input to the second optical input port  78  thereof. These combined signals are subsequently output at the optical output port  79  so as to provide combined signal  108  which is subsequently directed to the optical output port of branch D of the branching apparatus. 
   Similarly, odd-numbered channels output by intermediate drop interleaver D 5  at output port  76  thereof are directed to an optical input port  89  of the terminal add interleaver A 7  of the fourth interleaver triplet. 
   Optical signals  113  concurrently input to the second optical input port  90  of terminal add interleaver A 7  are combined thereby with the odd-numbered channels input thereto at the first optical input port  89  so as to provide at the optical output port  91  thereof a combined optical signal  114  for output at the optical output port A of the branching apparatus. 
   In a similar manner, even-numbered and odd-numbered optical channels input to terminal add interleavers A 6  and A 5  of the second and the third interleaver triplets respectively, are combined with other optical signals input to those respective terminal add interleavers so as to provide combined optical signals for output at the optical output port B and C of the branching apparatus. 
   Thus it will be appreciated that in interconnecting the four distinct interleaver triplets in this way a cascaded array of interleavers is provided which enables optical signals input at input branch E of the branching apparatus to be separated in to four distinct optical signal portions each of which may be subsequently combined with separate optical signals and output on one of four separate optical outputs of the branching apparatus. 
   It will be readily appreciated that the operation of the branching apparatus illustrated in  FIG. 5  may be reversed simply by reversing the operation of each interleaver in that apparatus. That is to say, by replacing each terminal add interleaver A 4 , A 5 , A 6  and A 7 , with a drop interleaver and similarly replacing each of the drop interleavers D 4 , D 5  and D 6  with an add interleaver, the function of each one of the four distinct interleaver triplets may be reversed. 
   In such a situation, optical signals may be directed in to the optical input of each initial drop interleaver of the four modified interleaver triplets from a respective one of the optical ports A, B, C or D of the branching apparatus. The intermediate and terminal add interleavers would then serve to combine the optical signals separately dropped by each of the four initial drop interleavers so as to provide a combined optical signal consisting of those four dropped portions for output at the optical port E of the modified branching apparatus. 
   By combining such a modified branching apparatus with the branching apparatus illustrated in  FIG. 5 , there is provided a branching unit having full interconnectivity between branch E and each of the other four branches A, B, C and D, of the branching unit. 
     FIG. 6  illustrates an optical network using just such an optical branching unit. The optical network  200  comprises a submerged network node  201  within which is located the branching unit, the network node interconnecting terminal E with each one of four other terminal nodes A, B, C and D. Optical interconnection from terminal E to each one of the other four optical terminals is achieved via optical transmission line  202  which passes from terminal E to an optical input port of the branching unit located within the submerged network node  201 , and via respective ones of the four optical output transmission lines  211 ,  206 ,  208  and  204  to respective terminals A, B, C and D. Optical branching apparatus of the form and configuration illustrated in  FIG. 5  achieves this interconnectivity and allows optical signals output from terminal E to be separated in to four distinct portions each one of which is directed to a respective one of the other four connected terminals. 
   The submerged network node  201  also includes modified optical branching apparatus as discussed above which permits optical signals input to the network node from terminals A, B, C or D, via optical transmission lines  210 ,  207 ,  209  and  205 , to be combined by the modified branching apparatus as discussed above and output to the terminal E along optical transmission line  203 . 
   In this way, a branching unit comprising branching apparatus as discussed with reference to  FIG. 5  may be used to interconnect terminal E with four other terminals each located in a different physical location. The optical network  200  also includes two full add and drop branching units BU 1  and BU 2 , which permit direct add and drop interconnectivity between terminals A and B, and between terminals C and D respectively, by way of respective add and drop optical transmission line-pairs  215  and  220 . 
   Thus, it will be appreciated that the present invention may provide a branching unit which permits optical interconnectivity between branches thereof by way of a branching apparatus comprising a plurality of interconnected interleavers. It will be readily appreciated that pluralities of interleavers may be interconnected in ways other than illustrated and described in the present embodiments, without departing from the scope of the present invention.