Abstract:
An upgradable optical router for use in an optical switching network. In an initial configuration, the optical router includes wavelength selective switches configured to switch optical signals having WDM wavelengths positioned in a grid having exactly 100 GHz (about 0.8 nm) spacing in optical frequency, aka fixed grid. The interface ports within the optical switch include an optical splitter and optical coupler and additionally space for a second selective switch. At a later point in time, a second wavelength selective switch can be added to provide additional capabilities such as switching wavelengths positioned in a flexible grid.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a National Phase entry of PCT Application No. PCT/GB2013/000209, filed May 10, 2013, which claims priority to EP 12250127.3, filed Jun. 29, 2012, the contents of which are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments relate to optical data transmission and in particular to an upgradable optical routing apparatus for switching optical signals using two optical carrier transmission schemes. 
       BACKGROUND 
       [0003]    In order to satisfy increasing demands for bandwidth, it is known to replace electrical core networks based on electrical signals over copper lines to optical core networks based on transmitting light pulses through optical fibres. 
         [0004]    In optical data transmission, a signal to be transmitted is sent as a sequence of light pulses over an optic fiber to a photo detector which converts the optical signal into an electronic one for subsequent processing. 
         [0005]    As with electrical data transmission, using a different fiber per transmission is expensive and therefore various techniques have been proposed to allow multiple signals to be transmitted over a single fiber. The two most common techniques are Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). 
         [0006]    In TDM, separate input signals are carried on a single fiber by allocating time transmission windows. The input signals are fed to a multiplexer which schedules use of the optical fiber so that each input signal is allowed to use the fiber in a specific time slot. At the receiver, synchronisation techniques are used to ensure that the different input signals are sent on to the appropriate destination. 
         [0007]    In WDM, the fiber is shared by sending each input signal at the same time, but on a different carrier wavelength, for example a first signal could be transmitted using a carrier wavelength of 1539 nm and another signal is transmitted using a carrier signal of 1560 nm. The two signals can be multiplexed onto the same line and provided the carrier wavelengths are sufficiently different, the signals will not interfere with each other. At the end of the optical fiber, a receiver or router will demultiplex the incoming light signals into the individual signals and process them as required. 
         [0008]    A grid of wavelengths is specified by the ITU so that compliant equipment from different manufacturers can operate together. The ITU has specified a number of Dense Wavelength Division Multiplexing grid sizes at 12.5 Ghz, 25 Ghz, 50 Ghz and 100 Ghz. 50 Ghz is currently the most popular channel with and using the DP-QPSK modulation format, it is possible to fit a 100 Gbit/s signal within a single channel in the 50 Ghz grid. 
         [0009]    However, research into optical transmission beyond 100 Gbit/s has shown that higher spectral efficiency formats have to be used, or the spectral width of the signals must be increased to support 400 Gbit/s or 1 Tbit/s transmission. Utilizing modulation formats with higher spectral efficiencies limits the distance the signal can propagate due to OSNR penalties, and increasing the spectral width means that the signal can no longer fit within the widely deployed 50 Ghz ITU grid. 
         [0010]    To overcome these problems, flexible grid or Flexgrid networks have been proposed. In this scheme, arbitrary sized wavelength blocks can be specified by the network owner and routed in Flexgrid Wavelength selective switches which can accommodate new bit rate services. 
         [0011]    However, existing equipment for fixed grid transmission is incompatible with Flexgrid and therefore Flexgrid networks would require a new range of optical switching and transmission component. This would be very expensive to implement and currently it is not clear whether it is better to invest in new Flexgrid networks or continue with networks based on the ITU grid. 
       SUMMARY 
       [0012]    Embodiments disclosed herein address the above issues. 
         [0013]    In one aspect, an embodiment provides an apparatus for routing an optical signal in an optical network, the signal having a plurality of independent wavelength channels, the apparatus comprising: at least three interface ports; and optical pathways for connecting each interface port to at least two other interface ports, wherein each interface port comprises: means for splitting said optical signal, first optical switch receiving means for receiving a first optical switch, second optical switch receiving means for receiving a second optical switch, and means for combining optical signals switched by at least one of said first or second switch so as to generate an output optical signal. 
         [0014]    In another aspect, an embodiment provides a method of reconfiguring an optical routing device having at least three interface ports, each interface port having a first optical switch and means for receiving a second optical switch, the method comprising: adding a second optical switch to at least one interface port of the optical routing device. 
         [0015]    In a further aspect, an embodiment provides an optical network for carrying optical data signals, comprising at least one apparatus according to claims  1  to  6 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Embodiments will now be described with reference to the accompanying Figures in which: 
           [0017]      FIG. 1  shows an overview of a data network in which one part of the network transports data signals optically; 
           [0018]      FIG. 2  shows a more detailed view of the optical transmission network in which data signals are routed via optical routers; 
           [0019]      FIG. 3  shows the internal structure of an optical router illustrated in  FIG. 2 ; 
           [0020]      FIG. 4  shows the initial configuration the three port optical router containing fixed grid WSSs; 
           [0021]      FIG. 5  shows the configuration of the three port optical router when some Flexgrid WSSs have been installed; and 
           [0022]      FIG. 6  shows the configuration of the three port optical router when fully converted to Flexgrid; and 
           [0023]      FIG. 7  shows the configuration of a four port optical router. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows an overview of a data network system  1  in which one part of the network  1  is configured to transport data signals using an optical signal. 
         [0025]    In  FIG. 1 , four clusters of electrical signal data networks  3  are shown containing a number of network devices such as computers  5  which generate, send and receive data packets in the form of electrical data signals. The electrical networks  3  are connected to an optical backbone network  7  via bundles of optical fibres  9  so that the data can be routed between the different electrical networks optically. Each electrical network contains an optoelectronic converter  11  for converting electrical signals into optical signals and vice versa in a conventional manner. 
         [0026]      FIG. 2  shows the main components of the optical backbone network  7 . Due to the higher data capacity offered by optical fibres over copper cables, the optical network  7  has a much higher bandwidth and therefore is used to carry data between networks  3 . 
         [0027]    The optical network  7  is connected to the electrical data networks  3  via the bundles of optical fibers  9 . In this embodiment, there are four bundles of optical fibers  9  carrying signals between the optical network  7  and four respective electrical data networks  3 . 
         [0028]    The optical network  7  contains a number of optical routers  13 ,  15 . For ease of explanation, in this embodiment, there are some optical routers  13  having three input/output ports whilst other optical routers  15  have four input/output ports. Interconnect optical fibers  17  link the three port and four port optical routers  13 ,  15 . 
         [0029]      FIG. 3  shows a more detailed view of a three port optical router  13 . In this router  13 , there are three input/output port  21  connected via an optical cross connect  23  and therefore optical signals entering via one port can leave the optical router  13  via one of two output ports. Input signals at port  21   a  can leave via port  21   b  or port  21   c , input signals at port  21   b  can leave via port  21   a  or  21   c  and input signals at port  21   c  can leave via port  21   a  or  21   b.    
         [0030]    Optical signals entering the optical router  13  on any of the input ports do not need to be converted into electrical signals in order to be routed to a destination port. The routing is performed in an optical manner on the basis of wavelength of the incoming optical signal and this is set by the optoelectronic converter  11  located at the interface between the electrical data network and the optical fibre bundles  9 . The optical routers  13  contain Wavelength Selective Switches  27 ,  29  in order to perform the optical routing on the basis of the wavelengths of the input light signal. 
         [0031]    In order to route both fixed grid and Flexgrid scheme transmissions, the optical router  13  can contain both fixed grid WSS  27  and Flexgrid WSSs  29 . A fixed grid WSS  27  operates to route optical signals having 50 Ghz channel widths while a Flexgrid WSSs  29  routes optical signals having variable channel widths based on multiples of 12.5 Ghz. 
         [0032]    Each input/output port  21  contains an optical splitter  25  which splits the incoming signal so that both the fixed grid WSS  27  and Flexgrid WSS  29  receive the input signal and can then switch the component wavelength signals to the appropriate output port via the optical cross connect  23 . Each input/output port  21  also has an optical coupler  31  which combines redirected signals before outputting them onto via an optical fiber bundle  9  to a different downstream optical router  13  or to the edge of the optical network. Since the splitter reduces the power of the input optical signal, an optical amplifier may be located between the optical routers in order to regenerate the optical signals. Each input/output port  21  provides space to fit a fixed grid WSS  27  and a Flexgrid WSS  29  regardless of whether it is actually fitted. Therefore each input/output port  21  will be in one of three configurations:
       fixed grid WSS  27  only;   fixed grid WSS  27  and Flex Grid WSS  29 ; or   Flex Grid WSS  29  only.       
 
         [0036]    This allows flexibility on the configuration of the optical router  13  and in particular allows the optical routers  13  to be upgraded as Flexgrid WSSs  29  fall in price. 
         [0037]    The configuration parameters for the WSS devices  27 ,  29  are controlled by a central controller  33 . 
         [0038]    An example of the operation of the optical router  13  will now be described in the case that an input optical signal containing two signals, a 50 Ghz fixed grid based signal A and a 12.5 Ghz flex grid signal B, arrives at the optical splitter  25   a  of input/output port  21   a . The optical splitter  25   a  splits the incoming signal into two identical but lower power signals onto the optical cross connect  23 . The optical cross connect  23  is configured so that it provides light paths which connect the two outputs of the optical splitter  25   a  to the respective inputs of the fixed grid WSS  27   a  and the Flexgrid WSS  29   a.    
         [0039]    The fixed grid WSS  27   a  and the Flexgrid WSS  29   a  both receive the input signal via the splitter. The fixed grid WSS  27   a  is configured to block the Flexgrid signal B but direct the fixed grid signal A to an output port which could be port  21   b  or  21   c . The fixed grid WSS therefore has two outputs which are connected via the optical cross connect  23  to optical coupler  31   b  of input/output port  21   b  and also optical coupler  31   c  of input/output port  21   c . In the example, the fixed grid WSS is configured to direct signal A to the coupler  31   b.    
         [0040]    The Flexgrid WSS  29   a  is configured to block the fixed grid component signal A, but route Flexgrid signal B to either output of input/output port  21   b  or  21   c . Flexgrid WSS  29   a  has two outputs onto the optical cross connect  23 . One is directed to the optical coupler  31   b  and the other to the optical coupler  31   c . In the example, the Flexgrid WSS is configured to direct signal B to the coupler  31   b.    
         [0041]    Each of the three fixed grid WSSs  27  has two outputs and each of the Flexgrid WSSs  29  has two outputs so therefore each optical coupler  31  has four inputs to receive each of the possible WSS outputs. In the example, signal A and signal B are received by the optical coupler  31   b . The signals are coupled onto the same output optical fibre bundle  9  towards the next optical router  13  or destination network. 
         [0042]    In the above description, the optical router  13  has the ability to contain both fixed grid and Flexgrid WSSs  27 ,  29 . However, Flexgrid technology is still fairly premature and therefore it is not expected that the optical routers  13  would be deployed in the configuration as shown in  FIG. 3 . 
         [0043]    The configuration of the optical routers  13  with groups of input/output ports  21  each having an optical splitter  25  and an optical coupler  31  allows the optical router  13  to be incrementally upgraded as Flexgrid WSSs mature. 
         [0044]      FIG. 4  shows an initial configuration for the optical router  13  in which received optical signals conform to the fixed grid scheme and therefore the optical routers contain conventional fixed grid WSS devices  25  to optically route the optical signals. In this configuration, the controller  31  sets the splitter  25  to redirect all incoming light signals to the installed fixed grid WSS  27 . Any fixed grid signals are routed to one of the couplers  29  of the other two ports  21 . The ports contain a space  35  for the Flexgrid WSSs which will eventually be installed. 
         [0045]    At a later point in time, when it is expected that Flexgrid has matured enough that Flexgrid WSS devices are available, the optoelectronic converters  11  are upgraded to support Flexgrid and therefore it is necessary to upgrade the core optical network  7  to support Flexgrid. 
         [0046]    Installing an entire new Flexgrid enabled core network would be expensive and time intensive due to the equipment and installation costs. The configuration of the optical routers  13 , however, allows the optical network to be upgraded incrementally with Flexgrid WSS  27  devices and the optical router  13  can switch to using Flexgrid without significant changes. 
         [0047]      FIG. 5  shows the optical router  13  with two of the input/output ports  21   a  and  21   c  upgraded with Flexgrid WSSs  29  while the third input/output port  21   b  has not been upgraded yet. 
         [0048]    With the partial upgrade, cost savings can be made while improving the functionality of the optical router  13 . In this partial upgrade configuration, the optical router  13  is able to carry both Flexgrid and fixed grid optical signals between ports  21   a  and  21   c  while fixed grid signals can be routed between ports  21   a ,  21   b  and  21   c . Therefore the optical router  13  has been improved without carrying out a full upgrade. 
         [0049]      FIG. 6  shows a later configuration in which the optical router  13  is switched entirely to Flexgrid operation. In this case the fixed grid WSSs  25  are not present in the optical router  13  and only Flexgrid WSSs  27  are used to route the optical signals based on wavelength. Each splitter  25  splits the incoming optical signals to two signals on the optical cross connect  23  but since only the Flexgrid WSSs  29  are connected, the signals which would previously have entered the fixed grid WSS are blocked and the component parts of input signals entering the FlexGrid WSS  29  are switched to an appropriate output port according to wavelength. 
         [0050]    The space  37  within the optical router  13  left by the removal of the fixed grid WSS  25  can be reutilized. For example, if industry moves beyond the capabilities of the Flexgrid scheme, then new switches based on wavelength switching or other technology can be replaced into the optical router  13 . An example could be switches which operate in the L frequency band (390 Mhz to 1.55 Ghz). 
         [0051]    For ease of explanation, the operation of a three input/output port optical router  13  has been described. However typically the optical routers would have more ports and therefore the number of inputs that the optical couplers can potentially combine and the number of optical paths provided within the optical cross connect are higher. 
         [0052]      FIG. 7  shows the structure of the four port optical router  15  when both fixed grid and Flexgrid WSSs are installed. 
         [0053]    Optical router  15  contains four sets of input/output ports  41 . Each input/output port  41  is connected to the other ports  41  via an optical cross connect  43  and each input/output port  41  has a two way splitter  43 , a fixed grid WSS  45 , a Flexgrid WSS  47  and instead of a four way coupler, now contains a six way coupler  49 . A controller  51  within the optical router  15  sets the configuration of the components. The operation of the optical router  15  and the upgrade process from initial installation to removal of the fixed grid WSSs  47  is the same as for three port optical routers  13 . 
       Alternatives and Modifications 
       [0054]    In embodiments, the optical routers include an optical cross connect for routing optical signals between the input/output ports. Using optical cross connects is advantageous because it allows for fast remote provisioning of Flexgrid and allows the fixed grid WSS to be freed and reused elsewhere. However, in an alternative configuration the optical cross connect is replaced with permanent light paths between the inputs and outputs of the optical router. Such a configuration provides a cheaper optical router while still providing the ability to upgrade to Flexgrid WSSs. 
         [0055]    In embodiments, Flexgrid WSSs are added to the optical routers. However, if an alternative optical transmission scheme is established which makes Flexgrid redundant, the configuration of optical routers allow WSSs based on the new scheme to be used instead of Flexgrid. 
         [0056]    In the embodiments, the fixed grid WSSs are removed from the optical routers, however, in an alternative, the fixed grid WSSs remain in the optical router and are used to route additional traffic arriving from the input ports. This provides extra capacity within the optical network. 
         [0057]    In embodiments, the output of an optical splitter is connected to the fixed grid and Flexgrid WSSs via the optical cross connect. In an alternative, the output of the optical splitter directly connects to the fixed grid or Flex grid WSS.