Abstract:
A light source package is disclosed for a Raman amplifier node having a primary optical fibre for carrying an optical signal and a secondary optical fibre for carrying the optical signal when the signal is rerouted from the primary optical fibre. The light source package includes a primary light source for emitting light into the primary optical fibre when the optical signal is carried by the primary optical fibre to induce Raman gain of the optical signal, and a secondary light source for emitting light into the secondary optical fibre when the optical signal is carried by the secondary optical fibre to induce Raman gain of the optical signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to optical path switching, and more particularly to switching between optical fibres when distributed Raman amplification is used. 
       BACKGROUND 
       [0002]    In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range. 
         [0003]    In optical networks, if equipment or a fibre of operating spans is damaged, it is possible to reroute traffic along another path while the damage is repaired. It is also possible to upgrade the equipment in the system without impacting the traffic, again by using alternative optical transmission paths. This has led in some part to optical networks being designed in mesh or ring topologies. In some networks, a secondary optical fibre may be available alongside a primary optical fibre used in normal operating conditions. This secondary optical fibre may be used for operations such as maintenance of the primary optical fibre or restoration if the primary optical fibre is damaged 
         [0004]    In optical networks, Raman amplifiers can be used to mitigate attenuation of the optical signal along the path. Raman amplifiers are optical amplifiers based on Raman gain, which results from the effect of stimulated Raman scattering. The optical signal is amplified by providing co-propagating and/or or counter-propagating pump light, usually provided by a pump laser or lasers. The wavelength of the pump light is typically around 100 nanometres shorter than the signal wavelength when using standard c-band channels in a Silica optical fibre as the gain medium. 
         [0005]    When optical path switching is used one piece of optical equipment could be used for both the primary and secondary optical fibres, as it is more efficient to be able to switch in these optical fibres to the same equipment, rather than using separate pieces of equipment.  FIG. 1  is a schematic illustration of such an amplifier node  100  in an optical network. The amplifier node  100  includes an optical fibre  110  coupled to an amplification device  101  such as an erbium-doped fibre amplifier (EDFA), which may be used in conjunction with the Raman amplifier. A switch  105  is coupled to the fibre  110  outside the amplifier node  100  and configured to switch optical paths between a primary optical fibre  102  and a secondary optical fibre  107 . The amplifier node  100  includes a Raman pump unit  120  including one or more light sources such as lasers  103 ,  104 . The lasers  103 ,  104  supply counter-propagating pump light  111  into the optical fibre  110  to induce Raman gain of an optical signal  108  passing along the primary or secondary optical fibre. The counter-propagating light travels in the opposite direction to the optical signal  108 . 
         [0006]    During the distributed Raman amplification process, very high optical powers are transmitted directly into the optical fibre  110 . As the switch  105  is placed outside the optical amplifier node  100  in  FIG. 1 , this requires the switch  105  to be able to cope with the high optical power from the lasers  103 ,  104 . In addition, the switch  105  introduces loss and therefore reduces the available pump power in the primary or secondary optical fibre. This arrangement also impacts the depolarisation of the lasers  103 ,  104 , increasing the Degree of Polarisation (DOP) of the lasers and reducing the effectiveness of the Raman amplification process. Furthermore, this arrangement adds risks and safety issues to operators working on or near the optical fibres  102 ,  107  where very high optical powers are used. 
         [0007]    An alternative solution to these problems is to locate the switch within the amplifier node  100 . However, in such a case, the switch would still need to support high optical powers, again resulting in high loss. This arrangement would also require a depolarisation technique for the lasers which is difficult to achieve and may need polarisation maintaining (PM) switches. 
       SUMMARY 
       [0008]    It would therefore be desirable to reduce the optical loss incurred by the switch between the primary and secondary optical fibres and to maintain the degree of depolarisation (DOP) of the lasers. 
         [0009]    According to one aspect of the present invention, there is provided a light source package for a Raman amplifier node having a primary optical fibre for carrying an optical signal and a secondary optical fibre for carrying the optical signal when the signal is rerouted from the primary optical fibre. The light source package comprises at least one primary light source coupled to the primary optical fibre for emitting light into the primary optical fibre when the optical signal is carried by the primary optical fibre to induce Raman gain of the optical signal. The package further comprises at least one secondary light source coupled to the secondary optical fibre for emitting light into the secondary optical fibre when the optical signal is carried by the secondary optical fibre to induce Raman gain of the optical signal, 
         [0010]    The at least one primary light source may be coupled only to the primary optical fibre and the at least one secondary light source may be coupled only to the secondary optical fibre. 
         [0011]    The light source package may be configured so that light can be emitted from either the primary or secondary light source. The light source package may be configured so that the at least one primary light source is not activated at the same time as the at least one secondary light source during normal operation. The light source may be configured so that the at least one secondary light source is activated at the same time as the at least one primary light source for a relatively short period while the output is switched from one light source to the other. 
         [0012]    The primary and secondary light sources may be configured as separate chips or stripes, or as a pair of stripes on the same chip. 
         [0013]    A Raman amplifier node may comprise the light source package described above. The amplifier node may comprise an optical switch for rerouting the optical signal from the primary optical fibre to the secondary optical fibre or vice versa. The switch may be arranged upstream of an injection point of the light injected by the primary and secondary light sources so that light injected by the primary and secondary sources does not pass through the switch. The amplifier node may further comprise a controller configured to control the operation of the primary and secondary light sources and the switch. 
         [0014]    The primary and secondary light sources may comprise lasers. 
         [0015]    According to another aspect of the present invention there is provided an optical apparatus comprising a primary optical fibre for carrying an optical signal. The primary optical fibre includes a primary Raman amplifier section. The apparatus further comprises a secondary optical fibre for carrying the optical signal when the signal is rerouted from the primary optical fibre. The secondary optical fibre includes a secondary Raman amplifier section. The apparatus further comprises a pump node comprising at least one primary light source coupled to the primary optical fibre for emitting light into the primary optical fibre when the optical signal is carried by the primary optical fibre to induce Raman gain of the optical signal. The pump node further comprises at least one secondary light source coupled to the secondary optical fibre for emitting light into the secondary optical fibre when the optical signal is carried by the secondary optical fibre to induce Raman gain of the optical signal. 
         [0016]    According to another aspect of the present invention there is provided a method of switching optical paths between a primary optical fibre and a secondary optical fibre in a Raman amplifier node, the primary optical fibre carrying an optical signal. The method comprises emitting light into the primary optical fibre from a primary light source coupled to the primary optical fibre to induce Raman gain of the optical signal in the primary optical fibre. The method further comprises rerouting the optical signal into the secondary optical fibre and emitting light into the secondary optical fibre from a secondary light source coupled to the secondary optical fibre to induce Raman gain of the optical signal in the secondary optical fibre. 
         [0017]    The primary light source may be deactivated when the secondary light source is activated. The secondary light source may be activated at the same time as the primary light source for a relatively short period while the output is switched from one light source to the other. 
         [0018]    The invention also provides a computer program, comprising computer readable code which, when run by a controller of a Raman amplifier node, causes a light source package to behave as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a schematic illustration of an amplifier node; 
           [0021]      FIG. 2  is a schematic illustration of an amplifier node with a Raman pump unit having two primary lasers and two secondary lasers; 
           [0022]      FIG. 3  is a schematic illustration of an alternative amplifier node; 
           [0023]      FIG. 4  is a schematic illustration of an alternative amplifier node; 
           [0024]      FIG. 5  is a schematic illustration of an optical apparatus; and 
           [0025]      FIG. 6  is a flow diagram illustrating the steps involved in emitting light into the primary and secondary optical fibres. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 2  is a schematic illustration of an amplifier node  200  with a Raman pump unit  220  having two primary lasers  203 ,  212  and two secondary lasers  204 ,  213 . The node  200  includes a switch  205 , optionally coupled to an EDFA or other amplification device  201 . The node also includes a primary optical fibre  202  and a secondary optical fibre  207 . The switch  205  is configured to switch optical paths between the primary optical fibre  202  and the secondary optical fibre  207 . An optical signal  208  normally passes along the primary optical fibre  202 . However, when a maintenance or restoration process is required for the primary optical fibre  202 , the switch  205  allows the optical signal  208  that was coming from the primary optical fibre  202  into the amplifier  202  to now pass the optical signal  208  on the secondary optical fibre  207  to pass into the amplifier  201 . The lasers  203 ,  204 ,  212 ,  213  are capable of supplying counter-propagating light  210 ,  211  into the optical fibres  202 ,  207 . The primary lasers  203 ,  212  are arranged to inject light  210  into the primary optical fibre  202 , and the secondary lasers  204 ,  213  are arranged to inject light into the secondary optical fibre  207 . During normal operating conditions (i.e. when the optical signal  208  is transmitted along the primary fibre  202 ), the primary lasers  203 ,  212  are activated to inject light into the primary optical fibre  202 . When any maintenance or restoration process is required and the signal  208  is transmitted along the secondary fibre  207 , the primary lasers  203 ,  212  are deactivated, and the secondary lasers  204 ,  213  are activated to inject light into the secondary fibre  207 . The operation of the lasers may be controlled by a controller (not shown in  FIG. 2 ). The switch  205  is arranged downstream of the lasers (in the direction of travel of the optical signal  208 ) so that the high power light injected into the optical fibres  202 ,  207  by the lasers  203 ,  204 ,  213 ,  214  does not pass through the switch  205 . This can also be considered to be upstream in the sense of the injected pump light  210 ,  211 . 
         [0027]    Since separate lasers are provided for the primary and secondary optical fibres  202 ,  207 , there is no need to have a switch outside the amplifier system  200  as in the case of  FIG. 1 . The arrangement of  FIG. 2  is advantageous because it does not have the high optical power loss and the degradation to DOP which are present in the arrangement of  FIG. 1 . However since four lasers  203 ,  204 ,  212 ,  213  are used, this arrangement may require extra physical space and added cost. 
         [0028]    It will be noted that the arrangement of  FIG. 2  uses four lasers  203 ,  204 ,  212 ,  213  which supply counter-propagating light  210 ,  211 . However, it will be appreciated that it is also possible to use any number of lasers supplying co- or counter-propagating light into the fibres  202 ,  207  (co-pump light is light which travels in the same direction as the optical signal  208  passing along the fibres  202 ,  207 ). 
         [0029]      FIG. 3  is a schematic illustration of an alternative amplifier node  300  incorporating a Raman pump unit  320 . Many features of the arrangement of  FIG. 3  have similar features to those of the arrangement of  FIG. 2 , i.e. an amplification device  301 , a primary optical fibre  302  and a secondary optical fibre  307 , and a switch  305  between the optical fibres  302 ,  307 . However, the lasers are configured differently in this arrangement. The amplifier node  300  includes two laser packages  315  and  316 . Each laser package  315 ,  316  includes a primary laser  303 ,  312  and a secondary laser  304 ,  313  integrated into the laser package  315 ,  316 . Essentially, there are two laser “stripes” integrated in the laser package. The design of the laser including laser chip, coupling optics, heat management and package could be of many varieties that best support the specific application. An example embodiment is incorporation of two independent 14XX laser stripes on a single thermo electric coupler (TEC) coupled submount with two independent fibres exiting a single industry standard  14 -pin butterfly package. 
         [0030]    In  FIG. 3 , the lasers  303 ,  304 ,  312 ,  313  are configured to inject counter-propagating light  310 ,  311  into the optical fibres  302 ,  307 . The primary lasers  303 ,  312  of both laser packages  315 ,  316  are configured to inject light  310  into the primary fibre  302 . The secondary lasers  304 ,  313  of both laser packages  315 ,  316  are configured to inject light  311  into the secondary fibre  307 . 
         [0031]    The amplifier node  300  of  FIG. 3  also includes a controller  350  operatively connected to the laser packages  315 ,  316  and the switch  305 . The controller  350  controls the operation of the lasers and the switch  305 . The controller  350  ensures that only one of the lasers of each laser package  315 ,  316  is activated at any one time in normal operation and controls how the lasers are switched. For example, when the primary optical fibre is in use, the primary lasers  303 ,  312  of both laser packages  315 ,  316  are activated by the controller  350 . When a maintenance or restoration process is required to the primary fibre  302 , the controller  300  operates the switch  305  to select the optical path through the secondary optical fibre  307 . The controller  350  also activates the secondary lasers  304 ,  313  of both laser packages  315 ,  316  and turns off the primary lasers  303 ,  312 . 
         [0032]    The laser packages  315 ,  316  of  FIG. 3  may also include a TEC  355  to control the temperature of the laser chips. It will be appreciated that there could be any number of laser packages in the Raman pump unit  320  in which each package may have at least one primary laser chip and at least one secondary laser chip. It will be also appreciated that there could be any number of optical fibres in the amplifier node  300  and the lasers would be able to inject light into any of those optical fibres as necessary. 
         [0033]    The arrangement of  FIG. 3  is advantageous because the package size is much smaller than the individual lasers used in the arrangement of  FIG. 2 . A laser package having two laser chips can be termed as a “dual chip pump”. The use of this dual chip pump is different from a conventional dual chip pump since only one laser chip needs to be activated at any time during normal use. As a result, the TEC  355  in the laser package needs to control the temperature only one laser chip at any one time, rather than controlling the temperature of both laser chips in the laser package at the same time. In addition, the switching time can be significantly faster than when using the optical switch outside the amplifier node (as with the case of  FIG. 1 ). 
         [0034]      FIG. 4  is a schematic illustration of an alternative amplifier node  400 . Many features of the arrangement of  FIG. 4  have features similar to those of the arrangement of  FIG. 3 , i.e. a Raman pump unit  420 , an amplification device  401 , a primary fibre  402  and a secondary fibre  407 , a switch  405  between the primary fibre  402  and the secondary optical fibre  407 , laser packages  415 ,  416  and two laser chips  403 ,  404 ;  412 ,  413  in each laser package  415 ,  416 . However, the lasers  403 ,  404 ;  413 ,  416  of each laser package  415 ,  416  are configured to inject co-propagating light  410 ,  411  into the fibres  402 ,  407 , which travel in the same direction as an optical signal  408  passing along the fibres  402 ,  407 . This arrangement provides the same advantages as those discussed with reference to the arrangement of  FIG. 3 . 
         [0035]      FIG. 5  is a schematic illustration of an optical apparatus  500 , which illustrates how the pump unit of  FIG. 4  interfaces with Raman amplifiers in practice. The optical apparatus includes a primary optical fibre  502  including a primary Raman amplifier section  550 , and a second optical fibre  507  including a second Raman amplifier section  551 . The apparatus  500  also includes a Raman amplifier node  560 . All the features of the amplifier node  560  of  FIG. 5  are the same as the amplifier node  400  of  FIG. 4 , i.e. a Raman pump unit  520 , an amplification device  501 , a switch  505  between the primary optical fibre  502  and the secondary optical fibre  507 , laser packages  515 ,  516  and two co-propagating laser chips  503 ,  504 ;  512 ,  513  in each laser package  515 ,  516 . This arrangement provides the same advantages as those discussed with reference to the arrangement of  FIG. 3  or  4 . 
         [0036]      FIG. 6  is a flow diagram illustrating an example of the steps involved in switching optical paths between the primary and secondary optical fibres in the arrangement of  FIG. 3  or  4 . 
         [0037]      600 : The primary optical fibre carries an optical signal and the primary laser emits light into the primary optical fibre. 
         [0038]      601 : A failure occurs in the primary fibre or a decision is taken to carry out maintenance of the primary fibre. 
         [0039]      602 : The switch reroutes the optical signal from the primary optical fibre to the secondary optical fibre. 
         [0040]      603 : The controller deactivates the primary laser and activates the secondary laser to emit light into the secondary fibre. 
         [0041]    It is noted that it could also be possible to turn on the secondary laser prior to turning off the primary laser during the process for a brief period of time. This will maintain Raman gain along the fibres for longer although the thermal management may need careful attention. 
         [0042]    It will be noted that the foregoing description is directed to arrangements having a switch downstream of the lasers (in the direction of travel of the optical signal). However, it will be appreciated that the Raman amplifier node may not have a switch. It is possible that a switch may be located in the optical network (not in the amplifier node) for rerouting the optical signal between optical fibres. In such a case, the amplifier node simply comprises the primary and secondary optical fibres, and the primary or secondary laser of the Raman pump unit is activated as required to inject light into the primary or secondary optical fibre. 
         [0043]    Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.