Abstract:
The invention provides an optical shutter ( 400 ) for a communication system of a type comprising first (B 1 , B 2 ) and second (A 1 , A 2 ) communication paths along which information-bearing radiation propagates in opposite directions. The shutter ( 400 ) comprises: an optical tap ( 440 ) and a power monitor ( 430 ) for monitoring power of information-bearing radiation propagating along the first path (B 1 , B 2 ) and for generating a corresponding radiation power indicative signal; a control unit ( 420 ) for comparing the indicative signal with a threshold value to generate a control signal (control); and shutter switch ( 410 ) for selectively substantially transmitting or blocking radiation propagating along the second path (A 1 , A 2 ) in response to the control signal.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an optical shutter, in particular, but not exclusively, to an optical shutter for shutting down one or more communication system optical paths in the event of system failure, for example in the event of an optical fibre waveguide break. 
     Contemporary optical communication systems comprise a plurality of spatially disposed nodes interlinked through optical fibre waveguides, the waveguides operable to convey modulated information-bearing optical radiation between the nodes. The information is typically partitioned into channels, each channel having a range of radiation wavelengths associated therewith; such partitioning is known as wavelength division multiplexing (WDM). 
     In the aforementioned communication systems, it is established practice to arrange the nodes in ring groupings. Optical fibre waveguides linking adjacent nodes in the ring groupings are known as sections. Where such groupings each have a diameter in the order of several kilometers, the groupings are known as metro-rings. The metro-rings are often of a sufficiently small diameter to circumvent the need for optical amplification therein, namely the rings are passive. When the metro-rings are somewhat larger, one or more optical amplifiers are typically incorporated therein. Optical amplifiers, for example erbium doped fibre amplifiers (EDFA), are costly items and therefore are only employed when absolutely necessary to ensure an acceptable system optical signal-to-noise ratio. 
     It is conventional practice for each optical communication system ring to employ two optical fibre loops, one loop for conveying WDM radiation in a clockwise direction therearound and another loop for conveying WDM radiation in a counter clockwise direction therearound. Duplication of loops assists to ensure that communication between nodes in the ring is sustained in the event of one of the loops being disabled. The loops are operable to convey WDM radiation which is often an aggregate of radiation from a plurality of modulated laser sources or an output of one or more EDFAs; this aggregate radiation can often have a power in the order of several 10&#39;s of mW which corresponds to Class IIIA or Class IIIB laser radiation power. Such power levels can be hazardous and hence, for safety reasons, it is conventional practice to have automatic shutdown facilities in the event of one or more of the loops breaking, for example due to fracture of an optical fibre waveguide being detected. 
     Automatic shutdown is normally achieved in an optical communication system by turning off an EDFA feeding radiation into a section when the system detects a severe reduction or loss of optical radiation power received from that section. 
     In the aforementioned metro-rings, it is conventional practice to insert passive nodes on optical fibre waveguides associated with sections, such passive nodes being devoid of optical amplification. Where normal conventional section shutdown operates on EDFAs feeding into these sections, this shutdown results in a cessation of radiation transmission into the sections. Where the WDM radiation includes a protection channel in addition to normal working channels, shutdown has the effect of disabling both working and protection channels so that all communication traffic to the passive nodes fails even in the event of one of the fibre paths to each of the passive nodes remaining intact. Thus, shutdown of whole sections in a ring grouping results in a potentially unnecessarily severe reduction of services provided through the passive nodes. 
     In ring groupings of nodes comprising passive nodes included in sections between active nodes responsible for implementing automatic shutdown, it is possible for the passive nodes to insert sufficient radiation power for the active nodes to interpret such radiation power as being a normal working situation in which case the active nodes will fail to apply automatic shutdown even though an optical fibre waveguide break has occurred. In this situation, active nodes do not shut down their optical amplifiers and so dangerous levels of radiation are launched into fibre waveguides towards breaks. 
     Thus, the inventor has appreciated that there is a problem as elucidated in the foregoing associated with conventional automatic shutdown, especially where passive nodes are incorporated into sections of optical communication systems. 
     The inventor has devised a solution which addresses the aforementioned problem, the solution involving the incorporation of one or more optical shutters into passive nodes inserted into sections of the optical communication systems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a node for a WDM communication system, the node being connectable to other nodes of the communication system by first and second optical paths along which WDM radiation propagates in opposite directions between the nodes, the node comprising: add/drop multiplexing means for adding and dropping at least one WDM channel at the node; characterised by at least one optical shutter comprising: measuring means for monitoring power of WDM radiation input to the node along the first path and for generating a corresponding radiation power indicative signal; controlling means for comparing the indicative signal with a threshold value to generate a control signal; and switching means for selectively substantially transmitting or blocking WDM radiation output from the node along the second path in response to the control signal. 
     The inclusion of the shutter provides the advantage that it is capable of attenuating dangerous levels of radiation being output from the node along the second path in an event of a break or fracture occurring in one or more of the first and second paths. 
     Preferably, the controlling means and the switching means are operable to block WDM radiation output along the second path when the indicative signal falls in magnitude to less than the threshold value. A condition where the indicative signal is less than the threshold value can correspond to a break in the first path, hence the shutter can be effective at preventing dangerous levels of radiation passing through the switching means towards such a break which may also have occurred in the second path. 
     Conveniently, when constructing the shutter, the measuring means includes an optical coupler and a radiation detector, the coupler being operable to divert a portion of the WDM radiation to the detector for detection therein to generate the indicative signal. Use of such a coupler can provide the benefit that only a small perturbation occurs to the WDM radiation propagating along the first path. 
     For practical reasons, the coupler conveniently comprises one or more of a fusion spliced fibre coupler, and a 1×N optical waveguide coupler for diverting a portion of the information-bearing radiation to the detector. These couplers are especially suited for use in the shutter for reasons of their low relative cost and relatively low insertion loss. 
     Preferably, the switching means comprises one or more of a liquid crystal optical attenuator, a thermally controlled optical attenuator, a charge carrier dispersion modulator, an electromechanical optical switch or an optical micromachined optical structure (MEMs) device. These attenuators and modulator are, for practical reasons, well suited for use in attenuating WDM radiation. 
     In WDM communication system, it is desirable also to communicate between nodes, supervisory information for supervising control of the system. To enable communication of such supervisory information, it is preferable that the measuring means is succeeded along the first path by radiation dropping means for diverting a portion of the WDM radiation corresponding to a supervisory channel bearing communication system supervisory information for the first path, the supervisory information for use in controlling the system. Additionally, the switching means is preferably succeeded along the second path by radiation adding means for adding radiation corresponding to a supervisory channel to the second path. The latter provides the advantage that in the event of WDM radiation being blocked by the switching means the node continues to attempt to transmit at least the supervisory channel. 
     Preferably, the multiplexing means is operable to add and drop a working channel and a protection channel from the first and second optical paths. The node thereby is capable of providing communication access to the system. Moreover, in order to improve reliability of the system, the node is operable to use the protection channel for communication when the working channel is inoperative. 
     Beneficially, to reduce cost, the node is advantageously a passive node, namely it is devoid of optical amplification for amplifying radiation propagating in the first and second optical paths. The node of the present invention finds particular application in WDM communication system comprising a ring configuration. In such system configuration the first and second optical paths to which the node is connectable comprise clockwise and anti-clockwise communications paths. Protected Communication rings incorporating clockwise and anti-clockwise paths provide the benefit of improved reliability in the event of one of the paths becoming defective, for example due to an optical fibre waveguide break. Preferably the communication ring is a metro ring. 
     Advantageously the node includes a respective optical shutter that is operable to selectively transmit or block WDM radiation output from the node along the first and second optical paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which: 
         FIG. 1  is an illustration of a prior art optical fibre waveguide section interlinking two active nodes of a communication system, the section including a passive node; 
         FIG. 2  is an illustration of the section in  FIG. 1  subject to a break in one of its fibre waveguides; 
         FIG. 3  is an illustration of the section in  FIG. 1  subject to a double break of its fibre waveguides; 
         FIG. 4  is a schematic diagram of an optical shutter according to the invention; 
         FIG. 5  is a schematic diagram of a passive node included in a section between active nodes, the passive node including two shutters of a type shown in  FIG. 4 ; and 
         FIG. 6  is a schematic diagram of the section in  FIG. 5  subject to two fibre breaks. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order that the invention is better understood the problems arising in known optical communication systems with automatic shutdown when a fibre waveguide break or optical amplifier failure occurs will firstly be described. 
     In  FIG. 1 , there is shown first and second active nodes  10 ,  20  respectively of an optical communication system, and a passive node  30  interposed in a section linking the active nodes  10 ,  20  together. The passive node  30  incorporates first and second passive add/drop optical multiplexers  40 ,  50  respectively. Moreover, the active nodes  10 ,  20  include first and second optical amplifiers  60 ,  70  respectively. 
     In the section, there are four optical fibre waveguides, namely a first waveguide  80 , a second waveguide  90 , a third waveguide  100  and a fourth waveguide  110 . The first waveguide  80  is connected from the first amplifier  60  to an optical input port of a first multiplexer  40 . The second waveguide  90  is connected from an optical output port of the first multiplexer  40  to an optical input port of the second node  20 . The third waveguide  100  is connected from an optical output port of the second amplifier  70  to an optical input port of the second multiplexer  50 . The fourth waveguide  110  is connected from an optical output of the second multiplexer  50  to an optical input port of the active node  10 . As will be appreciated the first and second waveguides  80 ,  90  constitute a first transmission path from the first node  10  to the second node  20  and the third and fourth waveguides  100 ,  110  constitute a second transmission path from the second node  20  to the first node  10 . 
     The multiplexers  40 ,  50  each comprise dielectric filters and are operable to divert a working channel present in WDM radiation propagating through the section out from the passive node  30  and also to add optical radiation at a wavelength range corresponding to the working channel. Moreover, each multiplexer  40 ,  50  is also operable to divert a protection channel present in the WDM radiation propagating through the section out from the passive node  30  and also to add optical radiation at a wavelength range corresponding to the protection channel. Furthermore, each multiplexer  40 ,  50  is additionally operable to divert at least part of the radiation of a supervisory channel present in the WDM radiation for reception at supervisory electronic circuits (not shown) included within the node  30 . 
     In operation, WDM information-bearing radiation comprising a number of channels is output from the first amplifier  60  and propagates along the first waveguide  80  to the add/drop multiplexer  40 . The multiplexer  40  isolates radiation corresponding to the worker channel, to the protection channel and also to the supervisory channel and outputs, namely drops, the radiation at the passive node  30 . Moreover, the multiplexer  40  also receives radiation input to the passive node  30  at wavelengths corresponding to the working channel, the protection channel and the supervisory channel, and then adds it to radiation propagating through the multiplexer  40  to provide output radiation which is launched into the second waveguide  90  for propagation to the second active node  20 . 
     In a similar manner, WDM information-bearing radiation comprising a number of channels is output from the second amplifier  70  and propagates along the third waveguide  100  to the second add/drop multiplexer  50 . The multiplexer  50  isolates radiation corresponding to the worker channel, to the protection channel and to the supervisory channel and outputs, namely drops, the radiation at the passive node  30 . Moreover, the multiplexer  50  also receives radiation input to the passive node  30  at wavelengths corresponding to the working channel, the protection channel and the supervisory channel, and then adds it to radiation propagating through the multiplexer  50  to provide output radiation which is launched into the fourth waveguide  110  for propagation to the first active node  10 . 
     The supervisory electronic circuits receive radiation of the supervisory channel dropped at the passive node  30  and interpret instructions conveyed therein which are pertinent to functioning of the node  30 . These instructions determine whether or not the protection channel is to be used and in which direction radiation corresponding to the working channel or the protection channel is to be directly, namely towards the first active node  10  via the second multiplexer  50  or towards the second active node  20  via the first multiplexer  40 . 
     Referring now to  FIG. 2 , the nodes  10 ,  20 ,  30  and the fibre waveguides  80 ,  90 ,  100 ,  110  are shown as in  FIG. 1  but with the waveguide  80  subject to a break indicated by  200 , the break  200  causing an interruption of radiation propagation from the amplifier  60  along the waveguide  80  to the first multiplexer  40 . A reduction, as a consequence of the break  200 , in radiation received at the second active node  20  is detected which causes it to switch off its amplifier  70  leaving the passive node  30  isolated without communication service provided thereto. The first active node  20  also detects a reduction in received radiation from the amplifier  70  which causes the node  10  to switch off its amplifier  60 , thereby preventing dangerous power levels of optical radiation being output at the break  200 . 
     In  FIG. 2 , if sufficient radiation is input to the passive node  30  such that the second active node  20  interprets this as normal radiation level, there is a risk that the second active node  20  fails to implement the automatic shutdown with a result that dangerous power levels of radiation are output at the break  200 . 
     Referring now to  FIG. 3 , the nodes  10 ,  20 ,  30  and the fibre waveguides  80 ,  90 ,  100 ,  110  are shown as in  FIG. 1  but with the waveguides  80 ,  110  subject to first and second breaks  300 ,  310  respectively. The first break  300  causes an interruption of radiation propagation from the amplifier  60  along the waveguide  80  to the first multiplexer  40 . Moreover, the second break  310  causes an interruption of radiation propagation from the second amplifier  70  via the second multiplexer  50  to the first active node  10 . 
     When the breaks  300 ,  310  occur, the second active node  10  detects a reduction in received power and proceeds to switch off its amplifier  70 . Likewise, the first active node  10  also detects a reduction in received power and switches off its amplifier  60 . 
     If, in  FIG. 3 , sufficient radiation power is input at the passive node  30  and conveyed to the second active node  20 , the second node  20  can interpret this radiation as arising from the amplifier  60 . As a consequence, the second node  20  would continue to output radiation at its amplifier  70  with a result that dangerous levels of radiation would be output at the second break  310 . 
     It will be appreciated from the foregoing with reference to  FIGS. 2 and 3  that conventional automatic shutdown in response to one or more fibre waveguide breaks can result in loss of communication to the passive node  30  even though some of the waveguides are still potentially available for conveying information traffic to the passive node  30 . Moreover, a dangerous situation can arise where sufficient radiation power is input at the passive node  30  to prevent automatic shutdown being applied which results in dangerous levels of radiation being sustained at fibre waveguide breaks. 
     In order to address shortcomings of conventional automatic shutdown, the inventors have devised an optical shutter which can be incorporated into the passive node  30 . The shutter is illustrated in  FIG. 4  and indicated generally by  400 . The shutter  400  comprises an optical shutter switch  410 , a control unit  420 , a power monitor  430  and an optical tap  440 . Moreover, the shutter  400  can optionally also include a supervisory channel add coupler  450  and a supervisory channel drop coupler  460 . 
     Interconnection within the shutter  400  will now be described. The shutter  400  includes first and second optical input ports A 1 , B 1  respectively and also first and second optical output ports A 2 , B 2  respectively. The first input port A 1  is coupled to an optical input port of the shutter switch  410 , and an output port of the switch  410  is connected via the supervisory channel add coupler  450  to the first output port A 2 . Moreover, the second input port B 1  is connected to an optical input port of the optical tap  440 . Furthermore, an optical output port of the optical tap  440  is coupled via a the supervisory drop coupler  460  to the second output port B 2 . 
     An optical monitor output of the tap  440  is connected to an optical input of the power monitor  430 . The monitor  430  includes a power monitor output which is connected to an input of the control unit  420 . A control output of the control unit  420  is connected to a control input of the optical shutter  410 . 
     The optical tap  440 , the supervisory channel add coupler  450  and the supervisory channel drop coupler  460  are preferably fusion-spliced couplers although other types of couplers can alternatively be used. The optical shutter  410  is preferably implemented as a liquid crystal shutter. Moreover, the control unit  420  is preferably implemented in hardware, although it can alternatively be a software function. Furthermore, the power monitor  430  includes an optoelectronic detector and conditioning circuits to provide an output for the control unit  420 . 
     Operation of the shutter  400  will now be described with reference to  FIG. 4 . WDM communication-traffic modulated radiation received at the first input port A 1  propagates through the shutter switch  410  suffering attenuation in the order of 0.5 dB therein and onwards to the supervisory channel add coupler  450 . Supervisory channel radiation is added to radiation from the shutter unit  410  at the coupler  450  and propagates as aggregate radiation to the first output port A 2 . 
     Radiation received at the second input port B 1  propagates to the optical tap  440  whereat a portion of the received radiation, for example 20% or less, is diverted to the power monitor  430 . A remaining portion of the received radiation, for example 80% or more, is transmitted to the supervisory drop coupler  460 . A portion of the radiation received at the coupler  460  corresponding to the supervisory channel is diverted at the coupler  460  whereas a remainder of the radiation propagates to the second output B 2 . 
     Radiation diverted at the optical tap  440  propagates to the power monitor  430  whereat it is received and a corresponding radiation power indicative signal generated. The indicative signal passes to the control unit  420  which is operable to compare the magnitude of the indicative signal with a power threshold value to determine whether or not a fibre break has occurred. If a fibre break is deemed to have occurred, the control unit  420  outputs a control signal to the shutter switch  410  to switch it from a substantially transparent state, namely preferably with less than 1 dB of attenuation therethrough, to a non-transmissive state, namely with the order of 25 dB or more attenuation therethrough. 
     Referring next to  FIG. 5 , there is shown the passive add/drop node  30  modified to include two shutters  500 ,  510 , each of the shutters  500 ,  510  being identical to the shutter  400  in  FIG. 4 . The shutters  500 ,  510  are included within the section between the active nodes  10 ,  20 , namely at both sides of the passive node  30 . If necessary, for example to reduce cost, one of the shutters  500 ,  510  can optionally be omitted. 
     In  FIG. 6 , there is depicted a situation where first and second breaks  600 ,  610  have occurred in the first and fourth fibre waveguides  80 ,  110 . The first amplifier  60  emits radiation into the first waveguide  80  which propagates to the break  600 . As a consequence of the break  600 , no radiation is received at the optical tap  440  of the shutter  500 ; the power monitor  430  of the shutter  500  detects a lack of radiation power and switches its associated shutter switch  410  of the shutter  500  from a substantially transmissive state to an opaque non-transmissive state, namely to a blocking state. The first active-node  10  detects a reduction of power received at its input port and proceeds to implement an automatic shutdown procedure to switch off the first amplifier  60 . As a result of the automatic shutdown and blocking by the shutter switch  410 , radiation is prevented from reaching the breaks  600 ,  610 . If sufficient radiation power is input at the passive node  30  to cause the second active node  20  to interpret the power received thereat as originating from the amplifier  60  of the first node  10 , the shutter  500  prevents radiation output from the amplifier  70  of the second active node  20  reaching the second break  610 . 
     In a situation of the two breaks  600 ,  610  occurring, communication between the second active node  20  and the passive node  30  can be maintained either by ensuring that the second node  20  receives sufficient radiation so that its automatic shutdown does not switch off the amplifier  70  or by sending instructions from the passive node  30  via the supervisory channel to the second active node  20  to override its automatic shutdown. By such an approach, communication to the passive node  30  can be maintained even in the event of the section on one side of the passive node  30  being disabled by the two breaks  600 ,  610 ; moreover, dangerous levels of radiation are also prevented from reaching the breaks  600 ,  610 . 
     When the breaks  600 ,  610  occur, the section is re-started by applying a special re-start procedure involving sending test pulses, either automatically or under operator control. The test pulses are generated by temporarily switching the shutter switches  410  subject to shutdown to be momentarily substantially transparent, namely with preferably less than 1 dB attenuation therethrough. For example, when the breaks  600 ,  610  have been repaired, the shutter switch  410  of the shutter  510  can be made momentarily substantially transparent to allow one or more pulses of radiation through the multiplexer  50  to the first active node  10 ; the first node  10  detects radiation pulses received thereat and, if the pulses are of a magnitude greater than a threshold level, deems the fibre waveguide  110  to be intact. The first node  10  then proceeds to emit radiation into the fibre waveguide  80  which the optical tap  440  of the shutter  500  and its associated power monitor  430  and control unit  420  detect as exceeding a threshold value; the control unit  420  of the shutter  500  then proceeds to switch the shutter switch  410  to be continuously substantially transparent, thereby reestablishing communication from the first active node  10  to the passive node  30 . 
     Alternatively, test pulses can be sent on the supervisory channel to re-establish communication. When supervisory radiation power is received, the shutter  500  is made transparent. Supervisory power is either detected by re-establishment of the supervisory channel or as optical power received at the power monitor  430  of the shutter  500 . 
     It will be appreciated by those skilled in the art of optical communication system design that modifications and variations can be made to the shutter  400  and also to the passive node  30  incorporating one or more of the shutters  400  without departing from the scope of the invention. For example, although the control unit  420  is implemented in hardware, it may alternatively be implemented as a software function. Moreover, the shutter switch  410  is implemented as a liquid crystal device but can be implemented using other types of technology, for example charge carrier dispersion modulators utilising the Kronig-Kramer absorption phenomenon, a thermally driven attenuator, an electromechanical optical switch or an optical micromachined structure (MEMs) device. Furthermore, the optical tap can be implemented as a fusion-spliced fibre coupler or, alternatively, as a 1×N optical waveguide coupler of a type as described in a United States patent U.S. Pat. No. 4,950,045. It is also envisaged that a coupler of the type described in United States patent U.S. Pat. No. 5,410,625 could be employed. 
     The shutter of the present invention finds particular application in the nodes of an optical fibre ring transmission system having first and second transmission paths, typically two optical fibres, interconnecting the nodes and in which radiation is carried along the paths in counter propagating directions to provide a protection path in the event of a fibre break. More especially, although not exclusively, the present invention is particularly suited to metro-rings which are of a sufficiently short circumference (path length) to circumvent the need for optical amplification within the ring.