Abstract:
Optical safety functions are incorporated into protection switching modules which maintain redundant pathways to avoid interruptions in optical network connections. The optical safety functions which lower optical power levels upon interruptions of optical connections are effectively combined with protection switching procedures which are also triggered by interruptions in optical network connections. The interoperation of protection and safety processes keep optical power levels below hazardous levels at system points which might be accessible to human operators.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is related to optical networks and, more particularly to, safety and protection measures and procedures in such networks. 
         [0002]    When a fault occurs in the transmission of signals from a source node to a destination node, such as a break in an optical fiber, the protection measures of the optical network cause the rerouting of the signals to ensure the delivery of optical signals to their destination. There are different ways to protect optical networks against faults. One protection mechanism used in point-to-point links is the 1+1 mechanism in which the source node sends duplicate signals on two separate fibers to the destination node. The destination node receives the optical signals over one fiber, called the working fiber, and switches to the other fiber, called the protection fiber, in case a fault occurs with the first fiber to continue receiving the signals. Another protection mechanism is the 1:1 mechanism (a special case of 1:N protection) in which the source node sends optical signals over the working fiber to the destination node. In case of a fault in the transmission, the source node then switches the transmission of optical signals to the protection fiber. (In the 1:N mechanism there is one protection fiber for N working fibers.) 
         [0003]    The sources of these signals for optical networks are lasers. While the lasers used in optical networks have relatively low power compared to, for example, industrial lasers, they are powerful enough to damage the human eye. Therefore safety measures are required for optical networks to avoid injury to human operators and service personnel. A typical safety protocol is OFC (Open Fiber Control) which has measures to detect cuts in optical fiber links, turn off the lasers connected to the link, and then allow low-level laser pulses intermittently on the cut fiber link for the link to be brought back into full operation upon the repair of the cut fiber. 
         [0004]    In optical networks modularization of components is highly desirable because of ease of maintenance and repair and these protection and safety measures have been installed in separate modules in accordance with the function of the modules. However, optical networks still have problems in implementing protection and safety measures. 
         [0005]    The present invention is directed toward the optimum and practical realization of protecting the integrity of network signal delivery and ensuring the safety of human operators. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  is a representation of an optical network node with amplifier module;  FIG. 1B  is a state diagram of the safety operations of the amplifier module. 
           [0007]      FIG. 2A  is a representational diagram of protection module for an optical network node;  FIG. 2B  is a state diagram of the  FIG. 2A  protection module. 
           [0008]      FIG. 3  is a representational diagram of a physical combination of the  FIG. 1A  amplifier module and the  FIG. 2A  protection module for an optical network node. 
           [0009]      FIG. 4  is a representational diagram of a combination of the  FIG. 1A  amplifier module and a protection module according to one embodiment of the present invention. 
           [0010]      FIG. 5  is a logic state diagram for the protection module in the  FIG. 4  combination for safety and protection operations which avoids the deficiencies of a straightforward combination of modules, in accordance to one embodiment of the present invention. 
           [0011]      FIG. 6  is a representational diagram of the control unit of the protection module of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Overview 
       [0012]    In an overview of the present invention: 
         [0013]    One aspect provides for a method of operating an interface between a network node and an optical fiber link to a remote node at an opposite end of the link. The link has first and second optical fiber loop and each optical fiber loop has a transmitting optical fiber and a receiving optical fiber. The method comprises the steps of: transmitting signals from the node through transmitting optical fibers of both of the first and second optical fiber loops; receiving signals for the node through one of the receiving optical fibers of the first and second optical fiber loops; monitoring receiving optical fibers of both of the first and second optical fiber loops; detecting a fault on one of the receiving optical fibers of the first and second optical fiber loops; switching off a transmitting optical fiber of the optical fiber loop to which the fault-detected receiving optical fiber belongs; maintaining reception of signals for the node through the one of the receiving optical fibers of said first and second optical fiber loops if the fault-detected receiving optical fiber is the other receiving optical fiber of the first and second optical fiber loops, or switching reception of signals for the node through the other receiving optical fiber of the first and second optical fiber loops if the fault-detected receiving optical fiber is the one receiving optical fiber of the first and second optical fiber loops; and maintaining transmission of signals from the node through a transmitting optical fiber of an optical fiber loop to which the fault-detected receiving optical fiber does not belong; whereby the interface performs both protection and safety measures for the network node. 
         [0014]    Another aspect provides for an interface between a network node and an optical fiber link to a remote node at an opposite end of the link. The link has a first and second optical fiber loops with each optical fiber loop having a transmitting optical fiber and a receiving optical fiber. The interface comprises: a network node input port for receiving optical signals from the network node; a network node output port for transmitting optical signals to the network node; first and second optical fiber link output ports for sending optical signals from the network node input port to the transmitting optical fibers of the first and second optical fiber loops to the remote node; first and second optical fiber link input ports for receiving optical signals from the receiving optical fibers of the first and second optical fiber loops and passing the optical signals to the network node output port; a plurality of VOAs (Variable Optical Attenuators) controlling the strength of signals on the first and second optical fiber link output ports; and a control unit controlling the plurality of VOAs so that upon detection of a fault on one of the receiving optical fibers of the first and second optical fiber loops, a transmitting optical fiber of the optical fiber loop to which the fault-detected receiving optical fiber belongs is switched off. The interface also has an optical switch connected between the first and second optical fiber link input ports, and the network node output port. The control unit controls the switch so that upon the detection of the fault on the one of the receiving optical fibers of the first and second optical fiber loops, the switch directs optical signals from the other of the receiving optical fibers of the first and second optical fiber loops to the network node output port. 
         [0015]    Still another aspect provides for an interface between a network node and an optical fiber link to a remote node at an opposite end of the link, the link having a first and second optical fiber loops and each optical fiber loop having a transmitting optical fiber and a receiving optical fiber. The interface comprises: means for transmitting signals from the node through transmitting optical fibers of both of the first and second optical fiber loops; means for receiving signals for the node through one of the receiving optical fibers of the first and second optical fiber loops; means for monitoring receiving optical fibers of both of the first and second optical fiber loops; means for detecting a fault on one of the receiving optical fibers of the first and second optical fiber loops; means for switching off a transmitting optical fiber of the optical fiber loop to which the fault-detected receiving optical fiber belongs; means for maintaining reception of signals for the node through the one of the receiving optical fibers of the first and second optical fiber loops if the fault-detected receiving optical fiber is the other receiving optical fiber of the first and second optical fiber loops, or switching reception of signals for the node through the other receiving optical fiber of the first and second optical fiber loops if the fault-detected receiving optical fiber is the one receiving optical fiber of the first and second optical fiber loops; and means for maintaining transmission of signals from the node through a transmitting optical fiber of an optical fiber loop to which the fault-detected receiving optical fiber does not belong; whereby the interface performs both protection and safety measures for the network node. 
       DESCRIPTION OF EXAMPLE EMBODIMENTS 
       [0016]      FIG. 1A  illustrates a node  10  connected to another node (not shown) at the remote end of an exemplary two-fiber link, which forms a fiber loop between the two nodes. The node  10  is represented by a multiplexer  12  which gathers signals from various sources to transmit to the remote node by a Tx fiber and a demultiplexer  11  which separates signals received from the remote node by an Rx fiber for various destinations. 
         [0017]    Optical signals deteriorate as they travel through optical networks and must be periodically amplified. To this end optical amplifiers are installed at different locations of an optical network to ensure that optical signals are not lost. In the exemplary and conventional arrangement of  FIG. 1A , a pre-amplifier  14  boasts the strength of incoming signals received through the Rx input port over the Rx line before passing the signals to the node  10 . For signals leaving the node  10 , a post-amplifier  15  boasts signal strength of signals being transmitted through the Tx line port over the Tx line and to the remote node. For ease of maintenance and replacement, the post-amplifier  15  is part of an amplifier module  13  which also provides a splitter  18  on the Rx line and a combiner  19  on the Tx line by which OSC (Optical Supervisory Channel) can be respectively received and sent by a network supervisory and management unit  16 . The module  13  provides an interface between the optical fibers of the link to the remote node and the node  10 . 
         [0018]    The amplifier module  13  can implement safety measures, such as the OFC (Open Fiber Control) protocol.  FIG. 1B  is a state diagram of optical safety protocols for the module  13 . The power of outgoing signal is controlled responsive to the power of the incoming signals which are transmitted over the link by the remote node also following OFC protocol. In the “normal” logic state  30 , the power of the signals on the Rx line are normal so that the post-amplifier  15  remains operative and the outgoing Tx line is active. The amplifier module  13  remains in that state  30  as long as the Rx line is “okay.” But in case of a fiber cut or failure on the Rx line, a loss of signal (Rx=LOS), and OSC is lost. Rather than the network supervisory and management unit  16  which provides the control for the optical network and its constituent elements, such as the amplifier module  13 , the amplifier module  13  itselfs shuts down the output of the post-amplifier  15  (the control lines to the amplifier are not shown) by an ALS (Automatic Laser Shutdown) command and the Tx line is “off.” This is represented by logic state  31 . The control unit of the module  13  is not shown in the drawings. As long as the Rx line is out and there are no OSC signals through the splitter  18 , the module  13  remains in state  31  and no optical signals are sent on the Tx line. 
         [0019]    When the fiber connectivity is restored, as detected by the return of the OSC signals or the Rx signals to the unit  16 , the post-amplifier  15  is switched-on at a reduced output power level, i.e., the APR (Automatic Power Reduction) level. The amplifier module  13  transitions to a logic state  32  and begins APR restart procedures. In this state the optical power launched into the Tx fiber is kept below the hazardous level for a period set by an APR timer and at this point the remote node is reciprocally sending incoming signals on the Rx optical fiber at an APR level. This ensures that the connectivity of the fiber loop of the Tx and Rx optical fibers is fully restored. If the Rx fiber is lost again, Rx=LOS, before the APR timer times out, the module  13  returns to the logic state  31 . If nothing happens in the logic state  32  before the APR timer times out, then the module  13  returns to the normal logic state  30 . 
         [0020]    For protection measures, redundancy is provided for the optical signals by two alternate fiber paths, i.e., the working fiber and the protection fiber. Currently, protection modules are conventionally deployed in optical networks to implement protection measures.  FIG. 2A  is a diagram of a conventional protection module for 1+1 protection. The protection module sits as an interface between the network node and the optical fibers which provide the link to another node on the remote of the link. Since the protection is 1+1, there are four fibers in the link, two transmitting fibers, a Tx-W line and Tx-P line, and two receiving fibers, a Rx-W line and Rx-P line. By convention, W stands for working and P stands for protection. 
         [0021]    The  FIG. 2A  protection module  20  has a transmission section which has an input Com-Rx port  47  which receives signals to be transmitted from the network node to the remote node across the link. The signals are split 50-50 by a splitter  21  for the working transmitting fiber Tx-W and for the protection transmitting fiber Tx-P. Before reaching the transmitting ports  49   w  and  49   p , the output each set of the split signals is controlled by a switch  23   w ,  23   p . The effectiveness of each switch  23   w ,  23   p  is monitored by a corresponding PD (Photo Diode)  25   w ,  25   p  which receives a small tapped off portion of the signals from the output of the switches  23   w ,  23   p.    
         [0022]    The protection module  20  also has receiving section which has two input ports,  46   w  for the working receiving fiber Rx-W and  46   p  for the protection receiving fiber Rx-P. The received signals from the two ports  46   p  and  46   w  and the remote node are fed into the input terminals of a 1×2 optical switch  26  which selects whether the signals from the input ports  46   w  or  46   p  are to be passed to the output Com-Tx port  46  and the network node. A VOA  24  controls the power of the signals to the output Com-Tx port  46 . These signals are monitored by a PD  22  which receives a small tapped off portion of the signals from the output of the VOA  24 . 
         [0023]    A control unit  45  receives the tapped off monitoring signals from the PDs  22 ,  25   w ,  25   p ,  28   w  and  28   p , and controls the switches  23   w ,  23   p  and VOA  24 . Control lines to and from these components are not shown. Alternatively, the operation of the protection module  20  could be controlled by the network supervisory and management unit  16  communicating over the OSC, but a control unit in the module  20  operates faster and more efficiently. 
         [0024]      FIG. 2B  illustrates the operation of the protection module  20  with a logic state diagram. In a “normal” logic state  40 , both receiving lines, Rx-W and Rx-P, are operative and the protection module  20  transmits outgoing signals on both transmitting lines, Tx-W and Tx-P. The switch  26  selects the signals of the receiving RX-W line and forwards them to the network node. By convention, the receiving line selected by the switch  26  is in the Active state (ACT), while the other is the Standby state (STB). Even though the two states, ACT and STB, are related to the receiving fibers, they are also applied to the transmitting fibers, Tx-W and Tx-P, even though even if both lines are transmitting the same optical signals in accordance with 1+1 protection procedures. Thus the transmitting Tx-W line is considered Active and the Tx-P line is Standby. 
         [0025]    In case of failure on one Rx line, the absence of optical power, say, LOS (Loss of Signal) on the Rx-P line, then there is a transition from logic state  40  to logic state  42 . In this state, the Tx-W line remains Active, but the transmitting Tx-P line corresponding to the receiving Rx-P line is shut down. This is done by the switch  23   p  and represented by an open command, i.e., a command to open the switch and break the optical circuit. This command corresponds to an ALS (Automatic Laser Shutdown) command. The module  20  remains in the logic state  42  as long as the Rx-P line is in a LOS state and even if the Rx-W line now falls into a LOS state. When both receiving lines, Rx-W and Rx-P, have recovered and are functioning, there is a transition from the logic state  42  back to logic state  40 . On the other hand, if the Rx-W line falls into a LOS state and the Rx-P line has recovered, the module  20  transitions from the logic state  42  to the logic state  43  in which the Tx-P is considered Active and the Tx-W line Standby. 
         [0026]    The logic state  43  can also be reached from the logic state  40 . If the receiving Rx-W line fails, i.e., LOS, while the Rx-P line remains functional, the module  20  transitions from the logic state  40  to the logic state  43 . The module  20  remains in the logic state  43  as long as the Rx-W line is in a LOS state and even if the Rx-P line now falls into a LOS state. When both receiving lines, Rx-W and Rx-P, have recovered and are functioning, there is a transition from the logic state  43  to the logic state  41  in which the Tx-P line remains Active and the Tx-W in Standby. As long as the two receiving lines remain functional, the module  20  remains in the logic state  41 . Transitions from the logic state  41  occur when the Rx-P is lost (transition to the logic state  42 ) and when the Rx-W line fails (transition to the logic state  43 ). This preserves the symmetry of the transitions between the logic states  40 - 43 . 
         [0027]    With modules for optical network safety measures, such as the amplifier module  13 , and modules for optical network protection measures, such as the protection module  20 , a combination of such modules can presumably be installed to obtain the benefits of such measures.  FIG. 3  shows the combination of the two modules  13  and  20  from  FIGS. 1A and 2A  respectively. The same reference numerals from the earlier drawings are used for ease of understanding. As shown in  FIG. 3 , working and protection lines are paired into loops so that the link to the remote node (not shown) is formed by a first fiber loop, transmitting working optical fiber  50   w  and receiving working optical fiber  51   w , and a second fiber loop, transmitting protection optical fiber  50   p  and receiving protection optical fiber  51   p . The Com-Rx input port  47  of the protection module  20  is fed directly by the post-amplifier  15  of the module  13 . The power levels of both outputs, the output port  49   w  for Tx-W line, and the output port  49   p  for the Tx-P line, should satisfy optical safety measures, such as the OFC protocol. The Com-Tx output port  46  of the protection module  20  feeds the input of the pre-amplifier  14 . This configuration provides protection for the line path (link fibers and possible intermediate amplifiers sites) but no protection is provided for the two end nodes, viz., the node  10  and its remote node. 
         [0028]    But there are problems with the separate implementation of optical safety measures in the module  13  and the protection measures in the module  20 . For example, if one assumes that the transmitting Tx-W line  50   w  is in the active (ACT) state, the switch  26  passes the signals on the receiving Rx-W line  51   w , which is correspondingly active, to the amplifier module  13 . If the received signals are lost, Rx-W=LOS, the optical safety protocol shuts down the POST amplifier  15 . Even though the protection module  20  has responded to the LOS state on the Tx-W line  50   w  and made the transmitting protection Tx-P line  50   p  active, it is also turned off since the POST amplifier  15  which drives both transmitting lines  50   w  and  50   p  and is now off. 
         [0029]    Furthermore, with the Tx-P line  50   p  now considered active, the switch  26  of the protection module  20  passes signals from the corresponding receiving Rx-P line  51   p  to the amplifier module  13 . Thus the module  13  is not monitoring the working Rx-W line  51   w  to determine its return to operating condition; the correct line restart cannot be performed for the corresponding transmitting Tx-W line  50   w . The APR procedures cannot be performed. 
         [0030]    To avoid these problems, the present invention provides for the close interoperation of safety measures with protection measures. The protection module  20  is modified so that the switches  23   w ,  23   p  are replaced by VOAs  33   w ,  33   p  and the module&#39;s operations are changed with a control unit  45 ′. The modified protection module is labeled  20 ′ with the same elements as the module  20  except the switches  23   w ,  23   p  and the control unit  45 ′. The connections of the amplifier module  13  and protection module  20 ′ are shown in  FIG. 4  and unchanged from those of  FIG. 3  in this embodiment of the present invention. Safety and protection measures are installed in the protection module  20 ′. Besides being able to control the output power of the signals leaving the transmitting ports  49   w  and  49   p  independently, the variable optical attenuators  33   w  and  33   p  are used to control the amplifier output in accordance with safety procedures. In the following description the AVS (Automatic VOA Shutdown) command/status for the module  20 ′ corresponds to as ALS (Automatic Laser Shutdown) command/status used for the amplifier module  13 . 
         [0031]    The protection module  20 ′ operates according to the logic state diagram of  FIG. 5 . Starting with the S 1 -W logic state  60 , both receiving lines, Rx-W line  51   w  and Rx-P line  51   p , have power levels above their thresholds (i.e. both fiber loops, working and protection, are OK). Since the S 1 -W logic state  60  (TX-W=ACT and TX-P=STB) is symmetric with the S 1 -P  61  logic state (TX-W=STB and TX-P=ACT) and the descending branches of logic states, logic states  62 ,  64  and  66  from logic state  60  and logic states  63 ,  65  and  67  from logic state  61 , only one branch is described in full detail. 
         [0032]    Starting with logic state  60 , the S 1 -W logic state  60  transitions to the S 2 -W logic state  62  upon a failure on the receiving Rx-P line  51   p  (Rx-P=LOS). The output of the transmitting Tx-P line  50   p  is turned off (Tx-P=AVS) following safety procedures. If, on the other hand, there is a failure of the receiving Rx-W line  51   w , the logic state  60  transitions to the S 2 -P logic state  63  in which protection switching is performed. The protection transmitting Tx-P line  50   p  become active (Tx-P=ACT) and the optical safety procedures are started by shutting down the working transmitting Tx-W line  50   w  (Tx-W=AVS). 
         [0033]    Returning to the S 2 -W logic state  62 , the state is a “timed” state where the transmitting Tx-P line  50   p  is kept in AVS for a time TAVS whatever the conditions detected on the receiving Rx lines  51   w ,  51   p  are. Only after the timer for TAVS has timed out is the receiving Rx-P line  51   p  checked. If the receiving Rx-P line  51   p  still fails (Rx-P=LOS), a re-start procedure is attempted by a transition to the S 3 -W logic state  64 . On the other hand, if the receiving Rx-P line  51   p  is operative (Rx-P=OK), the receiving protection line  51   p  is already restored (and a re-start procedure has been already started by the remote site), the logic state  62  transitions to the S 4 -W logic state  66 . It should be noted that in S 2 -W logic state  62 , the receiving Rx-W line  51   w  is not checked. Any failure on the Rx-W line  51   w  at this stage does not trigger anything because the logic state  62  is directed toward about the failure of the Rx-P line  51   p.    
         [0034]    The S 3 -W logic state  64  is also “timed” state where the module  20  is attempting a re-start procedure on the transmitting Tx-P line  50   p  (TX-P=APR). The transmitting line  50   p  is pulsed with its power kept below the hazardous level by APR procedures. This state is maintained for a time T APR1  to allow the remote node to detect the re-start procedure. After the timer T APR1  times out, there are two possibilities. If the receiving Rx-P line  51   p  remains non-operative (Rx-p=LOS), the re-start procedure fails and there is a transition back to the S 2 -W logic state (where TX-P=AVS). Alternatively, if the receiving Rx-P line  51   p  becomes operative (Rx-P=OK) and the protection loop is restored, the re-start procedure is proceeding successfully. There is a transition to the S 4 -W logic state  66  where the re-start is confirmed. 
         [0035]    The S 4 -W logic state  66  is another “timed” state where a timer for a period T APR2 &lt;T APR1  is used to confirm that the re-start procedure is successful. This is needed to avoid bouncing between APR and AVS power levels when the re-start procedure is started at the same time by the protection units  20  at the subject node, node  10 , and the remote node of the link. In the logic state  66  the Tx-P line  50   p  is still in APR. At the end of T APR2  if the receiving Rx-P line  51   p  is still operative (Rx-P=OK), the line  51   p  is fully powered and operative, i.e., there is a transition to the S 1 -W logic state  60  where Tx-P=STB). If, on the other hand, the receiving Rx-P line  51   p  is not operative (Rx-P=LOS) and the re-start procedure has not succeeded, the line  50   p  is set to AVS again (S 2 -W logic state  62 ). In case of a failure detected on the receiving Rx-W line  51   w  (Rx-W=LOS), there is a transition to the S 2 -P logic state  63  where protection switching is performed. The protection transmitting Tx-P line  50   p  becomes active (Tx-P=ACT) and the working transmitting Tx-W line  50   w  is shut down (Tx-W=AVS). This starts the optical safety procedure for the working line. In the S 4 -W logic state  66 , the transmitting Tx-P line  50   p  is considered “almost ready,” so that protection switching can be performed, viz, transitioning to the S 2 -P logic state  63 , even if the timer T APR2  has not yet expired. 
         [0036]    From the symmetry of the logic state branches, it is easy to see that the logic states  61 ,  63 ,  65  and  67  correspond to the described logic states  60 ,  62 ,  64  and  66 , except that the earlier branch of states deal principally with the protection transmitting Tx-P  50   p  being active and a failure on the receiving Rx=W line  51   w  (Rx-W=LOS). 
         [0037]    The components of the control unit  45 ′ for the protection module  20  are shown in  FIG. 6 . The control unit  45 ′ includes a memory subsystem  72  which can store and retrieve software programs incorporating computer code that implements aspects of the present invention, data for use with the invention and the like, and a central processor subsystem  71  which, among other functions, processes the instructions and data of the computer code. Example computer readable storage media for the memory  72  include semiconductor system memory and flash memory preferably, though other storage media, such as hard drive, CD-ROM, floppy disk, and tape, might be used. The control unit  45  might further include subsystems, such as fixed storage  74  (e.g., hard drive), removable storage  76  (e.g., CD-ROM drive), and one or more network interfaces  77 , all connected by a system bus  78 . The network interface  77  provides a pathway for the node to communicate with the network management system, i.e., the network supervisory and management unit  16 , and other nodes to synchronize operations. Additional or fewer subsystems in the control block may be used. For example, the control unit may include more than one processor  71  (i.e., a multi-processor system), or a cache memory. 
         [0038]    As described, the present invention receives the benefits of a conventional installation of power amplifier modules  13  and protection modules  20 ′ with only a minimum of modifications and existing arrangements. The memory  72  of the control unit  45 ′ is loaded with the computer code to carry out the operations described above and the network supervisory and management unit  16  is shut off from the safety measures. Of course, if more integration in the modules is desirable, the post amplifier  15  and possibly the pre-amplifier  14  can be combined with the protection module. 
         [0039]    With the present invention, optical safety on both lines is integrated with the protection switching state machine to guarantee the optical power level reduction when the fiber interruption is detected and also the correct line restart when the fiber loop is restored. The optical power level remains below the hazard level at any point of the optical network which might be accessible to the network operator or service personnel. 
         [0040]    This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.