Abstract:
Safety and power control arrangement and method for optical communication apparatus, where a first circuit pack emitting an optical signal (OS 1 ) and at least a second and a third circuit pack ( 2, 3 ) are connected in series via optical fibers ( 4, 5 ). A power monitor ( 26 ) connected to an output ( 27 ) of the at least second circuit pack ( 2 ) reduces the signal power (PW 2 ) output from the second circuit pack ( 2 ) to a pre-determined safe value if a loss-of signal monitor ( 34 ) of a next circuit pack ( 3 ) forwards a loss-of-signal control signal (LOC 3 ). The advantage is that the maximum allowable value is achieved at the input of an interrupted fiber section and the non-interrupted circuits can still receive the optical signals with a reasonable power level.

Description:
FIELD OF THE INVENTION 
     The invention relates to a safety and power control arrangement for optical fiber communication systems and a method therefore. 
     BACKGROUND OF THE INVENTION 
     In optical transmission systems optical signals are transmitted over an optical fiber. If the fiber is accidentally cut or disconnected and the optical signal would still be transmitted a service person would be harmed, if he happened to look at the concentrated light signal being emitted from the broken fiber. To avoid such harm, there is the need for a safety arrangement that automatically reduces the power of a light signal to avoid injuries. High optical power at the open fiber end must be also avoided before fiber resplicing or patchcord reconnecting. 
     The U.S. Pat. No. 6,359,708 B1 describes a facility which detects the loss of a high power optical signal in an optical transmission path and invokes a procedure which causes a transmitter connected to the cut fiber to reduce the power level of an optical signal emitted to a safe power level. 
     The same procedure may be applied to a multi stage amplifier if a connection between the stages is interrupted. 
     And the same safety arrangement is also necessary within an optical apparatus comprising a plurality of circuit packs (optical devices), which are connected via optical fibers. A possibility to avoid high optical power inserted into a broken fiber is to interrupt the line directly downstream of a transmitter/optical amplifier, or to shut down the transmitter/optical amplifier, or to strongly reduce the output power, if a LOS (loss-of-signal) is determined, or equivalently a “signal-present” of the signal, confirming a faultless connection, cannot be determined. If the fiber system is closed again, it is desirable that the signal-present signal is automatically detected and original power level is recovered. But especially in systems where the optical signal is split and sent to a plurality of circuit packs the strong reduction of power level of the transmitter results in interrupting or at least impairing all other connections. 
     Another possibility is to avoid an interruption and to control the power of the transmitter up to a predetermined safe value. Thereby the emitted power at the broken end further down the fiber is reduced to the safe power value or even below, and the power sent to other circuit packs may be far below a sufficient value required for signal transmission. 
     Therefore, in many cases, especially when a plurality of channels is transmitted, this method leads to not necessary impairments. 
     OBJECTS AND SUMMERY OF THE INVENTION 
     It is object of the invention to reduce the power of an emitted optical signal to an allowable amount if an optical fiber is interrupted, and to ensure that the optical signal can be transmitted with a reasonable power level via non interrupted fiber segments to non-affected circuits. 
     According to a first embodiment of the invention, there is provided a safety and power control arrangement for an optical communication apparatus with a first circuit pack emitting an optical signal, at least one further circuit pack, and at least one end circuit pack connected in series via optical fibers,
         at least one signal monitor for monitoring an output power of an output port of the at least one further circuit pack,   a control device being designed for receiving control signals from a signal monitor of the at least further circuit pack and from further monitors of further downstream circuit packs, and being designed for   decreasing the output power of the first circuit pack according to a power control signal of a signal monitor of said at least one further circuit pack receiving the optical signal till a measured output power of an optical signal emitted into an interrupted fiber is reduced to a save power value if loss-of-signal is determined for the next downstream circuit pack.       

     According to this invention an optical signal with a highest allowed power is always transmitted to the circuit packs. 
     Another embodiment comprises
         a splitter circuit pack, and at least a third and at least a fourth circuit pack connected via further fibers to outputs of the splitter circuit pack,   the splitter circuit pack comprising a first signal monitor and an at least second signal monitor connected to its outputs,   the control device being adapted for receiving control signals from the first signal monitor and the at least second signal monitor of the splitter circuit pack, and loss-of-signal control signals from at least third and fourth circuit packs, and being adapted for   decreasing the output power of the first circuit pack according to a power control signal of a last signal monitor of the splitter circuit pack receiving the optical signal till a measured output power of an optical signal emitted into an interrupted fiber is reduced to a save power value if loss-of-signal is determined for at least one of the next down-stream circuit packs.       

     A further embodiment comprises a signal monitor designed for measuring the output power of the at least further circuit pack and to transmit a corresponding power control signal, and the control device is designed for determining loss-of-signal and a power control signal by comparing the received power signal with a predetermined value and for determining the output power level of said at least further circuit pack. 
     All decisions are made by the control device and the control device can be easy adapted to different systems. 
     Another embodiment comprises a signal monitor designed to compare the measured output power with a predetermined value and to transmit an overpower signal as a power control signal to the control device if the output power is too high or to low. 
     Binary valued control signals (bits) are sufficient, to transmit the information for optimization of the output power. 
     A further embodiment comprises advanced signal monitors for determining a loss-of-signal and to transmit a loss-of-signal control signal, and to measure the output power level, to compare the output power level with a predetermined value, and to transmit a power control signal meaning the output power is too high or too low. 
     The number of signal monitors can be reduced if the advanced signal monitor also calculates the output power levels of optical devices comprising splitters, which are arranged downstream of his measuring point. Control signals comprising two bits are sufficient to transmit the states loss-of-signal and overpower. 
     Another type of the advanced signal monitor may transmit the measured power level and the calculations and decisions may be done by the control device. 
     A safety and power control method for an optical communication apparatus with a first circuit pack emitting an optical signal, at least one further circuit pack, and at least one end circuit pack connected in series via optical fibers comprises the steps of
         monitoring loss-of-signal of an optical signal to be received by a circuit pack, if loss-of-signal is determined for a circuit pack:   measuring an output power of a next upstream circuit pack emitting this optical signal to be received and transmitting a power control signal to a control device, and   decreasing an output power of the first circuit pack till the measured output power of said optical signal emitted into an interrupted fiber is reduced to a save power value.       

     More details of the invention are described in further depending claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the invention including a presently preferred embodiment are described below with reference to accompanying drawings, where 
         FIG. 1  is a schematic diagram of an optical transmission apparatus equipped with the safety and power control according to the invention, 
         FIG. 2  is a schematic diagram of an optical transmission apparatus for distributing signals, 
         FIG. 3  is a schematic diagram of an optical transmission apparatus equipped with advanced signal monitors, and 
         FIG. 4  is a further embodiment equipped with an advanced signal monitor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a simplified block diagram of a transmission apparatus comprising a safety and power control arrangement according to the invention with three circuit packs  1 ,  2 ,  3  connected in series. A first circuit pack (circuit assembly)  1  comprises, beside other equipment, an optical amplifier  12  and a control device  13  determining the output power PW 1  of the optical amplifier  12  by controlling a pump source  14 . An optical signal OS, e.g. a wavelength division multiplex signal, is received via a transmission fiber L 1  at an input  11  of the first circuit pack  1  and amplified. The optical signal is denoted OS 1 , OS 2 , OS 3 , OS 7  while travelling through the arrangement. A few channels of the optical signal; e.g. channel CD, may be dropped (here at an output port of circuit pack  2 ). 
     The output  18  of the first circuit pack  1  is connected via an optical fiber  4  to an input  21  of one further (second) circuit pack  2 . This circuit pack  2  comprises an optical device  22 , a loss-of-signal monitor  24  connected to the input  21  (or to another suitable point on the board), and a signal monitor  26 , connected to the output  27  (or to another suitable point on the board after the last optical circuit). Both monitors  24 ,  26  are connected via splitters  23  and  25  to the optical waveguides/fibers  4  and  5  respectively transmitting the optical signal. The output signal of a circuit pack has in most cases a lower power level than the input signal because of the attenuation of passive elements like filters, splitters, add-drop-devices etc. within that optical device. 
     A plurality of these type further circuit packs  2 , only one is shown in  FIG. 1 , may be connected in series. An output  27  of the further circuit pack  2  is connected via the optical fiber  5  to an input  31  of a last or an “end circuit pack”  3 . A LOS monitor  34  is connected via a splitter  33  to the input  31  (or another suitable point). Here, the end circuit pack does not need a power monitor, because—presuming the apparatus is a receiver—the optical signal is determined and converted into an electrical signal by the circuit device  32 , or because the device  32  is splitting the received optical signal into a plurality of optical tributary signals each having a uncritical power level from a laser safety point of view (dotted lines in  FIG. 1 ). 
     The signal monitor  26  of the further/second circuit pack  2  sends its power control signal POC 2  to the control device  13 , preferable situated within the first circuit pack  1 . The power control signal POC 2  can be a measured power level or, after comparing the measured power level with a predetermined value, a (binary) “overpower” signal indicating that the power is too high or too low. The output power PW 1  of the first circuit pack may be additional controlled by in internal control loop (connection from the amplifier output to the control device) to generate an accurate power signal. 
     The loss-of-signal monitors  24  and  34  send control signals LOC 2  and LOC 3  with (binary) logical information, e.g. via an electrical connection. The loss-off-signal is e.g. determined by measuring the power of the received optical signal. 
     If the optical fiber  4  between the first and the second circuit pack is interrupted, the loss-of-signal monitor  24  of the second circuit pack determines LOS (loss-of-signal). An emitted LOS control signal LOC 2  is emitted and received at the control device  13  and decreases, independent of the power control signal POC 2 , the output power PW 1  to an predetermined “allowed safe power value”. The loss-of-signal is always generated if the monitored signal is lower than a threshold level. 
     An interrupted/open control line  19  (dashed cross) regularly transmitting a LOS control signal is also evaluated as LOS by the control device  13  to avoid malfunction. 
     Now it is assumed that the optical fiber  5  between the further/second circuit pack  2  and the end/third circuit pack  3  is interrupted (X in the drawing). A power control signal POC 2  is emitted from the second circuit pack  2  and a LOS control signal LOS 3  is emitted from the third circuit pack  3 . The control signals are received by the control device  13  at inputs  15 ,  16 ,  17 . The control device  13  selects the first LOS control signal LOC 3  of the serial connection of the circuit packs, in this arrangement the LOS control signal LOC 3  of circuit pack  3 , and decreases the output power PW 1  of the first circuit pack  1  controlled by the power monitor  26  of the second circuit pack  2 . The optical output power PW 2  of signal OS 2  emitted into the interrupted fiber  5  is decreased to the allowed safe power value (or an other desired power limit). The “allowed safe power” is the allowed predetermined maximum light power inserted into an interrupted fiber. The foregoing packs, here only circuit pack  2  still receives the optical signal OS 1  with sufficient or at least the highest possible power level while the power emitted into the interrupted fiber  5  does not exceed the allowed safe power value. 
     An equivalent signal-present control signal may be generated instead of the loss-of-signal control signal by the LOS monitors. The advantage is that supervision of an open control line is not necessary. If this signal-present control signal is not received because a control line  19  is interrupted or open, e.g. because the next downstream circuit pack  3  is missing, the signal-present control signal is not received by the control device and the output power is reduced. 
     The signal monitors may compare, as mentioned before, the output power with the ‘allowed safe power value’ and forward only an ‘overpower’ signal. If the optical fiber  5  is interrupted (or the third circuit pack  3  is missing) and LOS is received from the next downstream circuit pack  3  (or detected by the control device), the output power PW 2  is decreased by the control device  13  till this ‘overpower’ control signal vanishes. If the output power is already lower than the safe power level it is not further decreased. The power measurement may be executed continuously or after determining LOS. 
     By way of example, the output power PW 2  of the second circuit pack  2  is decreased to the safe power value within 1 second. If the reduction of output power PW 2  to the safe power value is not executed within a guard time of 2 second because of a system failure, the output power PW 1  of the first circuit pack is reduced to the safe power value automatically. 
     In a further embodiment the output power PW 1  is increased again to the original power level not earlier than 10 seconds after detecting LOS, even if the loss-of-signal control signal has vanished earlier. 
     In a further improved embodiment, after the ‘overpower’ control signal has vanished, the original target power PW 2  is slowly resumed after e.g. 1000 seconds. 
       FIG. 2  illustrates a simplified block diagram of a further arrangement comprising the first circuit pack  1  with the optical amplifier  12  and the control device  13  and a modified further “splitter circuit pack”  6  connected to the output  18  of the first circuit pack  1 . The splitter circuit pack  6  comprises an optical splitter  62  receiving at its input  61  the optical signal OS 1 . Outputs  65  and  66  of the modified splitter circuit pack  6  are connected by fibers  51  and  52  to inputs of  31 ,  71  of two further circuit packs  3  and  7 , which may be end circuit packs. 
     The first circuit pack  1  has almost the same function as described before. The splitter  62  is e.g. a de-interleaver separating the channels of an optical wavelength division multiplex signal into two (or more) groups OS 3  and OS 7 . Each group is forwarded to one of two downstream end circuit packs  3  and  4 . 
     The splitter circuit pack  6  has two signal monitors  63  and  64 , each connected to an output  65  and  66  respectively. Their power control signals POC 63  and POC 64  are forwarded to the control device  13 , in this embodiment via a safety bus ring  8 . A LOS monitor at the input is not necessary if no internal data processing is carried out within this circuit pack, as will be explained below. 
     Each end circuit pack  3  and  7  comprises only a LOS monitor  34  and  74  respectively. The received optical signals OS 3  and OS 7  are converted into electrical signals in the devices  32  and  72 , or e.g. the devices  32  and  72  are optical devices splitting the received signals into a plurality of tributary signals each having a power level being low from a laser safety point of view. 
     If one of the LOS monitors  34  or  74  determines a LOS, e.g. LOS monitor  34  detects a LOS according to an interruption (X in the drawing) of the upper fiber  51 , it forwards a LOS control signal LOC 3  to the control device  13 . Then the emitted signal power PW 1  is reduced according to the power control signal POC 63  of the associated upstream signal monitor  63  of splitter circuit pack  6 . The control device decreases the output power PW 1  till the power PW 3  of signal OS 3  emitted into the interrupted upper fiber  51  is reduced to the safe power level. The signal power PW 7  in the other branch is unavoidable also reduced, but the associated power control signal POC 64  is not taken into account, because there is no LOS control signal emitted (or a signal-present control signal is emitted) by the associated next downstream LOS monitor  74 . According to different numbers of channels in both groups the total power in the undisturbed branch can be higher. Different signal power control circuits in the splitter circuit pack would allow an individual control of the output powers PW 3  and PW 7  but are usually not affordably. 
     If both LOS monitors  34  and  74  determine LOS, then it can be assumed that the fiber  4  is interrupted and the output power PW 1  of the first circuit pack  1  is reduced to the predetermined safe power value. Also, the signal monitors  63  and  64  can also substitute a LOS monitor connected to the input of the splitter pack. If both report a level of about zero the output power PW 1  is reduced to the safe level. 
     More than the shown circuit packs may be connected in series or in series and parallel. And also the branches may comprise two or more circuit packs connected in series. The control mechanism has to be adapted to the configuration. It is always the output power of the circuit pack upstream the interruption which has to be decreased to the predetermined safe power value and which is always controlled by the power monitor of that circuit pack inserting an optical signal into the interrupted fiber. 
       FIG. 3  shows a further embodiment of the invention. At least three circuit packs  1 ,  20 , and  30  are connected in series. 
     Each further circuit pack  20 ,  30  comprises only one advanced signal monitor  28  and  38  respectively. The standard signal monitor supervises only the output power whereas the advanced signal monitor has two functions: To determine loss-of-signal (or signal-received) by a first comparator C 1  and to supervise the output power (like the monitors of the splitter circuit pack) by a second comparator (or power measurement circuit) C 2 . Both control signals LOC 2 /POC 2 , LOC 3 /POC 3  are derived by the advanced signal monitor and transmitted to the first circuit pack  1 . Because the advanced signal monitor is connected to the output (or to a point suited to supervise the output power) this embodiment has in addition the advantage that also an interruption within the circuit pack  20  is detected. 
     Of course, another embodiment of the advanced signal monitor may transmit the measured power value. Then the control device  13  derives the LOS signal and a suitable control signal for power regulation itself. Both types of the advanced signal monitors can be also used in splitter circuit packs. 
     The LOS monitoring at the output has the advantage, that if fiber  4  or a connection on the second circuit pack  20  is interrupted—and the advanced signal monitor  28  determines LOS—the output power PW 1  of the first circuit pack is reduced to the save power level. 
     And if fiber  5  or a connection on the third circuit pack  30  is interrupted—and the advanced signal monitor  38  determines LOS—the output power PW 2  of the second circuit pack  2  is reduced to the save power value controlled by the advanced signal monitor  28 . 
       FIG. 4  illustrates an arrangement with a splitter circuit pack  60 . An advanced signal monitor  68  is connected to the input of a splitter device  62 . This advanced signal monitor  68  supervises loss-of-signal and also measures the received power. If one of the fibers  51  or  52  is broken, e.g.  51 , and LOS is reported from the following circuit pack, the output power PW 3  is calculated by a calculation and comparison circuit CC of advanced signal monitor  68 . This can be done because the attenuation of the splitter  62  (or another optical device) is known. All (binary) control signals LOC 2 , POC 2  are forwarded via a control bus  80  to the control device  13  of the first circuit pack. Only a single signal monitor is necessary to control LOS and to control the powers PW 3 , PW 7  of both (or more) outputs. 
     Of course, as mentioned before, also the measured power can be forwarded by the advanced signal monitor (and also by the LOS monitors) to the control device  13 , which may derive the suitable control signals. 
     E.g. the monitor  34  on the end circuit pack  3  can also be an advanced type monitor, supervising downstream outputs (dashed lines) with respect to overpower. 
     As mentioned before, to avoid malfunctions a guard time may be allocated to the control signals. To avoid control problems of amplifiers/receivers allocated in the end circuit packs  32 ,  72  the output power of the first circuit pack is increased and/or decreased continuously or in small steps. 
     REFERENCE SIGNS 
     
         
           1  first circuit pack (CP) 
           11  input 
           12  optical amplifier (EDFA) 
           13  control device 
           14  pump source 
           15  control signal input 
           16  control signal input 
           17  control signal input 
           18  output 
           2  second CP 
           21  input 
           22  optical device 
           23  splitter 
           24  LOS monitor 
           26  splitter 
           25  power monitor 
           28  advanced signal monitor 
           3  third/last CP 
           31  input 
           32  optical/opto-electrical device 
           33  splitter 
           34  LOS monitor 
           38  advanced signal monitor 
           4  optical fiber 
           5  optical fiber 
           51 ,  52  optical fiber 
           6  splitter CP 
           61  input 
           62  splitter device/deinterleaver 
           63  power monitor 
           64  power monitor 
           65  output 
           66  output 
           60  splitter circuit pack with advanced signal monitor 
           7  circuit pack 
           71  input 
           72  opto-electrical device 
           73  splitter 
           74  LOS monitor 
           20  second circuit pack 
           30  third circuit pack 
           28 ,  38 ,  68  advanced signal monitor 
         OS, OS 1  optical signal 
         OS 2 , OS 3  optical signal 
         OS 7  optical signal 
         C 1 , C 2  comparator 
         CC calculation and comparison circuit 
         CD dropped channel 
         LOS LOSS OF SIGNAL 
         LOC 2  LOS control signal of circuit pack  2   
         LOC 3  LOS control signal of CP 3   
         POC 2  power control signal of circuit pack  2   
         POC 63  first power control signal of CP 6   
         POC 64  second power control signal of CP 6   
         PW 1  output signal power of CP 1   
         PW 2  output signal power of CP 2