Abstract:
An optical transmission equipment includes an optical amplifier that is coupled to an optical transmission path and amplifies a first optical signal which is received from the optical transmission path, a first controller that controls the optical amplifier depending on a first optical power of output light from the optical amplifier and a second optical power of reflecting light to the optical amplifier, an optical coupler that branches a second optical signal from the optical amplifier into a first output and a second output, an optical demultiplexer that demultiplexes the first output of the optical coupler, an optical switch or attenuator that receives the second output of the optical coupler, and a second controller that controls the optical switch or attenuator depending on a third optical power of output light from the optical switch or attenuator and a fourth optical power of reflecting light to the optical switch or attenuator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of U.S. application Ser. No. 11/752,331, filed May 23, 2007, the contents of which are incorporated herein by reference. 
     
    
     CLAIM OF PRIORITY 
       [0002]    The present application claims priority from Japanese patent application serial no.  2006 - 173915 , filed on June  23 ,  2006 , the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to optical transmission equipment and an optical add-drop multiplexer (OADM) that use a wavelength multiplexing technique. More particularly, the invention relates to optical transmission equipment and an optical add-drop multiplexer that have excellent maintainability. 
         [0004]    With the increasing capacity of data system communication represented by the Internet technology, a rapid growth in data volume and an increase in the associated transmission capacity are expected in optical transmission systems. In order to meet such demands, the wavelength multiplexing technique is applied to perform communication by bundling plural signal lights of different wavelengths through one optical fiber. Recently, there is being established a communication network using optical add-drop multiplexers that can drop and add optical signals for each wavelength at plural locations, in addition to transmit large volume of data between two separate sites by means of the wavelength multiplexing technique. 
         [0005]      FIG. 1  shows a block diagram of an optical transmission network using optical add-drop multiplexers. An optical transmission network  1000  includes six optical add-drop multiplexers  101  that are connected in a ring configuration through an optical fiber transmission line  102 . The optical add-drop multiplexer  101  selects whether to add and drop plural optical signals of different wavelengths for each wavelength, or whether to transmit the optical signals through the multiplexer. In  FIG. 1 , there are shown the start points and end points of five optical signals λ 1  (lambda  1 ) to λ 5  each having different wavelengths, all of which are added and dropped at nodes that can be freely selected. 
         [0006]      FIG. 2  is a block diagram showing a principal part of the optical transmission network.  FIG. 2  shows the device configuration of a portion of the optical network shown in  FIG. 1 , from an optical add-drop multiplexer  101 - 1 , to an optical add-drop multiplexer  101 - 2 , and to an optical add-drop multiplexer  101 - 3 . However, in  FIG. 2 , there are only shown an East side function part for the optical add-drop multiplexer  101 - 1  and a West side function part for the optical add-drop multiplexer  101 - 3 . The optical add-drop multiplexer  101  includes an optical amplification function part (West)  202 - 2 , an optical amplification function part (East)  202 - 1 , an optical add-drop function part (West)  201 - 2 , and an optical add-drop function part (East)  202 - 1 . The optical amplification function part  202  includes: a reception optical amplifier  203  for amplifying an input optical signal from the optical fiber transmission line  102  and transmitting the amplified signal to the optical add-drop function part  201 ; and a transmission optical amplifier  204  for amplifying an input optical signal from the optical add-drop function part  201  and transmitting the amplified signal to the optical fiber transmission line  102 . The optical add-drop function part  201  includes: an optical drop part having an optical coupler  206 - 2  and an optical demultiplexer  207 ; and an optical transmission/add selection part having the optical demultiplexer  207 , an optical multiplexer  208 , an optical switch  209 , and a variable optical attenuator (VOA)  210 . 
         [0007]    Taking an example of the optical signal flow in a direction from West to East in the optical add-drop multiplexer  101 - 2 , the operation of the entire optical add-drop multiplexer will be described. A received optical signal from the optical add-drop multiplexer  101 - 1  is amplified by the reception optical amplifier  203  of the optical amplification function part (West)  202 - 1  of the optical add-drop multiplexer  101 - 2 . Then the amplified signal is transmitted to the optical add-drop function part (West)  201 - 2 . Incidentally, the operations of an optical coupler  206 - 1  and a laser safety part  205  will be described below with reference to  FIG. 4 . In the optical add-drop function part (West)  201 - 2 , the optical signal is split into two halves by the optical coupler  206 - 2 , one of which is further branched into lights at each wavelength by the optical demultiplexer  207  and is output from a drop optical port  260 - 2 . The other optical signal is transmitted as it is to the optical add-drop function part (East)  201 - 1  through an optical fiber  211  connecting the optical add-drop function parts. In the optical add-drop function part (East)  201 - 1 , the optical signal is branched into optical signals at different wavelengths by the optical demultiplexer  207 , and the signals are input to the optical switch  209 . The optical switch  209  selects and outputs either the transmitted optical signal from West or the added optical signal from an add optical port  250 - 2  of the optical add-drop multiplexer  101 - 2 . The variable optical attenuator  210  is provided in the later stage of the optical switch  209  to equally adjust all the optical power levels of each of the wavelengths. The light whose optical power levels are adjusted by the variable optical attenuator  210  is wavelength multiplexed by the optical multiplexer  208 , and is transmitted to the optical amplification function part (East)  202 - 1 . In the optical amplification function part (East)  202 - 1 , the wavelength multiplexed light is amplified by the transmission optical amplifier  204  and is transmitted to the optical fiber transmission line  102 . 
         [0008]      FIG. 3  is a view illustrating the optical signal flow from the optical add-drop multiplexer  101 - 1 , to the optical add-drop multiplexer  101 - 2 , and to the optical add-drop multiplexer  101 - 3  in the optical network of  FIG. 1 . The optical signal λ 1  is dropped and added in the optical add-drop function part (East)  201 - 1  of the optical add-drop multiplexer  101 - 1  and in the optical add-drop function part (West)  201 - 2  of the optical add-drop multiplexer  101 - 3 , while being transmitted through the optical add-drop multiplexer  101 - 2 . Similarly, the optical signal λ 2  is dropped and added in the optical add-drop multiplexers A, B. The optical signal λ 3  is transmitted through the optical add-drop multiplexer  101 - 1 , while being dropped and added in the optical add-drop multiplexer  101 - 2 . The optical signal λ 4  is dropped and added in the optical add-drop multiplexer  101 - 2 , while being transmitted through the optical add-drop multiplexer  101 - 3 . 
         [0009]    In  FIG. 2 , the reception optical amplifier  203  of each of the optical amplification function parts  202 - 1 ,  202 - 2  has a function of compensating the optical power reduction including not only a loss in the optical fiber transmission line but also a loss in the optical add-drop function part. Consequently the optical power level is high. Assuming that the optical power level for one wavelength is +6 dBm in the reception optical amplifier  203 , the optical power level for 40 wavelengths reaches +22 dBm which corresponds to a laser standard class 3B defined by JIS C 6082. There is a risk that the eyes will remain damaged by directly seeing such a laser beam. In order to avoid such a risk, the reception optical amplifier  203  includes a laser safety function for automatically reducing the optical power level to about an optical power level at one wavelength (about +5 dBm or less) which corresponds to a class  1  standard, by detecting an output open of the optical fiber by reflected light. In  FIG. 2 , the laser safety function is realized using the optical coupler  206 - 1  and the laser safety part  205 . The laser safety function in the optical amplifier as described above is disclosed in JP-A No. 200130/1997, JP-A No. 144687/2001, and JP-A No. 335214/2002. 
         [0010]    In the configuration of  FIG. 2 , it is assumed that a failure occurs in the optical add-drop function part (East)  201 - 1  of the optical add-drop multiplexer  101 - 2  and the relevant function part is needed to be replaced. In this case, main signal interruption occurs in the two signals λl, λ 4  of the optical signals shown in  FIG. 3 , and main signal interruption should not occur in the optical signals λ 2 , λ 3  that are originally not involved in the replacement. However, when the optical fiber  211  connected between the optical add-drop function part (West)  201 - 2  and the optical add-drop function part (East)  201 - 1  is removed, the laser safety part  205  detects an output open of the optical fiber by reflected light, thereby providing laser safety to the reception optical amplifier  203 . Given the optical level per wavelength of +6 dBm in the output of the reception optical amplifier  203 , the reception optical amplifier  203  of the optical amplification function part (West)  202 - 2  amplifies the three signals of λ 1 , λ 2 , λ 3 , so that the total optical power level of all the optical signals is +10.8 dBm. This will be reduced to +5 dBm because the laser safety functions due to removal of the optical fiber connected between the optical add-drop function parts. In other words, the optical power level per wavelength is reduced by 4.8 dB to +1.2 dBm, which has an impact on main signal continuity of the optical signals λ 2  and λ 3 , causing main signal interruption. The above description has been made on the optical signals of three wavelengths. However, assuming that the present system is a system supporting 40 wavelengths, the optical level is reduced by up to 16 dB according to the calculation in the same way as described above. 
         [0011]    The simplest way to solve the above problem is to insert an optical isolator into an input end from the reception optical amplifier of the optical add-drop function part . In this case, however, the laser safety does not function because the optical fiber is opened during the replacement of the optical add-drop function part. From the output of the reception optical amplifier to the fiber connected between the optical add-drop function parts, the optical loss occurs only in the optical isolator and the optical coupler, and their loss is at most about 2 dB in total. In the case of 40-wavelength system, the eyes may be damaged by directly seeing a maximum of +20 dBm during removal of the optical fiber  211  between the optical add-drop function parts. For this reason, in the method of inserting the optical isolator, it is necessary to have a structure in which a light blocking function such as an optical fiber connection shutter is provided to prevent the eyes from directly seeing the light from the optical fiber. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides optical transmission equipment and an optical add-drop multiplexer that have no impact on a main signal without being involved in a maintenance operation for package replacement necessary due to a failure in the optical add-drop multiplexer. 
         [0013]    The above described object can be achieved by optical transmission equipment including an optical amplification function part and an optical drop function part. The optical amplification function part includes: an optical amplifier connected to a transmission line to amplify the optical signal received from the transmission line; and a first laser safety part for controlling the optical amplifier based on the power of output light of the optical amplifier and on the power of reflected light to the first optical amplifier. The optical drop function part includes: an optical coupler connected to the optical amplification function part to split the optical signal received from the optical amplification function part; an optical demultiplexer for wavelength demultiplexing a first output optical signal of the optical coupler; a reflected light mask part for receiving a second output optical signal of the optical coupler as an input; and a second laser safety part for controlling the reflected light mask part based on the power of output light of the reflected light mask part and on the power of reflected light to the reflected light mask part. 
         [0014]    Further the above object can be achieved by an optical add-drop multiplexer including: an optical amplification function part; an optical drop function part; an optical add function part; and a second optical amplifier connected to the optical add function part to amplify an optical signal received from the optical add function part. The optical amplification function part includes: a first optical amplifier connected to a transmission line to amplify an optical signal received from the transmission line; and a first laser safety part for controlling the first optical amplifier based on the power of output light of the first optical amplifier and on the power of reflected light to the first optical amplifier. The optical drop function part includes: an optical coupler connected to the optical amplification function part to split the optical signal received from the optical amplification function part; a first optical demultiplexer for wavelength demultiplexing a first output optical signal of the optical coupler; a reflected light mask part for receiving a second output optical signal of the optical coupler as an input; and a second laser safety part for controlling the reflected light mask part based on the power of output light of the reflected light mask part and on the power of reflected light to the reflected light mask part. The optical add function part includes: a second optical demultiplexer connected to the optical drop function part to wavelength demultiplex the optical signal received from the optical drop function part; plural optical switches for receiving the outputs of the second optical demultiplexer and receiving from an add optical port, as inputs; and an optical multiplexer for wavelength multiplexing the outputs of the plural optical switches. 
         [0015]    Further the above object can be achieved by an optical add-drop multiplexer including: a reception optical amplification part including a first optical amplifier for amplifying received wavelength multiplexed light; an optical drop part for wavelength demultiplexing a portion of the wavelength multiplexed light; an optical add part for replacing the wavelength demultiplexed signals of the other portion of the wavelength multiplexed light; and a transmission optical amplification part including a second optical amplifier for amplifying the wavelength multiplexed light from the optical add part. The reception optical amplification part has an output portion provided with a first laser safety part for controlling the first optical amplifier by monitoring reflected light. Further the optical drop part has an output portion provided with a reflected light mask part for monitoring and masking reflected light, and with a second laser safety part for controlling the reflected light mask part. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  is a block diagram of an optical transmission network using an optical add-drop multiplexer; 
           [0018]      FIG. 2  is a block diagram showing a principal part of the optical transmission network; 
           [0019]      FIG. 3  is a view illustrating the main signal flow in the optical transmission network; 
           [0020]      FIG. 4  is a block diagram showing a principal part of an optical add-drop multiplexer; 
           [0021]      FIG. 5  is a time chart illustrating the device operation when a transmission optical fiber of an optical add-drop function part is removed; and 
           [0022]      FIG. 6  is a block diagram showing a principal part of the optical add-drop multiplexer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Modes for carrying out the invention will be described below based on preferred embodiments with reference to the accompanying drawings. Like parts are given like reference numbers and their description will not be repeated. 
       Embodiment 1 
       [0024]    A first embodiment will be described with reference to  FIGS. 4 and 5 . Here,  FIG. 4  is a block diagram showing a principal part of an optical add-drop multiplexer.  FIG. 5  is a time chart illustrating the device operation when a transmission optical fiber of an optical add-drop function part is removed. 
         [0025]      FIG. 4  shows an optical amplification function part and an optical add-drop function part, both only on West side in the optical add-drop multiplexer. As for optical parts, an optical switch  209 , an optical coupler  206 - 3 , a reflected light monitoring optical detector  401 - 3 , an output light monitoring optical detector  401 - 4 , and an optical terminator  406  are added to a transmitted light output end of an optical add-drop function part  201 - 2 . Here, the optical coupler  206 - 3  splits the optical signal to the optical fiber  211  and to the optical detector  401 - 4 , while splitting the reflected light to the optical switch  209  and to the optical detector  401 - 3 . The optical terminator  406  is an optical attenuator. 
         [0026]    In the optical add-drop function part  201 - 2 , the optical signal is converted by the output light monitoring optical detector  401 - 4  into an electrical signal that is proportionate to the output light power from the optical add-drop function part  201 - 2 , and the optical signal is converted by the reflected light monitoring optical detector  401 - 3  into an electrical signal that is proportionate to the reflected light power upon removal of the optical fiber  211  connected between the optical add-drop function parts. Then the amount of reflection is calculated in a reflection level calculation circuit  402 - 2  by the monitor results of the optical detectors. The reflection level calculation circuit  402 - 2  performs the following calculation: 
         [0000]      Amount of reflection=Reflected light power/Output light power . . .  (1) 
         [0000]    If no output light power is given, the calculation circuit outputs a certain fixed amount of reflection without performing the above calculation process. This is to continue warning even when the output light power is absent. A reflection warning determination circuit  403 - 2  periodically monitors the calculation result of the calculation circuit at an interval of 1 ms, and transmits a reflection warning to an optical switch selection circuit  405  when the amount of reflection exceeds a reference value for a predetermined time period. Upon detection of the warning, the optical switch selection circuit  405  performs switching of the optical switch  209  so that the optical signal proceeds to the optical terminator  406 . The principle of reflection warning detection in the reception optical amplifier  203  of the optical amplification function part  202 - 2  is exactly the same as in the case of the optical add-drop function part. The laser safety circuit  205  of the reception optical amplifier  203  is different from the laser safety circuit of the optical add-drop function part  201 - 2 , in only a point that an optical amplifier control circuit  404  is provided instead of the optical switch selection circuit  405 . The reception optical amplifier  203  controls the output power to be constant by reducing the output of the optical amplifier to a safe level by the optical amplifier control circuit  404 , instead of switching the optical switch upon detection of the reflection warning due to removal of the fiber. 
         [0027]    In  FIG. 5 , (a) shows the state of the output fiber of the optical add-drop function part, (b) shows the calculation result of the reflection calculation circuit, (c) shows the output of the reflection warning determination circuit, (d) shows the control signal output of the optical switch control circuit, (e) shows the transmitted light output level of the optical switch part, (f) shows the calculation result of the reflection calculation circuit of the optical amplification function part, (g) shows the output of the reflection warning determination circuit, (h) shows the control signal output of the optical amplifier control circuit, and (i) shows the output level of the optical amplifier. 
         [0028]    In  FIG. 5 , the optical fiber  211  connecting the optical add-drop function parts is removed at time  0 . At the same time of the removal of the optical fiber  211 , the calculation result of the reflection calculation circuit of the optical add-drop function part and the calculation result of the reflection calculation circuit in the optical amplification function part are raised to their reflection detections, respectively. After time t 1  when the reflection calculation result of the optical add-drop function part  201 - 2  reaches the reflection warning detection level, the warning detection is actually determined by the reflection warning determination circuit  403 - 2 . Here, t 1  represents a protection time. Similarly, time t 3  represents a protection time from when the reflection calculation result of the optical amplification function part  202 - 2  reaches the reflection warning determination level to when the warning detection is actually determined. Here, t 1  is sufficiently smaller than t 3 . Time t 2  represents a time when the switching of the optical switch is completed in the optical add-drop function part  201 - 2  and when the optical signal is terminated in the optical terminator. Accordingly the value of t 2 −t 1  is an actual switching time on the optical switch hardware. Incidentally t 1  and t 3  are set by a timer of the reflection warning determination circuit  403 . 
         [0029]    When the optical fiber is removed at time  0 , the optical add-drop function part  201 - 2  and the optical amplification part  202 - 2  detect at substantially the same time that the reflection calculation result reaches the reflection warning detection level in the reflection level calculation circuit  402 . The optical add-drop function part  201 - 2  and the optical amplification function part  202 - 2  operate differently after the refection calculation. The optical add-drop function part  201 - 2  transmits a reflection warning by the reflection warning determination circuit  403 - 2  after the protection time t 1  when the reflection result reaches the reflection warning detection level, and then moves to laser safety operation by optical switch selection operation. On the other hand, the optical amplification function part  201 - 2  does not detect the reflection warning because the reflection calculation result does not reach the protection time t 3  at the time when the protection time t 1  has passed, and remains in the normal mode of operation without moving to laser safety operation to the optical amplifier. At time t 2  when the optical switch selection operation is completed, the optical add-drop function part  201 - 2  is still detecting the reflection warning. However, the optical amplification function part  202 - 2  does not detect the reflected light as the light is terminated in the optical add-drop function part  201 - 2 , in which the reflection calculation result is below the reflection warning detection level. At time t 2  when the reflection calculation result does not reach the protection time t 3  of the reflection warning detection in the optical amplification function part  202 - 2 , the optical amplification function part continues normal operation without entering at all the laser safety operation which is performed in response to the detection of reflected light. 
         [0030]    Incidentally, when the transmitted light level in the optical switch part is reduced to the safe level and the output light power is “0” in the reflection calculation circuit of the optical add-drop function part, the reflection calculation circuit outputs a certain fixed amount of reflection. As a result, the optical add-drop function part continues to detect the warning. 
         [0031]    In the case of using a mechanical optical switch as the optical switch  209 , the switching time, t 2 −t 1 , in the optical switch  209  is about 1 ms. Assuming that t 1  is 5 ms, the time needed for the optical add-drop function part  201 - 2  to move to the laser safety operation is 6 ms. Accordingly the protection time t 3  of the reflection warning detection in the optical amplification function part  202 - 2  can be determined to 60 ms which is ten times larger than 6 ms. Incidentally 60 ms is a sufficiently short time as an exposure time of the eyes to the class 3B laser. 
         [0032]    By applying the embodiment to the optical add-drop multiplexer  101 - 2  in the network configuration shown in  FIG. 2 , even when the optical fiber  211  connecting the optical add-drop function parts is removed for package replacement necessary due to a failure in the optical add-drop function part (East)  201 - 1 , the maintenance operation can be performed without having any impact on the optical signals λ 2 , λ 3  that are originally not involved in the package replacement because the optical amplification function part (West)  202 - 2  does not enter the laser safety operation. In addition, there is no risk that the eyes will be damaged by directly seeing the optical fiber as the transmitted light output of the optical add-drop function part (West)  202 - 2  is terminated by the optical switch  209 . However, once the optical add-drop function part  202 - 2  enters the laser safety, the light is perfectly terminated and the reflection warning recovery will not be detected any more. For this reason, it is necessary to return the optical switch to the normal line by an operator operation when the maintenance operation is completed. 
         [0033]    Incidentally  FIG. 2  shows an example of the two-way network, but the embodiment can be applied to transmission equipment for one-way transmission lines. This is the same in a second embodiment. The optical switch  209  and the optical terminator  406  can be a reflected light mask part for preventing the reflected light from being seen by the optical amplification function part  202 - 2 . 
         [0034]    According to the embodiment, the laser safety can be operated solely by the optical add-drop function part, without being operated by the optical amplification function part. Because of this feature, even when the optical fiber is removed in order to replace the optical add-drop function part in the maintenance operation, there is no impact on the optical signals that are not involved in the maintenance operation at all as the laser safety is not operated in the optical amplification function part. In addition, it is possible to eliminate the risk of damaging the eyes by directly seeing high power output light during the maintenance operation. 
       Embodiment 2 
       [0035]    A second embodiment will be described with reference to  FIG. 6 . Here,  FIG. 6  is a block diagram showing a principal part of an optical add-drop multiplexer. The differences from the optical add-drop multiplexer shown in  FIG. 4  are that the optical termination function of the optical add-drop function part  201 - 2  is realized by a variable optical attenuator  270  instead of by the optical switch  209  and the optical terminator  406 , and that the optical switch selection circuit  405  of the laser safety  220  is replaced with a variable optical attenuator control circuit  407  of a laser safety  230 . 
         [0036]    The operation principle itself is substantially the same as in the first embodiment. Only the difference is that when the reflection warning is detected, the signal light is terminated by increasing the attenuation amount of the variable optical attenuator  270  to about 20 dB at once, instead of being terminated by switching the optical switch. 
         [0037]    When a thermo-optic variable optical attenuator is used as the variable optical attenuator  270 , the attenuation amount change time, t 2 −t 1 , in the variable optical attenuator is about 50 ms. Assuming that the protection time t 1  of the reflection warning detection in the optical add-drop function part is 5 ms, the time needed for the optical add-drop function part to move to the laser safety operation is 55 ms. Accordingly it is necessary to determine the protection time t 3  of the reflection warning detection in the optical amplification function part to about 550 ms which is about ten times larger than 55 ms. Incidentally 550 ms is a sufficiently short time as an exposure time of the eyes to the class 3B laser. The variable optical attenuator  270  can be a reflected light mask part for preventing the reflected light from being seen by the optical amplification function part  202 - 2 . 
         [0038]    Also according to the embodiment, the laser safety can be solely controlled by the optical add-drop function part, without being controlled by the optical amplification function part. Because of this feature, even when the optical fiber is removed in order to replace the optical add-drop function part in the maintenance operation, there is no impact on the optical signals that are not involved in the maintenance operation as the laser safety is not operated in the optical amplification function part. In addition, it is possible to eliminate the risk of damaging the eyes by directly seeing high power output light during the maintenance operation. 
         [0039]    As compared to the first embodiment, the advantage of the second embodiment is that the transmitted light is not perfectly blocked but is only attenuated by 20 dB in the termination by the variable optical attenuator  270 . Because the transmitted light is not perfectly blocked as described above, the refection warning recovery can be detected when the optical fiber, which has been removed during the maintenance, is returned to the original position. Thus, unlike the first embodiment, it is possible to automatically return to the normal operation without the need for operator operation, when the maintenance operation is completed. 
         [0040]    According to the present invention, it is possible to provide optical transmission equipment and an optical add-drop multiplexer that have no impact on a main signal without being involved in a maintenance operation for package replacement necessary due to a failure in the optical transmission equipment or in the optical add-drop multiplexer.