Abstract:
This apparatus of bidirectional optical recirculation loop transmission enables bidirectional transmission system to be tested in the long transmission distance. Unidirectional optical recirculation loop is composed of two optical modulators and one 4-port optical coupler. Bidirectional optical recirculation loop is composed of four optical switches, one 4-port optical coupler and six optical circulators. Two optical circulators at the entrance (simultaneously exit) of the loop enable transmitted (received) signals to be added (dropped). Four optical circulators enable forward (reverse) signal to bypass the optical switch set for reverse (forward) signal in the inner optical loop. Forward (reverse) signal can be transmitted simultaneously with the reverse (forward) signal without interference. Two independent optical recirculation loops exist on the same fiber link in each direction.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims the benefit of Korean Patent Application No. 10-2004-0103673, filed on Dec. 9, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a bidirectional optical recirculation loop transmitting device, and more particularly, to a bidirectional optical recirculation loop transmitting device capable of long-distance transmission using a small number of test samples in a wavelength division multiple optical transmission system.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 1  shows a configuration of a conventional unidirectional optical recirculation loop transmitting device. Referring to  FIG. 1 , the conventional unidirectional optical recirculation loop transmitting device includes a transmitting unit  100  for generating and transmitting an N-number of optical signals, a wavelength division multiplexer (WDM)  110  for multiplexing the N-number of optical signals, a first optical amplifier  121  for amplifying multiplexed optical signals, a second optical switch  132  for selectively passing an optical signal output from the first optical amplifier under the control of a controller  190  that is described later, an optical combiner  140  for separating an optical signal output from the second optical switch  132  into two optical signals and outputting the separated optical signals, an optical link  150  that is a path along which one of the separated optical signals is transmitted, a first optical switch  131  for selectively passing the optical signal transmitted along the optical link  150  under the control of the controller  190 , a second optical amplifier  122  for amplifying the other one of the two optical signal separated from the optical combiner  140 , a wavelength division demultiplexer  160  for separating an optical signal output from the second optical amplifier  122  according to the wavelength of each channel, a receiving unit  170  for receiving an optical signal output from the wavelength division demultiplexer  160 , a measuring instrument  180  connected to the receiving unit  170  and monitoring performance of an optical signal per channel of the unidirectional optical recirculation loop transmitting device, and the controller  190  for controlling the first optical switch  131 , the second optical switch  132 , and the measuring instrument  180 .  
         [0006]     The transmitting unit  100  consists of an N-number of transmitters from a first transmitter  1001  to an N th  transmitter  100 N. The receiving unit  170  consists of an N-number of receivers from a first receiver  1701  to an N th  receiver  170 N. The optical link  150 , which is a transmission path of an optical signal, consists of a plurality of nodes and optical fibers.  
         [0007]      FIG. 2  is a time diagram with respect to the first optical switch signal, the second optical switch signal, an optical recirculation loop output signal, and a gate trigger signal of  FIG. 1 . Referring to  FIG. 2 , the first optical switch  131  and the second optical switch  132  operate in the opposite states. That is, from a time point at which t=0 to a time point at which t=1T, the second optical switch  132  is in an “ON” state while the first optical switch  131  is in an “OFF” state. Also, from a time point at which t=1T to a time point at which t=nT, the second optical switch  132  is in the “OFF” state while the first optical switch  131  is in the “ON” state. It can be seen that a period when the second optical switch  132  remains in the “ON” state is equivalent to a time T needed for the optical signal to proceed in the optical link  150  in the optical recirculation loop.  
         [0008]     When the second optical switch  132  is in the “ON” state from the time point at which t=0 to the time point at which t=1T, the optical signal output from the transmitting unit  100  is input to a first port of the optical combiner  140  having four ports: half of the input optical signal is transmitted to the receiving unit  170  via a third port and the other half is transmitted to the optical link  150  via a fourth port. An optical signal equivalent to the time T output from the transmitting unit  100  from the time point at which t=0 to the time point at which t=1T is transmitted to the receiving unit  170  and the optical signal equivalent to the time T remains in the optical link  150 .  
         [0009]     When the second optical switch  132  is in the “OFF” state and the first optical switch  131  is in the “ON” state at the time point at which t=1T, the optical signal passing through the optical link  150  is input to a second port of the optical combiner  140  having four ports: half of the input optical signal is transmitted to the receiving unit  170  via the third port and the other half is transmitted to the optical link  150  again via the fourth port.  
         [0010]     According to the same operation principle, as time passes, optical signals having traveled a longer distance are sequentially received by the receiving unit  170 . As shown in  FIG. 2 , the receiving unit  170  can receive an optical signal transmitted after rotating an n-1 turn.  
         [0011]     When the length of the optical link  150  is M km, the receiving unit  170  can sequentially receive an optical signal transmitted 0 km to (n-1)M km. Since the optical signals sequentially arrive at the receiving unit  170 , in  FIG. 2 , the optical intensity at the receiving unit  170  represented by an optical recirculation loop output is detected to indicate that an optical signal is always present.  
         [0012]     When performance of an optical signal corresponding to a desired transmission distance is to be measured by the measuring instrument  180  connected to the receiving unit  170 , only a range of the optical signals corresponding to the number of rotations needs to be detected from the sequential signals. To do so, only a portion corresponding to a particular rotation number k is gated like the gate trigger in  FIG. 2  to use only a value from a time point at which t=kT to a time point at which t=(k+1)T as a measurement material while the other portion is excluded from the measurement material. Since there may be a contaminated optical signal at around a boundary region of the rotation number when gate trigger is performed, to avoid a measurement error, a protection time A is provided at either side of the boundary so that the performance of the optical signal is measured from a time point at which t=kT+Δ to a time point at which t=(k+1)T−Δ. By configuring the optical recirculation loop as above and conducting the test, a long distance transmission is made possible with a small number of test samples.  
         [0013]     However, the conventional unidirectional optical recirculation loop transmitting device described with reference to  FIGS. 1 and 2  has a problem that an optical signal proceeding in the opposite direction cannot be generated. That is, the conventional transmitting device can be used for a unidirectional optical recirculation loop transmitting device, but it cannot be used for a bidirectional optical recirculation loop transmitting system.  
       SUMMARY OF THE INVENTION  
       [0014]     To solve the above and/or other problems, the present invention provides a bidirectional optical recirculation loop transmitting device using an optical recirculation loop having a plurality of optical switches and a 4-port optical combiner, and an optical circulator for separating or inserting a forward directional optical signal and a reverse directional optical signal.  
         [0015]     The present invention provides a bidirectional optical recirculation loop transmitting device using an optical recirculation loop having a plurality of optical switches and a 4-port optical combiner, and an interleaver for separating or inserting a forward directional optical signal and a reverse directional optical signal.  
         [0016]     The present invention provides a bidirectional optical recirculation loop transmitting device using an optical recirculation loop having a plurality of optical switches and a 4-port optical combiner, and an band pass filter for separating or inserting a forward directional optical signal and a reverse directional optical signal.  
         [0017]     According to an aspect of the present invention, a bidirectional optical recirculation loop transmitting device for transmitting a forward directional optical signal generated from an end and a reverse directional optical signal generated from the other end to the other end and the end, respectively, comprises an optical combiner having four ports which, when receiving an optical signal from a first port and a second port, separately outputs one to a third port and the other to a fourth port and, when receiving an optical signal from the third port and the fourth port, separately outputs one to the first port and the other to the second port, a first optical circulator which receives the forward directional optical signal generated from the end and outputs the received forward directional optical signal to connect to the first port of the optical combiner, and receives the reverse directional optical signal from the first port of the optical combiner and outputs the received reverse directional optical signal to a receiving end of the reverse directional optical signal formed at the end, a second optical circulator which receives the reverse directional optical signal generated from the other end and outputs the received reverse directional optical signal to connect to the third port of the optical combiner, and receives the forward directional optical signal from the third port of the optical combiner and outputs the received forward directional optical signal to a receiving end of the forward directional optical signal formed at the other end, a first connection unit which allows an optical link and the second port of the optical combiner to be in an “ON” connection state during a period from a time point at which the forward directional optical signal is stopped from being input to the first optical circulator to a time point at which the input of the forward directional optical signal resumes, and a second connection unit which allows the optical link and the fourth port of the optical combiner to be in an “ON” connection state during a period from a time point at which the reverse directional optical signal is stopped from being input to the second optical circulator to a time point at which the input of the reverse directional optical signal resumes.  
         [0018]     According to another aspect of the present invention, a bidirectional optical recirculation loop transmitting device for transmitting a forward directional optical signal generated from an end and a reverse directional optical signal generated from the other end to the other end and the end, respectively, comprises an optical combiner having four ports which, when receiving an optical signal from a first port and a second port, separately outputs one to a third port and the other to a fourth port and, when receiving an optical signal from the third port and the fourth port, separately outputs one to the first port and the other to the second port, a first interleaver which receives the forward directional optical signal from a transmitting end of the forward directional optical signal and outputs the received forward directional optical signal to connect to the first port of the optical combiner, and receives the reverse directional optical signal from the first port of the optical combiner and outputs the received reverse directional optical signal to a receiving end of the reverse directional optical signal formed at the end, a second interleaver which receives the reverse directional optical signal from a transmitting end of the reverse directional optical signal and outputs the received reverse directional optical signal to connect to the third port of the optical combiner, and receives the forward directional optical signal from the third port of the optical combiner and outputs the received forward directional optical signal to a receiving end of the forward directional optical signal formed at the other end, a first connection unit which allows an optical link and the second port of the optical combiner to be in an “ON” connection state during a period from a time point at which the forward directional optical signal is stopped from being input to the first interleaver to a time point at which the input of the forward directional optical signal resumes, and a second connection unit which allows the optical link and the fourth port of the optical combiner to be in an “ON” connection state during a period from a time point at which the reverse directional optical signal is stopped from being input to the second interleaver to a time point at which the input of the reverse directional optical signal resumes.  
         [0019]     According to another aspect of the present invention, a bidirectional optical recirculation loop transmitting device for transmitting a forward directional optical signal generated from an end and formed of a lower band and a reverse directional optical signal generated from the other end and formed of an upper band to the other end and the end, respectively, comprises an optical combiner having four ports which, when receiving an optical signal from a first port and a second port, separately outputs one to a third port and the other to a fourth port and, when receiving an optical signal from the third port and the fourth port, separately outputs one to the first port and the other to the second port, a first band selection filter which receives the forward directional optical signal from a transmitting end of the forward directional optical signal and outputs the received forward directional optical signal to connect to the first port of the optical combiner, and receives the reverse directional optical signal from the first port of the optical combiner and outputs the received reverse directional optical signal to a receiving end of the reverse directional optical signal formed at the end, a second band selection filter which receives the reverse directional optical signal from a transmitting end of the reverse directional optical signal and outputs the received reverse directional optical signal to connect to the third port of the optical combiner, and receives the forward directional optical signal from the third port of the optical combiner and outputs the received forward directional optical signal to a receiving end of the forward directional optical signal formed at the other end, a first connection unit which allows an optical link and the second port of the optical combiner to be in an “ON” connection state during a period from a time point at which the forward directional optical signal is stopped from being input to the first band selection filter to a time point at which the input of the forward directional optical signal resumes, and a second connection unit which allows the optical link and the fourth port of the optical combiner to be in an “ON” connection state during a period from a time point at which the reverse directional optical signal is stopped from being input to the second band selection filter to a time point at which the input of the reverse directional optical signal resumes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The above and other features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0021]      FIG. 1  is a view illustrating a configuration of a conventional unidirectional optical recirculation loop transmitting device;  
         [0022]      FIG. 2  is a time diagram with respect to a first optical switch signal, a second optical switch signal, an optical recirculation loop output signal, and a gate trigger signal of  FIG. 1 ;  
         [0023]      FIG. 3  is a view illustrating a configuration of a bidirectional optical recirculation loop transmitting device according to an embodiment of the present invention;  
         [0024]      FIG. 4  is a view illustrating a configuration of a bidirectional optical recirculation loop transmitting device according to another embodiment of the present invention; and  
         [0025]      FIG. 5  is a view illustrating a configuration of a bidirectional optical recirculation loop transmitting device according to yet another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Referring to  FIG. 3 , to control a forward directional (clockwise) optical signal and a reverse directional (counterclockwise) optical signal, a control unit  390  generates a control signal (a pulse signal) to control a first optical switch  351 , a second optical switch  352 , a third optical witch  353 , and a fourth optical switch  354 , and a control signal (a pulse signal) to trigger an odd number channel measuring instrument  391  and an even number channel measuring instrument  392 . An even number channel is defined as a route along which a signal proceeds in a forward direction (clockwise) and an odd number channel is defined as a route along which a signal proceeds in a reverse direction (counterclockwise).  
         [0027]     The even number channel transmitting unit  300  generates and transmits a forward directional optical signal to proceed along the even number channel. The even number channel transmitting unit  300  includes a second transmitter  3002 , . . . , and an N th  transmitter  300 N, where “N” is an even number.  
         [0028]     An even number channel wavelength division multiplexer (WDM)  320  receives the forward directional optical signal from the even number channel transmitting unit  300  and multiplexes the received optical signal. A first optical amplifier  341  amplifies the forward directional optical signal multiplexed by the even number channel WDM  320 . A third optical switch  353  selectively passes the forward directional optical signal output from the first optical amplifier  341  under the control of the control unit  390 .  
         [0029]     A first optical circulator  361  connects the forward directional optical signal input through the third optical switch  353  to a first port of the optical combiner  370  and separates a reverse directional optical signal input through the first port of the optical combiner  370  to proceed toward an odd number channel receiving unit  315 .  
         [0030]     An odd number channel transmitting unit  305  generates and transmits the reverse directional optical signal to proceed along the odd number channel. The odd number channel transmitting unit  305  includes a first transmitter  3051 , . . . , and an (N−1) th  transmitter  305 N−1.  
         [0031]     An odd number channel wavelength division multiplexer (WDM)  325  receives the reverse directional optical signal from the odd number channel transmitting unit  305  and multiplexes the received optical signal. A third optical amplifier  343  amplifies the reverse directional optical signal multiplexed by the odd number channel WDM  325 . A fourth optical switch  354  selectively passes the reverse directional optical signal output from the third optical amplifier  343  under the control of the control unit  390 .  
         [0032]     A second optical circulator  362  connects the reverse directional optical signal input through the fourth optical switch  354  to a third port of the optical combiner  370  and separates the forward directional optical signal input through the third port of the optical combiner  370  to proceed toward an even number channel receiving unit  310 .  
         [0033]     A fourth optical amplifier  344  amplifies the reverse directional optical signal separated from the first optical circulator  361 . An odd number channel wavelength division demultiplexer  335  receives the reverse directional optical signal from the fourth optical amplifier  344  and demultiplexes the received optical signal. An odd number channel receiving unit  315  receives the reverse directional optical signal for each channel from the odd number channel wavelength division demultiplexer  335 . The odd number channel receiving unit  315  includes a first receiver  3151 , . . . , and an (N−1) th  receiver  315 N−1.  
         [0034]     The odd number channel measuring instrument  391  measures performance of an optical signal corresponding to a desired transmission distance with respect to the optical signal received through the odd number channel receiving unit  315  under the control of the control unit  390 .  
         [0035]     A second optical amplifier  342  amplifies the forward directional optical signal separated from the second optical circulator  362 . An even number channel wavelength division demultiplexer  330  receives the forward directional optical signal from the second optical amplifier  342  and demultiplexes the received optical signal.  
         [0036]     The even number channel receiving unit  310  receives the forward directional optical signal for each channel from the even number channel wavelength division demultiplexer  330 . The even number channel receiving unit  310  includes a second receiver  3102 , . . . , and an N th  receiver  310 N.  
         [0037]     The even number channel measuring instrument  392  measures performance of an optical signal corresponding to a desired transmission distance with respect to the optical signal received through the even number channel receiving unit  310  under the control of the control unit  390 .  
         [0038]     The optical combiner  370  having four ports and the first port, the second port, the third port, and the fourth port are connected to the first optical circulator  361 , a third optical circulator  363 , the second circulator  362 , and a fifth optical circulator  365 , respectively.  
         [0039]     The forward directional optical signal generated from the even number channel transmitting unit  300  in the forward direction passes through the third optical switch  353  and is input to the first port of the optical combiner  370  through the first optical circulator  361 .  
         [0040]     Half of the forward directional optical signal input to the optical combiner  370  passes through the second optical circulator  362  via the third port of the optical combiner  370  and is transmitted to the even number channel receiving unit  310  so as to be a zero-turn signal.  
         [0041]     The other half of the forward directional optical signal input to the optical combiner  370  is transmitted to a sixth optical circulator  366 , bypassing the second optical switch  362 , through the fourth port of the optical combiner  370  and the fifth optical circulator  365 . The forward directional optical signal passing through the sixth optical circulator  366  is transmitted along an optical link  380  in the optical recirculation loop, sequentially passes through a fourth optical circulator  364 , the first optical switch  351 , and the third optical circulator  363 , and arrives at the second port of the optical combiner  370 .  
         [0042]     Half of the forward directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted to the even number channel receiving unit  310  through the third port of the optical combiner  370  and the second optical circulator  362 , so as to be a one-turn signal. The other half of the forward directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted through the sixth optical circulator  366 , bypassing the second optical switch  352 , through the fourth port of the optical combiner  370  and the fifth optical circulator  365 . The optical signal transmitted through the sixth optical circulator  366  is transmitted along the optical link  380  and sequentially passes through the fourth optical circulator  364 , the first optical switch  351 , and the third optical circulator  363  to arrive again at the second port of the optical combiner  370 .  
         [0043]     By repeating the above process, the half of the forward directional optical signal input to the optical combiner  370  is transmitted to the second optical circulator  362  and then to the even number channel receiving unit  310 . The other half of the forward directional optical signal is transmitted to circulate in the forward direction through the fifth optical circulator  365  and the optical link  380 .  
         [0044]     The reverse directional optical signal generated from the odd number channel transmitting unit  305  in the reverse direction passes through the fourth optical switch  354  and is input to the third port of the optical combiner  370  through the second optical circulator  362 .  
         [0045]     Half of the reverse directional optical signal input to the optical combiner  370  passes through the first optical circulator  361  via the first port of the optical combiner  370  and is transmitted to the odd number channel receiving unit  315  so as to be a zero-turn signal.  
         [0046]     The other half of the reverse directional optical signal input to the optical combiner  370  is transmitted to the fourth optical circulator  364 , bypassing the first optical switch  361 , through the second port of the optical combiner  370  and the third optical circulator  363 . The reverse directional optical signal passing through the fourth optical circulator  364  is transmitted along the optical link  380 , sequentially passes through the sixth optical circulator  366 , the second optical switch  352 , and the fifth optical circulator  365 , and arrives at the fourth port of the optical combiner  370 .  
         [0047]     Half of the reverse directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted to the odd number channel receiving unit  315  through the first port of the optical combiner  370  and the first optical circulator  361 , so as to be a one-turn signal. The other half of the reverse directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted through the fourth optical circulator  364 , bypassing the first optical switch  351 , through the second port of the optical combiner  370  and the third optical circulator  363 . The optical signal transmitted through the fourth optical circulator  364  is transmitted along the optical link  380  and sequentially passes through the sixth optical circulator  366 , the second optical switch  352 , and the fifth optical circulator  365  to arrive again at the fourth port of the optical combiner  370 .  
         [0048]     By repeating the above process, the half of the reverse directional optical signal input to the optical combiner  370  is transmitted to the first optical circulator  361  and then to the odd number channel receiving unit  315 . The other half of the reverse directional optical signal is transmitted to circulate in the reverse direction through the third optical circulator  363  and the optical link  380 .  
         [0049]     In the performance of the respective optical circulators, the first optical circulator  361  outputs the forward directional optical signal output from the even number channel transmitting unit  300  in the optical combiner  370  and outputs the reverse directional optical signal output from the optical combiner  370  in the odd number receiving unit  315 . The second optical circulator  362  outputs the reverse directional optical signal output from the odd number channel transmitting unit  305  in the optical combiner  370  and outputs the forward directional optical signal output from the optical combiner  370  in the even number receiving unit  310 .  
         [0050]     The third optical circulator  363  and the fourth optical circulator  364  allow the reverse optical signal to bypass the first optical switch  351  while the fifth optical circulator  365  and the sixth optical circulator  366  allow the forward optical signal to bypass the second optical switch  352 . By including the first to sixth optical circulators  361 ,  362 ,  363 ,  364 ,  365 , and  366  as above, the forward directional optical signal and the reverse directional optical signal, although sharing a bidirectional link, independently circulate the optical recirculation loop so that the optical recirculation loop is available for a bidirectional transmission system.  
         [0051]     In the operation of the control unit  390 , the control unit  390  controls the third optical switch  353  and the first optical switch  351  to operate in the difference states. That is, from a time point at which t=0 to a time point at which t=1T, the third optical switch  353  is in the “ON” state while the first optical switch  351  is in the “OFF” state. From the time point at which t=1T to the time point at which t=nT, the third optical switch  353  is in the “OFF” state while the first optical switch  351  is in the “ON” state. It can be seen that a period when the third optical switch  353  remains in the “ON” state is equivalent to a time T needed for the optical signal to proceed in the optical link  380  in the optical recirculation loop.  
         [0052]     The control of the first, second, third, and fourth optical switches  351 ,  352 ,  353 , and  354  by the control unit  390  that are not described above will be described with reference to  FIG. 2 .  
         [0053]     The control unit  390  generates a control signal to control the odd number channel measuring instrument  391  and an even number channel measuring instrument  392 . Since the control unit  390  outputs the control signal to the odd number channel measuring instrument  391  and the even number channel measuring instrument  392 , only a portion corresponding to a particular rotation number k is gated like the gate trigger of  FIG. 2  so that only a value in a range from a time point at which t=kT to a time point at which t=(k+1)T is used as a test material. The other portion is excluded from the measuring material. Since there may be a contaminated optical signal at around a boundary region of the rotation number when gate trigger is performed, to avoid a measurement error, a protection time A is provided at either side of the boundary so that the performance of the optical signal is measured from a time point at which t=kT+Δ to a time point at which t=(k+1)T−Δ. By configuring the optical recirculation loop as above and conducting the test, a long distance transmission is made possible with a small number of test samples.  
         [0054]     The third optical circulator  363 , the first optical switch  351 , and the fourth optical circulator  364  constitute a first connection unit which allows the forward directional optical signal and the reverse directional optical signal to proceed as described above under the control of the control unit  390 . The fifth optical circulator  365 , the second optical switch  352 , and the sixth optical circulator  366  constitute a second connection unit which allows the forward directional optical signal and the reverse directional optical signal to proceed as described above under the control of the control unit  390 .  
         [0055]      FIG. 4  is a view illustrating a configuration of a bidirectional optical recirculation loop transmitting device according to another embodiment of the present invention. Referring to  FIG. 4 , when a bi-direction is divided by the even channel and the odd channel as shown in  FIG. 3 , the bidirectional optical recirculation loop transmitting device can be configured by using an interleaver (IL). The IL has a first port and a second port which respectively accommodate the even number channel and the odd number channel and is a passive device in which bidirectional proceeding of a signal is possible.  
         [0056]     For the even channel, an optical signal in a forward direction passes through the third optical switch  353  and is input to the first port of the optical combiner  370  having four ports, through the first port of a first IL  461 . Half of the forward directional signal input to the optical combiner  370  is transmitted to the even number channel receiving unit  310  through the first port of a second IL  462  so as to be a zero-turn signal.  
         [0057]     The other half of the forward directional optical signal input to the optical combiner  370  is transmitted to the first port of a sixth IL  466 , bypassing the second optical switch  362 , through the fourth port of the optical combiner  370  and the first port of a fifth IL  465 . The forward directional optical signal passing through the sixth IL  466  is transmitted along the optical link  380  in the optical recirculation loop, sequentially passes through the first port of a fourth IL  464 , the first optical switch  351 , and the first port of a third IL  463 , and arrives at the second port of the optical combiner  370 .  
         [0058]     Half of the forward directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted to the even number channel receiving unit  310 , passing through the third port of the optical combiner  370  and the first port of the second IL  462 , so as to be a one-turn signal. The other half of the forward directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted through the first port of the sixth IL  466 , bypassing the second optical switch  352 , through the fourth port of the optical combiner  370  and the first port of the fifth IL  465 . The optical signal transmitted through the sixth IL  466  is transmitted along the optical link  380  and sequentially passes through the first port of the fourth IL  464 , the first optical switch  351 , and the first port of the third IL  463  to arrive again at the second port of the optical combiner  370 .  
         [0059]     As described above, the half of the forward directional optical signal input to the optical combiner  370  is transmitted to the second IL  462  and then to the even number channel receiving unit  310  while the other half of the forward directional optical signal is transmitted to the fifth IL  465  to circulate along the optical link  380  of the optical recirculation loop.  
         [0060]     For the odd channel, an optical signal in a reverse direction passes through the fourth optical switch  354  and is input to the third port of the optical combiner  370 , through the second port of the second IL  462 . Half of the reverse directional optical signal input to the optical combiner  370  passes through the second port of the first IL  461  via the first port of the optical combiner  370  and is transmitted to the odd number channel receiving unit  315  so as to be a zero-turn signal.  
         [0061]     The other half of the reverse directional optical signal input to the optical combiner  370  is transmitted to the second port of the fourth IL  464 , bypassing the first optical switch  351 , through the second port of the optical combiner  370  and the second port of the third IL  463 . The reverse directional optical signal passing through the fourth IL  464  is transmitted along the optical link  380 , sequentially passes through the second port of the sixth IL  466 , the second optical switch  352 , and the second port of the fifth IL  465 , and arrives at the fourth port of the optical combiner  370 .  
         [0062]     Half of the reverse directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted to the odd number channel receiving unit  315  through the first port of the optical combiner  370  and the second port of the first IL  461 , so as to be a one-turn signal. The other half of the reverse directional optical signal arriving at the optical combiner  370  after making one turn along the optical link  380 , is transmitted through the second port of the fourth IL  464 , bypassing the first optical switch  351 , through the second port of the optical combiner  370  and the second port of the third IL  463 . The optical signal transmitted through the fourth IL  464  is transmitted along the optical link  380  and sequentially passes through the second port of the sixth IL  466 , the second optical switch  352 , and the second port of the fifth IL  465  to arrive again at the fourth port of the optical combiner  370 .  
         [0063]     As described above, the half of the reverse directional optical signal input to the optical combiner  370  is transmitted to the first IL  461  and then to the odd number channel receiving unit  315  while the other half of the reverse directional optical signal is transmitted to the third IL  463  to circulate along the optical link  380  of the optical recirculation loop.  
         [0064]     The third IL  463 , the first optical switch  351 , and the fourth IL  464  constitute the first connection unit to make the forward directional optical signal and the reverse directional optical signal proceed under the control of the control unit  390 . Although, in  FIG. 4 , all the optical circulators of  FIG. 3  are replaced with the interleavers, only a part of the optical circulators of  FIG. 3  can be replaced to form another embodiment. Please refer to the description of  FIG. 3  for portions not described in  FIG. 4 .  
         [0065]      FIG. 5  is a view illustrating a configuration of a bidirectional optical recirculation loop transmitting device according to yet another embodiment of the present invention. Referring to  FIG. 5 , the bi-direction is divided not into the even number channel and the odd number channel as shown in  FIGS. 3 and 4 , but into a lower band and an upper band. The bidirectional optical recirculation loop transmitting device according to yet another embodiment of the present invention can be configured by using a band selection filter (a band coupler or a band separator).  
         [0066]     The band selection filter has a first port and a second port which respectively accommodate the lower band and the upper band and is a passive device in which bidirectional proceeding of a signal is possible.  
         [0067]     A lower band transmitting unit  500  of  FIG. 5  corresponds to the even number channel transmitting unit  300  of  FIG. 3  or  4  while a lower band receiving unit  510  of  FIG. 5  corresponds to the even number channel receiving unit  310  of  FIG. 3  or  4 . An upper band transmitting unit  505  of  FIG. 5  corresponds to the odd number channel transmitting unit  305  of  FIG. 3  or  4  while an upper band receiving unit  515  of  FIG. 5  corresponds to the odd number channel receiving unit  315  of  FIG. 3  or  4 .  
         [0068]     The lower band transmitting unit  500  includes an (N+1) th  transmitter  500 N+1, . . . , and a 2N th  transmitter  5002 N while the lower band receiving unit  510  includes an (N+1) th  receiver  510 N+1, . . . , and a 2N th  receiver  5102 N. The upper band transmitting unit  505  includes a first transmitter  5051 , . . . , and an N th  transmitter  505 N while the upper band receiving unit  515  includes a first receiver  5151 , . . . , and an N th  receiver  515 N.  
         [0069]     The first band selection filter  561  corresponds to the first IL  461  of  FIG. 4 ; the second band selection filter  562  corresponds to the second IL  462  of  FIG. 4 ; the third band selection filter  563  corresponds to the third IL  463  of  FIG. 4 ; the fourth band selection filter  564  corresponds to the fourth IL  464  of  FIG. 4 ; the fifth band selection filter  565  corresponds to the fifth IL  465  of  FIG. 4 ; and the sixth band selection filter  566  corresponds to the sixth IL  466  of  FIG. 4 .  
         [0070]     The third band selection filter  563 , the first optical switch  351 , and the fourth band selection filter  564  constitute a first connection unit to make the forward directional optical signal and the reverse directional optical signal proceed under the control of the control unit  390  as shown in  FIGS. 3 and 4 . The fifth band selection filter  565 , the second optical switch  352 , and the sixth band selection filter  566  constitute a second connection unit to make the forward directional optical signal and the reverse directional optical signal proceed under the control of the control unit  390  as shown in  FIGS. 3 and 4 .  
         [0071]     Although, in  FIG. 5 , all the optical circulators of  FIG. 3  or all the interleavers of  FIG. 4  are replaced with the band selection filters, only a part of the optical circulators of  FIG. 3  or the interleavers of  FIG. 4  can be replaced to form another embodiment. Please refer to the description of  FIG. 3  or  4  for portions not described in  FIG. 5 .  
         [0072]     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.  
         [0073]     As described above, according to the bidirectional optical recirculation loop transmitting device according to the present invention, a drawback that the conventional unidirectional optical recirculation loop transmitting device measures performance of a uni-directional optical signal by forming an optical recirculation loop with respect to only a unidirectional transmission system and performing a long-distance transmission with a small number of test samples, is overcome, and an optical recirculation loop is formed with respect to a bidirectional transmission system so that the performance of the bidirectional optical signal can be simultaneously measured.  
         [0074]     Thus, in a wavelength division multiple optical transmission system, since a bidirectional optical recirculation loop transmission apparatus capable of performing a long-distance transmission using a small number of test samples can be provided, a bidirectional long-distance transmission test is made possible.