Abstract:
Disclosed is a bidirectional optical add/drop multiplexer that includes a first filter for reflecting a first optical signal pre-selected among input forward optical signals and transmitting the remaining forward optical signals, a second filter for reflecting a second optical signal pre-selected among input reverse optical signals and transmitting the remaining reverse optical signals, a first optical splitter for combining the first optical signal and the forward optical signals by reflection from the first filter, wherein the reflected second optical signal having passed through the first filter is outputted to a connected second drop terminal and the first optical signal input from a connected first add terminal is outputted to the first filter, and a second optical splitter for adding the second optical signal and the reverse optical signals by reflection from the second filter, wherein the reflected first optical signal having passed through the second filter is outputted to a connected first drop terminal and the second optical signal input from a connected second add terminal is outputted to the second filter.

Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “Bidirectional Optical Add/Drop Multiplexer and Wavelength Division Multiplexed Ring Network Using the Same,” filed in the Korean Intellectual Property Office on Sep. 24, 2004 and assigned Ser. No. 2004-77244, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical add/drop multiplexer, and more particularly to a bidirectional optical add/drop multiplexer and a wavelength division multiplexed ring network using the same. 
   2. Description of the Related Art 
   Recently, demands for Internet-based diverse multimedia services has increased rapidly, and research on an economic passive optical network (PON) that can provide a large capacity of information has been pursued actively. A passive optical network includes a central office (CO) for providing a service, a plurality of subscriber devices for receiving the service, and remote nodes (RNs), installed in areas adjacent to the subscribers, for connecting the central office with the subscriber devices. An outdoor network typically includes passive optical devices; however, it does not include the central office or subscriber devices. Since a network in which a single central office accommodates hundred of thousands to millions of subscribers distributed in a small region cannot be established, a metro-access network is typically used. In such network, a plurality of subscriber networks are constructed around remote nodes that can accommodate a specified number of subscribers, and a central office accommodates the nodes to communicate with them. 
   Recently, research efforts have been made regarding a ring structure that adopts a wavelength division multiplexed transmission technology, that secures the reliability of network, and that has an easy extensibility. Such research intends to accommodate the demand for increasing communication bandwidth in the metro-access network. In the metro-access network, the central office and the remote nodes are connected in a ring structure and communicate with one another using the optical signals having inherent wavelengths in the wavelength division multiplexed metro-access network. Since respective remote nodes communicate with a central office using such inherent wavelength optical signals, an add/drop function capable of (1) dropping and receiving the optical signal of the corresponding wavelength transmitted from the central office and (2) adding and transmitting the optical signal of the corresponding wavelength to be transmitted to the central office has been required. 
     FIG. 1  is a view illustrating the construction of a typical bidirectional optical add/drop multiplexer. Referring to  FIG. 1 , the bidirectional optical add/drop multiplexer  100  is arranged in transmission lines  170  and  175 . The bidirectional optical add/drop multiplexer  100  includes first to sixth circulators  110  to  120  and first and second fiber Bragg gratings (FBGs)  130  and  135 . The second and third circulators  112  and  114  and the first fiber Bragg grating  130  are arranged in a first optical path  140  that connects the first circulator  110  with the fourth circulator  116 . The fifth and sixth circulators  118  and  120  and the second fiber Bragg grating  135  are arranged in a second optical path  145  that connects the first circulator  110  with the fourth circulator  116 . 
   The first circulator  110  has first to third ports. The first port is connected to the sixth circulator  120 , the second port is connected to the transmission line  170 , and the third port is connected to the second circulator  112 . The first circulator  110  outputs a first optical signal λ 1  entering the second port to the third port and outputs a second optical signal λ 2  entering the first port to the second port. 
   The second circulator  112  has first to third ports. The first port is connected to the third port of the first circulator  110 , the second port is connected to the first fiber Bragg grating  130 , and the third port is connected to a first drop terminal  160 . The second circulator  112  outputs the first optical signal entering the first port to the second port and outputs the first optical signal entering the second port to the third port. 
   The first fiber Bragg grating  130  is arranged between the second port of the second circulator  112  and the second port of the third circulator  114  The first fiber Bragg grating  130  reflects the first optical signal. In other words, the first fiber Bragg grating  130  reflects the first optical signal from the second circulator  112  to the second circulator  112  and reflects the first optical signal from the third circulator  114  to the third circulator  114 . The first and second fiber Bragg gratings  130  and  135  reflect optical signals of pre-selected wavelengths and transmit optical signals with wavelengths other than the pre-selected wavelengths. 
   The third circulator  114  has first to third ports. The first port is connected to a first add terminal  150 , the second port is connected to the first fiber Bragg grating  130 , and the third port is connected to a first port of the fourth circulator  116 . The third circulator  114  outputs the first optical signal entering the first port to the second port and outputs the first optical signal entering the second port to the third port. 
   The fourth circulator  116  has first to third ports. The first port is connected to the third port of the third circulator  114 , the second port is connected to the transmission line  175 , and the third port is connected to the first port of the fifth circulator  118 . The fourth circulator  116  outputs the first optical signal entering the first port to the second port and outputs the second optical signal entering the second port to the third port. 
   The fifth circulator  118  has first to third ports. The first port is connected to a third port of the fourth circulator  116 , the second port is connected to the second fiber Bragg grating  135 , and the third port is connected to a second drop terminal  165 . The fifth circulator  118  outputs the second optical signal entering the first port to the second port and outputs the second optical signal entering the second port to the third port. 
   The second fiber Bragg grating  135  is arranged between the second port of the fifth circulator  118  and the second port of the sixth circulator  120  and reflects the inputted second optical signal. In other words, the second fiber Bragg grating  135  reflects the second optical signal from the fifth circulator  118  to the fifth circulator  118  and reflects the second optical signal from the sixth circulator  120  to the sixth circulator  120 . 
   The sixth circulator  120  has first to third ports. The first port is connected to a second add terminal  155 , the second port is connected to the second fiber Bragg grating  135 , and the third port is connected to the first port of the first circulator  110 . The sixth circulator  120  outputs the second optical signal entering the first port to the second port and outputs the second optical signal entering the second port to the third port. 
   The process of dropping the first optical signal from the transmission line  170  by the bidirectional optical add/drop multiplexer  100  will now be explained. 
   The first optical signal entering the bidirectional optical add/drop multiplexer  100  through the transmission line  170  passes through the first and second circulators  110  and  112 , in order, and the signal is inputted to the first fiber Bragg grating  130 . The first optical signal is then reflected by the first fiber Bragg grating  130  and exits to the first drop terminal  160  through the second circulator  112 . 
   The process of adding the first optical signal to the transmission line  175  by the bidirectional optical add/drop multiplexer  100  will now be explained. 
   The first optical signal entering the first add terminal  150  is inputted to the first fiber Bragg grating  130  through the third circulator  114 . The first optical signal is then reflected by the first fiber Bragg grating  130  and exits to the transmission line  175  through the third and fourth circulators  114  and  116 . 
   The process of dropping the second optical signal from the transmission line  175  by the bidirectional optical add/drop multiplexer  100  will now be explained. 
   The second optical signal entering the bidirectional optical add/drop multiplexer  100  through the transmission line  175  passes through the fourth and fifth circulators  116  and  118 , in order, and the second optical signal is inputted to the second fiber Bragg grating  135 . The second optical signal is then reflected by the second fiber Bragg grating  135  and exits to the second drop terminal  165  through the fifth circulator  118 . 
   The process of adding the second optical signal to the transmission line  170  by the bidirectional optical add/drop multiplexer  100  will now be explained. 
   The second optical signal entering the second add terminal  155  is inputted to the second fiber Bragg grating  135  through the sixth circulator  120 . The second optical signal is then reflected by the second fiber Bragg grating  135  and exits to the transmission line  170  through the sixth and first circulators  120  and  110 . 
   The bidirectional optical add/drop multiplexer  100 , as described above, has the problems that it employs six expensive circulators  110  to  120 , and the implementation cost, therefore, is high. In addition, the number of optical elements through which optical signals to be added or dropped pass is large, causing a great optical loss. Furthermore, crosstalk caused by optical signals having the same wavelength may occur due to the incomplete reflection by the fiber Bragg gratings  130  and  135 . Specifically, if the optical signals to be dropped are not completely reflected by the fiber Bragg gratings  130  and  135 , the transmitted optical signals cause crosstalk to the optical signals of the same wavelengths to be added after being reflected by the fiber Bragg gratings  130  and  135 . Conversely, if the optical signals to be added are not completely reflected by the fiber Bragg gratings  130  and  135 , the transmitted optical signals cause crosstalk to the optical signals of the same wavelengths to be dropped after being reflected by the fiber Bragg gratings  130  and  135 . In such situation, crosstalk between optical signals of same wavelength can be avoided only by heightening the reflection rate of the fiber Bragg gratings  130  and  135  greatly. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been designed to solve the above and other problems occurring in the prior art. One aspect of the present invention is to provide a bidirectional optical add/drop multiplexer and a wavelength division multiplexed ring network using the same that can reduce the number of elements to reduce optical loss and that can be implemented at low cost. 
   In one embodiment, a bidirectional optical add/drop multiplexer comprises: a first filter for reflecting first optical signal of pre-selected wavelength, among entering forward optical signals, and transmitting remaining forward optical signals; a second filter for reflecting second optical signal of pre-selected wavelength, among entering reverse optical signals, and transmitting remaining reverse optical signals; a first optical splitter for combining the first optical signal and the forward optical signals by reflection from the first filter, wherein the reflected second optical signal transmitted through the first filter is outputted to a connected second drop terminal and the first optical signal inputted from a connected first add terminal is outputted to the first filter; and a second optical splitter for combining the second optical signal and the reverse optical signals by reflection from the second filter, wherein the reflected first optical signal transmitted through the second filter is outputted to a connected first drop terminal and the second optical signal from a connected second add terminal is outputted to the second filter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a view illustrating the construction of a typical bidirectional optical add/drop multiplexer; 
       FIG. 2  is a view illustrating the construction of a bidirectional optical add/drop multiplexer according to one embodiment of the invention; 
       FIG. 3  is a view explaining a process of adding/dropping the first optical signal by the bidirectional optical add/drop multiplexer illustrated in  FIG. 2 ; 
       FIG. 4  is a view explaining a process of adding/dropping the (N+1)-th optical signal by the bidirectional optical add/drop multiplexer illustrated in  FIG. 2 ; 
       FIG. 5  is a view illustrating the construction of a bidirectional wavelength division multiplexed ring network using the bidirectional optical add/drop multiplexer according to the invention; 
       FIG. 6  is a view illustrating spectrums of optical signals being transmitted in the ring network illustrated in  FIG. 5 ; 
       FIG. 7  is a view illustrating spectrums of optical signals passing through the n-th wavelength division multiplexer  350 - n  illustrated in  FIG. 5 ; 
       FIG. 8  is a view illustrating the construction of the bidirectional optical add/drop multiplexer  200 - m  of the m-th remote node  400 - m  illustrated in  FIG. 5 ; and 
       FIG. 9  is a view illustrating the ring network of  FIG. 5  in which an impediment to transmission is present. 
   

   DETAILED DESCRIPTION 
   The present invention will be described in detail hereinafter with reference to the accompanying drawings. For the purposes of clarity and simplicity, only parts necessary for understanding the operation of the present invention will be explained but a detailed description of known functions and configurations incorporated herein will be omitted when it obscures the subject matter of the present invention. 
     FIG. 2  is a view illustrating the construction of a bidirectional optical add/drop multiplexer according to an embodiment of the invention. Referring to  FIG. 2 , the bidirectional optical add/drop multiplexer  200  is arranged in transmission lines and it includes first and second filter  210  and  220  and first and second circulators  230  and  240 . The circulators  230  and  240  act as optical splitters. The first and second filters  210  and  220  are arranged in the first optical path  270 . The first circulator  230 , the first and second filters  210  and  220 , and the second circulator  240  are arranged in the second optical path  275 . Both terminals of the first optical path  270  are connected to the transmission lines. The first and second optical paths  270  and  275  may be optical fibers or optical waveguides. The first and second optical paths  270  and  275  cross each other twice, and the corresponding filters  210  and  220  are arranged at respective cross points. In  FIG. 2 , the first and second optical paths  270  and  275  appear to cross each other three times; however, the two cross each other only twice. The first and second optical paths  270  and  275  may also cross each other three times. 
   The first filter  210  is arranged at the first cross point of the first and second optical paths  270  and  275 . The filter  210  reflects and outputs the first optical signal λ 1  to the second optical path  275  and transmits remaining optical signals λ 2  to λ 2N . The first and second filters  210  and  220  reflect an optical signal of a pre-selected wavelength and transmit optical signals of other wavelengths. The first filter  210 , along with the second filter  220 , includes a double-sided thin film filter composed of a transparent substrate and multilayer thin films coated on both surfaces of the substrate. At the first crossing point, the first and second optical paths  270  and  275  may cross at right angles, and the first filter  210  may be arranged at an angle of 45 degrees to the respective optical paths. The first to N-th optical signals may be forward optical signals in a certain communication network or those signals that propagate clockwise. The (N+1)-th to (2N)-th optical signals may be reverse optical signals on the communication network or those signals that propagate counterclockwise. 
   The second filter  220  is arranged at the second cross point of the first and second optical paths  270  and  275 . The second filter  220  reflects and outputs an inputted (N+1)-th optical signal λ N+1  to the second optical path  275  and transmits the remaining optical signals λ 1  to λ N  and λ N+2  to λ 2N . At the second crossing point, the first and second optical paths  270  and  275  may cross at right angles, and the second filter  220  may be arranged to be at an angle of 45 degrees to the respective optical paths. 
   The first circulator  230  is arranged at one terminal of the second optical path  275 , and it  230  has first to third ports. The first port is connected to the first add terminal  250 , the second port is connected to the first filter  210 , and the third port is connected to the second drop terminal  265 . The first circulator  230  outputs the first optical signal entering the first port to the second port and outputs the (N+1)-th optical signal entering the second port to the third port. 
   The second circulator  240  is arranged at the other terminal of the second optical path  275 , and it has first to third ports. The first port is connected to a second add terminal  255 , the second port is connected to the second filter  220 , and the third port is connected to a first drop terminal  260 . The second circulator  240  outputs the (N+1)-th optical signal entering the first port to the second port and outputs the first optical signal entering the second port to the third port. 
     FIG. 3  is a view explaining a process of adding/dropping the first optical signal by the bidirectional optical add/drop multiplexer  200 . 
   The process of dropping the first optical signal from the transmission line by the bidirectional optical add/drop multiplexer  200  will now be explained. 
   The second to N-th optical signals, among the first to N-th optical signals, enter the bidirectional optical add/drop multiplexer  200  through the transmission line and exit to the transmission line through the first and second filters  210  and  220 . The first optical signal is reflected by the first filter  210  and exits to the first drop terminal  260  through the second filter  220  and the second circulator  240 . 
   The process of adding the first optical signal to the transmission line by the bidirectional optical add/drop multiplexer  200  will now be explained. 
   The first optical signal inputted through the first add terminal  250  is transmitted to the first filter  210  through the first circulator  230 . The first optical signal reflected by the first filter  210  exits to the transmission line through the second filter  220 . 
     FIG. 4  is a view explaining a process of adding/dropping the (N+1)-th optical signal by the bidirectional optical add/drop multiplexer  200 . 
   The process of dropping the (N+1)-th optical signal from the transmission line by the bidirectional optical add/drop multiplexer  200  will now be explained. 
   The (N+2)-th to (2N)-th optical signals, among the (N+1)-th to (2N)-th optical signals, enter the bidirectional optical add/drop multiplexer  200  through the transmission line and exit to the transmission line through the second and first filters  220  and  210 . The (N+1)-th optical signal is reflected by the second filter  220 , and the signal exit to the second drop terminal  265  through the first filter  210  and the first circulator  230 . 
   The process of adding the (N+1)-th optical signal to the transmission line by the bidirectional optical add/drop multiplexer  200  will now be explained. 
   The (N+1)-th optical signal entering through the second add terminal  255  is transmitted to the second filter  220  through the second circulator  240 . The (N+1)-th optical signal is reflected by the second filter  220 , and the signal exits to the transmission line through the first filter  210 . 
     FIG. 5  is a view illustrating the construction of a bidirectional wavelength division multiplexed ring network using the bidirectional optical add/drop multiplexer according to the present invention. The ring network  300  includes a central office  310  and first to third remote nodes  400 - 1  to  400 - 3  in a ring structure using a transmission line  305 . The first to third remote nodes  400 - 1  to  400 - 3  include bidirectional optical add/drop multiplexers  200 - 1  to  200 - 3  having the construction as illustrated in  FIG. 2 . Normally, the respective remote nodes  400 - 1  to  400 - 3  add/drop the optical signals bidirectionally through the single transmission line  305 . Accordingly, they communicate with the central office  310  through two optical signals, and sufficient communication bandwidth can be secured. 
   The ring network  300  transmits first to third optical signals λ 1  to λ 3  of a high priority in counterclockwise direction and transmits fourth to sixth optical signals λ 4  to λ 6  of a low priority in clockwise direction. Here, the high-priority optical signal refers to an optical signal that carries high-priority data. In the embodiment of the present invention, the high-priority or low-priority optical signals may be changed according to presence of an impediment to transmission. For example, if an impediment is not present, the third optical signal is of high priority and the sixth optical signal is of low priority. However, if an impediment is present, the sixth optical signal is of high priority and the third optical signal is of low priority. 
   The central office  310  includes first to sixth switches (SW)  321  to  326 , first to sixth optical transmitters (TX)  331  to  336 , first to sixth optical receivers (RX)  341  to  346 , first to sixth wavelength division multiplexers (WDM)  350 - 1  to  350 - 6 , and first and second arrayed waveguide gratings (AWG)  362  and  364 . 
   The first to third switches  321  to  323 , transmitting-end switches, transmits data and the fourth to sixth switches  324  to  326 , receiving-end switches, receives data. The first to sixth switches  321  to  326  are kept in bar state during absence of any impediment. However, if an impediment is present, at least one of the switches is converted to cross state. In bar state, the m-th switches  321  to  323  connect high-priority terminals H, to which high-priority data is inputted, with the m optical transmitters  331  to  333  and connect low-priority terminals L, to which low-priority data is inputted, with the (m+3)-th optical transmitters  334  to  336 . Here, m denotes a natural number that is not more than 3. In cross state, the m-th switches  321  to  323  connect the high-priority terminals with the (m+3)-th optical transmitters  334  to  336 . In addition, m-th switches connect the low-priority terminals with the m-th optical transmitters  331  to  333 . In bar state, the (m+3)-th switches  324  to  326  connect high-priority terminals, which output high-priority data, with the m-th optical receivers  341  to  343  and connect low-priority terminals, which output low-priority data, with the (m+3)-th optical receivers  344  to  346 . In the cross state, the (m+3)-th switches  324  to  326  connect the high-priority terminals with the (m+3)-th optical receivers  334  to  336  and connect the low-priority terminals with the m-th optical receivers  341  to  343 . 
   The first to sixth optical transmitters  331  to  336  output corresponding optical signals generated by respective input data and the m-th to (m+3)-th optical transmitters  331 ,  334 ;  332 ,  335 ; and  333 ,  336  are connected with the m-th switches  321  to  323 . The n-th optical transmitters  331  to  336  output the n-th optical signals. Here, n denotes a natural number that is not more than 6. 
   The first to sixth optical receivers  341  to  346  convert the corresponding input optical signals into data and output such data. The m-th to (m+3)-th optical receivers  341 ,  344 ;  342 ,  345 ; and  343 ,  346  are connected with the (m+3)-th switches  324  to  326 . The n-th optical receivers  341  to  346  receive the n-th optical signals. 
   The first to sixth wavelength division multiplexers  350 - 1  to  350 - 6  have first to third ports. The first to third wavelength division multiplexers  350 - 1  to  350 - 3  are connected to the first arrayed waveguide grating  362  and the fourth to sixth wavelength division multiplexers  350 - 4  to  350 - 6  are connected to the second arrayed waveguide grating  364 . The first ports of the m-th wavelength division multiplexers  350 - 1  to  350 - 3  are connected to the m-th de-multiplexing ports of the first arrayed waveguide grating  362 , the second ports thereof are connected to the m-th optical transmitters  331  to  333 , and the third ports thereof are connected to the (m+3)-th optical receivers  344  to  346 . The m-th wavelength division multiplexers  350 - 1  to  350 - 3  output the (m+3)-th optical signals entering the first ports to the third ports, and subsequently, to the (m+3)-th optical receivers  344  to  346 . In addition, the m th  wavelength division multiplexers  350 - 1  to  350 - 3  output the m-th optical signals from the m-th optical transmitters  331  to  333  through the second ports and to the first ports. 
   The first ports of the (m+3)-th wavelength division multiplexers  350 - 4  to  350 - 6  are connected to the m-th de-multiplexing ports of the second arrayed waveguide grating  364 , the second ports thereof are connected to the (m+3)-th optical transmitters  334  to  336 , and the third ports thereof are connected to the m-th optical receivers  341  to  343 . The (m+3)-th wavelength division multiplexers  350 - 4  to  350 - 6  output the m-th optical signals entering the first ports through the third ports, and subsequently, to the m-th optical receivers  341  to  343 . In addition, the (m+3)-th wavelength division multiplexers  350 - 4  to  350 - 6  output the (m+3)-th optical signals from the (m+3)-th optical transmitters  334  to  336  through the second ports to the first ports. 
   The first arrayed waveguide grating  362  has a multiplexing port MP and first to third de-multiplexing ports. The multiplexing port is connected to the transmission line  305 , and the first to third de-multiplexing ports are connected to the first to third wavelength division multiplexers  350 - 1  to  350 - 3 . The first arrayed waveguide grating  362  de-multiplexes and outputs the fourth to sixth optical signals entering the multiplexing port to the first to third de-multiplexing ports. In addition, first arrayed waveguide grating  362  multiplexes and outputs the first to third optical signals entering the first to third de-multiplexing ports to the multiplexing port. 
   The second arrayed waveguide grating  364  has a multiplexing port MP and first to third de-multiplexing ports. The multiplexing port is connected to the transmission line  305 , and the first to third de-multiplexing ports are connected to the fourth to sixth wavelength division multiplexers  350 - 4  to  350 - 6 . The second arrayed waveguide grating  364  de-multiplexes and outputs the first to third optical signals entering the multiplexing port to the first to third de-multiplexing ports. In addition, the second arrayed waveguide grating  364  multiplexes and outputs the fourth to sixth optical signals entering the first to third de-multiplexing ports to the multiplexing port. 
     FIG. 6  is a view illustrating spectrums of the optical signals being transmitted in the ring network. As illustrated in  FIG. 6 , the wavelength bands of the first to third optical signals, transmitted counterclockwise, and the wavelength bands of the fourth to sixth optical signals, transmitted clockwise, are allocated differently. Also, the respective wavelength bands are set to correspond to a free spectral range of the respective arrayed waveguide gratings  362  and  364 . The band pass characteristic of the arrayed waveguide gratings  362  and  364  is periodic according to the free spectral range. Such periodicity enables the arrayed waveguide gratings  362  and  364  to process two wavelength bands. 
     FIG. 7  is a view illustrating spectrums of the optical signals passing through the n-th wavelength division multiplexer  350 - n . As illustrated in  FIG. 7 , the n-th wavelength division multiplexer  350 - n  separates two wavelength bands entering the first port and outputs the separated wavelength bands through the second and third ports. Conversely, the n-th wavelength division multiplexer  350 - n  combines the two wavelength bands entering the second and third ports and outputs the combined wavelength band through the first port. 
   Returning to  FIG. 5 , the first to third remote nodes  400 - 1  to  400 - 3  have similar construction; however, the nodes process different signals. That is, the first remote node  400 - 1  adds/drops the first and fourth optical signals, the second remote node  400 - 2  adds/drops the second and fifth optical signals, and the third remote nodes  400 - 3  adds/drops the third and sixth optical signals. 
   The first to third remote nodes  400 - 1  to  400 - 3  include bidirectional optical add/drop multiplexers  200 - 1  to  200 - 3 ; first and second optical transmitters  422 - 1 ,  424 - 1 ;  422 - 2 ,  424 - 2 ; and  422 - 3 ,  424 - 3 ; first and second optical receivers  432 - 1 ,  434 - 1 ;  442 - 2 ,  434 - 2 ; and  432 - 3 ,  434 - 3 ; and first and second switches  412 - 1 ,  414 - 1 ;  412 - 2 ,  414 - 2 ; and  412 - 3 ,  414 - 3 , respectively. 
   Hereinafter, the m-th remote node  400 - m  will be explained. 
   The first switch  412 - m , a transmitting-end switch, transmits data and the second switch  414 - m , a receiving-end switch, receives the data. The first and second switches  412 - m  and  414 - m  are kept in bar state if no impediment to transmission is present; however, if impediment is present, at least one of the switches is converted to cross state. In bar state, the first switch  412 - m  connects high-priority terminals, where high-priority data is inputted, with the first optical transmitter  422 - m  while connecting low-priority terminals, where low-priority data is inputted, with the second optical transmitter  424 - m . In cross state, the first switch  412 - m  connects the high-priority terminals with the second optical transmitter  424 - m  while connecting the low-priority terminals with the first optical transmitter  422 - m . In bar state, the second switch  414 - m  connects high-priority terminals, which output high-priority data, with the first optical receiver  432 - m  while connecting low-priority terminals, which output low-priority data, with the second optical receiver  434 - m.    
   In cross state, the second switch  414 - m  connects the high-priority terminals with the second optical receiver  434 - m  while connecting the low-priority terminals with the first optical receiver  432 - m.    
   The first and second optical transmitters  422 - m  and  424 - m  output optical signals generated by respective input data, and the first and second optical transmitters  422 - m  and  424 - m  are connected to the first switch  412 - m . The first optical transmitter  422 - m  outputs the m-th optical signals, and the second optical transmitter  424 - m  outputs the (m+3)-th optical signals. 
   The first and second optical receivers  432 - m  and  434 - m  convert the input optical signals to data and output the data. The first and second optical receivers  432 - m  and  434 - m  are connected to the second switch  414 - m . The first optical receiver  432 - m  receives the m-th optical signals, and the second optical receiver  434 - m  receives the (m+3)-th optical signals. 
     FIG. 8  is a view illustrating the construction of a bidirectional optical add/drop multiplexer  200 - m  of the m-th remote node  400 - m . Since the construction of the bidirectional optical add/drop multiplexer  200 - m  is similar to that in  FIG. 2 , only the process of adding/dropping optical signals will be explained. First, the process of dropping the m-th optical signal from the transmission line by the m-th bidirectional optical add/drop multiplexer  200 - m  will be explained. 
   The optical signals among the first to third optical signals entering the m-th bidirectional optical add/drop multiplexer  200 - m  through the transmission line exit to the transmission line through first and second filters  210 - m  and  220 - m . Such optical signals, however, exclude the m-th optical signal. The m-th optical signal, instead, is reflected by a first filter  210 - m  and exit to a first drop terminal  260 - m  through a second filter  220 - m  and a second circulator  240 - m.    
   The process of adding the m-th optical signal to the transmission line by the bidirectional optical add/drop multiplexer  200 - m  will now be explained. 
   The m-th optical signal entering through a first add terminal  250 - m  is transmitted to the first filter  210 - m  via a first circulator  230 - m . The m-th optical signal is then reflected by the first filter  210 - m  and exits to the transmission line through the second filter  220 - m.    
   The process of dropping the (m+3)-th optical signal from the transmission line by the bidirectional optical add/drop multiplexer  200 - m  will now be explained. 
   The optical signals among the fourth to sixth optical signals enter the bidirectional optical add/drop multiplexer  200 - m  through the transmission line and exit to the transmission line through the second and first filters  220 - m  and  210 - m . Such optical signals, however, exclude (M+3)-th optical signal. The (m+3)-th optical signal, instead, is reflected by the second filter  220 - m  and exit the second drop terminal  265 - m  through the first filter  210 - m  and the first circulator  230 - m.    
   The process of adding the (m+3)-th optical signal to the transmission line by the bidirectional optical add/drop multiplexer  200 - m  will now be explained. 
   The (m+3)-th optical signal entering through the second add terminal  255 - m  is transmitted to the second filter  220 - m  via the second circulator  240 - m . The (m+3)-th optical signal is then reflected by the second filter  220 - m  and exits to the transmission line through the first filter  210 - m.    
     FIG. 9  is a view illustrating the ring network of  FIG. 5  with impediment to the transmission.  FIG. 9  shows the case that the transmission line between the first remote node and the second remote node is cut off. If the impediment is present in the ring network  300 , half of the optical signals propagating in the transmission line in forward direction is lost. In such situation, the central office  310  and the respective remote nodes  400 - 1  to  400 - 3  can detect presence of impediment in and determine the position of the impediment by checking whether the optical signals entering the respective optical receivers are also exiting. 
   With presence of impediment, the second and third remote nodes  400 - 2  and  400 - 3  cannot receive the second and third optical signals, transmitted counterclockwise, from the central office  310 . In addition, the first remote node  400 - 1  cannot receive the fourth optical signal, transmitted clockwise, from the central office  310 . Furthermore, the presence of the hindrance will prohibit the central office  310  from receiving the first optical signal, transmitted counterclockwise, from the first remote node  400 - 1  and from receiving the fifth and sixth optical signals, transmitted clockwise, from the second and third remote nodes  400 - 2  and  400 - 3 . In absence of any impediment, the fourth optical signal carried the low-priority data; the second and third optical signals carried the high-priority data; the fifth and sixth optical signals carried the low-priority data; and the first optical signal carried the high-priority data. 
   Upon detecting impediment in the transmission line, the central office  310  converts the second and third switches  322  and  323  to cross state to restore the high-priority data. Accordingly, the high-priority data is carried in the fifth and sixth optical signals  325  and  326 . Moreover, converting the second switch  414 - 2  enables the second remote node  400 - 2  to receive the fifth optical signal and the third remote node  400 - 3  to receive the sixth optical signal. 
   Conversion to cross state also occurs with respect to the first switch  412 - 1 . Upon detecting the impediment, the first remote node  400 - 1  converts the first switch  412 - 1  to the cross state to restore the high-priority data. Accordingly, the high-priority data are carried in the fourth optical signal and the central office  310  receives the fourth optical signal by converting the fourth switch to cross state. 
   As described above, it will be apparent that the bidirectional optical add/drop multiplexer according to the present invention can be implemented at low cost and can reduce optical loss by reducing the number of elements. The wavelength division multiplexed ring network using the bidirectional optical add/drop multiplexer can also be implemented at low cost and can reduce optical loss. 
   Moreover, the wavelength division multiplexed ring network using the bidirectional optical add/drop multiplexer according to the present invention can expand the number of accommodated remote nodes and transmission distance by reducing the number of optical elements through which signals to be added/dropped pass, thereby reducing optical loss. 
   Furthermore, the wavelength division multiplexed ring network using the bidirectional optical add/drop multiplexer according to the present invention can reduce the cost for implementation by improving the transmission efficiency through the use of a bidirectional transmission line and can secure the reliability of network by promptly restoring the transmission line when an impediment to transmission is present. 
   While the present invention has been shown and described with reference to certain 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 present invention as defined by the appended claims.