Abstract:
Disclosed is a self-healing passive optical network comprising: a station, such as a central office, for outputting first and second multiplexed downstream optical signals to first and second feeder fibers; a remote node connected to the central office through the first and second feeder fibers to demultiplex each input multiplexed downstream optical signal into a plurality of downstream optical signals and to output the demultiplexed downstream optical signals; and a plurality of optical network units for receiving one or more downstream optical signals, each of the optical network units are connected to the remote node through at least one distribution fiber, wherein the station outputs the first and second multiplexed downstream optical signals to the first and second feeder fibers, respectively, and outputs the first and second multiplexed downstream optical signals to one of the first and second feeder fibers when a defect occurs in a fiber.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims to the benefit of an earlier application entitled “Self-Healing Passive Optical Network,” filed in the Korean Intellectual Property Office on Jan. 27, 2005 and assigned Serial No. 2005-7588, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a passive optical network (PON), and more particularly to a passive optical network capable of self-healing defects occurring in an optical fiber.  
         [0004]     2. Description of the Related Art  
         [0005]     Wavelength division multiplexing passive optical networks (WDM-PONs) provide ultra high-speed broadband communication service using specific wavelengths assigned to each subscriber unit. Consequently, WDM-PONs can ensure the communication security and easily accommodate special communication services or the enlargement of channel capacity required from each subscriber unit. They can also easily increase the number of subscriber units by adding specific wavelengths assigned to such new subscribers. However, in spite of these advantages, the WDM-PON has not yet been used practically. This is because a station, such as a central office (CO) and the like, and each optical network unit (ONU) require both light sources (having specific oscillation wavelengths) and additional wavelength stabilization circuits (for stabilizing the wavelengths of the light sources). These requirements put a heavy economic burden on the subscribers. In order to realize an economic WDM-PON, some conventional WDM-PON have tried using a spectrum-sliced broadband light source, which allows wavelength management to be facilitated, a Fabry-Perot laser diode wavelength-locked with inherent light or a reflective semiconductor optical amplifier, as a WDM light source.  
         [0006]     Generally, a WDM-PON uses a double star structure in order to minimize the length of an optical fiber (i.e. the optical line). That is, a central office (CO) and a remote node (RN) installed at an area adjacent to optical network units (ONUs) are connected through one feeder fiber in a PON. This remote node and each optical network unit (ONU) are connected through a separate distribution fiber. In the WDM-PON, a multiplexed downstream optical signal is transmitted to the remote node through the feeder fiber. Then, the multiplexed downstream optical signal is demultiplexed into a plurality of downstream signals by a wavelength division multiplexer located in the remote node. Each of the downstream signals is transmitted to a corresponding optical network unit through a corresponding distribution fiber. Upstream optical signals output from the optical network units are transmitted to the remote node, multiplexed by the wavelength division multiplexer located in the remote node, and then transmitted to the central office.  
         [0007]     In the WDM-PON, large amounts of data are transmitted at a high speed based on wavelengths assigned to each optical network unit. Accordingly, when an unexpected abnormality (such as a malfunction or deterioration) of an upstream/downstream light source, or a defect (such as a cut or deterioration) in feeder/distribution fibers occur, the transmitted data may be lost even if the defect occurs only for a short time. Thus, such a defect must be quickly detected and instantly corrected. However, when such a defect occurs, the direct optical line between the central office and the optical network units is cut. Therefore, the central office and the optical network units cannot report the occurrence of the defect to each other. For this situation, a separate low-speed communication line may be provided. However, in order to install the low-speed communication line, additional cost/investment is required for continuously managing and supervising the low-speed communication line. In addition, in order for the central office and each optical network unit to communicate, check a defect occurrence through the separate low-speed communication line, and report the defect occurrence, additional time is required.  
         [0008]     Therefore, there is a need in the art to develop a WDM-PON capable of quickly detecting a defect in the feeder fibers or distribution fibers and self-healing the defect. Particularly, in a PON in which each wavelength is shared by a plurality of subscriber units, to reduce the high cost required to realize a typical WDM-PON which allocates a specific wavelength to each subscriber unit.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention has been made to reduce or overcome the above-mentioned problems occurring in the prior art. One illustrative object of the present invention is to provide a passive optical network (PON) capable of self-healing a defect in feeder fibers or distribution fibers.  
         [0010]     In accordance with one aspect of the present invention, there is provided a self-healing passive optical network comprising: a station, such as a central office, to output first and second multiplexed downstream optical signals to first and second feeder fibers; a remote node connected to the station using the first and second feeder fibers to enable demultiplexing each multiplexed downstream optical signal into a plurality of downstream optical signals and to output the demultiplexed downstream optical signals; and a plurality of optical network units wherein each of the optical network units is connected to the remote node through at least one distribution fiber, wherein the station outputs the first and second multiplexed downstream optical signals to the first and second feeder fibers, respectively, and outputs the first and second multiplexed downstream optical signals to one of the first and second feeder fibers when a defect occurs in a fiber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a block diagram of a self-healing passive optical network (PON) according to an embodiment of the present invention;  
         [0013]      FIG. 2  is a diagram illustrating wavelength bands processed in the self-healing PON shown in  FIG. 1 ;  
         [0014]      FIG. 3  is a diagram illustrating the pass band of the Nth wavelength selective coupler of a first optical transceiver array shown in  FIG. 1 ;  
         [0015]      FIG. 4  is a diagram illustrating the pass band of a first optical coupler shown in  FIG. 1 ;  
         [0016]      FIG. 5  is a block diagram to explain the signal processing procedure when a defect occurs in a first feeder fiber in the PON shown in  FIG. 1 ; and  
         [0017]      FIG. 6  is a block diagram to explain the signal processing procedure when a defect occurs in a first working distribution fiber in the PON shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0018]     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may obscure the subject matter of the present invention.  
         [0019]      FIG. 1  is a block diagram of a self-healing passive optical network (PON) according to an embodiment of the present invention.  FIG. 2  is a diagram illustrating wavelength bands processed in the self-healing PON. The self-healing PON  100  includes a central office (CO)  110 , a remote node (RN)  200  connected to the central office  110  through first and second feeder fibers (FF)  190  and  195 , and a subscriber-side device (SSD)  250  connected to the remote node  200  through first to 2N th  pairs of distribution fibers (DF)  240 - 1 ,  245 - 1 , . . . ,  240 - 2 N,  245 - 2 N. The subscriber-side device  250  includes a beam splitting part (BSP)  260 , and first to 2N th  optical network unit groups (ONU groups)  270 - 1  to  270 - 2 N. The central office  110  transmits first and second multiplexed downstream optical signals and receives first and second multiplexed upstream optical signals. The remote node  200  demultiplexes the received first and second multiplexed downstream optical signals into downstream optical signals of first and second downstream wavelength bands  310  and  330 . The remote node  200  then transmits the demultiplexed downstream optical signals to the subscriber-side device  250 . Also, the remote node  200  multiplexes received upstream optical signals of first and second upstream wavelength bands  320  and  340  into first and second multiplexed upstream optical signals, and transmits the multiplexed upstream optical signals to the central office  110 . Each of ONUs  270 - 1 - 1  to  270 - 2 N-M receives a corresponding downstream optical signal from the remote node  200 , and transmits a corresponding upstream optical signal to the remote node  200 . As shown in  FIG. 2 , the first and second downstream wavelength bands  310  and  330  and the first and second upstream wavelength bands  320  and  340  are spaced from each other. The first downstream wavelength band  310  includes first to N th  wavelengths λ 1  to λ N . The first upstream wavelength band  320  includes (N+1) th  to 2N th  wavelengths λ (N+1)  to λ 2N . The second downstream wavelength band  330  includes (2N+1) th  to 3N th  wavelengths λ (2N+1)  to λ 3N . The second upstream wavelength band  340  includes (3N+1) th  to 4N th  wavelengths λ (3N+1)  to λ 4N .  
         [0020]     The central office  110  includes first and second optical transceiver arrays (TRXA)  120  and  130 , first and second wavelength division multiplexers (WDM)  140  and  150 , and a first switching part (SWP)  160 . The first switch part  160  switches the connection between the first and second wavelength division multiplexers  140  and  150  and the first and second feeder fibers  190  and  195 , respectively. Each of the first and second optical transceiver arrays  120  and  130  inputs/outputs optical signals of relevant wavelength bands. Each of the first and second wavelength division multiplexers  140  and  150  multiplexes or demultiplexes optical signals of relevant wavelength bands.  
         [0021]     The first optical transceiver array  120  includes first to N th  optical transceivers (TRX)  120 - 1  to  120 -N, which outputs downstream optical signals of the first downstream wavelength band  310  and receive upstream optical signals of the first upstream wavelength band  320 . The first to N th  optical transceivers  120 - 1  to  120 -N have the same or similar configuration. The N th  optical transceiver  120 -N includes an N th  downstream optical transmitter (DTX)  122 -N (to generate a downstream optical signal of the N th  wavelength), an N th  upstream optical receiver (URX)  124 -N (to photo-electrically convert an upstream optical signal of the 2N th  wavelength) and an N th  wavelength selective coupler (WSC)  126 -N (to output an input upstream optical signal or downstream optical signal to a corresponding output port). The N th  wavelength selective coupler  126 -N includes first to third ports. Herein, the first port is connected to an N th  demultiplexing port (DP) of the first wavelength division multiplexer  140 . The second port is connected to the N th  downstream optical transmitter  122 -N. The third port is connected to the N th  upstream optical receiver  124 -N. The N th  wavelength selective coupler  126 -N outputs a downstream optical signal of the N th  wavelength that has been input thereto through the second port to the first port. The N th  wavelength selective coupler  126 -N also outputs an upstream optical signal of the 2N th  wavelength that has been input thereto through the first port to the third port. Downstream optical signals of the N th  wavelength include first to M th  time slots forming one cycle, in which the M th  time slot is allocated to the M th  ONU  270 -N-M of the N th  ONU group  270 -N. Similarly, upstream optical signals of the 2N th  wavelength include first to M th  time slots forming one cycle, in which the M th  time slot is allocated to the M th  ONU  270 -N-M of the N th  ONU group  270 -N.  
         [0022]      FIG. 3  is a diagram illustrating the pass band of the N th  wavelength selective coupler  126 -N. As shown in  FIG. 3 , the N th  wavelength selective coupler  126 -N separates or combines signals of two different wavelength bands  310  and  320 . In particular, the first port allows signals of the first downstream wavelength band  310  and first upstream wavelength band  320  to be input/output. The second port allows signals of the first downstream wavelength band  310  to be input/output. The third port allows signals of the first upstream wavelength band  320  to be input/output.  
         [0023]     The first wavelength division multiplexer  140  includes a multiplexing port (MP) connected to a first switch (SW)  170 , and first to N th  demultiplexing ports connected one-to-one to the first to N th  optical transceivers  120 - 1  to  120 -N of the first optical transceiver array  120 . The first wavelength division multiplexer  140  multiplexes downstream optical signals of the first downstream wavelength band input from the first to N th  demultiplexing ports, into a first multiplexed downstream optical signal. It then outputs the first multiplexed downstream optical signal through the multiplexing port. Also, the first wavelength division multiplexer  140  demultiplexes a first multiplexed upstream optical signal input from the multiplexing port, into upstream optical signals of the first upstream wavelength band. It then outputs the demultiplexed upstream optical signals through the first to N th  demultiplexing ports. As shown in  FIG. 2 , each of the wavelength bands  310  to  340  are identical to the free spectral range (FSR) of the first wavelength division multiplexer  140 . This enables the first wavelength division multiplexer  140  to process signals of the first downstream and upstream wavelength bands  310  and  320 .  
         [0024]     The second optical transceiver array  130  includes first to N th  optical transceivers  130 - 1  to  130 -N, which output downstream optical signals of the second downstream wavelength band and receive upstream optical signals of the second upstream wavelength band. The first to N th  optical transceivers  130 - 1  to  130 -N have the same or similar configuration. The N th  optical transceiver  130 -N includes an N th  downstream optical transmitter  132 -N (to output a downstream optical signal of the 3N th  wavelength), an N th  upstream optical receiver  134 -N (to photo-electrically convert an upstream optical signal of the 4N th  wavelength) and an N th  wavelength selective coupler  136 -N (to output an input upstream optical signal or downstream optical signal to a corresponding output port). The N th  wavelength selective coupler  136 -N includes first to third ports. The first port is connected to an N th  demultiplexing port of the second wavelength division multiplexer  150 . The second port is connected to the N th  downstream optical transmitter  132 -N. The third port is connected to the N th  upstream optical receiver  134 -N. The N th  wavelength selective coupler  136 -N outputs a downstream optical signal of the 3N th  wavelength input from its second port to its first port. The N th  wavelength selective coupler  136 -N also outputs an upstream optical signal of the 4N th  wavelength input from its first port to its third port. Downstream optical signals of the 3N th  wavelength include first to M th  time slots forming one cycle, in which the M th  time slot is allocated to the M th  ONU  270 - 2 N-M of the 2N th  ONU group  270 - 2 N. Similarly, upstream optical signals of the 4N th  wavelength include first to M th  time slots forming one cycle, in which the M th  time slot is allocated to the M th  ONU  270 - 2 N-M of the 2N th  ONU group  270 - 2 N.  
         [0025]     The second wavelength division multiplexer  150  includes a multiplexing port connected to a second switch  175 , and first to N th  demultiplexing ports connected one-to-one to the first to N th  optical transceivers  130 - 1  to  130 -N of the second optical transceiver array  130 .  
         [0026]     The second wavelength division multiplexer  150  multiplexes downstream optical signals of the second downstream wavelength band input from its first to N th  demultiplexing ports, into a second multiplexed downstream optical signal. It then outputs the second multiplexed downstream optical signal through its multiplexing port. Also, the second wavelength division multiplexer  150  demultiplexes a second multiplexed upstream optical signal input from its multiplexing port, into upstream optical signals of the second upstream wavelength band. It then outputs the demultiplexed upstream optical signals through its first to N th  demultiplexing ports. The second wavelength division multiplexer  150  has a free spectral range identical or similar to that of the first wavelength division multiplexer  140 . This enables the second wavelength division multiplexer  150  to process signals of the second downstream and upstream wavelength bands  310  and  320 .  
         [0027]     The first switching part  160  includes first and second switches  170  and  175  (to switch the transmission paths of first and second multiplexed downstream optical signals) and first and second optical couplers (CP)  180  and  185  (to receive and transfer first and second multiplexed downstream optical signals to the first and second feeder fibers  190  and  195 ). The first switch  170  includes first to third ports. The first port is connected to the multiplexing port of the first wavelength division multiplexer  140 . The second port is connected to a second port of the first optical coupler  180 . The third port is connected to a third port of the second optical coupler  185 . The first switch  170  selectively connects its first port to either its second or third port.  
         [0028]     The second switch  175  includes first to third ports. The first port is connected to the multiplexing port of the second wavelength division multiplexer  150 . The second port is connected to a second port of the second optical coupler  185 . The third port is connected to a third port of the first optical coupler  180 . The second switch  175  selectively connects its first port to either its second or third port.  
         [0029]     The first optical coupler  180  includes first to third ports, in which its first port is connected to the first feeder fiber  190 . The first optical coupler  180  outputs first and second multiplexed downstream optical signals input through its second and third ports, respectively, to its first port. Also, the first optical coupler  180  outputs a first multiplexed upstream optical signal input through its first port, to its second port. In addition, the first optical coupler  180  outputs a second multiplexed upstream optical signal input through its first port, to its third port.  
         [0030]      FIG. 4  is a diagram illustrating the pass band of the first optical coupler. As shown in  FIG. 4 , the first optical coupler  180  separates or combines signals of four wavelength bands  310  to  340  different from each other. In the first optical coupler  180 , its first port allows signals of the first downstream and first upstream wavelength bands  310  and  320  and the second downstream and second upstream wavelength bands  330  and  340  to be input/output. The second port allows signals of the first downstream and first upstream wavelength bands  310  and  320  to be input/output. The third port allows signals of the second downstream and second upstream wavelength bands  330  and  340  to be input/output.  
         [0031]     The second optical coupler  185  includes first to third ports, in which its first port is connected to the second feeder fiber  195 . The second optical coupler  185  outputs second and first multiplexed downstream optical signals input through its second and third ports, respectively, to its first port. Also, the second optical coupler  185  outputs a second multiplexed upstream optical signal input through its first port, to its second port. In addition, the second optical coupler  185  outputs a first multiplexed upstream optical signal input through its first port, to its third port.  
         [0032]     The remote node  200  includes third and fourth wavelength division multiplexers  230  and  235 , and a second switching part  210 . The second switching part  210  switches optical signal transmission paths between the third and fourth wavelength division multiplexers  230  and  235  and the first and second feeder fibers  190  and  195  depending on wavelengths. Each of the third and fourth wavelength division multiplexers  230  and  235  multiplexes or demultiplexes optical signals of relevant wavelength bands. The second switching part  210  includes third and fourth optical couplers  220  and  225 .  
         [0033]     The third optical coupler  220  includes first to third ports. The first port is connected to the first feeder fiber  190 . The second port is connected to a working multiplexing port (WMP) of the third wavelength division multiplexer  230 . The third port is connected to a protection multiplexing port (PMP) of the fourth wavelength division multiplexer  235 . The third optical coupler  220  outputs a first multiplexed downstream optical signal input through its first port to its second port. It also outputs a second multiplexed downstream optical signal input through its first port to its third port. Also, the third optical coupler  220  outputs first and second multiplexed upstream optical signals input through its second and third ports, respectively, to its first port.  
         [0034]     The fourth optical coupler  225  includes first to third ports. The first port is connected to the second feeder fiber  195 . The second port is connected to a working multiplexing port of the fourth wavelength division multiplexer  235 . The third port is connected to a protection multiplexing port of the third wavelength division multiplexer  230 . The fourth optical coupler  225  outputs a second multiplexed downstream optical signal input through its first port to its second port. It also outputs a first multiplexed downstream optical signal input through its first port to its third port. Also, the fourth optical coupler  225  outputs second and first multiplexed upstream optical signals input through its second and third ports, respectively, to its first port.  
         [0035]     The third wavelength division multiplexer  230  includes working and protection multiplexing ports, first to N th  working demultiplexing ports (WDP), and first to N th  protection demultiplexing ports (PDP). The N th  working and protection demultiplexing ports are connected to an N th  distribution fiber pair  240 -N and  245 -N, which includes an N th  working distribution fiber  240 -N and an N th  protection distribution fiber  245 -N. The third wavelength division multiplexer  230  demultiplexes a first multiplexed downstream optical signal input through its working multiplexing port into downstream optical signals of the first downstream wavelength band  310 . It then outputs the demultiplexed downstream optical signals to its first to N th  working demultiplexing ports. The third wavelength division multiplexer  230  demultiplexes a first multiplexed downstream optical signal input through its protection multiplexing port into downstream optical signals of the first downstream wavelength band  310 . It then outputs the demultiplexed downstream optical signals to its first to N th  protection demultiplexing ports. In addition, the third wavelength division multiplexer  230  multiplexes upstream optical signals of the first upstream wavelength band  320  input through its first to N th  working demultiplexing ports into a first multiplexed upstream optical signal. It then outputs the first multiplexed upstream optical signal to its working multiplexing port. The third wavelength division multiplexer  230  multiplexes upstream optical signals of the first upstream wavelength band  320  input through its first to N th  protection demultiplexing ports into a first multiplexed upstream optical signal. It then outputs the first multiplexed upstream optical signal to its protection multiplexing port.  
         [0036]     The fourth wavelength division multiplexer  235  includes working and protection multiplexing ports, first to N th  working demultiplexing ports, and first to N th  protection demultiplexing ports. The N th  working and protection demultiplexing ports are connected to a 2N th  distribution fiber pair  240 - 2 N and  245 - 2 N, which includes a 2N th  working distribution fiber  240 - 2 N and a 2N th  protection distribution fiber  245 - 2 N. The fourth wavelength division multiplexer  235  demultiplexes a second multiplexed downstream optical signal input through its working multiplexing port into downstream optical signals of the second downstream wavelength band  330 . It then outputs the demultiplexed downstream optical signals to its first to N th  working demultiplexing ports. The fourth wavelength division multiplexer  235  demultiplexes a second multiplexed downstream optical signal input through its protection multiplexing port into downstream optical signals of the second downstream wavelength band  330 . It then outputs the demultiplexed downstream optical signals to its first to N th  protection demultiplexing ports. In addition, the fourth wavelength division multiplexer  235  multiplexes upstream optical signals of the second upstream wavelength band  340  input through its first to N th  working demultiplexing ports into a second multiplexed upstream optical signal. It then outputs the second multiplexed upstream optical signal to its working multiplexing port. The fourth wavelength division multiplexer  235  multiplexes upstream optical signals of the second upstream wavelength band  340  input through its first to N th  protection demultiplexing ports into a second multiplexed upstream optical signal. It then outputs the second multiplexed upstream optical signal to its protection multiplexing port.  
         [0037]     The subscriber-side device  250  includes a beam splitting part  260 , and first to 2N th  ONU groups  270 - 1  to  270 - 2 N. The beam splitting part  260  includes first to 2N th  beam splitters  260 - 1  to  260 - 2 N, which are connected one-to-one to the first to 2N th  distribution fiber pairs  240 - 1  and  245 - 1 , . . . ,  240 - 2 N and  245 - 2 N in regular sequence and have the same configuration.  
         [0038]     The N th  beam splitter (BS)  260 -N is connected to the N th  distribution fiber pair  240 -N and  245 -N. One side of the N th  beam splitter  260 -N includes first and second coupling ports, and another side of the N th  beam splitter  260 -N includes first to M th  split ports. The first coupling port is connected to the N th  working distribution fiber  240 -N. The second coupling port is connected to the N th  protection distribution fiber  245 -N. The first to M th  split ports are connected one-to-one to the first to M th  ONUs  270 -N−1 to 270-N-M of the N th  ONU group  270 -N in regular sequence. In the N th  beam splitter  260 -N, the first coupling port is connected to the N th  working distribution fiber  240 -N. The second coupling port is connected to the N th  protection distribution fiber  245 -N, and the first to M th  split ports are sequentially connected to the first to M th  ONUs  270 -N−1 to  270 -N-M of the N th  ONU group  270 -N. The Nth beam splitter  260 -N power-splits a downstream optical signal of the N th  wavelength input through its first or second coupling port into M downstream optical signals. It then outputs the split M downstream optical signals to its first to M th  split ports. Also, the N th  beam splitter  260 -N power-splits upstream optical signals of the 2N th  wavelength input through its first to M th  split ports into two upstream optical signals. It then outputs the two upstream optical signals to its first and second coupling ports.  
         [0039]     The 2N th  beam splitter  260 - 2 N is connected to the 2N th  distribution fiber pair  240 - 2 N and  245 - 2 N. One side of the 2N th  beam splitter  260 - 2 N includes first and second coupling ports, and another side of the 2N th  beam splitter  260 - 2 N includes first to M th  split ports. The first coupling port is connected to the 2N th  working distribution fiber  240 - 2 N. The second coupling port is connected to the 2N th  protection distribution fiber  245 - 2 N. The first to M th  split ports are connected one-to-one to the first to M th  ONUs  270 - 2 N−1 to  270 - 2 N-M of the 2N th  ONU group  270 - 2 N in regular sequence. The 2N th  beam splitter  260 - 2 N power-splits a downstream optical signal of the 3N th  wavelength input through its first or second coupling port into M downstream optical signals. It then outputs the split M downstream optical signals to its first to M th  split ports. Also, the 2N th  beam splitter  260 - 2 N power-splits upstream optical signals of the 4N th  wavelength input through its first to M th  split ports into two upstream optical signals. It then outputs the two upstream optical signals to its first and second coupling ports.  
         [0040]     The first to 2N th  ONU groups  270 - 1  to  270 - 2 N are connected one-to-one to the first to 2N th  beam splitters  260 - 1  to  260 - 2 N in regular sequence.  
         [0041]     The M th  ONU  270 -N-M of the N th  ONU group  270 -N includes an M th  upstream transmitter (UTX)  272 -N-M (to output an upstream optical signal of the 2N th  wavelength), an M th  downstream receiver (URX)  274 -N-M (to photo-electrically convert a downstream optical signal of the N th  wavelength), and an M th  wavelength selective coupler  276 -N-M (to output an input upstream optical signal or downstream optical signal to a corresponding port). The M th  wavelength selective coupler  276 -N-M includes first to third ports. The first port is connected to the M th  split port of the N th  beam splitter  260 -N. The second port is connected to the M th  upstream transmitter  272 -N-M. The third port is connected to the M th  downstream receiver  274 -N-M. The M th  wavelength selective coupler  276 -N-M outputs an upstream optical signal of the 2N th  wavelength input through its second port to its first port. It then outputs a downstream optical signal of the N th  wavelength input through its first port to its third port.  
         [0042]     The M th  ONU  270 - 2 N-M of the 2N th  ONU group  270 - 2 N includes an M th  upstream transmitter  272 -N-M (to output an upstream optical signal of the 4N th  wavelength), an M th  downstream receiver  274 - 2 N-M (to photo-electrically convert a downstream optical signal of the 3N th  wavelength), and an M th  wavelength selective coupler  276 - 2 N-M (to output an input upstream optical signal or downstream optical signal to a corresponding port). The M th  wavelength selective coupler  276 - 2 N-M includes first to third ports. The first port is connected to the M th  split port of the 2N th  beam splitter  260 - 2 N. The second port is connected to the M th  upstream transmitter  272 - 2 N-M. The third port is connected to the M th  downstream receiver  274 - 2 N-M. The M th  wavelength selective coupler  276 - 2 N-M outputs an upstream optical signal of the 4N th  wavelength input through its second port to its first port. It then outputs a downstream optical signal of the 3N th  wavelength input through its first port to its third port.  
         [0043]     In a steady state, the following is the procedure for processing downstream optical signals of the first downstream wavelength band  310  in the self-healing PON  100 . Each of the first and second switches  170  and  175  connects its first port to its second port.  
         [0044]     Downstream optical signals of the first downstream wavelength band  310  output from the first optical transceiver array  120  are multiplexed into a first multiplexed downstream optical signal by the first wavelength division multiplexer  140 . Then, the first multiplexed downstream optical signal passes through the first switch  170 , the first optical coupler  180 , the first feeder fiber  190  and the third optical coupler  220 , and are input to the third wavelength division multiplexer  230 . The third wavelength division multiplexer  230  demultiplexes the first multiplexed downstream optical signal into downstream optical signals of the first downstream wavelength band  310 . The demultiplexed downstream optical signals pass through the first to N th  working distribution fibers  240 - 1  to  240 -N, and are input to the first to N th  beam splitters  260 - 1  to  260 -N. Each of the first to N th  beam splitters  260 - 1  to  260 -N power-splits each downstream optical signal into M downstream optical signals. It then outputs the M downstream optical signals to corresponding first to N th  ONU groups  270 - 1  to  270 -N.  
         [0045]     In the steady state, the following is the procedure for processing upstream optical signals of the first upstream wavelength band  320  in the self-healing PON  100 .  
         [0046]     Upstream optical signals of the first upstream wavelength band  320  output from the first to N th  ONU groups  270 - 1  to  270 -N are input to the first to N th  beam splitters  260 - 1  to  260 -N. Each of the first to N th  beam splitters  260 - 1  to  260 -N couple and output upstream optical signals of a corresponding wavelength. Upstream optical signals of the first upstream wavelength band  320  output from the first to N th  beam splitters  260 - 1  to  260 -N pass through the first to N th  working distribution fibers  240 - 1  to  240 -N, and are input to the third wavelength division multiplexer  230 . The third wavelength division multiplexer  230  multiplexes the input upstream optical signals of the first upstream wavelength band  320  into a first multiplexed upstream optical signal and outputs the first multiplexed upstream optical signal. The first multiplexed upstream optical signal passes through the third optical coupler  220 , the first feeder fiber  190 , the first optical coupler  180  and the first switch  170 , and is input to the first wavelength division multiplexer  140 . The first wavelength division multiplexer  140  demultiplexes the first multiplexed upstream optical signal into upstream optical signals of the first upstream wavelength band  320 . It then outputs the demultiplexed upstream optical signals to the first optical transceiver array  120 .  
         [0047]     In the steady state, the procedures for processing the second multiplexed downstream optical signal and the second multiplexed upstream optical signal are similar to those described above, so a detailed description thereof will be omitted.  
         [0048]      FIG. 5  is a block diagram to explaining the signal processing procedure when a defect occurs in the first feeder fiber  190  in the PON  100  shown in  FIG. 1 .  
         [0049]     When a defect occurs in the first feeder fiber  190 , the central office  110  recognizes a defect in the first feeder fiber  190  and controls that the first switch  170  connects its first port to its third port, since the first optical transceiver array  120  cannot receive upstream optical signals of the first upstream wavelength band  320 .  
         [0050]     In this case, the following is the procedure for processing downstream optical signals of the first downstream wavelength band  310  in the self-healing PON  100 .  
         [0051]     Downstream optical signals of the first downstream wavelength band  310  output from the first optical transceiver array  120  are multiplexed into a first multiplexed downstream optical signal by the first wavelength division multiplexer  140 . The first multiplexed downstream optical signal passes through the first switch  170 , the second optical coupler  185 , the second feeder fiber  195  and the fourth optical coupler  225 , and are input to the third wavelength division multiplexer  230 . The third wavelength division multiplexer  230  demultiplexes the first multiplexed downstream optical signal into downstream optical signals of the first downstream wavelength band  310 . The demultiplexed downstream optical signals pass through the first to N th  protection distribution fibers  245 - 1  to  245 -N, and are input to the first to N th  beam splitters  260 - 1  to  260 -N. Each of the first to N th  beam splitters  260 - 1  to  260 -N power-splits each input downstream optical signal into M downstream optical signals. It then outputs the M downstream optical signals to a corresponding ONU group selected from among the first to N th  ONU groups  270 - 1  to  270 -N.  
         [0052]     In addition, in this case, the following is the procedure for processing upstream optical signals of the first upstream wavelength band  320  in the self-healing PON  100 .  
         [0053]     Upstream optical signals of the first upstream wavelength band  320  from the first to N th  ONU groups  270 - 1  to  270 -N are input to the first to N th  beam splitters  260 - 1  to  260 -N. Each of the first to N th  beam splitters  260 - 1  to  260 -N couple and output upstream optical signals of a corresponding wavelength. Upstream optical signals of the first upstream wavelength band  320  output from the first to N th  beam splitters  260 - 1  to  260 -N pass through the first to N th  protection distribution fibers  245 - 1  to  245 -N, and are input to the third wavelength division multiplexer  230 . The third wavelength division multiplexer  230  multiplexes the input upstream optical signals of the first upstream wavelength band  320  into a first multiplexed upstream optical signal, and outputs the first multiplexed upstream optical signal. The first multiplexed upstream optical signal passes through the fourth optical coupler  225 , the second feeder fiber  195 , the second optical coupler  185  and the first switch  170 , and is input to the first wavelength division multiplexer  140 . The first wavelength division multiplexer  140  demultiplexes the first multiplexed upstream optical signal input thereto into upstream optical signals of the first upstream wavelength band  320 . It then outputs the demultiplexed upstream optical signals to the first optical transceiver array  120 .  
         [0054]     In this embodiment, the procedures for processing the second multiplexed downstream optical signal and the second multiplexed upstream optical signal are similar to those described above, so a detailed description thereof will be omitted to avoid redundancy.  
         [0055]     In addition, when a defect occurs in the second feeder fiber  195 , procedures similar to those described above may be performed, so a detailed description thereof will be omitted.  
         [0056]      FIG. 6  is a block diagram to explain the signal processing procedure when a defect occurs in the first working distribution fiber  240 - 1  in the PON  100  shown in  FIG. 1 .  
         [0057]     In the case in which a defect occurs in the first working distribution fiber  240 - 1 , the central office  110  recognizes a defect in the first working distribution fiber  240 - 1  and controls that the first switch  170  connects its first port to its third port, since the first optical transceiver array  120  cannot receive an upstream optical signal of the (N+1) th  wavelength.  
         [0058]     In this case, the following is the procedure for processing downstream optical signals of the first downstream wavelength band  310  in the self-healing PON  100 .  
         [0059]     Downstream optical signals of the first downstream wavelength band  310  output from the first optical transceiver array  120  are multiplexed into a first multiplexed downstream optical signal by the first wavelength division multiplexer  140 . The first multiplexed downstream optical signal passes through the first switch  170 , the second optical coupler  185 , the second feeder fiber  195  and the fourth optical coupler  225 , and are input to the third wavelength division multiplexer  230 . The third wavelength division multiplexer  230  demultiplexes the first multiplexed downstream optical signal into downstream optical signals of the first downstream wavelength band  310 . The demultiplexed downstream optical signals pass through the first to N th  protection distribution fibers  245 - 1  to  245 -N, and are input to the first to N th  beam splitters  260 - 1  to  260 -N. Each of the first to N th  beam splitters  260 - 1  to  260 -N power-splits each input downstream optical signal into M downstream optical signals. It then outputs the M downstream optical signals to a corresponding ONU group selected from among the first to N th  ONU groups  270 - 1  to  270 -N.  
         [0060]     In addition, in this case, the following is the procedure for processing upstream optical signals of the first upstream wavelength band  320  in the self-healing PON  100 .  
         [0061]     Upstream optical signals of the first upstream wavelength band  320  output from the first to N th  ONU groups  270 - 1  to  270 -N are input to the first to N th  beam splitters  260 - 1  to  260 -N. Each of the first to N th  beam splitters  260 - 1  to  260 -N couple and output upstream optical signals of a corresponding wavelength. Upstream optical signals of the first upstream wavelength band  320  output from the first to N th  beam splitters  260 - 1  to  260 -N pass through the first to N th  protection distribution fibers  245 - 1  to  245 -N, and are input to the third wavelength division multiplexer  230 . The third wavelength division multiplexer  230  multiplexes the input upstream optical signals of the first upstream wavelength band  320  into a first multiplexed upstream optical signal, and outputs the first multiplexed upstream optical signal. The first multiplexed upstream optical signal passes through the fourth optical coupler  225 , the second feeder fiber  195 , the second optical coupler  185  and the first switch  170 , and is input to the first wavelength division multiplexer  140 . The first wavelength division multiplexer  140  demultiplexes the first multiplexed upstream optical signal input thereto into upstream optical signals of the first upstream wavelength band  320 . It then outputs the demultiplexed upstream optical signals to the first optical transceiver array  120 .  
         [0062]     In this embodiment, the procedures for processing the second multiplexed downstream optical signal and the second multiplexed upstream optical signal are similar to those described above, so a detailed description thereof will be omitted.  
         [0063]     As described above, according to the embodiment of the present invention, the self-healing PON outputs the first and second multiplexed downstream optical signals through either the first or second feeder fiber by using the first and second switching parts when a defect occurs in an optical fiber. Thus, the defect in the feeder fibers or distribution fibers can be self-healed.  
         [0064]     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 invention as defined by the appended claims. Accordingly, the scope of the invention is not to be limited by the above embodiments but by the claims and the equivalents thereof.