Abstract:
A passive optical network is disclosed. The network includes a plurality of subscriber units that generate upstream optical signals, respectively, reflect channels applied thereto in association with the subscriber units, respectively, and detect downstream optical signals associated with the subscriber units, respectively, and a central office that output a multiplexed downstream optical signal and a monitoring light, and detects a multiplexed channel signal. The network also includes a remote node that demultiplexes the monitoring light into different channels, outputs the channels to the subscriber units, respectively, multiplexes the channels, which are reflected from the subscriber units, generates the multiplexed channel signal, and outputs the multiplexed channel signal to the central office. The network further includes a first main optical fiber linking the central office and the remote node, and a plurality of second main optical fibers linking the remote node and the subscriber units, respectively.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “SELF-MONITORING PASSIVE OPTICAL NETWORK,” filed in the Korean Intellectual Property Office on Jun. 15, 2004 and assigned Serial No. 2004-43996, the contents of which are hereby incorporated 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 PON having self-monitoring and self-healing functions.  
         [0004]     2. Description of the Related Art  
         [0005]     A wavelength division multiplexed (WDM) PON can provide ultrahigh-speed broadband communication services, using particular wavelengths assigned to respective subscribers. Such WDM PONs can ensure communication security while being capable of accommodating separate communication services required by a subscriber. In addition, such WDM PONs can accommodate a subsciber&#39;s need for expansion of communication capacity. However, such WDM PONs require, for a central office (CO) associated therewith and each optical network unit (ONU) associated therewith, light sources having a particular oscillation wavelength, and wavelength stabilizing circuits adapted to stabilize the wavelength of the light sources, respectively. This results in an increased cost that is imposed on subscribers.  
         [0006]     Generally, WDM PONs use a double star type topology to minimize the length of optical lines used therein. In such double star type topologies, a remote node is installed in an area where a plurality of subscribers are distributed within a near distance. The remote node is connected to a central office via a feeder fiber. The subscribers are individually connected to the remote node via distribution fibers.  
         [0007]     The central office multiplexes a plurality of downstream optical signals having different wavelengths, and transmits the resultant multiplexed optical signal to the remote node. The remote node demultiplexes the multiplexed downstream optical signal, and sends the resultant demultiplexed optical signals to respective subscribers. The remote node also multiplexes upstream optical signals received from respective subscribers, and transmits the resultant multiplexed optical signal to the central office.  
         [0008]     Since the central office and remote node are connected by a single feeder fiber in the above-mentioned WDM PON, if the feeder fiber fails or is degraded, downstream and upstream optical signals, which are transmitted through the feeder fiber, are inevitably lost. In order to minimize damage caused by the failure or degradation of the feeder fiber, a separate low-speed communication network is typically installed between the central office and the remote node in general PONs.  
         [0009]     However, the low-speed communication network takes a considerable amount of time to check whether or not there is any abnormality between the central office and the remote node (due to the low-speed communication network used), and to inform a manager of the result of the checking. This means that the amount of time communication is interrupted between the central office and the remote node is prolonged. Therefore, the above-mentioned PON requires a self-healing function to rapidly and reliably heal abnormalities generated within the PON.  
         [0010]      FIG. 1  is a block diagram illustrating a conventional, self-healing, bi-directional, ring-type, optical network  100 . The conventional, self-healing, ring-type, optical network  100  includes a plurality of nodes  110 ,  120 ,  130 , and  140  to transmit/receive a first optical signal having wavelengths λ 1  to λ N  and a second optical signal having wavelengths λ N+1  to λ 2N  to/from one another, and first and second optical fibers  101  and  102  to link the nodes  110 ,  120 ,  130 , and  140  in the form of a ring. The first and second optical signals use different wavelength ranges respectively including the wavelengths λ 1  to λ N  and the wavelengths λ N+1  to λ 2N . Each of the nodes  110 ,  120 ,  130 , and  140  drops an associated channel from the first optical signal input thereto, and outputs the resultant first optical signal. Each of the nodes  110 ,  120 ,  130 , and  140  also adds a particular channel to the second optical signal, and outputs the resultant second optical signal.  
         [0011]     Each of the nodes  110 ,  120 ,  130 , or  140  includes a first switch  111 ,  121 ,  131 , or  141 , a second switch  112 ,  122 ,  132 , or  142 , a first optical add-drop multiplexer (OADM)  113 ,  123 ,  133 , or  143  to connect the first switch  111 ,  121 ,  131 , or  141  and the second switch  112 ,  122 ,  132 , and  142 , and a second OADM  114 ,  124 ,  134 , or  144  to connect the first switch  111 ,  121 ,  131 , or  141  and the second switch  112 ,  122 ,  132 , or  142 . Each of the switches may be a 2×2 switch.  
         [0012]     Each of the first OADMs  113 ,  123 ,  133 , and  143  drops an associated channel from the first optical signal, multiplexes the undropped remaining channels of the first optical signal, and outputs the resultant multiplexed optical signal. Each of the second OADMs  114 ,  124 ,  134 , and  144  adds a particular channel corresponding to a predetermined wavelength to the second optical signal, multiplexes the channel-added second optical signal, and outputs the resultant multiplexed optical signal. The first optical signal is sequentially input to and output from the nodes  110 ,  120 ,  130 , and  140  via the first optical fiber  101 . Similarly, the second optical signal is sequentially input to and output from the nodes  110 ,  120 ,  130 , and  140  via the second optical fiber  102 .  
         [0013]     Each of the first switch  111 ,  121 ,  131 , and  141  receives the first optical signal output from the node connected to an input terminal of the first switch, and outputs the received first optical signal to an associated one of the first OADM  113 ,  123 ,  133 , and  143 . Each of the first switch  111 ,  121 ,  131 , and  141  also receives the second optical signal from an associated one of the second OADMs  114 ,  124 ,  134 , and  144 , and outputs the received second optical signal to the node connected to an output terminal of the first switch.  
         [0014]     In the above-mentioned bi-directional, self-healing, ring-type, optical network  100 , even when there is a fault caused by a line failure generated in one of the first and second optical fibers  101  and  102  or a degradation of the constituent elements of the nodes  110 ,  120 ,  130 , and  140 , it is possible to transmit/receive the first and second optical signals through the nodes  110 ,  120 ,  130 , and  140  by circulating the other optical fiber  102  or  101  by the second switches  112 ,  122 ,  132 , and  142  or the first switches  111 ,  121 ,  131 , and  141 .  
         [0015]     However, if this self-healing optical network architecture is applied to the above-mentioned WDM PON, there are again problems cost burden and bulky size because a plurality of switches and a plurality of multiplexers/demultiplexers must be additionally used. Furthermore, conventional optical communication systems having a self-healing or monitoring means have a problem in that it is impossible to accurately identify a correct cause of optical signal disturbance.  
       SUMMARY OF THE INVENTION  
       [0016]     One aspect of the present invention relates to a PON capable of tracing a cause of signal disturbance and healing the cause.  
         [0017]     One embodiment of the present invention is directed to a passive optical network including a plurality of subscriber units that generate upstream optical signals, respectively, and reflect channels applied thereto in association with the subscriber units, respectively, and that receive downstream optical signals associated with respective subscriber units; a central office that outputs a multiplexed downstream optical signal and a monitoring light and that detects a multiplexed channel signal; a remote node that demultiplexes the monitoring light into different channels, outputs the channels to the subscribers, respectively, multiplex the channels from the subscriber units, generate the multiplexed channel signal, and output the multiplexed channel signal to the central office; a first main optical fiber linking the central office and the remote node; and a plurality of second main optical fibers linking the remote node and the subscriber units, respectively.  
         [0018]     Another embodiment of the present invention is directed to a passive optical network including a plurality of subscriber units that generate upstream optical signals, respectively, reflect channels applied thereto in association with the subscriber units, respectively, and detect downstream optical signals associated with the subscriber units, respectively; and a central office that outputs a multiplexed downstream optical signal and a monitoring light, and detects a multiplexed channel signal. The network also includes a remote node that demultiplexes the monitoring light into different channels, output the channels to the subscriber units, respectively, multiplexes the channels, which are reflected from the subscriber units, generates the multiplexed channel signal, and outputs the multiplexed channel signal to the central office; a first main optical fiber linking the central office and the remote node; a plurality of second main optical fibers linking the remote node and the subscribers, respectively; a first auxiliary optical fiber used to transmit the multiplexed downstream optical signal and the monitoring light to the remote node when a fault occurs in the first main optical fiber, and to transmit a multiplexed signal of the upstream optical signals and a multiplexed signal of the channels to the central office when the fault occurs; and a plurality of second auxiliary optical fibers each used to transmit an associated one of demultiplexed signals of the multiplexed downstream optical signal to an associated one of the subscriber units and transmit the upstream optical signal generated from the associated subscriber unit and the channel reflected from the associated subscriber unit to the remote node when a fault occurs in an associated one of the second main optical fibers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above aspects and embodiments of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which:  
         [0020]      FIG. 1  is a block diagram illustrating a conventional self-healing bi-directional ring type optical network;  
         [0021]      FIG. 2  is a block diagram illustrating a WDM PON having self-monitoring and self-healing functions in accordance with a first embodiment of the present invention; and  
         [0022]      FIG. 3  is a block diagram illustrating a WDM PON having self-monitoring and self-healing functions in accordance with a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Now, embodiments of the present invention will be described in detail with reference to the annexed drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.  
         [0024]      FIG. 2  is a block diagram illustrating a WDM PON having self-monitoring and self-healing functions in accordance with a first embodiment of the present invention. As shown in  FIG. 2 , the WDM PON  200  includes a plurality of subscribers  250  that generate upstream optical signals, and detect downstream optical signals associated with the subscribers  250 , respectively, a central office  210  that generates a multiplexed downstream optical signal and detects upstream optical signals, and a remote node  240  that performs a relay operation between the subscribers  250  and the central office  210 . The WDM PON  200  also includes a first main optical fiber  201  and a first auxiliary optical fiber  202  to link the central office  210  and the remote node  240 , and a plurality of second main optical fibers  203  and a plurality of second auxiliary optical fibers  204  to link the remote node  240  and respective subscribers  250 . The central office  210  generates a monitoring light to monitor generation of a fault in the WDM PON  200  and cause of the fault. The remote node  240  demultiplexes the monitoring light into channels of different wavelengths, and outputs the demultiplexed channels to the subscribers  250 , respectively. Each subscriber  250 , which receives the channel from the remote node  240 , reflects the received channel to the remote node  240 . The remote node  240  multiplexes the reflected channels received from respective subscribers  250 , and outputs the multiplexed channel signal to the central office  210 . Accordingly, the central office  210  can monitor whether or not there is a fault in the WDM PON  200  by determining whether or not the channel transmitted from each subscriber  250  has been detected.  
         [0025]     When a fault occurs in the first main optical fiber  201 , the first auxiliary optical fiber  202  transmits the multiplexed downstream optical signals and the monitoring light to the remote node  240 , while transmitting the multiplexed upstream optical signals and channels from the remote node  240  to the central office  240 .  
         [0026]     When there is a fault in one of the second main optical fibers  203 , which is connected between the remote node  240  and an associated one of the subscribers  250 , the second auxiliary optical fiber  204  associated with the faulty second main optical fiber  203  transmits the associated demultiplexed downstream optical signal to the associated subscriber  250 , while transmitting, to the remote node  240 , the upstream optical signal generated from the associated subscriber  250  and the channel reflected from the associated subscriber  250 .  
         [0027]     The central office  210  includes a plurality of downstream light sources  211 , a plurality of upstream photodetectors  212 , a monitor  230  generates the monitoring light and detects respective channels transmitted form the subscribers  250 , a first multiplexer/demultiplexer (MUX/DEMUX)  215 , first wavelength-selective couplers  213 , first switches  214 , and a broadband optical module  220  wavelength-lock the downstream light sources  211  and subscribers  250 .  
         [0028]     The first MUX/DEMUX  215  multiplexes the downstream optical signals input to the central office  210 , and outputs the resultant multiplexed downstream optical signal to the remote node  240 . The first MUX/DEMUX  215  also demultiplexes the multiplexed upstream optical signal input to the central office  210 , and outputs the resultant demultiplexed upstream optical signals to the upstream photodetectors  212 , respectively.  
         [0029]     The downstream light sources  211  generate wavelength-locked downstream optical signals, respectively. Each upstream photodetector  212  detects an associated one of the upstream optical signals demultiplexed by the first MUX/DEMUX  215 .  
         [0030]     Each first wavelength-selective coupler  213  outputs an associated one of the demultiplexed upstream optical signals to an associated one of the upstream photodetectors  212 . Each first wavelength-selective coupler  213  also outputs the downstream optical signal generated from an associated one of the downstream light source  211  to the first MUX/DEMUX  215 .  
         [0031]     Each first optical switch  214  is arranged between an associated one of the first wavelength-selective coupler  213  and the first MUX/DEMUX  215  to selectively connect the associated first wavelength-selective coupler  213  to a desired one of at least two ports of the fist MUX/DEMUX  215  assigned to the associated first wavelength-selective coupler  213 .  
         [0032]     The monitor  230  includes a monitoring light source  235 , a spectrum analyzer  236 , a second wavelength-selective coupler  231 , a third wavelength-selective coupler  232 , a second optical switch  233 , and a circulator  234 . Using this configuration, the monitor  230  can generate a monitoring light, and detects channels reflected from respective subscribers  250 .  
         [0033]     The monitoring light source  235  generates a monitoring light having a plurality channels with different wavelengths. The spectrum analyzer  236  demultiplexes the multiplexed channel signal received from the remote node  240 , and detects the resultant demultiplexed channels.  
         [0034]     The second wavelength-selective coupler  231  connects the first main optical fiber  201  and the monitor  230  so that the monitoring light can be output to the remote node  240  via the first main optical fiber  201 . The multiplexed channel signal received from the remote node  240  is also output to the second optical switch  233  via the second wavelength-selective coupler  231 . The second wavelength-selective coupler  231  also transmits the multiplexed downstream optical signal from the central office  230  to the remote node  240 , and transmits the multiplexed upstream optical signal from the remote node  240  to the central office  210 .  
         [0035]     The third wavelength-selective coupler  232  connects the first auxiliary optical fiber  202  and the monitor  230  so that the monitoring light can be output to the remote node  240  via the first auxiliary optical fiber  202 . The multiplexed channel signal received from the remote node  240  is also output to the second optical switch  233  via the third wavelength-selective coupler  232 . The third wavelength-selective coupler  232  also transmits the multiplexed downstream optical signal from the central office  230  to the remote node  240 , and transmits the multiplexed upstream optical signal from the remote node  240  to the central office  210 .  
         [0036]     The second optical switch  233  selectively outputs the monitoring light received from the circulator  243  to the second wavelength-selective coupler  231  or third wavelength-selective coupler  232 , and outputs, to the circulator  234 , the multiplexed channel signal received from the second or third wavelength-selective coupler  231  or  232 .  
         [0037]     The circulator  234  outputs the monitoring light generated from the monitoring light source  235  to the second optical switch  233 , and outputs the multiplexed channel signal received from the second optical switch  233  to the spectrum analyzer  236 .  
         [0038]     The spectrum analyzer  236  analyzes whether or not each channel has been detected. The result of the analysis is used to recognize whether or not there is a fault in the subscriber  250  associated with the channel. It is possible to determine whether the fault is based on a line failure generated in one of the first and second main optical fibers  201  and  203 , and first and second auxiliary optical fibers  202  and  204 , or a failure or degradation of one of the above-described constituent elements, by comparing the determination result as to whether or not the associated upstream optical signal has been detected and the determination result as to whether or not the associated channel has been detected.  
         [0039]     The broadband optical module  220  includes a first broadband light source  221  that generates a downstream light to induce wavelength-locking of the multiplexed downstream optical signal, a second broadband light source  222  that generates an upstream light to induce wavelength-locking of the multiplexed upstream optical signal, a first optical distributor  225 , a second optical distributor  226 , a fourth optical switch  223 , and a fifth optical switch  224 .  
         [0040]     The first optical distributor  225  is arranged on the first main optical fiber  201  so that the downstream light can be output to the first MUX/DEMUX  215  and the upstream light can be output to the second wavelength-selective coupler  231 . The second optical distributor  226  is arranged on the first auxiliary optical fiber  202  so that the upstream light can be output to the third wavelength-selective coupler  232  and the downstream light can be output to the first MUX/DEMUX  215 .  
         [0041]     The fourth optical switch  223  selectively connects the first broadband light source  221  to the first optical distributor  225  or second optical distributor  226 . The fifth optical switch  224  selectively connects the second broadband light source  222  to the first optical distributor  225  or second optical distributor  226 .  
         [0042]     The remote node  240  includes a second MUX/DEMUX  241 . The second MUX/DEMUX  241  is linked to the central office  219  via the first main optical fiber  201  and first auxiliary optical fiber  202 . The second MUX/DEMUX  241  is also linked to the subscribers  250  via the second main optical fibers  203  and second auxiliary optical fibers  204 , respectively.  
         [0043]     The second MUX/DEMUX  241  demultiplexes the multiplexed downstream optical signal output from the central office  210 , and outputs the resultant demultiplexed downstream optical signals to respective subscribers  250 . The second MUX/DEMUX  241  also multiplexes the upstream optical signals output from respective subscribers  250 , and outputs the resultant multiplexed upstream optical signal to the central office  210 . The second MUX/DEMUX  241  also demultiplexes the monitoring light into channels, and outputs the channels to respective subscribers  250 . In addition, the second MUX/DEMUX  241  multiplexes the channels reflected from respective subscribers  250 , and outputs the multiplexed channel signal to the central office  210 .  
         [0044]     Each subscriber  250  is linked to the remote node  240  via an associated one of the second main optical fibers  203  and an associated one of the second auxiliary optical fibers  204 . Each subscriber  250  includes an upstream light source  251 , a downstream photodetector  252 , a fifth optical switch  254 , a fourth wavelength-selective coupler  253 , and first and second reflection filters  256  and  257 .  
         [0045]     The upstream light source  251  generates an upstream optical signal wavelength-locked by the second broadband light source  222 . The downstream photodetector  252  detects an associated one of the demultiplexed downstream optical signals output from the remote node  240 .  
         [0046]     The fourth wavelength-selective coupler  253  receives the associated downstream optical signal from the fifth optical switch  254 , and outputs the received associated downstream optical signal to the downstream photodetector  252 . The fourth wavelength-selective coupler  253  also outputs the upstream optical signal generated from the upstream light source  251  to the fifth optical switch  254 . The fifth optical switch  254  selectively connects the fourth wavelength-selective coupler  253  to the first reflection filter  256  or second reflection filter  257 .  
         [0047]     The first and second reflection filters  256  and  257  transmit the upstream optical signal and downstream optical signal, while reflecting, to the remote node  240 , an associated one of the channels output from the remote node  240 .  
         [0048]      FIG. 3  is a block diagram illustrating a WDM PON  300  having self-monitoring and self-healing functions in accordance with a second embodiment of the present invention. The WDM PON  300  includes a plurality of subscribers  370  that generate upstream optical signals and can detect downstream optical signals associated with the subscribers  370 , respectively, a central office  310  that generates a multiplexed downstream optical signal and can detect upstream optical signals, and a remote node  400  that performs a relay operation between the subscribers  370  and the central office  310 . The WDM PON  200  also includes a first main optical fiber  301  and a first auxiliary optical fiber  302  to link the central office  310  and the remote node  400 , and a plurality of second main optical fibers  303  and a plurality of second auxiliary optical fiber  304  to link the remote node  400  and respective subscribers  370 .  
         [0049]     The central office  310  includes at least a first optical transmitting/receiving module  320 , at least a second optical transmitting/receiving module  330 , a downstream optical module  340 , an upstream optical module  350 , a monitor  360 , first optical switches  312 , a first optical distributor  314 , and a second optical distributor  313 .  
         [0050]     Each first optical transmitting/receiving module  320  includes a first downstream light source  321 , a first upstream photodetector  322 , and a first wavelength-selective coupler  323 . The first wavelength-selective coupler  323  of each first optical transmitting/receiving module  320  outputs an associated downstream optical signal to an associated one of the first optical switches  312 , and outputs an upstream optical signal received from the associated first optical switch  312  to an associated one of the first upstream photodetector  322 . In this way, the first downstream light source  321  of each first optical transmitting/receiving module  320  generates a wavelength-locked downstream optical signal, and the first upstream photodetector  322  detects an associated upstream optical signal.  
         [0051]     Each second optical transmitting/receiving module  330  operates when a fault occurs in an associated one of the first optical transmitting/receiving modules  320 . The second optical transmitting/receiving module  330  includes a second downstream light source  331  to generate a downstream optical signal, a second upstream photodetector  332  to detect an associated upstream optical signal, and a second wavelength-selective coupler  333  to output the downstream optical signal generated from the second downstream light source  331  to an associated one of the first optical switches  312 , and to output an upstream optical signal received from the associated first optical switch  312  to the second upstream photodetector  332 . In this way each second optical transmitting/receiving module  330  is substituted for the associated first optical transmitting/receiving module  320  when the associated first optical transmitting/receiving module  320  cannot perform normal operation due to a fault condition.  
         [0052]     The downstream optical module  340  includes first and second downstream light sources  341  and  342  that generate a downstream light, and a second optical switch  343  that outputs the downstream light generated from the first or second downstream light source  341  or  342  to the first optical distributor  314  or second optical distributor  313 . The second optical switch  343  outputs the downstream light generated from the second downstream light source  342  when there is a fault in the first downstream light source  341 , and outputs the downstream light generated from the first downstream light source  341  when there is a fault in the second downstream light source  342 .  
         [0053]     The upstream optical module  350  includes first and second upstream light sources  351  and  352  that generate an upstream light to wavelength-lock the subscribers  370 , and a third optical switch  353  that outputs the upstream light generated from the first or second upstream light source  351  or  352  to the first optical distributor  314  or second optical distributor  313 . The third optical switch  353  outputs the upstream light generated from the second downstream light source  352  when there is a fault in the first upstream light source  351 , and outputs the upstream light generated from the first upstream light source  351  when there is a fault in the second upstream light source  352 .  
         [0054]     The first MUX/DEMUX  311  multiplexes the downstream optical signals respectively received from the first optical switches  312 , and outputs the resultant multiplexed downstream optical signal to the remote node  400 . The first MUX/DEMUX  311  also demultiplexes a multiplexed upstream optical signal received from the remote node  400 , and outputs the resultant demultiplexed upstream optical signals to the first optical switches  312  associated therewith, respectively.  
         [0055]     The monitor  360  includes a monitoring light source  361  that generates a monitoring light, a spectrum analyzer  362 , a second wavelength-selective coupler  365  arranged on the first main optical fiber  301 , a third wavelength-selective coupler  366  arranged on the first auxiliary optical fiber  302 , and a fourth optical switch  364  to selectively connect the monitor  360  to the second wavelength-selective coupler  365  or third wavelength-selective coupler  366 . The monitor  360  also includes a circulator  363  that outputs the monitoring light generated from the monitoring light source  361  to the fourth optical switch  364 , and outputs a multiplexed channel signal including a plurality channels having different wavelengths received from the fourth optical switch  364  to the spectrum analyzer  362 .  
         [0056]     The monitoring light source  361  outputs the monitoring light generated therefrom to the circulator  363 . The spectrum analyzer  362  demultiplexes the multiplexed channel signal from the remote node  400  via the circulator  363 , and detects the resultant demultiplexed channels. The spectrum analyzer  362  may include a diffraction grating to split the multiplexed channel signal into channels having different wavelengths, and photodetectors to detect the split channels outputted from the diffraction grating, respectively. For the diffraction grating, a Bragg grating or hologram element may be used. For the photodetectors, photodiodes capable of detecting the channels of different wavelengths may be used, respectively.  
         [0057]     The second wavelength-selective coupler  365  outputs the monitoring light to the remote node  400  via the first main optical fiber  301 , and outputs the multiplexed channel signal received from the remote node  400  via the first main optical fiber  301  to the fourth optical switch  364 . The third wavelength-selective coupler  366  outputs the monitoring light to the remote node  400  via the first auxiliary optical fiber  302 , and outputs the multiplexed channel signal received from the remote node  400  via the first auxiliary optical fiber  302  to the fourth optical switch  364 . The fourth optical switch  364  selectively couples the circulator  363  to the second wavelength-selective coupler  365  or third wavelength-selective coupler  366 . The circulator  363  outputs the monitoring light generated from the monitoring light source  361  to the fourth optical switch  364 , and outputs the multiplexed channel signal received from the fourth optical switch  364  to the spectrum analyzer  362 .  
         [0058]     Each first optical switch  312  selectively connects the associated first optical transmitting/receiving module  320  or the associated second optical transmitting/receiving module  330  to the first MUX/DEMUX  311 .  
         [0059]     The first optical distributor  314 , which is arranged on the first main optical fiber  301 , is connected to both the downstream optical module  340  and the upstream optical module  350 , to output the downstream light to the first MUX/DEMUX  311 , and to output the upstream light to the remote node  400 .  
         [0060]     The second optical distributor  313 , which is arranged on the first auxiliary optical fiber  302 , is connected to both the downstream optical module  340  and the upstream optical module  350 , to output the downstream light to the first MUX/DEMUX  311 , and to output the upstream light to the remote node  400 .  
         [0061]     The remote node  400  includes a second MUX/DEMUX  401 . The second MUX/DEMUX  401  is linked to the central office  310  via the first main optical fiber  301  and first auxiliary optical fiber  302 . The second MUX/DEMUX  401  is also linked to the subscribers  370  via the second main optical fibers  303  and second auxiliary optical fibers  304 , respectively.  
         [0062]     Each subscriber  370  includes a first optical module  380  that generates a wavelength-locked upstream optical signal, a second optical module  390  that generates a wavelength-locked upstream optical signal, a fifth optical switch  371  that selectively connects the first optical module  380  or second optical module  390  to the remote node  400 , a first band-pass filter  373  arranged on an associated one of the second main optical fibers  303 , and a second band-pass filter  372  arranged on an associated one of the second auxiliary optical fibers  304 .  
         [0063]     The fifth optical switch  371  of each subscriber  370  outputs the upstream optical signal generated from the associated second optical module  390  when a fault occurs in the associated first optical module  380 , and outputs an associated one of the demultiplexed downstream optical signals output from the remote node  400  to the associated second optical module  390 . The fifth optical switch  371  also connects the associated second auxiliary optical fiber  304  to the first optical module  380  or second optical module  390  when a fault occurs in the associated second main optical fiber  303 .  
         [0064]     The first band-pass filter  373  of each subscriber  370  outputs the downstream optical signal received via the associated second main optical fiber  303  to the associated fifth optical switch  371 , and outputs the upstream optical signal generated from the associated first or second optical module  380  or  390  to the remote node  400  via the associated second main optical fiber  303 . The first band-pass filter  373  also reflects the associated channel received from the remote node  400  to the remote node  400 .  
         [0065]     The second band-pass filter  372  of each subscriber  370  outputs the downstream optical signal received via the associated second auxiliary optical fiber  304  to the associated fifth optical switch  371 , and outputs the upstream optical signal generated from the associated first or second optical module  380  or  390  to the remote node  400  via the associated second auxiliary optical fiber  304 .  
         [0066]     The first optical module  380  of each subscriber  370  includes a first downstream photodetector  382  that detects the associated downstream optical signal, a first upstream light source  381  that generates a wavelength-locked upstream optical signal, and a fifth wavelength-selective coupler  383 . The fifth wavelength-selective coupler  383  outputs the associated downstream optical signal received from the associated fifth optical switch  371  to the associated first downstream photodetector  382 , and outputs the upstream optical signal received from the associated first upstream light source  381  to the associated fifth optical switch  371 .  
         [0067]     The second optical module  390  of each subscriber  370  includes a second downstream photodetector  392  that detects the associated downstream optical signal, a second upstream light source  391  that generates a wavelength-locked upstream optical signal, and a sixth wavelength-selective coupler  393 . The sixth wavelength-selective coupler  393  outputs the associated downstream optical signal received from the associated fifth optical switch  371  to the associated second downstream photodetector  392 , and outputs the upstream optical signal received from the associated second upstream light source  391  to the associated fifth optical switch  371 . The second optical module  390  operates in place of the first optical module  380  when the first optical module  380  cannot perform normal operation due to a fault condition or cannot generate a desired upstream optical signal.  
         [0068]     While various embodiments of the present invention has been described above, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the appended claims.