Abstract:
Provided are an apparatus and method for tracking a wavelength in a passive optical subscriber network in which a central base station and at least one subscriber terminal are connected via a remote node. The apparatus includes a first wavelength aligning unit multiplexing and aligning wavelengths of optical signals from a plurality of single-mode optical sources of the central base station; a second wavelength aligning unit multiplexing and aligning wavelengths of optical signals transmitted to the remote node from a plurality of single-mode optical sources of the subscriber terminal; and a third wavelength aligning unit demultiplexing and aligning wavelengths of optical signals from the second wavelength aligning unit, the third wavelength aligning unit being included in the central base station. Accordingly, when the wavelengths of passbands of a multiplexer/demultiplexer (MUX/DEMUX) of a remote station change due to a change in the ambient temperature, wavelength tracking is performed by making aligned the wavelengths of optical sources of a central base station, a multiplexer/demultiplexer, and subscriber terminals, thereby minimizing optical channel loss and enabling reliable management of WDM-PON.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims the priority of Korean Patent Application No. 10-2005-121985, filed on Dec. 12, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a wavelength tracking apparatus and method in a wavelength-division multiplexed (WDM)-passive optical network (PON), and more particularly, to a reliable WDM-PON system by aligning wavelengths of an optical source in the central office, a pass band of multiplexer/demultiplexer in the central office, and an optical source in the subscriber terminal, with respect to a pass band of the multiplexer/demultiplexer in the remote node, which varies according to ambient temperature.  
         [0004]     2. Description of the Related Art  
         [0005]     A digital subscriber line (DSL) technique that uses a unshielded twisted pair (UTP) and a cable modem termination system (CMTS) technique that uses a hybrid fiber coaxial (HFC), which have been currently used, are not expected to guarantee a bandwidth and service quality enough to provide subscribers with a convergence service of voice, data, and broadcasting which will be widely popularized in a few years. To solve this problem, a great deal of research has been conducted all over the world to develop a fiber-to-the home (FTTH) technique that connects the subscriber&#39;s home to the network via an optical fiber.  
         [0006]     In a wavelength-division multiplexed (WDM)-passive optical network (PON), since communications are established between a central office and each subscriber by using a wavelength allocated to the subscriber, it is possible to provide a variety of independent communication services to each subscriber while guaranteeing quality of service and security. Also, unlike time division multiplexing (TDM), the WDM-PON assigns each wavelength to an individual subscriber who may use an optical source with low output power and a receiver with a narrow bandwidth.  
         [0007]     However, the WDM-PON employs optical sources corresponding to subscribers, each optical source having a unique wavelength, thus increasing installation costs, and is substantially difficult to be competitive in cost over the TDM based passive optical network accordingly. Thus, development of a low-cost optical source for the WDM-PON is critically important. Also, in terms of equipment management, preparing a stock of optical sources having different wavelengths for respective subscribers against mechanical and functional troubles may be too heavy a burden for a service provider. Therefore, it is very important to design a WDM-PON that can provide subscribers with the ONT (optical Network Terminal) of one kind with wavelength-independent optical source.  
         [0008]     For reliable management of the WDM-PON, it is important to monitor wavelengths of optical sources against aging of the componets or temperature changes, and optical fiber cut, and to align wavelengths of the multiplexer/demultiplexer whose pass band change according to ambient temperature.  
         [0009]     In particular, it is very important to align wavelengths of optical sources and the multiplexer/demultiplexer in the central office, and an optical source of a subscriber terminal (ONT) with respect to a pass band of the multiplexer/demultiplexer in the remote node (RN) whose pass bands vary on ambient temperature changes.  
         [0010]     For easy repair and management of the WDM-PON, electric current is not supplied to a remote node. However, in this case, the temperature of the optical multiplexer/demultiplexer in the remote node may change from −40° C. to 80° C., and particularly, to a maximum of 120° C., according to ambient temperature.  
         [0011]     Accordingly, misalignment of wavelengths of the WDM multiplexers/demultiplexers (WMD) of the central office (CO) and the remote node (RN), and wavelengths of the WDM multiplexer/demultiplexer (WMD) in the remote node and each of optical sources of ONTs, may cause not only optical loss in the optical channels but also performance degradation due to crosstalk occurring between optical channels.  
         [0012]     To solve these problems, a wavelength tracking method has been introduced to equalize a wavelength of an optical source for downward transmission with a passband of WMD, which varies upon ambient temperature change.  
         [0013]     Also, a method has been introduced to equalize a passband of WMD in the RN with that of WMD in the CO for a WDM-PON that uses a spectrum-sliced optical source. However, these methods do not disclose alignment of the wavelength of an optical source, a pass band of WMD in CO, a pass band of WMD in RN, and an optical source in ONT. These methods are not applicable to a WDM-PON that uses a general single-mode optical source.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a system and method for aligning wavelengths of an optical source and an optical multiplexer/demultiplexer of a central base station, an optical multiplexer/demultiplexer of a remote node, and an optical source of a subscriber terminal together in a wavelength-division multiplexing (WDM)-passive optical network (PON) that uses a single-mode optical source.  
         [0015]     According to an aspect of the present invention, there is provided an apparatus for tracking a wavelength in a passive optical subscriber network in which a central base station and at least one subscriber terminal are connected via a remote node, the apparatus comprising a first wavelength aligning unit multiplexing and aligning wavelengths of optical signals from a plurality of single-mode optical sources of the central base station; a second wavelength aligning unit multiplexing and aligning wavelengths of optical signals transmitted to the remote node from a plurality of single-mode optical sources of the subscriber terminal; and a third wavelength aligning unit being included in the central base station, and demultiplexing and aligning wavelengths of optical signals from the second wavelength aligning unit.  
         [0016]     According to another aspect of the present invention, there is provided a method of tracking a wavelength in a passive optical subscriber network in which a central base station and at least one subscriber terminal are connected via a remote node, the method comprising a first wavelength aligning operation in which wavelengths of optical signals from a plurality of single-mode optical sources of the central base station are multiplexed and aligned; a second wavelength aligning operation in which the remote node multiplexes and aligns wavelengths of optical signals from a plurality of single-mode optical sources of the subscriber terminal; and 
        a third wavelength aligning operation in which the central base station demultiplexes and aligns the optical signals being demultiplexed and aligned in the second wavelength aligning operation.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other aspects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0019]      FIG. 1  is a block diagram of a wavelength-division multiplexing (WDM)-passive optical network (PON) system that uses a single-mode optical source;  
         [0020]      FIG. 2  is a block diagram of a wavelength tracking apparatus included in a WDM-PON system according to an embodiment of the present invention;  
         [0021]      FIG. 3  is a block diagram of a bi-directional WDM-PON that uses a single optical fiber line;  
         [0022]      FIG. 4  is a block diagram of a WDM-PON illustrated in  FIG. 3  which uses a wavelength tracking apparatus, according to an embodiment of the present invention;  
         [0023]      FIG. 5  is a flowchart illustrating operations of controlling the temperatures of thermoelectric coolers and power monitors of the wavelength tracking apparatus shown in  FIG. 4 , according to an embodiment of the present invention;  
         [0024]      FIG. 6  illustrates graphs respectively showing variations in the optical power level and wavelength of an optical upstream signal received at a central base station when the temperature of a remote node changes, in the wavelength tracking apparatus illustrated in  FIG. 2 , according to an embodiment of the present invention;  
         [0025]      FIG. 7  illustrates graphs respectively showing variations in the optical power level and wavelength of an optical downstream signal received at a subscriber terminal when the temperature of a remote node changes, in the wavelength tracking apparatus of  FIG. 2 , according to an embodiment of the present invention;  
         [0026]      FIG. 8  is a flowchart illustrating a method of aligning a wavelength of an optical source of a central base station with respect to that of a multiplexer/demultiplexer of a central base station according to an embodiment of the present invention;  
         [0027]      FIG. 9  is a flowchart illustrating a method. of aligning a wavelength of a passband of a multiplexer/demultiplexer of a remote node with respect to that of an optical source of a subscriber terminal according to an embodiment of the present invention; and  
         [0028]      FIG. 10  is a flowchart illustrating a method of aligning a wavelength of a passband of a demultiplexer of a central base station with respect to that of a passband of a multiplexer of a remote node according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Throughout the drawings, whenever the same element reappears in a subsequent drawing, it is denoted by the same reference numeral.  
         [0030]      FIG. 1  is a block diagram of a general wavelength-division multiplexed (WDM)-passive optical network (PON) system that uses a single-mode optical source. Referring to  FIG. 1 , the system includes a central base station  110 , an optical fiber  120  for a downstream signal, an optical fiber  121  for an upstream signal, a remote node  130 , an optical fiber  140  for a downstream signal, an optical fiber  141  for an upstream signal, and N subscriber terminals  150 .  
         [0031]     The central base station  110  includes an array of N individual or integrated single-mode optical sources  111  (a DFB-LD, etc.), an array of individual or integrated optical receivers  113 , an optical multiplexer  114 , and an optical demultiplexer  115 .  
         [0032]     The single-mode optical sources  111  output a unique wavelength for one subscriber terminal  150 . Thus, N-optical sources build up N wavelengths, for the N subscriber terminals  150 , i.e., downstream signals D i  (i=1 to N). The array of the optical receivers  113  may be constructed with PIN-PDs orAPDs, and receive upstream signals U i  from the N subscriber terminal  150  (i=1 to N). The optical multiplexer  114  multiplexes signals from the N single-mode optical sources  111  and delivers the multiplexing result to the optical fiber  120 .  
         [0033]     N thermoelectric coolers  112  are respectively connected to the N single-mode optical sources  111  so as to control wavelengths of the N single-mode optical sources  111 .  
         [0034]     The remote node  130  also includes an optical multiplexer  131  and an optical demultiplexer  132 . The optical demultiplexer  131  distributes the downstream signals D i  to the N subscriber terminals  150  via the optical fiber  140  according to a wavelength.  
         [0035]     Each of the Nsubscriber terminals  150  includes a single-mode optical source  151  and an optical receiver  153 . Like in the central base station  110 , N thermoelectric coolers  152  are respectively connected to the N single-mode optical sources  151  to control wavelengths of the N single-mode optical sources  151 . The N optical receivers  153  respectively receive the downstream signals D i , and the N single-mode optical sources  151  respectively modulate the received downstream signals D i , into the upstream signals U i  and transmit the upstream signals U i  to the central base station  110 .  
         [0036]     The lights modulated into the upstream signals U i  are multiplexed by the optical multiplexer  132  of the remote node  130  via the optical fiber  141 , and the multiplexed lights are supplied to the central base station  110  via the optical fiber  121 . The supplied multiplexed lights are demultiplexed via the optical demultiplexer  115  according to a wavelength, and supplied to the optical receivers  113 .  
         [0037]     The optical receiver  113  finally receives the upstream signal U N . However, a change in a passband of the optical demultiplexer  131  and the optical demultiplexer  132  due to a change in the ambient temperature of the remote node  130  may cause not only loss in optical channels of the upstream and downstream signals but also performance degradation due to a crosstalk among the wavelength channels.  
         [0038]      FIG. 2  is a block diagram of a wavelength tracking apparatus for use in a WDM-PON system according to an embodiment of the present invention. Referring to  FIG. 2 , in order to maintain the system performance even when a passband of an optical demultiplexer  131  and an optical multiplexer  132  of a remote node  130  of  FIG. 1  change, power monitors  210 ,  211 , and  250  and partial reflectors  212  and  230  according to an embodiment of the present invention are installed into a central base station  110 , the remote node  130 , and subscriber terminals  150 .  
         [0039]     The installed power monitors  210 ,  211 , and  250  and partial reflectors  212  and  230  equalize a wavelength of an optical multiplexer  114  of the central base station  110  with those of optical sources  111  of the central base station  110 , a wavelength of an optical multiplexer  132  of the remote node  130  with those of subscriber optical sources  151 , and wavelengths of an optical multiplexer  114  and an optical demultiplexer  115  of the central base station  110  with those of the optical demultiplexer  131  and the optical multiplexer  132  of the remote node  130 .  
         [0040]     Thus, even if wavelengths of passbands of the optical multiplexer  131  and the optical demultiplexer  132  are changed due to a change in the temperature of the remote node  130 , optical downstream signals from the central base station  110  are transmitted to the subscriber terminals  150  and optical upstream signals from the subscriber terminals  150  are transmitted to the central base station  110  without optical loss.  
         [0041]     Aligning wavelengths of the optical multiplexer  114  and the optical sources  111  of the central base station  110 , lights emitted from the optical sources  111  pass through the optical multiplexer  114 , and some portion of the power of the lights are reflected from the partial reflector  212  and the other portion pass through the partial reflector  212  for transmission of the downstream signals.  
         [0042]     The lights reflected from the partial reflector  212  pass through the optical multiplexer  114  again, and some portion of the reflected power of the lights are feeded into the power monitors  210  via optical couplers  117 , respectively.  
         [0043]     Each of the power monitors  210  controls a thermoelectric cooler  112  connected to the corresponding optical source  111  to maximize the power of the received light. In particular, since the lights reflected from the partial reflector  212  pass through the optical multiplexer  114  twice, the lights are significantly affected by a change in a passband of the optical multiplexer  114 , and thus can be efficiently used for wavelength tracking.  
         [0044]     Aligning wavelengths of the optical multiplexer  132  of the remote node  130  with those of the optical sources  151 , lights emitted from the optical sources  151  pass through the optical multiplexer  132  via the optical fiber  141 , and some portion of the optica; power of the passing lights are reflected from the partial reflector  230  and the other portion of the power pass through the partial reflector  230  for transmission of upstream signals.  
         [0045]     The reflected power of the lights pass through the optical multiplexer  132  and the optical fiber  141  and travel into the power monitors  250  via optical couplers  155 , respectively. Then, each of the power monitors  250  controls the thermoelectric cooler  152  connected to the corresponding optical source  151  to maximize the power level of the received light.  
         [0046]     Lastly, the output power of upstream signals received at the central base station  110  are used in order to align wavelengths of the optical multiplexer  114  and the optical demultiplexer  115  of the central base station  110  with those of the optical multiplexer  131  and the optical demultiplexer  132  of the remote node  130 . Specifically, upstream signals from the subscriber terminals 150  sequentially pass through the remote node  130 , the optical fiber  121 , and the demultiplexer  115  of the central base station  110 , and are finally supplied to the optical receivers  113 . Some of the upstream signals are supplied to the power monitor  211  via an optical coupler  118  before the optical receiver  113 . The power monitor  211  maximizes the power level of the received light by controlling a thermoelectric cooler  115 - 2  of the demultiplexer  115 .  
         [0047]      FIG. 3  is a block diagram of a bi-directional WDM-PON system that uses a channel of an optical fiber. In the bidirectional WDM-POM system of  FIG. 3 , an optical fiber via which optical downstream signals and optical upstream signals are transmitted, is a single optical fiber  120  for economical efficiency.  
         [0048]     Referring to  FIG. 3 , an array of N single-mode optical sources  111  modulate lights having N unique wavelengths into downstream signals D i  (i=1 through N) to be transmitted to N subscriber terminals  150 . An array of optical receivers  113  may be constructed with PIN-PDs or APDs, and receives upstream signals U i  (i=1 to N) from subscriber terminals  150 . An optical demultiplexer/multiplexer  114  multiplexes the N single-mode optical sources  111  and outputs the multiplexed to the optical fiber  120 .  
         [0049]     Also, thermoelectric coolers  112  are respectively connected to the single-mode optical sources  111  to control wavelengths of the single-mode optical sources  111 .  
         [0050]     A remote node  130  includes an optical multiplexer/demultiplexer  131  that respectively distributes the downstream signals D i  to the subscriber terminals  150  via the optical fiber  140  according to a wavelength. The optical demultiplexer/multiplexer  114  of the central base station  110  and the optical demultiplexer/multiplexer  131  of the remote node  130  are respectively constructed as single AWGs, each acting as an optical multiplexer or an optical demultiplexer according to the direction of an optical signal. In this type of use, the key of optical demultiplexer/multiplexer  114  or  131  is the passing wavelength periodicity of AWG.  
         [0051]     Each of the subscriber terminals  150  includes single-mode optical source  151  and an optical receiver  153 . Like in the central base station  110 , a thermoelectric cooler  152  is connected to the single-mode optical source  151  to control a wavelength of the single-mode optical source  151 . The optical receivers  153  respectively receive the downstream signals D i , and the single-mode optical sources  151  respectively modulate the received optical signals D i  into the upstream signals U i  and transmit them to the central base station  110 .  
         [0052]     The upstream signals U i  are multiplexed by the optical multiplexer  131  of the remote node  130  via an optical fiber  140 , and the multiplexed lights are input to the central base station  110  via the optical fiber  120 . The input multiplexed lights are demultiplexed by the optical demultiplexer  114  according to a wavelength and the demultiplexed lights are input to the optical receivers  113 , respectively. Then, the nth optical receiver  113  finally receives the upstream signal U N .  
         [0053]     Compared to the WDM-PON system of  FIG. 1 , the bidirectional WDM-PON system of  FIG. 3  that uses a single optical fiber further includes a WDM filter  116  in the central base station  110  and a WDM filter  154  in the subscriber terminals  150  in order to separate the optical upstream signals from the optical downstream signals.  
         [0054]      FIG. 4  is a block diagram of a WDM-PON system, illustrated in  FIG. 3 , which uses a wavelength tracking apparatus, according to an embodiment of the present invention. Referring to  FIG. 4 , in order to maintain the system performance even when a passband of an optical multiplexer/demultiplexer  131  of a remote node  130  changes, power monitors  210 ,  211 , and  250  and partial reflectors  212  and  230  according to an embodiment of the present invention are installed into a central base station  110 , the remote node  130 , and subscriber terminals  150 .  
         [0055]     The installed power monitors  210 ,  211 , and  250  and partial reflectors  212  and  230  align wavelengths of an optical multiplexer/demultiplexer  114  of the central base station  110  with those of optical sources  111  of the central base station  110 , wavelengths of the optical multiplexer/demultiplexer  131  of the remote node  130  with those of subscriber optical sources  151 , and wavelengths of the optical multiplexer/demultiplexer  114  with those of the optical multiplexer/demultiplexer  131  of the remote node  130 .  
         [0056]     Thus, even if the wavelengths of passbands of the optical multiplexer/demultiplexer  131  of the remote node  130  are changed due to a change in the temperature of the remote node  130 , optical downstream signals from the central base station  110  are transmitted to the subscriber terminals  150  and optical upstream signals from the subscriber terminals  150  are transmitted to the central base station  110  without optical loss.  
         [0057]     Specifically, in order to equalize the wavelengths of the optical multiplexer/demultiplexer  114  and the optical sources  111  of the central base station  110 , lights emitted from the optical sources  111  pass through the optical multiplexer/demultiplexer  114 , and some portion of the optical power of the lights are reflected from the partial reflector  212  and the other portion of the optical power pass through the partial reflector  212  for transmission of the downstream signals.  
         [0058]     The lights reflected from the partial reflector  212  pass through the optical multiplexer  114  again, and some portion of the power of the reflected lights travel into the power monitors  210  via optical couplers  117 , respectively.  
         [0059]     Each of the power monitors  210  maximizes the power level of the received light by controlling a thermoelectric cooler  112  connected to the corresponding optical source  111 . In particular, since the lights reflected from the partial reflector  212  pass through the optical multiplexer  114  twice, the lights are significantly affected by a change in a passband of the optical multiplexer  114 , and thus can be efficiently used for wavelength tracking.  
         [0060]     Similarly, in order to equalize wavelengths of the optical multiplexer  131  of the remote node  130  with those of the optical sources  151 , lights emitted from the optical sources  151  pass through the optical multiplexer  131  via an optical fiber  140 , and some portion of the optical power of the passing lights are reflected from the partial reflector  230  and the other portion of the power pass through the partial reflector  230  for transmission of upstream signals.  
         [0061]     The reflected lights pass through the optical multiplexer  131  and the optical fiber  140  again and travel into the power monitors  250  via optical couplers  155 , respectively. Then, each of the power monitors  250  controls the thermoelectric cooler  152  connected to the corresponding optical source  151  to maximize the power level of the received light.  
         [0062]     Lastly, in order to equalize the wavelengths of the optical multiplexer/demultiplexer  114  of the central base station  110  with those of the optical multiplexer/demultiplexer  131  of the remote node  130 , upstream signals supplied to the central base station  110  are used. Specifically, the upstream signals from the subscriber terminals 150  sequentially pass through the remote node  130 , the optical fiber  120 , and the demultiplexer  114 , and are finally the optical receivers  113  via the optical demultiplexer  114 . Some of the upstream signals are supplied to the power monitor  211  via an optical coupler  118  before the optical receiver  113 . The power monitor  211  controls a thermoelectric cooler  115 - 1  of the demultiplexer  114  to maximize the power level of the received light.  
         [0063]      FIG. 5  is a flowchart illustrating operations of controlling the power monitors  210 ,  211 , and  250  and the temperatures of the thermoelectric coolers  112 ,  115 , and  152  of the wavelength tracking apparatus shown in  FIG. 4 , according to an embodiment of the present invention. Referring to  FIG. 5 , the power level of an optical signal P 0  is measured ( 510 ), and the changed optical power level of an optical signal P 1  is measured ( 530 ) after increasing or reducing the temperature by ΔT ( 520 ).  
         [0064]     Next, it is determined whether the power level of the optical signal P 1  is equal to or greater than that of the optical signal P 0 , i.e., P 1 ≧P 0  ( 540 ). If P 1 ≧P 0 , the changed temperature is maintained ( 541 ), and then, the level of an optical signal P 2  is measured ( 542 ).  
         [0065]     Similarly, it is determined whether P 2 ≧P 1 . If P 2 ≧P 1 , the changed temperature is maintained.  
         [0066]     However, if P 1 &lt;P 0 , the increased temperature is reduced or the reduced temperature is increased ( 543 ). In this way, it is possible to control the thermoelectric coolers  112 ,  115 , and  152  so that the level of an optical signal can be maximized.  
         [0067]      FIG. 6  illustrates graphs respectively showing variations in the optical power level and wavelength of an optical upstream signal received from the central base station  110  when the temperature of the remote node  130  changes, in the wavelength tracking apparatus illustrated in  FIG. 2 , according to an embodiment of the present invention. The graph (a) of  FIG. 6  shows a variation in the optical power level of the optical upstream signal received at the central base station  110  as the temperature of the remote node  130  changes. The graph (b) of  FIG. 6  shows a variation in the wavelength of the optical upstream signal received at the central base station  110  as the temperature of the remote node  130  changes.  
         [0068]     To measure the performance of the wavelength tracking apparatus, the temperature of the remote node  130  was periodically changed by about 30° C. at a rate of 0.88° C./min. As a result, a variation in the optical power level of the optical upstream signal received was just 0.25 dB or less when the temperature of the remote node  130  was changed by 30° C. The result shows that the optical upstream signal tracks down a variation in the wavelength of a passband of the optical multiplexer  132  of the remote node  130 .  
         [0069]      FIG. 7  illustrates graphs respectively showing variations in the optical power level and wavelength of an optical downstream signal received from one of the subscriber terminals  150  when the temperature of the remote node  130  changes, in the wavelength tracking apparatus of  FIG. 2 , according to an embodiment of the present invention. The graph (a) of  FIG. 6  shows a variation in the optical power level of the optical downstream signal received at the subscriber terminal  150  as the temperature of the remote node  130  changes. The graph (b) of  FIG. 6  shows a variation in the wavelength of the optical downstream signal received at the subscriber terminal  0  as the temperature of the remote node  130  changes.  
         [0070]     To measure the performance of a wavelength tracking method according to an embodiment of the present invention, the temperature of the remote node  130  was periodically changed by about 30° C. at a rate of 0.88° C./min. As a result, a variation in the optical power level of the optical downstream signal received was just 0.7 dB or less when the temperature of the remote node  130  was changed by 30° C. The graph shows that the optical downstream signal tracks down a variation in the wavelength of a passband of the optical multiplexer  131  of the remote node  130 .  
         [0071]      FIG. 8  is a flowchart illustrating a method of aligning a wavelength of to those of a plurality of single-mode optical sources of a central base station with respect to that of a multiplexer of a central base station according to an embodiment of the present invention. Referring to  FIG. 8 , optical signals from the single-mode optical sources of the central base station are multiplexed by the multiplexer/demultiplexer of the central base station and transmitted downward to the subscriber terminals (S 800 ).  
         [0072]     Next, some portion of the optical power of the multiplexed optical signals are reflected from a partial reflector and returned to the optical sources, and the other portion of the multiplexed optical signals are transmitted downward to the subscriber terminals (S 810 ).  
         [0073]     Some portion of the optical power of the optical signals that are reflected from the partial reflector, pass through the multiplexer, and then are returned are extracted by optical couplers (S 820 ).  
         [0074]     Next, a power monitor controls a thermoelectric cooler connected to each of the optical sources to maximize the optical power level of the optical signals extracted by the optical coupler, thereby aligning the wavelengths of the single-mode optical sources of the central base station with respect to the wavelengths of the multiplexer/demultiplexer (S 830 ).  
         [0075]      FIG. 9  is a flowchart illustrating a method of aligning wavelengths of passbands of a multiplexer/demultiplexer of a remote node with respect to those of a plurality of single-mode optical sources of subscriber terminals according to an embodiment of the present invention. Referring to  FIG. 9 , signals from the single-mode optical sources of the subscriber terminals are multiplexed by the multiplexer of the remote node and then transmitted upward to the central base station (S 900 ).  
         [0076]     Next, some portion of the optical power of the multiplexed optical signals are reflected from a partial reflector and returned to the optical sources of the subscriber terminals, and the other portion of the optical signals are transmitted upward to the central base station (S 910 ).  
         [0077]     Next, some portion of the optical power of the optical signals that are reflected from the partial reflector, pass through the multiplexer, and then are returned are extracted by optical couplers (S 920 ).  
         [0078]     Then, a power monitor controls a thermoelectric cooler connected to each of the optical sources so that the optical power level of the optical signals extracted by the optical coupler can be maximized, thereby aligning the wavelengths of the single-mode optical sources of the subscriber terminal with respect to those of the multiplexer of the remote node (S 930 ).  
         [0079]      FIG. 10  is a flowchart illustrating a method of aligning a wavelength of passbands of a demultiplexer of a central base station with respect to that of passbands of a multiplexer of a remote node according to an embodiment of the present invention. First, optical signals that are multiplexed by the multiplexer of the remote node and transmitted upward are demultiplexed by the demultiplexer of the central base station (S 1000 ).  
         [0080]     Next, some portion of the optical power of the demultiplexed optical signals are extracted by optical couplers (S 1010 ).  
         [0081]     Next, a power monitor controls a thermoelectric cooler connected to the demultiplexer to maximize the optical power level of the optical signals extracted by the optical coupler, thereby aligning the wavelength of the multiplexer of the remote node with respect to that of the demultiplexer of the central base station (S 1030 ).  
         [0082]     As described above, the present invention provides an apparatus and method for efficiently tracking a wavelength in a general WDM-PON system that uses a single-mode optical source. According to the present invention, even if the temperature of a remote node changes, the optical power levels of an optical upstream signal received at a central base station and an optical downstream signal received at a subscriber terminal can be maintained at 1 dB or less. Even if power is not supplied to a remote node, the optical downstream signal can be stably transmitted to the subscriber terminal and the optical upstream signal can be stably transmitted to the central base station, thereby increasing the reliability of the WDM-PON system.  
         [0083]     Also, it is possible to minimize optical loss in a channel caused by a change in the temperature of a remote node and the performance degradation of the system due to crosstalk among the optical channels.  
         [0084]     In a wavelength tracking apparatus according to the present invention, a signal reflected from a partial reflector passes through a multiplexer/demultiplexer twice to be adjusted according to a change in the wavelength of a passband of the multiplexer/demultiplexer, and thus can be effectively utilized for wavelength tracking.  
         [0085]     The wavelength tracking apparatus according to the present invention also equalizes a wavelength of an optical source of a central base station with that of an optical multiplexer of the central base station, a wavelength of an optical multiplexer of a remote node with that of a subscriber optical source, and a wavelength of an optical multiplexer/demultiplexer of the central base station with that of an optical multiplexer/demultiplexer of the remote node, thereby monitoring a cut occurrence of the optical fiber and increasing the reliability of the network.  
         [0086]     While this invention has been particularly shown and described with reference to exemplary 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.