Abstract:
An optical wavelength tracking apparatus and method in a wavelength division multiplexed (WDM) passive optical network (PON) in which a central office (CO) having a multi-frequency light source is connected to a plurality of optical network units (ONUs) having loop-back light sources through a WDM MUX/DEMUX in a remote node (RN). The power levels of downstream and upstream WDM optical signals are measured. The WDM wavelengths of the multi-frequency light source and the WDM MUX/DEMUX are controlled to be nearly identical in order to minimize the difference between the power levels of the downstream and upstream WDM optical signals.

Description:
[0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus and Method for Tracking Optical Wavelength in Wavelength Division Multiplexed Passive Optical Network Using Loop-back Light Source,” filed in the Korean Intellectual Property Office on Aug. 27, 2003 and assigned Serial No. 2003-59536, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a WDM (Wavelength Division Multiplexed) PON (Passive Optical Network) using loop-back light sources. More particularly, the present invention relates to an optical wavelength tracking apparatus and method for rendering the WDM wavelengths of a multi-frequency light source in a central office (CO) identical to those of a WDM multiplexer/demultiplexer (MUX/DEMUX) in a remote node (RN).  
         [0004]     2. Description of the Related Art  
         [0005]     A WDM PON provides very high-speed, wide-band communication service to subscribers by optical signals having different subscriber-specific wavelengths. Thus, the WDM PON helps to ensure the privacy of communications, and easily satisfies individual users&#39; demands for additional communication services or a larger communication capacity. This service accommodates more and more subscribers simply by adding specific wavelengths for new subscribers. Despite these advantages, the WDM PON has serious drawbacks that have delayed deployment because of the requirement of an additional wavelength stabilizing circuit for stabilizing the oscillation frequency of a light source that generates an optical signal in both a CO and each subscriber device. Therefore, the development of an economical WDM light source is essential to implementation of the WDM PON. Loop-back light sources, such as a Fabry-Perot laser injection-locked to the wavelength of an external optical signal or a reflective semiconductor optical amplifier (RSOA), have been proposed as economical WDM light sources for upstream data transmission from an optical network unit (ONU) as a subscriber device to a CO. A loop-back light source used for upstream data transmission in the ONU receives an optical signal from the CO and outputs an optical signal having the same wavelength as that of the received optical signal that is modulated according to the upstream transmission data. Hence, the loop-back light source does not need frequency selection and wavelength stabilization. In addition, the Fabry-Perot laser is a low-price light source that is popular because the laser outputs an optical signal injection-locked to an input optical signal and the injection-locked signal is a high-power signal with a narrow linewidth. Therefore, the Fabry-Perot laser can transmit modulated data at high speed. The RSOA amplifies an input optical signal to a high power level even though it is at a very low power level, and modulates the amplified signal according to an upstream transmission data signal. Hence, if the ROSA is used as a Loop-back light source in the ONU, a low price multi-frequency light source, which generates a WDM optical signal destined for loop-back light source in the ONU, is usable in the CO.  
         [0006]     In general, the physical configuration of the PON is that of a double star topology, thereby minimizing the length of the optical lines used. In other words, the CO is connected to an RN close-by to subscribers by a single optical fiber, and the RN is in turn connected to the individual subscribers by independent optical fibers. Therefore, the CO and the RN are provided with a WDM MUX for WDM-multiplexing optical signals having different wavelengths and a WDM DEMUX for WDM-demultiplexing the WDM optical signal. Arrayed waveguide gratings (AWGs) are usually used as the WDM MUX/DEMUX. The RN close-by to the subscribers does not require a device to maintain an internal temperature constant. As a result, the RN is affected by temperature changes between seasons, or between day and night. The WDM wavelengths of the AWG WDM MUX/DEMUX in the RN are subsequently changed by the temperature variations. The wavelength variation of the AWG with temperature is determined according to the material of the AWG. If the AWG is formed of an III-IV group compound, a typical semiconductor material, its wavelength variation with temperature is about 0.1 nm/° C. If the AWG is formed of silica (SiO 2 ), the wavelength variation with temperature is about 0.01 nm/° C.  
         [0007]     As the WDM wavelengths of the WDM MUX/DEMUX in the RN change with corresponding temperature changes, the WDM wavelengths of the multi-frequency light source in the CO are not exactly the same as compared with those of the WDM MUX/DEMUX in the RN. Similarly, the WDM wavelengths of the WDM DEMUX in the CO are at variance from those of the WDM MUX/DEMUX in the RN. The resulting increase in the output loss of upstream and downstream channels and crosstalk from adjacent channels degrades system transmission performance. Accordingly, there is a need for developing optical wavelength tracking techniques for making the WDM wavelengths of the multi-frequency light source and the WDM DEMUX identical to those of the WDM MUX/DEMUX in the RN in order to prevent the transmission performance degradation caused by the temperature change in the RN.  
         [0008]     In light of the above, there have been proposed some optical wavelength tracking schemes to render the wavelengths of the WDM light source for downstream transmission to be identical to those of the AWG varying with temperature in the RN in the WDM PON. One of them is U.S. Pat. No. 5,729,369 entitled “Method of Tracking a Plurality of Discrete Wavelengths of a Multi-Frequency Optical Signal for Use in a Passive Optical Network Telecommunications System”, invented by Martin Zirngibl, and granted on Mar. 17, 1998. According to the patent, the discrete wavelengths are tracked by removing one of ONUs connected to a multi-frequency router corresponding to the WDM MUX/DEMUX in the RN and reflecting back a wavelength corresponding to the removed ONU in the upstream direction.  
         [0009]     Another optical wavelength tracking scheme attempting to solve the aforementioned problems is found in U.S. Pat. No. 6,304,350 entitled “Temperature Compensated Multi-Channel Wavelength-Division-Multiplexed Passive Optical Network”, invented by Christopher Richard Doerr, et. al., and granted on Oct. 16, 2001. In this disclosure, the RN detects the power level of one of channels at a WGR (Waveguide Grating Router) corresponding to the WDM MUX/DEMUX and notifies the CO of the detected power level. The CO then changes the temperature of a multi-frequency laser (MFL) (which is a multi-frequency light source) in accordance with the change of the received power level, to thereby enable the frequencies of the MFL in the CO to track the channels of the WGR in the RN.  
         [0010]     The above optical wavelength tracking techniques are inefficient because one of WDM wavelengths is only used for tracking, and these techniques require an additional device by which the RN measures the power level of one channel and transmits the power level to the CO, thereby increasing cost.  
       SUMMARY OF THE INVENTION  
       [0011]     An object of the present invention is to provide an efficient, economical optical wavelength tracking apparatus and method in a WDM PON using loop-back light sources.  
         [0012]     The present invention provides an optical wavelength tracking apparatus in a WDM PON in which a CO having a multi-frequency light source is connected to a plurality of ONUs having loop-back light sources through a WDM MUX/DEMUX in an RN.  
         [0013]     In the optical wavelength tracking apparatus according to the present invention, a first optical power measurer measures the power level of a downstream WDM optical signal for the ONUs directed from the multi-frequency light source to the WDM MUX/DEMUX, a second optical power measurer measures the power level of an upstream WDM optical signal received from the loop-back light sources through the WDM MUX/DEMUX, and a control unit controls the WDM wavelengths of the multi-frequency light source and the WDM MUX/DEMUX to be nearly identical in order to minimize the difference of the power levels of the measured downstream and upstream WDM optical signals.  
         [0014]     In the optical wavelength tracking method according to the present invention, a section of a downstream WDM optical signal for the ONUs directed from the multi-frequency light source to the WDM MUX/DEMUX is branched off, the power level of the branched optical signal is measured as a reference voltage, a section of an upstream WDM optical signal received from the loop-back light sources through the WDM MUX/DEMUX is branched off, and the power level of the branched optical signal is measured as a monitoring voltage. The WDM wavelengths of the multi-frequency light source and the WDM MUX/DEMUX are controlled to be nearly identical by adjusting the temperature of the multi-frequency light source in order to minimize the difference between the reference voltage and the monitoring voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0016]      FIG. 1  illustrates the configuration of a WDM PON using loop-back light sources;  
         [0017]      FIG. 2  illustrates the configuration of a WDM PON using loop-back light sources, including an optical wavelength tracking apparatus according to one aspect of the present invention;  
         [0018]      FIG. 3  is a flowchart illustrating optical wavelength tracking in a controller illustrated in  FIG. 2 ; and  
         [0019]      FIG. 4  illustrates the configuration of a WDM PON using loop-back light sources, including an optical wavelength tracking apparatus according to another aspect of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present invention will be described herein below with reference to the accompanying drawings, which are provided for purposes of illustration and do not limit the present invention to the depiction in the drawings. In the following description, well-known functions or constructions that are known by a person of ordinary skill in the art may not be described in detail if their discussion would obscure the invention with unnecessary detail. Throughout the application it is repeated that the WDM wavelengths of the multi-frequency light source and the WDM MUX/DEMUX are controlled to be nearly identical. The term “nearly identical” for purposes of this application includes wavelengths that are completely identical in size as well as slight variations that would not impact the accuracy of the tracking apparatus.  
         [0021]      FIG. 1  illustrates the configuration of a WDM PON using loop-back light sources. Referring to  FIG. 1 , a CO  100  is connected to an RN  104  adjacent to subscribers by a single optical fiber  102 , and the RN  104  is connected to ONUs  110 ,  130  by independent optical fibers  106  to  108  An upstream data transmission structure in which the ONUs  110  to  130  transmit data to the CO  100  via a WDM MUX/DEMUX  124  in the RN  104 . There are n ONUs  110  to  130  and thus a multi-frequency light source  114  generates a WDM optical signal containing optical signals at n wavelengths λ 1  to λn. It is to be noted here that only components needed for understanding the subject matter of the present invention are shown between the CO  100  and the RN  104  and the RN  104  and the ONUs  110  to  130 , with the other components omitted.  
         [0022]     The CO  100  is comprised of the multi-frequency light source  114 , an optical circulator  116 , a WDM DEMUX  118 , and upstream optical data receivers  120  to  122 . The multi-frequency light source  114  generates a WDM optical signal destined for loop-back light sources  126  to  128  used for upstream data transmission in the ONUs  110  to  130 . The optical circulator  116  connects the multi-frequency light source  114  and the WDN DEMUX  118 . The circulator  116  is also connected to the optical fiber  102  which connects the CO to the WDM MUX/DEMUX  124  of the RN  104 , for outputting the WDM optical signal received from the multi-frequency light source  114  to the optical fiber  102  and outputting a WDM optical signal received from the optical fiber  102  to the WDM DEMUX  118 . The WDM DEMUX  118  is a 1×N WDM DEMUX that WDM-demultiplexes the WDM optical signal received from the optical circulator  116  into optical signals having the n wavelengths λ 1  to λn and feeds the optical signals to the upstream optical data receivers  120  to  122 . The WDM DEMUX  118  is usually an AWG. The upstream optical data receivers  120  to  122  generate electrical signals corresponding to the respective received optical signals.  
         [0023]     The RN  104  includes the 1×N WDM MUX/DEMUX  124 . The WDM MUX/DEMUX  124  WDM-demultiplexes the WDM optical signal from the multi-frequency light source  114  into optical signals having the n wavelengths λ 1  to λn and feeding the optical signals to the optical fibers  106  to  108 . It also WDM-multiplexes optical signals having the n wavelengths λ 1  to λn received from the optical fibers  106  to  108  and feeds the multiplexed optical signal to the optical fiber  102 . The ONUs  110  to  122  have their respective loop-back light sources  126  to  128  that generate optical signals at specific wavelengths for upstream data transmission. As described before, injection-locked Fabry-Perot lasers or RSOAs are used as the loop-back light sources  126  to  128 .  
         [0024]     The WDM optical signal output from the multi-frequency light source  114  in the CO  100  reaches the RN  104  via the optical circulator  116  and the optical fiber  102 . The WDM optical signal is then WDM-demultiplexed in the WDM MUX/DEMUX  124  in the RN  104 . The respective demultiplexed optical signals are applied to the input of the upstream light sources, the loop-back light sources  126  to  128  in the ONUs  110  to  130  via the optical fibers  106  to  108 . The loop-back light sources  126  to  128  output optical signals having the same wavelengths as those of the received optical signals and modulated according to upstream data signals. The signals from the loop-back light sources  126  to  128  are WDM-multiplexed in the WDM MUX/DEMUX  124  of the RN  104  and then transmitted to the CO  100 . The WDM optical signal is applied to the input of the WDM DEMUX  118  via the optical circulator  116 . The optical signal is WDM-demultiplexed in the WDM DEMUX  118  and the demultiplexed optical signals are fed to the respective upstream optical data receivers  120  to  122 .  
         [0025]      FIG. 2  illustrates the configuration of a WDM PON using loop-back light sources according to a first aspect of the present invention. The WDM PON further includes an optical wavelength tracking apparatus in a CO  200  in addition to the components of the WDM PON illustrated in  FIG. 1 . The optical wavelength tracking apparatus including a first optical power measurer with an optical divider  224  and an optical receiver  228 , a second optical power measurer with an optical divider  226 , an optical receiver  230 , and an electric amplifier  232 , and a control unit including a controller  234  and a temperature control unit (TCU)  236 .  
         [0026]     The CO  200  is connected to an RN  204  adjacent to subscribers by a single optical fiber  202 , and the RN  204  is connected to ONUs  210  to  250  by independent optical fibers  206  to  208 . An upstream data transmission structure in which the ONUs  210  to  250  transmit data to the CO  200  via a WDM MUX/DEMUX  238  in the RN  204  is illustrated in  FIG. 2 . There are n ONUs  210  to  250  and thus a multi-frequency light source  214  generates a WDM optical signal containing optical signals at n wavelengths λ 1  to λn. It is to be noted here that only components needed for understanding the subject matter of the present invention are shown between the CO  200  and the RN  204  and the RN  204  and the ONUs  210  to  250 , with the other components omitted.  
         [0027]     The optical dividers  224  and  226  are disposed in the optical signal path between the optical fiber  202  connected to the WDM MUX/DEMUX  238  of the RN  204  and an optical circulator  216 . A WDM optical signal output from the multi-frequency light source  214  reaches the RN  204  via the optical circulator  216 , the optical dividers  224  and  226 , and the optical fiber  202 . The WDM optical signal is then WDM-demultiplexed in the WDM MUX/DEMUX  238  in the RN  204 . The respective demultiplexed optical signals are applied to the input of loop-back light sources  240  to  242  in the ONUs  210  to  250  via the optical fibers  206  to  208 . The loop-back light sources  240  to  242  output optical signals having the same wavelengths as those of the received optical signals and modulated according to upstream data signals. The signals from the loop-back light sources  240  to  242  are WDM-multiplexed in the WDM MUX/DEMUX  238  of the RN  204  and then transmitted to the CO  200 . The WDM optical signal is applied to the input of the WDM DEMUX  218  via the optical dividers  224  and  226  and the optical circulator  216 . It is WDM-demultiplexed in the WDM DEMUX  218  and the demultiplexed optical signals are fed to respective upstream optical data receivers  220  to  222 .  
         [0028]     The first optical power measurer measures the power level of a downstream WDM optical signal directed from the multi-frequency light source  214  to the WDM MUX/DEMUX  238 . The optical divider  224  in the first optical power measurer branches off a section of the downstream WDM optical signal to the optical receiver  228 . The optical receiver  228  applies a voltage at the power level of the received optical signal as a reference voltage to the controller  234 .  
         [0029]     The second optical power measurer measures the power level of an upstream WDM optical signal received from the loop-back light sources  240  to  242  via the WDM MUX/DEMUX  238 . The optical divider  226  in the second optical power measurer branches off a section of the upstream WDM optical signal to the optical receiver  230 . The optical receiver  230  applies a voltage at the power level of the received optical signal as a monitoring voltage to the electric amplifier  232 . The electric amplifier  232  amplifies the monitoring voltage with an amplification gain that renders the monitoring voltage equal to the reference voltage when the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  are identical, and feeds the amplified voltage to the controller  234 .  
         [0030]     The control unit controls the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  to be nearly identical in order to minimize the difference between the power levels of the downstream and upstream optical signals measured in the first and second optical power measurers. For this purpose, the TCU  236  of the control unit is appended to the multi-frequency light source  214  and changes temperature under the control of the controller  234 . A thermo-electric cooler (TEC) can be used as the TCU  236 . The controller  234  controls the temperature of the multi-frequency light source  214  through the TCU  236  such that the difference between the reference voltage received from the optical receiver  228  and the monitoring voltage received from the optical receiver  230  through the electric amplifier  232  is minimized. Accordingly the optical wavelength tracking for the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  to be identical is possible. A microprocessor can be adopted as the controller  234 , for control by an optical wavelength tracking algorithm.  
         [0031]     To describe the optical wavelength tracking in more detail, the output of the multi-frequency light source  214  in the CO  200  is maintained constant. Therefore, the reference voltage detected at the optical receiver  228  that receives a section of the output of the multi-frequency light source  214  is also maintained constant. The power of the upstream WDM optical signal is maximal when the WDM wavelengths of the multi-frequency light source  214  are identical to those of the WDM MUX/DEMUX  238 . The upstream WDM optical signal is an optical signal that is generated in the downstream direction in the multi-frequency light source  214  of the CO  200 , WDM-demultiplexed in the RN  204 , modulated in the loop-back light sources  240  to  242  in the ONUs  210  to  250 , WDM-multiplexed in the RN  204 , and then transmitted to the CO  200 .  
         [0032]     Meanwhile, optical links connecting the CO  200  to the ONUs  210  to  212  undergo constant loss and the power of optical signals from the loop-back light sources  240  to  242  in the ONUs  210  to  250  is proportional to that of received optical signals. Hence, the maximum power of the upstream WDM optical signal transmitted to the CO  200  is dependent on that of the downstream WDM optical signal from the multi-frequency light source  214 . If the multi-frequency light source  214  and the WDM MUX/DEMUX  238  are identical in WDM wavelength, the difference between the reference voltage and the monitoring voltage is minimized. On the other hand, as the WDM wavelength difference between the multi-frequency light source  214  and the WDM MUX/DEMUX  238  gets broad, the difference between the reference voltage and the monitoring voltage also becomes large. The controller  234  monitors the voltage difference and minimizes the difference between the reference voltage and the monitoring voltage by control of the WDM wavelengths of the multi-frequency light source  214  through the TCU  236 . As a result, the controller  234  enables the WDM wavelengths of the multi-frequency light source  214  to track those of the WDM MUX/DEMUX  238  varying with temperature in the RN  204  so that they are identical all the time.  
         [0033]     The multi-frequency light source  214  may include an AWG that is used for determining its wavelength band. Such a multi-frequency light source may include an Erbium-doped lasing light source, a spectrum-division multi-frequency light source using an Erbium-doped optical fiber, or a multi-frequency light source using a semiconductor optical amplifier. Alternatively, the multi-frequency light source  214  may adopt a resonator structure including a gain medium for oscillating an optical signal, by which its wavelength band is determined.  
         [0034]     If the wavelength band of the multi-frequency light source  214  is determined by an AWG, a TEC used as the TCU  236  is mounted to the AWG of the multi-frequency light source  214 . In this case, if the controller  234  adjusts the strength and direction of a current flowing in the TEC, the AWG&#39;s temperature is controlled and thus the AWG&#39;s WDM wavelengths are changed. Hence, the controller  234  can control the WDM wavelengths of the multi-frequency light source  214 . On the other hand, if the wavelength band of the multi-frequency light source  214  is determined according to the resonator structure with a gain medium, the TEC is mounted to a light source in the multi-frequency light source  214 . In this case, the controller  234  can control the WDM wavelengths of the multi-frequency light source  214  by adjusting the strength and direction of a current flowing in the TEC and thus controlling the temperature of the gain medium.  
         [0035]     As described above, the power of the upstream WDM optical signal transmitted to the CO  200  can be maximized by enabling the WDM wavelengths of the multi-frequency light source  214  in the CO  200  to track those of the WDM MUX/DEMUX  238  in the RN  204 . In this process, one of the WDM wavelengths is neither confined to tracking, nor is an additional device for measuring the power level of one channel and notifying the CO  200  of the power level used in the RN  204 . The optical wavelength tracking is carried out using a section of a transmitted WDM optical signal and a simple control circuit. Thus, the present invention is more cost-effective than previously known in the art.  
         [0036]     To facilitate the control of optical wavelength tracking in the controller  234 , the reference voltage can be rendered equal to the monitoring voltage when the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  are identical. One way is by setting the dividing ratios of the optical dividers  224  and  226  to the same value, the electric amplifier  232  is positioned between the controller  234  and the optical receiver  230 , and the amplification gain of the electric amplifier  232  is set in the manner that makes the monitoring voltage equal to the reference voltage when the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  are identical.  
         [0037]     An implementation of an algorithm for optical wavelength tracking by control of the temperature of the multi-frequency light source  214  in the CO in the controller  234  is shown in steps  300  to  320  of  FIG. 3 . As the optical wavelength tracking according to the present invention is initiated, the controller  234  measures the reference voltage and the monitoring voltage in step  300 . For notational simplicity, the reference voltage is denoted by V 1  and the monitoring voltage, by V 2  in  FIG. 3 . In step  302 , the controller  234  stores the absolute difference between V 1  and V 2 , |V 1 −V 2 | as the present voltage difference. The controller  234  increases the temperature of the multi-frequency light source  214  by a predetermined temperature variation ΔT in step  304 .  
         [0038]     A predetermined time later that takes into consideration the time required for changing the temperature of the multi-frequency light source  214  in step  306 , the controller  234  measures V 1  and V 2  again in step  308  and stores the difference between V 1  and V 2 , |V 1 −V 2 | as the present voltage difference in step  310 .  
         [0039]     In step  312 , the controller  234  compares the present voltage difference with a threshold Vr. If the voltage difference is equal to or less than Vr, the controller  234  returns to step  306 , taking into consideration that the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  are identical. In this case, the temperature of the multi-frequency light source  214  is maintained unchanged. Since the amplification gain of the electric amplifier  232  is set to render the monitoring voltage equal to the reference voltage when the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  are identical, the present voltage different is 0. Even if the present voltage difference is not truly 0 because of errors, Vr is preset to a value that can make the WDM wavelengths of the multi-frequency light source  214  and the WDM MUX/DEMUX  238  almost identical to each other.  
         [0040]     If the present voltage difference exceeds Vr in step  312 , the controller  234  compares the present voltage difference with the present voltage difference in step  314 . If the present voltage difference is equal to or greater than the present voltage difference, this difference implies that the WDM wavelengths of the multi-frequency light source  214  are more discrepant from those of the WDM MUX/DEMUX  238 . Therefore, the controller  234  increases or decreases the temperature of the multi-frequency light source  214  by ΔT, by a value that is contrary to the present temperature change, in step  316 . That is, if the temperature of the multi-frequency light source  214  was increased by ΔT, it is decreased by ΔT at this time, and vice versa.  
         [0041]     On the other hand, if the present voltage difference is less than the present voltage difference, this difference implies that the WDM wavelengths of the multi-frequency light source  214  are not significantly different from those of the WDM MUX/DEMUX  238 . Therefore, the controller  234  increases or decreases the temperature of the multi-frequency light source  214  by ΔT, in the same manner as the previous temperature change, in step  318 . That is, if the temperature of the multi-frequency light source  214  was increased by ΔT, it is also increased by ΔT at this time. If the temperature of the multi-frequency light source  214  was decreased by ΔT, it is also decreased by ΔT at this time.  
         [0042]     After step  316  or  318 , the controller  234  stores the present voltage difference as the present voltage difference in step  320  and returns to step  306 .  
         [0043]     As described above, the controller  234  periodically measures |V 1 −V 2 |. According to |V 1 −V 2 |, it maintains the temperature of the multi-frequency light source  214  unchanged, or gradually increases/decreases the temperature of the multi-frequency light source  214  by ΔT so as to minimize |V 1 −V 2 |. Thus, the WDM wavelengths of the multi-frequency light source  214  become identical to those of the WDM MUX/DEMUX  238 .  
         [0044]     If the controller  234  dynamically sets ΔT according to |V 1 −V 2 | in such a manner that ΔT is set large as |V 1 −V 2 | is relatively large and small as |V 1 −V 2 | is relatively small, the optical wavelength tracking is done in a shorter time.  
         [0045]     Meanwhile, along with the temperature change of the RN  204 , the WDM wavelengths of the WDM DEMUX  218  in the CO  200  become varied from those of the WDM MUX/DEMUX  238  in the RN  204 . Therefore, the W DM wavelengths of the WDM DEMUX  218  as well as those of the multi-frequency light source  214  in the CO  200  can be enabled to track those of the WDM MUX/DEMUX  238  in the RN  204 .  
         [0046]      FIG. 4  illustrates the configuration of a WDM PON using loop-back light sources according to another aspect of the present invention, which allows the WDM wavelengths of a WDM DEMUX  418  as well as those of a multi-frequency light source  414  in a CO  400  to track those of a WDM MUX/DEMUX  442  in an RN  404 . Another embodiment of the optical wavelength tracking apparatus for the CO  400  according to the present invention is also comprised of first and second optical power measurers and a control unit. As in the optical wavelength tracking apparatus illustrated in  FIG. 2 , the first power measurer includes an optical divider  424  and an optical receiver  428 , and the second optical power measurer includes an optical divider  426 , an optical receiver  430 , and an electric amplifier  432 . Notably, the control unit further has an electric amplifier  438  and a TCU  440  in addition to a controller  434  and a TCU  436 .  
         [0047]     As in the WDM PON illustrated in  FIG. 2 , the CO  400  is connected to the RN  404  adjacent to subscribers by a single optical fiber  402 , and the RN  404  is connected to ONUs  410  to  460  by independent optical fibers  406  to  408 . An upstream data transmission structure in which the ONUs  410  to  460  transmit data to the CO  400  via a WDM MUX/DEMUX  442  in the RN  404  is illustrated in  FIG. 4 . There are n ONUs  410  to  460  and thus the multi-frequency light source  414  generates a WDM optical signal containing optical signals at n wavelengths λ 1  to λn.  
         [0048]     The optical dividers  424  and  426  are disposed in the optical signal path between the optical fiber  402  connected to the WDM MUX/DEMUX  442  of the RN  404  and an optical circulator  416 .  
         [0049]     The control unit controls the WDM wavelengths of the multi-frequency light source  414  and the WDM DEMUX  418  so as to be nearly identical to those of the WDM MUX/DEMUX  442  in order to minimize the difference between the power levels of the downstream and upstream optical signals measured in the first and second optical power measurers. For this purpose, the TCU  436  of the control unit is appended to the multi-frequency light source  414  and changes its temperature under the control of the controller  434 , and the other TCU  440  is mounted to the WDM DEMUX  418  and changes its temperature through the electric amplifier  438  under the control of the controller  434 . A TEC can also be used as the TCU  440 . If the WDM DEMUX  418  is an AWG, a TEC used as the TCU  440  is mounted to the AWG In this case, if the controller  434  adjusts the strength and direction of a current flowing in the TEC, the AWG&#39;s temperature is controlled and thus the AWG&#39;s WDM wavelengths are changed. Hence, the controller  434  can control the WDM wavelengths of the WDM DEMUX  418 .  
         [0050]     Here, the electric amplifier  438  is used for the controller  434  to control simultaneously the TCUs  436  and  438  through one output port because the multi-frequency light source  414  and the WDM DEMUX  418  exhibit different wavelength characteristics with respect to temperature. Accordingly, the amplification gain of the electric amplifier  438  is set in correspondence with the wavelength characteristics of the multi-frequency light source  414  and the WDM DEMUX  418  with respect to temperature. However, if the controller  434  is configured to control the TCUs  436  and  440  separately through different output ports, there is no need for the electric amplifier  438 .  
         [0051]     The controller  434  controls the temperature of the multi-frequency light source  414  to thereby minimize the difference between a reference voltage received from the optical receiver  428  and a monitoring voltage received from the optical receiver  430  through the electric amplifier  432 . At the same time, it controls the temperature of the WDM DEMUX  418  by means of the TCU  440 . Thus, optical wavelength tracking is carried out so that the WDM wavelengths of the multi-frequency light source  414  and the WDM DEMUX  418  are nearly identical to those of the WDM MUX/DEMUX  442 .  
         [0052]     Consequently, the WDM wavelengths of the WDM DEMUX  418 , which demultiplexes an upstream WDM optical signal and provides the demultiplexed signals to optical receivers  420  to  422 , as well as those of the multi-frequency light source  414 , also tracks output of the WDM MUX/DEMUX  442 .  
         [0053]     The power of the upstream WDM optical signal transmitted to the CO  400  is, therefore, maximized and degradation from adjacent channels is minimized in the upstream WDM optical signal WDM-demultiplexed in the WDM DEMUX  418 . In this process, none of the WDM wavelengths are confined to tracking, or are any additional devices required for measuring the power level of one channel and notifying the CO  400  of the power level used in the RN  404 . The optical wavelength tracking is carried out using a section of a transmitted WDM optical signal and a simple control circuit. Thus, the present invention is more cost-effective than known heretofore in the art In accordance with the present invention as described above, optical wavelength tracking is carried out so efficiently, economically that the WDM wavelengths of a multi-frequency light source and/or a WDM DEMUX in a CO can track those of a WDM MUX/DEMUX in an RN.  
         [0054]     While the invention has been shown and described with reference to certain preferred embodiments thereof, they are mere exemplary applications. For example, while an AWG is used as the WDM MUX/DEMUX  442 , the same thing occurs when a device having a wavelength variable with temperature is adopted as the WDM MUX/DEMUX  442 . Thus, 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.