Abstract:
Provided is a loopback-type wavelength division multiplexing passive optical network (WDM-PON) system including: a central office converting downstream signals into downstream optical subcarrier multiplexed (SCM) signals to wavelength-multiplex and transmit the downstream SCM signals, or receiving and demodulating upstream optical on-off keying (OOK) signals to convert the upstream optical OOK signals into upstream signals; a remote node receiving the wavelength-multiplexed downstream optical SCM signals and wavelength-demultiplexing and transmitting the downstream optical SCM signals, or receiving the upstream optical OOK signals and wavelength-multiplexing and transmitting the upstream optical OOK signals; and subscriber interface units dividing the downstream optical SCM signals into first and second optical SCM signals, converting the first optical SCM signals into downstream signals, and modulating the upstream signals to the optical OOK signals by using the second optical SCM signals to transmit the optical OOK signals to the remote node. Accordingly, stable transmission quality can be guaranteed and system implementation costs can be reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application Number 10-2006-120330 filed on Dec. 1, 2006 and the Korean Patent Application Number 10-2007-95259 filed on Sep. 19, 2007, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wavelength division multiplexing passive optical network (WDM-PON), and more particularly, to a loopback-type WDM-PON system. 
         [0004]    This work was supported by the IT R&amp;D program of MIC/IITA [2005-S-401-02, Optical Subscriber and Access Network Technology]. 
         [0005]    2. Description of the Related Art 
         [0006]    As a method of providing data services to subscribers over existing telephone wires, xDSL (digital subscriber line) technologies have been developed and widely used. In addition, a method of providing data services over a cable network using coaxial cables has been proposed and developed. 
         [0007]    The existing data service technologies can be applied in consideration of Internet traffic currently used by subscribers without significant problems. However, when super high speed services such as work-at-home services, teleconference, high-definition television (HDTV) level high quality images, remote-education, remote diagnosis, and the like are widely provided to general subscribers some time, it is predicted that it is difficult to provide adequate broadband high quality services since their band widths and distances are limited. 
         [0008]    As a subscriber access method of providing broadband services that users require, there is an optical network technology, called, a wavelength division multiplexing passive optical network (WDM-PON), which recently has been studied. 
         [0009]    The WDM-PON as a next-generation subscriber network for the information society has advantages in that a large amount of information can be provided to each subscriber and security can be guaranteed. However, it has to solve the problem in that a light source having a predefined wavelength is needed for each subscriber. 
         [0010]    In order to solve the problem of subscriber light source, a loopback-type WDM-PON where downstream optical signal is remodulated to be an upstream signal has been proposed and actively developed. 
         [0011]    In the loopback scheme, a central office transmits light along with a downstream signal to a subscriber unit, and the subscriber unit re-modulates the light transmitted from the central office to an upstream signal and transmits the upstream signal to the central office. 
         [0012]    In a paper tilted ‘Bidirectional WDM-PON Based on Gain-Saturated Reflective Semiconductor Optical Amplifiers’ (IEEE photonics technology letters, Vol. 17, No. 11, pp. 2462462, November 2005), in order to solve the aforementioned problem, a reflective semiconductor optical amplifier (RSOA) is used. 
         [0013]    In this case, an downstream optical signal is directly modulated in an on-off keying (OOK) scheme and transmitted, the RSOA of an subscriber interface unit receives the downstream optical signal and amplifies in a gain saturation region, so that data ‘0’ and ‘1’ of the downstream optical signal is amplified as if all of transmitted data is ‘1’. The downstream optical signal leveled by the RSOA is re-modulated to an upstream OOK signal so as to be transmitted. 
         [0014]    However, in this method, since a leveling degree of the downstream optical signal is determined by gain saturation characteristics of the RSOA, the leveling degree affects quality of the re-modulated upstream optical signal. 
         [0015]    For example, when power less than incident light power for operating the RSOA in a gain saturation region is received to the RSOA, since the RSOA does not operate in the gain saturation region, the downstream optical signal cannot be leveled. In this case, components of the downstream optical signal remain in the upstream optical signal, so that signal quality is significantly degraded, and according to cases, serious error floor may occur. 
         [0016]    In another paper titled ‘A Novel Hybrid WDM/SCM-PON Sharing Wavelength for Up- and Down-Link Reflective Semiconductor Optical Amplifier’ (IEEE photonics technology letters, Vol. 18, No. 3, pp. 502-504, February 2006), a method in which when the downstream optical signal is modulated in the OOK scheme and transmitted, the RSOA of the subscriber interface unit re-modulates and transmits the downstream optical signal modulated in the OOK scheme to a subcarrier multiplexed (SCM) signal in a frequency band higher than an upstream OOK signal frequency band, is proposed. 
         [0017]    However, the RSOA has to be modulated to the SCM signal in the frequency band higher than the upstream OOK signal frequency band, so that 3 dB modulation bandwidths of the RSOAs of all subscriber interface units have to include the SCM signal frequency band. Therefore, the RSOA having a high 3 dB modulation bandwidth is needed. 
         [0018]    In another paper titled ‘WDM Passive Optical Network With Subcarrier Transmission and Baseband Detection Scheme for Laser-Free Subscriber interface units’(IEEE photonics technology letters, Vol. 18, No. 11, pp. 1279-1281, June 2006), a method of transmitting a downstream signal in the SCM scheme and transmitting an upstream signal in the OOK scheme is proposed. In this method, there is a problem in that an expensive apparatus such as a Mach-Zehnder modulator used for the central office and the subscriber interface unit is needed. 
       SUMMARY OF THE INVENTION 
       [0019]    As described above, a conventional loopback-type wavelength division multiplexing passive optical network (WDM-PON) system has problems in that transmission quality cannot be stably guaranteed and high system implementation costs are required. 
         [0020]    According to an aspect of the present invention, there is provided a loopback-type WDM-PON system including: a central office converting downstream signals into downstream optical subcarrier multiplexed (SCM) signals to wavelength-multiplex and transmit the downstream SCM signals, or receiving and demodulating upstream optical on-off keying (OOK) signals to convert the upstream optical OOK signals into upstream signals; a remote node receiving the wavelength-multiplexed downstream optical SCM signals and wavelength-demultiplexing and transmitting the downstream optical SCM signals, or receiving the upstream optical OOK signals and wavelength-multiplexing and transmitting the upstream optical OOK signals; and subscriber interface units dividing the downstream optical SCM signals into first and second optical SCM signals, converting the first optical SCM signals into downstream signals, and modulating the upstream signals to the optical OOK signals by using the second optical SCM signals to transmit the optical OOK signals to the remote node. 
         [0021]    In the above aspect of the present invention, the central office may include: frequency up converters modulating the downstream signals into downstream SCM signals; light sources modulating the downstream SCM signals into the downstream optical SCM signals having unique wavelengths; a first optical wavelength multiplexer wavelength-multiplexing and transmitting the downstream optical SCM signals; a first optical wavelength demultiplexer wavelength-demultiplexing the wavelength-multiplexed upstream OOK signals into the upstream optical OOK signals; first optical receivers converting the upstream optical OOK signals into the upstream signals; and first low pass filters removing SCM signal components remaining in the upstream signals. 
         [0022]    In addition, the central office may further include a first circulator transmitting the wavelength-multiplexed downstream optical SCM signal to the remote node and transmitting the multiplexed upstream OOK signal to the optical wavelength demultiplexer. 
         [0023]    In addition, the central office may include: single mode laser diodes generating seed light sources having unique wavelengths; a second optical wavelength multiplexer wavelength-multiplexing and outputting the seed light sources; second frequency up converters modulating the downstream signals into downstream SCM signals; second reflective semiconductor optical amplifiers (RSOAs) receiving the seed light sources and modulating the downstream SCM signals to the downstream optical SCM signals having unique wavelengths; a second optical wavelength multiplexer/demultiplexer wavelength-demultiplexing the wavelength-multiplexed seed light sources to the seed light sources so as to be provided to the RSOAs, and wavelength-multiplexing and transmitting the downstream optical SCM signals; a second optical wavelength demultiplexer wavelength-demultiplexing the wavelength-multiplexed upstream optical OOK signals into the upstream optical OOK signals; a plurality of second optical receivers demodulating the upstream optical OOK signals into the upstream signals; and a plurality of second low pass filters removing SCM signal components remaining at the upstream signals. In some cases, the central office may further include a second circulator transmitting the wavelength-multiplexed downstream optical SCM signal to the remote node and transmitting the wavelength-multiplexed upstream optical OOK signals to the optical wavelength demultiplexer. 
         [0024]    In addition, the remote node may include an optical wavelength multiplexer/demultiplexer wavelength-demultiplexing the wavelength-multiplexed downstream optical SCM signal to the downstream optical SCM signals to transmit the downstream optical SCM signals to the subscriber interface units, and wavelength-multiplexing and transmitting the upstream optical OOK signals to the central office. In some cases, the remote node may further include a third circulator transmitting the wavelength-multiplexed downstream optical SCM signal to the optical wavelength multiplexer/demultiplexer, or transmitting the wavelength-multiplexed upstream optical OOK signal to the central office. In addition, the remote node may further include first couplers dividing and transmitting the downstream optical SCM signals output from the optical wavelength multiplexer/demultiplexer into two signals, or transferring the upstream optical OOK signals output from the subscriber interface units to the optical wavelength multiplexer/demultiplexer. 
         [0025]    In addition, each of the subscriber interface units may include: a second coupler dividing the downstream optical SCM signal transmitted from the remote node into first and second optical SCM signals; a second optical receiver converting the first optical SCM signal to an SCM signal; a frequency down converter converting the SCM signal to the downstream signal; and an RSOA generating the upstream signal by using the second optical SCM signal as the seed light source to transmit the upstream signal to the remote node. 
         [0026]    According to another aspect of the present invention, there is provided an subscriber interface unit including: a coupler dividing a downstream optical SCM signal transmitted from the remote node into first and second optical SCM signals; an optical receiver converting the first optical SCM signal to an SCM signal; a frequency down converter converting the SCM signal to a downstream signal; and an RSOA generating an upstream signal by using the second optical SCM signal as a seed light source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0028]      FIG. 1  is a structural view illustrating a loopback-type WDM-PON system according to a first embodiment of the present invention; 
           [0029]      FIG. 2  is a structural view illustrating a loopback-type WDM-PON system according to a second embodiment of the present invention; 
           [0030]      FIG. 3  is a structural view illustrating a loopback-type WDM-PON system according to a third embodiment of the present invention; 
           [0031]      FIG. 4  is a structural view illustrating a loopback-type WDM-PON system according to a fourth embodiment of the present invention; and 
           [0032]      FIG. 5  is a structural view illustrating a loopback-type WDM-PON system according to a fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the description, the detailed descriptions of well-known functions and structures may be omitted so as not to hinder the understanding of the present invention. 
         [0034]    Like reference numerals designate like elements throughout the specification. 
         [0035]      FIG. 1  is a structural view illustrating a loopback-type wavelength division multiplexing passive optical network (WDM-PON) system according to a first embodiment of the present invention. 
         [0036]    Referring to  FIG. 1 , the WDM-PON system according to the first embodiment of the present invention includes a central office  110 , a remote node (RN)  150 , and N subscriber interface units  130 - 1  to  130 -N. The central office  110  is connected to the remote node  150  through a first optical fiber  121 , and the remote node  150  is connected to the N subscriber interface units  130 - 1  to  130 -N through second optical fibers  141 - 1  to  141 -N. 
         [0037]    The central office  110  includes N frequency up converters  111 - 1  to  111 -N, N light sources  112 - 1  to  112 -N, an optical wavelength multiplexer  113 , an optical wavelength demultiplexer  114 , N optical receivers  115 - 1  to  115 -N, N low pass filters  116 - 1  to  116 -N, and a circulator  117 . 
         [0038]    Each of the frequency up converters  111 - 1  to  111 -N converts a downstream baseband signal into a downstream subcarrier multiplexed (SCM) signal. Each of the light sources  112 - 1  to  112 -N modulates the downstream SCM signal output from a corresponding frequency up converter  111 - 1  to  111 -N to a downstream optical signal having a unique wavelength. The optical wavelength multiplexer  113  wavelength-multiplexes and transmits N downstream optical SCM signals output from the N light sources  112 - 1  to  112 -N to the circulator  117 . 
         [0039]    Here, the light sources  112 - 1  to  112 -N according to the embodiment of the present invention may be single mode laser diodes (SMLs) such as distributed feedback laser diodes (DFB-LDs) and implemented individually or in an integrated array type. Otherwise, the light sources  112 - 1  to  112 -N may be implemented as an optical module packaged in a transmitter optical CAN (TO CAN) type. 
         [0040]    The optical wavelength demultiplexer  114  divides N upstream optical on-off keying (OOK) signals transmitted from the circulator  117  according to wavelengths to transmit the divided signal to each of the N optical receivers  115 - 1  to  115 -N. Each of the optical receivers  115 - 1  to  115 -N converts the input upstream optical OOK signal into an upstream OOK signal, that is, an electric signal. The low pass filters  116 - 1  to  116 -N remove SCM signal components remaining in the upstream OOK signals. 
         [0041]    The circulator  117  separates the wavelength-multiplexed downstream optical SCM signals transmitted from the optical multiplexer  113  and wavelength-multiplexed upstream optical OOK signals transmitted from the remote node  150  from each other in order to transmit the wavelength-multiplexed downstream optical SCM signals and the wavelength-multiplexed upstream optical OOK signals to the remote node  150  and the optical demultiplexer  114 , respectively. 
         [0042]    The remote node  150  includes an optical wavelength multiplexer/demultiplexer  151 . The optical multiplexer/demultiplexer  151  wavelength-demultiplexes and transmits the wavelength-multiplexed downstream optical SCM signals transmitted from the circulator in the central office  110  according to wavelengths, to the N subscriber interface units  130 - 1  to  130 -N, or wavelength-multiplexes and transmits the upstream optical OOK signals transmitted from the N subscriber interface units  130 - 1  to  130 -N to the central office  110 . 
         [0043]    In this case, the N downstream optical SCM signals wavelength-demultiplexed by the optical wavelength multiplexer/demultiplexer  151  are transmitted to the N subscriber interface units  130 - 1  to  130 -N through the N second optical fibers  141 - 1  to  141 -N, and the multiplexed upstream optical OOK signals are transmitted to the central office  110  through the first optical fiber  121 . 
         [0044]    The subscriber interface unit  130 - 1  to  130 -N includes couplers  131 - 1  to  131 -N, frequency down converters  133 - 1  to  133 -N, and reflective semiconductor optical amplifiers (RSOAs)  134 - 1  to  134 -N. 
         [0045]    The coupler  131 - 1  to  131 -N divides a downstream optical SCM signal transmitted from a corresponding second optical fiber  141 - 1  to  141 -N into first and second optical SCM signals. Each of the optical receivers  132 - 1  to  132 -N converts the first optical SCM signal into an electric signal, that is, an SCM signal. Each of the frequency down converters  133 - 1  to  133 -N converts the first SCM signal received from a corresponding optical receiver  132 - 1  to  132 -N to a baseband signal. Each RSOA  134 - 1  to  134 -N re-modulates an upstream baseband signal into an upstream OOK signal by using the second optical SCM signal as a seed light source. 
         [0046]    In this case, the SCM signal is not obtained by directly changing a signal level as the OOK signal but is a signal obtained changing a frequency in a frequency modulation method. Therefore, the RSOA  134 - 1  to  134 -N according to the embodiment of the present invention can use the received second SCM signal as the seed light source without an additional signal leveling operation. 
         [0047]    In addition, since frequency bands used by the upstream signal and the downstream signal are different from each other, although the downstream optical signal is used as the upstream light, signal interference does not occur. Here, SCM signal components included in the upstream signal have to be removed by the low pass filters  116 - 1  to  116 -N included in the central office  110 . 
         [0048]    As described above, by modulating and transmitting the downstream signal in the SCM method, the RSOAs  134 - 1  to  134 -N can generate upward signals having stable transmission quality regardless of incident light power. 
         [0049]    Now, operations of the WDM-PON illustrated in  FIG. 1  are described. 
         [0050]    First, the WDM-PON system transmits a baseband signal. 
         [0051]    Specifically, a downstream baseband signal to be transmitted to the subscriber interface unit  131 - 1  to  131 -N is converted by the frequency up counter  111 - 1  to  111 -N to a downstream SCM signal, and the downstream SCM signal is modulated by the light source  112 - 1  to  112 -N as a downstream optical SCM signal having a unique wavelength, and the downstream optical SCM signal is wavelength-multiplexed by the optical wavelength-multiplexer  113 . 
         [0052]    The wavelength-multiplexed downstream optical SCM signal is wavelength-demultiplexed by the optical wavelength multiplexer/demultiplexer  151  so as to be transmitted to a corresponding subscriber interface unit  131 - 1  to  131 -N. 
         [0053]    The downstream optical SCM signal input to the subscriber interface unit  131 - 1  to  131 -N is divided into first and second optical SCM signal by the coupler  134 - 1  to  134 -N, and the first optical SCM signal is converted into the original downstream baseband signal by the optical receiver  132 - 1  to  132 -N and the frequency down converter  133 - 1  to  133 -N. 
         [0054]    The RSOA  134 - 1  to  134 -N modulates an upstream baseband signal into an upstream optical OOK signal by using the second optical SCM signal as a seed light source. 
         [0055]    As described above, after the upstream OOK signal is generated by using the second optical SCM signal, the WDM-PON system according to the embodiment of the present invention transmits the upstream optical OOK signal to the central office  110  by performing the following operations. 
         [0056]    The upstream optical OOK signal modulated by using the second optical SCM signal is wavelength-multiplexed by the remote node  150 , the wavelength-multiplexed upstream optical OOK signal is wavelength-demultiplexed by the optical wavelength demultiplexer  114  so as to be transmitted to a corresponding optical receiver  115 - 1  to  115 -N. 
         [0057]    Thereafter, the upstream optical OOK signal is converted into an upstream OOK signal by the optical receiver  115 - 1  to  115 -N, and SCM signal components remaining in the upstream OOK signal are removed by the low pass filter  116 - 1  to  116 -N. 
         [0058]    The WDM-PON system illustrated in  FIG. 1  as described above may be modified as illustrated in  FIGS. 2 to 5 . 
         [0059]      FIG. 2  is a structural view illustrating a loopback-type WDM-PON system according to a second embodiment of the present invention. The optical fiber of the WDM-PON system of  FIG. 2  illustrated in  FIG. 1  is modified. 
         [0060]    Referring to  FIG. 2 , a remote node  200  includes a circulator  220  that is included in the central office  110  in  FIG. 1 , and the first optical fiber  121  is divided into a downstream optical fiber  121 - 1  and an upstream optical fiber  121 - 2 . 
         [0061]    Here, when the wavelength-multiplexed downstream optical SCM signal is transmitted through the downstream optical fiber  121 - 1 , the remote node  200  transmits the received signal to the optical wavelength multiplexer/demultiplexer  210  through the circulator  220 . The optical wavelength multiplexer/demultiplexer  210  wavelength-demultiplexes the received signal to N downstream optical SCM signals as illustrated in  FIG. 1  and allocate the N downstream optical SCM signals to N second optical fibers  141 - 1  to  141 -N. 
         [0062]    When N upstream optical OOK signals are transmitted through the N second optical fibers  141 - 1  to  141 -N, the remote node  200  receives and wavelength-multiplexes the N upstream optical OOK signals through the optical wavelength multiplexer/demultiplexer  210 . The wavelength-multiplexed upstream optical OOK signals are output to the upstream optical fiber  121 - 2  through the circulator  220 . 
         [0063]      FIG. 3  is a structural view illustrating a loopback-type WDM-PON system according to a third embodiment of the present invention. The loopback-type WDM-PON system in  FIG. 3  is used to accommodate a larger number of subscriber interface units. 
         [0064]    Referring to  FIG. 3 , a remote node  300  further includes N splitters  321  to  32 N in addition to an optical wavelength multiplexer/demultiplexer  310 . The N splitters  321  to  32 N divide N downstream optical SCM signals transmitted from the central office  110  into N×M downstream optical SCM signals, or combine N×M upstream optical OOK signals transmitted from N×M subscriber interface units  130 - 11  to  130 -NM into N upstream optical OOK signals by the optical wavelength. 
         [0065]    When wavelength-multiplexed downstream optical SCM signals are transmitted from the central office  110  to the remote node  300 , the wavelength-multiplexed downstream optical SCM signals are wavelength-demultiplexed to N downstream optical SCM signals by the optical wavelength multiplexer/demultiplexer  310  and divided into N×M downstream optical SCM signals by the N splitters  321  to  32 N. 
         [0066]    When N×M upstream optical OOK signals are transmitted from the N×M subscriber interface units  130 - 11  to  130 -NM, the N×M upstream optical OOK signals are combined by the N splitters  321 - 32 N of the remote node  300  according to wavelengths, combined into N upstream optical OOK signals, wavelength-multiplexed by the optical wavelength multiplexer/demultiplexer  310 , and transmitted to the central office  110 . 
         [0067]    As described above, the WDM-PON system illustrated in  FIG. 3  can accommodate the subscriber interface units  130 - 11  to  130 -NM of which the number is greater than that in the WDM-PON system illustrated in  FIG. 1 . 
         [0068]      FIG. 4  is a structural view illustrating a loopback-type WDM-PON system according to a fourth embodiment of the present invention. In this system, a remote node  400  controls a transmission direction of a signal and accommodates a larger number of subscriber interface units. 
         [0069]    Referring to  FIG. 4 , the remote node  400  further includes N splitters  421  to  42 N and a circulator  430  in addition to an optical wavelength multiplexer/demultiplexer  410 . 
         [0070]    When the remote node  400  receives wavelength-multiplexed downstream optical SCM signals from the central office  110 , the wavelength-multiplexed downstream optical SCM signals are transmitted to the optical wavelength multiplexer/demultiplexer  410  through the circulator  430 , demultiplexed by the optical multiplexer/demultiplexer  410  into N downstream optical SCM signals, and divided into N×M downstream optical SCM signals by the N splitters  421  to  42 N. 
         [0071]    When the remote node  400  receive N×M upstream optical OOK signals from N×M subscriber interface units  130 - 11  to  130 -NM, the N×M upstream optical OOK signals are combined according to wavelengths into N upstream optical OOK signals by the N splitters  421  to  42 N, wavelength-demultiplexed by the optical wavelength multiplexer/demultiplexer  410 , and transmitted to the optical demultiplexer  114  of the central office  110  through the circulator  430 . 
         [0072]      FIG. 5  is a structural view illustrating a loopback-type WDM-PON system according to a fifth embodiment of the present invention. In this system, downstream optical SCM signals are generated by using RSOAs. 
         [0073]    Referring to  FIG. 5 , a central office  500  N single mode lasers (SMLs)  511 - 1  to  511 -N, an optical multiplexer  512 , N frequency up converts  513 - 1  to  513 -N, N RSOAs  514 - 1  to  514 -N, an optical wavelength multiplexer/demultiplexer  515 , an optical demultiplexer  516 , N optical receivers  517 - 1  to  517 -N, N low pass filters  518 - 1  to  518 -N, and a circulator  519 . 
         [0074]    The N SMLs  511 - 1  to  511 -N generate seed lights corresponding to each of the N RSOAs  514 - 1  to  514 -N. The optical wavelength multiplexer  512  multiplexes and transmits the N seed lights to the N RSOAs  514 - 1  to  514 -N. 
         [0075]    The N frequency up converters  513 - 1  to  513 -N convert downstream baseband signals to downstream SCM signals. The N RSOAs  514 - 1  to  514 -N are provided with the seed lights from the SMLs  511 - 1  to  511 -N through the optical wavelength multiplexer  512  and modulates the downstream SCM signals to downstream optical SCM signals by using the seed lights. 
         [0076]    The optical wavelength multiplexer/demultiplexer  515  wavelength-demultiplexes and divides the wavelength-multiplexed seed lights transmitted from the optical wavelength multiplexer  512  into N seed lights and transmits the divided seed lights to each of the N RSOAs  514 - 1  to  514 -N, or multiplexes and transmits the N downstream optical SCM signals transmitted from the N RSOAs  514 - 1  to  514 -N to the circulator  519 . 
         [0077]    The optical wavelength demultiplexer  516  divides the N upstream optical OOK signals transmitted from the circulator  519  according to wavelengths to transmit the divided signal to each of the N optical receivers  517 - 1  to  517 -N. Each of the optical receivers  517 - 1  to  517 -N converts the input upstream optical OOK signal into an upstream OOK signal. The low pass filters  518 - 1  to  518 -N remove SCM signal components remaining in the upstream OOK signals. 
         [0078]    The circulator  519  transmits the seed light sources transmitted from the optical wavelength multiplexer  512  to the optical wavelength multiplexer/demultiplexer  515 , transmits the downstream optical SCM signals transmitted from the optical wavelength multiplexer/demultiplexer  515  to the remote node  150 , and transmits the upstream optical OOK signals transmitted from the remote node  150  to the optical wavelength demultiplexer  516 . 
         [0079]    According to the current embodiment illustrated in  FIG. 5 , the central office  500  provides the seed lights to the optical wavelength multiplexer/demultiplexer  515  through the second optical wavelength multiplexer  512 . However, the central office  500  may implement the seed light sources as a spectrum-sliced broad band light or a wavelength-multiplexed multi-wavelength light to provide the seed light to the optical wavelength multiplexer/demultiplexer  515  without the second optical wavelength multiplexer  512  as needed. 
         [0080]    As described above, the WDM-PON system illustrated in  FIG. 5  can generate the downstream optical signals as in the WDM-PON systems illustrated in  FIGS. 1 to 4 . Specifically, according to the present invention, the SCM signals can be converted into the optical signals in various methods, and any method of converting electric signals into optical signals can be applied to the present invention. 
         [0081]    In addition, the WDM-PON system illustrated in  FIG. 5  can be modified in various manners as illustrated in  FIGS. 2 to 4 . 
         [0082]    Accordingly, the loopback-type WDM-PON system uses the SCM as the downstream optical signal and generates the upstream optical signal by using the optical SCM signal as the seed light, so that an additional signal leveling operation is not needed for this downstream optical signal remodulation scheme. Significant transmission performance deterioration does not occur at low incident light power at which the RSOA is not operated in a gain saturation region, and accordingly a more power budget margin of a network can be achieved. Therefore, the more stable network can be constructed. 
         [0083]    In addition, an expensive apparatus such as a Mach-Zehnder modulator is not needed unlike in the conventional art, so that the loopback-type WDM-PON system can be implemented at lower costs. 
         [0084]    While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.