Abstract:
A WDM-PON system configured to perform an asymmetric wavelength division multiplexing and demultiplexing is proposed. A receiving end of WDM-PON system configured to receive a light transmitted from a transmission end including a first wavelength division multiplexer/demultiplexer includes a second wavelength division multiplexer/demultiplexer configured to demultiplex the light received rom the transmission end, where the second bandwidth of wavelength division multiplexer/demultiplexer is substantially greater than that of the first wavelength division multiplexer/demultiplexer.

Description:
[0001]    This application is a National Stage filing under 35 U.S.C. §371 of, and claims priority via, International Application No. PCT/KR2012/000450 for APPARATUS FOR SUPPRESSING NOISE IN INJECTION-LOCKED LIGHT SOURCE AND WDM-PON SYSTEM PROVIDED WITH SAME, filed Jan. 18, 2012, and pursuant to 35 U.S.C. §119 this application also claims the benefit of earlier filing date and right of priority to Korean Patent Application Nos. 10-2011-0126586 and 10-2012-0005880 filed on Nov. 30, 2011 and Jan. 18, 2012, respectively, and the contents of all such applications are hereby incorporated by reference in their entirety. 
       FIELD 
       [0002]    The teachings in accordance with the exemplary embodiments of this present disclosure generally relate to a noise suppression apparatus, and more particularly to a noise suppression apparatus used in a transceiver of a WDM-PON (Wavelength Division Multiplexed-Passive Optical Network) system, and a WDM-PON system having the noise suppression apparatus. 
       BACKGROUND 
       [0003]    Generally, a dense WDM-PON (Wavelength Division Multiplexing-Passive Optical Network) may be well understood as a next generation optical network. In WDM-PON technology, an optical transmission module may need to be non-wavelength dependent despite using a plurality of optical wavelengths. Thus, the WDM-PON system (technology) is perceived to be an ultimate alternative to cope with explosively increased data traffic because the WDM-PON system can satisfy an explosively high demand on a wider bandwidth in an access network and is excellent in security. 
         [0004]    One of the embodying methods for WDM-PON system is an injection seeded WDM-PON technology. This method has attracted considerable attention as a favorite solution of WDM-PON due to advantages in Giga-byte transmissibility and cost efficiency. However, this method suffers from difficulty in high Giga-byte transmission due to influence by noise of injection-seeded optical source. 
         [0005]    In relation thereto, a noise-reducing method using non-linear effect occurring in semiconductors or optical fibers has been proposed. However, the non-linear effect also suffers from disadvantages that require high-cost for high output optical source and management cost due to requirement of light power exceeding a predetermined level for non-linear effect. Thus, a method is demanded capable of effectively and economically reducing noise while a required light power level is low. 
       SUMMARY 
       [0006]    Therefore, the present disclosure has been made to solve the foregoing disadvantages/problems of the prior art and therefore an object of the disclosure is to provide a noise suppression apparatus for injection seeded optical source configured to suppress noise of seeded light generated from a light (optical) source in a feed forward method, and a WDM-PON (Wavelength Division Multiplexed-Passive Optical Network) system having the noise suppression apparatus. 
         [0007]    The present disclosure has been made to solve the foregoing disadvantages/problems of the prior art and therefore another object of the disclosure is to provide a noise suppression apparatus for injection seeded optical source configured to amplify a high frequency bandwidth of a optical source and to suppress noise of seeded light generated from the optical source in a feed forward method, and a WDM-PON (Wavelength Division Multiplexed-Passive Optical Network) system having the noise suppression apparatus. 
         [0008]    Technical subjects to be solved by the present disclosure are not restricted to the above-mentioned description, and any other technical problems not mentioned so far will be clearly appreciated from the following description by the skilled in the art. That is, the present disclosure will be understood more easily and other objects, characteristics, details and advantages thereof will become more apparent in the course of the following explanatory description, which is given, without intending to imply any limitation of the disclosure, with reference to the attached drawings. 
         [0009]    In one general aspect of the present invention, there is provided a noise suppression apparatus for injection seeded optical source in an optical transmitter configured to receive an injection light from an injection optical source, the apparatus comprising: 
         [0000]    an optical coupler configured to receive and split an injection light;
 
a converter unit configured to convert an optical signal received from the optical coupler to an electrical signal;
 
an inverter unit configured to invert a phase of the electrical signal converted by the converter unit; and
 
an optical source driving unit configured to receive the electrical signal inverted in phase by the converter unit and to provide the phase-inverted electrical signal to the optical source.
 
         [0010]    Preferably, but not necessarily, the optical source may be a reflective optical source. 
         [0011]    Preferably, but not necessarily, the optical source may be a non-reflective optical source. 
         [0012]    Preferably, but not necessarily, the apparatus may further comprise an amplifier connected between the inverter unit and the optical source driving unit to adjust the intensity of electrical signal phase-inverted by the inverter unit. 
         [0013]    Preferably, but not necessarily, the apparatus may further comprise an optical delayer connected between the optical coupler and the optical source to offset a time loss. 
         [0014]    Preferably, but not necessarily, the optical source driving unit may adjust a driving current to allow a driving current to grow smaller when a injection light increases, and to allow the driving current to grow larger when the injection light decreases. 
         [0015]    Preferably, but not necessarily, the apparatus may further comprise a polarization beam adjuster configured to allow a polarization state of the injection light and that of a light outputted from the optical source to match. 
         [0016]    Preferably, but not necessarily, the polarization adjuster may include a polarization beam splitter and a polarization beam controller. 
         [0017]    Preferably, but not necessarily, the polarization adjuster may be a polarization beam controller. 
         [0018]    In another general aspect of the present disclosure, there is provided a WDM-PON system having a noise suppression apparatus of claims  1  to  9 . 
         [0019]    In still another general aspect of the present disclosure, there is provided a noise suppression apparatus for injection seeded optical source in an optical transmitter configured to receive an injection light from an injection optical source, the apparatus comprising: 
         [0000]    an optical coupler configured to receive and split an injection light;
 
a converter unit configured to convert an optical signal received from the optical coupler to an electrical signal;
 
an inverter unit configured to invert a phase of the electrical signal converted by the converter unit;
 
a bandwidth offset unit configured to amplify a high frequency bandwidth of the electrical signal configured to generate a driving current that drives the optical source; and
 
an optical source driving unit configured to receive the electrical signal inverted in phase by the converter unit, to generate a driving current configured to drive the optical source by using the signal amplified in high frequency bandwidth by the bandwidth offset unit and to provide the driving current to the optical source.
 
         [0020]    Preferably, but not necessarily, the optical source may be a reflective optical source. 
         [0021]    Preferably, but not necessarily, the optical source may be a non-reflective optical source. 
         [0022]    Preferably, but not necessarily, the apparatus may further comprise an amplifier connected between the inverter unit and the optical source driving unit to adjust the intensity of electrical signal phase-inverted by the inverter unit. 
         [0023]    Preferably, but not necessarily, the apparatus may further comprise an optical delayer connected between the optical coupler and the optical source to offset a time loss. 
         [0024]    Preferably, but not necessarily, the bandwidth offset unit may amplify a signal having a 3-4 GHz bandwidth. 
         [0025]    Preferably, but not necessarily, the optical source unit may adjust a driving current to make the driving current smaller when the injection light increases, and to make the driving current greater when the injection light decreases. 
         [0026]    Preferably, but not necessarily, the bandwidth offset unit may amplify a data modulated by the optical source driving unit. 
         [0027]    In still further general aspect of the present disclosure, there is provided a WDM-PON system including a transmitter having a noise suppression apparatus of any one claim of  1  to  18 . 
       ADVANTAGEOUS EFFECTS 
       [0028]    In an advantageous effect, the present disclosure can promote economy of decreasing an external injection light power in an injection seeded optical source-based WDM-PON by adopting a feed forward noise suppression method configured to operate at a low optical power. 
         [0029]    In another advantageous effect, the present disclosure can enhance transmission speed and quality of an optical communication network by reducing noise using a feed forward method. 
         [0030]    In still another advantageous effect, the present disclosure can accomplish an easy upgrade from an existing optical communication network by replacing an optical transmission end module because the noise reducing process is performed at an optical communication end. 
         [0031]    In still further advantageous effect, the present disclosure can effectively transmit a signal of high frequency bandwidth noiselessly in a wavelength locked optical source-based WDM-PON system by adopting a low optical power operable feed forward method by offsetting bandwidth loss limited to an optical source and transmitting a signal of high frequency bandwidth. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the disclosure and together with the description serve to explain the principle of the disclosure. In the drawings: 
           [0033]      FIG. 1  is a block diagram illustrating a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure; 
           [0034]      FIG. 2  is a block diagram illustrating a noise suppression apparatus for injection seeded optical source according to another exemplary embodiment of the present disclosure. 
           [0035]      FIG. 3  is a graph illustrating RNI (Relative Noise Intensity) of a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure; 
           [0036]      FIG. 4  is a block diagram illustrating a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure; 
           [0037]      FIGS. 5   a  and  5   b  illustrate a spectrum of a signal outputted from an optical source according to prior art; 
           [0038]      FIGS. 6   a  and  6   b  illustrate a spectrum of a signal outputted from an optical source according an exemplary embodiment of the present disclosure; 
           [0039]      FIG. 7  is a block diagram illustrating a WDM-PON system having a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure; and 
           [0040]      FIG. 8  is a block diagram illustrating a WDM-PON system having a noise suppression apparatus for injection seeded optical source according to another exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Various exemplary embodiments will be described more fully hereinafter with reference to the accompanying drawings, in which some exemplary embodiments are shown. The present inventive concept may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein. Accordingly, the described aspect is intended to embrace all such alterations, modifications, and variations that fall within the scope and novel idea of the present disclosure. 
         [0042]    Other features and advantages of the disclosed embodiments will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within the scope of the disclosed embodiments, and protected by the accompanying drawings. 
         [0043]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first region/layer could be termed a second region/layer, and, similarly, a second region/layer could be termed a first region/layer without departing from the teachings of the disclosure. 
         [0044]    It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other elements or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0045]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the general inventive concept. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
         [0046]    It will be understood that the terms “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof That is, the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or the claims to denote non-exhaustive inclusion in a manner similar to the term “comprising”. 
         [0047]    Furthermore, “exemplary” is merely meant to mean an example, rather than the best. It is also to be appreciated that features, layers and/or elements depicted herein are illustrated with particular dimensions and/or orientations relative to one another for purposes of simplicity and ease of understanding, and that the actual dimensions and/or orientations may differ substantially from that illustrated. That is, in the drawings, the size and relative sizes of layers, regions and/or other elements may be exaggerated or reduced for clarity. Like numbers refer to like elements throughout and explanations that duplicate one another will be omitted. 
         [0048]    Now, the present disclosure will be described in detail with reference to the accompanying drawings. 
         [0049]      FIG. 1  is a block diagram illustrating a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure. 
         [0050]    Referring to  FIG. 1 , the noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure generally applied to an optical transmitter  1  includes an optical coupler  11 , an optical delayer  12 , an optical source  13 , a converter unit  14 , an inverter unit  15 , an amplifier  16  and an optical source driving unit  17 . 
         [0051]    The optical transmitter  1  applied to the present disclosure receives a light (hereinafter referred to as an injection light) injected from an injection optical source  2  to an optical source  13 . The optical coupler  11  splits an injection light by receiving the injection light. The converter unit  14  is connected to the optical coupler  11  to convert a noise of the injection light to an electrical signal. The converter unit  14  may be a photo diode, for example. 
         [0052]    The inverter unit  15  is connected to the converter unit  14  to inverse a phase of the noise converted to the electrical signal, and the amplifier  16  is connected to the inverter unit  15  to adjust intensity of phase-inverted noise and output the intensity -adjusted phase-inverted noise. The optical source driving unit  17  is connected to the amplifier  16  to receive the output of the amplifier  16 , and provide a current to the optical source  13  using the received output, and to modulate the light source  13 . 
         [0053]    The optical delayer  12  is connected to the optical coupler  11  to offset a time loss generated in the course of electrical processing. The optical source  13  is connected to the optical delayer  12  to allow injection of injection light, and is driven by the optical source driving unit  17 . The optical source  13  may be a reflective optical source, or a non-reflective optical source. 
         [0054]    Now, operation of noise suppression apparatus according to the present disclosure will be described in more detail. 
         [0055]    An injection light outputted from the injection optical source  2  and transmitted to the optical transmitter  1  is split into two lights through the optical coupler  11 . Some of the injection light is transmitted to the converter unit  14 . The converter unit  14  converts a noise of the injection light to an electrical signal by detecting the noise of the injection light. The noise converted to the electrical signal by the converter unit  14  is converted to a noise of inverted phase, adjusted in intensity by the amplifier  16  and transmitted to the optical source driving unit  17 . 
         [0056]    The optical source driving unit  17  receives the noise that is converted to reverse phase, where the noise is combined by a modulation current and a driving current, and outputs the combined noise to the optical source  13 . The optical source driving unit  17  adjusts the driving current in such a manner that the driving current grows smaller when the injection light increases by the intensity of noise and the driving current decreases when the injection light decreases. 
         [0057]    Meantime, another output of the optical coupler  11  is injected to the optical source  13 , where the noise is offset because noises are operated to mutually-offset directions, and a noise of light injected from the optical source  13  and a noise transmitted from the optical source driving unit  17  are mutually inversely phased. 
         [0058]    It should be apparent to the skilled in the art that the noise-suppressed output light of the optical source  13  becomes an output light of an optical transmitter (Tx) via the optical coupler when the optical source  13  is a reflection type optical source, while the light is outputted without being reflected and not via the optical coupler  11 , when the optical source  13  is a non-reflection type optical source. 
         [0059]      FIG. 2  is a block diagram illustrating a noise suppression apparatus for injection seeded optical source according to another exemplary embodiment of the present disclosure. 
         [0060]    Referring to  FIG. 2 , the noise suppression apparatus for injection seeded optical source according to another exemplary embodiment of the present disclosure generally applied to an optical transmitter  1  includes an optical coupler  11 , an optical delayer  12 , an optical source  13 , a converter unit  14 , an inverter unit  15 , an amplifier  16 , an optical source driving unit  17  and a polarization beam adjuster  18 . 
         [0061]    The exemplary embodiment of the present disclosure illustrated in  FIG. 2  further includes a polarization beam adjuster  18 . Description of other elements than the polarization beam adjuster  18  will be omitted, as other elements than the polarization beam adjuster  18  have the same functions as those in  FIG. 1 . 
         [0062]    The polarization beam adjuster  18  receives an injection light from an injection optical source  2  to match a polarization state of the injection light to that of a light outputted from an optical source  13 . The polarization beam adjuster may include a polarization beam splitter or may include a polarization beam controller. 
         [0063]    In comparison between the exemplary embodiment of  FIG. 1  and the exemplary embodiment of  FIG. 2 , the exemplary embodiment of  FIG. 2  intends to offset the polarization state when the polarization state of injection light source  2  and that of the injected seeded optical source  13  are different. The injection light outputted from the injection light source  2  is adjusted to be matched to the polarization state of the optical source  13  and then inputted to the optical coupler  11 . Other operational principles are same as those of  FIG. 1 . 
         [0064]    A part of light including the intensity of noise injected to the optical transmitter from outside in the noise suppression apparatus according to the present disclosure is converted to a current, and then converted to a phase-inverted current and amplified. This current is added to a modulation current of the optical source and injected to the optical source whereby an optical noise can be economically suppressed. 
         [0065]      FIG. 3  is a graph illustrating RNI (Relative Noise Intensity) of a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure, where the RNI indicates distribution of noise for unit intensity of each frequency, and where A defines an RIN prior to application of the noise suppression apparatus according to the present disclosure and B defines an RIN after application of the noise suppression apparatus according to the present disclosure. As illustrated in the graph, it can be noted that the intensity of noise is reduced by the noise suppression apparatus. 
         [0066]      FIG. 4  is a block diagram illustrating a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure. 
         [0067]    Referring to  FIG. 4 , the noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure generally applied to an optical transmitter  1  includes an optical coupler  11 , an optical delayer  12 , an optical source  13 , a converter unit  14 , an inverter unit  15 , an amplifier  16 , a bandwidth offset unit  19  and an optical source driving unit  17 . 
         [0068]    The optical transmitter  1  applied to the present disclosure receives a light (hereinafter referred to as an injection light) injected from an injection optical source  2  to an optical source  13 . The optical coupler  11  splits an injection light by receiving the injection light. The converter unit  14  is connected to the optical coupler  11  to convert a noise of the injection light to an electrical signal. The converter unit  14  may be a photo diode, for example. 
         [0069]    In order to transmit a signal of high Giga bit, the bandwidth offset unit  19  amplifies a high frequency bandwidth of a signal to be provided by the optical source driving unit to the optical source  13 , and offsets loss of high frequency bandwidth caused by limited bandwidth of the optical source  13 . 
         [0070]    A conventionally used RSIA (Reflective Semiconductor Optical Amplifier) or a optical source such as F-D LP (Fabry-Perot Laser Diode) is limited in bandwidth to 3-4 GHz. Thus, in order to transmit a signal of more than 3-4 GHz, when the bandwidth offset unit  17  according to the present disclosure amplifies a signal of high frequency bandwidth and transmits the amplified signal, it is possible to offset the loss of high frequency bandwidth. The bandwidth offset unit  19  preferably amplifies a signal of more than 3-4 GHz, for example, in the optical source  13 , and the bandwidth offset unit  19  may be an equalizer, for example. 
         [0071]    Although the exemplary embodiment of the present disclosure has explained the bandwidth offset unit  19  amplifies a signal of the feed forward optical source  13 , the present disclosure is not limited thereto, it is possible to amplify data modulated by the optical source driving unit  17 , in addition to the signal of the feed forward optical source  13  by being provided into the optical source driving unit  17 . 
         [0072]    The optical source driving unit  17  receives a phase-inverted noise from the amplifier  16  and receives a bandwidth-offset signal from the bandwidth offset unit  19 , provides a current to the optical source  13  using the same and modulates a signal provided to the optical source  13 . 
         [0073]    The optical delayer  12  is connected to the optical coupler  11  to offset a time loss generated in the course of electrical processing. The optical source  13  is connected to the optical delayer  12  to allow injection of injection light, and is driven by the optical source driving unit  17 . The optical source  13  is not limited to the exemplary embodiment thus described, and may be a reflective optical source, or a non-reflective optical source. 
         [0074]    Now, an operation of the noise suppression apparatus will be described in more detail of  FIG. 4 . 
         [0075]    An injection light outputted from the injection optical source  2  and transmitted to the optical transmitter  1  is split into two lights through the optical coupler  11 . A part of the injection light is transmitted to the converter unit  14 . The converter unit  14  converts a noise of the injection light to an electrical signal by detecting the noise of the injection light. The noise converted to the electrical signal by the converter unit  14  is converted to a noise of inverted phase by the inverter unit  15 , adjusted in intensity by the amplifier  16  and transmitted to the bandwidth offset unit  19 . 
         [0076]      FIGS. 5   a  and  5   b  illustrate a spectrum of a signal outputted from an optical source according to prior art, and  FIGS. 6   a  and  6   b  illustrate a spectrum of a signal outputted from an optical source according an exemplary embodiment of the present disclosure, where  FIGS. 5   a  and  6   a  illustrate a spectrum of an electrical signal inputted from the optical source. 
         [0077]    In the conventional system, an electrical signal inputted from the optical source loses a high frequency bandwidth component to be outputted in a biased signal as illustrated in  FIG. 5   b . However, when the bandwidth offset unit  19  amplifies the high frequency bandwidth to output a signal amplified as shown in  FIG. 6   a  according to the present disclosure and the signal amplified in the high frequency bandwidth by the bandwidth offset unit  19  is provided to the optical source  13  through the optical source driving unit  17 , it can be noted that a signal free from distortion can be outputted. 
         [0078]    The optical source driving unit  17  receives a phase-inverted noise from the amplifier  16 , and receives a high frequency bandwidth-amplified signal from the bandwidth offset unit  19 , combines a modulation current generated using the high frequency bandwidth-amplified signal with a driving current, and outputs the combined to the optical source  13 . The optical source driving unit  17  adjusts a driving current in such a manner that the driving current decreases when an injection light increases due to noise, and the driving current increases when the injection light decreases. 
         [0079]    Meanwhile, another output of the optical coupler  11  is injected to the optical source  13 , where the noise is offset because noises are operated to mutually-offset directions, and a noise of light injected from the optical source  13  and a noise transmitted from the optical source driving unit  17  are mutually inversely phased. Furthermore, because a current injected to the optical source  13  is a current amplified in high frequency bandwidth, noise in the high frequency bandwidth can be effectively reduced. 
         [0080]    It should be apparent to the skilled in the art that the noise-suppressed output light of the optical source  13  becomes an output light of an optical transmitter (Tx) via the optical coupler when the optical source  13  is a reflection type optical source, while the light is outputted without being reflected and not via the optical coupler  11 , when the optical source  13  is a non-reflection type optical source. 
         [0081]      FIG. 7  is a block diagram illustrating a WDM-PON system having a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure. 
         [0082]    Referring to  FIG. 7 , the WDM-PON system having a noise suppression apparatus for injection seeded optical source according to an exemplary embodiment of the present disclosure includes a CO (Central Office,  3 ), a RN (Remote Node,  4 ) and ONU (Optical Network Unit,  5 ). 
         [0083]    The CO  5  includes an injection optical source  3 , where the injection optical source  13  is classified into a downstream signal injection optical source  21  and an upstream signal injection optical source  22 . Furthermore, the optical source  2  may include a circulator  23 . 
         [0084]    The circulator  23  is an element having three ports, where a signal inputted to a first port is outputted to a second port, and a signal inputted to the second port is outputted to a third port. Furthermore, the CO  3  includes a plurality of OLTs (Optical Line Terminal,  31 ) and a first wavelength division multiplexer/demultiplexer  32 . The OLT  31  includes optical transmitters  1  having the noise suppression apparatus according to the present disclosure (indicated as ‘Tx 1 ’ . . . ‘Txn’ in  FIG. 4 ) and a WDM (Wavelength Division Multiplexer). 
         [0085]    The RN  4  is connected to the CO  3  through an SMF (Single Mode Fiber), and includes a second wavelength division multiplexer/demultiplexer  41  as a router installed in an apartment administration office. 
         [0086]    The ONU  5  is connected to the RN  4  via SMF, and includes an ONT (Optical Network End 51 ) as a unit provided to each household. The ONT  51  which is an OLT  31  at a user side includes an optical transmitter  1  formed with a noise suppression apparatus according to the present disclosure, an optical receiving end and a WDM. A signal directed to the ONU  5  is a downstream signal and a signal directed to the CO  3  from the ONU  5  is an upstream signal. 
         [0087]    Now, operation of the WDM-PON system according to the present disclosure will be described. The system in  FIG. 7  is symmetrically configured such that description will be made based on downstream. 
         [0088]    A light of broadband spectrum outputted from the downstream injection optical source  21  is inputted to a first port of the circulator  23 , outputted to a second port and transmitted to the second wavelength division multiplexer/demultiplexer  41  of the RN  40  via SMF. The second wavelength division multiplexer/demultiplexer  41  divides the received light to wavelength of each channel, whereby noise of injection light is greatly increased. Successively, the wavelength-divided light is inputted to WDM of ONT  51 . An L band (1565 nm˜1625 nm) is generally used for the downstream signal. The WDM of ONT  51  transmits a wavelength-different L band light to the optical transmitter  1  and transmits a light of another wavelength to an optical receiver Rx. Thus, the light received by the ONT  51  is injected to the optical transmitter  1  according to the present disclosure. 
         [0089]    The injection light injected to the optical transmitter  1  is divided to two by the optical coupler  11 . One light is transmitted to the optical source  13  and the other light is converted to an electrical signal by the converter unit  14 . 
         [0090]    The inverter unit  15  reverses the phase, and the amplifier  16  adjusts the size of phase-inverted signal. The optical source driving unit  17  receives a phase inverted noise to combine the noise with a modulated current and a driving signal, and outputs to the optical source  13 . 
         [0091]    Alternatively, the inverter unit  15  reverses the phase, and the amplifier  16  adjusts the size of phase-inverted signal. The optical source driving unit  17  receives a phase inverted noise to combine a modulated current generated by using noise phase-inverted by the bandwidth offset unit  19  and a driving signal, and outputs to the optical source  13 . 
         [0092]    The optical source  13  according to the exemplary embodiment of the present disclosure is actually operated as an optical amplifier. A driving signal and a gain in an optical source are substantially proportional, such that the gain decreases when the injection light increases, and the gain increases when the injection light decreases. Thus, it is possible to smooth an output light after passing through the optical source  13 . 
         [0093]    Thereafter, the light suppressed in noise by passing through the reflective light source  13  of the optical transmitter  1  is transmitted to the SMF by the second wavelength division multiplexer/demultiplexer  41  via the WDM of the ONT  51 , and transmitted to the receiver Rx via the first wavelength division multiplexer/demultiplexer  32 . 
         [0094]      FIG. 8  is a block diagram illustrating a WDM-PON system having a noise suppression apparatus for injection seeded optical source according to another exemplary embodiment of the present disclosure. 
         [0095]    The WDM-PON system of  FIG. 5  is different from the WDM-PON of  FIG. 4  in terms of position of optical source  20 , but the principle is same as that of WDM-PON system according to the exemplary embodiment of  FIG. 4 . 
         [0096]    A third wavelength division multiplexer/demultiplexer  33  divides the light outputted from a multiple wavelength injection optical source  20  and provides the divided light to the OTL  31 . Based on the downstream signal, the light outputted from the injection optical source  20  is divided by the third wavelength division multiplexer/demultiplexer  33  to be injected to the optical transmitter  1  of the OLT  31 , and received by the optical receiver Rx of the ONT  51  via the first wavelength division multiplexer/demultiplexer  32 , the SMF and the second wavelength division multiplexer/demultiplexer  41 . Other descriptions are omitted because description is same as those in  FIG. 7 . 
         [0097]    The previous description of the present disclosure is provided to enable any person skilled in the art to make or use the inventive disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to limit the examples described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.