Abstract:
Provided is an optical filtering apparatus which is applied to a Time Division Multiplexing Passive Optical Network (TDM-PON) based on a wavelength division multiplexing (WDM) method. The optical filtering apparatus includes: an optical signal distributing unit dividing an optical signal received from an optical line terminal into at least one optical signal with uniform output intensity, and distributing the at least one optical signal into at least one optical network unit; and a multiplexing unit combining a plurality of optical signals having different wavelengths received from the at least one optical network unit using a wavelength division multiplexing (WDM) method, and outputting the combined optical signal to the optical line terminal. Therefore, by using the optical filtering apparatus, it is possible to connect a large number of subscribers to an Optical Line Terminal (OLT) on a network such as Fiber To The Home (FTTH), thereby increasing efficiency in costs.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from Korean Patent Application No. 10-2007-0125681, filed on Dec. 5, 2007, and Korean Patent Application No. 10-2008-0025864, filed on Mar. 20, 2008, the disclosures of which are incorporated herein in their entireties by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical filtering apparatus, and more particularly, to an optical filtering apparatus which is used in a Time Division Multiplexing-based Passive Optical Network (TDM-PON) to which wavelength division multiplexing (WDM) is applied. 
         [0004]    2. Description of the Related Art 
         [0005]    In a Time Division Multiplexing Passive Optical Network (TDM-PON), a plurality of optical network units (ONUs) are connected to an optical line terminal (OLT). 
         [0006]    The plurality of ONUs share the OLT including an optical source and an optical transceiver and optical lines. Accordingly, the ONUs can share costs for installing optical fibers and costs for installing the OLT. That is, service costs per one ONU can be reduced. 
         [0007]    The greater the number of ONUs connected to an OLT, the less the service cost per one ONU. However, when a large number of ONUs are connected to an OLT, optical loss is introduced by an optical splitter. Also, since an optical receiver can detect an optical signal with low error rate only if its output intensity is higher than a reference output intensity, an optical source having high output intensity is needed in order to connect a large number of ONUs to an OLT. Increasing the number of ONUs capable of sharing an OLT in spite of an increase in costs of the OLT will be advantageous in view of total cost. Accordingly, by increasing the output intensity of an optical source utilized in the OLT, service costs per one ONU can be reduced. 
         [0008]    However, increasing the output intensity of optical sources utilized in ONUs is economically inefficient because it increases the service cost per one ONU in proportion to an increase in costs of the ONUs. Accordingly, in the case of upstream transmission, there is a limit in increasing the optical output intensity. 
         [0009]    Also, in a conventional TDM-PON, an optical splitter combines optical signals having the same wavelength, which are received from an ONU 1 , ONU 2 , . . . , ONUn. In this case, optical loss corresponding to optical loss of downstream signals is generated. 
         [0010]    In a TDM-PON method in which loss of original signals inevitably occurs, there are difficulties in restoring the original signals through time division multiplexing. That is, in order to compensate for optical loss generated in parts at which ONUs are coupled, receiver sensitivity of an optical receiver needs to be enhanced or appropriate processing needs to be performed. However, enhancing the receiver sensitivity of the optical receiver is very difficult, and reducing optical loss in the conventional TDM-PON also is not easy. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a method of connecting a large number of subscribers to an OLT on a Time Division Multiplexing Passive Optical Network (TDM-PON) based on Wavelength Division Multiplexing (WDM), thereby increasing efficiency of the TDM-PON. 
         [0012]    The present invention also provides a method of minimizing optical loss when combining optical signals transmitted as uplink signals from a plurality of ONUs and transferring the combined signal to an OLT. 
         [0013]    According to an aspect of the present invention, there is provided an optical filtering apparatus including: an optical signal distributing unit dividing an optical signal received from an OLT into at least one optical signal with uniform output intensity, and distributing the at least one optical signal into at least one optical network unit; and a multiplexing unit combining a plurality of optical signals having different wavelengths received from the at least one optical network unit using a wavelength division multiplexing (WDM) method, and outputting the combined optical signal to the OLT. 
         [0014]    According to another aspect of the present invention, there is provided an optical communication system including: an OLT including a light source, and at least one communication unit outputting an optical signal generated by the light source, and receiving an optical signal from the outside; at least one optical network unit outputting an optical signal having a predetermined wavelength; and an optical filtering apparatus including an optical signal distributing unit dividing an optical signal received from the OLT into at least one optical signal with uniform output intensity and distributing the at least one optical signal into the at least one optical network unit, and a multiplexing unit combining a plurality of optical signals having different wavelengths received from the at least one optical network unit using a wavelength division multiplexing (WDM) method and outputting the combined optical signal to the OLT. 
         [0015]    Therefore, it is possible to reduce coupling loss of uplink signals outputted from a plurality of ONUs on a TDM-PON. In a conventional TDM-PON system illustrated in  FIG. 8 , downlink signal patterns (a) and uplink signal patterns (b) are transmitted and received between an OLT  100  and an optical splitter  800  on a TDM-PON, and when signals having the same wavelength are combined through the optical splitter  800 , signal loss is introduced. 
         [0016]    However, according to the present invention, it is possible to reduce signal loss which can be generated by signal coupling based on the TDM-PON. Accordingly, since a large number of subscribers can be connected to an OLT on the TDM-PON, it is possible to further increase cost efficiency. 
         [0017]    Additional aspects of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
         [0018]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention, and together with the description serve to explain the aspects of the invention. 
           [0020]      FIG. 1  is a configuration view of an optical communication system according to an embodiment of the present invention; 
           [0021]      FIG. 2  is a configuration view of an optical communication system according to another embodiment of the present invention; 
           [0022]      FIG. 3  is a configuration view of an optical communication system according to still another embodiment of the present invention; 
           [0023]      FIG. 4  is a block diagram of an optical filtering apparatus according to an embodiment of the present invention; 
           [0024]      FIGS. 5 through 7  are configuration views of optical filtering apparatuses according to another embodiments of the present invention; and 
           [0025]      FIG. 8  is a configuration view of a conventional Time Division Multiplexing-Passive Optical Network (TDM-PON). 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0026]    The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
         [0027]      FIG. 1  is a configuration view of an optical communication system according to an embodiment of the present invention. As illustrated in  FIG. 1 , the optical communication system includes: an optical line terminal (OLT)  100  including a light source, and at least one communication unit which outputs an optical signal generated by the light source and receives an optical signal from the outside; at least one optical network unit (ONU) which outputs an optical signal having a predetermined wavelength ; and an optical filtering apparatus  200  including an optical signal distributing unit which divides an optical signal received from the OLT  100  into at least one optical signal with uniform output intensity and distributes at least one optical signal into the at least one ONU, and a multiplexing unit which combines a plurality of optical signals having different wavelengths received from the at least one ONU using a wavelength division multiplexing (WDM) method and outputs the combined optical signal to the OLT. 
         [0028]    The OLT  100 , which is an OLT of a service provider, is a multiservice apparatus for connecting an optical subscriber network to another system. The OLT  100  may be a service interface and protocol processing (SIPP) apparatus, a CATV apparatus, a transmission apparatus, or a network management apparatus. Functionally, the OLT  100  provides multiservice access functions: accessing a common channel signaling service (CCSS) in a personal switched telephone network (PSTN) and an integrated services digital network (ISDN), accessing a local switch in channel associated signaling (CAS), interfacing in a Packet Switched Public Data Network (PSPDN), accessing a headend in a CATV network, accessing an Asynchronous Transfer Mode (ATM) in a broadband convergence network, and accessing a broadband service such as the Internet. 
         [0029]    The ONU is a terminal of an optical communication network which provides a service interface to end users. The ONU uses Fiber To The Curb (FTTC), Fiber To The Building (FTTB), Fiber To The Floor (FTTF), Fiber To The Office (FTTO), etc., in order to provide high accessibility to users. Also, the ONU may be installed in a subscriber&#39;s house, and transforms an image signal interface or a communication interface, such as a user-network interface of a Narrowband Integrated Services Digital Network (N-ISDN), a user-network interface of a Broadband Integrated Service Digital Network (B-ISDN), etc., to access an optical fiber network. 
         [0030]    The filtering apparatus  200  outputs optical signals having a wavelength λ d  to N ONUs, respectively. That is, the wavelengths of downlink signals outputted to the N ONUs are the same. The transmittance T of an optical signal which is output to each ONU is 1/N of the transmittance of an optical signal outputted from OLT  100 . 
         [0031]    Meanwhile, as illustrated in  FIG. 1 , optical signals received from the ONUs by the optical filtering apparatus  200  have different wavelengths of λ u1 , λ u2 , . . . , λ un . Accordingly, the uplink signals having the different wavelengths of λ u1 , λ u2 , . . . , λ un  are combined by a wavelength filter such as a WDM coupler. The WDM method can reduce coupling loss compared to a TDM-PON method which combines signals having the same wavelength with each other. 
         [0032]      FIG. 2  is a configuration view of an optical communication system according to another embodiment of the present invention. 
         [0033]    As illustrated in  FIG. 2 , the optical communication system includes a plurality of optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  between an optical filtering apparatus  200  and a plurality of ONUs. In the current embodiment of the present invention, the optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  divide an optical signal of an OLT into a plurality of optical signals and output the plurality of signals to the plurality of ONUs, or transfer a plurality of optical signals received from the plurality of ONUs to the optical filtering apparatus  200 . 
         [0034]    Here, one or more ONUs can be grouped into one group, and ONUs belonging to the same group output optical signals having the same wavelength. For example, an optical signal having a wavelength λ d  which is output from the OLT is output to a plurality of optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  through the optical signal filtering apparatus  200 . The plurality of optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  transfer the optical signal to the plurality of ONUs. ONUs connected to an optical splitter can be set to a group. ONUs connected to an optical splitter, that is, ONUs belonging to a group output optical signals having the same wavelength to the optical splitter. That is, optical signals having different wavelengths are output from different ONU groups connected to different optical splitters. 
         [0035]    For example, when n optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  are connected to an optical filtering apparatus and m ONUs are connected to each optical splitter, the amplitude of a downlink optical signal transferred to each ONU is reduced to 1/(m*n) of that of an optical signal outputted from a light source of the OLT. In order to compensate for loss of the optical signal, the output intensity of the light source of the OLT is increased. Meanwhile, the intensity of optical signals that are to be transferred from the ONUs is reduced to 1/m by the optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n.    
         [0036]    Here, the wavelengths of optical signals transferred from the optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  to the optical filtering apparatus  200  are different from each other. That is, the optical filtering apparatus  200  combines optical signals having different wavelengths received from the optical splitters  210 - 1 ,  210 - 2 , . . . ,  210 - n  using a WDM method. In this case, since no coupling loss occurs, the amplitude of an optical signal transmitted to the OLT is reduced to 1/m of that of an optical signal outputted from a light source of the OLT. That is, since no coupling loss is introduced by the optical filtering apparatus  200 , light sources having relatively low output intensity can be utilized in the ONUs and more ONUs can be connected to the OLT. 
         [0037]      FIG. 3  is a configuration view of an optical communication system according to another embodiment of the present invention. 
         [0038]    As illustrated in  FIG. 3 , the optical communication system includes an optical signal distributing unit  300  between an OLT  100  and a plurality of optical filtering apparatuses  200 - 1 ,  200 - 2 , . . . ,  200 - k.  In order to connect the plurality of ONUs to the OLT  100 , an optical signal distributing unit  300  can be installed between the OLT  100  and the optical filtering apparatuses  200 - 1 ,  200 - 2 , . . . ,  200 - k.  Various modifications for efficiently connecting more ONUs to an OLT, other than the embodiments of the optical communication systems illustrated in  FIGS. 3 and 4 , are possible. 
         [0039]    Hereinafter, an optical filtering apparatus according to an embodiment of the present invention will be described in detail with reference to  FIG. 4 . 
         [0040]      FIG. 4  is a block diagram of an optical filtering apparatus  400  according to an embodiment of the present invention. As illustrated in  FIG. 4 , the optical filtering apparatus  400  includes a first signal divider  410  for separating uplink signals from downlink signals, an optical signal distributing unit  420 , a multiplexer  430 , and a plurality of second signal dividers  415   a,    415   b,  and  415   c.    
         [0041]    An optical signal received from an OLT through an input/output terminal is transferred to the optical signal distributing unit  420  through an upper output terminal of the first signal divider  410 . The optical signal distributing unit  420  divides the received optical signal into a plurality of optical signals and distributes the divided optical signals to a plurality of ONUs through a plurality of terminals. Here, the optical signal distributing unit  420  divides the optical signal received from the OLT into the plurality of optical signals with uniform output intensity, and distributes the plurality of optical signals to the plurality of ONUs. In the current embodiment, the optical signal distributing unit  420  may be an optical splitter. 
         [0042]    Also, the optical signals output from the optical signal distributing unit  420  are transferred to input/output terminals connected respectively to the ONUs, through the second signal dividers  415   a,    415   b,  and  415   c.  The optical signal distributing unit  420  can transfer optical signals with the same amplitude to the respective input/output terminals of the ONUs. 
         [0043]    The multiplexer  430  transfers a plurality of optical signals received from the plurality of ONUs to the OLT, according to a WDM method. The optical signals received from the plurality of ONUs through the second signal dividers  415   a,    415   b,  and  415   c  have different wavelengths. In the current embodiment, the multiplexing unit  430  may be a wavelength division multiplexing (WDM) filter, such as an arrayed waveguide grating (AWG), for multiplexing a plurality of optical signals having different wavelengths. Accordingly, the multiplexing unit  430  can transfer a plurality of optical signals to the OLT without coupling loss. 
         [0044]    Here, the first signal dividing unit  410  and the second signal dividers  415   a,    415   b,  and  415   c  may be WDM filters or circulators. In the current embodiment, the first signal divider  410  separates a path through which an optical signal that is to be transferred from the OLT to the optical signal distributing unit  420  passes, from a path through which an optical signal that is to be transferred from the multiplexing unit  430  to the OLT passes. 
         [0045]    Also, the second signal dividers  415   a,    415   b,  and  425   c  separate a path through which a signal that is to be transferred from the optical signal distributing unit  420  to the ONUs passes, from a path through which a signal that is to be transferred from the ONUs to the multiplexing unit  430  passes. 
         [0046]      FIGS. 5 through 7  are configuration views of optical filtering apparatuses according to another embodiments of the present invention. 
         [0047]    In detail, an optical filtering apparatus  500  illustrated in  FIG. 5  uses a thin film filter. In the optical filtering apparatus  500 , a thin film WDM filter  510  and a multiplexing unit  530  correspond respectively to the first signal divider  410  and the second signal dividers  415   a,    415   b  and  415   c  described above with reference to  FIG. 4 . Also, the multiplexing unit  530  may include a plurality of thin film WDM filters and a plurality of thin film mirrors. That is, as illustrated in  FIG. 5 , optical signals having different wavelengths λ u1 , λ u2 , λ u3 , and λ u4  are combined and provided to an OLT  100 . The optical signal distributing unit  520  may include a plurality of thin film mirrors. The numbers and arrangements of the thin film filters and thin film mirrors are not limited to the current embodiment shown in  FIG. 5 , and various modifications are possible. The number and arrangement of the thin film WDM filters included in the multiplexing unit  530 , and the number and arrangement of the thin film mirrors included in the optical signal distributing unit  520  can also vary according to the number of input/output terminals, that is, according to the number of ONUs connected to the OLT. 
         [0048]      FIG. 6  shows an optical filtering apparatus  600  including a photonic lightwave filter, such as an optical fiber or a photonic lightwave circuit (PLC). A first signal divider  610  and a plurality of second signal dividers  615  may be photonic waveguide bidirectional signal dividers. Also, an optical signal divider  620  may be a directional coupler, a waveguide splitter, a Y-branch, etc. A multiplexer  630  may be an AWG, diffraction lattices, a WDM filter, etc. In the current embodiment, a plurality of optical signals having different wavelengths can be received from an OLT  100 . 
         [0049]      FIG. 7  shows an optical filtering apparatus  700  including a photonic waveguide filter for transmitting a plurality of optical signals having various wavelengths to a large number of input/output terminals. A first signal divider  710  and a second signal divider  715  may be photonic waveguide bidirectional dividers. A signal received from an OLT is input to the first signal divider  710  and output through an upper terminal of the first signal divider  710 . In the current embodiment, an optical signal distributing unit  720  includes a wavelength divider  722  and a sub optical signal distributing unit  724 . Due to the configuration of the optical signal distributing unit  720 , it is possible to receive optical signals having various wavelengths. A multiplexing unit  730  may be an AWG, diffraction lattices, a WDM filter, etc. 
         [0050]    It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.