Abstract:
A wavelength division multiplexed passive optical network (WDM PON) includes a central office that has: a broadband light source, a first wavelength division multiplexer to spectrum-slice light outputted from the light source, semiconductor optical amplifiers or variable optical attenuators, each modulating an associated one of spectrum-sliced lights in accordance with input data, and a second wavelength division multiplexer to multiplex optical signals respectively outputted from the semiconductor optical amplifiers. A remote node, connected to the central office via a main optical fiber, distributes the optical signals to distribution optical fibers, and, in, turn, to respective optical network units.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “WAVELENGTH DIVISION MULTIPLEXED PASSIVE OPTICAL NETWORK,” filed in the Korean Intellectual Property Office on Jan. 27, 2004 and assigned Serial No. 2004-4989, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a passive optical network, and more particularly to a wavelength division multiplexed passive optical network using a spectrum-sliced light source.  
         [0004]     2. Description of the Related Art  
         [0005]     A wavelength division multiplexed (WDM) passive optical network (PON) can provide ultrahigh-speed broadband communication services, using particular wavelengths assigned to respective subscribers. Accordingly, the WDM PON ensures communication security, and easily accommodates a separate communication service required by a subscriber, and expansion of communication capacity. To this latter point, the WDM PON allows addition of a particular wavelength to be assigned to a new subscriber, so that it is possible to easily achieve an increase in the number of subscribers.  
         [0006]     In spite of the advantages, however, the WDM PON additionally requires, for its central office (CO) and each optical network unit (ONU), light sources with a particular oscillation wavelength, and wavelength stabilizing circuits adapted to stabilize the wavelength of the respective light sources. As a result, a heavy burden is imposed on subscribers. The WDM PON is therefore not yet in widespread use. Implementation of the WDM accordingly requires that an economical WDM light source be developed.  
         [0007]     Proposed light sources for a WDM PON include a distributed feedback (DFB) laser, a multi-frequency laser (MFL), a picosecond pulse light source, or the like.  
         [0008]      FIG. 1  is a schematic diagram illustrating a conventional WDM PON using a spectrum-sliced light source. The PON  100  in  FIG. 1  includes a central office (CO)  110 , a remote node (RN)  170  connected to the central office  110  via a main optical fiber (MF)  160 , and a plurality of optical network units (ONUs), ONU 1    200 - 1  to ONU n    200 - n , connected to the remote node  170  via a plurality of distribution optical fibers (DFs), DF 1    190 - 1  to DF n    190 - n , respectively.  
         [0009]     The central office  110  includes a broadband light source (BLS)  120 , a first wavelength division multiplexer (WDM 1 )  130 , n LiNbO 3  modulators (MOD 1  to MOD n )  140 - 1  to  140   n , and a second wavelength division multiplexer (WDM 2 )  150 .  
         [0010]     The WDM 1   130  has a multiplexing port MP, and n demultiplexing ports DP 1  to DP n . The multiplexing port MP of the WDM 1   130  is connected to the broadband light source  120 , whereas the n demultiplexing ports DP 1  to DP n  of the WDM 1   130  are connected to the LiNbO 3  MOD 1    140 - 1  to LiNbO 3  MOD n    140 - n , respectively. The WDM 1   130  spectrum-slices (or demultiplexes) broadband light outputted from the broadband light source  120  into n lights of different-wavelengths, and outputs the different-wavelength lights to the respective demultiplexing ports DP 1  to DP n . Specifically, the light having the k-th wavelength is outputted to the k-th demultiplexing port DP k  of the WDM 1   130 , where 1≦k≦n.  
         [0011]     The LiNbO 3  MOD 1    140 - 1  to LiNbO 3  MOD n    140 - n  are connected between the WDM 1   130  and the WDM 2   150  such that each of them connects demultiplexing ports of the WDM 1   130  to corresponding multiplexing ports of WDM 2   150 . Accordingly, each of the LiNbO 3  MOD 1    140 - 1  to LiNbO 3  MOD n    140 - n  generates a respective optical signal by using received external data to modulate light with an associated wavelength received from the WDM 1   130 . For example, the LiNbO 3  MOD n    140 - n  connects the n-th demultiplexing ports DP n  of the WDM 1   130  and WDM 2   150 . Accordingly, the LiNbO 3  MOD n    140 - n  modulates light with the n-th wavelength received from the WDM 1   130 , using external data that has been received, and thus, generates an n-th optical signal.  
         [0012]     The WDM 2   150  has a multiplexing port MP, and n demultiplexing ports DP 1  to DP n . The multiplexing port MP of the WDM 2   150  is connected to the main optical fiber  160 , whereas the n demultiplexing ports DP 1  to DP n  of the WDM 2   150  are connected to the LiNbO 3  MOD 1    140 - 1  to LiNbO 3  MOD n    140 - n , respectively. The WDM 2   150  multiplexes n optical signals respectively inputted to the demultiplexing ports DP 1  to DP n  thereof, and outputs the multiplexed optical signal through the multiplexing port MP.  
         [0013]     The remote node  170  is connected to the central office  110  via the main optical fiber  160 , while being connected to the ONU 1    200 - 1  to ONU n    200 - n  via the distribution optical fibers  190 - 1  to  190 - n , respectively. The remote node  170  includes a third wavelength division multiplexer (WDM 3 )  180 .  
         [0014]     The WDM 3   180  has a multiplexing port MP connected to the main optical fiber  160 , and n demultiplexing ports DP 1  to DP n  connected to the n distribution optical fibers  190 - 1  to  190 - n , respectively. The WDM 3   180  demultiplexes n optical signals inputted into its multiplexing port MP, and outputs the demultiplexed n optical signals to the n demultiplexing ports DP 1  to DP n  thereof, respectively.  
         [0015]     ONU 1    200 - 1  to ONU n    200 - n  are connected to the n distribution optical fibers  190 - 1  to  190 - n , respectively. For example, the ONU n    200 - n  is connected to the n-th distribution optical fiber  190 - n . Each ONU receives an optical signal from the associated distribution optical fiber, and opto-electrically detects the received signal. For example, the ONU n    200 - n  receives an optical signal from the n-th distribution optical fiber  190 - n , and opto-electrically detects the received signal.  
         [0016]     However, the LiNbO 3  modulators conventionally employed in the WDM PON are expensive. Additionally, they exhibit a severe characteristic variation depending on the polarized state of the incoming light, and suffer high insertion loss. It is accordingly necessary in some cases to provide additional optical amplifiers, e.g., in the central office, to compensate for a loss margin which varies with the transmission distance of the multiplexed optical signals. The resulting overhead leads to additional expense and lost competitiveness.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention has been made to address the above-mentioned shortcomings of the related art. An object of the invention is to provide a WDM PON that can be inexpensively implemented while using a spectrum-sliced light source involving simple wavelength management, and without using expensive modulators.  
         [0018]     In accordance with the present invention, this object is accomplished by providing a central office, of an optical network, that includes a broadband light source, a first wavelength division multiplexer configured to spectrum-slice light outputted from the broadband light source, semiconductor optical amplifiers each configured to modulate, in accordance with data that has been inputted, an associated one of spectrum-sliced lights outputted from the first wavelength division multiplexer and to output the modulated lights. The central office further includes a second wavelength division multiplexer configured to multiplex the outputted, modulated lights. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which the same or similar elements are identically annotated throughout the several views:  
         [0020]      FIG. 1  is a schematic diagram illustrating a conventional WDM PON;  
         [0021]      FIG. 2  is a schematic diagram illustrating a configuration of a WDM PON according to a first embodiment of the present invention, in which semiconductor optical amplifiers (SOAs) are used; and  
         [0022]      FIG. 3  is a schematic diagram illustrating a configuration of a WDM PON according to a second embodiment of the present invention, in which variable optical attenuators (VOAs) are used. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Preferred embodiments of the present invention are described below in detail with reference to the annexed drawings. Details of known functions and configurations incorporated herein are omitted for clarity of presentation.  
         [0024]      FIG. 2  illustrates a configuration of a WDM PON according to a first embodiment of the present invention, which differs from the prior art embodiment of  FIG. 1  in that the LiNbO 3  modulators  140 - 1  to  140 - n , and possibly additional loss-insertion-compensating amplifiers, are replaced with an array  340  of semiconductor optical amplifiers (SOAs) SOA 1    340 - 1  to SOA n    340 - n . The PON, which is designated by reference numeral  300  in  FIG. 2 , includes a central office (CO)  310 , a remote node (RN)  370  connected to the central office  310  via a main optical fiber (MF)  360 , and a plurality of optical network units (ONUs), ONU 1    400 - 1  to ONU n    400 - n , connected to the remote node  370  via a plurality of distribution optical fibers (DFs), DF 1    390 - 1  to DF n    390 - n , respectively.  
         [0025]     The central office  110  includes a broadband light source  320 , a first wavelength division multiplexer (WDM 1 )  330 , n semiconductor optical amplifiers (SOAs), SOA 1    340 - 1  to SOA n    340 - n , and a second wavelength division multiplexer (WDM 2 )  350 . The WDM 1   330  has a multiplexing port MP, and n demultiplexing ports DP 1  to DP n . The multiplexing port MP of the WDM 1   330  is connected to the broadband light source  320 , whereas the n demultiplexing ports DP 1  to DP n  of the WDM 1   330  are connected to the SOA 1    340 - 1  to SOA n    340 - n , respectively. The WDM 1   330  spectrum-slices (or demultiplexes) broadband light outputted from the broadband light source  320  and inputted at the multiplexing port MP thereof, into n lights of different-wavelengths, and outputs the different-wavelength lights to respective demultiplexing ports DP 1  to DP n  thereof. Specifically, the light having the k-th wavelength is outputted to the k-th demultiplexing port DP k  of the WDM 1   330 , where 1≦k≦n. Each of the WDM 1   330  and WDM 2   350  may include an arrayed waveguide grating (AWG).  
         [0026]     The SOA 1    340 - 1  to SOA n    340 - n  are connected between the WDM 1   330  and the WDM 2   350  such that each of them connects demultiplexing ports of the WDM 1   330  to corresponding multiplexing ports of WDM 2   350 . Accordingly, each of the SOA 1    340 - 1  to SOA n    340 - n  generates a respective optical signal by using received external data to modulate light with an associated wavelength received from the WDM 1   330 . For example, the SOA n    340 - n  connects the n-th demultiplexing ports DP n  of the WDM 1   330  and WDM 2   350 . Accordingly, the SOA n    340 - n  modulates light with the n-th wavelength received from the WDM 1   330 , using external data that has been received, and thus, generates an n-th optical signal.  
         [0027]     Each of SOA 1    340 - 1  to SOA n    340 - n  not only serves as a modulator, but also serves as an amplifier having a gain. Thus, the SOA 1    340 - 1  to SOA n    340 - n  can compensate for an insertion loss generated in each of the WDM 1   330  and WDM 2   350  and an insertion loss generated due to a difference between the central wavelengths of the WDM 1   330  and WDM 2   350 . Advantageously, the PON  300  can therefore be designed with a lower system margin.  
         [0028]     The WDM 2   350  has a multiplexing port MP, and n demultiplexing ports DP 1  to DP n . The multiplexing port MP of the WDM 2   350  is connected to the main optical fiber  360 , whereas the n demultiplexing ports DP 1  to DP n  of the WDM 2   350  are connected to the SOA 1    340 - 1  to SOA n    340 - n , respectively. The WDM 2   350  multiplexes n optical signals respectively inputted to the demultiplexing ports DP 1  to DP n  thereof, and outputs the multiplexed optical signals through the multiplexing port MP.  
         [0029]     The remote node  370  is connected to the central office  310  via the main optical fiber  360 , while being connected to the ONU 1    400 - 1  to ONU n    400 - n  via the distribution optical fibers  390 - 1  to  390 - n , respectively. The remote node  370  includes a third wavelength division multiplexer (WDM 3 )  380 .  
         [0030]     The WDM 3   380  has a multiplexing port MP connected to the main optical fiber  360 , and n demultiplexing ports DP 1  to DP n  connected to the n distribution optical fibers  390 - 1  to  390 - n , respectively. The WDM 3   380  demultiplexes n optical signals inputted into its multiplexing port MP, and outputs the demultiplexed n optical signals to the n demultiplexing ports DP 1  to DP n  thereof, respectively. The WDM 3   380  may include an AWG.  
         [0031]     ONU 1    400 - 1  to ONU n    400 - n  are connected to the n distribution optical fibers  390 - 1  to  390 - n , respectively. For example, the ONU n    400 - n  is connected to the n-th distribution optical fiber  390 - n . Each ONU receives an optical signal from the associated distribution optical fiber, and opto-electrically detects the received signal.  
         [0032]      FIG. 3  illustrates a configuration of a WDM PON according to a second embodiment of the present invention. The PON  500  in  FIG. 3  has a configuration similar to that of  FIG. 2 , except that it uses a variable optical attenuator (VOA) array  540  in place of the SOA array  340  used in the configuration of  FIG. 2 . The array  540  includes VOAs  540 - 1  to  540 - n.    
         [0033]     The PON  500  includes a central office (CO)  510  that incorporates the VOA array  540 , and, as in the previous embodiment, the remote node (RN)  370 , main optical fiber (MF)  360 , the distribution optical fibers (DFs)  390 - 1  to  390 - n  and the plurality of optical network units (ONUs)  400 - 1  to  400 - n.    
         [0034]     Each of the VOA 1    540 - 1  to VOA n    540 - n  generates a respective optical signal by using received external data to modulate light with an associated wavelength received from the WDM 1   530 . The VOAs  540 - 1  to  540 - n  of the second embodiment of the present invention and the SOAs  340 - 1  to  340 - n  of the first embodiment of the present invention both variably adjust the optical power of light, but differ in that the VOAs attenuate, rather than amplify, light.  
         [0035]     As apparent from the above description, the WDM PON of the present invention reduces operation and maintenance costs by using a spectrum-sliced light source that involves simple wavelength management. The WDM PON of the present invention advantageously allows configuration of an economical network, using inexpensive semiconductor optical amplifiers or variable optical attenuators, in place of expensive modulators.  
         [0036]     While this invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, to the contrary, is intended to cover various modifications within the spirit and scope of the appended claims.