Abstract:
Disclosed is a method for removing cross-talk in a wavelength division multiplexed passive optical network (WDM-PON). The WDM-PON and the method remove cross-talk between adjacent wavelength channels due to incomplete alignment of wavelength channels in a MUX/de-MUX between a central office and a remote node in the WDM-PON employing light-injected light sources. The WDM-PON includes at least two broadband light sources having different bands, which provide injection light to be injected to light-injected channels light sources, a transmitter receiving injection from the broadband light sources, injecting the injection light to odd channel light-injected light sources and even channel light-injected light sources, arraying odd and even channels in such a manner that the odd and even channels belong to different spectrum bands, multiplexing the signal according channels, and transmitting the multiplexed signal. The WDM-PON may also include a receiver for receiving the multiplexed signal transmitted from the transmitter and splitting the multiplexed signal according to the odd and even channels.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims the benefit of the earlier filing date of that patent application entitled “Wavelength Division Multiplexed Passive Optical Network (WDM-PON) without Cross-Talk and Method for Removing Cross-talk” filed in the Korean Intellectual Property Office on Feb. 1, 2005, and assigned Serial No. 2005-9101, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a wavelength division multiplexed passive optical network (WDM-PON), and more particularly to a wavelength division multiplexed passive optical network with reduced cross-talk between adjacent wavelengths.  
         [0004]     2. Description of the Related Art  
         [0005]     With increasing interest in a wavelength division multiplexed passive optical network (WDM-PON) as a next generation optical network for providing a future broadband communication service, efforts for economical realization of the WDM-PON are presently being undertaken.  
         [0006]     Since such a WDM-PON allocates an individual wavelength to each subscriber, it is necessary to employ WDM light sources for subscribers and a multiplexer/de-multiplexer (MUX/de-MUX) to process the plurality of wavelength channels generated from the WDM light sources. The economical realization of wavelength alignment between the WDM light sources and the MUX/de-MUX is an important factor of reducing maintenance costs of a WDM-PON network.  
         [0007]     Generally, light sources such as a distributed feedback laser array, a high-power light emitting diode, or a spectrum-sliced source are suggested as the WDM light sources. However, recently, light-injected light sources such as an external light-injected Fabry-Perot laser diode (FP-LD) or a wavelength-seeded reflective semiconductor amplifier, which have wavelengths determined by externally injected light have been suggested.  
         [0008]     Since these light-injected light sources have wavelengths determined by externally injected light, one type of a light source may be used for a plurality of wavelength channels without additional control. Accordingly, it is unnecessary to align wavelengths between the light source and a MUX/de-MUX, so it is possible to maintain and operate a network in a simple manner.  
         [0009]     The typical WDM-PON has several advantages such as having a large bandwidth, superior security, and protocol independence. However, since the typical WDM-PON requires a plurality of light sources, device costs increase. In addition, since the typical WDM-PON employs a MUX/de-MUX in order to multiplex a plurality of wavelength channels into one transmitted signal and demultiplex the transmitted signal into a plurality wavelength channels, the WDM-PON is susceptible to adjacent wavelength channel cross-talk.  
         [0010]     In particular, the WDM-PON employing light-injected light sources may have cross-talk due to adjacent wavelength channels when wavelength channels multiplexed/de-multiplexed in a MUX/D-MUX between a central office and a remote node are incompletely aligned.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a wavelength division multiplexed passive optical network having no cross-talk by removing cross-talk between adjacent wavelength channels due to incomplete alignment of wavelength channels in a MUX/de-MUX.  
         [0012]     According to one aspect of the present invention, there is provided a wavelength division multiplexed passive optical network (WDM-PON) employing a light-injected light source without cross-talk, which includes at least two broadband light sources having different spectrum bands, which provide injection light to be injected to light-injected channel light sources, a transmitter receiving injection light having different bands from the broadband light sources, injecting the injection light to odd channel light-injected light sources and even channel light-injected light sources, aligning odd and even channels such that the odd and even channels belong to different spectrum bands, multiplexing the signal channels and transmitting the multiplexed signal. Also included is a receiver for receiving the multiplexed signal transmitted from the transmitter and splitting the multiplexed signal according to the respective channels.  
         [0013]     According to another aspect of the present invention, there is provided a method for removing cross-talk in a wavelength division multiplexed passive optical network employing a light-injected light source, the method including the steps of providing at least two broadband light sources having different spectrum bands, the two broadband light sources provide injection light to be injected to light-injected channel light sources, receiving the injection light from the broadband light sources, injecting the injection light to odd channel light-injected light sources and even channel light-injected light sources, aligning the odd and even channels such that the odd and even channels belong to different spectrum bands, multiplexing the channels, transmitting the multiplexed signal and receiving the transmitted multiplexed signal and splitting the transmitted multiplexed signal according to the respective channels. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0015]      FIGS. 1A and 1B  are block diagrams illustrating an upstream transmission structure of a WDM-PON using external light-injected light sources according to an embodiment of the present invention;  
         [0016]      FIGS. 2A and 2B  are block diagrams illustrating a downstream transmission structure of a WDM-PON using external light-injected light sources according to an embodiment of the present invention;  
         [0017]      FIGS. 3A and 3B  are block diagrams illustrating an upstream and downstream transmission structure of a WDM-PON using externally light-injected light sources according to an embodiment of the present invention; and  
         [0018]      FIG. 4  is a block diagram illustrating the structure of a bi-directional transceiver shown in  FIG. 3A . 
     
    
     DETAILED DESCRIPTION  
       [0019]     Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. Note that the same or similar components in drawings are designated by the same reference numerals as far as possible although they are shown in different drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention unclear.  
         [0020]     The present invention relates to a structure for removing cross-talk between neighboring or adjacent channels due to an incomplete wavelength alignment of a MUX/de-MUX between a central office and a remote node in a wavelength division multiplexed passive optical network (WDM-PON) using a light-injected light source (e.g. a light-injected Fabry-Perot laser or a wavelength-seeded reflective semiconductor optical amplifier).  
         [0021]      FIG. 1A  is a block diagram illustrating an upstream transmission structure of a WDM-PON using external light-injected light sources according to an embodiment of the present invention.  
         [0022]     As shown in  FIG. 1A , the WDM-PON employs a structure using two wavelength bands separated from each other (mutually exclusive) by a free spectral range (FSR) in a multiplexer/de-multiplexer (MUX/de-MUX) used for a central office and a remote node.  
         [0023]     Light-injected upstream light sources include a first broadband light source  112  having a first band and a second broadband light source  113  having a second band.  
         [0024]     In the procedure of injecting light for the purpose of employing broadband light sources as upstream light sources, an injection light having a wide line-width generated from the first broadband light source  112  is delivered to a second interleaver  115  from a first WDM filter  120  through a circulator  110  and a transmission optical fiber. In this case, interleavers  107 ,  108 ,  114 , and  115  are elements for outputting an input light through two output ports by splitting the input light into odd channels and even channels. The interleavers  107 ,  108 ,  114 , and  115  employed according to an embodiment of the present invention operate based on channels identical to those of MUXs/de-MUXs  105 ,  106 ,  116 , and  117 . In addition, the MUXs/de-MUXs  105 ,  106 ,  116 , and  117  employed according to an embodiment of the present invention have two input/output ports at one side of the MUXs/de-MUXs  105 ,  106 ,  116 , and  117  in the 2×N shapes.  
         [0025]     In addition, injection light output to an even port of the second interleaver  115  is divided into channels according to its wavelength(s) and output in the first MUX/de-MUX  116  connected to the second interleaver  115 . Since the injection light is input to the second port of the first MUX/de-MUX  116 , the injection light is output as odd channels and input as injection light to a light-injected light source of the odd channels. The light-injected light sources include, for example, a Fabry-Perot laser or a reflective semiconductor optical amplifier.  
         [0026]     In the meantime, injection light output to an odd port of the second interleaver  115  is divided into channels according to its wavelength(s) and output in the second MUX/de-MUX  117  connected to the second interleaver  115 . Since the injection light is input to the first port of the second MUX/de-MUX  117 , the injection light is output as odd channels and input as injection light to a light-injected light source of the odd channels. The light-injected light sources include a Fabry-Perot laser or a reflective semiconductor optical amplifier, for example.  
         [0027]     In other words, the first broadband light source  112  having the first band fixes the wavelengths of odd channels  118 - 1  and  119 - 1  which are spectrum-split in the MUX/de-MUXs  116  and  117 , respectively.  
         [0028]     Similarly, injection light having a wide line-width generated from the second light source  113  occupying a separate second band is output to even and odd ports of the first interleaver  114 , output according to channels in the MUX/de-MUX  116  and  117 , and input as injection light to a light-injected light source for even channels. Herein, the light-injected light sources include a Fabry-Perot laser or a reflective semiconductor optical amplifier, for example.  
         [0029]     In other words, the second broadband light source  113  having a second band fixes the wavelengths of even channels  118 - 2  and  119 - 2  which are spectrum-split in the MUX/de-MUXs  116  and  117 , respectively.  
         [0030]     Through the above-described scheme, 2×N wavelength channels are arrayed by interleaving the first-band wavelength as odd channels and the second-band wavelength as even channels.  
         [0031]     The wavelength channels arrayed as described above are output from the Fabry-Perot laser or the reflective semiconductor optical amplifier, progress in a reverse direction, are multiplexed in the first WDM filter  120 , and then are transmitted to the central office through a transmission optical fiber. Wavelength channels are de-multiplexed and input in receivers while undergoing the same scheme in the second WDM filter  109 .  
         [0032]     A multiplexed upstream optical signal having the injection light of the first broadband light source  112  is delivered to the third interleaver  108  from the second WDM filter  109  through a transmission optical fiber.  
         [0033]     An upstream optical signal output to an even port of the third interleaver  108  is divided into channels according to its wavelength(s) and output in the fourth MUX/de-MUX  105  connected to the third interleaver  108 . However, since the upstream optical signal is input to the second port of the fourth MUX/de-MUX  105 , the upstream optical signal is output as odd channels and input to an optical receiver  101 - 1  of the odd channels. In this case, a first bandpass filter  103 - 1  passing the first band wavelength is installed at a front side of the optical receiver  101 - 1  so as to prevent cross-talk.  
         [0034]     In addition, an upstream optical signal output through an odd port of the third interleaver  108  is divided into channels according to its wavelength(s) and output in the third MUX/de-MUX  106  connected to the third interleaver  108 . Since the upstream optical signal is input to the first port of the third MUX/de-MUX  106 , the upstream optical signal is output as odd channels and input to an optical receiver  102 - 1  of the odd channels. In this case, a first bandpass filter  104 - 1  passing the first band wavelength is installed at a front side of the optical receiver  102 - 1  to prevent cross-talk.  
         [0035]     Meanwhile, an upstream optical signal output through an odd port of the fourth interleaver  107  is divided into channels according to its wavelength(s) and output in the third MUX/de-MUX  106  connected to the fourth interleaver  107 . Since the upstream optical signal is input to the second port of the third MUX/de-MUX  106 , the upstream optical signal is output as even channels and input to an optical receiver  102 - 2  of the even channels. In this case, a second bandpass filter  104 - 2  passing the second band wavelength is installed at a front side of the optical receiver  102 - 2  to prevent cross-talk.  
         [0036]     Furthermore, an upstream optical signal output to an even port of the fourth interleaver  107  is divided into channels according to its wavelength(s) and output in the fourth MUX/de-MUX  105  connected to the fourth interleaver  107 . Since the upstream optical signal is input to the first port of the fourth MUX/de-MUX  105 , the upstream optical signal is output as even channels and input to an optical receiver  101 - 2  of the even channels. In this case, a second bandpass filter  103 - 2  passing the second band wavelength is installed at a front side of the optical receiver  101 - 2  so as to prevent cross-talk.  
         [0037]     As described above, it is possible to efficiently prevent cross-talk due to optical signals of adjacent channels by fixing the wavelengths of adjacent channels using injection light having different bands. In other words, even though channels are adjacent to each other, wavelengths of adjacent channels belong to mutually exclusive wavelength bands. Accordingly, even though wavelengths are incompletely aligned in a MUX/de-MUX, it is possible to prevent light of the adjacent channels from being received in receivers by employing bandpass filters.  
         [0038]      FIG. 1B  illustrates broadband light sources having mutually different bands in an upstream transmission structure of the WDM-PON using an external light-injected light source according to an embodiment of the present invention.  
         [0039]     As shown in  FIG. 1B , the spectral band of light source  112  (first band) and the spectral band of light source  113  (second band) are separated from each other by a free spectral range (FSR). The first band and the second band include odd channel upstream signals and even channel upstream signals, respectively.  
         [0040]      FIG. 2A  is a block diagram illustrating a downstream transmission structure of the WDM-PON using external light-injected light sources according to an embodiment of the present invention.  
         [0041]     The WDM-PON shown employs a structure using two wavelength bands separated from each other by a free spectral range (FSR) in a multiplexer/de-multiplexer (MUX/de-MUX) used for a central office and a remote node.  
         [0042]     Light sources for injecting downstream light include a first broadband light source  210  having a first band and a second broadband light source  211  having a second band.  
         [0043]     In the procedure of injecting light for the purpose of employing broadband light sources as downstream light sources, injection light having a wide line-width generated from the first broadband light source  210  is delivered to a second interleaver  206  from a first WDM filter  207  through a circulator  208 . In this case, interleavers  205 ,  206 ,  213 , and  214  are elements for outputting an input light through two output ports by splitting the input light into odd channels and even channels. The interleavers  205 ,  206 ,  213 , and  214  employed according to an embodiment of the present invention operate based on channels substantially identical to channels of MUXs/de-MUXs  203 ,  204 ,  215 , and  216 . In addition, the MUXs/de-MUXs  203 ,  204 ,  215 , and  216  employed according to this embodiment of the present invention have two input/output ports at one side of the MUXs/de-MUXs  203 ,  204 ,  215 , and  216  in the 2×N shape.  
         [0044]     In addition, injection light output to an even port of the second interleaver  206  is divided into channels according to its wavelength(s) and output in the first MUX/DE-MUX  203  connected to the second interleaver  206 . Since the injection light is input to the second port of the first MUX/de-MUX  203 , the injection light is output as odd channels and input as injection light to a light-injected light source  201 - 1  of the odd channels. The light-injected light sources may include a Fabry-Perot laser or a reflective semiconductor optical amplifier, for example.  
         [0045]     The injection light output to an odd port of the second interleaver  206  is divided into channels according to its wavelength(s) and output in the second MUX/de-MUX  204  connected to the second interleaver  206 . Since the injection light is input to the first port of the second MUX/de-MUX  204 , the injection light is output as odd channels and input as injection light to a light-injected light source  202 - 1  of the odd channels. The light-injected light sources include a Fabry-Perot laser or a reflective semiconductor optical amplifier.  
         [0046]     In other words, the first broadband light source  210  having the first band fixes the wavelengths of odd channels  201 - 1  and  202 - 1  spectrum-split in the MUX/de-MUXs  203  and  204 , respectively.  
         [0047]     Similarly, injection light having a wide line-width generated from the second light source  211  having the second band is output to even and odd ports of the first interleaver  205 , output according to channels in the MUX/de-MUX  203  and  204 , and input to light-injected light sources  201 - 2  and  202 - 2  for even channels as injection light. Herein, the light-injected light sources may include a Fabry-Perot laser or a reflective semiconductor optical amplifier, for example.  
         [0048]     In other words, the second broadband light source  211  having the second band fixes the wavelengths of even channels  201 - 2  and  202 - 2  spectrum-split in the MUX/de-MUXs  203  and  204 , respectively.  
         [0049]     Through the above-described scheme, 2×N wavelength channels are aligned by interleaving the first-band wavelength including odd channels and the second-band wavelength including even channels.  
         [0050]     The wavelength channels aligned as described above are output from the Fabry-Perot laser or the reflective semiconductor optical amplifier, progress in a reverse direction, are multiplexed in the first WDM filter  207 , and then are transmitted to the remote node through a transmission optical fiber. The wavelength channels are de-multiplexed and input to receivers  219 - 1 ,  219 - 2 ,  220 - 1 , and  220 - 1  while undergoing the same scheme in the second WDM filter  212 .  
         [0051]     A multiplexed downstream optical signal having the injection light of the first broadband light source  210  is delivered to the third interleaver  214  from the second WDM filter  212  through a transmission optical fiber.  
         [0052]     In addition, a downstream optical signal output to an even port of the third interleaver  214  is divided into channels according to its wavelength(s) and output in the fourth MUX/DE-MUX  215  connected to the third interleaver  214 . Since the downstream optical signal is input to the second port of the fourth MUX/DE-MUX  215 , the downstream optical signal is output as odd channels and input to an optical receiver  219 - 1  of the odd channels. In this case, a first bandpass filter  217 - 1  passing the first band wavelengths is installed at a front side of the optical receiver  219 - 1  so as to prevent cross-talk.  
         [0053]     In addition, a downstream optical signal output through an odd port of the third interleaver  214  is divided into channels according to its wavelength(s) and output in the third MUX/de-MUX  216  connected to the third interleaver  214 . Since the downstream optical signal is input to the first port of the third MUX/de-MUX  216 , the downstream optical signal is output as odd channels and input to an optical receiver  220 - 1  of the odd channels. In this case, a first bandpass filter  218 - 1  passing the first band wavelengths is installed at a front side of the optical receiver  220 - 1  to prevent cross-talk.  
         [0054]     Meanwhile, a downstream optical signal output through an odd port of the fourth interleaver  213  is divided into channels according to its wavelength(s) and output in the third MUX/de-MUX  216  connected to the fourth interleaver  213 . However, since the downstream optical signal is input to the second port of the third MUX/de-MUX  216 , the downstream optical signal is output as even channels and input to an optical receiver  220 - 2  of the even channels. In this case, a second bandpass filter  218 - 2  passing the second band wavelengths is installed at a front side of the optical receiver  220 - 2  so as to prevent cross-talk.  
         [0055]     Furthermore, a downstream optical signal output to an even port of the fourth interleaver  213  is divided into channels according to its wavelength(s) and output in the fourth MUX/de-MUX  215  connected to the fourth interleaver  213 . Since the downstream optical signal is input to the first port of the fourth MUX/de-MUX  215 , the downstream optical signal is output as even channels and input to an optical receiver  219 - 2  of the even channels. In this case, a second bandpass filter  217 - 2  passing the second band wavelengths is installed at a front side of the optical receiver  219 - 2  to prevent cross-talk.  
         [0056]     As described above, it is possible to efficiently prevent cross-talk in optical signals of adjacent channels by fixing the adjacent channels using injection light in different bands. In other words, even though channels are adjacent to each other, wavelengths of adjacent channels belong to mutually different wavelength bands. Accordingly, even though wavelengths are incompletely aligned in a MUX/de-MUX, it is possible to prevent light of the adjacent channels from being received to receivers by employing bandpass filters.  
         [0057]      FIG. 2B  illustrates broadband light sources having mutually different bands in a downstream transmission structure of the WDM-PON using an external light-injected light source according to an embodiment of the present invention.  
         [0058]     As shown in  FIG. 2B , according to an embodiment of the present invention, the light source  112  having a first band and the light source  113  having a second band are separated from each other by a free spectral range (FSR). The first band and the second band include odd channel signals and even channel signals, respectively.  
         [0059]      FIG. 3A  is a block diagram illustrating an upstream and downstream transmission structure of the WDM-PON using externally light-injected light sources according to an embodiment of the present invention.  
         [0060]     The operation of the upstream and downstream transmission structure of the WDM-PON shown in  FIG. 3A  is identical to those of the upstream transmission structure shown in  FIG. 1A  and the downstream transmission structure shown in  FIG. 2A  except that the upstream and the downstream transmission structure of the WDM-PON includes bi-directional transceivers  301 - 1  to  301 - 4 ,  302 - 1  to  302 - 4 ,  320 - 1  to  320 - 4 , and  321 - 1  to  321 - 4 , and upstream and downstream injection light is input to a transmission optical fiber using a directional coupler  308  instead of circulators  110  and  208 .  
         [0061]     According to an embodiment of the present invention, as shown in  FIG. 3B , a first broadband light source  310  and a second broadband light source  311  for upstream transmission are separated from each other by a free spectral range (FSR), a first broadband light source  313  and a second broadband light source  314  for downstream transmission are separated from each other by FSR, and upstream and downstream bands are separated from each other by an integer multiple of the FSR.  
         [0062]      FIG. 4  is a block diagram illustrating the structure of the bi-directional transceiver shown in  FIG. 3A .  
         [0063]     As shown in  FIG. 4 , the bi-directional transceiver shown in  FIG. 3A  includes a receiver  42  and a light-injected light source  41  and connects the receiver  42  to the light-injected light source  41  by means of a WDM filter  413 .  
         [0064]     According to the present invention, a bandpass filter is further included at a front side of the receiver in order to prevent cross-talk.  
         [0065]     As described above, according to the present invention, a wavelength division multiplexed passive optical network (WDM-PON) is described to effectively prevent cross-talk due to incomplete alignment of wavelength channels in a MUX/DE-MUX between a central office and a remote node.  
         [0066]     In addition, according to the present invention, it is unnecessary to align wavelengths in the MUX/de-MUX, and it is possible to make conditions for the wavelength alignment easier to realize an economical WDM-PON.  
         [0067]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. Consequently, the scope of the invention is not limited to the embodiments described herein, but is to be defined by the appended claims and equivalents thereof.