Abstract:
Provided are a photonic cross-connector system, a wavelength division multiplexing (WDM) system using the photonic cross-connector system, and an optical communication network based on the WDM system. The photonic cross-connector system includes: an optical coupler branching an input optical signal into a plurality of paths; a wavelength selective switch (WSS) extracting at least one wavelength signal from the input optical signal and outputting the extracted wavelength signal to at least one port; a WDM multi-casting apparatus simultaneously copying and reproducing the input optical signal into different wavelengths and changing modulation methods of the input optical signal into different types of modulation methods; an optical transmitter and/or receiver branching and coupling the input optical signal; and a control system controlling the optical coupler, the WSS, the WDM multicasting apparatus, and the optical transmitter and/or receiver.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2006-0096456, filed on Sep. 29, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical communication, and more particularly, a photonic cross-connector system connecting paths of wavelength division multiplexing (WDM) signals, a WDM system using the photonic cross-connector system, and an optical communication network based on the WDM system. 
         [0004]    2. Description of the Related Art 
         [0005]    Recent information communication user environments have been suddenly ubiquitous. Thus, information society-based networks have been developed from existing fixed forms to variable forms. In addition, inventions of network node apparatuses have been increased to simply install, operate, and maintain network apparatuses. These apparatuses may be realized through a minimization of an optical-to-electric-to-optical transformation, efficiency of routing and wavelength allocation, optical switching and branching/coupling of channels of a control system. 
         [0006]      FIG. 1  is schematic view illustrating a wavelength division multiplexing (WDM) optical communication network. Referring to  FIG. 1 , the WDM optical communication network may include a plurality of networks having different physical characteristics such as a used wavelength and a modulation method, i.e., a network A  10  and a network B  20 . A node used inside a network having the same physical characteristic, i.e., a photonic cross-connector system, requires a switching function of transmitting signals input from different ports to a desired output port. When a signal of a network having a different physical characteristic, i.e., the network A  10 , is connected to the network B  20 , a node at a boundary between the networks A and B  10  and  20  requires a wavelength transformation, an optical reproduction, a change of a modulation method, etc. A node  8  shown in  FIG. 1  must perform such a function. 
         [0007]    There is increased a demand for a multicasting method by which a signal generated in a departure is simultaneously transmitted to a plurality of destinations with the arrival of an Internet protocol television (IP-TV), a teleconference, a virtual private network (VPN), etc. In the multicasting method, a plurality of signals are not generated in a departure and not transmitted to a plurality of destinations. However, a signal is generated and then transmitted a predetermined distance through a common path and diverges from a divergence node to each destination. Thus, traffic of the optical communication network can be reduced. 
         [0008]    For example, when a signal generated by a node  1  of the network A  10  is to be simultaneously transmitted to nodes  6  and  7 , the signal is first transmitted through nodes  1 ,  4 , and  5  and then copied and diverged from the node  5  into the nodes  6  and  7 . Since an existing optical communication network through which an IP signal is transmitted does not provide a switching function, such multicasting is performed by an electrical IP router. Thus, a minimization of an optical-to-electric-to-optical transformation and efficiency of routing and wavelength allocation cannot be realized. 
         [0009]    There is suggested a photonic cross-connector system which divides signals input through an optical fiber according to each wavelength, transmits the same wavelength of various wavelengths divided from different optical fibers to a switch, switches the wavelength according to each path, and transmits the wavelength. However, such a photonic cross-connector system does not perform an optical multicasting function and a modulation method conversion function. There is suggested a photonic cross-connector system which realizes a switch switching the same wavelengths to different paths to support a multicasting function with a split and delivery (SaD) including a plurality of optical couplers, a gate optical switch, and a 1:N optical switch. However, the photonic cross-connector system uses optical couplers, wherein the number of optical couplers is equal to the number of wavelengths. Thus, loss is very large, and a large number of optical switches must be used. Also, like the previously described photonic cross-connector system, the present photonic cross-connector system does not provide a function of multicasting to different wavelengths and a function of transforming a modulation method. 
         [0010]    There is suggested a photonic cross-connector system which divides an input signal to two paths using a coupler, connects the one of the two paths to a drop path and the other one to a transmission path, and disallows the signal to be transmitted to the transmission path using a wavelength blocker or simultaneously transmits the signal to the drop and transmission paths so as to perform a multicasting function. However, the photonic cross-connector system limits each of input and output ports and does not provide a function of multicasting to different wavelengths and a function of transforming a modulation method. 
         [0011]    An optical signal is frequently deteriorated during its transmission due to a chromatic dispersion of an optical fiber, a polarization mode dispersion, noise of an optical amplifier, etc. Thus, each node, i.e., a photonic cross-connector system, requires a function of regenerating a signal, transforming the signal into a clear signal, and transmitting the clear signal to a destination port. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a photonic cross-connector system capable of performing optical multicasting to switch a wavelength from a random input port to an output port or branch or copy a wavelength from an input port to a plurality of output ports and regenerating a deteriorated signal into a clear signal. 
         [0013]    The present invention also provides a wavelength division multiplexing (WDM) system using the photonic cross-connector system to regenerate a deteriorated signal so as to generate a signal from which noise has been removed, transforming a wavelength to prevent a collision between different wavelengths so as to increase network efficiency, and changing a modulation method without a photoelectric transformation when different networks are connected to each other, and an optical communication network based on the WDM system. 
         [0014]    According to an aspect of the present invention, there is provided a photonic cross-connector system including: an optical coupler branching an input optical signal into a plurality of paths; a wavelength selective switch (WSS) extracting at least one wavelength signal from the input optical signal and outputting the extracted wavelength signal to at least one port; a WDM multicasting apparatus simultaneously copying and reproducing the input optical signal into different wavelengths and changing modulation methods of the input optical signal into different types of modulation methods; an optical transmitter and/or receiver branching and coupling the input optical signal; and a control system controlling the optical coupler, the WSS, the WDM multicasting apparatus, and the optical transmitter and/or receiver. 
         [0015]    According to another aspect of the present invention, there is provided a WDM system using the photonic cross-connector system. 
         [0016]    According to another aspect of the present invention, there is provided a WDM optical communication network formed based on the WDM system. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0018]      FIG. 1  is a schematic view illustrating a wavelength division multiplexing (WDM) optical network; 
           [0019]      FIGS. 2A and 2B  are block diagrams illustrating switching functions of photonic cross-connector systems according to embodiments of the present invention; 
           [0020]      FIGS. 3A through 3D  are block diagrams illustrating optical multicasting functions of photonic cross-connector systems according to embodiments of the present invention; 
           [0021]      FIG. 4  is a block diagram illustrating a photonic cross-connector system having an optical multicasting function according to an embodiment of the present invention; 
           [0022]      FIG. 5  is a detailed block diagram illustrating a WDM multicasting apparatus illustrated in  FIG. 4 ; 
           [0023]      FIG. 6  is a detailed block diagram illustrating a multi-wavelength transforming and optical regenerating apparatus illustrated in  FIG. 5 ; 
           [0024]      FIGS. 7A through 7E  are graphs illustrating phase shift patterns and input and output waveforms generated by a Mach-Zehnder interference semiconductor optical amplifier (SOA) of the multi-wavelength transforming and optical regenerating apparatus illustrated in  FIG. 6 ; and 
           [0025]      FIGS. 8A through 8C  are graphs illustrating waveforms which are copied to a plurality of different wavelengths and of which modulation methods have been transformed through the multi-wavelength transforming and optical regenerating apparatus of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
         [0027]      FIGS. 2A and 2B , and  FIGS. 3A through 3D  are block diagrams and a graph illustrating basic functions of photonic cross-connector systems according to embodiments of the present invention.  FIGS. 2A through 3D  are introduced to simply describe a basic structure of a photonic cross-connector system of the present invention before the detailed structure of the photonic cross-connector system is described. 
         [0028]      FIGS. 2A and 2B  are block diagrams illustrating switching functions of photonic cross-connector systems according to embodiments of the present invention. Here, the switching functions are the most basic functions of the photonic cross-connector systems used in a network. 
         [0029]    Referring to  FIG. 2A , a network node  30 , i.e., a photonic cross-connector system, switches a signal of a first wavelength λ 1  input from a south port to a west port and a signal of a second wavelength λ 2  input from a north port to an east port. 
         [0030]    Referring to  FIG. 2B , a network node  30   a  switches a signal of a first wavelength λ 1  input from a south port to a west port and transforms the first wavelength λ 1 . In other words, the network node  30   a  transforms the first wavelength λ 1  into a third wavelength λ 3  and then outputs the signal of the third wavelength λ 3  to the west part. In this case, the first wavelength λ 1  has been already used in a path of the south port. Thus, the photonic cross-connector system requires such a wavelength transformation function to prevent a wavelength collision. 
         [0031]      FIGS. 3A through 3D  are block diagrams and a graph illustrating optical multicasting functions of photonic cross-connector systems according to embodiments of the present invention. Such a multicasting function contributes to reducing traffic in a network or between different networks so as to further efficiently transmit a signal. 
         [0032]    Referring to  FIG. 3A , a network node  40 , i.e., a photonic cross-connector system, copies an optical signal of a first wavelength λ 1  input from a south port into a plurality of identical wavelengths, i.e., a plurality of first wavelengths λ 1 , and then outputs the plurality of wavelengths to different destination ports such as east, west, and north ports. 
         [0033]    Referring to  FIG. 3B , a network node  40   a  copies a plurality of identical wavelengths to a first wavelength λ 1  input from a south port and transforms a plurality of identical wavelengths into a plurality of different wavelengths, i.e., first, second, and third wavelengths λ 1 , λ 2 , and λ 3 , and then outputs the first, second, and third wavelengths λ 1 , λ 2 , and λ 3 . 
         [0034]    Referring to  FIG. 3C , a network node  40   b  reproduces a signal, which has been deteriorated during its transmission, as a clear signal and then outputs the clear signal to a destination port. In other words, an optical signal is generally deteriorated due to a chromatic dispersion of an optical fiber, a polarization mode dispersion, noise of an optical amplifier, etc. As a result, a photonic cross-connector system requires a function of regenerating a deteriorated optical signal as a clear signal. 
         [0035]      FIG. 3D  is a graph illustrating two methods of modulating a signal. An upper graph A illustrates a return-to-zero (RZ) modulation method, and a lower graph B illustrates a Non-Return-to-Zero (NRZ) modulation method. In general, networks having different physical characteristics, i.e., the network A  10  and the network B  20  shown in  FIG. 1 , use different modulation methods. A photonic cross-connector system must perform a modulation method transformation function for an optical transmission performed between the networks. In addition, the photonic cross-connector system requires a wavelength transformation function or an optical regeneration function of a new signal as illustrated in  FIG. 3B  or  3 C. For example, the node  8  shown in  FIG. 1  must perform these functions. 
         [0036]    A photonic cross-connector system of the present invention must perform functions illustrated in  FIGS. 2A through 3D  and will now be described in detail. 
         [0037]      FIG. 4  is a block diagram illustrating a photonic cross-connector system having an optical multicasting function according to an embodiment of the present invention. Referring to  FIG. 4 , the photonic cross-connector system includes optical couplers  100 , wavelength selective switches (WSSs), WDM multicasting apparatuses  300 , optical transmitters and/or receivers  400 , and a control system  500 . Each of the optical couplers  100  branches an input optical signal into a plurality of paths. Each of the WSSs extracts at least one wavelength signal from an input signal and transmits the at least one wavelength signal to at least one port. Each of the WDM multicasting apparatuses  300  simultaneously copies and regenerates an input signal as different wavelength signals and transforms the different wavelengths signals using different modulation methods. The optical transmitters and/or receivers  400  branch and couple signals. The control system  500  controls the optical couplers, the WSSs, the WDM multicasting apparatuses  300 , and the optical transmitters and/or receivers  400 . 
         [0038]    The photonic cross-connector system of the present embodiment has a function of switching a signal input from an input port to three different destination ports. The function of the photonic cross-connector system will now be described in brief. A WDM signal input from a north input port branches into three paths through the optical coupler  100  and then transmitted to the north branching WSS  200   a , the west coupling WSS  230   b , and the east coupling WSS  220   b . The signal to be transmitted to the north branching WSS  200   a  is transmitted to the north branching WSS  200   a  through the WDM multicasting apparatus  300 . 
         [0039]    Here, a WSS is an apparatus which selects a random wavelength and transmits the random wavelength to a random port, i.e., transmits a single wavelength or a plurality of wavelengths to a port. Also, the WSS may adjust an intensity of an input signal according to each wavelength and determine blocking and a transmission of a signal. Thus, the WSS may branch or couple a signal to a transmitter and/or receiver and transmit the signal to a desired WSS. 
         [0040]    The north branching WSS  200   a  branches a wavelength to a branching receiver according to a command of the control system  500  and transmits other channel signals to the south coupling WSS  210   b , the west coupling WSS  230   b , and the east coupling WSS  220   b  according to predetermined destinations. The south coupling WSS  210   b  transmits only signals of a signal connected to a coupling transmitter, a signal connected to the north branching WSS  220   a  through a wavelength transformation or a simple branch and/or copy, and a signal connected to the east and west input couplers  100  through a simple branch and/or copy, and a signal connected to the east branching WSS  220   a  through a multi-wavelength copy or an optica regeneration I to output ports according to a command of the control system  500 , wherein the only signals are set to be output to a south direction, and blocks the other signals. 
         [0041]    A process of inputting a signal from a north input port and then transmitting the signal to a south output port has been described in brief. However, a signal may be input from any one of east, west, south, and north directions and output to each of the east, west, south, and north directions according to the above-described principle. In other words, a signal may be input from any direction, and a signal or a plurality of signals may be output to a specific port or to a plurality of ports. The coupling WSSs  200   b ,  210   b ,  220   b , and  230   b  transmit only set signals to output ports, respectively, and block other signals during an output to each port. 
         [0042]    In the photonic cross-connector system of the present embodiment, each of the optical couplers  100  performs a multicasting function of simultaneously copying and/or branching an identical wavelength as illustrated in  FIG. 3A . Each of the WDM multicasting apparatuses  300  and the branching WSSs  200   a ,  210   a ,  220   a , and  230   a  performs a multicasting function of simultaneously copying and/or branching a wavelength into a plurality of different wavelengths as illustrated in  FIG. 3B . 
         [0043]      FIG. 5  is a detailed block diagram illustrating the WDM multicasting apparatus  300  illustrated in  FIG. 4 . Referring to  FIG. 5 , the WDM multicasting apparatus  300  includes an input WSS  310 , an output WSS  320 , and at least one or more multi-wavelength transforming and optical regenerating apparatuses  330 . The WDM multicasting apparatus  300  transmits a signal, which does not need to be wavelength transformed or optical regenerated, from the input WSS  310  to the output WSS  320  but switches a signal, which is required to be wavelength transformed or optical regenerated, to a path to which the multi-wavelength transforming and optical reproducing apparatuses  330  are connected. Each of the multi-wavelength transforming and optical regenerating apparatuses  330  copies an input wavelength into a wavelength or a plurality of different wavelengths and outputs the wavelength or the plurality of different wavelengths to the output WSS  320  and has optical regenerating and modulation method transforming functions. 
         [0044]      FIG. 6  is a block diagram illustrating the multi-wavelength transforming and optical regenerating apparatus  330  illustrated in  FIG. 5 . Referring to  FIG. 6 , the multi-wavelength transforming and optical regenerating apparatus  330  includes an optical coupler  331 , a time delayer  332 , WDM light sources  333 , a multiplexer  334 , and a Mach-Zehnder interference semiconductor optical amplifier (SOA)  335 . The optical coupler  331  splits an input signal. The time delayer  332  delays one of the signals split by the optical coupler  331  for a predetermined period of time and outputs the signal. The WDM light sources  333  generate a plurality of wavelength signals. The multiplexer  334  combines the plurality of wavelength signals output from the WDM light sources  333 . The Mach-Zehnder interference SOA  335  receives the split input signals and a signal output from the multiplexer  334 . Here, the Mach-Zehnder interference SOA  335  has a dual-arm structure in which two arms shift phases. 
         [0045]    A function of the multi-wavelength transforming and optical reproducing apparatus  330  will now be described in brief. A signal of a single wavelength λ i#1  is divided into two signals through the optical coupler  331 , and then one of the two signals is input to a Mach-Zehnder interferometer SOA  335 - 1 , and the other one is input to a Mach-Zehnder interferometer SOA  335 - 2  through the time delayer  332 . A signal of the WDM light source  333  having a plurality of wavelengths λ in#1  through λ in#n  are combined into a path using the multiplexer  334  and input to the Mach-Zehnder interferometer SOAs  335 - 1  and  335 - 2  through an optical attenuator  336  and a Mach-Zehnder interference central path. Here, a signal output from the WDM light sources  333  has a continuous wave form or a pulse form. 
         [0046]    The signal of the single wavelength λ i#1  input to the Mach-Zehnder interferometer SOA  335  changes a carrier density in the Mach-Zehnder interferometer SOA  335  so as to induce phase shifts Φ of the plurality of wavelengths λ in#1  through λ in#n  input from the WDM light sources  333 . Since a difference of phase shifts between wavelengths is small in the Mach-Zehnder interferometer SOA  335 , the same phase shift may be induced to a plurality of wavelengths. The signal having passed the time delayer  332  is delayed for a delay time τ to induce the phase shifts Φ of the plurality of wavelengths λ in#1  through λ in#n . The signal of the single wavelength λ i#1  is removed using a filter or the like, and thus a signal having a plurality of wavelengths λo #1  through λo #n  is output as an output signal. 
         [0047]      FIGS. 7A through 7E  are graphs illustrating phase shift patterns and input and output waveforms generated by the Mach-Zehnder interferometer SOA  335  of the multi-wavelength transforming and optical reproducing apparatus  330  illustrated in  FIG. 6 . 
         [0048]      FIG. 7A  is a graph illustrating a pulse form wavelength generated by the WDM light source  333 . Here, a horizontal axis denotes a time, and a vertical axis denotes an input optical power P in .  FIG. 7   b  is a graph illustrating a signal of which phase has been shifted through a Mach-Zehnder interferometer. Here, a horizontal axis denotes a time, and a vertical axis denotes a phase Φ. 
         [0049]      FIG. 7C  is a graph illustrating fast and slow phase shift parts of a signal which have been shifted through a Mach-Zehnder interferometer. Here, the fast and slow phase shift parts are denoted by Φ fast  and Φ slow . The slow phase shift part Φ slow  is shifted with a delay time τ to the fast phase shift part Φ fast . Signals of which phases are shifted at different delay times are combined at an output port of a Mach-Zehnder interferometer and destructively interfered. Thus, when an output is represented as an intensity, i.e., an optical power, a difference between phase shift patterns is reflected on the output. Here, a form of the output depends on the delay time τ. 
         [0050]      FIG. 7D  is a graph illustrating an output waveform Φ fast -Φ slow  when a delay time τ is smaller than a bit rate of an input digital signal. Here, a vertical axis denotes a unit of an optical power. If the delay time τ is smaller than the bit rate of the input digital signal, and an RZ modulation signal is input, the RZ modulation signal is output. 
         [0051]      FIG. 7E  is a graph illustrating an output waveform Φ fast -Φ slow  when a delay time τ is equal to a bit rate of an input signal. If an RZ modulation signal is input in this case, a signal, which has been transformed to an NRZ modulation method, is output. In other words, a modulation method of the input signal is changed. As a result, the delay time τ may vary to vary the output waveform Φ fast -Φ slow . 
         [0052]    A phase shift pattern is formed non-linearly to an intensity of an input signal with a constant threshold. Thus, the multi-wavelength transforming and optical regenerating apparatus  300  may reduce intensity noise of a digital signal. Also, if a signal of a WDM light source is a pulse, and a light source synchronizing with the pulse input to the WDM light source, timing noise may be reduced. 
         [0053]    A laser may be turned on and/or off to adjust a number of WDM light sources input to a Mach-Zehnder interferometer according to a command of the control system  500 . An optical power of a light source input to the Mach-Zehnder interference may be maintained constant using an optical attenuator. The process of using an RZ modulation method as it is or changing the RZ modulation method into an NRZ modulation method has been described with reference to  FIGS. 7A through 7E . However, this is only an example. Thus, the NRZ modulation method may be used as it is or may be changed in to the RZ modulation method. 
         [0054]      FIGS. 8A through 8C  are graphs illustrating waveforms which are copied to a plurality of different wavelengths and of which modulation methods are transformed through the multi-wavelength transforming and optical reproducing apparatus  330  of  FIG. 7 . 
         [0055]      FIG. 8A  is a graph illustrating an input wavelength signal which has an RZ modulation method and is simultaneously copied and output into a plurality of wavelengths.  FIG. 8B  is a graph illustrating an input wavelength signal of which RZ modulation method is transformed into an NRZ modulation method and which is simultaneously copied and output into a plurality of wavelengths.  FIG. 8C  is a graph illustrating an input wavelength signal which has an NRZ modulation method and is simultaneously copied and output into a plurality of wavelengths. 
         [0056]    As shown in  FIGS. 8A through 8C , output signals having more considerably reduced noises than input signals are generated. In other words, a multi-wavelength transforming and optical regenerating apparatus reduces intensity noise from a digital signal. 
         [0057]    As described above, a photonic cross-connector system of the present invention can perform an optical multicasting function of switching a wavelength from an input port to an output port or branching or copying the wavelength from an input port into a plurality of output ports. The photonic cross-connector system can perform a function of regenerating a deteriorated signal to a clear signal. 
         [0058]    The photonic cross-connector system having such functions can be used in a WDM system, and an optical communication network including the WDM system can be established. Thus, an optical signal can be accurately transmitted, and a collision between different wavelengths can be prevented through a wavelength transformation to increase efficiency of the optical communication network. In addition, when networks having different modulation methods are connected to each other, the different modulation methods can be easily changed without a photoelectric transformation. Also, multicasting can be performed on an optical signal layer. Thus, a network apparatus can be easily installed, operated, and maintained compared to an existing optical communication network. 
         [0059]    As described above, a photonic cross-connector system according to the present invention can perform an optical multicasting function of switching a wavelength from an input port to an output port or branching or copying a wavelength from an input port to a plurality of output ports. The photonic cross-connector system can also regenerate a deteriorated signal into a clear signal. 
         [0060]    In addition, a WDM system and an optical communication network including the WDM system according to the present invention can use the photonic cross-connector system to accurately transmit an optical signal. Also, a collision between different wavelengths can be prevented through a wavelength transformation to increase efficiency of the optical communication network. In addition, when networks having different modulation methods are connected to each other, the different modulation methods can be easily changed without a photoelectric transformation. 
         [0061]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.