Abstract:
A method and apparatus for providing a supervisory channel in a wavelength division multiplexing (WDM) fiber-optic communication system uses a controlled optical attenuator disposed in an optical path between a demultiplexer (DMUX) and a multiplexer (MUX) of an Optical Add-Drop Multiplexer (OADM.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the field of fiber-optic communication systems and, in particular, to a method and apparatus for a supervisory channel in a fiber-optic communication system.  
       BACKGROUND OF THE INVENTION  
       [0002]     Advanced fiber-optic communication systems using a wavelength division multiplexing (WDM) technique typically require supervisory channels (SUPVYs) to facilitate exchange of service information between the nodes of the system. Conventionally, injection of a SUPVY in the WDM system is associated with a generation of a dedicated SUPVY transmission channel outside of the WDM band, incorporation of the SUPVY data in a payload WDM channel, or additional low frequency modulation of the a payload WDM channel with the data of the SUPVY.  
         [0003]     However, such means of providing the SUPVY add to already high cost of the fiber-optic components and complexity of modulation and demodulation schemes used in the payload channels of the WDM systems.  
       SUMMARY OF THE INVENTION  
       [0004]     Various deficiencies of the prior art are addressed by the present invention of a method and apparatus for providing a supervisory channel (SUPVY) in a WDM fiber-optic communication system.  
         [0005]     One aspect of the invention is a method for providing a SUPVY in the WDM fiber-optic communication system. According to this method, the SUPVY is formed by modulating attenuation of a variable optical attenuator disposed in an optical path between a demultiplexer (DMUX) and a multiplexer (MUX) of a downstream Optical Add-Drop Multiplexer (OADM) of the system. Information transmitted by the SUPVY is extracted and selectively demodulated in an OADM of the downstream node, thus facilitating exchange of service-related data between adjacent nodes of the WDM system.  
         [0006]     In other embodiments of the invention are disclosed the apparatuses and communication systems suitable for executing the inventive method. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0008]      FIG. 1  depicts a high-level schematic diagram of a WDM fiber-optic communication system suitable for use with the present invention;  
         [0009]      FIG. 2  depicts a high-level schematic diagram of a section of a node of the WDM fiber-optic communication system of  FIG. 1  according to one embodiment of the present invention; and  
         [0010]      FIG. 3  depicts a flow diagram of a method for providing a supervisory channel (SUPVY) in the WDM fiber-optic communication system of  FIG. 1  according to one embodiment of the present invention. 
     
    
       [0011]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
         [0012]     It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The present invention will be generally described within the context of a long-haul terrestrial WDM fiber-optic communication system. It will be appreciated by those skilled in the art that the invention may also be utilized within the context of undersea WDM fiber-optic communication systems and short-haul WDM fiber-optic communication networks.  
         [0014]     Hereafter, similar devices, transmission paths, and communication channels are identified using the same numeric and/or alphabetic references, except that the suffixes may be added, when appropriate, to differentiate between the specific devices, paths, and channels.  
         [0015]      FIG. 1  depicts a high-level schematic diagram of a WDM fiber-optic communication system suitable for use with the present invention. Specifically, the system  100  of  FIG. 1  comprises a plurality of N nodes  102 , where N is an integer greater than 1. The nodes  102  are coupled to one another using fiber-optic links  104  and  108 . Each of the nodes  102  may optionally be coupled, using links  106  and  114 , to a node of another communication system (not shown).  
         [0016]     Generally, the links  104  and  108  are reduced to practice in a form of one or more fiber-optic cables. A node  102  typically comprises amplifying, transmitting, and receiving units for the payload WDM channels, as well as computing means that, in operation, facilitate functioning of the node. In the depicted embodiment, the system  100  is illustratively shown as an open chain of the nodes  102 . In another embodiment, the system  100  may comprise the nodes  102  forming at least one branch and/or at least one closed loop.  
         [0017]     In one embodiment, each of the links  104  and  108  comprises at least one optical fiber (e.g., single-mode optical fiber) and propagates a plurality of M spectrally separated optical transmission channels (i.e., WDM channels), where M is, illustratively,  32 - 128 . Additionally, each of the links  104  and  108  carries a supervisory channel (SUPVY)  110  and  112 , respectively (shown using broken lines).  
         [0018]     The SUPVYs  110   K  and  112   K  are used to provide a local information exchange between adjacent nodes  102   K  and  102   K+1 , of the system  100 , where K&lt;N. Typically, the SUPVY transmits information related to an operational status of the nodes and equipment associated with the nodes, signal levels in the optical transmission path between the nodes, and the like. For example, in a two node system (i.e., N=2, K=1), only nodes  102   1  and  102   2  are in communication.  
         [0019]      FIG. 2  depicts a high-level schematic diagram of a section of a node (e.g., a node  102   K ) of the WDM fiber-optic communication system of  FIG. 1 . For better understanding of this embodiment of the invention, the reader should refer simultaneously to  FIGS. 1 and 2 .  
         [0020]     Specifically, the section  200  may be associated with one of optical fibers in the link  104  or  108 . In one embodiment, the section  200  of  FIG. 2  comprises a controller  240 , a fiber-optic module  202 , transmitting modules  206  of added WDM channels, and receiving modules  206  of dropped WDM channels.  
         [0021]     In operation, the controller  240  (e.g., a general purpose computer associated with the corresponding input/output devices) monitors performance and transmission properties of components of section  200 , as well as facilitates execution of pre-determined communication algorithms and programs.  
         [0022]     In the depicted embodiment the controller  240  illustratively comprises a modulator  242  and a demodulator  260  of the SUPVY of the node  102   K  (e.g., SUPVY  110   K ) of the system  100  of  FIG. 1 . Alternatively, the modulator  242  and demodulator  260  may be stand-alone devices functioning under control of the controller  240 .  
         [0023]     Referring to  FIGS. 1 and 2 , an input  218  of the optical module  102   K  is coupled to an output of an optical fiber propagating from the upstream node  102   K−1  of the system  100 . Accordingly, an output  224  of the module  102   K  is coupled to an input of the optical fiber propagating to the downstream node  102   K+1 . These couplings, as well as other interconnections between optical components within the section  200  may be performed using optical connectors, fiber splicing techniques, or techniques employing optical splitters, blockers and couplers.  
         [0024]     In the depicted embodiment, the module  202  includes an input unit  246 , an output unit  248 , and an Optical Add-Drop Multiplexer (OADM)  204 . In another embodiment, the units  246  and  248  may be portions of the OADM  204 .  
         [0025]     The input unit  246  generally comprises a tap coupler (TP)  212  directing a small portion of an optical power of an incoming WDM signal to a photo detector  214 , and an input optic amplifier (OA)  210  (e.g., erbium-doped OA) that, using a feedback signal of an input power monitor  216  or other criteria, is controlled by the controller  240 . In alternate embodiments, the photo detector  214  may be an integral part of the coupler  212  or the input power monitor  216 . In operation, the OA  210  amplifies the incoming WDM signal to compensate for losses of optical power in the transmission span between the upstream node  102   K−1  and the node  102   K .  
         [0026]     The output unit  248  includes a booster OA  220  and a power monitor  222 , which is further coupled (not shown) to the controller  240 . In operation, the OA  220  compensates for losses of optical power in components of the OADM  204  and sets a pre-determined level of the optical power of the WDM signal at the output  224  of the module  202 .  
         [0027]     The OADM  204  generally comprises a demultiplexer (DMUX)  238  of the DWM channels; couplers  230  and  232 , blockers  226 , variable optical attenuators (Vs)  236  and tapped couplers  254  of B through channels; variable optical attenuators  234  and tapped couplers  256  of A (or less) added channels; and a multiplexer (MUX)  244  of the DWM channels. Herein, A and B are integers, and A+B≦M.  
         [0028]     In alternate embodiments, at least a portion of the attenuators  234  and corresponding couplers  256  may be associated with the transmitting module  206 .  
         [0029]     Through outputs of the couplers  230  are connected to inputs of blockers  226 , tap output of the couplers  230  are connected to inputs  250  of the receiving modules  208 , while through outputs of the couplers  232  are connected to optical inputs of the respective variable attenuators  236 . The blockers  236  are generally electrically controlled pass/block optical attenuators. In a “through” state of the OADM  204 , a blocker is set to minimum attenuation to allow propagation of the respective DWM channel. Accordingly, in the “add/drop” state, the blocker is set to maximum attenuation to restrict propagation of that channel, while the channel receiver and transmitter are activated for accessing and/or transmitting the data, respectively.  
         [0030]     The receiving module  208  may either transfer, using a link  114  of  FIG. 1 , the dropped WDM channel to a node of another fiber-optic system (not shown) or convert an information content of that channel in an electrical domain compatible with an attached wired or wireless communication network.  
         [0031]     The transmitting module  206  typically comprises a means (not shown) for converting information to be transmitted by the channels of such wired or wireless communication network from the electrical domain to the optical domain, thus forming the added WDM channel(s) of the system  100  of  FIG. 1 . Outputs  252  of the transmitting module  206  containing the added WDM channels are coupled to the optical inputs of the respective variable attenuators  234 .  
         [0032]     In another embodiment, the transmitting module  206  may also include adapters (not shown) for receiving, via the link  106  of  FIG. 1 , and passing through DWM channels of another fiber-optic system (not shown).  
         [0033]     Optical outputs of the attenuators  234  and  236  are coupled to the corresponding inputs of the MUX  244  via the tapped couplers  256  and  254 , respectively. Each of the couplers  256  and  254  directs a small portion of an optical power of the respective DWM channel to a photo detector acting as a power monitor (both not shown) that is further coupled (not shown) to the controller  240 .  
         [0034]     In one embodiment, the variable optical attenuators  234  and  236  are electrically-controlled attenuators having, with respect to a control signal, an electrical bandwidth of about 1 MHz or greater. Such variable attenuators, as well as other optical components of the section  200  are commercially available.  
         [0035]     The attenuators  234  and  236  are controlled by the controller  240  in a manner facilitating equalization of an optical power between the DWM channels. In the present invention, attenuation of at least one of the attenuators  234  and  236  (illustratively, attenuator  254   B ) is modulated by a modulator  242  of a SUPVY (e.g., SUPVY  108   K ) that superimposes low-frequency data of the SUPVY upon the high bit rate (e.g., 2.5-40 Gbs) payload information of the corresponding WDM channel. Preferably, such an attenuator is selected from plurality of the attenuators  234  of the through WDM channels of the OADM  204 . Due to the small modulation depth of the low frequency SUPVY data, the data transmission of the channel is not or only minimally impaired.  
         [0036]     Specifically, such an attenuator is provided with a control signal that is equal to a sum of a modulating signal of the SUPVY and a setting for a power equalizing attenuation for the WDM channel. The modulator  242  may utilize various modulation formats, e.g., an amplitude modulation format, a frequency modulation format, and/or a phase modulation format in a manner that an electrical bandwidth of the modulating signal is equal to or smaller than an electrical bandwidth of the attenuator being modulated.  
         [0037]     An output signal of the modulated attenuator is a WDM channel carrying both the payload information and the data of the SUPVY. In the MUX  244 , that WDM channel is combined with other WDM channels of the transmission fiber. Then, the combined optical signal is amplified by the booster OA  220  and transmitted (e.g., via the link  104   K ) from the module  200  of a node  102   K  to the downstream node  102   K+1  of the system  100  of  FIG. 1 .  
         [0038]     In the downstream node  102   K+1 , the transmission fiber of the link  104   K  that carries the optical signal comprising the WDM channel with the superimposed data of the SUPVY is coupled to an input module  246  of the corresponding module  202  of the upstream node. A portion of that signal is tapped by an input coupler  212  and then detected, demodulated, and analyzed by the controller of that module  202 , thus forming the SUPVY between the upstream-node  102   K  and the downstream node  102   K+1  of the system  100 .  
         [0039]     In one embodiment, the modulator  242  of the upstream node and the demodulator of downstream node are selectively modified and/or configured to facilitate independent SUPVYs between the nodes of the system  100  and to limit a length of each SUPVY to a distance between the adjacent downstream and upstream nodes. Specifically, at least one modulating property of the modulator  242  (e.g., a modulation format, a carrier frequency of the modulating signal, and the like) of the downstream node and a configuration of the demodulator  260  of the upstream node may be chosen such that only the SUPVY between such nodes is selectively detected and/or demodulated in the upstream node, while signals corresponding to SUPVYs between other nodes are suppressed at the demodulator.  
         [0040]     In one embodiment, that is accomplished by selecting a carrier frequency of SUPVY of the downstream node and configuring the demodulator of the upstream node to selective demodulate the SUPVY having that carrying frequency to suppress other SUPVYs, as well as cross-channel talks between SUPVYs in the system  100 .  
         [0041]      FIG. 3  depicts a flow diagram of a method for providing a SUPVY in the WDM fiber-optic communication system of  FIG. 1 . Specifically, the method of  FIG. 3  contemplated several system functions suitable for use in accordance with the present invention.  
         [0042]     The method of  FIG. 3  starts at step  310 , where the OADMs  204  having the coupler  212  and at least one variable optical attenuator  236  are provided within the nodes  102  of system  100  of  FIG. 1 .  
         [0043]     At step  320 , the optical attenuators  236  are selectively modulated using the modulators  242  with the data of the SUPVYs between the respective downstream and upstream nodes of the system  100 .  
         [0044]     Referring to a box  315 , the modulator  242  applies to the attenuator  236  a sum of the modulating signal of the SUPVY and a setting for attenuating optical power of the WDM channel and properties of the modulating signal are selectively controlled to suppress cross-channel talks between the SUPVYs of the system  100 .  
         [0045]     At step  330 , the data content of the SUPVY established between the upstream and downstream nodes is selectively demodulated by the demodulator  260  of the downstream node.  
         [0046]     Referring to a box  325 , the demodulator  260  of the downstream node may choose a SUPVY channel created by an arbitrary upstream node by selectively demodulating the information entered in that upstream node of the system  100 .  
         [0047]     While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.