Abstract:
A system and method for securing DWDM networks that includes adding background (e.g., white) noise to a level up to the larger of the cross-talk signal or the background channel noise (e.g., amplified spontaneous emission level). The system comprises a white noise generator appropriately placed so that any leaked signals are masked by the white noise generated. Accordingly, only signals intended to be dropped are recoverable because all other signals would not be recoverable from the background white noise.

Description:
RELATED APPLICATIONS 
     The priority date for this Utility Patent Application is established by Provisional Patent Application No. 60/108,127, for which the filing date was Nov. 12, 1998 and the application was entitled “Method and System for Securing Wavelength Division Multiplex Systems”. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to Dense Wavelength Multiplex Systems (DWDMs) and specifically to securing information or channels in these systems. 
     BACKGROUND OF THE INVENTION 
     Dense Wavelength Division Multiplex (DWDM) technology has provided a cost-effective solution to fiber exhaust in communications networks by increasing the data throughput of the network without requiring the installation of new fiber and is the enabling technology for the emerging all optical networking. In a DWDM system each of several input signals enter a DWDM node or network element and is assigned or converted to a specific wavelength, typically, in the 1550 nanometer (nm) band. After wavelength conversion each individual signal wavelength or channel is then multiplexed by wavelength division multiplexing and transmitted onto the same fiber. Consequently, a single fiber carries more than one wavelength. In fact each wavelength carried by a DWDM system may be considered a virtual fiber. 
     In order for DWDM technology to be truly viable as a network solution, DWDM systems must be secure. As opposed to re al fibers, the signal carried on the virtual fibers of DWDM systems may be susceptible to eavesdropping. In DWDM systems different channels travel through the same fiber and the same components. As a result of cross-talk, nonlinearity, etc., at the receiving end, there is a residual of signal(s) from other channels that can be isolated, amplified, and detected. 
     The potential for eavesdropping may be better appreciated by reference to FIG.1 where there is depicted a receiving node  100  in a DWDM network. Receiving node  100  may be an optical demultiplexer or add drop multiplexer, a wavelength converter, or an optical cross-connect that serves as a drop off or interchange point for one or more channels. FIG. 2 shows, on a logarithmic scale, the optical spectrum of channel  10  in FIG. 1 as it dropped from node  100 . As FIG. 2 shows, although the goal was to drop only channel  10 , channel  11  is clearly visible. In FIG. 3, I used a notch filter to reduce the optical signal to noise ratio (OSNR) for channel  10 . As FIG. 3 shows, channel  11  is still present with enough power to be recoverable. In fact, in FIG. 4, I have turned off the channel  10  transmitter and as FIG. 4 shows there is a significant amount of residual power still present from channel  11 . I have also achieved similar results shown in FIG. 4 by introducing a second filter to attenuate channel  10  in the received spectrum. In either case, in FIG. 4, channel  11  is leaked with large enough OSNR to be recoverable after optical amplification. I have achieved better than 20 dB OSNR for the leaked signal for this particular optically amplified DWDM system. I expect better eavesdropping performance (larger OSNR than 20 dB for the leaked signal) for DWDM systems without the amplified stimulated emission (ASE) associated with the optical amplification process. Accordingly, the user of channel of  10  may be able to recover channel  11  without the network operator ever knowing of the breach in security. On another level, residual power from each channel may be available on all the channels thereby providing for security akin to having a party line. 
     Of utility then would be a method and system for securing DWDM networks against potential eavesdropping. 
     SUMMARY OF THE INVENTION 
     My invention is a method and system for securing DWDM networks by introducing noise into the fiber channel or cable up to the level of cross-talk (leakage) or ASE noise, which ever is larger, so that unauthorized recovery of channels is prevented or not permitted. 
     In accordance with my invention, a white noise source inputs white noise into the fiber channel up to amplified spontaneous emission level so that only the signal intended to be dropped or terminated can be recovered. In accordance with my invention the added noise masks the leaked signal without affecting the performance of the channel intended to be dropped or terminated. 
     By using only a noise source the network is secured against eavesdropping without the need of any sophisticated monitoring or processing software. Accordingly, a DWDM system designed in accordance with my invention will not incur a substantial increase in cost. 
     In one aspect of my invention the noise source is included as part of the DWDM node at the point the multiwavelength signal is being received, i.e., at the point within the equipment before the multiwavelength signal is optically demultiplexed. Although the noise can be injected anywhere along the path of the multiwavelength signals, it is most effective if injected at the receiving end. 
     In another aspect of my invention the noise source is coupled onto the fiber after the optical demultiplexer and just before the single channel optical signal is being handed over. In this case only the channels that have to be secure get the noise injection. In accordance with this aspect of my invention, DWDM systems that have been already deployed may be protected by my invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustratively depicts a receiving node in a DWDM network; 
     FIG. 2 is a spectral plot of a received channel that is dropped from the DWDM node of FIG. 1, channel  11  appears as cross-talk in the spectral plot of FIG. 2; 
     FIG. 3 is a spectral plot of the spectrum in FIG. 2 after filtering; 
     FIG. 4 depicts the leaked channel after channel  10  in FIG. 3 is turned off or after a second filter is applied to FIG. 3; 
     FIG. 5 depicts the optical spectrum of the dropped channel with white noise added in accordance with my invention; 
     FIG. 6 illustratively depicts a node in a secure DWDM network designed in accordance with an aspect of my invention; 
     FIG. 7 illustratively depicts a secure DWDM network in accordance with another aspect of my invention; and 
     FIG. 8 illustratively depicts an optical cross-connect node in a multiwavelength network implemented in accordance with my invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to FIG. 6, there is illustratively depicted a receiving node in a DWDM network that is dropping signals. In accordance with my invention a white noise source  610  may be placed at some point before a DWDM receiving node  620  as depicted in FIG.  6 . The white noise source  610  includes a white noise generator  612  and coupler  613 . In accordance with this aspect of my invention white noise is added to all the channels on fiber  615  at approximately detail A by the same source  610  or, specifically, by white noise generator  612 . 
     The spectrum that results on channel  10  is illustratively shown in FIG.  5 . As FIG. 5 shows, white noise is added only up to the amplified spontaneous emission level of the amplifiers deployed in the system. As FIG. 5 also shows, the added noise masks channel  11  while allowing recovery of channel  10 . Note also that although FIG. 5 clearly shows channel  11 , on an actual oscilloscope trace channel  11  would not at all be identifiable, i.e., it&#39;s presence is not detectable at channel  10 . Accordingly, the intended recipient or carrier of channel  10  is not able to recover channel  11 . Therefore, a DWDM system employing my invention in this manner allows the network operator to provide security. As such, by my invention DWDM systems already deployed can be secured by the network provider without any redesign or reengineering of these systems. This is the case because the noise or security code is added to fiber  615  at detail A either before or after the signal or information is internal to DWDM node or supplier equipment  620  and not in the node or any supplier equipment. 
     In accordance with another aspect of my invention, white noise may be coupled only onto the channels being dropped before those signals are handed off to another carrier or a customer. Tuning now to FIG. 7, there is depicted a secure WDM node  710  of a DWDM system designed in accordance with this aspect of my invention. In FIG. 7, incoming signals on fiber span  705  are demultiplexed at WDM node  710  by optical demultiplexer  711 . One of the signals that is demultiplexed is dropped at node  710  and is illustratively depicted as signal or channel  10 . Typically, optical signal or channel  10  is handed over to another carrier. As can be seen by reference to FIG. 2 when channel  10  is dropped, other channels, such as channel  11 , are leaked, thereby allowing for eavesdropping. However, in accordance with this aspect of my invention, noise generator  712  residing in node  710  injects white noise onto the dropped signal of channel  10  after the signal is demultiplexed by demultiplexer  711  but before channel  10  exits node  710 . Accordingly, the spectrum shown in FIG. 5 results. In the absence of my invention the intended recipient or carrier of channel  10  has access to the signal leaked from channel  11 . That leaked signal can be filtered, optically amplified, and recovered using conventional techniques. In addition to masking the leaked residual signal from channel  11 , noise generator  712  can also mask a residual signal that is the result of nonlinear optical coupling. The particular application of my invention depicted in FIG. 7 may be particularly advantageous in scenarios where the owner of node  710  is precluded from adding noise generator  712  to fiber  705  owing to regulatory considerations. 
     At node  710  the inverse operation to demultiplexing may also take place. That is, signal  110  enters node  710  so as to be multiplexed and transmitted to node  790 . At node  710  the multiplexing operation is carried out by wavelength division multiplexer  720 . The multiplexed signal from multiplexer  720  is then coupled on fiber span  780  for transmission to node  790 . At node  790  an operation similar to that which took place at node  710  also occurs, i.e., signals are added and dropped as required by customers. As such, by addition of a noise source in accordance with my invention only the channels intended to be dropped are decipherable by their respective recipient. Accordingly, by my invention a low cost solution is provided which prevents eavesdropping in DWDM networks. 
     My security method is also particularly advantageous in optical-cross connect nodes of a multiwavelength optical network because security can be done where it is most convenient. For example, and with reference to FIG. 8, an optical cross-connect  810  that is being used to groom traffic from several DWDM nodes  820  may also be used to secure traffic to and from the nodes  820 . Specifically, after the signals are de-multiplexed in cross-connect  810  a white noise generator  821  injects noise onto the signal up to the ASE level. Although FIG. 8 illustratively depicts the placement of noise generator  821  after the incoming signals to cross-connect  810  are de-multiplexed, the addition of noise may also be done right at the input of the cross-connect multiplexers. This implementation is depicted by the placement of noise generator  822  in FIG.  8 . As discussed above the signal or information would 
     Although the above description is directed to DWDM systems, i.e., systems having optical amplifiers, my invention is also applicable to systems not having optical amplifiers. In those systems, noise would be added only up to the level necessary to mask leaked channels. 
     The above description is exemplary of my invention. Numerous modifications and variations may be made by those skilled in the art without departing from the scope and spirit of my invention.