Abstract:
An optical communication system is provided that includes first and second optical transmitters/receivers remotely located with respect to one another and which are coupled together by first and second optical transmission paths for bidirectionally transmitting optical information therebetween. First and second optical amplifiers are respectively disposed in the first and second optical transmission paths. At least one loop-back path optically couples a portion of a WDM optical signal from the first to the second transmission path. The loop-back path includes a filter for transmitting a monitoring channel but not a data channel included in the optical signal portion traversing the loop-back path.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical communication system and more particularly to a system for monitoring an optical communication system. 
     BACKGROUND OF THE INVENTION 
     In long distance fiber optic communication systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system. 
     Prior art monitoring techniques include the use of a testing system which generates a monitoring signal and modulating the monitoring signal onto a single channel (or wavelength) with the transmitted data signal. For example, the data signal may be amplitude modulated by the monitoring signal. A loop-back coupler within an optical amplifier pair or repeater located downstream is used to return a portion of the transmitted signal (data signal plus monitoring signal modulation) to the testing system. The testing system then separates the monitoring signal from the data signal and processes the monitoring signal to examine the health of the transmission system. U.S. Pat. Nos. 4,586,186 and 4,633,464 C. Anderson et al. disclose a similar technique to modulate monitoring response information from a repeater onto the main data signal to monitor the health of the system. 
     Optical time domain reflectometry (OTDR) is another technique used to remotely detect faults in optical communication systems. In OTDR, an optical pulse is launched into an optical fiber and backscattered signals returning to the launch end are monitored. In the event that there are discontinuities such as faults or splices in the fiber, the amount of backscattering generally changes and such change is detected in the monitored signals. Since backscattering and reflection also occur from elements such as couplers, the monitored signals are usually compared with a reference record, new peaks and other changes in the monitored signal level being indicative of changes in the fiber path, normally indicating a fault. The time between pulse launch and receipt of a backscattered signal is proportional to the distance along the fiber to the source of the backscattering, thus allowing the fault to be located. In a wavelength division multiplexing (WDM) system, one wavelength is usually assigned as the OTDR channel. 
     Typically, line monitoring equipment (LME) detecting a returned portion of the transmission signal is employed when the transmission system is in-service and OTDR is employed when the system is out-of-service. Therefore, crosstalk between the OTDR channel and the data channels is not a concern. Since the line monitoring equipment is used in-service, however, crosstalk is a concern in this case. Specifically, crosstalk arises between the returning portion of the signal and the data channels traveling on the opposite-going transmission path. 
     Accordingly, it would be desirable to provide line monitoring equipment for an optical transmission system that reduces crosstalk between the data channels and the returning portion of the signal that is to be monitored. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an optical communication system is provided that includes first and second optical transmitters/receivers remotely located with respect to one another and which are coupled together by first and second optical transmission paths for bidirectionally transmitting optical information therebetween. First and second optical amplifiers are respectively disposed in the first and second optical transmission paths. At least one loop-back path optically couples a portion of a WDM optical signal from the first to the second transmission path. The loop-back path includes a filter for transmitting a monitoring channel but not a data channel included in the optical signal portion traversing the loop-back path. 
     The loop-back path may include first and second optical couplers disposed in the first and second transmission paths, respectively. In one particular embodiment of the invention, the transmission system also includes an OTDR path for coupling a backscattered signal from the first to the second transmission path. In this embodiment, the loopback path and the OTDR path overlap at least in part and the first and second couplers further couple the backscattered signal from the first to the second transmission path. 
     In another embodiment of the invention, the WDM optical signal includes a plurality of data channels that occupy a given data bandwidth and the monitoring channel is located at a wavelength outside of the given data bandwidth. In some cases the monitoring channel is located at a wavelength below the given data bandwidth, or alternatively, above the given data bandwidth. 
     In accordance with another aspect of the invention, a method is provided for monitoring an optical communication system that includes first and second optical transmission paths coupling a first transmitter/receiver to a second transmitter/receiver for bidirectionally transmitting optical information therebetween. The first and second optical transmission paths respectively include first and second optical amplifiers. In accordance with the method, a WDM signal is first transmitted. The WDM signal includes a monitoring channel and at least one data channel through the first optical transmission path. Next, a portion of the WDM optical signal is coupled from the first transmission path and filtered so that the monitoring channel but not the data channel is transmitted. The filtered portion of the WDM optical signal is coupled to the second transmission path. Finally, the monitoring channel is detected to access the status of the transmission system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an optical transmission system having a monitoring system in accordance with the present invention. 
     FIG. 2 shows an alternative embodiment of the monitoring system shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a monitoring system  10  in accordance with the present invention. Monitoring system  10  includes (LME)  12  for monitoring the health of a telecommunications transmission system, such as a fiber optic transmission system. LME  12  includes pseudo-random sequence (PRS) tone generator  14  connected to laser transmitter  16  for generating and outputting a PRS used to modulate a tone. Laser transmitter  16  generates a low level AM signal  18  on the data signal based on the tones generated by PRS tone generator  14 . 
     LME  12  also includes a delay system  20  connected to PRS tone generator  14  for delaying the tones received from PRS tone generator  14 . LME  12  further includes an optical filter  26  for selectively passing one or more wavelengths or channels, while blocking the transmission of other wavelengths. 
     Comparator/correlator  22  is connected to delay system  20  and optical filter  26 . Comparator/correlator  22  correlates the outputs of optical filter  26  and delay system  20  using well known digital signal processing techniques. Comparator/correlator  22  outputs a result of the correlation operation, which is used by a computer or other systems (not shown) to diagnose faults or problems in the optical transmission system. 
     LME  12  is connected to a portion of an optical transmission system. The optical transmission system includes a laser transmitter  30  and an optical fiber pair, including fibers  28  and  29 , for carrying optical signals. Fibers  28  and  29  can be the long distance optical fiber lines for deployment, for example, under the ocean. Optical fibers  28  and  29  are unidirectional fibers and carry signals in opposite directions. Fibers  28  and  29  together provide a bi-directional path for transmitting signals. While the monitoring system according to a disclosed embodiment of the present invention monitors a transmission system that includes two unidirectional fibers  28  and  29 , the present invention may be used to monitor transmission systems employing a single bi-directional fiber. 
     Laser transmitter  30  transmits optical data on a plurality of channels (or wavelengths) over fiber  29 . Laser transmitter  30  can comprise a plurality of laser transmitters each transmitting an optical data signal over fiber  29  using a different channel or wavelength. A plurality of data signals each at a different wavelength are sent over fiber  29  using wavelength division multiplexing (WDM). Alternatively, only a single channel of data may be carried on fiber  29 . Similarly WDM data signals may be carried over fiber  28 , but traveling in a direction opposite of those signals on fiber  29 . A coupler  34  combines the WDM data  32  from transmitter  30  and the LME tone  18  from transmitter  16  and outputs this combined signal for transmission onto fiber  29 . A first optical repeater  36  receives the combined signal from coupler  34 . Repeater  36  includes amplifiers  38  and  40  for amplifying optical signals transmitted over fiber  28  and  29 , respectively. Repeater  36  also includes a loop-back path  42 , which returns a portion of the signal being transmitted on fiber  29  to fiber  28  (via high loss couplers  46  and  48 ) for transmission to LME  12 . Similarly, repeater  36  includes a loop-back path  44 , which returns a portion of the signal being transmitted on fiber  28  to fiber  29  (via high loss couplers  48  and  46 ) to an LME (not shown) located at the receiver terminal along fiber  29 . If, as in FIG. 1, an OTDR path is also employed (discussed below), loop-back paths  42  and  44  may traverse a portion of the OTDR path via additional couplers  47  and  49 , respectively. In this way only a single coupler is required in each of the transmission paths  28  and  29 . Specifically, coupler  49  receives both the OTDR signal and the LME tone from optical fiber  28 . Likewise, coupler  47  receives both the OTDR signal and the LME tone from optical fiber  29 . Additional optical repeaters (not shown), including their associated loop-back couplers, may be connected to fibers  28  and  29  for periodically amplifying and returning signals thereon. 
     Signal  52  arrives at the end of fiber  28  and carries all signals present on fiber  28 , including the combined WDM data  32  and the amplitude modulated tones  18  returned by loop-back path  42 . Signal  52  is input to optical filter  26 . Optical filter  26  is wavelength selective and passes only the wavelength of LME tone  18 . Comparator/correlator  22  then correlates the returned LME tone with the delayed PRS tones. Comparator/correlator  22  may correlate electrical signals or optical signals. Where comparator/correlator  22  correlates electrical signals, LME  12  further includes an optical decoder connected between optical filter  26  and the comparator/correlator  22  for converting the optical signals output by filter  26  into electrical signals. 
     Comparator/correlator  22  correlates the PRS tones output by the PRS tone generator  14  with each of the returned LME tones. To perform this correlation, delay system  20  receives the PRS tones from the PRS tone generator  14  and outputs a plurality of delayed PRS tones to comparator/correlator  22 . Delay system  20  outputs each PRS tone after the time delays corresponding to each repeater. In other words, delay system  20  delays the PRS tones based on the location of each repeater. This process is repeated for each PRS tone received by the delay system  20 . Comparator/correlator  22  compares or correlates the delayed LME tone returned from each repeater with correspondingly delayed PRS tones generated by PRS tone generator  14 . 
     As previously mentioned, LME tone  18  is typically generated within the bandwidth of the data channels. To avoid the adverse effects of noise caused by the LME tone  18 , the present invention employs an LME tone that is outside the bandwidth of the data channels. For example, if the data channels occupy a bandwidth between 1543 and 1557 nm, the LME tone will be located at a wavelength greater than 1557 nm or less than 1543 nm. As shown in FIG. 1, filter  41  is inserted in loopback path  42  to selectively remove the data channels so that only the LME tone is transmitted. That is, the filter  41  has a passband centered about the LME tone and a stop band centered about the data channels. By ensuring that the returned signal only includes the LME tone  18  and not the data channels, the filter  41  effectively eliminates crosstalk that may occur between the returned signal and the data signal directed along fiber  28 . 
     Similar to filter  41  employed in loop-back path  42 , the present invention provides a filter  43  located in loop-back path  44 . Like filter  41 , filter  43  has a passband centered about the LME tone and a stop band centered about the data channels. Filter  43  eliminates cross-talk between the returned signal and the data signal directed along fiber  29 . 
     Referring again to FIG. 1, repeater  36  includes an OTDR path  45  through which a portion of the signal reflected by Rayleigh scattering may be tapped and returned along the opposite-traveling fiber path so that OTDR may be performed. Because couplers  46  and  48  are located at the outputs of (i.e., downstream from) optical amplifiers  40  and  38 , respectively, the backscattered signal is coupled to the opposite-going fiber before undergoing amplification in the optical amplifiers. In operation, a portion of the backscattered signal traveling along optical fiber  29  is coupled to optical path  28  via OTDR path  45  and returned to terminal  30  where OTDR may be performed. Similarly, a portion of the backscattered signal along optical fiber  28  is coupled to optical path  29  via OTDR path  45  and returned to terminal  31 . 
     FIG. 2 shows an alternative embodiment of the invention in which a single loop-back path  58  is used for coupling both the LME tone and the OTDR signal to the opposite-going transmission path. In FIGS. 1 and 2, like reference numerals refer to like elements. Rather than the transmissive filters  41  and  43  employed in FIG. 1, however, in FIG. 2 reflective filters  54  and  56  are used. Reflective filters  54  are  56  reflect the LME tone and pass all other wavelengths into a non-reflective termination device. For example, if the LME tone on optical fiber  29  is located at a wavelength λ hi  that is greater than the wavelengths of the data channels, reflective filter  54  will only reflect λ hi  back along loop-back path  58 . Likewise, if the LME tone on optical fiber  28  is located at a wavelength λ low  that is less than the wavelengths of the data channels, reflective filter  56  will only reflect λ low  back along loop-back path  58 . 
     In operation, coupler  46  receives the LME tone directed along optical fiber  29  and couples it to reflective filter  54 , which in turn reflects the LME tone along loop-back path  58  so that it is coupled to optical fiber  28  by coupler  48 . Coupler  46  also receives the backscattered signal from optical fiber  29  and couples it to loop-back path  58  so that it can be coupled to optical fiber  28  by coupler  48 . Coupler  48  receives the LME tone directed along optical fiber  28  and couples it to reflective filter  56 , which in turn reflects the LME tone along loop-back path  58  so that it is coupled to optical fiber  29  by coupler  46 . Coupler  48  also receives the backscattered signal from optical fiber  28  and couples it to loop-back path  58  so that it can be coupled to optical fiber  29  by coupler  46 . 
     The reflective filters  54  and  56  may be formed from any appropriate device such as a fiber Bragg grating, for example. Other devices that may be employed include thin film reflectors.