Abstract:
A method and apparatus is provided for applying an electroding tone to a power conductor located in an undersea optical cable employed in an undersea optical transmission system having at least one repeater. The method begins by applying a DC line current to the power conductor. The DC line current has a value sufficient to power the repeater and less than a value sufficient to power the repeater and to perform electroding. An AC current is impressed on the DC line current at a prescribed frequency and a peak-to-peak amplitude ranging from zero and a prescribed value to generate the tone. The impression or application of the AC current is terminated after a period of time at least sufficient to locate the optical cable or an optical cable fault.

Description:
STATEMENT OF RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/658,321, filed Mar. 3, 2005, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to undersea optical transmission systems, and more particularly to a cable station for an optical transmission system to which electrical power is supplied such as an undersea optical transmission system. 
       BACKGROUND OF THE INVENTION 
       [0003]    An undersea or submarine optical transmission system consists of land-based cable stations interconnected by a cable that is installed on the ocean floor. The cable contains optical fibers that carry Dense Wavelength Division Multiplexed (DWDM) optical signals between the terminals. The cable stations contain power supplies for the undersea cable, transmission equipment to insert and remove DWDM signals from the fibers and associated monitoring and control equipment. Over long distances the strength and quality of a transmitted optical signal diminishes. Accordingly, repeaters are located along the cable, which contain optical amplifiers to provide amplification to the optical signals to overcome fiber loss. The cables also contain a power conductor to deliver electrical power from the cable stations to the repeaters. 
         [0004]    Since the cables are relatively inaccessible on the ocean floor repair solutions can be complex and repair timelines and costs quite significant. Once impediment within the submarine telecommunications industry to effectuating repairs to the undersea cable and repeaters is the ability to reliably and efficiently locate and track the cables to identify the position of the fault. It is often necessary to identify the location of the cable itself because it may have been physically moved from its as-laid position by external aggression. Rapid location of the cable results in substantial costs savings largely from the reduced ship time required to make the repair. Cost savings are also achieved because of reductions in system downtime. In addition to repair costs, it is important to locate cables for other purposes such as end-of-life removals and to satisfy regulatory requirements. 
         [0005]    One technique used to locate undersea cables and the position of faults therein is electroding. Electroding can be performed in-service or out-of-service by modulating the electric current supplied to the power conductor within the cable to generate a low frequency tone. Generally the current is modulated by around +/−100 mA at frequencies between about 4 and 50 Hz. This allows cables to be traced and located accurately using ship-borne, low frequency electromagnetic field sensing equipment so that repairs can be made. Once the cable has been located in this manner, it is followed to the point at which the tone disappears or is greatly attenuated. This point typically marks the position of the fault. The additional current necessary to generate the low frequency tone is generally added to the total fixed line current that is provided to the cable. This additional current is generally not a concern when traditional repeaters are employed, which are relatively large in size. However, small form factor repeaters have recently been developed in which thermal management issues require that the electrical power they consume be minimized. In this case the additional current traversing the repeaters leads to additional ohmic losses that can be problematic. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) optical transmission system in which the present invention may be employed. 
           [0007]      FIG. 2  is a functional block diagram of a cable station employed in the system of  FIG. 1 . 
           [0008]      FIG. 3  shows the current as a function of time when electroding is performed in accordance with a conventional method. 
           [0009]      FIG. 4  shows the current as a function of time when electroding is performed in accordance with the present invention. 
           [0010]      FIG. 5  is a side-by-side comparison of the current as a function of time when electroding is performed with a conventional method and with the present invention. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0011]    In accordance with the present invention, a method and apparatus is provided for applying an electroding tone to a power conductor located in an undersea optical cable employed in an undersea optical transmission system having at least one repeater. The method begins by applying a DC line current to the power conductor. The DC line current has a value sufficient to power the repeater and less than a value sufficient to power the repeater and to perform electroding. An AC current is impressed on the DC line current at a prescribed frequency and a peak-to-peak amplitude ranging from zero and a prescribed value to generate the tone. The impression or application of the AC current is terminated after a period of time at least sufficient to locate the optical cable or an optical cable fault. 
         [0012]    In accordance with one aspect of the invention, when the AC current is impressed, the DC line current is effectively increased by an amount equivalent to an rms value of the prescribed amplitude of the AC current. 
         [0013]    In accordance with another aspect of the invention, the prescribed frequency of the AC current is between about 4 and 50 Hz. 
         [0014]    In accordance with another aspect of the invention, the repeater is a small form factor repeater. 
         [0015]    In accordance with another aspect of the invention, the prescribed amplitude of the AC current is between about 100 and 300 mA. 
         [0016]    In accordance with another aspect of the invention, the DC line current prior to application of the AC current is between about 600 and 1000 mA. 
         [0017]    In accordance with another aspect of the invention, a land-based cable station is provided for an undersea optical transmission system having power feed equipment (PFE) performing any of the aforementioned methods 
       DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) transmission system in which the present invention may be employed. The transmission system serves to transmit a plurality of optical channels over a pair of unidirectional optical fibers  106  and  108  between cable stations  200  and  202 . Optical fibers  106  and  108  are housed in an optical cable that also includes a power conductor for supplying power to the repeaters. Cable stations  200  and  202  are of the type depicted in  FIG. 2 . The transmission path is segmented into transmission spans or links  130   1 ,  130   2 ,  130   3 , . . .  130   n+1 . The transmission spans  130 , which are concatenated by repeaters  112   1 ,  112   2 , . . .  112   n , can range from 40 to 120 km in length, or even longer if Raman amplification is employed. The repeaters include optical amplifiers  120  that connect each of the spans  130 . It should be noted that the invention is not limited to point-to-point network architectures such as shown in  FIG. 1  but more generally may encompass more complex architectures such as those employing branching units, optical mesh networks, and ring networks, for example. 
         [0019]    A functional block diagram of a cable station is shown in  FIG. 2 . The cable station  10  includes submarine line terminal equipment (SLTE)  12 , power feed equipment (PFE)  18 , and an element management system (EMS)  16  and a cable termination box (CTB)  14 . The SLTE  12  converts terrestrial traffic into an optical signal that is appropriate for an undersea transmission line. The power-feed equipment  18  electrically powers all the active undersea equipment, most notably the repeaters. The EMS  16  allows the system operator to configure the system and to obtain information regarding its status. The CTB  14  terminates the undersea cable and physically separates the cable into optical fibers and the power-feed line and may also serve as a monitoring point for the cable. Additional details concerning cable stations may be found in chapter 10 of “Undersea Fiber Communication Systems,” J. Chesnoy, ed. (Academic Press, 2002). 
         [0020]    On the transmit side, the SLTE  12  receives traffic such as an STM signal from a terrestrial terminal that is generally located in a Point of Presence (PoP). The SLTE  12  converts each wavelength of the optical signal to an electrical signal and encodes it with FEC. An electrical to optical unit modulates a continuous wave light from a laser with the electrical signal to generate an optical line signal at each wavelength, which is then optically amplified. The amplified wavelengths may undergo signal conditioning such as dispersion compensation before (or after) being multiplexed together and sent out on the undersea transmission cable. The receive side of the SLTE  12  operates in a complementary manner. The SLTE  12  may also performing line monitoring to determine the status and health of the transmission path. For example, the SLTE  12  may employ a COTDR arrangement to monitor and measure the optical loss of the transmission path. 
         [0021]    The PFE  18  is designed to provide a stable DC line current to the submerged portion of the transmission system. The repeaters  112  are powered in series by the PFE  18  located in the cable stations. The entire submerged plant operates at the same DC line current and the PFE must provide sufficient voltage to power all devices at that line current. Line currents and system voltages are typically up to 2000 mA and 15 kV, respectively. The power is delivered to the submerged plant along a copper conductor located within the optical cable, which typically has an impedance of between about 0.5 and 1.5 ohm/km. A large fraction of the power provided by the PFE is wasted as ohmic heating in the cable and repeaters. By way of example, in a long-haul transmission system 7000 km in length with a system voltage of about 16 kV and a line current of 1000 mA, about 7 kW of the 16 kW system load would be lost to ohmic heating. Zener diodes located in the repeaters  112  convert the line current to voltage to power the electronics associated with the optical amplifiers located in the repeaters. 
         [0022]    As previously mentioned, electroding is a technique that can be used for cable and cable fault location, which is independent of the optical characteristics of the transmission system. Specifically, a low frequency (e.g., 4-50 Hz) electric signal of about 100-300 mA p-p is impressed on the DC line current and the resulting time-varying electromagnetic field can be remotely detected aboard a ship that is sent out to repair or inspect the cable. 
         [0023]    The ability to perform in-service electroding is conventionally achieved by providing to the undersea optical cable a constant line current that comprises two components. The first component is the DC operating current required to operate to power the repeaters. The second component is the root-mean-square (rms) value of the AC current that is required when electroding is performed. This is illustrated in  FIG. 3 , which shows the total constant current I T  comprising a first component I nominal  for powering the repeaters and a second component I electroding  for performing electroding. 
         [0024]    Since electroding as conventionally performed requires an additional DC component to the current, the amount of power that is lost through ohmic heating is also increased. In conventional undersea transmission systems the repeaters may be greater than 5 feet in diameter and comprise substantial amounts of thermally conductive materials such as copper-beryllium or steel. In such repeaters ohmic heating is not a concern because the heat can be easily dissipated. Recently, however, small form factor repeaters have been developed that are substantially smaller than conventional repeaters. These repeaters can have dimensions on the order of 10 centimeters. The small form factor repeaters are typically configured to consume very low power to increase the inherent reliability of the pump lasers, reduce thermal loads, and lessen the power producing and carrying requirements on the PFE and undersea optical cable, respectively. Such a design not only increases overall amplifier reliability, but also substantially lowers costs in the cable because both the power conductor (typically formed from copper) and the dielectric sheathing (typically a medium or high-density polyethylene) can be made smaller in size. For instance, some examples of small form factor repeaters are shown in U.S. patent application Ser. Nos. 10/687,547 and 10/800,424, which are hereby incorporated by reference in their entirety. In undersea transmission systems employing small form factor repeaters, heat dissipation can become a critical issue. This can be a particularly challenging problem because the various optical pump sources and zener diodes employed in the repeaters generate a significant amount of heat that must be dissipated over a relatively small area to ensure that the temperature of the various components do not exceed their operational limits. The additional current needed to perform electroding only exacerbates this problem. 
         [0025]    In accordance with the present invention, the aforementioned thermal management issues are ameliorated by applying the AC current needed to perform electroding only when the cable or cable fault is to be located. (i.e., only when electroding is to be performed). At other times the current applied to the cable is a DC current maintained at the level required to power the repeaters (e.g., I Nominal  in  FIG. 3 ). That is, the AC current I etectroding  is only applied when electroding is to be performed. This is illustrated in  FIG. 4 , which shows the total current I T  increasing from I nominal  to I nominal +I electroding  at the time electroding is to be performed and decreasing back to I nominal  after completion of the electroding process. The AC current I E  has an amplitude that varies between zero and some appropriate value (e.g., 100-300 mA) sufficient to generate an electromagnetic field sufficient to generate a field that can be detected aboard ship. That is, the AC current I E  is only modulated between values of a single sign that is the same as the sign of the DC current I Nominal . A side-by-side comparison of  FIGS. 2 and 3 , shown in  FIG. 5 , indicates the current savings A that is achieved under normal system operating conditions when electroding is not being performed, which leads to a commensurate savings in power. In this way the thermal management burden placed on the repeaters is reduced. 
         [0026]    To perform electroding the repeaters need to have a relatively low insertion loss for the low frequencies that are employed. When electroding is performed in-service, the zener diodes present a low impedance path and thus application of the electroding tone over the transmission path is generally not a problem. When performed out-of-service, the zener diodes have very little voltage across them and thus present a relatively high impedance path to the electroding tone. This problem can be overcome by placing an appropriately sized capacitor in parallel with the zener diodes. Alternatively, a bypass resistor may be placed in parallel with the zener diodes to allow current to pass through the repeaters before the zener diodes have turned on.