Abstract:
An optical transmission system includes a transmitter unit, a receiver unit, and an optical transmission path interconnecting the transmitter and receiver units. A plurality of optical repeaters are situated along the transmission path. Adjacent ones of the repeaters are interconnected by transmission spans that collectively constitute a majority of the optical transmission path. Each of the transmission spans comprises substantially identical lengths of cabled optical fiber having substantially identical prescribed path average dispersions. At least one adjustable dispersion trimming element is located in the optical repeater and optically couples one of the transmission spans to an optical amplifier located in the repeater. The adjustable dispersion trimming element has an adjustable path average dispersion selected such that a total path average dispersion of the transmission span to which it is coupled plus the adjustable dispersion trimming element has a desired value.

Description:
STATEMENT OF RELATED APPLICATIONS  
       [0001]    This application claims the benefit of priority to U.S. Provisional Patent Application No. 60/404,616, filed Aug. 20, 2002, entitled “Dispersion Map Design.” 
         [0002]    This application is also related to copending U.S. patent application Ser. No. ______ [Docket No. 9005/20] entitled “Optical Repeater Employed In An Optical Communication System Having A Modular Dispersion Map,” filed on even date herewith. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention relates generally to optical transmission systems, and more particularly to a dispersion map for an undersea optical transmission system.  
         BACKGROUND OF THE INVENTION  
         [0004]    The introduction of multigigabit, multiwavelength optical communication systems operating over long distances (e.g., transoceanic) and high average powers has resulted in the exploration of fiber designs that can minimize signal degradation. In the last decade several new and useful fiber designs have become commercially available. These fibers come with a variety of dispersion, loss, and effective core area values. The goal of all transmission line design is to reduce the deleterious effects of a number of phenomena, including accumulation amplified spontaneous emission (ASE) noise accumulation, group velocity dispersion, and Kerr effect nonlinearities.  
           [0005]    It turns out there is no one fiber that reduces all these effects at once. For example if the signal travels at the zero dispersion wavelength it will not suffer any temporal distortions. However, at the zero dispersion wavelength the signal and the ASE noise generated by the optical amplifiers and the signal and adjacent signals are well phase matched. Thus they have the opportunity to interact, via four wave mixing and cross phase modulation, over long distances. The result is the transfer of power out of the signal and into unwanted wavelengths and/or the phase modulation of one signal by another. The end result of all this can be a severe degradation in signal fidelity. Conversely if the signal propagates at a wavelength for which the dispersion is large then there is a large phase mismatch (i.e., a group velocity difference) between the signal and noise, which greatly reduces the efficiency of four wave mixing. However, large values of dispersion result in increased inter-symbol interference due to the temporal spreading of the signal  
           [0006]    An important advance in the implementation of multi-channel WDM systems has been the use of dispersion management techniques. In view of the above mentioned conflicting demands, the basic principle of dispersion management is to keep local dispersion non-zero but make the overall system dispersion substantially zero. This can be accomplished by using a dispersion map in which the zero dispersion wavelengths of the constituent fibers are chosen so that they are appropriately far from the system&#39;s operating wavelengths. Constituent fibers with different zero dispersion wavelengths are then arranged in some periodic fashion so that the path average dispersion for the whole transmission line is appropriately small. For example, the transmission line may be divided into two or more sections approximately equal length. In one section, the optical fiber has a zero dispersion wavelength less than the operating wavelengths. The following section has optical fiber with a zero dispersion wavelength greater than the operating wavelengths. The overall transmission line is thus constructed in a periodic manner from a concatenation of fiber sections having different zero dispersion wavelengths. By constructing the transmission line out of alternating lengths of positive and negative dispersion fiber, the path average dispersion can be adjusted so that it causes minimal temporal distortion. Moreover, by selecting the local dispersions of the constituent fibers to be large in magnitude, nonlinear interactions can be suppressed. The path-average dispersion of a fiber span of length L may be mathematically denoted as:  
         D   average     =       1   L            ∫     z   =   0       z   =   L              D        (   z   )                          z                                 
 
           [0007]    For applications involving the transmission of non-return-to-zero (NRZ) data, the desired D aveage  is zero, while, for soliton data transmission, the desired D average  is in the range of about 0.05 to 0.5 picoseconds per nanometer-kilometer.  
           [0008]    Undersea optical communication systems have been traditionally custom-designed on a system-by-system basis. Fundamental design parameters such as amplifier spacing, amplifier gains and bandwidths, dispersion maps, data rate, wavelength count and constituent fiber are often significantly different from system to system. For example, amplifier span length (i.e., the length of fiber between consecutive amplifiers) varies from about 33 km to 80 km. Hence the amplifier gains vary from about 8 dB to 16 dB, requiring amplifiers with very different designs. Dispersion maps have also varied in length and in composition of the constituent fiber.  
           [0009]    One problem that arises when the dispersion map of undersea communication systems differs from system to system is that a great variety of optical fiber must be available that have the proper length and dispersion for the segments of each different dispersion map. The need for such a variety of different fibers increases their manufacturing costs and therefore system costs. Moreover, the cost to maintain a supply of replacement fibers in inventory is increased when so many different fibers must be maintained.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the present invention, an optical transmission system includes a transmitter unit, a receiver unit, and an optical transmission path interconnecting the transmitter and receiver units. A plurality of optical repeaters are situated along the transmission path. Adjacent ones of the repeaters are interconnected by transmission spans that collectively constitute a majority of the optical transmission path. Each of the transmission spans comprises substantially identical lengths of cabled optical fiber having substantially identical prescribed path average dispersions. At least one adjustable dispersion trimming element is located in the optical repeater and optically couples one of the transmission spans to an optical amplifier located in the repeater. The adjustable dispersion trimming element has an adjustable path average dispersion selected such that a total path average dispersion of the transmission span to which it is coupled plus the adjustable dispersion trimming element has a desired value.  
           [0011]    In accordance with one aspect of the invention, the adjustable dispersion trimming element includes a plurality of adjustable dispersion trimming elements respectively located in the plurality of optical repeaters and which are optically coupled to a respective one of the transmission spans.  
           [0012]    In accordance with another aspect of the invention, each of the optical repeaters includes an optical amplifier. The adjustable dispersion trimming element is located at an input to the optical amplifier. Alternatively, the adjustable dispersion trimming element may be located at an output to the optical amplifier.  
           [0013]    In accordance with another aspect of the invention, the prescribed path average dispersion of each of the transmission spans is approximately equal to zero.  
           [0014]    In accordance with another aspect of the invention, the adjustable dispersion trimming element comprises spooled optical fiber.  
           [0015]    In accordance with another aspect of the invention, the adjustable dispersion trimming element comprises a Bragg grating.  
           [0016]    In accordance with another aspect of the invention, at least one of the transmission spans comprises a cabled optical fiber having a single value of dispersion.  
           [0017]    In accordance with another aspect of the invention, at least one of the transmission spans comprises a plurality of cabled optical fibers each having a different value of dispersion.  
           [0018]    In accordance with another aspect of the invention, at least one of the transmission spans comprises a cabled optical fiber having a single value of dispersion.  
           [0019]    In accordance with another aspect of the invention, the spooled optical fiber has a dispersion value substantially greater than the single dispersion value of the cabled optical fiber.  
           [0020]    In accordance with another aspect of the invention, a method is provided for establishing a dispersion map for an optical transmission system. The transmission system includes an optical transmission path having a plurality of optical amplifiers interconnected by respective transmission spans. The method begins by selecting a desired path average dispersion for each period of the dispersion map. The desired path average dispersion has a first fixed component arising from a respective one of the transmission spans associated with each period and a second adjustable component associated with each period. For a given period, a path average dispersion is adjusted to achieve the desired path average dispersion by trimming the second adjustable component associated with the given period.  
           [0021]    In accordance with another aspect of the invention, the adjusting step is performed by at least one adjustable dispersion trimming element associated with one of the optical amplifiers.  
           [0022]    In accordance with another aspect of the invention, a method is provided for assembling an optical transmission system. The method begins by providing a plurality of optical repeaters each having an input and output. Each of the repeaters includes an optical amplifier and an adjustable dispersion trimming element. A plurality of spans of cabled optical fiber are also provided. Each of the spans comprises substantially identical lengths of optical fiber having substantially identical prescribed path average dispersions. The input and output of each of the repeaters are optically coupled to an end of one of the spans of cabled optical fiber to form a transmission path having a concatenation of optical repeaters such that each of the spans of cabled optical fiber is associated with one of the adjustable dispersion trimming elements. A path average dispersion of the adjustable dispersion trimming elements is adjusted to achieve a desired total path average dispersion for the cabled optical fiber span and the adjustable trimming element associated therewith. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 shows a simplified block diagram of an exemplary wavelength division multiplexed transmission system in accordance with the present invention.  
         [0024]    [0024]FIG. 2 shows a single transmission span of the transmission system depicted in FIG. 1 to which optical repeaters are connected.  
         [0025]    [0025]FIG. 3 shows an exemplary transmission span comprising a cabled optical fiber having two components with length L 1  and L 2  and dispersions D 1  and D 2 , respectively.  
         [0026]    [0026]FIG. 4 shows a schematic diagram of a repeater constructed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]    The present invention provides a modular, single span, dispersion map with an adjustable path average dispersion. A modular dispersion map eliminates many design problems associated with multispan dispersion maps, most significantly matching the period of the dispersion map to some multiple of the amplifier span length. Such a modular adjustable dispersion map can be made to accommodate most modulation formats quite easily.  
         [0028]    In particular, the present inventors have recognized that significant advantages and cost savings can be achieved by using a dispersion map that comprises two components and has a period that is equal to the amplifier span length. The first is a fixed combination of two fibers of chosen dispersions and lengths. The second component is an adjustable portion of the dispersion map that is used to trim the fixed periodic portion as needed on a system-by-system or span-by-span basis. The optical fiber of the transmission path comprises the fixed, periodic component. By deliberate and judicious design choices, the fixed periodic component is the same from system to system, thereby reducing the number of different optical fibers that are required. The fixed period of the dispersion map is preferably selected to be as small as is practical to enhance the flexibility of the design. For example, in one particular embodiment of the invention, the fixed periodic component has a length equal to the span of optical fiber that connects adjacent amplifiers.  
         [0029]    [0029]FIG. 1 shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) transmission system in accordance with the present invention. The transmission system serves to transmit a plurality of optical channels over a single path from a transmitting terminal to a remotely located receiving terminal. While FIG. 1 depicts a unidirectional transmission system, it should be noted that if a bi-directional communication system is to be employed, two distinct transmission paths are used to carry the bi-directional communication. The optical transmission system may be an undersea transmission system in which the terminals are located on shore and one or more repeaters may be located underwater  
         [0030]    Transmitter terminal  100  is connected to an optical transmission medium  200 , which is connected, in turn, to receiver terminal  300 . Transmitter terminal  100  includes a series of encoders  110  and digital transmitters  120  connected to a wavelength division multiplexer  130 . For each WDM channel, an encoder  110  is connected to a digital transmitter  120 , which, in turn, is connected to the wavelength division multiplexer  130 . In other words, wavelength division multiplexer  130  receives signals associated with multiple WDM channels, each of which has an associated digital transmitter  120  and encoder  110 . Transmitter terminal  100  also includes a chromatic dispersion compensator  140  that precompensates for dispersion arising in transmission medium  200 .  
         [0031]    Digital transmitter  120  can be any type of system component that converts electrical signals to optical signals. For example, digital transmitter  120  can include an optical source such as a semiconductor laser or a light-emitting diode, which can be modulated directly by, for example, varying the injection current. WDM multiplexer  130  can be any type of device that combines signals from multiple WDM channels. For example, WDM multiplexer  130  can be a star coupler, a fiber Fabry-Perot filter, an inline Bragg grating, a diffraction grating, cascaded filters and a wavelength grating router, among others.  
         [0032]    Receiver terminal  300  includes a series of decoders  310 , digital receivers  320  and a wavelength division demultiplexer  330 . WDM demultiplexer  330  can be any type of device that separates signals from multiple WDM channels. For example, WDM demultiplexer  330  can be a star coupler, a fiber Fabry-Perot filter, an in-line Bragg grating, a diffraction grating, cascaded filters and a wavelength grating router, among others. Receiver terminal  300  also includes a chromatic dispersion compensator  340  that provides post-compensation for dispersion arising in transmission medium  200 .  
         [0033]    Optical transmission medium  200  includes rare-earth doped optical amplifiers  210   1 - 210   n  interconnected by transmission spans  240   1 - 240   n+1  of optical fiber. If a bi-directional communication system is to be employed, rare-earth doped optical amplifiers are provided in each transmission path. Moreover, in a bi-directional system each of the terminals  100  and  300  include a transmitter and a receiver. In a bi-directional undersea communication system a pair of rare-earth doped optical amplifiers supporting opposite-traveling signals is often housed in a single unit known as a repeater. While only four rare-earth optical amplifiers are depicted in FIG. 1 for clarity of discussion, it should be understood by those skilled in the art that the present invention finds application in transmission paths of all lengths having many additional (or fewer) sets of such amplifiers.  
         [0034]    Each of the transmission spans  240   1 - 240   n+1  comprise optical fiber enclosed in a cable designed to withstand the undersea environment. As previously mentioned, in one embodiment of the invention each transmission span, and therefore each span of cabled optical fiber, constitutes the fixed, periodic component of the dispersion map. Each transmission span may comprise one or more types of optical fiber having different zero dispersion wavelengths so that the path average dispersion of each span, and hence the path average dispersion of the fixed component of the dispersions map, is either zero or some other appropriate value determined in part by the modulation format that is employed.  
         [0035]    In accordance with the present invention, the adjustable portion of the dispersion map is provided by an adjustable dispersion trimming element having a given dispersion value so that the path average dispersion of the transmission span plus the adjustable dispersion trimming element is tailored to some precise value that is appropriate for the particular modulation format and transmission distance that is employed in any given system.  
         [0036]    The adjustable dispersion trimming element, which may be spooled fiber or a discrete device such as a Bragg grating, for example, may be conveniently located in the housing of the repeaters. For example, FIG. 2 shows a single transmission span  340  interconnected by adjacent repeaters  3101  and  3102 . Transmission span  340  comprises cabled fiber  320 . The adjustable dispersion trimming element  330  is shown as spooled fiber that is located in repeater  3102  and extends from the termination of the cabled fiber  320  to the input of the optical amplifier  3322 .  
         [0037]    One advantage of the present invention is that it achieves the cost savings and simplicity in design that arises from the use of a common transmission span that is the same for each and every span within a given system as well as among different systems, combined with the flexibility to trim the dispersion map on a system by system and/or a span by span basis. That is, when the system is initially installed, all that is needed are multiple units of a single cabled fiber having a prescribed length and path average dispersion. Any adjustments to the dispersion map can be readily performed within the housings of the repeaters, either by trimming spooled fiber to the appropriate length or by appropriate adjustment of a discrete device.  
         [0038]    [0038]FIG. 3 shows an exemplary transmission span comprising a cabled fiber having two components  22  and  24  with lengths L 1  and L 2  and dispersions D 1  and D 2 , respectively. A dispersion trimming element  26  has a length L trim  and a dispersion D trim . The path average dispersion of the transmission span  20  plus the dispersion trimming element  26 , D average total , is  
           D   average total =( D   1   L   1   +D   2   L   2   +D   trim   L   trim )/( L   1   +L   2   +L   trim )  
         [0039]    The path average dispersion of the transmission span should be selected so that the requisite dispersion trimming element does not significantly degrade the overall performance of the system. In particular, the optical loss, PMD and PDL associated with the dispersion trimming element should be minimized. Accordingly, the path average dispersion of the transmission span should be selected so that the dispersion trimming element only needs to make a small contribution to D average total . Hence the path average dispersion of the transmission span is preferably close to zero. This is not a significant constraint since most long haul systems operate best at small absolute values of dispersion, typically between about 0.1 and 1.0 ps/nm-km in magnitude.  
         [0040]    As a numerical example, assume the path average of the fixed portion of the dispersion map is D 1 =+0.3 ps/nm-km with a period of 50 km, and D trim =−100 ps/nm-km. The addition of 150 m of dispersion trimming fiber can reduce the total path average dispersion D average total  to zero. If an additional 150 m of dispersion trimming fiber is added, D average total  will be changed to −0.3 ps/nm-km. This additional fiber only adds at most an extra fiber loss of about 0.1 dB and perhaps another 0.15 dB for splice losses. The total loss can be directly built into the amplifier design budget.  
         [0041]    As the example illustrates, the dispersion trimming fiber is preferably a high dispersion fiber so that the total path average dispersion can be appropriately adjusted with small a length of fiber as possible. Since high dispersion fiber has a relatively small core area (e.g., about 25 μm 2  for the aforementioned −100 ps/nm-km fiber), the dispersion trimming fiber is preferably added at the end of transmission span, where the signal intensity is lowest, rather than at the beginning of the span where the signal intensity is highest. In this way nonlinear penalties are reduced because the power density in the dispersion trimming fiber will be less when it is positioned at the end of the transmission span. For example, in FIG. 2, dispersion trimming fiber  26  is located at the end of transmission span  20 . Similarly, FIG. 4 shows a schematic diagram of a repeater  40  for a bidirectional transmission system having unidirectional fibers  30  and  32 . The repeater includes optical amplifiers  34  and  36  for providing amplification to signals traveling along fibers  30  and  32 , respectively. As shown, the dispersion trimming fibers  42  and  44  are each located at the respective inputs to the optical amplifiers  34  and  36 , and thus at the end of their respective transmission spans. Of course, in other embodiments of the invention the dispersion trimming fibers (or other adjustable dispersion trimming element) may be located at the output of the optical amplifier preceding a given transmission span.