Abstract:
A dispersion compensating element having a substantially uniform spectral nonlinearity is described. The dispersion compensating unit is thus suitable for incorporation into high speed wavelength division multiplexed (WDM) optical communication systems. Several examples of dispersion compensating elements are discussed, each of which comprising at least one segment of dispersion compensating fiber.

Description:
The present invention is directed toward a wavelength division multiplexed communication system having a dispersion compensation element that provides spectrally uniform nonlinearity behavior. 
     BACKGROUND OF THE INVENTION 
     Optical signals transmitted in a fiber optic communication system typically constitute a series of pulses of digital information. Although the pulses are usually at a single nominal wavelength, each pulse is actually composed different spectral components. These spectral components propagate through the transmission fiber at different speeds with higher frequency components traveling slower than lower frequency components. This effect, known as “chromatic dispersion”, can result in spectral components of one pulse arriving at a receiver at substantially the same time as a succeeding pulse, thereby causing degraded receiver sensitivity. Chromatic dispersion becomes increasingly pronounced at higher bit rates, e.g. those associated with synchronous optical network (SONET) OC-192 transmission speeds. 
     Dispersion compensated fiber, commercially available from Corning, for example, can be used to offset chromatic dispersion. It is known, however, that dispersion compensated fiber has a nonlinearity coefficient γ, which is related to a nonlinearity property of the refractive index of the fiber, n 2 , and the mode field diameter, otherwise referred to as the fiber effective area A eff  (see Agrawal, “Nonlinear Fiber Optics”, Academic Press, Inc., 1995, pp. 37-43). In particular, γ can be expressed as follows:        γ   =       2      π                   n   2         λ                   A   eff                                
     In general, n 2  depends on the fiber composition and dopants, e.g., fluoride doped fibers have a different n 2  value than germanium doped fibers. A eff , however, is related to fiber geometry, doping profile and waveguiding characteristics. For single wavelength transmission applications, n 2  and A eff  have fixed values associated with the transmission wavelength λ. 
     Recently, wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. The wavelengths are typically within a narrow range about 1550 nm, the absorption minimum of silica fiber. At high data speeds, such as OC-192 rates, the dispersion associated with each channel must be compensated. Moreover, the nonlinearity coefficient γ for each channel should be substantially the same, i.e., γ should be spectrally uniform. Otherwise, some channels may have more errors than others, thereby degrading system performance. 
     SUMMARY OF THE INVENTION 
     The nonlinearity coefficient γ of dispersion compensated fiber has been assumed to be spectrally uniform over a relatively broad range of optical wavelengths. It has been found, however, that γ can vary substantially over a relatively narrow wavelength range of wavelengths. In particular, for wavelengths within a narrow range about 1550 nm, n 2 /A eff  values of dispersion compensated fiber as high as 1.7×10 −9  (1/W) (at about 1543 nm) and as low as low as 1.2×10 −9  (1/W) (at about 1557 nm) have been measured. Accordingly, in WDM systems in which optical signals carry high-speed data at these wavelengths, selected channels can have a 40% higher γ than other channels. As a result, the higher γ channels may exhibit different system performance as characterized by different error rates. Accordingly, system design is made difficult. 
     Accordingly, the present invention provides a WDM optical communication apparatus comprising a dispersion compensating element having at least one dispersion compensating fiber. The dispersion compensating element is configured to mitigate the effects of a spectrally nonuniform nonlinearity coefficient behavior of the fiber, thereby giving an effectively uniform nonlinearity behavior for the plural optical signals in a WDM system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates a functional block diagram of an optical communication system comprising an optical amplifier in accordance with a feature of the present invention; 
     FIG. 2 illustrates the optical amplifier shown in FIG. 1 in greater detail; 
     FIG. 3 a  illustrates a dispersion compensating element in accordance with an embodiment of the present invention; 
     FIG. 3 b  illustrates a plot of power vs. wavelength for a dispersion compensating element with spectrally nonuniform γ resulting in spectrally uniform nonlinearity behavior; 
     FIG. 4 a  illustrates a dispersion compensating element in accordance with an additional embodiment of the present invention comprising segments of dispersion compensated fiber; 
     FIG. 4 b  illustrates plots of nonlinearity coefficient γ vs. wavelength for the segments of dispersion compensated fiber shown in FIG. 4 a;    
     FIG. 5 a  illustrates a dispersion compensating element in accordance with a further embodiment of the present invention comprising segments of dispersion compensated; 
     FIG. 5 b  illustrates plots of nonlinearity coefficient γ as a function of wavelength for the segments of dispersion compensated fiber shown in FIG. 5 a;    
     FIG. 6 a  illustrates a dispersion compensating element in accordance with an additional embodiment of the present invention; and 
     FIG. 6 b  illustrates a plot of dispersion compensating fiber length L as a function of wavelength for a given nonlinearity coefficient γ. 
    
    
     DETAILED DESCRIPTION 
     Turning to the drawings in which like reference characters indicate the same or similar elements in each of the several views, FIG. 1 illustrates a WDM communication system  100  in accordance with a feature of the present invention. WDM communication system  100  comprises a plurality of optical transmitters  110 - 1  to  110 -n, each including a directly or externally modulated laser and outputting optical signals at a respective one of wavelengths λ 1  to λ n . The optical signals are fed to combiner  120 , described, for example, in U.S. Pat. No. 5,504,609 incorporated by reference herein, for supplying the optical signals to an optical communication path  150  including an optical fiber. In order to compensate for optical fiber loss, one or more optical amplifiers  130  can be provided along optical communication path  150  for amplifying the optical signals. The optical signals are next input to optical demultiplexer  140 , comprising, for example, an arrayed waveguide grating or in-fiber Bragg gratings, for separating the optical signals based on wavelength. Each optical signal wavelength is supplied to a corresponding one of receivers  150 - 1  to  150 -n. Receivers  150 - 1  to  150 -n each include a photodetector for generating electrical signals for further transmission and/or processing. 
     Optical amplifier  130  is shown in greater detail in FIG.  2 . Optical amplifier  130  includes first and second segments of erbium-doped optical fiber  210  and  220 , each of which being pumped with a known laser source operating at 980 nm and/or 1480 nm to provide appropriate gain. Optical amplifier  130  further comprises a dispersion compensating element  230  having a substantially uniform spectral nonlinearity coefficient γ. As a result, each optical signal has substantially the same error rate, thereby improving system performance and simplifying system design. 
     FIG. 3 a  illustrates a first example of dispersion compensating element  130  comprising a single dispersion compensating fiber  310 . Dispersion compensating fiber  310  is constructed in a known manner to have an optical power vs. wavelength characteristic shown in FIG. 3 b.  In this case, the effective nonlinearity experienced by signals propagating in the dispersion compensating element  310  at relatively low wavelengths and power levels is substantially the same as the effective nonlinearity at higher wavelengths and power levels. Accordingly, in this example, lower wavelength transmitters shown in FIG. 1 should output at lower power levels than high wavelength transmitters in order to obtain a spectrally uniform nonlinearity. 
     A second example of dispersion compensating element  230  will next be described with reference to FIGS. 4 a  and  4   b.  As seen in FIG. 4 a,  dispersion compensating element  230  in this example includes an optical demultiplexer  410 , such as a dielectric filter element, having an input port  410 - 1  that receives the optical signals from erbium-doped fiber segment  210  and outputs higher wavelength optical signals to dispersion compensating fiber  420  via port  410 - 2 . Optical demultiplexer  410  further outputs lower wavelength optical signals via output port  410 - 3  to dispersion compensating fiber  430 . Both dispersion compensating fibers  420  and  430  serve to offset chromatic dispersion present in the transmitted optical signals. After propagating through fibers  420  and  430 , the optical signals are combined by optical combiner  440  and output to erbium-doped fiber segment  220 . 
     Moreover, as shown in FIG. 4 b,  dispersion compensating fiber  420  has a γ vs. wavelength curve  450 , while dispersion compensating fiber  430  has a γ vs. wavelength curve  460 . Typically, optical signals having wavelengths to the right of dashed line  470  in FIG. 4 b,  i.e., the higher wavelength optical signals, are supplied to dispersion compensating fiber  420 . Accordingly, these optical signals having corresponding γ values on the lower portion of curve  450 . On the other hand, optical signals having wavelengths to the left of dashed line  470 , the lower wavelength optical signals, are fed to dispersion compensating fiber  430 . As a result, these optical signals have γ values lying on the upper portion of curve  460  which are substantially the same as the higher wavelength γ values of curve  450 . Thus, both higher and lower wavelength optical signals have substantially the same γ, and substantially uniform nonlinearity can be achieved over a range of wavelengths. 
     In accordance with a third example, dispersion compensating element  230  includes first and second dispersion compensating fibers  510  and  520  shown in FIG. 5 a.  Both fibers effectively offset chromatic dispersion present in the optical signals. However, dispersion compensating fiber  510  is fabricated in a known manner to have a γ vs. wavelength characteristic represented by curve  540  in FIG. 5 b,  while fiber  520  has a characteristic corresponding to curve  530  having a slope opposite of curve  540 . Since fibers  510  and  520  typically have equal length, the nonlinearity coefficients of the two fibers effectively cancel each other, so that the net γ of element  230  is substantially uniform over the range of wavelengths input to dispersion compensating element  230 , thereby providing spectrally uniform nonlinearity behavior. 
     In a fourth example shown in FIG. 6 a,  optical signals output from erbium-doped segment  210  are supplied to an optical demultiplexer  610 , separated, and supplied to a respective one of dispersion compensated fibers  620 - 1  to  620 -n. Each of these fibers has a particular length associated with a wavelength output from demultiplexer  610 , and is configured to effectively offset chromatic dispersion present in a corresponding one of the optical signals. The lengths are chosen so that each of fibers  620 - 1  to  620 -n experience substantially the same nonlinearity. In particular, the lengths are adjusted in accordance with plot  640  shown in FIG. 6 b,  which illustrates dispersion compensating fiber lengths L vs. wavelength. As seen in FIG. 6 b,  lower wavelength optical signals propagating through shorter lengths L of dispersion compensating fiber experience substantially the same nonlinearity as higher wavelength optical signals transmitted through longer sections of dispersion compensating fiber. Thus, by providing longer length dispersion compensating fibers for longer wavelengths and shorter length fiber for shorter wavelengths, substantially uniform spectral nonlinearity behavior can be obtained. As further shown in FIG. 6 a,  each optical signal, having substantially the same effective nonlinearity, is passed to an optical combiner  630  and output to erbium-doped fiber segment  220 . 
     While the foregoing invention has been described in terms of the embodiments discussed above, numerous variations are possible. Accordingly, modifications and changes such as those suggested above, but not limited thereto, are considered to be within the scope of the following claims. For example, in the above examples, the dispersion compensating element is provided within an optical amplifier. It is noted, however, that the dispersion compensating element can be provided at any location within an optical communication system suitable for performing dispersion compensation.