Abstract:
The present invention relates to a dispersion-compensating module having a structure which compensates for the dispersion of an optical transmission line in a wavelength band of 1.55 μm and adjusts loss fluctuations among wavelengths in the wavelength band of 1.55 μm into an appropriate range. The dispersion-compensating module comprises a structure adapted to be installed in an already installed optical fiber transmission line, and has a loss slope with a polarity opposite to that of the optical fiber transmission line in the wavelength band of 1.55 μm. A configurational example of the dispersion-compensating module comprises a dispersion-compensating optical fiber as a dispersion-compensating device, and an optical fiber doped with a transition metal element as a loss-equalizing device. As a consequence, the loss fluctuations among individual wavelengths in the whole transmission line including the dispersion-compensating module are adjusted by the loss-equalizing device in the dispersion-compensating module so as to fall within an appropriate range.

Description:
RELATED APPLICATIONS  
       [0001]    This is a Continuation-In-Part application of a continuation application failed on April 2, 2002 from patent application Ser. No. 09/452,103 filed on Dec. 2, 1999, now pending. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a dispersion-compensating module which improves the transmission quality of large-capacity, high-speed optical transmission systems of WDM (Wavelength Division Multiplexing) type.  
           [0004]    2. Related Background Art  
           [0005]    A WDM type optical transmission system is a system which transmits a plurality of signal light components within a 1.55-μm wavelength band (1.53 μm to 1.57 μm) by way of an optical fiber transmission line network, and enables large-capacity, high-speed optical communications. This optical transmission system comprises an optical amplifier for collectively optically amplifying a plurality of signal light components, and the like, in addition to an optical fiber line which is a transmission medium. In such WDM communications, various studies and developments are under way in order to enable communications with further larger capacity and higher speed.  
           [0006]    One of important subjects to be studied concerning the optical transmission line is reduction of dispersion in its signal wavelength band. Namely, each signal light component has a certain bandwidth even though it is monochromatic. As a result, when dispersion occurs in the wavelength band of the signal light in the optical transmission line, the signal light having reached a receiving station by way of the optical transmission line after being sent out from a transmitting station deforms its waveform, thereby deteriorating its reception. Therefore, in the signal light wavelength band, it is desirable that the dispersion of the optical transmission line be as small as possible.  
           [0007]    However, standard single-mode optical fibers (hereinafter referred to as SMF), having a zero-dispersion wavelength in a 1.3-μm wavelength band, already installed as an optical transmission line have a dispersion of about 16 ps/nm/km in a wavelengthband of 1.53 μm to 1.57 μm which is used in WDM communications. Many of already installed optical transmission lines are constituted by such an SMF. Therefore, a dispersion-compensating module is disposed within a repeater in order to compensate for the dispersion of the optical transmission line, while making use of such an already installed optical transmission line.  
           [0008]    With respect to the dispersion over the whole length of the optical transmission line to be compensated for, the dispersion-compensating module generates a dispersion having an opposite polarity with substantially the same absolute value. Specifically, the dispersion-compensating module comprises a dispersion-compensating optical fiber having a dispersion with a polarity opposite to that of the dispersion of the optical transmission line (including SMF), and compensates for the dispersion of the optical transmission line (including SMF) by adjusting the dispersion-compensating optical fiber to an appropriate length. Also, for reducing the size of the dispersion-compensating module, it is a common practice to wind the dispersion-compensating optical fiber into a coil having a small diameter.  
         SUMMARY OF THE INVENTION  
         [0009]    The inventors have studied conventional dispersion-compensating modules and, as a result, have found out the following problems.  
           [0010]    Recently, in optical amplifiers, the wavelength bandwidth of signal light which can collectively be optically amplified has been expanded, and gain deviations in optically amplifiable signal wavelength bands have been reduced in order to improve the versatility thereof. On the other hand, inter-wavelength loss deviations (fluctuations in loss among wavelengths) occurring in the optical transmission line in the signal light wavelength band are too large to neglect. Also, since inter-wavelength loss deviations in the signal light wavelength band occur to a certain extent in a dispersion-compensating optical fiber as well, it is necessary to improve loss deviations in the whole transmission line including the conventional dispersion-compensating modules.  
           [0011]    When a plurality of stages of conventional dispersion-compensating modules and optical amplifiers are disposed on such an optical transmission line; even if a plurality of signal light components sent out from a transmitting station exhibit no inter-wavelength optical power deviation at the time when sent out, they will generate an optical power deviation, due to inter-wavelength loss deviations, while they are propagating through the optical transmission line and conventional dispersion-compensating modules, and the optical power deviation is expanded by optical amplifiers having a small gain deviation. Since the plurality of signal light components reaching the receiving station constitute signal light in which inter-wavelength optical power deviations are accumulated, the signal light power may become weak depending on wavelength, thereby generating reception errors.  
           [0012]    In order to overcome the above-mentioned problems, it is an object of the present invention to provide a dispersion-compensating module having a structure which compensates for the dispersion of optical transmission lines in a 1.55-μm wavelength band (1.53 μm to 1.57 μm) and adjusts loss fluctuations among signal light components into an appropriate range.  
           [0013]    The dispersion-compensating module according to the present invention is generally installed between repeaters together with an optical amplifier, whereas the object to be compensated for thereby is an SMF which is laid between a transmitting station and a receiving station, between repeater stations, between the transmitting station and a repeater station, or between a repeater station and the receiving station. For being installed on an already installed optical fiber transmission line, the dispersion-compensating module comprises an input end for capturing signal light propagating through the optical fiber transmission line and an output end for sending out the signal light into the optical fiber transmission line, has a positive loss slope in a 1.55-μm wavelength band, and further comprises a structure for allowing the dispersion generated in a predetermined length of the optical fiber transmission line to be compensated for and loss deviations among individual wavelengths to be adjusted into an appropriate range.  
           [0014]    Specifically, the dispersion-compensating module according to the present invention comprises a dispersion-compensating device and a loss-equalizing device. The dispersion-compensating device compensates for the dispersion of the above-mentioned optical fiber transmission line in the 1.55-μm wavelength band. Also, for compensating for the wavelength dependence of loss in at least the above-mentioned optical fiber transmission line and dispersion-compensating device, the loss-equalizing device adjusts the total loss slope of the dispersion-compensating module such that the loss deviations among individual wavelengths in the 1.55-μm wavelength band caused by propagation in the optical fiber transmission line and dispersion-compensating device falls within the appropriate range.  
           [0015]    In this specification, “loss slope” refers to the gradient of a graph indicating the wavelength dependence of transmission loss. Also, the above-mentioned optical fiber transmission line is an SMF having a zero-dispersion wavelength in a 1.3-μm wavelength band; and, letting L be the length of the above-mentioned SMF, and α be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the dispersion-compensating module in the 1.55-μm wavelength band is greater than 0 but not greater than 0.000175×L+α. In general, when an SMF having a zero-dispersion wavelength in a 1.3-μm wavelength band is employed as an optical fiber transmission line, the loss slope per unit length of the SMF is about −0.000175 (dB/nm/km =dB/(nm·km)). Therefore, the loss slope of the dispersion-compensating module is ideally +0.000175×L when an SMF having a length of L is concerned. In practice, however, since the manufacturing error α cannot be neglected, the loss slope (dB/nm) of the dispersion-compensating module in the 1.55-μm wavelength band is greater than 0 but not greater than 0.000175×L+α.  
           [0016]    The loss-equalizing device includes an optical fiber, comprising a core region doped with a transition metal element and a cladding region disposed at an outer periphery of the core region, in which a single mode is secured in the 1.55-μm wavelength band. Preferred examples of the transition metal element include Cr and Co, and the amount of compensation of loss in the loss-equalizing device can be adjusted when the kind of the transition metal element and the amount of addition thereof are appropriately regulated.  
           [0017]    The above-mentioned loss-equalizing device may include an optical fiber formed with a long-period grating in which a propagation mode and a radiation mode are coupled to each other. Alternatively, the above-mentioned dispersion-compensating device may be an optical device having, as the above-mentioned loss-equalizing device, a long-period grating in which a propagation mode and a radiation mode are coupled to each other. In each of these configurations, the long-period grating, which is the loss-equalizing device, enables loss deviations among individual wavelengths to be adjusted in the whole optical transmission line without increasing the transmission loss of the whole dispersion-compensating module. In particular, in the configuration in which the long-period grating, which is the loss-equalizing device, is formed in the optical fiber acting as the dispersion-compensating device, the dispersion-compensating device does not have a connecting portion which may yield loss. Consequently, it is not necessary to take account of influences of transmission loss in the connecting portion, whereby loss fluctuations among individual wavelengths can be adjusted more easily. Here, as explicitly shown in U.S. Pat. No. 5,703,978, the long-period grating is a grating which induces coupling (mode coupling) between a core mode and a cladding mode which propagate through an optical fiber, and is clearly distinguished from a short-period grating for reflecting light centered at a predetermined wavelength. Also, in the long-period grating, for yielding a strong power conversion from the core mode to the cladding mode, the grating period (pitch) is set such that the optical path difference between the core mode and the cladding mode becomes 2π. As a consequence, since the long-period grating acts so as to couple the core mode to the cladding mode, the core mode attenuates over a narrow band centered at a predetermined wavelength (hereinafter referred to as “loss wavelength”).  
           [0018]    The above-mentioned loss-equalizing device can also be realized by connecting respective end portions of a pair of optical fibers by fusion. In this case, the fused portion of the pair of optical fibers functions as the loss-equalizing device.  
           [0019]    Preferably, the pair of optical fibers are connected by fusion at the fused portion while their optical axes are shifted from each other. This fused portion may also be realized by connecting the pair of optical fibers by fusion while their core regions are bent. Also, when a pair of optical fibers each having a core region with an outside diameter expanding toward the opposed portion is connected to each other by fusion, the fused portion functions as the loss-equalizing device.  
           [0020]    The loss-equalizing device may include a fiber coupler, or an optical fiber bent at one or more parts thereof. A desirable loss wavelength characteristic can be obtained in each of these cases.  
           [0021]    Further, the loss-equalizing device may include any one of a slant type fiber grating, which comprises an optical fiber and a grating formed in the optical fiber and being inclined at a predetermined angle with respect to an optical axis of the optical fiber, and a dielectric multilayered filter. The loss-equalizing device preferably have a valiable wavelength chanracteristic, and includes, for example, a variable loss-equalizing device having a planar waveguide or a variable loss-equalizing device with MEMS (Micro Electro Mechanical Systems).  
           [0022]    The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.  
           [0023]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a view showing a schematic configuration of a first embodiment of the dispersion-compensating module according to the present invention;  
         [0025]    [0025]FIGS. 2A to  2 E are graphs showing relationships between loss and wavelength in respective parts in each of dispersion-compensating modules according to first, second, fourth, fifth, and sixth embodiments;  
         [0026]    [0026]FIG. 3A is a graph showing the loss wavelength characteristic of a standard single-mode optical fiber having a zero-dispersion wavelength in a 1.3-μm wavelength band, whereas FIG. 3B is a graph showing, to a larger scale, the loss wavelength characteristic of the single-mode optical fiber in the vicinity of a 1.5-μm wavelength band in the graph shown in FIG. 3A;  
         [0027]    [0027]FIG. 4A is a graph showing an example of the loss wavelength characteristic of a loss-equalizing optical fiber whose core region is doped with Co element (transition metal element), whereas FIG. 4B is a graph showing, to a larger scale, the loss wavelength characteristic of the loss-equalizing optical fiber in the vicinity of the 1.5-μm wavelength band in the graph shown in FIG. 4A;  
         [0028]    [0028]FIG. 5 is a view showing a schematic configuration of a second embodiment of the dispersion-compensating module according to the present invention;  
         [0029]    [0029]FIG. 6 is a graph showing an example of the loss wavelength characteristic of a long-period grating;  
         [0030]    [0030]FIG. 7 is a view showing a schematic configuration of a third embodiment of the dispersion-compensating module according to the present invention;  
         [0031]    [0031]FIGS. 8A to  8 D are graphs showing relationships between loss and wavelength in respective parts in the dispersion-compensating module according to the third embodiment shown in FIG. 7;  
         [0032]    [0032]FIG. 9 is a view showing a schematic configuration of a fourth embodiment of the dispersion-compensating module according to the present invention;  
         [0033]    [0033]FIGS. 10A to  10 C are views for explaining specific configurations of the fused portion as a loss-equalizing device in the dispersion-compensating module according to the fourth embodiment shown in FIG. 9;  
         [0034]    [0034]FIG. 11 is a graph showing an example of the loss wavelength characteristic of the fusedportion shown in FIGS. 10A to  10 C;  
         [0035]    [0035]FIG. 12 is a view showing a schematic configuration of a fifth embodiment of the dispersion-compensating module according to the present invention;  
         [0036]    [0036]FIG. 13 is a view showing a schematic configuration of a sixth embodiment of the dispersion-compensating module according to the present invention; and  
         [0037]    [0037]FIG. 14 is a view showing a schematic common configuration of seventh to tenth embodiments of the dispersion-compensating module according to the present invention; and  
         [0038]    [0038]FIGS. 15A to  15 F are views for explaining specific configurations of loss-equalizing devices in the dispersion-compensating modules according to the seventh to tenth embodiments. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    In the following, embodiments of the dispersion-compensating module according to the present invention will be explained with reference to FIGS. 1, 2A to  4 B,  5  to  7 ,  8 A to  8 D,  9 ,  10 A to  10 C,  11  to  14  and  15 A to  15 F. In the explanation of the drawings, constituents identical to each other will be referred to with numerals or letters identical to each other without repeating their overlapping descriptions.  
         [0040]    First Embodiment  
         [0041]    To begin with, a first embodiment of the dispersion-compensating module according to the present invention will be explained. FIG. 1 is a view showing a schematic configuration of the dispersion-compensating module according to the first embodiment. This drawing shows, in addition to the dispersion-compensating module  10  according to this embodiment, a repeater  1  disposed upstream of the dispersion-compensating module  10 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  10 .  
         [0042]    The dispersion-compensating module  10  according to this embodiment has an input end  10   a  and an output end  10   b,  and is disposed while in a state where a dispersion-compensating device and a loss-equalizing device are optically connected to each other in the optical path between the input end  10   a  and the output end  10   b.  In particular, the dispersion-compensating module  10  is constituted by a dispersion-compensating optical fiber  11 , as the dispersion-compensating device, and an optical fiber  12  doped with a transition metal element, as the loss-equalizing device, which are connected to each other by fusion at a connecting portion  13 .  
         [0043]    The dispersion-compensating optical fiber  11  is an optical device which compensates for the chromatic dispersion in the WDM signal wavelength band of the optical transmission line  2  into which the dispersion-compensating module  10  is inserted. On the other hand, the transition-metal-element-doped optical fiber  12  is an optical fiber, basically comprising a core region and a cladding region disposed at the outer periphery of the core region, in which a transition metal element such as Cr element, Co element, or the like is added at least into the core region. When the kind and amount of the transition metal element added to the core region are appropriately selected, then the loss wavelength characteristic of the optical fiber  12  itself is adjusted so as to compensate for wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  11 . As a consequence, the total loss fluctuation in the signal wavelength band of the optical transmission line  2  provided with the dispersion-compensating module  10  decreases.  
         [0044]    [0044]FIGS. 2A to  2 E are graphs showing relationships between transmission loss and wavelength in respective parts in the dispersion-compensating module according to first embodiment. In particular, FIG. 2A is a graph showing the relationship between transmission loss and wavelength in a wavelength band of 1.53 μm to 1.57 μm in the optical transmission line  2  employing an SMF having a zero-dispersion wavelength in a 1.3-μm wavelength band. FIG. 2B is a graph showing the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the dispersion-compensating optical fiber  11  acting as the dispersion-compensating device. FIG. 2C is a graph showing the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the transition-metal-element-doped optical fiber  12  acting as the loss-equalizing device. FIG. 2D is a graph showing the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the whole dispersion-compensating module  10 . FIG. 2E is a graph showing the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the whole optical transmission line provided with the dispersion-compensating module  10 .  
         [0045]    As shown in FIGS. 2A and 2B, each of the optical transmission line  2  and dispersion-compensating optical fiber  11  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. By contrast, as shown in FIG. 2C, the transition-metal-element-doped optical fiber  12  is a single-mode optical fiber whose core region is doped with Co element at a concentration of about 10 ppm, which is designed such that its transmission loss becomes greater as the wavelength is longer, so as to be able to compensate for wavelength-dependent loss deviations in view of the loss slopes of the optical transmission line  2  and dispersion-compensating optical fiber  11 .  
         [0046]    Therefore, as shown in FIG. 2D, the total loss in the dispersion-compensating module  10  is the sum of respective losses in the dispersion-compensating optical fiber  11  and the transition-metal-element-doped optical fiber  12 , and becomes greater as the wavelength is longer in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown in FIG. 2E, the total loss in the optical transmission line  2  and the dispersion-compensating module  10  is the sum of the irrespective losses, and yields a deviation of 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm, whereby its wavelength dependence is weaker than that of the loss deviation of each constituent.  
         [0047]    [0047]FIGS. 3A and 3B are graphs showing the loss wavelength characteristic of a standard SMF having a zero-dispersion wavelength in a 1.3-μm wavelength band. The graph of FIG. 3A shows the loss characteristic within a wavelength range of 1200 μm to 1700 μm; whereas the graph of FIG. 3B enlarges a part of FIG. 3A, so as to show the loss characteristic within a wavelength range of 1480 μm to 1620 μm. This SMF has a stepped index type refractive index profile, whose core region is doped with Ge element while silica is used as a base. As shown in these graphs, the loss in this SMF per unit length (km) varies about 0.007 dB/km between wavelengths of 1530 nm and 1570 nm. In the wavelength band having a width of 40 nm (=1570-1530), the loss slope of the SMF per unit length is approximately −0.007/40=−0.000175 dB/nm/km (whereby the loss on the longer wavelength side tends to become smaller).  
         [0048]    [0048]FIGS. 4A and 4B are graphs showing an example of the loss wavelength characteristic of a loss-equalizing optical fiber whose core region is doped with Co element. The graph of FIG. 4A shows the loss characteristic within a wavelength range of 600 μm to 1800 μm; whereas the graph of FIG. 4B enlarges a part of FIG. 4A, so as to show the loss characteristic within a wavelength range of 1500 μm to 1600 μm. This loss-equalizing optical fiber has a stepped index type refractive index profile, whose core region is doped with Co element while silica is used as a base. As shown in these graphs, the loss slope of this loss-equalizing optical fiber is positive in the wavelength band of 1.53 μm to 1.57 μm. This loss slope can be adjusted by the amount of addition of Co element and the like.  
         [0049]    When the SMF is used as the optical transmission line  2 , the loss slope of the optical transmission line  2  in the wavelength band of 1.53 μm to 1.57 μm is negative as mentioned above. Therefore, if the loss slope of the whole dispersion-compensating module  10  is set positive, then the total loss in the optical transmission line  2  and dispersion-compensating module  10  can fall within an appropriate range.  
         [0050]    Specifically, since the SMF employed as the optical transmission line  2  has a loss slope per unit length of about −0.000175 (dB/nm/km=dB/(nm km)) in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiber length of the optical transmission line  2 , the loss slope (dB/nm) of the whole dispersion-compensating module  10  in the wavelength band of 1.53 μm to 1.57 μm is ideally a value which is greater than 0 but not greater than 0.000175×L.  
         [0051]    The loss slope of the loss-equalizing optical fiber (loss-equalizing device) is set to an appropriate value by adjusting the amount of addition of Co element or the like, such that, while the loss slope of the dispersion-compensating optical fiber (dispersion-compensating device) is taken into consideration, the loss slope of the whole dispersion-compensating module  10  falls within the range mentioned above. In practice, however, the manufacturing error of the loss-equalizing optical fiber must be taken into consideration, whereby the loss slope value S (dB/nm) of the whole dispersion-compensating module  10  becomes 0&lt;S≦0.000175×L+α where α is the absolute value of manufacturing error of the loss-equalizing optical fiber, which is specifically about 000 dB/nm.  
         [0052]    When the loss slope of the loss-equalizing optical fiber is controlled as such, the loss deviations among individual wavelengths occurring in the optical transmission line  2  exceeding, for example, 80 km and the dispersion-compensating optical fiber can fall within an appropriate range in the dispersion-compensating module  10  as a whole.  
         [0053]    Second Embodiment  
         [0054]    A second embodiment of the dispersion-compensating module according to the present invention will now be explained. FIG. 5 is a view showing a schematic configuration of the dispersion-compensating module according to the second embodiment. This drawing shows, in addition to the dispersion-compensating module  20  according to this embodiment, a repeater  1  disposed upstream of the dispersion-compensating module  20 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  20 .  
         [0055]    The dispersion-compensating module  20  according to this embodiment has an input end  20   a  and an output end  20   b,  and is disposed while in a state where a dispersion-compensating device and a loss-equalizing device are optically connected to each other in the optical path between the input end  20   a  and the output end  20   b.  In particular, the dispersion-compensating module  20  is constituted by a dispersion-compensating optical fiber  21 , as the dispersion-compensating device, and an optical fiber  23  formed with a long-period fiber grating  22 , as the loss-equalizing device, which are connected to each other by fusion at a connecting portion  24 . The optical fiber  23  is preferably an SMF or dispersion-compensating optical fiber having a zero-dispersion wavelength in a 1.3-μm wavelength band.  
         [0056]    The dispersion-compensating optical fiber  21  is an optical device for compensating for the chromatic dispersion in the WDM signal wavelength band of the optical transmission line into which the dispersion-compensating module  20  is inserted. The long-period fiber grating  22  is obtained when a refractive index change having a predetermined period is generated in at least a core region of the optical fiber  23 , in which the period of refractive index change is a long period on the order of several hundreds of micrometers, whereby the core-mode light propagating through the core region and the cladding-mode light radiated to the cladding region are coupled together. By appropriately selecting the period of refractive index change and length, the long-period fiber grating  22  is designed such that, for example, the transmission loss at a wavelength of 1520 nm is minimized, while the transmission loss at a wavelength of 1570 nm is maximized, whereby wavelength-dependent loss deviations of the optical transmission line  2  and the dispersion-compensating optical fiber  21  are compensated for.  
         [0057]    Therefore, the wavelength dependence of the total loss in the optical transmission line  2  and dispersion-compensating module  20  is weaker than that of the loss deviation in each of the dispersion-compensating optical fiber  21  and long-period fiber grating  22 . When the long-period fiber grating  22  is thus used as the loss-equalizing device, loss deviations among individual signal light components can fall within an appropriate range without greatly decreasing the transmission loss in the whole dispersion-compensating module  20 . Also, desirable transmission characteristics can easily be obtained in a wide wavelength band. Here, the long-period fiber grating  22  is an optical component which is clearly distinguished from a short-period fiber grating which reflects only a signal light component having a predetermined wavelength (see U.S. Pat. No. 5,703,978).  
         [0058]    Since graphs showing the relationships between transmission loss and wavelength in the dispersion-compensating module  20  according to the second embodiment are similar to FIGS. 2A to  2 E, operations of the dispersion-compensating module  20  according to this embodiment will be explained with reference to these graphs.  
         [0059]    As shown in FIGS. 2A and 2B, each of the optical transmission line  2  and dispersion-compensating optical fiber  21  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. By contrast, as shown in FIG. 2C, the long-period fiber grating  22 , which is the loss-equalizing device, is designed such that its transmission loss becomes greater as the wavelength is longer, so as to be able to compensate for wavelength-dependent loss deviations in view of the loss slopes of the optical transmission line  2  and dispersion-compensating optical fiber  21 .  
         [0060]    Therefore, as shown in FIG. 2D, the total loss in the dispersion-compensating module  20  is the sum of respective losses in the dispersion-compensating optical fiber  21  and the long-period grating  22 , and becomes greater as the wavelength is longer in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown in FIG. 2E, the total loss in the optical transmission line  2  and the dispersion-compensating module  20  is the sum of their respective losses, and yields a deviation of 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm.  
         [0061]    [0061]FIG. 6 is a graph showing an example of the loss wavelength characteristic of a long-period fiber grating. For making this long-period fiber grating, a silica-based optical fiber having a stepped index type refractive index profile, whose core region is doped with Ge element, is irradiated with ultraviolet rays through an intensity-modulating mask, so as to generate a refractive index modulation in the core region. As shown in this graph, the loss slope of the long-period fiber grating is positive in the wavelength band of 1.53 μm to 1.57 μm. This loss slope can be adjusted by the period of refractive index change and the length.  
         [0062]    As mentioned in the foregoing, when the SMF is used as the optical transmission line  2 , the loss slope of the optical transmission line  2  in the wavelength band of 1.53 μm to 1.57 μm is negative in this embodiment as well. Therefore, if the loss slope of the whole dispersion-compensating module  20  is set positive, then the total loss in the optical transmission line  2  and dispersion-compensating module  20  can fall within an appropriate range.  
         [0063]    Also, since the SMF employed as the optical transmission line  2  has a loss slope per unit length (km) of about −0.000175 (dB/nm/km=dB/(nm km)) in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiber length of the optical transmission line  2 , and α be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the whole dispersion-compensating module  20  in the wavelength band of 1.53 μm to 1.57 μm is preferably a value which is greater than 0 but not greater than 0.000175×L+α.  
         [0064]    Here, the loss slope of the long-period grating  22  is adjusted by appropriately setting the grating period and length such that the loss slope of the whole dispersion-compensating module  20  falls within the range mentioned above in view of the loss slope of the dispersion-compensating optical fiber  21 .  
         [0065]    Third Embodiment  
         [0066]    A third embodiment of the dispersion-compensating module according to the present invention will now be explained. FIG. 7 is a view showing a schematic configuration of the dispersion-compensating module according to the third embodiment. This drawing shows, in addition to the dispersion-compensating module  30  according to this embodiment, a repeater  1  disposed upstream of the dispersion-compensating module  30 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  30 .  
         [0067]    The dispersion-compensating module  30  according to this embodiment has an input end  30   a  and an output end  30   b,  and is disposed while in a state where a dispersion-compensating device and a loss-equalizing device are optically connected to each other in the optical path between the input end  30   a  and the output end  30   b.  In particular, the dispersion-compensating module  30  is constituted by a dispersion-compensating optical fiber  31 , as the dispersion-compensating device, and a long-period fiber grating  32 , as the loss-equalizing device, directly formed in the dispersion-compensating optical fiber  31 .  
         [0068]    The dispersion-compensating optical fiber  31  is an optical device for compensating for the chromatic dispersion in the WDM signal wavelength band of the optical transmission line into which the dispersion-compensating module  30  is inserted. The long-period fiber grating  32  is obtained when a refractive index change having a predetermined period is generated in at least a core region of the dispersion-compensating optical fiber  31 , in which the period of refractive index change is a long period on the order of several hundreds of micrometers, whereby the core-mode light propagating through the core region and the cladding-mode light radiated to the cladding region are coupled together. By appropriately selecting the period of refractive index change and the length, the long-period fiber grating  32  is designed such that, for example, the transmission loss at a wavelength of 1520 nm is minimized, while the transmission loss at a wavelength of 1570 nm is maximized, whereby wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  31  are compensated for.  
         [0069]    Therefore, the total loss in the optical transmission line  2  and dispersion-compensating module  30  is the sum of the transmission loss in the optical transmission line  2 , the original transmission loss in the dispersion-compensating optical fiber  31 , and the transmission loss in the formed long-period fiber grating  32 , thereby weakening the wavelength dependence as a whole. When the long-period fiber grating  32  is thus used as the loss-equalizing device, loss deviations among individual signal light components can fall within an appropriate range without greatly decreasing the transmission loss in the whole dispersion-compensating module  30 . Also, desirable loss characteristics can easily be obtained in a wide wavelength band. Further, in the third embodiment, since the long-period fiber grating  32 , as the loss-equalizing device, is directly formed in the dispersion-compensating optical fiber  31 , there is no connecting portion which may yield a loss, whereby it is unnecessary to take account of the influence of the loss in the connecting portion.  
         [0070]    [0070]FIGS. 8A to  8 D are graphs showing relationships between transmission loss and wavelength in the dispersion-compensating module  30  according to the third embodiment. FIG. 8A is a graph showing the relationship between transmission loss and wavelength in a wavelength band of 1.53 μm to 1.57 μm in the optical transmission line  2  employing the SMF. FIG. 8B is a graph showing the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the dispersion-compensating optical fiber  31  before the long-period fiber grating  32  is formed. FIG. 8C is a graph showing the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the dispersion-compensating optical fiber  31  after the long-period fiber grating  32  is formed, i.e., the relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the dispersion-compensating module  30 . FIG. 8D is a graph showing the total relationship between transmission loss and wavelength in the wavelength band of 1.53 μm to 1.57 μm in the optical transmission line  2  and dispersion-compensating module  30 .  
         [0071]    As shown in FIG. 8A, the optical transmission line  2  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. Also, as shown in FIG. 8B, the dispersion-compensating optical fiber  31  before the formation of the long-period fiber grating  32  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope.  
         [0072]    On the other hand, the long-period fiber grating  32  has a loss which becomes greater as the wavelength is longer, thereby compensating for the original loss deviations among individual wavelengths of the optical transmission line  2  and dispersion-compensating optical fiber  31 . As shown in FIG. 8C, the total loss in the dispersion-compensating optical fiber  31  formed with the long-period fiber grating  32 , i.e., the whole dispersion-compensating module  30 , is the sum of the original transmission loss in the dispersion-compensating optical fiber  31  and the transmission loss in the long-period fiber grating  32 , and has a positive loss slope in the wavelength band of 1.53 μm to 1.57 μm. As shown in FIG. 8D, the total loss in the optical transmission line  2  and the dispersion-compensating module  30  is the sum of their respective losses, and yields a deviation of 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm.  
         [0073]    As mentioned in the foregoing, when the SMF is used as the optical transmission line  2 , the loss slope of the optical transmission line  2  in the wavelength band of 1.53 μm to 1.57 μm is negative in this embodiment as well. Therefore, if the loss slope of the whole dispersion-compensating module  30  is set positive, then the total loss in the optical transmission line  2  and dispersion-compensating module  30  can fall within an appropriate range.  
         [0074]    Also, since the SMF employed as the optical transmission line  2  has a loss slope per unit length (km) of about −0.000175 (dB/nm/km=dB/(nm·km)) in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiber length of the optical transmission line  2 , and a be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the whole dispersion-compensating module  30  in the wavelength band of 1.53 μm to 1.57 μm is preferably a value which is greater than 0 but not greater than 0.000175×L+α.  
         [0075]    Here, the loss slope of the whole dispersion-compensating module  30  is adjusted by appropriately setting the grating period and length of the long-period grating  32 .  
         [0076]    Fourth Embodiment  
         [0077]    A fourth embodiment of the dispersion-compensating module according to the present invention will now be explained. FIG. 9 is a view showing a schematic configuration of the dispersion-compensating module according to the fourth embodiment. This drawing shows, in addition to the dispersion-compensating module  40  according to this embodiment, a repeater  1  disposed upstream of the dispersion-compensating module  40 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  40 .  
         [0078]    The dispersion-compensating module  40  according to this embodiment is constituted by a dispersion-compensating optical fiber  41 , as a dispersion-compensating device, and a single-mode optical fiber  42  which are connected to each other by fusion at a fused portion  43 . In this configuration, the dispersion-compensating optical fiber  41  is an optical device which compensates for the chromatic dispersion in the signal light wavelength band of the optical transmission line into which the dispersion-compensating module  40  is inserted. Though the fused portion  43  generates a loss, its wavelength characteristic varies depending on fusion conditions such as the heating temperature upon connecting by fusion and the amount of intrusion of the fiber, whereby the wavelength dependence of transmission loss in the fused portion  43  can be adjusted by appropriately setting these fusion conditions.  
         [0079]    [0079]FIGS. 10A to  10 C are views showing specific examples of the dispersion-compensating module according to the fourth embodiment. A specific structure of the fused portion  43  can be realized when, as shown in FIG. 10A for example, the core region  41   a  of the dispersion-compensating optical fiber  41  and the core region  42   a  of the single-mode optical fiber  42  are fused together while their optical axes AX 1 , AX 2  are shifted from each other by a predetermined distance D. It can also be realized when, as shown in FIG. 10B, the dispersion-compensating optical fiber  41  and the single-mode optical fiber  42  are connected to each other by fusion while each of the core region  41   b  of the dispersion-compensating optical fiber  41  and the core region  42   b  of the single-mode optical fiber  42  is minutely bent. Further, as shown in FIG. 10C, the core region  41   c  of the dispersion-compensating optical fiber  41  and the core region  42   c  of the single-mode optical fiber  42  may be configured so as to expand their diameters toward the fused portion  43 . These specific examples can be combined. For example, in the fused portion  43 , the radius of bend of the core region may be expanded, or structures of bending the core region may be combined together. In each of these cases, the deviation of total loss in the optical transmission line  2  and dispersion-compensating module  40  can be kept at 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm.  
         [0080]    Since graphs showing the relationships between transmission loss and wavelength in the dispersion-compensating module  40  according to the fourth embodiment are similar to FIGS. 2A to  2 E, operations of the dispersion-compensating module  40  will be explained with reference to these graphs.  
         [0081]    As shown in FIGS. 2A and 2B, each of the optical transmission line  2  and dispersion-compensating optical fiber  41  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. By contrast, as shown in FIG. 2C, one of the amount of shift of optical axes, amount of bending of optical axes, and expanded core diameter of the fused portion  43 , which is the loss-equalizing device, is designed such that its transmission loss becomes greater as the wavelength is longer, so as to be able to effectively compensate for wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  41 .  
         [0082]    Therefore, as shown in FIG. 2D, the total loss in the dispersion-compensating module  40  is the sum of respective losses in the dispersion-compensating optical fiber  41  and the fused portion  43 , and becomes greater as the wavelength is longer in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown in FIG. 2E, the total loss in the optical transmission line  2  and the dispersion-compensating module  40  is the sum of their respective losses, and yields a deviation of 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm.  
         [0083]    [0083]FIG. 11 is a graph showing an example of the loss wavelength characteristic of the fused portion. As shown in this graph, the loss slope of this fusedportion is positive in the wavelength band of 1.53 μm to 1.57 μm. This loss slope can be adjusted by the amount of shift of optical axes, the amount of bending of optical axes, and the expanded core diameter.  
         [0084]    As mentioned in the foregoing, when the SMF is used as the optical transmission line  2 , the loss slope of the optical transmission line  2  in the wavelength band of 1.53 μm to 1.57 μm is negative in this embodiment as well. Therefore, if the loss slope of the whole dispersion-compensating module  40  is set positive, then the total loss in the optical transmission line  2  and the dispersion-compensating module  40  can fall within an appropriate range.  
         [0085]    Also, since the SMF employed as the optical transmission line  2  has a loss slope per unit length (km) of about −0.000175 dB/nm/km in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiber length of the optical transmission line  2 , and α be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the whole dispersion-compensating module  40  in the wavelength band of 1.53 μm to 1.57 μm is preferably a value which is greater than 0 but not greater than 0.000175×L+α.  
         [0086]    Though the fusion connection between the dispersion-compensating optical fiber  41  and the SMF  42  is explained in the fourth embodiment, the configuration of the fused portion  43  should not be restricted thereto. For example, an SMF may be used in place of the dispersion-compensating optical fiber  41 , and a dispersion-compensating optical fiber or other optical fibers may be used in place of the SMF  42 . In any case, if the wavelength dependence of transmission loss in the fused portion therebetween is adjusted, then the wavelength dependence of the total loss in the optical transmission line and dispersion-compensating module can be weakened.  
         [0087]    Fifth Embodiment  
         [0088]    A fifth embodiment of the dispersion-compensating module according to the present invention will now be explained. FIG. 12 is a view showing a schematic configuration of the dispersion-compensating module according to the fifth embodiment. This drawing shows, in addition to the dispersion-compensating module  50  according to this embodiment, a repeater  1  disposed upstream of the dispersion-compensating module  50 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  50 .  
         [0089]    The dispersion-compensating module  50  according to this embodiment is disposed in a state where a dispersion-compensating device and a loss-equalizing device are optically connected to each other in the optical path between an input end  50   a  and an output end  50   b.  Specifically, this embodiment comprises a dispersion-compensating optical fiber  51  as the dispersion-compensating device and a fiber fusion type coupler (WDM coupler)  52  as the loss-equalizing device. The WDM coupler  52  preferably has a polarization-dependent loss (PDL) of 0.2 dB or less.  
         [0090]    The dispersion-compensating optical fiber  51  is an optical device which compensates for the chromatic dispersion in the WDM signal wavelength band of the optical transmission line into which the dispersion-compensating module  50  is inserted. The WDM coupler  52  is obtained by fusing together two optical fibers disposed in parallel; and, its fusion conditions and coupling length are appropriately selected such that, for example, the transmission loss at a wavelength of 1520 nm is minimized, while the transmission loss at a wavelength of 1570 nm is maximized, whereby wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  51  are compensated for. As a consequence, the wavelength dependence of the total loss in the optical transmission line  2  and dispersion-compensating module  50  is weakened as a whole.  
         [0091]    Since graphs showing the relationships between transmission loss and wavelength in the dispersion-compensating module  50  according to the fifth embodiment are similar to FIGS. 2A to  2 E, operations of the dispersion-compensating module  50  will be explained with reference to these graphs.  
         [0092]    As shown in FIGS. 2A and 2B, each of the optical transmission line  2  and dispersion-compensating optical fiber  51  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. By contrast, as shown in FIG. 2C, the fusion conditions and coupling length of the WDM coupler  52 , which is the loss-equalizing device, are designed such that its transmission loss becomes greater as the wavelength is longer, so as to be able to effectively compensate for wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  51 .  
         [0093]    Therefore, as shown in FIG. 2D, the total loss in the dispersion-compensating module  50  is the sum of respective losses in the dispersion-compensating optical fiber  51  and the WDM coupler  52 , and becomes greater as the wavelength is longer in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown in FIG. 2E, the total loss in the optical transmission line  2  and the dispersion-compensating module  50  is the sum of their respective losses, and yields a deviation of 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm.  
         [0094]    As mentioned in the foregoing, when the SMF is used as the optical transmission line  2 , the loss slope of the optical transmission line  2  in the wavelength band of 1.53 μm to 1.57 μm is negative in this embodiment as well. Therefore, if the loss slope of the whole dispersion-compensating module  50  is set positive, then the total loss in the optical transmission line  2  and dispersion-compensating module  50  can fall within an appropriate range.  
         [0095]    Also, since the SMF employed as the optical transmission line  2  has a loss slope per unit length (km) of about −0.000175 (dB/nm/km=db/(nm·km)) in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiber length of the optical transmission line  2 , and a be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the whole dispersion-compensating module  50  in the wavelength band of 1.53 μm to 1.57 μm is preferably a value which is greater than 0 but not greater than 0.000175×L+α.  
         [0096]    Here, the loss slope of the WDM coupler  52  is adjusted by appropriately setting the fusion conditions and coupling length such that the loss slope of the whole dispersion-compensating module  50  falls within the range mentioned above in view of the loss slope of the dispersion-compensating optical fiber  51 .  
         [0097]    Sixth Embodiment  
         [0098]    A sixth embodiment of the dispersion-compensating module according to the present invention will now be explained. FIG. 13 is a view showing a schematic configuration of the dispersion-compensating module according to the sixth embodiment. This drawing shows, in addition to the dispersion-compensating module  60  according to this embodiment, a repeater  1  disposed upstream of the dispersion-compensating module  60 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  60 .  
         [0099]    The dispersion-compensating module  60  according to this embodiment has an input end  60   a  and an output end  60   b,  and is disposed while in a state where a dispersion-compensating device and a loss-equalizing device are optically connected to each other in the optical path between the input end  60   a  and the output end  60   b.  In particular, the dispersion-compensating module  60  is constituted by a dispersion-compensating optical fiber  61 , as the dispersion-compensating device, and an optical fiber  63  having a bent portion  62 , as the loss-equalizing device, which are connected to each other by fusion at a connecting portion  64 . The optical fiber  63  is preferably an SMF or dispersion-compensating optical fiber having a zero-dispersion wavelength in a 1.3-μm wavelength band. It is also preferable that the optical fiber  63  be common with the dispersion-compensating optical fiber  61 .  
         [0100]    The dispersion-compensating optical fiber  61  is an optical device for compensating for the chromatic dispersion in the WDM signal wavelength band of the optical transmission line into which the dispersion-compensating module  60  is inserted. In the bent portion  62 , a plurality of parts of the optical fiber  63  are bent at a predetermined curvature over a predetermined length. The bent portion  62  is designed by appropriately selecting the length and curvature such that, for example, the transmission loss at a wavelength of 1520 nm is minimized, while the transmission loss at a wavelength of 1570 nm is maximized, whereby wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  61  are compensated for. As a consequence, the wavelength dependence of the total loss in the optical transmission line  2  and dispersion-compensating module  60  is weaker than that of the respective loss deviations of the dispersion-compensating optical fiber  61  and the bent portion  62 .  
         [0101]    Since graphs showing the relationships between transmission loss and wavelength in the dispersion-compensating module  60  according to the sixth embodiment are similar to FIGS. 2A to  2 E, operations of the dispersion-compensating module  60  will be explained with reference to these graphs.  
         [0102]    As shown in FIGS. 2A and 2B, each of the optical transmission line  2  and dispersion-compensating optical fiber  61  has a transmission loss which becomes smaller as the wavelength is longer in general in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. By contrast, as shown in FIG. 2C, the bent portion  60 , which is the loss-equalizing device, is designed such that its transmission loss becomes greater as the wavelength is longer, so as to be able to effectively compensate for wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber  61 .  
         [0103]    Therefore, as shown in FIG. 2D, the total loss in the dispersion-compensating module  60  is the sum of respective losses in the dispersion-compensating optical fiber  61  and the bent portion  62 , and becomes greater as the wavelength is longer in the wavelength band of 1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown in FIG. 2E, the total loss in the optical transmission line  2  and the dispersion-compensating module  60  is the sum of their respective losses, and yields a deviation of 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm.  
         [0104]    As mentioned in the foregoing, when the SMF is used as the optical transmission line  2 , the loss slope of the optical transmission line  2  in the wavelength band of 1.53 μm to 1.57 μm is negative in this embodiment as well. Therefore, if the loss slope of the whole dispersion-compensating module  60  is set positive, then the total loss in the optical transmission line  2  and dispersion-compensating module  60  can fall within an appropriate range.  
         [0105]    Also, since the SMF employed as the optical transmission line  2  has a loss slope per unit length (km) of about −0.000175 (dB/nm/km=db/(nm·km)) in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiber length of the optical transmission line  2 , and a be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the whole dispersion-compensating module  60  in the wavelength band of 1.53 μm to 1.57 μm is preferably a value which is greater than 0 but not greater than 0.000175×L+α.  
         [0106]    Here, the loss slope of the whole dispersion-compensating module  60  is set by appropriately setting the length and curvature of the bent portion  62  so as to adjust the loss slope of the bent portion  62 .  
         [0107]    In each of the above-mentioned first to sixth embodiments, either the dispersion-compensating device or the loss-equalizing device may be disposed upstream of the other. In view of influences of nonlinear optical phenomena (four-wave mixing in particular), however, it is preferable that the loss-equalizing device be disposed upstream of the dispersion-compensating device. Namely, as a consequence, signal light enters the dispersion-compensating device after incurring a loss due to the loss-equalizing device, whereby nonlinear optical phenomena such as four-wave mixing and the like are effectively restrained from occurring.  
         [0108]    Seventh to Tenth Embodiments  
         [0109]    [0109]FIG. 14 is a view showing a schematic common configuration of seventh to tenth embodiments of the dispersion-compensating module according to the present invention. This drawing shows, in addition to the dispersion-compensating module  70  according to each of seventh to tenth embodiments, a repeater  1  disposed upstream of the dispersion-compensating module  70 , and an optical transmission line  2  between the repeater  1  and the dispersion-compensating module  70 .  
         [0110]    The dispersion-compensating module  70  has an input end  70   a  and an output end  70   b,  and comprises a dispersion-compensating device  71  and a loss-equalizing device  72  optically connected to each other at a connection portion  73 . These devices  71  and  72  are disposed in the optical path between the input end  70   a  and the output end  70   b.    
         [0111]    The dispersion-compensating device  71  is preferably a dispersion-compensating optical fiber as an optical device which compensates for the chromatic dispersion in the WDM signal wavelength band of the optical transmission line  2  into which the dispersion-compensating module  70  is inserted. On the other hand, the loss-equalizing device  72  has a loss wavelength characteristic adjusted so as to compensate for wavelength-dependent loss deviations of the optical transmission line  2  and dispersion-compensating optical fiber as the dispersion-compensating device  71 . As a consequence, the total loss fluctuation in the signal wavelength band of the optical transmission line  2  provided with the dispersion-compensating module  70  decreases.  
         [0112]    In the dispersion-compensating module  70  according to the seventh embodiment, the loss-equalizing device  72  includes a slant type fiber grating. The slant type fiber grating  721 , as shown in FIG. 15A, comprises a optical fiber and a grating formed in the optical fiber while being inclined at a predetermined angle with respect to an optical axis OP of the optical fiber. It is known that a loss of the slant type fiber grating  721  as the loss-equalizing device  72  has a wavelength dependency as described in Isabelle Riant, et al. “36 NM AMPLIFIER GAIN EQUALIZER BASED ON SLANTED BRAGG GRATING TECHNOLOGY FOR MULTICHANNEL TRANSMISSION”, SubOptic 2001 International Convention, P.4.3.10. The Isabelle reference teaches the use of the slant type fiber grating for EDFAs as a gain equalizer, but a positive loss slope of the whole dispersion-compensating module  70  can be realized by combining the dispersion-compensating optical fiber as the dispersion-compensating device  71  and this slant type fiber grating  721  with desirably modified design. The slant type fiber grating  721  in the seventh embodiment includes a single slanted grating and a combination of a plurality of slanted gratings.  
         [0113]    Next, the loss-equalizing device  72  in the dispersion-compensating module according to the eighth embodiment includes a dielectric multilayered filter  722  shown in FIG. 15B. A loss wavelength dependency of the dielectric multilayered filter  722  can be adjusted by adjusting a thickness, a refractive index of layer, the number of layer thereof. Therefore, a positive loss slope of the dispersion-compensating module  70  can be realized by combining the dispersion-compensating optical fiber as the dispersion-compensating device  71  and this dielectric multilayered filter  722 .  
         [0114]    Furthermore, the loss-equalizing device  72  of FIG. 14 may has a structure with a variable loss wavelength characteristic. The loss-equalizing device  72  in the ninth embodiment, as sown in FIG. 15C, includes a variable loss-equalizing device having a planar waveduide (see Hitoshi Hatayama, et al., “Low loss variable attenuation slope compensator with high slope linearity based on planar lightwave circuit”, ECOC 2000, 26 th  European Conference on Optical Communication, pp.287-288). The variable loss-equalizing device  723  of FIG. 15C comprises a substrate  723   a,  a waveguide  723   b  formed on the substrate  723   a,  and a heater  723   c.  As be understood from the Hitoshi reference, the loss-equalizing device  723  can modify it&#39;s loss slope. For example, the reference shows the loss wavelength characteristic from 1570 nm to 1605 nm. Therefore, a positive loss slope of the whole dispersion-compensating module  72  according to the ninth embodiment can be realized by desirably changing a design of this variable loss-equalizing device  723  and by combining the dispersion-compensating optical fiber as the dispersion-compensating device  71  and this variable loss-equalizing device  723 .  
         [0115]    Further, the loss-equalizing device  72  may includes a variable loss-equalizing device with MEMS (Micro Electro Mechanical Systems). FIGS. 15D to  15 F show a configuration of the variable loss-equalizing device in tenth embodiment. The loss-equalizing device is shown in James A. Walker, “Telecommunications Applications of MEMS”, mstnews 3/00, pp. 6-9 and J. E. Ford, et al., “PASSBAND-FREE DYNAMIC WDM EQUALIZATION”, ECOC&#39;98, 20-24 September 1998, pp. 317-318. In the loss-equalizing device of the tenth embodiment, input light is demultiplexed at the grating, and the reflected light from the grating is introduced to the device plane of the optical MEMS device through λ/4 plate and focus lens. On the device plane of the optical MEMS device, a plurality ofmirror devices, as shown in FIGS. 15E and 15F, areprovided. Each of these mirror devices can change it&#39;s reflectance on the based on an added voltage. The light from the device plate is multiplexed at the grating and outputted through the mirror and collimator. By adjusting voltages added to the plurality of mirror devices respectively, a loss wavelength characteristic of components in the demultiplexed wavelength range can be changed, and therefore the combination of the dispersion-compensating optical fiber as the dispersion-compensating device  71  and this variable loss-equalizing device of FIGS. 15D to  15 F can make a loss slope of the whole dispersion-compensating module  70  positive.  
         [0116]    According to the present invention, as explained in the foregoing, the dispersion of the optical transmission line in the wavelength band of 1.53 μm to 1.57 μm is compensated for by the dispersion-compensating device, whereas the loss deviations of the optical transmission line and dispersion-compensating device in the wavelength band of 1.53 μm to 1.57 μm are compensated for by the loss-equalizing device. Namely, not only the dispersion of the optical transmission line is compensated for, but also the wavelength dependence of the total loss in the optical transmission line and dispersion-compensating module is weaker. As a consequence, the intensity level deviations among individual wavelengths of the WDM signal reaching the receiving station are small, and each wavelength component of the WDM signal reaches the receiving station with a sufficient intensity level and SN ratio, whereby no reception error occurs in the receiving station.  
         [0117]    In particular, since the dispersion-compensating module as a whole has a positive loss slope in the wavelength band of 1.53 μm to 1.57 μm with respect to an optical transmission line made of an SMF having a zero-dispersion wavelength in the wavelength band of 1.3 μm (whereas the optical transmission line has a negative loss slope in the wavelength band of 1.53 μm to 1.57 μm), the total loss in the optical transmission line and dispersion-compensating module can fall within an appropriate range.  
         [0118]    Also, letting L (km) be the fiber length of the optical transmission line, and a be the absolute value of a permissible manufacturing error, the loss slope (dB/nm) of the dispersion-compensating module in the wavelength band of 1.53 μm to 1.57 μm is a value which is greater than 0 but not greater than 0.000175×L+α. Since the optical transmission line made of an SMF having a zero-dispersion wavelength in the wavelength band of 1.3 m is about −0.000175 dm/nm/km in the wavelength band of 1.53 μm to 1.57 μm; even if the loss slope of the optical transmission line in the wavelength band of 1.53 μm to 1.57 μm has a fluctuation, the total loss in the optical transmission line and dispersion-compensating module can fall within an appropriate range when the loss slope of the whole dispersion-compensating module is set to a value within the range mentioned above.  
         [0119]    From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.