Abstract:
In a wavelength-division multiplexing system with an OADM, part of the chromatic dispersion on a transmission line is compensated for by a chromatic dispersion compensator for a dropped wavelength of the wavelength-division multiplexing system. Also, chromatic dispersion is compensated for by a chromatic dispersion compensator for an added wavelength of the wavelength-division multiplexing system. The chromatic dispersion compensator for dropped wavelength acts on the signal dropped by the OADM, and the chromatic dispersion compensator for added wavelength acts on the added signal. Both the chromatic dispersion compensators act on the passing signal. With the chromatic dispersion compensators being mounted in the optical transmission apparatus before the system is upgraded to OADM, it is not necessary to alter the chromatic dispersion compensating method and the variation of the communication quality can be suppressed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an optical transmission apparatus and compensating method with capabilities of effective compensation for the chromatic dispersion of optical fibers as transmission lines, and of avoiding the dispersion compensation method alteration and adjustment when the relay unit is upgraded to the optical add drop multiplexer (abbreviated as OADM) in the wavelength-division multiplexing optical transmission system. 
     As means for a large transmission capacity of optical transmission system, the wavelength-division multiplexing (abbreviated WDM) transmission system is practically used in which a single optical fiber is used to transmit a plurality of optical signals with different wavelengths. In addition, an optical fiber amplifier such as an erbium-doped fiber amplifier (hereafter, referred to as optical amplifier) exhibits a characteristic of amplifying in block a plurality of optical signals over a wide range of wavelengths. Therefore, a combination of the WDM and optical amplifier can achieve the simultaneous amplification of a plurality of optical different-wavelength signals, with the result that a large-capacity, long-distance transmission can be realized economically and with simple construction. 
     However, the optical fiber as a transmission line has a chromatic dispersion characteristic under which the wavelengths of light propagating within the fiber transmit with different velocities. It follows that the signal waveforms deteriorate as the light propagates in the optical fiber. Therefore, a dispersion compensation fiber having a dispersion value opposite to that of the transmission line is introduced to combine with the transmission line, thereby reducing the effect of the chromatic dispersion of the transmission line. Thus, this technique reshapes the deteriorated signal waveforms to be correct waveforms. 
     Recently, demand has increased to change from the simple communication type in which communication is simply made between two points as represented by the point-to-point system. One approach would use a bus-shape OADM mode, in which communication is made between a plurality of points with a plurality of branches and inserts provided between the two opposite points, as represented by the bus-shape system, so that the optical signals can be transmitted therein as they are. Another approach would use a ring-shape OADM mode, in which communication is made between a plurality of points connected by branches and inserts in a ring shape, as represented by the ring-shape system, so that the optical signals can be transmitted therein as they are. 
     In this OADM mode, too, the dispersion compensation is extremely important in order to achieve an excellent transmission characteristic. Thus, the dispersion compensation technique is expected to be simpler and more excellent. 
     There is known a conventional dispersion compensation method in the wavelength-division multiplexing system. This conventional method considers the self-phase modulation effect in the relay transmission using the optical amplifier (for example, see JP-A-7-74699, the fourth to fifth items in  FIG. 1 . 
     In the wavelength-division multiplexing system of the OADM mode, a system-upgrading method is advantageous to reduce the introduction cost and increase the efficiency. In this upgrading method, the point-to-point system is built at the time of the initial introduction of the system. The OADM function is added later, with the increase of communication demand. At this time, before and after the system has been upgraded to add the OADM function, it is desired that there be no need to particularly alter or adjust other portions than the added OADM function. 
     However, the addition of OADM function actually causes the communication quality to degrade together with the reduction of optical signal-to-noise (S/N) ratio, and the system&#39;s performance to go down at the time of addition. In addition, the alteration of the dispersion compensation method greatly affects the system construction and network so as to change the communication quality. 
     In the conventional wavelength-division multiplexing system, the point-to-point system is dominantly demanded, but the bus-shape system or ring-shape system using the OADM equipment is not demanded so much. However, recently the wavelength-division multiplexing system has also been demanded to have high efficiency, and flexibility of network as the communication traffic and different kinds of data are increased, and the users have had an interest in the alteration of system construction due to the addition of OADM function. Particularly, the alteration of dispersion compensation method that has close relationship with the communication quality of the system becomes a factor of reducing the communication quality at the time of adding the OADM function, and thus it is one of the items to which utmost attention must be paid in the system construction. 
     In the technique described in the above-mentioned JP-A-7-74699, the waveform deterioration and timing jitter due to the nonlinear effect have been reduced by cutting to zero the total dispersion value after the points where the nonlinear effects such as the above self-phase modulation effect or mutual phase modulation effect occurred. However, this chromatic dispersion compensation method is described mainly about the application to the wavelength-division multiplexing system in the point-to-point system, but not about the wavelength-division multiplexing system having the OADM function and the upgrading to that system. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a dispersion compensation method and apparatus capable of making stable dispersion compensation without giving any change to the existing equipment even when the point-to-point system is upgraded to an OADM system. 
     An optical transmission apparatus, according to one feature of the invention, transmits wavelength-division multiplexed light from a first optical transmission line to a second optical transmission line. The apparatus includes a first dispersion compensator for compensating for the chromatic dispersion that occurs while the wavelength-division multiplexed light is transmitted from a first point on the first optical transmission line to the optical transmission apparatus. A second dispersion compensator compensates for the waveform dispersion that occurs while the wavelength-division multiplexed light propagates from the optical transmission apparatus to a second point on the second optical transmission line, thereby carrying out dispersion compensation. 
     Thus, an add drop portion for realizing the OADM function can be mounted between the first and second dispersion compensators or demounted from between them. In addition, just before and after the mounting or demounting, those compensators do not affect the transmission characteristics of other signals including the dropped or added signal. Therefore, change of the communication quality due to the upgrading from the relay equipment to the OADM unit can be suppressed. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram to which reference is made in explaining the principle of the first embodiment of the invention. 
         FIG. 2  is a diagram to which reference is made in explaining the first embodiment of the invention. 
         FIG. 3  is a diagram to which reference is made in explaining the first embodiment of the invention. 
         FIG. 4  is a diagram to which reference is made in explaining the principle of the second embodiment of the invention. 
         FIG. 5  is a diagram to which reference is made in explaining the second embodiment of the invention. 
         FIG. 6  is a diagram to which reference is made in explaining the second embodiment of the invention. 
         FIG. 7  is a diagram to which reference is made in explaining the second embodiment of the invention. 
         FIG. 8  is a detailed diagram to which reference is made in explaining the second embodiment of the invention. 
         FIG. 9  is a diagram to which reference is made in explaining the problems with the third embodiment of the invention. 
         FIG. 10  is a diagram to which reference is made in explaining the problems with the third embodiment of the invention. 
         FIG. 11  is a diagram to which reference is made in explaining the third embodiment of the invention. 
         FIG. 12  is a detailed diagram to which reference is made in explaining the third embodiment of the invention. 
         FIG. 13  is a diagram to which reference is made in explaining other modifications of the third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  diagrammatically shows the basic principle of the dispersion compensation method in the wavelength-division multiplexing system as the first embodiment of the invention. A transmission terminal  110  has an optical transmitter  111  and a preamplifier  112 . In the transmission terminal  110 , the optical transmitter  111  is actually formed of a plurality of optical transmitters the number of which corresponds to that of the wavelengths of the wavelength-division multiplexed optical signal emitted from the transmission terminal  110 . In addition, a wavelength multiplexer not shown is provided to multiplex the wavelengths of the optical signals produced from the optical transmitters. The above description is also applicable to other embodiments which will be described later. 
     The optical signals generated from the optical transmitter  111  are amplified by the preamplifier  112 , and supplied to an optical fiber  100 - 1  as a transmission line. A relay device  120  is provided to compensate for the light intensity lost during the time in which the optical signals are propagated in the optical fiber  100 . In this case, the relay device  120  amplifies the attenuated optical signals, and again supplies the intensity-increased optical signals to the following-stage optical fiber  100  provided on the downstream side. 
     In this construction, the chromatic dispersion in the optical fiber  100  is compensated for by a dispersion compensator  123  mounted on the following relay device  120 . For example, the dispersion in the optical fiber  100 - 1  is compensated for by a dispersion compensator  123 - 1  mounted on a relay device  120 - 1 , and the dispersion in an optical fiber  100 - 2  by a dispersion compensator  123 - 2  mounted on a relay device  120 - 2 . 
     After repetition of necessary stages according to the transmission distance or the like, the optical signals arrive at a receiving terminal  130 . The receiving terminal  130  has a post-amplifier  131 , a dispersion compensator  132 , and an optical receiver  133  mounted thereon. The arriving optical signals are amplified by the post-amplifier  132 . Then, after the dispersion in the optical fiber  100 - 3  is compensated for by the dispersion compensator  132 , the optical signals are received by the optical receiver  133 . In the receiving terminal  130 , the optical receiver  133  is actually formed of a plurality of optical receivers the number of which corresponds to that of the wavelengths of the wavelength-division multiplexed optical signal. In addition, a wavelength separator not shown is provided therein to separate the optical signals of different wavelengths. This is also applicable to other embodiments which will be described later. 
       FIG. 2  shows an example of the construction of the relay device  120  having an OADM function as when the point-to-point system of  FIG. 1  is upgraded to the OADM system. The upgraded relay device  120  has a wavelength dropping portion  200  and a wavelength adding portion  201  provided in addition to the non-upgraded construction in which the dispersion compensator  123  is provided between the separate optical amplifiers  121 ,  122 . 
     In this upgraded relay device  120 , like the non-upgraded one, the dispersion compensator  123  is mounted to compensate for the chromatic dispersion in the optical fiber  100  connected on the upstream side. In addition, the wavelength dropping portion  200  causes some of the wavelengths to be branched off, or dropped in the direction indicated by  220 , or optical signals  210  of a certain wavelength band are separated and taken out. In addition, the wavelength adding portion  201  causes some wavelengths to be inserted, or added in the direction indicated by  221 , or optical signals  211  of a certain wavelength band are multiplexed with the light fed from the dropping portion  200 . 
     In this case, the transmission characteristic to which the optical signals fed directly through the relay device  120  to the receiver  133  are subjected can be made equal to the transmission characteristics exhibited to the optical signal  210  of a wavelength band fed from the transmitter  101  and dropped at the upgraded relay device  120  and to the optical signal  211  of a wavelength band added at this relay device  120  until they are thereafter transmitted and arrive at the corresponding receiver  133 . 
       FIG. 3  shows the construction of an upgraded one of the point-to-point system of  FIG. 1  in the case where the relay device  120 - 1  is changed to an upgraded relay device having the OADM function. The received wavelength-division multiplexed light is fed to the upgraded relay device  120 - 1 , where the lost part of the light attenuated in the transmission line  100 - 1  is recovered by the optical amplifier  121 - 1  and then the chromatic dispersion caused in the transmission line  100 - 1  is compensated for by the chromatic dispersion compensator  123 - 1 . At a wavelength dropping portion  301 , an optical signal of a predetermined band is dropped from the compensated wavelength-division multiplexed light, and fed to an external device including an optical receiver  310 . An optical signal from an external device including an optical transmitter  311  that produces the optical signal of a predetermined band is supplied to the upgraded relay device, where a wavelength adding portion  302  multiplexes it with the wavelength-division multiplexed light and an optical amplifier  122 - 2  amplifies the additionally-multiplexed light and supplies it to the transmission line  100 - 2 . 
     As described above, the OADM function can be added to simply upgrade the relay device  120  with the same transmission characteristic kept, without changing the construction of other devices (such as the transmitting terminal  110 , relay device  120 - 2  and receiving terminal  130 ) and with requiring no special adjustment. 
     However, when the transmission distance between the transmitting terminal  110  and receiving terminal  130  becomes long, the number of the stages of relay device  120  to be provided between them increases, resulting in the accumulation of the noise occurring in each relay device and the nonlinear effects of the optical fibers of the transmission line. This accumulation will sometimes limit the transmission capacity and transmission distance. 
       FIG. 4  is a diagram useful for explaining the chromatic dispersion in the second embodiment of the invention that considers the above nonlinear effect. A chromatic dispersion compensator  113  is provided in the transmitting terminal  110  in order to compensate for the chromatic dispersion occurring when the signal is transmitted from the transmitting terminal  110  up to a predetermined distance La  400 - 1  on the transmission line  100 - 1 . Also, a chromatic dispersion compensator  125 - 1  is mounted in the relay device  120 - 1  in order to compensate for both the chromatic dispersions occurring in the remaining part of the transmission line  100 - 1  after the distance La  400 - 1  and in the range from the relay device  120 - 1  up to a predetermined distance Lb  400 - 2  on the transmission line  100 - 2 . 
     A chromatic dispersion compensator  125 - 2  similarly mounted in the relay device  120 - 2  compensates for the chromatic dispersion occurring in the remaining part after the distance Lb  400 - 2  on the transmission line  100 - 2  and for the chromatic dispersion caused in the range from the relay device  120 - 2  up to a predetermined distance Lc  400 - 3  on the transmission line  100 - 3 . A chromatic dispersion compensator  135  mounted in the receiving terminal  130  compensates for the chromatic dispersion caused in the remaining part of the transmission line  100 - 3  after the distance Lc  400 - 3 . 
     Each of the predetermined distances La  400 - 1 , Lb  400 - 2 , Lc  400 - 3  is substantially uniquely determined in consideration of the optical fiber&#39;s characteristics, and is about 20 km. How to determine this distance is described in, for example, the above document 1. 
     Since the nonlinear effect can be reduced in the system in which the chromatic dispersion in the optical fiber is compensated for in this way, even a system having a large number of optical relay devices provided along a long distance fiber can be built to have excellent characteristics. 
     Here, let it be considered to upgrade the relay device  120  to a device having the OADM function. Under the upgrading method in which the wavelength dropping portion  301  and wavelength adding portion  302  are simply added after the chromatic dispersion compensator  123 , the transmission characteristics of the respective optical wavelength signals before the upgrading cannot be made the same as those after the upgrading, because the amounts of the chromatic dispersion compensation in the above chromatic dispersion compensator  123  consider the chromatic dispersion occurring in the remaining part of the optical fiber provided before the corresponding relay device and the chromatic dispersion caused in the range from this relay device up to a predetermined distance on the next optical fiber provided after this relay device. Therefore, the optical signal of a predetermined wavelength band dropped at the wavelength dropping portion  301  is excessively compensated for its chromatic dispersion. In addition, the optical signal of a wavelength band added by the wavelength adding portion  302  is insufficiently compensated for its chromatic dispersion by the amount corresponding to the range from this relay device up to the predetermined distance on the optical fiber provided after this relay device. 
     Thus, the relay device  120  that is expected to be upgraded to the device having the OADM function is previously constructed as shown in  FIG. 5  at the relay device  120 - 2 . In other words, referring to  FIG. 5 , two chromatic dispersion compensators  500 - 2 ,  501 - 2  are mounted in the relay device  120 - 2 . The chromatic dispersion compensator  500 - 2  is constructed to compensate for the chromatic dispersion occurring in the remaining part of the transmission line  100 - 2  after the predetermined distance Lb  400 - 2  (on the relay device  120 - 2  side). The chromatic dispersion compensator  501 - 2  is constructed to compensate for the chromatic dispersion caused in the range from the relay device up to the predetermined distance Lc  400 - 3  on the transmission line  100 - 3 . That is, the function of the chromatic dispersion compensator  125 - 2  in  FIG. 4  is replaced by the two chromatic dispersion compensators  500 - 2 ,  501 - 2 . Therefore, since the nonlinear effect can be reduced like the case shown in  FIG. 4 , even a system having a large number of optical relay devices along a long distance fiber can be built to have excellent characteristics. 
     A method of upgrading the optical relay device  120 - 2  in  FIG. 5  to a device having the OADM function will be described below. As shown in  FIG. 6 , a wavelength dropping portion  601  and a wavelength adding portion  602  are added between the chromatic dispersion compensators  500 ,  501  of the relay device  120 - 2 . The relay device  120 - 2  is previously divided in its function into modules so that these wavelength dropping and adding portions can be added and that connectors for connecting other modules to be added thereto can be provided. This module structure will help the later upgrading operation be made with ease. 
       FIG. 7  shows the construction of a transmission system that has the OADM function achieved by using the device  120 - 2  graded as in  FIG. 6 . The wavelength-division multiplexed light received from the transmission line  100 - 2  is fed to an optical amplifier  121 - 2  where its loss caused in the fiber is recovered by the amplification. The amplified wavelength-division multiplexed light is supplied to the chromatic dispersion compensator  500 , and compensated thereby for its chromatic dispersion caused in the remaining part of the transmission line  100 - 2  after the predetermined distance Lb  400 - 2 . The compensated wavelength-division multiplexed light is fed to the waveform dropping portion  601 , where an optical signal of a predetermined band is extracted from it. The extracted optical signal is supplied to an external device including an optical receiver  610 . An optical signal from an external device including an optical transmitter  611  for producing the optical signal of a predetermined band is supplied to the wavelength adding portion  602 , where it is multiplexed with the wavelength-division multiplexed light. The additionally multiplexed light is amplified by the optical amplifier  122 - 2 , and then fed to the transmission line  100 - 3 . 
     Under the above construction, the wavelength-division multiplexed light produced from the previous-stage chromatic dispersion compensator  500  has no chromatic dispersion because the chromatic dispersion the light had just when it was supplied to the upgraded relay device  120 - 2  was all compensated for by the compensator  500 . Therefore, the optical signal received by the external device through the wavelength dropping portion  601  has its chromatic dispersion already similarly compensated for, or can be received as an excellent-characteristic optical signal with the chromatic dispersion compensated for. In addition, the following chromatic dispersion compensator  501  compensates the optical signal produced from the optical transmitter  611  for its chromatic dispersion caused in the range from the relay device  120 - 2  to the predetermined distance Lc  400 - 3  on the optical transmission line  100 - 3 , and a chromatic dispersion compensator  135  of the receiving terminal  130  compensates this optical light for its chromatic dispersion caused in the remaining part of the transmission line  100 - 3  after the predetermined distance Lc  400 - 3 . Therefore, the optical receiver  133  can receive the excellent-characteristic multiplexed light having all the chromatic dispersion compensated for. 
     A method of canceling out the chromatic dispersions caused in the wavelength dropping portion  601  and wavelength adding portion  602  will be described below. Even in the wavelength dropping and adding portions  601 ,  602 , the optical signals that pass therethrough or that are dropped or added sometimes cause chromatic dispersion. This chromatic dispersion will be caused by mismatching among the connected optical fibers or among the components used in the optical amplifiers, and wavelength dropping and adding portions. The chromatic dispersion caused in the devices and components is in some case, not completely removed with ease, but can be adjusted to a predetermined amount by designing the group delay of each optical component. Therefore, if the chromatic dispersion occurring in a certain portion is tried to compensate by adjusting the chromatic dispersion in other portions, the chromatic dispersion can be probably overall compensated for. 
     For example, it is assumed that chromatic dispersion of +D occurs in the signal passing through the wavelength dropping portion  601  and that chromatic dispersion of +d occurs in the dropped signal. At this time, if the chromatic dispersion occurring in the signal passing through the following-stage wavelength adding portion  602  can be adjusted to be −D, the chromatic dispersions caused in the optical signals passing through the upgraded relay device  120 - 2  can be cancelled out to be zero. 
     In addition, if the optical signal that is to be dropped at the wavelength dropping portion  601  of the relay device  120 - 2  can be adjusted for its chromatic dispersion to be −d when it passes through the wavelength-division multiplexer that is, though not shown, mounted in the transmitting terminal  110 , the chromatic dispersions caused in the dropped optical signal can be cancelled out to be zero at the wavelength dropping portion  601 . 
       FIG. 8  shows an example of the detailed construction of the upgraded optical relay device  120  in which the OADM function is mounted as illustrated in  FIG. 6 . 
     In the pre-stage amplifier  121 , an optical supervisory channel signal (OSC light) that includes wavelength number information and so on is extracted from the input optical wavelength-division multiplexed signal by a wavelength divider  121 - 11 , and the optical supervisory channel signal is fed to and converted by an opto-electric converter  121 - 12  into an electric signal, which is then fed to a controller  121 - 20 . The input wavelength-division multiplexed light passed through the wavelength divider  121 - 11  is fed to a power splitter  121 - 13 , where the optical power is separated. The optical power is converted into an electric signal by use of an opto-electric converter  121 - 14 , and then fed to the controller  121 - 20 . 
     Similarly, the output optical power is separated by use of a power splitter  121 - 18 . The separated optical power is converted into an electric signal by using an opto-electric converter  121 - 19 , and then fed to the controller  121 - 20 . In addition, the pumping light from a pumping laser diode  121 - 16  is multiplexed with the wavelength-division multiplexed light by a pumping light multiplexer  121 - 15 . The wavelength-division multiplexed light is then fed to an amplifying doped fiber  121 - 17 , where it is amplified. 
     The controller  121 - 20  of the input-side optical amplifier  121  uses the optical power of the input signal, the optical power of the output signal, the wavelength number information included in the OSC light or the control signal from a device controller  800  which will be described later to control the pumping laser diode  121 - 16  to generate the pumping power so that an optimum gain can be obtained in the amplifying doped fiber  121 - 17 . 
     Similarly, in the output-side amplifier  122 , the optical power is separated by a power splitter  122 - 11 , and converted by an opto-electric converter  122 - 12  into an electric signal, which is then supplied to a controller  122 - 20 . In addition, the output optical power is separated by a power splitter  122 - 16 . The separated optical power is converted by an opto-electric converter  122 - 17  into an electric signal, which is then fed to the controller  122 - 20 . The pumping light from a pumping laser diode  122 - 14  is multiplexed with the wavelength-division multiplexed light by a pumping light multiplexer  122 - 13 . The resulting wavelength-division multiplexed light is amplified by an amplifying doped fiber  122 - 15 . 
     The controller  122 - 20  uses the optical power of the input signal, the optical power of the output signal or a control signal from the device controller  800  which will be described later to control the pumping laser diode  122 - 14  to generate the pumping power so that an optimum gain can be obtained in the amplifying doped fiber  122 - 15 . In addition, the control information from the device controller and other control information for use in the following devices are supplied through an electro-optical converter  122 - 19  to a wavelength multiplexer  122 - 18 , where it is multiplexed with the wavelength-division multiplexed light. 
     In an add drop portion  600  added for upgrade between the chromatic dispersion compensators  500 ,  501 , the input optical power is separated by a power splitter  600 - 1 . The separated optical power is converted by an opto-electric converter  600 - 2  into an electric signal, which is then transmitted to a controller  600 - 11 . Similarly, the optical power of the output light is separated by use of a power splitter  600 - 9 . The separated optical power is converted by an opto-electric converter  600 - 10  into an electric signal, which is then supplied to the controller  600 - 11 . 
     The signal light of a band to be dropped by this add drop portion  600  is separated by a demultiplexer  600 - 3 , and supplied through a power splitter  600 - 4  to the outside as a dropped signal  621 . In addition, the optical power of the branched signal is extracted by the power splitter  600 - 4 , and converted by an opto-electric converter  600 - 8  into an electric signal, which is then transmitted to the controller  600 - 11 . 
     Signal light  620  of a band to be added from the outside by the add drop portion  600  is multiplexed by a multiplexer  600 - 7  through a power splitter  600 - 5 . In addition, the optical power of the added signal  620  is extracted by the power splitter  600 - 5 , and converted by an opto-electric converter  600 - 6  into an electric signal, which is then fed to the controller  600 - 11 . The optical supervisory channel signals from the pre-stage amplifier  121 , post-stage amplifier  122  and add drop portion  600  are transmitted to the device controller  800 . 
     Since it cannot be decided, by only monitoring the optical power, whether the optical power is insufficient or the optical power is observed to be small because of, originally, a small wavelength multiplex number, the wavelength number information received by the input-side amplifier  121  is used, making it possible to control the input-side amplifier  121  to properly amplify the power. The wavelength multiplex number to the output-side amplifier  122  can be computed on the device controller  800  by using the wavelength number information to the input-side amplifier  121 , and the number of dropped wavelengths and the number of added wavelengths processed by the add drop portion  600  as follows.
 
(Number of multiplexed wavelengths to the output-side amplifier)=(Number of multiplexed wavelengths to the input-side amplifier)−(Number of dropped wavelengths)+(Number of added wavelengths)  [Equation 1]
 
     A method for the chromatic dispersion compensation further using the function to compensate for a gain tilt will be described as the third embodiment of the invention. 
       FIG. 9  is a diagram to which reference is made in explaining the gain tilt caused in the optical amplifier. Since the gain of the optical amplifier depends on the wavelength, the light intensities of the multiplexed optical signals sometimes have a difference depending on their wavelengths. If a wavelength  902  at around the center of the wavelength band of the optical signals multiplexed as multiplexed light is compared with a wavelength  901  that is shorter than the wavelength  902  as, for example, shown in  FIG. 9 , it will be seen that the signal intensity of the wavelength  901  is weaker than the wavelength  902  because an intensity difference  910  occurs between them. As a result, the optical S/N ratio might be reduced. 
     In addition, it will be seen that the intensity of the wavelength  903  becomes larger than that of wavelength  902 , thus causing a light-intensity difference  911 . Thus, the wavelength  903  is more influenced by the nonlinear effect within the optical fiber. Because of these phenomena, it is difficult to make the signal quality uniform over all the wavelengths that are bundled within the wavelength-division multiplexed light. 
       FIG. 10  is a diagram to which reference is made in explaining the operation of a gain tilt equalizer for reducing the inter-wavelength gain tilt shown in  FIG. 9 . The gain tilt equalizer equalizes the above light-intensity difference between the wavelengths. When the light intensity of longer wavelengths is larger than that of shorter wavelengths (upward-sloping characteristic) as shown in  FIG. 9 , control is made so that a characteristic to decrease the light intensity of longer wavelength side as indicated by  1004  or  1005  in  FIG. 10  can be obtained to cancel out the upward-sloping characteristic. Contrarily, when the light intensity of shorter wavelengths is larger than that of longer wavelengths (downward-sloping characteristic), control is carried out so that a characteristic to increase the light intensity of longer wavelength side as indicated by  1002  or  1003  can be obtained to cancel out the downward-sloping characteristic. In addition, the amount of control in the gain tilt equalizer is changed as at  1004  or  105  according to the magnitude of the light-intensity difference caused in the optical amplifier. 
       FIG. 11  shows one example of the optical amplifier with the gain tilt equalizer of  FIG. 10  mounted. A gain tilt equalizer  1100 - 8  is mounted to equalize the gain tilt caused in an amplifying doped fiber  1100 - 5 . The output light from the amplifying doped fiber  1100 - 5  is supplied to a power splitter  1100 - 6 , where the optical power is extracted. The extracted optical power is supplied through an opto-electric converter  1100 - 7  to a controller  1100 - 9  as optical power information of the output light. The controller  1100 - 9  controls the output power of a pumping laser diode  1100 - 4  so that the power of the output light is a predetermined value. A pumping multiplexer  1100 - 3  multiplexes it with the wavelength-division multiplexed light, and supplies the resulting light to the amplifying doped fiber  1100 - 5 . 
     The gain tilt equalizer  1100 - 8  is controlled by a control signal from the controller  1100 - 9 . The gain tilt of the amplifying doped fiber  1100 - 5  is generally dependent on the power of the input light. In addition, the power of the input light can be observed by using a power splitter  1100 - 1  and an opto-electric converter  1100 - 2  as is the optical power of the output light. Therefore, if the gain tilt characteristic of the amplifying doped fiber  1100 - 5  responsive to the input light power is previously acquired by measurement, simulation or the like and stored in a parameter memory, not shown, within the controller  1100 - 9 , the gain tilt of amplifying doped fiber  1100 - 5  based on the optical power of the input light can be automatically adjusted. 
       FIG. 12  shows an example of an add drop unit having means for directly observing the gain tilt caused in the amplifying doped fiber and applying it to the control of the gain tilt equalizer. This add drop unit has the optical amplifier  121  on the input side and the add drop portion  600  on the output side like the construction of  FIG. 8 . The optical amplifier  121  has a gain tilt equalizer  121 - 30  at its output end. In  FIG. 12 , the details of the control of the gain tilt equalizer  121 - 30  is particularly shown. The internal arrangement and operation of the optical amplifier  122  are the same as described previously. 
     The add drop portion  600  has the optical power detecting opto-electric converter  600 - 2  to which the input signal is fed, and the optical power detecting opto-electric converter  600 - 8  to which the dropped signal is supplied. The optical power signals from those converters are supplied through the controller  600 - 11  to the device controller  800 . 
     Moreover, in the input-side optical amplifier  121 , the multiplexed-wavelengths number information included in the optical supervisory channel light (OSC light) that the wavelengths multiplexed light includes is extracted by the supervisory channel wavelength divider  121 - 11  and opto-electric converter  121 - 12 , and fed through the controller  121 - 20  to the device controller  800 . 
     In the device controller  800 , the input optical power detected by the opto-electric converter  600 - 2  of the add drop portion  600  is divided by the multiplexed-wavelengths number information from the input-side optical amplifier  121 , thus making it possible to compute the average light intensity of all the wavelengths-multiplexed light. For example, the average light intensity corresponds to the light intensity  902  at around the central wavelength in  FIG. 9 . 
     In addition, the light intensity of the optical signal dropped at the add drop portion  600  corresponds to, for example, the light intensity  901  of the shorter wavelength side light shown in  FIG. 9  if the band of the dropped signal is on the shorter wavelength side of the multiplexed light. If the band is on the longer wavelength side of the multiplexed light, the light intensity corresponds to the light intensity  903  of the longer wavelength side light. Therefore, in the device controller  800 , the gain tilt  910  or  911  can be computed by using the light intensity information fed from the controller  600 - 11  of the add drop portion  600 . 
     The above gain tilt is supplied from the device controller  800  to the controller  121 - 20  of the input-side optical amplifier  121 , thus controlling the gain tilt equalizer  121 - 30  to reduce the gain tilt to zero. Thus, the signal quality can be kept uniform over all the wavelengths bundled within the wavelength-division multiplexed light. 
       FIG. 13  shows an example of a construction having a gain tilt equalizer  600 - 30  provided in the add drop portion  600 . Even with this construction, the gain tilt of the multiplexed light to the add drop portion  600  can be estimated by the device controller  800  in the same way as in  FIG. 12 . 
     The gain tilt is supplied from the device controller  800  to the controller  600 - 11 , thus controlling the gain tilt equalizer  600 - 30  to reduce the gain tilt to zero. Thus, the signal quality can be kept uniform over all the wavelengths bundled within the multiplexed light. 
     Thus, according to the invention, the wavelength-division multiplexing system having the optical add drop multiplexer (OADM) can make the signal qualities before and after the connection of OADM the same over all the wavelengths bundled within the multiplexed light without altering the chromatic dispersion method. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.