Patent Publication Number: US-2012045212-A1

Title: Optical amplification device, communication system, and amplification method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-183040, filed on Aug. 18, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an amplification device configured to amplify light, a communication system, and an amplification method. 
     BACKGROUND 
     As the multimedia network technology moves forward, the communication traffic is in increasing demand and wavelength division multiplexing (WDM) communication systems transferring a WDM signal subjected to the WDM are used. In the WDM communication system, an optical amplifier using an optical fiber such as an erbium doped fiber (EDF) as an amplification medium is used. Further, because of increasing demand for the transfer capacity, communication systems with a data rate of about 40 [Gbps] have been studied. 
     In the WDM communication system, collective compensation for losses occurring in a path provided to transfer the WDM signal subjected to the WDM is made with a WDM amplifier, and the wavelength multiplexing/demultiplexing is performed with an arrayed waveguide grating (AWG) for example. After that, a variable dispersion compensator (VDC) makes compensation for the wavelength dispersion for each of optical signals that are obtained through the wavelength multiplexing/demultiplexing. The VDC is often referred to as a tunable dispersion compensator (TDC). 
     When a single VDC makes collective compensation for the dispersion of each of WDM signals as mentioned above, dispersion compensation errors occur by wavelengths. Since a small dispersion compensation error is permissible in a high-speed (e.g., about 40 [Gbps]) communication system, the VDC may make the dispersion compensation for each wavelength. 
     Since the loss of the WDM signal, which occurs in the VDC or the AWG, is significant, a single-wave amplifier is provided in the anteceding stage and compensation for the loss is made. After that, the WDM signal is transmitted to a receiver. The single-wave amplifier may be a single-wave amplifier provided for each of signal wavelengths of the WDM signal in view of the stocks of customers. 
     Further, related technologies of keeping the gain characteristic constant and attaining a wide input dynamic range have been available (see Japanese Laid-Open Patent Publication No. 2005-192256, for example). The technology disclosed in Japanese Laid-Open Patent Publication No. 2005-192256 provides a gain adjuster configured to detect a deviation from the target gain of a first variable gain of a first optical amplifier and adjust a second variable gain of a second optical amplifier so that the sum of the first and second variable gains is kept constant, which makes compensation for the detected deviation. 
     According to the above-described related technology, however, the signal (S)/amplified spontaneous emission (ASE) ratio of a signal light is deteriorated due to an ASE light occurring in the preceding stage of a receiver. The S/ASE ratio is the ratio between the power of the signal and that of the ASE light. The ASE light occurs in, for example, an optical amplifier. When the S/ASE ratio is deteriorated, the reception quality of the receiver is decreased. 
     SUMMARY 
     According to an aspect of the invention, an amplification device includes an amplifier configured to amplify a signal light by inputting the signal light and an excitation light to a rare-earth doped amplification medium, a wavelength arrangement monitor configured to acquire wavelength arrangement information indicating a wavelength of the signal light, and a light power controller configured to control power of the input excitation light based on the acquired wavelength arrangement information. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary configuration of an amplification device according to an embodiment. 
         FIG. 2  illustrates exemplary operations that are performed with an amplification device according to an embodiment. 
         FIG. 3  illustrates an exemplary wavelength gain characteristic of an EDF. 
         FIG. 4  illustrates an exemplary relationship between a signal wavelength and the target value of output power of the EDF. 
         FIG. 5A  illustrates a wavelength output characteristic of the EDF, the wavelength output characteristic being attained when the signal wavelength is relatively short. 
         FIG. 5B  illustrates another wavelength output characteristic of the EDF, the wavelength output characteristic being attained when the signal wavelength is relatively long. 
         FIG. 6A  illustrates a wavelength output characteristic of a VOA, the wavelength output characteristic being attained when the signal wavelength is relatively short. 
         FIG. 6B  illustrates another wavelength output characteristic of the VOA, the wavelength output characteristic being attained when the signal wavelength is relatively long. 
         FIG. 7A  illustrates the relationship between the signal wavelength and the S/ASE ratio. 
         FIG. 7B  illustrates the improvement amount of the S/ASE ratio. 
         FIG. 8  illustrates a specific example of the relationship between the signal wavelength and the output power of the EDF. 
         FIG. 9  illustrates the relationship between the signal wavelength and the attenuation amount of the VOA. 
         FIG. 10  illustrates a communication system according to an embodiment. 
         FIG. 11  illustrates an exemplary modification  1  of an amplification device according to an embodiment. 
         FIG. 12  illustrates an exemplary modification  2  of an amplification device according to an embodiment. 
         FIG. 13  illustrates an exemplary modification  3  of an amplification device according to an embodiment. 
         FIG. 14  illustrates an exemplary wavelength attenuation characteristic of a wavelength filter. 
         FIG. 15  illustrates an exemplary modification  4  of an amplification device according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred embodiments of disclosed technologies will be described in detail with reference to the attached drawings. 
     EMBODIMENTS 
       FIG. 1  illustrates an exemplary configuration of an amplification device  100  according to an embodiment. As illustrated in  FIG. 1 , the amplification device  100  includes an excitation light source  111 , a wavelength division multiplexer  112 , an EDF  113 , a wavelength arrangement monitor  121 , a splitter  131 , an optical detector  132 , a correspondence information memory  133 , an EDF output level supplier  134 , a power controller  135 , a variable optical attenuator (VOA)  141 , a target output memory  142 , an attenuation amount supplier  143 , and an attenuator controller  144 . 
     The excitation light source  111  and the multiplexer  112  function as an amplifier configured to input a signal light and an excitation light into a rare-earth doped amplification medium to amplify the signal light. More specifically, the excitation light source  111  generates and outputs the excitation light to the multiplexer  112 . The power (inverted distribution ratio) of the generated excitation light is controlled by, for example, the power controller  135 . The excitation light source  111  may include, for example, a laser diode (LD). 
     The multiplexer  112  multiplexes the signal light input to the amplification device  100  and the excitation light output from the excitation light source  111 . The multiplexer  112  outputs light obtained through the multiplexing to the EDF  113 , which is a rare earth-doped amplification medium. The EDF  113  lets the output light pass therethrough so that the light is output to the splitter  131 . Thus, the EDF  113  amplifies the signal light based on the power of the excitation light and outputs the amplified light to the splitter  131 . 
     The wavelength arrangement monitor  121  may acquire wavelength arrangement information indicating the wavelength of the signal light input to the amplification device  100  (signal wavelength λ). The wavelength arrangement monitor  121  outputs the acquired wavelength arrangement information to the EDF output level supplier  134 . For example, the wavelength arrangement information may be stored in the memory of the amplification device  100  and the wavelength arrangement monitor  121  may acquire the wavelength arrangement information stored in the memory of the amplification device  100 . 
     In another case, the wavelength arrangement monitor  121  may acquire the wavelength arrangement information from an external control device. For example, the wavelength arrangement monitor  121  may obtain wavelength arrangement information via an optical supervisory channel. 
     The splitter  131  and the optical detector  132  function as a monitor configured to monitor the power of the signal light amplified with the EDF  113 . More specifically, the splitter  131  causes the signal light output from the EDF  113  to branch and outputs signal lights that are obtained through the branching to the individual VOA  141  and optical detector  132 . The optical detector  132  converts the signal light output from the splitter  131  into an electric signal. The optical detector  132  outputs the electric signal obtained through the conversion to the power controller  135 . The optical detector  132  may include, for example, a photodiode. 
     The correspondence information memory  133  is a power memory provided to store correspondence information making the signal light wavelength (signal wavelength λ) and a target value Pedf of the output power of the EDF  113  correspond to each other. The correspondence information may be provided as table data indicating the correspondence between the signal wavelength λ, and the target value Pedf or a relational expression calculating the target value Pedf of the output power based on the signal wavelength λ. Further, according to the correspondence information stored in the correspondence information memory  133 , a target value Pedf corresponding to a relatively short signal wavelength λ is larger than that corresponding to a relatively long signal wavelength λ. 
     The EDF output level supplier  134  determines the target value Pedf of the output power of the EDF  113  based on the wavelength arrangement information output from the wavelength arrangement monitor  121  and the correspondence information stored in the correspondence information memory  133 . More specifically, the EDF output-level supplier  134  acquires the target value Pedf corresponding to a signal wavelength λ indicated by the wavelength arrangement information from the correspondence information. The EDF output-level supplier  134  supplies information about the acquired target value Pedf to each of the power controller  135  and the attenuation amount supplier  143 . 
     The power controller  135  controls the power of the excitation light output from the excitation light source  111  so that the value of power of the electric signal output from the optical detector  132  becomes that of the target value Pedf information output from the EDF output-level supplier  134 . Consequently, it becomes possible to control the power of the excitation light input to the EDF  113  based on the wavelength arrangement information acquired with the wavelength arrangement monitor  121 . 
     The VOA  141  attenuates (loses) the signal light output from the splitter  131  by as much as a variable attenuation amount. 
     The VOA  141  outputs the attenuated signal light to the anteceding stage of the amplification device  100 . The attenuation amount attained with the VOA  141  is controlled with the attenuation controller  144 , for example. 
     The target output memory  142 , the attenuation amount supplier  143 , and the attenuation controller  144  function as an attenuation controller provided to control the attenuation amount attained with the VOA  141  so that the power of the signal light output from the VOA  141  becomes constant. More specifically, the target output memory  142  stores information about the target value Pout of power of the signal light output from the amplification device  100 . 
     The attenuation amount supplier  143  determines an attenuation amount Lvoa attained with the VOA  141  based on the target value Pout information stored in the target output memory  142  and the target value Pedf information output from the EDF output-level supplier  134 . For example, the attenuation amount supplier  143  calculates the attenuation amount Lvoa in accordance with the equation Lvoa=Pedf−Pout. Consequently, the attenuation supplier  13  can determine the attenuation amount Lvoa by which the power of the signal light output from the VOA  141  becomes the target value Pout. The attenuation amount supplier  143  supplies information about the determined attenuation amount Lvoa to the attenuation controller  144 . 
     The attenuation controller  144  controls the attenuation amount attained with the VOA  141  based on the attenuation amount Lvoa information output from the attenuation amount supplier  143 . Thus, the attenuation amount supplier  143  and the attenuation controller  144  control the attenuation amount attained with the VOA  141  based on the difference between the target value Pedf of the output power of the EDF  113  and the target value Pout of the power of the signal light output from the amplification device  100 . 
     Consequently, it becomes possible to keep the output power of the amplification device  100  constant at the target value Pedf even though the power of the excitation light is controlled with the power controller  135 . Further, since the attenuation amount attained with the VOA  141  is controlled based on the target value Pedf of the output power of the EDF  113 , it becomes possible to keep the output power of the amplification device  100  constant without providing, for example, a monitor configured to monitor the output power of the VOA  141 . Accordingly, the electronic circuit scale is reduced. 
     Each of the above-described EDF output-level supplier  134 , power controller  135 , attenuation amount supplier  143 , and attenuation controller  144  may include at least one circuit such as a field-programmable gate array (FPGA) and/or at least one processor such as a digital signal processor (DSP), for example. Further, each of the correspondence information memory  133  and the target output memory  142  may include at least one memory. 
     According to the above-described configuration, the EDF  113  is provided in the anteceding stage of the multiplexer  112  to achieve forward excitation. According to a different configuration, however, the EDF  113  may be provided in the preceding stage of the multiplexer  112  to achieve backward excitation. In that case, the multiplexer  112  inputs the excitation light output from the excitation light source  111  to the EDF  113  so that the signal light and the excitation light pass through the EDF  113  in the reverse direction. The latter configuration can also amplify the signal light. 
       FIG. 2  illustrates a series of exemplary operations performed with an amplification device according to an embodiment. The wavelength arrangement monitor  121  may comprise a processor. As illustrated in  FIG. 2 , first, the wavelength arrangement monitor  121  acquires the wavelength arrangement information indicating the signal wavelength λ (operation S 201 ). Next, the EDF output-level supplier  134  determines the target value Pedf of the output power of the EDF  113  based on the wavelength arrangement information acquired at operation S 201  and the correspondence information stored in the correspondence information memory  133  (operation S 202 ). 
     Next, the power controller  135  controls the power of the excitation light so that the power of the electric signal output from the optical detector  132  attains the target value Pedf determined at operation S 202  (operation S 203 ). 
     Next, the attenuation amount supplier  143  determines the attenuation amount Lvoa attained with the VOA  141  based on the target value Pedf determined at operation S 202  and the target value Pout information stored in the target output memory  142  (operation S 204 ). Next, the attenuation controller  144  controls the attenuation amount attained with the VOA  141  so that the attenuation amount Lvoa determined at operation S 204  is attained (operation S 205 ). Then, the series of exemplary operations is finished. The above-described operations allow for controlling the power of the excitation light based on the signal wavelength  2 , and controlling and keeping the power of the signal light output from the amplification device  100  constant. The above-described operations are performed when the amplification device  100  is started. Further, the above-described operations may be performed repeatedly while the amplification device  100  is operated. 
       FIG. 3  illustrates exemplary wavelength gain characteristics of the EDF  113 . In  FIG. 3 , the horizontal axis indicates the optical wavelength and the vertical axis indicates the optical gain. Each of wavelength gain characteristics  301 ,  302 , and  303  indicates the gain characteristic corresponding to the wavelength of a light input to the EDF  113 . According to the wavelength gain characteristic  301 , the gain of the EDF  113  is increased as the wavelength becomes longer. 
     The wavelength gain characteristic  302  indicates a wavelength gain characteristic attained when the power of an excitation light (excitation state) input to the EDF  113  is higher than that corresponding to the wavelength gain characteristic  301 . According to the wavelength gain characteristic  302 , the gain of the EDF  113  becomes substantially constant with reference to the wavelength. The wavelength gain characteristic  303  indicates a wavelength gain characteristic attained when the power of an excitation light input to the EDF  113  is higher than that corresponding to the wavelength gain characteristic  302 . According to the wavelength gain characteristic  303 , the gain of the EDF  113  is decreased as the wavelength becomes longer. 
     Thus, each of the wavelength gain characteristics of the EDF  113  is changed based on the power of an input excitation light. Therefore, when the signal wavelength λ is obtained on the short-wavelength side of the signal band, it becomes possible to make the gain of the signal wavelength λ relatively higher by increasing the excitation light power so that the wavelength gain characteristic of the EDF  113  becomes substantially or exactly equivalent to the wavelength gain characteristic  303 . Further, when the signal wavelength λ is obtained on the long-wavelength side of the signal band, it becomes possible to make the gain of the signal wavelength λ, relatively higher by decreasing the excitation light power so that the wavelength gain characteristic of the EDF  113  becomes substantially or exactly equivalent to the wavelength gain characteristic  301 . 
       FIG. 4  illustrates an exemplary relationship between the signal wavelength and the target value of the output power of the EDF  113 . 
     In  FIG. 4 , the horizontal axis indicates the signal wavelength λ and the vertical axis indicates the target value Pedf of the output power of the EDF  113 . A relationship  400  indicates the relationship between the signal wavelength  2  and the target value Pedf. 
     According to the relationship  400 , the target value Pedf is expressed as a linear gradient with reference to the signal wavelength λ, and the target value Pedf is increased as the signal wavelength becomes shorter. The correspondence information memory  133  stores correspondence information indicating the relationship  400 . 
     The correspondence information indicating the relationship  400  is correspondence table data making the signal wavelength λ, and the target value Pedf correspond to each other, where the correspondence table data is generated by discretizing the relationship  400 , for example. Further, the correspondence information indicating the relationship  400  may be the relational expression Pedf=aλ+b indicating the relationship  400  as a linear function. Each of the signs a and b is a coefficient, and the sign a is a negative coefficient in the above-described embodiment. Accordingly, the power of the excitation light is increased as the signal wavelength λ becomes shorter, and the gain attained on the short wavelength-side of the signal band becomes larger than that attained on the long wavelength-side of the signal band. Further, the power of the excitation light is decreased as the signal wavelength λ becomes longer, and the gain attained on the long wavelength-side of the signal band becomes larger than that attained on the short wavelength-side of the signal band. 
     In the above-described embodiment, the target value Pedf is continuously increased as the signal wavelength λ becomes shorter. However, when the signal wavelength is relatively short, the target value Pedf may be increased more than that attained when the signal wavelength λ is relatively long. For example, when the signal wavelength λ is obtained on the long wavelength-side relative to the center wavelength of the signal band, the target value Pedf may be determined to be a constant value A, and when the signal wavelength λ is obtained on the short wavelength-side relative to the center wavelength of the signal band, the target value Pedf may be determined to be a constant value B (&gt;A). 
       FIG. 5A  is a graph illustrating a wavelength output characteristic of the EDF  113 , the wavelength output characteristic being attained when the signal wavelength is relatively short. 
     In  FIG. 5A , the horizontal axis indicates the wavelength and the vertical axis indicates the optical power (ditto for each of  FIGS. 5B ,  6 A, and  6 B). A signal light  510  indicates a signal light included in the light output from the EDF  113 . An ASE light  520  indicates an ASE light included in the light output from the EDF  113 . When the signal wavelength λ is relatively short, the target value Pedf of the output power of the EDF  113  is determined to be a relatively high value. Consequently, the power of the excitation light input to the EDF  113  is increased. 
     Accordingly, the wavelength gain characteristic of the EDF  113  becomes substantially or exactly equivalent to the wavelength gain characteristic  303  (see  FIG. 3 ), for example. In consequence, the short wavelength-side gain of the light input to the EDF  113  becomes larger than the long wavelength-side gain of the light input to the EDF  113 . Therefore, the gain of the signal light  510  which is a short wavelength is increased and the long wavelength-side gain of the ASE light  520  is reduced. Accordingly, it becomes possible to increase the S/ASE ratio. 
       FIG. 5B  is a graph illustrating another wavelength output characteristic of the EDF  113 , the wavelength output characteristic being attained when the signal wavelength is relatively long. 
     In  FIG. 5B , the same properties as those of  FIG. 5A  are designated by the same reference numerals, and the descriptions thereof will not be furnished. When the signal wavelength λ is relatively long, the target value Pedf of the output power of the EDF  113  is determined to be a relatively low value. Consequently, the power of the excitation light input to the EDF  113  is decreased so that the wavelength gain characteristic of the EDF  113  becomes substantially or exactly equivalent to the wavelength gain characteristic  301  (see  FIG. 3 ), for example. 
     In consequence, the long wavelength-side gain of the light input to the EDF  113  becomes larger than the short wavelength-side gain of the light input to the EDF  113 . Therefore, the gain of the signal light  510  which is a long wavelength is increased and the short wavelength-side gain of the ASE light  520  is reduced. Accordingly, it becomes possible to increase the S/ASE ratio. According to the above-described configuration, the target value Pedf of the output power of the EDF  113  is determined to be a relatively low value. Therefore, the power of a light output from the EDF  113  becomes generally lower than in the case illustrated in  FIG. 5A . 
       FIG. 6A  is a graph illustrating a wavelength output characteristic of the VOA  141 , the wavelength output characteristic being attained when the signal wavelength is relatively short. 
       FIG. 6B  is another graph illustrating a wavelength output characteristic of the VOA  141 , the wavelength output characteristic being attained when the signal wavelength is relatively long. In  FIGS. 6A and 6B , the same properties as those of  FIGS. 5A and 5B  are designated by the same reference numerals, and the descriptions thereof will not be furnished. 
     Since the excitation light power attained when the signal wavelength λ is relatively short is different from that attained when the signal wavelength λ is relatively long, the power of the light output from the EDF  113 , which corresponds to the relatively short signal wavelength λ, is different from that corresponding to the relatively long signal wavelength λ (see  FIGS. 5A and 5B ). In relation to the above-described power difference, the attenuation amount supplier  143  determines the attenuation amount Lvoa attained with the VOA  141  so that the power of light output from the VOA  141  becomes constant. Consequently, the power of the light output from the VOA  141  becomes constant irrespective of the signal wavelength λ as illustrated in  FIGS. 6A and 6B . 
       FIG. 7A  is a graph illustrating the relationship between the signal wavelength and the S/ASE ratio. In  FIG. 7A , the horizontal axis indicates the signal wavelength λ [nm] of a signal light input to the amplification device  100 , and the vertical axis indicates the S/ASE ratio [dB] of a signal light output from the amplification device  100 . A relationship  711  indicates the relationship between the signal wavelength λ and the S/ASE ratio, which is attained when the output power of the EDF  113  is controlled based on the signal wavelength λ as is the case with the amplification device  100 . A relationship  712  indicates the relationship between the signal wavelength λ and the S/ASE ratio for reference, which is attained based on the assumption that the output power of the EDF  113  is not controlled based on the signal wavelength λ as is the case with the amplification devices achieved through the related technologies. 
       FIG. 7B  is a graph illustrating the improvement amount of the S/ASE ratio. In  FIG. 7B , the horizontal axis indicates the signal wavelength λ [nm] of a signal light input to the amplification device  100 , and the vertical axis indicates the improvement amount [dBm] of the S/ASE ratio of a signal light output from the amplification device  100 . A relationship  720  indicates the difference between the relationship  711  and the relationship  712  that are illustrated in  FIG. 7A , and indicates the amount of improvement which is made to the S/ASE ratio corresponding to the relationship  712  to attain the S/ASE ratio corresponding to the relationship  711 . As indicated by the relationship  720 , the amplification device  100  can improve the S/ASE ratio by as much as, for example, 1.5 [dB] when the signal wavelength λ is obtained on the long wavelength-side or the short wavelength-side. 
     Next, an exemplary method of generating the correspondence information making the signal light wavelength λ and the target value Pedf correspond to each other will be described. According to the exemplary method, the amplification device  100  is applied to a single-wave amplifier having an input of −20 [dBm], an output of 3 [dBm], and a gain of 23 [dB], the single-wave amplifier being used for a WDM communication system operated at wavelength intervals of 50 [GHz] within the C band, where the number of wavelengths is eighty-eight. The excitation light source  111  outputs an excitation light having a wavelength of 0.98 or 1.48 [μm], for example. In the above-descried embodiment, however, the output excitation light has a wavelength of 1.48 [μm]. 
       FIG. 8  is a graph illustrating a specific example of the relationship between the signal wavelength and the output power of the EDF  113 . In  FIG. 8 , the horizontal axis indicates the signal wavelength λ [nm] and the vertical axis indicates the output power [dBm] of the EDF  113 . 
     First, assuming that the signal wavelength λ is the center wavelength of the signal band, the value of an attenuation amount Lvoa attained based on the above-described assumption is determined to be 5 [dB], for example. According to the assumption, a gain of 23 [dB]+5 [dB]=28 [dB] is appropriate for the EDF  113 . Further, the EDF  113  has a length of, for example, 24 [m] so that the wavelength gain characteristic (see  FIG. 3 ) is leveled off as much as possible when the gain value is 28 [dB]. According to the assumption, the target output of the EDF  113  (target value Pedf) is expressed as the equation 3 [dBm]+5 [dBm]=8 [dBm] (plot point  801 ). 
     Next, assuming that the signal wavelength λ is the longest wavelength of the signal band, the value of an attenuation amount Lvoa attained based on the above-described assumption is determined to be 0 [dB], for example. According to the assumption, the target output of the EDF  113  (target value Pedf) is expressed as the equation 3 [dBm]+0 [dBm]=3 [dBm] (plot point  802 ). 
     Next, assuming that the signal wavelength λ is the shortest wavelength of the signal band, the value of an attenuation amount Lvoa attained based on the above-described assumption is determined to be 5 [dBm]×2=10 [dB] which is twice as large as the loss of the center wavelength, for example. According to the assumption, the target output of the EDF  113  (target value Pedf) is expressed as the equation 3 [dBm]+10 [dBm]=13 [dBm] (plot point  803 ). 
     Next, the line joining the plot points  801  to  803  is determined to be the relationship  800  between the signal wavelength λ and the target value Pedf (as with the relationship  400  illustrated in  FIG. 4 ). Next, correspondence information indicating the relationship  800  is stored in the correspondence information memory  133 . Consequently, correspondence information indicating that the target value Pedf is increased as the signal wavelength λ, becomes shorter is generated. 
       FIG. 9  is a graph illustrating the relationship between the signal wavelength and the attenuation amount of the VOA  141 . In  FIG. 9 , the horizontal axis indicates the signal wavelength λ, [nm] and the vertical axis indicates the attenuation amount Lvoa [dB] of the VOA  141 . A relationship  900  illustrates the relationship between the signal wavelength λ and the attenuation amount Lvoa of the VOA  141 . 
     In the amplification device  100 , the output power of the EDF  113  is increased as the signal wavelength λ becomes shorter as indicated by the relationship  800  illustrated in  FIG. 8 , whereas the attenuation amount Lvoa of the VOA  141  is increased as the signal wavelength becomes shorter as indicated by the relationship  900 . Therefore, the power of a signal light output from the amplification device  100  is made constant even though the output power of the EDF  113  is changed due to the signal wavelength λ. 
       FIG. 10  illustrates a communication system  1000  according to an embodiment. As illustrated in  FIG. 10 , the communication system  1000  includes a WDM amplifier  1010 , an AWG  1020 , VDCs  1031 ,  1032 ,  1033 ,  1034 , and  1035 , optical amplifiers  1041 ,  1042 ,  1043 ,  1044 , and  1045 , and optical receivers  1051 ,  1052 ,  1053 ,  1054 , and  1055 . The communication system  100  is a system configured to receive optical signals, each of which is obtained by performing the wavelength multiplexing for a WDM optical signal transmitted via a transfer path  1001 . 
     The WDM amplifier  1010  amplifies the WDM optical signal output from the transfer path  1001  and makes compensation for a loss occurring in the transfer path  1001 , for example. The WDM amplifier  1010  outputs the amplified WDM optical signal to the AWG  1020 , the AWG  1020  being a demultiplexer performing wavelength multiplexing/demultiplexing for the WDM optical signal output from the WDM amplifier  1010 . The AWG  1020  outputs optical signals that are subjected to the wavelength multiplexing/demultiplexing to the individual VDCs  1031  to  1035 . 
     The VDCs  1031  to  1035  are dispersion compensators that are configured to make compensation for the wavelength dispersion of the optical signals that are output from the AWG  1020  and output the optical signals that are subjected to the wavelength dispersion compensation to the individual amplifiers to  1045 . The amplifiers  1041  to  1045  amplify the optical signals that are output from the individual VDCs  1031  to  1035  and output the amplified optical signals to the individual receivers  1051  to  1055 . The receivers  1051  to  1055  receive the optical signals that are output from the individual amplifiers  1041  to  1045 . 
     Each of the amplifiers  1041  to  1045  may include the amplification device  100  to increase the S/ASE ratio. Consequently, the reception quality of each of the receivers  1051  to  1055  is increased. 
     In the above-described embodiment, the amplification device  100  is applied to the communication system  1000 , which is a WDM communication system. However, without being limited to the WDM communication system, the amplification device  100  can be applied to the reception side of a communication system configured to transmit/receive an optical signal having a single wavelength. In that case, the S/ASE ratio can also be increased with the amplification device  100  and the reception quality can also be increased. 
       FIG. 11  illustrates an exemplary modification  1  of an amplification device according to an embodiment. In  FIG. 11 , the same components as those illustrated in  FIG. 1  are designated by the same reference numerals and the descriptions thereof will not be furnished. As illustrated in  FIG. 11 , the amplification device  100  may include a demultiplexer  1101  and an optical detector  1102  in addition to the components that are illustrated in  FIG. 1 . In the above-described embodiment, a signal light input to the amplification device  100  includes wavelength arrangement information indicating the wavelength of the signal light as a control signal. 
     The demultiplexer  1101  demultiplexes the wavelength of the control signal from each of the signal lights that are input to the amplification device  100  and that are output to the multiplexer  112 , and outputs the demultiplexed control signals to the optical detector  1102  configured to convert a control signal output from the demultiplexer  1101  into an electric signal. The optical detector  1102  outputs the control signal converted into the electrical signal to the wavelength arrangement monitor  121  configured to acquire the wavelength arrangement information from the control signal output from the optical detector  1102 . Thus, the optical detector  1102  may receive the wavelength arrangement information as control information transmitted from an external device. 
       FIG. 12  illustrates an exemplary modification  2  of an amplification device according to an embodiment. In  FIG. 12 , the same components as those illustrated in  FIG. 1  are designated by the same reference numerals and the descriptions thereof will not be furnished. As illustrated in  FIG. 12 , the amplification device  100  may include a splitter  1201  and an optical detector  1202  in addition to the components that are illustrated in  FIG. 1 . In the above-described configuration, the attenuation amount supplier  143  illustrated in  FIG. 1  may be eliminated. 
     The splitter  1201  causes a signal light output from the VOA  141  to the anteceding stage of the amplification device  100  to branch, and outputs a signal light obtained through the branching to the optical detector  1202 , which is configured to convert the signal light output from the splitter  1201  into an electric signal. The optical detector  1202  outputs the electric signal obtained through the conversion to the attenuation controller  144 . 
     The attenuation controller  144  controls the amount of attenuation performed with the VOA  141  so that the power of the electric signal output from the optical detector  1202  attains the target value Pout of which information is stored in the target output memory  142 . Thus, the output power of the VOA  141  may be monitored and the amount of attenuation performed with the VOA  141  may be controlled to make the monitored power constant. In that case, the power of the signal light output from the amplification device  100  can also be made constant (Pout). 
       FIG. 13  illustrates an exemplary modification  3  of an amplification device according to an embodiment. In  FIG. 13 , the same components as those illustrated in  FIG. 1  are designated by the same reference numerals and the descriptions thereof will not be furnished. As illustrated in  FIG. 13 , the amplification device  100  may include a splitter  1301 , a wavelength filter  1302 , an optical detector  1303 , and a correspondence information memory  1304  in addition to the components that are illustrated in  FIG. 1 . The splitter  1301  is a first splitter configured to cause a signal light that is input to the amplification device  100  and that is output to the multiplexer  112  to branch, and output a signal light obtained through the branching to the wavelength filter  1302 . 
     The wavelength filter  1302  lets the signal light output from the splitter  1301  pass therethrough and outputs the signal light to the optical detector  1303 . Further, the wavelength filter  1302  has such a wavelength attenuation characteristic that the attenuation amount varies from one wavelength to another. The optical detector  1303  is a first monitor configured to monitor the power of the signal light that had passed through the wavelength filter  1302 . More specifically, the optical detector  1303  converts the signal light output from the wavelength filter  1302  into an electric signal, and outputs the electric signal to the wavelength arrangement monitor  121 . 
     The correspondence information memory  1304  is configured to store correspondence information making the power of the electric signal output from the optical detector  1303  and the signal wavelength λ correspond to each other. The wavelength arrangement monitor  121  acquires the wavelength arrangement information by retrieving information about the signal wavelength λ corresponding to the power of the electric signal output from the optical detector  1303  from the correspondence information stored in the correspondence information memory  1304 . 
     Since the wavelength filter  1302  has the wavelength attenuation characteristic which makes the attenuation amount vary from one wavelength to another, the power of the signal light output from the wavelength filter  1302  and the signal wavelength λ are in the ratio 1:1 so long as the power of a signal light input to the amplification device  100  is constant. Accordingly, the wavelength arrangement monitor  121  can determine the signal wavelength λ based on the correspondence information making the power of the electric signal output from the optical detector  1303  and the signal wavelength λ correspond to each other and the power of the electric signal output from the optical detector  1303 . As a consequence, the wavelength arrangement information indicating the signal wavelength λ can be autonomously acquired without setting the wavelength arrangement information in advance or acquiring the wavelength arrangement information from an external device. 
     The correspondence information stored in the correspondence information memory  1304  can be generated by, for example, inputting a signal light with a known wavelength into the amplification device  100  and monitoring the power of an electric signal output from the optical detector  1303 . 
       FIG. 14  is a graph illustrating an exemplary wavelength attenuation characteristic  1400  of the wavelength filter  1302 . In  FIG. 14 , the horizontal axis indicates the signal wavelength λ and the vertical axis indicates the amount of attenuation performed with the wavelength filter  1302  illustrated in  FIG. 13 . The wavelength attenuation characteristic  1400  is the characteristic of the attenuation amount corresponding to the wavelength of a light passing through the wavelength filter  1302 . As indicated by the wavelength attenuation characteristic  1400 , the wavelength filter  1302  has such a wavelength attenuation characteristic that the attenuation amount against the wavelength becomes a linear gradient. 
     Accordingly, the power of the signal light output from the wavelength filter  1302  and the signal wavelength λ are in the ratio 1:1 so long as the power of the signal light input to the amplification device  100  is constant. However, without being limited to the wavelength attenuation characteristic  1400 , the wavelength filter  1302  may have any wavelength attenuation characteristic so long as the attenuation amount varies from one wavelength to another. 
       FIG. 15  illustrates an exemplary modification  4  of an amplification device according to an embodiment. In  FIG. 15 , the same components as those illustrated in  FIG. 13  are designated by the same reference numerals and the descriptions thereof will not be furnished. As illustrated in  FIG. 15 , the amplification device  100  may include a splitter  1501  and an optical detector  1502  in addition to the components that are illustrated in  FIG. 13 . 
     The splitter  1501  is a second splitter configured to cause a signal light to branch, the signal light being output from the splitter  1301  to the wavelength filter  1302 , and output a signal light obtained through the branching to the wavelength filter  1502 . The optical detector  1502  is a second monitor configured to monitor the power of the signal light obtained through the branching performed with the splitter  1501 . More specifically, the optical detector  1502  converts the signal light output from the splitter  1501  into an electric signal, and outputs the electric signal to the wavelength arrangement monitor  121 . 
     The correspondence information memory  1304  stores correspondence information making the difference between the power of an electric signal output from the optical detector  1303  and that of an electric signal output from the optical detector  1502 , and the signal wavelength λ correspond to each other. The wavelength arrangement monitor  121  acquires the wavelength arrangement information by retrieving information about the signal wavelength  7  corresponding to the above-described difference from the correspondence information stored in the correspondence information memory  1304 . 
     The difference between the power of the electric signal output from the optical detector  1303  and that of the electric signal output from the optical detector  1502  indicates the attenuation amount corresponding to each of the wavelengths of a signal light, the attenuation amount being attained with the wavelength filter  1302 . Further, since the wavelength filter  1302  has the wavelength attenuation characteristic which makes the attenuation amount vary from one wavelength to another, the above-described difference and the signal wavelength λ are in the ratio 1:1. 
     Accordingly, the wavelength arrangement monitor  121  can determine the signal wavelength λ based on the correspondence information making the power of the electric signal output from the optical detector  1303  and the signal wavelength λ correspond to each other, and the difference between the power of the electric signal output from the optical detector  1303  and that of the electric signal output from the optical detector  1502 . As a consequence, the wavelength arrangement information indicating the signal wavelength λ can be autonomously acquired even though the power of a signal light input to the amplification device  100  is not constant. 
     The correspondence information stored in the correspondence information memory  1304  can be generated by, for example, inputting a signal light with a known wavelength into the amplification device  100  and monitoring the difference between the power of the electric signal output from the optical detector  1303  and that of the electric signal output from the optical detector  1502 . 
     Further, the amplification device  100  may be configured to monitor the power of an excitation light input to the EDF  113  and control the power of the excitation light so that the monitoring result attains a target value. In that case, the splitter  131  is provided between the excitation light source  111  and the multiplexer  112 , for example. Further, in that case, the correspondence information memory  133  stores correspondence information making the signal wavelength λ and the power of the excitation light input to the EDF  113  correspond to each other. 
     Even though the above-described amplification device  100  includes the EDF  113  as an optical fiber causing the stimulated emission phenomenon, a different rare-earth doped optical fiber causing the stimulated emission phenomenon may be used in place of the EDF  113 . In addition, thulium, praseodymium, and so forth are known as other rare-earth elements that are used for the doping. 
     Thus, the amplification device  100  according to an embodiment changes the wavelength gain characteristic of an optical fiber by controlling the output power of the EDF  113  based on the signal wavelength, where the output power and a signal light are input to an optical fiber, so that the gain of the signal wavelength is relatively increased. As a consequence, the signal-to-noise ratio (e.g., the S/ASE ratio) can be increased so that the reception quality of a receiver provided in the anteceding stage can be increased. 
     Thus, the above-described amplification device, communication system, and amplification method can increase the signal-to-noise ratio. 
     The amplification device, the communication system, and the amplification method that are disclosed herein can increase the signal-to-noise ratio on the optical amplification. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.