Abstract:
An optical receiver includes a semiconductor optical amplifier configured to amplify an optical signal in which an optical signal with a first wavelength and an optical signal with a second wavelength are multiplexed. The receiver includes an optical demultiplexer configured to receive the optical signal amplified by the semiconductor optical amplifier and include a first filter configured to transmit the optical signal with the first wavelength with a transmission rate T 1  and a second filter configured to transmit the optical signal with the second wavelength with a transmission rate T 2 . The receiver includes a first optical detector configured to receive the optical signal with the first wavelength from the optical demultiplexer. The receiver includes a second optical detector configured to receive the optical signal with the second wavelength from the optical demultiplexer. Further, the transmission rate T 1  and the transmission rate T 2  satisfy a relation T 1 &gt;T 2.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of International Application PCT/JP2013/053875 filed on Feb. 18, 2013 and designated the U.S., the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiments discussed herein are related to an optical receiver. 
       BACKGROUND 
       [0003]    A WDM (Wavelength Division Multiplexing) scheme, in which a plurality of wavelengths are bundled and transmitted via an optical fiber, have been employed as a method of transmitting a large amount of information in the optical communication field. In 100 Gbps Ethernet (100GE), of which the standardization has been achieved, a WDM scheme in which four 25.8 Gbps signals are used has been employed. And CFP (100G Form-factor Pluggable) optical transceivers as optical modules have been developed. The types of CFP modules for 100GE are divided into LR4 (10 km) and ER4 (40 km) according to transmission distance. The CFP modules for ER4 employ an amplifier for optical intensity compensation. 
       PRIOR ART DOCUMENTS 
       [0004]    Patent document 1: Japanese Patent Application Laid-Open Publication No. 2003-283463 
         [0005]    Patent document 2: Japanese Patent Application Laid-Open Publication No. 2005-27210 
         [0006]    Patent document 3: Japanese Patent Application Laid-Open Publication No. 2010-98166 
       SUMMARY 
       [0007]    According to one embodiment, it is provided an optical receiver, including a semiconductor optical amplifier configured to amplify an optical signal in which an optical signal with a first wavelength and an optical signal with a second wavelength are multiplexed, an optical demultiplexer configured to receive the optical signal amplified by the semiconductor optical amplifier and include a first filter configured to transmit the optical signal with the first wavelength with a transmission rate T 1  and a second filter configured to transmit the optical signal with the second wavelength with a transmission rate T 2 , a first optical detector configured to receive the optical signal with the first wavelength from the optical demultiplexer, and a second optical detector configured to receive the optical signal with the second wavelength from the optical demultiplexer, wherein the transmission rate T 1  and the transmission rate T 2  satisfy a relation T 1 &gt;T 2 . 
         [0008]    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 
         [0009]      FIG. 1  is a diagram schematically illustrating an example of an optical receiver included in a CFP module for 100GE ER4 according to an embodiment; 
           [0010]      FIG. 2  is a diagram illustrating an example of a relation between wavelength of light input into an SOA and its gain; 
           [0011]      FIG. 3  is a diagram illustrating an example of an optical communication system according to an embodiment; 
           [0012]      FIG. 4  is a diagram illustrating an example of an optical receiver; 
           [0013]      FIG. 5  is a diagram illustrating an example of control of a semiconductor optical amplifier of an optical receiver according to an embodiment; 
           [0014]      FIG. 6  is a diagram illustrating an example of a configuration of an optical demultiplexer according to an embodiment; 
           [0015]      FIG. 7  is a diagram illustrating an example of a configuration of an optical detector according to an embodiment; 
           [0016]      FIG. 8  is a diagram illustrating an example of a configuration of a filter of an optical distributor according to an embodiment; 
           [0017]      FIG. 9  is a diagram illustrating an example of an optical transmission path between the optical distributor and the optical detector in Configuration Example 3; 
           [0018]      FIG. 10  is a diagram illustrating an example (1) of the intensity of light signals in the optical receiver; 
           [0019]      FIG. 11  is a diagram illustrating an example (2) of the intensity of light signals in the optical receiver; and 
           [0020]      FIG. 12  is a diagram illustrating an example (3) of the intensity of light signals in the optical receiver. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]      FIG. 1  is a diagram illustrating an optical receiver included in a CFP module for 100GE ER4. The optical receiver illustrated in  FIG. 1  includes an optical fiber into which optical signals with a plurality of wavelengths are multiplexed and input, an SOA (Semiconductor Optical Amplifier), an optical demultiplexer, an optical fiber for connecting the SOA and the optical demultiplexer and an optical detector. The SOA is a small amplifier which can be installed in an optical transceiver. The optical demultiplexer outputs demultiplexed light which a multiplexed optical signal is demultiplexed into a plurality of wavelength channels. In an example illustrated in  FIG. 1 , the optical demultiplexer demultiplexes an optical signal into four wavelength channels. The optical detector detects the demultiplexed optical signals by wavelength channels and converts the optical signals into electric signals. In the optical receiver illustrated in  FIG. 1 , an optical signal with four wavelengths (λ 0 , λ 1 , λ 2  and λ 3  in ascending order of wavelength) transmitted by an optical fiber is passed through the SOA and divided according to the wavelengths by the optical demultiplexer. The optical detector detects each demultiplexed optical signal with each wavelength and converts the detected optical signals into electric signals. The number of wavelength channels is four for example. It can be understood that the number of wavelength channels is not limited to four. 
         [0022]    The SOA is a device which injects electric current into a semiconductor and amplifies light by stimulated emission. When the SOA is used as an optical module such as CFP module, the electric current injected into the SOA is adjusted according to the power of transmitted light input into an optical receiver to control the optical power of the light input into an optical detector. The optical detector in the optical receiver has a minimum receiving sensitivity (Pmin) and a maximum receiving sensitivity (Pmax). When the intensity of the transmitted light is not between Pmin and Pmax, the transmitted information cannot be reproduced correctly since a bit-error-free operation cannot be achieved. 
         [0023]      FIG. 2  is a diagram illustrating an example of a relation between wavelength and gain regarding light input into an SOA. The horizontal axis of the graph in  FIG. 2  represents the wavelength of the light input into the SOA and the vertical axis represents the gain of the SOA.  FIG. 2  illustrates a case in which the electric current injected into the SOA (SOA electric current) is I 1  and a case in which the SOA electric current is I 2  (&lt;I 1 ). As the intensity of the light input into the SOA increases, the amount of the light injected into the SOA is reduced not to let the intensity of the light input into the optical detector exceed the maximum reception sensitivity of the optical detector. As the amount of the electric current injected into the SOA decreases, a tilt occurs in the gain spectrum as illustrated by “SOA electric current I 2 ” in  FIG. 2  and the difference of gains from several dB to a dozen dB emerges between the shortest wavelength (λ 0 ) and the longest wavelength (λ 3 ) in the transmission wavelength range. This occurrence cannot be avoided as long as the SOA in which the gain spectrum can be changed according to the injected electric current is employed. The gain tilt widens the difference of the intensity of the light on the short wavelength side and on the long wavelength side in the transmission wavelength range. In this case, even when the SOA electric current is adjusted, the bit-error-free operation cannot be achieved in each wavelength channel if the intensity of the light of the shortest wavelength is equal to or smaller than Pmin of the optical detector or the intensity of the light of the longest wavelength is equal to or larger than Pmax. Namely, a bit error can occur in any one of the wavelength channels. Thus, the optical power in each wavelength channel should be equalized to solve the problem. That is, the optical power in each wavelength channel should be equal to or larger than Pmin of the optical detector and should be equal to or smaller than Pmax of the optical detector. 
         [0024]    A technique has been employed for providing distributors to compensate the tilts of light transmission characteristics between light wavelengths occurred according to the number of reflections at filters by achieving the reversed tilts and equalizing the optical power for each wavelength channel. However, since the tilt of transmission characteristics occurred due to light loss by reflection at filters is smaller than 1 dB, using the above characteristics cannot sufficiently equalize the non-uniformity of the optical power occurred due to the gain tile of the SOA. 
         [0025]    Another technique has been employed for using VOA (Variable Optical Attenuator) and variable spectrum equalizer to equalize the optical power. A CFP module is required to include optical devices such as alight source for four channels, an optical receiver, an SOA, an optical multiplexer and an optical demultiplexer in limited space. When a variable optical attenuator and a variable spectrum equalizer are employed, the number of optical elements increases and the cost of the optical receiver. In addition, control circuits for the optical elements are also used for controlling the optical receiver according to the input optical power. Therefore, the footprint of the optical elements increases. Thus, it is required to achieve a small footprint for optical elements and equalize the non-uniformity of the optical power, that is, the intensity of light signal for each wavelength channel occurred due to the gain tilt of the SOA. 
         [0026]    With the above in mind, it is an object to provide an optical receiver for adjusting the optical power for each wavelength channel. 
         [0027]    An embodiment is described below with reference to the drawings. A configuration of the following embodiment is an exemplification, and the present apparatus is not limited to the configuration of the embodiment. 
         [0028]    In the present embodiment, it is assumed that there are four channels as wavelength channels. The wavelengths corresponding to the four channels are denoted as λ 0 , λ 1 , λ 2  and λ 3  in ascending order of wavelength. 
         [0029]    The number of wavelength channels is not limited to four. For example, the number of wavelength channels may be two or five. When a configuration in which the number of wavelength channels is two is employed, the wavelengths λ 0  and λ 3  can be used. 
       EMBODIMENTS 
     Configuration Example 
       [0030]      FIG. 3  is a diagram illustrating an example of an optical communication system according to the present embodiment. The optical communication system  10  includes an optical transmitter  100 , an optical receiver  200  and an optical transmission path  300 . The optical transmission path  300  connects the optical transmitter  100  with the optical receiver  200 . An optical fiber exemplifies the optical transmission path  300 . 
         [0031]    The optical transmitter  100  transmits optical signals in a plurality of wavelength channels to the optical receiver  200  via the optical transmission path  300 . 
         [0032]    The optical receiver  200  receives the optical signals in the plurality of wavelength channels from the optical transmitter  100  via the optical transmission path  300 . The optical signals in the plurality of wavelength channels are multiplexed. The optical receiver  200  demultiplexes the received optical signals by wavelength channels and converts the demultiplexed optical signals into electric signals. 
         [0033]      FIG. 4  is a diagram illustrating an example of the optical receiver. The optical receiver  200  in  FIG. 4  includes an optical transmission path  211 , a semiconductor optical amplifier (SOA)  220 , an optical transmission path  231 , an optical transmission path  251 , an optical transmission path  252 , an optical transmission path  253 , an optical transmission path  254 , an optical detector  261 , an optical detector  262 , an optical detector  263  and an optical detector  264 . 
         [0034]    Optical signals transmitted from the optical transmitter  100  etc. are input into the optical receiver  200 . The optical signals are input into the optical receiver  200  via the optical transmission path  211 . 
         [0035]    The optical transmission path  211  connects external devices with the semiconductor optical amplifier  200 . Each optical transmission path propagates optical signals. For example, optical fibers provide the transmission paths. 
         [0036]    The semiconductor optical amplifier  220  amplifies the optical signals input via the optical transmission path  211  and outputs the amplified optical signals to the optical demultiplexer  240  via the optical transmission path  231 . 
         [0037]    For example, the semiconductor optical amplifier  220  includes an active layer, a p-type semiconductor layer and an n-type semiconductor layer sandwiching the active layer, a substrate, an electrode for electric current injection. The amplification factor of the semiconductor optical amplifier  220  varies according to the injected electric current. Specifically, the amplification factor of the semiconductor optical amplifier  220  depends on the electric current value and the wavelength of the injected electric current. Namely, when the injected electric current decreases as illustrated in  FIG. 2 , a tilt occurs in the gain spectrum of the semiconductor optical amplifier  220 . In addition, the semiconductor optical amplifier  220  includes an electric current injection terminal, a TEC electric current terminal and a temperature monitoring terminal for temperature control, for example. And an optical transmission path for optical input and output is connected to the semiconductor optical amplifier  200 . 
         [0038]      FIG. 5  is a diagram illustrating an example of the control of the semiconductor optical amplifier in the optical receiver. The optical receiver  200  in  FIG. 5  includes a configuration which is omitted in the optical receiver  200  illustrated in  FIG. 4 . The optical receiver in  FIG. 5  includes the semiconductor optical amplifier  220 , a drive unit  222 , a control unit  224 , a storage unit  226  and an optical input intensity monitor unit  270 . 
         [0039]    The optical input intensity monitor unit  270  measures the intensity of the optical signals transmitted from the optical transmitter  100  etc. The optical input intensity monitor unit  270  notifies the control unit  224  of the measured optical input intensity of the received signals. The drive unit  222  injects electric current for driving the semiconductor optical amplifier  220  based on the information notified by the control unit  224 . The optical detectors  261 ,  262 ,  263  and  264  measure the intensity of the received optical signals by wavelength and notify the control unit  224  of the measured intensity. The storage unit  226  stores the intensity of the optical signals measured by the optical input intensity monitor unit  270  and the intensity of the optical signals by wavelength measured by the optical detectors  261 ,  262 ,  263  and  264 . 
         [0040]    The control unit  224  calculates the amount of electric current injected into the semiconductor optical amplifier  220  based on the intensity of the optical signal measured by the optical input intensity monitor unit  270 . The control unit  224  can calculate the amount of electric current injected into the semiconductor optical amplifier  220  based on the intensity of the optical signal measured by the optical input intensity monitor unit  270  and the intensity of the optical signal by wavelength measured by the optical detectors  261 ,  262 ,  263  and  264 . The control unit  224  notifies the drive unit  222  of the calculated amount of electric current. The control unit  224  calculates the amount of electric current for which the intensity of the optical signal with the wavelength λ 0  measured by the optical detector  261  can be larger than Pmin of the optical detector  261 . In addition, the control unit  224  calculates the amount of electric current for which the intensity of the optical signal with the wavelength λ 3  measured by the optical detector  264  can be smaller than Pmax of the optical detector  261 . 
         [0041]    The optical receiver  200  can be fabricated using a general-purpose computer such as Personal Computer (PC) or a dedicated computer such as server machine. The control unit  224  can be fabricated using a Central Processing Unit (CPU) or a Digital Signal Processor (DSP). The storage unit  226  can be fabricated using Random Access Memory (RAM), Erasable Programmable ROPM (EPROM) and Hard Disk Drive (HDD), for example. Further, the storage unit  226  may be a removable medium, namely, portable storage medium. Such a removable medium includes Universal Serial Bus (USB) memory or a disk storage medium such as Compact Disc (CD) and Digital Versatile Disc (DVD), for example. The storage unit  226  is a computer-readable storage medium. 
         [0042]    The optical transmission path  231  connects the semiconductor optical amplifier  220  with the optical demultiplexer  240 . The optical demultiplexer  240  demultiplexes the input optical signals into optical signals with wavelength channels λ 0  to λ 3 . Further, the optical demultiplexer  240  outputs the demultiplexed optical signals with wavelength channels λ 0  to λ 3  to the optical detectors  261  to  263 , respectively. 
         [0043]      FIG. 6  is a diagram illustrating an example of a configuration of the optical demultiplexer. The optical demultiplexer  240  includes an input-side lens  241 , filters  242 - 1 ,  242 - 2 ,  242 - 3  and  242 - 4 , mirrors  243 - 1 ,  243 - 2  and  243 - 3 , output-side lenses  244 - 1 ,  244 - 2 ,  244 - 3  and  244 - 4 . 
         [0044]    The input-side lens condenses optical signals input via the optical transmission path  231  and outputs the condensed optical signals to the filter  242 - 1 . 
         [0045]    The filter  242 - 1  transmits optical signals with wavelength λ 0  and reflects optical signals with wavelengths except for λ 0 . 
         [0046]    The filter  242 - 2  transmits optical signals with wavelength λ 1  and reflects optical signals with wavelengths except for λ 1 . The configuration of the filter  242 - 2  is similar to the configuration of the filter  242 - 1 . The filter  242 - 3  transmits optical signals with wavelength λ 2  and reflects optical signals with wavelengths except for λ 2 . The configuration of the filter  242 - 3  is similar to the configuration of the filter  242 - 1 . The filter  242 - 4  transmits optical signals with wavelength λ 3  and reflects optical signals with wavelengths except for λ 3 . The configuration of the filter  242 - 4  is similar to the configuration of the filter  242 - 1 . 
         [0047]    The mirror  243 - 1  reflects optical signals reflected by the filter  242 - 1 . The optical signals reflected by the mirror  243 - 1  are input to the filter  242 - 2 . The mirror  243 - 2  reflects optical signals reflected by the filter  242 - 2 . The optical signals reflected by the mirror  243 - 2  are input to the filter  242 - 3 . The mirror  243 - 3  reflects optical signals reflected by the filter  242 - 3 . The optical signals reflected by the mirror  243 - 3  are input to the filter  242 - 4 . 
         [0048]    The optical signals transmitted through the filter  242 - 1  are guided to the optical transmission path  251  by the output-side lens  244 - 1 . The optical signals transmitted through the filter  242 - 2  are guided to the optical transmission path  252  by the output-side lens  244 - 2 . The optical signals transmitted through the filter  242 - 3  are guided to the optical transmission path  253  by the output-side lens  244 - 3 . The optical signals transmitted through the filter  242 - 4  are guided to the optical transmission path  254  by the output-side lens  244 - 4 . 
         [0049]    The optical transmission path  251  connects the optical demultiplexer  240  with the optical detector  261 . The functions of the optical transmission paths  252 ,  253  and  254  are similar to the function of the optical transmission path  251 . 
         [0050]    The optical detector  261  receives optical signals with wavelength channel of λ 0  via the optical transmission path  251  and converts the received optical signals into electric signals. The optical detector  261  can be fabricated using a lens and a photodiode (PD), for example. The converted electric signals are processed by an electronic circuit provided at a subsequent stage to the optical detector  261 , for example. 
         [0051]      FIG. 7  is a diagram illustrating an example of a configuration of the optical detector. The optical detector  261  in  FIG. 7  includes a lens  261 - 1  and a PD  261 - 2 . The optical detector  262  includes a lens  262 - 1  and a PD  262 - 2 . The optical detector  263  includes a lens  263 - 1  and a PD  263 - 2 . The optical detector  264  includes a lens  264 - 1  and a PD  264 - 2 . 
         [0052]    The optical detector  261  is, for example, a Receiver Optical Sub-Assembly (ROSA), which includes a PD chip and an amplifier (Trans-Impedance Amplifier: TIA) for amplifying electric signals to which photoelectric conversion is applied by the PD. The PD chip is, for example, a PIN-PD for wavelength of 1300 nm band and made of InP series material. 
         [0053]    The optical detector  262  receives optical signals with wavelength channel of λ 1  via the optical transmission path  252  and converts the received optical signals into electric signals. The optical detector  263  receives optical signals with wavelength channel of λ 2  via the optical transmission path  253  and converts the received optical signals into electric signals. The optical detector  264  receives optical signals with wavelength channel of λ 3  via the optical transmission path  254  and converts the received optical signals into electric signals. The optical detectors  262 ,  263  and  264  can be fabricated similar to the optical detector  261 . 
         [0054]    When the intensity of the optical signal input into the PD of each optical detector is equal to or larger than Pmin and equal to or smaller than Pmax, each optical detector can achieve an bit-error-free operation for processing the optical signal. Therefore, the intensity of the optical signal input into the PD of each optical detector should be equal to or larger than Pmin and equal to or smaller than Pmax. 
         [0055]    In the present embodiment, it is assumed that the intensity of the optical signal with wavelength λ 1  input into the PD  261 - 2  of the optical detector  261  to the intensity of the optical signal with wavelength λ 0  output from the SOA  220  is transmission rate T 0  of the optical signal with wavelength λ 0 . It is also assumed that the intensity of the optical signal with wavelength λ 1  input into the PD  262 - 2  of the optical detector  262  to the intensity of the optical signal with wavelength λ 1  output from the SOA  220  is transmission rate T 1  of the optical signal with wavelength λ 1 . It is further assumed that the intensity of the optical signal with wavelength λ 2  input into the PD  263 - 2  of the optical detector  263  to the intensity of the optical signal with wavelength λ 2  output from the SOA  220  is transmission rate T 2  of the optical signal with wavelength λ 2 . Moreover, it is assumed that the intensity of the optical signal with wavelength λ 3  input into the PD  263 - 2  of the optical detector  263  to the intensity of the optical signal with wavelength λ 3  output from the SOA  220  is transmission rate T 3  of the optical signal with wavelength λ 3 . In addition, the transmission rates T 0 , T 1 , T 2  and T 3  is determined by the configuration of the optical demultiplexer  240 , the optical transmission paths  251 ,  252 ,  253  and  254 , the optical detectors  261 ,  262 ,  263  and  264 . And the transmission rates T 0 , T 1 , T 2  and T 3  are determined to satisfy the following conditions (1-1) and (1-2). Since the condition (1-1) includes equal signs, the transmission rates of the two adjacent channels can be the same. When the transmission rates of the two adjacent channels are the same, the transmission rates can be increased without attenuating the optical signals. The condition (1-2) indicates that the transmission rate T 0  becomes larger than the transmission rate T 3 . That is, the optical signal having the longest wavelength is attenuated more strongly than the optical signal having the shortest wavelength. Therefore, the more an optical signal with a long wavelength is amplified due to gain tilt, the more strongly the intensity of the optical signal can be attenuated. For example, the values of the transmission rates are stored in the storage unit  226  and the control unit  224  uses the transmission rate to calculate the amount of electric current injected into the semiconductor optical amplifier  220 . When the number of wavelength channels is two, for example, the wavelengths λ 0  and λ 3  should satisfy the condition (1-2). 
         [0000]        T 0≧ T 1≧ T 2≧ T 3  (1-1)
 
         [0000]        T 0&gt; T 3  (1-2)
 
       Configuration Example 1 
       [0056]    In Configuration Example 1, a metal thin film or a dielectric multilayer is formed in the optical demultiplexer  240  to adjust the optical transmission rates T 0 , T 1 , T 2  and T 3 . 
         [0057]      FIG. 8  is a diagram illustrating a configuration example of the filter of the optical demultiplexer. The filter  242  includes, for example, a substrate  242 - 11  which decreases the transmission loss for wavelength ranging from λ 0  to λ 3  and a dielectric multilayer  242 - 12 . Optical signals are input from the side of the dielectric multilayer  242 - 12 . The substrate  242 - 11  is a glass substrate, for example. The wavelength transmitted by the dielectric multilayer  242 - 12  and the transmission rate of the wavelength of the dielectric multilayer  242 - 12  can set by changing the film thickness and the design of the layer configuration of the dielectric multilayer  242 - 12 . For example, SiO2/TiO2 or SiO2/Ta2O5 multilayer provided on a glass substrate can be used as the material of the optical filter for a 1300 nm wavelength band. The transmission rate of the filter is adjusted by forming a metal thin film for increasing the transmission loss on the opposite side of the side of the substrate  242 - 11  on which the electric multilayer  242 - 12  is formed. For example, the metal thin film is a Ni or Cr film. In addition, the transmission rate of the filter is adjusted by forming a SiO2/TiO2 or SiO2/Ta2O5 multilayer on the opposite side of the side of the substrate  242 - 11  on which the electric multilayer  242 - 12  is formed. 
         [0058]    It is assumed here that T 11  is the optical transmission rate for the wavelength λ 0  of the filter  242 - 1  of the optical demultiplexer  240 , T 12  is the optical transmission rate for the wavelength λ 1  of the filter  242 - 2  of the optical demultiplexer  240 , T 13  is the optical transmission rate for the wavelength λ 2  of the filter  242 - 3  of the optical demultiplexer  240  and T 14  is the optical transmission rate for the wavelength λ 3  of the filter  242 - 4  of the optical demultiplexer  240 . Each optical transmission rate of each filter is set to satisfy the following conditions (2-1) and (2-2). 
         [0000]        T 11≧ T 12≧ T 13≧ T 14  (2-1)
 
         [0000]        T 11&gt; T 14  (2-2)
 
         [0059]    When each optical transmission rate of each filter satisfies the above conditions, the transmission rates T 0 , T 1 , T 2  and T 3  satisfy the conditions (1-1) and (1-2). A metal thin film or a dielectric multilayer as described above can be formed on the surfaces of the output-side lens  244 - 1 ,  244 - 2 ,  244 - 3  and  244 - 4  instead of adjusting the transmission rate of each filter of the optical demultiplexer  240 . In this case, the optical transmission rate of each output-side lens should be set to satisfy the above conditions (2-1) and (2-2). In addition, a metal thin film or a dielectric multilayer as described above can be formed on the end surface of the optical transmission paths  251 ,  252 ,  253  and  254  facing the optical demultiplexer  240  instead of adjusting the transmission rate of each filter of the optical demultiplexer  240 . In this case, the optical transmission rate at each end surface should be set to satisfy the above conditions (2-1) and (2-2). 
         [0060]    According to Configuration Example 1, the intensity of optical signals can be adjusted without changing the configurations of the optical transmission paths  251 ,  252 ,  253  and  254  and the optical detectors  261 ,  262 ,  263  and  264 . 
       Configuration Example 2 
       [0061]    In Configuration Example 2, the coupling efficiency of the lens of the optical demultiplexer  240  is changed to adjust the optical transmission rates T 0 , T 1 , T 2  and T 3 . It is assumed here that the optical transmission rates of the filters in the optical demultiplexer are equal with each other. 
         [0062]    Optical signals transmitted through the filters are coupled to the optical transmission paths by the corresponding output-side lenses on the output side in the optical demultiplexer  240 . For example, the optical signals with wavelength λ 0  transmitted through the filter  242 - 1  are coupled to the optical transmission path  251  by the output-side lens  244 - 1 . It is assumed here that n 11  is the coupling efficiency of the output-side lens  244 - 1 . Similarly, the optical signals with wavelength λ 1  transmitted through the filter  242 - 2  are coupled to the optical transmission path  252  by the output-side lens  244 - 2 . It is assumed here that η 12  is the coupling efficiency of the output-side lens  244 - 2 . In addition, the optical signals with wavelength λ 2  transmitted through the filter  242 - 3  are coupled to the optical transmission path  253  by the output-side lens  244 - 3 . It is assumed here that η 13  is the coupling efficiency of the output-side lens  244 - 3 . Further, the optical signals with wavelength λ 3  transmitted through the filter  242 - 4  are coupled to the optical transmission path  254  by the output-side lens  244 - 4 . It is assumed here that η 14  is the coupling efficiency of the output-side lens  244 - 4 . The respective coupling efficiency is set to satisfy the following conditions (3-1) and (3-2). 
         [0000]      η11≧η12≧η13≧η14  (3-1)
 
         [0000]      η11&gt;η14  (3-2)
 
         [0063]    When each coupling efficiency satisfies the above conditions, the transmission rates T 0 , T 1 , T 2  and T 3  satisfy the conditions (1-1) and (1-2). The adjustment of the respective coupling efficiency can be achieved by modifying the position at which each output-side lens is fixed to cause defocus. Generally, a lens is fixed by YAG laser welding in the optical demultiplexer for matching the focus of the lens with the light condensing position such as the end surface of the optical fiber and the light receiving surface of the PD. When the light condensing position is modified, the lens is fixed by YAG laser welding at a position at which the focus of the lens and the light condensing position are intentionally displaced with each other to achieve the above defocus, for example. 
         [0064]    In Configuration Example 2, the intensity of the optical signal can be adjusted without changing the configuration of the optical transmission paths  251 ,  252 ,  253  and  254  and the optical detectors  261 ,  262 ,  263  and  264 . 
       Configuration Example 3 
       [0065]    In Configuration Example 3, connection points are provided on the optical transmission paths between the optical demultiplexer and the optical detectors to adjust the optical transmission rates T 0 , T 1 , T 2  and T 3 . 
         [0066]      FIG. 9  is a diagram illustrating an example of the optical transmission paths between the optical demultiplexer and the optical detectors in Configuration Example 3. The optical transmission path  251  connecting the optical demultiplexer  240  with the optical detector  261  includes the connection point  251 - 1 . The optical transmission path  252  connecting the optical demultiplexer  240  with the optical detector  262  includes the connection point  252 - 1 . The optical transmission path  253  connecting the optical demultiplexer  240  with the optical detector  263  includes the connection point  253 - 1 . The optical transmission path  254  connecting the optical demultiplexer  240  with the optical detector  264  includes the connection point  254 - 1 . Each optical transmission path is spliced at the respective connection point to displace the optical axes of the optical transmission paths optically connected with each other. The larger of the displacement of the optical axes of the optical coupling at the connection point of the optical transmission path becomes, the smaller the optical transmission rate of the optical transmission path becomes. The optical transmission rate of the optical transmission path is adjusted by adjusting the displacement of the axes of the optical coupling at the connection point of the optical transmission path. It is assumed here that the optical transmission rates of the optical transmission paths  251 ,  252 ,  253  and  254  are T 21 , T 22 , T 23  and T 24 , respectively. Each transmission rate is adjusted to satisfy the following conditions (4-1) and (4-2). 
         [0000]        T 21≧ T 22≧ T 23≧ T 24  (4-1)
 
         [0000]        T 21&gt; T 24  (4-2)
 
         [0067]    The optical transmission path  251  may not include the connection point  251 - 1 . When the optical transmission path  251  does not include the connection point  251 - 1 , the optical transmission rate T 21  of the optical transmission path  251  is almost 1 and the above conditions are satisfied. 
         [0068]    According to Configuration Example 3, the intensity of the optical signals can be adjusted without modifying the configurations of the optical demultiplexer  240  and the optical detectors  261 ,  262 ,  263  and  264 . 
       Configuration Example 4 
       [0069]    In Configuration Example 4, the optical transmission rates T 0 , T 1 , T 2  and T 3  are adjusted by the respective configurations of the optical detectors. 
         [0070]    The optical signals with wavelength λ 0  input into the optical detector  261  are coupled with the PD  261 - 2  by the lens  261 - 1 . It is assumed that the coupling efficiency is η 21 . Similarly, the optical signals with wavelength λ 1  input into the optical detector  262  are coupled with the PD  262 - 2  by the lens  262 - 1 . It is assumed that the coupling efficiency is n 22 . In addition, the optical signals with wavelength λ 2  input into the optical detector  263  are coupled with the PD  263 - 2  by the lens  263 - 1 . It is assumed that the coupling efficiency is n 23 . Further, the optical signals with wavelength λ 3  input into the optical detector  264  are coupled with the PD  264 - 2  by the lens  264 - 1 . It is assumed that the coupling efficiency is η 24 . In this case, each coupling efficiency is adjusted to satisfy the following conditions (5-1) and (5-2). 
         [0000]      η21≧η22≧η23≧η24  (5-1)
 
         [0000]      η21&gt;η24  (5-2)
 
         [0071]    When each coupling efficiency satisfies the above conditions, the transmission rates T 0 , T 1 , T 2  and T 3  satisfy the conditions (1-1) and (1-2). 
         [0072]    The adjustment of each coupling efficiency can be achieved by modifying the position at which the lens is fixed, for example. 
         [0073]    Metal thin films or dielectric multilayers for causing transmission loss can be formed on the end surface of the optical transmission path  251 , the surface of the lens  261 - 1  of the optical detector  261  and the detection surface of the PD  261 - 2  to adjust the coupling efficiency to satisfy the conditions (1-1) and (1-2). 
         [0074]    According to Configuration Example 4, the intensity of the optical signals can be adjusted without modifying the configurations of the optical demultiplexer  240  and the optical transmission paths  251 ,  252 ,  253  and  254 . 
       Specific Example 1 
       [0075]      FIG. 10  is a diagram illustrating an example (1) of the intensity of the optical signals received by the optical receiver according to the present embodiment. The horizontal axis of each graph represents the wavelengths of the optical signals and the vertical axis of each graph represents the intensity of the optical signals. The graph A 1  in  FIG. 10  represents the intensity of the optical signals input into the semiconductor optical amplifier  220 . Since the intensity of the optical signals themselves illustrated in the graph A 1  is greatly smaller than Pmin of the optical detectors, a larger electric current as illustrated by the electric current I 1  in  FIG. 2  is input into the semiconductor optical amplifier  220 . The graph A 2  in  FIG. 10  represents the intensity of the optical signals when the optical signals as illustrated in the graph A 1  are amplified by and output from the semiconductor optical amplifier  220 . Since the electric current such as I 1  as illustrated in  FIG. 2  is input into the semiconductor optical amplifier  220 , a gain tilt does not occur and each optical signal is amplified almost equally. The graph A 3  in  FIG. 10  represents the intensity of the optical signals detected by the optical detectors when the optical signals as illustrated in the graph A 2  are demultiplexed by the optical demultiplexer  240 . In the optical demultiplexer  240 , each optical transmission path between the optical demultiplexer and each optical detector and each optical detector, the transmission rate is set to be smaller as the wavelength of the optical signal becomes longer. Therefore, the intensity of the optical signals with wavelength λ 0 , λ 1 , λ 2  and λ 3  received by the optical detectors becomes smaller in this order. In addition, the intensity of each optical signal is equal to or larger than Pmin of the respective optical detector and equal to or smaller than Pmax of the respective optical detector. 
       Specific Example 2 
       [0076]      FIG. 11  is a diagram illustrating an example (2) of the intensity of the optical signals received by the optical receiver according to the present embodiment. The horizontal axis of each graph represents the wavelengths of the optical signals and the vertical axis of each graph represents the intensity of the optical signals. The graph B 1  in  FIG. 11  represents the intensity of the optical signals input into the semiconductor optical amplifier  220 . Since the intensity of the optical signals themselves illustrated in the graph B 1  is similar to Pmin of the optical detectors, a smaller electric current as illustrated by the electric current I 2  in  FIG. 2  is input into the semiconductor optical amplifier  220 . The graph B 2  in  FIG. 11  represents the intensity of the optical signals when the optical signals as illustrated in the graph B 1  are amplified by and output from the semiconductor optical amplifier  220 . Since the electric current such as I 2  as illustrated in  FIG. 2  is input into the semiconductor optical amplifier  220 , a gain tilt occurs. In addition, the larger the wavelength of the optical signal is, the larger the intensity of the optical signal becomes. The graph B 3  in  FIG. 11  represents the intensity of the optical signals detected by the optical detectors when the optical signals as illustrated in the graph B 2  are demultiplexed by the optical demultiplexer  240 . In the optical demultiplexer  240 , each optical transmission path between the optical demultiplexer and each optical detector and each optical detector, the transmission rate is set to be smaller as the wavelength of the optical signal becomes longer. Therefore, the intensity of the optical signals received by the respective optical detectors becomes almost equal. In addition, the intensity of each optical signal is equal to or larger than Pmin of the respective optical detector and equal to or smaller than Pmax of the respective optical detector. 
       Specific Example 3 
       [0077]      FIG. 12  is a diagram illustrating an example (3) of the intensity of the optical signals received by the optical receiver according to the present embodiment. The horizontal axis of each graph represents the wavelengths of the optical signals and the vertical axis of each graph represents the intensity of the optical signals. The graph C 1  in  FIG. 12  represents the intensity of the optical signals input into the semiconductor optical amplifier  220 . Since the intensity of the optical signals themselves illustrated in the graph C 1  is similar to Pmin of the optical detectors, a smaller electric current as illustrated by the electric current I 2  in  FIG. 2  is input into the semiconductor optical amplifier  220 . The graph C 2  in  FIG. 12  represents the intensity of the optical signals when the optical signals as illustrated in the graph C 1  are amplified by and output from the semiconductor optical amplifier  220 . Since the electric current such as I 2  as illustrated in  FIG. 2  is input into the semiconductor optical amplifier  220 , a gain tilt occurs. In addition, the larger the wavelength of the optical signal is, the larger the intensity of the optical signal becomes. The graph C 3  in  FIG. 12  represents the intensity of the optical signals detected by the optical detectors when the optical signals as illustrated in the graph C 2  are demultiplexed by the optical demultiplexer  240 . In the optical demultiplexer  240 , each optical transmission path between the optical demultiplexer and each optical detector and each optical detector, the transmission rate is set to satisfy the following condition. 
         [0000]        T 0= T 1= T 2&gt; T 3  (6-1)
 
         [0078]    The above transmission rates satisfy the conditions (1-1) and (1-2) as described above. In the present example, the intensity of the optical signals with wavelength λ 3  which is above Pmax of the optical detector is attenuated more greatly than the optical signals with other wavelengths at the stage of the output from the semiconductor optical amplifier  220  and then the intensity of the optical signals with wavelength λ 3  becomes equal to or smaller than Pmax. In addition, the intensity of each optical signal is equal to or larger than Pmin of the respective optical detector and equal to or smaller than Pmax of the respective optical detector. 
         [0079]    It is noted that the configurations as described above can be combined as long as possible. 
       Advantageous Effects of Embodiments 
       [0080]    The optical receiver  200  receives optical signals in which optical signals with a plurality of wavelength channels are multiplexed and the semiconductor optical amplifier  220  amplifies the received optical signals. The optical receiver  200  demultiplexes the amplified optical signals by wavelength channels. The optical receiver  200  employs different transmission rates for different wavelength channels to adjust the intensity of the optical signals. By using the optical receiver  200 , the intensity of the optical signals detected by the optical detectors can be within a predetermined range even when the gain tilts occur in the semiconductor optical amplifier  220  and then a bit-error-free operation can be achieved for each wavelength channel. The optical receiver according to the above embodiments can suppress the variations of the intensity of the optical signals occurred due to the gain tilts in the semiconductor optical amplifier  220  without increasing the number of parts and the footprint of the optical receiver  200 . 
         [0081]    All example 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 embodiments 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.