Abstract:
In the wavelength division multiplexing optical transmission system according to the present invention, when setting the transmission wavelengths of the wavelength-variable optical transmitter in case of addition or expansion of the optical transmitter, each transmission wavelength is set automatically so as to match with each port wavelength of the input port in the multiplexer. Therefore, the setting workload of the optical transmitter is remarkably reduced, and occurrence of incorrect setting can be avoided. Also, with a simple and low-cost structure, the cost is restrained from increasing.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a wavelength division multiplexing optical transmission system and a method for controlling the transmission wavelengths of a plurality of wavelength-variable optical transmitters used in the wavelength division multiplexing optical transmission system.  
       BACKGROUND OF THE INVENTION  
       [0002]     The wavelength division multiplexing (WDM) optical transmission system transmitting signals by multiplexing different wavelengths on a single optical fiber has been put into practical use as long haul and large capacity communication medium.  
         [0003]      FIG. 1  shows a diagram illustrating an overall configuration example of the wavelength division multiplexing optical transmission system. A transmission unit  10  is constituted of a plurality of optical transmitters  12 - 1 ,  12 - 2 , . . .  12 -n. From each optical transmitter, light of different wavelengths is transmitted and input into a multiplexer unit  22  in a WDM unit  20  on the transmission side. Multiplexer unit  22  has a plurality of ports which receives a plurality of different wavelengths (λ 1 , λ 2 , . . . λn). Optical signals input to each port are multiplexed in multiplexer unit  22 . The multiplexed optical signals are then amplified by an optical amplifier  24  in WDM unit  20 , and forwarded to a transmission line  30 . The optical signals are amplified in optical relay units  32  disposed in the middle of transmission line  30 , and reach a demultiplexer  42  of a WDM unit  40  on the receiving side. In demultiplexer  42 , the optical signals are demultiplexed to each optical signal of different wavelength.  
         [0004]     To introduce the wavelength division multiplexing optical transmission system, complicated wiring and a multiplicity of optical transmitters are necessary. Therefore, a vast amount of time and work are necessary for such setting and wiring at the time of installation and expansion.  
         [0005]     Here, an exemplary installation procedure of a new channel according to the conventional method is described in the following.  
         [0006]     (1) Inserting an optical transmitter into a vacant slot of a shelf for mounting a plurality of optical transmitters in the transmission unit.  
         [0007]     (2) Connecting and setting a WDM unit (such as optical multiplexer and optical amplifier), and initiating the WDM unit.  
         [0008]     (3) Connecting the optical transmitter to a designated port of the multiplexer in the WDM unit, while verifying using a wiring diagram, etc.  
         [0009]     (4) Setting an optical transmitter wavelength to the wavelength of the connected port.  
         [0010]     In particular, in the above work (4), when connecting a multiplicity of optical transmitters to the respective ports in the multiplexer of the WDM unit, it is necessary to set the transmission wavelength of each transmitter to the wavelength corresponding to the connected port. Conventionally, a worker sets this work manually by referring to the diagram.  
         [0011]     However, this manual work of setting each transmission wavelength to the transmitter is time-consuming and may possibly induce incorrect setting. Once such an incorrect setting occurs, various problems may occur, such as delayed start of normal operation and extra operation cost.  
         [0012]     As such, when installing or expanding the wavelength division multiplexing optical transmission system, and when setting a multiplicity of channels is required, it is necessary to install, or expand, a multiplicity of optical transmitters corresponding to the number of channels. This requires an increased load of workers on site, and therefore reduction of the work has been desired.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, it is an object of the present invention to provide a wavelength multiplexing optical transmission system and a transmission wavelength control method therefor, thereby enabling easy setting of the transmission wavelengths in an optical transmitter in the wavelength multiplexing optical transmission system, and a reduced workload therefor.  
         [0014]     In order to attain the aforementioned object, according to the present invention, a wavelength division multiplexing optical transmission system includes: a plurality of wavelength-variable optical transmitters having a wavelength-variable light source; a multiplexer multiplexing a plurality of optical transmission signals having different wavelengths respectively input from the plurality of wavelength-variable optical transmitters, and outputting the multiplexed optical transmission signal; and a return optical signal generator generating a return optical signal corresponding to each optical transmission signal output from the multiplexer, and transmitting the return optical signal toward the wavelength-variable optical transmitters via the multiplexer. The aforementioned wavelength-variable optical transmitter includes a first sensor sensing the return optical signal; and a controller controlling a transmission wavelength of the optical transmission signal output from the light source, based on the return optical signal detected in the first sensor corresponding to the optical transmission signal, when the transmission wavelength of the optical transmission signal output from the light source is set so as to match a port wavelength proper to each multiplexer input port receiving the optical transmission signal to enable wavelength multiplexing.  
         [0015]     Namely, the optical transmission signal wavelength is controlled and set based on the correspondence relation between the optical transmission signal from the wavelength-variable optical transmitter and the return optical signal generated from the optical transmission signal passing through the multiplexer.  
         [0016]     Preferably, when the first sensor does not detect the return optical signal corresponding to the optical transmission signal having a predetermined wavelength output from the light source, the controller controls to retransmit the optical transmission signal from the light source after changing the transmission wavelength of the optical transmission signal to another wavelength, while when the first sensor detects the return optical signal, the controller sets the transmission wavelength to the optical transmission signal wavelength corresponding to the detected return optical signal.  
         [0017]     Further, preferably, in order to enable both the optical transmission signal of the predetermined wavelength output from the light source and the return optical signal corresponding to the optical transmission signal to be identified from respective optical transmission signals output from other wavelength-variable transmitters and return optical signals corresponding to the optical transmission signals, the controller controls to superpose predetermined information to the optical transmission signal having the predetermined wavelength output from the light source, and transmit the optical transmission signal, and in case the first sensor detects the return optical signal, the controller compares the information superposed onto the return optical signal with the information transmitted in the optical transmission signal, and when both the information sets are inconsistent, the controller controls to retransmit the optical transmission signal from the light source with predetermined information being superposed after changing the transmission wavelength of the optical transmission signal to another wavelength, while when both the superposed information sets are consistent, the controller sets the transmission wavelength to the optical transmission signal wavelength corresponding to the detected return optical signal.  
         [0018]     Further, in case that the optical transmission signal is intensity-modulated with a predetermined frequency, the aforementioned information is the frequency concerned.  
         [0019]     Also, as a transmission wavelength control method according to the present invention to achieve the above-mentioned object, in a wavelength division multiplexing optical transmission system including a plurality of wavelength-variable optical transmitters generating wavelength-variable optical transmission signals; and a multiplexer multiplexing a plurality of optical transmission signals having different wavelengths respectively input from the plurality of wavelength-variable optical transmitters, and outputting the multiplexed optical transmission signal, the transmission wavelength control method of an optical transmission signal to be transmitted from the above-mentioned wavelength-variable transmitter includes: a first step generating a return optical signal corresponding to each optical transmission signal output from the multiplexer, and transmitting the return optical signal toward the wavelength-variable optical transmitters via the multiplexer; and a second step controlling the transmission wavelength of the optical transmission signal based on the return optical signal detected in the wavelength-variable optical transmitter, when the transmission wavelength of the optical transmission signal output from the light source is set so as to match a port wavelength proper to each multiplexer input port receiving the optical transmission signal, to enable wavelength multiplexing.  
         [0020]     In the wavelength division multiplexing optical transmission system according to the present invention, when setting the transmission wavelengths of the wavelength-variable optical transmitter in case of addition or expansion of the optical transmitter, each transmission wavelength is set automatically so as to match with each port wavelength of the input port in the multiplexer. Therefore, the setting workload of the optical transmitter is remarkably reduced, and occurrence of incorrect setting can be avoided. Also, with a simple and low-cost structure, the cost is restrained from increasing.  
         [0021]     Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  shows an overall configuration example of a wavelength division multiplexing optical transmission system.  
         [0023]      FIG. 2  shows a first exemplary configuration of a wavelength division multiplexing optical transmission system according to an embodiment of the present invention.  
         [0024]      FIG. 3  shows a flowchart illustrating the operation of a wavelength controller  128  in the first exemplary configuration.  
         [0025]      FIG. 4  shows a diagram illustrating a modification example of the first exemplary configuration.  
         [0026]      FIG. 5  shows a second exemplary configuration of a wavelength division multiplexing optical transmission system according to an embodiment of the present invention.  
         [0027]      FIG. 6  shows a third exemplary configuration of a wavelength division multiplexing optical transmission system according to an embodiment of the present invention.  
         [0028]      FIG. 7  shows a diagram illustrating a table indicative of correspondence relations between output wavelengths and port wavelengths.  
         [0029]      FIG. 8  shows a flowchart illustrating the operation of a wavelength controller  128  in the third exemplary configuration.  
         [0030]      FIG. 9  shows a diagram illustrating a wavelength setting method when the transmission wavelengths are set simultaneously for a plurality of optical transmitters.  
         [0031]      FIG. 10  shows an operation flowchart of a wavelength controller in case of setting the transmission wavelength of an optical transmitter using an intensity-modulated optical signal.  
         [0032]      FIG. 11  shows another exemplary configuration of an optical transmitter according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     The preferred embodiment of the present invention is described hereinafter referring to the charts and drawings. However, it is noted that the scope of the present invention is not limited to the embodiments described below, but instead embraces all equivalents to the claims described.  
         [0034]      FIG. 2  shows a first exemplary configuration of a wavelength division multiplexing optical transmission system according to an embodiment of the present invention. In this  FIG. 2 , configurations of an optical transmitter  12  provided in a transmission unit and a multiplexer unit  22  provided in a WDM unit, respectively equipped on the transmission side of a wavelength division multiplexing optical transmission system according to the embodiment of the present invention. Optical transmitter  12  includes wavelength-variable (tunable) laser diode (TLD)  122 , optical circulator  124 , photodiode (PD)  126 , and wavelength controller  128 . Also, multiplexer unit  22  includes multiplexer  222 , optical coupler  224 , photodiode (PDb)  226  and light emitting diode (LED)  228 .  
         [0035]     The operation of the first exemplary configuration is explained in the following. An optical signal wavelength output from TLD  122  can be changed according to a control signal given from wavelength controller  128 , and the output optical signal is input into a port Px of multiplexer  222 , to which optical transmitter  12  is connected, via optical circulator  124 . The port Px is a port which transmits an optical signal of wavelength λx. Therefore, when the wavelength of an optical signal from TLD  122  is λx, an optical signal having the wavelength λx is output from multiplexer  222 . The optical signal of wavelength λx then reaches photodiode (PDb)  226  via optical coupler  224  of multiplexer unit  22 . On receipt of the optical signal of wavelength λx, photodiode (PDb)  226  outputs a detection signal (electric current) corresponding the optical signal level (and a frequency in case that a predetermined frequency is superposed onto the optical signal, as described later). The detection signal is then supplied to light emitting diode (LED)  228 . LED  228  has a light emission spectrum which covers the entire wavelengths received by the respective ports of multiplexer  222 , and emits the light corresponding to each detected signal level. The light emitted from LED  228  produces return light proceeding, through optical coupler  224 , in the reverse direction to the traveling direction of the light output from TLD  122 . This return light enters the output portion of multiplexer  222 . Then, the return light is made-to branch into each wavelength, and is output from each port including the port Px. From the port Px, the light having wavelength λx is input into optical transmitter  12 . Through optical circulator  124  of optical transmitter  12 , the return light of wavelength λx from the port Px is received by photodiode (PD)  126 , and the detection signal of the return light is supplied to wavelength controller  128 . Here, in  FIG. 2 , optical circulator  124  is used to make the return light branch. However, it may also be possible to use an optical coupler, in place of optical circulator  124 , when an isolator is incorporated in TLD  122 .  
         [0036]     Namely, when the optical signal wavelength output from TLD  122  matches the wavelength (port wavelength) passing through the port P of multiplexer  222  to which optical transmitter  12  is connected, photodiode (PD)  126  in the optical transmitter receives the return light. Based on the output signal from photodiode (PD)  126 , wavelength controller  128  can recognize that the optical signal wavelength from TLD  122  matches the port wavelength of the port P of multiplexer  222   
         [0037]     On the other hand, when the optical signal wavelength output from TLD  122  does not match the port wavelength of the port P of multiplexer  222  connected therein, since the optical signal cannot pass through the port P, the optical signal does not reach photodiode (PDb)  226  in multiplexer unit  22 . Also, since there is no return light output from light emitting diode (LED)  228 , photodiode (PD)  126  in optical transmitter  12  does not receive any return light, and also wavelength controller  128  does not receive any signal from photodiode (PD)  126 . In this case, wavelength controller  128  can recognize that the optical signal wavelength output from TLD  122  does not match the port wavelength of the port P of multiplexer  222 .  
         [0038]      FIG. 3  shows a flowchart illustrating the operation of wavelength controller  128  in the first exemplary configuration. Wavelength controller  128  sets the wavelength in TLD  122  to an arbitrary initial value (step S 10 ), and transmits an optical signal having the wavelength set above (S 12 ). Then, in the step S 14 , when no detection signal from photodiode (PD)  126  based on the return light corresponding to the optical signal having the wavelength of interest is received, wavelength controller  128  sets the wavelength of TLD  122  to another wavelength (S 16 ), and transmits an optical signal having the other wavelength set above (S 12 ). Wavelength controller  128  repeats the above-mentioned operation until a detection signal is received from photodiode (PD)  126 . On receipt of an output signal from photodiode (PD)  126 , wavelength controller  128  fixes the transmission wavelength of TLD  122  to the wavelength having been set above, without modification (S 18 ).  
         [0039]     In such a way, according to the embodiment of the present invention, the optical signal wavelength to be transmitted from optical transmitter  12  can automatically be matched the port wavelength of the port P of multiplexer unit  22 . Thus, the setting work of a transmission wavelength into the optical transmitter becomes quite easy.  
         [0040]      FIG. 4  shows a diagram illustrating a modification example of the first exemplary configuration, in which optical transmitter  12  is connected to a multiplexer port via an optical cross-connect switch  50 . Optical cross-connect switch  50  enables arbitrarily switching connections between a multiple of inputs and outputs. Even when optical cross-connect switch  50  is disposed between optical transmitter  12  and multiplexer unit  22  as shown in  FIG. 4 , the operation of each configuration element is quite the same as the operation in the first exemplary configuration, except for the optical signal being passed through the wiring of optical cross-connect switch  50  in this case.  
         [0041]      FIG. 5  shows a second exemplary configuration of the wavelength division multiplexing optical transmission system according to an embodiment of the present invention. In this second exemplary configuration, a total-reflection mirror  223  is used, in place of photodiode (PDb)  226  and light emitting diode (LED)  228  in multiplexer unit  22  of the aforementioned first exemplary configuration. Namely, the light from TLD  122  through multiplexer  222  is reflected by total-reflection mirror  223 , by which the return light is generated. This return light is received by photodiode (PD)  126  in optical transmitter  12 , which enables determination whether the transmission wavelength matches the port wavelength. The operation of wavelength controller  128  in this case is the same as that shown in the flowchart of  FIG. 3 .  
         [0042]      FIG. 6  shows a third exemplary configuration of the wavelength division multiplexing optical transmission system according to an embodiment of the present invention. In  FIG. 6 , an array waveguide grating (AWG) is employed as multiplexer  222  of multiplexer unit  22 , and only multiplexer unit  22  is shown, while optical transmitter  12  is omitted in this figure. The structure of optical transmitter  12  is similar to that shown in the above first and second exemplary configuration.  
         [0043]     On the input side of the array waveguide grating (AWG) a plurality of ports (P 1 , P 2 , . . . Pn) are provided corresponding to different wavelengths (λ 1 , λ 2 , . . . λn). On the output side, in addition to an output port Q 1  from which a multiplexed optical signal is output, a monitor port Q 2  and an LED input port Q 3  are provided. Here, in the case an optical signal having a different wavelength from the corresponding port wavelength is input to a certain port, the optical signal having the above different wavelength is output to the monitor port Q 2 . To the LED input port Q 3 , the light output from the light emitting diode (LED) is input. A photodiode (PD) is attached to the monitor port Q 2 . When the optical signal is output from the monitor port Q 2 , the photodiode (PD) receives the optical signal. Caused by the current being output corresponding to the optical signal level, the light emitting diode (LED) emits light. Return light from the light emitting diode (LED) is input to the LED input port Q 3 , and output from each port P on the input side of the array waveguide grating (AWG).  
         [0044]     For example, assuming that the port wavelength of the port P 1  on the input side is λ 1 , in case an optical signal of wavelength λ 1  is input into the port P 1 , the optical signal of wavelength λ 1  is output from the output port Q 1 . Meanwhile, in case an optical signal of wavelength λ 2  is input into the port P 1 , the optical signal of wavelength λ 2  is output from the monitor port Q 2 .  
         [0045]     Under the above circumstance, in case the optical signal of wavelength λ 2  is input into the port P 1  of which port wavelength is λ 1 , photodiode (PD)  126  in optical transmitter  12  receives the return light. Wavelength controller  128  is provided in advance with a table indicative of correspondence relations between the transmission wavelengths from TLD  122  and the port wavelengths. By referring to this table, wavelength controller  128  recognizes the port wavelength based on the return light received by the photodiode (PD).  
         [0046]     Here, in this third exemplary configuration using the array waveguide grating, when the total-reflection mirror is used to generate return light, as shown in the third exemplary configuration of  FIG. 5 , the LED input port Q 3  is not necessary. A return optical signal generated by reflecting a transmitted optical signal is input from the monitor port Q 2 .  
         [0047]      FIG. 7  shows an example of the table indicative of correspondence relations. According to the relation shown in  FIG. 7 , in case of the example shown above, since photodiode (PD)  126  receives return light when an optical signal of wavelength λ 2  is input to the port P 1 , wavelength controller  128  recognizes the port wavelength of the port P 1  as λ 1 , and accordingly wavelength controller  128  changes the output wavelength of TLD  122  from wavelength λ 2  to wavelength λ 1 , and fixes the output wavelength as such.  
         [0048]     As shown in  FIG. 7 , in case that an optical signal of wavelength λ 3  is input to the port P 2  of port wavelength λ 2 , an optical signal of wavelength λ 4  is input to the port P 3  of port wavelength λ 3 , or an optical signal of wavelength λ 1  is input to the port P 4  of port wavelength λ 4 , the array waveguide grating (AWG) is structured so as to output each optical signal being input to each port to the monitor port Q 2 . Therefore, in the same way as described above, wavelength controller  128  recognizes the port wavelength of each port, and sets each transmission wavelength of TLD  122 .  
         [0049]      FIG. 8  shows a flowchart illustrating the operation of a wavelength controller  128  in the third exemplary configuration. Wavelength controller  128  sets the wavelength in TLD  122  to an arbitrary initial value (S 10 ), and transmits an optical signal having the wavelength set above (S 12 ). Next, in the step S 14 , when no output signal according to the return light corresponding to the optical signal of the set wavelength is received from photodiode (PD)  126 , wavelength controller  128  sets the wavelength of TLD  122  to a different wavelength (S 16 ), and then retransmits the optical signal with the set different wavelength (S 12 ). Wavelength controller  128  repeats the above-mentioned operation until any output signal is received from photodiode (PD)  126 . On receipt of the output signal from photodiode (PD)  126 , wavelength controller  128  refers to the table exemplified in  FIG. 7 , acquires the port wavelength corresponding to the transmission wavelength (S 20 ), and fixes the transmission wavelength of TLD  122  to the wavelength acquired above (S 22 ).  
         [0050]     Here, when the transmission wavelengths are set for a plurality of optical transmitters at a time, there is a case that the wavelength setting of the optical transmitters cannot be set only by the return light detection. The reason for this is as follows: A particular return light proceeds through multiplexer unit  22  in the reverse direction, and is made to branch into the port wavelength of each port, and then reaches the entire optical transmitters  12 . Therefore, when setting the transmission wavelengths for the plurality of optical transmitters at a time, it is not possible to identify from which optical transmitter the optical signal returns. To solve this problem, in order to enable identifying a transmission source of an optical signal corresponding to return light, as an example, optical signals to be output from optical transmitters  12  are intensity-modulated with predetermined frequencies ω, which are differently assigned on a signal-by-signal basis.  
         [0051]      FIG. 9  shows a diagram illustrating a wavelength setting method when the transmission wavelengths are set for the plurality of optical transmitters at a time. By way of example, the wavelength division multiplexing optical transmission system shown in  FIG. 9  is constituted of the aforementioned first exemplary configuration ( FIG. 3 ), and a case that optical signals are transmitted simultaneously from an optical transmitter  12 - 1  and an optical transmitter  12 - 2  is considered. The transmission wavelength (λ 1 ) of the optical signal from optical transmitter  12 - 1  is consistent with the port wavelength (λ 1 ), and this optical signal is intensity-modulated with a frequency ω 1 . Also, the transmission wavelength (λ 3 ) of the optical signal from optical transmitter  12 - 2  is not consistent with the port wavelength (λ 2 ), and this optical signal is intensity-modulated with a frequency ω 2 . Further, each frequency ω 1 , ω 2  is set corresponding to each signal wavelength.  
         [0052]     In this case, since the wavelength λ 3  of the optical signal output from optical transmitter  12 - 2  is not consistent with the port wavelength λ 2 , the optical signal cannot pass through multiplexer  222  of multiplexer unit  22 , and also the return light is not detected. Meanwhile, the wavelength λ 1  of the optical signal output from optical transmitter  12 - 1  is consistent with the port wavelength λ 1 . Therefore, the optical signal can pass through multiplexer  222  of multiplexer unit  22 , and is received by photodiode (PDb)  226 . At this time, since the optical signal is superposed and modulated with a frequency ω 1 , the detection current of photodiode (PDb)  226  having the frequency ω 1  is output. Because light emitting diode (LED)  228  also emits light with the frequency ω 1  of the detection current, the frequency ω 1  is also superposed in the return light.  
         [0053]     The return light intensity-modulated with this frequency ω 1  proceeds through multiplexer  222  in the reverse direction. Then, the return light is made to branch into the port frequency of each port, and input to each optical transmitter including optical transmitters  12 - 1 ,  12 - 2 , and then detected by each photodiode (PD)  126 - 1 ,  126 - 2 . Each wavelength controller  128 - 1 ,  128 - 2  in each optical transmitter  12 - 1 ,  12 - 2  compares the frequency of the detection current supplied from each photodiode (PD)  126 - 1 ,  126 - 2  with the frequency superposed onto the optical signal transmitted from TLD  122 - 1 ,  122 - 2  using the synchronous detection. As a result of the comparison, when the frequencies match each other, each wavelength controller  128 - 1 ,  128 - 2  can recognize that the return light is based on the optical signal having been transmitted from the optical transmitter of interest. In the example shown in  FIG. 9 , wavelength controller  128 - 2  in optical transmitter  12 - 2  compares the frequency ω 1  of the detection current from photodiode (PD)  126 - 2  with the frequency ω 2  of the optical signal transmitted from TLD  122 - 2 , and determines these two frequencies do not match in this case. As a result, wavelength controller  128 - 2  recognizes that this return light does not correspond to the optical signal transmitted from optical transmitter  12 - 2 . Namely, wavelength controller  128 - 2  determines that the transmission wavelength does not match the port wavelength. Meanwhile, wavelength controller  128 - 1  compares the frequency ω 1  of the detection current from photodiode (PD)  126 - 1  with the frequency ω 1  of the optical signal transmitted from TLD  122 - 1 . In this case, since these frequencies match each other, wavelength controller  128 - 1  recognizes that this return light corresponds to the optical signal transmitted from optical transmitter  12 - 1  concerned, and determines that the transmission wavelength matches the port wavelength. As a result, wavelength controller  128 - 1  sets the transmission wavelength of TLD  122 - 1  to the wavelength λ 1 .  
         [0054]      FIG. 10  shows an operation flowchart of the wavelength controller in case of setting the transmission wavelength of an optical transmitter using an intensity-modulated optical signal. As mentioned earlier, wavelength controller  128  sets the wavelength in TLD  122  to an arbitrary initial value (S 10 ), superposes a modulation signal corresponding to the wavelength, and then retransmits the optical signal having the wavelength set above (S 12 ). Next, in the step S 14 , when no detection signal based on the return light corresponding to the optical signal of the set wavelength is received from photodiode (PD)  126  within a predetermined time period, wavelength controller  128  sets the wavelength of TLD  122  to a different wavelength (S 16 ) superposes a modulation signal corresponding to the different wavelength set above, and then retransmits an optical signal with the set different wavelength (S 12 ). Wavelength controller  128  repeats the above-mentioned operation until a detection signal is received from photodiode (PD)  126 . On receipt of the detection signal from photodiode (PD)  126  (S 17 ), wavelength controller  128  checks whether the detection signal frequency matches the frequency superposed onto the optical signal transmitted from TLD  122  using the synchronous detection. If the frequencies match, wavelength controller  128  sets the transmission wavelength of TLD  122  to the frequency being set at present, in the case of the first and second exemplary configurations (S 18 ). Meanwhile, in the case of the third exemplary configuration, wavelength controller  128  refers to the table exemplified in  FIG. 7 , and acquires a port wavelength corresponding to the transmission wavelength (S 20 ), and fixes the transmission wavelength of TLD  122  to the frequency acquired above (S 22 ).  
         [0055]     In the above description, the method of the present invention is not limited to the system using the intensity modulation of the carrier wave (optical transmission signal) being intensity-modulated by a predetermined frequency. For example, a digital signal corresponding to the wavelength may be superposed as information. In such a case, this signal is used for the comparison and the determination of consistency or inconsistency.  
         [0056]      FIG. 11  shows another exemplary configuration of the optical transmitter according to an embodiment of the present invention. In this configuration, optical circulator  124  is not employed in optical transmitter  12  to make transmission light and return light branch. Instead, an optical isolator  125  is employed for branching. Optical isolator  125  (which is shown as being enclosed by the dotted line) is structured of an ordinary optical isolator, in which a Faraday rotator and a polarization rotator are sandwiched by birefringent prisms, and lenses are disposed on the outer sides of the birefringent prisms. An optical signal being input in the normal direction from TLD  122  disposed on the light axis is separated into two polarization components by the birefringent prisms, although not shown in  FIG. 11 . By making the two polarization components pass through the polarization rotator and the Faraday rotator, and incident to the birefringent prisms, each polarization component can focus on the core portion of the optical fiber disposed on the optical axis. Thus the optical signal can pass through optical isolator  125  in the normal direction, and can proceed in the optical fiber further.  
         [0057]     On the other hand, the return light is input into optical isolator  125  in the reverse direction from the optical fiber side. The path of the polarized wave component of the return light in this case is shown in  FIG. 11 . Namely, after separated into two polarization components by the birefringent prism, the return light is made to pass through the polarization rotator and the Faraday rotator. In the reverse direction, since the polarization plane is different from that in the normal direction by 90 degrees, when the polarization component is input to the birefringent prism, both polarization components do not focus to one point even after collected by the lens, in a different way from the case of the normal direction. The two polarization components are output from the points deviating from the optical axis. Accordingly, the return light can be received by photodiode (PD)  126  disposed on either one of the output points deviating from the optical axis.  
         [0058]     In the wavelength division multiplexing optical transmission system according to the embodiments of the present invention, the optical transmitter may be provided with a display unit for displaying the transmission wavelength set in the above manner. According to the embodiments of the present invention, the optical transmitter automatically fixes the transmission wavelength according to the correspondence relation between the transmission wavelength successively set and the port wavelength. In other words, without externally displaying the transmission wavelength value, a worker on site cannot recognize the transmission wavelength having automatically been set. For this reason, it is of use to provide the display unit on the optical transmitter so as to enable the worker to confirm that the transmission wavelength having been set is a particular predetermined wavelength.  
         [0059]     Additionally, in a wavelength division multiplexing optical transmission system already put into service, in such a case as installing an additional optical transmitter for expansion, it is preferable that the transmission level of the optical signals for setting the transmission wavelength according to the present invention be set lower than the transmission level of the optical signals used in the normal operation.  
         [0060]     The foregoing description of the embodiments is not intended to limit the invention to the particular details of the examples illustrated. Any suitable modification and equivalents may be resorted to the scope of the invention. All features and advantages of the invention which fall within the scope of the invention are covered by the appended claims.