Abstract:
In an optical transmitter, an optical receiver and an optical wavelength multiplexing system, they can reduce the number of expensive optical parts, and can also protect a mutual interference with multiplexed optical signals of other channels, even if a wavelength interval of a signal light source is extremely narrow. Output lights of a plurality of signal laser modules and a stabilzed light source having a wavelength stableness higher than them are coupled with one wave of an adjacent wavelength. A photo-electric conversion and a heterodyne detection are performed thereon to thereby obtain a beat signal. Then, a wavelength of a signal laser module is controlled such that a frequency of the beat signal is constant. If a wavelength stabilzed light source is not used, only a relative wavelength stabilization through the heterodyne detection is carried out, and a fluctuation in an absolute wavelength is detected in a wavelength routing unit. Consequently, it is compensated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical transmitter, an optical receiver and an optical wavelength multiplexing system for stabilizing and controlling a plurality of different laser wavelengths and multiplexing and transmitting them. z 1  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, the rapid spread of the Internet leads to the explosive increase of data traffics in a main communication network. This explosive increase requires a large capacity communication system in which a larger number of information can be transmitted at a high speed. In the above-mentioned circumstance, an optical wavelength multiplexing system, such as WDM (Wavelength Division Multiplexing), DWDM (Dense Wavelength Division Multiplexing) and the like, which transmits a plurality of different optical signals through one optical fiber, can sharply increase an information transmission amount by using an existing system in its original state without newly laying the optical fiber. Thus, it is considered as the strongest method to make the capacity of the communication network larger.  
           [0005]    In this system, a fluctuation of a certain optical signal causes a mutual interference with a multiplexed optical signal of a different channel. Thus, the wavelength of a signal light must be kept extremely stable. A technique noted in Japanese Laid Open Patent Application (JP-A-Heisei, 7-202311) is listed as a technique for attaining it.  
           [0006]    [0006]FIG. 7 is a block diagram showing a wavelength stabilizing method of an apparatus of a semiconductor laser noted in the above-mentioned gazette. A semiconductor laser module  201  includes: a semiconductor laser  202  for converting an input electric signal into an optical signal; a monitoring potodiode  203  for monitoring a backward emitting light of the semiconductor laser  202 ; and a Peltier cooling element  204  serving as a cooling element placed near the semiconductor laser  202 . A forward emitting light of the semiconductor laser  202  is sent through a lens (not shown) to an optical fiber. A control circuit  205  is connected to the semiconductor laser module  201 . An injection current to the semiconductor laser  202  is controlled so as to keep a monitoring current of the monitoring potodiode  203  constant.  
           [0007]    The optical signal emitted from the semiconductor laser module  201  is inputted to an optical fiber amplifier  206 , and amplified thereby, and then sent out to a transmission path. An amplified light sent out by the optical fiber amplifier  206  is branched by a first optical branch  208 , and this branched light is further branched by a second optical branch  209 . One of the lights branched by this second optical branch  209  is converted into an electric signal by a first light receiving module  210 . As for the other light, a wavelength component of a part thereof is selected by a wavelength filter  211  for passing only the light having a wavelength slightly different from a peak wavelength of the amplified light, and it is converted into an electric signal by a second light receiving module  212 . The electric signals obtained by the first light receiving module  210  and the second light receiving module  212  are inputted to a temperature control circuit  213 , and an electric power ratio is calculated. The temperature control circuit  213  controls a value of a current to the cooling element  204  so that the electric power ratio becomes contact.  
           [0008]    In this configuration, if an atmosphere temperature of the semiconductor laser module  201  is increased and the wavelength is shifted to the side of the longer wavelength, the wavelength of the amplified light is also shifted to the side of the longer wavelength. Thus, a rate of a transmitted light intensity of the wavelength filter  211  to an intensity of the entire amplified light is increased. At this time, the monitoring potodiode  203  decreases a temperature of the cooling element  204 . On the contrary, if the atmosphere temperature of the semiconductor laser module  201  is decreased, the wavelength of the amplified light is shifted to the shorter wavelength. Thus, the rate of the transmitted light intensity of the wavelength filter  211  to the intensity of the entire amplified light is decreased. At this time, it increases the temperature of the cooling element  204 .  
           [0009]    However, the above-mentioned conventional wavelength control method requires the optical parts for adjusting the wavelength, such as the wavelength filter  211  and the optical fiber amplifier  206 , which are very high in accuracy and stableness, and the like, for each of several tens of signal light sources. Thus, this method brings about a problem of an increase in a cost of the entire system. Also, if the further advance in the optical multiplexing causes a wavelength interval between the signal lights to be narrower, in the case of the conventional method of stabilizing the wavelengths of the respective signal lights through the controls independent of each other, it is difficult to keep the wavelengths of the respective signal lights at the wavelength interval which does not involve the interference with other signal lights.  
           [0010]    In view of the above-mentioned problems, it is therefore an object of the present invention to provide an optical transmitter, an optical receiver, and an optical wavelength multiplexing system, which can reduce the number of expensive optical parts, and can also protect a mutual interference with multiplexed optical signals of other channels, even if a wavelength interval of a signal light source is extremely narrow.  
         SUMMARY OF THE INVENTION  
         [0011]    In order to attain the above-mentioned objects, the optical transmitter of the present invention includes:  
           [0012]    a plurality of signal light sources for respectively emitting lights each having different wavelength from one another;  
           [0013]    a unit for obtaining a beat signal by coupling an emitting light from the signal light source with one wave of an adjacent wavelength and carrying out a photo-electric conversion; and  
           [0014]    a unit for controlling the wavelength of the signal light source so that a frequency of the beat signal is constant.  
           [0015]    Also, in order to attain the above-mentioned objects, the optical transmitter of the present invention comprises:  
           [0016]    a plurality of signal light sources for respectively emitting lights each having different wavelength from one another;  
           [0017]    a reference light source;  
           [0018]    a unit for obtaining a beat signal by coupling emitting lights from the reference light source and the signal light source with one wave of an adjacent wavelength and carrying out a photo-electric conversion; and  
           [0019]    a unit for controlling the wavelength of the signal light source so that a frequency of the beat signal is constant.  
           [0020]    In order to attain the above-mentioned objects, an optical receiver of the present invention comprises:  
           [0021]    a unit for branching a light into three directions and sending to an optical branching filter and a first wavelength filter and a second wavelength filter which are different in transmission property;  
           [0022]    a unit for detecting transmitted light intensities of the first wavelength filter and the second wavelength filter; and  
           [0023]    a unit for calculating a ratio between the transmitted light intensity of the first wavelength filter and the transmitted light intensity of the second wavelength filter, and calculating a deviation amount of this ratio from a predetermined standard value, and then shifting peaks of all transmission wavelengths of the optical branching filter by an equal amount, in accordance with the deviation amount.  
           [0024]    Also, in order to attain the above-mentioned objects, an optical receiver of the present invention comprises:  
           [0025]    a unit for branching a light into two directions, and sending to a variable wavelength filter and an optical branching filter, respectively;  
           [0026]    a unit for detecting a transmitted light intensity of the variable wavelength filter;  
           [0027]    a unit for sweeping a transmission wavelength of the variable wavelength filter with a predetermined wavelength as an origin, and after the transmitted light intensity of the variable wavelength filter passes a first peak, detecting a transmission wavelength when it becomes firstly smaller by a certain rate than the peak; and  
           [0028]    a unit for shifting the peaks of all of the transmission wavelengths of the optical branching filter by an equal amount in accordance with the detected transmission wavelength.  
           [0029]    Due to the above-mentioned configuration, it is possible to send and receive the multiplexed wavelength signal which is extremely stable, only by adjusting a relative wavelength through a heterodyne detection, without any absolute wavelength control using the expensive optical elements such as a wavelength filter, an optical resonator and the like, for each laser for a signal. Thus, it is possible to reduce the number of the expensive optical elements, and also possible to protect a mutual interference with multiplexed optical signals of other channels, even if a wavelength interval of a signal light source is extremely narrow.  
           [0030]    Also, the signal light source is a semiconductor laser, and its wavelength is controlled by changing a temperature of the semiconductor laser.  
           [0031]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0032]    Also, the signal light source is a semiconductor laser, and its wavelength is controlled by changing a bias current of the semiconductor laser.  
           [0033]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0034]    Also, the signal light source is a semiconductor laser, and its wavelength is controlled by changing a temperature of the semiconductor laser, and its optical intensity is stabilized and controlled by changing a bias current.  
           [0035]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0036]    Also, the light outputted to the transmission path is a forward emitting light of the semiconductor laser, and the light used to control the wavelength of the signal light source is a backward emitting light of the semiconductor laser.  
           [0037]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0038]    Also, the light used to control the wavelength of the signal light source is a light obtained y branching the forward emitting light of the semiconductor laser.  
           [0039]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0040]    Also, the wavelength controls for the respective signal light sources are all carried out at different speeds.  
           [0041]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0042]    Also, the first wavelength filter and the second wavelength filter have characteristics so as to transmit a light having the longest wavelength among emitting lights of the signal light source, and transmission wavelength peaks of the first wavelength filter and the second wavelength filter are set on a longer wavelength side than a wavelength fluctuation range of the light having the longest wavelength among the emitting lights of the signal light source.  
           [0043]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0044]    Also, the first wavelength filter and the second wavelength filter have characteristics so as to transmit a light having the shortest wavelength among the emitting lights of the signal light source, and the transmission wavelength peaks of the first wavelength filter and the second wavelength filter are set on a shorter wavelength side than the wavelength fluctuation range of the light having the shortest wavelength among the emitting lights of the signal light source.  
           [0045]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0046]    Also, the wavelength of the reference light source is longer than the wavelength of any of the signal lasers, and the origin from which the transmission wavelength of the variable wavelength filter is swept is located on a longer wavelength side than a wavelength fluctuation range of the reference light source, and the sweeping direction is a direction to a shorter wavelength side from a longer wavelength side.  
           [0047]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0048]    Also, the wavelength of the reference light source is shorter than the wavelength of any of the signal lasers, and the origin from which the transmission wavelength of the variable wavelength filter is swept is located on a shorter wavelength side than the wavelength fluctuation range of the reference light source, and the sweeping direction is a direction to a longer wavelength side from a shorter wavelength side.  
           [0049]    Due to the above-mentioned configuration, it is possible to reduce the number of the expensive optical elements, and also possible to protect the mutual interference with the multiplexed optical signals of the other channels, even if the wavelength interval of the signal light source is extremely narrow.  
           [0050]    On of the above-mentioned transmitters and one of the above-mentioned receiver may be combined to provide an optical wavelength multiplexing system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]    These and other objects and features will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0052]    [0052]FIG. 1 is a schematic block diagram showing an optical transmitter, an optical receiver and an optical wavelength multiplexing system in a first embodiment of the present invention;  
         [0053]    [0053]FIG. 2 is a schematic block diagram showing an optical transmitter, an optical receiver and an optical wavelength multiplexing system in a second embodiment of the present invention;  
         [0054]    [0054]FIG. 3 is an explanatory view showing wavelength properties of a first wavelength filter and a second wavelength filter of FIG. 2;  
         [0055]    [0055]FIG. 4 is an explanatory view showing a relation between wavelengths of lights incident to the first wavelength filter and the second wavelength filter and V n /V n+1 ;  
         [0056]    [0056]FIG. 5 is a schematic block diagram showing an optical transmitter, an optical receiver and an optical wavelength multiplexing system in a third embodiment of the present invention;  
         [0057]    [0057]FIG. 6 is an explanatory view showing a relation between a transmission wavelength of a variable wavelength filter of FIG. 5 and an optical intensity detected by an (n+1)-th light receiver; and  
         [0058]    [0058]FIG. 7 is a block diagram showing a hardware configuration for stabilizing a wavelength in a conventional light wavelength multiplexing system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0059]    &lt;First Embodiment&gt; 
         [0060]    Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic block diagram showing an optical transmitter, an optical receiver and an optical wavelength multiplexing system in a first embodiment of the present invention. In FIG. 1, a stabilzed light source  101  is a laser light source, which is controlled such that a fluctuation in a wavelength is extremely smaller than those of first, second, . . . signal laser modules  102 ,  103 , . . . The wavelength of the stabilzed light source  101  is controlled by using, for example, a wavelength filter and an optical resonator, which is high in stableness, and the like and then adjusting a temperature so that the intensities of laser emitting lights transmitted through them are constant.  
         [0061]    The first, second, . . . signal laser modules  102 ,  103 , . . . are the semiconductor laser modules in which wavelengths λ 1 , λ 2 , . . . are set to be slightly different from each other, and a modulation is performed on a bias current. They emit laser lights emitted from front and rear end planes of a laser chip, to the side of optical fibers (F 11 , F 12 , . . . ) and the side of optical fibers (F 21 , F 22 , . . . ), respectively. The number of the signal laser modules is, for example,  64  in a DWDM system. However, any number may be used.  
         [0062]    Forward emitting lights from the respective signal laser modules  102 ,  103 , . . . are coupled by an optical coupler  105 , and amplified by a booster amplifier  106 , and then inputted through a single transmission path F 1  to a pre-amplifier  118 . The light amplified by the pre-amplifier  118  is branched into different transmission paths for each wavelengths λ 1 , λ 2 , . . . by an optical branching filter  119 , and received by first, second, . . . receivers  120 ,  121 , . . . . Incidentally, this may be designed such that the emitting from the optical branching filter  119  is not inputted to the first, second receivers  120 ,  121  and it is inputted to a transponder, and after the wavelength is converted thereby, it is inputted to an existing SDH transmitting apparatus.  
         [0063]    Each of backward emitting lights from the respective signal laser modules  102 ,  103 , . . . and an emitting light from the stabilzed light source  101  is coupled with an adjacent wave of a different wavelength by an optically coupling distributor  107 , and heterodyne-detected by each of light receivers, such as a first light receiver  108 , a second light receiver  111  and the like. The reason why the combination of the coupled lights is done between the lights of the wavelengths adjacent to each other is that a beat frequency obtained by the heterodyne detection is suppressed to a small value, which does not exceed a band of a processing circuit. By the way, the light, in which the beat frequency obtained by the synthesis with the emitting light from the stabilzed light source  101  does not exceed the band of the processing circuit, may be synthesized with the emitting light from the stabilzed light source  101 . Also, in the above-mentioned explanation, the light coupled in the optically coupling distributor  107  is used as the backward emitting light of the respective signal laser modules  102 ,  103 , . . . . However, instead of it, the light obtained by branching the forward emitting light may be used. Then, the backward emitting light may be used for a power monitor of the signal laser modules  102 ,  103 , . . . .  
         [0064]    The beat signal obtained in the first light receiver  108  is divided by a first divider  109 , and inputted to a first demodulator  110 . The first demodulator  110  converts a frequency signal into a voltage, and outputs to a wavelength correction amount calculating circuit  117 . The process similar to the output of the first light receiver  108  is performed on the beat signals obtained in the second, third, . . . light receivers  111 , . . . by using the second, third to nth dividers  112  and the second, third to nth demodulators  113 . Incidentally, the demodulators  110 ,  113 , . . . may be any type if the frequency can be detected. For example, a frequency counter may be used instead of them. Also, if the beat frequencies obtained in the light receivers  108 ,  111 , . . . are within the bands of the demodulators  110 ,  113 , . . . , the dividers  109 ,  112 , . . . between the demodulators  110 ,  113 , . . . and the light receivers  108 ,  111 , . . . may be omitted.  
         [0065]    The outputs of the respective demodulators  110 ,  113 , . . . are inputted to the wavelength correction amount calculating circuit  117 , which calculates a correction amount for a temperature of the signal laser. A method of calculating a correction amount will be described below by exemplifying the first signal laser.  
         [0066]    When an optical frequency difference between the stabilzed light source  101  and the first signal laser module  102  is assumed to be ν 1 , an input frequency f 1  to the first demodulator  110  is represented by αν 1 . Here, α is a constant determined correspondingly to a division number of the divider  109  inserted between the light receiver  108  and the demodulator  110 . For example, if the divider  109  is not inserted, α=1. If the division number is 2, α=2. Also, if an output of the first demodulator  110  is V 1 , a target value of a frequency interval between the signal laser modules is ν step , an input frequency of the demodulator  110  when the input frequency of the demodulator  110  becomes αν step  is f 0 , and an output of the demodulator  110  when the input frequency of the demodulator  110  becomes f 0  is V 0 , a deviation amount (V 1 -V 0 ) of V 1  from V 0  is represented by β(f-f 0 ). Here, β is a constant (dV 1 /df 1 ) determined from the property of the demodulator  108 . Since f 1 , f 0  are represented by αν 1 , αν step , respectively, the deviation from the ideal value ν step  of the optical frequency difference ν 1  between the first signal laser module  102  and the stabilzed light source  101  is represented by:  
         (1/α/β)(V 1 −V 0 )  
         [0067]    On the other hand, when the bias current of the first signal laser module  102  is constant, a temperature change amount of the laser required to change ν 1  by Δν 1  is substantially γΔν 1  if a temperature change T 1  is within several ° C. Thus, a temperature correction amount T c1  required to make the optical frequency difference ν 1  between the stabilzed light source  101  and the first signal laser module  102  agree with the ideal value ν step  is −γ(ν 1 −ν step ). Here, γ is a constant (dT 1 /dν 1 ) determined from the temperature property of the signal laser and the wavelength relation between the first signal laser module  102  and the stabilzed light source  101 . If the wavelength of the first signal laser module  102  is longer, γ is a plus value, if the wavelength of the stabilzed light source  101  is longer, γ is a minus value. Since (ν 1 −ν step ) is represented by (1/α/β)(V 1 −V 0 ), the temperature correction amount T c1  is represented by (−γ/α/β)(V 1 −V 0 ). The wavelength correction amount calculating circuit  117  stores in advance the values of (γ/α/β) and V 0 , and calculates T c1  from V 1  inputted from the demodulator  110 , and then outputs to a laser driving circuit  104 . The temperature correction amounts are similarly calculated for the second to nth signal laser modules  103 , . . . , and outputted to the laser driving circuit  104 .  
         [0068]    The laser driving circuit  104  sends the laser bias current to each of the first, second, . . . signal laser modules  102 ,  103 , . . . and controls the temperature. Each of temperature control target values of the signal lasers is the value corrected in accordance with the correction amount calculated by the wavelength correction amount calculating circuit  117 . Incidentally, the correction is done from the laser having the wavelength that is the closest to the stabilzed light source  101 . When a correction of a next laser is done, the correction amount measured and calculated after the correction of the previous laser is ended is used. Also, if the correction is done by a simple loop control using an analog circuit without any execution of the above-mentioned timing control, a time constant of the circuit is set to be longer as the wavelength is farther from the stabilzed light source.  
         [0069]    The above-mentioned method is the method of feeding the fluctuation amount in the output from the frequency detector back to a temperature setting value. However, it is allowable to directly feed back to an amount of a current flowing through a cooling element such as Peltier or the like. Incidentally, if the fluctuation amount in the bias current of the laser is within a range of several tens of milli-amperes, a fluctuation amount in the optical frequency of the laser light is proportional. Thus, the correction may be performed on the bias current instead of the temperature. Also, the optical intensity stabilizing control of the laser may be carried out by using the bias current, and the wavelength control may be carried out by using the temperature. At this time, the backward emitting light of each signal laser may be used as the power monitor.  
         [0070]    &lt;Second Embodiment&gt; 
         [0071]    [0071]FIG. 2 is a block diagram showing an optical transmitter, an optical receiver and an optical wavelength multiplexing system in a second embodiment. In FIG. 2, (n−1) signal laser modules  102 ,  103 , . . . are the semiconductor laser modules in which wavelengths are set to be slightly different from each other, and the modulation is performed on the bias current. They output the laser lights emitted from the front and rear end planes of the laser chip, to the side of the optical fibers (F 11 , F 12 , . . . ) and the side of the optical fibers (F 21 , F 22 , . . . ), respectively. Each of the backward emitting lights from the respective signal laser modules is coupled with one wave of the wavelength of the adjacent light by the optically coupling distributor  107 , and heterodyne-detected by each of the light receivers  111 ,  114 , . . . . The beat signal obtained by the heterodyne detection is processed by the method similar to the first embodiment. On the basis of the thus-obtained correction amount, the target value of the temperature control or the target value of the laser bias current control is corrected.  
         [0072]    Moreover, in the second embodiment, the forward emitting lights from the respective signal laser modules  102 ,  103 , . . . , are coupled by the optical coupler  105 , and amplified by the booster amplifier  106 , and then inputted through a single transmission path to the pre-amplifier  118 . The light amplified by the pre-amplifier  118  is branched into three directions by an optical distributor  122 , and inputted to the optical branching filter  119  and a first wavelength filter and a second wavelength filter  124  having the properties, respectively, as shown in FIG. 3. Incidentally, in the above-mentioned explanation, the light coupled by the optically coupling distributor  107  is the backward emitting light of each signal laser. However, it may be the light obtained by branching the forward emitting light, and the backward emitting light may be used as the power monitor of each signal laser.  
         [0073]    The optical branching filter  119  is the variable wavelength filter using, for example, dielectric multiple-layer film, AWG, fiber grating, LNbO3. The input light is branched into a different transmission path for each wavelength, and outputted and received by the first, second, . . . receivers  120 ,  121 , . . . . Both of the first wavelength filter  123  and the second wavelength filter  124  are designed so as to transmit the first signal laser light having the longest wavelength. As for the transmission wavelength range, the range of the first wavelength filter  123  is narrower, as shown in FIG. 3. Incidentally, the transmission wavelength peaks of the two wavelength filters  123 ,  124  are set on the longer wavelength side than the fluctuation range of the wavelength λ 1  of the first signal laser having the longest wavelength.  
         [0074]    The output lights of the two wavelength filters  123 ,  124  are inputted to the n-th light receiver  125  and the (n+1)-th light receiver  126 , respectively. When the output of the n-th light receiver  125  is V n  and the output of the (n+1)-th light receiver  126  is V n+1 , if λ 1  is within the range of the transmission wavelength of the two wavelength filters  123 ,  124  and shorter than the transmission wavelength peaks of the two wavelength filters  123 ,  124 , V n /V n+1  is monotonically increased as the λ 1  is increased, as shown in FIG. 4. A transmission wavelength correction amount calculating circuit  127  stores in advance this property, and uses it to calculate the λ 1  from the value V n /V n+1 , and further calculates a deviation amount Δλ 1  of the λ 1  from a predetermined wavelength λ 0 , and then outputs to an optical branching filter controller  128 .  
         [0075]    The optical branching filter controller  128  is used to control the transmission wavelength of the optical branching filter  119 . For example, if the optical branching filter  119  is the variable wavelength filter using the LiNbO3, it functions as a high frequency voltage generator for generating an elastic surface wave in a LiNbO3 crystal. If the optical branching filter  119  is the fiber grating or the like, it functions as an apparatus for controlling a temperature or a pressure. The optical branching filter controller  128  sets all of the transmission wavelength peaks of the optical branching filter  119 , respectively, as follows:  
         [0076]    λ 0 +Δλ 1 ,  
         [0077]    λ 0 +Δλ 1 −λ step ,  
         [0078]    λ 0 +Δλ 1 −2λ step ,  
         [0079]    . . .  
         [0080]    λ 0 +Δλ 1 −(n−1)λ step    
         [0081]    Here, λ step  is the interval between the wavelengths of the signal lasers, (λ 0 +Δλ 1 ) is the transmission wavelength corresponding to the first signal laser, (λ 0 +Δλ 1 −λ step ) is the transmission wavelength corresponding to the second signal laser, (λ 0 +Δλ 1 −2λ step ) is the transmission wavelength corresponding to the third signal laser, and (λ 0 +Δλ 1 −(n−1)λ step ) is the transmission wavelength corresponding to the n-th signal laser.  
         [0082]    In this system, the relative wavelength between the signal lasers is very stabilized by the heterodyne detection. Thus, the respective wavelength change amounts of the signal lasers are substantially equal to each other. Thus, if all of the transmission wavelength peaks of the optical branching filter  119  are changed correspondingly to the change in the wavelength λ 1  of the first signal laser, each signal laser light is normally branched by the optical branching filter  119 .  
         [0083]    By the way, in the above-mentioned explanation, the wavelength λ 1  of the first signal laser is set to be longer than any of the signal lasers. However, it may be set to be shorter than any of the signal lasers. At this time, the transmission peaks of the first and second wavelength filters  123 ,  124  are set on the side of the shorter wavelength than the fluctuation range of the wavelength λ 1  of the first signal laser. All of the transmission wavelength peaks of the optical branching filter  119  are set to λ 1 +Δλ 1 , λ 0 +Δλ 1 +λ step , λ 0 +Δλ 1 +2λ step , . . . , λ 0 +Δλ 1 +(n−1) A step, respectively.  
         [0084]    &lt;Third Embodiment&gt; 
         [0085]    [0085]FIG. 5 is a block diagram showing an optical transmitter, an optical receiver and an optical wavelength multiplexing system in a third embodiment. In FIG. 5, a reference light source  129  is a laser light source of non-modulation, and its central wavelength is set to be longer than the wavelengths of any of the signal lasers. N signal laser modules  102 ,  103 , . . . are the semiconductor laser modules in which wavelengths are set to be slightly different from each other, and the modulation is performed on the bias current. They output the laser lights emitted from the front and rear end planes of the laser chip, to the side of the optical fibers (F 11 , F 12 , . . . ) and the side of the optical fibers (F 21 , F 22 , . . . ), respectively.  
         [0086]    Each of the backward emitting lights from the respective signal laser modules  102 ,  103 , . . . and a backward emitting light from the reference light source  129  is coupled with one wave of the wavelength of the adjacent light by the optically coupling distributor  107 , and heterodyne-detected by each of the light receivers. The beat signal obtained by the heterodyne detection is processed by the method similar to the first embodiment. On the basis of the thus-obtained correction amount, the target value of the temperature control or the target value of the laser bias current control is corrected.  
         [0087]    The forward emitting light from the reference light source  129  and the forward emitting lights from the respective signal laser modules are coupled by the optical coupler  105 , and amplified by the booster amplifier  106 , and then inputted through a single transmission path to the pre-amplifier  118 . The light amplified by the pre-amplifier  118  is branched into two directions by the optical distributor  122 , and inputted to the optical branching filter  119  and a variable wavelength filter  130 , respectively. The light inputted to the optical branching filter  119  is branched into the different transmission path for each wavelength, and outputted and received by the first, second, . . . receivers  120 ,  121 , . . . Incidentally, in the above-mentioned explanation, the light coupled by the optically coupling distributor  107  is the backward emitting light of the laser. However, it may be the light obtained by branching the forward emitting light, and the backward emitting light may be used as the power monitor of each laser.  
         [0088]    The output light of the variable wavelength filter  130  is inputted to the (n+1)-th light receiver  126 . An optical intensity detected thereby is inputted to a transmission wavelength correction amount calculating circuit  132 . A variable wavelength filter controller  131  usually sets a transmission wavelength peak λ f  of the variable wavelength filter  130  to λ 0  on a longer wavelength side than a fluctuation range of a central wavelength λ r  of the reference laser, and periodically sweeps it in a short wavelength direction with the λ 0  as an origin, and also outputs the signal, which indicates a present transmission wavelength setting value of the variable wavelength filter  130  and also indicates that it is presently being swept, to the transmission wavelength correction amount calculating circuit  132 .  
         [0089]    While it is swept, the transmission optical intensity of the variable wavelength filter  130  detected by the (n+1)-th light receiver  126  is as shown in FIG. 6. In FIG. 6, λ r  is the central wavelength of the reference laser, λ 1  is the central wavelength of the first signal laser, and λ 2  is the central wavelength of the second signal laser. However, since the signal lasers are modulated, there may be a case that the optical intensities in the vicinities of the λ 1  and the λ 2  are actually different from those shown in FIG. 6.  
         [0090]    The transmission wavelength correction amount calculating circuit  132  calculates the wavelength λ r  of the reference laser, from the output of the (n+1)-th light receiver  126  and the output of the variable wavelength filter controller  131 . The calculating method will be described below.  
         [0091]    While the variable wavelength filter controller  131  sends the signal indicating that the transmission wavelength is being swept, the transmission wavelength correction amount calculating circuit  132  records an optical intensity I detected by the (n+1)-th light receiver  126 . While it is swept or after it is swept, the transmission wavelength correction amount calculating circuit  132  calculates the maximum value I a  of the optical intensities I firstly observed after the start of the sweeping operation. In succession, the transmission wavelength correction amount calculating circuit  132  calculates a transmission wavelength setting value λ b  when the I is reduced by a predetermined rate, with respect to the maximum value I a . Since the λ b  is made shorter by a certain wavelength Δλ than the wavelength λ r  of the reference laser, the transmission wavelength correction amount calculating circuit  132  calculates λ r  from the following equation:  
         λ r =λ b +Δλ 
         [0092]    By the way, the Δλ is a constant determined from the wavelength property of the variable wavelength filter  130 , a line width of the reference laser and a property of the booster amplifier  106 , and it is stored in advance in the transmission wavelength correction amount calculating circuit  132 . The thus-obtained λ r  is outputted to the optical branching filter controller  128 .  
         [0093]    The optical branching filter controller  128  sets the respective transmission wavelength peaks of the optical branching filter  119  to λ r −λ step , λ r −2λ step , . . . , λ r −n λ step , respectively. Here, the λ step  is the interval between the wavelengths of the signal lasers, the (λ r −λ step ) is the transmission wavelength corresponding to the first signal laser, the (λ r −2λ step ) is the transmission wavelength corresponding to the second signal laser, and the (λ r −nλ step ) is the transmission wavelength corresponding to the n-th signal laser. In this system, the relative wavelength between the lasers is very stabilized by the heterodyne detection. Thus, the wavelength change amounts of the respective lasers are substantially equal to each other. Thus, if all of the transmission wavelength peaks of the optical branching filter  119  are changed correspondingly to the change in the λ r , each signal laser light is normally branched by the optical branching filter  119 .  
         [0094]    By the way, in the above-mentioned explanation, the wavelength of the reference light source  129  is set to be longer than any of the signal lasers. However, it may be set to be shorter than any of the signal lasers. At this time, the variable wavelength filter controller  131  usually sets the transmission wavelength peak λ f  of the variable wavelength filter  130  to λ 0  on the side of a shorter wavelength than the fluctuation range of the central wavelength λ r  of the reference laser, and periodically sweeps it in a long wavelength direction with the λ 0  as an origin. The λ r  is determined from λ r =λ b −Δλ. The respective transmission wavelength peaks of the optical branching filter  119  are set to λ r +λ step , λ r +2λ step , . . . , λ r +nλ step , respectively.  
         [0095]    As mentioned above, according to the present invention, it is possible to send and receive the multiplexed wavelength signal which is extremely stable, only by adjusting the relative wavelength through the heterodyne detection, without any absolute wavelength control using the expensive optical elements such as the wavelength filter, the optical resonator and the like.  
         [0096]    Also, the resolution in the wavelength interval measurement using the heterodyne detection is very high, which enables the fluctuation to be measured until the order of several MHz. Thus, the wavelength interval can be precisely adjusted over the case when the absolute wavelength control is performed on each of the signal lasers. Hence, it is possible to minimize the interference with the lights of the other wavelengths on the transmission path.