Abstract:
At least two light-emitting elements emit light to a branching device, and the branching device divides the light that entered into an input port into at least two and emits the light from M number of output ports. An optical modulator individually modulates the M number of light beams that were emitted. When a first light-emitting element driven normally fails, the first light-emitting element is stopped and a second light-emitting element that was stopped is driven, thereby maintaining the emission of the modulated M number of light beams.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical transmitter that emits a plurality of optical modulation signals in parallel and to an optical communication device including the optical transmitter. 
       BACKGROUND ART 
       [0002]    Currently, an optical communication device may be packaged in the interior of a so-called electronic instrument. Such an optical communication device includes, for example, an optical transmitter, M optical waveguides, and an optical receiver. The optical transmitter converts M electric signals, which are externally input, into M optical modulation signals and emits the optical modulation signals as light beams in parallel. 
         [0003]    The M optical waveguides allow the M optical modulation signals emitted as the light beams in parallel to be transmitted in parallel. The optical receiver receives the M optical modulation signals, transmitted in parallel, through the M optical waveguides, to convert the M optical modulation signals into M electric signals. 
         [0004]    An optical transmitter in such an optical communication device as described above simply includes M light-emitting elements, such as semiconductor lasers, and M optical modulators. In such an optical transmitter, M optical modulators individually modulate light beams individually emitted from M light-emitting elements. 
         [0005]    However, current semiconductor lasers has insufficient reliability compared to usual electronic circuits and may become unusable at unexpected timing. In such a configuration as mentioned above, an overall optical transmitter becomes unusable even if one of M semiconductor lasers becomes unusable. 
         [0006]    In order to solve the above, there has been proposed an optical transmitter in which one semiconductor laser is used, a light beam from the laser is optically distributed into M light beams in parallel through optical fibers and the like, and the M light beams are individually modulated by M optical modulators, to make optical modulation signals (for example, see Patent Literature 1). 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL 1] Japanese Patent Laid-Open No. 2007-256716 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    The number of the semiconductor laser of the optical transmitter of Patent Literature 1 is one which is one Mth that of an optical transmitter in which M light-emitting elements and M optical modulators are combined. Therefore, a probability that the overall optical transmitter becomes unusable because the one semiconductor laser becomes unusable is also one Mth. 
         [0009]    However, the fact that the overall optical transmitter becomes unusable when the one semiconductor laser becomes unusable remains unchanged. Therefore, there has been a problem that high redundancy has been unable to be achieved. 
         [0010]    The present invention is directed at providing: an optical transmitter that solves such a problem as mentioned above; and an optical communication device including the optical transmitter. 
       Solution to Problem 
       [0011]    An optical transmitter according to one exemplary embodiment of the present invention includes: at least two light-emitting elements for emitting beams; a branching device including input ports on which beams emitted by the light-emitting elements are incident and output ports for branching beams, incident on the input ports, into at least two beams and for emitting the beams; and optical modulators for individually modulating beams emitted from the output ports. 
         [0012]    An optical communication device according to one exemplary embodiment of the present invention includes: the optical transmitter of the present invention; M optical waveguides for transmitting beams, individually emitted from the M optical modulators of the optical transmitter, in parallel; and an optical receiver for receiving M beams in parallel from the M optical waveguides. 
       Advantageous Effects of Invention 
       [0013]    The optical transmitter of the present invention can achieve high redundancy. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a schematic block diagram illustrating the circuit structure of an optical transmitter according to a first exemplary embodiment of the present invention. 
           [0015]      FIG. 2  is a schematic block diagram illustrating the circuit structure of an optical communication device according to a second exemplary embodiment of the present invention. 
           [0016]      FIG. 3  is a schematic block diagram illustrating the circuit structure of the optical transmitter of the optical communication device according to the second exemplary embodiment of the present invention. 
           [0017]      FIG. 4  is a schematic circuit diagram illustrating the structure of an optical element device which is the optical transmitter of the optical communication device according to the second exemplary embodiment of the present invention. 
           [0018]      FIG. 5  is a schematic circuit diagram illustrating the structure of a directional coupler type 3 dB coupler which is a 2×2 branching device that forms the N×M branching device of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0019]      FIG. 6  is a schematic plan view illustrating the circuit structure of the directional coupler type 3 dB coupler of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0020]      FIG. 7  illustrates the inner structure of the directional coupler type 3 dB coupler of the optical transmitter according to the second exemplary embodiment of the present invention, in which  FIG. 7  ( a ) is a cross-sectional view taken along the line a-a′ of  FIG. 6 , and  FIG. 7  ( b ) is a cross-sectional view taken along the line b-b′ of  FIG. 6 . 
           [0021]      FIG. 8  is a schematic circuit diagram illustrating the structure of the MZI optical modulator of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0022]      FIG. 9  is a schematic plan view illustrating the circuit structure of the MZI optical modulator of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0023]      FIG. 10  illustrates the inner structure of the MZI optical modulator of the optical transmitter according to the second exemplary embodiment of the present invention, in which  FIG. 10  ( a ) is a cross-sectional view taken along the line a-a′ of  FIG. 9 , and  FIG. 10  ( b ) is a cross-sectional view taken along the line b-b′ of  FIG. 9 . 
           [0024]      FIG. 11  is a characteristic diagram indicating the relationships between the wavelengths and spectral intensities of a first semiconductor laser and a second semiconductor laser which are the light-emitting elements of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0025]      FIG. 12  is a characteristic diagram indicating the variations with time of the outputs of the first semiconductor laser and the second semiconductor laser in the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0026]      FIG. 13A  is a schematic block diagram illustrating the circuit structure of a first alternative example of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0027]      FIG. 13B  is a schematic block diagram illustrating the circuit structure of a second alternative example of the optical transmitter according to the second exemplary embodiment of the present invention. 
           [0028]      FIG. 14  is a schematic circuit diagram illustrating the structure of an optical transmitter according to a third exemplary embodiment of the present invention. 
           [0029]      FIG. 15  is a schematic circuit diagram illustrating the structure of an optical element device which is an optical transmitter according to a fourth exemplary embodiment of the present invention. 
           [0030]      FIG. 16  is a schematic circuit diagram illustrating the structure of an optical element device which is an optical transmitter according to a fifth exemplary embodiment of the present invention. 
           [0031]      FIG. 17  illustrates the circuit structure of a laser array which is the light-emitting element of the optical element device according to the fifth exemplary embodiment of the present invention, in which  FIG. 17  ( a ) is a schematic plan view, and  FIG. 17  ( b ) is a cross-sectional view taken along the line a-a′ of  FIG. 17  ( a ). 
           [0032]      FIG. 18  is a schematic circuit diagram illustrating the structure of an optical element device which is an optical transmitter according to a sixth exemplary embodiment of the present invention. 
           [0033]      FIG. 19  illustrates the circuit structure of a laser array which is the light-emitting element of the optical element device according to the sixth exemplary embodiment of the present invention, in which  FIG. 19  ( a ) is a schematic plan view, and  FIG. 19  ( b ) is a cross-sectional view taken along the line a-a′ of  FIG. 19  ( a ).  FIG. 19  is a schematic circuit diagram illustrating the structure of an alternative example of the optical transmitter according to the exemplary embodiment of the present invention. 
           [0034]      FIG. 20  is a schematic circuit diagram illustrating the structure of an optical element device which is an optical transmitter in an alternative example of the second exemplary embodiment. 
           [0035]      FIG. 21  is a schematic circuit diagram illustrating the structure of a multimode interference coupler type 3 dB coupler which is a 2×2 branching device that forms an N×M branching device of an alternative example of an optical transmitter in an alternative example of the second exemplary embodiment of the present invention. 
           [0036]      FIG. 22  is a schematic plan view illustrating the circuit structure of the multimode interference coupler type 3 dB coupler of the alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
           [0037]      FIG. 23  illustrates the inner structure of the multimode interference coupler type 3 dB coupler of the alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention, in which  FIG. 23  ( a ) is a cross-sectional view taken along the line a-a′ of  FIG. 22 , and  FIG. 23  ( b ) is a cross-sectional view taken along the line b-b′ of  FIG. 22 . 
           [0038]      FIG. 24  is a schematic circuit diagram illustrating the structure of an N×M star coupler that forms the N×M branching device of the alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
           [0039]      FIG. 25  is a schematic circuit diagram illustrating the structure of a ring optical modulator of an alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
           [0040]      FIG. 26  is a schematic plan view illustrating the circuit structure of the ring optical modulator of the alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
           [0041]      FIG. 27  is a cross-sectional view that illustrates the inner structure of the ring optical modulator of the alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention and that is taken along the line a-a′ of  FIG. 26 . 
           [0042]      FIG. 28  is a schematic circuit diagram illustrating the structure of the ring optical modulator of the alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
           [0043]      FIG. 29  is a schematic block diagram illustrating the structure of an electro-absorption type optical modulator of an alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
           [0044]      FIG. 30  is a characteristic diagram indicating the variations with time of the outputs of a first semiconductor laser and a second semiconductor laser of an alternative example of the optical transmitter in the alternative example of the second exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0045]    The first exemplary embodiment of the present invention will be explained below with reference to  FIG. 1 .  FIG. 1  is a schematic block diagram illustrating the circuit structure of the optical transmitter of the present exemplary embodiment. 
         [0046]    The optical transmitter  100  of the present exemplary embodiment includes N (N is a natural number of 2 or more) light-emitting elements  111  to  11 N, an N×M branching device  120 , and M (M is a natural number of 2 or more) optical modulators  131  to  13 M. 
         [0047]    The N light-emitting elements  111  to  11 N include semiconductor lasers, high-brightness LEDs (Light Emitting Diodes), and the like, and each emits light beams. The N light-emitting elements  111  to  11 N are individually connected to, for example, N driver circuits, and the N driver circuits are connected to one control circuit (not illustrated). 
         [0048]    The N×M branching device  120  includes N input ports  141  to  14 N and M output ports  151  to  15 M. The N×M branching device  120  branches a light beam, incident from one of the N light-emitting elements  111  to  11 N to one of the N input ports  141  to  14 N, equally into M beams, to emit the beams in parallel from the M output ports  151  to  15 M. 
         [0049]    The M optical modulators  131  to  13 M individually modulate light beams incident individually from the M output ports  151  to  15 M. 
         [0050]    In such a configuration as mentioned above, in the optical transmitter  100  of the present exemplary embodiment, for example, the control circuit selectively controls only one of the N driver circuits. Therefore, only one of the N light-emitting elements  111  to  11 N connected to the one driver circuit is selectively normally driven, and the remaining light-emitting elements  111  to  11 N other than the one are stopped as backups. 
         [0051]    Thus, for example, one light beam emitted by the first light-emitting element  111  as one driven as mentioned above is allowed to be incident on the first input port  141  of the N×M branching device  120 . The light beam incident on the first input port  141  is branched equally into M light beams, which are emitted in parallel from the M output ports  151  to  15 M. 
         [0052]    The M light beams emitted equally in parallel in such a manner are individually modulated by the M optical modulators  131  to  13 M. Therefore, the optical transmitter  100  of the present exemplary embodiment can output optical modulation signals including the M light beams in parallel. 
         [0053]    For example, when a malfunction occurs in the first light-emitting element  111  which is normally driven as mentioned above, the emission of the M modulated light beams can be maintained by stopping the light-emitting element  111  and by driving the stopped second light-emitting element  112 . 
         [0054]    Since the light-emitting elements  111  to  11 N as mentioned above can be switched up to (N−1) times corresponding to the number of the light-emitting elements  111  to  11 N, a probability that the optical transmitter  100  of the present exemplary embodiment becomes unusable can be further allowed to be one Mth that of the optical transmitter of Patent Literature 1. 
         [0055]    Therefore, the optical transmitter  100  of the present exemplary embodiment can achieve high redundancy. 
         [0056]    The second exemplary embodiment of the present invention will be explained below with reference to  FIG. 2  to  FIG. 12 .  FIG. 2  is a schematic block diagram illustrating the circuit structure of the optical communication device of the present exemplary embodiment,  FIG. 3  is a schematic block diagram illustrating the circuit structure of the optical transmitter of the optical communication device, and  FIG. 4  is a schematic circuit diagram illustrating the structure of an optical element device which is the optical transmitter. 
         [0057]      FIG. 5  is a schematic circuit diagram illustrating the structure of a directional coupler type 3 dB coupler which is a 2×2 branching device that forms the N×M branching device of the optical transmitter,  FIG. 6  is a schematic plan view illustrating the circuit structure of the directional coupler type 3 dB coupler, and  FIG. 7  illustrates the inner structure of the directional coupler type 3 dB coupler, in which  FIG. 7  ( a ) is a cross-sectional view taken along the line a-a′ of  FIG. 6 , and  FIG. 7  ( b ) is a cross-sectional view of taken along the line b-b′. 
         [0058]      FIG. 8  is a schematic circuit diagram illustrating the structure of the MZI optical modulator of the optical transmitter,  FIG. 9  is a schematic plan view illustrating the circuit structure of the MZI optical modulator, and  FIG. 10  illustrates the inner structure of the MZI optical modulator, in which  FIG. 10  ( a ) is a cross-sectional view taken along the line a-a′ of  FIG. 9 , and  FIG. 10  ( b ) is a cross-sectional view taken along the line b-b′ of  FIG. 9 . 
         [0059]      FIG. 11  is a characteristic diagram indicating the relationships between the wavelengths and spectral intensities of a first semiconductor laser and a second semiconductor laser which are the light-emitting elements of the optical transmitter, and  FIG. 12  is a characteristic diagram indicating the variations with time of the outputs of the first semiconductor laser and the second semiconductor laser. 
         [0060]    The optical communication device  10  of the second exemplary embodiment of the present invention includes an optical transmission device  500 , which is also an optical transmitter, an optical waveguide array  300 , and an optical receiver array  400 , as illustrated in  FIG. 2 , and the optical transmission device  500  includes an optical transmitter  200  as illustrated in  FIG. 4 . 
         [0061]    The optical transmitter  200  of the second exemplary embodiment of the present invention includes a laser array  210 , an 8×8 branching device  220 , which is an N×M branching device, and a modulator array  230 , as illustrated in  FIG. 2  to  FIG. 4 . 
         [0062]    In the optical transmitter  200  of the second exemplary embodiment of the present invention, the laser array  210  includes semiconductor lasers  211  to  218  which are light-emitting elements of N=8, and the eight semiconductor lasers  211  to  218  individually emit laser beams as light beams. 
         [0063]    The 8×8 branching device  220  is formed of twelve directional coupler type 3 dB couplers  260  as illustrated in  FIG. 4  and  FIG. 7 , and the twelve 3 dB couplers  260  are connected in a 4 by 3 matrix as illustrated in  FIG. 3  and  FIG. 4 . 
         [0064]    The 3 dB couplers  260  equally branch a light beam incident on one of two input ports  261  and  262  and emit such light beams from two output ports  263  and  264 , as illustrated in  FIG. 5 . 
         [0065]    Such an optical coupler  260  includes, for example, a SiO 2  lower cladding layer  26   b  and a SiO 2  upper cladding layer  26   c  which are layered in turn on a Si (silicon) substrate  26   a ; and Si optical waveguides  26   d  which are formed on the SiO2 lower cladding layer  26   b  and sealed with the SiO 2  upper cladding layer  26   c , as illustrated in  FIG. 6  and  FIG. 7 . 
         [0066]    The 8×8 branching device  220  includes input ports  241  to  248  of N=8 and output ports  251  to  258  of M=8 since such 3 dB couplers  260  as mentioned above are connected in a 4 by 3 matrix, as illustrated in  FIG. 3  and  FIG. 4 . 
         [0067]    Therefore, the 8×8 branching device  220  branches light beams, individually incident from the eight semiconductor lasers  211  to  218  to the eight input ports  241  to  248 , equally into eight light beams and emits the light beams in parallel from the eight output ports  251  to  258 . 
         [0068]    The modulator array  230  includes MZI (Mach-Zehnder interferometer) type optical modulators  231  to  238  of M=8. The eight optical modulators  231  to  238  individually modulate light beams individually incident from the eight output ports  251  to  258  of the 8×8 branching device  220 . 
         [0069]    Each of such optical modulators  231  to  238  includes, for example, a Si substrate  23   a , a SiO 2  lower cladding layer  23   b , a SiO 2  upper cladding layer  23   c , a Si optical waveguide  23   d , p+−Si  23   e , n+−Si  23   f , and an electrode  23   g , as illustrated in  FIG. 4  and  FIG. 8  to  FIG. 10 . 
         [0070]    In general, an N×M branching device can be realized in a structure, in which 3 dB couplers  260  of which the number is 2 n −1×n are connected in a matrix, or a portion thereof, in the case of N≦M and M=2 n  (n is a natural number). The optical transmission device  500  which is an optical transmitter partially including the optical transmitter  200  of the present exemplary embodiment will be explained in more detail below. 
         [0071]    The optical transmission device  500  of the second exemplary embodiment of the present invention includes a detector array  290  which is a monitoring unit, and the detector array  290  includes germanium photoreceivers  291  to  298  which are eight photodetectors, as illustrated in  FIG. 4 . 
         [0072]    The eight germanium photoreceivers  291  to  298  are individually optically connected to eight optical waveguides through which the eight semiconductor lasers  211  to  218  and the input ports  241  to  248  of the 8×8 branching device  220  are optically connected to each other. 
         [0073]    The eight germanium photoreceivers  291  to  298  individually detect light beams emitted from the semiconductor lasers  211  to  218  to the eight input ports  241  to  248  of the 8×8 branching device  220 . 
         [0074]    The optical transmission device  500  of the second exemplary embodiment includes a driver array  510  which is a driving unit, and the driver array  510  selectively drives the eight semiconductor lasers  211  to  218 . 
         [0075]    The driver array  510  includes eight driver circuits  511  to  518 , and the eight driver circuits  511  to  518  are individually connected to the eight semiconductor lasers  211  to  218 , respectively. The driver circuits  511  to  518  are connected to one control circuit  520 , and the eight germanium photoreceivers  291  to  298  are connected to the one control circuit  520 . 
         [0076]    The one control circuit  520  selectively drives one of the driver circuits  511  to  518  in response to the detection results of the eight germanium photoreceivers  291  to  298 , whereby one of the eight semiconductor lasers  211  to  218  selectively emits a light beam. 
         [0077]    In the semiconductor lasers  211  to  218  of the optical transmission device  500  of the second exemplary embodiment, it is assumed that, for example, the center wavelength of the ath semiconductor laser  21   a  which is normally driven is λ1, the full-width at half maximum of the wavelength spectrum of the ath semiconductor laser  21   a  is w1, the center wavelength of the (a+1)th semiconductor laser  21 ( a +1) which is normally stopped is λ2, and the full-width at half maximum of the wavelength spectrum of the (a+1)th semiconductor laser  21 ( a +1) is w2, as indicated in  FIG. 11 . 
         [0078]    In addition, the ath semiconductor laser  21   a  and the (a+1)th semiconductor laser  21 ( a +1) satisfy |λ1−λ2|≧(w1+w2)/2. 
         [0079]    In the optical transmission device  500  of the second exemplary embodiment, it is specified that the lower limit of the output of a light beam from the ath semiconductor laser  21   a  which is normally driven is P1 and the normal output thereof is P2, as indicated in  FIG. 12 . 
         [0080]    In addition, the control circuit  520  decreases the output of the ath semiconductor laser  21   a  which is normally driven to zero (0) with time and increases the output of the (a+1)th semiconductor laser  21 ( a +1) as a backup, which is stopped, from zero (0) to P2 with time when it is detected that the output of a light beam from the ath semiconductor laser  21   a  which is normally driven decreases from P2 to P1. 
         [0081]    As illustrated in  FIG. 2 , eight optical waveguides  301  to  308  in the optical waveguide array  300  are optically connected to the optical transmission device  500  as mentioned above, and the eight optical waveguides  301  to  308  are optically connected to eight optical receivers  401  to  408  in the optical receiver array  400 . 
         [0082]    In the optical transmitter  200  of the second exemplary embodiment, the eight semiconductor lasers  211  to  218 , the twelve 3 dB couplers  260 , the eight optical modulators  231  to  238 , and the eight germanium photoreceivers  291  to  298  as mentioned above are integrated on a silicon substrate  201  which is one semiconductor substrate. The eight semiconductor lasers  211  to  218  are individually separated and placed as illustrated in  FIG. 4 . 
         [0083]    In the optical transmission device  500  of the second exemplary embodiment, the eight driver circuits  511  to  518  and the one control circuit  520  may also be integrated on the silicon substrate  201  which is one semiconductor substrate in the optical transmitter  200  or may also be formed on a silicon substrate (not illustrated) which is another semiconductor substrate. 
         [0084]    In such a configuration as mentioned above, the optical transmitter  200  of the second exemplary embodiment of the present invention functions in a manner similar to that of the optical transmitter  100  mentioned above as the first embodiment. In the optical transmission device  500  which is an optical transmitter partially including the optical transmitter  200 , the control circuit  520  normally drives the first driver circuit  511  which is one of the eight driver circuits  511  to  518  and normally stops the remaining seven backup driver circuits  512  to  518 . 
         [0085]    Therefore, one light beam emitted by one of the first semiconductor laser  211  which is normally driven in the first driver circuit  511  is branched into eight light beams in parallel by the 8×8 branching device  220 , and the light beams are individually modulated by the eight optical modulators  231  to  238 . The eight optical modulation signals emitted in parallel from the optical transmission device  500  are transmitted in parallel through the eight optical waveguides  301  and individually received by the eight optical receivers  401  to  408 . 
         [0086]    In this case, a light beam emitted from the first semiconductor laser  211  to the first input port  241  of the 8×8 branching device  220  is detected by the first germanium photoreceiver  291 , and the detection results of the first germanium photoreceiver  291  are always monitored by the control circuit  520 . 
         [0087]    As mentioned above, it is specified that the lower limit of the output of a light beam from the ath semiconductor laser  21   a  which is normally driven is P1 and the normal output thereof is P2, as indicated in  FIG. 12 , in the optical transmission device  500  of the second exemplary embodiment. 
         [0088]    The control circuit  520  decreases the output of the first semiconductor laser  211  which is normally driven to zero (0) with time and increases the output of the second semiconductor laser  212  as a backup, which is stopped, from zero (0) to P2 with time when the output of the first semiconductor laser  211  is detected decreasing from P2 to P1 by the first germanium photoreceiver  291  as mentioned above. 
         [0089]    In this case, the center wavelength of the first semiconductor laser  211  which is normally driven is λ1, the full-width at half maximum of the wavelength spectrum thereof is w1, the center wavelength of the second semiconductor laser  212  which is normally stopped is λ2, and the full-width at half maximum of the wavelength spectrum thereof is w2, as indicated in  FIG. 11 , 
         [0000]      |λ1−λ2|≧( w 1 +w 2)/2
 
         [0090]    is satisfied, and therefore, a problem such as interference does not occur when the semiconductor lasers  211  and  212  are switched with time as mentioned above. 
         [0091]    When the second semiconductor laser  212  is in the state of being normally driven as mentioned above, the output thereof is always detected by the second germanium photoreceiver  292  and monitored by the control circuit  520 , and the third semiconductor laser  213  is used as a backup. 
         [0092]    Since the eight semiconductor lasers  211  to  218  as mentioned above can be switched up to seven times corresponding to the number of the semiconductor lasers  211  to  218 , a probability that the optical transmission device  500  of the second exemplary embodiment becomes unusable can be further allowed to be one eighth that of the optical transmitter of Patent Literature 1. Therefore, the optical transmitter  200  of the present exemplary embodiment can achieve high redundancy. 
         [0093]    In the optical transmitter  200  of the second exemplary embodiment, the eight semiconductor lasers  211  to  218 , the twelve 3 dB couplers  260 , the eight optical modulators  231  to  238 , and the eight germanium photoreceivers  291  to  298  are integrated on the silicon substrate  201  which is one semiconductor substrate. 
         [0094]    Therefore, the eight semiconductor lasers  211  to  218 , the twelve 3 dB couplers  260 , the eight optical modulators  231  to  238 , and the eight germanium photoreceivers  291  to  298  can be reliably optically connected, and the productivity and packaging density thereof can be improved. 
         [0095]    Further, the eight semiconductor lasers  211  to  218  are individually separated and placed in the optical transmitter  200  of the second exemplary embodiment. Therefore, crosstalk or the like does not occur in the eight semiconductor lasers  211  to  218 . 
         [0096]    In the optical transmitter  200  of the second exemplary embodiment, light beams emitted from the semiconductor lasers  211  to  218  to the eight input ports  241  to  248  of the 8×8 branching device  220  are individually detected by the eight germanium photoreceivers  291  to  298 . 
         [0097]    Therefore, light beams emitted from the semiconductor lasers  211  to  218  to the eight input ports  241  to  248  of the 8×8 branching device  220  can be individually directly detected by the eight germanium photoreceivers  291  to  298 . 
         [0098]    The present invention is not limited to the second exemplary embodiment, but various modifications may be made without departing from the spirit and scope of the present invention. For example, it was exemplified in the above embodiment that the 8×8 branching device  220  which is the N×M branching device of the optical transmitter  200  of the optical transmission device  500  includes the 2×2 3 dB couplers  260  connected in a 4 by 3 matrix, and the eight semiconductor lasers  211  to  218  and the eight optical modulators  231  to  238  are optically connected to each other through the one 8×8 branching device  220  including the twelve 3 dB couplers  260 . 
         [0099]    However, two semiconductor lasers  211  and  212  and two optical modulators  231  and  232  may be optically connected through one optical modulator  601  including one 3 dB coupler  260  as in the case of an optical transmitter  600  exemplified in  FIG. 13A . 
         [0100]    Four semiconductor lasers  211  to  214  and four optical modulators  231  to  234  may be optically connected through one 4×4 optical modulator  611  including four 3 dB couplers  260  as in the case of an optical transmitter  610  exemplified in  FIG. 13B . 
         [0101]    Next, an optical communication device which is an optical transmitter of the third exemplary embodiment of the present invention will be explained below with reference to  FIG. 14 .  FIG. 14  is a schematic circuit diagram illustrating the structure of the optical transmitter of the present exemplary embodiment. 
         [0102]    In the optical transmitter  700  of the third exemplary embodiment of the present invention, semiconductor lasers  711  to  718  in a laser array  710  include leakage ports, through which leakage beams leak, in sides opposite to ports for emission of a light beam to an 8×8 branching device  220 . 
         [0103]    Thus, eight germanium photoreceivers  291  to  298  in a detector array  290  are individually connected to the leakage ports of the eight semiconductor lasers  711  to  718 , and the outputs of light beams are detected based on the outputs of leakage beams that individually leak from the leakage ports of the eight semiconductor lasers  711  to  718 . 
         [0104]    Even in the optical transmitter  700  of the third exemplary embodiment, the eight semiconductor lasers  711  to  718 , twelve 3 dB couplers  260 , eight optical modulators  231  to  238 , and the eight germanium photoreceivers  291  to  298  as mentioned above are integrated on a silicon substrate  201  which is one semiconductor substrate. 
         [0105]    Unlike the optical transmitter  200  mentioned above, the eight germanium photoreceivers  291  to  298  in the detector arrays  290  detect the outputs of light beams based on the outputs of leakage beams that individually leak from the leakage ports of the eight semiconductor lasers  711  to  718 , in the optical transmitter  700  of the third exemplary embodiment in such a configuration as mentioned above. 
         [0106]    Therefore, it is not necessary to branch light beams transmitted from the semiconductor lasers  711  to  718  to an 8×8 branching device  220  and to allow the light beams to be detected by the germanium photoreceivers  291  to  298 , and the outputs of light beams transmitted from the semiconductor lasers  711  to  718  to the 8×8 branching device  220  can be prevented from decreasing. 
         [0107]    Further, it is not necessary to form the eight germanium photoreceivers  291  to  298  in gaps between optical waveguides through which the eight semiconductor lasers  711  to  718  and eight input ports  241 - 248  in the 8×8 branching device  220  are optically connected to each other. Therefore, the spacings between the eight semiconductor lasers  711  to  718  can be minimized to improve a packaging density. 
         [0108]    In particular, the eight semiconductor lasers  711  to  718  are integrally formed in the optical transmitter  700  of the third exemplary embodiment. Therefore, the packaging density of the eight semiconductor lasers  711  to  718  can be easily improved, and the productivity thereof can be improved. 
         [0109]    Next, an optical transmitter of the fourth exemplary embodiment of the present invention will be explained below with reference to  FIG. 15 .  FIG. 15  is a schematic circuit diagram illustrating the structure of an optical transmission device which is the optical transmitter of the present exemplary embodiment. 
         [0110]    The optical transmission device  530  which is the optical transmitter of the fourth exemplary embodiment includes an optical transmitter  720 , as illustrated. A laser array  210  in the optical transmitter  720  of the present exemplary embodiment includes semiconductor lasers  211  to  218  which are light-emitting elements of N=8, and a modulator array  230  includes MZI type optical modulators  231  to  238  of M=8. 
         [0111]    However, the eight semiconductor lasers  211  to  218  are divided into two groups of four odd-numbered semiconductor lasers and four even-numbered semiconductor lasers. Therefore, an N×M branching device  531  is formed of four one-stage 3 dB couplers  260 . 
         [0112]    The four semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group are individually connected to the first input ports  261  of the first to fourth 3 dB couplers  260 , respectively, and the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group are individually connected to the second input ports  262  of the first to fourth 3 dB couplers  260 , respectively. 
         [0113]    In addition, two driver circuits  511  and  512  in a driver array  510  which is a driving unit are individually connected to the two groups of the odd-numbered four semiconductor lasers and the even-numbered four semiconductor lasers of the eight semiconductor lasers  211  to  218 , respectively. 
         [0114]    In the semiconductor lasers  211  to  218  of the optical transmission device  530  of the fourth exemplary embodiment, it is assumed that, for example, the center wavelengths of the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group, which are normally driven, are λ1, the full-widths at half maximum of the wavelength spectra of the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group are w1, the center wavelengths of the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group, which are normally stopped, are λ2, and the full-widths at half maximum of the wavelength spectra of the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group are w2, as indicated in  FIG. 11 . 
         [0115]    In addition, 
         [0000]      |λ1−λ2|≧( w 1 +w 2)/2
 
         [0116]    is satisfied by the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group and the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group. 
         [0117]    In the optical transmission device  530  of the fourth exemplary embodiment, it is specified that the lower limit of the output of a light beam from the semiconductor laser  21   a  of the first group, which is normally driven, is P1 and the normal output thereof is P2, as indicated in  FIG. 12 . 
         [0118]    When the output of a light beam from the semiconductor laser  21   a  of the first group, which is normally driven, is detected decreasing from P2 to P1, a control circuit  520  decreases the output of the semiconductor laser  21   a  of the first group, which is normally driven, to zero (0) with time and increases the outputs of the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group, as backups, which have been stopped, from zero (0) to P2 with time. 
         [0119]    In such a configuration as mentioned above, the optical transmitter  720  of the fourth exemplary embodiment functions in a manner similar to that of the optical transmitter  200  mentioned above as the second embodiment. In the optical transmission device  530  which is an optical transmitter partially including the optical transmitter  720 , the control circuit  520  normally drives the first driver circuit  511  which is one of the two driver circuits  511  and  512  and normally stops the remaining one backup second driver circuit  512 . 
         [0120]    Therefore, four light beams emitted in parallel by the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group, which are normally driven in the first driver circuit  511 , are branched into eight light beams in parallel by the N×M branching device  531 , and the light beams are individually modulated by the eight optical modulators  231  to  238 . 
         [0121]    Even in the optical transmission device  530  of the fourth exemplary embodiment, the control circuit  520  decreases the outputs of the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group, which are normally driven, to zero (0) with time and increases the outputs of the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group, as backups, which have been stopped, from zero (0) to P2 with time when the output of any of the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group is detected decreasing from P2 to P1, as indicated in  FIG. 12 . 
         [0122]    In this case, the center wavelengths of the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group, which are normally driven, are λ1, the full-widths at half maximum of the wavelength spectra of the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group are w1, the center wavelengths of the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group, which are normally stopped, are λ2, the full-widths at half maximum of the wavelength spectra of the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group are w2, as indicated in  FIG. 11 , 
         [0000]      |λ1−λ2|≧( w 1 +w 2)/2
 
         [0123]    is satisfied, and therefore, a problem such as interference does not occur when the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group and the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group are switched with time as mentioned above. 
         [0124]    Therefore, in the optical transmission device  530  of the fourth exemplary embodiment, the semiconductor lasers  211 ,  213 ,  215 , and  217  of the first group and the semiconductor lasers  212 ,  214 ,  216 , and  218  of the second group, as mentioned above, can be switched up to twice corresponding to the number of the groups. 
         [0125]    Thus, a probability that the optical transmission device  530  and the optical transmitter  720  of the fourth exemplary embodiment become unusable can be further allowed to be one half that of the optical transmitter of Patent Literature 1, and the optical transmission device  530  and the optical transmitter  720  of the fourth exemplary embodiment can achieve high redundancy. 
         [0126]    In the optical transmission device  530  of the present exemplary embodiment, eight optical modulation signals are output as mentioned above, and the driver circuits  511  and  512  for driving the eight semiconductor lasers  211  to  218  may be two. Therefore, the productivity and packaging density thereof can be improved. 
         [0127]    Next, an optical element device which is an optical transmitter of the fifth exemplary embodiment of the present invention will be explained below with reference to  FIG. 16  and  FIG. 17 .  FIG. 16  is a schematic circuit diagram illustrating the structure of the optical element device which is the optical transmitter of the present exemplary embodiment, and  FIG. 17  illustrates the circuit structure of a laser array which is a light-emitting element in the optical element device of the present exemplary embodiment, in which  FIG. 17  ( a ) is a schematic plan view and  FIG. 17  ( b ) is a cross-sectional view taken along the line a-a′ of  FIG. 17  ( a ). 
         [0128]    The optical transmission device  540  which is the optical transmitter of the fifth exemplary embodiment includes an optical transmitter  730  as illustrated. A laser array  740  in the optical transmitter  730  of the present exemplary embodiment includes semiconductor lasers  741  to  748  which are light-emitting elements of N=8. 
         [0129]    However, the eight semiconductor lasers  741  to  748  are divided into two groups of the first to fourth semiconductor lasers which are four and the fifth to eighth semiconductor lasers which are four. Therefore, based on the two groups, the first to fourth semiconductor lasers  741  to  744  are integrally formed, and the fifth to eighth semiconductor lasers  745  to  748  are also integrally formed. 
         [0130]    Such semiconductor lasers  741  to  748  include a Si substrate  74   a , an electrode  74   b  (on Si substrate), a solder bump  74   c , an electrode  74   d  (lower portion of laser), a Si base  74   e , a laser active region  74   f , an electrode  74   g  (upper portion of laser), and the like, as illustrated in  FIG. 17 . 
         [0131]    The first to fourth semiconductor lasers  741  to  744  are integrally formed and therefore share the electrode  74   b  (on Si substrate) while the fifth to eighth semiconductor lasers  745  to  748  are integrally formed and therefore share the electrode  74   b  (on Si substrate). 
         [0132]    Further, an optical waveguide  74   h  optically connected to such semiconductor lasers  741  to  748  is formed between a SiO 2  lower cladding layer  74   i  and a SiO 2  cladding layer  74   j  on the Si substrate  74   a.    
         [0133]    Thus, an N×M branching device  531  in the optical transmitter  730  of the fifth exemplary embodiment is formed of four 3 dB couplers  260  as in the case of the optical transmitter  540  mentioned above. However, the first to fourth semiconductor lasers  741  to  744  of the first group are individually optically connected to the first input ports  261  of the first to fourth 3 dB couplers  260 , respectively, and the fifth to eighth semiconductor lasers  745  to  748  of the second group are individually optically connected to the second input ports  262  of the first to fourth 3 dB couplers  260 , respectively. 
         [0134]    In addition, the first to fourth semiconductor lasers  741  to  744  and the fifth to eighth semiconductor lasers  745  to  748  are selectively connected to a driver circuit  511 , which is one driving unit, via one change-over switch  541 , and a control circuit  520  is connected to the change-over switch  541 . 
         [0135]    In such a configuration as mentioned above, the optical transmission device  540  of the fifth exemplary embodiment functions in a manner similar to that of the above-mentioned optical transmission device  530  of the fourth exemplary embodiment, or the like. In addition, in the optical transmission device  540 , the one driver circuit  511  is always driven, and the control circuit  520  allows the driver circuit  511  to be normally connected to the semiconductor lasers  741  to  714  of the first group through the change-over switch  541 . 
         [0136]    Thus, four light beams emitted in parallel by the semiconductor lasers  741  to  714  of the first group are branched into eight light beams in parallel by the N×M branching device  531 , and the light beams are individually modulated by eight optical modulators  231  to  238 . 
         [0137]    Therefore, in the optical transmission device  540  of the fifth exemplary embodiment, the semiconductor lasers  741  to  744  and  745  to  748  of the first group and the second group as mentioned above can be switched up to twice corresponding to the number of the groups. 
         [0138]    Thus, a probability that the optical transmission device  540  and the optical transmitter  730  of the fifth exemplary embodiment become unusable can be further allowed to be one half that of the optical transmitter of Patent Literature 1, and therefore, the optical transmission device  540  and the optical transmitter  730  of the fifth exemplary embodiment can achieve high redundancy. 
         [0139]    In the optical transmission device  540  of the fifth exemplary embodiment, eight optical modulation signals are output as mentioned above, and the eight semiconductor lasers  741  to  748  are divided into the two groups each including four semiconductor lasers, which are integrally formed. Therefore, the productivity and packaging density thereof can be improved. 
         [0140]    Next, an optical element device which is an optical transmitter of the sixth exemplary embodiment of the present invention will be explained below with reference to  FIG. 18  and  FIG. 19 .  FIG. 18  is a schematic circuit diagram illustrating the structure of the optical element device which is the optical transmitter of the sixth exemplary embodiment, and  FIG. 19  ( a ) and  FIG. 19  ( b ) illustrate the circuit structure of a laser array which is a light-emitting element in the optical element device of the sixth exemplary embodiment, in which  FIG. 19  ( a ) is a schematic plan view, and  FIG. 19  ( b ) is a cross-sectional view taken along the line a-a′ of  FIG. 19  ( a ). 
         [0141]    The optical transmission device  550  which is also the optical transmitter of the sixth exemplary embodiment includes an optical transmitter  750  as illustrated. A laser array  760  in the optical transmitter  750  of the sixth exemplary embodiment includes semiconductor lasers  761  to  768  which are light-emitting elements of N=8. 
         [0142]    However, the eight semiconductor lasers  761  to  768  are divided into two groups of four odd-numbered semiconductor lasers and four even-numbered semiconductor lasers. However, the eight semiconductor lasers  761  to  768  are integrally formed and are divided into the first group and the second group depending on connection of first and second common electrodes  771  and  772 . 
         [0143]    Such semiconductor lasers  761  to  768  includes a Si substrate  76   a , an electrode  76   b  (on Si substrate), a solder bump  76   c , an electrode  76   d  (lower portion of laser), a Si base  76   e , a laser active region  76   f , an electrode  76   g  (upper portion of laser), and the like, as illustrated in  FIG. 19  ( b ). 
         [0144]    For example, in the semiconductor lasers  762 ,  764 ,  766 , and  768  of the second group, the second common electrode  772  and the electrode  76   g  (upper portion of laser) are electrically connected to each other through a contact hole  761  formed in an insulating layer  76   k.    
         [0145]    Therefore, the eight semiconductor lasers  761  to  768  are integrally formed and are electrically connected to the first and second common electrodes  771  and  772  through the contact hole  761 , whereby the semiconductor lasers are divided into the two groups of the odd-numbered semiconductor lasers and the even-numbered semiconductor lasers. 
         [0146]    Further, an optical waveguide  76   h  optically connected to such semiconductor lasers  761  to  768  is formed between a SiO 2  lower cladding layer  76   i  and a SiO 2  lower cladding layer  76   j  on the Si substrate  76   a.    
         [0147]    An N×M branching device  531  is formed of four 3 dB couplers  260 , the four semiconductor lasers  761 ,  763 ,  765 , and  767  of the first group are individually connected to the first input ports  261  of the first to fourth 3 dB couplers  260 , respectively, and the semiconductor lasers  762 ,  764 ,  766 , and  768  of the second group are individually connected to the second input ports  262  of the first to fourth 3 dB couplers  260 , respectively. 
         [0148]    In addition, two driver circuits  511  and  512  in a driver array  510  which is a driving unit are individually connected to the two groups of the odd-numbered four semiconductor lasers and the even-numbered four semiconductor lasers of the eight semiconductor lasers  761  to  768 . 
         [0149]    In such a configuration as mentioned above, the optical transmitter  750  of the sixth exemplary embodiment functions in a manner similar to that of the optical transmitter  200  mentioned above as the second exemplary embodiment, or the like. In addition, in the optical transmission device  550  which is an optical transmitter partially including the optical transmitter  750 , a control circuit  520  normally drives the first driver circuit  511  which is one of the two driver circuits  511  and  512  and normally stops the remaining one second driver circuit  512  as a backup. 
         [0150]    Therefore, four light beam emitted in parallel by the semiconductor lasers  761 ,  763 ,  765 , and  767  of the first group, which are normally driven by the first driver circuit  511 , are branched into eight light beams in parallel by the N×M branching device  531 , and the light beams are individually modulated by eight optical modulators  231  to  238 . 
         [0151]    Even in the optical transmission device  550  of the sixth exemplary embodiment, the control circuit  520  stops the semiconductor lasers  761 ,  763 ,  765 , and  767  of the first group, which are normally driven, and starts driving of the semiconductor lasers  762 ,  764 ,  766 , and  768  of the second group, as backups, which have been stopped, when the output of any of the semiconductor lasers  761 ,  763 ,  765 , and  767  of the first group is detected decreasing. 
         [0152]    Therefore, in the optical transmission device  550  of the sixth exemplary embodiment, the semiconductor lasers  761 ,  763 ,  765 , and  767  of the first group and the semiconductor lasers  762 ,  764 ,  766 , and  768  of the second group as mentioned above can be switched up to twice corresponding to the number of the groups. 
         [0153]    Thus, a probability that the optical transmission device  550  and the optical transmitter  750  of the sixth exemplary embodiment become unusable can be further allowed to be one half that of the optical transmitter of Patent Literature 1, and therefore, the optical transmission device  550  and the optical transmitter  750  can achieve high redundancy. 
         [0154]    In the optical transmission device  550  of the sixth exemplary embodiment, eight optical modulation signals are output as mentioned above, and the eight semiconductor lasers  761  to  768  are integrally formed. Therefore, the productivity and packaging density thereof can be improved. 
         [0155]    The present invention is not limited to the first to sixth exemplary embodiments and the alternative examples, described above, but various modifications can be made without departing from the spirit and scope of the present invention. For example, it is exemplified that N=M=2 n  (n is a natural number) is satisfied in the optical transmitters  200  and  600  of the second exemplary embodiment, and the like. 
         [0156]    Further, in an optical transmitter  790  exemplified, as an alternative example of the second exemplary embodiment, in  FIG. 20 , a laser array  791  includes semiconductor lasers  711  to  713  which are three light-emitting elements of which the number is not N. Thus, a 3×8 branching device  792  which is an N×M branching device is formed of two first-stage, two second-stage, and four third-stage 3 dB couplers  260 . 
         [0157]    Therefore, the semiconductor laser  711  is not connected to the second input port  262  of the second 3 dB coupler  260  of the first step. Such an optical transmitter  790  also functions in a manner similar to that of the optical transmitter  200  of the second exemplary embodiment, or the like. 
         [0158]    In addition, a probability that the optical transmitter  790  becomes unusable can be further allowed to be one third that of the optical transmitter of Patent Literature 1. Thus, a probability that the optical transmitter  790  becomes unusable can be further allowed to be one half that of the optical transmitter of Patent Literature 1, and therefore, the optical transmitter  790  can achieve high redundancy. 
         [0159]    In addition, it is exemplified that the 2×2 branching device is formed of a directional coupler type 3 dB coupler  260  in the optical transmitter  200  of the second exemplary embodiment, or the like. Further, in an alternative example of the second exemplary embodiment, a 2×2 branching device may be formed of a multimode interference coupling type 3 dB coupler  780  as illustrated in  FIG. 21  to  FIG. 23 . 
         [0160]    Such a 3 dB coupler  780  includes, for example, a Si substrate  78   a , a SiO 2  lower cladding layer  78   b , and a Si slab optical waveguide  78   c  as illustrated in  FIG. 23 . 
         [0161]    Further, it is exemplified that the N×M branching device is formed of the plural 3 dB couplers  260  in the optical transmitter  200  of the second exemplary embodiment, or the like. Further, in an alternative example of the second exemplary embodiment, an N×M branching device may be formed of an N×M star coupler  810  as illustrated in  FIG. 24 . 
         [0162]    In addition, it is exemplified that the optical modulators  231  to  238  are formed in MZI form in the optical transmitter  200  of the second exemplary embodiment, or the like. Further, in an alternative example of the second exemplary embodiment, an optical waveguide may be formed in one-ring form in an optical modulator  820  as illustrated in  FIG. 25  to  FIG. 27 . 
         [0163]    Such an optical modulator  820  in ring form includes, for example, a Si substrate  82   a , a SiO 2  lower cladding layer  82   b , an n+_Si unit  82   c , a p+_Si unit  82   d , a Si optical waveguide  82   e , an electrode  82   f , and a SiO 2  upper cladding layer  82   g  as illustrated in  FIG. 26  and  FIG. 27  in an alternative example of the second exemplary embodiment. 
         [0164]    In general, an N×M branching device can be realized in a structure, in which 3 dB couplers  260  of which the number is 2 n −1×n are connected in a matrix, or a portion thereof, in the case of N≦M and M=2 n  (n is a natural number). 
         [0165]    Similarly, in alternative examples of the second exemplary embodiment, an optical modulator may be formed in an optical modulator  830  in ring form, in which optical waveguides are two, as illustrated in  FIG. 28 , and may be formed in an electro-absorption type optical modulator  840  as illustrated in  FIG. 29 . 
         [0166]    Further, it is exemplified that the ath semiconductor laser  21   a  which is normally driven and the (a+1)th semiconductor laser  21 ( a +1) which is normally stopped satisfy 
         [0000]      |λ1−λ2|≧( w 1 +w 2)/2
 
         [0167]    as indicated in  FIG. 11  and  FIG. 12 , and the output of the ath semiconductor laser  21   a  which is normally driven is decreased to zero (0) with time and the output of the (a+1)th semiconductor laser  21 ( a +1), as a backup, which has been stopped is increased from zero (0) to P2 with time when the output of a light beam from the ath semiconductor laser  21   a  which is normally driven is detected decreasing from P2 to P1, in the optical transmission device  500  of the second exemplary embodiment, or the like. 
         [0168]    However, in an alternative example of the second exemplary embodiment, when the decrease of the output of an ath semiconductor laser  21   a  which is normally driven is detected by a detector array  290 , the driving of the ath semiconductor laser  21   a  of which the decrease of the output is detected and of an (a+1)th semiconductor laser  21 ( a +1), as a backup, which shares the output ports of a 3×8 branching device  792  and has been stopped, may be controlled to changelessly complement the outputs of light beams emitted from the shared output ports, as indicated in  FIG. 30 , 
         [0169]    In this case, the lower limit of the output of a light beam from the ath semiconductor laser  21   a  which is normally driven is P1, and the normal output thereof is P2; and when the output of a light beam from the ath semiconductor laser  21   a  which is normally driven is detected decreasing from P2 to P1, the output of the (a+1)th semiconductor laser  21 ( a +1), as a backup, which has been stopped, is controlled to allow the total output of light beams emitted from the shared output ports to be P2. 
         [0170]    It is exemplified that the plural semiconductor lasers  211  to  218 , the plural 3 dB couplers  260 , and the plural optical modulators  231  to  238  are integrated on the silicon substrate  201 , which is one semiconductor substrate, in the optical transmitter  200  of the embodiment described above, and the like. However, the semiconductor lasers  211  to  218  may be exchangeably formed (not illustrated). 
         [0171]    This application claims the priority based on Japanese Patent Application No. 2012-168219, which was filed in Japan on Jul. 30, 2012 and the content of which is incorporated herein. 
       INDUSTRIAL APPLICABILITY 
       [0172]    In accordance with the present invention, it is applicable to provide an optical transmitter that can achieve high redundancy. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Optical communication device 
           100  Optical transmitter 
           111  to  11 N Light-emitting element 
           120  N×M branching device 
           131  to  13 M Optical modulator 
           141  to  14 N Input port 
           151  to  15 M Output port 
           200  Optical transmitter 
           201  Silicon substrate as semiconductor substrate 
           211  to  218  Semiconductor laser as light-emitting element 
           220  8×8 branching device as N×M branching device 
           231  to  238  Optical modulator 
           241  to  248  Input port 
           251  to  258  Output port 
           260  3 dB Coupler as 2×2 branching device 
           290  Detector array as monitoring unit 
           291  to  298  Photodetector 
           301  to  308  Optical waveguide 
           401  to  408  Optical receiver 
           500  Optical transmission device as optical transmitter 
           510  Driver array as driving unit 
           511  to  518  Driver circuit 
           520  Control circuit as control unit 
           530  Optical transmission device as optical transmitter 
           531  N×M branching device 
           540  Optical transmission device as optical transmitter 
           550  Optical transmission device as optical transmitter 
           600  Optical transmitter 
           601  2×2 Branching device as N×M branching device 
           610  Optical transmitter 
           611  4×4 Branching device as N×M branching device 
           720  Optical transmitter 
           730  Optical transmitter 
           741  to  748  Semiconductor laser as light-emitting element 
           750  Optical transmitter 
           761  to  768  Semiconductor laser as light-emitting element 
           790  Optical transmitter 
           792  3×8 Branching device as N×M branching device 
           800  3 dB Coupler as 2×2 branching device 
           810  N×M star coupler as N×M branching device 
           820  Optical modulator 
           830  Optical modulator 
           840  Optical modulator