Patent Publication Number: US-9906306-B2

Title: Optical transmission system, transmitter, receiver, and optical transmission method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-133897, filed on Jun. 30, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical transmission system, a transmitter, a receiver, and an optical transmission method. 
     BACKGROUND 
     In known optical transmission systems, an optical element made of a material containing silicon transmits a multiplexed optical signal composed of optical signals of a plurality of different wavelengths (Refer to Japanese Laid-Open Patent Publication Nos. 2013-041143, 2009-139734, 2013-157722, and Japanese National Publication of International Patent Application No. 2013-513825, for example). Such optical transmission systems can be implemented, for example, using the silicon photonics technique. According to Wikipedia published on the Internet, “Silicon photonics is the study and application of photonic systems which use silicon as an optical medium”. 
     For example, the optical transmission system transmits an optical signal between electric circuits such as CPUs and LSIs. The term CPU is an abbreviation of central processing unit. The term LSI is an abbreviation of large scale integration. The optical transmission system is also referred to as an optical interconnect, for example. 
     The optical signal transmitted using an optical waveguide is less subjected to degradation in waveform than the electric signal transmitted using a metal wire. Therefore, the transmission of the optical signal allows for a larger transmission line capacity than the transmission of the electric signal. 
     In the above-mentioned optical transmission system, a light source may be formed of a compound semiconductor such as gallium arsenide, for example. Further, the light source may be disposed near another optical element. Therefore, the light source tends to become hot, degrading itself due to thermal stress and the like. This lowers the optical output level of the light source. For example, as the optical output level of the light source that outputs light of certain wavelength is smaller, the transmission rate, that is, the amount of information transmitted in the light of the certain wavelength per unit time decreases. Accordingly, the transmission rate that is the amount of information transmitted in the multiplexed optical signal per unit time also decreases. 
     SUMMARY 
     According to an aspect of the embodiments, an optical transmission system for transmitting a multiplexed optical signal including optical signals of a plurality of different wavelengths by using an optical element made of a material containing silicon, includes: a first light source configured to output a light wave of a first wavelength among the plurality of wavelengths; a second light source configured to output a light wave of a second wavelength; and a first detection section configured to detect abnormality in the light wave from the first light source, wherein the first light source, the second light source, and the first detection section each are made of a material containing silicon, and upon detection of the abnormality, the multiplexed optical signal including an optical signal of a modulated light wave generated using the light wave from the second light source in place of an optical signal of a modulated light wave generated using the light wave from the first light source is transmitted. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration example of an optical transmission system according to a first embodiment; 
         FIG. 2  is a block diagram illustrating a configuration example of a transmitter in  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating a configuration example of a receiver in  FIG. 1 ; 
         FIG. 4  is a flowchart illustrating an example of processing executed by the transmitter in  FIG. 1 ; 
         FIG. 5  is a block diagram illustrating a configuration example of a transmitter according to a second embodiment; 
         FIG. 6  is a block diagram illustrating a configuration example of a receiver according to the second embodiment; 
         FIG. 7  is a flowchart illustrating an example of processing executed by the transmitter in  FIG. 5 ; 
         FIG. 8  is a flowchart illustrating an example of processing executed by the receiver in  FIG. 6 ; 
         FIG. 9  is a block diagram illustrating a configuration example of a transmitter according to a third embodiment; 
         FIG. 10  is a block diagram illustrating a configuration example of a receiver according to the third embodiment; 
         FIG. 11  is a block diagram illustrating a configuration example of a transmitter in a first modification example of the third embodiment; and 
         FIG. 12  is a block diagram illustrating a configuration example of a receiver in the first modification example of the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described below with reference to drawings. However, the embodiments described below are exemplary. Accordingly, various modifications and arts that are not specifically disclosed may be applied to the embodiments. The same reference numerals through the drawings referred in the embodiments denote the same or similar components unless otherwise specified. 
     First Embodiment 
     Configuration 
     As illustrated in  FIG. 1 , an optical transmission system  1  according to a first embodiment includes a first transmission device  30 , an optical transmission line  40 , and a second transmission device  50 . The first transmission device  30  is communicably connected to the second transmission device  50  via the optical transmission line  40 . One example of the optical transmission line  40  is an optical fiber. 
     The first transmission device  30  is connected to a first electric circuit  10  via a plurality of wires  20 . Similarly, the second transmission device  50  is electrically connected to a second electric circuit  70  via a plurality of wires  60 . 
     In this embodiment, the first electric circuit  10  and the second electric circuit  70  each are an integrated circuit (IC). The IC may be a CPU. Note that the first electric circuit  10  and the second electric circuit  70  each may be an LSI. 
     In this embodiment, the optical transmission system  1  is used as an optical interconnect for communicably interconnecting the plurality of electric circuits  10  and  70 . 
     In this embodiment, in the optical transmission system  1 , the first transmission device  30  bidirectionally communicates with the second transmission device  50 , but the first transmission device  30  may unidirectionally communicate with the second transmission device  50 . 
     For convenience of explanation, it will be now described the configuration and operation of the optical transmission system  1  in which the first transmission device  30  transmits a signal to the second transmission device  50 . Transmission of a signal from the second transmission device  50  to the first transmission device  30  is performed in the same manner and thus, description thereof is omitted. 
     To facilitate understanding, the first transmission device  30 , the second transmission device  50 , the first electric circuit  10 , and the second electric circuit  70  may be referred to as the transmitter  30 , the receiver  50 , the transmission-side IC  10 , and the reception-side IC  70 , respectively. 
     The transmitter  30  receives N transmission electric signals from the transmission-side IC  10  via the plurality of wires  20 . In this embodiment, N is 4. N is not limited to 4, and may be any integer of 2 or more. N may be referred to as the number of lanes. The transmitter  30  converts the input N transmission electric signals into N optical signals of different wavelengths. The transmitter  30  transmits a multiplexed optical signal composed of the optical signals of N wavelengths to the receiver  50  via the optical transmission line  40 . In this embodiment, the transmission electric signals are differential signals. 
     The receiver  50  receives the multiplexed optical signal from the transmitter  30 . The receiver  50  separates the optical signals of N wavelengths from the received multiplexed optical signal. The receiver  50  converts the optical signals of N wavelengths into N reception electric signals. The receiver  50  outputs the N reception electric signals to the reception-side IC  70  via the plurality of respective wires  60 . In this embodiment, the reception electric signals are differential signals. 
     The transmitter  30  will be further described. 
     As illustrated in  FIG. 2 , the transmitter  30  includes a first light source section  311 , a second light source section  312 , an optical switch  320 , a multiplexer  330 , a waveguide  340 , a modulation section  350 , and a control section  360 . 
     The first light source section  311  outputs light waves of N different wavelengths. In this embodiment, the first light source section  311  is a laser that oscillates at each of N different wavelengths. Examples of the laser include an array laser and a vertical cavity surface emitting laser (VCSEL). The laser may be formed of a compound semiconductor containing crystals of indium gallium arsenide (InGaAs) or gallium arsenide (GaAs). 
     The “light source” refers to a part of the “light source section”, which outputs a light wave of one wavelength. Consequently, in this embodiment, the first light source section  311  includes N different light sources. 
     Like the first light source section  311 , the second light source section  312  outputs light waves of N different wavelengths. In this embodiment, the N wavelengths of light waves output from the second light source section  312  are the same as the N wavelengths of light waves output from the first light source section  311 . 
     Note that the wavelengths of the light waves output from the light sources in the first light source section  311  and the second light source section  312  may vary depending on individual difference and temperature. 
     The light source in the first light source section  311  is an example of a first light source. The light source in the second light source section  312  is an example of a second light source. 
     The optical switch  320  receives light waves of N wavelengths from the first light source section  311  and light waves of N wavelengths from the second light source section  312 . The optical switch  320  outputs either the light wave input from the first light source section  311  or the light wave input from the second light source section  312  for each of N wavelengths to the multiplexer  330 . In other words, the optical switch  320  switches a light wave to be output to the multiplexer  330  between the light wave input from the first light source section  311  and the light wave input from the second light source section  312  for each of N wavelengths. 
     In this embodiment, at activation of the optical transmission system  1 , the optical switch  320  outputs each of the light waves of N wavelengths input from the first light source section  311  to the multiplexer  330 . 
     The optical switch  320  is an example of a first switch. 
     The multiplexer  330  combines the light waves of N wavelengths from the optical switch  320 , and outputs a combined light wave to the waveguide  340 . 
     The waveguide  340  propagates the light wave input from the multiplexer  330  to the optical transmission line  40 . The light wave from the multiplexer  330  is a multiplexed light wave composed of unmodulated light waves of N wavelengths. Multiplexing may be expressed as combining or coupling. 
     The modulation section  350  includes N modulators  351  to  354 . 
     The N modulators  351  to  354  are aligned along the waveguide  340 . The N modulators  351  to  354  modulate light waves of N wavelengths # 1  to #N in light waves propagated from the waveguide  340  according to the N transmission electric signals input from the transmission-side IC  10 . 
     The modulator  351  includes a ring resonator  3511 , a driving circuit  3512 , a detector  3513 , and a heater  3514 . The modulator  351  may include a cooler such as a Peltier element in addition to or in place of the heater  3514 . The heater  3514  is an example of a temperature regulator. 
     Like the modulator  351 , the modulators  352  to  354  include ring resonators  3521  to  3541 , driving circuits  3522  to  3542 , detectors  3523  to  3543 , and heaters  3524  to  3544 , respectively. The modulators  352  to  354  each are the same as the modulator  351  and thus, description thereof is omitted. 
     The ring resonator  3511  is an annular waveguide. The ring resonator  3511  may be a circular or elliptic waveguide. The ring resonator  3511  resonates a light wave of a wavelength corresponding to the length of the waveguide. The wavelength corresponding to the length of the waveguide may be expressed as resonant wavelength. 
     The driving circuit  3512  amplifies the input transmission electric signals, and feeds the amplified transmission electric signals to the ring resonator  3511 . In this embodiment, the transmission electric signals are voltage signals. In this embodiment, the amplification factor of the driving circuit  3512  is preset according to properties of the modulator  351 . 
     The ring resonator  3511  has a pn junction. In this embodiment, feeding of the transmission electric signal is application of a voltage to the pn junction. By applying a voltage to the pn junction, a current flows through the pn junction. Since the current flowing through pn junction changes the refractive index, the intensity of the light wave of the resonant wavelength is modulated according to the transmission electric signal. The intensity of the light wave may be expressed as amplitude of the light wave. 
     The detector  3513  detects the intensity of light propagated from the ring resonator  3511 . In this embodiment, the detector  3513  outputs a current corresponding to the intensity of the light propagated through the ring resonator  3511 . The detector  3513  is a photodiode, for example. The photodiode may be formed of a compound semiconductor containing crystals of germanium (Ge) or indium gallium arsenide (InGaAs). 
     The detector  3513  to  3543  each are an example of a first detection section. 
     The temperature of the heater  3514  changes with the applied voltage. In this embodiment, the temperature of the heater  3514  becomes higher as the applied voltage is larger. With the change in the temperature of the heater  3514 , the temperature of the ring resonator  3511  also changes. The ring resonator  3511  expands more as the temperature of the ring resonator  3511  is higher. Accordingly, the resonant wavelength of the ring resonator  3511  changes with the change in the temperature of the heater  3514 . 
     Based on the light intensity detected by the detector  3513 , the control section  360  controls the voltage applied to the heater  3514 , such that the resonant wavelength of the ring resonator  3511  matches wavelength # 1 . Note that the control section  360  controls each of the modulators  352  to  354  as well in the same manner. 
     This can keep the state where the resonant wavelengths of the modulator  351  to  354  match N wavelengths # 1  to #N of light waves output from the multiplexer  330 , respectively. 
     In this manner, the N modulators  351  to  354  modulate the light waves of N wavelengths # 1  to #N in the light waves propagated through the waveguide  340  according to the input N transmission electric signals. Each of the modulated light waves of N wavelengths # 1  to #N may be expressed as optical signal. Note that N optical signals are multiplexed on an area of the waveguide  340  on the side of the optical transmission line  40  from the modulation section  350 . The optical signals thus multiplexed may be expressed as multiplexed optical signal. 
     The control section  360  may feed a bias current to the pn junctions of the ring resonators  3511  to  3541  and adjust the amount of the bias current, thereby controlling the resonant wavelengths of the ring resonators  3511  to  3541 . 
     In this embodiment, the multiplexer  330 , the waveguide  340 , and the modulator  351  to  354  each are an optical element made of a material containing silicon. In this embodiment, the transmitter  30  is implemented by silicon photonics. 
     Based on the light intensity detected by the detector  3513  to  3543 , the control section  360  detects abnormality in each of the light waves of N wavelengths. In this embodiment, if the detected light intensity is smaller than a predetermined threshold, the control section  360  determines that the light wave is abnormal. In this embodiment, if the intensity of the detected light intensity is larger than the threshold, the control section  360  determines that the light wave is normal. 
     If abnormality in the light wave of wavelength #i is detected, the control section  360  outputs a switch instruction to instruct switching of the light source for wavelength #i to the optical switch  320 . i represents an integer from 1 to N. In this embodiment, the switch instruction includes an identifier for identifying wavelength #i. 
     When receiving the switch instruction from the control section  360 , the optical switch  320  switches the light wave of wavelength #i identified by the switch instruction, which is to be output to the multiplexer  330 , from the light wave input from the first light source section  311  to the light wave input from the second light source section  312 . 
     Next, the receiver  50  will be described. 
     As illustrated in  FIG. 3 , the receiver  50  includes a waveguide  510 , a demultiplexing section  520 , and a control section  530 . 
     The waveguide  510  propagate the multiplexed optical signal from the optical transmission line  40 . 
     The demultiplexing section  520  includes N demultiplexers  521  to  524 . 
     The N demultiplexers  521  to  524  are aligned along the waveguide  510 . The N demultiplexers  521  to  524  separate optical signals of N wavelengths # 1  to #N from the multiplexed optical signal propagated through the waveguide  510 . The N demultiplexers  521  to  524  output reception electric signals corresponding to the separated optical signals of N wavelengths # 1  to #N to the reception-side IC  70 . 
     The demultiplexer  521  include a ring resonator  5211 , a detector  5212 , a converter  5213 , and a heater  5214 . The demultiplexer  521  may include a cooler such as a Peltier element in addition to the heater  5214  or in place of the heater  5214 . 
     Like the demultiplexer  521 , the demultiplexers  522  to  524  includes ring resonators  5221  to  5241 , detectors  5222  to  5242 , converters  5223  to  5243 , and heaters  5224  to  5244 , respectively. The demultiplexers  522  to  524  each are the same as the demultiplexer  521  and thus, description thereof is omitted. 
     The detector  5212  to  5242  each are an example of a first detection section. 
     The ring resonator  5211 , the detector  5212 , and the heater  5214  have the same configuration as the ring resonator  3511 , the detector  3513 , and the heater  3514 , respectively. 
     The converter  5213  converts a current output from the detector  5212  into a voltage. The converter  5213  amplifies the converted voltage according to a predetermined amplification factor. The converter  5213  outputs the amplified voltage change as the reception electric signal to the reception-side IC  70 . In this embodiment, the converter  5213  is a transimpedance amplifier (TIA). 
     Like the control section  360 , based on the light intensity detected by the detector  5212 , the control section  530  controls a voltage applied to the heater  5214  such that the resonant wavelength of the ring resonator  5211  matches wavelength # 1 . Note that the control section  530  controls each of the demultiplexers  522  to  524  as well in the same manner as the demultiplexer  521 . 
     This can keep the state where the resonant wavelengths of the demultiplexers  521  to  524  match N wavelengths # 1  to #N of light waves output from the multiplexer  330 , respectively. 
     In this manner, the N demultiplexers  521  to  524  separate the optical signals of N wavelengths # 1  to #N from the multiplexed optical signal propagated through the waveguide  510 , and output the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     The control section  530  may feed a bias current to pn junctions of the ring resonators  5211  to  5241  and adjust the amount of the bias current, thereby controlling the resonant wavelengths of the ring resonators  5211  to  5241 . 
     In this embodiment, the waveguide  510  and the demultiplexers  521  to  524  each are an optical element made of a material containing silicon. In this embodiment, the receiver  50  is implemented by silicon photonics. 
     (Operation) 
     Next, the operation of the optical transmission system  1  will be described with reference to  FIG. 4 . 
     In this embodiment, at activation of the optical transmission system  1 , the optical switch  320  outputs each of the light waves of N wavelengths from the first light source section  311  to the multiplexer  330 . Accordingly, at activation of the optical transmission system  1 , the light sources included in the first light source section  311  may be referred to as current light sources. At activation of the optical transmission system  1 , the light sources included in the second light source section  312  may be referred to as auxiliary light sources or redundant light sources. 
     The operation of the transmitter  30  will be first described. 
     Upon activation of the optical transmission system  1 , the transmitter  30  modulates light waves of N wavelengths # 1  to # 4  from the first light source section  311 , and transmits a multiplexed optical signal composed of the modulated optical signals. 
     Further, upon activation of the optical transmission system  1 , the transmitter  30  executes processing illustrated in  FIG. 4 . 
     The control section  360  waits until detection of abnormality in at least one of light waves of N wavelengths (“No” in Step S 101  in  FIG. 4 ). Here, it is assumed that abnormality in the light wave of wavelength # 1  is detected. 
     In this case, the control section  360  selects “Yes”, and determines whether or not the auxiliary light source for the light wave of wavelength #i having abnormality is present (Step S 102  in  FIG. 4 ). 
     In this embodiment, the control section  360  holds information on the number of the auxiliary light sources for each of N wavelengths. In this embodiment, at activation of the optical transmission system  1 , the control section  360  holds “1” as the information on the number of the auxiliary light sources for each of N wavelengths. In this embodiment, as described below, when switching the light source for wavelength #i, the control section  360  subtracts “1” from the number of the auxiliary light sources, which is indicated by the information for wavelength #i. 
     Consequently, at this time, the control section  360  holds “1” as the information on the number of the auxiliary light sources for the light wave of wavelength #i having abnormality. Thus, the control section  360  selects “Yes”, and outputs a switch instruction to switch the light source for wavelength #i to the optical switch  320 . In response to this, the optical switch  320  switches the light wave of wavelength #i identified by the switch instruction, which is to be output to the multiplexer  330 , from the light wave from the first light source section  311  to the light wave from the second light source section  312  (Step S 103  in  FIG. 4 ). 
     Then, the transmitter  30  retransmits the optical signal that was not normally received by the receiver  50  due to abnormality in the light wave of wavelength #i (Step S 104  in  FIG. 4 ). The transmitter  30  may transmit an optical signal corresponding to data of predetermined size, an optical signal transmitted at a predetermined time, or an optical signal corresponding to a unit of transaction processing. 
     Then, the transmitter  30  returns to Step S 101 , and repeats the processing in Step S 101  to Step S 104 . 
     If the auxiliary light source for the light wave of wavelength #i having abnormality is not present, the control section  360  selects “No” in Step S 102 , and returns to Step S 101 . 
     Next, the operation of the receiver  50  will be described. 
     Upon activation of the optical transmission system  1 , the receiver  50  receives a multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4 . The receiver  50  separates the optical signals of N wavelengths # 1  to # 4  from the received multiplexed optical signal, and outputs reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     As described above, when abnormality is detected in the light wave from the first light source, the optical transmission system  1  according to the first embodiment transmits the multiplexed optical signal including the optical signals of modulated light waves from the second light source in place of the optical signals of modulated light waves from the first light source. In this embodiment, the first light source is included in the first light source section  311 , and the second light source is included in the second light source section  312 . 
     Thus, when abnormality is detected in the light wave from the first light source, the multiplexed optical signal including the optical signals of the modulated light waves from the second light source is transmitted. As a result, when abnormality is detected in the light wave from the first light source, the multiplexed optical signal can restrain a decrease in the transmission rate that is the amount of information transmitted per unit time. 
     The optical transmission system  1  according to the first embodiment includes the optical switch  320  that switches the light wave to be output to the modulation section  350  from the light wave from the first light source to the light wave from the second light source when abnormality is detected in the light wave from the first light source. 
     Such switching of the light wave can be performed more rapidly than switching of the path of the electric signal. As a result, when abnormality occurs in the light wave from the first light source, a decrease in the transmission rate can be restrained. 
     In addition, when abnormality is detected in any one of a plurality of wavelengths, the optical transmission system  1  according to the first embodiment switches the light wave to be output to the modulation section  350 , from the light wave from the first light source section  311  to the light wave from the second light source section  312 . 
     Such switching of the light wave can be performed more rapidly than switching of the path of the electric signal. As a result, when abnormality occurs in the light wave from the first light source  311 , a decrease in the transmission rate can be restrained. Even when abnormality occurs in any of the plurality of wavelengths, a decrease in the transmission rate can be restrained. 
     Note that the optical transmission system  1  includes only one auxiliary light source for each wavelength, but may include a plurality of auxiliary light sources for each wavelength. The optical transmission system  1  includes the auxiliary light source for each of N wavelengths, but does not have to include the auxiliary light sources for some of N wavelengths. 
     Second Embodiment 
     Next, an optical transmission system according to a second embodiment will be described. The optical transmission system according to the second embodiment is different from the optical transmission system according to the first embodiment in an auxiliary modulator that modulates the light wave of auxiliary wavelength when abnormality is detected. The difference will be mainly described below. The same or substantially similar components in the second embodiment are given the same reference numerals as those in the first embodiment. 
     (Configuration) 
     As illustrated in  FIG. 5 , a transmitter  30 A in the second embodiment includes a first light source section  311 A, a modulation section  350 A, and a control section  360 A in place of a first light source section  311 , a modulation section  350 , and a control section  360  in  FIG. 2 . 
     The first light source section  311 A outputs light waves of N+1 (in this embodiment, 5) different wavelengths # 1  to # 5  to the multiplexer  330 . 
     The multiplexer  330  combines the light waves of N+1 wavelength # 1  to # 5  from the first light source section  311 A, and outputs the combined light wave to the waveguide  340 . 
     The waveguide  340  propagates the light wave from the multiplexer  330  to the optical transmission line  40 . The light from the multiplexer  330  is a multiplexed light wave composed of the unmodulated light waves of N+1 wavelength # 1  to # 5 . 
     The modulation section  350 A includes a modulator  355  and a switch  371  in addition to the modulation section  350  in  FIG. 2 . Like the modulator  351 , the modulator  355  includes a ring resonator  3551 , a driving circuit  3552 , a detector  3553 , and a heater  3554 . 
     The modulator  355  has the same function as the modulator  351  except for the resonant wavelength. The modulator  351  is an example of a first modulator. The modulator  355  is an example of a second modulator. 
     Based on the light intensity detected by the detector  3553 , the control section  360 A controls a voltage applied to the heater  3554  such that the resonant wavelength of the ring resonator  3551  matches wavelength # 5 . 
     Transmission electric signals are input from the transmission-side IC  10  to the switch  371  via the wires  20 . The switch  371  outputs the input transmission electric signals to either of the modulator  351  or the modulator  355 . In other words, the switch  371  switches the destination for the input transmission electric signals between the modulator  351  and the modulator  355 . 
     In this embodiment, at activation of the optical transmission system  1 , the switch  371  outputs the input transmission electric signals to the modulator  351 . The switch  371  is an example of a second switch. 
     Thus, at activation of the optical transmission system  1 , the modulator  351  may be referred to as current modulator. Further, at activation of the optical transmission system  1 , the modulator  355  may be referred to as an auxiliary modulator or a redundant modulator. At activation of the optical transmission system  1 , wavelengths # 1  to # 4  may be referred to as current wavelengths. At activation of the optical transmission system  1 , wavelength # 5  may be referred to as auxiliary wavelength. 
     Thus, at activation of the optical transmission system  1 , the transmitter  30 A transmits the multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4  and the unmodulated light wave of wavelength # 5 . Wavelength # 1  is an example of a first wavelength. Wavelength # 5  is an example of a second wavelength. 
     Based on the light intensity detected by the detector  3513 , the control section  360 A detects abnormality in the light wave of the wavelength # 1 . In this embodiment, if the detected light intensity is smaller than a predetermined threshold, the control section  360 A detects that the light wave of the wavelength # 1  is abnormal. In this embodiment, if the detected light intensity is larger than the threshold, the control section  360 A determines that the light wave is normal. 
     If abnormality is detected in the light wave of the wavelength # 1 , the control section  360 A outputs a switch instruction to switch wavelength to the switch  371 . 
     When receiving the switch instruction from the control section  360 A, the switch  371  switches the destination for the input transmission electric signals from the modulator  351  to the modulator  355 . Thereby, the transmitter  30 A transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 2  to # 5  and the unmodulated light wave of wavelength # 1 . 
     As illustrated in  FIG. 6 , a receiver  50 A in the second embodiment includes a demultiplexing section  520 A and a control section  530 A in place of the demultiplexing section  520  and the control section  530  in  FIG. 3 . 
     The demultiplexing section  520 A includes a demultiplexer  525  in place of the demultiplexing section  520  in  FIG. 3 , and a switch  541 . Like the demultiplexer  521 , the demultiplexer  525  includes a ring resonator  5251 , a detector  5252 , a converter  5253 , and a heater  5254 . 
     The demultiplexer  525  has the same function as the demultiplexer  521  except that the resonant wavelength is different from that of the demultiplexer  521 . The demultiplexer  521  is an example of a first demultiplexer. The demultiplexer  525  is an example of a second demultiplexer. 
     Based on the light intensity detected by the detector  5252 , the control section  530 A controls a voltage applied to the heater  5254  such that the resonant wavelength of the ring resonator  5251  matches wavelength # 5 . 
     The switch  541  receives reception electric signals from either the demultiplexer  521  of the demultiplexer  525 . In other words, the switch  541  switches the source for the reception electric signal between the demultiplexer  521  and the demultiplexer  525 . 
     The switch  541  outputs the input reception electric signals to the reception-side IC  70  via the wires  60 . In other words, the switch  541  switches electric signals output as the reception electric signals between electric signals corresponding to the optical signals separated by the demultiplexer  521  and electric signals corresponding to the optical signals separated by the demultiplexer  525 . 
     In this embodiment, at activation of the optical transmission system  1 , the switch  541  outputs the reception electric signals input from the demultiplexer  521  to the reception-side IC  70 . The switch  541  is an example of a third switch. 
     Thus, at activation of the optical transmission system  1 , the demultiplexer  521  may be referred to as a current demultiplexer. Further, at activation of the optical transmission system  1 , the demultiplexer  525  may be referred to as auxiliary demultiplexer or redundant demultiplexer. 
     Thus, at activation of the optical transmission system  1 , the receiver  50 A separates optical signals of N wavelengths # 1  to # 4  from a received multiplexed optical signal, and outputs reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     Based on the light intensity detected by the detector  5212 , the control section  530 A detects abnormality in the light wave of the wavelength # 1 . In this embodiment, if the detected light intensity is smaller than a predetermined threshold, the control section  530 A detects that the light wave of the wavelength # 1  is abnormal. In this embodiment, if the detected light intensity is larger than the threshold, the control section  530 A detects that the light wave of the wavelength # 1  is normal. 
     Upon detection of abnormality in the light wave of the wavelength # 1 , the control section  530 A outputs a switch instruction to switch wavelength to the switch  541 . 
     When receiving the switch instruction from the control section  530 A, the switch  541  switches a source for the reception electric signals from the demultiplexer  521  to the demultiplexer  525 . Thereby, the receiver  50 A separates optical signals of N wavelengths # 2  to # 5  from the received multiplexed optical signal, and outputs the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     (Operation) 
     Next, the operation of the optical transmission system  1  according to the second embodiment will be described with reference to  FIG. 7  and  FIG. 8 . 
     In this embodiment, at activation of the optical transmission system  1 , the switch  371  outputs the input transmission electric signals to the modulator  351 . In this embodiment, at activation of the optical transmission system  1 , the switch  541  outputs the reception electric signals input from the demultiplexer  521  to the reception-side IC  70 . 
     The operation of the transmitter  30 A will be first described. 
     Upon activation of the optical transmission system  1 , the transmitter  30 A transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4  and the unmodulated light wave of wavelength # 5 . 
     Upon activation of the optical transmission system  1 , the transmitter  30 A executes processing illustrated in  FIG. 7 . 
     The control section  360 A waits until detection of abnormality in the light wave of the wavelength # 1  (“No” in Step S 201  in  FIG. 7 ). Here, it is assumed that abnormality is detected in the light wave of the wavelength # 1 . 
     In this case, control section  360 A selects “Yes”, and determines whether or not the auxiliary wavelength is present (Step S 202  in  FIG. 7 ). 
     In this embodiment, the control section  360 A holds information on the number of the auxiliary wavelengths. In this embodiment, at activation of the optical transmission system  1 , the control section  360 A holds “1” as the number of the auxiliary wavelengths. In this embodiment, when switching wavelength as described later, the control section  360 A subtracts “1” from the number of the auxiliary wavelengths, which is indicated by the held information. 
     Consequently, at this time, the control section  360 A holds “1” as the information on the number of the auxiliary wavelengths. Thus, the control section  360 A selects “Yes”, and outputs a switch instruction to switch wavelength to the switch  371 . 
     Thereby, the switch  371  switches the destination for the input transmission electric signals from the modulator  351  to the modulator  355  (Step S 203  in  FIG. 7 ). Accordingly, the transmitter  30 A modulates the light wave of wavelength # 5  in place of the light wave of the wavelength # 1 . Thus, the transmitter  30 A transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 2  to # 5  and the unmodulated light wave of wavelength # 1 . 
     Then, the transmitter  30 A retransmits the optical signal that was not received by the receiver  50 A due to abnormality in the light wave of the wavelength # 1  (Step S 204  in  FIG. 7 ). The transmitter  30 A may transmit an optical signal corresponding to data of predetermined size, an optical signal transmitted at a predetermined time, or an optical signal corresponding to a unit of transaction processing. 
     Then, the transmitter  30 A returns to Step S 201 , and repeats the processing in Step S 201  to Step S 204 . 
     If the auxiliary wavelength is not present, the control section  360 A selects “No” in Step S 202 , and returns to Step S 201 . 
     Next, the operation of the receiver  50 A will be described. 
     Upon activation of the optical transmission system  1 , the receiver  50 A receives a multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4  and the unmodulated light wave of wavelength # 5 . The receiver  50 A separates the optical signals of N wavelengths # 1  to # 4  from the received multiplexed optical signal, and outputs reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     Further, upon activation of the optical transmission system  1 , the receiver  50 A executes processing illustrated in  FIG. 8 . 
     The control section  530 A waits until detection of abnormality in the light wave of the wavelength # 1  (“No” in Step S 301  in  FIG. 8 ). Here, it is assumed that abnormality is detected in the light wave of the wavelength # 1 . 
     In this case, the control section  530 A selects “Yes”, and determines whether or not the auxiliary wavelength is present (Step S 302  in  FIG. 8 ). 
     In this embodiment, like the control section  360 A, the control section  530 A holds information on the number of the auxiliary wavelengths. 
     Consequently, at this time, the control section  530 A holds “1” as the information on the number of the auxiliary wavelengths. Thus, the control section  530 A selects “Yes”, and outputs a switch instruction to switch wavelength to the switch  541 . 
     Thereby, the switch  541  switches the source for the reception electric signals from the demultiplexer  521  to the demultiplexer  525  (Step S 303  in  FIG. 8 ). As a result, the receiver  50 A separates optical signals of N wavelengths # 2  to # 5  from the received multiplexed optical signal, and outputs the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     Then, the receiver  50 A returns to Step S 301 , and repeats the processing in Step S 301  to Step S 303 . 
     If the auxiliary wavelength is not present, the control section  530 A selects “No” in Step S 302 , and returns to Step S 301 . 
     As described above, when abnormality is detected in the light wave from the first light source, the optical transmission system  1  according to the second embodiment transmits the multiplexed optical signal including optical signals of modulated light waves from the second light source in place of optical signals of modulated light waves from the first light source. In this embodiment, the first light source is a part of the first light source section  311 A, which that outputs the light wave of the wavelength # 1 , and the second light source is a part of the first light source section  311 A, which that outputs the light wave of wavelength # 5 . 
     Thus, when abnormality is detected in the light wave from the first light source, the multiplexed optical signal including the optical signals of modulated light waves from the second light source. For this reason, when abnormality is detected in the light wave from the first light source, the multiplexed optical signal can restrain a decrease in the transmission rate that is the amount of information transmitted per unit time. 
     Further, when abnormality is detected in the light wave of the wavelength # 1 , the demultiplexing section  520 A in the second embodiment switches the electric signals output as the reception electric signals, from the electric signals corresponding to the separated optical signals for wavelength # 1  to electric signals corresponding to the separated optical signals for wavelength # 5 . 
     That is, when abnormality is detected in the light wave of the wavelength # 1 , the electric signals corresponding to the separated optical signals for wavelength # 5  are output as the reception electric signals. This can restrain a decrease in the transmission rate at occurrence of abnormality in the light wave of the wavelength # 1 . 
     The optical transmission system  1  according to the second embodiment includes only one auxiliary wavelength, and may include a plurality of auxiliary wavelengths. The optical transmission system  1  includes the auxiliary modulator and the auxiliary demultiplexer for one wavelength # 1 , but may include the auxiliary modulator and the auxiliary demultiplexer for each of N wavelengths # 1  to # 4 . The optical transmission system  1  may include the auxiliary modulator and the auxiliary demultiplexer for some of N wavelengths # 1  to # 4 . 
     Third Embodiment 
     Next, an optical transmission system according to a third embodiment of the present disclosure will be described. The optical transmission system  1  according to the third embodiment is different from the optical transmission system according to the first embodiment in that the temperature of the modulator in that the light wave of the auxiliary wavelength is modulated when abnormality is detected. The difference will be mainly described. The same or substantially similar components in the third embodiment are given the same reference numerals as those in the first embodiment. 
     (Configuration) 
     As illustrated in  FIG. 9 , a transmitter  30 B in the third embodiment includes a first light source section  311 B and a control section  360 B in place of the first light source section  311  and the control section  360  in  FIG. 2 . The control section  360 B is an example of a first control section. 
     The first light source section  311 B outputs light waves of N+1 (in this embodiment, 5) different wavelengths # 1  to # 5  to the multiplexer  330 . 
     The multiplexer  330  combines the light waves of N+1 wavelength # 1  to # 5  from the first light source section  311 B, and outputs the combined light wave to the waveguide  340 . 
     The waveguide  340  propagates the light wave from the multiplexer  330  to the optical transmission line  40 . The light wave from the multiplexer  330  is a multiplexed light wave composed of unmodulated light waves of N+1 wavelengths # 1  to # 5 . 
     In this embodiment, at activation of the optical transmission system  1 , the control section  360 B controls a voltage applied to the heater  3514  to  3544  such that the resonant wavelengths of the N modulators  351  to  354  match N wavelengths # 1  to # 4 , respectively. Accordingly, at activation of the optical transmission system  1 , the N modulators  351  to  354  modulate light waves of N wavelengths # 1  to # 4  propagated by the waveguide  340  according to the input N transmission electric signals. In this embodiment, at activation of the optical transmission system  1 , wavelengths # 1  to # 4  may be referred to as current wavelengths. Further, at activation of the optical transmission system  1 , wavelength # 5  may be referred to as auxiliary wavelength. 
     Thus, at activation of the optical transmission system  1 , the transmitter  30 B transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4  and the unmodulated light wave of wavelength # 5 . Wavelengths # 1  to # 4  are an example of a first wavelength. Wavelength # 5  is an example of a second wavelength. 
     Based on the light intensity detected by the detectors  3513  to  3543 , the control section  360 B detects abnormality in each of light waves of N wavelengths # 1  to # 4 . In this embodiment, if the detected light intensity is smaller than predetermined threshold, the control section  360 B determines that the light wave is abnormal. In this embodiment, if the detected light intensity is larger than the threshold, the control section  360 B determines that the light wave is normal. 
     When abnormality is detected in the light wave of wavelength # 1 , the control section  360 B controls the temperature of the ring resonator  3511  to  3541  having the resonant wavelength of wavelength #i such that the resonant wavelength matches wavelength # 5 . i represents an integer of 1 to N. Thus, the modulator  351  to  354  that have modulated the light wave of wavelength #i before detection of abnormality in the light wave of wavelength #i becomes to modulate the light wave of wavelength # 5 . 
     As a result, when abnormality is detected in the light wave of wavelength # 1 , the transmitter  30 B transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 2  to # 5  and the unmodulated light wave of wavelength # 1 . 
     As illustrated in  FIG. 10 , a receiver  50 B in the third embodiment includes a control section  530 B in place of the control section  530  in  FIG. 3 . The control section  530 B is an example of a second control section. 
     In this embodiment, at activation of the optical transmission system  1 , the control section  530 B controls a voltage applied to the heaters  5214  to  5244  such that resonant wavelengths of the N demultiplexers  521  to  524  match N wavelengths # 1  to # 4 , respectively. Accordingly, at activation of the optical transmission system  1 , the N demultiplexers  521  to  524  separate the optical signals of N wavelengths # 1  to # 4  from the received multiplexed optical signal, and outputs the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     Based on the light intensity detected by the detector  5212  to  5242 , the control section  530 B detects abnormality in each of light waves of N wavelengths # 1  to # 4 . In this embodiment, if the detected light intensity is smaller than a predetermined threshold, the control section  530 B determines that the light wave is abnormal. In this embodiment, if the detected light intensity is larger than the threshold, the control section  530 B determines that the light wave is normal. 
     When abnormality is detected in the light wave of wavelength # 1 , the control section  530 B controls the temperature of the ring resonators  5211  to  5241  having the resonant wavelength of wavelength #i such that the resonant wavelength matches wavelength # 5 . i represents an integer from 1 to N. Thus, the demultiplexers  521  to  524  that have separated the optical signal of wavelength #i before detection of abnormality in the light wave of wavelength #i becomes to separate the optical signal of wavelength # 5 . 
     As a result, for example, when abnormality is detected in the light wave of wavelength # 1 , the receiver  50 B separates optical signals of N wavelengths # 2  to # 5  from the received multiplexed optical signal, and outputs the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     (Operation) 
     Next, the operation of the optical transmission system  1  according to the third embodiment will be described with reference to  FIG. 7  and  FIG. 8 . 
     In this embodiment, optical transmission system  1 , the control section  360 B controls the voltage applied to the heaters  3514  to  3544  such that the resonant wavelengths of the N modulators  351  to  354  match N wavelengths # 1  to # 4 , respectively. In this embodiment, at activation of the optical transmission system  1 , the control section  530 B controls the voltage applied to the heaters  5214  to  5244  such that the resonant wavelengths of the N demultiplexers  521  to  524  match N wavelengths # 1  to # 4 , respectively. 
     The operation of the transmitter  30 B will be first described. 
     Upon activation of the optical transmission system  1 , the transmitter  30 B transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4  and the unmodulated light wave of wavelength # 5 . 
     Further, upon activation of the optical transmission system  1 , the transmitter  30 B executes processing illustrated in  FIG. 7 . 
     The control section  360 B waits until detection of abnormality in at least one of the light waves of N wavelengths (“No” in Step S 201  in  FIG. 7 ). Here, it is assumed that abnormality in the light wave of wavelength # 1  is detected. 
     In this case, control section  360 B selects “Yes”, whether or not the auxiliary wavelength is present (Step S 202  in  FIG. 7 ). 
     In this embodiment, the control section  360 B holds information on the number of the auxiliary wavelengths. In this embodiment, at activation of the optical transmission system  1 , the control section  360 B holds “1” as the information on the number of the auxiliary wavelengths. In this embodiment, when switching wavelength as described later, the control section  360 B subtracts “1” from the number of the auxiliary wavelengths, which is indicated by the held information. 
     Consequently, at this time, the control section  360 B holds “1” as the information on the number of the auxiliary wavelengths. Thus, the control section  360 B selects “Yes”, and controls the temperature of the ring resonator  3511  having the resonant wavelength of wavelength # 1  such that the resonant wavelength matches wavelength # 5  (Step S 203  in  FIG. 7 ). Thereby, the transmitter  30 B modulates the light wave of wavelength # 5  in place of the light wave of the wavelength # 1 . As a result, the transmitter  30 B transmits a multiplexed optical signal composed of the optical signals of N wavelengths # 2  to # 5  and the unmodulated light wave of wavelength # 1 . 
     Then, the transmitter  30 B retransmits the optical signal that was not received by the receiver  50 B due to abnormality in the light wave of the wavelength # 1  (Step S 204  in  FIG. 7 ). The transmitter  30 B may transmit an optical signal corresponding to data of predetermined size, an optical signal transmitted at a predetermined time, or an optical signal corresponding to a unit of transaction processing. 
     Then, the transmitter  30 B returns to Step S 201 , and repeats the processing in Step S 201  to Step S 204 . 
     If the auxiliary wavelength is not present, the control section  360 B selects “No” in Step S 202 , and returns to Step S 201 . 
     Next, the operation of the receiver  50 B will be described. 
     Upon activation of the optical transmission system  1 , the receiver  50 B receives a multiplexed optical signal composed of the optical signals of N wavelengths # 1  to # 4  and the unmodulated light wave of wavelength # 5 . The receiver  50 B separates the optical signals of N wavelengths # 1  to # 4  from the received multiplexed optical signal, and outputs the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     Further, upon activation of the optical transmission system  1 , the receiver  50 B execute processing illustrated in  FIG. 8 . 
     The control section  530 B waits until detection of abnormality in at least one of the light waves of N wavelengths (“No” in Step S 301  in  FIG. 8 ). According to the above-mentioned assumption, abnormality in the light wave of the wavelength # 1  is detected. 
     Thus, the control section  530 B selects “Yes”, and determines whether or not the auxiliary wavelength is present (Step S 302  in  FIG. 8 ). 
     In this embodiment, like the control section  360 B, the control section  530 B holds information on the number of the auxiliary wavelengths. 
     Consequently, at this time, the control section  530 B holds “1” as the information on the number of the auxiliary wavelengths. Thus, the control section  530 B selects “Yes”, and controls the temperature of the ring resonator  5211  having the resonant wavelength of wavelength # 1  such that the resonant wavelength matches wavelength # 5  (Step S 303  in  FIG. 8 ). Thereby, the receiver  50 B separates the optical signal of wavelength # 5  from the received multiplexed optical signal in place of the optical signal of wavelength # 1 . 
     As a result, the receiver  50 B separates the optical signals of N wavelengths # 2  to # 5  from the received multiplexed optical signal, and outputs the reception electric signals corresponding to the separated optical signals to the reception-side IC  70 . 
     Then, the receiver  50 B returns to Step S 301 , and repeats the processing in Step S 301  to Step S 303 . 
     If the auxiliary wavelength is not present, the control section  530 B selects “No” in Step S 302 , and returns to Step S 301 . 
     As described above, when abnormality is detected in the light wave from the first light source, the optical transmission system  1  according to the third embodiment transmits the multiplexed optical signal including the optical signal of the modulated light wave from the second light source in place of the optical signal of the modulated light wave from the first light source. In this embodiment, the first light source is a part of the first light source section  311 B, which outputs the light wave of the wavelength # 1 , and the second light source is a part of the first light source section  311 B, which outputs the light wave of wavelength # 5 . 
     Thus, when abnormality is detected in the light wave from the first light source, the multiplexed optical signal including the optical signal of the modulated light wave from the second light source is transmitted. For this reason, at occurrence of abnormality in the light wave from the first light source, the multiplexed optical signal can restrain a decrease in the transmission rate that is the amount of information transmitted per unit time. 
     Further, when abnormality is detected in the light wave of the wavelength # 1 , the control section  360 B in the third embodiment controls the temperature of the ring resonator  3511  such that the resonant wavelength of the ring resonator  3511  matches wavelength # 5 . 
     Thus, when abnormality is detected in the light wave of the wavelength # 1 , the wavelength of the modulated light wave can be rapidly switched from wavelength # 1  to the wavelength # 5  without switching the signal path. This can restrain a decrease in the transmission rate at occurrence of abnormality in the light wave of the wavelength # 1 . 
     In addition, when abnormality is detected in the light wave of the wavelength # 1 , the control section  530 B in the third embodiment controls the temperature of the ring resonator  5211  such that the resonant wavelength of the ring resonator  5211  matches wavelength # 5 . 
     Thus, when abnormality is detected in the light wave of the wavelength # 1 , the wavelength of the separated optical signal can be rapidly switched from wavelength # 1  to wavelength # 5  without switching the signal path. This can restrain a decrease in the transmission rate at occurrence of abnormality in the light wave of the wavelength # 1 . 
     The optical transmission system  1  according to the third embodiment include only one auxiliary wavelength, but may be include a plurality of auxiliary wavelengths. 
     First Modification Example of Third Embodiment 
     Next, an optical transmission system in a first modification example of the third embodiment of the present disclosure will be described. The optical transmission system in the first modification example of the third embodiment is different from the optical transmission system  1  according to the third embodiment in that a receiver detects temperature, and the voltage applied to the heater is determined based on the detected temperature. The difference will be mainly described below. The same or substantially similar components in the first modification example of the third embodiment are given the same reference numerals as those in the third embodiment. 
     (Configuration) 
     As illustrated in  FIG. 11 , a transmitter  30 C in the first modification example of the third embodiment includes a control section  360 C and a temperature sensor  380 C in place of the control section  360 B in  FIG. 9 . The control section  360 C is an example of a first control section. 
     The wavelength of the light wave output from the first light source section  311 B varies depending on the temperature of the first light source section  311 B. The temperature of the first light source section  311 B is closely correlated with the temperature of the transmitter  30 C. Therefore, a relation between the temperature of the transmitter  30 C and the voltage applied to the heaters  3514  to  3544  when the resonant wavelengths of the modulators  351  to  354  match the auxiliary wavelength is previously found, and the voltage applied to the heaters  3514  to  3544  can be determined based on the relation. 
     The temperature sensor  380 C detects the temperature of the transmitter  30 C. 
     The control section  360 C holds a first relation between the temperature detected by the temperature sensor  380 C and the voltage applied to the heaters  3514  to  3544  when the resonant wavelengths of the modulators  351  to  354  match the auxiliary wavelength. 
     The first relation may be previously held. The control section  360 C may acquire the first relation at manufacturing of the optical transmission system  1 , at shipment of the optical transmission system  1 , or a predetermined time before operation of the optical transmission system  1 . For example, the first relation a plurality of different temperatures and voltages corresponding to the temperatures. The first relation may be held as information in a table, and information on a formula for calculating voltage, or combination thereof. 
     When abnormality is detected in the light wave of wavelength #i, the control section  360 C determines the voltage applied to the heaters  3514  to  3544  for wavelength #i, based on the held first relation and the temperature detected by the temperature sensor  380 C at detection of the abnormality. 
     As illustrated in  FIG. 12 , a receiver  50 C in the first modification example of the third embodiment includes a control section  530 C and a temperature sensor  540 C in place of the control section  530 B in  FIG. 10 . The control section  530 C is an example of a second control section. The temperature sensor  540 C is an example of a second detection section. 
     The receiver  50 C does not detect the temperature of the transmitter  30 C. The temperature of the receiver  50 C is closely correlated with the temperature of the transmitter  30 C. Thus, the voltage applied to the heaters  5214  to  5244  can be determined based on the temperature of the receiver  50 C. 
     The temperature sensor  540 C detects the temperature of the demultiplexing section  520 . 
     The control section  530 C holds a first temperature detected by the temperature sensor  540 C at a predetermined first time. The first time may be at the time of manufacturing of the optical transmission system  1 , at the time of shipment of the optical transmission system  1 , or at a predetermined time before operation of the optical transmission system  1 . 
     Further, the control section  530 C acquires the voltage applied to the heaters  5214  to  5244  in the case where the resonant wavelengths of the demultiplexers  521  to  524  match the auxiliary wavelength at the first time, and holds the acquired voltage. 
     When abnormality is detected in the light wave of wavelength #i, the control section  530 C acquires a voltage V r (t 2 ) applied to the heaters  5214  to  5244  for wavelength #i, based on a second temperature detected by the temperature sensor  540 C at a second time when abnormality is detected, and Formula 1.
 
 V   r ( t   2 )= V   r ( t   1 )+ F ( T ( t   2 )− T ( t   1 ))  [Formula 1]
 
     t 1  and t 2  denote the first time and the second time, respectively. T(t 1 ) and T(t 2 ) denote the first temperature and the second temperature, respectively. V r (t 1 ) denotes the voltage applied to the heaters  5214  to  5244  for wavelength #i in the case where the resonant wavelengths of the demultiplexers  521  to  524  for wavelength #i match the auxiliary wavelength at the first time. F denotes a function that represents the second relation between the temperature of the demultiplexers  521  to  524  and the voltage applied to the heaters  5214  to  5244 , and converts temperature into voltage. 
     The control section  530 C may previously hold information on F. Alternatively, the control section  530 C may acquire the information on F at manufacturing of the optical transmission system  1 , at shipment of the optical transmission system  1 , or a predetermined time before operation of the optical transmission system  1 . In this case, the temperature detected by the temperature sensor  540 C may be used as the temperature of the demultiplexers  521  to  524 . 
     Therefore, when abnormality is detected in the light wave of wavelength # 1 , the optical transmission system  1  in the first modification example of the third embodiment can rapidly match the resonant wavelength of the ring resonator  3511  in the modulator  351  with the auxiliary wavelength. 
     Further, when abnormality is detected in the light wave of wavelength # 1 , the optical transmission system  1  in the first modification example of the third embodiment can rapidly match the resonant wavelength of the resonator  5211  in the demultiplexer  521  with the auxiliary wavelength. 
     This can restrain a decrease in the transmission rate. 
     The control section  530 C may determine the voltage Vr(t2) applied to the heaters  5214  to  5244  for wavelength #i having abnormality according to Formula 2 in place of Formula 1.
 
 V   r ( t   2 )= V   r ( t   1 )+ F ( F   −1 ( V   k ( t   2 )− V   k ( t   1 ))− T ( t   2 )− T ( t   1 ))  [Formula 2]
 
     F −1  is an inverse function of F. V k (t 1 ) denotes the voltage applied to the heaters  5214  to  5244  for wavelength #i in the case where the resonant wavelengths of the demultiplexers  521  to  524  for wavelength #i match the current wavelength (in other words, wavelength #i) at the first time. V k (t 2 ) denotes the voltage applied to the heaters  5214  to  5244  for wavelength #i in the case where the resonant wavelengths of the demultiplexers  521  to  524  for wavelength #i match the current wavelength (in other words, wavelength #i) at the second time. V k (t 1 ) and V k (t 2 ) may be acquired by the control section  530 C, and the acquired values may be held in the control section  530 C. 
     Therefore, even when the wavelength of the light wave from the first light source section  311 B varies with a change in the temperature of the first light source section  311 B, the resonant wavelengths of the ring resonators  5211  to  5241  can be rapidly matched with the auxiliary wavelength. 
     The control section  530 C may determine the voltage V r (t 2 ) applied to the heaters  5214  to  5244  for wavelength #i having abnormality according to Formula 3 in place of Formula 2. 
     
       
         
           
             
               
                 
                   
                     
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     dλ r /dT denotes a change rate of the wavelength of the light wave from the first light source section  311 B as the light wave of redundant wavelength with respect to the temperature. dλ k /dT denotes a change rate of the wavelength of the light wave from the first light source section  311 B as the light wave of wavelength #i with respect to the temperature. 
     dλ r /dT and dλ k /dT may be acquired by the control section  360 C of the transmitter  30 C, and the acquired values may be held in the control section  360 C. dλ r /dT and dλ k /dT may be acquired at manufacturing of the optical transmission system  1 , at shipment of the optical transmission system  1 , or a predetermined time before operation of the optical transmission system  1 . Information on dλ r /dT and dλ k /dT may be transmitted from the transmitter  30 C to the receiver  50 C at a predetermined time before activation of the optical transmission system  1 . The information on dλ r /dT and dλ k /dT may be transmitted by Inter-Integrated Circuit (I2C) communication. 
     Therefore, even when the change rate of the wavelength of the light wave from the first light source section  311 B with respect to temperature varies depending on wavelength, the resonant wavelengths of the ring resonators  5211  to  5241  can be rapidly matched with the auxiliary wavelength. 
     The control section  530 C may determine the voltage V r (t 2 ) applied to the heaters  5214  to  5244  for wavelength #i having abnormality according to Formula 4 in place of Formula 3. 
     
       
         
           
             
               
                 
                   
                     
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     F k  denotes a function that represents the second relation of the demultiplexers  521  to  524  for the wavelength #i having abnormality among the plurality of demultiplexers  521  to  524 , and converts temperature into voltage. 
     The control section  530 C may previously hold information on F k . The control section  530 C may acquire the information on F k  at manufacturing of the optical transmission system  1 , at shipment of the optical transmission system  1 , or a predetermined time before operation of the optical transmission system  1 . In this case, the temperature detected by the temperature sensor  540 C may be used as the temperature of the demultiplexers  521  to  524 . 
     Therefore, even when the second relation varies among the demultiplexers, the resonant wavelengths of the ring resonators  5211  to  5241  can be rapidly matched with the auxiliary wavelength. 
     In the optical transmission system  1  according to the first embodiment, the wavelength of the light wave from the first light source section  311  may be slightly different from the wavelength of the light wave from the second light source section  312 . In this case, to adjust wavelength after switching of the light source, temperature control in the first modification example of the third embodiment can be performed. 
     The optical transmission system  1  according to each embodiment may be combined with at least one of the optical transmission systems  1  in the other embodiments. 
     The optical transmission system  1  according to each embodiment is used in the optical interconnect, but may be used for optical communication. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 
     Additional Note 1. An optical transmission system for transmitting a multiplexed optical signal including optical signals of a plurality of different wavelengths by using an optical element made of a material containing silicon, the optical transmission system comprising: a first light source configured to output a light wave of a first wavelength among the plurality of wavelengths; a second light source configured to output a light wave of a second wavelength; and a first detection section configured to detect abnormality in the light wave from the first light source, wherein upon detection of the abnormality, the multiplexed optical signal including an optical signal of a modulated light wave generated using the light wave from the second light source in place of an optical signal of a modulated light wave generated using the light wave from the first light source is transmitted. 
     Additional Note 2. The optical transmission system according to additional note 1, further comprising a modulation section configured to modulate at least one of the input light waves of the plurality of wavelengths to generate the optical signals of the plurality of wavelengths, and upon detection of the abnormality, modulate the light wave from the second light source in place of the light wave from the first light source. 
     Additional Note 3. The optical transmission system according to additional note 2, wherein the second wavelength is identical to the first wavelength, and the optical transmission system further comprises a first switch configured to switch the light wave to be output to the modulation section from the light wave from the first light source to the light wave from the second light source, upon detection of the abnormality. 
     Additional Note 4. The optical transmission system according to additional note 3, further comprising: a first light source section including the first light source and configured to output each of the light waves of the plurality of wavelengths; and a second light source section including the second light source and configured to output each of the light waves of the plurality of wavelengths, wherein the first detection section detects abnormality in each of the light waves of the plurality of wavelengths from the first light source section, and upon detection of the abnormality in any one of the plurality of wavelengths, the first switch switches the light wave of the wavelength to be output to the modulation section from the light wave from the first light source section to the light wave from the second light source section. 
     Additional Note 5. The optical transmission system according to additional note 2, wherein the second wavelength is a wavelength other than the first wavelength among the plurality of wavelengths, the light wave from the first light source and the light wave from the second light source are input to the modulation section, and the optical transmission system further comprises a demultiplexing section configured to separate an optical signal from the transmitted multiplexed optical signal for each of at least one of the plurality of wavelengths, and output reception electric signals corresponding to the separated optical signals, and upon detection of the abnormality, switch the electric signals output as the reception electric signals from the electric signals corresponding to the separated optical signals for the first wavelength to the electric signals corresponding to the separated optical signals for the second wavelength. 
     Additional Note 6. The optical transmission system according to additional note 5, wherein the modulation section includes: a first modulator configured to modulate the light wave of the first wavelength according to an input transmission electric signal; a second modulator configured to modulate the light wave of the second wavelength according to the input transmission electric signals; and a second switch configured to switch the modulator receiving the transmission electric signals from the first modulator to the second modulator, upon detection of the abnormality. 
     Additional Note 7. The optical transmission system according to additional note 5, wherein the demultiplexing section includes: a first demultiplexer configured to separate an optical signal of the first wavelength from the transmitted multiplexed optical signal; a second demultiplexer configured to separate an optical signal of the second wavelength from the transmitted multiplexed optical signal; and a third switch configured to switch the electric signals output as the reception electric signals from the electric signal corresponding to the optical signal separated by the first demultiplexer to the electric signal corresponding to the optical signal separated by the second demultiplexer, upon detection of the abnormality. 
     Additional Note 8. The optical transmission system according to additional note 5, wherein the modulation section include a modulator including a ring resonator and configured to modulate the light wave of the first wavelength, the ring resonator being configured to resonate with the first wavelength, and the optical transmission system further comprises a first control section configured to control a temperature of the ring resonator included in the modulator, upon detection of the abnormality, such that the resonant wavelength of the ring resonator matches the second wavelength. 
     Additional Note 9. The optical transmission system according to additional note 5, wherein the demultiplexing section includes a demultiplexer including a ring resonator and configured to separate the optical signals of the first wavelength from the transmitted multiplexed optical signal, the ring resonator being configured to resonate with the first wavelength, and the optical transmission system further comprises a second control section configured to control a temperature of the ring resonator included in the demultiplexer, upon detection of the abnormality, such that the resonant wavelength of the ring resonator matches the second wavelength. 
     Additional Note 10. The optical transmission system according to additional note 9, further comprising a second detection section configured to detect a temperature of the demultiplexing section, wherein the second control section includes a temperature regulator configured to change in temperature according to an applied voltage, and the voltage applied to the temperature regulator is determined based on a first temperature detected at a predetermined first time, a second temperature detected at a second time when the abnormality is detected, and the voltage applied to the temperature regulator in the case where the resonant wavelength of the ring resonator included in the demultiplexer matches the second wavelength at the first time. 
     Additional Note 11. The optical transmission system according to additional note 10, wherein the voltage is determined based on the voltage applied to the temperature regulator in the case where the resonant wavelength of the ring resonator included in the demultiplexer matches the first wavelength at each of the first time and the second time. 
     Additional Note 12. The optical transmission system according to additional note 10, wherein the second control section previously holds a relation between the voltage applied to the temperature regulator and the temperature of the demultiplexing section, and the voltage is determined based on the held relation. 
     Additional Note 13. The optical transmission system according to additional note 10, wherein the voltage is determined based on a change rate of the wavelength of the light wave output from the first light source with respect to temperature, and a change rate of the wavelength of the light wave output from the second light source with respect to temperature. 
     Additional Note 14. A transmitter for transmitting a multiplexed optical signal including optical signals of a plurality of different wavelengths by using an optical element made of a material containing silicon, the transmitter comprising: a first light source configured to output a light wave of a first wavelength among the plurality of wavelengths; a second light source configured to output a light wave of a second wavelength; and a first detection section configured to detect abnormality in the light wave from the first light source, wherein upon detection of the abnormality, the multiplexed optical signal including an optical signal of a modulated light wave generated using the light from the second light source in place of an optical signal of a modulated light wave generated using the light from the first light source is transmitted. 
     Additional Note 15. The transmitter according to additional note 14, further comprising a modulation section configured to modulate at least one of the input light waves of the plurality of wavelengths to generate the optical signals of the plurality of wavelengths, and upon detection of the abnormality, modulate the light wave from the second light source in place of the light wave from the first light source. 
     Additional Note 16. The transmitter according to additional note 15, wherein the second wavelength is identical to the first wavelength, and the transmitter further comprises a first switch configured to switch the light wave to be output to the modulation section from the light wave from the first light source to the light wave from the second light source, upon detection of the abnormality. 
     Additional Note 17. The transmitter according to additional note 15, wherein the second wavelength is a wavelength other than the first wavelength among the plurality of wavelengths, and the light wave from the first light source and the light wave from the second light source are input to the modulation section. 
     Additional Note 18. The transmitter according to additional note 17, wherein the modulation section includes: a first modulator configured to modulate the light wave of the first wavelength according to an input transmission electric signal; a second modulator configured to modulate the light wave of the second wavelength according to the input transmission electric signal; and a second switch configured to switch the modulator receiving the transmission electric signal from the first modulator to the second modulator, upon detection of the abnormality. 
     Additional Note 19. The transmitter according to additional note 17, wherein the modulation section include a modulator including a ring resonator and configured to modulate the light wave of the first wavelength, the ring resonator being configured to resonate with the first wavelength, and the transmitter further comprises a first control section configured to control a temperature of the ring resonator included in the modulator, upon detection of the abnormality, such that the resonant wavelength of the ring resonator matches the second wavelength. 
     Additional Note 20. A receiver for receiving a multiplexed optical signal including optical signals of a plurality of different wavelengths by using an optical element made of a material containing silicon, the receiver comprising a first detection section configured to detect abnormality in a light wave of a first wavelength among the plurality of wavelengths, wherein upon detection of the abnormality, the receiver receives the multiplexed optical signal including an optical signal of the modulated light wave of a second wavelength other than the first wavelength among the plurality of wavelengths in place of the optical signal of the modulated light wave of the first wavelength. 
     Additional Note 21. The receiver according to additional note 20, further comprising a demultiplexing section configured to separate an optical signal from the transmitted multiplexed optical signal for each of at least one of the plurality of wavelengths, and output reception electric signals corresponding to the separated optical signals, and upon detection of the abnormality, switch the electric signal output as the reception electric signal from the electric signal corresponding to the separated optical signal for the first wavelength to the electric signal corresponding to the separated optical signal for the second wavelength. 
     Additional Note 22. The receiver according to additional note 21, wherein the demultiplexing section includes: a first demultiplexer configured to separate an optical signal of the first wavelength from the transmitted multiplexed optical signal; a second demultiplexer configured to separate an optical signal of the second wavelength from the transmitted multiplexed optical signal; and a third switch configured to switch the electric signal output as the reception electric signal from the electric signal corresponding to the optical signal separated by the first demultiplexer to the electric signal corresponding to the optical signal separated by the second demultiplexer, upon detection of the abnormality. 
     Additional Note 23. The receiver according to additional note 21, wherein the demultiplexing section includes a demultiplexer including a ring resonator and configured to separate the optical signals of the first wavelength from the transmitted multiplexed optical signal, the ring resonator being configured to resonate with the first wavelength, and the receiver further comprises a second control section configured to control a temperature of the ring resonator included in the demultiplexer, upon detection of the abnormality, such that the resonant wavelength of the ring resonator matches the second wavelength. 
     Additional Note 24. An optical transmission method for transmitting a multiplexed optical signal composed of optical signals of a plurality of different wavelengths by using an optical element made of a material containing silicon, the optical transmission method comprising: detecting abnormality in light from a first light source configured to output light of a first wavelength among the plurality of wavelengths; and upon detection of the abnormality, transmitting the multiplexed optical signal including an optical signal of a modulated light generated using the light from a second light source configured to output light of a second wavelength in place of an optical signal of a modulated light generated using the light from the first light source.