Patent Publication Number: US-2023135622-A1

Title: Optical transmission apparatus and optical transmission method

Description:
TECHNICAL FIELD 
     The present invention relates to an optical transmission apparatus and an optical transmission method. 
     BACKGROUND ART 
     As a video communication network system that delivers video to a house of a subscriber, there is known a system that performs relay broadcasting by using an optical signal.  FIG.  7    is a diagram illustrating a configuration example of a video communication network system  900  using a technology in the related art. For example, a frequency modulation (FM) collective conversion method is used for the video communication network system  900  (see NPL 1). The video communication network system  900  includes, for example, a transmitter  910 , a transmitter  911 , one or more transmission apparatuses  920 , one or more transmission apparatuses  921 , a WDM filter  930 , and a receiver  950  and a receiver  951  installed in a house of each subscriber or the like. While one receiver  950  and one receiver  951  are illustrated in the same figure, the number of houses of subscribers, and the number of receivers  950  and the number of receivers  951  in a house of each subscriber are arbitrary. The WDM filter  930  and the receiver  950  are connected via an access network  940 . The access network  940  performs wavelength multiplexing transmission. 
     The transmitter  910  inputs a video signal of terrestrial digital broadcasting, or a video signal of broadcasting by broadcasting satellites (BS) or broadcasting by communication satellites (CS) which is delivered to the ground from a satellite using a right-hand circular polarization. In the following, the video signal is also described as a terrestrial digital/BSCS right-hand video signal. The transmitter  911  inputs a video signal of the BS broadcasting or the CS broadcasting that is delivered to the ground from a satellite using a left-hand circular polarization. This is a video signal for new 4K/8K satellite broadcasting. In the following, the video signal is also described as a BSCS left-hand video signal. Each of the transmitter  910  and the transmitter  911  collectively converts input multi-channel video signals into wideband FM signals and then converts the wideband FM signals into optical signals to output the optical signals. An optical signal SG 90  having a wavelength λ 1  (e.g., λ 1  is 1558 nm) output by the transmitter  910  is relayed and transmitted by the transmission apparatus  920 , and is then input to the WDM filter  930 . An optical signal SG 91  having a wavelength λ 2  output by the transmitter  910  (e.g., λ 2  is 1552 nm) is relayed and transmitted by the transmission apparatus  920 , and is then input to the WDM filter  930 . Note that in the current state, the number of channels for the left-hand circular polarization is small, and thus λ 2 =1558 nm is used. 
     The WDM filter  930  outputs a multiplexed signal SG 92  obtained by multiplexing the optical signal SG 90  for video relayed by the transmission apparatus  920 , the optical signal SG 91  for video relayed by the transmission apparatus  921 , and an optical signal for communication input from a transmission apparatus (not illustrated) to the access network  940 . The multiplexed signal SG 92  transmitted in the access network  940  is demultiplexed, the receiver  950  receives a multiplexed signal SG 93  for video having the wavelength λ 1 , and the receiver  951  receives a multiplexed signal SG 94  for video having the wavelength λ 2 . Note that the receiver  950  and the receiver  951  cut signals having wavelengths other than a wavelength band supported by the receivers themselves with a filter. 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: “ITU-T J.185: Transmission equipment for transferring multi-channel television signals over optical access networks by frequency modulation conversion,” International Telecommunication Union, June 2012. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In the video communication network system illustrated in  FIG.  7   , the terrestrial digital/BSCS right-hand video signal and the BSCS left-hand video signal are relayed in different systems. As a result, relay transmission equipment is required for each of the terrestrial digital/BSCS right-hand video signal and the BSCS left-hand video signal. As such, it is conceived to multiplex the terrestrial digital/BSCS right-hand video signal and the BSCS left-hand video signal using different wavelengths to transmit the multiplexed signal in one system. However, some video signals of video signals that have been multiplexed may be deteriorated, and video watched by a viewer may be interrupted. 
     In view of the above circumstances, an object of the present invention is to provide an optical transmission apparatus and an optical transmission method capable of transmitting a multiplexed signal obtained by multiplexing optical signals having different wavelengths with good quality. 
     Means for Solving the Problem 
     An aspect of the present invention is an optical transmission apparatus including: an optical input unit configured to input a first multiplexed signal obtained by multiplexing a plurality of optical signals having different wavelengths and a second multiplexed signal obtained by multiplexing a plurality of the optical signals having different wavelengths; a demultiplexing unit configured to demultiplex the first multiplexed signal and the second multiplexed signal by wavelength; a measurement unit configured to measure qualities of a plurality of optical signals obtained by demultiplexing the first multiplexed signal and qualities of a plurality of optical signals obtained by demultiplexing the second multiplexed signal; and a switching unit configured to perform switching between the first multiplexed signal and the second multiplexed signal to be output to a subsequent stage based on a result of measurement by the measurement unit. 
     An aspect of the present invention is an optical transmission method including: inputting a first multiplexed signal obtained by multiplexing a plurality of optical signals having different wavelengths and a second multiplexed signal obtained by multiplexing a plurality of the optical signals having different wavelengths; demultiplexing the first multiplexed signal and the second multiplexed signal by wavelength; measuring qualities of a plurality of optical signals obtained by demultiplexing the first multiplexed signal and qualities of a plurality of optical signals obtained by demultiplexing the second multiplexed signal; and performing switching between the first multiplexed signal and the second multiplexed signal to be output to a subsequent stage based on a result of measurement by the measuring. 
     Effects of the Invention 
     According to the present invention, it is possible to transmit a multiplexed signal obtained by multiplexing optical signals having different wavelengths with good quality. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a configuration example of a video communication network system according to a first embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a configuration of an optical transmission system according to the first embodiment. 
         FIG.  3    is a diagram illustrating a configuration example of a signal selection unit according to the first embodiment. 
         FIG.  4    is a diagram illustrating a configuration example of a signal selection unit according to a second embodiment. 
         FIG.  5    is a diagram illustrating a configuration example of a signal selection unit according to a third embodiment. 
         FIG.  6    is a diagram illustrating a configuration example of a signal selection unit according to a fourth embodiment. 
         FIG.  7    is a diagram illustrating a configuration example of a video communication network system according to a technology in the related art. 
         FIG.  8    is a diagram illustrating a configuration example of a signal selection unit according to the technology in the related art. 
         FIG.  9    is a diagram illustrating a relationship between a power of a multiplexed signal and powers of optical signals that are multiplexed. 
         FIG.  10    is a diagram illustrating a relationship between a power of a multiplexed signal and powers of optical signals that are multiplexed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. A redundant optical transmission system according to an embodiment has a redundant configuration in which a transmission signal is transmitted through a duplex transmission line. The transmission signal is a multiplexed signal obtained by multiplexing optical signals of a plurality of wavelength bands. In the redundant optical transmission system, a signal quality in each wavelength band before multiplexing of the transmission signal is measured and retained in advance internally or externally. The signal quality is represented, for example, by a level or a level difference. A signal selection apparatus of the redundant optical transmission system inputs two transmission signals transmitted through transmissions lines that have been duplexed, and demultiplexes each of the transmission signals into optical signals by wavelength. The signal selection apparatus detects presence or absence of deterioration in quality for each of the optical signals obtained by demultiplexing based on the retained information. When deterioration in quality is detected in one of the transmission signals, a switching unit of the signal selection apparatus selects and outputs the other of the transmission signals in which deterioration in quality has not been detected. As a result, it is possible to detect an abnormal condition more accurately than in signal transmission in a multiplexed state, and perform switching of the transmission signal to be output to a subsequent stage. Hereinafter, a case in which the redundant optical transmission system is a video communication network system that transmits a video signal will be described as an example. 
     First Embodiment 
       FIG.  1    is a diagram illustrating a configuration example of a video communication network system  100  according to a first embodiment. The video communication network system  100  includes a transmitter  110 , a transmitter  111 , a WDM filter  120 , one or more transmission apparatuses  130 , a transmission apparatus  140 , a WDM filter  150 , and a receiver  170  and a receiver  171  installed in a house of each subscriber or the like. While one receiver  170  and one receiver  171  are illustrated in the same figure, the number of houses of subscribers, and the number of receivers  170  and the number of the receivers  171  in a house of each subscriber are arbitrary. The WDM filter  150 , and the receiver  170  and receiver  171  are connected via an access network  160 . The access network  160  performs wavelength multiplexing transmission. The transmitter  110 , the transmitter  111 , the access network  160 , the receiver  170 , and the receiver  171  are the same as the transmitter  910 , the transmitter  911 , the access network  940 , the receiver  950 , and the receiver  950  illustrated in  FIG.  7   . 
     Similarly to the transmitter  910  illustrated in  FIG.  7   , the transmitter  110  inputs a video signal of the terrestrial digital broadcasting or a video signal of the BS broadcasting or the CS broadcasting delivered to the ground from a satellite using the right-hand circular polarization, and collectively converts the video signal into a wideband FM signal. The transmitter  110  converts the collectively converted FM signal into an optical signal SG 10  having a wavelength λ 1  and inputs the optical signal SG 10  to the WDM filter  120 . Similarly to the transmitter  910  illustrated in  FIG.  7   , the transmitter  111  inputs a video signal of the BS broadcasting or the CS broadcasting delivered to the ground from a satellite using the left-hand circular polarization, and collectively converts the video signal into a wideband FM signal. The transmitter  111  converts the collectively converted FM signal into an optical signal SG 11  having a wavelength λ 2  and inputs the optical signal SG 11  to the WDM filter  120 . 
     The WDM filter  120  multiplexes the optical signal SG 10  having the wavelength λ 1  input from the transmitter  110  and the optical signal SG 11  having the wavelength λ 2  input from the transmitter  111 , and outputs a multiplexed signal SG 12 . The multiplexed signal SG 12  output by the WDM filter  120  is relayed and transmitted by the one or more transmission apparatuses  130  and the transmission apparatus  140 , and is then input to the WDM filter  150 . 
     The WDM filter  150  multiplexes the multiplexed signal SG 12  input from the transmission apparatus  140  and an optical signal for communication input from a transmission apparatus (not illustrated), and outputs a multiplexed signal SG 13  to the access network  160 . The multiplexed signal SG 13  transmitted in the access network  160  is demultiplexed, the receiver  170  receives an optical signal SG 14  for video having the wavelength λ 1 , and the receiver  171  receives an optical signal SG 15  for video having the wavelength λ 2 . 
     In the video communication network system  100 , dual wavelength transmission from the WDM filter  120  to the WDM filter  150  is duplexed. With reference to  FIG.  2   , a configuration of the duplexed dual wavelength transmission applied to the video communication network system  100  will be described. 
       FIG.  2    is a diagram illustrating a configuration of an optical transmission system  200  according to the present embodiment. The optical transmission system  200  includes a head end  210 , a head end  211 , an optical transmission unit  220 , an optical transmission unit  221 , an optical transmission unit  230 , an optical transmission unit  231 , a multiplexing unit  240 , a multiplexing unit  241 , one or more amplification units for relay  250 , one or more amplification units for relay  251 , a signal selection apparatus  260 , one or more amplification units  270 , one or more distribution units  280 , and one or more optical reception units  290 . A 0-system is an operation system, and a 1-system is a preliminary system. The head end  210 , the optical transmission unit  220 , and the optical transmission unit  230  are operation systems. The head end  211 , the optical transmission unit  221 , and the optical transmission unit  231  are preliminary systems. 
     The head end  210 , the optical transmission unit  220 , and the optical transmission unit  221  correspond to the transmitter  110  illustrated in  FIG.  1   , and the head end  211 , the optical transmission unit  230 , and the optical transmission unit  231  correspond to the transmitter  111  illustrated in  FIG.  1   . The multiplexing unit  240  and the multiplexing unit  241  correspond to the WDM filter  120  illustrated in  FIG.  1   . The amplification-for-relay unit  250  may be the transmission apparatus  130  illustrated in  FIG.  1   , or may be an amplifier connected to the transmission apparatus  130 . The signal selection apparatus  260  corresponds to the transmission apparatus  140  illustrated in  FIG.  1   . The amplification unit  270  is connected to a preceding stage of the WDM filter  150  illustrated in  FIG.  1   , and the distribution unit  280  is connected to a subsequent stage of the WDM filter  150  illustrated in  FIG.  1   . The optical reception units  290  correspond to the receiver  170  and the receiver  171  illustrated in  FIG.  1   . 
     The head end  210  receives, by radio waves, a video signal of the terrestrial digital broadcasting transmitted from a broadcast station, or a video signal of the BS broadcasting or the CS broadcasting delivered to the ground from a satellite using the right-hand circular polarization. Wavelengths of video signals are different for each channel. The head end  210  converts a video signal of each channel into an electrical signal and transmits the electrical signal to the optical transmission unit  220  and the optical transmission unit  221 . The optical transmission unit  220  and the optical transmission unit  221  each convert the received electrical signal of each channel into the optical signal SG 10  having the wavelength λ 1 . FM collective conversion is used for the conversion. The optical transmission unit  220  outputs the optical signal SG 10  after conversion to the multiplexing unit  240 , and the optical transmission unit  221  outputs the optical signal SG 10  after conversion to the multiplexing unit  241 . 
     The head end  211  receives, by radio waves, a video signal of the new 4K/8K satellite broadcasting transmitted from the broadcast station. Wavelengths of video signals are different for each channel. head end  211  converts a video signal of each channel into an electrical signal and transmits the electrical signal to the optical transmission unit  230  and the optical transmission unit  231 . The optical transmission unit  230  and the optical transmission unit  231  each convert the received electrical signal of each channel into the optical signal SG 11  having the wavelength λ 2 . FM collective conversion is used for the conversion. The optical transmission unit  230  outputs the optical signal SG 11  after conversion to the multiplexing unit  240 , and the optical transmission unit  231  outputs the optical signal SG 11  after conversion to the multiplexing unit  241 . 
     The multiplexing unit  240  multiplexes the optical signal SG 10  having the wavelength λ 1  received from the optical transmission unit  220  and the optical signal SG 11  having the wavelength λ 2  received from the optical transmission unit  230 , and outputs a 0-system multiplexed signal SG 20  to the transmission line  245 . The multiplexed signal SG 20  of the optical signals is amplified in the one or more amplification units for relay  250  provided in the transmission line  245 , and relayed to a subsequent stage. 
     The multiplexing unit  241  multiplexes the optical signal SG 10  having the wavelength λ 1  received from the optical transmission unit  221  and the optical signal SG 11  having the wavelength λ 2  received from the optical transmission unit  231 , and outputs a 1-system multiplexed signal SG 21  to the transmission line  246 . The multiplexed signal SG 21  of the optical signals is amplified in the one or more amplification units for relay  251  provided in the transmission line  246 , and relayed to a subsequent stage. 
     The signal selection apparatus  260  is an example of the optical transmission apparatus. The signal selection apparatus  260  includes an amplification-for-relay unit  261 , an amplification-for-relay unit  262 , and a signal selection unit  263 . The amplification-for-relay unit  261  amplifies the 0-system multiplexed signal SG 20  transmitted through the transmission line  245  and outputs the amplified 0-system multiplexed signal SG 20  to the signal selection unit  263 . The amplification-for-relay unit  262  amplifies the 1-system multiplexed signal SG 21  transmitted through the transmission line  246  and outputs the amplified 1-system multiplexed signal SG 21  to the signal selection unit  263 . The signal selection unit  263  selects either the multiplexed signal SG 20  or the multiplexed signal SG 21 , and outputs the selected signal as a multiplexed signal SG 22  to a subsequent stage. 
     The amplification unit  270  amplifies the multiplexed signal SG 22  output by the signal selection apparatus  260  and outputs the amplified multiplexed signal SG 22  to the access network. The distribution unit  280  distributes the multiplexed signal SG 22  received via the access network. The optical reception unit  290  receives the multiplexed signal SG 22  distributed by the distribution unit  280  and receives the optical signal SG 10  having the wavelength λ 1  or the optical signal SG 11  having the wavelength λ 2  multiplexed into the multiplexed signal SG 22 . 
     Here, a signal selection unit using a technology in the related art will be described.  FIG.  8    is a diagram illustrating a configuration example of a signal selection unit  980  according to the technology in the related art. The signal selection unit  980  includes an optical input unit  981 , an optical input unit  982 , a detection unit  983 , a detection unit  984 , a switching unit  985 , and an optical output unit  986 . The optical input unit  981  inputs the 0-system multiplexed signal SG 20 , and the optical input unit  982  inputs the 1-system multiplexed signal SG 21 . The detection unit  983  is configured to perform processing of outputting the 0-system multiplexed signal SG 20  input by the optical input unit  981  to the switching unit  985 , and processing of, when a level reduction of the 0-system multiplexed signal SG 20  is detected, outputting a switching request signal SG 80  for switching to the 1-system to the switching unit  985 . The detection unit  983  is configured to perform processing of outputting the 1-system multiplexed signal SG 21  input by the optical input unit  982  to the switching unit  985  and processing of, when a level reduction of the 1-system multiplexed signal SG 21  is detected, outputting a switching request signal SG 81  for switching to the 0-system to the switching unit  985 . The switching request signal SG 80  and the switching request signal SG 81  each are a control signal. 
     When receiving the switching request signal SG 80  for switching to the 1-system, the switching unit  985  outputs the 1-system multiplexed signal SG 21  to the optical output unit  986 . When receiving the switching request signal SG 81  for switching to the 0-system, the switching unit  985  outputs the 0-system multiplexed signal SG 20  to the optical output unit  986 . The optical output unit  986  outputs the 0-system multiplexed signal SG 20  or the 1-system multiplexed signal SG 21  output by the switching unit  985  as the multiplexed signal SG 22  to a subsequent stage. Note that the signal selection unit  980  may implement a level adjustment function, but the level adjustment function is omitted in  FIG.  8   . 
     The multiplexed signal SG 20  and the multiplexed signal SG 21  each are a signal obtained by multiplexing the optical signal SG 10  having the wavelength λ 1  and the optical signal SG 11  having the wavelength λ 2 . The detection unit  983  detects a level reduction in the multiplexed signal SG 20 , and the detection unit  984  detects a level reduction in the multiplexed signal SG 21 . However, neither the detection unit  983  nor the detection unit  984  can detect which of the optical signal SG 10  having the wavelength λ\ 1  and the optical signal SG 11  having the wavelength λ 2  is abnormal. 
       FIG.  9    and  FIG.  10    each illustrate a relationship between a power of a multiplexed signal obtained by multiplexing two optical signals and a power of a second optical signal that is multiplexed into the multiplexed signal. A threshold of signal deterioration is given for the power after multiplexing and not given for a power of each wavelength. As such, it is conceivable that switching due to a power reduction of one of the two optical signals is actually unnecessary or vice versa. An ideal threshold is a value based on a condition in which there is no deterioration in power for each of the two optical signals. At this time, as illustrated in  FIG.  9   , if a set threshold is lower than the ideal threshold, when a power after multiplexing is a value of P 1  exceeding the set threshold, switching may not be performed even when the power of the second optical signal is low. Furthermore, as illustrated in  FIG.  10   , if the set threshold is higher than the ideal threshold, when the power after multiplexing is a value of P 2  smaller than the set threshold, switching may be performed even when there is no problem in the power of the second optical signal. 
     In the related art, in a transmission system having a plurality of transmission lines, in which optical signals of a plurality of wavelength bands are multiplexed in each transmission line to be transmitted through one optical fiber, when the switching unit switches the transmission lines, no processing is performed to demultiplex the multiplexed signal in the transmission system into optical signals having original wavelengths. Typically, demultiplexing is performed before an access or at an optical network unit (ONU) at an end of an access section. Thus, the signal selection unit in the related-art technology detects a level reduction in the multiplexed signal as described above. This makes it difficult to accurately set the threshold of the level reduction for each wavelength. As a result, switching may not occur in the vicinity of the threshold, or conversely, even when switching is not required, switching may occur. 
     The signal selection unit according to the present embodiment demultiplexes the multiplexed signal back to signals having the original wavelengths, and then performs level measurement and level reduction detection of the optical signals for determination of switching. The signal selection unit measures a level and detects a level reduction for each wavelength, and can thus detect deterioration of a signal having an individual wavelength more accurately than measuring a level and detecting a level reduction for the multiplexed signal as is. Thus, it is possible to improve a determination accuracy of the switching processing. 
       FIG.  3    is a diagram illustrating a detailed configuration of a signal selection unit  300  of the present embodiment. The signal selection unit  300  illustrated in  FIG.  3    is used as the signal selection unit  263  included in the signal selection apparatus  260  illustrated in  FIG.  2   . The signal selection unit  300  includes an optical input unit  310 , an optical input unit  311 , a demultiplexing unit  320 , a demultiplexing unit  321 , a storage unit  330 , measurement detection units  340 - 1  to  340 -N (N is an integer of 2 or greater), measurement detection units  341 - 1  to  341 -N (N is an integer of 2 or greater), a multiplexing unit  350 , a multiplexing unit  351 , a switching unit  360 , and an optical output unit  370 . 
     The optical input unit  310  inputs the 0-system multiplexed signal SG 20 , and the optical input unit  311  inputs the 1-system multiplexed signal SG 21 . The demultiplexing unit  320  demultiplexes the 0-system multiplexed signal SG 20  into N optical signals having different wavelengths SG 30 - 1  to SG 30 -N. The demultiplexing unit  320  outputs the optical signal SG 30 - n  (n is an integer of 1 or greater and N or less) to the measurement detection unit  340 - n . The demultiplexing unit  321  demultiplexes the 1-system multiplexed signal SG 21  into N optical signals having different wavelengths SG 31 - 1  to SG 31 -N. The demultiplexing unit  321  outputs the optical signal SG 31 - n  (n is an integer of 1 or greater and N or less) to the measurement detection unit  341 - n.    
     The storage unit  330  stores a setting table in advance. The setting table indicates thresholds of optical signal levels of wavelengths required for detection for performing switching. The thresholds of the optical signal levels for the N wavelengths are described as optical signal level thresholds P( 1 ) to P(N). The measurement detection units  340 - 1  to  340 -N and  341 - 1  to  341 -N, by using a control signal SG 40 , reads information from the storage unit  330 . 
     The measurement detection unit  340 - n  includes a measurement unit  3401 , a detection unit  3402 , and an instruction unit  3403 . The measurement unit  3401  measures an optical signal level M 0 ( n ) of the input optical signal SG 30 - n , and outputs the optical signal SG 30 - n  after measurement to the multiplexing unit  350 . The detection unit  3402  reads the optical signal level threshold P(n) stored in the storage unit  330  and compares it to the optical signal level M 0 ( n ). When M 0 ( n )≤P(n) or M 0 ( n )&lt;P(n) is satisfied, the detection unit  3402  detects deterioration in quality. When the detection unit  3402  detects deterioration in quality, the instruction unit  3403  outputs a switching instruction SG 50  for switching to the 1-system to the switching unit  360 . The switching instruction SG 50  is a control signal. 
     The measurement detection unit  341 - n  includes a measurement unit  3411 , a detection unit  3412 , and an instruction unit  3413 . The measurement unit  3411  measures an optical signal level M 1 ( n ) of the input optical signal SG 31 - n , and outputs the optical signal SG 31 - n  after measurement to the multiplexing unit  351 . The detection unit  3412  reads the optical signal level threshold P(n) stored in the storage unit  330  and compares it to the optical signal level M 1 ( n ). When M 1 ( n )≤P(n) or M 1 ( n )&lt;P(n) is satisfied, the detection unit  3412  detects deterioration in quality. When the detection unit  3412  detects deterioration in quality, the instruction unit  3413  outputs a switching instruction SG 51  for switching to the 0-system to the switching unit  360 . The switching instruction SG 51  is a control signal. 
     The multiplexing unit  350  inputs the optical signals SG 30 - 1  to SG 30 -N from the measurement detection units  340 - 1  to  340 -N, respectively. The multiplexing unit  350  outputs the multiplexed signal SG 20  obtained by multiplexing the optical signals SG 30 - 1  to SG 30 -N to the switching unit  360 . 
     The multiplexing unit  351  inputs the optical signals SG 31 - 1  to SG 31 -N from the measurement detection units  341 - 1  to  341 - n , respectively. The multiplexing unit  351  outputs the multiplexed signal SG 21  obtained by multiplexing the optical signals SG 31 - 1  to SG 31 -N to the switching unit  360 . 
     When receiving the switching instruction, the switching unit  360  performs switching between the multiplexed signal SG 20  and the multiplexed signal SG 21  to be output. When receiving the switching instruction SG 50  for switching to the 1-system from at least one of the measurement detection units  340 - 1  to  340 -N, the switching unit  360  selects the multiplexed signal SG 21  and outputs the selected multiplexed signal SG 21  to the optical output unit  370 . When receiving the switching instruction SG 51  for switching to the 0-system from at least one of the measurement detection units  341 - 1  to  341 -N, the switching unit  360  selects the multiplexed signal SG 20  and outputs the selected multiplexed signal SG 20  to the optical output unit  370 . The optical output unit  370  outputs the multiplexed signal SG 20  or the multiplexed signal SG 21  output from the switching unit  360  as the multiplexed signal SG 22  to a subsequent stage. 
     Note that the storage unit  330  may be provided in an apparatus outside the signal selection apparatus  260 , or may be provided outside the signal selection unit  300  in the signal selection apparatus  260 . Furthermore, some of the functions of the measurement detection units  340 - 1  to  340 -N and  341 - 1  to  341 -N, and the storage unit  330  may be included in an apparatus outside the signal selection unit  300  or the signal selection apparatus  260 . For example, an external control apparatus that monitors and controls the video communication network system  100  or the optical transmission system  200  may have some of the functions of the measurement detection units  340 - 1  to  340 -N and  341 - 1  to  341 -N. In this case, the control apparatus may include the detection units  3402  and the instruction units  3403  of the measurement detection units  340 - 1  to  340 -N, and the detection units  3412  and the instruction units  3413  of the measurement detection units  341 - 1  to  341 -N. The measurement detection unit  340 - n  measures the optical signal level M 0 ( n ) of the optical signal SG 30 - n  and notifies the control apparatus of the measured level, and the measurement detection unit  341 - n  measures the optical signal level M 1 ( n ) of the optical signal SG 31 - n  and notifies the control apparatus of the measured level. When M 0 ( n )≤P(n) or M 0 ( n )&lt;P(n) is satisfied, the control apparatus outputs the switching instruction SG 50  for switching to the 1-system to the switching unit  360 , and when M 1 ( n )≤P(n) or M 1 ( n )&lt;P(n) is satisfied, the control apparatus outputs the switching instruction SG 51  for switching to the 0-system to the switching unit  360 . 
     Note that in order to compensate for level reduction due to demultiplexing or multiplexing, the signal selection unit  300  may include a level adjustment unit (AMP)  380  that amplifies the multiplexed signal between the multiplexing unit  350  and the switching unit  360  and between the multiplexing unit  351  and the switching unit  360 . 
     Second Embodiment 
     The signal selection apparatus according to the first embodiment demultiplexes a main signal output to a subsequent stage, performs level measurement, and then multiplexes the demultiplexed signals. A signal selection apparatus according to a present embodiment branches an input multiplexed signal, demultiplexes the branched multiplexed signal, and performs level measurement, so that multiplexing and demultiplexing of a main signal are not performed. Hereinafter, in the present embodiment, differences from the first embodiment will be mainly described. 
       FIG.  4    is a diagram illustrating a detailed configuration of a signal selection unit  400  according to the present embodiment. The signal selection unit  400  illustrated in  FIG.  4    is used as the signal selection unit  263  included in the signal selection apparatus  260  illustrated in  FIG.  2   . The signal selection unit  400  includes an optical input unit  410 , an optical input unit  411 , a branching unit  420 , a branching unit  421 , a demultiplexing unit  430 , a demultiplexing unit  431 , a storage unit  440 , measurement detection units  450 - 1  to  450 -N (N is an integer of 2 or greater), measurement detection units  451 - 1  to  451 -N, a switching unit  460 , and an optical output unit  470 . 
     The optical input unit  410  inputs a 0-system multiplexed signal SG 20  and outputs the signal to the branching unit  420 . The optical input unit  411  inputs a 1-system multiplexed signal SG 21  and outputs the signal to the branching unit  421 . The branching unit  420  branches the multiplexed signal SG 20  input from the optical input unit  410 . The branching unit  420  outputs the branched multiplexed signal SG 20  to the demultiplexing unit  430 , and outputs the multiplexed signal SG 20  of the main signal to the switching unit  460 . The branching unit  421  branches the multiplexed signal SG 21  input from the optical input unit  411 . The branching unit  421  outputs the branched multiplexed signal SG 21  to the demultiplexing unit  431 , and outputs the multiplexed signal SG 21  of the main signal to the switching unit  460 . 
     The demultiplexing unit  430  demultiplexes the 0-system multiplexed signal SG 20  into optical signals SG 30 - 1  to SG 30 -N having N wavelengths. The demultiplexing unit  430  outputs the optical signal SG 30 - n  (n is an integer of 1 or greater and N or less) to the measurement detection unit  450 - n . The demultiplexing unit  431  demultiplexes the 1-system multiplexed signal SG 21  into optical signals SG 31 - 1  to SG 31 -N having N wavelengths. The demultiplexing unit  431  outputs the optical signal SG 31 - n  (n is an integer of 1 or greater and N or less) to the measurement detection unit  451 - n.    
     The storage unit  440  stores a setting table similar to that of the storage unit  330  of the first embodiment illustrated in  FIG.  3   . The measurement detection units  450 - 1  to  450 -N and  451 - 1  to  451 -N, by using a control signal SG 40 , reads information from the storage unit  440 . 
     The measurement detection unit  450 - n  includes a measurement unit  4501 , a detection unit  4502 , and an instruction unit  4503 . The measurement unit  4501  measures an optical signal level M 0 ( n ) of an input optical signal SG 30 - n . The detection unit  4502  reads an optical signal level threshold P(n) stored in the storage unit  440  and compares it to the optical signal level M 0 ( n ). When M 0 ( n )≤P(n) or M 0 ( n )&lt;P(n) is satisfied, the detection unit  4502  detects deterioration in quality. When the detection unit  4502  detects deterioration in quality, the instruction unit  4503  outputs a switching instruction SG 50  for switching to the 1-system to the switching unit  460 . 
     The measurement detection unit  451 - n  includes a measurement unit  4511 , a detection unit  4512 , and an instruction unit  4513 . The measurement unit  4511  measures an optical signal level M 1 ( n ) of the input optical signal SG 31 - n . The detection unit  4512  reads the optical signal level threshold P(n) stored in the storage unit  440  and compares it to the optical signal level M 1 ( n ). When M 1 ( n )≤P(n) or M 1 ( n )&lt;P(n) is satisfied, the detection unit  4512  detects deterioration in quality. When the detection unit  4512  detects deterioration in quality, the instruction unit  4513  outputs a switching instruction SG 51  for switching to the 0-system to the switching unit  460 . 
     When receiving the switching instruction SG 50  for switching to the 1-system from at least one of the measurement detection units  450 - 1  to  450 -N, the switching unit  460  selects the multiplexed signal SG 21  input from the branching unit  421  and outputs the selected multiplexed signal SG 21  to the optical output unit  470 . When receiving the switching instruction SG 51  for switching to the 0-system from at least one of the measurement detection units  451 - 1  to  451 -N, the switching unit  460  selects the multiplexed signal SG 20  input from the branching unit  420  and outputs the selected multiplexed signal SG 20  to the optical output unit  470 . The optical output unit  470  outputs the multiplexed signal SG 20  or the multiplexed signal SG 21  output from the switching unit  460  as a multiplexed signal SG 22  to a subsequent stage. 
     Note that the storage unit  440  may be provided in an apparatus outside the signal selection apparatus  260 , or may be provided outside the signal selection unit  400  in the signal selection apparatus  260 . Furthermore, some of the functions of the measurement detection units  450 - 1  to  450 -N and  451 - 1  to  451 -N, and the storage unit  440  may be included in an apparatus outside the signal selection unit  400  or the signal selection apparatus  260 . For example, an external control apparatus that monitors and controls the video communication network system  100  or the optical transmission system  200  may have some of the functions of the measurement detection units  450 - 1  to  450 -N and  451 - 1  to  451 -N. In this case, the control apparatus may include the detection units  4502  and the instruction units  4503  of the measurement detection units  450 - 1  to  450 -N, and the detection units  4512  and the instruction units  4513  of the measurement detection units  451 - 1  to  451 -N. The measurement detection unit  450 - n  measures the optical signal level M 0 ( n ) of the optical signal SG 30 - n  to notify the control apparatus of the measured level, and the measurement detection unit  451 - n  measures the optical signal level M 1 ( n ) of the optical signal SG 31 - n  and notifies the control apparatus of the measured level. When M 0 ( n )≤P(n) or M 0 ( n )&lt;P(n) is satisfied, the control apparatus outputs the switching instruction SG 50  for switching to the 1-system to the switching unit  460 , and when M 1 ( n )≤P(n) or M 1 ( n )&lt;P(n) is satisfied, the control apparatus outputs the switching instruction SG 51  for switching to the 0-system to the switching unit  460 . 
     Note that in order to compensate for level reduction due to branching or demultiplexing, the signal selection unit  400  may include a level adjustment unit (AMP)  480  that amplifies a multiplexed signal between the branching unit  420  and the demultiplexing unit  430 , between the branching unit  420  and the switching unit  460 , between the branching unit  421  and the demultiplexing unit  431 , or between the branching unit  421  and the switching unit  460 . 
     The present embodiment has a structure somewhat more complex than that of the first embodiment, but a main signal is not demultiplexed and multiplexed, so that deterioration in quality of the main signal can be suppressed compared to the first embodiment. 
     Third Embodiment 
     The signal selection apparatus according to the first embodiment demultiplexes the 0-system and 1-system multiplexed signals, and compares an optical signal level of each wavelength obtained by demultiplexing to a threshold to detect deterioration of an optical signal. A signal selection apparatus according to a present embodiment detects deterioration of an optical signal based on an optical signal level difference between adjacent wavelengths. 
     If the optical signal level difference between adjacent wavelengths deviates from a range of specified values, a quality of a weaker optical signal may decrease. Thus, in the present embodiment, a threshold B ij  (i and j are each an integer of 1 or greater and N or less) is provided for an optical signal level difference between the ith wavelength and the jth wavelength adjacent to each other. The signal selection apparatus performs switching between a 0-system multiplexed signal and a 1-system multiplexed signal to be output based on comparison between a measurement value D ij  (i and j are each an integer of 1 or greater and N or less) of an optical signal level difference between the two adjacent wavelengths and the threshold B ij . Hereinafter, in the present embodiment, differences from the first embodiment will be mainly described. 
       FIG.  5    is a diagram illustrating a detailed configuration of a signal selection unit  500  according to the present embodiment. The signal selection unit  500  illustrated in  FIG.  5    is used as the signal selection unit  263  included in the signal selection apparatus  260  illustrated in  FIG.  2   . In  FIG.  5   , the same portions as those of the signal selection unit  300  according to the first embodiment illustrated in  FIG.  3    are denoted by the same reference numerals and signs, and description thereof will be omitted. The signal selection unit  500  includes an optical input unit  310 , an optical input unit  311 , a demultiplexing unit  320 , a demultiplexing unit  321 , measurement units  510 - 1  to  510 -N (N is an integer of two or greater), measurement units  511 - 1  to  511 -N, a storage unit  520 , a calculation instruction unit  530 , a multiplexing unit  350 , a multiplexing unit  351 , a switching unit  540 , and an optical output unit  370 . 
     The demultiplexing unit  320  outputs an optical signal SG 30 - n  (n is an integer of 1 or greater and N or less) obtained by demultiplexing a 0-system multiplexed signal SG 20  input from the optical input unit  310  to the measurement unit  510 - n . The demultiplexing unit  321  outputs an optical signal SG 31 - n  (n is an integer of 1 or greater and N or less) obtained by demultiplexing a 1-system multiplexed signal SG 20  input from the optical input unit  311  to the measurement unit  511 - n.    
     The measurement unit  510 - n  measures an optical signal level M 0 ( n ) of the input optical signal SG 30 - n , and outputs the optical signal SG 30 - n  after measurement to the multiplexing unit  350 . The measurement unit  510 - n  notifies the calculation instruction unit  530  of the measured optical signal level M 0 ( n ) by a control signal SG 41 . The measurement unit  511 - n  measures an optical signal level M 1 ( n ) of the input optical signal SG 31 - n , and outputs the optical signal SG 31 - n  after measurement to the multiplexing unit  351 . The measurement unit  511 - n  notifies the calculation instruction unit  530  of the measured optical signal level M 1 ( n ) by the control signal SG 41 . 
     The storage unit  520  stores a setting table. The setting table indicates the threshold B ij  (i and j are each an integer of 1 or greater and N or less) of the optical signal level difference between two wavelengths of the i-th wavelength and the j-th wavelength adjacent to each other. The calculation instruction unit  530  reads information from the storage unit  520  by a control signal SG 42 . 
     The calculation instruction unit  530  receives notification of optical signal levels M 0 ( 1 ) to M 0 (N) of the respective wavelengths from the measurement units  510 - 1  to  510 -N, and receives notification of optical signal levels M 1 ( 1 ) to M 1 (N) of the respective wavelengths from the measurement units  511 - 1  to  511 -N, by the control signal SG 41 . The calculation instruction unit  530  includes a calculation unit  5301 , a detection unit  5302 , and an instruction unit  5303 . The calculation unit  5301 , by using the optical signal levels M 0 ( 1 ) to M 0 (N), calculates a level difference D 0   ij  (=M 0 ( i )−M 0 ( j )) between optical signal levels of a wavelength i and a wavelength j adjacent to each other for the 0-system. The detection unit  5302  reads the threshold B ij  from the setting table stored in the storage unit  520  and compares the threshold to the level difference DN. The detection unit  5302  detects deterioration in quality of the 0-system when any level difference D 0   ij  is greater than or equal to the threshold B ij  (D 0   ij ≥B ij ), or when any level difference D 0   ij  is equal to or smaller than the threshold B ij  (D 0   ij ≤B ij ). The detection unit  5302  may detect deterioration in quality of the 0-system when any level difference D 0   ij  is greater than the threshold B ij  (D 0   ij &gt;B ij ), or when any level difference D 0   ij  is smaller than the threshold B ij  (D 0   ij &lt;B ij ). When the detection unit  5302  detects deterioration in quality of the 0-system, the instruction unit  5303  outputs a switching instruction SG 52  for switching to the 1-system to the switching unit  540 . 
     The calculation instruction unit  530  performs processing similar to that of the 0-system for the 1-system. In other words, the calculation unit  5301 , by using the optical signal levels M 1 ( 1 ) to M 1 (N), calculates a level difference D 1   ij  (=M 1 ( i )−M 1 ( j )) of optical signal levels of a wavelength i and a wavelength j adjacent to each other. The detection unit  5302  compares the threshold B ij  to the level difference D 1   ij . The detection unit  5302  detects deterioration in quality of the 1-system when any level difference D 1   ij  is greater than or equal to the threshold D 1   ij  (D 1   ij ≥B ij ), or when any level difference D 1   ij  is equal to or smaller than the threshold B ij  (D 1   ij ≤B ij ). The detection unit  5302  may detect deterioration in quality of the 1-system when any level difference D 1   ij  is greater than the threshold B ij  (D 1   ij &gt;B ij ), or when any level difference D 1   ij  is smaller than the threshold B ij  (D 1   ij &lt;B ij ). When the detection unit  5302  detects deterioration in quality of the 1-system, the instruction unit  5303  outputs a switching instruction SG 52  for switching to the 0-system to the switching unit  540 . 
     Note that which is an appropriate condition for detecting deterioration in quality, a case where the level differences D 0   ij , D 1   ij  are greater than or equal to B ij  or greater than B ij , or a case where the level differences D 0   ij , D 1   ij  are equal to or smaller than the threshold B ij  or smaller than the threshold B ij , varies depending on an appropriate optical signal level for a service transmitted by each wavelength, and the like. 
     The multiplexing unit  350  outputs a multiplexed signal SG 20  obtained by multiplexing the optical signals SG 30 - 1  to SG 30 -N output from the measurement units  510 - 1  to  510 -N to the switching unit  540 . The multiplexing unit  351  outputs a multiplexed signal SG 21  obtained by multiplexing the optical signals SG 31 - 1  to SG 31 -N output from the measurement units  511 - 1  to  511 -N to the switching unit  540 . 
     When receiving the switching instruction SG 52  for switching to the 1-system from the calculation instruction unit  530 , the switching unit  540  selects the multiplexed signal SG 21  and outputs the selected multiplexed signal SG 21  to the optical output unit  370 , and when receiving the switching instruction SG 52  for switching to the 0-system from the calculation instruction unit  530 , the switching unit  540  selects the multiplexed signal SG 20  and outputs the selected multiplexed signal SG 20  to the optical output unit  370 . The optical output unit  370  outputs the multiplexed signal SG 20  or the multiplexed signal SG 21  output from the switching unit  540  as a multiplexed signal SG 22  to a subsequent stage. 
     Note that the storage unit  520  may be provided in an apparatus outside the signal selection apparatus  260 , or may be provided outside the signal selection unit  500  in the signal selection apparatus  260 . The storage unit  520  and the calculation instruction unit  530  may be included in an apparatus outside the signal selection unit  300  or the signal selection apparatus  260 . For example, an external control apparatus that monitors and controls the video communication network system  100  or the optical transmission system  200  may have the storage unit  520  and the calculation instruction unit  530 . 
     Note that in order to compensate for level reduction due to demultiplexing or multiplexing, the signal selection unit  500  may include a level adjustment unit (AMP)  380  that amplifies a multiplexed signal between the multiplexing unit  350  and the switching unit  540  and between the multiplexing unit  351  and the switching unit  540 . 
     Fourth Embodiment 
     The signal selection apparatus according to the third embodiment demultiplexes a main signal to be output to a subsequent stage, performs level measurement, and then multiplexes the demultiplexed signals. Similarly to the second embodiment, a signal selection apparatus according to a present embodiment branches an input multiplexed signal and demultiplexes the branched multiplexed signal to perform level measurement. Hereinafter, in the present embodiment, differences from the second embodiment will be mainly described. 
       FIG.  6    is a diagram illustrating a detailed configuration of a signal selection unit  600  according to the present embodiment. The signal selection unit  600  illustrated in  FIG.  6    is used as the signal selection unit  263  included in the signal selection apparatus  260  illustrated in  FIG.  2   . The signal selection unit  600  includes an optical input unit  410 , an optical input unit  411 , a branching unit  420 , a branching unit  421 , a demultiplexing unit  430 , a demultiplexing unit  431 , measurement units  610 - 1  to  610 -N (N is an integer of 2 or greater), measurement units  611 - 1  to  611 -N, a storage unit  620 , a calculation instruction unit  630 , a switching unit  640 , and an optical output unit  470 . 
     The demultiplexing unit  430  outputs an optical signal SG 30 - n  (n is an integer of 1 or greater and N or less) obtained by demultiplexing a 0-system multiplexed signal SG 20  branched by the branching unit  420  to the measurement unit  610 - n . The demultiplexing unit  431  outputs an optical signal SG 30 - n  (n is an integer of 1 or greater and N or less) obtained by demultiplexing a 1-system multiplexed signal SG 21  branched by the branching unit  421  to the measurement unit  611 - n.    
     The measurement unit  610 - n  measures an optical signal level M 0 ( n ) of the input optical signal SG 30 - n , and notifies the calculation instruction unit  630  of the measured optical signal level M 0 ( n ) by a control signal SG 41 . The measurement unit  611 - n  measures an optical signal level M 1 ( n ) of the input optical signal SG 31 - n , and outputs the measured optical signal level M 1 ( n ) to the calculation instruction unit  630  by the control signal SG 41 . 
     The storage unit  620  stores a setting table similar to that of the storage unit  520  of the third embodiment. The setting table indicates a threshold B ij  (i and j are each an integer of 1 or greater and N or less) of an optical signal level difference between two wavelengths of the i-th wavelength and the j-th wavelength adjacent to each other. The calculation instruction unit  630  reads information from the storage unit  620  by a control signal SG 42 . 
     The calculation instruction unit  630  performs processing similar to that of the calculation instruction unit  530  of the third embodiment. The calculation instruction unit  630  includes a calculation unit  6301 , a detection unit  6302 , and an instruction unit  6303 . The calculation unit  6301 , by using optical signal levels M 0 ( 1 ) to M 0 (N) of which are notified by the measurement units  610 - 1  to  610 -N, calculates a level difference D 0   ij  (=M 0 ( i )−M 0 ( j )) of optical signal levels of a wavelength i and a wavelength j adjacent to each other. The detection unit  6302  compares a threshold B ij  read from the setting table to the level difference D 0   ij , and detects deterioration in quality of the 0-system when any level difference D 0   ij  is greater than or equal to the threshold B ij  (D 0   ij ≥B ij ), or when any level difference D 0   ij  is equal to or smaller than the threshold B ij  (D 0   ij ≤B ij ). The detection unit  6302  may detect deterioration in quality of the 0-system when any level difference D 0   ij  is greater than the threshold B ij  (D 0   ij &gt;B ij ), or when any level difference D 0   ij  is smaller than the threshold B ij  (D 0   ij &lt;B ij ). When the detection unit  6302  detects deterioration in quality of the 0-system, the instruction unit  6303  outputs a switching instruction SG 52  for switching to the 1-system to the switching unit  460 . 
     The calculation instruction unit  630  performs processing similar to that of the 0-system for the 1-system. In other words, the calculation unit  6301 , by using the optical signal levels M 1 ( 1 ) to M 1 (N), calculates a level difference D 1   ij  (=M 1 ( i )−M 1 ( j )) between optical signal levels of a wavelength i and a wavelength j adjacent to each other. The detection unit  6302  detects deterioration in quality of the 1-system when any level difference D 1   ij  is greater than or equal to the threshold B ij  (D 1   ij ≥B ij ), or when any level difference D 1   ij  is equal to or smaller than the threshold B ij  (D 1   ij ≤B ij ). The detection unit  6302  may detect deterioration in quality of the 1-system when any level difference D 1   ij  is greater than the threshold B ij  (D 1   ij &gt;B ij ), or when any level difference D 1   ij  is smaller than the threshold B ij  (D 1   ij &lt;B ij ). When the detection unit  6302  detects deterioration in quality of the 1-system, the instruction unit  6303  outputs the switching instruction SG 52  for switching to the 0-system to the switching unit  640 . 
     When receiving the switching instruction SG 52  for switching to the 1-system from the calculation instruction unit  630 , the switching unit  640  selects the multiplexed signal SG 21  and outputs the selected multiplexed signal SG 21  to the optical output unit  470 , and when receiving the switching instruction SG 52  for switching to the 0-system from the calculation instruction unit  630 , the switching unit  640  selects the multiplexed signal SG 20  to output the selected multiplexed signal SG 20  to the optical output unit  470 . The optical output unit  470  outputs the multiplexed signal SG 20  or the multiplexed signal SG 21  output from the switching unit  640  as a multiplexed signal SG 22  to a subsequent stage. 
     Note that the storage unit  620  may be provided in an apparatus outside the signal selection apparatus  260 , or may be provided outside the signal selection unit  600  in the signal selection apparatus  260 . The storage unit  620  and the calculation instruction unit  630  may be included in an apparatus outside the signal selection unit  600  or the signal selection apparatus  260 . For example, an external control apparatus that monitors and controls the video communication network system  100  or the optical transmission system  200  may have the storage unit  620  and the calculation instruction unit  630 . 
     Note that in order to compensate for level reduction due to branching or demultiplexing, the signal selection unit  600  may include a level adjustment unit (AMP)  480  that amplifies a multiplexed signal between the branching unit  420  and the demultiplexing unit  430 , between the branching unit  420  and the switching unit  460 , between the branching unit  421  and the demultiplexing unit  431 , and between the branching unit  421  and the switching unit  460 . 
     Typically, a multiplexed signal subjected to multiplexing performed to transmit a plurality of signals through one optical fiber is not demultiplexed until signal transmission in the optical transmission system is completed. This signal transmission completion means, for example, until signal transmission of an access section is completed, that is, until the ONU receives signals. This is for avoiding deterioration in signal quality and complexity of an internal structure of an apparatus that performs transmission in the optical transmission system due to the demultiplexing processing. 
     The signal selection apparatus according to the present embodiment achieves an improvement in switching accuracy by demultiplexing a multiplexed optical signal into optical signals having original wavelengths internally and then performing level measurement. Alternatively, the signal selection apparatus according to the present embodiment branches a signal for measurement, and demultiplexes the branched optical signal into optical signals having original wavelengths to measure a level so as not to affect original signal transmission itself. Furthermore, the signal selection apparatus has a configuration in which amplification is performed by level adjustment to prevent signal level reduction due to branching, so that it is possible to perform main signal transmission without impairing original signal transmission quality. 
     In addition, the signal selection apparatus may have a structure in which a main signal lead wire and a lead wire for measuring and detecting are further branched by the branching unit to suppress deterioration of a main signal, or a structure in which measurement and detection are performed by a level difference among a plurality of wavelengths as a countermeasure when an optical output level is kept constant. 
     According to the above-described embodiments, it is possible to transmit a multiplexed signal obtained by multiplexing optical signals having different wavelengths to a subsequent stage with good quality. Thus, when the present embodiments are applied to transmission of a video signal, video watched by a viewer can be difficult to interrupt. In addition, an accuracy of switching determination of the transmission signal can be improved, so that it is possible to suppress unnecessary signal switching processing. 
     According to the above-described embodiments, the optical transmission apparatus includes an optical input unit, a demultiplexing unit, a measurement unit, and a switching unit. The optical input unit inputs a first multiplexed signal obtained by multiplexing a plurality of optical signals having different wavelengths and a second multiplexed signal obtained by multiplexing a plurality of the optical signals having different wavelengths. The demultiplexing unit demultiplexes the first multiplexed signal and the second multiplexed signal by wavelength. The measurement unit measures qualities of a plurality of optical signals obtained by demultiplexing the first multiplexed signal and qualities of a plurality of optical signals obtained by demultiplexing the second multiplexed signal. The switching unit performs switching between the first multiplexed signal and the second multiplexed signal to be output to a subsequent stage based on a result of measurement by the measurement unit. 
     The optical transmission apparatus may further include a multiplexing unit configured to perform processing of multiplexing a plurality of optical signals obtained by demultiplexing the first multiplexed signal after measurement by the measurement unit and outputting the multiplexed signal to the switching unit, and processing of demultiplexing a plurality of optical signals obtained by demultiplexing the second multiplexed signal after measurement by the measurement unit and outputting demultiplexed signals to the switching unit. 
     The optical transmission apparatus may further include a branching unit configured to perform processing of branching the first multiplexed signal input by the input unit and outputting the branched first multiplexed signal to the demultiplexing unit and the switching unit, and processing of branching the second multiplexed signal input by the input unit and outputting the branched second multiplexed signal to the demultiplexing unit and the switching unit. 
     The optical transmission apparatus may further include an instruction unit configured to instruct the switching unit to output the second multiplexed signal when a level of any of a plurality of optical signals obtained by demultiplexing the first multiplexed signal satisfies a condition indicating deterioration in quality, and instruct the switching unit to output the first multiplexed signal when a level of any of a plurality of optical signals obtained by demultiplexing the second multiplexed signal satisfies the condition indicating deterioration in quality. 
     The optical transmission apparatus may further include an instruction unit configured to instruct the switching unit to output the second multiplexed signal when a level difference between optical signals having adjacent wavelengths satisfies a condition indicating deterioration in quality among a plurality of optical signals obtained by demultiplexing the first multiplexed signal, and instruct the switching unit to output the first multiplexed signal when a level difference between optical signals having adjacent wavelengths satisfies the condition indicating deterioration in quality among a plurality of optical signals obtained by demultiplexing the second multiplexed signal. 
     Although embodiments of the present invention have been described above in detail with reference to the drawings, the specific configurations thereof are not limited to those of the embodiments and also include designs or the like without departing from the spirit of the present invention. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  900  Video communication network system 
           110 ,  111 ,  910 ,  911  Transmitter 
           120 ,  150 ,  930  WDM filter 
           130 ,  140 ,  920 ,  921  Transmission apparatus 
           160 ,  940  Access network 
           170 ,  171 ,  950 ,  951  Receiver 
           200  Optical transmission system 
           210 ,  211  Head end 
           220 ,  221 ,  230 ,  231  Optical transmission unit 
           240 ,  241 ,  350 ,  351  Multiplexing unit 
           245 ,  246  Transmission line 
           250 ,  251 ,  261 ,  262  Amplification-for-relay unit 
           260  Signal selection apparatus 
           263 ,  300 ,  400 ,  500 ,  600 ,  980  Signal selection unit 
           270  Amplification unit 
           280  Distribution unit 
           290  Optical reception unit 
           310 ,  311 ,  410 ,  411 ,  981 ,  982  Optical input unit 
           320 ,  321 ,  430 ,  431  Demultiplexing unit 
           330 ,  440 ,  520 ,  620  Storage unit 
           340 - 1  to  340 -N,  341 - 1  to  341 -N,  450 - 1  to  450 -N,  451 - 1  to  451 -N Measurement detection unit 
           360 ,  460 ,  540 ,  640 ,  985  Switching unit 
           370 ,  470 ,  986  Optical output unit 
           380 ,  480  Level adjustment unit 
           420 ,  421  Branching unit 
           510 - 1  to  510 -N,  511 - 1  to  511 -N,  610 - 1  to  610 -N,  611 - 1  to  611 -N,  3401 ,  3411 ,  4501 ,  4511  Measurement unit 
           530 ,  630  Calculation instruction unit 
           983 ,  984 ,  3402 ,  3412 ,  4502 ,  4512 ,  5302 ,  6302  Detection unit 
           3403 ,  3413 ,  4503 ,  4513 ,  5303 ,  6303  Instruction unit 
           5301 ,  6301  Calculation unit