Abstract:
A wavelength division multiplexing (WDM) transmission system for transmitting a wavelength division multiplexed signal light from a sender transmission apparatus to a receiver transmission apparatus is provided. The system comprises a computing unit that subtracts from a first optical signal noise ratio (OSNR) of the signal light measured by the receiver transmission apparatus a second OSNR ascribed to a sideband of the signal light measured by the sender transmission apparatus so as to compute a corrected OSNR of an amplified spontaneous emission (ASE) noise light with a reduction of an effect of the sideband.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wavelength division multiplexing transmission system, a wavelength division multiplexing transmission apparatus, and an optical signal noise ratio calculation method used for the system and apparatus. 
     2. Description of the Related Art 
     Due to an increased volume of communicated information, a high capacity and low cost optical fiber communication system has been developed. In such a system, a wavelength division multiplexing (WDM) method for multiplexing multiple wavelengths and transmitting the multiplexed signal has been adopted and the degree of wavelength-multiplexing tends to increase higher and higher in recent years. 
     Channel spacing, an index of the degree of wavelength-multiplexing, has been standardized by the International Telecommunications Union Telecommunications Standardization Sector (ITU-T). It is known that a standard WDM system wavelength-multiplexes a signal whose the signal transmission capacity per one channel is a 10-gigabit per second (Gbps) with a 100-gigahertz (GHz) (approximately 0.8 nano meters (nm)) spacing or a 50-GHz (approximately 0.4 nm) spacing. 
     In WDM systems, an erbium doped fiber amplifier (EDFA) is commonly used as a repeater to offset optical fiber line loss. In a system employing EDFA, amplified spontaneous emission (ASE) is generated becoming noise causing bit error rate (BER) increases. As such, the optical signal noise ratio (OSNR) evaluation becomes important. 
     Since the WDM system transmits multiple channels simultaneously, the OSNR for the receiving end (after transmission) of each channel differs for each channel. Additionally, the BER of each channel also varies. As such, the quality of transmission among channels becomes unequal. Hence, to optimize the transmission level of each channel such that the transmission quality becomes equivalent, a pre-emphasis method is commonly employed. 
     In the pre-emphasis process, since the level of each channel on the transmission side is determined based on the OSNR, the OSNR must be accurately measured. A method for measuring the spectrum of the signal and the ASE component by using a spectrum analyzer and calculating the OSNR based on the measurement results provides a relatively high accuracy. 
     The following patent document proposes an OSNR measurement apparatus and an OSNR measurement method that can accurately measure the OSNR in an optical fiber communication of high bit rate and high-density channel spacing. 
     [Patent document 1] Japanese Patent Application Laid-Open Publication No. 2008-85883. 
     In recent years, due to an increasing demand for a much larger amount of the transmission capacity, the transmission wavelength becomes narrower and the bit rate of the transmission wavelength becomes still higher. Hence, it is hard to acquire a wavelength point where only the noise component can be measured without suffering from the effect of the modulated sideband component of the transmission signal so that an accurate noise optical power cannot be obtained. Thus there exists such a problem that the OSNR cannot be accurately measured under these circumstances. 
     SUMMARY OF THE INVENTION 
     In this background, a general purpose of the present invention is to provide a transmission system or the like that can measure the OSNR accurately. 
     One embodiment of the present invention relates to a wavelength division multiplexing (WDM) transmission system for transmitting a wavelength division multiplexed signal light from a sender transmission apparatus to a receiver transmission apparatus. The system comprises: a computing unit that subtracts from a first optical signal noise ratio (OSNR) of the signal light measured by the receiver transmission apparatus a second OSNR ascribed to a sideband of the signal light measured by the sender transmission apparatus so as to compute a corrected OSNR of an amplified spontaneous emission (ASE) noise light with a reduction of an effect of the sideband. 
     Preferably, the sender transmission apparatus may comprise a wavelength multiplexer that wavelength-multiplexes the signal light; and an optical amplifier that amplifies the signal light wavelength-multiplexed by the wavelength-multiplexer. The sender transmission apparatus may measure the second OSNR from the signal light branched between the wavelength multiplexer and the optical amplifier and may notify the measured second OSNR to the receiver transmission apparatus. 
     Preferably, the receiver transmission apparatus may comprise a WDM monitor that measures the first OSNR of the signal light. The WDM monitor may subtract from the first OSNR the second OSNR notified from the sender transmission apparatus. 
     Preferably, the sender transmission apparatus may transmit the second OSNR to the receiver transmission apparatus via a transmission channel for transmitting the signal light. 
     The receiver transmission apparatus may comprise: a WDM monitor that measures the first OSNR of the signal light; and a monitor/control unit that is connected to a communication network. The monitor/control unit may subtract from the first OSNR provided by the WDM monitor the second OSNR notified by the sender transmission apparatus. 
     Preferably, the sender transmission apparatus may transmit the second OSNR to the receiver transmission apparatus via the communication network. 
     Preferably, the sender transmission apparatus may comprise: a measuring unit that measures the OSNRs′ that contains the ASE component of an optical amplifier based on an output signal light of the optical amplifier, which amplifies the wavelength multiplexed signal; and a computing unit that subtracts from the measured OSNRs′ an OSNRs″ that corresponds to the ASE component of the optical amplifier, which is obtained beforehand, so as to compute the second OSNR. 
     Preferably, the sender transmission apparatus may comprise: a WDM monitor that measures the OSNRs′ of the output signal light branched from an output terminal of the optical amplifier; and a storage unit that stores the OSNRs″. 
     Preferably, the OSNRs″ may be a value measured beforehand and stored in the storage unit at the time of manufacturing the optical amplifier. 
     Preferably, the storage unit may store OSNRs″n_min and OSNRs″n_max measured beforehand. The sender transmission apparatus computes the OSNRS″ based on the OSNRs″n_min and OSNRs″n_max stored in the storage unit. The OSNRs″n_min and the OSNRs″n_max are the values of the OSNR of channel n measured at an output terminal of the optical amplifier when a light source of a wavelength of the channel n is input to the optical amplifier at a minimum input optical power and a maximum input optical power, respectively. 
     Another embodiment of the present invention relates to an optical signal noise ratio calculation method employed in a wavelength division multiplexing (WDM) transmission system for transmitting a wavelength division multiplexed signal light from a sender transmission apparatus to a receiver transmission apparatus. The method comprises: measuring a first optical signal noise ratio (OSNR) of the signal light in the receiver transmission apparatus; measuring a second OSNR ascribed to a sideband of the signal light in the sender transmission apparatus; and subtracting the second OSNR from the first OSNR so as to compute a corrected OSNR of an amplified spontaneous emission (ASE) noise light with a reduction of an effect of the sideband. 
     Preferably, the method may further comprises: measuring the second OSNR from the signal light branched between a wavelength multiplexer and an optical amplifier in the sender transmission apparatus, the wavelength multiplexer wavelength-multiplexing the signal light, the optical amplifier amplifying the signal light wavelength-multiplexed by the wavelength-multiplexer; and notifying the measured second OSNR to the receiver transmission apparatus. 
     Yet another embodiment of the present invention relates to a wavelength division multiplexing (WDM) transmission apparatus for transmitting a wavelength division multiplexed signal light. The apparatus comprises: a wavelength multiplexer that wavelength-multiplexes a signal light; an optical amplifier that amplifies the signal light wavelength-multiplexed by the wavelength multiplexer; an optical branching unit that branches the signal light between the wavelength multiplexer and the optical amplifier; and a WDM monitor that measures an optical signal noise ratio of the signal light branched by the optical branching unit. The apparatus notifies the optical signal noise ratio measured by the WDM monitor to a destination of the signal light. 
     Yet another embodiment of the present invention relates to a wavelength division multiplexing (WDM) transmission apparatus for transmitting a wavelength division multiplexed signal light. The apparatus comprises: an optical amplifier that amplifies a transmitted signal light; an optical branching unit that branches the signal light amplified by the optical amplifier; and a WDM monitor that measures a first optical signal noise ratio (OSNR) of the signal light branched by the optical branching unit. The apparatus subtracts from the first OSNR measured by the WDM monitor a second OSNR ascribed to a sideband of the signal light provided from the source of the signal light so as to compute an OSNR of an ASE noise light with a reduction of an effect of the sideband. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of examples only, with reference to the accompanying drawings, which are meant to be exemplary, not limiting and wherein like elements are numbered alike in several Figures in which: 
         FIG. 1A  is a conceptual diagram illustrating a typical example of a wavelength-multiplexed signal light spectrum when a control/monitor light does not exist; 
         FIG. 1B  is a conceptual diagram illustrating a typical example of a wavelength-multiplexed signal light spectrum when a control/monitor light exists; 
         FIG. 2  is a block diagram illustrating a WDM transmission system that transmits a signal light having no control/monitor light; 
         FIG. 3  is a block diagram illustrating a structure of a WDM transmission system that transmits a signal light having a control/monitor light; 
         FIG. 4  is a block diagram illustrating a structure of a WDM transmission system according to an embodiment of the present invention; 
         FIG. 5  is a block diagram illustrating a structure of a WDM transmission system according to another embodiment of the present invention; 
         FIG. 6  is a block diagram illustrating a structure of a WDM transmission system according to yet another embodiment of the present invention; 
         FIG. 7  is a block diagram illustrating a structure of a WDM monitor that performs an OSNR measurement for a WDM signal light; 
         FIGS. 8A and 8B  illustrate a typical example of how to compute OSNR from a computed optical power distribution; 
         FIGS. 9A and 9B  illustrate OSNR measurement error ascribed to narrow wavelength spacing; 
         FIGS. 10A and 10B  illustrate the OSNR measurement error ascribed to the high bit rate; 
         FIGS. 11A and 11B  illustrate OSNRsn information detected by a transmission apparatus at a sending site; 
         FIGS. 12A and 12B  illustrate how the difference between the OSNRsn information and the OSNRrn information is detected; 
         FIG. 13  is a flow diagram illustrating a process for computing corrected OSNRn information in a WDM transmission system; 
         FIG. 14  is a block diagram illustrating a typical structural example of a WDM monitor; 
         FIG. 15  is a block diagram illustrating a WDM transmission system where a light branched from at an output terminal of an optical amplifier is input to a WDM monitor; 
         FIG. 16  is a flowchart illustrating a measurement process and a storing process of OSNRs″n_min and OSNRs″n_max information; 
         FIG. 17  is a flowchart showing an example of how to compute OSNRs″n; and 
         FIG. 18  is a block diagram illustrating a structure of an input/output optical power monitor of an optical amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     Before explaining some characteristic structures of the embodiments, the structure and function of the WDM transmission system is briefly explained.  FIG. 2  and  FIG. 3  show a structural example of the WDM transmission system that handles the wavelength-multiplexed signal as shown in  FIGS. 1A and 1B .  FIG. 1A  is a conceptual diagram illustrating a typical example of the wavelength-multiplexed signal light spectrum when a control/monitor light (it is also referred to as a control/monitor channel) does not exist.  FIG. 1B  is a conceptual diagram illustrating a typical example of the wavelength-multiplexed signal light spectrum when a control/monitor light exists. 
     As shown in  FIG. 1 , the signal light is a multiplexed light of n wavelengths from channel  1  to channel n (typically n=40). The optical power of the signal light has an ASE noise light superimposed therein, which is mainly caused by the optical amplifier. The control/monitor light has a wavelength different from the normal signal light of n wavelengths from channel  1  to channel n. Various types of control information are attached to the control/monitor light, such as the number of wavelengths of the optical amplifier and the level control or the like related to the WDM transmission apparatus. 
       FIG. 2  is a block diagram illustrating the WDM transmission system  2000  that transmits the signal light having no control/monitor light. The WDM transmission system  2000  includes the transmission apparatus  2100  of a sending site, the transmission apparatus  2200  of a relay site and the transmission apparatus  2300  of a receiving site. The transmission apparatus  2200  of the relay site is provided for compensating the degradation of the strength level of the signal light in the event of the long transmission distance. 
     The transmission apparatus  2100  of the sending site has a plurality of transponders  2110 : the transponder  2110 ( 1 ) to the transponder  2110 ( n ) corresponding to wavelengths to be multiplexed. The plurality of transponders  2110  wavelength-converts so-called a SDH signal and/or an Ethernet signal into a WDM signal. 
     The wavelength-converted signal light is multiplexed by the wavelength multiplexer  2120  and amplified by the optical amplifier  2130 . A part of the signal light amplified by the optical amplifier  2130  is branched by the coupler  2140  and the WDM monitor  2150  obtains the OSNR information (it is also referred to as the optical signal noise ratio information) from the branched signal light. The monitor/control unit  2160  receives an input of the OSNR information and notifies the OSNR information via the network  2400  to the control/monitor center  2500  that monitors and controls a plurality of sites. 
     On the other hand, the signal light that is not branched by the coupler  2140  is transmitted to the transmission apparatus  2200  of the relay site via the fiber transmission channel  2600 . The transmission apparatus  2200  of the relay site amplifies the transmitted signal light at the optical amplifier  2230  and transmits the amplified signal light to the transmission apparatus  2300  of the receiving site via the fiber transmission channel  2700 . 
     A part of the signal light amplified by the optical amplifier  2230  is branched by the coupler  2240  and input to the WDM monitor  2250 . Then the WDM monitor  2250  obtains the OSNR information from the branched signal light. The monitor/control unit  2260  receives an input of the OSNR information and notifies the OSNR information via the network  2400  to the control/monitor center  2500  that monitors and controls a plurality of sites. 
     The signal light transmitted to the transmission apparatus  2300  of the receiving site via the fiber transmission channel  2700  is amplified by the optical amplifier  2330  and input to the wavelength multiplexer  2320  through the coupler  2340 . The signal light demultiplexed by the wavelength multiplexer  2320  is input to a transponder (not shown) for each wavelength (each channel). 
     The WDM monitor  2350  obtains the OSNR information from the signal light branched from the coupler  2340 . The monitor/control unit  2360  notifies the OSNR information via the network  2400  to the control/monitor center  2500  that monitors and controls a plurality of sites. 
       FIG. 3  is a block diagram illustrating a structure of the WDM transmission system  3000  that transmits the signal light having a control/monitor light. The same reference numerals used in  FIG. 3  denote the same elements of the WDM transmission system  2000  and thus a detailed explanation of such elements is omitted. 
     The difference between the WDM transmission system  3000  and the WDM transmission system  2000  is mainly in that the former has a function for controlling communication between the sites by using a control/monitor light but the latter does not. The control/monitor light is used for transmitting management information for monitoring the WDM system and control information for the optical amplifier or the like. 
     For this purpose, each transmission apparatus  2100 ,  2200 ,  2300  of the WDM transmission system  3000  is provided with the control/monitor light transmission unit  2170 ,  2270 ,  2370  for transmitting the control/monitor light between the transmission apparatuses. Each transmission apparatus  2100 ,  2200 ,  2300  of the WDM transmission system  3000  is also provided with the filter (FIL)  2180 ,  2280 ,  2290 ,  2390  for branching and superimposing the control/monitor light between the transmission apparatuses. 
     As described above, the WDM transmission systems  2000 ,  3000  are provided with the WDM monitors  2150 ,  2250 ,  2350  for each site, which measures the OSNR for each wavelength of the amplified signal light. The control/monitor center  2500  monitors the OSNR information of all the sites  2100 ,  2200 ,  2300  received from the monitor/control units  2160 ,  2260 ,  2360  via the network  2400 . 
     The purpose of monitoring by the control/monitor center  2500  is to prevent the errors in the main signal while monitoring the deterioration of the OSNR. Namely, by monitoring the deterioration status of the OSNR, typically, the status of the ASE noise light, the control/monitor center  2500  not only can detect and prevent the communication failure but also can analyze the cause of the communication failure if it occurs. In other words, the OSNR can be utilized as data for checking the transmission quality of each wavelength. 
       FIG. 7  is a block diagram illustrating a typical structure of the WDM monitors  2150 ,  2250 ,  2350  that perform the OSNR measurement for the WDM signal light. The WDM monitor  700  shown in  FIG. 7  emits the signal light subjected to spatial dispersion by a spectroscope  710  to a photodiode (PD) array  720  and detects the optical power for every constant wavelength interval (for each channel). 
     The optical power photoelectric-converted by the PD array  720  is current-voltage converted by a current/voltage converter  730  and further converted into a digital signal by an A/D converter  740  and the resultant signal is input to a computing unit  750 . The computing unit  750  computes an optical power distribution (spectrum) by applying a curve approximation to digital values of the detected optical power. 
     The structure of the WDM monitor  700  is not limited to the above-mentioned configuration. For instance, the WDM monitor  700  may be configured by using the spectroscope  710 , a wavelength-variable filter (e.g., spectrum analyzer), and a single PD.  FIGS. 8A and 8B  illustrate a typical example of how to compute the OSNR from the optical power distribution (spectrum) computed as described above. The signal light as shown in  FIG. 8A  can be measured and computed by the WDM monitor  700  as shown  FIG. 8B . 
     As shown in  FIG. 8B , the peak optical power S[dBm] for each wavelength and the noise optical power N 1 , N 2 [dBm] are computed. The noise optical power is detected at such a location that is distant by a constant wavelength from the both end of the peak wavelength, preferably, at the middle point of the wavelength grid or a location close thereto. The OSNR is computed according to the following expression (1), by using the correction coefficient C[dB] that is specific to the measurement device in order to make the noise light lower bandwidth to be 0.1 nm.
 
OSNR= S− 10×log((10 − ( N 1/10)+10 − ( N 2/10))/2)+ C   (1)
 
     The WDM transmission systems  2000 ,  3000  do not have a means for correcting the OSNR by canceling the OSNR measurement error ascribed to the narrow wavelength spacing as shown in  FIGS. 9A and 9B  and/or the OSNR measurement error ascribed to the high bit rate as shown in  FIGS. 10A and 10B .  FIGS. 9A and 9B  illustrate the OSNR measurement error ascribed to the narrow wavelength spacing and  FIGS. 10A and 10B  illustrate the OSNR measurement error ascribed to the high bit rate. 
     Due to the increase in the amount of communicated information, the density of the wavelengths increases from 40 waves to 80 waves and the bit rate increases from 10 Gbps to 40 Gbps. The WDM transmission systems need to cope with the high density and high bit rate of the signal light. It is noted that when the bit rate rises from 10 Gbps to 40 Gbps, the modulation frequency becomes high. 
     [First Embodiment] 
       FIG. 4  to  FIG. 6  are block diagrams illustrating the structure of WDM transmission systems according to some embodiments of the present invention. The same reference numerals used in  FIG. 4  to  FIG. 6  denote the same elements in  FIG. 2  and  FIG. 3  and thus a detailed explanation of such elements is omitted. In the WDM transmission systems shown in  FIG. 4  to  FIG. 6 , a wavelength multiplexer (also referred to as a mux/demux) at a sender site wavelength-multiplexes each signal light from transponders, and a coupler (also referred to as an optical branching unit), which is arranged in front of an optical fiber, branches the wavelength-multiplexed optical signal to give the branched signal to a WDM monitor. 
     In the WDM transmission system  4000  in  FIG. 4 , the transmission apparatus  4100  at the sender site obtains OSNRsn information from the signal light branched by the coupler  2140  in front of the optical amplifier  2130 . Hereinafter, the lowercase character “s” indicates a sender and the lowercase character “n” indicates the number of channels (wavelengths). More specifically, the WDM monitor  2150  obtains the OSNRsn information from the signal light branched by the coupler  2140 . The control/monitor transmission unit  4170  adds the OSNRsn information obtained by the WDM monitor  2150  to a control/monitor light. The FIL  2180  further adds the control/monitor light having the OSNRsn information to the signal light and then transmit the resultant signal light. 
     The OSNRsn measured by the WDM monitor  2150  is the OSNR information of OSNRs 1  to OSNRs 40 , if the number of the wavelength multiplexing is 40. The measurement unit of OSNRsn value is dB. The letter “n” indicates the channel number: wavelength  1  to wavelength n. The OSNRsn information measured by the WDM monitor  2150  does not include the ASE noise light component of the optical amplifier, as shown in  FIGS. 11A and 11B . Therefore, the measured value corresponds to the OSNR information in which the sideband component of the transmission signal is digitalized.  FIGS. 11A and 11B  illustrate the OSNRsn information detected by the transmission apparatus  4100  at the sending site. 
     In the transmission apparatus  4200  at a relay site, the control/monitor light transmission unit  4270  receives the OSNRrn information and further transmits it to the downstream transmission apparatus  4300  of a receiving site. In addition, the transmission apparatus  4200  at the relay site notifies the OSNRrn information to its own WDM monitor  4250 . 
     The WDM monitor  4250  computes a corrected OSNR, in which the effect of the sideband is reduced or eliminated, based on both the notified OSNRsn information and the OSNRrn information measured from the signal light branched by the coupler  2240 . Hereinafter, the lowercase character “r” indicates a receiver. The computed corrected OSNR mainly reflects the effect by the ASE noise signal of the optical amplifier  2130 . By using the corrected OSNR, the WDM monitor  4250  can obtain an accurate noise component. The monitor/control unit  2260  notifies the corrected OSNR to the control/monitor center  2500  via the network  2400 . 
     The transmission apparatus  4300  of the receiving site only notifies the OSNRsn information received in the control/monitor light transmission unit  4370  to its own WDM monitor  4350  but does not forward the information downstream. Therefore, the downstream transfer process is not required. 
     The WDM monitor  4350  computes a corrected OSNR, in which the effect of the sideband is reduced or eliminated, based on both the notified OSNRsn information and the OSNRrn information measured from the signal light branched by the coupler  2340 . The computed corrected OSNR mainly reflects the effect by the ASE noise signal of both the optical amplifier  2130  and the optical amplifier  2230 . By using the corrected OSNR, the WDM monitor  4350  can obtain an accurate noise component. The monitor/control unit  2360  notifies the corrected OSNR to the control/monitor center  2500  via the network  2400 . 
     In the transmission apparatus  4200  of the relay site and the transmission apparatus  4300  of the receiving site, the couplers  2240 ,  2340  are provided at the output terminal of the optical amplifiers  2230 ,  2330  respectively and the WDM monitors  4250 ,  4350  are assigned to the branch destination of the couplers  2240 ,  2340  respectively. The WDM monitor  4250 ,  4350  may measure OSNR according to any well-known methods and the measured value is OSNRrn, where the measurement unit is dB and “n” indicates the channel number: wavelength  1  to wavelength n. In this case, the couplers (optical branching units)  2240 ,  2340  may be provided in front of the optical amplifiers  2230 ,  2330 ; however there is concern that the OSNR characteristics could deteriorate to a certain degree. 
     As described above, there is a difference, as shown in  FIGS. 12A and 12B , between the OSNRsn information and the OSNRrn information, the former is received by the WDM monitors  4250 ,  4350  of the transmission apparatus  4200  of the relay site and the transmission apparatus  4300  of the receiving site and the latter is measured by the same. The difference is the ASE noise component.  FIGS. 12A and 12B  illustrate how the difference between the OSNRsn information and the OSNRrn information is detected. 
     Hence, the WDM monitors  4250 ,  4350  can compute a relatively accurate corrected OSNRn in which the modulated sideband component of each channel has been eliminated according to the following expression (2), wherein the measurement unit is dB and “n” is the channel number (1 to the number of wavelengths).
 
OSNR n=− 10×log(10 (−OSRNrn/10) −10 (−OSNRsn/10) )  (2)
 
WHERE n IS THE CHANNEL NUMBER.
 
     The monitor/control unit  2260  notifies the corrected OSNRn information computed for each channel according to the above-mentioned expression (2) to the control/monitor center  2500  via the network  2400 . With reference to  FIG. 13 , a flow of the computation of the corrected OSNRn information executed by the WDM transmission system  4000  is explained.  FIG. 13  is a flow diagram illustrating the process for computing the corrected OSNRn information in the WDM transmission system  4000 . 
     (Step S 130 ) 
     In the transmission apparatus  4100  of the sending site, the WDM monitor  2150  performs OSNRsn measurement based on the WDM signal optically branched by the coupler  2140  in front of the optical amplifier  2130  and obtains the digitalized OSNRsn information. The WDM monitor  2150  then notifies the digitalized OSNRsn information to the control/monitor light transmission unit  4170 . 
     (Step S 140 ) 
     The control/monitor light transmission unit  4170  sends the OSNRsn information notified from the WDM monitor  2150  to the downstream transmission apparatus  4200  of the relay site by a communication function using the control/monitor light. 
     It is preferable that the WDM monitor  2150  should coordinate the notification of the OSNRsn information with the OSNR measurement cycle (for instance several tens of milliseconds to hundreds of milliseconds) and should continuously notify the updated OSNRsn information at all times. Thereby, the WDM transmission system  4000  can monitor the up-to-date transmission quality. 
     (Step S 150 ) 
     If the destination of the notified OSNRsn information is the transmission apparatus  4200  of the relay site, Step S 160  is performed, but if not, Step S 170  is performed instead. 
     (Step S 160 ) 
     The transmission apparatus  4200  of the relay site transfers the notified OSNRsn information to the downstream receiving site without modification. If there is another relay site downstream, the transmission apparatus  4200  of the relay site transfers it to the downstream relay site. More specifically, the control/monitor light transmission unit  4270  transfers the notified OSNRsn information downstream. 
     (Step S 170 ) 
     The control/monitor light transmission unit  4270  notifies the notified OSNRsn information to the WDM monitor  4250 . It is noted that Step  160  and Step S 170  could be executed in any order and the control/monitor light transmission unit  4270  might perform Step  160  and Step  170  simultaneously. 
     (Step S 180 ) 
     The WDM monitor  4250  performs OSNR measurement based on the signal light branched by the coupler  2240  to obtain the digitized OSNRrn information. 
     (Step S 190 ) 
     The WDM monitor  4250  computes the corrected OSNRn information by performing the calculation of the above-mentioned expression (2). The WDM monitor  4250  may be configured, for instance, as a structure explained in  FIG. 14 .  FIG. 14  is a block diagram illustrating a typical structural example of the WDM monitor  4250 . 
     More specifically, the computing unit  4252  performs the calculation process of the above-mentioned expression (2) using both the OSNRrn information, which the OSNRrn measuring unit  4251  has obtained and digitalized through the OSNR measurement based on the signal light branched from the coupler  2240 , and the OSNRsn information provided from the control/monitor light transmission unit  4270 , so as to obtain the corrected OSNRn information. 
     (Step S 200 ) 
     The WDM monitor  4250  notifies the computed corrected OSNRn information to the monitor/control unit  2260 . The monitor/control unit  2260  notifies the computed corrected OSNRn information to the control/monitor center  2500  via the network  2400 . 
     [Second Embodiment] 
     The WDM transmission systems  4000  and  5000  shown in  FIG. 4  and  FIG. 5  respectively are of a typical structure for notifying the OSNRsn information to the downstream transmission apparatus  4200 ,  4300  of the relay site or the receiving site by using the control/monitor light. On the other hand, the WDM transmission system  6000  of  FIG. 6  is of a typical structure for notifying the OSNRsn information to the downstream relay site or receiving site via the monitor/control units  2260 ,  2360  and the network  240 . 
     The WDM transmission system  5000  of  FIG. 5  differs from the WDM transmission system  4000  in that the OSNRsn information received via the control/monitor light transmission units  4170  and  4270  is notified to the monitor/control unit  2260  and the process for computing the corrected OSNRn is performed in the monitor/control unit  2260 . 
     In the WDM transmission system  5000 , the WDM monitor  4250  is not notified of the OSNRsn information and the WDM monitor  4250  does not compute the corrected OSNRn according to the above-mentioned expression (2). It is noted that the structure and process of the WDM transmission system  5000  is the same as those of the WDM transmission system  4000  in that the control/monitor light transmission unit  4270  of the transmission apparatus  4200  of the relay site transfers the OSNRsn information, and thus a detailed explanation thereof is omitted. 
     The WDM transmission system  6000  of  FIG. 6  differs from the WDM transmission systems  4000  and  5000  in that the monitor/control units  2260  and  2360  in the transmission apparatuses  4200  and  4300  of both the relay site and the receiving site receive the OSNRsn information via the network  2400 . The WDM transmission system  6000  is the same as the WDM transmission system  5000  in that the monitor/control units  2260  and  2360  computes the corrected OSNRn according to the above-mentioned expression (2). 
     In the WDM transmission system  6000 , the control/monitor light transmission unit  4170  does not embed the OSNRsn information notified from the WDM monitor  2150  into the control/monitor light, but notifies it to the monitor/control unit  2160  instead. The monitor/control unit  2160  further notifies the notified OSNRsn information to the transmission apparatus  4200  of the relay site and the transmission apparatus  4300  of the receiving site via the network  2400 . Thus, the WDM transmission system  6000  is preferably applicable to a transmission system where communication is performed by a signal light with no control/monitor light. 
     On the other hand, according to the WDM transmission system  4000 , since the transmitted signal light with the addition of the OSNRsn information is transmitted via the same fiber transmission channel, the failure of the signal light and the notification failure of the OSNRsn information could be synchronized. Therefore, it can preferably prevent such a situation that the OSNRsn information is not notified, despite the fact that the signal light is properly transmitted. 
     [Third Embodiment] 
       FIG. 15  is a block diagram illustrating the WDM transmission system  14000  where the light is branched from the coupler  2140  attached to the output terminal of the optical amplifier  2130  in the transmission apparatus e 100  of the sending site and the branched light is input to the WDM monitor  2150 . The WDM transmission system  14000  is preferable in that the common arrangement and structure of the optical amplifiers  2230 ,  2330  and the couplers  2240 ,  2340  in the transmission apparatuses e 200 , e 300  of the relay site and the receiving site can be adopted. 
     In the WDM transmission system  14000  shown in  FIG. 15 , the OSNRs′n measured by the WDM monitor  2150  contains the ASE component of the optical amplifier  2130  that corresponds to a sending amplifier. The measurement unit is dB and “n” indicates the channel number (1 to the number of the wavelengths). 
     Therefore, the value of OSNR″n that corresponds to the ASE component of the optical amplifier  2130  is subtracted from the value of OSNRs′n measured by the WDM monitor  2150  so as to compute the sideband component OSNRsn of the main signal as follows. 
                           Bn   =         10   ⋀     ⁢     (       -     OSNRs   ′       ⁢     n   /   10       )       -       10   ⋀     ⁢     (       -     OSNRs   ′′       ⁢     n   /   10       )                     OSNRsn   =       -   10     ×     log   ⁡     (   Bn   )                 }           (   3   )               
WHERE n IS THE CHANNEL NUMBER.
 
     The value of OSNRs″n is calculated from OSNRs″n_min information and OSNRs″n_max information, which has been measured beforehand and stored in the optical amplifier  2130  at the time of manufacturing the optical amplifier  2130 . 
     The OSNRs″n_min and OSNRs″n_max information are stored in the optical amplifier  2130  according to the flow described in  FIG. 16 . For this purpose, the optical amplifier  2130  may have a memory for storing the OSNRs″n_min and OSNRs″n_max information.  FIG. 16  is a flowchart illustrating the measurement process and the storing process of the OSNRs″n_min and OSNRs″n_max information. 
     (Step S 1510 ) 
     At the time of manufacture, the light source of the wavelength of the channel  1  is input to the optical amplifier  2130  in the transmission apparatus e 100  of the sending site at the minimum input optical power Pmin[dB]. The OSNR of channel  1  at the output of the optical amplifier  2130  is measured by using the light spectrum analyzer measurement device. 
     Then the value measured by the light spectrum analyzer measurement device is stored as OSNRs″ 1 _min[dB] information in a storage unit such as a flash memory provided in the optical amplifier  2130 . 
     (Step S 1520 ) 
     The light source of the wavelength of the channel  1  is input to the optical amplifier  2130  in the transmission apparatus e 100  of the sending site at the maximum input optical power Pmax[dB]. The OSNR of channel  1  at the output of the optical amplifier  2130  is measured by the light spectrum analyzer measurement device. 
     Then the value measured by the light spectrum analyzer measurement device is stored as the OSNRs″ 1 _max[dB] information in a storage unit such as a flash memory provided in the optical amplifier  2130 . 
     (Step S 1530 ) 
     The measurement of the above-mentioned steps S 1510  and S 1520  is repeatedly performed for each channel from the channel  2  to n. Thus, OSNRs″ 1 _min[dB] to OSNRs″n_min[dB] and OSNRs″ 1 _min[dB] to OSNRs″n_min[dB] are stored in a storage unit in the optical amplifier  2130 . 
     The optical amplifier  2130  notifies the OSNRs″n_min and OSNRs″n_max stored beforehand to the WDM monitor  2150 . Upon receiving the notification from the optical amplifier  2130 , the WDM monitor  2150  can compute the OSNRs″n according to the flow described in  FIG. 17 .  FIG. 17  is a flowchart showing an example of how to compute the OSNRs″n. 
     In the WDM transmission system  14000 , the optical input level is normally within the range of plus or minus 3 dB. For this reason, it is possible to precisely compute the value of the OSNRs″n by using a linear approximation described in the process flow explained with reference to  FIG. 17 . Alternatively, the computation is performed by using a polynomial approximation based on the measured OSNR of a plurality of input levels. 
     (Step S 1610 ) 
     The optical amplifier  2130  notifies the total optical input power value Pin_total[dBm] of the optical amplifier  2130  in the transmission apparatus e 100  of the sending site to the WDM monitor  2150 . 
     (Step S 1620 ) 
     The WDM monitor  2150  in the transmission apparatus e 100  of the sending site measures the peak power Pout_n[dBm] for each wavelength and computes the averaged optical power Pave according to the following expression (4). 
     
       
         
           
             
               
                 
                   
                     Pave 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       dBm 
                       ) 
                     
                   
                   = 
                   
                     10 
                     × 
                     
                       log 
                       ( 
                       
                         
                           ( 
                           
                             1 
                             / 
                             n 
                           
                           ) 
                         
                         × 
                         
                           ( 
                           
                             
                               ∑ 
                               
                                 n 
                                 = 
                                 1 
                               
                               
                                 
                                   NUMBER 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   OF 
                                 
                                 ⁢ 
                                 
                                   
 
                                 
                                 ⁢ 
                                 WAVELENGTH 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 10 
                                 ⋀ 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   Pout_n 
                                   / 
                                   10 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     (Step S 1630 ) 
     By substituting the total optical input power Pin_total[dBm], the peak power Pout_n[dBm] of each wavelength, and the average optical power Pave into the following expression (5), the optical power value Pin_n[dBm] of each channel input to the optical amplifier  2130  in the transmission apparatus e 100  is calculated.
 
 P in —   n  (dBm)= P in_out —   n−P ave+ P in_total−10×log (NUMBER OF WAVELENGTH)   (5)
 
     (Step S 1640 ) 
     By using OSNRs″n_min[dB] and OSNR″n_max[dB] stored beforehand in the optical amplifier  2130 , the WDM transmission system  14000  calculates the OSNRs″n[dB] of each cannel as follows.
 
OSNR s″n =OSNR s″n _min+(OSNR s″n _max−OSNR s″n _min)×(( P in —   n−P min)/( P max− P min))  (6)
 
     It is noted that the Pmin[dB], Pmax[dB] of  FIG. 16  and Pin_n[dBm] in Step S 1630  are selected to meet Pmin&lt;Pin_n&lt;Pmax. 
     As shown in  FIG. 18 , the optical amplifier  2130  generally includes an input-terminal optical coupler  1810  arranged at the input terminal and an output-terminal optical coupler  1820  arranged at the output terminal. Furthermore, the optical amplifier  2130  includes photo diodes (PD)  1830 ,  1840  arranged at the branch terminal of the input-terminal optical coupler  1810  and the output-terminal optical coupler  1820  so that the amplifier has a function for photoelectric-converting the total optical power including the wavelength-multiplexed optical signal and the ASE noise into the electrical signal and monitoring the signal.  FIG. 18  is a block diagram illustrating a structure of the input/output optical power monitor of the optical amplifier  2130 . 
     By using the function for monitoring the total optical power, the optical amplifier  2130  measures the Pin_total in the process flow explained with reference to  FIG. 17 . Furthermore, the WDM transmission system  14000  determines how the input power to the optical amplifier  2130  varies channel by channel by alternatively evaluating the output power of the optical amplifier for each channel. 
     For this reason, the value of the OSNRs″n of each channel will have a slight error. Therefore, the WDM transmission systems  4000 ,  5000  and  6000  explained with reference to  FIG. 4 ,  FIG. 5  and  FIG. 6  are preferable in that they can measure the OSNRs″n more precisely than the WDM transmission system  14000 . 
     Alternatively, although it is not explained above, the calculation of the OSNRs″n may be performed in the monitor/control unit  2160 , but such calculation is not restricted only to the monitor/control unit  2160 . The calculation may be performed in any other units of the WDM transmission system  14000 . 
     The structure of the WDM transmission system  14000  where the transmission apparatus e 200  of the relay site and/or the transmission apparatus e 300  of the receiving site compute the corrected OSNRn from the OSNRsn calculated as described above is the same as that of the WDM transmission systems  4000 ,  5000 ,  6000  with reference to  FIG. 4  to  FIG. 6 . 
     The method in which the transmission apparatus e 200  of the relay site and/or the transmission apparatus e 300  of the receiving site in the WDM transmission system  14000  compute the corrected OSNRn from the OSNRsn is realized as a method similar to the process flow explained with reference to  FIG. 13 . To avoid repetition in a description of the structure of the WDM transmission system  14000 , the same reference numerals denote the elements corresponding to the WDM transmission system  6000  or the like and thus a detailed explanation of such elements is omitted. 
     The WDM transmission systems illustrated in the embodiments can measure the OSNR of each wavelength relatively precisely even when the transmission wavelength becomes narrower and the bit rate of the transmission wavelength becomes higher. Furthermore, even when the ASE noise light and the sideband are not distinguishable on the display monitor, more accurate noise component can be detected by using the above-mentioned structure and process. 
     The WDM transmission system and the WDM transmission apparatus according to the present invention are not restricted only to the description of the embodiments. It is understood by those skilled in the art that various modifications to the combination of each component and process thereof are possible and that such modifications are also within the scope of the present invention.