Abstract:
According to an aspect of an embodiment, a signal detector device includes a first monitor unit, a second monitor unit, and a discriminator unit. 
     The discriminator unit discriminates whether an inputted light includes a signal light on the bases of the first monitor unit for monitoring an intensity of the inputted light and the second monitor unit for monitoring an alternating current component intensity of the inputted light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-087848, filed on Mar. 29, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present invention relates to an input signal detection device for detecting whether or not an optical signal has been inputted to an optical device such as an optical amplifier in an optical communication system and an optical device control apparatus using the result of the detection. 
         [0003]      FIG. 28  shows a configuration example of a related art optical communication system using the wavelength division multiplexing (WDM) technology. This optical communication system includes an optical transmission device  11 , transmission line fibers  12 ,  14 , and  16 , optical amplification relays  13  and  15 , and an optical reception device  17 . 
         [0004]    Among these, the optical transmission device  11  includes optical transmitters  21 - 1  to  21 - n , an optical multiplexer  22 , and an optical amplifier  23 . The optical reception device  17  includes an optical amplifier  24 , an optical demultiplexer  25 , optical amplifiers  26 - 1  to  26 - n , variable wavelength dispersion compensators  27 - 1  to  27 - n , and optical receivers  28 - 1  to  28 - n . The optical amplification relays  13  and  15  and the optical amplifiers  23  and  24  each amplify a WDM signal as a single unit, while the optical amplifiers  26 - 1  to  26 - n  each amplify an optical signal of one wavelength. 
         [0005]    One of optical amplifiers that are currently most widely used is an erbium-doped fiber amplifier (EDFA) that uses an induced emission of a rare earth element, erbium, which is added to the core of an optical fiber. The optical amplification relays  13  and  15  amplify optical signals that have been transmitted through the transmission line fibers  12  and  14 , respectively, and have reduced their power. 
         [0006]    At this time, simultaneously with the amplification of each optical signal, an amplified spontaneous emission (ASE) occurs that has a random amplitude, phase, polarized wave, and the like due to the induced emission. Thus, the optical signal to noise ratio (OSNR) is deteriorated. This ASE is amplified and accumulated each time it passes through an optical amplification relay, and is finally inputted to the optical reception device  17  together with an optical signal. 
         [0007]    In an example shown in  FIG. 28 , light including a WDM signal  31  and an ASE  32  is outputted from the optical transmission device  11 . Then, light including a WDM signal  33  and an ASE  34  is inputted to the optical demultiplexer  25  of the optical reception device  17  and light including an optical signal  35  of one wavelength and an ASE  36  is inputted to the optical amplifier  26 - 2 . 
         [0008]    A tolerance to wavelength dispersion is significantly reduced in a high-speed optical transmission system having a transmission speed per wavelength of 40 Gbit/s; therefore, a highly accurate wavelength dispersion compensation is needed. For this reason, the variable dispersion compensators  27 - 1  to  27 - n  are provided in the optical reception device  17 . This allows a highly accurate wavelength dispersion compensation for each channel, as well as allows constant optimization of the amount of dispersion compensation while following temporal variations in wavelength dispersion value with time during operation of the system. Also, if signal quality significantly deteriorates due to polarization mode dispersion (PMD), a PMD compensator may be disposed between the optical demultiplexer  25  and the optical receivers  28 - 1  to  28 - n  in order to compensate for such deterioration. 
         [0009]    However, application of the variable wavelength dispersion compensators  27 - 1  to  27 - n  or the PMD compensator may increase optical loss, thereby causing lack of light power over the input dynamic ranges of the optical receivers  28 - 1  to  28 - n  that are disposed after these components. In this case, input power to the optical receivers  28 - 1  to  28 - n  is secured by amplifying the optical signals using the optical amplifiers  26 - 1  to  26 - n.    
         [0010]      FIG. 29  shows a system for controlling such an optical amplifier for loss compensation. An optical amplifier  42  amplifier is provided before an optical receiver  43  so as to amplify input light. An optical coupler  41  and a photodiode (PD)  44  are provided on the input side of the optical amplifier  42  so as to monitor input light. According to a monitor signal from the PD  44 , a processor  45  determines whether or not an optical signal has been inputted. The controller  46  controls operations of the optical amplifier  42  according to the result of the determination. 
         [0011]    As shown in  FIG. 30 , the processor  45  sets a shutdown threshold Pth of light power near the lower limit value of the signal input range. If monitored light power is higher than the Pth, the processor  45  determines that a signal has been inputted. If monitored light power is lower than the Pth, it determines that no signal has been inputted. If a signal has been inputted, the processor  46  causes the optical amplifier  42  to operate; if no signal has been inputted, it causes the optical amplifier  42  to stop operating (that is, it shuts down the optical amplifier  42 ). 
         [0012]    Therefore, if an optical signal is turned off at a time t 1  and input light power  51  of the optical amplifier  42  falls below the Pth, the optical amplifier  42  is shut down and output light power  52  of the optical amplifier  42  comes close to zero. 
         [0013]    Japanese Laid-open Patent Publication No. 2004-112427 relates to a method for monitoring the OSNR in an optical transmission system. 
         [0014]    The above-mentioned related art optical amplifier control method has the following problem. 
         [0015]    As shown in  FIG. 31 , if only one channel of a WDM signal is turned off due to breakage, removal, or the like of a optical fiber of the optical transmitter  21 - 2  during operation of the WDM communication system having n channels, only an ASE that has occurred and accumulated in the optical amplification relays disposed between the optical transmission unit and the optical reception unit is inputted to the optical amplifier  26 - 2  corresponding to that channel. 
         [0016]    If this ASE power is larger than the lower limit value of the signal input dynamic range of the optical amplifier  26 - 2 , input light power  61  does not fall below the shutdown threshold Pth even if the signal is turned off at the time t 1 , as shown in  FIG. 32 . As a result, a distinction cannot be made between the signal and the ASE, whereby the optical amplifier  26 - 2  will not be shut down. 
         [0017]    Then, if the signal is turned on at a time t 2  with the optical amplifier  26 - 2  operational and the optical signal is inputted to the optical amplifier  26 - 2 , an optical surge  63  occurs as shown in output light power  62 . Thus the optical receiver  28 - 2  disposed after the optical amplifier  26 - 2  will be broken. 
         [0018]    Also, if the optical amplifier is mistakenly started when only an ASE has been inputted at an initial start of the WDM communication system, an optical surge occurs at an instant when an optical signal is actually inputted afterward. Thus, the optical receiver will be broken as well. 
         [0019]    To prevent such an erroneous determination, a method is considered in which an input signal detection device as shown in  FIG. 33  is used. An input signal detection device includes an optical coupler  72 , a high-speed PD  73 , a band path filter (BPF)  74 , and an intensity monitor  75 . A monitor signal outputted from the high-speed PD  73  is transferred to the intensity monitor  75  via the BPF  74 , and the intensity monitor  75  monitors the intensity of components of the signal and outputs the monitor signal to the controller  76 . According to the monitor signal from the intensity monitor  75 , the controller  76  determines whether or not an optical signal has been inputted, and controls the operation of the optical amplifier  42 . 
         [0020]    Monitoring the intensity of the signal components at the input terminal of the optical amplifier  42  in this way allows determination whether a signal has been inputted or only an ASE has been inputted. However, disposing the input signal detection device  71  by the number of wavelengths requires use of many high-frequency parts. This will make the system very costly. 
       SUMMARY 
       [0021]    Accordingly, an object of an aspect of present invention is to detect whether or not inputted light including a signal. 
         [0022]    According to an aspect of an embodiment, a signal detector device includes a first monitor unit, a second monitor unit, and a discriminator unit. 
         [0023]    The discriminator unit discriminates whether an inputted light includes a signal light on the bases of the first monitor unit for monitoring an intensity of the inputted light and the second monitor unit for monitoring an alternating current component intensity of the inputted light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a principle diagram of an input signal detection device. 
           [0025]      FIG. 2  shows a configuration example of a first input signal detection device. 
           [0026]      FIG. 3  is graphs showing the spectrums and temporal waveforms at the input terminals of two intensity monitors. 
           [0027]      FIG. 4  is a graph showing the dependence of the total intensity on the input light power. 
           [0028]      FIG. 5  a graph showing the dependence of the AC intensity on the input light power. 
           [0029]      FIG. 6  is a configuration diagram of a first optical amplifier control system. 
           [0030]      FIG. 7  is a flowchart of first optical amplifier start control. 
           [0031]      FIG. 8  is a flowchart of first optical amplifier stop control. 
           [0032]      FIG. 9  is a configuration diagram of a second input signal detection device. 
           [0033]      FIG. 10  is a graph showing the dependence of the monitor intensity on the input light power in a case where a loss part is provided. 
           [0034]      FIG. 11  is a flowchart of second optical amplifier start control. 
           [0035]      FIG. 12  is a flowchart of second optical amplifier stop control. 
           [0036]      FIG. 13  is a configuration diagram of a third input signal detection device. 
           [0037]      FIG. 14  is a diagram showing two thresholds. 
           [0038]      FIG. 15  is a flowchart of third optical amplifier start control. 
           [0039]      FIG. 16  is a flowchart of third optical amplifier stop control. 
           [0040]      FIG. 17  shows a configuration example of a fourth input signal detection device. 
           [0041]      FIG. 18  shows a configuration diagram of a second optical amplifier control system. 
           [0042]      FIG. 19  is a configuration diagram of a third optical amplifier control system. 
           [0043]      FIG. 20  is a graph showing the spectrum of input light in a case where no optical filter is provided. 
           [0044]      FIG. 21  is a graph showing the spectrum of input light in a case where an optical filter is provided. 
           [0045]      FIG. 22  is a graph showing the dependence of the AC intensity on the input light power in a case where no optical filter is provided. 
           [0046]      FIG. 23  is a configuration diagram of a fourth optical amplifier control system. 
           [0047]      FIG. 24  is a graph showing the dependence of the AC intensity on the OSNR. 
           [0048]      FIG. 25  is a graph showing the difference in the AC intensity between modulation systems. 
           [0049]      FIG. 26  is a graph showing variations in AC intensity according to the number of wavelengths in the case of phase modulation. 
           [0050]      FIG. 27  is a graph showing variations in AC intensity according to the number of wavelengths in the case of intensity modulation. 
           [0051]      FIG. 28  is a configuration diagram of a related art optical communication system. 
           [0052]      FIG. 29  is a configuration diagram of a related art optical amplifier system. 
           [0053]      FIG. 30  is a diagram showing a related art optical amplifier control method. 
           [0054]      FIG. 31  is a diagram showing a state in which a signal having one channel is turned off. 
           [0055]      FIG. 32  is a graph showing occurrence of an optical surge. 
           [0056]      FIG. 33  is a diagram showing an improvement measure to detect whether or not a signal has been inputted. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0057]    A preferred embodiment to carry out the present invention will now described in detail with reference to the accompanying drawings. 
         [0058]      FIG. 1  is a diagram of an input signal detection device according to an embodiment. The input signal detection device  71  or  603  or  1806  shown in  FIG. 1  includes first monitoring unit  101 , second monitoring unit  102 , and determination unit  103 . 
         [0059]    The first monitoring unit  101  monitors the intensity of input light and outputs a first monitor signal indicating the intensity of the input light. The second monitoring unit  102  monitors the intensity of the alternating current component of the input light and outputs a second monitor signal indicating the alternating current intensity. Using the first and second monitor signals, the determination unit  103  determines whether or not signal light is contained in the input light. 
         [0060]    An alternating current (AC) component and a direct current (DC) component are contained in input light, and the DC intensity is dominant in the intensity of the input light. Therefore, it is difficult to distinguish between a state in which signal light and an ASE are contained in the input light and a state in which an ASE is contained therein, according to the intensity of the input light. However, monitoring the AC intensity as well as the input light intensity allows these two input states to be easily distinguished from each other. 
         [0061]    Also, if light on the input side or output side of the optical device is inputted to an input signal detection device and it is determined whether or not signal light is contained in the inputted light, the optical device is properly controlled according to the result of the determination. 
         [0062]    The first monitoring unit  101  includes, for example, an intensity monitor  205  to be discussed later, and the second monitoring unit  102  includes, for example, a DC block  203  and an intensity monitor  204 . The determination unit  103  corresponds to, for example, a determination processor  206 . 
         [0063]    In this embodiment, after input light is photoelectric-converted, the AC intensity of the low frequency range is monitored. Then, the input state is determined using a fact that there is a difference in monitor intensity between a case where an optical signal has been inputted and a case where an ASE has been inputted. Then, the operation of an optical amplifier is controlled using the result of the determination. 
         [0064]      FIG. 2  shows a configuration example of such an input signal detection device  71  or  603  or  1806 . This input signal detection device  71  or  603  or  1806  includes a low-speed PD  201 , a transimpedance amplifier (TIA)  202 , a DC block  203 , intensity monitors  204  and  205 , and a determination processor  206 . 
         [0065]    The low-speed PD  201  converts input light into an electric signal. The transimpedance amplifier (TIA)  202  amplifies the PD output current to convert the voltage. The DC block  203  blocks the DC component of the output of the TIA  202  and outputs the AC component thereof to the intensity monitor  204 . The intensity monitor  204  outputs the signal intensity of the inputted AC component. The intensity monitor  205  outputs the signal intensity of the output of the TIA  202  as total intensity. The determination processor  206  determines the input state using the AD intensity from the intensity monitor  204  and the total intensity from the intensity monitor  205 , and outputs the result of the determination as control information. 
         [0066]      FIG. 3  shows results of simulations of the spectrum and temporal waveform at the input terminal of each intensity monitor. It is assumed in these simulations that the response speed of the low-speed PD  201  is a speed sufficiently lower than the speed of the optical signal and that the DC block  203  includes, for example, a capacitor. 
         [0067]    From an example shown in  FIG. 3 , it is understood that a DC component is not contained in any of the spectrum and temporal waveform at the input terminal of the AC intensity monitor  204  and that a DC component is contained in each of the spectrum and temporal waveform at the input terminal of the total intensity monitor  205 . 
         [0068]      FIG. 4  shows the dependence of the total intensity on the input light power, and  FIG. 5  shows the dependence of the AC intensity on the input light power. Since the DC intensity is dominant in the total intensity, the dependence of the total intensity on the input light power in a case where only an ASE has been inputted and that in a case where an optical signal and an ASE have been inputted are nearly matched with each other. On the other hand, the dependence of the AC intensity on the input light power varies according to the input state. 
         [0069]    In  FIG. 5 , the dependence of the AC intensity on the input light power in a case where only an ASE has been inputted and that in a case where an optical signal and an ASE have been inputted differ from each other. The dependence of the AC intensity on the input light power also varies according to the OSNR. Therefore, monitoring the AC intensity and total intensity allows a distinction between a state in which only an ASE has been inputted and a state in which an optical signal has been inputted. 
         [0070]    A specific method for determining the input state and a method for controlling an optical amplifier using the result of the determination will now be described using specific examples. 
         [0071]      FIG. 6  shows a configuration example of an optical amplifier control system. This system includes an optical coupler  601 , an optical amplifier  602 , a input signal detection device  603 , and a controller  604 . The input signal detection device  603  has the configuration shown in  FIG. 2  and determines the input state in the input terminal of the optical amplifier  602  via the optical coupler  601 . The controller  604  controls the operation of the optical amplifier  602  using the result of the determination. 
         [0072]    In this case, only an ASE is previously inputted to the input signal detection device  603  so as to obtain the relation between the AC intensity and total intensity, and the relation is stored as a data table in the determination processor  206  of the input signal detection device  603 . 
         [0073]      FIG. 7  is a flowchart of optical amplifier start control in a state in which an optical amplifier is stopping. The intensity monitors  204  and  205  of the input signal detection device  603  measure the AC intensity and total intensity (step  701 ). 
         [0074]    Next, the determination processor  206  calculates AC intensity corresponding to the measured value of the total intensity using the data table (step  702 ), and compares the obtained calculated value with the measured value of the AC intensity (step  703 ). If the measured value of the AC intensity is equal to or larger than the calculated value, the determination processor  206  determines that no signal has been inputted, that is, determines that only an ASE has been inputted (step  704 ), and repeats the operations in steps  701  and later. 
         [0075]    If the measured value of the AC intensity is smaller than the calculated value, the determination processor  206  determines that a signal has been inputted (step  705 ), and transfers information indicating that a signal has been inputted, to the controller  604  (step  706 ). Then, the controller  604  starts the optical amplifier  602  according to the information indicating that a signal has been inputted (step  707 ). 
         [0076]      FIG. 8  is a flowchart of optical amplifier stop control in a state in which an optical amplifier is operating. Operations in steps  801  to  803  are similar to those in steps  701  to  703 . 
         [0077]    If the measured value of the AC intensity is smaller than the calculated value in step  803 , the determination processor  206  determines that a signal has been inputted (step  804 ), and repeats the operations in steps  801  and later. 
         [0078]    If the measured value of the AC intensity is equal to or larger than the calculated value, the determination processor  206  determines that no signal has been inputted, that is, determines that only an ASE has been inputted (step  805 ), and transfers information indicating that no signal has been inputted, to the controller  604  (step  806 ). Then, the controller  604  causes the optical amplifier  602  to stop operating, according to the information indicating that no signal has been inputted (step  807 ). 
         [0079]      FIG. 9  shows another configuration example of the input signal detection device. This input signal detection device has a configuration in which an attenuator  901  is provided between the TIA  202  and the intensity monitor  205  in the input signal detection device shown in  FIG. 2 , so as to assign weights to the AC intensity and total intensity. For example, an attenuator is used as the attenuator  901 . 
         [0080]    In this case, attenuating an input signal of the intensity monitor  205  by the attenuator  901  allows a reduction in measured value of the total intensity. As a result, the dependence of the AC intensity on the input light power and the dependence of the total intensity on the input light power become what are shown in  FIG. 10 . The amount of loss produced by the attenuator  901  is adjusted so that total intensity is detected between the AC intensity in a case where only an ASE has been inputted and that in a case where a signal and an ASE have been inputted. 
         [0081]    Thus, the input state is determined by only comparing the magnitude relation between the AC intensity and total intensity. This makes the data table for determination unnecessary, thereby simplifying the configuration of the determination processor  206 . 
         [0082]      FIG. 11  is a flowchart of optical amplifier start control in a case where the configuration shown in  FIG. 9  is used as the input signal detection device  603  shown in  FIG. 6 . First, the intensity monitors  204  and  205  measure the AC intensity and total intensity (step  1101 ). 
         [0083]    Next, the determination processor  206  compares the measured value of the AC intensity with the measured value of the total intensity (step  1102 ). If the measured value of the AC intensity is larger than the measured value of the total intensity, the determination processor  206  determines that no signal has been inputted (step  1103 ), and repeats the operations in steps  1101  and later. 
         [0084]    If the AC intensity is smaller than the total intensity, the determination processor  206  determines that a signal has been inputted (step  1104 ), and transfers information indicating that a signal has been inputted, to the controller  604  (step  1105 ). Then, the controller  604  starts the optical amplifier  602  according to the information indicating that a signal has been inputted (step  1106 ). 
         [0085]      FIG. 12  is a flowchart of optical amplifier stop control in a case where the input signal detection device shown in  FIG. 9  is used. Operations in steps  1201  and  1202  are similar to those in steps  1101  and  1102  shown in  FIG. 11 . 
         [0086]    If the AC intensity is smaller than the total intensity, the determination processor  206  determines that a signal has been inputted (step  1203 ), and repeats the operations in steps  1201  and later. 
         [0087]    If the AC intensity exceeds the total intensity, the determination processor  206  determines that no signal has been inputted (step  1204 ), and transfers information indicating that no signal has been inputted, to the controller  604  (step  1205 ). Then, the controller  604  causes the optical amplifier  602  to stop, according to the information indicating that no signal has been inputted (step  1206 ). 
         [0088]    While a weight is assigned to each monitor intensity in the input signal detection device shown in  FIG. 9  by providing the attenuator  901  in the apparatus, TIAs having different gains may be provided near the AC intensity monitor and near the total intensity monitor as another method, as shown in  FIG. 13 . 
         [0089]    In the input signal detection device shown  71  or  603  or  1806  in  FIG. 13 , a TIA  1301  having a gain G 1  is disposed between the low-speed PD  201  and the DC block  203 , and a TIA  1302  having a gain G 2  is disposed between the low-speed PD  201  and the intensity monitor  205 . If the G 2  is set to be sufficiently smaller than G 1 , a characteristic similar to that shown in  FIG. 10  is realized. 
         [0090]    While the input state is determined in the input signal detection devices shown in  FIGS. 7 and 8  by comparing the measured value of the AC intensity with the calculated value thereof, the input state is also determined by additionally using the threshold of light power. In this case, as shown in  FIG. 14 , thresholds Pth 1  and Pth 2  are previously set near the lower limit value of the signal input range and near the upper limit value of the ASE input range, respectively. 
         [0091]    If the total intensity is larger than the Pth 2 , it is determined that a signal has been inputted. If the total intensity lies between the Pth 1  and Pth 2 , a determination is made additionally using the AC intensity. If the total intensity is smaller than the Pth 1 , it is determined that no signal has been inputted. 
         [0092]      FIG. 15  is a flowchart of such optical amplifier start control. First, the intensity monitor  205  measures total intensity Pmon (step  1501 ). 
         [0093]    Next, the determination processor  206  compares the Pmon with the Pth 1  (step  1502 ). If the Pmon is equal to or smaller than the Pth 1 , the determination processor  206  determines that no signal has been inputted (step  1503 ), and repeats operations in steps  1501  and later. 
         [0094]    If the Pmon is larger than the Pth 1 , the determination processor  206  compares the Pmon with the Pth 2  (step  1504 ). If the Pmon is smaller than the Pth 2 , the determination processor  206  performs operations similar to those in steps  701  to  704  (steps  1505  to  1508 ). 
         [0095]    If the measured value of the AC intensity is smaller than the calculated value thereof in step  1507 , the determination processor  206  determines that a signal has been inputted (step  1509 ) and performs operations similar to those in steps  706  and  707  (steps  1510  to  1511 ). 
         [0096]    If the Pmon is equal to or larger than the Pth 2  in step  1504 , the determination processor  206  determines that a signal has been inputted (step  1509 ) and performs operations in steps  1510  and  1511 . 
         [0097]      FIG. 16  is a flowchart of optical amplifier stop control. First, the intensity monitor  205  measures the total intensity Pmon (step  1601 ). 
         [0098]    Next, the determination processor  206  compares the Pmon with the Pth 2  (step  1602 ). If the Pmon is equal to or larger than the Pth 2 , the determination processor  206  determines that a signal has been inputted (step  1603 ), and repeats the operations in steps  1601  and later. 
         [0099]    If the Pmon is smaller than the Pth 2 , the determination processor  206  compares the Pmon with the Pth 1  (step  1604 ). If the Pmon is larger than the Pth 1 , the determination processor  206  performs operations similar to those in steps  801  to  804  (steps  1605  to  1608 ). 
         [0100]    If the measured value of the AC intensity is equal to or larger than the calculated value thereof in step  1607 , the determination processor  206  determines that no signal has been inputted (step  1609 ) and performs operations similar to those in steps  806  and  807  (steps  1610  to  1611 ). 
         [0101]    If the Pmon is equal to or smaller than Pth 1  in step  1604 , the determination processor  206  determines that no signal has been inputted (step  1609 ) and performs operations similar to those in steps  1610  and  1611 . 
         [0102]    While the input state is determined from the total intensity and AC intensity in the above embodiment, the DC intensity may be monitored instead of the total intensity, since the DC intensity is dominant in the total intensity. 
         [0103]      FIG. 17  shows a configuration example of such an input signal detection device  71  or  603  or  1806 . In this input signal detection device  71  or  603  or  1806 , an AC block  1701  is provided between the TIA  202  and the intensity monitor  205 . The AC block  1701  extracts a DC component from an output of the TIA  202  and outputs the DC component to the intensity monitor  205 . The AC block  1701  includes, for example, a low-path filter including a coil. 
         [0104]    The order, number, or the like of components may be changed in the configuration examples of the above-mentioned input signal detection devices if each apparatus is provided with a function of monitoring desired intensity. Also, these input signal detection devices may be disposed inside an optical amplifier rather than outside. 
         [0105]      FIG. 18  shows a configuration example of a system for controlling multiple optical amplifiers using one input signal detection device. This system includes an optical demultiplexer  1801 , optical couplers  1802 - 1  to  1802 - n , optical amplifiers  1803 - 1  to  1803 - n , optical receivers  1804 - 1  to  1804 - n , an optical switch  1805 , an  1806 , and a controller  1807 . 
         [0106]    The optical demultiplexer  1801  divides a WDM signal into n channels of optical signals. The optical switch  1805  selects any one of the n channels of optical signals inputted via the optical couplers  1802 - 1  to  1802 - n  and outputs the selected optical signal to the input signal detection device  1806 . The controller  1807  controls the operation of the optical amplifiers  1803 - 1  to  1803 - n  using the result of the determination with respect to each channel. 
         [0107]      FIG. 19  shows a configuration example of a system for limiting the bandwidth of light inputted to an input signal detection device using an optical filter. This system has a configuration in which a variable or fixed optical filter  1901  is provided between the optical coupler  601  and the input signal detection device  603  in the optical amplifier control system shown in  FIG. 6 . 
         [0108]    If the optical filter  1901  is not provided, a signal  2001  and an ASE  2002  extending over a wide wavelength range are contained in input light of the signal input detection system  603 , as shown in  FIG. 20 . On the other hand, if the optical filter  1901  is provided, a signal  2101  and an ASE  2102  corresponding to the bandwidth are contained in input light thereof, as shown in  FIG. 21 . Therefore, as shown in  FIG. 22 , the difference in AC intensity between a case where only an ASE has been inputted and a case where a signal and an ASE have been inputted is increased. Thus, the input state is easily determined. 
         [0109]      FIG. 23  shows another configuration example of the optical amplifier control system using an optical filter. This system has a configuration in which a variable or fixed optical filter  2301  is provided on the input side of the optical coupler  601  in the optical amplifier control system shown in  FIG. 6 . Also in this case, an advantage in that the input state is easily determined is obtained, as in the configuration shown in  FIG. 19 . 
         [0110]    An OSNR may be calculated from the measured value of the AC intensity using the above-mentioned input signal detection device. In this case, data on a curve indicating the dependence of the AC intensity on the OSNR is previously obtained for each value of the input light power while changing the power, and the obtained data is stored as a data table in the determination processor  206 . 
         [0111]    During operation of the system, the determination processor  206  selects a curve  2401  according to the measured value of the total intensity (input light power), as shown in  FIG. 24 , and calculates an OSNR corresponding to the measured value of the AC intensity using data on the curve  2401 . 
         [0112]    Incidentally, the AC intensity measured by the input signal detection device varies with the modulation system of an optical signal. The AC intensity in a case where a signal modulated according to the intensity modulation system has been inputted is larger than that in a case where only an ASE has been inputted. The AC intensity in a case where a signal modulated according to the phase modulation system has been inputted is smaller than that in a case where only an ASE has been inputted. Therefore, it is determined which modulation system&#39;s signal has been inputted, by comparing the measured value of the AC intensity and the AC intensity in a case where only an ASE has been inputted. 
         [0113]      FIG. 25  shows the difference in the dependence of the AC intensity on the input light power between the two modulation systems. The AC intensity in a case where non-return to zero (NRZ) modulation (intensity modulation) is used is larger than that in the case of only an ASE, and the AC intensity in a case where return to zero-differential quadrature phase shift keying (RZ-DQPSK) modulation (phase modulation) is used is smaller than that in the case of only an ASE. 
         [0114]    If phase modulation is used, the input state is determined by using the control methods shown in  FIGS. 7 ,  8 ,  11 ,  12 ,  15 , and  16 . On the other hand, if intensity modulation is used, the direction of the inequality sign must be reversed in steps  703 ,  803 ,  1102 ,  1202 ,  1507 , and  1607  to make a determination. Also, if the control methods shown in  FIGS. 11 and 12  are used, the amount of. loss produced by the attenuator  901  or the gains of the TIA  1301  and  1302  must be adjusted so that total intensity is detected above the AC intensity in a case where only an ASE has been inputted, unlike in  FIG. 10 . 
         [0115]    While an optical signal of one wavelength is selected and inputted to the input signal detection device in the optical amplifier control system shown in  FIG. 18 , an optical signal of multiple wavelengths, such as a WDM signal, may directly be inputted. 
         [0116]      FIG. 26  shows variations in AC intensity according to the number of wavelengths in a case where phase modulation is used. In this case, the AC intensity in a case where a signal of the m+1 (m is a natural number) number of wavelengths has been inputted is smaller than that in a case where a signal of one wavelength has-been inputted. That is, the AC intensity becomes smaller as the number of wavelengths becomes larger. Therefore, the AC intensity in a case where a signal of the m+1 number of wavelengths has been inputted is easily distinguished from the AC intensity in the case of only an ASE. 
         [0117]      FIG. 27  shows variations in AC intensity according to the number of wavelengths in a case where intensity modulation is used. In this case, the AC intensity in a case where a signal of the m+1 number of wavelengths has been inputted is larger than that in a case where a signal of one wavelength has been inputted. That is, the AC intensity becomes larger as the number of wavelengths becomes larger. Therefore, the AC intensity in a case where a signal of the m+1 number of wavelengths has been inputted is easily distinguished from the AC intensity in the case of only an ASE. 
         [0118]    The input signal detection device is provided at the input terminal of the optical amplifier in the above-described embodiment; however, without being limited to this, the input signal detection device may be provided at the input terminal or output terminal of another optical device. For example, by disposing the input signal detection device before any optical receiver or at the input or output terminal of the optical switch and determining the state of an input/output signal of these optical devices, the optical receiver or optical switch is properly controlled. 
       ADVANTAGES 
       [0119]    The relatively simple and low-cost configuration according to the embodiment allows determination whether or not a signal has been inputted to the input terminal of an optical device, that is, it allows a distinction between a state in which a signal has been inputted and a state in which only an ASE has been inputted. Thus, the optical device is started and stopped safely and reliably. 
         [0120]    Also, it is detected whether or not a signal has been inputted, without depending on the bit rate or format of an optical signal; therefore, systems that have different bit rates or formats are also flexibly supported.