Patent Publication Number: US-2007109630-A1

Title: Optical amplifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2005-331269, filed on Nov. 16, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to optical amplifiers, and particularly to an optical amplifier including a variable optical attenuator and an external attenuating medium connected in series.  
      2. Description of the Related Art  
      In optical wavelength multiplex transmission systems, an erbium-doped fiber amplifier (EDFA) is generally used as an optical amplifier used for a transmitter or a repeater. The EDF optical amplifier has a set signal gain determined by the signal gain characteristics of the EDF (such as dependence of gain on the wavelength). Accordingly, the amplifier includes a variable optical attenuator (VOA) for absorbing changes in amplifier gain and interstage losses such as an insertion loss of a dispersion compensating optical fiber (DCF) (refer to Japanese Unexamined Patent Publication Nos. 2004-72062 and H10-51057). When the amplifier gain or interstage loss decreases, the loss by the VOA is increased by the amount of decrease.  
      The demand for optical amplifiers having a wide dynamic range (7 to 16 dB, for instance) has been growing to keep up with a wide variety of user requirements for system gain. The VOA loss range must be increased accordingly, but a great VOA loss range would worsen the noise factor. To avoid the problem, two stages of VOAs are configured.  
       FIG. 13A  is a block diagram of a conventional optical amplifier. The shown optical amplifier has two stages of VOAs. Signal light is input to an input terminal  201  shown in  FIG. 13A . The input signal light is output through a coupler  202  to an EDF  204 . The coupler  202  combines the signal light with pumped light from a laser diode (LD)  203  and outputs the combined light to the EDF  204 . Now, the signal light has a gain depending on the power of the pumped light. A VOA  205  attenuates the signal light output from the EDF  204 . A coupler  206 , an LD  207 , and an EDF  208  are analogous to the coupler  202 , the LD  203 , and the EDF  204 . Couplers  212  and  216 , LDs  213  and  217 , EDFs  214  and  218 , and a VOA  215  are analogous to the corresponding elements between the input terminal  201  and an output terminal  209 . Amplified signal light is output from an output terminal  219 . A DCF  210  is connected to terminals  209  and  211  and compensates for wavelength dispersion of the signal light output from the EDF  208 .  
       FIG. 13B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 13A . The height of waveforms A and B in  FIG. 13B  represents the level of light (in units of dBm, for instance), and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 13A .  
      If the level of signal light input to the input terminal  201  is originally represented by waveform A and increased as represented by waveform B, the gain of the EDF  204  decreases to reduce the level of signal light (the inclination of waveform B is smaller than the inclination of waveform A). Because the total gain of the EDF  204  and the EDF  208  must be kept constant because of the EDF signal gain characteristics, the gain of the EDF  208  needs to be increased by the amount of decrease in gain of the EDF  204 . (The inclination of waveform B becomes greater than the inclination of waveform A, between the input and output of the EDF  208 .) Squares or circles in the figure represent that the marked segments have the same inclination, which means that the total gain of the EDF  204  and the EDF  208  is kept constant.  
      The level of light at the terminal  209  must also be kept constant, so that the amount of loss (attenuation) by the VOA  205  becomes as represented by waveform B, which is greater than that of waveform A. The same configuration is provided between the terminal  211  and the output terminal  219 . The VOA loss range collectively provided by the VOA  205  and the VOA  215  prevents the noise factor from becoming worse.  
      When a change in interstage loss (change in loss across the DCF  210 ) becomes  10  dB or greater, the loss range of the VOA  215  would increase in the configuration shown in  FIG. 13A .  
      Suppose that the amount of loss in level of light between the input and output of the DCF  210  becomes as represented by waveform B, which is smaller than that represented by waveform A (the inclination of waveform B becomes smaller than the inclination of waveform A). In other words, suppose that the level of signal light input to the terminal  211  increases. The gain of the EDF  214  would decrease because of the upper limit of power of the LD  213 , then the gain of the EDF  218  should be increased under constant sum gain control. Because a constant (target) level of signal light must be obtained at the output terminal  219 , the amount of loss by the VOA  215  increases by the amount of decrease in loss by the DCF  210 . This would worsen the noise factor, depending on the amount of decrease in power input to the EDF  218 , and the LD  217  should have a great power. To avoid this problem, a series connection between the VOA and the DCF has been increasing.  
       FIG. 14A  is a block diagram of an optical amplifier in which a VOA and a DCF are connected in series. In the optical amplifier shown in  FIG. 14A , a VOA  229  and a DCF  231  are connected in series. The DCF  231  is connected to terminals  230  and  232 . Couplers  222  and  226 , LDs  223  and  227 , EDFs  224  and  228 , and a VOA  225  are analogous to the couplers  202  and  206 , the LDs  203  and  207 , the EDFs  204  and  208 , and the VOA  205  in  FIG. 13A . In the optical amplifier shown in  FIG. 14A , the single VOA  225  absorbs the dynamic range. No VOA is connected after the DCF  231 , between an EDFA including a coupler  233 , an LD  234 , and EDF  235  and another EDFA including a coupler  236 , an LD  237 , and an EDF  238 . Signal light is input to an input terminal  221  and output from an output terminal  239 .  
       FIG. 14B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 14A . The height of waveforms A and B in  FIG. 14B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 14A .  
      The operation from the input terminal  221  to the EDF  228  is the same as the operation of the amplifier shown in  FIG. 13 . If the amount of loss by the DCF  231  becomes as represented by waveform B, decreased from the amount represented by waveform A, the amount of loss by the VOA  229  is increased by the amount of decrease so that the level of signal light is kept constant at the terminal  232 . The noise factor can be prevented from becoming worse by connecting the VOA  229  and the DCF  231  in series to keep the level of light after the DCF  231  constant.  
      The DCF  231  is directly connected and disconnected by the user. The connection or disconnection of the DCF  231  must be confirmed by checking the amount of loss before and after the DCF  231 . Accordingly, an optical detection section for detecting the level of light, such as a photo diode (PD), must be provided before and after the DCF  231 .  
       FIG. 15  is a block diagram of an optical amplifier which can detect the connection or disconnection of a DCF. In the shown optical amplifier, signal light input to an input terminal  241  passes one EDF  246 , a DCF  253 , and another EDF  259  and is output from an output terminal  262 .  
      Couplers  242  and  247  branch signal light into a PD  243  and a PD  248  respectively, and the PD  243  and the PD  248  convert the light to an electric signal. An AGC  271  adjusts the pumped light of an LD  245  in accordance with the electric signals of the PD  243  and the PD  248 , or the levels of signal light before and after the EDF  246 . The pumped light is input through a coupler  244  to the EDF  246 . Couplers  255 ,  257 , and  260 , PDs  256  and  261 , an LD  258 , and an AGC  273  after the DCF  253  function in the same way. The AGC  271  and the AGC  273  bring the signal light to a target level at the output terminal  262 . An interstage loss control block  272  controls a VOA  249  to keep the total gain of the EDF  246  and the EDF  259  constant.  
      The interstage loss control block  272  monitors the level of light before and after the DCF  253 , in accordance with the level of light branched by a coupler  250  and detected by a PD  251  and the level of light branched by the coupler  255  and detected by the PD  256 . Then, the interstage loss control block  272  detects a disconnection (of the DCF  253  from terminals  252  and  254 ) or a connection (of the DCF  253  to the terminals  252  and  254 ), in accordance with the level of light before and after the DCF  253 .  
      To detect the connection or disconnection of an external attenuating medium (in this example, the DCF  253 ) to be connected or connected in series with a variable optical attenuator (in this example, the VOA  249 ), an optical detection section (in this example, the PD  251  and the PD  256 ) must be provided before and after the external attenuating medium. If the optical detection section is provided before the variable optical attenuator and after the external attenuating medium, the varying amount of attenuation by the variable optical attenuator makes it impossible to check the connection or disconnection of the external attenuating medium in accordance with the correct amount of loss by the external attenuating medium.  
      The detection section provided before and after the external attenuating medium, however, requires that the signal light is branched into the detection section, and the loss depending on the branching ratio would lead to SN degradation.  
      For instance, the PD  251  provided before the DCF  253  shown in  FIG. 15  would provide a loss depending on the branching ratio and cause SN degradation accordingly.  
      To prevent SN degradation, signal light must be amplified by the amount of loss depending on the branching ratio, and a higher LD power is required. This would reduce the cost effectiveness and would increase power consumption because of LD temperature control.  
     SUMMARY OF THE INVENTION  
      In view of the foregoing, it is an object of the present invention to provide an optical amplifier which eliminates the need for an optical detection section before and after an external attenuating medium, does not cause SN degradation, and does not increase the power of the LD.  
      To accomplish the above object, according to one aspect of the present invention, there is provided an optical amplifier including a variable optical attenuator and an external attenuating medium connected in series. This optical amplifier includes the following elements: an attenuation amount detection section for detecting the amount of signal light attenuation caused by the variable optical attenuator and the external attenuating medium, by means of a front optical detection section of a front optical amplification block provided before the variable optical attenuator and the external attenuating medium and a back optical detection section of a back optical amplification block provided after the variable optical attenuator and the external attenuating medium; an attenuation amount control section for controlling the variable optical attenuator to keep the amount of signal light attenuation constant; and a connection detection section for detecting a connection or disconnection of the external attenuating medium in accordance with the amount of signal light attenuation obtained when the amount of attenuation caused by the variable optical attenuator is minimized.  
      The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a view showing an overview of an optical amplifier.  
       FIG. 2  is a block diagram of an optical amplifier of a first embodiment.  
       FIGS. 3A and 3B  show the operation of the optical amplifier when an interstage loss increases.  
       FIGS. 4A and 4B  show the operation of the optical amplifier when signal light increases.  
       FIGS. 5A and 5B  show the operation of the optical amplifier in which a disconnection is detected.  
       FIGS. 6A and 6B  show the operation of the optical amplifier in which safety optical level control is performed and a connection is restored.  
       FIG. 7  shows a VOA varying with temperature.  
       FIG. 8  is a block diagram of an optical amplifier of a second embodiment.  
       FIGS. 9A and 9B  show the operation of the optical amplifier.  
       FIG. 10  shows an example hardware configuration of the optical amplifier.  
       FIG. 11  shows an optical amplifier of a third embodiment.  
       FIGS. 12A and 12B  show the operation of the optical amplifier.  
       FIGS. 13A and 13B  show the operation of a conventional optical amplifier.  
       FIGS. 14A and 14B  show the operation of an optical amplifier including a VOA and a DCF connected in series.  
       FIG. 15  is a block diagram of an optical amplifier which can detect the connection or disconnection of a DCF. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The principle of the present invention will be described in detail with reference to drawings.  
       FIG. 1  is a view showing an overview of an optical amplifier. As shown in the figure, the optical amplifier includes a front optical amplification block  1 , a back optical amplification block  2 , an attenuation amount detection section  3 , an attenuation amount control section  4 , a connection detection section  5 , a variable optical attenuator  6 , and an external attenuating medium  7 , which is a DCF, for instance, connected to terminals  8   a  and  8   b.    
      The front optical amplification block  1  includes an EDF  1   b  for amplifying signal light and a front optical detection section  1   a  for monitoring the gain of the EDF  1   b  in an output stage.  
      The back optical amplification block  2  includes an EDF  2   b  for amplifying signal light and a back optical detection section  2   a  for monitoring the gain of the EDF  2   b  in an input stage.  
      The attenuation amount detection section  3  detects the amount of signal light attenuation caused by the variable optical attenuator  6  and the external attenuating medium  7 , by means of the front optical detection section  1   a  of the front optical amplification block  1  provided before the variable optical attenuator  6  and the external attenuating medium  7  connected in series and the back optical detection section  2   a  of the back optical amplification block  2  provided after the variable optical attenuator  6  and the external attenuating medium  7 .  
      The attenuation amount control section  4  controls the variable optical attenuator  6  so that the attenuation amount detection section  3  detects a constant amount of signal light attenuation caused by the variable optical attenuator  6  and the external attenuating medium  7 .  
      The connection detection section  5  detects a connection or disconnection of the external attenuating medium  7  in accordance with the amount of signal light attenuation obtained when the attenuation amount of the variable optical attenuator  6  is minimized.  
      When the external attenuating medium  7  is disconnected from the terminal  8   a  or  8   b,  the amount of attenuation detected by the front optical detection section  1   a  and the back optical detection section  2   a  increases. The attenuation amount control section  4  controls the variable optical attenuator  6  and decreases the amount of attenuation by the variable optical attenuator  6  to keep the amount of signal light attenuation by the variable optical attenuator  6  and the external attenuating medium  7  constant. Because the external attenuating medium  7  is disconnected, the amount of attenuation by the variable optical attenuator  6  is reduced to the minimum value. If the amount of signal light attenuation by the variable optical attenuator  6  and the external attenuating medium  7  exceeds a certain threshold while the attenuation amount of the variable optical attenuator  6  is minimized, the connection detection section  5  determines that the external attenuating medium  7  is disconnected. If the amount of signal light attenuation by the variable optical attenuator  6  with the minimum amount of attenuation and the external attenuating medium  7  falls below the certain threshold, the connection of the external attenuating medium  7  is detected.  
      The variable optical attenuator  6  has a lower limit of attenuation amount, below which the amount of attenuation cannot be reduced. Accordingly, the amount of attenuation by the external attenuating medium  7  can be recognized by the front optical detection section  1   a  of the front optical amplification block  1  and the back optical detection section  2   a  of the back optical amplification block  2 , without providing any optical detection section before or after the external attenuating medium  7 . In other words, a disconnection or connection of the external attenuating medium  7  can be detected in accordance with the amount of attenuation before and after the external attenuating medium  7 , without any optical detection section before or after the external attenuating medium  7 , SN degradation can be avoided, and the power consumption can be reduced.  
      A first embodiment of the present invention will be described in detail with reference to drawings.  
       FIG. 2  is a block diagram of an optical amplifier of the first embodiment. As shown in the figure, the optical amplifier includes a front optical amplification block  10 , a back optical amplification block  20 , a DCF  32  connected to terminals  31  and  33 , automatic gain controllers (AGCs)  41  and  43 , an interstage loss control block  42 , an EDF gain control block  44 , and an AMP gain control block  45 . The front optical amplification block  10  includes an input terminal  11 , couplers  12 ,  14 , and  17 , PDs  13  and  18 , an LD  15 , an EDF  16 , and a VOA  19 . The back optical amplification block  20  includes couplers  21 ,  23 , and  26 , PDs  22  and  27 , an LD  24 , an EDF  25 , and an output terminal  28 .  
      The input terminal  11  of the front optical amplification block  10  receives wavelength division multiplexing (WDM) signal light, for instance. The signal light input to the input terminal  11  passes the couplers  12  and  14  and is output to the EDF  16 . A part of the signal light input to the input terminal  11  is branched to the PD  13  by the coupler  12 . The PD  13  converts the power (optical intensity) of the input light to an electric signal and outputs the signal to the AGC  41 , the EDF gain control block  44 , and the AMP gain control block  45 .  
      The LD  15  outputs pumped light as controlled by the AGC  41 . The coupler  14  combines the pumped light output from the LD  15  with the signal light input from the input terminal  11  and outputs the combined light to the EDF  16 . Now, the signal light has a gain depending on the power of the pumped light.  
      A part of the signal light output from the EDF  16  is branched to the PD  18  by the coupler  17 . The PD  18  converts the power of the input light to an electric signal and outputs the signal to the AGC  41 , the interstage loss control block  42 , and the EDF gain control block  44 . The EDF  16  also outputs the signal light to the VOA  19 . The VOA  19  attenuates the input signal light as controlled by the interstage loss control block  42  and outputs the light to the terminal  31 .  
      The DCF  32  is connected to the terminals  31  and  33  and compensates for wavelength dispersion of the signal light output from the front optical amplification block  10 . The DCF  32  can be replaced by the user, and a DCF satisfying the user requirements is connected to the terminals  31  and  33 .  
      A part of the signal light output from the DCF  32  is branched to the PD  22  by the coupler  21  of the back optical amplification block  20 . The PD  22  converts the power of the input light to an electric signal and outputs the signal to the interstage loss control block  42 , the AGC  43 , and the EDF gain control block  44 . The DCF  32  also outputs the signal light through the couplers  21  and  23  to the EDF  25 .  
      The LD  24  outputs pumped light as controlled by the AGC  43 . The coupler  23  combines the pumped light output from the LD  24  with the signal light and outputs the combined light to the EDF  25 . Now, the signal light has a gain depending on the power of the pumped light.  
      A part of the signal light output from the EDF  25  is branched to the PD  27  by the coupler  26 . The PD  27  converts the power of the input light to an electric signal and outputs the signal to the AGC  43 , the EDF gain control block  44 , and the AMP gain control block  45 . The EDF  25  also outputs the signal light to the output terminal  28 .  
      The AGC  41  controls the pumped light of the LD  15  with reference to the electric signals representing the power of light, output from the PD  13  and the PD  18 , so that the gain of the EDF  16  becomes a target value G 1 . The target value G 1  is calculated by the AMP gain control block  45 .  
      The interstage loss control block  42  controls the VOA  19  with reference to electric signals representing the power of light output from the PD  18  of the front optical amplification block  10  and the PD  22  of the back optical amplification block  20 , so that the amount of loss by the VOA  19  and the DCF  32  becomes a target value L 1 . The target value L 1  is calculated by the EDF gain control block  44 . The interstage loss control block  42  also detects a connection or disconnection in accordance with the electric signals representing the power of light output from the PD  18  and the PD  22 . If a disconnection is detected, the interstage loss control block  42  controls the AGC  41  so that a safety level of signal light is output from the VOA  19  and turns off the AGC  43 .  
      The AGC  43  controls the pumped light of the LD  24  with reference to the electric signals representing the power of light, output from the PD  22  and the PD  27  so that the gain of the EDF  25  becomes a target value G 2 . The target value G 2  is a fixed value, and the AGC  43  performs control to bring the gain of the EDF  25  to the target value G 2 .  
      The EDF gain control block  44  obtains the target value L 1  in accordance with the electric signals of the PD  13 , the PD  18 , the PD  22 , and the PD  27  such that the total gain of the EDF  16  and the EDF  25  is kept constant. If the total gain of the EDF  16  and the EDF  25  changes, the waveform characteristics of the gain to the signal light would change. The VOA  19  absorbs the gain of the EDF  16  changed by the AGC  41 . The target value L 1  is given by expressions (1) and (2) below:
 
 L 1 =L 1 −ΔL 1  (1)
 
Δ L 1=( PD 18 −PD 13)+( PD 27 −PD 22)− EDFfg   (2)
 
      “PD 18 −PD 13 ” of the expression (2) represents the gain of the EDF  16  calculated from the electric signals of the PD  18  and the PD  13 . “PD 27 −PD 22 ” represents the gain of the EDF  25  calculated from the electric signals of the PD  27  and the PD  22 . The value of “PD 27 −PD 22 ” is constant because the gain of the EDF  25  is kept constant by the AGC  43 .  
      EDFfg (fg: flat gain) represents the total gain of the EDF  16  and the EDF  25 , which must be kept constant, and the value is stored in memory in advance. Accordingly, ΔL 1  represents a difference between the total gain of the EDF  16  and the EDF  25 , which must be kept constant, and the actual gain. The difference obtained in the expression (2) is subtracted from the target value L 1 , and a new target value L 1  is obtained in the expression (1).  
      The AMP gain control block  45  calculates the target value G 1  in accordance with the electric signals of the PD  13  and the PD  27  so that the target level of light is obtained at the output terminal  28 . The target value G 1  used to control the AGC  41  is given by expressions (3) and (4) below:
 
 G 1 =G 1 −ΔG 1  (3)
 
Δ G 1=( PD 27 −PD 13)−target-AMP-gain  (4)
 
      “PD 27 −PD 13 ” in the expression (4) represents a gain between the stage preceding the EDF  16  and the stage following the EDF  25 . The target AMP gain represents a target gain of signal light required to obtain a constant target level of light at the output terminal  28 . ΔG 1  represents a difference between the actual gain and the target gain of signal light required to obtain the target level of light at the output terminal  28 . The difference of the expression (4) is subtracted from the target value G 1 , and a new target value G 1  is obtained in the expression (3).  
      The operation will be described with reference to  FIG. 2 : The operation performed when-an interstage loss increases will be described first; the operation performed when signal light input to the input terminal  11  increases will be described next; the operation for detecting a disconnection without providing a PD before the DCF  32  will be described next; and the operation of safety optical level control and the operation to restore the connection will be described last. The first operation performed when an interstage loss increases will next be described.  
       FIGS. 3A and 3B  are views given to describe the operation of the optical amplifier when an interstage loss increases.  FIG. 3A  shows the optical amplifier shown in  FIG. 2 , omitting the EDF gain control block  44  and the AMP gain control block  45 .  
       FIG. 3B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 3A . The height of waveforms in  FIG. 3B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 3A .  
      Suppose that the amount of loss by the VOA  19  and the amount of loss by the DCF  32  are initially stable as represented by waveform A and that the amount of loss by the DCF  32  increases as represented by waveform B.  
      The AMP gain control block  45  increases the target value G 1  of the AGC  41  by the amount of loss caused by the DCF  32  so that the target level of signal light can be obtained at the output terminal  28 .  
      The increase in target value G 1  increases the gain of the EDF  16 , and the total gain of the EDF  16  and the EDF  25  cannot be kept constant. The interstage loss control block  42  then reduces the amount of loss caused by the VOA  19  as represented by waveform C so that the gain of the EDF  16  can be reduced. In other words, to reduce the gain of the EDF  16 , the amount of loss by the VOA  19  is reduced by the amount of reduction to be made in the gain of the EDF  16  to absorb the loss by the DCF  32 .  
      If the interstage loss increases, the optical amplifier operates as described above to keep a constant amount of loss caused by the VOA  19  and the DCF  32 . The gain of the EDF  16  and the EDF  25  is kept constant, and the target level of light is obtained at the output terminal  28 . The AMP gain control block  45  and the interstage loss control block  42  repeat the calculation of the expressions (1) to (4) under PID control to obtain final target values G 1  and L 1  (it seems ultimately that just the amount of loss of the VOA  19  increases).  
      The operation performed when the signal light input to the input terminal  11  increases will be described next.  
       FIGS. 4A and 4B  are views given to describe the operation of the optical amplifier when signal light increases.  FIG. 4A  shows the optical amplifier shown in  FIG. 2 , omitting the EDF gain control block  44  and the AMP gain control block  45 .  
       FIG. 4B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 4A . The height of waveforms in  FIG. 4B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 4A .  
      Suppose that the level of signal light input to the input terminal  11  is as represented by waveform A and that the optical level per wave increases to increase the level of signal light input to the input terminal  11  as represented by waveform B.  
      The AMP gain control block  45  reduces the target value G 1  of the AGC  41  so that the target level of signal light is kept at the output terminal  28 .  
      The decrease in the target value G 1  decreases the gain of the EDF  16 , and the gain of the EDF  16  and the EDF  25  is not kept constant. The interstage loss control block  42  increases the amount of loss by the VOA  19  as represented by waveform C so that the gain of the EDF  16  can be increased. In other words, to increase the gain of the EDF  16 , the amount of loss by the VOA  19  is increased by the amount of increase to be made in the gain of the EDF  16  to absorb the increase of the level of the input signal light.  
      If the optical level per wave increases in signal light, the optical amplifier keeps the amount of loss by the VOA  19  and the DCF  32  constant through the operation described above. The gain of the EDF  16  and EDF  25  is kept constant, and the target level of light is obtained at the output terminal  28 . The AMP gain control block  45  and the interstage loss control block  42  repeat the calculation of the expressions (1) to (4) to obtain the final target values G 1  and L 1 .  
      The operation to detect a disconnection without using a PD before the DCF  32  will be described next.  
       FIGS. 5A and 5B  are views given to describe the operation of the optical amplifier to detect a disconnection.  FIG. 5A  shows the optical amplifier shown in  FIG. 2 , omitting the EDF gain control block  44  and the AMP gain control block  45 .  
       FIG. 5B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 5A . The height of waveforms in  FIG. 5B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 5A .  
      Suppose that signal light is input to the optical amplifier and the level of signal light is stable as represented by waveform A. When the DCF  32  comes off from the terminal  33 , the amount of loss by the DCF  32  increases, and the interstage loss control block  42  decreases the amount of loss by the VOA  19  so that the total amount of loss caused by the DCF  32  and the VOA  19  is kept constant. Because the DCF  32  is disconnected from the terminal  33 , the amount of loss by the VOA  19  continues decreasing as indicated by an upward arrow in the figure, and the interstage loss control block  42  finally opens the VOA  19  (minimizes the amount of loss caused by the VOA  19 ) as represented by waveform B.  
      The amount of loss caused by the DCF  32 , disconnected from the terminal  33 , is large as represented by waveform C, and the power of light detected by the PD  22  becomes small even if the VOA  19  is open. As a result, the amount of loss caused by the open VOA  19  and the DCF  32  exceeds a threshold, and the interstage loss control block  42  detects a disconnection (of the DCF  32 ). The threshold is given by expression (5) below:
 
 Lth=Ldm+Lvdl+Lm   (5)
 
 where, Lth represents the threshold at which a disconnection is detected; Ldm represents the maximum allowable value of loss in the stage between the terminals  31  and  33  or the maximum value of loss which DCF, an external attenuating medium, allows for; Lvdl is the value of loss when the VOA  19  is open, that is, when the amount of attenuation by the VOA  19  is minimized; and Lm is a disconnection detection margin. The interstage loss control block  42  outputs a control signal to the VOA  19  and can determine that the VOA  19  is open. When the voltage controlling the VOA  19  reaches the level at which the VOA  19  opens, the interstage loss control block  42  recognizes that the VOA  19  is open. The amount of loss by the open VOA  19  can be known beforehand in the design phase and the like and is stored in memory. The values of Ldm and Lm are also determined in advance as requirements and stored in memory. 
 
      The amount of loss produced when the DCF  32  is connected can be detected from the PD  18  and the PD  22 . This is because the loss of the open VOA  19  is constant. Accordingly, a disconnection can be detected from the amount of loss before and after the DCF  32 , without providing a PD before the DCF  32 . When a disconnection is detected, the interstage loss control block  42  turns off the AGC  43  to reduce the gain of the EDF  25  as represented by waveform D. In the example shown in  FIGS. 5A and 5B , the target value G 1  of the AGC  41  would increase if the AMP gain control block  45  operates to keep the level of light constant at the output terminal  28 , as given by the expressions (3) and (4). Therefore, when a disconnection is detected, the operation of the AMP gain control block  45  is stopped to make the AGC  41  stop updating the target value G 1 , preventing the output level at the terminal  33  from increasing.  
      If the AMP gain is changed by a single VOA  19  as shown in  FIGS. 2 and 5 , the VOA  19  would have a margin for reducing the loss further when the interstage loss is maximized. In this case, the VOA  19  opens accordingly (to reduce the loss), and the threshold at which a disconnection is detected varies with the AMP gain. The threshold is given by expression (6) below:
 
 Lth=Ldm+Lvdl+Lm− (AMP-gain−maximum-AMP-gain)  (6)
 
 where, AMP-gain is the current AMP gain, or a gain between the stage preceding the EDF  16  and the stage following the EDF  25 ; and maximum-AMP-gain is the maximum gain of the optical amplifier, or an AMP gain when the VOA  19  is widest open to provide Lvdl. 
 
      The operation of safety optical level control and the operation to restore a connection without using a PD before the DCF  32  will be described next.  
      To restore a connection is to connect the external attenuating medium again between the front optical amplifier block and the back optical amplifier block.  
       FIGS. 6A and 6B  are views given to describe the operation of safety optical level control and the operation of the optical amplifier to restore a connection.  FIG. 6A  shows the optical amplifier shown in  FIG. 2 , omitting the EDF gain control block  44  and the AMP gain control block  45 .  
       FIG. 6B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 6A . The height of waveforms in  FIG. 6B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 6A .  
      When a disconnection is detected, the interstage loss control block  42  performs safety optical level control so that a great level of light is not output to the stage (terminal  31 ) preceding the user-detachable DCF  32 . More specifically, the PD  18  monitors the level of light before the VOA  19 , and the AGC  41  controls the gain of the EDF  16  so that a safety level of light is obtained at the terminal  31 .  
      When a disconnection is detected, the VOA  19  is open, and the interstage loss control block  42  determines the level of light that should be detected by the PD  18  by using the amount of loss by the open VOA  19  stored in memory. The level of light that should be detected by the PD  18  to ensure a safety level of signal light before the DCF  32  is given by expression (7) below.
 
 PDs=P safe+ Lvdl   (7)
 
 where, PDs represents the level of light that should be detected by the PD  18 ; Psafe represents a safety level of light that should be output from the VOA  19 ; and Lvdl represents the amount of loss obtained when the VOA  19  is open. When a disconnection is detected, the interstage loss control block  42  controls the AGC  41  so that the level of light meeting the expression (7) is detected by the PD  18 . The value of Psafe is stored in memory in advance. 
 
      The operation to restore a connection will be, described next.  
      Suppose that the interstage loss control block  42  shown in  FIG. 6  detects a disconnection and controls the AGC  41  so that a safety level of light is obtained before the DCF  32 , as represented by waveform A. When the connection is restored (the DCF  32  is connected to the terminals  31  and  33 ), the amount of loss across the DCF  32  decreases. The interstage loss control block  42  monitors the amount of loss caused by the VOA  19  and the DCF  32  by means of the PD  18  and the PD  22  and recognizes that the connection is restored when the amount of loss falls below a restoration threshold. The restoration threshold is given by expression (8) below:
 
 Lret=Ldm+Lvdl−Lm   (8)
 
 where, Lret represents the restoration threshold at which the restoration of a connection is detected; Ldm represents the maximum allowable value of loss between the terminals  31  and  33 ; Lvdl represents the value of loss when the VOA  19  is open; and Lm represents a connection restoration detection margin. These values are stored in memory in advance. 
 
      When the connection of the DCF  32  is restored and when the amount of loss caused by the VOA  19  and the DCF  32  falls below the restoration threshold Lret, as represented by waveform B, the interstage loss control block  42  recognizes that the connection is restored. The interstage loss control block  42  stops the safety optical level control of the AGC  41 , enabling usual gain control of the EDF  16 , and starts the operation of the AGC  43  again. Signal light is amplified as represented by waveform C and is output from the output terminal  28  at a desired level.  
      The amount of loss by the DCF  32  can be detected by the PD  18  and the PD  22  because the loss is constant when the VOA  19  is open. Accordingly, the restoration of the connection can be detected in accordance with an accurate amount of loss caused by the DCF  32 , without providing a PD before the DCF  32 .  
      When the VOA is open, the amount of loss varies little with temperature. Accordingly, a disconnection or a restored connection can be detected in accordance with the accurate amount of loss caused by the DCF.  
       FIG. 7  shows the VOA varying with temperature. The horizontal axis of the graph represents a control signal for controlling the amount of loss by the VOA, and the vertical axis represents the transmission loss of the VOA. As shown in the figure, the transmission loss varies with temperature even if the value of the control signal is the same.  
      An arrow X 1  represents that the transmission loss widely varies with temperature at a great distance from the open VOA. An arrow X 2  represents that the transmission loss varies a little with temperature when the VOA is open. Because the loss varies a little with temperature when the VOA is open, a connection and disconnection can be detected from the accurate amount of loss caused by the DCF.  
      Accordingly, a connection and disconnection of the DCF  32  can be detected from the amount of attenuation by the DCF  32 , by means of the PD  18  of the front optical amplification block  10  and the PD  22  of the back optical amplification block  20 , without providing a PD before the DCF  32 . Therefore, SN degradation can be avoided, and the power consumption can be reduced.  
      Because no PD is required before the DCF  32 , the cost can be reduced.  
      The restoration of the connection is determined from the interstage loss when the VOA  19  is open, so that the optical amplifier will not be frozen without attaining the target value. If the restoration of the connection is recognized just by monitoring the loss caused by the DCF  32  without opening the VOA  19 , the target value might not be attained because of a monitoring error even when the VOA is fully open. The optical amplifier shown in  FIG. 2  detects the restoration of connection in accordance with the value of loss caused by the DCF, by means of the PD  18  and the PD  22  while the VOA  19  is open. Because the VOA is open while the limit value is known, the optical amplifier will not be frozen without attaining the target value.  
      A second embodiment of the present invention will be described in detail with reference to drawings. Whereas the first embodiment uses a VOA connected in series with a DCF to absorb the dynamic range, the second embodiment provides another VOA to absorb the dynamic range.  
       FIG. 8  is a block diagram of an optical amplifier of the second embodiment. As shown in the figure, the optical amplifier includes a front optical amplification block  50 , a back optical amplification block  70 , a DCF  82  connected to terminals  81  and  83 , AGCs  91  and  93 , an interstage loss control block  92 , an EDF gain control block  94 , a constant loss control block  95 , and an AMP gain control block  96 . The front optical amplification block  50  includes an input terminal  51 , couplers  52 ,  54 ,  57 ,  60 ,  62 , and  65 , PDs  53 ,  58 ,  61 , and  66 , LDs  55  and  63 , EDFs  56  and  64 , and VOAs  59  and  67 . The back optical amplification block  70  includes couplers  71 ,  73 , and  76 , PDs  72  and  77 , an LD  74 , an EDF  75 , and an output terminal  78 .  
      The input terminal  51  of the front optical amplification block  50  receives a WDM signal light, for instance. The signal light input to the input terminal  51  is output through the couplers  52  and  54  to the EDF  56 . The signal input to the input terminal  51  is also branched off to the PD  53  by the coupler  52 . The PD  53  converts the power of the input light to an electric signal and outputs the signal to the AGC  91 , the EDF gain control block  94 , and the AMP gain control block  96 .  
      The LD  55  outputs pumped light as controlled by the AGC  91 . The coupler  54  combines the pumped light output from the LD  55  with the signal light and outputs the combined light to the EDF  56 . Now, the signal light has a gain depending on the power of the pumped light.  
      The signal light output from the EDF  56  is branched off to the PD  58  by the coupler  57 . The PD  58  converts the power of input light to an electric signal and outputs the signal to the EDF gain control block  94 . The signal light from the EDF  56  is also output to the VOA  59 . The VOA  59  attenuates the input signal light as controlled by the EDF gain control block  94 , and outputs the attenuated light to the coupler  60 .  
      The PD  61  converts the power of signal light output from the VOA  59  to an electric signal and outputs the signal to the EDF gain control block  94 .  
      The ID  63  outputs pumped light as controlled by the AGC  91 . The coupler  62  combines the pumped light output from the LD  63  with the signal light and outputs the combined light to the EDF  64 . Now, the signal light has a gain depending on the power of the pumped light.  
      The signal light output from the EDF  64  is branched off to the PD  66  by the coupler  65 . The PD  66  converts the power of the input light to an electric signal and outputs the signal to the AGC  91 , the interstage loss control block  92 , the EDF gain control block  94 , and the constant loss control block  95 . The signal light is also output from the EDF  64  to the VOA  67 . The VOA  67  attenuates the input signal light as controlled by the interstage loss control block  92  and outputs the attenuated light to the terminal  81 .  
      The DCF  82  is connected to the terminals  81  and  83  and compensates for wavelength dispersion of the signal light output from the front optical amplification block  50 . The DCF  82  can be replaced by the user, and a DCF satisfying the user requirements is connected to the terminals  81  and  83 .  
      The signal light output from the DCF  82  is branched off to the PD  72  by the coupler  71  of the back optical amplification block  70 . The PD  72  converts the power of the input light to an electric signal and outputs the signal to the interstage loss control block  92 , the AGC  93 , and the constant loss control block  95 . The signal light output from the DCF  82  is also output through the couplers  71  and  73  to the EDF  75 .  
      The LD  74  outputs pumped light as controlled by the AGC  93 . The coupler  73  combines the pumped light output from the LD  74  with the signal light and outputs the combined light to the EDF  75 . Now, the signal light has a gain depending on the power of the pumped light.  
      The signal light output from the EDF  75  is branched off to the PD  77  by the coupler  76 . The PD  77  converts the power of the input light to an electric signal and outputs the signal to the AGC  93  and the AMP gain control block  96 . The signal light output from the EDF  75  is also output to the output terminal  78 .  
      The AGC  91  controls the pumped light of the LD  55  and the LD  63  with reference to the electric signals output from the PD  53  and the PD  66  so that the gain between the stage preceding the EDF  56  and the stage following the EDF  64  becomes a target value G 1 . The target value G 1  is calculated by the AMP gain control block  96 .  
      The interstage loss control block  92  performs control to bring the amount of loss caused by the VOA  67  and the DCF  82  to a target value L 2 , with reference to the electric signals output from the PD  66  and the PD  72 . The target value L 2  is calculated by the constant loss control block  95 . The interstage loss control block  92  detects a connection or disconnection in accordance with the electric signals output from the PD  66  and the PD  72 . When a disconnection is detected, the AGC  91  is controlled so that a safety level of signal light is output from the VOA  67 , and the AGC  93  is turned off.  
      The AGC  93  controls the pumped light of the LD  74  with reference to the electric signals output from the PD  72  and the PD  77  so that the gain of the EDF  75  becomes a target value G 2 . The target value G 2  is a fixed value, and the AGC  93  performs control to keep the gain of the EDF  75  to G 2  always.  
      The EDF gain control block  94  calculates a target value L 1  with which the VOA  59  is controlled to keep the total gain of the EDF  56  and the EDF  64  constant, in accordance with the electric signals of the PD  53 , the PD  58 , the PD  61 , and the PD  66 . If the total gain of the EDF  56  and the EDF  64  changes, the waveform characteristics of the signal light would change. The VOA  59  absorbs the gain of the EDF  56  and the EDF  64  varied by the AGC  91 . The target value L 1  of the VOA is given by expressions (9) and (10) below:
 
 L 1 =L 1 −ΔL 1  (9)
 
Δ L 1=( PD 58 −PD 53)+( PD 66 −PD 61)− EDFfg   (10)
 
      “PD 58 −PD 53 ” of the expression (10) represents the gain of the EDF  56  calculated from the electric signals of the PD  58  and the PD  53 . “PD 66 −PD 61 ” represents the gain of the EDF  64  calculated from the electric signals of the PD  66  and the PD  61 . EDFfg is the total gain of the EDF  56  and the EDF  64 , which must be kept constant, and is stored in memory in advance. ΔL 1  represents a difference between the total gain of the EDF  56  and the EDF  64 , which must be kept constant, and the actual gain. The difference of the expression (10) is subtracted from the target value L 1 , and a new target value L 1  is obtained, as given by the expression (9).  
      The constant loss control block  95  calculates a target value L 2  from the electric signals of the PD  66  and the PD  72  such that the amount of loss caused by the VOA  67  and the DCF  82  becomes constant. The calculated target value L 2  is output to the interstage loss control block  92 . The target value L 2  is given by expression (11) below:
 
 L 2 =Ldm+Lvdl   (11)
 
 where, Ldm represents the maximum allowable value of loss in the stage between the terminals  81  and  83 ; and Lvdl represents the value of loss produced when the VOA  67  is open. The VOA  67  can sufficiently absorb changes in the loss caused by the DCF  82  by setting the target value L 2  to the sum of the maximum allowable loss of the stage and the value of loss produced when the VOA  67  is open. 
 
      The AMP gain control block  96  controls the AGC  91  in accordance with the electric signals of the PD  53  and the PD  77  such that a target level of light is kept at the output terminal  78 . The target value G 1  with which the AGC  91  is controlled is given by expressions (12) and (13) below:
 
 G 1 =G 1 −ΔG 1  (12)
 
Δ G 1=( PD 77 −PD 53)−target-AMP-gain  (13)
 
      “PD 77 −PD 53 ” in the expression (12) represents the gain between the stage preceding the EDF  56  and the stage following the EDF  75 . The signal light must obtain the target AMP gain so that the target level of light is obtained at the output terminal  78 . ΔG 1  represents a difference between the actual gain and the target gain of the signal light for obtaining the target level of light at the output terminal  78 . The difference given by the expression (13) is subtracted from the target value G 1 , and a new target value G 1  is obtained.  
      The operation of the optical amplifier shown in  FIG. 8  will next be described.  
       FIGS. 9A and 9B  are views given to describe the operation of the optical amplifier.  FIG. 9A  shows the optical amplifier shown in  FIG. 8 , omitting the AGCs  91  and  93 , the interstage loss control block  92 , the EDF gain control block  94 , the constant loss control block  95 , and the AMP gain control block  96 .  
       FIG. 9B  shows the level of signal light varying in different stages of the optical amplifier shown in  FIG. 9A . The height of waveforms in  FIG. 9B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 9A .  
      Suppose that the level of signal light input to the input terminal  51  is as represented by waveform A and that the optical level per wave increases to increase the level of signal light input to the input terminal  51  as represented by an upward arrow given in the figure.  
      The AMP gain control block  96  changes the gain of the EDF  56  and the EDF  64  so that the constant level of signal light is kept at the output terminal  78  as represented by waveform B. The amount of loss by the VOA  59  is also changed to keep the total gain of the EDF  56  and the EDF  64  constant.  
      If the amount of loss by the DCF  82  decreases as represented by a downward arrow given in the figure, the constant loss control block  95  increases the amount of loss caused by the VOA  67  so that the amount of loss by the VOA  67  and the DCF  82  is kept constant.  
      Accordingly, the optical amplifier operates in such a manner that the constant gain of the EDFs is kept and the target level of light is obtained at the output terminal  78 .  
      The operation to detect a disconnection will next be described. When a disconnection occurs, the amount of loss by the DCF  82  increases. The constant loss control block  95  functions to keep a constant amount of loss, reducing the amount of loss by the VOA  67  and finally opening the VOA  67 . When the amount of loss by the open VOA  67  and the DCF  82  exceeds a threshold, the interstage loss control block  92  recognizes that an interstage loss appears. The threshold of interstage loss is given by expression (14) below:
 
 Lth=Ldm+Lvdl+Lm   (14)
 
 where, Lth represents a threshold at which a disconnection is detected; Ldm represents the maximum allowable value of loss between the terminals  81  and  83 ; Lvdl represents the value of loss obtained when the VOA  67  is open; and Lm represents a disconnection detection margin. 
 
      The interstage loss control block  92  outputs a control signal to the VOA  67  and can recognize that the VOA  67  is open. If the voltage controlling the VOA  67  reaches a level for opening the VOA  67 , the interstage loss control block  92  recognizes that the VOA  67  is open. The amount of loss obtained when the VOA  67  is open can be known in advance, such as in the design phase, and stored in memory. The values of Ldm and Lm are determined in advance as requirements and stored in memory.  
      In the configuration given above, the VOA  59  alone absorbs the AMP gain, but the VOA  67  may also be used to absorb the AMP gain. In that case, the target value L 2  is given by expression (15) below:
 
 L 2 =Ldm+Lvdl+VOA -absorption  (15)
 
      VOA-absorption represents the amount of AMP gain absorbed by the VOA  67 . The threshold of the expression (14) is given by expression (16) below:
 
 Lth=Ldm+Lvdl+Lm+VOA -absorption  (16)
 
      For instance, the lower limit Gm of the gain between the PD  53  and the PD  66  is stored in memory in advance, and if the target value G 1  of the gain between the PD  53  and the PD  66  falls below the lower limit Gm, VOA-absorption is set to Gm−G 1  and the gain is absorbed by the VOA  67 . This Gm is output as the target value G 1  to the AGC  91 . Even if the target value of gain falls below the lower limit, a corresponding amount of loss is absorbed by the VOA  67 , and NF degradation of the VOA  59  can be avoided.  
      Safety optical level control and the restoration of connection will next be described. When a disconnection is detected, the interstage loss control block  92  performs safety optical level control so that a great level of light is not output to the stage (terminal  81 ) preceding the user-detachable DCF  82 . More specifically, the PD  66  monitors the level of light before the VOA  67 , and the AGC  91  controls the gain of the EDF  56  and the EDF  64  so that a safety level of light is obtained at the terminal  81 .  
      When a disconnection is detected, the VOA  67  is open, and the interstage loss control block  92  determines the level of light to be detected by the PD  66  in accordance with the amount of loss caused by the open VOA  67  stored in memory. The level of light that should be detected by the PD  66  when a safety level of light is obtained before the DCF  82  is given by expression (17) below:
 
 PDs=P safe+ Lvdl   (17)
 
 where, PDs represents the level of light that should be detected by the PD  66 ; Psafe represents a safety level of light that should be output from the VOA  67 ; and Lvdl represents an amount of loss produced when the VOA  67  is open. When a disconnection is detected, the interstage loss control block  92  controls the AGC  91  so that the level of light meeting the expression (17) is detected by the PD  66 . The value of Psafe is stored in memory in advance. 
 
      When a disconnection is detected, the interstage loss control block  92  also turns off the AGC  93 . This prevents the output from increasing excessively at the terminal  81 , so that the operation of the AMP gain control block  96  stops and the updating of the target value G 1  of the AGC  91  also stops.  
      When the DCF  82  is connected to the terminals  81  and  83 , the amount of loss across the DCF  82  decreases. The interstage loss control block  92  monitors the amount of loss caused by the VOA  67  and the DCF  82  by means of the PD  66  and the PD  72 . When the amount of loss falls below the restoration threshold, the interstage loss control block  92  recognizes that the connection is restored. The restoration threshold is given by expression (18) below.
 
 Lret=Ldm+Lvdl−Lm   (18)
 
 where, Lret represents the restoration threshold at which the restoration of connection is detected; Ldm represents the maximum allowable value of loss in the stage between the terminals  81  and  83 ; Lvdl represents a value of loss produced when the VOA  67  is open; and Lm represents a connection restoration detection margin. These values are stored in memory in advance. The optical amplifier shown in  FIG. 8  operates as described above. 
 
      A connection and disconnection of the DCF  82  can be detected in accordance with the amount of attenuation by the DCF  82  by means of the PD  66  of the front optical amplification block  50  and the PD  72  of the back optical amplification block  70 , without providing a PD before the DCF  82 . Accordingly, SN degradation can be avoided, and the power consumption can be reduced.  
      Because no PD is required before the DCF  82 , the cost can be reduced.  
      An example hardware configuration of the optical amplifier will be described next.  
       FIG. 10  shows a hardware configuration of the optical amplifier. Elements identical to elements shown in  FIG. 8  are denoted by the same reference numerals, and a description of those identical elements will be omitted. Although some elements shown in  FIG. 10  do not correspond to the elements shown in  FIG. 8 , the overall function is the same: the VOA  59  absorbs the AMP gain; a constant level of interstage loss is kept by the VOA  67 ; and a target level of signal light is output from the output terminal  78 .  
      A computation device  120  shown in  FIG. 10  is a single semiconductor chip or a microprocessor. The computation device  120  may also include analog-to-digital converters (ADCs)  101 ,  103 ,  105 ,  107 ,  109 , and  111 , and digital-to-analog converters (DACs)  102 ,  104 ,  106 ,  108 , and  110 .  
      A LOG conversion block  121  in the computation device  120  converts digitally converted PD data to a log value. In the shown example, digitally converted data of the PDs  53 ,  58 ,  61 ,  66 ,  72 , and  77  are converted log values P 1 , P 2 , P 3 , P 4 , P 5 , and P 6  (dBm).  
      An AGC  121 a calculates a gain C_G 1  between the stage preceding the EDF  56  and the stage following the EDF  64 . The gain C_G 1  is given by expression (19) below:
 
 C   —   G 1 =P 4 −P 1  (19)
 
      An AMP gain control block  125  calculates such a target value T_G 1  of the AGC  121   a  that a target level of light is kept after the EDF  64 . The target value T_G 1  is given by expressions (20) and (21) below:
 
 T   —   G 1 =T   —   G 1 +ΔG 1  (20)
 
Δ G 1 =C   —   G 1 =C   —   L 2 +C   —   G 2−target-AMP-gain  (21)
 
 where, C_L 2  represents an interstage loss caused by the VOA  67  and the DCF  82 , and C_G 2  represents the gain of the EDF  75 . C_L 2  and C_G 2  are kept to a constant level, that is, C_G 1  is kept to a constant level. An external device  130  calculates such a target AMP gain that a constant level of output is kept by the optical amplifier, and the AMP gain control block  125  may have the same function. 
 
      The AGC  121   a  controls the ID  55  and the LD  63  so that the gain C_G 1  matches the target value T_G 1  calculated by the AMP gain control block  125 .  
      A loss control block  122  calculates the amount of loss C_L 1  before and after the VOA  59 . C_L 1  is given by expression (22) below:
 
 C   —   L 1 =P 2 −P 3  (22)
 
      An EDF gain control block  126  calculates such an amount of loss caused by the VOA  59  that the total gain of the EDF  56  and the EDF  64  is kept constant. The target value T_L 1  of the amount of loss is given by expressions (23) and (24) below:
 
 T   —   L 1 =T   —   L 1 −ΔL 1  (23)
 
Δ L 1 =C   —   G 1 +C   —   L 1 −EDFfg   (24)
 
      EDFfg is stored in an internal memory  128  in advance.  
      The loss control block  122  controls the VOA  59  so that the amount of loss C_L 1  matches the target value T_L 1  calculated by the EDF gain control block  126 .  
      An interstage loss control block  123  calculates an amount of loss C_L 2  between the VOA  67  and the DCF  82 . C_L 2  is given by expression (25) below:
 
 C   —   L 2 =P 4 −P 5  (25)
 
      An AGC  124  calculates the gain C_G 2  of the EDF  75 . C_G 2  is given by expression (26) below:
 
 C   —   G 2 =P 6 −P 5  (26)
 
      The AGC  124  controls the LD  74  so that the gain C_G 2  matches the target gain T_G 2  (fixed value) stored in the internal memory  128 .  
      An EDF gain control block  127  calculates such an amount of loss caused by the VOA  67  that the gain of the EDF  75  is kept constant. The target value T_L 2  of the amount of loss is given by expressions (27) and (28) below:
 
Δ L 2 =T   —   L 2 −ΔL 2  (27)
 
Δ T   —   L 2 =C   —   G 2 +C   —   L 2 −EDFflg   (28)
 
      EDFflg is stored in the internal memory  128  in advance.  
      The interstage loss control block  123  controls the VOA  67  so that the amount of loss C_L 2  matches the target value T_L 2  calculated by the EDF gain control block  127 . The gain C_G 2  included in the expression (28) is a fixed value, and the interstage loss control block  123  functions to keep the amount of loss caused by the VOA  67  and the DCF  82  constant.  
      The gain of signal light is controlled in this way to keep the amount of loss between the VOA  67  and the DCF  82  constant.  
      A third embodiment of the present invention will be described in detail with reference to drawings. The third embodiment differs from the second embodiment in that the front optical amplification block  50  and the back optical amplification block  70  are interchanged.  
       FIG. 11  shows an optical amplifier of the third embodiment. As shown in the figure, the optical amplifier includes a front optical amplification block  140 , a back optical amplification block  150 , a DCF  172  connected to terminals  171  and  173 , AGCs  181  and  183 , an interstage loss control block  182 , a constant loss control block  184 , an EDF gain control block  185 , and an AMP gain control block  186 .  
      The front optical amplification block  140  includes an input terminal  141 , couplers  142 ,  144 , and  147 , PDs  143  and  148 , an LD  145 , and an EDF  146 . The back optical amplification block  150  includes couplers  151 ,  153 ,  156 ,  159 ,  161 , and  164 , PDs  152 ,  157 ,  160 , and  165 , LDs  154  and  162 , EDFs  155  and  163 , a VOA  158 , and an output terminal  166 . The front optical amplification block  140  is analogous to the back optical amplification block  70  shown in  FIG. 8 , and a detailed description of the front optical amplification block  140  will be omitted. The back optical amplification block  150  is analogous to the front optical amplification block  50  shown in  FIG. 8 , and a detailed description of the back optical amplification block  150  will be omitted.  
      The AGC  181  operates so that the gain of the EDF  146  matches a fixed target value G 1 . The interstage loss control block  182  controls the VOA  149  so that the amount of loss caused by the VOA  149  and the DCF  172  matches a target value L 1 . The AGC  183  controls pumped light of the LD  154  and the LD  162  so that the gain between the stage preceding the EDF  155  and the stage following the EDF  163  matches a target value G 2 .  
      The constant loss control block  184  calculates the target value L 1  such that the amount of loss caused by the VOA  149  and the DCF  172  becomes constant. The EDF gain control block  185  calculates such a target value L 2  of the VOA  158  that the total gain of the EDF  155  and the EDF  163  is always kept constant. The AMP gain control block  186  calculates the target value G 2  of the AGC  183  such that the constant level of light is kept at the output terminal  166 .  
      The target value L 1  calculated by the constant loss control block  184  is given by expression (29) below.
 
 L 1 =Ldm+Lvdl   (29)
 
      Ldm represents the maximum allowable value of loss between the terminals  171  and  173 . Lvdl represents the amount of loss when the VOA  149  is open.  
      The target value L 2  calculated by the EDF gain control block  185  is given by expressions (30) and (31) below: 
      ti  L 2 =L 2 −L 2  (30)
 
Δ L 2 =G 2 +L 2 −EDFfg   (31)
 
 where, G 2  represents a gain between the stage preceding the EDF  155  and the stage following the EDF  163 ; and L 2  represents an amount of current loss caused by the VOA  158 . “G 2 +L 2 ” represents the total gain of the EDF  155  and the EDF  163 . EDFfg represents the total gain of the EDF  155  and the EDF  163 , which must be kept constant. 
 
      The target value G 2  calculated by the AMP gain control block  186  is given by expressions (32) and (33):
 
 G 2 =G 2 =ΔG 2  (32)
 
Δ G 2=( P 6 −P 1)−target-AMP-gain  (33)
 
      “P 6 −P 1 ” in the expression (33) represents a gain between the stage preceding the EDF  146  and the stage following the EDF  163 , and target-AMP-gain represents a gain for obtaining a target level of signal light at the output terminal  166 .  
      The operation of the optical amplifier shown in  FIG. 11  will next be described.  
       FIGS. 12A and 12B  are views given to describe the operation of the optical amplifier.  FIG. 12A  shows the optical amplifier shown in  FIG. 11 , omitting the AGCs  181  and  183 , the interstage loss control block  182 , the constant loss control block  184 , the EDF gain control block  185 , and the AMP gain control block  186 .  
       FIG. 12B  shows the level of signal light varying in different stages of the optical amplifier shown in FIG.  12 A. The height of waveforms in  FIG. 12B  represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in  FIG. 12A .  
      Suppose that the level of signal light input to the input terminal  141  is as represented by waveform A and that the optical level per wave increases to increase the level of signal light input to the input terminal  141  as represented by waveform B.  
      Because the gain of the EDF  146  is kept constant, the gain of signal light at the EDF  146  is the same in waveforms A and B. The amount of loss caused by the VOA  149  and the DCF  172  is kept constant by the constant loss control function. Accordingly, different levels of signal light are input to the back optical amplification block  150 . The AMP gain control block  186  controls the EDF  155  and the EDF  163  such that a target level of signal light is output from the output terminal  166 . The EDF gain control block  185  controls the VOA  158  such that the total gain of the EDF  155  and the EDF  163  is kept constant.  
      If the amount of loss by the DCF  172  is reduced as shown in the figure from waveform C to waveform D, then the constant loss control block  184  increases the amount of loss by the VOA  149  such that the amount of loss caused by the VOA  149  and the DCF  172  is kept constant.  
      The optical amplifier operates in this way to obtain a constant EDF gain and the target level of signal light.  
      The operation to detect a disconnection will next be described. When a disconnection occurs, the amount of loss by the DCF  172  increases. This causes the constant loss control block  184  to function, decreasing the amount of loss by the VOA  149  and finally opening the VOA  149 . If the amount of loss by the open VOA  149  and the DCF  172  exceeds a threshold, the interstage loss control block  182  recognizes that an interstage loss occurs. The threshold of interstage loss is given by expression (34) below:
 
 Lth=Ldm+Lvdl+Lm   (34)
 
 where, Lth represents a threshold at which a disconnection is detected; Ldm represents the maximum allowable value of loss between the terminals  171  and  173 ; Lvdl represents the amount of loss produced when the VOA  149  is open; and Lm represents a disconnection detection margin. The interstage loss control block  182  outputs a control signal to the VOA  149  and can recognize that the VOA  149  is open. When the voltage for controlling the VOA  149  reaches the voltage at which the VOA  149  opens, the interstage loss control block  182  recognizes that the VOA  149  is open. The amount of loss produced when the VOA  149  is open can be known in advance, such as in the design phase, and stored in memory. The values of Ldm and Lm are determined in advance as requirements and stored in memory in advance. 
 
      In the configuration described above, the VOA  158  absorbs the AMP gain of the optical amplifier. The VOA  149  may also be used to absorb the AMP gain. In that case, the target value L 2  is given by expression (35) below:
 
 L 2 =Ldm+Lvdl+VOA -absorption  (35)
 
      VOA-absorption represents the amount of loss absorbed by the VOA  149 . The threshold of the expression (34) is given by expression (36) below:
 
 Lth=Ldm+Lvdl+Lm+VOA -absorption  (36)
 
      For instance, the lower limit Gm of the gain between the PD  152  and the PD  156  is stored in memory in advance, and if the target value G 2  of the gain between the PD  152  and the PD  165  falls below the lower limit Gm, VOA-absorption is set to Gm−G 2  and the gain is absorbed by the VOA  149 . This Gm is output as the target value G 2  to the AGC  183 . Even if the target value of the gain falls below the lower limit, a corresponding amount of loss is absorbed by the VOA  149 , and NF degradation of the VOA  158  can be avoided.  
      Safety optical level control and the restoration of connection will next be described. When a disconnection is detected, the interstage loss control block  182  performs safety optical level control such that a great level of light is not output to the stage (terminal  171 ) preceding the user-detachable DCF  172 . More specifically, the PD  148  monitors the level of light before the VOA  149 , and the AGC  181  controls the gain of the EDF  146  such that a safety level of light is obtained at the terminal  171 .  
      The interstage loss control block  182  determines the level of light that should be detected by the PD  148 , in accordance with the amount of loss of the open VOA  149  stored in memory. The light of level that should be detected by the PD  148  such that a safety level of light is obtained in the stage preceding the DCF  172  is given by expression (37) below:
 
 PDs=P safe +Lvdl   (37)
 
 where, PDs represents the level of light that should be detected by the PD  148 ; Psafe represents a safety level of light that should be output from the VOA  149 ; and Lvdl represents the amount of loss produced when the VOA  149  is open. When a disconnection is detected, the interstage loss control block  182  controls the AGC  181  such that the PD  148  detects the level of light meeting the expression (37). 
 
      When the DCF  172  is connected to the terminals  171  and  173 , the amount of loss across the DCF  172  decreases. The interstage loss control block  182  monitors the amount of loss caused by the VOA  149  and the DCF  172  by means of the PD  148  and the PD  152 . When the amount of loss falls below a restoration threshold, the interstage loss control block  182  recognizes that the connection is restored. The restoration threshold is given by expression (38) below:
 
 Lret=Ldm+Lvdl−Lm   (38)
 
 where, Lret represents the restoration threshold at which the restoration of the connection is detected; Ldm represents the maximum allowable amount of interstage loss between the terminals  171  and  173 ; Lvdl represents the amount of loss produced when the VOA  149  is open; and Lm represents a restoration detection margin. 
 
      A connection or disconnection of the DCF  172  can be detected in accordance with the amount of attenuation by the DCF  172  by means of the PD  148  of the front optical amplification block  140  and the PD  152  of the back optical amplification block  150 , eliminating the need for providing a PD before the DCF  172 . Accordingly, SN degradation can be avoided, and the power consumption can be reduced.  
      Because no PD is required before the DCF  172 , the cost can also be reduced.  
      An optical amplifier according to the present invention detects the amount of attenuation caused by a variable optical attenuator and an external attenuating medium by means of a front optical detection section of a front optical amplification block connected before the variable optical attenuator and the external attenuating medium connected in series and a back optical detection section of a back optical amplification block connected after the variable optical attenuator and the external attenuating medium, and performs control to keep a constant amount of signal light attenuation. A connection or disconnection of the external attenuating medium is detected in accordance with the amount of attenuation caused by the variable optical attenuator and the external attenuating medium when the amount of attenuation caused by the variable optical attenuator is minimized. Because the amount of attenuation caused by the variable optical attenuator is constant after it reaches the minimum, the connection or disconnection of the external attenuating medium can be detected in accordance with the amount of attenuation before and after the external attenuating medium, without providing any optical detection section before or after the external attenuating medium. SN degradation can be prevented, the cost effectiveness will not be degraded because of an increase in LD power, and the power consumption can be reduced by LD temperature control.  
      The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.