Abstract:
An optical amplifier includes: a rare-earth doped fiber configured to amplify signal light to thereby produce a amplified signal light; a gain control circuit configured to control an optical gain of the rare-earth doped fiber; a photodetector configured to detect intensities of different wavelength of light obtained from the amplified signal light; and an abnormality detection circuit configured to detect an abnormality of the signal light in accordance with a ratio or a difference between the intensities of the different wavelength.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-163629 filed on Jul. 24, 2012, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to the detection of the abnormality of an optical signal that occurs in an optical amplifier. 
       BACKGROUND 
       [0003]    In an optical amplifier including a rare-earth doped fiber and an excitation light source, a ratio between light levels detected in a light input monitoring unit and an output monitoring unit that include optical branching couplers and photodiodes, namely, an optical gain, is maintained at a given level owing to automatic gain control. 
         [0004]    An amplifier alarm circuit detects an increase in spontaneous emission light output from the optical amplifier. A signal component in a predetermined wavelength band passes through a band pass filter to remove the wavelength component of an optical signal and the power of the signal component is compared with a threshold value. When power exceeding the threshold value has been detected, it is determined that spontaneous emission light has increased, and an output abnormality alarm is output. 
         [0005]    In an optical transmission system, a case where the optical power of light having been received is smaller than a predetermined threshold value is determined as an abnormality. Each of a signal light wavelength component and a predetermined noise light component is extracted from transmitted light, the ratio of optical powers of the signal light wavelength component and the predetermined noise light component is compared with a predetermined value, and the deterioration of transmission quality is detected. 
         [0006]    A related technique is disclosed in Japanese Laid-open Patent Publication No. 2002-368698, Japanese Laid-open Patent Publication No. 2004-6887, Japanese Laid-open Patent Publication No. 10-209967, or Japanese Laid-open Patent Publication No. 11-186962. 
       SUMMARY 
       [0007]    According to an aspect of the embodiments, an optical amplifier includes: a rare-earth doped fiber configured to amplify signal light to thereby produce a amplified signal light; a gain control circuit configured to control an optical gain of the rare-earth doped fiber; a photodetector configured to detect intensities of different wavelength of light obtained from the amplified signal light; and an abnormality detection circuit configured to detect an abnormality of the signal light in accordance with a ratio or a difference between the intensities of the different wavelength. 
         [0008]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are examples and are explanatory in nature and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  illustrates an example of a hardware configuration of an optical amplifier; 
           [0011]      FIG. 2A  and  FIG. 2B  illustrate examples of an output spectrum; 
           [0012]      FIG. 3  illustrates an example of a gain-wavelength characteristic; 
           [0013]      FIG. 4A  and  FIG. 4B  illustrate examples of a spectrum of wavelength-multiplexed signal light; 
           [0014]      FIG. 5A ,  FIG. 5B , and  FIG. 5C  illustrate examples of a spectrum of equalized light; 
           [0015]      FIG. 6A  and  FIG. 6B  illustrate examples of pass bands of band pass filters; 
           [0016]      FIG. 6C  illustrates an example of an equalization characteristic; 
           [0017]      FIG. 7  illustrates an example of an abnormality detection circuit; 
           [0018]      FIG. 8  illustrates an example of an operation of an optical amplifier; 
           [0019]      FIG. 9A  and  FIG. 9B  illustrate examples of a wavelength region; and 
           [0020]      FIG. 10  illustrates an example of a hardware configuration of an optical amplifier. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    When a failure caused by increase in optical-loss has occurred in an optical component in an optical amplifier, a light output may be decreased or the ratio of the power of a spontaneous emission light (amplified spontaneous emission: ASE) component to the power of a signal component may be increased. The ratio of the power of the ASE component to the power of the signal component may be increased based on an increase in a transmission path loss between optical amplifiers. Therefore, based on a power ratio between the signal component and the ASE component, another factor other than the optical amplifier may be detected as the abnormality of the optical amplifier. 
         [0022]      FIG. 1  illustrates an example of the hardware configuration of an optical amplifier. An optical amplifier  1  includes optical branching couplers  10 ,  11 ,  12 , and  21 , photodiodes (PD)  13 ,  14 ,  24 , and  25 , an excitation light source (PS)  15 , an optical coupler  16 , and a rare-earth doped fiber  17 . The optical amplifier  1  includes an equalizer (EQ)  18 , optical isolators  19  and  20 , and band pass filters (BPF)  22  and  23 . The optical amplifier  1  includes an abnormality detection circuit  30  and a control circuit  31 . 
         [0023]    The abnormality detection circuit  30  and the control circuit  31  may include a logic circuit such as an application specific integrated circuit (ASIC) or a field-programming gate array (FPGA). The abnormality detection circuit  30  and the control circuit  31  may also include amplifier circuits and analog-digital converter circuits, used for reading the detection signals of the photodiodes  13 ,  14 ,  24 , and  25 , and a digital-analog converter circuit and a drive circuit, used for driving the excitation light source  15 . 
         [0024]    In the drawing, the photodiode, the excitation light source, the equalizer, and the band pass filter may be expressed as “PD”, “PS”, “EQ”, and “BPF”, respectively. The hardware configuration illustrated in  FIG. 1  is just exemplified, and the configuration thereof may be arbitrary. 
         [0025]    The photodiode  13  detects and supplies, to the control circuit  31 , the optical power of the input optical signal of the optical amplifier  1 , which has branched from the optical branching coupler  10 . The photodiode  14  detects and supplies, to the control circuit  31 , the optical power of the output optical signal of the optical amplifier  1 , which has branched from the optical branching coupler  11  and passed through the optical coupler  12 . The control circuit  31  feedback-controls the excitation light source  15  so that a ratio in optical power between the input optical signal and the output optical signal, for example, an optical gain, becomes a given level. The optical coupler  16  multiplexes and causes signal light, input from the optical branching coupler  10  through the optical isolator  19 , and excitation light from the excitation light source  15 , to enter the rare-earth doped fiber  17 . 
         [0026]    The equalizer  18  equalizes the wavelength characteristic of the signal light amplified by the rare-earth doped fiber  17 . To the equalizer  18 , a transmission characteristic is assigned whose characteristic is opposite to the gain-wavelength characteristic of the rare-earth doped fiber  17  according to a population inversion rate corresponding to a preliminarily defined optical gain. The output of the equalizer  18  passes through the optical isolator  20  and the optical branching coupler  11  and is output from the optical amplifier  1 . 
         [0027]    The output optical signal of the optical amplifier  1  having branched from the optical branching coupler  11  is caused to further branch by the optical branching coupler  12  and enters the optical branching coupler  21 . The optical branching coupler  21  causes the incident light to further branch and enter the band pass filters  22  and  23 . The band pass filters  22  and  23  individually extract and input the wavelength components of different wavelengths λ1 and λ2 to the photodiodes  24  and  25 . 
         [0028]    The photodiodes  24  and  25  detect the optical powers of these incident lights, for example, the wavelength components of the wavelengths λ1 and λ2 in the output optical signal of the optical amplifier  1 , and supplies the optical powers to the abnormality detection circuit  30 . Based on the optical powers of these wavelength components, the abnormality detection circuit  30  detects the abnormality of an optical signal, which has occurred based on the failure caused by increase in optical-loss of an optical component in the optical amplifier  1 . 
         [0029]      FIG. 2A  and  FIG. 2B  illustrate examples of an output spectrum.  FIG. 2A  illustrates the output spectrum of the optical amplifier  1 . The output spectrum of the optical amplifier  1  includes a plurality of signal components  35 ,  . . . , and  36  and an ASE component  37 . When the failure caused by increase in optical-loss of an optical component within the optical amplifier  1  has occurred, the signal components  35 , . . . , and  36  become reduced. For example, when a faulty optical component is an optical component between the optical branching coupler  11  and the rare-earth doped fiber  17 , the control circuit  31  increases the gain of the rare-earth doped fiber  17  so as to maintain output light power. Therefore, the ASE component is increased. This state is illustrated in  FIG. 2B . 
         [0030]    As illustrated in  FIG. 2B , the ASE component having been increased based on the failure caused by increase in optical-loss of the optical component has a characteristic of decreasing with an increase in a wavelength.  FIG. 3  illustrates an example of a gain-wavelength characteristic. In  FIG. 3 , the gain-wavelength characteristics of a rare-earth doped fiber are illustrated in various population inversion rates. In the wavelength bands of a C band (1530 to 1565 nm) and an L band (1565 nm to 1625 nm) used in optical communication, a gain decreases with an increase in a wavelength, and that tendency increases in a state where the population inversion rate is larger. 
         [0031]    When, at the time of the failure caused by increase in optical-loss of the optical component, the gain of the rare-earth doped fiber has increased, for example, the population inversion rate has increased, equalization due to the equalizer  18  becomes insufficient. Therefore, a gain-wavelength characteristic occurs where a gain decreases with an increase in a wavelength, and the power of the ASE component decreases with an increase in a wavelength. 
         [0032]      FIG. 4A  and  FIG. 4B  illustrate examples of the spectrum of wavelength-multiplexed signal light. In  FIG. 4A  and  FIG. 4B , the spectra of wavelength-multiplexed signal light are individually illustrated that are subjected to a small transmission path loss and a large transmission path loss. As illustrated in  FIG. 4A  and  FIG. 4B , the ratio of a signal to ASE decreases with an increase in the transmission path loss. A characteristic may not occur where the ASE component decreases in a long wavelength region such as when the population inversion rate of the rare-earth doped fiber has increased. 
         [0033]    Based on a ratio in optical power between the wavelength components of the different wavelengths λ1 and λ2 in the light amplified by the rare-earth doped fiber  17 , the abnormality detection circuit  30  detects the abnormality of an optical signal.  FIG. 5A ,  FIG. 5B , and  FIG. 5C  illustrate examples of the spectrum of equalized light.  FIG. 5A  illustrates the pattern diagram of the spectrum of light amplified by the rare-earth doped fiber  17  and equalized by the equalizer  18 . The light equalized by the equalizer  18  is led into the band pass filters  22  and  23  by the optical branching couplers  11 ,  12 , and  21 , and only the wavelength components of the wavelengths λ1 and λ2 are individually extracted. 
         [0034]      FIG. 5B  and  FIG. 5C  individually illustrate the pattern diagrams of the spectra of the wavelength components of the wavelengths λ1 and λ2 extracted by the band pass filters  22  and  23 . The optical power p1 and the optical power p2 of the wavelength components of the wavelengths λ1 and λ2 are detected by the photodiodes  24  and  25 , and input to the abnormality detection circuit  30 . 
         [0035]    Based on the optical power p1 and the optical power p2, the abnormality detection circuit  30  determines whether or not the ASE component decreases with an increase in a wavelength. When the ASE component decreases with an increase in a wavelength, the abnormality detection circuit  30  detects the failure caused by increase in optical-loss of an optical component. For example, when an optical power ratio r=p1/p2 is larger than a predetermined threshold value, the abnormality detection circuit  30  detects the failure caused by increase in optical-loss of an optical component. 
         [0036]      FIG. 6A  and  FIG. 6B  illustrate examples of the pass bands of the band pass filters  22  and  23 . So as to extract the wavelength components of the wavelengths λ1 and λ2, the band pass filters  22  and  23  individually cause only the wavelength regions of the wavelengths λ1 and λ2 to pass therethrough. The λ1 and λ2 may also be set in wavelength regions including no signal component. For example, in  FIGS. 6A and 6B , the wavelength λ1 may be a wavelength shorter than a wavelength region including a signal component, and the wavelength λ2 may be a wavelength longer than the wavelength region including the signal component. 
         [0037]      FIG. 6C  illustrates an example of an equalization characteristic. In  FIG. 6C , the equalization characteristic of the equalizer  18  is illustrated. An equalization region due to the equalizer  18  covers the wavelengths λ1 and λ2. Therefore, a difference between the optical power p1 and the optical power p2 becomes large in a state where the failure caused by increase in optical-loss of an optical component does not occur, and the failure caused by increase in optical-loss of an optical component may not be erroneously detected. 
         [0038]      FIG. 7  illustrates an example of an abnormality detection circuit. The abnormality detection circuit  30  includes power detection signal reception units  40  and  41 , a comparison unit  42 , a determination unit  43 , and an alarm output unit  44 . The power detection signal reception units  40  and  41  receive, from the photodiodes  24  and  25 , power detection signals indicating the values p1 and p2 of the optical power of the wavelengths λ1 and λ2. 
         [0039]    The comparison unit  42  compares the values p1 and p2 of the optical power of the wavelengths λ1 and λ2 with each other, and calculates a ratio r=p1/p2 of the optical power p1 to the optical power p2. When the optical power ratio r exceeds a threshold value Th, the determination unit  43  determines that the ASE component decreases with an increase in a wavelength and an abnormality has occurred in an optical signal based on the occurrence of the failure caused by increase in optical-loss of an optical component. When the optical power ratio r does not exceed the threshold value Th, the determination unit  43  may not determine that an abnormality has occurred in an optical signal. 
         [0040]    When the abnormality of an optical signal has been detected, the alarm output unit  44  outputs a predetermined alarm signal indicating that the failure caused by increase in optical-loss of an optical component has occurred and an abnormality has occurred in an optical signal. The alarm signal may be a visual signal visually giving notice of the occurrence of an abnormality. For example, the alarm signal may be the flashing of a lamp or an LED or the displaying of a message or an icon due to an image display device or a character display device. The alarm signal may also be an audible signal such as a message or a buzzer, which audibly gives notice of the occurrence of an abnormality. The alarm signal may also be an electromagnetic signal used for interrupting the operation of the optical amplifier  1  or notifying another device of the occurrence of an abnormality. 
         [0041]      FIG. 8  illustrates an example of the operation of an optical amplifier. A series of operations illustrated in  FIG. 8  may also include a plurality of procedures. 
         [0042]    In an operation AA, the optical branching couplers  11 ,  12 , and  21  cause the light amplified by the rare-earth doped fiber  17  to branch and be led into the band pass filters  22  and  23 . In an operation AB, the band pass filters  22  and  23  individually extract only the wavelength components of the wavelengths λ1 and λ2 from within the incident light. In an operation AC, the photodiodes  24  and  25  detect the optical power p1 and the optical power p2 of the wavelength components of the wavelengths λ1 and λ2. 
         [0043]    In an operation AD, the comparison unit  42  calculates the ratio r=p1/p2 of the optical power p1 to the optical power p2. In an operation AE, the determination unit  43  determines whether or not the optical power ratio r exceeds the threshold value Th. When the optical power ratio r exceeds the threshold value Th (the operation AE: Y), the processing proceeds to an operation AG. When the optical power ratio r does not exceed the threshold value Th (the operation AE: N), the processing proceeds to an operation AF. 
         [0044]    In an operation AF, the determination unit  43  determines that an optical component is normal. The processing returns to the operation AA. In the operation AG, the determination unit  43  determines that the failure caused by increase in optical-loss of an optical component has occurred and an optical signal is abnormal. The processing proceeds to an operation AH. In the operation AH, the alarm output unit  44  outputs the predetermined alarm signal. The processing returns to the operation AA. 
         [0045]    It is determined whether the abnormality of an optical signal detected based on an increase in the ASE component is caused by the failure of an optical component within the optical amplifier or caused by another factor. Therefore, the false detection of an optical signal abnormality occurring in the optical amplifier may be reduced, and it may be easy to specify a failure point. 
         [0046]    The wavelengths λ1 and λ2 whose optical power values are compared in the comparison unit  42  may be wavelengths within a wavelength region including no signal component.  FIG. 9A  and  FIG. 9B  illustrate examples of a wavelength region. In  FIG. 9A , wavelength regions including no signal component are illustrated. The wavelengths λ1 and λ2 may be selected from a region R 21  whose wavelength is shorter than the wavelength of a wavelength region R 1  including a signal and a region R 22  whose wavelength is longer than the wavelength of the wavelength region R 1  including the signal. 
         [0047]    In  FIG. 9B , wavelength regions including no signal component are illustrated. A wavelength region R 23  including no signal component may be provided in, for example, a region sandwiched between wavelength regions R 11  and R 12  including a signal. For example, the wavelengths λ1 and λ2 may be wavelengths located between the wavelength regions R 11  and R 12  including the signal. A wavelength region including the signal may be temporally switched, and the values p1 and p2 of the optical power may be detected during a time period when no signal is included in the wavelengths λ1 and λ2. 
         [0048]    For example, in the wavelength bands of an O band (1260 to 1360 nm) and an S band (1460 nm to 1530 nm), at the time of the failure caused by increase in optical-loss of an optical component, the power of the ASE component increases with an increase in a wavelength. Therefore, in an optical amplifier used in these wavelength bands, when the ASE component increases with an increase in a wavelength, the abnormality detection circuit  30  may determine that an abnormality has occurred in an optical signal based on the occurrence of the failure caused by increase in optical-loss of an optical component. For example, when the ratio r=p2/p1 of the optical power p2 to the optical power p1 has exceeded a threshold value, the abnormality detection circuit  30  may determine that an abnormality has occurred in an optical signal based on the occurrence of the failure caused by increase in optical-loss of an optical component. In any case of a case where the power of the ASE component decreases with an increase in a wavelength and a case where the power of the ASE component increases with an increase in a wavelength, the abnormality detection circuit  30  may also detect the occurrence of the failure caused by increase in optical-loss of an optical component. For example, in accordance with the wavelength-intensity characteristic of the ASE component included in light amplified by the rare-earth doped fiber  17  and equalized by the equalizer  18 , the abnormality detection circuit  30  may also detect the occurrence of the failure caused by increase in optical-loss of an optical component. For example, when a ratio between the optical power p1 and the optical power p2 has exceeded a predetermined acceptable range, it may be determined that an abnormality has occurred in an optical signal based on the occurrence of the failure caused by increase in optical-loss of an optical component. 
         [0049]    When, in place of the ratio between the optical power p1 and the optical power p2, a difference between the optical power p1 and the optical power p2 has exceeded a threshold value, it may be determined that an abnormality has occurred in an optical signal based on the occurrence of the failure caused by increase in optical-loss of an optical component. The band pass filters  22  and  23  individually detecting the wavelength components of λ1 and λ2 may be separated filters and may also be integrated filters. 
         [0050]      FIG. 10  illustrates an example of the hardware configuration of an optical amplifier. An optical amplifier  1  includes an input port  50 , optical branching couplers  51 ,  52 ,  60 ,  61 ,  62 , and  90 , and photodiodes  53 ,  54 ,  63 ,  64 ,  93 , and  94 . The optical amplifier  1  includes excitation light sources  55  and  65 , optical couplers  56  and  66 , rare-earth doped fibers  57  and  67 , and an output port  70 . The optical amplifier  1  includes an equalizer  58 , optical isolators  59  and  69 , a variable optical attenuator  68 , and band pass filters  91  and  92 . The optical amplifier  1  includes amplifier circuits  71 ,  72 ,  73 ,  74 ,  95 , and  97  and analog-digital converter circuits  75 ,  76 ,  77 ,  78 ,  96 , and  98 . The optical amplifier  1  includes drive circuits  79 ,  80 , and  83  and digital-analog converter circuits  81 ,  83 , and  84 . 
         [0051]    The optical amplifier  1  includes an automatic gain control circuit  100 , an automatic level control circuit  101 , and an abnormality detection circuit  102 . The automatic gain control circuit  100 , the automatic level control circuit  101 , and the abnormality detection circuit  102  include logic circuits such as ASICs or FPGAs. In the drawing, an analog-digital converter circuit, a digital-analog converter circuit, automatic gain control, and automatic level control may be expressed as “ADC”, “DAC”, “AGC”, and “ALC”, respectively. A variable optical attenuator may be expressed as “VOA”. 
         [0052]    A preceding-stage optical amplification unit may include the optical branching couplers  51  and  52 , the photodiodes  53  and  54 , the excitation light source  55 , the optical coupler  56 , the rare-earth doped fiber  57 , the optical isolator  59 , and the automatic gain control circuit  100 . The preceding-stage optical amplification unit may also include the amplifier circuits  71  and  72 , the analog-digital converters  75  and  76 , the drive circuit  79 , and the digital-analog converter  81 . 
         [0053]    The photodiodes  53  and  54  detect and supply, to the automatic gain control circuit  100 , the optical power of the input light and the optical power of the output light of the preceding-stage optical amplification unit, which have branched from the optical branching couplers  51  and  52 . The automatic gain control circuit  100  feedback-controls the excitation light source  55  so that a ratio in optical power between the input light and the output light of the preceding-stage optical amplification unit becomes a given level. The optical coupler  56  multiplexes and causes signal light, input from the optical branching coupler  51  through the optical isolator  59 , and excitation light from the excitation light source  55 , to enter the rare-earth doped fiber  57 . 
         [0054]    The amplifier circuits  71  and  72  amplify the detection signals of the photodiodes  53  and  54 , and the analog-digital converters  75  and  76  convert the amplified detection signals into digital signals, and supply the digital signals to the automatic gain control circuit  100 . The digital-analog converter  81  converts the control signal of the excitation light source  55 , output by the automatic gain control circuit  100 , into a driving signal having an analog form. The drive circuit  79  amplifies and supplies the driving signal to the excitation light source  55 . 
         [0055]    A subsequent-stage optical amplification unit may include the optical branching couplers  60 ,  61 , and  62 , the photodiodes  63  and  64 , the excitation light source  65 , the optical coupler  66 , the rare-earth doped fiber  67 , the optical isolator  69 , and the automatic gain control circuit  100 . The subsequent-stage optical amplification unit may also include the amplifier circuits  73  and  74 , the analog-digital converters  77  and  78 , the drive circuit  80 , and the digital-analog converter  82 . 
         [0056]    The photodiode  63  detects and inputs, to the automatic gain control circuit  100 , the optical power of the subsequent-stage optical amplification unit, which has branched from the optical branching coupler  60 . The photodiode  64  detects and inputs, to the automatic gain control circuit  100 , the optical power of the output light of the subsequent-stage optical amplification unit, which has branched from the optical branching coupler  61  and passed through the optical branching coupler  62 . The automatic gain control circuit  100  feedback-controls the excitation light source  65  so that a ratio in optical power between the input light and the output light of the subsequent-stage optical amplification unit becomes a given level. The optical coupler  66  multiplexes and causes signal light, input from the optical branching coupler  60  through the optical isolator  69 , and excitation light from the excitation light source  65 , to enter the rare-earth doped fiber  67 . 
         [0057]    The amplifier circuits  73  and  74  amplify the detection signals of the photodiodes  63  and  64 , and the analog-digital converters  77  and  78  convert the amplified detection signals into digital signals, and supply the digital signals to the automatic gain control circuit  100 . The digital-analog converter  82  converts the control signal of the excitation light source  65 , output by the automatic gain control circuit  100 , into a driving signal having an analog form. The drive circuit  80  amplifies and supplies the driving signal to the excitation light source  65 . 
         [0058]    The equalizer  58  equalizes the wavelength characteristic of the signal light amplified by the rare-earth doped fibers  57  and  67 . To the equalizer  58 , transmission characteristics are assigned whose characteristics are opposite to the gain-wavelength characteristics of the rare-earth doped fibers  57  and  67  according to population inversion rates corresponding to preliminarily defined optical gains. 
         [0059]    In the optical amplifier  1 , the VOA  68  is disposed between the preceding-stage amplification unit and the subsequent-stage amplification unit (an interstage VOA configuration). Based on the power of the output light of the optical amplifier  1 , detected by the photodiode  64 , the automatic level control circuit  101  increases or decreases the attenuation of the optical signal, and hence, maintains the output light of the optical amplifier  1  at a given level. The digital-analog converter  84  converts the control signal of the VOA  68 , output by the automatic level control circuit  101 , into a driving signal having an analog form. The drive circuit  83  amplifies and supplies the driving signal to the VOA  68 . 
         [0060]    The output optical signal of the optical amplifier  1  having branched from the optical branching coupler  61  is caused to further branch by the optical branching coupler  62  and enters the optical branching coupler  90 . The optical branching coupler  90  causes the incident light to further branch and enter the band pass filters  91  and  92 . The band pass filters  91  and  92  individually extract and input the wavelength components of the different wavelengths λ1 and λ2 to the photodiodes  93  and  94 . 
         [0061]    The photodiodes  93  and  94  detect the optical powers of these incident lights, for example, the wavelength components of the wavelengths λ1 and λ2 in the output optical signal of the optical amplifier  1 , and supplies the optical powers to the abnormality detection circuit  102 . The amplifier circuits  95  and  97  amplify the detection signals of the photodiodes  93  and  94 . The analog-digital converters  96  and  98  convert the amplified detection signals into digital signals, and supply the digital signals to the abnormality detection circuit  102 . 
         [0062]    Based on the optical powers of these wavelength components, the abnormality detection circuit  102  detects the abnormality of an optical signal, which has occurred based on the failure caused by increase in optical-loss of an optical component in the optical amplifier  1 . The processing of the abnormality detection circuit  102  may be substantially the same as or similar to the processing of the abnormality detection circuit  30  illustrated in  FIG. 7  and  FIG. 8 . 
         [0063]    When the input light has been reduced based on the abnormal loss of a transmission path fiber transmitting light to enter the optical amplifier  1 , the optical amplifier  1  having the interstage VOA configuration may maintain the level of the output light at a given level by changing the attenuation of the VOA  68 . Therefore, the level of the output light may be maintained at a given level without changing the signal gains of the rare-earth doped fibers  57  and  67  in the preceding-stage amplification unit and the subsequent-stage amplification unit. 
         [0064]    When the abnormal loss of the transmission path has occurred and the attenuation of the VOA  68  has been reduced, the ASE component of the output light of the optical amplifier  1  increases and the ratio of a signal to ASE decreases, as illustrated in  FIG. 4B . Since the signal gains of the rare-earth doped fibers  57  and  67  do not change from gains in a state where an abnormality does not occur, a characteristic of becoming smaller in a long wavelength region may not occur in the ASE component. Therefore, the abnormality detection circuit  102  may not determine that the loss-increase failure of an optical component has occurred. 
         [0065]    When the failure caused by increase in optical-loss of an optical component has occurred, the characteristic of becoming smaller in a long wavelength region occurs in the ASE component of the output light of the optical amplifier  1 . Therefore, the abnormality detection circuit  102  may determine that the failure caused by increase in optical-loss of an optical component has occurred, and may determine that an abnormality has occurred in the optical signal. 
         [0066]    When the abnormality of an optical signal has occurred in an optical amplifier having an interstage VOA configuration, it may be determined whether the abnormality of the optical signal is caused by the failure of an optical component within the optical amplifier or caused by another factor. Therefore, the false detection of an optical signal abnormality occurring in the optical amplifier may be reduced, and it may be easy to specify a failure point. 
         [0067]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.