Abstract:
An apparatus includes a first optical amplifier that uses a rare-earth-doped optical medium, an isolator that inputs amplified light amplified by the first optical amplifier, a second optical amplifier that uses a rare-earth-doped optical medium to amplify a light output from the isolator, and a first light router that routes amplified spontaneous emission light generated by the first optical amplifier or the second optical amplifier to input, by a second light router, the routed amplified spontaneous emission light to the optical rare-earth-doped medium other than the optical rare-earth-doped medium where the routed amplified spontaneous emission light is generated.

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. 2009-142331, filed on Jun. 15, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    Technologies described herein relate to optical amplifiers using rare-earth-doped optical mediums. 
       BACKGROUND 
       [0003]    Relay stations in optical transmission systems included in optical communication networks have adopted optical amplifiers that amplify optical signals as they are instead of using regenerative relaying involving photoelectric conversion so as to support faster (wider-bandwidth) optical signals. Optical amplifiers commonly used nowadays include those using rare-earth-doped optical fibers as amplifying mediums. In particular, erbium-doped fiber amplifiers (EDFAs) using erbium-doped fibers (EDFs) as amplifying mediums have mainly been used. 
         [0004]    Since recent networks require a longer relay distance, higher gains are required for optical amplifiers of relay stations. When EDFAs are used, excellent amplification can be achieved by using serially connected (cascaded) EDFAs with two stages rather than EDFAs with single stages with consideration of noise figure (NF).  FIG. 7  illustrates an optical amplifier including an EDFA  1  serving as a first optical amplifier and an EDFA  2  serving as a second optical amplifier serially connected to each other with an optical isolator  3  for preventing loop oscillation interposed therebetween. 
         [0005]    The first EDFA  1  is of the forward pumping type, and includes a first EDF  1   a , a first excitation light source  1   b  that generates excitation light, and a first optical multiplexer  1   c  that is disposed upstream of the first EDF  1   a  and multiplexes input light and excitation light generated by the first excitation light source  1   b  so as to supply the resultant light to the first EDF  1   a . Moreover, a second EDFA  2  is of the same forward pumping type, and includes a second EDF  2   a , a second excitation light source  2   b  that generates excitation light, and a second optical multiplexer  2   c  that is disposed upstream of the second EDF  2   a  and multiplexes input light and excitation light generated by the second excitation light source  2   b  so as to supply the resultant light to the second EDF  2   a.    
         [0006]    In the optical amplifier illustrated in  FIG. 7 , signal light is input to the first EDF  1  through the first optical multiplexer  1   c  first. Excitation light is also supplied from the first excitation light source  1   b  to the first EDF  1   a  via the first optical multiplexer  1   c , and the signal light input to the first EDF  1   a  is amplified by stimulated emission from erbium excited by the excitation light. The signal light amplified by and output from the first EDFA  1  is input to the second EDF  2   a , to which excitation light is supplied from the excitation light source  2   b , through an optical isolator  3  and the second optical multiplexer  2   c , and is amplified in a manner similar to that in the first EDF  1   a . The optical isolator  3  transmits light in only one direction from the first EDFA  1  to the second EDFA  2 . With this, the isolator prevents a resonator structure from being formed, the structure having connecting points of an optical path using, for example, optical connectors serving as reflective ends at an input port IN and an output port OUT of the optical amplifier, and prevents loop oscillation of the optical amplifier. 
         [0007]    When optical signals are amplified and relayed using the optical amplifier illustrated in  FIG. 7 , polarization dependent gain (PDG) occurs due to polarization hole burning (PHB) that arises in the EDFs  1   a  and  2   a  of the first and second EDFAs  1  and  2 , respectively. The effect of polarization dependent gain may accumulate and may have an adverse effect when a system includes a plurality of relay stations using the optical amplifier illustrated in  FIG. 7  on transmission paths thereof. For example, when signal light in the C band (approximately from 1,528 nm to 1,565 nm) is amplified and relayed, the optical signal-to-noise ratio (OSNR) of signal components in a short wavelength region in the C band is often measurably reduced. 
         [0008]    Polarization hole burning is a phenomenon that causes the gain of signal light input to EDFs to vary in accordance with the polarization state of excitation light and the signal light input to the EDFs (Shoichi Sudo, Erbium-doped optical fiber amplifier; Optronics Co., Ltd.: Tokyo, 1999; pp 59-61). When signal light with a high intensity and a high degree of polarization (DOP) is input to EDFs, gain of light components in a polarization direction parallel to the polarization direction of the signal light is reduced due to polarization hole burning. Variations in gain in the EDFs also affect amplified spontaneous emission (ASE) light arising inside the EDFs in addition to the signal light. ASE light is not polarized, and includes polarized components parallel to the polarization direction of the signal light and those perpendiculars to the polarization direction. Therefore, only the polarized components parallel to the signal light among those in the ASE light are affected by variations in gain caused by the polarization hole burning. That is, the polarization hole burning causes a reduction in gain of the signal light and a reduction in gain of the polarized components parallel to the signal light among those in the ASE light while the gain of the polarized components perpendicular to the signal light is not reduced. Therefore, a difference between the gain of polarized components parallel to the signal light with a high degree of polarization and the gain of polarized components perpendicular to the signal light among those in the ASE light arising in the EDFs serves as a polarization dependent gain. This reduces the OSNR of the output light after being amplified compared with the case without polarization hole burning as a result of a relative increase in the proportion of the polarized components perpendicular to the signal light among those in the ASE light. That is, when signal light in a short wavelength region in the C band has a high intensity and a high degree of polarization, the light is affected by the polarization dependent gain caused by the polarization hole burning, and the OSNR thereof after amplification is reduced. 
         [0009]    Herein, the polarization dependent gain caused by the polarization hole burning depends on the degree of polarization of the light in the EDFs, and is suppressed as the degree of polarization is reduced (For example, see Bruere F. Measurement of polarization-dependent gain in EDFAs against input degree of polarization and gain compression; Electron. Lett. 1995, 31, No. 5, pp 401-403). The term “degree of polarization” refers to a ratio of the light power of completely polarized components to total light power at a specific wavelength. When the degree of polarization is zero, it refers to a non-polarized state, and when it is one, it refers to a completely polarized state. 
         [0010]    In  FIG. 7 , input light O 1  including signal light S in a short wavelength region in the C band and noise light N 1  with wavelengths over the entire C band may be input to the first EDFA  1 . In this case, the input light O 1  before being amplified has an OSNR depending on the power of the signal component with the wavelength of the signal light and the power of the noise component. 
         [0011]    When the degree of polarization of the signal light S included in the input light O 1  is high, the signal light is affected by the polarization dependent gain caused by the above-described polarization hole burning in the first EDFA  1 . As a result, output light O 2  after amplification by the first EDFA  1  has a higher proportion of a noise component N 2  with wavelengths, in particular, adjacent to that of the signal light S, and the OSNR of the signal light S is reduced. 
         [0012]    The output light O 2  after amplification by the first EDFA  1  is subsequently input to the second EDFA  2 , and further amplified by the second EDFA  2 . The input light O 2  is also affected by the polarization dependent gain caused by the polarization hole burning in the second EDFA  2 . Therefore, output light O 3  after amplification by the second EDFA  2  has a still higher proportion of a noise component N 3  with wavelengths adjacent to that of the signal light S, and the OSNR of the signal light S is further reduced. 
         [0013]    Although an optical amplifier of the forward pumping type is illustrated in  FIG. 7 , the OSNR may be similarly reduced even with an optical amplifier of the bidirectional pumping type. 
         [0014]    For example, Japanese Unexamined Patent Application Publication No. 2003-315739 describes a technology for passing transmission light through a polarization scrambler in a transmitting station and sending the transmission light to a transmission path so that the transmission light is made non-polarized in order to reduce the degree of polarization of the input light to zero with consideration of polarization dependent gain in EDFAs. 
         [0015]    When a polarization scrambler is used, the polarization scrambler needs to be controlled in synchronization with, for example, an optical modulator that generates signal light as described in Japanese Unexamined Patent Application Publication No. 2003-315739. However, the control of the polarization scrambler becomes difficult as the speed of signal light is increased, and presents problems for practical application. In addition, this leads to an increase in costs. 
         [0016]    In view of the above-described background, apparatuses and methods for suppressing polarization dependent gain without using polarization scramblers are required. 
         [0017]    Herein, a structure for suppressing polarization dependent gain of an optical amplifier, including a first optical amplifier and a second optical amplifier serially connected to each other on an optical path between an input port and an output port with an optical isolator interposed therebetween, will be described. 
       SUMMARY 
       [0018]    According to an aspect of the invention, an apparatus includes a first optical amplifier that uses a rare-earth-doped optical medium; an isolator that inputs amplified light amplified by the first optical amplifier; a second optical amplifier that uses a rare-earth-doped optical medium to amplify a light output from the isolator; and a first light router that routes amplified spontaneous emission light generated by the first optical amplifier or the second optical amplifier to input, by a second light router, the routed amplified spontaneous emission light to the optical rare-earth-doped medium other than the optical rare-earth-doped medium where the routed amplified spontaneous emission light is generated. 
         [0019]    The object and advantages of the various embodiments will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0020]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the various embodiments, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  illustrates an example optical transmission system. 
           [0022]      FIGS. 2A and 2B  illustrate optical amplifiers according to a first embodiment. 
           [0023]      FIG. 3  illustrates a spectral shape of amplified spontaneous emission light that arises in an erbium-doped fiber (EDF). 
           [0024]      FIG. 4  illustrates polarization dependent gain (PDG) of an EDF to an optical signal in the C band. 
           [0025]      FIG. 5  illustrates an optical amplifier according to a second embodiment. 
           [0026]      FIG. 6  illustrates an optical amplifier according to a third embodiment. 
           [0027]      FIG. 7  illustrates a known optical amplifier. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]      FIG. 1  illustrates an example optical transmission system including relay stations with optical amplifiers. 
         [0029]    A transmitting station  10  and a receiving station  11  are connected by an optical-fiber transmission path  12 , and a large number of relay stations  13  are disposed on the optical-fiber transmission path  12 . Each relay station  13  includes an optical amplifier, and optical signals transmitted through the optical-fiber transmission path  12  are amplified and relayed. Optical signals transmitted between the transmitting station  10  and the receiving station  11  can include wavelength-division-multiplexed (WDM) signal light or single-wavelength signal light. 
         [0030]      FIG. 2A  illustrates an example optical amplifier according to a first embodiment included in the relay stations  13 . 
         [0031]    The optical amplifier illustrated in  FIG. 2A  includes a first amplifier  20  and a second amplifier  30  that are serially connected to each other (cascaded), and an optical isolator  40  disposed between the first amplifier  20  and the second amplifier  30 . Although an optical amplifier including two cascaded amplifiers is described as an example herein, the optical amplifier can include three or more cascaded amplifiers. 
         [0032]    The first amplifier  20  of the forward pumping type according to an embodiment includes a first EDF  21  serving as a first rare-earth-doped optical fiber, a first excitation light source  22  that generates excitation light for pumping erbium added to the first EDF  21  as a rare earth element, and a first optical multiplexer  23  that is disposed upstream of the first EDF  21  and supplies the excitation light from the first excitation light source  22  to the first EDF  21 . When WDM signal light is amplified, the amplifier can include a gain equalizer that approximates the wavelength-gain characteristic to a flat shape. The first excitation light source  22  includes, for example, a laser diode that generates excitation light with a wavelength of, for example, 0.98 μm or 1.48 μm. The first optical multiplexer  23  includes, for example, a WDM coupler. 
         [0033]    The second amplifier  30  of the forward pumping type according to an embodiment includes a second EDF  31  serving as a second rare-earth-doped optical fiber, a second excitation light source  32  that generates excitation light for pumping erbium added to the second EDF  31 , and a second optical multiplexer  33  that is disposed upstream of the second EDF  31  and supplies the excitation light from the second excitation light source  32  to the second EDF  31 . Furthermore, the amplifier can include a gain equalizer. As in the first amplifier  20 , the second excitation light source  32  includes, for example, a laser diode that generates excitation light with a wavelength of, for example, 0.98 μm or 1.48 μm, and the second optical multiplexer  33  includes, for example, a WDM coupler. 
         [0034]    The first excitation light source  22  and the second excitation light source  32  can be a single light source, and can supply excitation light to the EDFs  21  and  31 . Herein, the amplifiers  20  and  30  are also referred to as first and second erbium-doped fiber amplifiers (EDFAs), respectively. 
         [0035]    The optical isolator  40  interposed between the EDFAs  20  and  30  has a characteristic of subjecting light traveling forward from the first EDFA  20  to the second EDFA  30  to low loss and subjecting light traveling backward from the second EDFA  30  to the first EDFA  20  to high loss, that is, serves as an optical device that allows passage of light only in one direction. The optical isolator  40  prevents a resonator structure from being formed, the substrate having connecting points of an optical path using, for example, optical connectors serving as reflective ends at an input port IN and an output port OUT of the optical amplifiers  20  and  30 , and prevents loop oscillation of the optical amplifier. 
         [0036]    A light supplying unit (or light router)  51  is disposed on an optical path  50  that transmits input light to the first EDF  21  of the first EDFA  20 . A light router  51  has an input port and at least two output ports and may change a first light path into a second light path different from the first light path. The light supplying unit  51  includes, for example, an optical circulator. While the input light traveling forward to the first EDF  21  is transmitted as it is through the optical path  50 , amplified spontaneous emission (ASE) light arising in the first EDF  21  and traveling in a direction opposite to the traveling direction of the input light is supplied from the optical path  50  to an ASE-light transmission path (for example, an optical fiber)  52 . The light supplying unit  51  using an optical circulator can, for example, transmit the input light from a first port to a second port, and can transmit the ASE light from the EDF  21  from the second port to a third port. Although the light supplying unit  51  can be disposed on an optical path between the first optical multiplexer  23  and the first EDF  21 , excitation efficiency may be reduced due to the loss of the excitation light at the connecting points of the light supplying unit  51  (loss from, for example, connecting state of connectors). Accordingly, the supplying unit can be disposed upstream of the first optical multiplexer  23  as illustrated in  FIG. 2A . 
         [0037]    A light input unit (light router)  61  is disposed on an optical path  60  that transmits output light from the second EDF  31  of the second EDFA  30 . The light input unit  61  includes, for example, an optical circulator. The output light output from the second EDF  31  and traveling forward on the optical path  60  is transmitted as it is through the light input unit  61 . On the other hand, ASE light supplied by the light supplying unit  51  and transmitted through the ASE-light transmission path  52  is input to the second EDF  31  from downstream of the second EDF  31  through the light input unit  61 . The light input unit  61  using an optical circulator can, for example, transmit the output light from a first port to a second port, and can transmit the ASE light from the ASE-light transmission path  52  from a third port to the first port. 
         [0038]    The light supplying unit  51  and the light input unit  61  are not limited to the optical circulators, and can be a combination of, for example, an optical coupler and an optical isolator. However, an optical circulator may be more preferable since an optical coupler causes an insertion loss of about 3 dB, which is more than that of the optical circulator. 
         [0039]    The ASE light arising in the EDFs  21  and  31  and traveling backward has a spectral shape illustrated in  FIG. 3 . In  FIG. 3 , the abscissa represents the wavelength (nm), and the ordinate represents the light power (mW). The spectrum of the ASE light arising in the EDFs  21  and  31  and traveling backward extends in a certain range and has a peak at about 1,530 nm. That is, the ASE light arising in the EDFs  21  and  31  and traveling backward has a spectral band including the C band approximately from 1,528 nm to 1,565 nm. 
         [0040]    It is conceivable that the ASE light traveling backward arises in the EDFs  21  and  31  as follows. 
         [0041]    First, the forward-pumping excitation light is input to the EDF from an input end thereof. With this, the population inversion factor (excitation state) of erbium in the vicinity of the input end is increased in the EDF. When the population inversion factor is increased in the EDF, a large amount of ASE light arises in a short wavelength region in the C band. The ASE light travelling in a direction toward the input end of the EDF among the generated ASE light is emitted from the input end of the EDF while being amplified inside the EDF. The emitted ASE light travels on the optical path  50  in the direction opposite to that of the input light. The ASE light exhibits a spectral shape having a maximum peak power in a short wavelength region in the C band. 
         [0042]    In  FIG. 2A , the ASE light arising in the first EDF  21  and traveling backward is transmitted from the optical path  50  to the ASE-light transmission path  52  by the light supplying unit  51 . The ASE light output to the ASE-light transmission path  52  is transmitted to the light input unit  61  through the ASE-light transmission path  52 , and sent from the light input unit  61  to the second EDF  31 . The ASE light input to the second EDF  31  from the downstream thereof by the light input unit  61  travels in the direction opposite to that of the input light and is amplified inside the second EDF  31 . 
         [0043]    The ASE light input from the light input unit  61  and traveling backward inside the second EDF  31  is non-polarized light including a large amount of short wavelength components in the C band as illustrated in  FIG. 3 . Therefore, the ASE light reduces the degree of polarization (DOP) of the input light including the signal light in a short wavelength region in the C band traveling forward in the second EDF  31 . That is, when non-polarized light with wavelengths in the vicinity of that of the signal light is input to the rare-earth-doped optical fiber through which the signal light with a high degree of polarization is transmitted, the degree of polarization of the signal light is reduced. For example, when non-polarized light with wavelengths in a range of ±2.5 nm of the wavelength of signal light with a high degree of polarization is input to an EDF, the degree of polarization of the signal light is reduced. As a result of a reduction in the degree of polarization of the input light to be amplified traveling forward in the second EDF  31 , polarization dependent gain (PDG) caused by polarization hole burning (PHB) is suppressed. 
         [0044]      FIG. 2A  schematically illustrates spectral distributions O 10 , A 10 , O 11 , A 13 , A 12 , and O 12  of the input light and the ASE light at principal parts in the optical amplifier according to the first embodiment. 
         [0045]    In the optical amplifier illustrated in  FIG. 2A , input light O 10  including signal light S in a short wavelength region in the C band and noise light N 10  over the entire C band is input to the first EDFA  20 . In this case, the input light O 10  before being amplified has an OSNR depending on the power of the signal component at the signal light wavelength and the power of the noise component. 
         [0046]    When the degree of polarization of the signal light S included in the input light O 10  is high, the signal light is affected by the polarization dependent gain caused by the polarization hole burning in the first EDFA  20 . As a result, output light O 11  after amplification by the first EDFA  20  has a higher proportion of a noise component N 11  with wavelengths, in particular, adjacent to that of the signal light S, and the OSNR of the signal light S is reduced. 
         [0047]    During amplification by the first EDFA  20 , ASE light A 10  that travels in a direction opposite to that of the input light O 10  arises. The ASE light A 10  arising in the first EDFA  20  and traveling backward on the optical path  50  is supplied to the ASE-light transmission path  52  by the light supplying unit  51 . The supplied ASE light A 11  is transmitted to the light input unit  61  through the ASE-light transmission path  52 . 
         [0048]    Subsequently, the output light O 11  after amplification by the first EDFA  20  is input to the second EDFA  30 , and is further amplified by the second EDFA  30 . At this moment, ASE light A 12  traveling backward is input from the light input unit  61  to the second EDFA  30 . The ASE light A 12  is the ASE light A 11  transmitted to the light input unit  61  through the ASE-light transmission path  52 , and is in a non-polarized state with a spectral shape having a peak in a short wavelength region in the C band as illustrated in  FIG. 3 . Therefore, the degree of polarization of the input light O 11  amplified in the second EDF  31  is reduced in the vicinity of the wavelength of the signal light S, and the polarization dependent gain caused by the polarization hole burning is suppressed. As a result, the noise component N 12  in the vicinity of the wavelength of the signal light S in the output light O 12  output from the second EDFA  30  is prevented from being increased, and the OSNR of the signal light S is improved. 
         [0049]    As illustrated in  FIG. 4 , the polarization dependent gain occurring in the EDF on the signal light in the C band prominently appears in, in particular, light component with wavelengths shorter than 1,540 nm (Davidson C. R., et al. “Spectral Dependence of Polarization Hole-Burning”Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest. CD-ROM. Optical Society of America: Washington, D.C., 2006; paper OThC3). In  FIG. 4 , the abscissa represents the wavelength (nm), and the ordinate represents the polarization dependent gain (dB). 
         [0050]    However, deviation of the polarization dependent gain during amplification of the input light O 11  in the second EDF  31  is suppressed since the ASE light A 12  having a peak at a wavelength adjacent to 1,530 nm as illustrated in  FIG. 3  is input to the second EDF  31  and the degree of polarization is reduced in the vicinity of 1,530 nm. 
         [0051]    The ASE light A 12  input to the second EDFA  30  from downstream thereof travels backward, is amplified in the second EDF  31 , and is output from upstream of the second EDFA  30 . However, the ASE light A 13  output from the second EDFA  30  is blocked by the optical isolator  40 . 
         [0052]      FIG. 2B  illustrates an optical amplifier similar to that according to the first embodiment illustrated in  FIG. 2A  except that the optical amplifier includes a band pass filter  53  that allows passage of the ASE light on the ASE-light transmission path  52 . 
         [0053]    The band pass filter  53  allows passage of the ASE light A 11  with wavelengths in a required band among the ASE light A 11  supplied by the light supplying unit  51 . That is, the band pass filter  53  allows passage of light only in a band in the vicinity of the wavelength of the signal light S of the input light O 11  illustrated in  FIG. 2A  within the band of the ASE light A 11 . ASE light A 11 ′ is generated by blocking the other unnecessary bands. The band-limited ASE light A 11 ′ is input to the second EDFA  30  from downstream thereof through the light input unit  61 . 
         [0054]    As illustrated in  FIG. 4 , when the polarization dependent gain caused by the polarization hole burning prominently appears in a part of wavelengths, the polarization dependent gain during amplification of the input light O 11  in the second EDF  31  can be suppressed by providing the band-limited ASE light A 11 ′, whose band is limited by the band pass filter  53  in accordance with the wavelength at which the polarization dependent gain prominently appears, to the second EDFA  30 . Furthermore, energy of the excitation light consumed by the amplification of the ASE light A 11 ′ in the second EDF  31  can be reduced. 
         [0055]      FIG. 5  illustrates an optical amplifier according to a second embodiment. The optical amplifier according to the second embodiment inputs ASE light arising in the second EDFA  30  and traveling backward to the first EDF  21  from downstream of the first EDFA  20 . The first EDFA  20 , the second EDFA  30 , and the optical isolator  40  are the same as those in the first embodiment. 
         [0056]    The optical amplifier illustrated in  FIG. 5  includes an optical path  70  that transmits input light to the second EDF  31  and a light supplying unit (light router)  71  disposed on the path. The light supplying unit  71  supplies the ASE light arising in the second EDF  31  and traveling in a direction opposite to that of the input light from the optical path  70  to an ASE-light transmission path  72 . The light supplying unit  71  includes, for example, an optical circulator. While the input light traveling to the second EDF  31  is transmitted as it is through the optical path  70 , the ASE light traveling backward from the second EDF  31  is extracted from the optical path  70  to the ASE-light transmission path  72 . That is, the light supplying unit  71  using an optical circulator can, for example, transmit the input light from a first port to a second port, and can transmit the ASE light from the second port to a third port. 
         [0057]    Moreover, the optical amplifier illustrated in  FIG. 5  includes an optical path  80  that transmits output light from the first EDF  21  and a light input unit (light router)  81  on the path. The light input unit  81  inputs the ASE light supplied by the light supplying unit  71  and transmitted through the ASE-light transmission path  72  to the first EDF  21  from downstream of the first EDFA  20 . The light input unit  81  includes, for example, an optical circulator. The light input unit  81  using an optical circulator can, for example, transmit the output light from a first port to a second port, and can transmit the ASE light from a third port to the first port. 
         [0058]    The first excitation light source  22  and the second excitation light source  32  can be a single light source, and can split and supply excitation light to the EDFs  21  and  31 . 
         [0059]    In the optical amplifier according to the second embodiment, the ASE light arising in the second EDF  31  and traveling backward has a spectral shape illustrated in  FIG. 3 . Therefore, the degree of polarization of components of the input light amplified in the first EDF  21 , the components having wavelengths adjacent to that of the signal light S in a short wavelength region in the C band, is reduced by the ASE light extracted by the light supplying unit  71  and input from the light input unit  81  to the first EDFA  20 . As in the first embodiment, the OSNR of the light output from the optical amplifier according to the second embodiment after amplification is improved since the polarization dependent gain caused by the polarization hole burning in the first EDF  21  is suppressed due to a reduction in the degree of polarization of the input light. 
         [0060]    As illustrated by dotted lines in  FIG. 5 , the optical amplifier according to the second embodiment can also include a band pass filter  73  on the ASE-light transmission path  72  so as to allow passage of the ASE light in a desired band. 
         [0061]      FIG. 6  illustrates an optical amplifier according to a third embodiment. The optical amplifier according to the third embodiment differs from the optical amplifier according to the first embodiment illustrated in  FIG. 2A  in that a first EDFA  100  and a second EDFA  110  are of the bidirectional pumping type. The optical isolator  40 , the light supplying unit  51 , the ASE-light transmission path  52 , and the light input unit  61  are the same as those in the first embodiment. Moreover, the optical amplifier can include the band pass filter  53  illustrated in  FIG. 2B  as illustrated by dotted lines in  FIG. 6 . 
         [0062]    The first EDFA  100  of the bidirectional pumping type includes a first EDF  101 , a first forward-pumping light source  102  that generates excitation light for forward pumping of the first EDF  101 , and a first forward-pumping optical multiplexer  103  that is disposed upstream of the first EDF  101  and supplies the excitation light generated by the first forward-pumping light source  102  to the first EDF  101 . The first EDFA  100  further includes a first backward-pumping light source  104  that generates excitation light for backward pumping of the first EDF  101  and a first backward-pumping optical multiplexer  105  that is disposed downstream of the first EDF  101  and supplies the excitation light generated by the first backward-pumping light source  104  to the first EDF  101 . When WDM signal light is amplified, the amplifier can include a gain equalizer downstream of the backward-pumping optical multiplexer  105 . The first forward-pumping light source  102  and the backward-pumping light source  104  include laser diodes that generate excitation light with a wavelength of, for example, 0.98 μm or 1.48 μm, and the first forward-pumping optical multiplexer  103  and the backward-pumping optical multiplexer  105  include, for example, WDM couplers. The first excitation light source  102  and the second excitation light source  104  can be a single light source, and can split and supply excitation light to the EDF  101  for bidirectional pumping. 
         [0063]    Similarly, the second EDFA  110  of the bidirectional pumping type includes a second EDF  111 , a second forward-pumping light source  112  that generates excitation light for forward pumping of the second EDF  111 , and a second forward-pumping optical multiplexer  113  that is disposed upstream of the second EDF  111  and supplies the excitation light generated by the second forward-pumping light source  112  to the second EDF  111 . The second EDFA  110  further includes a second backward-pumping light source  114  that generates excitation light for backward pumping of the second EDF  111  and a second backward-pumping optical multiplexer  115  that is disposed downstream of the second EDF  111  and supplies the excitation light generated by the second backward-pumping light source  114  to the second EDF  111 . The first excitation light source  112  and the second excitation light source  114  can be a single light source, and can split and supply excitation light to the EDF  111  for bidirectional pumping. 
         [0064]    Herein, the amplifier can include, for example, a gain equalizer for WDM downstream of the backward-pumping optical multiplexer  115 . As in the first EDFA  100 , the second forward-pumping light source  112  and the backward-pumping light source  114  include laser diodes that generate excitation light with a wavelength of, for example, 0.98 μm or 1.48 μm, and the second forward-pumping optical multiplexer  113  and the backward-pumping optical multiplexer  115  include, for example, WDM couplers. 
         [0065]    In the optical amplifier according to the third embodiment, the ASE light traveling backward and extracted by the light supplying unit  51  has a spectral shape similar to that of the ASE light illustrated in  FIG. 2A . Therefore, the degree of polarization of components of the input light amplified in the second EDF  111 , the components having wavelengths adjacent to that of the signal light S in a short wavelength region in the C band, is reduced by the ASE light extracted by the light supplying unit  51  and input from the light input unit  61  to the second EDFA  111 . As in the first embodiment, the OSNR of the light output from the optical amplifier according to the third embodiment after amplification is improved since the polarization dependent gain caused by the polarization hole burning in the second EDF  111  is suppressed due to a reduction in the degree of polarization of the input light. 
         [0066]    The optical amplifier according to the third embodiment can include the light supplying unit disposed upstream of the second EDF  111  and the light input unit disposed downstream of the first EDF  101  as in the second embodiment illustrated in  FIG. 5 . 
         [0067]    In the above-described embodiments, the EDFs are excited by forward pumping or bidirectional pumping in which excitation light is input to the EDFs from upstream thereof. Aside from these, the EDFAs of the backward pumping type to which excitation light is input from downstream of the EDFs can also reduce the degree of polarization of the input light inside the EDFs by extracting the ASE light arising in one of the EDFAs and inputting the light to the other EDFA. 
         [0068]    Moreover, although the EDFs are used for amplifying the signal light including the signal components in a short wavelength region in the C band in the above-described embodiments, the first and second amplifiers are not limited to the EDFAs. Other optical amplifiers using other rare-earth-doped optical fibers can also reduce the degree of polarization by using the ASE light traveling backward, and can be effective when the ASE light traveling backward includes components with wavelengths in the vicinity of that of the signal light. 
         [0069]    A simulation result obtained by specifying numerical values in the optical amplifier illustrated in  FIG. 2A  will now be described. It was hypothesized that input light O 10  of −20.4 dBm was input to the optical amplifier, that the wavelength of the signal light S included in the input light O 10  was 1,531.9 nm, and that the OSNR of the input light O 10  was 34.00 dB (0.1 nm resolutions). Moreover, it was hypothesized that the gain of the entire optical amplifier was 22.9 dB, and that the signal light output from the optical amplifier was amplified to 2.5 dBm. Furthermore, it was hypothesized that the length of the first EDF  21  was 11 m, that the length of the second EDF  31  was 14 m, that the wavelength of the excitation light generated by the excitation light sources  22  and  32  were 0.98 μm, and that the intensity of the excitation light was set such that the gain of the first EDF  21  became 25.8 dB and the gain of the second EDF  31  became 9 dB. 
         [0070]    According to the simulation result, ASE light A 10  having a spectral shape illustrated in  FIG. 3  and a power of 0.2 dBm arose in the first EDF  21 . The ASE light A 10  traveling in a direction opposite to that of the input light O 10  was input from downstream of the second EDF  31  to the second EDF  31  through the light supplying unit  51  and the light input unit  61 . Input light O 11  of −1.4 dBm and ASE light of −9.8 dBm traveling forward through the optical isolator  40  were input to the second EDF  31  from upstream thereof. The degree of polarization of the input light O 11  was reduced from 87% to 39% in the second EDF  31  due to the ASE light A 12  input from the light input unit  61 , and the polarization dependent gain caused by the polarization hole burning was suppressed from 0.17 dB to 0.09 dB. With this, the polarization dependent gain of the entire optical amplifier was suppressed from 0.34 dB to 0.26 dB. 
         [0071]    Due to the suppression of the polarization dependent gain, the OSNR of the output light O 12  output from the optical amplifier was improved from 30.84 dB to 30.87 dB. Although the improvement in the OSNR for each optical amplifier was small, the effect of the improvement accumulates and grows significantly after multiple relay transmission. For example, the OSNR of the signal light after transmission of 15 spans was improved from 20.40 dB to 20.95 dB by providing the optical amplifier illustrated in  FIG. 2A  for each relay station. 
         [0072]    In accordance with the optical amplifier and the method for suppressing the polarization dependent gain according to the above-described technology, ASE light traveling in a direction opposite to that of signal light among ASE light arising in a rare-earth-doped optical fiber in one of the cascaded optical amplifiers is input to a rare-earth-doped optical fiber in the other optical amplifier. In the rare-earth-doped optical fiber to which the ASE light traveling backward is input, the degree of polarization of the light with wavelengths in the vicinity of that of the input ASE light is reduced, and the polarization dependent gain caused by the polarization hole burning is suppressed. 
         [0073]    The many features and advantages of the embodiments are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the embodiments that fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the inventive embodiments to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope thereof.