Abstract:
Disclosed is an optical amplifier, comprising: at least one optically amplifying unit having: a forward excitation light source for generating a forward excitation light; a forward excitation light coupler for coupling the forward excitation light with an input optical signal; an optically amplifying medium for amplifying the input optical signal combined with the forward excitation light; a backward excitation light source for generating a backward excitation light; a backward excitation light coupler for coupling the backward excitation light with the amplified optical signal; a first controller for controlling the forward excitation light source to generate the forward excitation light with a power up to a maximum power thereof; and a second controller for controlling the backward excitation light source to generate the backward excitation light in such a way that the backward excitation light compensates gain deficiency as compared with a desired gain of the optical amplifier.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical amplifier and an optically amplifying method, and in particular, to those that are applicable for an optical amplification for use with an optical communication system and that have a bidirectional excitation light source.  
           [0003]    2. Description of the Prior Art  
           [0004]    Conventionally, an optical amplifier and an optically amplifing method are applied to for example an optical amplification for use with an optical communication system. Recently, as an explosive increase of the users of the Internet, it is necessary to drastically increase the transmission capacity of backbone transmission systems. As a means for increasing the transmission capacity effectively using the existing infrastructure, WDM (Wavelength Division Multiplexing), in particular, DWDM (Dense Wavelength Division Multiplexing) is becoming attractive. As a means for totally amplifying an optical signal that has been wavelength multiplexed, an optical amplifier and an optically amplifying method are becoming important.  
           [0005]    [0005]FIG. 1 shows an example of the structure of a conventional optical amplifier. A part of an optical signal  901  that is input from a transmission path is branched to a photoelectrically converting device  933  by a directional coupler  921 . The photoelectrically converting device  933  converts an output signal of the directional coupler  921  into an electric signal corresponding to the input power. A part of an output signal of an optically amplifying medium  920  is branched to a photoelectrically converting device  934  by a directional coupler  924 . The photoelectrically converting device  934  converts an output signal of the directional coupler  924  into an electric signal corresponding to an output power to the transmission path.  
           [0006]    A controlling circuit  950  controls the power of excitation light of a forward excitation light source  931  or a backward excitation light source  932  so that the output power to the transmission path becomes a predetermined power.  
           [0007]    There are two types of excitations of an optically amplifying medium. One is a forward excitation in which the optically amplifying medium is excited from an input end and the other is a backward excitation in which the optically amplifying medium is excited from an output end. The forward excitation has an advantage of a high S/N ratio, whereas the backward excitation has an advantage of a high gain. In the conventional C band (1530 nm to 1565 nm), regardless of the forward excitation and the backward excitation, a desired gain can be obtained. However, as the number of wavelengths to be multiplexed increases, the L band (1570 nm to 1610 nm) has been used.  
           [0008]    However, according to the prior art, since the L band requires a higher gain than the C band, when a desired gain is obtained with the backward excitation, a sufficient S/N ratio is not obtained. In contrast, when a desired S/N ratio is obtained with the forward excitation, a sufficient gain is not obtained.  
         SUMMARY OF THE INVENTION  
         [0009]    Therefore, an object of the present invention is to provide an optical amplifier and an optically amplifying method that allows a higher S/N ratio and a high gain to be obtained.  
           [0010]    According to a first aspect of the present invention, there is provided an optical amplifier, comprising: at least one optically amplifying unit having: a forward excitation light source for generating a forward excitation light; a forward excitation light coupler for coupling the forward excitation light with an input optical signal; an optically amplifying medium for amplifying the input optical signal combined with the forward excitation light; a backward excitation light source for generating a backward excitation light; a backward excitation light coupler for coupling the backward excitation light with the amplified optical signal; a first controller for controlling the forward excitation light source to generate the forward excitation light with a power up to a maximum power thereof, and a second controller for controlling the backward excitation light source to generate the backward excitation light in such a way that the backward excitation light compensates gain deficiency as compared with a desired gain of the optical amplifier.  
           [0011]    In the optical amplifier, the first controller may control the forward excitation light source to generate the forward excitation light with the maximum power thereof.  
           [0012]    In the optical amplifier, the optical amplifying unit may further comprise: a first directional coupler for deriving a first branch signal of the input optical signal; and a second directional coupler for deriving a second branch signal of the amplified optical signal, and the second controller may control the backward excitation light source on the basis of the first branch signal and the second branch signal.  
           [0013]    In the optical amplifier, the optical amplifier may comprises a plurality of the optically amplifying unit connected in series.  
           [0014]    The optical amplifier may further comprise: an optical output adjuster inserted between two of the optically amplifying units, wherein the optical output adjuster may adjust the level of the optical signal which passes therethrough.  
           [0015]    The optical amplifier may further comprise: a transmission path characteristic compensator inserted between two of the optically amplifying units, wherein the transmission path characteristic compensator may compensate dispersion and/or gain of the optical signal which passes therethrough.  
           [0016]    According to a second aspect of the present invention, there is provided an optically amplifying method, comprising the steps of: generating a forward excitation light; coupling the forward excitation light with an input optical signal; amplifying the input optical signal combined with the forward excitation light; generating a backward excitation light; coupling the backward excitation light with the amplified optical signal; controlling the forward excitation light to be generated with a power up to a maximum power thereof; and controlling the backward excitation light in such a way that the backward excitation light compensates gain deficiency as compared with a desired gain of the optical amplifier.  
           [0017]    In the optically amplifying method, the forward excitation light may be controlled to be generated with the maximum power thereof.  
           [0018]    The optically amplifying method may further comprise: deriving a first branch signal of the input optical signal; and deriving a second branch signal of the amplified optical signal, and wherein the backward excitation light source may be controlled on the basis of the first branch signal and the second branch signal.  
           [0019]    These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]    [0020]FIG. 1 is a block diagram showing the structure of a conventional optical amplifier;  
         [0021]    [0021]FIG. 2 is a block diagram showing the structure of an optical amplifier according to an embodiment of the present invention;  
         [0022]    [0022]FIG. 3 is a block diagram showing the structure of an optical amplifier according to a second embodiment of the present invention;  
         [0023]    [0023]FIG. 4 is a block diagram showing an example of the structure of optically amplifying portions  201  to  204  according to the second embodiment of the present invention;  
         [0024]    [0024]FIG. 5 is a block diagram showing an example of the structure of an optically amplifying portion according to a third embodiment of the present invention;  
         [0025]    [0025]FIG. 6 is a block diagram showing an example of the structure of an optically amplifying portion according to a fourth embodiment of the present invention; and  
         [0026]    [0026]FIG. 7 is a block diagram showing an example of the structure of an optically amplifying portion according to a fifth embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]    Next, with reference to FIGS.  2  to  7 , optical amplifiers and optically amplifying methods according to embodiments of the present invention will be described.  
       First Embodiment  
       [0028]    [0028]FIG. 2 shows an example of the structure of an optical amplifier according to a first embodiment of the present invention. The optical amplifier according to the first embodiment has directional couplers  21  and  24 , excitation light coupler  22  and  23 , an optically amplifying medium  20 , a forward excitation light source  31 , a backward excitation light source  32 , photoelectrically converting devices  33  and  34 , and controlling circuits  40  and  50 . Main signals  101  and  102  are input and output signals of the optical amplifier.  
         [0029]    The directional coupler  21  branches a part of the optical signal  101  that is input from a transmission path (not shown) to the photoelectrically converting device  33 . The directional coupler  21  outputs the rest of the optical signal  101  to the excitation light coupler  22 . The excitation light coupler  22  couples excitation light that is input from the forward excitation light source  31  with the optical signal  101  that is input from the directional coupler  21  and outputs the resultant signal to the optically amplifying medium  20 . The optically amplifying medium  20  amplifies the optical signal that is input from the excitation light coupler  22 . The excitation light coupler  23  is connected to an output terminal of the optically amplifying medium  20 . The excitation light coupler  23  couples excitation light that is input from the backward excitation light source  32  with the amplified signal of the optically amplifying medium  20  and outputs the resultant signal to the output terminal of the optically amplifying medium  20 .  
         [0030]    The directional coupler  24  branches a part of the optical signal  101  that is input from the excitation light coupler  23  to the photoelectrically converting device  34 . The directional coupler  24  outputs the rest of the optical signal  101  as the optical signal  102  to the transmission path (not shown).  
         [0031]    The photoelectrically converting device  33  converts the optical signal branched by the directional coupler  21  into an electric signal. The photoelectrically converting device  34  converts the optical signal branched by the directional coupler  24  into an electric signal.  
         [0032]    The forward excitation light source  31  is composed of one or a plurality of light sources. Likewise, the backward excitation light source  32  is composed of one or a plurality of light sources. The controlling circuit  40  controls the forward excitation light source  31  so that it generates light with the maximum power thereof. However, the controlling circuit  40  may control the forward excitation light source  31  so that it generates light with a power slightly less than the maximum power thereof or with a power which marginally satisfies a desired S/N ratio of the output optical signal.  
         [0033]    The optically amplifying portion controlling circuit  50  compares an electric signal corresponding to an input power of the non-amplified optical signal that is input from the photoelectrically converting device  33  with an electric signal corresponding to an output power of an amplified optical signal that is output from the photoelectrically converting device  34 .  
         [0034]    Corresponding to the compared result, the optically amplifying portion controlling circuit  50  controls the backward excitation light source  32  so that a predetermined gain and an output power to the transmission path are maintained.  
         [0035]    Example of Operation  
         [0036]    Next, with reference to FIG. 2, an example of the operation of the present invention will be described. An optical signal that is input from a transmission path (not shown) has been multiplexed in wavelength division.  
         [0037]    A part of the optical signal  101  that is input from the transmission path (not shown) is branched to the photoelectrically converting device  33  by the directional coupler  21 . The photoelectrically converting device  33  converts the optical signal into an electric signal corresponding to an input power of the non-amplified optical signal that is input from the directional coupler  21 . The rest of the optical signal  101  is coupled with excitation light of the forward excitation light source  31  by the excitation light coupler  22  and supplied to the optically amplifying medium  20 . The output signal of the optically amplifying medium  20  is coupled with excitation light of the backward excitation light source  32  by the excitation light coupler  23  and supplied to the output terminal of the optically amplifying medium  20 .  
         [0038]    A part of the optical signal amplified by the optically amplifying medium  20  is branched to the photoelectrically converting device  34  by the directional coupler  24 . The rest of the main signal is output as the optical signal  102  to the transmission path (not shown).  
         [0039]    The controlling circuit  40  controls the forward excitation light source  31  so that it generates light with the maximum power thereof, thereby gaining a desired S/N ratio. The controlling circuit  50  compares an electric signal corresponding to an input power of the non-amplified optical signal that is input from the photoelectrically converting device  33  with an electric signal corresponding to an output power of the amplified optical signal that is input from the photoelectrically converting device  34 . Corresponding to the compared result, the controlling circuit  50  compensates the gain deficiency of the forward excitation light source  31  and controls the backward excitation light source  32  so that a predetermined gain of the optical amplifier, that is, ratio of the amplitude of the input optical signal and the amplitude of the output optical signal, and an output power to the transmission path are maintained.  
         [0040]    Effect  
         [0041]    Since the forward excitation light source and the backward excitation light source are controlled so that the output of the forward excitation light source that excites the optically amplifying medium is maximized and the gain deficiency is compensated by the backward excitation light source, the desired S/N ratio and the desired gain of the optical amplifier can be obtained.  
       Second Embodiment  
       [0042]    According to a second embodiment of the present invention, a plurality of optically amplifying portions  201  to  204  are disposed, differently from the first embodiment. According to the second embodiment, when a desired gain cannot be obtained by one optically amplifying portion, the desired gain can be obtained by the plurality of optically amplifying portions.  
         [0043]    [0043]FIG. 3 is a block diagram showing an example of the combination of the optically amplifying portions  201  to  204 . FIG. 4 is a block diagram showing an example of the structure of each of the optical amplifiers  201  to  204  according to the second embodiment.  
         [0044]    Next, the relation of the structures of the structural portions according to the first embodiment shown in FIG. 1 and the second embodiment shown in FIGS. 3 and 4 will be described. According to the first embodiment and the second embodiment, the amplifying portion is composed of the directional coupler  21 / 211 , the excitation light coupler  22 / 212 , the excitation light coupler  23 / 213 , the directional coupler  24 / 214 , the optically amplifying medium  20 / 210 , the forward excitation light source  31 / 215 , the backward excitation light source  32 / 216 , the photoelectrically converting device  33 / 217 , the photoelectrically converting device  34 / 218 , and the controlling circuit  40 / 240 .  
       Third Embodiment  
       [0045]    [0045]FIG. 5 is a block diagram showing an example of the structure of an optically amplifying portion according to a third embodiment of the present invention. In the structure of the third embodiment, an optical output adjuster  81 ,  82 ,  83  such as an optical fixed attenuator or an optical variable attenuator is added to the structure of the second embodiment. For example, when the optical output adjuster  81  is disposed between the optically amplifying portion  201  and the optically amplifying portion  202 , the optical input level to the optically amplifying portion  202  can be adjusted for an optimum gain.  
       Fourth Embodiment  
       [0046]    [0046]FIG. 6 is a block diagram showing an example of the structure of an optical amplifier according to a fourth embodiment of the present invention. In the structure of the fourth embodiment, a transmission path characteristic compensator  80  is added to the structure of the second embodiment.  
         [0047]    The transmission path characteristic compensator  80  is for example a dispersion compensation fiber that compensates a dispersion characteristic of the transmission path and an equalizing filter that equalizes a gain characteristic of each wavelength of the transmission band. When the transmission path characteristic compensator  80  is disposed in the optical amplifier, the loss of the transmission path can be alleviated.  
       Fifth Embodiment  
       [0048]    [0048]FIG. 7 is a block diagram showing an example of the structure of an optical amplifier according to a fifth embodiment of the present invention. The structure of the fifth embodiment is a combination of the structure of the third embodiment and the structure of the fourth embodiment. In an optical amplifier according to the fifth embodiment shown in FIG. 7, an optical signal is amplified at four stages of optically amplifying portions  201 ,  202 ,  203 , and  204 . An optical output adjuster  81  is disposed between the optically amplifying portions  201  and  202 . In addition, an optical output adjuster  82  is disposed between the optically amplifying portions  203  and  204 . A transmission path characteristic compensator  80  is disposed between the optically amplifying portions  202  and  203 .  
         [0049]    Features  
         [0050]    In WDM and DWDM, for an optimum S/N ratio and a desired amplification factor, a bidirectional exciting system is used to allot respective gains in such a manner that an output of a forward excitation light source that excites an optically amplifying medium is maximized and the gain deficiency is compensated by a backward excitation light source.  
         [0051]    As was described above, in the optical amplifier and optically amplifying method according to the present invention, a gain controlling function in which an output of a forward excitation light source is maximized for an amplification of an optical signal and the gain deficiency is compensated by a backward excitation light source is provided. Thus, an S/N ratio can be improved while realizing a desired gain.  
         [0052]    Although the present invention has been shown and described with respect to the best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.