Abstract:
A bidirectional wavelength division multiplex transmission apparatus includes first and second optical circulators, an optical amplifier, and a demultiplexing unit. The first optical circulator is connected to a first optical transmission line to branch/insert reverse and forward wavelength division multiplex optical signals having different wavelengths. The second optical circulator is connected to a second optical transmission line to branch/insert reverse and forward wavelength division multiplex optical signals having different wavelengths. The multiplexer performs wavelength division multiplexing of the signals from the optical circulators. The optical amplifier amplifies the optical signal output from the multiplexer. The demultiplexing unit demultiplexes the optical signal amplified by the optical amplifier on the basis of the wavelengths, and outputs the demultiplexed optical signal to one of the first and second optical transmission lines. The demultiplexing unit includes an optical demultiplexer for demultiplexing the optical signal from the amplifier into two optical signals, and optical filters for allowing optical signals, of the demultiplexed optical signals, which have specific wavelengths to pass therethrough, and outputting the signals to one of the first and second circulators.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a bidirectional wavelength division multiplex transmission apparatus using a wavelength division multiplex optical amplifier. 
     FIG. 7 shows the first example of the conventional bidirectional transmission apparatus using the bidirectional wavelength division multiplex optical amplifier. As shown in FIG. 7, this apparatus uses WDM (Wavelength Division Multiplex) couplers  21  to  23  as optical branching/inserting means for signal light beams to be transmitted bidirectionally. Reference numeral  2  denotes an optical multiplexer; and  3 , a wavelength division multiplex transmission optical amplifier. 
     FIG. 8 shows the second example of the apparatus using the bidirectional wavelength division multiplex optical amplifier. As shown in FIG. 8, this apparatus uses optical circulators  31  and  32  to make only the optical transmission lines bidirectional. The apparatus amplifies branched wavelength division multiplex signals by using wavelength division multiplex transmission amplifiers  41  and  42  separately used for reverse and forward links. 
     In the apparatus as the first example using the WDM couplers, the respective wavelengths must be spaced apart from each other by at least about 10 nm, as indicated by the wavelength transmission range characteristics in FIG.  9 . For this reason, the number of wavelengths which can be multiplexed is limited, and optical transmission lines for transmitting optical signals cannot be effectively used. 
     The apparatus as the second example, which makes only the optical transmission lines bidirectional, requires two optical amplifiers. This increases the cost of the apparatus and degrades maintainability. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a bidirectional wavelength division multiplex transmission apparatus which effectively uses optical transmission lines. 
     It is another object of the present invention to provide a bidirectional wavelength division multiplex transmission apparatus which prevents an increase in apparatus cost and degradation in maintainability. 
     In order to achieve the above objects, according to the present invention, there is provided a bidirectional wavelength division multiplex transmission apparatus comprising first optical branching/inserting means, connected to a first optical transmission line through which reverse and forward wavelength division multiplex optical signals having different wavelengths are transmitted, for branching/inserting the optical signals, second optical branching/inserting means, connected to a second optical transmission line through which reverse and forward wavelength division multiplex optical signals having different wavelengths are transmitted, for branching/inserting the optical signals, multiplexing means for performing wavelength division multiplexing of the optical signals output from the first and second branching/inserting means, an optical amplifier for amplifying the wavelength division multiplex optical signal output from said multiplexing means, and demultiplexing means for demultiplexing the wavelength division multiplex optical signal amplified by the optical amplifier on the basis of the wavelengths, and outputting the demultiplexed optical signal to one of the first and second optical transmission lines which corresponds to a propagating direction, the demultiplexing means being constituted by an optical demultiplexer for demultiplexing the optical signal from the optical amplifier into two optical signals, and optical filters for allowing optical signals, of the demultiplexed optical signals, which have specific wavelengths to pass therethrough, and outputting the signals to one of the first and second optical branching/inserting means which corresponds to the propagating direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a bidirectional wavelength division multiplex transmission apparatus according to the first embodiment of the present invention; 
     FIG. 2 is a block diagram showing a bidirectional wavelength division multiplex transmission apparatus according to the second embodiment of the present invention; 
     FIG. 3 is a block diagram showing a bidirectional wavelength division multiplex transmission apparatus according to the third embodiment of the present invention; 
     FIG. 4 is a block diagram showing a bidirectional wavelength division multiplex transmission apparatus according to the fourth embodiment of the present invention; 
     FIG. 5 is a block diagram showing a bidirectional wavelength division multiplex transmission apparatus according to the fifth embodiment of the present invention; 
     FIG. 6 is a block diagram showing a bidirectional wavelength division multiplex transmission apparatus according to the sixth embodiment of the present invention; 
     FIG. 7 is a block diagram showing a conventional bidirectional wavelength division multiplex transmission apparatus; 
     FIG. 8 is a block diagram showing another conventional bidirectional wavelength division multiplex transmission apparatus; and 
     FIG. 9 is a graph showing how the number of wavelengths which can be multiplexed is limited in the conventional apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail below with reference to the accompanying drawings. 
     FIG. 1 shows a bidirectional wavelength division multiplex transmission apparatus according to the first embodiment of the present invention. Referring to FIG. 1, the apparatus of this embodiment is constituted by an optical multiplexer  102  for multiplexing two optical signals input to the first and second input terminals, a wavelength division multiplex transmission amplifier  103  for amplifying an output from the optical multiplexer  102 , an optical demultiplexer  104  for demultiplexing an output from the wavelength division multiplex transmission amplifier  103  into two signals, a plurality of optical fiber gratings  151  to  154  which receive one output from the optical demultiplexer  104  and respectively have reflection characteristics corresponding to wavelengths λ1 to λ4, a plurality of optical fiber gratings  155  to  158  which receive the outer output from the optical demultiplexer  104  and respectively have reflection characteristics corresponding to wavelengths λ5 to λ8, an optical circulator  111  having three ports to which the output terminal of the optical fiber grating  154 , the first input terminal of the optical multiplexer  102 , and an optical transmission line A are respectively connected, and an optical circulator  112  having three ports to which the output terminal of the optical fiber grating  158 , the second input terminal of the optical multiplexer  102 , and an optical transmission line B are respectively connected. 
     The input ports of the optical circulators  111  and  112  are connected to the output terminals of the optical fiber gratings  154  and  158 . The output ports of the optical circulators  111  and  112  are connected to the first and second input terminals of the optical multiplexer  102 . The input/output ports of the optical circulators  111  and  112  are respectively connected to the optical transmission lines A and B. The optical transmission lines A and B are constituted by optical fibers. 
     The operation of the bidirectional wavelength division multiplex transmission apparatus having the above arrangement will be described next. Assume that the wavelengths of wavelength division multiplex signal light beams transmitted through the optical transmission line A are respectively represented by λ1 to λ4, and those of wavelength division multiplex light beams transmitted through the optical transmission line B are respectively represented by λ5 to λ8. The wavelength division multiplex signal light beams having the wavelengths λ1 to λ4 transmitted through the optical transmission line A are input to the first input terminal of the optical multiplexer  102  through the optical circulator  111 . The wavelength division multiplex signal light beams having the wavelengths λ5 to λ8 transmitted through the optical transmission line B are input to the second input terminal of the optical multiplexer  102  through the optical circulator  112 . The optical multiplexer  102  multiplexes the two input optical signals to output a wavelength division multiplex signal light beam having the wavelengths λ1 to λ8. The wavelength division multiplex signal light beam having the wavelengths λ1 to λ8 output from the optical multiplexer  102  is amplified by the wavelength division multiplex transmission amplifier  103  altogether. The optical demultiplexer  104  and optical fiber gratings  151  to  154  and  155  to  158  send the amplified wavelength division multiplex signal light beam to the optical transmission line B on the opposite side (transmission side) to the optical transmission line A side (transmission side). 
     The optical fiber gratings  151  to  154  and  155  to  158  select wavelengths corresponding to the optical transmission line on the transmission side from the wavelength division multiplex signal light beam demultiplexed by the optical demultiplexer  104 , thereby extracting a wavelength division multiplex signal light beam. With this operation, the wavelength division multiplex signal light beam extracted by the optical fiber gratings  155  to  158  is sent to the optical transmission line B through the optical circulator  112 . 
     Selection of wavelengths from a wavelength division multiplex signal light beam will be described in detail next. The wavelength division multiplex signal light beam having the wavelengths λ1 to λ8 output from the wavelength division multiplex transmission amplifier  103  is demultiplexed into two signal light beams by the optical demultiplexer  104 . The wavelengths to be sent to the optical transmission lines A and B are selected from the two wavelength division multiplex signal light beams by using the optical filter characteristics of the optical fiber gratings  151  to  158 . 
     More specifically, the wavelength division multiplex signal light beam having the wavelengths λ1 to λ4 from the optical transmission line A is demultiplexed into two signal light beams by the optical demultiplexer  104  to be input to the optical fiber gratings  151  to  154  and the optical fiber gratings  155  to  158 . The optical fiber gratings  155  to  158  have reflection characteristics corresponding to the wavelengths λ5 to λ8, and hence transmit the wavelength division multiplex signal light beam having the wavelengths λ1 to λ4 from the optical transmission line A. Since the optical fiber gratings  151  to  154  have reflection characteristics corresponding to the wavelengths λ1 to λ4, the wavelength division multiplex signal light beam having the wavelengths λ1 to λ4 from the optical transmission line A is reflected by each of the optical fiber gratings  151  to  154 . As a result, the transmission of the signal light beam is blocked. With this operation, the wavelength division multiplex signal light beam having the wavelengths λ1 to λ4 selected/extracted by the optical fiber gratings  155  to  158  is sent to the optical transmission line B through the optical circulator  112 . 
     The wavelength division multiplex signal light beam from the optical transmission line B is demultiplexed into two signal light beams by the optical demultiplexer  104 . The two signal light beams are input to the optical fiber gratings  151  to  154  and the optical fiber gratings  155  to  158 . The optical fiber gratings  151  to  154  have reflection characteristics corresponding to the wavelengths λ1 to λ4, and hence transmit the wavelength division multiplex signal light beam having the wavelengths λ5 to λ8 from the optical transmission line B. Since the optical fiber gratings  155  to  158  have reflection characteristics corresponding to the wavelengths λ5 to λ8, the wavelength division multiplex signal light beam having the wavelengths λ5 to λ8 from the optical transmission line B is reflected by each of the optical fiber gratings  155  to  158 . As a result, the transmission of this signal light beam is blocked. 
     A bidirectional wavelength division multiplex transmission apparatus according to the second embodiment of the present invention will be described next with reference to FIG.  2 . 
     Referring to FIG. 2, optical circulators  161  and  162 , each having three ports, are connected between the two output terminals of an optical demultiplexer  104  and optical fiber gratings  155  and  151 . The input port of the optical circulator  161  is connected to the first output terminal of the optical demultiplexer  104 . The output port of the optical circulator  161  is connected to the input port of an optical circulator  111 . The input/output port of the optical demultiplexer  104  is connected to the input terminal of the optical fiber grating  155 . The input port of the optical circulator  162  is connected to the second output terminal of the optical demultiplexer  104 . The output port of the optical circulator  162  is connected to the input port of an optical circulator  112 . The input/output port of the optical circulator  162  is connected to the input terminal of the optical fiber grating  151 . 
     In this arrangement, wavelength division multiplex signal light having wavelengths λ1 to λ4 from an optical transmission line A is demultiplexed into two signal light beams by the optical demultiplexer  104 . The two signal light beams are respectively input to a group of optical fiber gratings  151  to  154  and a group of optical fiber gratings  155  to  158  through the optical circulators  161  and  162 . The optical fiber gratings  151  to  154  respectively have reflection characteristics corresponding to the wavelengths λ1 to λ4, and hence reflect only the wavelength division multiplex signal light having the wavelengths λ1 to λ4. In this case, the optical fiber gratings  151  to  154  are processed to have reflectionless terminations so as to prevent signal light beams other than those having the wavelengths λ1 to λ4 from returning as reflected light beams. The wavelength division multiplex signal light having the wavelengths λ1 to λ4 reflected by the optical fiber gratings  151  to  154  is input to the optical circulator  162  and sent to an optical transmission line B through the optical circulator  112 . 
     Since the optical fiber gratings  155  to  158  have reflection characteristics corresponding to wavelengths λ5 to λ8, the wavelength division multiplex signal light having the wavelengths λ1 to λ4 branched from the optical transmission line A passes through these gratings without being reflected. 
     The wavelength division multiplex signal light having λ5 to λ8 from the optical transmission line B is demultiplexed into two signal light beams by the optical demultiplexer  104 . The two signal light beams are respectively input to the group of the optical fiber gratings  151  to  154  and the group of the optical fiber gratings  155  to  158  through the optical circulators  161  and  162 . The optical fiber gratings  155  to  158  respectively have reflection characteristics corresponding to the wavelengths λ5 to λ8, and hence reflect only the wavelength division multiplex signal light having the wavelengths λ5 to λ8 from the optical transmission line B. In this case, the optical fiber gratings  155  to  158  are processed to have reflectionless terminations so as to prevent signal light beams other than those having the wavelengths λ5 to λ8 from returning as reflected light beams. The wavelength division multiplex signal light having the wavelengths λ5 to λ8 reflected by the optical fiber gratings  155  to  158  is input to the optical circulator  161  and sent to the optical transmission line A through the optical circulator  111 . 
     Since the optical fiber gratings  151  to  154  respectively have reflection characteristics corresponding to the wavelengths λ1 to λ4, the wavelength division multiplex signal light having the wavelengths λ5 to λ8 from the optical transmission line B passes through these gratings without being reflected. 
     A bidirectional wavelength division multiplex transmission apparatus according to the third embodiment of the present invention will be described next with reference to FIG.  3 . 
     Referring to FIG. 3, waveguide type optical multiplexing/demultiplexing units  171 ,  172 , and  173  are used in place of the optical circulators  111  and  112  and the optical multiplexer  102  in FIG. 1, and a waveguide type optical multiplexing/demultiplexing unit  174  is used in place of the optical demultiplexer  104  and the optical fiber gratings  155  to  158 . The waveguide type optical multiplexing/demultiplexing units  171 ,  172 ,  173 , and  174  process the wavelengths of optical signals constituting wavelength division multiplex signal light. 
     The waveguide type optical multiplexing/demultiplexing units  171  and  172  respectively demultiplex wavelength division multiplex signal light having wavelengths λ1 to λ4 and wavelength division multiplex signal light having wavelengths λ5 to λ8 from optical transmission lines A and B in units of wavelengths, and output the resultant signal light beams to the waveguide type optical multiplexing/demultiplexing unit  173 . The waveguide type optical multiplexing/demultiplexing unit  173  multiplexes the wavelength division multiplex signal light having the wavelengths λ1 to λ4 and the wavelength division multiplex signal light having the wavelengths λ5 to λ8, and outputs the resultant signal light to the waveguide type optical multiplexing/demultiplexing unit  174  through an amplifier  103 . The waveguide type optical multiplexing/demultiplexing unit  174  demultiplexes the output from the amplifier  103  in units of wavelengths, and outputs the wavelength division multiplex signal light having the wavelengths λ1 to λ4 to the optical transmission line B through the waveguide type optical multiplexing/demultiplexing unit  172 . Similarly, the waveguide type optical multiplexing/demultiplexing unit  174  outputs the wavelength division multiplex signal light having the wavelengths λ5 to λ8 to the optical transmission line A through the waveguide type optical multiplexing/demultiplexing unit  171 . 
     The first to third embodiments (FIGS. 1 to  3 ) described above exemplify optical inline amplifiers. The fourth to sixth embodiments (FIGS. 4 to  6 ) exemplify optical booster amplifiers/optical preamplifiers. FIGS. 4 to  6  correspond to FIGS. 1 to  3 , and the same reference numerals denote the same parts throughout the drawings. 
     The fourth embodiment shown in FIG. 4 differs from the embodiment shown in FIG. 1 in that a transmission system constituted by a plurality of optical transmitters  181  to  184  and a waveguide type optical multiplexing/demultiplexing unit  113  serving as a multiplexing means and a reception system constituted by a waveguide type optical multiplexing/demultiplexing unit  114  serving as a demultiplexing means and a plurality of optical receivers  191  to  194  are separately arranged in place of the bidirectional optical transmission line A and the optical circulator  111 . In this embodiment, the output terminal of the optical fiber grating  154  is connected to the waveguide type optical multiplexing/demultiplexing unit  114 . 
     In the fifth embodiment shown in FIG. 5, as in the fourth embodiment in FIG. 4, a transmission system having a waveguide type optical multiplexing/demultiplexing unit  113  serving as a multiplexing means and a reception system having a waveguide type optical multiplexing/demultiplexing unit  114  serving as a demultiplexing means are separately arranged. In this embodiment, the output port of an optical circulator  161  is connected to the waveguide type optical multiplexing/demultiplexing unit  114 . 
     The sixth embodiment shown in FIG. 6 differs from the embodiment shown in FIG. 3 in that the input ports, of a waveguide type optical multiplexing/demultiplexing unit  173 , located on the optical transmission line A side are directly connected to optical transmitters  181  to  184 , and the input ports, of a waveguide type optical multiplexing/demultiplexing unit  174 , located on the optical transmission line A side are directly connected to the optical transmitters  181  to  184 . 
     As has been described above, according to the bidirectional wavelength division multiplex transmission apparatus of the present invention, since narrow-band wavelength division multiplexing of signal light can be performed, the optical transmission lines can be effectively used, and an increase in transmission capacity can be attained. 
     In addition, since bidirectional wavelength division multiplex signal light beams can be amplified by one wavelength division multiplex optical amplifier altogether, the apparatus arrangement can be simplified, and the apparatus cost can be reduced. In addition, the maintenance of the apparatus is facilitated.