Abstract:
An optical transmission system is composed of an optical transmitter, an optical fiber and an optical receiver. An input optical signal having a first transmission rate is inputted to the optical transmitter. The optical transmitter includes a converter converting the input optical signal into a plurality of optical transmission signals having second transmission rate, and an optical multiplexer multiplexing the plurality of optical transmission signals into a multiplexed optical transmission signal. The optical fiber transmitting the multiplexed optical transmission signal. The optical receiver receiving the multiplexed optical transmission signal. The second transmission rate is smaller than the first transmission rate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical transmission system and a method of optical transmission. More particularly, the present invention relates to an optical transmission system and a method of optical transmission, using optical wavelength division multiplexing.  
           [0003]    2. Description of the Related Art  
           [0004]    In order to transmit many information, an optical wavelength division multiplexing technique is used. In an optical wavelength division multiplexing system, optical signals having different wavelengths different from each other are transmitted through one optical fiber. An optical wavelength division multiplexing system is disclosed in Japanese Laid Open Patent Application (JP-A-Showa, 62-292030). As shown in FIG. 1. , electric digital signals  102   11  to  102   1n  from transmission terminals  101   11  to  101   1n  are inputted to an electrical-optical converters  103   1 . The electrical-optical converter  103   1  converts the electric digital signals  102   11  to  102   1n  into optical signals  104   11  to  104   1n . Wavelengths of the optical signals  104   11  to  104   1n  are λ 11  to λ 1n , respectively.  
           [0005]    The optical signals  104   11  to  104   1n  are multiplexed into an optical transmission signal by an wavelength division multiplexer  105 . The optical transmission signal is transmitted through an optical fiber  106 . The optical transmission signal is separated into optical signals  108   11 ′ to  108   1n ′ by using an wavelength division demultiplexer  107 . Wavelengths of the optical signals  108   11  to  108   1n  are λ 11  to λ 1n ′ respectively. The optical signals  108   11  to  108   1n  are inputted to an optical-electric converter  109   1 . The optical-electric converter  109   1  converts the optical signals  108   11  to  108   1n  into electric digital signals  110   11  to  110   1n . The digital signals  110   11  to  110   1n  arrive at reception terminals  111   11  to  111   1n .  
           [0006]    Also, electric digital signals  102   21  to  102   2n  from transmission terminals  101   21  to  101   2n  arrive at reception terminals  111   21  to  111   2n  in the same way.  
           [0007]    The optical signal suffers from an influence caused by a disturbance at a time of the transmission through the optical fiber. When the optical signal is transmitted through the optical fiber, an optical attenuation induced in the optical fiber, an optical dispersion phenomenon and a non-linearly optical effect result in the disturbance, which brings about the deterioration in the optical signal. Moreover, other factors besides them may result in the disturbance.  
           [0008]    The optical transmission system desirably has the strong durability against the disturbance.  
           [0009]    Moreover, the optical transmission system desirably has a long transmission distance.  
           [0010]    Another optical transmission system having basically the same architecture is disclosed in Japanese Laid Open Patent Application (Jp-A-Heisei 11-331132).  
           [0011]    Still another optical transmission system which may be related to the present invention is disclosed in Japanese Laid Open Patent Application (Jp-A-Heisei 1-144832). In the other optical transmission system, a synchronization optical signal and a data optical signal having different wavelengths are multiplexed and are transmitted.  
         SUMMARY OF THE INVENTION  
         [0012]    Therefore, an object of the present invention is to provide an optical transmission system having a strong durability against a disturbance.  
           [0013]    Another object of the present invention is to provide an optical transmission system having a long transmission distance.  
           [0014]    In order to achieve an aspect of the present invention, an optical transmission system is composed of an optical transmitter, an optical fiber and an optical receiver. An input optical signal having a first transmission rate is inputted to the optical transmitter. The optical transmitter includes a converter converting the input optical signal into a plurality of optical transmission signals having second transmission rate, and an optical multiplexer multiplexing the plurality of optical transmission signals into a multiplexed optical transmission signal. The optical fiber transmitting the multiplexed optical transmission signal. The optical receiver receiving the multiplexed optical transmission signal. The second transmission rate is smaller than the first transmission rate.  
           [0015]    Generally, the durability against the disturbance is increased as the transmission rate is small. The smaller second transmission rate improve the durability against the disturbance. Also, this results in long transmission distance.  
           [0016]    Desirably, it holes  
             n≧s   1   /s   2 ,  
           [0017]    where s 1  is the first transmission rate, and s 2  is the second transmission rate, and n is a number of the plurality of optical transmission signals. In this case, the effective transmission rate of the optical transmission system is not reduced.  
           [0018]    More desirably, number of the optical transmission signals is substantially equal to s 1 /s 2 , where s 1  is the first transmission rate, and s 2  is the second transmission rate.  
           [0019]    The plurality of optical transmission signals desirably have different wavelengths from each other.  
           [0020]    The converter is desirably composed of a transmitting side optical-electrical converter, a divider, and a transmitting side electrical-optical converter. The transmitting side optical-electrical converter converts the optical input signal into a first electrical signal. The divider divides the first electrical signal into a plurality of second electrical signals. The transmitting side electrical-optical converter respectively converts the plurality of second electrical signals into the plurality of optical transmission signals.  
           [0021]    The direct conversion of the optical input signal into the plurality of the optical transmission signal may be technically difficult. The electrical signal processing is generally easier than the optical signal processing. The conversion of the optical input signal into the first electrical signal facilitates the signal processing.  
           [0022]    The optical receiver may convert the multiplexed optical transmission signal into an optical output signal substantially identical to the optical input signal to output the optical output signal.  
           [0023]    In this case, the optical receiver includes a demultiplexer, receiving side optical-electrical converter, an electrical multiplexer, and a receiving side electrical-optical converter. The demultiplexer demultiplexes the multiplexed optical transmission signal. The receiving side optical-electrical converter respectively converts the plurality of transmitted optical transmission signal into a plurality of third electrical signals. The electrical multiplexer multiplexes the plurality of third electrical signals into a fourth electrical signal. The receiving side electrical-optical converter converts the fourth electrical signal into the optical output signal.  
           [0024]    In this case, the electrical multiplexer is desirably composed of a storage unit and a combining unit. The storage unit stores a plurality of data respectively transmitted over the plurality of third electrical signals and outputs a plurality of fifth electrical signals respectively indicative of the plurality of data at predetermined timing. The combining unit combines the plurality of fifth electrical signals to generate the fourth electrical signal.  
           [0025]    In order to achieve another aspect of the present invention, an optical transmitter is composed of a converter, and an optical multiplexer. An input optical signal having a first transmission rate is inputted to the converter. The converter converts the input optical signal into a plurality of optical transmission signals having second transmission rate. The optical multiplexer multiplexes the plurality of optical transmission signals into a multiplexed optical transmission signal. The second transmission rate is smaller than the first transmission rate.  
           [0026]    In order to achieve still another aspect of the present invention, an optical receiver is composed of a demultiplexer and a converting unit. The demultiplexer demultiplexes a multiplexed optical transmission signal into a plurality of optical transmission signals. Transmission rates of the plurality of optical transmission signals are substantially equal to a second transmission rate. The converting unit converts the plurality of optical transmission signals into a optical output signal having a first transmission rate. The second transmission rate is smaller than the first transmission rate.  
           [0027]    In order to achieve still another aspect of the present invention, an optical transmission method is composed of:  
           [0028]    inputting an input optical signal having a first transmission;  
           [0029]    converting the input optical signal into a plurality of optical transmission signals having second transmission rate;  
           [0030]    multiplexing the plurality of optical transmission signals into a multiplexed optical transmission signal;  
           [0031]    transmitting the multiplexed optical transmission signal over an optical fiber; and  
           [0032]    receiving the multiplexed optical transmission signal, wherein the second transmission rate is smaller than the first transmission rate.  
           [0033]    In order to achieve still another aspect of the present invention, an operating method of an optical transmitter is composed of:  
           [0034]    inputting an input optical signal having a first transmission;  
           [0035]    converting the input optical signal into a plurality of optical transmission signals having second transmission rate;  
           [0036]    multiplexing the plurality of optical transmission signals into a multiplexed optical transmission signal, wherein the second transmission rate is smaller than the first transmission rate.  
           [0037]    In order to achieve still another aspect of the present invention, an operating method of an optical receiver is composed of:  
           [0038]    receiving a multiplexed optical transmission signal from a optical fiber;  
           [0039]    demultiplexing the multiplexed optical transmission signal into a plurality of transmitted optical signals, wherein transmission rates of the plurality of transmitted optical signals are substantially equal to a second transmission rate; and  
           [0040]    converting the plurality of transmitted optical signals into an optical output signal having first transmission rate, wherein the second transmission rate is smaller than the first transmission rate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    [0041]FIG. 1 shows a conventional optical transmission system using wavelength division multiplexing;  
         [0042]    [0042]FIG. 2 shows a configuration of an optical transmission system of an embodiment according to the present invention; and  
         [0043]    [0043]FIG. 3 shows a course in which SONET/SDH frames #1 to #25 are sent by the optical transmission system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    With reference to the attached drawings, a dividing unit is provided with a wavelength division multiplexer (hereafter, referred to as a WDM). The dividing unit  1  is connected to a WDM  2 , as shown in FIG. 2.  
         [0045]    An optical input signal a is inputted to the dividing unit  1 . A transmission rate of the optical input signal a is 10 (Gbit/s). The dividing unit  1  converts the optical input signal a into four optical signals b 1  to b 4 . Transmission rates of the optical signals b 1  to b 4  are approximately 2.5 (Gbit/s). Wavelengths of the optical signals b 1  to b 4  are λ 1 , λ 2 , λ 3  and λ 4 , respectively. The wavelengths λ 1 , λ 2 , λ 3  and λ 4  are different from each other. Here, a plurality of dividing units  1  may be connected to the WDM  2 . FIG. 1 shows only one of the plurality of dividing units  1 .  
         [0046]    The WDM  2  multiplexes the 2.5-G optical signals b 1  to b 4  to generates a WDM signal c. The WDM  2  outputs the WDM signal c to an optical fiber  3 . The optical fiber  3  transmits the WDM signal c to a wavelength division demultiplexer (referred as WDD, hereinafter)  4 . Transmission rate of the WDM signal c is approximately 2.5 (Gbit/s).  
         [0047]    The WDD  4  demultiplexes the WDM signal c into 2.5-G optical signals d 1  to d 4 . The 2.5-G optical signals d 1  to d 4  are substantially identical to the 2.5-G optical signals b 1  to b 4 , respectively. The WDD  4  outputs the 2.5-G optical signals d 1  to d 4  to a multiplexing unit  5 .  
         [0048]    The multiplexing unit  5  integrates the 2.5-G optical signals d 1  to d 4  into one optical output signal e. The optical output signal e is substantially identical to the optical input signal a. In this case, when a plurality of dividing units  1  are mounted, one multiplexing unit  5  is mounted correspondingly to each of the plurality of dividing units  1 . FIG. 1 shows only one of the plurality of the multiplexing unit  5 .  
         [0049]    The configuration of the dividing unit  1  is described below in detail. The dividing unit  1  includes a 10-G optical/electrical converter  6 . The optical input signal a is inputted to the 10-G optical/electrical converter  6 . As mentioned above, the transmission rate of the optical input signal a is 10 (Gbit/s). A plurality of SONET/SDH frames are sequentially sent by the optical input signal a. The SONET/SDH frames respectively have framing bytes indicative of the heads of the SONET/SDH frames.  
         [0050]    The 10-G optical/electrical converter  6  converts the optical input signal a into an electric signal f to output to a 10-G frame synthesizer  7 . The 10-G frame synthesizer  7  detects the framing bytes included in the input electric signal f to generates a synchronous signal g indicative of a timing synchronous with each of the SONET/SDH frames. The 10-G frame synthesizer  7  further generates an electric signal f′ substantially identical to the electric signal f. The 10-G frame synthesizer  7  outputs the electric signal f′ and the synchronous signal g to a divider  8 . The divider  8  fetches the SONET/SDH frames from the electric signal f′ in synchronization with the synchronous signal g.  
         [0051]    The divider  8  outputs a first input SONET/SDH frame after a start of an input of the electric signal f′ to a 2.5-G electrical/optical converter  9   a  over a frame signal h 1 . The divider  8  outputs a next input SONET/SDH frame to a 2.5-G electrical/optical converter  9   b  over a frame signal h 2 . The divider  8  outputs a next input SONET/SDH frame to a 2.5-G electrical/optical converter  9   c  as a frame signal h 3 . The divider  8  outputs a further next input SONET/SDH frame to a 2.5-G electrical/optical converter  9   c  as a frame signal h 4 . After that, when a SONET/SDH frame is further inputted, the SONET/SDH frame is sequentially outputted to the 2.5-G electrical/optical converters  9   a  to  9   d  in accordance with the above-mentioned processes. As mentioned above, the divider  8  sequentially outputs the input SONET/SDH frames to the 2.5-G electrical/optical converters  9   a  to  9   d.    
         [0052]    The 2.5-G electrical/optical converters  9   a  to  9   d  respectively convert the frame signal h 1  to h 4  into the 2.5-G optical signals b 1  to b 4 , respectively. All of the transmission rates of the 2.5-G optical signals b 1  to b 4  are approximately 2.5 (Gbit/s). The 2.5-G electrical/optical converters  9   a  to  9   d  outputs the 2.5-G optical signals b 1  to b 4  to the WDM  2 , respectively. As mentioned above the WDM  2  multiplexes the 2.5-G optical signals b 1  to b 4  into the WDM signal c.  
         [0053]    The configuration of the multiplexing unit  5  will be described below in detail. The multiplexing unit  5  includes 2.5-G optical/electrical converters  10   a  to  10   d . The 2.5-G optical/electrical converters  10   a  to  10   d  respectively receive the 2.5-G optical signals d 1  to d 4  from the WDD  4 , which demultiplexes the WDM signal c. The 2.5-G optical/electrical converters  10   a  to  10   d  convert the 2.5-G optical signals d 1  to d 4  into electric signals i 1  to i 4 , respectively. The 2.5-G optical/electrical converters  10   a  to  10   d  output the electric signals i 1  to i 4  to a 2.5-G frame synthesizer  11 .  
         [0054]    The 2.5-G frame synthesizer  11  detects the framing bytes included in the electric signals i 1  to i 4  to generate a synchronous signal j indicative of a timing synchronous with each of the SONET/SDH frames sent through the electric signals i 1  to i 4 . The 2.5-G frame synthesizer  11  further generates electric signals i 1 ′ to i 4 ′ substantially identical to the electric signals i 1  to i 4 . The 2.5-G frame synthesizer  11  outputs the electric signals i 1 ′ to i 4 ′ and the synchronous signal j to an FIFO  12 .  
         [0055]    The FIFO  12  accumulates the respective SONET/SDH frames sent through the electric signals i 1 ′ to i 4 ′ while carrying out the synchronization between them on the basis of the synchronous signal j. The FIFO  12  outputs the respective SONET/SDH frames in the input order, at a predetermined timing.  
         [0056]    The reason of the installation of the FIFO  12  is as follows. As mentioned above, the 2.5-G optical signals b 1  to b 4 , multiplexed into the WDM signal c, have the wavelengths different from each other. When the 2.5-G optical signals b 1  to b 4  multiplexed into the WDM signal c are transmitted through the optical fiber  3 , transmission delay times in the optical fiber  3  is different depending on the wavelengths because of the optical dispersion phenomenon in the optical fiber  3 . This causes the transmission rates of the 2.5-G optical signal b 1  to b 4  in the WDM signal c to be slightly different from each other. The electric signals i 1 ′ to i 4 ′ outputted from the 2.5-G frame synthesizer  11  are also slightly out of timing, correspondingly to the difference of the delay times of the 2.5-G optical signal b 1  to b 4 . Therefore, the respective SONET/SDH frames sent through the electric signals i 1 ′ to i 4 ′ are once accumulated in the FIFO  12 . Then, the SONET/SDH frames are outputted from the FIFO  12  at the predetermined timing. In this way, the difference of the delay times of the 2.5-G optical signal b 1  to b 4  in the WDM signal c is absorbed.  
         [0057]    The FIFO  12  outputs the SONET/SDH frames transmitted through the electric signal i 1 ′ to a multiplexer  13  by an electric signal k 1  in the input order. Similarly, the FIFO  12  outputs the SONET/SDH frames transmitted through the electric signals i 2  to i 4  to the multiplexer  13  by electric signals k 2  to k 4  in the input order.  
         [0058]    The timing when the FIFO  12  outputs the SONET/SDH frame is not limited to the certain temporal interval. The timing when the FIFO  12  outputs the SONET/SDH frame may be a predetermined timing other than the certain temporal interval, at which the difference of the delay times of the 2.5-G optical signal b 1  to b 4  is absorbed.  
         [0059]    The multiplexer  13  couples the respective SONET/SDH frames transmitted through the electric signals k 1  to k 4  in the input order to generate the electric signal m. The multiplexer  13  outputs the electric signal m to a 10-G electric/optical converter  14 . A transmission rate of the electric signal m is 10 (Gbit/s). The 10-G electric/optical converter  14  converts the electric signal m into the optical output signal e to output it. The transmission rate of the optical output signal e is 10 (Gbit/s) as mentioned above.  
         [0060]    [0060]FIG. 3 shows a course in which SONET/SDH frames #1 to #25, inputted to the dividing unit  1 , are sent to the multiplexing unit  5  by the optical transmission system. FIG. 3 shows the momentary states of the SONET/SDH frames #1 to #25 immediately before the #5 is sent to the multiplexing unit  5  after the frames #1 to #4 are already sent to the multiplexing unit  5 .  
         [0061]    The SONET/SDH frames #1 to #25 are inputted to the dividing unit  1  in the order of the ascending number. The SONET/SDH frame #1 is converted into the 2.5-G optical signals b 1  having a wavelength of λ 1 , and then sent to the multiplexing unit  5 . Similarly, the SONET/SDH frames #2 to #4 are converted into the 2.5-G optical signals b 2  to b 4  having wavelengths of λ 2  to λ 4 , respectively, and then sent to the multiplexing unit  5 .  
         [0062]    [0062]FIG. 3 shows the SONET/SDH frames #1 to #4 are outputted as the optical output signal e in the order of the SONET/SDH frames #1, #2, #3 and #4.  
         [0063]    Also, FIG. 3 shows the SONET/SDH frames #5, #9, #13 and #17 are converted into the 2.5-G optical signals b 1  having a wavelength are λ 1 , and they are being sent to the multiplexing unit  5 . The SONET/SDH frames #6, #10, #14 and #18 are converted into the 2.5-G optical signals b 2  having the wavelength are λ 2 , and they are being sent to the multiplexing unit  5 . The SONET/SDH frames #7, #11, #15 and #19 are converted into the 2.5-G optical signals b 3  having the wavelength of λ 3 , and they are being sent to the multiplexing unit  5 . The SONET/SDH frames #5, #9, #13 and #17 are converted into the 2.5-G optical signals b 1  having the wavelength are λ 1 , and they are being sent to the multiplexing unit  5 .  
         [0064]    In addition, FIG. 3 shows the SONET/SDH frame #21 is being converted into the 2.5-G optical signals b 1 . Moreover, FIG. 3 shows the fact that the SONET/SDH frame #22 is being converted into the 2.5-G optical signals b 2 .  
         [0065]    Furthermore, FIG. 3 shows the SONET/SDH frames #23 to #25 are inputted to the dividing unit  1  after the SONET/SDH frame #22. The SONET/SDH frames #23, #24 and #25 are transmitted through the 2.5-G optical signals b 3 , b 4  and b 1  respectively having wavelengths of λ 3 , λ 4  and λ 1 , after the moment shown in FIG. 3.  
         [0066]    As mentioned above, the SONET/SDH frames #1 to #25 are sequentially inputted through the optical input signal a to the dividing unit  1  of the optical transmission system in this embodiment. The transmission rate of the optical input signal a is 10-Gb/s. The SONET/SDH frame #(4p 1 +1) is converted into the 2.5-G optical signals b 1  having wavelength of λ 1 , and sent to the multiplexing unit  5 . Here, the p 1  is the integer between 0 and 6. Also, the SONET/SDH frame #(4p 2 +2) is converted into the 2.5-G optical signals b 2  having wavelength of λ 2 , and sent to the multiplexing unit  5 . Here, the p2 is the integer between 0 and 5. Moreover, the SONET/SDH frame #(4p 3 +3) is converted into the 2.5-G optical signals b 3  having wavelength of λ 3  , and sent to the multiplexing unit  5 . Here, the p 3  is the integer between  0  and  5 . Moreover, the SONET/SDH frame #(4p 4 +4) is converted into the 2.5-G optical signals b 4  having wavelength of λ 4 , and sent to the multiplexing unit  5 . Here, the p 4  is the integer between 0 and 5. All of the transmission rates of the 2.5-G optical signals b 1  to b 4  having wavelength of λ 2  are approximately 2.5 (Gbits ). The multiplexing unit  5  sequentially outputs the SONET/SDH frames #1 to #25 as the optical output signal e having the transmission rate of 10 (Gbit/s).  
         [0067]    In the embodiments, the transmission rate of the optical input signal a is not limited to 10 (Gbit/s). Also, the transmission rates of the 2.5-G optical signals b 1  to b 4  are not approximately limited to 2.5 (Gbit/s) if they are lower than that of the optical input signal a. Moreover, the number of the 2.5-G optical signals b 1  to b 4  are not limited to four if they are at least plural.  
         [0068]    However, it desirably holds:  
         
       n≧s 
       1 
       /s 
       2  
     
         [0069]    where s 1  is the transmission rate of the optical input signal a, s 2  is the transmission rates of the 2.5-G optical signals b 1  to b 4 , n is the number of the 2.5-G optical signals b 1  to b 4  is n. This is because the data inputted through the optical input signal a can be sent without any drop in the effective transmission rate.  
         [0070]    In particular, n is desired to be substantially equal to s 1 /s 2 . This is because the circuit for converting the optical input signal a into the WDM signal c and the circuit for converting the WDM signal c into an optical output signal e can be reduced to the necessary minimum.  
         [0071]    The present invention provides the optical transmission system having the high durability against the disturbance.  
         [0072]    Moreover, the present invention provides the optical transmission system having the long transmission distance.  
         [0073]    Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.