Abstract:
An optical code division multiplexing communication method includes the steps of: producing a multi-wavelength optical pulse train from wavelength multiplexing pulse; transmitting the multi-wavelength optical pulse train through a transmission line using a time-spreading/wavelength-hopping method; decoding wavelength multiplexing pulse from the multi-wavelength optical pulse train transmitted through the transmission line; compensating delay time differences between individual optical pulses of the multi-wavelength optical pulse train, the delay time differences occurring in the step of transmitting the multi-wavelength optical pulse train through the transmission line; and compensating optical pulse spread in a time direction, which occurs in each of the optical pulses of the multi-wavelength optical pulse train in the step of transmitting the multi-wavelength optical pulse train through the transmission line.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical code division multiplexing communication method using a time-spreading/wavelength-hopping code, an optical code division multiplexing communication system capable of implementing the optical code division multiplexing communication method, an encoding device constructing the optical code division multiplexing communication system, and a decoding device constructing the optical code division multiplexing communication system, and in particular, to chromatic dispersion compensation in optical code division multiplexing communication.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 1  is a diagram for describing a principle of encoding and decoding in an optical code division multiplexing communication using a time-spreading/wavelength-hopping method. As shown in  FIG. 1 , an encoder  10  has a structure in which FBGs (Fiber Bragg Gratings)  10   a , . . . ,  10   d  of multiple wavelengths (for example, reflecting wavelengths are λ 1 , λ 3 , λ 4 , and λ 2 ) are connected in series. When wavelength multiplexing pulse  30  corresponding to the reflecting wavelengths of the FBGs  10   a , . . . ,  10   d  is inputted to the encoder  10  via an optical circulator  34 , a multi-wavelength optical pulse train including four optical pulses  31   a , . . . ,  31   d  of different wavelengths is produced in a spreading time determined by a code set in the encoder  10  (that is, structure of the encoder  10 ) from one wavelength multiplexing pulse  30  and is outputted to a transmission line  35  via an optical circulator  34 . This multi-wavelength optical pulse train is an encoded optical signal. A decoder  20  of the same codes as the encoder  10  (that is, having a structure of handling the same codes as the encoder  10 ) has a series structure of FBGs  20   d , . . . ,  20   a  which is opposite to the series structure of the encoder  10  in which the FBGs  10   a , . . . ,  10   d  are connected in series. Hence, the decoder  20  has a group delay time characteristic which is opposite to the group delay time characteristic of the encoder  10 . Therefore, when the encoded signal (optical pulses  31   a , . . . ,  31   d ) is inputted to the decoder  20  via an optical circulator  36 , time-spread optical pulses of four wavelengths are aligned at the same timing, whereby wavelength multiplexing pulse  33  of auto-correlation waveform is produced and outputted via the optical circulator  36 .  
         [0005]     When the encoded signal is inputted to the decoder  20 , if the codes agree with each other (the series structures of the multi-wavelength FBGs of the encoder  10  and the decoder  20  are related as object and mirror image), the relative time arrangement of the time-spread multi-wavelength optical pulses  31   a , . . . ,  31   d  is corrected to produce a wavelength multiplexing pulse  33  having an auto-correlation waveform. If the codes do not agree with each other (the series structures of the multi-wavelength FBGs of the encoder  10  and the decoder  20  are not related as object and mirror image), the relative time arrangement of the time-spread multi-wavelength optical pulses  31   a , . . . ,  31   d  is further spread to produce an cross-correlation waveform (not shown).  
         [0006]     In general, the optical code division multiplexing communication using a time-spreading/wavelength-hopping code, as disclosed in the non-patent document 1 (Wei et al., “BER Performance of an Optical Fast Frequency-Hopping CDMA System with Multiple Simultaneous Users”, OFC2003, Technical Digest, Vol. 2, ThQ1, pp. 544-546), is characterized in that because an optical signal of a plurality of wavelengths spread in a time range (having a wide frequency band) is used as an encoded signal, as a transmission distance is longer and a transmission signal rate is faster, the optical signal is more susceptible to the chromatic dispersion of the transmission line. Because a transmission line formed of a standard SMF has a chromatic dispersion characteristic of approximately 17 ps/(nm·km), when an encoded optical signal is transmitted through the transmission line, the relative time arrangement between the multi-wavelength optical pulses constructing the encoded optical signal varies according to the transmission distance. Hence, even if a decoder of the same codes as an encoder is used, the auto-correlation waveform as shown in  FIG. 1  cannot be produced (that is, the encoded optical signal cannot be excellently decoded). Therefore, when optical code division multiplexing communication using a time-spreading/wavelength-hopping code is applied to a transmission line having chromatic dispersion, if the chromatic dispersion is not compensated to a sufficiently smaller level with respect to the width of an optical signal pulse inputted to the encoder, an excellent auto-correlation waveform cannot be produced. However, when the chromatic dispersion of an individual transmission line is compensated by known means such as a chromatic dispersion compensating fiber, the communication system needs to be increased in size, which results in increasing transmission loss and costs.  
         [0007]     As to the problem like this, to reduce the effect of the chromatic dispersion with ease, a non-patent document 2 (Iwamura et al., “FBG based Optical Code En/Decoder for long distance transmission without dispersion compensating devices”, OFC2004, Technical Digest, WK6) discloses a technology of compensating a delay time difference in a wavelength band (frequency band) in an optical encoded signal, of the effects caused by the chromatic dispersion of an FBG type decoder, by the construction of the FBG type encoder. According to this technology, it is achieved to transmit an encoded optical signal through the SMF of 40 km at a transmission rate of 10 Gbps.  FIG. 2  is a diagram for describing an encoder and a decoder having a delay time compensating function in an optical code division multiplexing communication using a time-spreading/wavelength-hopping method. As shown in  FIG. 2 , when the encoded optical signal is transmitted through the SMF of 40 km, an optical pulse train (optical pulses  31   a , . . . ,  31   d ) immediately after produced by the encoder  10  is turned to an optical pulse train (optical pulses  32   a , . . . ,  32   d ) having the delay time difference between wavelengths increased by the transmission through the SMF. Therefore, a decoder  21  has a delay time characteristic in which a delay time characteristic of compensating delay time differences between wavelengths caused by the chromatic dispersion of the SMF transmission line is added to a delay time characteristic opposite to that of the encoder  10 .  
         [0008]     However, even if the decoder  21  shown in  FIG. 2  is used, it is only a wavelength difference and a delay time difference caused by a transmission distance that are compensated, and the optical pulse spread in a time direction of individual optical pulse, which is caused by the chromatic dispersion of the fiber, cannot be compensated. Hence, as the transmission distance is longer, the widths of auto-correlation waveforms are more expanded and the optical pulses finally overlap neighboring optical pulses, thereby making it impossible to receive the optical signal. In this manner, even if the decoder  21  shown in  FIG. 2  is used, it is only delay time differences between optical signal wavelengths that are compensated and the expansion of width of the individual optical signal pulse, which is caused by the dispersion slope of the transmission line, is not compensated. Therefore, the transmission distance cannot be more extended.  
         [0009]     Further, a non-patent document 3 (Buryak et al., “Optimization of Refractive Index Sampling for Multichannel Fiber Bragg Gratings”, IEEE Journal of Quantum Electronics, Vol. 39, No. 1, pp. 91-98, January 2003) discloses a technology related to dispersion slope compensation but does not disclose a technology applied to the compensation of encoded waveforms degraded by the transmission line of the optical code division multiplexing communication of a time-spreading/wavelength-hopping method using an optical signal of a plurality of wavelengths for one communication channel.  
       SUMMARY OF THE INVENTION  
       [0010]     The object of the present invention is to provide an optical code division multiplexing communication method capable of accomplishing the long-distance transmission of an optical signal using a time-spreading/wavelength-hopping method, an optical code division multiplexing communication system capable of implementing this method, and an encoding device and a decoding device that construct this system.  
         [0011]     An optical code division multiplexing communication method of the present invention includes the steps of: producing a multi-wavelength optical pulse train from wavelength multiplexing pulse; transmitting the multi-wavelength optical pulse train through a transmission line using a time-spreading/wavelength-hopping method; decoding wavelength multiplexing pulse from the multi-wavelength optical pulse train transmitted through the transmission line; compensating delay time differences between individual optical pulses of the multi-wavelength optical pulse train, the delay time differences occurring in the step of transmitting the multi-wavelength optical pulse train through the transmission line; and compensating optical pulse spread in a time direction, which occurs in each of the optical pulses of the multi-wavelength optical pulse train in the step of transmitting the multi-wavelength optical pulse train through the transmission line.  
         [0012]     Further, an optical code division multiplexing communication system of the present invention includes: an encoder for producing a multi-wavelength optical pulse train from wavelength multiplexing pulse; a decoder for decoding the wavelength multiplexing pulse from the multi-wavelength optical pulse train after the multi-wavelength optical pulse train is transmitted through a transmission line using a time-spreading/wavelength-hopping method; a delay time difference compensator for compensating delay time differences between individual optical pulses of the multi-wavelength optical pulse train, the delay time differences occurring when the multi-wavelength optical pulse train is transmitted through the transmission line; and a chromatic dispersion compensator for compensating optical pulse spread in a time direction, which occurs in each of optical pulses of the multi-wavelength optical pulse train when the multi-wavelength optical pulse train is transmitted through the transmission line.  
         [0013]     Furthermore, an encoding device includes: an encoder for producing a multi-wavelength optical pulse train to be transmitted through a transmission line in an optical code division multiplexing communication, in which the multi-wavelength optical pulse train produced from wavelength multiplexing pulse is transmitted through the transmission line using a time-spreading/wavelength-hopping method and the wavelength multiplexing pulse is decoded from the multi-wavelength optical pulse train transmitted through the transmission line; and a chromatic dispersion compensator for compensating optical pulse spread in a time direction, which occurs in each of the optical pulses of the multi-wavelength optical pulse train when the multi-wavelength optical pulse train is transmitted through the transmission line.  
         [0014]     Moreover, a decoding device includes: a decoder for decoding wavelength multiplexing pulse from a multi-wavelength pulse train transmitted through a transmission line in an optical code division multiplexing communication, in which the multi-wavelength optical pulse train produced from wavelength multiplexing pulse is transmitted through the transmission line using a time-spreading/wavelength-hopping method and the wavelength multiplexing pulse is decoded from the multi-wavelength optical pulse train transmitted through the transmission line; a delay time difference compensator for compensating delay time differences between individual optical pulses of the multi-wavelength optical pulse train, the delay time differences occurring when the multi-wavelength pulse train is transmitted through the transmission line; and a chromatic dispersion compensator for compensating optical pulse spread in a time direction of individual optical pulse of the multi-wavelength optical pulse train, the optical pulse spread occurring when the multi-wavelength optical pulse train is transmitted through the transmission line.  
         [0015]     According to the present invention, delay time differences between individual optical pulses of a multi-wavelength optical pulse train and the optical pulse spread in a time direction of the individual optical pulse, which are caused when the multi-wavelength optical pulse train is transmitted through a transmission line using a time-spreading/wavelength-hopping method, are compensated and hence the wavelength multiplexing pulse can be excellently decoded. Therefore, the present invention can produce an effect of further elongating a transmission distance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0017]      FIG. 1  is a diagram for describing a principle of encoding and decoding in an optical code division multiplexing communication using a time-spreading/wavelength-hopping method;  
         [0018]      FIG. 2  is a diagram for describing an encoder and a decoder with a delay time compensating function using a time-spreading/wavelength-hopping method;  
         [0019]      FIG. 3  is a block diagram showing the construction of an optical code division multiplexing communication system of the first embodiment (that is, a system capable of implementing an optical code division multiplexing communication method of the first embodiment) of the present invention;  
         [0020]      FIG. 4  is a diagram schematically showing the structure of an encoder;  
         [0021]      FIG. 5  is a diagram schematically showing the structure of a delay time compensating decoder;  
         [0022]      FIGS. 6A  to  6 C are diagrams showing a reflectivity characteristic, a group delay time characteristic, and the structure of a dispersion slope compensator;  
         [0023]      FIGS. 7A  to  7 D are diagrams showing the waveform of pulses inputted to an encoder, the waveform of pulses outputted from the encoder, the waveform of pulses decoded by a decoder, and the waveform of pulses compensated by a dispersion slope compensator;  
         [0024]      FIG. 8  is a block diagram showing the construction of an optical code division multiplexing communication system of the second embodiment (that is, a system capable of implementing an optical code division multiplexing communication method of the second embodiment) of the present invention; and  
         [0025]      FIG. 9  is a block diagram showing the construction of an optical code division multiplexing communication system of the third embodiment (that is, a system capable of implementing an optical code division multiplexing communication method of the third embodiment) of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications will become apparent to those skilled in the art from the detailed description.  
       First Embodiment  
       [0027]      FIG. 3  is a block diagram showing the construction of an optical code division multiplexing communication system of the first embodiment (that is, a system capable of implementing an optical code division multiplexing communication method of the first embodiment) of the present invention. As shown in  FIG. 3 , the optical code division multiplexing communication system of the first embodiment includes a transmitter  41 , a decoding device  43  connected to the transmitter  41  by a transmission line  42 , and a receiver  44 .  
         [0028]     As shown in  FIG. 3 , the transmitter  41  includes a multi-wavelength optical pulse light source  41   a  for producing a wavelength multiplexing pulse, a data signal generator  41   b , a wavelength multiplexing pulse modulator  41   c  which modulates a wavelength multiplexing pulse on the basis of a control signal from the data signal generator  41   b , and an encoder which produces a multi-wavelength optical pulse train (for example, corresponding to optical pulses  31   a , . . . ,  31   d  in  FIG. 1  and  FIG. 2 ) from the modulated wavelength multiplexing pulse (for example, corresponding to wavelength multiplexing pulse  30  in  FIG. 1 ).  
         [0029]      FIG. 4  is a diagram schematically showing an example of a structure of the encoder  41   d . As shown in  FIG. 4 , the encoder  41   d  has a structure in which an optical circulator  45  and FBGs  46   a , . . . ,  46   d , which have reflecting wavelengths λ 11 , λ 12 , λ 13 , and λ 14 , respectively, are connected in series. This structure is similar to the structure shown in  FIG. 5 ( b ) of JAPANESE PATENT KOKAI (LAID-OPEN) PUBLICATION NO. 2003-244101. Further, the number of FBGs connected in series may be more than or less than four.  
         [0030]     As shown in  FIG. 3 , the transmission line  42  includes a single-mode fiber (SMF)  42   a  and an optical amplifier  42   b  which compensates the loss of the SMF  42   a.    
         [0031]     Moreover, as shown in  FIG. 3 , the decoding device  43  includes a delay time difference compensating decoder  43   a  and a dispersion slope compensator  43   b.    
         [0032]      FIG. 5  is a diagram schematically showing an example of the structure of the delay time difference compensating decoder  43   a . As shown in  FIG. 5 , the delay time difference compensating decoder  43   a  has a structure in which an optical circulator  47  and FBGs  48   d , . . . ,  48   a  that have reflecting wavelengths λ 14 , λ 13 , λ 12 , and λ 11 , respectively, are connected in series. The FBGs  48   d , . . . ,  48   a  are arranged at intervals determined on the basis of delay time differences between individual optical pulses of an inputted multi-wavelength optical pulse train. The delay time difference compensating decoder  43   a  has a function of decoding wavelength multiplexing pulse from the multi-wavelength optical pulse train (corresponding to encoded signals  32   a , . . . ,  32   d  in  FIG. 2 ) after the multi-wavelength optical pulse train is transmitted through the transmission line  42  and a delay time difference compensating function of compensating delay time differences between the individual optical pulses of the multi-wavelength optical pulse train produced when the multi-wavelength optical pulse train is transmitted through the transmission line  42 . The structure of the delay time difference compensating decoder  43   a  is similar to the structure shown in  FIG. 6  of JAPANESE PATENT KOKAI (LAID-OPEN) PUBLICATION NO. 2003-244101.  
         [0033]      FIGS. 6A  to  6 C are diagrams schematically showing the reflectivity characteristic, the group delay time characteristic, and a structure of the dispersion slope compensator  43   b . As shown in  FIG. 6C , the dispersion slope compensator  43   b  includes, for example, an optical circulator  49   a  and a sampled FBG  49   b . The dispersion slope compensator  43   b  compensates such an optical pulse spread in a time direction of individual optical pulse of the multi-wavelength pulse train that is caused by the chromatic dispersion when the multi-wavelength pulse train is transmitted through the transmission line  42 . The sampled FBG  49   b  has a plurality of refractive index modulating structures formed in an optical waveguide (for example, the core of an optical fiber) so as to have a chirp structure and phase shift structures formed between the plurality of refractive index modulating structures, and is also referred to as an SS (Super Structure) FBG. The sampled FBG  49   b  has a characteristic of hardly producing delay time differences between the wavelengths. The reflectivity characteristic of the dispersion slope compensator  43   b  used for a multi-wavelength optical pulse train of 8 wavelengths at intervals of 100 GHz is shown in  FIG. 6A . As shown in a wavelength range R 1  in  FIG. 6B , the dispersion slope compensator  43   b  has a chromatic dispersion slope (approximately −680 ps/nm) opposite to the chromatic dispersion slope of an SMF transmission line of 40 km within a wavelength band corresponding to an optical signal of one wavelength. It is also recommended to employ such a compensator of other principle and structure that has the characteristics shown in  FIGS. 6A and 6B  as the dispersion slope compensator  43   b.    
         [0034]     In this regard, in  FIG. 3  is shown an example in which the dispersion slope compensator  43   b  is provided in the latter stage of the delay time difference compensating decoder  43   a . However, the dispersion slope compensator  43   b  may be provided in the former stage of the delay time difference compensating decoder  43   a.    
         [0035]     The operation of the optical multiplexing encoding system of the first embodiment (that is, optical multiplexing encoding method of the first embodiment) will be described below.  FIGS. 7A  to  7 D are diagrams based on actual data, wherein  FIG. 7A  shows a waveform of pulses inputted to the encoder  41   d ,  FIG. 7B  shows a waveform of pulses outputted from the encoder  41   d ,  FIG. 7C  shows a waveform of pulses decoded by the decoder  43   a , and  FIG. 7D  shows a waveform of pulses compensated by the dispersion slope compensator  43   b.    
         [0036]     First, an RZ wavelength division multiplexing optical pulse train of four wavelengths (λ 11 , λ 12 , λ 13 , and λ 14 ) at intervals of 100 GHz from the multi-wavelength optical pulse light source  41   a  is modulated by the modulator  41   c  to, for example, an RZ wavelength multiplexing pulse train of a period of 10 Gbps according to desired data from the data signal generator  41   b . At this time, it is assumed that RZ optical pulses constructing the RZ wavelength multiplexing pulse train have a width of, for example, 18 ps. When the RZ wavelength multiplexing pulse is inputted to the encoder  41   d , the RZ wavelength multiplexing pulse is reflected by the FBGs corresponding to the individual wavelengths, whereby an optical pulse train, in which optical pulses have arbitrary delay time differences between the wavelengths, respectively, is produced. Here, the order of FBGs of the respective wavelengths, which are connected in series, and the intervals between the respective FBGs are determined by a desired code. This optical pulse train is an encoded signal and is transmitted through the transmission line  42  in this state.  
         [0037]     A standard SMF has a dispersion characteristic of approximately 17 ps/(nm·km). Hence, when optical wavelength multiplexing pulses at intervals of 0.8 nm are transmitted through the 40 km SMF, a transmission time difference of approximately 544 ps (=0.8×17×40) is produced between the optical pulses by the chromatic dispersion of the SMF. Further, the widths of individual optical pulses are also expanded. As a result, when the encoded signal is transmitted through the SMF, the delay time difference between the wavelengths and such an optical pulse spread in a time direction of the optical pulse that is caused by the chromatic dispersion are superimposed to the delay time difference between the optical pulses caused by the encoding.  
         [0038]     The encoded signal affected by this chromatic dispersion is inputted to the decoding device (decoder module)  43 . In the decoding device  43 , the encoded signal affected by this chromatic dispersion is inputted to the delay time difference compensating decoder  43   a . When the codes match each other, the delay time differences between the wavelengths, provided at the time of encoding, and the delay time differences between the wavelengths, produced by the chromatic dispersion of the transmission line  42 , are compensated, whereby the optical pulses of individual wavelengths are aligned at the same timing. However, such an optical pulse spread in a time direction of the individual optical pulse that is caused by the chromatic dispersion of the transmission line  42  is not compensated by the delay time difference compensating decoder  43   a . Moreover, when the codes do not match each other, the optical pulses are further spread in a time direction.  
         [0039]     Next, when the optical pulses from the delay time difference compensating decoder  43   a  are inputted to the sampled FBG type dispersion slope compensator  43   b , the widths of the individual optical pulses are compressed to a state close to a state before transmission through the transmission line  42  by the negative dispersion slope of this dispersion slope compensator  43   b  (see  FIGS. 7A and 7D ). In this manner, such an optical pulse spread in a time direction of the individual optical signal pulse that is caused by the chromatic dispersion of the transmission line  42  is compensated by the sampled FBG type dispersion slope compensator  43   b  and hence the wavelength multiplexing pulse is excellently decoded. Therefore, there is an effect that a transmission distance can be elongated.  
         [0040]     Further, a system (conventional system) using a dispersion compensating fiber to compensate a dispersion slope is suitable for compensating a wide wavelength range in a collective manner but requires a large-sized and expensive apparatus. For this reason, a method and a system in accordance with the present invention are more advantageous for a node of receiving a specific number of wavelengths from the viewpoint of reducing the size of apparatus and reducing costs.  
       Second Embodiment  
       [0041]      FIG. 8  is a block diagram showing the construction of an optical code division multiplexing communication system of the second embodiment (that is, a system capable of implementing an optical code division multiplexing communication method of the second embodiment) of the present invention. As shown in  FIG. 8 , the optical code division multiplexing communication system of the second embodiment is different, in that a dispersion slope compensator  51   e  is provided in the former stage of a transmission line  52 , from the optical code division multiplexing communication system of the first embodiment in which the dispersion slope compensator ( 43   b  in  FIG. 3 ) is provided in the latter stage of the transmission line ( 42  in  FIG. 3 ).  
         [0042]     A multi-wavelength pulse light source  51   a , a data signal generator  51   b , a modulator  51   c , and an encoder  51   d  in  FIG. 8  correspond to the multi-wavelength pulse light source  41   a , the data signal generator  41   b , the modulator  41   c , and the encoder  41   d  in  FIG. 3  (the first embodiment), respectively. Further, a transmission line  52  in  FIG. 8  corresponds to the transmission line  42  in  FIG. 3 . Furthermore, a delay time difference compensating decoder  53   a  in  FIG. 8  corresponds to the delay time difference compensating decoder  43   a  in  FIG. 3 . Moreover, a receiver  54  in  FIG. 8  corresponds to the receiver  44  in  FIG. 3 . Still further, in  FIG. 8 , a dispersion slope compensator  51   e  is provided in place of the dispersion slope compensator  43   b  in  FIG. 3 . Here, the dispersion slope compensator  51   e  may be provided in the former stage of the encoder  51   d.    
         [0043]     Because the dispersion slope compensator  51   e  is provided in a transmitter  51  in the optical code division multiplexing communication system of the second embodiment, all optical pulses are transmitted in a state where the optical pulses are pre-chirped in response to the chromatic dispersion of the transmission line  52 . When the pre-chirped optical encoded signal is transmitted through the transmission line  52 , the chirp of the optical pulses is returned to the original state by the chromatic dispersion of the transmission line  52  but the delay time differences between the wavelengths occur as is the case with the first embodiment. Hence, by compensating the delay time differences by the delay time difference compensating decoder  53   b , an excellent decoded waveform (auto-correlation waveform) can be obtained.  
         [0044]     As described above, according to the second embodiment, the similar effect to the first embodiment can be produced. Moreover, according to the second embodiment, the transmitter  51  compensates the dispersion slope and the decoding device  53  on the receiver side compensates the delay time difference (that is, the transmitting side and the receiving side share the compensation of the chromatic dispersion), and hence a dispersion characteristic in which the transmitted optical pulses are pre-chirped can be obtained. Therefore, this makes it difficult for the third party trying an unauthorized access to make a corresponding decoder and hence can improve the concealment of information in an encoding communication system.  
         [0045]     Except for the above-described respects, the second embodiment is the same as the first embodiment.  
       Third Embodiment  
       [0046]      FIG. 9  is a block diagram showing the construction of an optical code division multiplexing communication system of the third embodiment (that is, a system capable of implementing an optical code division multiplexing communication method of the third embodiment) of the present invention. As shown in  FIG. 9 , the optical code division multiplexing communication system of the third embodiment is different, in that a dispersion slope compensator  61   e  is provided in the former stage of a transmission line  62  and in that a dispersion slope compensator  63   b  is provided in the latter stage of the transmission line  62 , from the optical code division multiplexing communication system of the first embodiment in which the dispersion slope compensator ( 43   b  in  FIG. 3 ) is provided only in the latter stage of the transmission line ( 42  in  FIG. 3 ).  
         [0047]     A multi-wavelength pulse light source  61   a , a data signal generator  61   b , a modulator  61   c , and an encoder  61   d  in  FIG. 9  correspond to the multi-wavelength pulse light source  41   a , the data signal generator  41   b , the modulator  41   c , and the encoder  41   d  in  FIG. 3  (the first embodiment), respectively. Further, the transmission line  62  in  FIG. 9  corresponds to the transmission line  42  in  FIG. 3 . Furthermore, a delay time difference compensating decoder  63   a  in  FIG. 9  corresponds to the delay time difference compensating decoder  43   a  in  FIG. 3 . Moreover, a receiver  64  in  FIG. 9  corresponds to the receiver  44  in  FIG. 3 . Still further, in  FIG. 9 , dispersion slope compensators  61   e  and  63   b  are provided in place of the dispersion slope compensator  43   b  in  FIG. 3 . Here, the dispersion slope compensator  61   e  may be provided in the former stage of the encoder  61   d . Moreover, the dispersion slope compensator  63   b  may be provided in the former stage of the delay time difference compensating decoder  63   b.    
         [0048]     The dispersion slope compensator  61   e  corresponding to an SMF transmission line of 40 km is provided in a transmitter  61  in the optical code division multiplexing communication system of the third embodiment and hence all optical pulses are transmitted in a state where the optical pulses are pre-chirped in response to the half of chromatic dispersion of the transmission line  62  of 80 km. When the pre-chirped optical encoded signal is transmitted through the transmission line  62 , the pre-chirp of the optical pulses is returned to the original state by the chromatic dispersion of the transmission line  62  and further is affected by the chromatic dispersion corresponding to the SMF of 40 km. That is, the optical pulse spread in a time direction of individual optical pulse is the same as that in the case of the SMF of 40 km but delay time differences between the wavelengths are caused by the transmission line of the SMF of 80 km. Hence, in accordance with the decoding device  63  including the dispersion slope compensator  63   b  corresponding to the SMF of 40 km and the delay time difference compensating decoder  63   b  corresponding to the SMF of 80 km, excellent decoded waveform (auto-correlation waveform) can be obtained.  
         [0049]     As described above, according to the third embodiment, the similar effect to the first embodiment can be produced. Moreover, according to the third embodiment, the transmitter  61  compensates the dispersion slope and the decoding device  63  on the receiver side compensates the delay time difference and the dispersion slope (that is, the transmitting side and the receiving side share the compensation of the chromatic dispersion) and hence a dispersion characteristic in which the transmitted optical pulses are pre-chirped can be obtained. Therefore, this makes it difficult for the third party trying an unauthorized access to make a corresponding decoder and hence can improve the concealment of information in an encoding communication system.  
         [0050]     Further, to respond to long-distance transmission, the quantity of compensation of the dispersion slope compensator needs to be increased. This results in elongating the total length of FBGs constructing a dispersion slope compensator and hence increasing difficulty in the manufacturing of the FBGs. However, according to the third embodiment, by employing the dispersion slope compensators of length to be easily manufactured in combination at the transmitting side and the receiving side, the still longer distance transmission of the optical signal can be achieved.  
         [0051]     Except for the above-described respects, the third embodiment is the same as the first or second embodiment.  
         [0052]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of following claims.