Patent Publication Number: US-7899336-B2

Title: Optical communications device and optical communications system

Description:
This application is the National Stage Entry of PCT Application No. PCT/JP2005/010676 filed Jun. 10, 2005, and claims priority from Japanese Patent Application No. 2004-172734 filed Jun. 10, 2004, which applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an optical communications device that multiplexes optical signals for picture communication and optical signals for data communication by WDM method and an optical communications system having the above optical communications device. 
     BACKGROUND ART 
     Japanese Patent Laid-Open Publication No. 2001-244528 discloses an optical fiber communication that combines optical signals for data communication on, for instance, Internet, with optical signals for picture communication on, for instance, CATV (Cable Television), by WDM method (Wavelength-Division Multiplexing) for the purpose of effective use of an optical fiber in FTTH (Fiber To The Home) and others. 
     Data communication on Internet is commonly done with the use of optical signals on direct digital modulation (e.g. “1” for light-on and “0” for light-off (vice versa)) at a data rate such as 622 Mbps and 1.25 Gbps. The optical signals for data communication are formed by successive time-series data composed of packet signals representing communication data such as telegraphic message and idle signals embedded in between the packet signals. That is, as shown in  FIG. 1 , the communication data such as telegraphic message is transmitted in the form of packets (i.e. packet signals P 1 , P 2 , . . . ) intermittently. However, since respective intervals between the packet signals are filled up with idle signals A 101 , A 102 , A 103 , . . . representing “absence of a packet signal”, the optical signals for data communication are transmitted continuously. 
     An idle signal is commonly provided with a cyclic signal pattern. For instance, according to 1000BASE.X (so-called “gigabit Ethernet” (registered trademark)) defined in ISO/IEC8802.3, the idle signal has a cycle of 20 bits represented as
         [00111110101001000101].       

       FIG. 2  shows this idle signal in the form of a modulation waveform of optical signals. 
     In Internet applications, such as website browsing, download of files and e-mail receiving, a downward communication request from a provider to a user discretely occurs in temporal direction. Consequently, in data communication on Internet, much of communication time is occupied with idle signals. Note here that in the data communication on Internet a direction from a provider&#39;s side to a user&#39;s side will be referred to as “downward direction” and additionally, the reverse direction from a user&#39;s side to a provider&#39;s side will be referred to as “upward direction”. 
     Meanwhile, in picture communication on CATV, it is necessary to transmit picture signals of about 100 channels in their multiplexed condition. The current transmission method in popular use is represented by WDM method where picture signals in multichannel are divided with respect to each specified frequency. This is a method where picture signals at individual channels are modulated in analog and thereupon, the modulated picture signals of each channel are arranged with respect to each constant frequency range in sequence. Of course, frequency components of the picture signals at a certain channel concentrate on a frequency range allocated to the same channel. 
     DISCLOSURE OF THE INVENTION 
     In the above-mentioned optical fiber communications by WDM method, in order to improve convenience in processing (amplifying, branching, etc.) multiplexed optical signals in block, it is frequently carried out to allow respective wavelengths of optical signals, which are transmitted in a same communication direction, to come close to each other. When transmitting the optical signals having adjacent wavelengths through a same optical fiber, a phenomenon that part of energy of optical signals having a short wavelength makes the transition to optical signals having a long wavelength, occurs due to the influence of the induced Raman scattering, which is one of nonlinear optical effects of the optical fiber. The induced Raman scattering is a scattering caused by an interaction between incident signal light and an optical phonon produced by the incident signal light. Thus, if the induced Raman scattering occurs, then energy of optical signals having a short wavelength amplifies optical signals having a long wavelength adjacent to the short wavelength. 
     The picture communication commonly adopts optical signals having a wavelength of 1.55 μm, while the data communication adopts optical signals having a wavelength of 1.49 μm. Therefore, when the Raman scattering occurs, the energy of the optical signals for data communication amplifies the optical signals for picture communication. 
     Such an amplification causes noise for optical signals having a long wavelength, in this case, optical signals for picture communication. Particularly, if the proportion of idle signals to optical signals for data communication gets longer temporarily, the frequency component of a modulation waveform of the relevant optical signals comes closer to concentrating on a specific frequency. Because the idle signal is constructed in the form of a cyclic signal pattern having the cycle of short bit-string, as described above. Whereat, the influence of the Raman scattering on picture signals of a channel including the relevant specified frequency becomes remarkable, so that the quality of picture signals deteriorates. 
     We now explain this problem more closely. 
     Data communication on Internet is commonly carried out between an apparatus on a provider&#39;s side and a device on a user&#39;s side interactively. On the other hand, picture communication on CATV is carried out from a provider&#39;s side to a user&#39;s side in one direction. Accordingly, two signals in the downward direction and one signal in the upward direction, that is, three signals in total are transmitted in an identical optical fiber. Then, the above problem occurs in between the two signals in the downward direction saliently, that is, a data signal for Internet and others and a picture signal for CATV and others. 
     In data communication on Internet, since direct digital modulation is applied to optical signals as mentioned above, the same signals come under the influence of the Raman scattering only at the lighting of the optical signals for data communication. In short, intensity-modulation waveforms of the optical signals for data communication are superimposed on picture signals at a certain rate. Although the degree of influence of the Raman scattering depends on a variety of parameters besides a difference in wavelength, for example, fiber&#39;s length, temperature, polarization of light and so on, the occurrence of the Raman scattering is unavoidable. 
     In data communication on Internet, the contents of communication data differ from each other on a case-by-case basis in general. Therefore, since the bit-strings of packet signals forming the optical signals for communication data differ from each other with respect to each communication data, the frequency components of modulation waveforms of the packet signals disperse to some degree in the process of data communication. Meanwhile, since each of the idle signals as the other component forming the relevant optical signals is constructed with a cyclic signal pattern having the cycle of a short bit string, the frequency components of modulation waveforms of the relevant idle signals concentrate on a specific frequency. 
     Supposing the waveform of the idle signals as a square waves as shown in  FIG. 2 , we analyze their frequency components. The analysis result is shown in  FIG. 3 . Here the data rate of 1000BASE.X defined in ISO/IEC8802.3 mentioned above is 1.25 Gbps and additionally, 20 bit-string of 1.25 Gbps is repeated at a frequency of 62.5 MHz. Thus, as obvious from  FIG. 3 , the frequency spectrum of the idle signals shown in  FIG. 2  is intensified at frequencies of integer multiples of the fundamental frequency: 6.25 MHz. In other words, the frequency components of the modulation waveform of the idle signals concentrate on frequencies of integer multiples of the fundamental frequency. 
     Thus, if the data communication on Internet multiplexed by WDM method in an optical fiber is 1000BASE.X defined in ISO/IEC8802.3, the influence of the induced Raman scattering on optical signals for picture communication on CATV, also concentrates on frequencies of  FIG. 3  whose spectral intensities are intensive. 
     Since respective channels of picture signals on CATV concentrate on a specified range of frequencies, the influence of the induced Raman scattering by 1000BASE.X optical signals becomes remarkable in each channel of the picture signals on CATV, thereby causing a possibility that the picture signal of the channel deteriorates. 
     Considering the above-mentioned problems in prior art, an object of the present invention is to provide an optical communications device and an optical communication system, both of which can reduce a baneful influence of the induced Raman scattering of optical signals for data communication on optical signals for picture communication in an optical fiber communication by WDM method, whereby the quality of picture optical communication can be improved in a low price. 
     In order to accomplish the above object, according to a first aspect of the present invention, there is provided an optical communications device for generating optical signals for data communication, comprising: a packet-signal generator that generates packet signals in the optical signals for data communication; an idle-signal generator that generates an idle signal to be inserted between the packet signals generated by the packet-signal generator; and a coder that disperses a frequency component of a modulation waveform of a signal pattern of the idle signal generated by the idle-signal generator. 
     Additionally, according to a second aspect of the present invention, there is provided an optical communications device for generating optical signals for data communication, comprising: a packet-signal generator that generates packet signals in the optical signals for data communication; a dummy packet-signal generator that generates a dummy packet signal to be inserted between the packet signals generated by the packet-signal generator; an idle-signal generator that generates an idle signal to be inserted between the packet signal generated by the packet-signal generator and the dummy packet signal generated by the dummy packet-signal generator; and a coder that disperses a frequency component of a modulation waveform of a signal pattern of the dummy packet-signal generated by the dummy packet-signal generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pattern diagram showing the contents of optical signals in a data communication in prior art. 
         FIG. 2  is a waveform chart showing modulation waveforms of idle signals in prior art. 
         FIG. 3  is a graph showing a frequency spectrum of the idle signals shown in  FIG. 2 . 
         FIG. 4  is a block diagram showing a constitution of an optical communications system having a data communication transmitter in accordance with a first embodiment. 
         FIG. 5  is a view showing one example of the signal pattern of the idle signals used in the first embodiment. 
         FIG. 6  is a graph showing a frequency spectrum of the idle signals shown in  FIG. 5 . 
         FIG. 7  is a pattern diagram showing the contents of optical signals outputted from the data communication transmitter in accordance with the first embodiment. 
         FIG. 8  is a block diagram showing a constitution of an optical communications system having a data communication transmitter in accordance with a second embodiment. 
         FIG. 9  is a pattern diagram showing the contents of optical signals outputted from the data communication transmitter in accordance with the second embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of an optical communications device and an optical communications system of the present invention will be described with reference to drawings. Note that the optical communications device of the present invention is applied to a data communication transmitter in the following embodiments. 
     1 st . Embodiment 
     The first embodiment is characterized by an alteration of the signal patterns of idle signals in optical signals for data communication. By this alteration, a frequency component of a modulation waveform of the relevant optical signals is dispersed to avoid a concentration on a specific frequency. 
       FIG. 4  is a block diagram showing a constitution of an optical communications system having an optical communications device in accordance with the first embodiment of the present invention. 
     This optical communications system comprises a data communication transmitter transmitting optical signals (e.g. wave length of 1.49 μm) for data communication such as internet, a picture signal transmitter (picture signal generator)  20  transmitting optical signals (e.g. wave length of 1.55 μm) for picture communication such as CATV, and a WDM unit  30  which produces optical signals where respective optical signals transmitted from the data communication transmitter  10  and the picture signal transmitter  20  are superimposed on each other by WDM method and which transmits the optical signals to an optical line composed of an optical fiber. 
     The data communication transmitter  10  includes an 8-bit counter (idle signal generator)  11 , a packet signal transmitting circuit (packet-signal generator)  12 , a selector  13  and a switching control circuit  17 . The 8-bit counter  11  is formed by a circuit that outputs sixteen 8-bit codes stretching from “0x00” to “0x0F” in hexadecimal form cyclically, while the packet signal transmitting circuit  12  is formed by a circuit that outputs transmission data in the form of a packet signal. The selector  13  selects either the 8-bit counter  11  or the packet signal transmitting circuit  12 . The switching control circuit  17  outputs a switching control signal S 1 . 
     Additionally, to an output side of the selector  13 , there are sequentially connected an 8B10B coder  14 , a parallel/serial converter  15  and an electro-optic converter  16 , so that optical signals outputted from the electro-optic converter  16  are inputted to the WDM unit  30 . Here, the 8B10B coder  14  is formed by a circuit that realizes 8B10B code rule being an optical-signal generation code rule defined by ISO/IEC8802.3, putting data of 8 bits into code for transmitting in the form of 10 bits. The parallel/serial converter  15  is formed by a circuit that converts parallel data outputted from the 8B10B coder  14  into serial data. The electro-optic converter  16  is formed by light emission diodes etc. and converts the serial data outputted from the parallel/serial converter  15  into optical signals. 
     According to the data communication transmitter  10  constructed above, the switching control circuit  17  receives a signal from the packet signal transmitting circuit  12 , further judges the presence or absence of a packet signal transmitted from the packet signal transmitting circuit  12  and outputs a switching control signal S 1  to allow the selector  13  to select an output from the packet signal transmitting circuit  12  while the packet signal is transmitted from the packet signal transmitting circuit  12 . On the other hand, the switching control circuit  17  outputs a switching control signal S 1  to allow the selector  13  to select an output from the 8B10B coder  14  while the packet signal is not transmitted from the packet signal transmitting circuit  12 . 
     Consequently, during the absence of a packet signal, sixteen 8-bit codes, that is, “0x00”, “0x01”, “0x02”, . . . and “0x0F” in hexadecimal form are sequentially outputted from the 8-bit counter  11  to an 8-bit input node of the 8B10B coder  14  through the selector  13 . Thus, as a signal pattern to be inserted between the packet signals, a signal pattern having a cycle of 160 bit-string shown in  FIG. 5  is outputted from the 8B10B coder  14 . 
       FIG. 6  shows a frequency component of the idle signals constructed in the cyclic signal pattern having this cycle of 160 bit-string. As obvious from the same figure, the frequency component is remarkably dispersed without concentrating on a specific frequency in comparison with the idle signals constructed in the conventional cyclic signal pattern having a cycle of 20 bit-string shown in  FIG. 3 . 
     Accordingly, as shown in  FIG. 7 , the optical signals for data communication transmitted from the data communication transmitter  10  have a format where idle signals A 1 , A 2 , . . . repeating the cyclic pattern having the above cycle of 160 bit-string are embedded throughout intervals having no place for the packet signals P 1 , P 2 , . . . . 
     Consequently, when the optical signals for data communication from the data communication transmitter  10  are supplied into the optical fiber through the WDM unit  30 , a baneful influence of the induced Raman scattering by the relevant optical signals for data communication on optical signals for picture communication transmitted from the picture signal transmitter  20  is reduced to allow the quality of the optical signals for picture communication to be maintained in good condition. Additionally, since the present invention can be embodied by such a very simple method, the effect of the invention can be accomplished at a low price. 
     Note that this embodiment is mainly directed to disperse a frequency component of a modulated waveform of the optical signals in the data communication and therefore, the signal pattern of idle signals to be altered for this dispersion is not limited to the pattern shown in  FIG. 5  only. 
     2 nd . Embodiment 
     The second embodiment is characterized by a formation to shorten a time occupied by normal idle signals in the optical signals for data communication under situation that it is difficult to alter the signal pattern of the normal idle signals in by reason of ensuring the connectivity with a conventional device. By this formation, a frequency component of a modulation waveform of the optical signals for data communication is dispersed to avoid a concentration on a specific frequency. 
       FIG. 8  is a block diagram showing a constitution of an optical communication system having an optical communication device in accordance with the second embodiment of the present invention. Elements in common with those of  FIG. 4  are indicated with the same reference numerals respectively and their explanations are eliminated. 
     The optical communication system of this embodiment differs from the first embodiment in the constitution of a data communication transmitter  50 . That is, the data communication transmitter  50  is equivalent to the constitution of the data communication transmitter  10  plus a dummy packet-signal generating circuit (dummy packet-signal generator)  51  for generating e.g. dummy packet signals consisting of the cyclic signal pattern having the cycle of 160 bit-string shown in  FIG. 5  and further a selector  52  for selecting either outputs of the packet signal transmitting circuit  12  or outputs of the dummy packet-signal generating circuit  51 . Additionally, a switching control circuit  54  outputs switching control signals S 11 , S 12  to the selector  52  and the selector  13 , respectively. Further, in this embodiment, the device includes an idle signal generator  53  which generates normal idle signals (see  FIG. 2 ) and which corresponds to the 8-bit counter  11  of the first embodiment. The selector  13  is constructed so as to select either outputs of the idle signal generator  53  or outputs of the selector  52 . Again, the device of this embodiment includes the idle signal generator  53  corresponding to the 8-bit counter  11  of the first embodiment. 
     According to the data communication transmitter  50  constructed above, the switching control circuit  54  receives a signal from the packet signal transmitting circuit  12 , further judges the presence or absence of a packet signal transmitted from the packet signal transmitting circuit  12  and outputs a switching control signal S 11  to allow the selector  13  to select an output from the packet signal transmitting circuit  12  while the packet signal is transmitted from the packet signal transmitting circuit  12 . On the other hand, the switching control circuit  54  outputs a switching control signal S 11  to allow the selector  52  to select an output from the dummy packet-signal generating circuit  51  in a constant time zone during that period when the packet signal is not transmitted from the packet signal transmitting circuit  12 . At this time, as described in the first embodiment, the outputs from the dummy packet-signal generating circuit  51  are converted to dummy packet signals DP 1 , DP 2 , . . . consisting of the cyclic signal pattern having the cycle of 160 bit-string by the 8B10B coder  14 . 
     Additionally, the switching control circuit  54  outputs a switching control signal S 12  to allow the selector  13  to select an output of the idle-signal generator  53  only during that period when the packet signal and the dummy packet-signal are not outputted from the selector  52 . 
     Consequently, as shown in  FIG. 9 , the optical signals for data communication transmitted from the data communication transmitter  50  have a format where dummy packet signals DP 1 , DP 2 , . . . consisting of the cyclic signal pattern having the cycle of 160 bit-string are inserted into periods that the packet signals P 1 , P 2  are absent and furthermore, normal idle signals A 1 - 1 , A 1 - 2 , A 2 - 1 , A 2 - 2  are embedded both in front and in the rear of these dummy packet signals DP 1 , DP 2 , . . . , respectively. 
     In the optical signals for data communication, thus, by inserting the dummy packet signals DP 1 , DP 2 , . . . consisting of the cyclic signal pattern having the cycle of 160 bit-string into the periods that the packet signals P 1 , P 2  are absent, it is possible to disperse the frequency component of the modulation waveform in the periods to insert the dummy packet signals. Further, since a time occupied by the normal idle signal is shortened, it is also possible to avoid a concentration of the frequency component of the idle signal on a specific frequency. 
     Consequently, as similar to the first embodiment, a baneful influence of the induced Raman scattering, which is caused by the optical signals for data communication transmitted from the data communication transmitter  50 , on the optical signals for picture communication transmitted from the picture signal transmitter  20  is reduced to allow the quality of the relevant optical signals for picture communication to be maintained favorably. 
     Note that the content of a dummy packet signal is not limited to the cyclic signal pattern having the cycle of 160 bit-string shown in  FIG. 5  so long as it is possible to disperse a frequency component of a modulation waveform of the signal pattern. In this embodiment, since the alteration of the content of the dummy packet signal inserted between the original packet signals allows the frequency component of the modulation waveform of the optical signals in data communication to disperse, the baneful influence of stimulated Raman dispersion could be reduced even if it is hard to alter the signal pattern of the idle signal, allowing the optical communication to be improved in its quality. 
     Further, by techniques, such as (1) transmitting the dummy packet signal in the form of a packet signal which is shorter than a prescribed packet length and (2) making a nonexistent destination display (address etc.) of the packet signal, a receiving end could dispose the dummy packet signal as invalid data. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, in the optical fiber communication of WDM method, it is possible to reduce a baneful influence of the induced Raman scattering by optical signals for data communication on optical signals for picture communication, improving the quality of picture optical communication in a low price. 
     Additionally, with the embodiment using the dummy packet signal, it is possible to ensure the connectivity with an existing device.