Abstract:
An optical communication transmission system including an optical amplifier lumped repeater system of the present invention includes, for the purpose of preventing degradation of the transmission characteristic arising from wavelength dispersion of optical fibers due to raised power of the optical signal, transmission optical fibers provided for all or most of the repeating sections and having wavelength dispersion values set to different values from zero, and optical fibers provided for the individual sections to compensate for the sum of wavelength dispersion of the sections so as to reduce the total wavelength dispersion to zero. The optical fiber for compensation for each section may be replaced by a substitutive compensation element. Alternatively, very small wavelength dispersion which remains due to failure in compensating to zero dispersion may be compensated for using a dispersion equalizer of an electric system in the reception section.

Description:
This is a Continuation of application Ser. No. 08/079,554 filed Jun. 22, 1993 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a high-speed, long-haul communication transmission circuit by an optical fiber, and more particularly to an optical communication transmission system which is expected to be developed as a communication system for a transmission network for advanced information service and which can transmit a large amount of information with a high degree of quality over a long distance. 
     2. Description of the Related Art 
     An optical communication transmission system makes use of the broad band feasibility of light to permit high-speed, very high-capacity, high-quality communications which cannot be realized readily with conventional communications using the microwave band or the millimeter wave band. For example, the following reports have been provided with regard to elements for use with communication of, for example, 10 Gbit/s: 
     by T. Suzaki et al., &#34;10-Gbit/s Optical Transmitter Model with Multiquantum Well DFB LD and Doped-channel Hetero-MISFET Driver IC,&#34; 1990 Optical Fiber Communication Conference, Technical Digest TUI2, and 
     by T. Suzaki et al., &#34;Ten-Gbit/s Optical Transmitter Module Using Modulator Driver IC and Semiconductor Modulator,&#34; Optical Fiber Communication Conference 1992, Technical digest TUI6. 
     An optical communication transmission system of the optical amplifier lumped repeater system which uses erbium-doped optical fiber amplifiers will be described with reference to FIG. 1. 
     An optical transmitter 3 modulates optical power outputted from a semiconductor laser source 1 by intensity modulation by an external modulator 2 of lithium niobate LiNbO3 which is driven by a signal of 10 Gbit/s outputted from a modulation signal source 5 and outputs the modulated optical power to an optical power amplifier 11. The optical power amplifier 11 consists of an erbium-doped optical fiber amplifier and amplifies a signal light level and outputs the amplified optical signal to a first optical fiber 101 for a transmission line of an optical amplifier lumped repeater system. In this instance, when the signal light level exceeds 10 dBm, in order to avoid the influence of Brillouin scattering in the transmission fiber, the line width of the semiconductor laser is expanded in advance using the well-known technique of direct FM modulation of the semiconductor laser or a like technique. After passing the optical fiber 101, the optical signal is amplified again by a direct optical amplifier repeater 12 which consists of an erbium-doped optical fiber amplifier and is then outputted to a second stage optical fiber 111 for transmission. The signal light inputted into the transmission line at the second stage is amplified by a second stage optical amplifier repeater 13 and outputted to a third transmission line 121. The signal light is thereafter processed in a similar manner and transmitted finally to a last transmission line 191. In an optical receiver 53 on the reception side, the optical signal is amplified by an optical preamplifier 21 and converted into an electric signal using a PIN photodiode 51, which is a photoelectric transducer. The electric signal, and consequently, the signal of 10 Gbit/s transmitted from the modulation signal source 5, is then reproduced by an equalizer amplifier regeneration circuit 52. 
     In the high-speed, high-capacity communication system described above, however, it is known that waveform distortion after transmission due to such causes as chromatic dispersion of the optical fibers strongly degrades the transmission characteristic through a very long distance transmission. 
     Therefore, the following countermeasures are conventionally taken: 
     First, as a countermeasure to chromatic dispersion of an optical fiber, which is conventionally considered to be the most significant cause of degradation of the transmission characteristic, a transmission line is constructed using an optical fiber which has no chromatic dispersion in the waveband of the light source of the optical transmitter. In other words, the optical fiber employed has zero chromatic dispersion. 
     For example, as a communication system for a long-distance submarine cable, transmission systems wherein the dispersion value of an optical fiber for transmission is reduced substantially to zero have been proposed by: 
     N. S. Bergano et al., &#34;9000 km, 5 Gbit/s NRZ Transmission Experiment Using 274 Erbium-doped Fiber-Amplifiers,&#34; Technical Digest of Topical Meeting on Optical Amplifiers and Their Applications, Santa Fe, Jun. 24-26, 1992, postdeadline paper PD11, and 
     T. Imai et al., &#34;Over 10,000 km Straight Line Transmission System Experiment at 2.5 Gbit/s Using In-Line Optical Amplifiers,&#34; Technical Digest of Topical Meeting on Optical Amplifiers and their Applications, Santa Fe, Jun. 24-26, 1992, postdeadline paper, PDI2. 
     In an actual transmission line, however, the requirement for zero chromatic dispersion cannot be fully satisfied over the entire length of the optical fiber, and very small level of chromatic dispersion exists. In order to suppress the influence of the very small dispersion, several techniques for compensating for the chromatic dispersion in the transmitter side and the receiver side have been proposed, for example, in Japanese Patent Laid-open No. 1987-65529 and Japanese Patent Laid-open No. 1987-65530, and by: 
     A. H. Gnauck et al., &#34;Optical Equalization of Fiber Chromatic Dispersion in a 5 Gbit/s Transmission System,&#34; Optical Communication Conference, San Francisco, Jan. 22-26, 1990, postdeadline paper PD7, and 
     N. Henmi et al., &#34;A Novel Dispersion Compensation Technique for Multigiga-bit Transmission with Normal Optical Fiber at 1.5 Micron Wavelength,&#34; Optical Fiber Communication Conference 1990, postdeadline paper PD8. 
     Further, in a coherent communication system, such techniques as equalizing an electric signal in the receiver side by using a delay equalizer at the stage of an intermediate frequency of the electric signal have been reported by: 
     K. Iwashita et al., &#34;Chromatic Dispersion Compensation in Coherent Optical Communications&#34;, IEEE, Journal of Lightwave Technology, Vol. 8, NO. 3, March 1990, pp. 367-375. 
     It is known that the causes for degradation of the transmission characteristic of an optical amplifier lumped repeater system include, in addition to wavelength dispersion of the optical fiber described above, a noise accumulation effect caused by spontaneous emission light and a noise increase effect caused by a non-linear effect in the optical fiber through multistage optical amplifier repeaters. In order to decrease the influence of the accumulation effect of noise of spontaneous emission light, the outputs of the optical amplifier repeaters must be set high. On the other hand, in order to suppress the non-linear effect in the optical fiber, the outputs of the optical amplifier repeaters must necessarily be set low. Due to these two contradictory requirements, it is conventionally difficult to simultaneously control both the noise accumulation effect and the non-linear effect. Therefore, in order to obtain a very long-haul transmission system or achieve an increase of the repeating distance, it is necessary to increase the repeater output while decreasing the non-linear effect in the optical fiber. 
     However, little is known of the non-linear effect in an optical fiber, and the causes of degradation have not been specifically identified as yet. 
     SUMMARY OF THE INVENTION 
     It is believed that a self-phase modulation effect is a major factor in the non-linear effect in an optical fiber. However, as recently reported by S. Saito et al.  &#34;2.5 Gbit/s, 80-100 km Spaced In-line Amplifier Transmission Experiments Over 2,500-4,500 km,&#34; Technical Digest of European Conference on Optical Communication 1991, postdeadline paper 3!, in addition to the self-phase modulation effect, noise is increased by the influence of a 4 wave-mixing effect between signal light and spontaneous emission light outputted from the optical amplifier, resulting in the degradation of the transmission characteristic. 
     Further, in addition to the self phase modulation effect, a noise increase believed to arise from a non-linear effect in an optical fiber for each section of a multistage optical amplifier lumped repeater system was discovered in experiments conducted by the inventors of the present application which will be hereinafter described. 
     It has been made clear that those noise-increasing effects, other than the self-phase modulation effect, increase with the increase of the signal power and the increase of the transmission distance, and noise is produced over the full length of the transmission line, resulting in a greater spectrum spread and a greater degradation of the signal-to-noise ratio than the self-phase modulation effect. Accordingly, it has become clear that the transmission limit is restricted by the non-linear effect in the optical fiber. 
     It has become apparent through experiments that the non-liner effect in the optical fiber occurs when the transmission light power is high but is deterred when the optical fiber for transmission does not have a zero dispersion wavelength at the wavelength of the optical signal. Therefore, if an optical fiber which does not have a zero dispersion wavelength at the wavelength of the optical signal is employed as the optical fiber for transmission, the non-linear effect in the optical fiber can be suppressed even when the transmission light power is high. 
     It is an object of the present invention to provide an optical communication transmission system including an optical amplifier lumped repeater system wherein very high-speed, high-capacity and long-haul optical communications can be realized with a high degree of quality. 
     In order to attain the object described above, an optical communication transmission system of the present invention includes transmission optical fiber means having a zero dispersion wavelength of a value different from the transmission wavelength of the optical transmitter means with at least two connections between the optical transmitter means and the optical receiver means, and dispersion compensation means for making the sum total of wavelength dispersion substantially equal to zero when the sections are arranged in cascade connection. 
     In an embodiment of the present invention, the dispersion compensation means is included in each of the sections of the transmission optical fiber means or in the optical transmitter means or the optical receiver means. Further, an optical signal is modulated by the optical transmitter means and received in a coherent system by the optical receiver means, and the influence of wavelength dispersion upon the optical signal over the entire transmission line is compensated by the electric dispersion equalization means. The type of modulation by the optical transmission may be optical frequency modulation, phase modulation or polarization modulation. 
     Further, the present invention can be applied readily to a conventional system by installing a dispersion compensation optical fiber for a transmission optical fiber, which is conventionally provided on the outside, inside an optical repeater and replacing the optical repeater. Alternatively, it is possible to install a small dispersion compensator such as a grating pair in the apparatus in place of the dispersion compensation optical fiber. 
     In summary, according to the present invention, in order to suppress the non-linear effect in an optical fiber, the zero dispersion wavelength of the transmission optical fibers, which is conventionally made to coincide with the transmission wavelength, is shifted from the transmission wavelength for each section. By virtue of this means, the present invention has the advantage that the transmission optical power of an optical amplifier lumped repeater system can be increased so as to improve the transmission characteristic, and consequently, a very high-speed, very long-haul optical communication transmission system can be realized readily. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate the examples of the preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a conventional optical amplifier lumped repeater system; 
     FIG. 2 is a spectrum diagram of the received signal when, in the repeater system of FIG. 1, the wavelength of the semiconductor laser source is set to the conventional standard of 1.552 μm and the power of the transmitter optical signal is raised; 
     FIG. 3 is a spectrum diagram of the received signal when the wavelength of the semiconductor laser source is set to 1.547 μm and the power of the transmission optical signal is raised; 
     FIG. 4 is a spectrum diagram of the received signal when the wavelength of the semiconductor laser source is set to 1.557 μm and the power of the transmission optical signal is raised; 
     FIG. 5 is a diagrammatic view of an optical amplifier lumped repeater system of a first embodiment of the present invention; 
     FIG. 6 is a diagrammatic view of an optical amplifier lumped repeater system of a third embodiment of the present invention; and 
     FIG. 7 is a diagram illustrating a code error ratio characteristic when the present invention is used. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The results of experiments with the conventional optical communication transmission system shown in FIG. 1 will first be described for comparison with the present invention. 
     In the transmission system of FIG. 1, the wavelength of the semiconductor laser source was set to 1.552 μm and a 1,000-km transmission experiment using a stage optical amplifier repeater was conducted using, for the optical fibers 101, 111, 121, 131, . . . and 191, a dispersion shifted fiber of 100 km whose zero dispersion wavelength was 1.552 μm. Here, the transmission loss of the dispersion shifted fiber per 100 km was 22 to 23 dB, and the noise figures of the optical power amplifier 11, the optical amplifier repeaters 12, 13, 14, 15, . . . and 20 and the optical preamplifier 21 were 8 to 9 dB. When the output levels of the optical power amplifier 11 and the optical amplifier repeaters 12 to 20 were set to approximately 1 dBm, the power levels of the input signal light power into the optical amplifiers dropped to -21 to -22 dBm, and consequently, the reception failed due to noise increase by noise accumulation of the spontaneous emission line. Thus, the optical amplifier repeater output power level was increased, but a good transmission characteristic was not obtained even when the signal level was raised to +11 to +12 dBm. FIG. 2 shows the reception signal spectra. It can be seen that when the transmission signal light level is raised, the signal-to-noise ratio of the signal light level is degraded conversely due to a non-linear effect in the optical fiber. 
     As a preliminary experiment of the present invention, the same experiment was conducted with the same transmission system as that of FIG. 1 changing the 1.552 μm wavelength of the semiconductor laser source to 1.547 μm and 1.557 μm. Here, the dispersion shifted fibers 101, 111, 121, . . . and 191 had the dispersion values of D=-0.35 ps/km/nm and D=+0.35 ps/km/nm, respectively, for the two wavelengths. Observation of the reception spectra after transmission line revealed that, as can be seen in FIGS. 3 and 4, the signal-to-noise ratio after transmission is improved at each of the anomalous (D&gt;0) and normal (D&lt;0) dispersion values. However, since the amount of dispersion of the entire transmission line was great, the waveform distortion after transmission was too great to receive the signal. 
     Conventionally, it is believed that the transmission characteristic degradation by a non-linear effect in an optical fiber arises from waveform distortion by self-phase modulation, but according to the experiments, a noise-increasing effect due to the non-linearity in the optical fiber has been observed. 
     While the cause of the noise-increasing effect is unknown, the inventors have clearly shown, based on the experiments, that the noise increase is great when the signal light has the same wavelength as the zero dispersion wavelength in the optical fiber but is small when the signal light does not have the same wavelength as the zero dispersion wavelength in the optical fiber. Also it has been observed that as the transmission distance increases, the noise component also increases, and it has been found out that the noise is produced over the entire length of the optical fibers constituting the transmission line and suppression of the noise increase is significant in the normal (D&lt;0) dispersion region. 
     The first embodiment of the present invention will next be described with reference to FIG. 5. 
     The wavelength of a semiconductor laser source 1 is set to 1.547 μm, and optical fibers 101, 111, 121, 131, . . . and 191 of a transmission line are constituted from dispersion shifted fibers whose zero dispersion wavelength is 1.552 μm. Conventional fibers 102, 112, 122, 132, . . . and 192 which have anomalous dispersion (D&gt;0) are inserted after the dispersion shifted fibers 101 to 191 of the individual transmission sections for compensating for the wavelength dispersion of the respective fibers 101 to 191. Since the amount of dispersion of the dispersion shifted fiber for each section is -35 ps/nm per 100 km, the conventional (D&gt;0) fibers of about 2 km (dispersion value 35 ps/nm) were arranged in cascade connection to set the total amount dispersion of each section to a value in the proximity of 0 ps/nm. As a result, when the repeater output was higher than +8 dBm, a good transmission characteristic was obtained wherein the reception sensitivity degradation after transmission was approximately 1 dB. 
     Further, as a second embodiment, in place of the conventional (D&gt;0) fiber of the first embodiment, dispersion compensators of -35 ps/nm were constituted from grating pairs, and the dispersion compensators were built into the optical repeaters, following which a transmission experiment similar to the first embodiment was conducted. In this experiment, a good result of approximately 1 dB was obtained for the amount of deterioration of reception sensitivity after transmission. In the present embodiment, optical fibers for dispersion compensation may be mounted in the optical repeaters in place of the dispersion compensators. 
     Next, the third embodiment of FIGS. 6(A) and 6(B) will be described. 
     An optical transmitter 3 drives the current to be supplied to a semiconductor laser source 1 with an electric signal of 5 Gbit/s outputted from a modulation signal source 5 and outputs a CPFSK (Continuous-Phase Frequency-Shift-Keying) optical signal modulation light waveform. The CPFSK modulated optical signal is amplified to +6 dBm by a first erbium-doped optical fiber amplifier 11 and outputted to a first transmission optical fiber 101. The transmission line optical fiber 101 is an optical fiber of 100 km which has a normal dispersion (D&lt;0) amount of -0.4 ps/km/nm and a loss of 21 dB at an oscillation wavelength of 1.552 μm of the semiconductor laser source 1. The signal transmitted through the transmission line optical fiber 101 is again amplified to +6 dBm by a second erbium-doped optical fiber amplifier 12 and outputted to a second transmission line optical fiber 111. The output light of the optical fiber 111 is amplified by a third optical amplifier repeater 13 and outputted to a third transmission line 121. In this manner, an optical amplifier lumped repeater system of 100 stages having a total distance of 10,000 km is constructed. In the optical amplifier lumped repeater system, an optical fiber of 100 km of normal (D&lt;0) dispersion similar to optical fiber 101 is employed for transmission optical fibers 111, 121, . . . 
     An optical receiver 200 mixes the signal light that has passed the last optical transmission line 191 with the output of a local oscillation light source 201 having a frequency that differs from that of the semiconductor laser source 1 by 10 GH z  and detects the mixture signal by heterodyne detection by a PIN photodiode 51, which is a photoelectric transducer. The heterodyne-detected signal is passed through a delay detector 300 to reproduce it as an electric signal of 5 Gbit/s. Here, a delay equalizer 301 shown in FIG. 6(B) is not used. 
     The dispersion of the transmission optical fibers is not limited to -0.4 ps/km/nm, and an optical fiber having a normal (D&lt;0) or anomalous (D&gt;0) dispersion region other than that value may be employed. It is to be noted, however, that taking the distribution of dispersion values in the longitudinal direction of the optical fibers, it is effective to set the dispersion to a value in a somewhat excessively normal (D&lt;0) dispersion region in advance so that the zero dispersion of the optical fiber may not occur at the signal light wavelength. 
     According to the above-mentioned experiments by Saito et al., when transmission was performed with the signal wavelength set to coincide with the zero dispersion wavelength, an error rate floor phenomenon was observed when the transmission distance is over approximately 2,500 km. However, when a transmission optical fiber was set to a normal (D&lt;0) dispersion region as in the present invention, the noise-increasing effect due to a non-linear effect was suppressed and no floor phenomenon was observed. However, reception sensitivity was degraded by approximately 7 to 8 dB due to the influence of the dispersion of the transmission line, as indicated by an alternate long and short dashes in FIG. 7. Further, while some influence of self-amplitude modulation peculiar to coherent communications was observed, no significant waveform degradation was found because the dispersion of the transmission optical fiber was set to a value in a normal (D&lt;0) dispersion region and the dispersion value was low. 
     Further, it was attempted to compensate for the influence of dispersion of a transmission line upon a heterodyne-detected electric signal in an intermediate frequency band using a delay equalizer 301, as shown in FIG. 6(B). A conventional strip line circuit was used for dispersion compensation. The amount of compensation of the strip line circuit was set to 4,000 ps/nm so as to compensate for the total amount of transmission line dispersion. By detecting the electric signal by delay detection after the electric signal passed the delay equalizer, the sensitivity degradation amount was suppressed to below 3 dB, as indicated by a broken line in FIG. 7. 
     The present invention may be modified in numerous ways in addition to those described above. For example, it is possible to set the transmission wavelength to a value in a anomalous (D&gt;0) dispersion wavelength band of a transmission optical fiber and employ a normal (D&lt;0) dispersion optical fiber as the optical fiber for compensating for the anomalous (D&gt;0) dispersion or to use a normal (D&lt;0) dispersion optical fiber and a anomalous (D&gt;0) dispersion optical fiber having equal absolute dispersion values and equal distances. Further, the number of kinds of optical fibers used for each section is also not limited to two but may be three or more. If the total amount of dispersion for each section is set to a value in the proximity of zero, the lengths of anomalous (D&gt;0) and normal (D&lt;0) dispersion optical fibers can be set freely for each section. Also, the number of repeating stages is not limited to 10 stages, but may be more or less than 10 stages, including for example 20 or 100 stages. Further, the length of each section may be greater or smaller than 100 km, including for example 50 km or 150 km, and the bit rate used may also be higher or lower than 10 Gbit/s, including for example 2.5 Gbit/s, 5 Gbit/s or 20 Gbit/s. 
     Further, the modulation system is not limited to intensity modulation but may also be frequency modulation or phase modulation. Also, the reception system is not limited to a direct detection system, and a heterodyne detection system may be employed. In addition, the optical amplifier for use with the optical amplifier lumped repeater system is not limited to an erbium-doped optical fiber amplifier but may be a semiconductor laser amplifier, a praseodymium-doped (Pr+3) optical fiber amplifier or an optical Raman amplifier. Also, the wavelength band of the transmission light source is not limited to the 1.5 μm band, but the 1.3 μm band may be used instead. 
     It is to be understood that variations and modifications of &#34;Optical Communication Transmission System&#34; disclosed herein will be evident to one skilled in the art. It is intended that all such modifications and variations be included within the scope of the appended claims.