JPH0685758A - Optical communication transmission system - Google Patents

Abstract

PURPOSE:To provide the optical communication transmission system at a low cost in which the production of a nonlinear effect in an optical fiber is avoided and the transmission range of over 10,000km is attained in the coherent communication. CONSTITUTION:A CPFSK optical signal subjected to phase modulation by a 5Gb/s signal being an output of an optical transmitter 3 is amplified by nearly +6dBm at an optical fiber amplifier 11 and the amplified signal is inputted to an optical fiber 101 being a 1st transmission line. The total length of the optical fiber 101 is nearly 100km and a fiber having a negative dispersion with respect to a signal optical wavelength. The signal light is amplified by nearly +6dBm at a 2nd optical amplifier repeater 12 and is inputted to an optical fiber 111 being a 2nd transmission line having a negative dispersion characteristic. Thus, the optical amplifier repeater and the optical multi-stage amplifier relay system whose length is 10,000km comprising a negative dispersion optical fiber are built up. The total transmission line dispersion is compensated by a delay equalizer 301 at an intermediate frequency band to equalize the dispersion in the transmission line at the receiver side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a high speed and ultra long distance transmission system for an optical fiber transmission system.

2. Description of the Related Art Since an optical communication system can transmit a large amount of information by utilizing the wide band property of light, it is highly expected as a communication system for a transmission network in the future advanced information society. In fact, ultra-high capacity communication of 10 Gb / s, which has been difficult in conventional microwave or millimeter wave communication, has been reported. For example, 1992 Optical Co
Mmmunication Conference Article No. TuI6 "Ten-Gbit / s Optical
transmitter module using
module or driver IC and
semiconductor modulato
r ", (T. Susuki et al.).

However, in such a large capacity communication,
It is also known that chromatic dispersion of an optical fiber, which is a transmission line, causes a large waveform distortion after transmission and greatly deteriorates transmission characteristics. For this reason, transmission using an optical fiber in which chromatic dispersion is zero in a wavelength band of a light source used for an optical transmitter has been reported. Further, when the optical fiber dispersion does not become completely zero, it is considered to insert an optical fiber for dispersion compensation into the transmission line in order to compensate for this dispersion amount (Japanese Patent Laid-Open No. 62-65530). . Further, in the coherent communication system, a technique of using a delay equalizer at an intermediate frequency stage of an electric signal after reception is used (“Chromatic Dispersers”).
ion Compensation in Coher
entOptical Communicatio
n, "IEEE, Journalof Lightwa
ve Technology, vol. 8, No. 3,1
990, pp367-375))).

With the advent of erbume fiber amplifiers in recent years, optical direct amplification repeater systems have been actively studied.
In such an optical transmission system, the influence of chromatic dispersion of the optical fiber is very large, and therefore transmission using a zero wavelength dispersion optical fiber is being studied. Especially, regarding the communication method for the submarine cable, transmission in which the dispersion value of the transmission optical fiber is set to zero is considered (“9000 km, 5 Gb / s NRZ
TRANSMISSION EXPERIMENT
Using 274 Erbium-Doped Fi
ber Amplifiers, "NS Berga
no et al. , Technical Diges
to of Topical meeting on O
optical Amplifiers and The
ir Applications 1992 post
deadline paperPD11, and "O
ver 10,000km Straight Lin
eTransmission System Exp
orientation at2.5 Gb / s Using
In-Line Optical Amplifier
s, "T. Imai et. al., Technica.
l Digest of Topical meet
ng on Optical Amplifiers
and Their Applications 19
92 postdeadline paper PD1
2).

[0004]

The cause of degrading the transmission characteristics in the optical direct amplification repeater system is not only the above-mentioned chromatic dispersion in the optical fiber. It is also known that the noise accumulation effect in the multistage direct amplification repeater system and the nonlinear effect in the optical fiber greatly deteriorate the transmission characteristics. In order to suppress the influence of the former noise accumulation effect, it is required to increase the transmission light power, and in order to suppress the influence of the latter nonlinear effect, it is required to reduce the transmission light power. Since these two effects require satisfying contradictory conditions, it is difficult to suppress both of them at the same time. In addition, there are many unclear points especially regarding the nonlinear effect in the optical fiber, and the cause of deterioration has not been specified.

Recent transmission experiments (eg "2.5 Gbi")
t / s, 80-100 km Spaced In-Li
ne Amplifier Transfer mission
Experiences Over 2,500-
4,500 km, "S. Saito et. Al., T.
echnecal Digest of Europe
an Conference on Optical
Communication 1991 postde
According to the result of ad-line paper, it is shown that in the transmission using the zero-dispersion fiber, the influence of the four-wave mixing effect generated between the signal and the spontaneous emission light emitted from the optical amplifier after the transmission is large and the transmission becomes difficult.

[0006]

An optical direct amplification repeater transmission system according to a first aspect of the present invention is an optical multistage direct optical system using an optical transmitter, an optical fiber for transmission, an optical direct amplification repeater and an optical receiver. In the amplification repeater system, the dispersion value of the transmission optical fiber in each section connecting the transmitter-optical amplification repeater, the optical amplification repeater, or the optical amplifier-optical receiver is equal to that of the optical transmitter. This is an optical communication transmission system characterized in that there is little area where the wavelength of the transmitted signal light is different from zero or becomes zero.

An optical direct amplification repeater transmission system according to a second aspect of the present invention is the optical communication transmission system according to the first aspect, in which information is transmitted to an optical signal by optical frequency, phase or polarization modulation in an optical transmitter. In addition, the optical communication transmission system is characterized in that the coherent optical reception is used on the optical receiving side, and the influence of the total chromatic dispersion in all transmission lines is compensated by using the dispersion equalizer of the electric system.

[0008]

The optical direct amplification repeater system of the present invention is an optical transmission system in which the region where the dispersion value of the transmission optical fiber becomes zero at the signal light wavelength is eliminated or reduced.

The factors that deteriorate the transmission characteristics of the optical multistage direct amplification repeater system are the accumulation effect of spontaneous emission optical noise after the multistage repeater,
Wavelength dispersion of optical fiber, nonlinear effect in optical fiber
Is one.

It is necessary to increase the output of each repeater in order to avoid the effect of the accumulation effect of spontaneous emission noise, but it is required to reduce the output of the repeater in order to suppress the nonlinear effect in the optical fiber. To be done. The former accumulated noise effect is theoretically unavoidable.

Therefore, in order to carry out ultra-long-distance transmission or increase the repeater interval, it is essential to suppress nonlinear effects in the optical fiber and increase the repeater output level for transmission. The self-phase modulation effect is considered to be large as the nonlinear effect in the optical fiber. However, the above S
It has been clarified through a transmission experiment such as aitoh that noise is increased due to the effect of four-wave mixing between the signal light and the spontaneous emission light output from the optical amplifier in addition to the self-phase modulation effect, and as a result, the transmission characteristics are greatly deteriorated. ing. This noise increase increases as the signal power and the transmission distance increase, and causes a large spectrum spread and a deterioration in the signal-to-noise ratio compared to the self-phase modulation effect. Therefore, the transmission limit of this optical transmission system is It is mainly limited by non-linear effects in the fiber.

However, this four-wave mixing effect is unlikely to occur in fibers other than zero dispersion where the matching condition of wave number is not satisfied. However, when a large signal light power exists in the optical fiber, the wave number matching condition may be satisfied even in the positive dispersion region.

Therefore, in order to suppress this noise increase, it is most effective to transmit in the negative dispersion region, and as a next best measure, it is also effective to transmit in a large positive dispersion region in which the matching condition is difficult to be satisfied. I understand.

Therefore, in the first aspect of the present invention, an optical fiber having a non-zero signal light wavelength is used as the transmission optical fiber. This makes it possible to suppress the noise increasing effect even when the transmission light power increases or the transmission distance increases. This makes it possible to suppress S / N ratio deterioration after transmission. The present invention is effective when the influence of waveform distortion due to transmission line dispersion or waveform distortion due to self-phase modulation is small.

In coherent communication, information can be placed on the frequency or phase of light so that the communication can be performed with a constant amplitude, so that there is an advantage that the influence of the self-phase modulation can be avoided. In addition, by setting the signal light wavelength in the negative dispersion region, the line width increase caused by four-wave mixing as in the past is eliminated,
As a result, the floor phenomenon caused by the decrease in S / N ratio after transmission is eliminated. However, fixed deterioration due to waveform deterioration due to dispersion will occur. (The effect of eliminating this floor phenomenon is effective even in the large positive dispersion region as described above.)
In the second aspect of the present invention, coherent communication in which the wavelength of the transmitted light is selected in a dispersion region different from such zero dispersion is performed, and fixed deterioration caused by chromatic dispersion in the optical fiber after transmission is delayed at the intermediate frequency. Compensation is performed using an equalizer. As a result, it becomes possible to perform transmission with fixed deterioration suppressed after transmission.

As described above, by using the optical direct amplification repeater system of the present invention, high speed and ultra long distance transmission is possible.

The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an optical direct amplification repeater system of an embodiment of the present invention. FIG. 1A shows an optical transmitter, an optical fiber for transmission, and an optical fiber amplifier.
(B) is a diagram showing a portion of the optical receiver.

The optical transmitter 3 drives the injection current to the semiconductor laser light source 1 with an electric signal of 5 Gb / s output from the modulation signal source 5, and outputs a CPFSK optical signal modulation wave. The CPFSK optical signal modulated wave is amplified to +6 dBm by the first elbum doped optical fiber amplifier 11 and output to the first transmission line optical fiber 101.
The optical fiber 101 for the transmission line has a negative dispersion of -0.4 ps / km / nm and a loss of 2 when the oscillation wavelength of the semiconductor laser light source 1 is 1.552 μm.
It is a 100 km optical fiber with 1 dB. The signal transmitted through the transmission line optical fiber 101 is again +6 dBm in the second elbum doped optical fiber amplifier 12.
And is output to the second transmission line optical fiber 111. Further, the output light of the optical fiber 111 is amplified by the third optical amplification repeater 13 and output to the third transmission line 121, thus forming an optical multistage direct amplification repeater system with a total extension distance of 100 steps of 10,000 km. Here, the transmission optical fibers 111, 121. ． ． Uses a negative dispersion fiber 100 km similar to the optical fiber 101.

In the receiver 200, the last optical transmission line 191
The signal light having passed through was mixed with the semiconductor laser light source 1 and a local light source 201 having a frequency different by 10 GHz, and heterodyne detection was performed using a PIN photodiode 51 which is a photoelectric conversion element. Further, this heterodyne detection signal was passed through the delay detector 300 to reproduce an electric signal of 5 Gb / s. Here, the delay equalizer 301 in the figure is not used.

As described above, according to the experiment of Saito et al., When the signal light wavelength is matched with zero dispersion and transmitted, a floor phenomenon is observed in which the code error rate is not improved even if the received power is increased from about 2500 km. ing.

However, by setting the transmission fiber in the negative dispersion region as in the first aspect of the present invention, the noise increasing effect due to the nonlinear effect is suppressed and the floor phenomenon is not observed. However, due to the influence of the dispersion of the transmission line, the receiving sensitivity deteriorated by about 7-8 dB as shown by the alternate long and short dash line in FIG. Although some effects of self-amplitude modulation peculiar to coherent communication were observed, no significant waveform deterioration was observed for two reasons: the negative dispersion region and the small dispersion value.

Further, as shown in FIG. 1B, the electrical signal heterodyne-detected is delayed and equalized 30 in the intermediate frequency band.
1 was used to try to compensate for the effect of dispersion on the transmission line. A conventionally known strip line circuit 301 is used for dispersion compensation. The stripline circuit has a compensation amount of 4000 ps / nm, which is set to compensate for the total dispersion amount of the transmission line. By performing delay detection after passing through this delay equalizer, the amount of sensitivity deterioration was suppressed to 3 dB or less as shown by the broken line in FIG.

The present invention has many other variations. The dispersion of the optical fiber for transmission is not limited to the value of -0.4 ps / km / nm, and the negative dispersion region above this value can be used. However, in consideration of the distribution of dispersion values in the longitudinal direction of the optical fiber, it is more effective to set a slightly negative dispersion region in advance so that zero dispersion does not occur at the signal light wavelength. Further, the dispersion value of the transmission fiber is not limited to negative dispersion, and a positive dispersion optical fiber can also be used. Optical direct amplification repeater output light level is + 6d
Not limited to Bm, it may be less than that, for example, +3 dBm or less, or more than that, such as +10 dBm or more. The number of relay stages is not limited to 100 stages, for example, 20 stages,
It may be 200 steps or more or less. The length of each optical direct amplification repeater section is not limited to 100 km but 50 km, 1
It may be 50 km or more or less. The total transmission length is not limited to 10000 km, but is 1000 km, for example.
It may be more or less than this, such as 2000 km and 20000 km.

The bit rate to be used is not limited to 5 Gb / s. For example, 2.5 Gb / s may be 10 Gb / s.
However, it may be 20 Gb / s or higher or lower.

The modulation method is not limited to CPFSK modulation, but generally frequency modulation, phase modulation, or polarization modulation can also be used. As the receiving method, not only heterodyne detection but also direct detection or homodyne detection can be used. The optical amplifier used in the multistage direct amplification repeater system is not limited to the erbume-doped fiber amplifier, but may be a semiconductor laser amplifier, a praseodymium-doped optical fiber amplifier, or an optical Raman amplifier. Further, the wavelength band of the transmission light source is not limited to the 1.5 μm band, and the 1.3 μm band can be used.

[0025]

As described above, by using the present invention,
Since the dispersion value of the optical fiber that is the transmission line in the optical direct amplification repeater system is not matched to zero at the transmission wavelength, the nonlinear effect in the optical fiber is suppressed, so the transmission power of the optical direct amplification repeater system is increased. As a result, it is possible to easily realize an ultrafast, ultralong distance optical direct amplification repeater system.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram for explaining a code error rate characteristic when the transmission system of the present invention is used.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 semiconductor laser light source 3 optical transmitter 5 modulation signal source 11, 12, 13 Elbium-doped optical fiber amplifier 101, 111, 121, 191 transmission optical fiber 51 PIN photodiode 200 optical receiver 201 local light source 300 delayed detection Unit 301 Delay equalizer

Claims (2)

[Claims] 1. In an optical multistage direct amplification repeater system using an optical transmitter, a transmission optical fiber, an optical direct amplification repeater and an optical receiver, between the transmitter and the optical amplification repeater and between the optical amplification repeaters. , Or the dispersion value of the transmission optical fiber in each section connecting between the optical amplifier and the optical receiver has a value different from zero at the wavelength of the signal light transmitted from the optical transmitter, or there is little area where the value becomes zero. An optical communication transmission system characterized in that 2. An optical transmitter carries information on an optical signal by optical frequency, phase, or polarization modulation, uses coherent optical reception on the optical receiving side, and influences the total chromatic dispersion in all transmission lines by an electrical system. The optical communication transmission system according to claim 1, wherein compensation is performed using a dispersion equalizer.