JPH0821908B2 - Optical communication transmission system - Google Patents

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.

[0002]

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 was difficult with conventional microwave or millimeter wave communication, was reported in the Optical Communication Conference Technical Digest (1990) (Tetsuyuki Saki et al., “10 Gb / s optical tr”).
ansmitter module with mul
tiquantum well DFB LD and
doped-channel hetero-MIS
FET driver IC, "1990 Opti
cal Fiber Communication C
onference technical digest TUI2
1990).

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 having zero chromatic dispersion in the wavelength band of the light source used for the optical transmitter has been reported. Further, when the optical fiber dispersion does not become completely zero, it is necessary to insert an optical fiber for dispersion compensation into the transmission line in order to compensate for this dispersion amount.
2-65530 and JP-A-62-65529.

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. Further, in an actual transmission line, the zero-dispersion condition cannot be completely satisfied over the entire length of the optical fiber. Therefore, a dispersion compensation technique on the transmitting side and the receiving side for suppressing the influence of this minute dispersion has been proposed. For example, Optical Fiber Communication Conference (1
990) PD7 and PD8. (A.H.Gnoke and others, "Optical Equal"
ization of Fiber Chromati
c Dispersion in a 5-Gb / s
Transmission system, 1990 Optical Fiber Communicat
Ion Conference Post Deadline Paper PD7 and Naoya Henmi et al., “A Nov.
el Dispersion Compensation
Technique for Multigigab
it Transmission with Norm
al OpticalFiber at 1.5 mi
cron Wavelength, "1990 Op
tial fiber communication
(Conference Post Deadline Paper PD8)

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 necessary to increase the transmission optical power, and in order to suppress the influence of the latter nonlinear effect, it is required to reduce the transmission optical power. At the same time it was difficult to suppress. 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.

[0005]

An optical multistage direct amplification repeater transmission system of the present invention is an optical multistage direct amplification repeater system using an optical transmitter, an optical fiber for transmission, an optical direct amplification repeater and an optical receiver. , The transmitter-optical amplification repeater, the optical amplification repeater, or the optical amplifier-optical receiver in each of the sections connecting the optical amplifier, the chromatic dispersion is zero at the transmission wavelength of the optical transmitter. The optical communication transmission system is characterized in that it is constituted by a plurality of cascaded optical fiber groups having different values from each other, and that the total chromatic dispersion of the optical fiber group in each section of the transmission optical fiber becomes zero.

[0006]

In the optical direct amplification repeater system of the present invention, the total chromatic dispersion amount is set to be zero for each transmission section, and the dispersion value of the transmission optical fiber is zero at the signal light wavelength. It is an optical transmission system that is not used as a path or has a short distance.

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
One. It is necessary to increase the output of each direct amplification repeater in order to avoid the effect of the accumulation effect of spontaneous emission noise, but it is necessary to reduce the repeater output in order to suppress the nonlinear effect in the optical fiber. To be done. Since the former accumulated noise effect is theoretically unavoidable, it is necessary to suppress the nonlinear effect in the optical fiber to a small level even if the repeater output level is increased. The self-phase modulation effect is considered to be large as the nonlinear effect in the optical fiber. However, through the transmission experiments described below, we have found, in addition to the self-phase modulation effect, the noise increase effect that is considered to be caused by the nonlinear effect in the optical fiber for each section of the multistage direct amplification system. It was also found that this noise increase increases as the transmission distance increases, and the noise is generated over the entire length of the transmission line. It was found that the transmission limit is mainly limited by this nonlinear effect in the optical fiber, because this effect causes a larger spectrum spread than the self-phase modulation effect and causes deterioration of the signal-to-noise ratio.

It has also been empirically clarified experimentally that this non-linear effect in the optical fiber occurs when the transmission light power is large, and is unlikely to occur when the transmission optical fiber is not zero at the signal light wavelength.

Therefore, if an optical fiber having a non-zero signal light wavelength is used as the transmission optical fiber, it is possible to suppress the nonlinear effect in the optical fiber even when the transmission light power is large.

At this time, however, it is necessary to avoid the influence of chromatic dispersion in the optical fiber.

As for the chromatic dispersion of the optical fiber, it is sufficient that the total chromatic dispersion over the entire length is zero when the self-phase modulation effect is not taken into consideration. A method of inserting a fiber for compensating the total dispersion of the transmission line on the receiving side is considered. To be However, the effect of self-phase modulation cannot be ignored in high-speed, ultra-long-distance transmission,
Such a conventionally known optical fiber for compensation only at the receiving end cannot be used for compensation. In order to suppress the influence of this self-phase modulation, it is necessary to make the total chromatic dispersion amount zero for each short transmission distance. Therefore, in the present invention, an optical fiber that makes the total chromatic dispersion amount zero is inserted in each section to avoid the influence of the self-phase modulation effect.

Further, since the noise (spectral spread) due to the nonlinear effect in the optical fiber occurs over the entire length of the transmission line as described above, if the compensation fiber is used only on the receiving side, The noise generated at the beginning is received at the same time as the signal light, but the noise generated at the rear is received at different times and becomes noise, which deteriorates the reception characteristics.
In the present invention, the noise is not generated because the compensating optical fibers are arranged in a distributed manner.

As described above, by using the optical direct amplification repeater system of the present invention, high speed and ultra long distance transmission is possible. Before describing the present invention, a conventional optical communication transmission system will be described. FIG. 5 shows a conventional optical direct amplification repeater system.

First, the operation of the conventional optical direct amplification repeater system will be specifically described with reference to FIG.

The optical transmitter 3 outputs the optical power output from the semiconductor laser light source 1 to the optical power output from the modulation signal source 5.
The intensity is modulated by the external modulator 2 of lithium niobate driven by the Gb / s signal and output to the optical power amplifier 11.
The optical power amplifier 11 is an Erbium-doped optical fiber amplifier, which amplifies the signal light level using this amplifier and outputs it to the first transmission line optical fiber 101.
At this time, when the signal light level is 10 dBm or more, in order to avoid the influence of Brillouin scattering in the transmission fiber, a well-known technique such as direct FM modulation of a semiconductor laser is used, The line width is widened in advance. The optical signal that has passed through the optical fiber 101 is amplified again by the optical direct amplification repeater 12 which is an erbume-doped optical fiber amplifier, and the second transmission line optical fiber 11
It is output to 1. The optical multistage amplification repeater system is configured in such a manner that the signal light of the second transmission line is amplified by the second optical amplification repeater 13 and output to the third transmission line 121.

On the receiving side, the signal light transmitted through the last optical transmission line 191 is amplified by the optical preamplifier 21 and received by the optical receiver 53. The optical receiver 53 uses the PIN photodiode 51, which is a photoelectric conversion element, to convert the received signal light into an electric signal, and then equalizes and amplifies and reproduces the 10 Gb / s signal from the modulation signal source 5 transmitted by the reproduction circuit 52. Playing the signal.

In this system, first, the wavelength of the semiconductor laser light source is set to 1.552 μm, and the optical fibers 101 and 11 are
Nine-stage multistage direct amplification 1000 km relay transmission was performed using 100 km of dispersion-shifted fiber having zero dispersion at 1.552 μm in 1, 121, 131, and 191. Here, the transmission loss of the dispersion-shifted fiber 100 km is 22-23 dB.
And the optical power amplifier 11, the optical amplification repeater 12, 1
The noise figure of 3, 14, 15, 20 and the optical preamplifier 21 was 8-9 dB. The output level of the optical power amplifier 11 and the optical amplification repeaters 12, 13, 14, 15, 20 is 1 dB.
When it is set to about m, the power level of the input signal light to each optical amplification repeater is lowered to -21 to -22 dBm, so that noise cannot be received due to noise increase due to spontaneous emission noise accumulation. Therefore, the transmission was performed by increasing the output level of the direct amplification repeater, but the signal level was + 11- + 12 dBm.
It was not possible to obtain good transmission characteristics even when the temperature was raised to. FIG. 2 shows the received signal spectrum when the transmission signal light power is increased. It can be seen that when the level of the transmitted signal light is increased, the signal light spectrum is returned due to the non-linear effect in the optical fiber and the signal to noise ratio is reduced.

Next, as a preliminary experiment for the present invention, the wavelength of the semiconductor laser light source was 1.547, 1.
The same experiment was performed by changing the thickness to 557 μm. Here, the dispersion shift fibers 101, 111, 121, 131, 1
91 is -0.35 ps / km / nm at each wavelength,
It has a dispersion value of +0.35 ps / km / nm. Observing the spectrum after transmission as shown in FIGS. 3 and 4, it can be seen that the signal-to-noise ratio after transmission is recovered for both positive and negative variance values. However, since the amount of dispersion in the entire transmission line is large, the waveform distortion after transmission was large and could not be received. The deterioration of the transmission characteristics due to the nonlinear effect in the optical fiber has been conventionally considered to be the waveform distortion due to the self-phase modulation, but in this transmission experiment, the noise increasing effect due to the nonlinearity in the optical fiber is observed. The cause of this noise increase effect is unknown at present, but the authors have clarified by experiments that it is large when there is signal light at the zero dispersion wavelength in the optical fiber and small when the dispersion value is not zero. Also,
It was also observed that the noise component increased as the transmission distance increased, and it was also found that this noise is generated over the entire length of the optical fiber that is the transmission line, but this noise increase suppression is remarkable in the negative dispersion region. .

[0019]

FIG. 1 is a block diagram of an optical direct amplification repeater system of an embodiment of the present invention.

The operation of the second embodiment will be described with reference to FIG. The wavelength of the semiconductor laser light source 1 is set to 1.547 μm, and the optical fibers 101, 111, 121, which are transmission lines,
131, ... 191 are 1.55 used in the experiments of FIGS.
A dispersion shift fiber having an atmospheric dispersion of 2 μm was used.
Further, in the relay system of FIG. 5, a normal dispersion fiber 102 for compensating the chromatic dispersion of each dispersion shift fiber,
, 192 are inserted after the dispersion shift fiber for each transmission path section. The dispersion amount of the dispersion shift fiber for each section is -35 ps / nm at 100 km.
Therefore, the normal dispersion fiber is about 2 km (dispersion value 35 p
s / nm) in cascade connection, and the total dispersion amount of each section is 0 ps
/ Nm was set. As a result, in the case of a repeater output of +8 dBm or more, it was possible to obtain a good transmission characteristic with reception sensitivity deterioration of about 1 dB after transmission.

From the above results, it has been shown that it is difficult to realize a high-speed, ultra-long-distance optical direct amplification repeater system with the conventional optical direct amplification repeater system, but it can be realized by using the present invention.

The present invention has many other variations. It is possible to set the transmission wavelength to the positive dispersion wavelength band of the transmission optical fiber and use a negative dispersion fiber as an optical fiber that compensates for this dispersion, or use negative dispersion and positive dispersion fibers with equal absolute dispersion values at equal distances. You can also Moreover, the number of optical fibers used in each section is not limited to two, and may be three or more. If the total dispersion amount for each section is set near zero, the positive and negative optical fiber lengths can be freely set for each section. The number of relay stages is not limited to 9, but 20 stages, 1
It may be 00 steps, or more or less. Each section length is 10
The distance is not limited to 0 km, and may be 50 km, 150 km, or more or less. The bit rate used is 1
Not limited to 0 Gb / s, 2.5 Gb / s is 5 Gb
/ S or 20 Gb / s or more or less.

The modulation method is not limited to intensity modulation, but may be frequency modulation or phase modulation. Not only the direct detection method but also the heterodyne detection method can be used as the reception method. 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.

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 an embodiment of the present invention.

FIG. 2 Optical signal spectrum after multistage direct amplification repeater transmission

FIG. 3 is a diagram of an optical signal spectrum after multistage direct amplification relay transmission.

FIG. 4 is a diagram of an optical signal spectrum after multistage direct amplification relay transmission.

FIG. 5 is a diagram for explaining a conventional technique.

[Explanation of symbols]

1 semiconductor laser light source 2 external modulator 3 optical transmitter 5 modulation signal source 11 optical power amplifier 12, 13, 14, 15, 20 optical direct amplification repeater 21 optical preamplifier 101, 111, 121, 131, 191 dispersion shift Fibers 102, 112, 122, 132, 192 Ordinary dispersion fiber 51 PIN photodiode 52 Equalization amplification reproduction circuit 53 Optical receiver

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H04B 10/16 10/18 H04B 9/00 M

Claims (1)

[Claims] 1. In an optical multistage direct amplification repeater transmission system using an optical transmitter, an optical fiber for transmission, an optical direct amplification repeater and an optical receiver, between the transmitter and the optical amplification repeater, the optical amplification repeater. Or a transmission optical fiber in each section connecting the optical amplifier and the optical receiver is constituted by a plurality of optical fiber group cascade connections having chromatic dispersion different from zero at the transmission wavelength of the optical transmitter. And an optical fiber group chromatic dispersion sum in each section of the transmission optical fiber is zero.