JP2001036468A - Wavelength multiplex transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the deterioration of a waveform caused by both quantities and to increase permitted fiber input power by setting optimum total dispersion quantity with respect to SPM to zero dispersion being optimum total dispersion quantify with respect to XPM. SOLUTION: Light-type repeaters 7 and 8 arranged for compensating the loss of a transmission fiber are arranged between a transmitter 1 and a receiver 13, which are connected to each other through a transmission optical fiber. The chirp coefficient of a transmission signal is set to be zero or positive and the dispersion value of the transmission fiber is set so that a local dispersion value becomes negative with respective to signal light wavelengths in a range from the input terminals of respective transmission periods to valid mutual operation length. Dispersion equalizers 16-1 to 16-n equalize total dispersion quantity in the respective wavelengths so that it becomes almost zero. Thus, a power limit by the non-linear optical effect of the transmission fiber can largely be relieved in an area where the chirp coefficient of the transmission signal is zero or positive and the local dispersion value of the transmission fiber is negative dispersion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a long-distance wavelength-division multiplexing optical transmission system, and more particularly, to a high-quality transmission system with a low error rate by increasing transmission fiber input power. The present invention relates to a wavelength division multiplexing transmission system that can be economically constructed by extending a repeater interval.

[0002]

2. Description of the Related Art In recent years, in the long-distance optical transmission system, a large-capacity transmission system using a wavelength division multiplex system has been realized with the advent of a broadband optical amplifier capable of simultaneously amplifying multiple wavelengths. In order to further increase the capacity, reduce the cost, and simplify the maintenance and operation of the wavelength division multiplexing transmission system,

(1) The distance between linear optical repeaters used between transmission lines is increased. (2) Increase the transmission speed per wavelength. (3) The interval between each signal wavelength is narrowed. Etc. need to be changed.

In response to the above changes (1) and (2),
In order to keep the S / N ratio at the receiving end constant, it is necessary to increase the transmission fiber input power for each wavelength. Input power is limited due to dispersion values and characteristic degradation due to nonlinear optical effects in the fiber.

(A) Four-wave mixing near zero-dispersion wavelength (F
Crosstalk due to self phase modulation effect (SPM) and fiber chromatic dispersion (c) Crosstalk due to cross phase modulation effect (XPM) and fiber chromatic dispersion Conventionally, for these degradation factors, The following countermeasures were taken respectively.

(A) Prevent the zero dispersion wavelength of a transmission fiber from overlapping the wavelength band of signal light (Reference: M. Jinno
et al., IEEE Photon.Technol. Lett., vol. 10, p. 454, 199
8) Alternatively, the wavelength intervals are set at irregular intervals so that even if FWM occurs, it does not interfere with other channels.
ol. 6, p. 754, 1994).

(B) at the transmitting end, in the linear optical repeater, or
The total dispersion is adjusted by the dispersion equalizer arranged at the receiving end so that the pulse spread is minimized (Reference: Y. Miyamoto et al., Electron. Lett., Vol. 30, p.7).
97, 1994).

(C) Use of a fiber having a large local dispersion value or widening the wavelength interval to increase the walk-off between wavelengths to reduce the cross-phase modulation effect. Minimize the intensity modulation component by performing dispersion equalization to bring the total dispersion to zero (Reference: RASaun
ders et al., Electron. Lett., vol. 32, p. 2206, 1996).

In a transmission system having a relatively low transmission rate per wavelength, these measures can suppress the above-mentioned deterioration factor.

[0010]

However, a large-capacity wavelength division multiplexing transmission system having a high transmission rate per wavelength has the following problems. Due to chirping due to the combined effect of SPM and chromatic dispersion, the total amount of dispersion that minimizes pulse spread is generally different from zero dispersion, and depends on the chirping of the transmission signal and the local dispersion value of the transmission fiber.

On the other hand, phase modulation by XPM is converted into intensity modulation by the PM-AM conversion effect as the total dispersion shifts from zero, and this causes crosstalk. That is, since the optimal total variance for SPM and the optimal total variance for XPM are different, it is not possible to equalize the waveform distortions due to both at the same time.

The present invention relates to a large-capacity wavelength-division multiplexing transmission system comprising such an ultra-high-speed transceiver,
An object of the present invention is to make the optimum total dispersion amount for M zero dispersion, which is the optimum total dispersion amount for XPM, thereby suppressing waveform deterioration due to both, and increasing the allowable fiber input power.

[0013]

According to the present invention, the above object is achieved by the means described in the claims. That is, a wavelength division multiplexing transmission system according to the present invention has a plurality of different transmission systems. A wavelength comprising an optical transmitter for wavelength, a wavelength division multiplexing circuit, a transmission fiber, an optical linear repeater for compensating for the loss of the transmission fiber, a wavelength separation circuit, and an optical receiver corresponding to each wavelength. In a division multiplex optical transmission system,

The chirp coefficient α of the optical transmitter is set to 0 or positive.
And the effective interaction length z from the input end _effStations in the range up to
The dispersion value is normal dispersion (negative dispersion) for all wavelengths.
So that the total dispersion at each wavelength is almost zero
It is characterized in that means for dispersion equalization are provided.

Here, α is given by “ _Equation 1” using the phase φ and the amplitude E of the electric field, and z _eff is given by “ _Equation 2” using the section length L of the transmission fiber and the loss coefficient a. .

[0016]

(Equation 1)

[0017]

(Equation 2)

For a transmission fiber already laid, a wavelength variable means for setting each signal light wavelength to a shorter wavelength side with respect to a local zero dispersion wavelength in a range from an input end of the transmission fiber to an effective interaction length. Is preferably provided in the transmitting device.

Further, since the dispersion value of the transmission fiber has a dispersion slope, it is necessary to prepare a dispersion equalizer for each wavelength in order to make the total dispersion amount zero for all signal light wavelengths. However, by using the dispersion slope equalizer, the total dispersion amount can be made zero for all signal light wavelengths by using one dispersion equalizer and dispersion slope equalizer.

According to a second aspect of the present invention, in the wavelength division multiplexing optical transmission system according to the first aspect, an optical dispersion equalizer is provided for each optical receiver corresponding to each wavelength. .

According to a third aspect of the present invention, in the wavelength division multiplexing optical transmission system according to the first aspect, a dispersion equalizer and a dispersion slope equalizer common to each wavelength are provided at a transmission end or in an optical linear repeater. Alternatively, it is configured to be provided at the receiving end.

According to a fourth aspect of the present invention, in the wavelength-division multiplexing transmission system according to any one of the first to third aspects, an optical transmitter is connected between at least an input end of a transmission fiber and an effective interaction length. The wavelength tunable means which can be set on the short wavelength side with respect to the local zero dispersion wavelength in the range is provided.

According to a fifth aspect of the present invention, in the wavelength division multiplex transmission system according to any one of the first to fourth aspects, the optical transmitter has a chirp coefficient of 0 or more and 1 or less. It is.

According to a sixth aspect of the present invention, in the wavelength division multiplexing transmission system according to any one of the first to fifth aspects, a local dispersion value in a range from at least an input end of the transmission fiber to an effective interaction length. Is -3 for all wavelengths
The configuration is such that it is not less than ps / nm / km and not more than 0 ps / nm / km.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below.
A description will be given based on examples. FIG. 1 is a diagram showing a first embodiment of the present invention. In the figure, numeral 1 is a transmitting device, 2-1 to 2-n are transmitters (Tx), 3 is a multiplexer, 4, 14 are optical amplifiers, 5, 9, and 11 are dispersion-shifted fibers ( Negative dispersion), 6, 10 and 12 are dispersion-shifted fibers (negative dispersion, positive dispersion and zero dispersion), 7 and 8 are optical linear repeaters, 13 is a receiving device, 15 is a duplexer, 16-1 to 16
-N is a dispersion equalizer, and 17-1 to 17-n are receivers (R
x).

The wavelength division multiplexing transmission system shown in FIG. 1 is an optical linear optical system installed between a transmission device 1 and a reception device 13 connected via a transmission optical fiber to compensate for a loss in the transmission fiber. This is a configuration in which repeaters 7 and 8 are arranged.

A feature of the present invention is that the chirp coefficient of a transmission signal is 0 or positive, and the local dispersion value is negative for all signal light wavelengths in the range from the input end of each transmission section to the effective interaction length. Set the dispersion value of the transmission fiber so that
In addition, the equalization is performed by a dispersion equalizer so that the total amount of dispersion at each wavelength becomes substantially zero.

The transmitting apparatus 1 includes several optical transmitting circuits (Tx1 to Txn) having different wavelengths, a multiplexer 3 for multiplexing signals of different wavelengths, and a post-optical amplifier for amplifying the multiplexed signal. Consists of four.

The receiving device 13 includes a pre-amplifier 14 for amplifying an optical signal after transmission, a demultiplexer 15 for separating a wavelength-multiplexed signal, a dispersion for compensating a total dispersion amount of a transmission line at each wavelength, and the like. 16-1 to 16-n and an optical receiving circuit (R) for receiving an optical signal of each wavelength and outputting a reproduced electric signal.
x1 to Rxn) 17-1 to 17-n.

As the optical amplifier and the optical linear repeater, for example, an erbium-doped fiber amplifier is used. For the multiplexer and the demultiplexer, for example, an arrayed waveguide grating filter is used. For example, a chirped fiber grating is used for the dispersion equalizer. The chirp of the transmission signal can be controlled by bias control of the drive signal even when a Mach-Zehnder modulator or an electro-absorption modulator is used as the modulator.

FIG. 2 shows a simulation result of the dependence of the eye opening degree on the fiber input power with respect to the local dispersion value and the total dispersion amount of the transmission fiber when transmission is performed at one wavelength. Here, each curve indicates a contour line at which the eye opening degree is 1 dB at each input power, and the inner part surrounded by the curve is a transmittable area at each input power. In this simulation, a linear repeater interval of 100
km, a transmission path of a total of 400 km is assumed, and the dispersion value in each section is assumed to be uniform in the longitudinal direction.

As can be seen from this calculation result, when the chirp parameter α of the transmitter shown in (c) is +0.7, the local dispersion value is negative even in the high power region where the fiber input power is +7 or +10 dBm. In a region where the dispersion is present, there is a transmittable region where the total dispersion amount is near zero dispersion.

On the other hand, when α shown in (a) is −0.7, it can be understood that the transmittable area exists in the area where the total variance is the positive variance at any local variance value. Also, when α = 0 shown in (b), the local variance value is in the negative region and the transmittable region exists near zero dispersion, but the optimal total variance is
It can be seen that it is shifted to the positive dispersion side as compared with the case of α = + 0.7.

This result can be explained as follows. That is, in the linear transmission region where the input power to the fiber is low, when α> 0, pulse compression occurs on the negative dispersion side, so that the eye aperture is good. On the other hand, as the input power increases, the amount of dispersion in which this pulse compression occurs shifts to the positive dispersion side due to the influence of nonlinear chirp due to SPM.

On the other hand, when α <0, pulse compression occurs on the positive dispersion side even in the linear transmission region, so that even if the power is increased, the vicinity of zero dispersion does not become an optimum value. Although this result is a calculation result at one wavelength, the effect of SPM is the same as that of one wavelength even when extended to a wavelength division multiplexing system, and the effects of FWM and XPM are superimposed on this.

It is a well-known fact that the influence of the FWM can be avoided by avoiding the use of the zero-dispersion wavelength region for the signal light wavelength or by making the signal wavelength intervals unequal as described above. However, for XPM, XP
In order to minimize the conversion of the phase modulation component generated in M into intensity modulation due to the PM-AM conversion effect, it is necessary to equalize the total dispersion value to zero dispersion.

Thus, under the condition that α is negative, the optimal value of the total variance when only SPM is considered is different from the zero variance that is the optimal value taking into account the effect of XPM.
The input power is limited as compared with the case of 0. On the other hand, in the region where α is positive or 0 and the local dispersion value of the transmission fiber is negative dispersion, it can be seen that the power limitation due to the nonlinear optical effect of the transmission fiber can be greatly eased.

FIG. 3 is a diagram showing a second embodiment of the present invention. In the figure, numeral 21 indicates a transmitting device, 22-1.
22 to n are transmitters (Tx), 23 is a multiplexer, 24 and 34 are optical amplifiers, 25, 29, and 31 are dispersion-shifted fibers (negative dispersion), and 26, 30, and 32 are dispersion-shifted fibers ( (Negative dispersion, positive dispersion, zero dispersion), 27 and 28 are optical linear repeaters, 33 is a receiver, 35 is a duplexer, and 36-
1 to 36-n represent dispersion equalizers, 37-1 to 37-n represent receivers (Rx), and 38-1 to 38-n represent wavelength variable means.

When upgrading a single-wavelength transmission system to a wavelength-division multiplexing transmission system by using an existing transmission line, the wavelength band of the signal light wavelength is fixed because the zero dispersion wavelength of the transmission fiber has already been determined. It is difficult for such a transmitting device to obtain a desired dispersion value.

In this case, a wavelength tunable means is provided in the transmitting device, and the signal light wavelength is set to a shorter wavelength side than the zero dispersion wavelength of the transmission line, so that the same as in the first embodiment described above. The effect of can be obtained. That is, assuming that the zero dispersion wavelength in each transmission section is λm, the signal light wavelength λ _sj is λ
_sj <λ _0i (for j = 1 to n, i = 1 to N).

When a dispersion-shifted fiber is laid as a transmission fiber, for example, the zero-dispersion wavelength of the fiber is about 1.55 μm, so that the signal light wavelength may be shorter than this. As the wavelength variable means, for example, a DBR-LD or an LD with an external resonator can be used for each transmission light source.

As the optical linear repeater, as the signal light, 1.
If it is about 53 μm, an erbium-doped fiber optical amplifier or a semiconductor optical amplifier can be used. On the shorter wavelength side, a thulium-doped fiber optical amplifier or a semiconductor optical amplifier can be used. By setting the signal light wavelength to the shorter wavelength side of the zero dispersion wavelength of the transmission line, the local dispersion value becomes negative dispersion, and it is needless to say that the same effect as in the first embodiment can be obtained. .

FIG. 4 is a diagram showing a third embodiment of the present invention. In the figure, numeral 41 indicates a transmitting device, 42-1
42 to n are transmitters (Tx), 43 is a multiplexer, 44 and 54 are optical amplifiers, 45, 49, and 51 are dispersion-shifted fibers (negative dispersion), and 46, 50, and 52 are dispersion-shifted fibers ( (Negative dispersion, positive dispersion, zero dispersion), 47 and 48 are optical linear repeaters, 53 is a receiving device, 55 is a duplexer, 56 is a dispersion equalizer, and 57-1 to 57-n are receivers (Rx). , 5
Reference numeral 8 denotes a slope equalizer.

Since the dispersion value of the transmission fiber is not constant with respect to the wavelength but has a dispersion slope, the total dispersion after transmission takes a different value with respect to the wavelength. In an ultra-high-speed transmission system of about 40 Gbit / s, the total dispersion allowed for a received waveform is very small, about several tens of ps / nm. Therefore, a dispersion equalizer is provided for each wavelength to make the total dispersion zero. There is a need.

On the other hand, by using a dispersion slope equalizer having a slope opposite to that of the transmission fiber at the transmitting end, in the linear repeater, or at the receiving end, a set of dispersion equalizer and dispersion slope etc. It is possible to make the total dispersion amount zero for all signal light wavelengths.

As the dispersion slope equalizer, a chirped fiber grating nonlinearly chirped, a PLC dispersion equalizer composed of a multi-stage asymmetric Mach-Zehnder interferometer, or the like can be used. By using this dispersion slope equalizer, the number of dispersion equalizers can be reduced, and effects such as a reduction in the configuration of the receiving apparatus and the apparatus size can be obtained.

FIG. 5 shows a simulation result of the dependence of the received waveform on the eye opening benality with respect to the chirp coefficient of the transmitter. In this simulation, a normal dispersion fiber having a local dispersion value of 3 ps / nm / km is used as a transmission fiber, a repeater interval is 100 km, and a total transmission distance is 400 km.
This is the result when one wavelength transmission is performed under the condition of m, and the total dispersion value is 0 ps / nm.

As shown in FIG. 5, the chirp coefficient of the transmitter is 0.
It can be seen that good reception characteristics can be obtained by taking a value of 1 or less. Furthermore, when the local dispersion value is negative and the absolute value is −3 ps / nm / km or less, it is clear that better characteristics can be obtained because the waveform distortion in the transmission fiber is reduced. is there.

As can be seen from the above description, when the local dispersion value of the transmission line is negative and the total dispersion value is equalized to 0 ps / nm by setting the chirp coefficient of the transmitter to 0 or more and 1 or less. In addition, better reception characteristics can be obtained.

FIG. 6 shows a simulation result of the dependence of the received waveform on the eye opening penalty with respect to the local dispersion value of the transmission fiber. This simulation is a result of one wavelength transmission performed under the conditions of a transmitter chirp coefficient of +0.7, a repeater interval of 100 km, and a total transmission distance of 400 km, and the total dispersion value is 0 ps / nm. FIG. 6 shows that better reception characteristics can be obtained by setting the local dispersion value to 0 to -3 ps / nm7 km.

[0051]

As described above, according to the present invention, waveform distortion due to nonlinear optical effects (SPM and XPM) in a fiber can be suppressed. Therefore, since the input power to the fiber can be increased, there is an advantage that a high quality transmission system with a low error rate can be constructed. On the other hand, there is also an advantage that an economical transmission system can be constructed by extending the repeater interval for the same reason.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a calculation result of a contour line degrading an eye opening degree by 1 dB;

FIG. 3 is a block diagram showing a second embodiment of the present invention.

FIG. 4 is a block diagram showing a third embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of a calculation result of the dependence of an eye opening penalty on local dispersion of a transmission fiber.

FIG. 6 is a diagram illustrating an example of a calculation result of dependence of an eye opening penalty on a local dispersion value of a transmission fiber.

[Explanation of symbols]

1,21,41 Transmission devices 2-1 to 2-n, 22-1 to 22-n, 42-1 to 42
-N transmitter 3,23,43 multiplexer 4,14,24,34,44,54 optical amplifier 5,9,11,25,29,31,45,49,51
Dispersion shift fiber 6,10,12,26,29,32,46,50,52
Dispersion shift fiber 13, 33, 53 Receiver 7, 8, 27, 28, 47, 48 Optical linear repeater 15, 35, 55 Demultiplexer 16-1 to 16-n, 36-1 to 36-n, 56
Distributed equalizer 17-1 to 17-n, 17-1 to 17-n, 57-1
57-n receiver

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shoichiro Kuwahara 2-3-1 Otemachi, Chiyoda-ku, Tokyo F-term within Nippon Telegraph and Telephone Corporation (reference) 5K002 AA01 AA03 AA06 BA05 CA01 CA13 DA02 FA01 5K046 AA08 CC30 DD01 DD02 EE01

Claims (6)

[Claims] 1. A plurality of optical transmitters having wavelengths different from each other, a wavelength multiplexing circuit, a transmission fiber, an optical linear repeater for compensating for a loss of the transmission fiber, and a wavelength separation circuit, each of which corresponds to each wavelength. In a wavelength division multiplexing optical transmission system including an optical receiver, the chirp coefficient of the optical transmitter is zero or positive, and the local dispersion value of at least the range from the input end of the transmission fiber to the effective interaction length is all It is characterized by providing means for dispersion equalization so that the dispersion is normal to the wavelength and the total dispersion, which is the sum of the dispersion of the transmission fiber and the dispersion equalizer, is zero for all wavelengths. Wavelength multiplexing transmission system. 2. The wavelength division multiplexing optical transmission system according to claim 1, wherein an optical dispersion equalizer is provided for each optical receiver corresponding to each wavelength. 3. The wavelength multiplexing system according to claim 1, wherein a dispersion equalizer and a dispersion slope equalizer common to each wavelength are provided at a transmitting end, an optical linear repeater, or a receiving end. Transmission system. 4. The optical transmitter according to claim 1, further comprising a wavelength tunable unit that can be set to a short wavelength side with respect to a local zero dispersion wavelength in a range from at least an input end of the transmission fiber to an effective interaction length. The wavelength division multiplexing transmission system according to claim 1. 5. The wavelength division multiplexing transmission system according to claim 1, wherein a chirp coefficient of the optical transmitter is 0 or more and 1 or less. 6. The transmission fiber according to claim 1, wherein a local dispersion value in a range from at least an input end to an effective interaction length is -3 ps / nm / km or more and 0 ps / nm / km for all wavelengths. The wavelength division multiplexing transmission system according to claim 1.