JPH11103286A - Wavelength multiplexed light transmitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the characteristic of a transmission line by arranging all channels in a normal distribution area without increasing the size of a dispersion compensating device. SOLUTION: A transmission line is composed of a dispersion compensating fiber (a 2nd dispersion compensating device) arranged near a transmitting/ receiving terminal and a transmission line optical fiber 1 occupying a large part of the transmission line. Since the transmission characteristic of a normal dispersion area in the transmission line is higher than that of an abnormal dispersion area, the fiber 1 is set up so that all channels are arranged in the normal dispersion area. The dispersion compensating device arranged on the transmitting/receiving terminal is composed of plural 1st dispersion compensating devices 1 to N and a 2nd dispersion compensating device consisting of an optical fiber having positive dispersion for compensating large negative dispersion stored due to the transmission of all channels in the normal dispersion area. Since the 2nd dispersion compensating device can be used also as a transmission line, the size of the device is not increased as compared with a conventional system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a long distance (thousands of km)
The present invention relates to a wavelength division multiplexing optical transmission system, and more particularly, to a wavelength division multiplexing optical transmission device including a configuration of a transceiver and an optimal dispersion arrangement of the entire transmission line.

[0002]

2. Description of the Related Art With the advent of high-performance optical fiber amplifiers, electrical 3R operation (re_timing, re_shaping, re_ge
nerating) without transmitting optical signals.
As a result, optical transmission technology has been greatly improved in terms of system reliability, cost, and the like.

Further, as a result of research on wavelength division multiplexing communication having a plurality of different wavelengths in one transmission line, transmission capacity exceeding 1 Tb / s has been realized with one fiber.

[0004] Such an improvement in performance with respect to transmission distance and transmission capacity is due to the ability of the optical amplifier to amplify signal light of a single channel or a plurality of channels at once. Research for realizing long-distance and large-capacity optical communication by using such an optical fiber amplifier and the wavelength multiplexing technique has been actively conducted.

As a conventional long-distance WDM optical transmission system, the number of channels is 32, and the transmission rate per channel is 5
Gb / s (total capacity 160 Gb / s), transmission distance 10.0
Experiments on optical amplification repeater transmission of less than 00 km have been reported ([1] NealS. Bergano et al, OFC'97, PD16, [2] N.Sh
imojoh et al Electronics Letters, vol. 33, No. 10, pp, 8
77-879, 1997. ). In the transmission experiment described in Document [1], the amount of dispersion accumulated during transmission in an optical transmission device using a transmission line optical fiber using a normal dispersion region and an abnormal dispersion region is compensated for collectively at the receiving end. The transmission experiment described in the document [2] is based on a compensation method for compensating the dispersion amount accumulated in an optical fiber transmission device using a similar transmission line optical fiber at both the transmission end and the reception end. Regarding those that adopted the law.

[0006] By the way, important problems in the long-distance WDM communication as described above include improvement of characteristics (noise figure, band expansion, flattening, etc.) of the relay optical amplifier, dispersion arrangement of transmission lines, dispersion compensation method, and transmission. There is suppression of various nonlinear optical effects occurring in the road.

[0007] Among the nonlinear optical effects, a problem peculiar to wavelength multiplex communication is waveform deterioration due to the SPM-GVD effect, which is an interaction between the self-phase modulation effect and the group velocity dispersion. The SPM-GVD effect is a waveform deterioration due to repetition of “spread of signal light spectrum due to self-phase modulation + waveform change due to chromatic dispersion”, and it is difficult to recover the waveform due to an irreversible process. This effect is more pronounced in channels away from the zero dispersion wavelength of the transmission line.

[0008] In order to suppress the SPM-GVD effect,
It is desirable that the signal light intensity in the transmission path be low. However, a decrease in the signal light intensity leads to a deterioration in the signal-to-noise ratio, and the lower limit thereof is determined depending on the noise characteristics (that is, noise figure) of the optical amplifier and cannot be extremely reduced.

SPM-GVD other than lowering signal light intensity
As a method of improving the effect tolerance, a method of superimposing a bit-synchronous sinusoidal phase modulation on the intensity-modulated signal light is known (for example, [3] NealS. Bergano et al, OFC'96, T
uN1, 1996). Such superposition of phase modulation is also employed in the transmission experiments described in the above-mentioned documents [1] and [2]. However, in the above experiment, the phase modulation is superimposed on the signal light in the form of polarization modulation instead of phase modulation.

The following two reasons are considered to improve the transmission characteristics due to the superposition of the bit synchronous phase modulation. First, adding an appropriate amount of dispersion to the bit-phase modulated signal light has the effect of improving the waveform. The other is that the signal light spectrum is excessively expanded by the phase modulation, so that the waveform changes drastically due to dispersion in the transmission path, and as a result, the peak power decreases and the SPM decreases.

Regarding the improvement of the transmission characteristics by the superposition of the bit-synchronous phase modulation, the above document [3] describes the phase relationship between the data component and the phase modulation, and the transmission end accumulates the amount of dispersion accumulated in the transmission path. Japanese Patent Application Laid-Open No. 9-46318 ([4]) describes an optimal configuration for a storage dispersion compensation method that employs a compensation method for compensating at both the receiving end and the receiving end.

The noise characteristics of the used optical repeater amplifier are inferior,
Despite the signal light intensity in the transmission path being set high,
Compared with the transmission experiment described in Reference [1], the transmission experiment described in Reference [2] shows superior transmission characteristics. This is largely due to the improved dispersion compensation method relating to the storage dispersion compensation described in the publication [4].

The improved dispersion compensation method proposed in the above publication [4] is to compensate for the residual dispersion that each channel receives and accumulates in the transmission path by dividing the residual dispersion by 50% at the transmitting end and the receiving end. It is to do. As described above, in the transmission experiment described in Reference [1], a method is adopted in which dispersion compensation is collectively compensated at the receiving end.

In document [2], signal light of 32 channels is arranged at intervals of 0.5 nm between 1545.0 nm and 1560.5 nm. Further, although the zero dispersion of the transmission line is not specified, it is estimated from around 1552.5 nm from the spectrum spread due to the nonlinear effect of each channel. Since this is near the center of the signal light band to be used, the channels are distributed by the same number on the normal dispersion side and the abnormal dispersion side as proposed in the above publication [4].

[0015]

Since the transmission characteristics are different between the normal dispersion region and the extraordinary dispersion region of the transmission line, the phase modulation (chirp) superimposed on the signal light causes the extraordinary dispersion in the positive direction in the normal dispersion region. In the negative direction in the area,
The necessity of turning the both sides in the opposite direction is described in the above publication [4].

In the vicinity of zero dispersion, although the transmission characteristic itself is excellent, the spectrum spread due to the self-phase modulation effect becomes remarkable. Therefore, the channel interval is widened to avoid coherent crosstalk caused by the overlap of the spectrum with the adjacent channel. There is a need. Further, it is necessary to suppress the signal light intensity to some extent in order to suppress the spectrum spread, and there is a limit in improving the signal-to-noise ratio after transmission.

Further, the SPM by the bit synchronization phase modulation
-The improvement of the GVD effect resistance is not as effective in the abnormal dispersion region as in the normal dispersion region.

As described above, in order to use both the normal dispersion region and the abnormal dispersion region with the transmission line zero dispersion in between, it is necessary to change the operating condition of the transmitter for each channel.
There is a problem that it is necessary to pay attention to the spectrum spread near zero dispersion.

Based on the above results, it is considered preferable that all channels are transmitted in the normal dispersion region while avoiding the abnormal dispersion region and the vicinity of zero dispersion of the transmission line.

However, in this case, the shortest wavelength channel is
Since the wavelength is farther away from the transmission line zero dispersion wavelength, it is necessary to perform extra dispersion compensation, and if the configuration proposed in the official gazette [4] is used, the dispersion compensator disposed at the transmission / reception end becomes large. The title arises.

(Object of the Invention) It is an object of the present invention to provide a multi-wavelength multiple transmission device capable of improving transmission path characteristics such as suppression of various nonlinear optical effects.

Another object of the present invention is to provide a multi-wavelength optical multi-transmission system capable of operating transmitters of all channels under the same conditions without having to adjust operating conditions of the transmitter for each channel. It is to provide a device.

Still another object of the present invention is to provide a dispersion compensator disposed at a transmitting end and a receiving end without increasing the size thereof and for transmitting all channels in a normal dispersion area of a transmission path. It is an object of the present invention to provide a wavelength-division multiplexing optical transmission device capable of performing the above.

[0024]

SUMMARY OF THE INVENTION A wavelength division multiplexing optical transmission apparatus according to the present invention is a wavelength division multiplexing transmission system for transmitting a plurality of signal lights having different wavelengths in one transmission line optical fiber.
Set the dispersion of the transmission line so that all channels are located in the normal dispersion region of the transmission line optical fiber, and set the transmission line so that the channels near the center just compensate for the amount of dispersion accumulated in the transmission line. A positive dispersion optical fiber is arranged at both ends of the optical fiber. Further, in the wavelength division multiplexing optical transmission device having the above configuration,
The dispersion compensating positive dispersion optical fiber is arranged so as to be substantially equally distributed at the transmitting end and the receiving end.

Further, in the wavelength division multiplexing optical transmission device,
In order to compensate for the remaining amount of dispersion obtained by subtracting the amount of dispersion compensated by the dispersion-compensating positive dispersion optical fiber from the amount of dispersion accumulated in the transmission path of each channel, optical multiplexing for wavelength multiplexing is performed from the output of the transmitter. An optical fiber for dispersion compensation is arranged for each channel between the devices and between the optical demultiplexer for wavelength separation and the input of the receiver of each channel. Further, in the wavelength division multiplexing optical transmission apparatus having the above-mentioned configuration, the optical fiber for dispersion compensation for each channel is approximately 5 at each of the transmitting end and the receiving end.
Distribute by 0%.

(Operation) By setting all channels in the normal dispersion region, various nonlinear optical effects on transmission signals can be suppressed, and adjustment of transmitter operating conditions and the like for each channel can be facilitated and channel spacing can be shortened. By distributing the increased amount of dispersion compensation by the setting to the transmitting end and the receiving end, it is possible to avoid an increase in the size of the dispersion compensator.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the wavelength division multiplexing optical transmission device of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a basic configuration of a wavelength division multiplexing optical transmission device at a transmitting end and a receiving end according to an embodiment of the present invention, and a dispersion map of a transmission line.

In the figure, the transmitting ends are transmitters _{1 to N} of the used wavelengths λ _{1 to} λ _N , respectively,
N, an optical multiplexer for wavelength multiplexing and a second dispersion compensator in front. The receiving end includes a post-dispersion compensator for the received optical signal, optical demultiplexers for demultiplexing 1 to N, dispersion compensators 1 to N, and receivers 1 to N.

The transmission line 1 is configured so that the average dispersion is slightly negative (downward to the right) at the signal light wavelengths of all the channels. For example, an optical fiber 2 having a negative dispersion as an optical fiber to be used, specifically, a dispersion value of -2 ps / nm / km
NZDSF (Non Zero Dispersion Shift Fi
ber) and an optical fiber 3 having a positive dispersion of about +17 ps / nm / km, for example, a 1.3 μm zero-dispersion optical fiber, so that the dispersion becomes slightly negative at the signal light wavelengths of all the channels. Can be configured.

In this case, assuming that the dispersion value of the 1.3 μm zero-dispersion optical fiber at the working wavelength is +17 ps / nm / km, the transmission line connects about 1.3 km of 1.3 μm zero-dispersion optical fiber to 500 km of NZDSF. By repeating this, the transmission path can be configured such that the average dispersion of the transmission path becomes negative (downward to the right) in all the channels. That is, in such a transmission line, each wavelength-multiplexed signal light receives a different average dispersion due to higher-order dispersion, but since all channels are transmitted in the normal dispersion region, the average dispersion of the transmission line in all channels is The ratio between NZDSF and 1.3 μm zero-dispersion optical fiber is adjusted so that becomes negative. More specifically, the average zero dispersion of the transmission line is set to be on the longer wavelength side by about 1 to 2 nm than the longest wavelength channel.

By the way, since such a transmission line is set closer to the normal dispersion with respect to the signal wavelength, it is necessary to perform positive dispersion compensation at both ends of the transmission line. The publication [4]
Compared with the described method, the amount of dispersion received on the transmission path is large, and thus adopting such a method would increase the size of the dispersion compensator.

In the present embodiment, in order to overcome the problem that the size of the dispersion compensator is increased, the dispersion compensators disposed at the transmitting end and the receiving end are each composed of two parts.
Further, the dispersion compensating devices arranged at the transmitting end and the receiving end,
It is realized by the same configuration except for the direction of the optical amplifier.

Therefore, a dispersion compensating device arranged at the transmitting end will be described as the dispersion compensating device in the present embodiment. The dispersion compensator of the present embodiment includes a first dispersion compensator for each channel and a second dispersion compensator for wavelength division multiplexing.

The first dispersion compensator is for compensating for the difference in the amount of accumulated dispersion between the channels caused by the presence of higher-order dispersion in the transmission path. A dispersion compensating optical fiber having a negative value is used. Specifically, a dispersion compensating optical fiber having negative dispersion at the signal light wavelength is used for the channel on the long wavelength side with respect to the central channel, and an optical fiber having positive dispersion is used for the channel on the short wavelength side. use.

The second dispersion compensator is for using all channels in the normal dispersion region of the transmission line, and is composed of a dispersion compensating optical fiber having a positive value. The amount of dispersion is set to 50% of the amount of dispersion accumulated in the transmission path in the central channel (100% in total at the sending and receiving ends).

As described above, at the transmitting end, each channel is multiplexed by the optical multiplexer after passing through the first dispersion compensator determined by the signal light wavelength, the distance of the transmission path, and the magnitude of the higher-order dispersion. And then through the optical fiber of a second dispersion compensator with positive dispersion.

On the other hand, at the receiving end, the received signal light is constituted by a dispersion compensating optical fiber having the same configuration as the transmitting end and having a positive value, and the central channel is set to 50% of the amount of dispersion accumulated in the transmission line. After passing through the second post-dispersion compensator, it is demultiplexed by the optical demultiplexer, and then each channel passes through the first post-dispersion compensator that compensates for the difference in the amount of accumulated dispersion between channels.

According to the present embodiment, the second dispersion compensator can be used as a transmission path. Therefore, only the first dispersion compensating device is disposed at the transmitting end and the receiving end, and the dispersion compensating device does not increase in size and complexity as compared with the configuration described in the official gazette [4]. .

Further, for example, +2 ps /
When NZDSF of about nm / km is adopted, the maximum residual dispersion amount of the shortest wavelength channel can be reduced, and as a result, transmission characteristics can be improved.

Next, the configuration of the transmitting end in the present embodiment, in particular, the specific configuration of the dispersion compensator will be described.

FIG. 2A is a diagram showing a first configuration example of the dispersion compensator and the like at the transmitting end.

The configuration of the transmitter of each channel includes a light source (LD) 10 for outputting a desired signal light wavelength, and an intensity modulator 11 connected to the light source for superimposing a data signal on the signal light.
A phase modulator 12 connected to the intensity modulator for superposing bit-synchronous phase modulation on the signal light; and a first dispersion compensator 13 connected to the phase modulator. The first dispersion compensator 13 is composed of dispersion compensating optical fibers 1 to N having different amounts of dispersion for each channel, and an optical multiplexer that bundles them. If an optical amplifier needs to be inserted in the middle because the dispersion compensating optical fiber is long, the optical amplifier may be inserted into the dispersion compensating optical fiber.

The second dispersion compensator 14 is composed of an optical fiber having positive dispersion, and is connected to the output of the optical multiplexer in the first dispersion compensator 13.

The amount of dispersion added in the first and second dispersion compensators 13 and 14 is obtained as follows.

First, the amount of dispersion compensation of the second dispersion compensator 14 is determined in consideration of the amount of dispersion accumulated in the transmission path. The wavelength of the center channel is λC [nm], and the zero dispersion wavelength of the transmission line is λ0.
[nm], the magnitude of the average dispersion slope in the transmission path is a [ps / nm]
² km] and the transmission distance is L [km].

In this case, the magnitude of the accumulated dispersion received by the central channel is a ^* (λC-λ0) ^* L [ps / nm].
Therefore, the amount of dispersion to be added by the second dispersion compensator 14 is-
a ^* (λc−λ0) ^* L / 2 [ps / nm].

Subsequently, the amount of dispersion added to each channel in the first dispersion compensator 13 is obtained. Accumulated dispersion amount channel of wavelength lambda is subjected in the transmission path ^{a * * (λ-λ0)} * * L [p
s / nm]. Among them, the sum of −a ^* (λc−) is obtained in each of the second dispersion compensators arranged at the transmitting end and the receiving end.
λ0) ^* L [ps / nm] is compensated. Therefore, the remaining -a ^* (λ
−λc) ^{* It} is necessary to compensate by L [ps / nm], and considering two times of the transmitting end and the receiving end, −a ^* (λ−λ0) ^* L / 2 [ps / n
m].

FIG. 2B is a diagram showing a second configuration example of the dispersion compensator and the like at the transmitting end. This configuration example is shown in FIG.
In this embodiment, the phase modulator 12 having the configuration shown in FIG. In long-distance optical transmission systems, it is known that effects depending on the polarization state of signal light, such as polarization-dependent loss included in each component and polarization hole burning in an optical amplifier, deteriorate transmission characteristics. Therefore, the polarization state of the signal light is scrambled in advance, and the signal light is transmitted in a depolarized state on purpose. The bit-synchronous polarization scrambling, in which the signal light is depolarized in one bit of the signal, has the same effect as the bit-synchronous phase modulation by the phase modulator because the chirp is simultaneously superimposed on the signal light.

FIG. 3 is a diagram showing another configuration example of the first dispersion compensator at the transmitting end and the like. This embodiment employs a configuration in which a pre-dispersion compensator connected in series is used in common for the transmitter of each channel. In this case, the amount of dispersion of each pre-dispersion compensator is determined by the difference in the amount of accumulated dispersion between adjacent channels. With this configuration, the dispersion compensating optical fiber is shared, so that the dispersion compensating device can be downsized.

[0051]

According to the present invention, it is possible to avoid an increase in the size and cost of a dispersion compensator in long-distance wavelength division multiplexing optical communication, and to set all channels to a normal dispersion region having excellent transmission characteristics. It is possible to arrange.

Further, since each channel does not use the zero dispersion region or the anomalous dispersion region of the transmission line, it is possible to sufficiently increase the light intensity in the transmission line as compared with a transmission system using such a region. it can.

In general, in wavelength division multiplexing optical communication, it is preferable that the channel spacing be as narrow as possible because it greatly affects the amplification band, flatness, and compensation amount of the relay amplifier. According to the present invention, it is not necessary to consider the spectrum spread near zero dispersion, so that it is possible to shorten the channel interval in addition to improving the signal light intensity as compared with the conventional method.

Further, according to the present invention, since all the channels are set in the normal dispersion area, it is not necessary to adjust the operating conditions of the transmitters for each channel. Therefore, the transmitters of all the channels operate under the same conditions. And simplification of the system can be realized.

As described above, the present invention is suitable for constructing a transmission system suitable for long-distance WDM optical transmission.

[0056]

[Brief description of the drawings]

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

FIG. 2 is a configuration example of a dispersion compensation device according to an embodiment of the present invention.

FIG. 3 is another configuration example of the dispersion compensating apparatus according to the embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 transmission line 2 negative dispersion NZDSF 3 1.3 μm zero dispersion optical fiber 10 light source (LD) 11 intensity modulator 12 phase modulator 13 first dispersion compensator 14 second dispersion compensator 15 polarization modulator

Claims (5)

[Claims] 1. A wavelength division multiplexing optical transmission device for transmitting a plurality of signal lights having different wavelengths in one transmission line optical fiber, wherein all channels are transmitted in a normal dispersion region of the transmission line optical fiber. Wavelength dispersion, wherein a dispersion compensating positive dispersion optical fiber is disposed at both ends of the transmission line so that the dispersion of the transmission line is set, and a channel near the center is compensated for the amount of dispersion accumulated in the transmission line. Multiplexed optical transmission device. 2. The dispersion-compensating positive dispersion optical fiber,
2. The wavelength division multiplexing optical transmission device according to claim 1, wherein the transmission end and the reception end are distributed in substantially equal amounts. 3. A wavelength division multiplexing optical transmission apparatus for transmitting a plurality of signal lights having different wavelengths in one transmission line optical fiber, wherein all channels are transmitted in a normal dispersion region of the transmission line optical fiber. Set the dispersion of the transmission line, and arrange the dispersion-compensating positive dispersion optical fibers at both ends of the transmission line so that the amount of dispersion accumulated in the transmission line near the center is compensated. In order to compensate for the remaining amount of dispersion obtained by subtracting the amount of dispersion compensated by the dispersion-compensating positive dispersion optical fiber from the amount of dispersion accumulated in the path, between the output of the transmitter and the optical multiplexer for wavelength multiplexing and A wavelength-division multiplexing optical transmission device comprising: an optical fiber for dispersion compensation arranged for each channel between an input of a wavelength demultiplexer and an input of a receiver for each channel. 4. The wavelength division multiplexing optical transmission device according to claim 3, wherein the dispersion compensating optical fiber for each channel is distributed to the transmitting end and the receiving end by approximately 50%. 5. The transmission line according to claim 1, wherein the transmission line comprises a transmission line optical fiber in which optical fibers having a negative dispersion value and optical fibers having a positive dispersion value are alternately connected.
5. The wavelength division multiplexing optical transmission device according to 2, 3, or 4.