JP2003514433A - Channel synchronization by dispersion fiber in wavelength division multiplex transmission system - Google Patents

Abstract

(57) [Summary] The present invention relates to a wavelength division multiplexing transmission system using an optical fiber. The invention proposes to play the channel by synchronous modulation at a point in the system where the channel is not synchronized. To this end, the regenerator has a dispersive fiber section and a synchronous modulator, the length and dispersion of which are selected to synchronize the bit times of the channels at the input of the synchronous modulator.

Description

Detailed Description of the Invention

The present invention relates to a wavelength division multiplexing transmission system using optical fibers, and more particularly to optical modulation in such a transmission system.

The transmission of soliton pulses or solitons is a known phenomenon. These pulses are RZ pulses with a time width narrower than the bit time (FWHM, pulse width of half the maximum power), and have a predetermined relationship between power, spectral width, and time width, and are generally , Propagated to the so-called abnormal dispersion part of the optical fiber. Such soliton envelope changes in single-mode fibers can be modeled by the nonlinear Schrodinger equation. This propagation is based on the balance between anomalous dispersion and non-linearity of the fiber. Various solutions have been proposed to control the jitter of such soliton signals. For example EP-A-05
It is known to use slide guide filter systems, such as 76208. It has also been proposed to implement synchronous modulation of soliton signals. To that end, various types of modulators can be used, in particular amplitude or phase synchronous modulators using the Kerr effect. H. Kubota, M .; “Solitary Transmission Control in T” by Nakazawa.
"IME AND FREQUENCY DOMAINS" (IEEE Jour
nal of Quantum Electronics Vol. 29, No. 3, 21
89), or N.W. J. Smith, N.M. J. Doran's "Evalua
toning the Capacity of Phase Modulator
-Controlled Long-Haul Soliton Transm
"Ission" (Optical Fibers Technology I,
218-235, 1995) introduces various techniques for controlling or reproducing soliton signals. K. "40 Gbit / s single by Suzuki I et al.
e channel optical soliton transmissi
on over 70,000 km using in-line synchr
once modulation and optical filteri
ng "(Electronics Letters, Vol. 34, No. 1, 1998).
, Pp. 98-100) experimentally show that narrow band filtering combined with intensity and phase modulation enables long range single channel transmission. The propagation type for soliton signals proposed in this document cannot be directly applied to wavelength division multiplexing systems due to collisions between solitons of different channels.

N. J. Smith et al., "Enhanced power solito"
ns in optical fibers with period d
"Ispersion management" (Electronics Le
(Tter, Vol. 32, No. 1, pp. 54-55, 1996) describes a configuration for correcting the propagation of soliton signals in order to overcome these limitations. By alternating positive and negative dispersive fiber sections, the average dispersion is reduced (there is little jitter), the local area dispersion is increased and the WDM effect is reduced (channels slide faster relative to each other). Such a distributed management soliton propagation configuration enables a wavelength division multiplexing transmission system.

In a wavelength division multiplexing transmission system using an optical fiber, it has been proposed to regularly use synchronous modulation of signals. Such modulation is suitable for light, especially for high bit rate systems. In that case, the problem of group velocity difference between channels is raised due to the difference between wavelengths.

Various solutions to this problem have been proposed. E. Desurvire
, O. Leclerc and O.L. The document "Synchro" by Audouin
nous in-line regeneration of wavelen
gth-division multiplexed soliton si
gnals in optical fibers "(Optics Lett
ers Vol. 21, No. 14, pp. 1026-1028) describes a wavelength allocation arrangement compatible with the use of synchronous modulators for soliton signals. This document
For a given spacing Z _R between repeaters, the signals of each channel, or more precisely the bit times of each multiplex channel, are approximately synchronized as they arrive at the repeater. It is proposed to assign wavelengths to. Thus,
The use of the discrete synchronous modulator enables in-line synchronous modulation of all channels at predetermined intervals. Such multiplexing wavelength allocation technique is also described in FR-A-274394 in the name of Alcatel Submarine Network. In this patent application, it is proposed to select a subgroup of channels which is not only synchronized to the spacing Z _R, but also to the spacing by a divisor of Z _R.

O. Leclerc, E.I. Desurvir, O.I. "Synchronous WDM soliton regeneration" by Audouin
n: towers 80-160 Gbit / s transoceanic
systems "(Optical Fiber Technology, 3
97-116, 1997) specify that this wavelength allocation configuration causes the spacing Z _R between the synchronous modulators to be too wide, or the spacing between the multiplexed channels to be too wide. In order to solve this problem, the document describes synchronizing the bit times of a subset of multiplexed channels to a divisor interval of Z _R in such a wavelength allocation configuration. As a result, this document proposes to reproduce a subset of multiplexed channels at shorter intervals.
However, such a solution requires filtering of the reconstructed subset of channels, causing the transmission system to lose the single periodicity for all channels.

FR-A-2770001 describes the use of synchronous modulators to modulate the soliton signals of all channels of a wavelength division multiplexing transmission system at a frequency N / T which is a multiple of the signal clock frequency 1 / T. is suggesting. Therefore, the constraint of synchrony can be relaxed by imposing the synchrony of each channel at a divisor of the bit time rather than imposing the synchrony of the bit time.

[0008] FR-A-2759516 proposes demultiplexing each channel to give the required delay and resynchronization. After multiplexing, each channel can be modulated by a single optical modulator. French patent application No. 9706590 proposes to resynchronize each channel with appropriate delay by using a network chain formed in the fiber, which requires demultiplexing and remultiplexing. There is no.

In the case of a non-soliton signal RZ, the synchronous modulation technique proposed by FR-A-2759830 delivers the power necessary for the conversion of the pulse into a Schrodinger standard soliton via a fiber section after the amplifier, By converting a non-soliton signal into a soliton signal and then synchronously modulating the signal in phase or intensity. The soliton signal is then reinjected into the transmission line, thereby causing the non-soliton RZ
Inverse conversion to a signal is performed. This patent uses a path to a strongly dispersive optical fiber with the appropriate input power corresponding to the soliton power for conversion. P. "" Black-box "optical re by Brindel et al.
generator for RZ transmission system
s "(Electronics Letters, Vol. 35, No. 6, 1999, 480-481), optical regeneration of signals in such devices is
It is said to be more effective when the phase and intensity modulation are separate, with the phase modulation occurring before the conversion fiber and the intensity modulation occurring at the output of the conversion fiber. B. Dany et al., “Transoceanic 4 × 40 Gbit /
s system combining dispersion-management
d soliton transmission and new "blac
k-box "in-line optical regeneration"
(Electronics Letters, Vol. 35, No. 5, 1999, 4
18-420), this reproduction technique is applied to a four-channel transmission system using distributed management soliton propagation with channel separation and a channel-corresponding regenerator.

The present invention proposes a solution to the problem of channel synchronization in wavelength division multiplexing transmission systems, enabling synchronous modulation of the channels. The present invention also provides FR-A-2.
In a reproducing device of the type 759830, a solution to the problem of channel synchronization for phase modulation as well as amplitude modulation is proposed.

More specifically, the present invention proposes a regenerator which receives and regenerates a plurality of channels of asynchronous wavelength division multiplexing at the input of the regenerator, the regenerator including a dispersive optical fiber section and a synchronous modulator, The length and dispersion of the intervals are chosen to synchronize the bit times of the channels at the input of the synchronous modulator.

In an embodiment, the regenerator further comprises a second distributed fiber optic section at the output of the modulator.

Advantageously, the product of the length and the chromatic dispersion for the second optical fiber section is the inverse of the product of the length and the chromatic dispersion for the optical fiber section.

[0014]   Preferably, the dispersive optical fiber has a negative dispersion.

The invention also proposes a wavelength division multiplexing transmission system comprising at least one such regenerator in the position of the system where the channels are asynchronous. In this case, it is preferred that the section has a dispersion with a sign opposite to the average dispersion in the line fiber of the transmission system.

The invention also includes a first synchronous modulator, a conversion fiber section, and a second synchronous modulator, the length and wavelength of said section being the bit of the channel at the input of the second synchronous modulator. We propose a regenerator that is selected to be time synchronized.

Preferably, the first modulator is a phase modulator and the second modulator is an intensity modulator.

An amplifier is provided between the first modulator and the conversion fiber section, and the chromatic dispersion of the conversion fiber is selected so that the maximum power of the signal in the conversion fiber is less than or equal to the output power of the amplifier.

Advantageously, the signal in the conversion fiber section is a soliton signal and the length of the conversion fiber section is a multiple of the period of the soliton signal.

In another embodiment, the first and second modulators have a common clock regenerator.

The invention further proposes a wavelength division multiplexing transmission system comprising at least one such regenerator.

Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, given by way of example with reference to the accompanying drawings.

The present invention proposes to synchronize the bit time of each channel with distributed fiber sections in order to enable synchronous modulation of all channels or parts of channels in a wavelength division multiplexing transmission system. The present invention relates to FIGS. 1-4 in which the dispersion slope (or third order dispersion) is an inverse dispersion fiber ("Reverse").
Dispersion Fiber "or RDF) is described for the case of a transmission system.

The invention also applies to regenerators using conversion fibers. In such a case, the bit time of each channel can be synchronized not only for phase modulation but also for amplitude modulation by appropriately selecting the dispersion of the conversion fiber as described with reference to FIGS. .

The present invention allows simultaneous passive synchronization of channels without the need for channel demultiplexing. The present invention can remove the constraints imposed by the prior art due to the relationship between the wavelength of the channel and the periodicity of the synchronous modulation. Further, as described in the following example, the position of the modulator can be freely selected regardless of the channel wavelength ratio.

FIG. 1 shows a distributed configuration in a transmission system implementing the present invention. As mentioned above, the system considered as an example for explaining the invention uses an inverse dispersion fiber to compensate the dispersion slope. Reproduction devices with synchronous modulation are provided along the system for all channels or only part of the channels at intervals Z _R. As mentioned above, the problem in this case is to synchronize the channels to allow modulation.

The abscissa of FIG. 1 describes the distance z along the transmission system. On the ordinate, the chromatic dispersion D (z) is described as a function of distance. As shown, the transmission system consists of alternating positive and negative dispersive fiber sections between the abscissa 0 and the spacing Z _R. For example, for the odd sections of the figure, a shift dispersion fiber (“DSF” or “Dis” with positive dispersion and negative slope).
version shifted fiber ") can be used. To compensate for the dispersion slope, the odd sections consist of fibers with negative dispersion and negative slope. The relative lengths of the even and odd intervals are chosen to cancel the cumulative dispersion slope in the chromatic dispersion range of the multiplexing, which is L _P · (dD / d
z) _P = -Lj * (dD / dz) _l . The letters "P" and "l" indicate even and odd intervals, respectively. Under these conditions, after a transmission distance Z _R , an almost zero cumulative dispersion slope is obtained, and the average value of the dispersion is equal to the value D _m or D− described in FIG.

Under these assumptions, after propagating a distance z, the kth channel, which is separated from the 0th channel by k · Δλ, _undergoes a time shift of T _k0 with respect to the 0th channel.
It is written as T _k0 = T _k −T ₀ = k · Δλ · D _m · z

The present invention proposes to compensate for this shift to ensure bit time synchronization and to enable synchronous modulation of the channel. For this reason, the invention proposes to insert a fiber section called compensation fiber in front of the modulator. The compensating fiber length Z _comp and dispersion D _comp are selected to synchronize the bit time of each channel at the input of the modulator. From this point of view, it is not necessary to compensate for the entire time shift between channels to enable synchronous modulation, it is sufficient to compensate for part of the shift expressed as a multiple of the bit time. .
In other words, the length and dispersion of the compensating fiber can be chosen such that the slide between channels is an integer multiple of the bit time T _bit . The bit time is referred to here as the period of each information element, as is generally the case in transmission systems.

In the embodiment of FIG. 1, a synchronous modulator is inserted after the compensating fiber section, which performs the modulation of a channel or a part of the channel. The modulator is
It may be an intensity modulator or a phase modulator. Furthermore, after this modulator,
A second fiber section is provided that can eliminate the effect of the compensating fiber section on chromatic dispersion. Typically, a fiber having a dispersion-D _comp and a length Z _comp can be used. By inserting the second fiber section in this way, the chromatic dispersion in the transmission system can be kept unchanged, which is particularly advantageous in the case of a transmission system using soliton pulses. Therefore, as mentioned above, the propagation of soliton pulses also occurs as a result of dispersion control. It is also possible not to use such a second fiber section after the modulator, as in the embodiment of FIG. 2 described below.

Hereinafter, the two adjacent channels will be referred to as a distance Z _s that is shifted by the bit time T _bit . This distance can be expressed as: Z _s = T _bit / (D _m · Δλ)

By using the above equation, the shift T _k0 between the kth-order channel and the 0th-order channel can be expressed as follows. T _k0 = kT _bit z / Z _s

To enable synchronous modulation of the distance z, it is sufficient to compensate for the time shift corresponding to a part of the shift expressed as a multiple of the bit time. The following shifts may be compensated for between the kth-order channel and the 0th-order channel. τ _k0 = T _ko −T _bit E (T _k0 / T _bit )

Here, E (x) is a function known per se in which the integer part is combined with the number x.
This shift can be compensated by inserting an optical fiber section with a dispersion D _comp and a length Z _comp into the transmission system. In this case, the shift that occurs between the two channels of the compensating optical fiber section can be expressed by analogy as: T ₁₀ = ΔλD _comp Z _comp

Such a shift that occurs in the compensating fiber allows synchronous modulation if the bit time synchronization of each channel is restored. The characteristics of the compensating fiber section depend on the position where this fiber is placed.

It is thus clear that the bit times of each channel are synchronized at distances which are multiples of the above distance Z _s , and there is no need to provide a compensating fiber section.

For a system of the type of FIG. 1, a second fiber section is placed after the modulator. It is clear that the time shift between each channel has a spatial period Z _s ,
As a result, it suffices to consider a modulator arranged between z = 0 and z = Z _s .

[0038]   Distance Z of the transmission system and distance Z in the compensating fiber section_compThe biography of
After seeding, the sufficient condition for the bit time to be synchronized is the compensating fiber section.
The propagation in is to compensate for the time shift between two adjacent channels.
Ie   T_bitZ / Z_s= -ΔλD_compZ_comp   Where Z <Z_sOr Z> Z_sin the case of,   T_bit(Z / Z_s-E (Z / Z_s)) =-ΔλD_compZ_compIs
.

From these two equations, in a transmission system in which the dispersion slope is zero on average and a second fiber section is provided after each modulator, the same result is obtained in terms of the periodicity of the shift between the channels. can get.

[0040]   For example, the interval Δλ between channels is 0.5 nm, and the bit time T_bit Is 100 ps corresponding to a bit rate of 100 Gbit / s.
You can Average variance D_mIf the system is 0.5 ps / (nm ・ km), Z_s Value of 400 km. It is possible to arrange a regenerator by synchronous modulation every 100 km.
To achieve this, after propagation of 100 km, correct the shift of quarter bit time to 200 k
After propagating m, corrected the shift of half bit time, after propagating 300 km, quadrant
Correct the 3-bit time shift of. After 400 km of propagation, the bit time is
Be synchronized. The system is 400 km cycle. After 100 km of propagation, shift
To compensate for the distance Z_comp1 km of fiber can be used. in this case
, Obtained D_compThe value of is −100 ps / (nm · km), which is
It is feasible. In this case, after propagation of 200 km and 300 km,
The same fibers with lengths of 2 and 3 km can be used. Signal 40 Gbit /
s, that is T_bit= 25 ps, the value D obtained after propagation of 100 km_{c omp} Is -12.5 ps / (nm · km), which is the current RDF file.
It can be achieved with Iber. In this case, after propagation of 200 km or 300 km, the length Z _comp Can use the same fiber for a longer length, or
However, fibers with higher chromatic dispersion values can be used.

FIG. 2 shows a distributed configuration in another transmission system embodying the present invention. The system of FIG. 2 is the same as the system of FIG. 1 except that there is no second fiber section after the synchronous modulator that allows the initial value of the chromatic dispersion to be reset. In other words, in the embodiment of FIG. 1, the insertion of the modulator and the corresponding two fiber sections is transparent in terms of chromatic dispersion accumulated in the transmission system. On the contrary, in the embodiment of FIG. 2, the insertion of the modulator causes a disruption in the accumulated chromatic dispersion.

As FIG. 2 shows, in this case the compensating fiber is arranged to be split, some before the modulator, the rest after the preceding modulator, or the line. It is advantageous to be at the beginning of the preceding section of fiber. This separation of compensating fibers can limit the perturbations that the soliton propagation mode introduces to solitons due to propagation in fiber sections where dispersion is not suitable for soliton propagation. In the embodiment of FIG. 2, the bit times are synchronized at the output of each modulator, and the chromatic dispersion accumulated along the transmission system is partially compensated by the compensating fiber. In other words, when the distance between the two modulators is Z _R , the average dispersion value D _m shifts to the average dispersion D _m + D _comp / Z _R.

The embodiment of FIG. 2 also has the advantage of limiting the length of the compensating fiber.
For example, the values proposed above can be considered again. In order to insert the modulator every 100 km, the dispersion D _comp with respect to the bit time of 100 ps is −100 ps / (nm · km) and the interval Δλ0.
A fiber section of 5 nm and a length Z _comp of 1 km can be used. After each modulator, the bit time of each channel is synchronized, and the time shift introduced from the previous modulator is not the time shift introduced from the beginning of the transmission system as in the embodiment of FIG. It may be compensated by the modulator. In other words, giving an example of numerical values, 100k
For each m, the length Z _comp is 1 km and the dispersion D _comp is −100 ps / (nm · k).
m) fiber sections can be used. In the embodiment of FIG. 2, using the length Z _{comp / 2} interval before the modulator, to use the length Z _{comp / 2} interval after the modulator. Assuming that the fiber length before and after each modulator is 1.5 km, a dispersion fiber having D _comp of -16.6 ps / (nm · km) can be used using the same numerical value. The dispersion of such a fiber is close to that of an SMF-compensated RDF fiber. In the transmission example of 40 Gbit / s, the fiber length Z _comp / 2 is 0.5 km and the chromatic dispersion D _comp is -25 ps / (nm.
km) fibers can be inserted.

For comparison, FIG. 3 shows the quality factor values in the prior art soliton signal transmission system as a function of distance, and FIG. 4 shows the quality factor values in the transmission system according to the invention. FIG. 1 corresponds to a 4-channel system of 40 Gbit / s each, the wavelengths of the channels being chosen so as to guarantee a natural synchronicity with a period Z _s of 150 km. An optical modulator having a period Zr of 150 km is arranged. The quality factor value asymptotically approaches the minimum value of 9 when the distance is 6000 km. FIG. 4 shows that the modulator period Zr is 180 k for this same system.
m, ie the shift is about 20% of the bit time. To compensate for the delay between the channels, a fiber with a dispersion value Z _comp of −17 ps / (nm · km) is used. The length Z _comp is 600 m. In a system of the type of FIG. 2, the dispersion introduced by the compensating fiber is not eliminated and a fiber section with a length Z _comp / 2 of 300 m can be provided before and after each modulator.

In FIG. 4, first, even if a compensation fiber for channel synchronization is inserted, the influence on the quality factor value is only secondary, and the quality factor value is comparable to the value in FIG. It is accepted that there is. Therefore, the embodiment of FIG. 2 can be implemented not only for RZ signals, but also for soliton signals whose dispersion is managed in the transmission system. Moreover, this solution is still valid for conventional soliton pulses whose dispersion is not managed in the transmission system.

In the examples above, for the sake of simplicity, the case of a system with a zero dispersion slope was considered. The invention applies as well for systems in which the average value D'm of the dispersion slope is not zero. In this case, for channel synchronization,
It suffices to choose a compensating fiber with a dispersion slope D' _comp which demonstrates that:

For modulators where the distance Z is located below Z _s , if D ′ _comp · Z _comp = D′ m · Z Z> Z _s then there are two systems of equations to solve. can get. T _bit (Z / Z _S −E (Z / Z _S )) = − ΔλD _comp Z _comp (1
) D' _comp Z _comp = D ' _m Z (2)

The first equation is the same as in the previous case and corresponds to the correction of part of the shift between bit times caused by dispersion. The second equation shows the additional shift introduced by the dispersion slope. This additional shift is generally smaller than the shift caused by the variance.

The present invention is applicable even when the synchronization between channels is not perfect. The invention also applies in the above-proposed formula, even when the chromatic dispersion slope is not completely zero. A sync defect of about ± 5% of the bit time is tolerated, allowing sync modulation.

FIG. 5 is a schematic diagram of a regenerator according to another embodiment of the present invention. The regenerator of FIG. 5 is of the type described in the P. Brindel document above. This regenerator comprises a coupler 4 for sampling a part of the signal sent to the clock regenerator 6.
Has as input. The phase modulator 8 provided after the coupler receives the clock signal sent from the clock regenerator 6. At the output of the phase modulator, the modulated signal is amplified by the amplifier 10 and then injected into the conversion fiber 11 to convert the signal into a soliton signal. As shown in the above-mentioned document or patent application, a strictly positive dispersion fiber of generally 2 to 17 ps / nm · km can be used as the conversion fiber. A narrowband filter 12 is provided after the dispersive fiber, and a coupler 14 then samples a portion of the signal and sends it to a clock regenerator 16.
The filtered signal is sent to the intensity modulator 18, which further receives the signal sent from the clock recovery device.

The above-mentioned literature proposes such a regenerator for a single channel.
The present invention can use this regenerator to synchronously regenerate the signal sent from each channel of the wavelength division multiplexing transmission system, as described with respect to FIGS. The invention further proposes, in such a regenerator, to select the properties of the conversion fiber so as to synchronize the bit time of each channel at the input of the intensity modulator. The invention also allows the clock signal for intensity modulation to be derived from the clock signal for phase modulation, since only one clock regenerator can be used, especially if the length of the conversion fiber is not very long. If the length of the conversion fiber is too long, it is advantageous, as in the prior art, to keep two clock recovery circuits and in some cases to be able to take into account the phase shift of the conversion fiber.

The length Z _comp and the dispersion D _comp of the conversion fiber are chosen by assuming that the bit times of each channel are synchronized at the output of the amplifier, such that the following equation is satisfied. Z _comp D _comp = kT _bit / Δλ (1)

Here, the symbol notation is the same as above, and k is an integer. These conditions ensure that the shift between the two channels in the conversion fiber is an integer multiple of the bit time. This condition applies when the chromatic dispersion slope of the conversion fiber can be ignored.

When the wavelength dispersion slope D ′ _comp of the conversion fiber is taken into consideration, it is as follows. kT _bit = Z _comp · D ' _comp · Δλ (Δλ + 2Δλ ₁₀ ) / 2 (
2)

Here, Δλ ₁₀ is the spectral interval between the wavelength λ ₁ of the channel having the shortest wavelength and the wavelength λ ₀ where the dispersion (or average dispersion of the section) of the fiber is zero.

The above equation guarantees bit time synchronization at the input of the intensity modulator. Similar to the previous embodiments, about 1-2% of the bit time synchronization defects between channels can be tolerated.

Furthermore, it is advantageous to maximize the chromatic dispersion of the conversion fiber so as to minimize the period Z ₀ of the “pseudo” soliton signal obtained at the output of the conversion fiber. This cycle Z ₀ can be expressed as follows.

[Equation 1]

At this time, Δt is the time width of the RZ pulse at the input of the conversion fiber, and λ is the wavelength of the channel (which channel?). The conversion can be improved for a given length of conversion fiber by minimizing the period Z ₀ . Moreover, in order to facilitate the conversion, it is advantageous that the conversion fiber length Z _{com p} is a multiple of the period Z ₀ of the “pseudo” soliton signal.

Another constraint that may be considered in choosing a conversion fiber is the constraint on the maximum power of the soliton pulse in the conversion fiber. The maximum power can be expressed as follows.

[Equation 2]

Here, n ₂ and A _eff are the nonlinear index and the effective area of the conversion fiber. The maximum power of the soliton is preferably chosen so that it does not exceed the power at the output of the amplifier. The limit value of 20 dBm corresponds to the allowable output power of the amplifier. Such constraints limit the chromatic dispersion value of the conversion fiber.

Therefore, the choice of chromatic dispersion of the conversion fiber is made within the range defined by equations (3) and (4). The length of the conversion fiber and the chromatic dispersion in this range are selected using equations (1), (2) to ensure bit time synchronization for intensity modulation.

The table below shows the amplitude and time quality factors, Qa and Qt, in a regenerator of the type of FIG. 5, used for synchronous regeneration of 4 × 40 Gbit / s multiplexing. These quality factors are measured in a manner known per se. The example uses channels with wavelengths 1548, 1550, 1552, 1554 nm and the sinusoidal type RZ pulse is raised by 12.5 ps, which is one half of the maximum width. The conversion fiber is NZDSF fiber and has a length Z _comp of 2.5.
The wavelength dispersion is 3 km, and the chromatic dispersion is +5 ps / nm · km. The effective cross section of the fiber is 80
μm ² and the non-linear index is 2.710 ⁻²⁰ m ² / W. Under such conditions, the period Z ₀ of the pseudo soliton is approximately 2.5 km. Therefore, the length of the conversion fiber is at least 0.7 times this period.

[0063]

[Table 1] From the results in the above table, it can be seen that the regenerator of the present invention can be used for synchronous regeneration of wavelength division multiplexed channels.

FIG. 6 shows the quality factors in a transmission system using a regenerator of the same type as in FIG. The abscissa is the mileage in the transmission system, and the ordinate is
The quality factor for each channel is shown. The quality factor is the smaller of the two quality factors, Qa and Qt. The transmission system is a 4x40 Gbit / s system with the same wavelengths used to obtain the results in the table. The pulse is a distributed managed soliton pulse. The system has an amplifier with a period of 40 km and a regenerator of the same type as in FIG. 5 with a period of 240 km. In the figure, the distance is 8000k
It was found that the quality factor asymptotically approached the values 8-10 for m, demonstrating the effectiveness of the regenerator of the invention.

By way of comparison, FIG. 7 shows the quality factors in a system in which the channels are played separately. The transmission system is the same as that shown in FIG. 6, except that the channel is the P. Reproduced separately by a regenerator of the type described in Brindel et al. It can be seen that after propagation of 8000 km, the quality factor reaches the same value as in FIG.

[0066]   Thus, the present invention enables synchronous playback of each channel of the multiplex.

Of course, the present invention is not limited to the examples and embodiments described and illustrated, but comprises numerous variants which can be considered by the person skilled in the art. The example considered the simplest case where all channels are synchronized so as to allow synchronous modulation of all channels. It is also possible to modulate only a subset of the channels or at frequencies above the bit frequency, as described in the above patent application. As a result, the length and dispersion of the compensating fiber changes.

In the example of FIGS. 1 to 4, the compensating fiber is arranged near the regenerator. The fiber can also be located elsewhere, for example split into sections where amplifiers are located.

In the embodiments of FIGS. 5-7, the placement of the intensity modulator after the phase modulator was considered. The configuration can be reversed. However, effectiveness is reduced. This is because intensity modulation requires "non-linear" pulses for the most efficient control of jitter and noise levels. Therefore, it is desirable to arrange the signal conversion before the intensity modulator, as in the above embodiment. Intensity and phase modulation can also be performed by the same component or at the same place in the line. That is, it can be performed after the conversion fiber or after the narrow band filter having periodicity.

[Brief description of drawings]

[Figure 1]   It is a figure which shows the structure of dispersion | distribution in the transmission system which implements this invention.

[Fig. 2]   It is a figure which shows the structure of dispersion | distribution in another transmission system which implements this invention.

[Figure 3]   It is a figure which shows the quality factor in the transmission system of a prior art.

[Figure 4]   It is a figure which shows the quality factor in the transmission system by this invention.

[Figure 5]   FIG. 6 is a schematic diagram of a regenerator according to another embodiment of the present invention.

6 is a diagram showing quality factors in a transmission system using a regenerator of the type of FIG.

FIG. 7 is a diagram showing quality factors in a system in which channels are reproduced separately.

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Claims (13)

[Claims] 1. A regenerator that receives and regenerates a plurality of wavelength division multiplexed channels that are asynchronous at the input, including a dispersion fiber optic section and a synchronous modulator, wherein the length and dispersion of the section are synchronously modulated. Regenerator selected at the input of the generator to ensure synchronization of the bit times of the channels. 2. The regenerator according to claim 1, further comprising a second dispersion fiber section at the output of the modulator. 3. The regenerator according to claim 1, wherein the product of the length and the chromatic dispersion of the second optical fiber section is the inverse of the product of the length and the chromatic dispersion for the optical fiber section. 4. The regenerator according to claim 1, 2 or 3, wherein the dispersion optical fiber has a negative dispersion. 5. A wavelength division multiplexing transmission system comprising a regenerator according to any one of claims 1 to 4 in the position of the system where the channels are asynchronous. 6. The system according to claim 5, wherein the section has a dispersion of a sign opposite to the average dispersion in the line fiber of the transmission system. 7. A first synchronous modulator (8), a conversion fiber section (11) and a second synchronous modulator (18), the length and dispersion of said section being the second synchronous modulator. A regenerator selected to ensure the bit time synchronization of the channels at the input of. 8. A method according to claim 7, wherein the first modulator is a phase modulator.
Regenerator described in. 9. The second modulator is an intensity modulator.
Or the regenerator described in 8. 10. An amplifier (10) between the first modulator and the conversion fiber section.
Regenerator according to claim 7, 8 or 9, characterized in that the chromatic dispersion of the conversion fiber is further selected such that the maximum power of the signal in the conversion fiber is less than or equal to the output power of the amplifier. 11. The signal according to claim 7, wherein the signal in the conversion fiber section is a soliton signal, and the length of the conversion fiber section is a multiple of the period of the soliton signal. Regenerator. 12. Regenerator according to claim 7, characterized in that the first and second modulators have a common clock regeneration device. 13. A regenerator according to any one of claims 7 to 12,
Wavelength division multiplex transmission system.