Optical signal for a soliton optical transmission system

In a soliton transmission system, the invention proposes sending a signal constituted by a periodic series of solitons, each having a width lying in the range 0.20 times to 0.33 times the period of the signal. In such a signal, interaction between the solitons compensates the jitter caused by the Gordon-Haus effect. A signal is thus obtained that can be used as a clock, having an amplitude or a time Q-factor Q.sub.a or Q.sub.t that is high, over large distances z.

The present invention relates to an optical signal for a soliton optical 
transmission system, and to an optical clock formed by such a signal. The 
invention also relates to a method of generating an optical clock signal 
in an optical data transmission system, and to an optical transmission 
system including such a clock signal. 
BACKGROUND OF THE INVENTION 
The transmission of soliton pulses or "solitons" in the portion of an 
optical fiber that has abnormal dispersion is a known phenomenon. The 
transmission of so-called "black" solitons constituted by pulse "holes" in 
a continuous signal, in the normal dispersion portion of an optical fiber 
is also known; in this case, the solitons have a wavelength such as to 
propagate with negative chromatic dispersion. Both for "white" solitons 
and for "black" solitons, to compensate dispersion of the optical signal, 
use is made of the non-linearity in the corresponding portion of the 
fiber. Soliton transmission is modelled in known manner by the non-linear 
Schrodinger equation. 
Various effects limit the transmission of such pulses, such as jitter 
induced by solitons interacting with the noise present in the transmission 
system, e.g. as described in the article by J. P. Gordon and H. A. Haus, 
published in Optical Letters, Vol. 11, No. 10, pp. 665-667. This effect, 
known as the Gordon-Haus effect, puts a theoretical limit on the quality 
or the bit rate of transmission by means of solitons. 
Because of the deformations induced on solitons during transmission, and in 
particular because of the jitter induced by the Gordon-Haus effect, 
considerable efforts are needed to ensure that a signal encoded by 
solitons is transmitted, and to make it possible to recover the necessary 
clock frequency from pseudo-random signals. Thus, sliding guiding filter 
systems have been proposed that enable the jitter of transmitted solitons 
to be controlled, as have various clock recovery systems, both optical and 
optoelectronic. Such systems are relatively expensive and complex, in 
particular because of the need to eliminate the effects of jitter before 
recovering the clock. An example of such a clock recovery system is 
described in FR-A-2 706 710. 
As described by F. M. Mitschke and L. F. Mollenauer, Optical Letters, Vol. 
12, No. 5, pp. 355-357, adjacent solitons interact. This interaction 
appears as attraction between adjacent solitons in the absence of 
modulation, i.e. for solitons that are in phase. It appears as repulsion 
between adjacent solitons when they are in phase opposition. 
This interaction is generally considered as being a harmful phenomenon 
since it leads to deformation of transmitted solitons that can lead to 
loss of information, see for example N. J. Smith et al., Optical Letters, 
Vol. 19, No. 1, pp. 16-18, which presents such interaction as "one of the 
major constraints in the design of soliton optical fiber communications 
systems". In the prior art, proposals have been made to avoid this 
interaction by imposing a constraint on the time "distance" between two 
transmitted solitons, thereby limiting the effects of interaction between 
solitons. A commonly accepted value for the minimum separation between two 
solitons is 0.2.times.Dt where Dt is the width of the solitons, 
conventionally defined by energy being equal to half the maximum energy, 
and known as "full width at half maximum" (FWHM). 
OBJECTS AND SUMMARY OF THE INVENTION 
The present invention proposes an original and simple solution to the 
problem of transmitting a clock in a soliton transmission system. The 
invention makes it possible to simplify transmission of a clock in a 
transmission system, in particular by overcoming the jitter caused by the 
Gordon-Haus effect. 
More precisely, the invention proposes an optical signal constituted by a 
periodic series of solitons, each having a time width lying in the range 
0.20 times to 0.33 times the period of the signal. 
In one embodiment, the solitons are black solitons. 
Adjacent solitons can be in phase or in phase opposition. If they are in 
phase opposition, the period of the power envelope is taken into 
consideration independently of the changes of sign in the field. 
The invention also provides an optical clock formed by such an optical 
signal. 
The invention also provides an optical data transmission system comprising 
such an optical signal or such an optical clock. 
Finally, the invention proposes a method of generating an optical clock 
signal in an optical data transmission system, comprising emitting a 
periodic series of solitons, each having a time width lying in the range 
0.20 times to 0.33 times the period of the signal. 
In an implementation, the solitons are black solitons. 
The method may also include O-p modulation of the emitted solitons. 
In an implementation of the invention, the solitons are emitted in the 
working data bandwidth of the optical data transmission system. 
It is also possible for the solitons to be emitted outside the working data 
bandwidth of the optical data transmission system.

MORE DETAILED DESCRIPTION 
FIGS. 1A, 1B, 1C, and 1D show examples of signals on various channels of a 
soliton transmission system using the invention. Typically such a soliton 
transmission system comprises emitter means connected to an optical fiber 
having amplifiers and/or filters disposed thereon at regular intervals. 
As already known, the working bandwidth of an optical fiber for soliton 
transmission is subdivided into a plurality of channels for transmitting 
different signals .lambda..sub.1, to .lambda..sub.n, each constituted by 
solitons corresponding to bits at logic value one and blanks corresponding 
to logic value zero. FIGS. 1A, 1B, and 1C show the appearance of these 
signals at three wavelengths .lambda..sub.1, .lambda..sub.2, and 
.lambda..sub.n, respectively, with amplitude being plotted up the ordinate 
and time along the abscissa. 
In addition to these various signals, the invention also proposes 
transmitting a signal .lambda..sub.s as shown in FIG. 1D, constituted by 
an uninterrupted run of solitons emitted at a clock frequency to be 
transmitted in the system or at the bit rate of the transmission system, 
and of a width such that interaction between solitons compensates the 
jitter induced by the Gordon-Haus effect. Because the solitons are 
transmitted in the form of an uninterrupted run, each soliton having any 
tendency to deform is pushed back by its interaction with adjacent 
solitons, thereby opposing deformation of the signal. 
It is possible to emit a series of solitons in phase, making use of the 
attractive interaction between adjacent solitons; alternatively, it is 
possible to emit a series of solitons in phase opposition, using O-p 
modulation, making use of the repulsive interaction between adjacent 
solitons. 
The invention thus overcomes the prejudice of the prior art as to the 
negative effects of interaction between solitons. It goes against the 
teaching of the state of the art by proposing to make use of the 
interaction between solitons to overcome deformation of the signal in the 
fiber, and in particular to correct the jitter induced by the Gordon-Haus 
effect. 
Using the conventional definition of the "width" (FWHM) of a soliton pulse, 
the invention proposes that the width of each soliton should lie in the 
range 0.20 times to 0.33 times the period of solitons in the signal. The 
word "period" is used herein for the power envelope, independently of any 
changes of sign in the field that may be used with solitons in phase 
opposition. The lower limit of this preferred range makes it possible to 
ensure that the solitons of the signal interact sufficiently to compensate 
jitter due to the Gordon-Haus effect and do not behave like isolated 
solitons in known transmission systems. The upper limit of this range 
makes it possible to ensure that the transmitted signals conserve their 
soliton nature in spite of the interaction between adjacent solitons, 
thereby enabling the stability of the signal to be conserved in non-linear 
propagation. 
The signal of the invention behaves like a run of solitons, and therefore 
passes through all of the passive elements that may be disposed on a 
fiber, such as guiding filters, etc. 
The signal of the invention can be transmitted for various applications. 
The following can be mentioned by way of example: 
in-line control of a synchronous modulator for regenerating solitons; 
in-line synchronization of an active component such as a wavelength 
converter; 
in-line control of demultiplexing; and 
carrying a clock signal. 
Use of the signal of the invention makes it possible to distribute a clock 
in a soliton transmission system, or merely to transmit a clock from one 
point in the system to another. 
It has the following advantages over apparatuses and methods known in the 
prior art. Firstly, the clock formed by the signal of the invention has 
greater spectral purity, or lower radiofrequency phase noise, than do 
clocks obtained in conventional manner from pseudo-random pulse trains. 
For bit rates of the order of 1 Gbit/s to 10 Gbit/s, the signal of the 
invention constitutes a clock having a spectrum width of a few hundreds of 
hertz. By way of comparison, a clock derived in conventional manner from 
pseudo-random pulse trains typically has a spectrum width that is greater 
than 1 MHz. Also, the signal of the invention makes it possible to use an 
optical clock directly, without any prior transformation or processing. 
Such direct use of the clock makes it possible to envisage operating at 
very high bit rates, such a 100 Gbit/s, at which conventional clock 
recovery methods present great difficulties. 
As will appear immediately to the person skilled in the art, it is possible 
to transmit the signal of the invention in one of the channels in the 
working bandwidth. In addition, because the signal is robust as can be 
seen clearly from the description of FIG. 2, and because the structure of 
the signal is known in advance, it is also possible to use the edges of 
the working band normally used with solitons. Attenuations that would be 
too great for conventional data signals remain acceptable for the signal 
of the invention. The phase information of the solitons is more important 
than their intensity. It is thus possible to accept poorer transmission 
gain performance than for a conventional soliton signal. 
The invention proposes not only a signal constituted by an uninterrupted 
run of solitons, such as the signal .lambda..sub.s as shown in FIG. 1D, it 
also proposes transmitting the inverse signal, i.e. a signal having black 
solitons emitted at a clock frequency to be transmitted in the system, and 
of a width such that the interaction between the solitons compensates the 
limiting effects of the jitter induced by the Gordon-Haus effect. Such a 
continuous signal with pulse holes forming black solitons presents the 
advantage of being capable of being transmitted in the portion of the 
optical fiber that has normal dispersion. This avoids the need to transmit 
the signal of the invention in the band that is of use for transmitting 
solitons, or even at the edges of that band. 
FIG. 2 is a graph of Q-factor Q as a function of distance z for a signal of 
the invention. The digital simulation of FIG. 2 is based on a train of 
solitons emitted at a rate of 10 Gbit/s, i.e. having a period of 100 ps, 
into a fiber of dispersion D=0.55 ps/nm.km. Each soliton was of width 30 
ps, i.e. 0.3 times the soliton period. The solitons were modulated by O-p 
modulation, which corresponds to interaction that is repulsive. Guiding 
filters were disposed on the fiber at an interval Zn=50 km. The 
propagation distance is plotted along the abscissa in Mm, and the Q-factor 
is plotted up the ordinate in dB. In addition, a bit error rate (BER) of 
10.sup.-9 is marked on the ordinate, corresponding to a Q-factor of about 
15 dB. 
Curve B shows the amplitude Q-factor Q.sub.a and curve A shows the time 
Q-factor Q.sub.t, as computed digitally from the non-linear Schrddinger 
equation. Both of these factors have a value greater than 20 dB for 
propagation distances of 10 Mm, thus ensuring a BER that is considerably 
smaller than 10.sup.-9. 
FIG. 2 shows clearly that the signal of the invention is robust, and 
confirms that making use of interaction between solitons, against the 
teaching of the prior art, makes it possible to compensate for the jitter 
induced by the Gordon-Haus effect. 
FIG. 3 shows the appearance of the eye diagram for the FIG. 2 signal for 
propagation over 10 Mm. The opening of the diagram shows the transmission 
quality of the signal of the invention. 
Naturally, the present invention is not limited to the examples and 
embodiments described and shown, and the person skilled in the art is 
capable of varying it in numerous ways. In particular, an "uninterrupted" 
run of solitons does not mean a run of solitons having a number of 
solitons that is infinite, but merely that is large enough to restrict 
deformations to the ends of the run. It is clear that jitter compensation 
on a soliton by its interaction with adjacent solitons is not as effective 
at the beginning or at the end of transmission of the signal as it is in 
the middle of such transmission. The number of solitons required for a 
given transmission system can be determined by the person skilled in the 
art merely by routine testing. It is also clear that the bit rate of the 
signal of the invention can be different from the bit rate of the data 
channels in the optical fiber, and can, for example, be a multiple or a 
sub-multiple of said bit rate.