WO1999008406A1

WO1999008406A1 - System and method of high-speed transmission and appropriate transmission apparatus

Info

Publication number: WO1999008406A1
Application number: PCT/EP1998/004782
Authority: WO
Inventors: Pierluigi Franco; Giorgio Grasso; Matjaz Vidmar
Original assignee: Pirelli Cavi E Sistemi S.P.A.
Priority date: 1997-08-04
Filing date: 1998-07-31
Publication date: 1999-02-18
Also published as: EP1002386B1; AU8733298A; BR9811127A; DE69808302D1; AR018999A1; CA2299724A1; DE69808302T2; NZ502778A; JP2001512861A; EP1002386A1

Abstract

Soliton or soliton-like optical pulses with characteristics adapted to propagation in an optical line for RZ type transmission are generated by modulating a continuous optical signal (1) by means of a modulator (2) which is substantially devoid of chirping, in which the modulator drive signal comprises at least one frequency (4) and one harmonic (5) thereof superimposed on each other (7). A high-speed optical transmission system is rendered independent of the type of transmitter or of signals sent to it if it comprises an adaptation unit (32) receiving the original optical pulses (30) and capable of generating corresponding pulsed signals (63) of RZ type.

Description

System and method of high-speed transmission and appropriate transmission apparatus.

In the field of digital type fibre-optic telecommunications, in addition to the possibility of using techniques of a conventional type (usually referred to as "Non-Return-to-Zero", or NRZ, in which essentially a 1 or 0 value is transmitted for the whole period corresponding to the speed of encoding adopted) , the possibility exists of utilizing a transmission technology of the pulsed type, for example of the "soliton" or "soliton-like" type (usually referred to as "Return-to-Zero", or RZ) , in .which essentially a sequence of pulses is transmitted, each lasting less than the period corresponding to the speed of encoding adopted, and modulated on the basis of the digital information to be transmitted.

One of the major problems encountered in the design of an RZ transmission system consists of the difficulty of generating pulses with an RZ profile which are suitable for this type of transmission, and, in particular, which are of sufficient time duration and are unaffected by "chirp" .

Among the techniques used for this purpose are known, for example, mode-locking fibre lasers and electroabsorption modulators, as well as other techniques. A characteristic common to the aforesaid techniques consists of the fact that they are efficient for generating particularly short pulses suitable for transmission over dispersion-shifted or DS fibres, i.e. fibres with chromatic dispersion which approaches zero within the wavelength band employed for telecom-munications, round about 1550 run, as for example defined by the ITU-T Recommendation G653 1993, and for time-division optical multiplexing.

In the case of transmission over so-called step- index fibres, or SI fibres, (as for example described in ITU-T Recommendations G650 1993 and G652 1993) and with dispersion compensation, it is useful to have available fairly long pulses (for example lasting from 20 to 60 ps for a transmission frequency of 10 Gbit/s) while it is observed that with shorter-lasting pulses in the said SI fibre systems, with high dispersion, phenomena of dispersive wave generation are observed leading, ultimately, to an increase in the error rate of the transmission (BER) .

Electroabsorption modulators, moreover, are intrinsically prone to generate pulses affected by "chirp".

The term "chirp" is understood to mean a variation in the frequency of the signal during its amplitude modulation, so that there is a (central) frequency of the signal which is different at the start of the pulse from the (central) frequency of the signal at the end of the said pulse.

The Patent WO 9616345 describes apparatus which uses two amplitude modulators controlled by two phase- locked modulating voltages, one having double the frequency of the other, in which the larger is the speed of pulse repetition.

The article from the IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 2, June 1996

(Veselka et al.) describes an apparatus which comprises several sinusoidally driven intensity modulators linked in series for forming pulses.

The Patent EP 622916 describes a soliton generator which comprises a phase modulator and an amplitude modulator, respectively driven at the frequency of pulse repetition and at a harmonically correlated lower frequency.

The Patent EP 718990 describes a device for converting a data stream of the NRZ type into an RZ stream, which employs a modulator with Mach-Zehnder interferometer or a directional coupler.

The Patent US 5157744 describes a soliton generator which comprises an amplitude modulator with Mach-Zehnder interferometer with a multiple series of distributed electrodes, driven at harmonically correlated frequencies. The Patent states that the process of combining several high-frequency signals into a single signal involves large attenuations and requires amplification, and that the transmission and processing of the final signal, which is a composite of many high-frequency signals, is extremely difficult. Moreover, if the composite signal requires amplification, a very expensive amplifier is required, able to amplify many very high frequencies uniformly. The invention of US 5157744 is aimed at a soliton generator which avoids these problems.

D. Le Guen et al., in OFC 97 PD17(l-3), describe an experiment on a WDM soliton system with 10 channels at 20 Gbit/s, with compensation for chromatic dispersion and pre- chirping, in which a 1000 kilometre line of step-index fibre with 100 kilometre stretches was simulated by means of a 102 kilometre recirculation ring. The transmission uses electroabsorption modulators to modulate the emission from the laser sources so as to generate 20 ps pulses, subsequently coded by a lithium niobate modulator.

F.M. Knox et al., in ECOC 96, WeC3.2, 3.101-3.104, describe an experiment in 10 Gbit/s soliton transmission, with compensation for chromatic dispersion, over more than 2022 kilometres of step-index fibre; the experiment employed sech² (t) pulses of around 20 ps at 2.5 GHz, generated by an active mode-locking erbium fibre ring and modulated with a pseudo-random bit sequence by a lithium niobate amplitude modulator and twice interleaved to give a data stream at 10 Gbit/s, and injected into a 33 kilometre recirculating ring with an appropriate module for compensating chromatic dispersion.

According to one aspect of the present invention, it is found that, by applying to a modulator of an optical signal a drive signal consisting of a periodic signal at one frequency, combined with at least one harmonic of the said periodic signal, it is possible to generate pulses of an amplitude suitable for pulsed optical communication, of the soliton type or the like.

Within the scope of the present invention, it has been found that optical pulses, of the soliton or similar type, can be used in a line comprising high-dispersion fibre (for example the aforesaid SI fibres) and chromatic dispersion compensation means, in which there is a first stretch with high signal power, in which this signal propagates under self-phase modulation (SPM) , essentially without undergoing the effects of chromatic dispersion; and a subsequent stretch, in which the signal propagates under linear conditions, allowing compensation of its chromatic dispersion with conventional means of compensation.

In this way, in the stretch in which, on account of the power of the signal, it would not be possible to compensate dispersion effects liable to cause non-linear phenomena such as to jeopardize the possibility of compensating for dispersion, such compensation is not required thanks to the use of pulses propagating under soliton conditions; on the other hand, in the stretch in which the power of the signal has dropped below the level such as to allow temporal reconfiguration of the pulses (or soliton "reshaping"), compensation for dispersion may take place. The term self-phase modulation is understood to mean a combination of non-linear effects associated with the propagation of a signal of intensity greater than a certain value under guided conditions in a dispersive optical conductor means, such that the chromatic dispersion of the means is essentially compensated and the temporal profile of any given pulse stays unaltered.

In the case of the fibre propagation of a signal of given power P, the signal intensity is I = P/A_f, where A_f is the area of the cross-section associated with the propagation of the signal in the fibre.

According to the present invention it has been found that optical pulses with characteristics suitable for propagation in an optical line for RZ type transmission were able to be generated by modulating a continuous optical signal by means of a modulator essentially devoid of chirping, provided that the modulator drive signal comprises at least one frequency and one harmonic thereof superimposed on one another. According to the present invention it has moreover been found that a high-speed optical transmission system can be rendered independent of the type of transmitter or of signals sent to it if it comprises an adaptation unit receiving the original optical pulses and capable of generating corresponding pulsed signals of the RZ type.

In particular, in a first aspect, the present invention relates to a high-speed optical pulse transmitter, which comprises: - a first optical signal modulator;

- a second optical pulse modulator, optically linked to the said first optical signal modulator;

- a generator of a continuous optical signal, optically linked to the said first and second optical modulators; - means of driving the said first optical signal modulator with an electrical signal bearing coded information with a preset repetition frequency; means of driving the said second modulator, these comprising an element for combining a first periodic electrical signal at the said preset frequency and at least one second periodic electrical signal at a second frequency which is a harmonic of the said preset frequency.

Preferably, the said means of driving the said second modulator comprise a circuit for generating the said first periodic electrical signal at the said preset frequency, driven by a clock signal associated with the said electrical signal bearing information, and a circuit for generating the said second periodic electrical signal at the said second frequency, a harmonic of the said preset frequency.

In particular, the said circuit for generating the said second periodic electrical signal at the said second frequency, a harmonic of the said preset frequency, comprises a frequency multiplier, linked to the said circuit for generating the said first periodic electrical signal.

Preferably, the said means of driving the said first optical signal modulator with an electrical signal bearing coded information with a preset repetition frequency consist essentially of a circuit for supplying an electrical signal bearing coded information with a preset repetition frequency to the said first optical modulator. In one embodiment, the said means of driving the said first optical signal modulator with an electrical signal bearing coded information with a preset repetition frequency comprise a circuit for generating the said electrical signal bearing coded information, in response to an external signal.

Preferably, the said circuit for generating the said first periodic electrical signal at the said preset frequency, driven by the said clock signal, comprises an output bearing a synchronization signal, in a preset time relationship with the said clock signal, linked to the said means of driving the said first optical signal modulator.

Preferably, the said means of driving the said first optical signal modulator with an electrical signal bearing coded information with a preset repetition frequency comprise a decision circuit, receiving the said electrical signal bearing coded information with a preset repetition frequency and the said clock signal.

Preferably, the said combining element is a distributed-constants circuit. In another aspect, the present invention relates to a pulsed transmission system, comprising at least one transmission station, one reception station, one fibre- optic line linking the said transmission station and the said reception station and at least one optical amplifier serially linked along the said fibre-optic line, characterized in that the said transmission station comprises a unit for generating signals which comprises:

- a first optical signal modulator, able to modulate an optical signal with a series of pulses bearing coded information with a preset repetition frequency;

- a second optical pulse modulator, optically linked to the said first optical signal modulator, able- to modulate an optical signal with a first sequence of periodic pulses of preset duration, with a preset repetition frequency;

- a generator of a continuous optical signal, optically linked to the said first and second optical modulators, with preset wavelength;

- means of driving the said optical pulse modulator, comprising an element for combining a first periodic electrical signal at the said preset frequency and at least one second periodic electrical signal at a second frequency which is a harmonic of the said preset frequency.

In particular, the said fibre-optic line linking the said transmission station and the said reception station has overall chromatic dispersion greater than zero at the wavelength of the said optical signal. Preferably, the said fibre-optic line linking the said transmission station and the said reception station comprises chromatic dispersion compensation means able to compensate a fraction of the chromatic dispersion of the line, and which are such that the total chromatic dispersion of the line is between 100 and 120% of the compensated dispersion.

In a particular embodiment, the said transmission station comprises

- several units for generating signals, each of which comprises a respective generator of a continuous optical signal at a respective wavelength, different from that of the other units, which are each able to generate an appropriate pulsed optical signal at a wavelength; and

- means of multiplexing the said pulsed optical signals. Preferably, the said reception station comprises means of wavelength demultiplexing the said pulsed optical signals.

In another aspect, the present invention relates to a pulsed transmission system, comprising at least one transmission station, one reception station, one fibre- optic line linking the said transmission station and the said reception station and at least one optical amplifier serially linked along the said fibre-optic line, characterized in that the said transmission station comprises a unit for generating signals which comprises:

- a second optical pulse modulator, optically linked to the said first optical signal modulator, able to modulate an optical signal with a first sequence of periodic pulses of preset duration T_{F HM}, with the said preset repetition frequency;

- in which the ratio bit/TjrwHM between the inverse of the said preset repetition frequency T_bι_t and the said preset duration T_{F HM} of the pulses, is between 6 and 10.

According to another aspect, the present invention, relates to a pulsed transmission system, comprising at least one transmission station, one reception station, one fibre-optic line linking the said transmission station and the said reception station and at least one optical amplifier serially linked along the said fibre-optic line, characterized in that it comprises:

- a station for transmitting at least two external optical signals, having respective first spectral parameters and each bearing information according to a first digital code;

- a respective interfacing unit allied with each of the said external optical signals, comprising a unit for receiving the said external optical signals and a unit for emitting corresponding optical work signals at preset wavelengths, digitally coded with the information from the said external optical signals, in the form of RZ pulses; a first optical conductor element, having a first chromatic dispersion at the wavelengths of the said work signals;

- a second optical conductor element, having a second chromatic dispersion at the wavelengths of the said work signals, of opposite sign to the said first chromatic dispersion, serially linked to the said first optical conductor element;

- the said first chromatic dispersion and the said second chromatic dispersion being of preset values, such that the overall chromatic dispersion is greater than zero at the wavelengths of the said work signals.

In particular, the said pulsed work signals possess, for at least one portion of their propagation path in one of the said first and second optical conductor elements, an intensity of a value such as to cause phase self modulation of the said work signals.

Preferably, the said optical amplifier possesses output power for each channel of a value such as to determine, in a portion of one of the said first and second optical conductor elements, an intensity of a value such as to cause phase self modulation of the said work signals.

In a preferred embodiment, the said first optical conductor element is a step-index optical fibre; alternatively, the said first optical conductor element is an optical fibre with non-zero dispersion.

According to a further aspect, the present invention relates to a method of high-speed optical transmission, comprising the phases of:

- generating an optical signal; - modulating the said optical signal with a first periodic signal at a preset transmission frequency;

- modulating the said optical signal with a second signal, bearing coded information at the said preset frequency;

- in which the said phase of modulating the said optical signal with a first periodic signal at a preset frequency involves applying to an optical modulator a drive signal comprising the said periodic signal at the said preset frequency and at least one harmonic of the said preset frequency. According to a further aspect, the present invention relates to a method of high-speed optical transmission, comprising the phases of: receiving a first modulated optical signal bearing information;

- converting the said optical signal into an electrical signal bearing the said information; - modulating a second optical signal with a sequence of pulses, of preset time duration;

- modulating the said second optical signal with the said electrical signal bearing the said information;

- supplying the said second modulated optical signal with the said sequence of pulses and the said electrical signal in an optical transmission line.

More details may be gleaned from the following description, with reference to the appended figures, in which are shown: In Figure 1 a general diagram of a generator device according to the present invention;

In Figure 2 a diagram of a generator device according to the present invention, as embodied for experimental purposes; In Figures 3a, 3b, 3c respectively the time graphs

(in arbitrary units) of the amplitude of the optical pulses obtained in the presence of a main frequency and its first harmonic, under various conditions of phase-shift and amplitude ratio; In Figure 4 an example of a combining filter for high frequencies as embodied for the device of Figure 2;

In Figure 5a the time graph of the pulses output by the first modulator of the device of Figure 2, in the presence of the fundamental frequency alone; In Figure 5b the time graph of the pulses output by the first modulator of the device of Figure 2, in the presence of the fundamental frequency and its first harmonic;

In Figure 6 the eye diagram on output from the device of Figure 2, in the presence of the fundamental frequency and its first harmonic, after the second modulation; In Figure 7 a general diagram of a high-speed transmission system according to the present invention;

In Figure 8 a dispersion compensation device suitable for use in the high-speed transmission system; In Figure 9 a diagram of a device for transforming signals according to the present invention;

In Figure 10 an illustrative diagram of a PLL circuit adapted as a synchronization circuit. Apparatus for generating pulses . As shown in Fig. 1, a continuous-emission laser 1 is linked by a first amplitude modulator 2, hereafter referred to as pulse modulator for example of the Mach- Zehnder interferometer type, driven by a composite electrical signal 3, consisting of a first periodic electrical signal 4, preferably sinusoidal, with frequency fi equal to the desired transmission frequency (for example 10 GHz) , by a second periodic electrical signal 5, also preferably sinusoidal, at a frequency consisting of the second harmonic f₂ of the signal 3 (for example 20 GHz) and, possibly, by one or more periodic electrical signals 6, also preferably sinusoidal, at frequencies consisting of higher harmonics f₃, f₄, ... (30, 40, ... GHz) of the transmission frequency fj_..

For the purposes of the present description, the term second harmonic of a signal of given frequency is understood to mean a signal with double the frequency of the said given frequency, the said fundamental frequency; the terms third harmonic, fourth harmonic, etc. are understood to mean signals at frequencies respectively triple, quadruple etc. the said given fundamental frequency.

For the purposes of the present invention, the term frequency of a periodic signal is understood to mean the frequency of the sinusoid, in the case in which the periodic signal is a sinusoidal signal, or else the frequency of the fundamental sinusoid in the Fourier series expansion of the signal, in the case in which it has a non- sinusoidal temporal profile, and the term higher harmonics is understood to mean whole multiple frequencies of the said sinusoid or of the said fundamental frequency.

Hereafter, unless otherwise specified, the terms "sinusoidal signal" and "harmonic of the frequency of the sinusoidal signal" are used to mean that these comprise either signals with sinusoidal time profile and appropriate harmonics or signals with a different time profile, for example with a triangular, square or similar wave, or else with a more complex profile, for example with a sech² (t) profile, (typical of soliton pulses) , and signals at harmonic frequencies of the fundamental frequency of the said signals, having the same or a different time profile.

Electrical signals with sinusoidal profile are preferred and can beneficially be generated with known electronic devices, as described hereafter.

Such electrical signals with different frequencies are combined together by means of a combining filter 7 (described hereafter) possibly after amplifica-tion by respective amplifiers 8, 9, 10. The amplifiers 8, 9, 10 are beneficially narrowband amplifiers (one for each harmonic) , which are very simple to produce and inexpensive (compared with wide-band amplifiers which would be needed to amplify a multifrequency signal such as that at the output of the combining filter 7) ; this is possible in the case in which the amplification is performed before the combining filter. The pulse modulator 2 moreover receives, beneficially, an electrical bias signal generated by a bias circuit 11. The pulse modulator 2 emits a pulsed modulated optical signal which is supplied to a second amplitude modulator 12 (also for example of the Mach-Zehnder interferometer type) , referred to hereafter as the signal modulator, driven by an electrical signal 13 containing the data to be transmitted, possibly amplified by a wide-band amplifier 14. The signal modulator 11 moreover receives, beneficially, an electrical bias signal generated by a bias circuit 15.

It is also possible to exchange the order of the modulators 2 and 12, placing the signal modulator, modulated with the data 12, before the pulse modulator 2, modulated with the sinusoids 4, 5, 6.

It is also possible to integrate both modulators 2 and 12, the pulse and signal modulators respectively, for example on the same LiNb0₃ "chip", obtaining an advantage in terms of the output power of the device.

Alternatively, moreover, as illustrated with dashed lines in Figure 1, it is also possible to interpose an optical amplification stage 16 between the two modulators, should the optical losses from the assembly be too high. The modulator 2 can, where beneficial, adopt a (narrowband) resonating-electrode structure.

Although the use has been described of Mach-Zehnder interferometer modulators, preferably made of LiNb0₃, it is possible to apply the present invention to various types of modulating means, for example, other than LiNb0₃ modulators, as well as electroabsorption modulators, those made of fibre subjected to "poling", those made of organic optical crystals or polymers, and similar devices, which are able to apply amplitude modulation to an optical signal and are driven by a preset input signal.

The signals 4, 5, 6 are in a preset phase and amplitude relation.

In particular, in the case of two frequencies, respectively denoted f₀ and 2f₀ (higher harmonic) , the resulting frequency is given by: fi = Aιsin(2πf₀) + A₂sin(4πf₀ + α) ; in which Ai and A₂ are the respective amplitudes of the two frequencies f₀ and 2f₀ used and a is the relevant phase difference.

Under this condition, under the assumption of an ideal frequency response of an electro-optical modulator, as shown in Figures 3a, 3b, 3c, respectively representing the pulse train output by the modulator for three different phase relations (corresponding to values of α equal to 0

(fig. 3a), π/4 (fig. 3b), π/2 (fig. 3c)), pulses of gradually smaller width may be obtained by increasing the ratio A₂/Aχ, as indicated by the corresponding curves 16,

17, 18, 19, 20 relating respectively to A₂/Aι = 0 (no second harmonic); A₂/Aι = 0.25; A₂/Aι = 0.5; A₂/Aχ = 0.75; A₂/Aι = 1.

A limit to the increase in the ratio A₂/Aχ is provided by the growth of a secondary peak 21, at double the frequency of the fundamental frequency applied; for example, under the conditions adopted, it is observed that this peak is of negligible amplitude for a ratio A₂/Aι of between 0.25 and 0.5, (curves 17, 18) with which a pulse amplitude is obtained which is already satisfactorily reduced as compared with the presence of the fundamental frequency alone.

It is known that the said secondary peak, if its value is too high, could be detected as a 1 value in the digital transmission, even if the corresponding main peak has been deleted following the prescribed modulation.

In general, in an actual system, the relationship between the amplitudes and the phase is influenced by the response characteristics of the modulators employed and should be defined from time to time, operationally, depending on the duration of the pulse which it is desired to obtain at the output of the device and the noise in the zero values at reception, by for example controlling the error rate (Bit Error Rate or BER) at reception as a function of the aforesaid parameters (relationship between amplitudes and phase) , in such a way that it is below the desired value (for example at least less than 10^"9) .

Synchronization of the phase of all the signals can be achieved with microwave phase adjustors, for example as described hereafter. The regular train of pulses at the fundamental frequency, this train being generated by the modulator 2, is supplied to the second modulator 12. This modulator, driven by an electrical signal containing the information to be transmitted, codes the information in the pulse train (digital optical system with external modulation) .

The non-linear transfer characteristic of the modulator 2, in response to the sinusoidal signals at different frequencies supplied to it, is such that at the output of the system is obtained a continuous train of pulses of the RZ type, essentially unaffected by " chirp" , and which are suitable for transmission under SPM at least in part of the line. One experiment utilized the apparatus represented in Fig. 2, in which the corresponding components are labelled with the same numerical references as Figure 1.

A DFB laser with output power of 10 mW and wavelength 1549 ran was used as continuous-wave laser 1. The pulse modulator 2 was modulated with a composite signal 3a, comprising the fundamental frequency 4a of 5 GHz, which was extracted from the system clock and suitably amplified with a microwave amplifier, and its second harmonic 5a at 10 GHz, obtained by multiplying the likewise amplified clock frequency by 2.

The combination, obtained by means of the combining filter 7, of the two frequencies 4a, 5a was then sent to the pulse modulator 2.

The optical signal which was obtained at the output of the pulse modulator 2 (obtained with a sampling oscilloscope) is plotted in Figures 5a, 5b, respectively in the case in which the 5 GHz frequency alone was supplied and in the presence of the two frequencies at 5 and at 10 GHz combined. For the purposes of the present description the term "duration" of a pulse is understood to mean its total duration at 1/2 height, known in the art as T_J-_WHM (Full Width Half Maximum) .

As is apparent from Figure 5b, the measured duration of the resulting pulses in the presence of the two frequencies at 5 and at 10 GHz combined was around 50 ps; by suitably controlling the bias of the modulator 2 and the amplitudes and relative phases of the two signals at 5 and at 10 GHz it was moreover possible to vary the duration of the pulses in the range 50-100 ps.

This adjustment, moreover, made it possible moreover to optimize the time profile of the pulses, rendering it as symmetrical as possible (i.e. with equal slope for the rising edge and for the falling edge of the pulse) .

With the 5 GHz frequency alone, however, it was possible to obtain a pulse train with a minimum duration of not less than 75 ps, which is much higher than that above.

The use of the two harmonic frequencies combined as input to the modulator thus makes it possible essentially to eliminate the restriction of the range of values obtainable, within the scope of values of duration which is of more interest in practical cases.

It is in fact useful, in RZ type transmissions, to use fairly short pulse durations as compared with the repetition period (in this case 200 ps) .

Pulses of excessive duration (for example 75 ps or more) , in fact, would be too close temporally and could interact with one another in the propagation along the line, giving rise to signal distortion stemming from the non linear effects associated with their propagation in the fibre . The duration for which two consecutive pulses are apt to collide, after a certain distance travelled in-fibre depends on the time intervening between these same pulses, i.e. on the transmission frequency (or bit rate), as for example described by Govind P. Agrawal, in "Nonlinear Fiber Optics", Academic Press, 2nd edition, 1995, pp. 170-172.

Typically, for fibre runs of che order of a thousand kilometres, a ratio Tbit/Trmm of greater than 6 and preferably greater than 8 is deemed to be acceptable. Preferably this ratio is less than 10. The notation T_bι_t is understood to mean the inverse of the transmission frequency, or "bit rate", adopted.

The extra degree of freedom made possible by introducing the second frequency renders the transmitter with the two frequencies very much more versatile than that with single frequency, and hence adaptable to the requirements of all systems of practical use.

The introduction of further harmonic frequencies, where beneficial, enables the system to be adapted moreover to particular specific requirements.

The pulse durations reported previously are the actual durations of the pulses, obtained by deconvolving the band effects of the instrument from the measurements. The optical pulses generated by the pulse modulator

2 were then sent to the second modulator 12, or signal modulator, passing through the optical amplifier 16, in such a way as to compensate the losses introduced by the pulse modulator. Then, the signal modulator 12 introduces the coding of the data at 5 Gbit/s, giving rise to the signal, represented in Fig. 6.

Beneficially the length of the electrical conductors linking together the electronic apparatuses and the modulators is sized so as to synchronize the pulse train generated by the first modulator with the electronic data signal which supplies the second modulator.

Beneficially the synchronization of all the signals may be obtained with microwave phase adjusters. Combining filter.

The combining filter 7, represented in Fig. 4, consists of a microstrip or distributed-constants circuit, consisting of a substrate 21a made of insulating material, preferably ceramic, on which are produced several conductive tracks or strips 22, 23, 24.

The strip 22 has two ends, 25, 26, respectively linked to an input conductor 25a, bearing a signal input at the main frequency (5 GHz in the example) and to an output conductor 26a, bearing the composite output signal consisting of the main frequency and its second harmonic. The signal at the frequency of the second harmonic of the main frequency (10 GHz in the example) is supplied by a second input conductor 27a linked to the strip 24. As shown in the Figure, the strips 22, 23, 24 have portions with respective lengths £χ, £₂, £ _f £A , i which depend on the wavelength of the signals; in particular, in the example illustrated, £χ, £₂, £₃, £₄ , are identical and equal to λ/4 and £₅ is equal to λ/2, where λ is the wavelength of the signal at 10 GHz.

In particular, a filter centred at 10 GHz (made up by the portion 28 of the strip 24, by the strip 23 and by the portion 29 of the strip 22, having respective lengths λ/4, λ/2, λ/4) has been placed on the 10 GHz input arm 24, so as to prevent passage of the 5 GHz frequency to the 10 GHz input 27a.

Furthermore, in respect of the 5 GHz input, the 10 GHz frequency sees an open circuit (made up by three stretches 29, 48, 31 of length λ/4) . Thus the 10 GHz frequency is conveyed entirely to the output 26. Adaptation to the output has been obtained on the 5 GHz input arm 25

(by means of a filter which blocks the 5 GHz frequency and by means of the three stretches 29, 48, 31 of length λ/4) : therefore, the 5 GHz frequency is also conveyed entirely to the output 26.

The loss introduced by the device has been shown to be around 0.5 dB for each frequency.

The signal thus obtained is forwarded to the modulator 2.

The Applicant has observed that, whereas if a microwave coupler is used for combining the frequencies, at least 6 dB are lost, the use of a device (such as the combining filter described in Fig. 4 by way of example) allows two or more different frequencies to be combined with negligible losses for each frequency, giving rise to a combination ideally having zero losses (apart from the excess losses, which may be of the order of 0.5 dB for each frequency) . Furthermore, according to the present invention, by virtue of the essential absence of losses in the combining operation, each frequency can be amplified separately ahead of the combiner, thus using narrow-band amplifiers which are inexpensive and easy to produce. Transmission system

As shown in Figure 7, a high-speed optical telecommunication system, with wavelength multiplexing, according to an example embodiment of the present invention receives several initial optical signals 30a, 30b, 30c, 30d, 30e, 30f etc. (for example 16) , each of which signals, referred to as the "external signals", possesses its own transmission characteristics, in particular wavelength, power, type of modulation and transmission frequency (bit rate), for example 10 Gbit/s.

The signals, generated by local external sources or originating from further portions of optical network, are each supplied to a respective interfacing unit 32a, 32b, 32c, 32d, 32e, 32f, etc., able to receive the initial external optical signals, detect them and reproduce them anew with characteristics matched to the high-speed transmission system. In particular, the said interfacing units generate respective optical work signals having wave-lengths λi, λ₂, λ₃, λ₄, λ₅, λ₆, and so on, included within the useful working band of the amplifiers arranged subsequently in the system, having, in addition, RZ pulsed modulation characteristics. In the Patent US 5267073, from the same Applicant, the description of which is incorporated for reference, interfacing units are described which comprise in particular a transmission adaptor, able to convert an optical signal input into a form matched to the optical transmission line, and a reception adaptor, able to reconvert the transmitted signal into a form matched to a reception unit.

In the case in which the signals of the various channels to be transmitted are signals of electrical rather than optical type, they are each supplied directly to a respective transmission unit, at the appropriate wavelength, incorporating the pulse generation apparatus described previously. The optical work signals generated by the interfacing units 32 or generated directly at the envisaged wavelengths, are then supplied to a signal combiner 33 able simultaneously to send down a single optical output fibre 34 the work signals at the wavelengths λi, λ_2/ λ₃, λ₄, λ₅, X₆, etc.

In general, the signal combiner 33 is a passive optical device by means of which the optical signals transmitted over respective optical fibres are superimposed in a single fibre; devices of this kind consist for example of fusible-fibre couplers, made in planar optics, microoptics and the like.

By way of example, combiners of this kind are sold by E-TEK Dynamics Inc., 1885 Lundy Ave., San Jose, CA (USA) .

Via the fibre 34 the said work signals are sent to a power amplifier 35, which raises their level to a value sufficient to traverse a subsequent stretch of intervening optical fibre ahead of fresh means of amplification retaining at the end a power level which is sufficient to guarantee the required transmissive quality.

A first stretch 36a of optical line, preferably consisting of a single-mode optical fibre inserted into a suitable optical cable, is then linked to the power amplifier 35; typically, for the transmission characteristics of the system according to the present invention, the line is of the order of around 100 kilometres long (for example 80-120 kilometres with the power levels indicated below and the dispersion compensation devices described) . At the end of the said first stretch 36a of optical line is a first line amplifier 37a, able to receive the signals, attenuated in their journey down the fibre, and to amplify them up to a level sufficient to supply them to a second stretch of fibre-optic line 36b, with similar characteristics to those of the previous line.

Subsequent line amplifiers 37b, 37c, 37d and so on, and respective stretches of optical fibre 36c, 36d, 36e, 36f, 36g and so on span the required overall transmission distance, reaching a reception station 38, which comprises a preamplifier 39, able to receive the signals and amplify them, compensating for the loss given by the subsequent demultiplexing equipment, up to a power level matched to the sensitivity of the reception devices.

Typically, in a preferred embodiment of the present system, the overall distance of the link between a transmission station, housing the interface units, and the reception station, may be of the order of around 1000-2000 kilometres, preferably about 1000 kilometres (having regard to the required safety margins) .

In a preferred embodiment, the single-mode optical fibres employed in the various stretches 36 described above are of the step-index type, with which satisfactory transmission at 10 Gbit/s is obtained over the aforesaid distance of around 1000-2000 kilometres.

Although step-index fibres are preferred for the purposes of the present invention, in relation to specific requirements, such as for example for systems with large distances, or higher values of encoding frequency (for example 40 Gbit/s) , it is possible to employ fibres with a lower value of chromatic dispersion, for example fibres with non-zero dispersion, described in ITU-T Recommendation G655 1997, or else of the dispersion shifted type, described in the already cited ITU-T Recommendation G653 1993 (for example in the case in which the Four Wave Mixing phenomena are not critical) , or combinations of fibres with different values of dispersion, provided that overall the propagation of soliton or soliton-like pulses is made possible.

From the preamplifier 39 the signals are sent to a demultiplexer 40, by which the same signals are separated depending on their relative wavelengths, and then sent to respective reception or interfacing units 41a, 41b, 41c, 41d, 41e, 41f, etc., which are able to receive the optical signals and use them as such, or else, if required, to regenerate them with the optical characteristics matched to the subsequent equipment envisaged (not represented) . The demultiplexer 40 can be produced by multifold technologies, for example employing signal dividers in association with Bragg grating filters, interference filters or combinations thereof, or else arrayed grating devices (Arrayed Waveguide Gratings, or AWGs) , or the like. The configuration described lends itself in a particularly satisfactory manner to transmissions over distances of the order of around 1000 km, with high transmission speed, for example 10 Gbit/s (achieving with sixteen channels at different multiplexed wavelengths an overall transmission capacity of 160 Gbit/s) .

For the purposes of the present invention and for the use described above, the power amplifier 35 is, preferably, an erbium-doped-fibre optical amplifier, with one or more stages; in the system illustrated, in the presence of 16 wavelength-multiplexed channels, the power amplifier 35 typically has the following characteristics: Input power from -5 to +2 dBm

Output power + 20 dBm

Working wavelength 1530-1560 nm.

Amplifiers of this type are for example sold by the Applicant. The term power amplifier is understood to mean an amplifier operating under conditions of saturation, in which the power output depends on the pumping power, as described in detail in European Patent No. EP 439,867 incorporated herein for reference. For the purposes of the present invention and for the use described above, the preamplifier 39 is, preferably, an erbium-doped-fibre optical amplifier, with one or more stages; in the system illustrated, in the presence of 16 wavelength-multiplexed channels, the preamplifier 39 typically has the following characteristics:

Input power from -5 to +2 dBm

Output power from +7 to +10 dBm

Working wavelength 1530-1560 nm. For the purposes of the present invention and for the use described above, the term preamplifier is understood to mean an amplifier placed at the end of the line, capable of raising the signal to be supplied to the receiver to a value suitably above the sensitivity threshold of this receiver (for example from -26 to -11 dBm on input to the receiver) , while at the same time introducing the least possible noise and retaining the equalization of the signals. Suitable preamplifiers are sold by the Applicant.

The line amplifiers are, preferably, erbium-doped- fibre optical amplifiers, preferably with several stages, able to output an overall power of at least 20 dBm and to operate with a working wavelength of 1530-1560 nm. Beneficially, at least one of the line amplifiers

37 and/or the preamplifier 39 are associated with a respective chromatic dispersion compensation unit 42, able to compensate at least part of the chromatic dispersion of the line or of a stretch of line relative thereto. Preferably, all the line amplifiers 37 and the preamplifier 39 are associated with a chromatic dispersion compensation unit 42.

Alternatively, it is possible to insert chromatic dispersion compensation units every 200-500 kilometres (for example every 2 or more amplifiers) , or even to insert one or more dispersion compensation units at the beginning or at the end of the entire link.

This choice is tied, among other factors, to the overall length of the line: for example, for a line with overall length of about 1000 kilometres it is possible to install a compensation unit every 100-200 kilometres roughly, whereas for lines of lesser overall length, for example 300-400 kilometres roughly, a single compensation unit can be installed. An illustrative embodiment of a transmission system according to the invention, over a distance of around 1000 km, comprising 10 stretches of step-index SI optical fibre of around 100 kilometres each, allows a maximum overall chromatic dispersion of around 18000 ps/nm and a minimum overall chromatic dispersion of around 15500 ps/nm (essentially dependent on the characteristics of the SI fibres employed) , of which at least 15500 ps/nm is compensated by the chromatic dispersion compensation units 42.

Preferably, in the presence of a total of 10 between line amplifiers 37 and preamplifier 39, each of the said chromatic dispersion compensation units 42 is designed to compensate around 1550 ps/nm.,

A diagram of a line amplifier is represented in Figure 8 by way of illustration.

This amplifier comprises a first stage 43 and a second stage 44, between which is sited the chromatic dispersion compensation unit 42.

Each of the stages 43 and 44 comprises an erbium- doped active fibre 45 and pumping means 46.

One or more optical isolators 47 are moreover present, preferably at the input and output of each stage. Preferably, at least one stretch of the active fibre 45 and the pumping means 46 are arranged in such a way as to supply a pump wavelength to the active fibre directed in the same sense as the signal in the first stage and in the sense opposite to the signal in the second stage.

In a preferred embodiment the pumping means comprise at least one pump wavelength source (typically a laser, in the case in which spatially coherent pumping is desired, or else, for example, a laser diode array, in the case in which the fibre is designed in such a way as to accept this type of pumping) , associated with appropriate means of coupling to the active fibre (for example fusible- fibre couplers or interference filters, or multimode type couplers, in association with double cladding fibres or the like) .

Beneficially, the chromatic dispersion compensation unit 42 is sited inside the amplifier, in a position intermediate between the two stages, in this way ensuring that the attenuation afforded thereby does not penalize the performance of the amplifier (in terms of signal/ noise ratio or output power) .

Alternatively, in the case in which the characteristics of the system so permit or so advise, the chromatic dispersion compensation unit 42 can also be arranged upstream or downstream of an amplifier, or else in a position independent of the latter.

The chromatic dispersion compensation unit 42 comprises, for example, a stretch of preset length of dispersion-compensating fibre (i.e., for example, a fibre having strongly negative chromatic dispersion in the wavelength band employed for transmission, such as to wholly or partly compensate the positive dispersion in this band of the SI fibres employed for the line) as described for example in the Patent US 5361319, or one or more fibre stretches bearing a "chirped" grating, (i.e. having a non- constant grating spacing) linked into the line by means of a circulator, a coupler or the like, so as to reflect in a suitable time sequence the various spectral components of the signal, for example as described in the Patent US 4239336, so as to cause a modification to the time profile of the pulse opposite to that caused by the fibre of the line and of such a magnitude as to compensate at least part thereof.

The typical characteristics of a system of the type described are summarized in Table 1 below.

TABLE 1

Transmission capability 16 channels x 10 Gbit/s

Length of link ~ 1000 km

Maximum allowable attenuation 10 x 25 dB

Minimum allowable attenuation 10 x 20 dB

Maximum allowable chromatic dispersion 18000 ps/nm

Minimum allowable chromatic dispersion 15500 ps/nm

Chromatic dispersion compensation 10 x 1550 ps/nm Output power from line amplifiers ~ 20 dBm Number of channels 8 - 16

Maximum allowable PMD ~ 0.5-i ps/km¹²

Total chromatic dispersion 100-120% of the compensated dispersion

A diagram of a transmission interfacing unit, according to an example embodiment of the present invention, is illustrated in Figure 9. The interfacing unit comprises a photodetector

(photodiode) 51, able to receive the optical signal 30, and to emit an electrical signal, which is supplied to an electronic amplifier 52.

The electronic amplifier 52 possesses an output line, bearing the amplified electrical signal, linked to a power divider node 53, having two respective output arms, the first of which linked to a decision circuit 54, able to generate an electrical signal 14 for driving a signal modulator 12, linked with a continuous-emission laser 1, and a second output arm linked to a second electronic amplifier 55 and from there to a clock extraction unit 56, able to generate a synchronous time signal with the digital information input.

Clock extraction units are sold, for example, by Veritech Microwave, Inc. (NJ, USA) .

A synchronization circuit 57, which receives the clock signal generated by the clock extraction unit 56, generates a synchronization signal 58, supplied to the decision circuit 54, and a periodic signal 4, preferably sinusoidal, at the frequency of encoding of the optical signal input, as indicated with reference to Figures 1 and 2.

In a preferred embodiment the synchronization circuit 57 is a PLL (Phase Locked Loop) circuit, a diagram of which is represented, for illustrative purposes, in Figure 10. The 10 GHz signal from the clock extraction circuit 56 is supplied to a first input of a mixer circuit 101, which moreover receives an electrical signal generated by a voltage-controlled oscillator 101, whose output frequency (for example 100 MHz) is preferably supplied to a frequency multiplier 102 (which generates at output a frequency of 10 GHz, multiplying the input by 100) the signal from which is sent both as output and to a second input of the mixer circuit 101. The output signal from the mixer circuit 101, consisting of a signal containing the sum frequency and the difference frequency of the frequencies input, is supplied, via a low-pass filter 103, to control the output frequency of the voltage-controlled oscillator 101, thus keeping this frequency clamped to that of the signal from the clock extraction circuit 56.

PLL circuits suitable for this purpose are known in the art.

Although PLL type synchronization circuits are preferred, it is also possible to use dielectric-resonant filters, such as for example the circuit which forms part of the O/E Converter device MP 9S042, sold by ANRITSU WILTRON S.p.A., Rome, constructed and marketed for use as an optical reception unit, operating at 10 Gbit/s, or equivalent technologies.

The components from the photodiode 51 to the synchronization circuit 57 and to the decision circuit 54 constitute, as a whole, an optical/electrical conversion unit 59; the function of this unit can also be carried out by employing the O/E Converter device MP 9S042 ANRITSU mentioned above (employing a dielectric-resonator filter) , or similar component devices of optical reception units.

Appropriate phase adjustor circuits 58a, 60 are moreover provided at the output of the synchronization circuit 57 along the lines of the respective signals 58 and 4, for the purpose of carrying the synchronization signal 58 and the sinusoidal signal 4 in a preset relative phase relation (taking into account the response characteristics of the various components of the circuit and of the modulators) .

The sinusoidal signal 4 is then sent, via an arm of a power divider 61, to a first narrow-band amplifier 8 and then to the input at the fundamental frequency of the combining filter 7; the sinusoidal signal 4 is moreover sent, via the other arm of the power divider 61, to a frequency multiplier 62, able to generate at output a signal 5 with a frequency which is a multiple of that input and is in an adjustable phase relation with it, to a second narrow-band amplifier 9, with a preferably adjustable output power, and from there to the second-harmonic input of the combining filter 7.

The signal 3 output by the combining filter 7, consisting of the combination of the signals 4, 5, is then sent to the pulse modulator 2, linked to the output of the laser 1 in series with the signal modulator 12.

The frequency multiplier 62 may moreover beneficially comprise further outputs, at frequencies which are multiples of the input frequency (higher harmonics) , which can in turn be supplied to the combining filter 7.

The optical output 63 of the modulator 12 (or of the modulator 2 if they are fitted in the reverse order) , constitutes the RZ pulsed transmission signal, at the typical wavelength of the laser 1.

The interfacing circuit moreover comprises elements, not illustrated, for circuit bias control, circuits for driving and controlling the emission wavelength of the laser 1, which are able to keep it constant at the preselected value, while compensating for any external disturbances such as temperature, as also the circuits for controlling the working point (bias) of the modulators 2, 12 and the like, on the basis of the specific requirements of the system. In the case in which the signal to be transmitted is a signal of electrical type, at the encoding rate envisaged (for example 10 Gbit/s), instead of the interfacing unit described above a transmission unit is used, having essentially the same structure as the interfacing unit, but without the photodiode 51, hence in which the electrical signal input is supplied directly to the input of the amplifier 52. Moreover in the case in which the time profile of the electrical signal bearing the information available, whether it be generated in this form directly or produced by the photodiode 51, meets sufficient requirements as to lead to an acceptable value of error rate, as defined above, the same electrical signal can also be supplied directly (or after amplification) as input to the modulator 12 to constitute the drive signal therefor.

Moreover in the case in which the aforesaid electrical signal bearing the information is generated close to the optical transmitter, or in which the appropriate clock signal is available anyway from an external origin (for example the same signal generation equipment) , this clock signal can be supplied directly to the synchronizing circuit 57, or even to its output. The system according to the invention thus makes it possible to receive optical signals having the characteristics typical of the transmission units to which this system is linked, and to generate, allied with these signals, RZ pulsed signals essentially devoid of chirp, of intensity and duration which are suitable for allowing propagation in the line without interactions between pulses and between different-wavelength signals, under conditions of self phase modulation for a stretch of this line, in which the intensity of the signal in the optical conductor means (for example the line optical fibre) exceeds a preset value and under substantially linear conditions able to allow compensation for the chromatic dispersion arising therein, in a second stretch of line.

Claims

1. High-speed optical pulse transmitter, characterized in that it comprises: - a first optical signal modulator;

- a second optical pulse modulator, optically linked to said first optical signal modulator;

- a generator of a continuous optical signal, optically linked to said first and second optical modulators; - means of driving said first optical signal modulator with an electrical signal bearing a coded information with a preset repetition frequency; and

- means of driving said second modulator, these comprising a combining element for combining a first periodic electrical signal at said preset frequency and at least one second periodic electrical signal at a second frequency which is a harmonic of said preset frequency.

2. High-speed optical pulse transmitter, according to Claim 1, characterized in that said means of driving said second modulator include a circuit for generating said first periodic electrical signal at said preset frequency, driven by a clock signal associated with said electrical signal bearing a coded information, and a circuit for generating said second periodic electrical signal at said second frequency.

3. High-speed optical pulse transmitter, according to Claim 1, characterized in that said circuit for generating said second periodic electrical signal comprises a frequency multiplier, linked to said circuit for generating said first periodic electrical signal.

4. High-speed optical pulse transmitter, according to Claim 1, characterized in that said means of driving said first optical signal modulator include a circuit for supplying an electrical signal bearing a coded information with a preset repetition frequency to said first optical modulator.

5. High-speed optical pulse transmitter, according to Claim 1, characterized in that said means of driving said first optical signal modulator include a circuit for generating said electrical signal bearing a coded information, in response to an external signal.

6. High-speed optical pulse transmitter, according to Claim 3, characterized in that said circuit for generating said first periodic electrical signal comprises an output bearing a synchronization signal, said synchronization signal being in a preset time relationship with said clock signal, said output being linked to said means of driving said first optical signal modulator.

7. High-speed optical pulse transmitter, according to Claim 5, characterized in that said means of driving said first optical signal modulator comprise a decision circuit receiving said electrical signal bearing a coded information and said clock signal.

8. High-speed optical pulse transmitter, according to Claim 5, characterized in that said combining element is a distributed-constants circuit.

9. Pulsed transmission system, comprising at least one transmission station, one reception station, one fibre- optic line linking said transmission station and said reception station and at least one optical amplifier serially linked along said fibre-optic line, characterized in that said transmission station comprises a unit for generating signals which comprises: - an optical signal modulator, able to modulate an optical signal with a series of pulses bearing a coded information and with a preset repetition frequency; - an optical pulse modulator, optically linked to said optical signal modulator, able to modulate an optical signal with a first sequence of periodic pulses of preset duration T_FWHM and with said preset repetition frequency; - a generator of a continuous optical signal, optically linked to said optical signal modulator and to said optical pulse modulator, with preset wavelength; and

- means of driving said optical pulse modulator, comprising an element for combining a first periodic electrical signal at said preset frequency and at least one second periodic electrical signal at a second frequency which is a harmonic of said preset frequency.

10. Pulsed transmission system, according to Claim 9, characterized in that said fibre-optic line has overall chromatic dispersion greater than zero at the wavelength of said optical signal.

11. Pulsed transmission system, according to Claim 10, characterized in that said fibre-optic line comprises chromatic dispersion compensation means able to compensate a fraction of the chromatic dispersion of the line, and which are such that the total chromatic dispersion of the line is between 100 and 120% of the compensated dispersion.

12. Pulsed transmission system, according to Claim 11, characterized in that said transmission station comprises:

- several units for generating signals, each of which comprises a respective generator of a continuous optical signal at a respective wavelength, different from that of the other units, each unit being able to generate an appropriate pulsed optical signal at a respective wavelength; and

- means of multiplexing said pulsed optical signals.

13. Pulsed transmission system, according to Claim 12, characterized in that said reception station comprises means for wavelength demultiplexing said pulsed optical signals .

14. Pulsed transmission system, comprising at least one transmission station, at least one reception station, one fibre-optic line linking said at least one transmission station and said at least one reception station and at least one optical amplifier serially linked along said fibre-optic line, characterized in that said at least one transmission station comprises a unit for generating signals which comprises:

- a first optical signal modulator, able to modulate an optical signal with a series of pulses bearing a coded information with a preset repetition frequency; - a second optical pulse modulator, optically linked to said first optical signal modulator, able to modulate an optical signal with a first sequence of periodic pulses of preset duration _{F HM}, with said preset repetition frequency; and - a generator of a continuous optical signal, optically linked to said first and second optical modulators, with preset wavelength; in which the ratio T_blt/TFWHM, between the inverse of said preset repetition frequency T_blt and said preset duration _FW_HM of the pulses, is between 6 and 10.

15. Transmission system, comprising at least one transmission station, at least one reception station, one fibre-optic line linking said at least one transmission station and said at least one reception station and at least one optical amplifier serially linked along said fibre-optic line, characterized in that said transmission station includes at least an interfacing unit adapted to be optically coupled to an external optical source, said at least an interfacing unit including:

- a receiving and converting device to receive, from said external optical source, a first optical signal bearing a coded information, and to convert said first optical signal into an electrical signal bearing said coded information, and

- an emitting device, electrically coupled to said receiving and converting unit to receive said electrical signal, and adapted to feed to said fibre-optic line a second optical signal of RZ type bearing said coded information.

16. Transmission system according to Claim 15, characterized in that said fibre-optic line includes: a first optical conductor element, having a first chromatic dispersion at the wavelength of said second optical signal; a second optical conductor element, having a second chromatic dispersion at the wavelength of said second optical signal, said second chromatic dispersion being of opposite sign with respect to said first chromatic dispersion, said second optical conductor element being serially linked to said first optical conductor element; said first chromatic dispersion and said second chromatic dispersion being of preset values such that the overall chromatic dispersion is greater than zero at the wavelength of said second optical signal.

17. Transmission system according to Claim 16, characterized in that said second optical signal has, for at least one portion of his propagation path in one of said first and second optical conductor elements, an intensity of a value such as to cause phase self modulation of said second optical signal.

18. Transmission system according to Claim 16, characterized in that said optical amplifier has amplification characteristics such as said second optical signal has, in at least one portion of his propagation path in one of said first and second optical conductor elements, an intensity of a value such as to cause phase self modulation of said second optical signal.

19. Transmission system according to one of Claims 15 to 18, characterized in that said first optical conductor element is a step-index optical fibre.

20. Transmission system according to one of Claims 15 to 18, characterized in that said first optical conductor element is an optical fibre with non-zero dispersion.

21. Method of high-speed optical transmission, comprising the steps of:

- generating an optical signal;

- modulating said optical signal with a periodic first signal at a preset frequency; - modulating said optical signal with a second signal, said second signal bearing a coded information at said preset frequency; in which said step of modulating said optical signal with a first periodic signal involves applying to an optical modulator a drive signal comprising said first periodic signal at said preset frequency and at least one harmonic of said preset frequency.

22. Method of high-speed optical transmission, comprising the steps of:

- receiving a first modulated optical signal bearing a coded information;

- converting said first modulated optical signal into an electrical signal bearing said coded information; - modulating a second optical signal with a sequence of pulses, of preset time duration; modulating said second optical signal with said electrical signal bearing said coded information;

- supplying said second optical signal modulated with said sequence of pulses and said electrical signal in an optical transmission line.

23. Method according to claim 22, characterized in that said step of modulating a second optical signal with a sequence of pulses comprises modulating a second optical signal with a first sequence of periodic pulses of preset duration _{F HM} and with a preset repetition frequency; and in that the ratio T_bi_t/TFWHM, between the inverse T_b╬╣_t of said preset repetition frequency and said preset duration T_FWHM of the pulses, is between 6 and 10.

24. Method according to claim 22 or 23, characterized in that said step of modulating a second optical signal with a sequence of pulses involves generating said sequence of pulses by combining a first periodic electrical signal at a first preset repetition frequency and at least one second periodic electrical signal at a second frequency which is a harmonic of said first preset repetition frequency.

25. Transmission system comprising at least one transmission station, one reception station and one fibre- optical line optically linking said transmission station and said reception station, characterized in that said transmission station comprises a first interfacing unit adapted to be optically coupled to an external optical source to receive a first optical signal bearing a coded information and adapted to feed to said fibre-optical line a second optical signal of RZ type bearing said coded information; and in that said reception station comprises a second interfacing unit optically coupled to said fibre- optical line to receive said second optical signal and adapted to be optically coupled to a subsequent optical equipment to transmit to said subsequent optical equipment a third optical signal bearing said coded information.

26. Transmission system according to claim 25, characterized in that said transmission system is a portion of an optical network and said external optical source is comprised in a further portion of said optical network.

27. Transmission system according to claim 25, characterized in that said second optical signal includes a sequence of periodic pulses, said periodic pulses having a preset duration

and a preset repetition frequency, the ratio T_bit/T_FWHM between the inverse T_bit of said preset repetition frequency and said preset duration T_FWHM of said periodic pulses being between 6 and 10.

28. Method of high-speed optical transmission, comprising the steps of:

- receiving, from a fibre-optic line, a modulated optical signal including a sequence of periodic pulses bearing a coded information, and - regenerating said optical signal to obtain a further modulated optical signal with optical characteristics matched to a subsequent optical equipment .

29. Method according to claim 28, characterized in that said step of regenerating comprises the steps of:

- converting said modulated optical signal into an electrical signal bearing said coded information;

- modulating a further optical signal with said electrical signal to obtain said further modulated optical signal.

30. Method according to claim 28, characterized in that said periodic pulses have a preset duration T_f-^ and a preset repetition frequency, the ratio T^/T^ between the inverse T_bit of said preset repetition frequency and said preset duration

of said periodic pulses being between 6 and 10.