EP0900407A1

EP0900407A1 - Optical regeneration for optical fibre transmission systems with soliton signals

Info

Publication number: EP0900407A1
Application number: EP98914923A
Authority: EP
Inventors: Sébastien Bigo; Emmanuel Desurvire
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel CIT SA; Alcatel Lucent SAS
Priority date: 1997-03-17
Filing date: 1998-03-16
Publication date: 1999-03-10
Also published as: FR2760855B1; FR2760855A1; CA2255595A1; JP2001506016A; WO1998041900A1

Abstract

The invention concerns a regenerator for transmission systems with solitons pulses, comprising a cavity laser, for instance a ring fibre laser, wherein is inserted a modulator (5), means for coupling the input signal to be regenerated in the modulator input (7), to ensure laser mode-locking, and means for coupling the regenerated signal in the modulator output (9). The soliton signal to be regenerated ensures the cavity laser locking-mode; moreover, the signal circulating in the cavity ensures a soliton signal modulation, which is thereby regenerated. Phase or intensity modulations, or both, can be used. The invention is characterised in that a ring cavity laser fibre, or a Fabry Perot cavity laser is used. The dispersion fibre non-linear loop mirror can be used as modulator, or an amplifier with semiconductor.

Description

OPTICAL REGENERATION FOR SOLITON SIGNAL FIBER OPTIC TRANSMISSION SYSTEMS

The subject of the present invention is an optical regenerator for a soliton pulse transmission system, and more particularly an optical regenerator in which the clock signal is obtained by mode locking of a cavity laser.

The invention also relates to a method for the optical regeneration of soliton pulses. Finally, it relates to a transmission system comprising such a regenerator.

The transmission of soliton or soliton pulses is a known phenomenon. These pulses are RZ pulses of temporal width (FWHM) small with respect to bit time, which have a determined relationship between power, spectral width and temporal width, and which generally propagate in the abnormally dispersed part of a optical fiber. The evolution of the envelope of such a soliton pulse in a single-mode fiber can be modeled by the nonlinear Schrodinger equation; propagation is based on a balance between the dispersion of the fiber and its non-linearity.

Various effects limit the transmission of such pulses, such as the jitter induced by the interaction of solitons with the noise present in the transmission system, described for example in the article by J. P. Gordon and H. A. Haus, Optical Letters, vol. 11 n ° 10 pages 665-667. This effect, called the Gordon-Haus effect, imposes a theoretical limit on the quality or bit rate of transmissions by solitons. In order to exceed this limit, it is possible to use synchronous modulation of the soliton signals, by a clock or clock signal, to correct their temporal jitter, as explained for example in an article by H. Kubota, IEEE Journal of Quantum Electronics, vol. 29 no 7 (1994), p. 2189 et s., With regard to intensity modulation, and in an article by N. J. Smith and N. J. Doran, Optical Fiber Technology, 1, p. 218 (1995), with regard to phase modulation.

Such modulation involves clock recovery from the soliton signal received. This clock recovery consists on the one hand of recovering a signal at a given frequency, and on the other hand of synchronizing the phase of this signal with the phase of the solitons received. Various solutions have already been proposed to ensure this clock recovery. One can in particular carry out an optoelectronic conversion, a phase recovery by electronic means, for example using a phase locked loop, then carry out an electronic-optical reconversion of the signal obtained to modulate the solitons. This solution intrinsically limits the throughput of the solitons link, due to the upper limit of the bandwidth of the semiconductor devices used. This solution is also complicated and expensive for high bit rates.

Another known solution consists in recovering the clock by blocking the modes of a ring fiber laser, and in applying the clock thus recovered to a synchronous modulator. An article by W. A. Pender et al., Electronics Letters, vol. 31 no. 18 (31.08.95) describes a device for recovering a clock by blocking the modes of a ring fiber laser. This article proposes to send half of the incident signal in a fiber laser doped with erbium in a ring, so as to ensure a mode lock of the laser at the frequency of the incident signal. The clock signal is extracted from the ring laser by a coupler, and is used in a Kerr gate acting as a modulator. To ensure the synchronization of the phases of the clock and of the signal to be regenerated, this article proposes to use a system of mechanical elongation of the fiber at the output of the ring laser.

T. Ono et al., OFC '94 Technical Digest, ThM3, p. 233 and s. proposes a clock recovery circuit using a Fabry-Perot laser diode, which is locked in lateral mode by the incident signal to supply a clock. This device involves a bit frequency corresponding exactly to the frequency of the lateral mode of the laser.

K. Smith et al., Electronics Letters, vol. 28 n ° 19, p. 1814 and following, proposes another clock recovery circuit, in which the transmission fiber and a cavity laser share a non-linear optical modulator, so that the signal of the transmission fiber modulates the light of the cavity laser, by phase transmodulation (XPM or "cross phase modulation"). The laser cavity undergoes a mode lock at the frequency of the solitons; a coupler extracts a clock signal from the cavity.

DM Patrick et al., Electronics Letters, vol. 30 No. 2 describes a clock recovery circuit of the same kind, in which the incident signal modulates the light of an erbium-doped fiber ring laser in a traveling wave semiconductor amplifier (TW- SCA). The clock signal is extracted from the cavity by a coupler. These three articles do not mention any particular method for ensuring the modulation of the signal by the recovered clock.

The problem with these different all-optical clock recovery circuits is that synchronization of the phases of the clock and of the solitons in line is difficult to achieve in all-optics other than by mechanical delay lines, as in the assembly. from W. A. Pender et al.

The present invention provides an original and simple solution to the problem of clock recovery and synchronous modulation of solitons. It makes it possible to avoid the problem of synchronization of the phases of the clock and of the signal to be modulated, which arises in the prior art: in fact, the invention combines in a single device the synchronous modulator and the clock recovery . The invention also has the advantage of being very compact.

More specifically, the invention proposes a regenerator for a soliton pulse transmission system, comprising a cavity laser in which a modulator is inserted, means for coupling the input signal to be regenerated to the input of the modulator so as to ensure blocking. mode of the laser, and means for coupling the regenerated signal at the output of the modulator.

The laser is advantageously a Fabry Perot cavity laser or a ring fiber laser. Dispersive means can be provided in the laser, such as a section of dispersive fiber.

The modulator can be a phase modulator or an intensity modulator. It can for example include a non-linear loop mirror, or a semiconductor amplifier, preferably with traveling waves.

The modulator may include a section of dispersive fiber providing modulation by the Kerr effect.

In one embodiment, the regenerator comprises an amplifier of the input signal to be regenerated, upstream of the modulator.

Advantageously, the regenerator has a filter filtering the regenerated signal. In the case of a cavity laser, the input mirror of the Fabry Perot cavity is preferably selective in wavelength, so as to separate the regenerated signal from the laser signal.

The invention also relates to an optical transmission system, comprising at least one such regenerator. Finally, it relates to a method for regenerating a soliton signal, comprising: - the creation of a clock by mode blocking of a cavity laser in which a modulator is inserted, by modulating the laser signal by the solitons signal to be regenerated;

- the extraction at the output of this modulator of the solitons signal regenerated by clock modulation.

The soliton signal can perform phase modulation of the laser signal in the modulator and / or intensity modulation of the laser signal.

The clock can perform in the modulator a phase modulation of the solitons signal, and / or an intensity modulation of the solitons signal. In one embodiment, the method comprises spreading the clock signal in the cavity laser.

It is also possible to provide a filtering of the signal extracted at the output of the modulator, so as to separate the regenerated signal from the clock.

Other characteristics and advantages of the invention will appear on reading the following description of embodiments of the invention, given by way of example and with reference to the appended drawings which show:

- Figure 1 a schematic representation of the principle of a regenerator according to the invention;

- Figure 2, below the shape of the soliton signal to be regenerated, and above, the shape of the clock signal in the cavity laser;

- Figure 3 a schematic representation of a regenerator according to a first embodiment of the invention;

- Figure 4 a schematic representation of a regenerator according to a second embodiment of the invention; - Figure 5 a schematic representation of a regenerator according to a third embodiment of the invention;

- Figure 6 a schematic representation of a regenerator according to a fourth embodiment of the invention;

- Figure 7 a schematic representation of a regenerator according to a fifth embodiment of the invention.

The invention proposes to associate in the same circuit the clock recovery function, by blocking modes of a laser, and the modulation function. To this end, it is based on the surprising observation that the modulator used to block the mode of the laser by the soliton signal to be regenerated at the same time modulates this soliton signal by the light of the laser. In a circuit formed by a cavity laser in which is inserted a modulator, and means for coupling the input signal to be regenerated at the input of the modulator, the invention therefore proposes to provide, not means for extracting a clock signal constituted by the light of the laser, but means for extract, at the output of the modulator, the regenerated signal. As in the known devices of the prior art, the soliton signal to be regenerated is coupled in the modulator inserted in the cavity laser, so as to ensure active blocking of the laser modes, at the rate of the bits of the received signal. This makes it possible to generate in the ring laser a clock signal at the bit frequency of the soliton signal to be regenerated. But this clock signal circulates in the cavity of the laser, and therefore in the modulator which is inserted there, and interacts in the modulator with the incident soliton signal to modulate it at the rate of the clock. It is obvious that the synchronization of the clock signal and of the soliton signal to be modulated is ensured automatically by the very structure of the regenerator of the invention, without it being necessary to provide mechanisms such as delay lines or the like.

The length of the cavity is adjusted in a manner known per se to ensure that the free spectral interval of the cavity is an integer sub-multiple of the bit frequency of the soliton signal. The invention therefore proposes to extract from the modulator the soliton signal used for blocking laser modes, which constitutes a regenerated signal.

Figure 1 shows a schematic representation of the principle of a regenerator according to the invention, operating according to this principle. The regenerator of FIG. 1 comprises a cavity laser, in this case a ring fiber laser, comprising a fiber 1, forming a ring; on the fiber are arranged an insulator 2, a filter 3, and means 4 for optical signal amplification. The arrow of the modulator shows the direction of light circulation in the cavity. In the cavity is inserted a modulator 5, which receives as input not only the signal from the fiber 1, on an input 6, but also, on an input 7, the soliton signal to be regenerated. At the output, the modulator is connected to the fiber 1, on an output 8, so as to allow an active mode blocking of the laser by the soliton signal. The modulator also supplies on an output 9 the regenerated soliton signal.

Dispersive means 10 can be provided on the fiber 1, which make it possible to control the width of the pulses of the clock signal in the fiber, and in particular to widen the clock pulses. This improves the correction of the time jitter of the soliton signal by modulation by the clock signal. We can use as dispersion means a fiber with a high dispersion coefficient, or a Bragg grating.

The modulator 5 of FIG. 1 can be a phase modulator, an intensity modulator, or a combined phase and intensity modulator, as explained in the various embodiments of FIGS. 3 to 6.

FIG. 2 shows, at the bottom, the shape of the soliton signals received on the input 7 of the modulator 5, and at the top the shape of the corresponding clock signal obtained when the modulator is a semiconductor amplifier as in the embodiment of Figure 5; this signal flows through the cavity laser. It is noted that the active mode blocking of the laser makes it possible to recover a quality clock signal.

FIG. 3 shows a schematic representation of the modulator of a regenerator according to a first embodiment of the invention. FIG. 3 does not show the fiber ring 1, the insulator 2, the filter 3, the means 4 for optical amplification of the signal, nor the dispersive means 10. In the embodiment of the Figure 3, the modulator 5 is formed of a nonlinear loop mirror (NOLM). This is formed from a length of highly dispersive fiber 20, for example a length of the order of 10 km. The two ends of the fiber 20 are coupled through a 2/2 coupler 21, and are connected to the fiber 1 so as to form the inlet 6 and the outlet 8 of the modulator. On the fiber loop 20 are provided two 2/2 couplers 22 and 23, arranged symmetrically with respect to the coupler 21, and in opposite directions. The coupler 22 couples in the NOLM the soliton signal to be regenerated, and constitutes the input 7 of the modulator. The coupler 23 extracts from the modulator the regenerated soliton signal and the clock. Downstream of the coupler 23 is provided a low pass filter 24 which blocks the clock. The output of the filter 24 constitutes the output 9 of the modulator 5.

The operation of the NOLM in FIG. 3 is as follows. The soliton signal induces a mode blockage of the laser, by intensity and phase modulation on the signal circulating in the fiber of the NOLM. The signal from fiber 1 is coupled in the NOLM by coupler 21, where it is separated into two signals propagating in opposite directions, as indicated by arrows 26 and 27. It is recombined at coupler 21, and is reflected at output 6. The signal propagating in the direction of arrow 26 is intensity-modulated by the incident soliton signal introduced into the NOLM by the coupler 22. At the output of the coupler 23, the regenerated soliton signal and the signal are obtained. clock. The filter 24 cuts the clock signal. Furthermore, the clock signal provides modulation synchronous phase of the incident soliton signal, and thus ensures the correction of its temporal jitter. For more details on the operation of the NOLM as a modulator, reference may be made to the article by S. Bigo et al., Electronics Letters, vol. 31 no.21 (1995). In the assembly of FIG. 3, an amplifier 25 can also be provided, upstream of the coupler 22, for the incident soliton signals.

The arrangement of FIG. 3 is particularly advantageous, in that the mode biocage is ensured by an intensity modulation, while the modulation of the soliton signal by the clock is ensured by a phase modulation. It is therefore possible to independently adjust the depth of modulation of the laser signal by the soliton signal, and the depth of modulation of the soliton signal by the laser signal. The adjustment of the phase modulation depth can be carried out by varying the power of the clock signal in the cavity; the intensity modulation depth can be adjusted by adjusting the coupling rate of the input coupler, for a given power of the clock signal in the cavity.

Thus, it ensures the mode blocking of the laser and a good regeneration which does not degrade the solitons.

In the embodiment of FIG. 3, the NOLM can be used in its conventional mirror configuration, with an asymmetrical coupler. You can also set the NOLM in "transmitter" mode, for example by using polarization controllers in the cavity, or by using a birefringent blade with correctly aligned axes. This makes the modulator non-blocking for the clock in the presence of zeros, so as to avoid any loss of the clock during the zeros of the soliton signal. In this case you can use a balanced input coupler. The power efficiency is then maximum.

Figure 4 shows a schematic representation of a regenerator according to a second embodiment of the invention; as in FIG. 3, FIG. 4 shows only the modulator 5. The modulation, in the embodiment of FIG. 4, is a phase modulation by Kerr effect in a section of fiber with high dispersion. _ For this purpose, the modulator 5 comprises a section of fiber 30 with high dispersion, a length which can be of the order of 10 km, and which is connected to the fiber 1 to form the ring of the laser, so forming input 6 and output 8 of the modulator. _ one end of the fiber 30, a coupler 31 couples the soliton signal to be regenerated in the fiber; at the other end of the fiber 30, a coupler 32 couples the soliton signal and the clock to a filter 33. The filter 33 blocks the clock signal, and the output of the filter 33 constitutes the output 9 of the modulator 5. Upstream of the coupler 31, one can provide, as in the case of FIG. 3, an amplifier whose input constitutes the input 7 of the modulator. The assembly of Figure 4 operates as follows. The soliton signal induces a mode blockage of the laser, by phase modulation by Kerr effect in the fiber 30. The clock signal provides synchronous phase modulation of the incident soliton signal. The filter 33 blocks the clock signal.

In the assembly of FIG. 4, the modulation of the soliton signal, like the blocking of mode are carried out by the same modulator. The modulation depth is chosen to be large enough to guarantee that the modes of the ring laser are blocked; it is however maintained at a level such that the modulation by the light of the laser ensures correct regeneration, and does not degrade the solitons.

Figure 5 shows a schematic representation of a regenerator according to a third embodiment of the invention; as in FIGS. 3 and 4, FIG. 5 shows only the modulator 5. The modulation, in the embodiment of FIG. 5, is an intensity and phase modulation in a semi-amplifier conductor, for example a traveling wave semiconductor amplifier (TW-SCA). The modulator of FIG. 5 therefore comprises a traveling wave semiconductor amplifier 40, the input and output of which are connected to the ends of the fiber 1. Upstream of the TW-SCA 40, a coupler 41 couples in the fiber 1 the incident soliton signal. Downstream of the TW-SCA 40, a coupler 42 supplies the modulated soliton signal and the clock signal. The coupler 42 is connected to a filter 43 which blocks the clock signal. The output of the filter 43 constitutes the output 9 of the modulator 5. Upstream of the coupler 41, one can provide, as in the case of FIGS. 3 and 4, an amplifier 44 whose input constitutes the input 7 of the modulator.

The assembly of Figure 5 works as follows. The soliton signal induces a mode lock of the laser, by phase and intensity modulation in the TW-SCA 40. The clock signal provides synchronous phase and intensity modulation of the incident soliton signal. The filter 43 blocks the clock signal. The modulation depth is adjusted as shown in Figure 4.

Figure 6 shows a schematic representation of a regenerator according to a fourth embodiment of the invention; as in the previous figures, only the modulator 5 is shown in FIG. 6. The device of FIG. 6 corresponds to that of FIG. 3, and the same reference numbers are used. However, the device of FIG. 6 further comprises in the NOLM a traveling wave semiconductor amplifier 50, between the couplers 22 and 23. This device makes it possible to limit the size of the NOLM, by replacing the propagation fiber by a semiconductor modulator. The operation of the device in FIG. 6 is similar to that of the device in FIG. 3. In particular, the respective modulation depths of the clock and of the soliton signal can be independently adjusted.

Figure 7 shows a schematic representation of a regenerator according to a fifth embodiment of the invention. In the embodiment of Figure 7, is used as a cavity not by a ring, but a Fabry Perot cavity. A modulator is inserted into the cavity, for example a semiconductor amplifier, such as a traveling wave semiconductor amplifier.

The device of FIG. 7 therefore comprises a circulator 55 with three terminals, which receives on an input on a first input terminal 56 the soliton signal to be regenerated. This signal is output at a second output and input terminal 57 and is transmitted to a Fabry Perot cavity 58 formed by two mirrors 59, 60; the first mirror 59 is a selective wavelength mirror. In the cavity 58 is arranged a modulator 61, typically a semiconductor amplifier, preferably with traveling waves. The third terminal 62 of the circulator 55 is an output terminal which supplies the signal received as input on the second terminal 57.

The operation of the device of Figure 7 is as follows. The soliton signal to be regenerated is received on the first terminal 56 of the selector 55, and is transmitted through the second terminal 57 to the cavity 58. The soliton signal enters the cavity 58, and modulates in phase and in intensity in the modulator 61 laser light, so as to ensure a mode lock. The clock signal modulates in the modulator 61 the soliton signal. The wavelength selective mirror 59 blocks the clock signal and lets the modulated soliton signal pass. This signal then arrives at the input on the second terminal 57 of the circulator, and is transmitted at the output to the third terminal 58.

The embodiment of Figure 7 has the advantage of extreme compactness. As in the other embodiments, an amplifier can be provided upstream of the circulator 55. One could also, instead of the circulator 55, use a coupler. One could also in the embodiment of Figure 7 provide in the cavity of the dispersive means. Of course, the present invention is not limited to the examples and embodiments described and shown, but it is susceptible of numerous variants accessible to those skilled in the art. For example, it is clear that the assembly of FIGS. 3 and 4 is symmetrical, and that the position of the transmission fiber and the laser fiber could be reversed at the coupler 31 and 32, or 41 and 42.

Claims

1. A regenerator for soliton impulse transmission system, comprising a cavity laser in which a modulator (5, 61) is inserted, means for coupling the input signal to be regenerated at the input of the modulator so as to ensure blocking mode of the laser, and means for coupling the regenerated signal at the output of the modulator.

2. A regenerator according to claim 1, characterized in that the laser is a Fabry Perot cavity laser (58).

3. A regenerator according to claim 1, characterized in that the laser is a ring fiber laser.

4. A regenerator according to claim 2 or 3, characterized in that the laser comprises dispersive means (10), such as a section of dispersive fiber.

5.- A regenerator according to one of claims 1 to 4, characterized in that the modulator is a phase modulator.

6. A regenerator according to one of claims 1 to 5, characterized in that the modulator is an intensity modulator.

7. A regenerator according to one of claims 1 to 6, characterized in that the modulator comprises a non-linear loop mirror (20).

8.- A regenerator according to one of claims 1 to 7, characterized in that the modulator comprises a semiconductor amplifier (40, 50, 61), preferably with traveling waves.

9. A regenerator according to one of claims 1 to 5, characterized in that the modulator comprises a section of dispersive fiber (30) providing modulation by the Kerr effect.

10.- A regenerator according to one of claims 1 to 9, characterized by an amplifier (25, 34, 44) of the input signal to be regenerated, upstream of the modulator (5).

11. A regenerator according to one of claims 1 to 10, characterized by a filter (24, 33, 43) filtering the regenerated signal.

12. A regenerator according to one of claims 2 to 11, characterized in that the input mirror of the Fabry Perot cavity is selective in wavelength, so as to separate the regenerated signal from the laser signal.

13. Optical transmission system, comprising at least one regenerator according to one of claims 1 to 12.

14.- A process for regenerating a soliton signal, comprising:

- the creation of a clock by mode blocking of a cavity laser in which a modulator (5) is inserted, by modulating the laser signal by the solitons signal to be regenerated;

- the extraction at the output of this modulator (5) of the solitons signal regenerated by modulation by the clock.

15.- A method according to claim 14, characterized in that the soliton signal performs in the modulator (5) a phase modulation of the laser signal.

16.- A method according to claim 14 or 15, characterized in that the soliton signal performs in the modulator (5) an intensity modulation of the laser signal.

17.- A method according to one of claims 14 to 16, characterized in that the clock performs in the modulator (5) a phase modulation of the soliton signal.

18.- A method according to one of claims 14 to 17, characterized in that the clock performs in the modulator (5) an intensity modulation of the solitons signal.

19.- A method according to one of claims 14 to 18, characterized by spreading the clock signal in the cavity laser.

20.- A method according to one of claims 14 to 19, characterized by filtering the signal extracted at the output of the modulator, so as to separate the regenerated signal from the clock.