EP1142168A1

EP1142168A1 - Device for generating carriers for wavelength-division multiplexing optical transmission system with rz signals

Info

Publication number: EP1142168A1
Application number: EP00968017A
Authority: EP
Inventors: Olivier Leclerc; Patrick Brindel
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel CIT SA; Alcatel Lucent SAS
Priority date: 1999-10-14
Filing date: 2000-10-12
Publication date: 2001-10-10
Also published as: WO2001028138A1; JP2003511966A; FR2803399B1; CA2355355A1; FR2803399A1

Abstract

The invention concerns the generation of optical carriers in optical transmission systems with RZ signals. It concerns a device for generating RZ signals, comprising optical sources delivering wavelength multiplexed input signals, an optical clock delivering a clock signal, and an optical gate receiving on its input the multiplexed optical signals and on its control input the clock signal. The optical gate delivers in output RZ signals. The optical gate is a loop non-linear mirror with one or two optical amplifiers, and one or two control inputs for injecting into the loop the clock signal. The invention enables to generate quality RZ carriers, from a single optical clock.

Description

CARRIER GENERATING DEVICE FOR RZ SIGNAL WAVELENGTH MULTIPLEXED OPTICAL TRANSMISSION SYSTEM

The present invention relates to the field of transmission by optical fibers, and in particular optical transmission systems with wavelength multiplexing. It more particularly relates to signal sources for transmission systems with RZ (return to zero) signals.

The transmission of signals in wavelength multiplexing involves generating optical signals at different wavelengths, to transport different information The generation of optical carriers is called the emission of light, followed by the shaping of pulses This generation is followed by a coding of the pulses before their transmission

The simplest and most widespread solution for generating optical carriers for a wavelength-division multiplex transmission system consists in using one light source per channel, in formatting the pulses at bit frequency, for each of the channels. , and finally to code the signals of the different channels of the wavelength comb separately This solution involves having one element of each type - source, pulse shaping device and coding device - for each channel The cost of the solution therefore increases with the number of channels in the multiplex, the cost is higher the higher the bit frequency - 40 Gbit / s for example - and the specific components are rare, as in the case RZ pulses

Y Takushima, 1 0-GHz, over 20-channel multiwavelength source source by slicing super-continuum generated in normal-dispersion fiber, IEEE Photonics Technology Letters, vol 1 1 no. 3, March 1 999, proposes to generate a super-continuum in a fiber with normal dispersion by a semiconductor laser locked in mode, and to cut this super-continuum into channels thanks to a waveguide network (in English AWG or arrayed waveguide grating) In this device, the pulse generated by the laser is spectrally widened by a non-linear effect in the fiber to form a broadband signal which is substantially continuous over the entire bandwidth and called the super continuum. This device provides a signal to the properties spectro-temporal which make it difficult to use for subsequent propagation.

B. Mikkelsen et al., All-optical wavelength converter scheme for high speed RZ signal formats, Electronics Letters, vol. 33 no. 25, December 1 997, proposes a wavelength conversion device for RZ signals, which consists of two semiconductor optical amplifiers (SOA), arranged in a Mach-Zender interferometer which is monolithically integrated. The conversion is carried out by differential cross-phase modulation between the two amplifiers. This device is used in this document as a wavelength converter. This device has losses of the order of 20 dB and would therefore be difficult to use to generate a carrier having the spectro-temporal qualities necessary for a transmission at wavelength multiplexing at high speed, of the 40 Gbit / s per channel type. or more.

Furthermore, FR-A-2 742 887 (folio 21 069) describes a non-linear loop mirror (in English NOLM or nonlinear optical loop mirror) with two control inputs, which is used as a regenerator for soliton signals. This document also describes the operating principle of the non-linear loop mirror. S. Bigo et al., IEE Electronics Letters, vol. 31 no 2, p. 21 91-21 93, or S. Bigo et al., Optics Letters, vol. 21 no.1 8, p. 1,463-1,465 describe the use of a non-linear optical mirror as a phase modulator.

There is therefore a need for a simple and efficient solution for generating optical carriers for wavelength division multiplex transmission systems.

The invention provides a solution to this need; it allows simple generation of carriers from a single RZ clock.

More specifically, the invention provides a device for generating RZ signals, comprising a plurality of optical sources each supplying an optical input signal, an optical clock supplying a clock signal, and an optical gate receiving the signals at an input. optical input and on a control input the clock signal and providing at output for each input signal an RZ signal. Preferably, the optical gate has losses of less than 15 dB.

Each optical source can provide an input signal in the form of a continuous wave, or an NRZ coded input signal. In one embodiment, the optical door comprises a non-linear loop mirror with at least one optical amplifier. One can use a semiconductor optical amplifier, or two optical amplifiers.

In another embodiment, the optical door comprises a loop non-linear mirror with at least one means for generating non-linearity, such as a non-linear fiber.

It is advantageous for the non-linear mirror to include a control input by which the clock signal is injected into the loop of the non-linear mirror. It is also possible to provide two control inputs by which the clock signal is injected into the loop of the non-linear mirror in two directions of propagation.

In yet another embodiment, the optical door comprises a Mach Zender interferometer in which at least one optical amplifier is arranged. In all cases, it is possible to provide at the output of the optical gate a filter adapted to filter the clock signal, or a demultiplexer.

The invention also provides a method for generating RZ signals, comprising the steps of applying a plurality of input signals each supplied by an optical source to the input of an optical gate; application to the control input of the optical door of a clock signal supplied by an optical clock. Preferably, the optical gate has losses of less than 15 dB. Each input signal supplied by an optical source can be a continuous wave or an NRZ coded signal. The input signal can also be multiplexed in wavelengths.

In one embodiment, the optical gate comprises a nonlinear mirror in a loop with at least one optical amplifier, typically a semiconductor optical amplifier. It is also possible to provide two optical amplifiers.

In another embodiment, the optical door comprises a loop non-linear mirror with at least one means for generating non-linearity, such as a non-linear fiber. It is possible that the non-linear mirror comprises two control inputs, and that the step of applying the clock signal comprises applying the clock signal to the two control inputs.

In yet another embodiment, the optical gate comprises a Mach Zender interferometer in which is arranged at least one optical amplifier.

Advantageously, the method comprises a step of filtering the clock signal at the output of the optical gate, or a step of demultiplexing of the signals supplied by the optical gate. 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 accompanying drawings, which show FIG. 1, a schematic representation of 'a device for generating non-coded RZ signals; - Figure 2, a schematic representation of a device for generating coded RZ signals; FIG. 3, a schematic representation of an embodiment for generating a carrier on a single wavelength, useful for understanding the invention; - Figure 4, a schematic representation of another embodiment of the invention, in which the optical door comprises a non-linear loop mirror; FIG. 5, a schematic representation of yet another embodiment of the invention, in which the optical door comprises a non-linear loop mirror with two heads; FIG. 6, a schematic representation of yet another embodiment of the invention, in which the optical door comprises an interferometer. The invention proposes, in order to generate several carriers, to switch continuous waves using an optical clock RZ. The use of a single optical clock makes it possible to use a good quality clock, and makes it possible to obtain output signals which are also of good quality.

FIG. 1 shows a schematic representation of a device for generating non-coded RZ signals; the device of Figure 1 comprises a series of continuous wave sources at different wavelengths λ _1; λ ₂ , ... λ _n , and a multiplexer 2 for multiplexing the continuous waves coming from the different sources. One can use as sources all the sources known per se, and in particular semiconductor lasers, or diodes; one of the commercially available components can be used as a multiplexer. Continuous waves at the various wavelengths of the multiplex are obtained at the output of the multiplexer.

Multiplexed continuous waves are applied to an input of an optical door 4. This is controlled by a control input to which an optical clock is applied, as symbolized in FIG. 1 by the arrow 6. The output of the optical gate of the non-coded carriers, in other words for each wavelength, a series of pulses of the same shape as the optical clock. Preferably, the optical gate has losses of less than 15 dB, which ensures a sufficient signal-to-noise ratio at the output to allow transmission by wavelength multiplexing of the generated channels.

As the invention in fact proposes to replicate the same optical clock over different wavelengths, it makes it possible to use only one good quality clock. It is ensured that the carriers generated are of equivalent quality, and have the spectrotemporal characteristics allowing good subsequent propagation.

In the example of FIG. 1, the signals supplied are not coded. FIG. 2 shows on the contrary a schematic representation of a device for generating coded RZ signals. The device comprises, like the device in FIG. 1, comprises a series of sources at different wavelengths λ _1; λ ₂ , ... λ _n ; these sources are, unlike FIG. 1, NRZ coded sources, and not sources producing continuous waves. One can for example use sources such as diodes, with a modulated emission current, or any other NRZ source known per se.

As in the device in FIG. 1, the signals from the sources at the different wavelengths are multiplexed by a multiplexer 2. Multiplex NRZ signals are obtained at the output of the multiplexer.

These signals are applied to an input of an optical door 4. This is controlled by a control input to which a clock is applied optical, as symbolized in FIG. 2 by the arrow 6. At the output of the optical gate, coded carriers are obtained, in other words for each of the wavelengths, a series of pulses of the same shape as the optical clock, and which correspond to the high level of NRZ signals. The embodiment of FIG. 2 avoids subsequent coding of the signals.

It has the same advantages as the embodiment of FIG. 1, and in particular a good quality of the RZ signals.

FIG. 3 shows a schematic representation of an embodiment for generating a carrier on a single wavelength, which is useful for understanding the embodiment of the invention represented in FIG. 4.

The example of FIG. 3 uses a nonlinear loop mirror (NOLM) 1 0. This consists of a fiber loop 1 2 connected to two inputs of a coupler 1 4 with four inputs. The coupler is for example a 50/50 coupler, or more generally a η / 1 -η coupler. The two other inputs of the coupler are respectively the input and the output of the non-linear mirror. The mirror has a head or coupler 1 6 for injecting into the fiber loop a control signal, which in the device of Figure 3 is the reference clock signal.

Finally, there is a semiconductor optical amplifier 18 on the fiber loop. This SOA is offset on the fiber, that is to say that it is not equidistant from the two ends of the fiber. On the contrary, we denote by Δt the offset between the position of the SOA and the point of symmetry of the mirror. The SOA can be of the type marketed by Alcatel Optronics.

At the output of the mirror is provided a filter 1 9 for rejection of the clock signal, for example a Fabry Perot filter centered on the wavelength of the signal. . The operation of the device of Figure 3 is as follows. A continuous signal is injected into the non-linear loop mirror, through the input of the coupler; it is separated by the coupler into two signals which propagate in the loop in opposite directions. The signals propagating in the two opposite directions cross the fiber, are recombined in the coupler, and leave the nonlinear mirror. Furthermore, the coupler 1 6 injects into the fiber loop a reference clock signal; this reference signal excites the semiconductor optical amplifier. Signals that propagate in reverse directions in the fiber loop are modulated by the semiconductor amplifier; the phase modulation crossed on the two signals leads to an optical clock replication on the continuous signal initially ιn | ecté The position of the optical amplifier in the loop makes it possible to control the phase shift between the two signals which propagate in directions opposite, and therefore the efficiency of cross-phase modulation

There is thus obtained at the output of the non-linear mirror a signal at the wavelength of the continuous input signal, on which the output clock is replicated. The filter eliminates from the output signal the clock used to control the optical door

One could ιn | ect in the device of FIG. 3 a signal signal NRZ instead of a continuous wave In this case, as explained with reference to FIG. 2, one would obtain at the output of the device a coded carrier

Figure 4 shows a schematic representation of an embodiment of the invention for generating a plurality of carriers The device of Figure 4 has a configuration similar to that of Figure 1 or 2, except that the optical door is as in the case of FIG. 3, a non-linear loop mirror. The device therefore comprises a plurality of sources at different wavelengths of carriers to be emitted; these sources can be sources of continuous waves, as represented in the figure, but they could also be sources coded NRZ, the various sources are ιn | ected in a multiplexer 20, whose output is connected to an input of a non-linear loop mirror 22. This mirror, like that of FIG. 3, comprises a coupler 24, a fiber 25 connected to two inputs of the coupler, a coupler 26 for the input of a control signal, and a semiconductor optical amplifier 28

At the output of the mirror is provided in the embodiment shown in the figure a demultiplexer 30 for demultiplexing the different carriers obtained, and allowing coding of the signals

The operation of the device in FIG. 4 is similar to that of the device in FIG. 3. the clock ιn | ected in the non-linear mirror is replicated on the various continuous waves, and a plurality of carriers at different wavelengths is obtained at the output of the mirror. The multiplexer allows to separate the different wavelengths for further coding. In addition, in this embodiment, it ensures rejection of the clock.

FIG. 5 shows a schematic representation of yet another embodiment of the invention, in which the optical door is formed of a non-linear mirror with two heads, like that described in the patent application FR-A- 2,742,887. The device of FIG. 5 is similar to that of FIG. 4, however, the non-linear mirror has two heads or control inputs; in addition the non-linear mirror has two optical amplifiers 28 and 38. The device therefore has a coupler 32, to duplicate the control signal to be injected. The first outing of

1 0 coupler 32 is connected to the first coupler for injecting the control signal, which is numbered 26 as in FIG. 4. The second output of coupler 32 is connected to a second coupler 34 for injecting the control signal, which injects the signal it receives in the fiber of the non-linear mirror, in a direction opposite to that of the first injection coupler. Is provided between coupler 32 and the second coupler

1 5 34 a phase control device, for example a phase shifter 36. This phase shifter makes it possible to control the relative phase of the control signals injected into the two heads of the non-linear mirror. When the two injection couplers have, as shown in FIG. 5, symmetrical positions in the non-linear mirror, the phase control device ensures that the phases of the signals

20 injected are identical. More generally, if the positions of the two couplers are not symmetrical, the phase control device ensures that the sum of the phases is zero near the input coupler of the non-linear mirror.

We note as in the previous figures Δt the offset between the position of the first amplifier and the point of symmetry of the mirror, and Δt 'the offset between the

25 position of the second amplifier and the point of symmetry of the mirror. The amplifiers are arranged on either side of the point of symmetry of the mirror. The symmetry of the device from the point of view of co-propagating and counter-propagating waves makes it possible to average the imperfections of the optical amplifier (in the case of FIGS. 3 and 4) or of the two optical amplifiers (in the case of FIG. 5 ). We can

30 so better control the pulses than in the case where the optical gate is formed by a Mach-Zender interferometer.

The operation of the device of FIG. 5 is identical to that of FIG. 4. The non-linear mirror functions as described in the patent application above the two series of signals at different wavelengths ιn | ectected in the non-linear mirror are modulated by the two amplifiers, and we obtain at the output of the non-linear mirror of the carriers on which the signal is reproduced clock The modulation by the second amplifier is exerted mainly on the signals propagating anti-clockwise, which allows to improve the efficiency of the mirror, by acting on the signals propagating in both directions

FIG. 6 also shows an embodiment of the invention, in which the optical door consists of a Mach Zender interferometer 40 with an optical amplifier 42 The operation of the device of FIG. 4 is identical to that of FIG. 3

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. Thus, the embodiments of FIGS. 4 and 5 have a demultiplexer to separate the different carriers and allow their coding, it would of course be possible to carry out other processing on all of the signals before their coding, or to transmit the carriers before coding them The position of the couplers allowing the signal input control in non-light mirrors can be different from that shown in the figures We can in the case of Figure 5 a | uster the fiber length between the coupler 32 and the ιn | ectιon couplers to fix the relative phases signals in the two heads of the non-linear mirror

The embodiments of FIGS. 4 and 5 use an optical semiconductor amplifier in the non-linear mirror as means for generating non-linearities. Another device could also be used to generate these non-linearities, and for example strongly or weakly non-linear fiber An SMF fiber or a chalcogenide fiber are suitable The accumulated non-linearity depends on the fiber length This solution is less advantageous in that it can generate instabilities of mechanical origin, especially if the length fiber is important

Claims

1. A device for generating RZ signals, comprising a plurality of optical sources each providing an optical input signal, an optical clock providing a clock signal, and an optical gate (4) receiving on an input the optical input signals and on a control input (6) the clock signal and providing at output for each input signal an RZ signal.

2. The device of claim 1, characterized in that the optical door (4) has losses of less than 1 5 dB.

3. The device of claim 1 or 2, characterized in that each optical source provides an input signal in the form of a continuous wave.

4. The device of claim 1 or 2, characterized in that each optical source provides an NRZ coded input signal.

5. The device of one of claims 1 to 4, characterized in that it comprises a multiplexer (2) for multiplexing the input signals.

6. The device of one of claims 1 to 5, characterized in that the optical door comprises a non-linear loop mirror (1 0.22) with at least one optical amplifier (1 8, 28, 38).

7. The device of claim 6, characterized in that the optical amplifier is a semiconductor optical amplifier.

8. The device of claim 6 or 7, characterized in that the non-linear mirror (22) has two optical amplifiers (28, 38).

9. The device of one of claims 1 to 5, characterized in that the optical door comprises a non-linear loop mirror with at least one means for generating non-linearity, such as a non-linear fiber.

10. The device of one of claims 6 to 9, characterized in that the non-linear mirror (1 0, 22) comprises a control input (1 6, 26) by which the clock signal is injected into the loop of the non-linear mirror.

1 1. The device of one of claims 6 to 1 0, characterized in that the non-linear mirror comprises two control inputs (26, 34) by which the clock signal is injected into the loop of the non-linear mirror in two directions of propagation

12. The device of one of claims 1 to 5, characterized in that the optical door comprises a Mach Zender interferometer (40) in which is arranged at least one optical amplifier (42).

13. The device of one of claims 1 to 1 2, characterized in that it has at the output of the optical door a filter (1 9) adapted to filter the clock signal.

14. The device of one of claims 5 to 1 3, characterized in that it has at the output of the optical door a demultiplexer (30).

1 5. A method for generating RZ signals, comprising the steps of - applying a plurality of input signals each supplied by an optical source to the input of an optical gate (4); - application to the control input (6) of the optical door of a clock signal supplied by an optical clock.

16. The method of claim 1 6, characterized in that the optical gate has losses of less than 1 5 dB

1 7. The method of claim 1 6 or 1 7, characterized in that the input signals are continuous waves.

1 8. The method of claim 1 6 or 1 7, characterized in that the input signals are coded NRZ.

5 1 9. The method of one of claims 1 5 to 1 8, characterized in that the input signals are multiplexed in wavelengths.

20. The method of one of claims 1 5 to 1 9, characterized in that the optical door comprises a non-linear loop mirror (1 0, 22) with at least one optical amplifier (1 8, 28, 38 ).

1 0 21. The method of claim 20, characterized in that the optical amplifier is a semiconductor optical amplifier.

22. The method of claim 20 or 21, characterized in that the non-linear mirror has two optical amplifiers (28, 38).

23. The method of one of claims 1 5 to 1 9, characterized in that the optical door 1 5 comprises a non-linear loop mirror with at least one means for generating non-linearity, such as a non-linear fiber. linear.

24. The method of one of claims 20 to 23, characterized in that the non-linear mirror comprises two control inputs (26, 34), and in that the step of applying the clock signal comprises the application of

20 clock signal at the two control inputs.

25. The method of one of claims 1 5 to 1 9, characterized in that the optical door comprises a Mach Zender interferometer (40) in which is disposed at least one optical amplifier (42). 1

26. The method of one of claims 1 5 to 25, characterized in that it further comprises a step of filtering the clock signal at the output of the optical gate.

27. The method of one of claims 1 9 to 26, characterized in that it further comprises a step of demultiplexing of the signals supplied by the optical gate.