Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0223—Conversion to or from optical TDM
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0221—Power control, e.g. to keep the total optical power constant

Definitions

• the present invention relates to an optical device, and a method, for converting WDM signals, comprising simultaneous pulses carried by distinct wavelengths, into an OTDM signal, the components of which are carried by a single wavelength and offset temporally, and vice versa.
• the invention is in the field of optical telecommunications and more particularly telecommunications over long distances.
• the increase in speed in the transmission channels is inevitable because it allows a reduction in the size of the end equipment and above all a reduction in their cost.
• the transport networks of telecommunications operators should see the deployment of the first WDM equipment ("Wavelength Division Multiplexing" in Anglo-Saxon literature) operating at 40 Gbit / s per wavelength and, at longer term, at 160 Gbits / s per wavelength.
• the OTDM / WDM conversion consists in doing optical time demultiplexing then in wavelength conversion.
• Optical temporal demultiplexing is carried out for example by using cross-phase modulation in a fiber. This technology is however very complex to implement.
• Optical temporal demultiplexing can also be achieved by means of non-linear optical mirrors using Mach-Zehnder, Michelson or Sagnac interferometers. Non-linear optical mirrors, however, have the disadvantage of being unstable, their stability depending on the temperature.
• the wavelength conversion for its part, is carried out using SOA semiconductor optical amplifiers ("Semiconductor Optical Amplifier" in English terminology).
• a laser, placed behind the SOA, provides the wavelength into which the signal must be converted.
• this solution involves the use of as many SOA and lasers as there are wavelength conversions to be carried out, so that the cost price of this solution remains very high and does not allow implantation. on a large scale, in networks that are currently booming.
• SOAs are not completely transparent to the data rate and distortions can appear and affect the signal.
• WDM / OTDM conversion consists in converting the length of each WDM signal into a single wavelength and then in optical time multiplexing.
• the wavelength conversion again requires the use of as many SOA and lasers as there are WDM signals, so that the cost price of this solution is very high.
• the solutions which have just been presented for the two types of OTDM / WDM and WDM / OTDM conversion have the advantage of being all optical, which simplifies the processing chain on the signals, they cannot work. only for low bit rates, less than 40Gbits / s.
• the technical problem to be solved by the present invention is to provide an optical device for converting WDM signals, the pulses of which are simultaneous and carried by distinct wavelengths, into an OTDM signal, the components of which are carried by a same wavelength and time shifted, which would allow operating at very high speeds to be able to be installed in long-distance optical transmission networks operating at very high speeds, typically greater than or equal to 40 Gbit / s.
• said device comprises: - shifting means, capable of introducing a time difference between the pulses supported by the optical carriers of the WDM signals,
• - modulation means capable of modifying the optical power of the WDM signals, - an optical spectral and temporal multiplexer / demultiplexer,
• absorption means capable of introducing optical losses on the components of the OTDM signal.
• the device according to the invention uses the well-known phenomenon of solitonic trapping (or "soliton trapping" in Anglo-Saxon literature) in a birefringent propagation medium, which makes it possible to create a shift in the optical frequency of the carrier, proportional to the optical power of a signal.
• solitonic trapping makes it possible to shift the wavelength of these pulses towards a so-called “target” wavelength of the optical carrier finally having to carry the information.
• Another technical problem to be solved by the present invention is to propose an optical device capable of making the reverse conversion, that is to say capable of converting an OTDM signal, the components of which are carried by the same wavelength ( ⁇ 4). and time-shifted (tl, t2, t3, t4), in WDM signals, the pulses of which are carried by distinct wavelengths ( ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4) which would make it possible to operate at very high bit rates so that they can be installed in long distance optical transmission networks.
• said device comprises: absorption means, capable of introducing optical losses on the components of the OTDM signal, a birefringent propagation medium into which the OTDM signal is injected so as to ensure a solitonic trapping phenomenon, an optical spectral and temporal multiplexer / demultiplexer, modulation means capable of modifying the optical power of the WDM signals.
• FIG. 1 a device according to the invention, used as a WDM / OTDM converter, FIG. 2, the WDM signals propagating at the input of the device of FIG. 1 and at the output of the spectral and time multiplexer,
• FIG. 3 the signals propagating at the input and at the output of the birefringent propagation medium of the device of FIG. 1,
• absorption means used in the device of FIG. 1 and the propagating signals at the input and at the output of these absorption means
• FIG. 5 other absorption means used, according to an alternative embodiment, in the device of FIG. 1, and the signals propagating at the input and at the output of these absorption means
• FIG. 6 a device according to the invention, used as an OTDM / WDM converter, and a diagram of the signals propagating at each stage of the conversion.
• it is a question of converting four WDM signals, carried by four channels operating for example at 40 Gbit / s, whose wavelengths are distinct, into an OTDM signal, carried by a single channel on a single optical carrier, operating at 160Gbit / s, and vice versa.
• the invention can of course be applied to signals having any bit rate. Preferably, it applies to signals having bit rates of 40, 160 or even 640 Gbit / s.
• the WDM / OTDM and OTDM / WDM conversion device is implemented for signals comprising “RZ” type data, according to the terminology commonly used to say “return to zero” or “reset to zero”. These data of the RZ type can be of the solitonic type or not.
• RZ signal is a digital signal comprising two states 0 and 1, bits at 1 corresponding to pulses and bits at 0 corresponding to the absence of pulse in bit time.
• the device referenced 100 is used as a WDM / OTDM converter. It is intended to convert, in this example, the four WDM signals, carried by four channels 10, 20, 30, 40 operating for example at 40 Gbit / s and whose wavelengths ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 are distinct , in an OTDM signal, carried by a single channel, on a single optical carrier ⁇ 4, and operating at 160Gbit / s.
• offset means 102, 103, 104, and modulation means 112, 113, 114 At the output of the four WDM channels, there are offset means 102, 103, 104, and modulation means 112, 113, 114.
• the offset means constituted for example by delay lines, make it possible to introduce a difference time between the pulses supported by the optical carriers of the WDM signals. This phase shift between the pulses is necessary in order to then be able to time multiplex the signals.
• 3 channels 20, 30, 40 are provided with these delay lines since it suffices that each carrier has a different offset with respect to the others. It is therefore not necessary to introduce a delay on the first channel 10, but of course there is nothing to prevent it either.
• delay lines 102, 103, 104 can be fixed and designed to shift each optical carrier by a fixed period of time for each signal. It is however preferable to use variable delay lines, in order to be able to adjust the offsets and refine them.
• the optical modulation means 112, 113, 114 allow, for their part, to modulate the optical power of the WDM signals.
• the modulation means are for example constituted by variable attenuators. So, we for example introduces different optical losses on each of the WDM signals to attenuate them. WDM signals are then obtained carried by distinct wavelengths ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 with different powers II, 12, 13, 14. These optical powers are adjusted so as to allow the desired later effect of solitonic trapping.
• variable optical attenuators are used to be able to adjust the power of each WDM signal.
• the delay lines 102, 103, 104 are arranged in front of the optical attenuators 112, 113, 114, but the order is really of no importance at this stage. It suffices that at the input of the optical multiplexer / demultiplexer 120 the WDM signals have been shifted and modulated.
• the optical spectral and temporal multiplexer / demultiplexer 120 then makes it possible to multiplex the WDM signals so as to have only one WDM multiplex comprising pulses of wavelength ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4, of powers II, 12, 13 , 14 different and offset (tl, t2, t3, t4) in time.
• the multiplex thus obtained is then injected into a birefringent propagation medium 130, such as a birefringent optical fiber for example, so as to ensure a phenomenon of solitonic trapping and to obtain a time multiplex signal carried by a single wavelength, ⁇ 4 in the example, constituting an OTDM signal.
• a birefringent propagation medium 130 such as a birefringent optical fiber for example
• Absorption means 140 then make it possible to equalize the optical power of the different components constituting the final OTDM signal.
• Figures 2 to 5, more detailed, allow a better understanding of the operation of this device, during the WDM / OTDM conversion.
• each WDM signal comprises pulses which are carried by a distinct wavelength ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4. These pulses of the different WDM signals all have the same intensity II and operate simultaneously.
• the multiplex presents pulses of wavelengths ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 distinct, of intensities II, 12, 13, 14 different and temporally offset tl, t2, t3, t4.
• the pulses of the OTDM signal which it is desired to obtain at the output of the device must be interleaved.
• the distance between two pulses must therefore be identical each time.
• the pulses are offset from each other by a difference of 6.25 ps.
• the offset between the pulses is therefore previously set and adjusted by means of the variable delay lines 102, 103, 104.
• the optical power II, 12, 13, 14 of each pulse of the WDM multiplex is previously set, by means of the variable attenuators 112 , 113, 114 to exacerbate the non-linear effects in the birefringent optical fiber 130 and thus promote the desired solitonic trapping effect and as illustrated in FIG. 3.
• a birefringent propagation medium has two main axes of propagation.
• the multiplex is injected according to a polarization at 45 ° relative to the main axes of propagation of the medium birefringent 130.
• a polarization controller can for example be placed in front of the optical fiber 130. This controller makes it possible to transform any incoming polarization into another polarization and in particular a linear polarization at 45 ° from the main axes of birefringent fiber.
• a soliton is a light pulse sufficiently intense to excite a non-linear effect which will compensate for the effects of chromatic dispersion during journeys over long distances.
• the pulses 1 to 4 injected retain their integrity and do not deform over time.
• their frequency spectrum is deformed and a frequency shift occurs with respect to the initial frequency of the spectrum of each of these pulses at the input of the propagation medium. This phenomenon, during which the pulse does not deform temporally but where the spectrum shifts in frequency, is known as solitonic trapping.
• the frequency shift ⁇ vi is proportional to the light power Ii of the pulse i injected into the propagation medium.
• the frequency offset ⁇ vi induced by the solitonic trapping phenomenon on the pulse i of the WDM multiplex can be adjusted to allow a perfect spectral correspondence of the spectrum displacements of the WDM channels.
• This precise adjustment is obtained thanks to the variable delay lines and the variable attenuators placed in front of the multiplexer 120.
• the pulses 1, 2, 3 of respective intensity II, 12, 13, each undergo a offset ⁇ vl, ⁇ v2, ⁇ v3 so that their wavelengths all coincide with the wavelength ⁇ 4 of the fourth pulse.
• an OTDM signal is therefore obtained, the components of which are carried by a single wavelength ⁇ 4 and are temporally offset (tl, t2, t3, t4).
• the components of the OTDM signal obtained do not have the same light power II, 12, 13, 14.
• Absorption means 140 are therefore provided for restoring an identical optical power level between all the components of the OTDM signal.
• This power equalization is for example based on the use of a MEA electro-absorption modulator which applies selective optical losses to the components of the OTDM channel, as illustrated in FIG. 4.
• the temporal profile of the losses Pos can be in stair steps 142, or a linear ramp 143 as illustrated on the curves of the applied voltage V and the optical losses at output Pos as a function of time t.
• the curve relating to the applied voltage V is in solid lines while the curve relating to the optical losses at output Pos is in broken lines.
• the absorption of the MEA being a function of the applied voltage V and of time, and the components of the injected signal each having a different intensity and being themselves shifted with respect to each other over time, each of them they do not see the same absorption when passing through the MEA.
• the different components 1, 2, 3, 4 then have an identical optical power Is.
• An alternative embodiment, to perform this power equalization, consists in using a saturable absorbent as illustrated in FIG. 5.
• the transfer function of a saturable absorbent has two distinct states: a blocking state when the power input le is less than a threshold power It, and a completely transparent state, when the input power is greater than this threshold power.
• the signal at the output of the saturable absorbent has a constant output power Is. If the various components of the OTDM signal obtained all have a power II, 12, 13, 14 greater than the threshold power It, they all have, at the output of the absorbent, an identical output power Is. If, on the other hand, the components of the OTDM signal have a power lower than the threshold power, then they are completely absorbed.
• the device 100 can also be used to carry out the reverse conversion, that is to say the conversion of an OTDM signal into WDM signals.
• This reverse conversion uses the same device in reverse. It is therefore described more succinctly, with reference to FIG. 6 which represents the device used as an OTDM / WDM converter and the signals propagating at each stage of the conversion.
• the OTDM signal passes through absorption means 140 so that selective optical losses are applied to its components.
• absorption means are for example constituted by the electro-absorbing modulator MEA as described above.
• the components of the OTDM signal do not see the same absorption and therefore undergo different optical losses.
• the OTDM signal obtained is then injected into the birefringent optical fiber 130 so as to ensure the solitonic trapping effect described above.
• the components of the OTDM spectrum undergo a frequency shift ⁇ vi proportional to their optical power.
• We therefore obtain a WDM multiplex whose pulses 1, 2, 3, 4 are carried by lengths wave ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 distinct, have different optical powers II, 12, 13, 14 and are temporally offset from each other.
• a polarization controller can for example be placed in front of the optical fiber 130 to facilitate the injection of the signal according to a polarization at 45 ° from the main axes of the optical fiber.
• the next step then consists in passing the WDM multiplex into the multiplexer / demultiplexer 120, in order to demultiplex it spectrally and temporally and to obtain four " signals carried by wavelengths ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4.
• the last step finally consists in modifying the optical power of the pulses of the WDM signals, in order to equalize them.
• This modification is done by means of modulation 112, 113, 114, which are for example constituted by variable attenuators as previously described.
• modulation 112, 113, 114 which are for example constituted by variable attenuators as previously described.
• the offset means 102 to 104 of FIG. 1 it is not essential to use the offset means 102 to 104 of FIG. 1.
• delay lines for example, they make it possible to temporally offset the pulses supported by the optical carriers of the WDM signals, so as to make them simultaneous.
• the device which has just been described is only an illustration and is in no way limited to this example. It finds its application in long distance high speed optical telecommunications.

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• 2002
  • 2002-04-19 FR FR0204968A patent/FR2838836B1/fr not_active Expired - Fee Related
• 2003
  • 2003-03-13 EP EP03727586A patent/EP1497939A1/de not_active Withdrawn
  • 2003-03-13 AU AU2003233375A patent/AU2003233375A1/en not_active Abandoned
  • 2003-03-13 US US10/511,227 patent/US7577363B2/en active Active
  • 2003-03-13 WO PCT/FR2003/000810 patent/WO2003090392A1/fr not_active Application Discontinuation
• 2009
  • 2009-05-04 US US12/434,796 patent/US7844180B2/en not_active Expired - Fee Related

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