Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-03-19
2001-11-27
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06323979
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a regenerator with distributed optical modulation for a transmission system conveying soliton pulses.
The invention also relates to a method of regenerating a soliton signal by distributed optical modulation.
Finally, the invention also relates to a transmission system including such a regenerator.
BACKGROUND OF THE INVENTION
The transmission of soliton pulses or “solitons” is a known phenomenon. These pulses are return-to-zero (RZ) pulses of time width (full width at half maximum or FWHM) that is small compared with the bit time, that present a determined relationship between power, spectrum width, and time width, and that generally propagate in that portion of an optical fiber which has abnormal dispersion. The way the envelope of such a soliton pulse varies in a monomode fiber can be modelled using the non-linear Schrödinger equation; propagation relies on equilibrium between fiber dispersion and fiber non-linearity.
The transmission of such pulses is limited by various effects such as jitter induced by solitons interacting with the noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, published in Optical Letters, Vol. 1, No. 10, pp. 665-667. This effect, known as the Gordon-Haus effect, puts a theoretical limit on the quality or on the rate of soliton transmission. To exceed this limit, it is possible to make use of synchronous modulation of soliton signals by means of a clock signal or “clock” for the purpose of correcting their time jitter, as explained for example in an article by H. Kubota, published in IEEE Journal of Quantum Electronics, Vol. 29, No. 7 (1994), p. 2189 et seq.
To provide such synchronous modulation, it has been proposed to use the Kerr effect in synchronous phase modulators. Thus, the fiber itself can be used for phase modulation purposes. A presentation by S. Bigo, P. Brindel, and O. Leclerc at the Oct. 30, 1996 symposium on guided optics (“Journées nationales de l'Optique guidées”) held at Nice (France) describes soliton signal regeneration by all-optical phase modulation. An optical clock is superposed on the soliton signal, thereby imparting a non-linear phase shift to the soliton signal pulses by copropagating with them in an optical fiber that includes a length which has been selected to minimize the effects of slip between the soliton signal and the optical clock. Reference may be made to an article by T. Widdowson et al., entitled “Soliton shepherding: all-optical active soliton control over global distance”, published in IEE Electron. Letters, Vol. 30, No. 12, p. 990 (1994).
It has also been shown by S. Bigo, in a thesis, University of Bensancon, 1996 entitled “Traitement de signal tout-optique pour la transmission {grave over (a )} très haut débit de solitons par fibre optique” [All-optical signal processing for very high rate transmission of solitons by optical fiber] that an all-optical modulator using the Kerr effect, such as a non-linear optical loop mirror (NOLM) or a fiber, can be considered as a discrete sinusoidal modulator synchronized with the soliton train in spite of the slip or “walkoff” due to chromatic dispersion and to losses, providing the clock used is sinusoidal and the time offset between the signal to be modulated and the clock is appropriately adjusted.
One of the problems that arises with synchronous phase modulation is that of synchronizing phase between the clock and the soliton signal to be regenerated. In a conventional semiconductor modulator, such synchronization is conventionally achieved by deriving a signal whose intensity is representative of the phase difference between the modulating signal and the signals to be modulated. Feedback is then used to adjust the phase difference. Nevertheless, that solution is not applicable to distributed optical phase modulator devices using the Kerr effect in which there is no signal available of intensity that enables the phase of the modulator signal to be determined.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention proposes an original and simple solution to the problem of synchronizing the clock in an optical phase modulator device, and in particular one using distributed optical phase modulation by the Kerr effect. The invention makes it possible by means of a simple device to obtain a signal whose intensity can be used to control synchronization, e.g. in a feedback loop. In the case of distributed optical phase modulation using the Kerr effect, this signal is representative of the non-linear phase profile induced by the modulator signal and integrated over the modulation length.
More precisely, the invention provides a regenerator for a soliton pulse transmission system, including a device for optically modulating the soliton signal that is to be regenerated with an optical clock signal, wherein the modulator device is included in an interferometer, and by means for synchronizing the soliton signal to be regenerated and the optical clock as a function of the intensity of the output signal from the interferometer.
The modulator device may be a phase modulator device, or a semiconductor modulator. The modulator device may also be a distributed optical modulator device, preferably distributed in the transmission fiber of the transmission system.
In an embodiment, the interferometer is a Sagnac interferometer, preferably a non-linear optical loop mirror. In which case, the interferometer advantageously includes polarization-maintaining fiber.
Preferably, the synchronization means adjust the phase of the clock signal in such a manner as to maximize the intensity of the output signal from the interferometer. Preferably, the clock signal is derived from the soliton signal to be regenerated.
The invention also provides an optical transmission system including at least one such regenerator.
Finally, the invention provides a method of regenerating a soliton signal, the method comprising:
optically modulating the soliton signal to be regenerated with an optical clock in a modulator contained within an interferometer; and
synchronizing the optical clock and the soliton signal to be regenerated as a function of the intensity of the output signal from the interferometer.
Advantageously, the modulation is phase modulation, preferably distributed optical phase modulation. In an implementation, the interferometer is a Sagnac interferometer, preferably a non-linear optical loop mirror.
In an implementation, neutrality relative to birefringence is ensured by means of a polarization-maintaining fiber.
Advantageously, synchronization is performed by adjusting the phase of the clock signal so as to maximize the intensity of the output signal from the interferometer. It is also possible to provide a step of deriving the clock signal from the soliton signal to be regenerated.
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M. Nakazawa et al, “Nolm Oscillator and Its Injection Locking Technique for Timing Clock Extraction and Demultiplexing”, Electronics Letters, vol. 32, No. 12, Jun. 6, 1996, p. 1122/1123.
Jinno, Masahiko, “All Optical Signal Regularizing/Regeneration Using a Nonlinear Fiber Sagnac Interferometer Switch with Signal-Clock Walk-Off”, IEEE Journal of Lightwave Technology, vol. 12, No. 9, Sep. 1994.*
Bigo et al., “20 GHz all-optical clock recovery based on fibre laser mode-locking with fibre nonlinear loop mirror as variable intensity/phase modulator”, IEEE Electronic Letters, vol. 31 No. 21, Oct. 12, 1995.*
Brun-Maunand, “Regenerated Transoceanic soliton System: A Study of Intensity Versus Phase Synchronous Modulation”, IEEE 1996.*
Bigo et al., “All-optical Fiber Signal Processing and Regeneration for Soli
Alcatel
Pascal Leslie
Singh Dalzid
Sughrue Mion Zinn Macpeak & Seas, PLLC
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