Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-02-17
2001-03-13
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06201621
ABSTRACT:
BACKGROUND OF THE INVENTION
Transmitting Return-to-Zero (RZ) pulses is currently in common use in terrestrial optical-fiber transmission systems. One of the problems encountered, in particular for existing systems, is that of increasing the data rate or the transmission distance without giving rise to error. Various solutions have been proposed. A first approach consists in reducing the duration of the RZ pulses, and in using time-division multiplexing. That approach is limited by the jitter caused by the propagation and by the various optical components of the transmission system. Similar problems are encountered for transmission systems using Non-Return-to-Zero (NRZ) pulses.
In order to mitigate that problem, it is known that the optical signal can be converted into an electronic signal in the regenerators and the electronic signal can then be regenerated. That approach is intrinsically limited by the pass-band of the semiconductor components used. It is also limited in terms of the maximum length of the transmission system. In addition, it is costly when the data rate is increased.
It has also been proposed to perform wavelength conversion, relative to a local clock of different wavelength and without jitter, by applying the RZ signal with jitter as a control signal to the clock. In addition to requiring a local clock, that technique also suffers from the drawback of being difficult to apply when wavelength-division multiplexing is used, or, more generally, when it is awkward to change wavelength.
Transmitting soliton pulses or solitons is known. Such pulses are RZ pulses of time width (Full Width at Half Maximum (FWHM)) that is narrow relative to bit time, they have a determined relationship between power, spectrum width, and time width, and they propagate generally in the “abnormal” dispersion portion of an optical fiber. The variation in the envelope of such a soliton pulse in a monomode fiber can be modelled by the non-linear Schrödinger equation: propagation relies on a balance between the abnormal dispersion of the fiber and its non-linearity. In order to control the jitter of such soliton signals, various solutions have been proposed. It is known that sliding guiding filter systems can be used (see, for example, EP-A-0 576 208). It has also been proposed to perform synchronous modulation on the soliton signals. For that purpose, it is possible to use modulators of various types, and in particular synchronous amplitude or phase modulators using the Kerr effect. Descriptions of the various techniques for controlling or regenerating soliton signals can be found in the article entitled “Soliton Transmission Control in Time and Frequency Domains” by H. Kubota and M. Nakazawa, IEEE Journal of Quantum Electronics, vol. 29, No. 3,2189, or in the article entitled “Evaluating the Capacity of Phase Modulator-Controlled Long-Haul Soliton Transmission” by N. J. Smith and N. J. Doran, Optical Fibers Technology I, 218-235 (1995).
Those techniques are not limited by the pass-band of electronic components. Unfortunately, they cannot be applied directly to non-soliton RZ pulses because the pulses or their spectrums are different from soliton signals.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention provides an original and simple solution to the problem of controlling jitter in non-soliton RZ optical signals. The term “non-soliton optical signals” is used to mean signals having one or more of the following characteristics: wide time width (FWHM) relative to bit time, i.e. greater than approximately in the range 30% thereof to 40% thereof; no determined relationship between power, spectrum width, and time width; propagation generally or on average in the normal dispersion or zero dispersion portion of an optical fiber; and no balance between the dispersion and the non-linearity during propagation.
In certain embodiments, the invention also makes it possible to correct not only the jitter or the phase noise, but also the amplitude noise of the non-soliton RZ signals. The invention is advantageously applicable to existing terrestrial transmission systems, for which it makes it possible to increase the data rate or the maximum transmission distance without requiring any action to be taken on the transmission medium. The invention thus makes it possible, merely by adding or modifying control systems, to increase the capacity of existing links.
More precisely, the invention provides a method of regenerating non-soliton RZ optical signals, the method comprising the following steps:
a compression step whereby the RZ signals are compressed into soliton-type signals;
a modulation step whereby synchronous optical modulation is performed on the soliton-type signals, by using a clock; and
a decompression step whereby the modulated soliton signals are decompressed into non-soliton RZ signals.
It is possible to provide a clock recovery step whereby the clock is recovered from the non-soliton RZ signals, from the soliton-type signals, or from the regenerated non-soliton RZ signals.
The compression step advantageously comprises at least one of the following steps:
performing spectrum or time processing on encoded optical signals, in particular by spectrum filtering;
amplification; and
causing the signals to propagate along a highly dispersive non-linear optical medium.
The decompression step preferably comprises at least one of the following steps:
time spreading; and
post-amplification.
In one implementation, the modulation step whereby synchronous optical modulation is performed on the soliton-type signals is repeated so that it is performed at least twice.
It is also possible to provide at least one filtering step whereby the soliton-type signals are filtered by a filter chosen from the group formed of guiding filters and sliding guiding filters.
The invention also provides a method of regenerating NRZ optical signals, the method comprising:
an NRZ-to-RZ conversion step whereby the NRZ optical signals are converted into non-soliton RZ signals;
a regeneration step whereby the non-soliton RZ signals are regenerated using the method of the invention; and
an RZ-to-NRZ conversion step whereby the regenerated RZ optical signals are converted into NRZ signals.
The invention also provides a method of regenerating multiplexed non-soliton RZ optical signals, the method comprising:
a demultiplexing step whereby the signals are demultiplexed;
a regeneration step whereby the demultiplexed signals are regenerated using the method of the invention; and
a multiplexing step whereby the regenerated signals are multiplexed.
The invention also provides a method of regenerating multiplexed non-soliton RZ optical signals, the method comprising:
a synchronization step whereby the channels of the multiplex are synchronized; and
a regeneration step whereby the signals of the synchronized channels are regenerated using the method of the invention.
In one implementation, the invention provides a method of regenerating NRZ optical signals, the method comprising:
an NRZ-to-RZ conversion step whereby the NRZ optical signals are converted into non-soliton RZ optical signals;
a regeneration step whereby the non-soliton RZ optical signals are regenerated using the method of the invention; and
an RZ-to-NRZ conversion step whereby the regenerated RZ optical signals are converted into NRZ optical signals.
In another implementation, the invention provides a method of regenerating multiplexed NRZ optical signals, the method comprising:
a demultiplexing and NRZ-to-RZ conversion step whereby the NRZ optical signals are demultiplexed and converted into non-soliton RZ optical signals;
a regeneration step whereby the RZ optical signals are regenerated using the method of the invention; and
a multiplexing and RZ-to-NRZ conversion step whereby the regenerated RZ optical signals are multiplexed and converted into multiplexed NRZ optical signals.
In addition, the invention provides a method of regenerating multiplexed NRZ optical signals, the method comprising:
an NRZ-to-RZ conversion step whereby the NRZ optical signals are converte
Desurvire Emmanuel
Maunand Elisabeth
Alcatel
Bello Agustin
Chan Jason
Sughrue Mion Zinn Macpeak & Seas, PLLC
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