Optical waveguides – With optical coupler
Reexamination Certificate
2001-05-10
2003-09-23
Kim, Robert H. (Department: 2882)
Optical waveguides
With optical coupler
C359S199200
Reexamination Certificate
active
06625338
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention is “all-optic” converters to convert an input signal in the NRZ format to an output signal in the RZ format. It also relates to a conversion process.
TECHNOLOGICAL BACKGROUND
All-optic transmission systems, and particularly wavelength division multiplexed WDM networks use different modulations and different data formats. Two standard data formats are fairly widespread. These are the Non Return to Zero (NRZ) format and the Return to Zero (RZ) format. As explained in page 359 of the manual entitled “Optique et Télécommunication—Transmission et traitement optique de l'information (Optics and Telecommunications—Transmission and optical information processing)” by A. COZANET et al. Published by EYROLLES, 1983, in a Non Return to Zero system a “one” level is transmitted by a high level throughout the duration of the bit transmission time, and a “zero” is transmitted by a low level during this duration; in a Return to Zero system, a “one” is transmitted by a high level for part of the period, usually the first half of the transmission time of the bit followed by a low level, and a “zero” is transmitted by a low level throughout the duration of the bit.
This simple description shows that the number of pulses transmitted for a set of data will be greater for the RZ format than for the NRZ format. In an NRZ format, a continuous sequence of “ones” will require a high level signal throughout the duration of the transmission of this sequence, namely one long pulse.
In the Return to Zero format, a sequence of “ones” will be represented by a sequence of pulses. Similarly, an isolated “one” in the RZ format will produce a pulse half as long as the length of the pulse in the NRZ format, assuming that the bit time is of the same duration for the two formats, such that the transmission of the RZ format will require a larger pass bandwidth.
For all these reasons, the pass band necessary to transmit the same data is twice as wide for the RZ format as for the NRZ format.
Therefore, it can be seen that the NRZ format does have an undoubted advantage compared with other formats that require a greater bandwidth for the same transmission speed.
However, the RZ format is also useful in some applications, for example multiplexing, demultiplexing by passive time division, soliton generation and deletion of the BRILLOUIN stimulated dispersion.
This is why converters from one format to the other are necessary to benefit from the advantages of both formats.
For example, one first known example of this type of converter is briefly described in an article in “Electronics Letters” on Feb. 13 1992 (vol. 28, No. 4, pages 405, 406) written by A. D. Ellis and D. A. Cleland and entitled “Commutation tout optique ultra rapide dans un miroir boucle optique non lineaire (NOLM) (Ultra rapid all-optic switching in a non-linear optical loop mirror)”. As shown in
FIG. 1
in this article, the device comprises a SAGNAC anti-resonance interferometer.
Inputs to the interferometer consist firstly of a regular pulse stream and secondly a signal in the Non Return to Zero format representative of a data signal.
The power of the two signals is chosen such that if only one of these signals is present, in other words is at the high level, a destructive interference occurs such that the signal at the output from the interferometer is low which corresponds to normal operation of a SAGNAC interferometer considering the phase delay between the signal in the forward direction and the signal in the reverse direction resulting from the birefringent nature of the fibre. However, when the two signals are high, which has the effect of doubling the total optical power, an additional phase of &pgr; is caused by modulation of the high power signal propagating in the reverse direction and therefore by constructive interference at the output from the interferometer.
The device thus behaves like an “and” gate.
Depending on the pass band of a filter located at the output from the interferometer, an RZ signal resulting from conversion of the NRZ signal can be obtained either on a carrier at the wavelength carrying the pulse stream, or at the wavelength carrying the NRZ signal.
A second embodiment of the NRZ-RZ converter is described in an article by S. BIGO, E. DESURVIRE, S. GAUCHARD and E. BRUN entitled “Amélioration de débit par conversion optique NRZ-RZ et multiplexage par division temporelle passive pour les systèmes à transmission soliton (Improvement of flow by NRZ-RZ optical conversion and multiplexing by passive time division for soliton transmission systems)” published in the “Electronics Letters” journal Jun. 9, 1994 (vol. 30, No. 12, pages 984-985). In the following, we will only consider the NRZ-RZ conversion described in this article. As in the previous case, a non-linear optical loop mirror is used. Also as in the previous case, two signals are input into the loop forming a SAGNAC interferometer. Firstly, a control signal is input at a wavelength &lgr;c, and secondly an NRZ signal is input at wavelength &lgr;s. An interferometer polarization controller is adjusted to minimize the output signal when the control signal is low in order to obtain a SAGNAC interferometer. After amplification, the control signal is input into the loop forming an interferometer through an 80/20 coupler located close to the point of sharing between the forward wave and the reverse wave.
The authors report that with this passive loop, in other words a loop that does not comprise an optical amplifier in the loop as in the previous example, they can obtain an RZ signal after conversion of the input NRZ signal.
The pulse width of the RZ signal is also slightly less than the width of the control pulses at wavelength &lgr;c.
However, the difference in level between the low level and the high level is more than 20 dB, representing an improvement by a factor of 10 compared with the previous example with active loop.
Note that in the two examples described above, the fibre loop mirrors are very sensitive to the light polarization and temperature fluctuations.
A third example embodiment is mentioned in an article by David NORTE and Allan E. WILLNER entitled “Démonstration expérimentale d'une conversion tout optique entre des données aux formats RZ et NRZ incorporant des changements non inverseurs de longueur d'onde et conduisant á une transparence du format (Experimental demonstration of an all-optic conversion between data in the RZ and NRZ formats including changes that do not invert the wavelength and leading to format transparency)” published in “IEEE photonics technology Letters” (vol. No. 8, No. 5, May 1996, pages 712-714).
The device mentioned in this article applies to an RZ-NRZ converter according to the diagram shown in
FIG. 1
of this article. The description of the device and its operation is not very clear because an optical amplifier SOA1 mentioned in the text is not shown in the figure. However, it can be understood that the first step is to recover the clock signal from the RZ signal. It is also explained that a device not described is used to change from an NRZ format to an RZ format such that either of the two formats can be used.
Each of the devices described or mentioned in these articles are subsystems that include several opto-electronic or electronic devices and require assembly work.
As mentioned above, devices based on SAGNAC interferometers are sensitive to fluctuations in temperature and light polarization. All devices described in these articles require a clock signal or a clock recuperation.
Therefore, there is a need for an NRZ-RZ conversion device that is easy to design and make.
BRIEF DESCRIPTION OF THE INVENTION
The basic idea of the invention is to use an interferometric structure, for example a Mach-Zehnder structure with two arms (a first arm and a second arm). At least the first or the second arm comprises an element in which the optical index can vary as a function of the optical power present in this element, for example a semi conducting optical amplifier (SOA).
An
Devaux Fabrice
Schlak Michael
Shen Alexandre
Tekin Tolga
Alcatel
Kim Richard
Kim Robert H.
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