Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
1999-11-19
2001-04-10
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S002000, C385S008000, C385S040000, C385S045000, C385S014000
Reexamination Certificate
active
06215935
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention lies in the field of semi-conductor optical components used for optical transmission or for methoding of digital optical data.
It more particularly relates to all optical components having a number of semi-conductor optical amplifiers, known in what follows as “SOA” (for “Semiconductor optical amplifier” in the Anglo-Saxon literature), such as for instance wavelength shifters, multi-arm amplifiers, all-optical switches or multiplexers.
To simplify the exposition of the invention, in what follows mention will be made only of all-active wavelength shifters, the subject matter of the invention also extending, as has been said, to all optical components based on a number of SOA.
Wavelength shifters are used in the field of telecommunications to shift a transmitted optical signal from one wavelength to another wavelength while maintaining its characteristics. Such changes of wavelength are used in particular during routing of signals to resolve problems of contention.
In such shifters the information is in the form of binary data represented by pulses modulating an optical carrier wave. A binary value is therefore determined dependent on the level of amplitude (or of power) of the modulated optical wave.
A common method of producing all-optical wavelength shifters is to use an interferometric structure of Mach-Zehnder type or equivalent.
An interferometric structure of this type is shown in plan view in FIG.
1
. It is made up of two guiding branches
1
and
2
. At least one of these branches is equipped with a semi-conductor optical amplifier SOA
1
. However in general it is preferred to position a second semi-conductor optical amplifier SOA
2
on the other guiding branch for reasons of symmetry. This is because the presence of a second semi-conductor optical amplifier SOA
2
makes it possible to maintain substantially the same level of amplification in the two branches and thus to have substantially identical power at the output from the branches of the interferometer.
A first coupler K
1
allows one end of each of these branches to be coupled to a peripheral semi-conductor optical amplifier, also called input amplifier SOA
5
. In a counter-directional functional configuration, an external laser source
7
also enables the supply to this amplifier SOA
5
an output carrier wave M of wavelength &lgr;s.
A second coupler K
1
is disposed so as to couple the other end of branch
1
to another peripheral semiconductor optical amplifier SOA
4
. This coupler K
2
enables introduction into the first amplifier SOA
1
, of an input signal E of wavelength &lgr;e which has been amplified by the input amplifier SOA
4
.
A third coupler K
3
, connected to coupler K
2
, to the second amplifier SOA
2
and to another peripheral semi-conductor optical output amplifier SOA
3
, is disposed in such a way as to supply an output signal resulting from the coupling of auxiliary waves AM
1
, and AM
2
supplied respectively by the first and second amplifiers SOA
1
and SOA
2
.
Waves AM
1
, and AM
2
correspond to waves M
1
and M
2
output from the coupler K
1
amplified respectively by amplifiers SOA
1
and SOA
2
. The output signal S, of wavelength &lgr;s, is then amplified by the peripheral output amplifier SOA
3
. It results from constructive or destructive interference of the waves AM
1
, and AM
2
, depending on the phase difference between the two branches
1
and
2
of the interferometer.
Another peripheral amplifier SOA
6
is also provided to maintain the symmetry of the structure and to enable replacement of one of the amplifiers SOA
3
or SOA
4
in case of any breakdown.
Amplification currents are injected respectively into the amplifiers SOA
1
, SOA
2
, SOA
3
, SOA
4
, and SOA
5
by means of electrodes E
1
, E
2
, E
3
, E
4
, and E
5
. To make this type of interferometric structure operate it is therefore necessary to use five independent electrodes E
1
, E
2
, E
3
, E
4
, E
5
, and five different current sources are necessary to inject an amplification current on each electrode. On the other hand, in a component with equivalent active/passive structure, only three current sources are necessary to make it work. This is because in this case the waveguides
3
and
5
are passive, so that it is not necessary to position electrodes above these guides to amplify the signal.
SUMMARY OF THE INVENTION
The problem which the invention sets out to solve thus consists in reducing the number of independent electrodes in all-active components based on a number of semi-conductor optical amplifiers, in order to reduce the number of current sources necessary to control them. Such a reduction in the number of current sources will make it possible to simplify the use of this type of component and to facilitate its insertion into optical telecommunications systems.
To this end, the invention provides more particularly a semi-conductor optical component having different regions with the same vertical structure, wherein an active waveguide is buried between the lower and upper buffer layers, the said regions having upper and lower electrodes to enable injection of equal or varying values of current density, characterized in that at least one of the said electrodes covers several different regions and has distributed transverse resistivity which is adjusted according to the region under consideration.
According to another characteristic of the invention, the upper electrodes are constituted by a contact layer on which is deposited a metallization layer, and the transverse resistivity of the electrode common to a number of regions is adjusted, depending on the region under consideration, by locally interrupting the contact layer. The local interruptions of this layer are for example realized by engraving over a width at least identical to that of the active waveguide.
According to another characteristic of the invention, the metallization layer of the common electrode is also interrupted locally, by engraving, over a width at least identical to that of the active waveguide and less than that of the said common electrode.
According to another characteristic of the invention, the upper electrodes are constituted of a doped contact layer on which is deposited a metallization layer, and the transverse resistivity of the electrode common to a number of regions is adjusted, depending on the region under consideration, by local modification of the doping of the contact layer. This local modification of the doping is realized, for example, by ion implantation over a width at least identical to that of the active waveguide.
According to another characteristic of the invention, the optical component is an all-active wavelength shifter.
According to yet another characteristic of the invention, this wavelength shifter has an electrode common to a guiding branch, to an input, and to an output; and two control electrodes covering respectively another guiding branch and another input.
Thanks to the invention it is possible to reduce the number of independent electrodes in an optical component, specifically in an all-active component. In the example of an all-active wavelength shifter this number of independent electrodes is reduced from five to three. This reduction in the number of electrodes makes it possible to reduce the number of current sources necessary to the functioning of the component and consequently to facilitate the use of the component and its insertion into an optical telecommunication system.
REFERENCES:
patent: 5146518 (1992-09-01), Mak et al.
patent: 5150436 (1992-09-01), Jaeger et al.
patent: 6005708 (1999-12-01), Leclerc et al.
patent: 6035078 (1999-12-01), Dagens et al.
patent: 06273816 (1994-09-01), None
patent: 10223989 (1998-08-01), None
Dagens Beatrice
Janz Christopher
Alcatel
Palmer Phan T. H.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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