Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2003-08-05
2004-07-27
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227280
Reexamination Certificate
active
06768097
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of selective optical filtering by electrical modulation of the spectral transfer function.
It relates more particularly, although not exclusively, to electrically tunable optoelectronic filter devices intended for the wavelength demultiplexing of the channels of certain optical telecommunication installations, and especially installations using multiplexing systems known as WDM (Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing).
BACKGROUND OF THE INVENTION
To provide this type of demultiplexing, Fabry-Pérot type filters have been proposed that include a resonant air cavity placed between partial reflectors. The wavelength tunability of these filters is obtained by displacing at least one of the partial reflectors, which is mounted in an elastic manner, under the effect of an electrostatic pressure.
Devices of this type have been described, for example, in the scientific articles mentioned below:
M. S. Wu, G. S. Yuen and C. J. Chang-Hasnain, “
Widely tunable
1.5
&mgr;m micromechanical optical filter using AlOx/AlGaAs DBR
”, Electronics Letters 33, 1702 (1997);
P. Tayebati, P. Wang, D. Vakhshoori and R. N. Sacks, “
Microelectromechanical tunable filters with
0.47
nm linewidth and
70
nm tuning range
”, Electronics Letters 34, 76 (1998), and “
Widely tunable Fabry
-
Perot filter using Ga
(
Al
)
As
-
AlOx deformable mirrors
”, IEE Photonics Technology Letters 10, 394 (1998);
P. Tayebati, P. Wang, A. Azimi, L. Maflah and D Vahshoori, “
Microelectromechanical tunable filters with stable half symmetric cavity
”, Electronics Letters 34, 1967 (1998);
D. Rondi, R. Blondeau, G. Guillot and P. Viktorovitch, “
Highly selective
1.55
&mgr;m InP/Air
-
gap micromachined Fabry-Perot Filter For Optical Communications
”, Electronics Letters 39, 453 (1998); and
A. Spisser, R. Ledantec, C. Seassal, J. L. Leclercq, T. Benyattou, D. Rondi, R. Blondeau, G. Guillot and P. Viktorovitch, “
Highly selective and widely tunable
1.55
&mgr;a InP/Air
-
gap micromachined Fabry
-
Perot Filter For Optical Communications
”, IEEE Photonics Technology Letters, 10 (9), 1259 (1998).
However, these devices having a single resonant cavity cannot provide simultaneously, for theoretical reasons, suitable separation (typically greater than −20 dB) between adjacent channels and a useful passband sufficient for transmission, without attenuation, of a rapid modulation (typically greater than 10 GHz) of the light signal.
It is known that suitable filters, that is to say those possessing a more “rectangular” passband, are obtained using several Fabry-Pérot cavities coupled together. A filter having two coupled cavities makes it possible, for example, to meet the specifications of the current WDM systems for a given wavelength. Solutions have already been proposed for achieving tunability of filters having two coupled cavities.
Thus, document U.S. Pat. No. 5,103,340 proposes the coupling of two resonant air cavities, called “thick” cavities because they have optical thicknesses of m&lgr;/2 and n&lgr;/2, m and n being integers of around 100 and close enough to have spaced-apart common resonance wavelengths. The thickness of each cavity is controlled here by a common piezoelectric actuator placed so that the displacements of the two cavities are proportional to the integers m and n. However, it is quite difficult to tune the two cavities simultaneously and the overall size of these devices is an obstacle to a high level of integration. In addition, these devices require high control voltages, typically of the order of a few hundred volts, which make them difficult, or even impossible, to use in environments in which the energy consumption levels are low.
SUMMARY OF THE INVENTION
The object of the invention is to remedy all or some of the aforementioned drawbacks.
For this purpose, the invention provides an optoelectronic device comprising i) first means that define a first (thick) resonant cavity, the thickness and the composition of which are chosen to offer a multiplicity of resonant transmission modes in a chosen wavelength range, and ii) second means that define a second (thin) resonant cavity, the thickness and the composition of which are chosen to offer a single resonant transmission mode in the chosen wavelength range, the device being characterized in that it also includes means for optically coupling said first and second means, and electrostatic means able to apply an electrical voltage to said second means, said electrical voltage making it possible to vary the thickness of the second cavity and the spectral position of the associated resonant mode so that this mode coincides with any one of the resonant modes of the first cavity (said mode being chosen from all those that it has). The device may thus transmit an incident light wave whose wavelength is that of a resonant mode common to the thin and thick cavities.
In the device according to the, invention, it is sufficient to tune the second cavity to a resonant mode that coincides with a resonant mode of the first cavity, which is much simpler than tuning both cavities simultaneously and can be carried out with simple means. The optical coupling of the two cavities allows transmission of really “rectangular” signals, in accordance with the standards of the WDM and DWDM systems.
The expression “optical coupling means” is understood here to mean a means allowing optical interaction between the thin and thick cavities. Furthermore, “any one” is understood to mean the fact of selecting, according to the requirements, one or other mode of the first cavity by controlling the thickness of the second cavity. In other words, any one of the modes of the first cavity may be chosen, depending on the requirements.
Thus, the two, thin and thick, coupled cavities produce a filter whose transmission wavelength may be adjusted in a discontinuous manner over each (or at least a certain number) of the wavelengths of the various transmission modes of the thick cavity. The spectral characteristic of the transmission function thus produced is that of a filter having two coupled cavities, and therefore has rejection and passband characteristics that are greatly superior to those of a filter with a single cavity.
In one advantageous embodiment, the thickness and the composition of the first (thick) cavity are chosen so that the multiplicity of its resonant transmission modes defines a comb, the position of the modes and the distance between adjacent modes (or intermode spacing) of which are chosen so as to coincide respectively with the position and distance of the wavelengths of the light signal that it is desired to demultiplex, such as those that are defined by the international standards (such as the ITU).
Preferably, the first means that define the first (thick) cavity comprise two approximately parallel partial reflectors spaced apart by a first layer of material (preferably semiconductor material), the thickness of which fixes the position of the resonant modes of the first cavity, and which ensure the resonance of this first cavity.
Also preferably, the second means that define the second (thin) cavity comprise at least two approximately parallel partial reflectors spaced apart by a second layer of material (preferably an air layer), the thickness of which defines the position of the resonant mode of the second cavity, and which ensure the resonance of this second cavity.
According to another feature of the invention, the electrostatic means are produced by electrically connecting each of the two partial mirrors of the second means to a first electrode and a second electrode in such a way that, when a potential difference is applied between said first and second electrodes, the thickness of the air cavity located between the two partial reflectors (or mirrors) changes. In one particular embodiment, the second means are configured so as to define at least one substructure of the pin or nip junction type. In this case, the pin junction, or alternatively the nip junction, is rever
Garrigues Michel
Leclercq Jean-Louis
Viktorovitch Pierre
Alston & Bird LLP
Centre National de la Recherche Scientifique
Le Que T.
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