Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-03-30
2001-02-06
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C359S257000, C385S039000, C385S040000
Reexamination Certificate
active
06185033
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electro-optical modulators.
More particularly, the invention proposes electro-optical modulators that are fast, i.e. that make it possible to obtain modulation frequencies of the order of 1 MHz or higher.
This type of modulator is, in particular, used advantageously in the field of optical telecommunications to transform an electrical signal into a modulated optical signal.
Such modulation is obtained either by modulating the transmitter laser internally, or else by performing modulation externally. External modulation may be performed by discrete components or by integrated optical components which use various physical effects such as, for example, acousto-optical effects, electro-absorption effects, or electro-optical effects. Among the latter effects, the most commonly used effect is the transverse electric Pockels effect which makes it possible to modulate the refractive index of the propagation medium in proportion to the electric field via an electro-optical coefficient. The electro-optical coefficient depends on the non-linear optical properties of the material used. Such a material is said to be “active” because its refractive index varies significantly with the electric field to which it is subjected.
The Pockels effect may be obtained with various types of material, and in particular with light guides made using:
suitable semiconductors;
inorganic or organic crystals that are not centrally symmetrical;
electro-optical polymers; or
passive materials associated with electro-optical materials.
The invention relates to Pockels-effect electro-optical modulators whose light guides are made using passive materials associated with electro-optical materials.
The advantage of this type of structure is that it can be connected easily to other passive components on an optical integrated circuit platform.
For examples of Pockels-effect modulator structures using passive materials, reference may advantageously be made to the following publications:
M. Wilkinson et al., Electronics Letters, vol. 27 (11), p. 979, (1991);
R. A. Hill et al., Proceedings OFC'96, Technical Digest, WL1, P. 166, (1996); and
G. Fawcett et al., Electronics Letters, vol. 28 (11), p. 985, (1992).
Usually, such modulator structures are constituted by passive waveguides which are made of silica or of glasses treated by ion exchange and on which respective polymer films are deposited. Such a structure takes advantage of the evanescent wave coupling that takes place between the core and the polymer at well-defined wavelengths, since the polymer has a refractive index that is considerably higher than the refractive index of the core.
Unfortunately, such a structure suffers from large losses at those wavelengths, and modulation takes place with power levels that are low.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to propose an electro-optical modulator of the above-mentioned type that does not suffer from that drawback and that makes it possible, with small optical losses, to obtain optical phase modulation analogous to the optical phase modulation obtained with cores made of active materials.
Another object of the invention is to propose a method that is simple to implement for manufacturing such a modulator.
To these ends, the invention provides a Pockels-effect electro-optical modulator including a light guide having a core made of an inorganic amorphous material, and on which a structure is superposed that is made up of two electrode-forming layers between which an electro-optical polymer is interposed, said structure being superposed directly on the material of the core of the light guide, said modulator being characterized in that it includes a layer of cladding-forming material which extends over the core material on either side of the structure comprising the two electrode-forming layers and the electro-optical polymer, and which, in the vicinity of said structure, has transition zones in which its thickness decreases down to the electrode superposed on the core material, the electro-optical polymer covering said cladding-forming material in the transition zones defined in this way.
Such a modulator may advantageously further include the following characteristics, taken singly or in any technically-feasible combination:
the electro-optical polymer is covered in a protective layer whose refractive index is lower than the refractive index of the core;
the electrode which is superposed on the core of the light guide is made of a material that is both electrically conductive and also transparent at the operating wavelength; and
said material is a material based on tin oxide; and
the structure comprising the two electrode-forming layers and the electro-optical polymer superposed on the core is of a length lying in the range 5 mm to 30 mm.
The invention also provides a method of making an electro-optical modulator of the above-mentioned type, said method being characterized by the following steps:
a structure is made that includes a light guide having a core that is unclad and that is made of an inorganic amorphous material;
a layer of a material serving to constitute cladding for said core is deposited on said structure;
said layer is etched through a mask;
an electrode is deposited in the bottom of the etched zone; and
an electro-optical polymer is deposited on the electrode and on the edges of the etched zone.
In particular, the etching is performed by plasma etching, with the mask being spaced apart from the cladding.
Preferably, in order to space the mask apart from the cladding, shims are disposed between said mask and said cladding.
Advantageously, the polymer is deposited on the electrode and on the transition zones defined on the cladding during said etching.
In another variant, the following steps are implemented:
a structure is made that includes a light guide having a core that is unclad and that is made of an inorganic amorphous material;
cladding is deposited by plasma on said structure while interposing a mask facing the zone in which the electro-optical polymer is to be deposited, which mask has an uninterrupted portion facing said zone;
an electrode is deposited in the bottom of the depression obtained in this way;
an electro-optical polymer is deposited on the electrode and on the edges of the depression.
Other characteristics and advantages of the invention appear from the following description.
REFERENCES:
patent: 4957655 (1990-09-01), Khanarian et al.
patent: 5265178 (1993-11-01), Braun et al.
patent: 5396363 (1995-03-01), Valette
patent: 5400416 (1995-03-01), Enokihara et al.
patent: 5729641 (1998-03-01), Chandonnet et al.
patent: 5930412 (1999-07-01), Toussaere
patent: 0961154A1 (1999-12-01), None
patent: 2238879 (1991-06-01), None
Hill R A et al.; “Polymeric in-line fiber modulator using novel processing techniques” Optical Fiber Communication 1996, San Jose, Feb. 25-Mar. 1, 1996 vol. 2, No. CONG. 19, Feb. 25, 1996, pp. 166-167.
Wilkinson M et al: “Optical Fibre Modulator Using Electro-Optic Polymer Overlay” Electronics Letters, May 23, 1991, UK vol. 27, No. 11, pp. 979-981.
Fawcett G et al.: “In-line Fibre-Optic Intensity Modulatory Using Electro-Optic Polymer” Electronics Letters, May 21, 1992, UK vol. 28, No. 11, pp. 985-986.
Benech Pierre
Bosc Dominique
Morand Alain
Tedjini Smaïl
Blakely & Sokoloff, Taylor & Zafman
Epps Georgia
France Telecom
Spector David N.
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