Optical waveguides – Temporal optical modulation within an optical waveguide
Patent
1993-12-27
1995-05-16
Bovernick, Rodney B.
Optical waveguides
Temporal optical modulation within an optical waveguide
385 2, 385 9, 359254, G02B 610
Patent
active
054168589
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to devices which allow an optical signal to be modulated on the basis of an electrical signal. Such devices are most often integrated on the surface of a substrate consisting of a material whose optical properties change under the influence of an electric field which is applied to it. These devices make it possible in particular to modulate a light signal in order to make it possible to transmit analogue signals, for example vestigial side band amplitude -modulated (VSBAM) television signals over cabled television networks employing optical fibres. In order to enhance the linearization of the electrooptical response of an integrated optical modulator, the applicant has filed a French Patent Application under No. 2 670 589. This invention is based on the simultaneous modulation of two distinct regions of a Y-junction coupler. The modulation of a "coupler"-type region has the drawback of not being very effective, since it necessitates a two-conductor electrode structure and thus a vertical electric field (on Z-cut LiNbO.sub.3).
SUMMARY OF THE INVENTION
Similar structures are also known from two articles, one which appeared in IEEE Photonics Technology Letters, vol. 3, February 1991, New York U.S.A., pages 144 to 146, and the other which appeared in NTZ Nachrichttechnishe Zeitschrift, vol. 42, No. 2, February 1989, Berlin, Germany, pages 68 to 71. These structures exhibit the same drawbacks.
The object of the present invention is to further enhance the linearization of the electrooptical response of an integrated optical modulator, and also to reduce the control voltage necessary for the modulation. To this end, the present invention is based on the simultaneous modulation of two different regions of an interferometer, that is to say of two regions in which the guides are sufficiently far apart. This allows the use of a three-electrode structure, more effective from an electronic point of view, and thus a reduced control voltage. This structure is compatible with a X-cut on lithium niobate LiNbO.sub.3, and moreover gives several advantages inherent in this cut, and especially less sensitivity to temperature and the possibility of integration of a polarizer allowing monomode, monopolarization operation.
The invention more particularly relates to an electrooptical modulation device of the type comprising, on an electrooptical substrate:
a first section 1 comprising a Y-junction, intended to receive the optical signal to be modulated and to divide it into two equal parts,
a first modulation region 2 of length L.sub.1 comprising two parallel optical guides 104, 105 for receiving the two parts of the optical signal coming out of the first section, the said guides 104, 105 being spaced sufficiently to prevent optical coupling between them, the said region also comprising three electrodes 106, 107, 108 extending over the substrate along the guides in order to apply a modulation electric field as a function of a modulation electrical signal of amplitude V.sub.1 applied to the said electrodes 106, 107, 108,
a third, coupler-forming section 3 in which the optical guides 110, 111 extend, while remaining mutually parallel over a length L.sub.2, and determining a coupling length l.sub.c2, with ##EQU1##
a section 6 for separating the optical guides 115, 116 and delivering two optical signals P.sub.R, P.sub.S modulated in a complementary way; this device being characterized in that the third section is not subjected to modulation, and in that it further comprises:
a second modulation region 4 of length L.sub.3 comprising two parallel optical guides 112, 113 for receiving the two parts of the optical signal coming out of the third section 3, the said guides 112, 113 being spaced apart sufficiently to prevent optical coupling between them, the said region also comprising three electrodes 106', 107', 108' extending over the substrate along the guides for applying a modulation electric field as a function of a modulation electrical signal of amplitude V.sub.2 applied
REFERENCES:
patent: 4679893 (1987-07-01), Ramer
patent: 4900115 (1990-02-01), Heuring et al.
patent: 5101450 (1992-03-01), Olshansky
patent: 5161206 (1992-11-01), Djupsjobacka
patent: 5168534 (1992-12-01), McBrien et al.
patent: 5278923 (1994-01-01), Nazarathy et al.
IEEE Photonics Technology Letters, Feb. 1991, "In Search of a Linear Electrooptic Amplitude Modulator", Liu et al., vol. 3 pp. 144-146.
"Thomson-CSF"
Bovernick Rodney B.
Sanghavi Hemang
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