Integrated electro-optical modulation device

Optical waveguides – Directional optical modulation within an optical waveguide – Electro-optic

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385 3, 385 8, 385 14, 385 15, 385 40, 385 45, 385129, 385132, G02B 612, G02B 610

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active

053596803

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This application is a 371 of PCT/FR91/00994, filed Dec. 10, 1991.
The present invention relates to devices allowing modulation of an optical signal from an electric signal. Devices of this kind are most frequently incorporated into the surface of a substrate made from a material whose optical properties are changed under the effect of an electric field applied to it. These devices make it possible, in particular, to modulate a light signal in order to be able to transmit analog signals, e.g., television signals, over an optical fiber.
2. Discussion of Background
To modulate an optical signal using an analog signal, use must be made of a modulator whose electro-optical response is the most linear possible, in order to avoid excessive distortion of the signal to be transmitted. Linearity tolerances may be greater of lesser, depending on the signals to be transmitted; however, they are especially reduced in the very important instance of a set of television channels transmitted under amplitude modulation with reduced sideband (AMRS).
Various modulator structures are known, some of which make it possible to cancel out even non-linearities using the symmetry of the electro-optical response. On this subject, IEEE Transactions: Microwave Theory and Techniques, Vol. 28, No. 5, May 1990, article of Halemane and Korotky, p. 673, may be consulted.
In an article published in IEEE Journal of Quantum Electronics, Vol. 22, No. 12, December 1986, pp. 2191-2194, Kwang Tsai Koai et al. suggested a modulator structure based on a directional coupler allowing good linearity. This structure improves in their entirety the properties of the directional coupler, but does not make it possible to cancel out the even-numbered or third-order terms.


SUMMARY OF THE INVENTION

To obtain good linearity while canceling out even-numbered and at least third-order terms, the invention puts forth an electro-optical device characterized mainly by the fact that it comprises, on an electro-optical substrate: signal to be modulated and to divide it into two identical parts; the two parts of the optical signal emerging from the first section, these guides being coupled over a first length L1 so as to determine a coupling length lc1 and the second section further comprising two first electrodes extending on the substrate and along and to the outside of the guides, in order to receive the electric modulation signal and to apply an electric modulation field to the guides; parallel and coupled over a second length L2 so as to determine a second coupling length lc2; and modulated optical signals of different intensities.


BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will emerge clearly from the following description provided as a non-restrictive example with reference to the attached drawings, in which:
FIG. 1 is a layout diagram of a modulator according to the invention;
FIG. 2 illustrates modulator-response curves;
FIGS. 3 to 7 illustrate layout diagrams of variants of the modulator in FIG. 1.


DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment illustrated in FIG. 1, the coupler according to the invention consists of four parts 1 to 4.
The first part 1 comprises a convention Y-junction in which the light signal arriving through an input guide 101 is divided into two substantially-equal parts by means of two arms 102 and 103 which branch off from the guide 101.
These arms feed two parallel guides 104 and 105 which pass through parts 2 and 3.
In the part 2, two electrodes 106 and 107 extend along and to the outside of these guides. The term "outside" as used here has a purely conventional meaning used to describe the invention. The exact position in relation to the guides depends, in conventional fashion, on the material and the cut used. Thus, using LiNbO.sub.3 in the Z cut, the electrodes partially overlap the guides. The electrode 107 is grounded, while the electrode 106 is connected, on one side, to a modulation source which applies to it an elec

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Applied Physics Letters, vol. 53, No. 20, 14 Nov. 1988, New York US, pp. 1908-1910; Y. Bourbin et al.: `Intrinsically resonant traveling-wave Y-fed directional couplers at high frequency`.
IEEE Transactions on Microwave Theory and Techniques. vol. 38, No. 5, May 1990, New York US, pp. 669-673, T. R. Halemane, et al.: `Distortion characteristics of optical directional coupler modulators`.
IEEE Journal Of Quantum Electronics. vol. QE-22, No. 12, Dec. 1986, New York US, pp. 2191-2194; K. T. Koai, et al.: "Digital and quasi-linear electrooptic modulators synthesized from directional couplers".

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