Linearized Y-fed directional coupler modulators

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

Reexamination Certificate

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Reexamination Certificate

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06236772

ABSTRACT:

FIELD OF INVENTION
The subject invention pertains to electro-optic devices and in particular, to an integrated-optic modulator with a linearized transfer curve. The subject invention is further concerned with realizing modulators with suppressed nonlinear distortions for analog optical links, including but not limited to fiber-optic communication links and cable television systems. In a specific embodiment, the subject invention relates to a coupled-waveguide modulator with a linearized transfer curve. The subject invention is also concerned with realizing high-speed modulators.
BACKGROUND OF THE INVENTION
Currently, information transmission, for example data and video transmission for cable television (CATV) and wireless communication systems, often utilizes long-haul fiber-optic links. In order to take advantage of the enormous bandwidth that the optical fiber provides, the optical carrier has to be modulated and transmitted at high rates. Directly modulated laser diodes have been used for this purpose, however, these modulated laser diodes are known to be susceptible to frequency chirping. This frequency chirping problem can be avoided by employing external modulators. Accordingly, analog systems based on efficient external modulators are highly desirable for many commercial and military applications, for example sensor systems, fiber-optic telecommunication links, and microwave antenna remote detection systems.
Guided-wave electro-optic modulators in which light is confined within a waveguiding area of small transversal size are promising candidates for such applications. Although several commercial integrated-optic (IO) modulators are currently available, distortions caused by nonlinearities in their modulation curves severely degrade their performance. Typically, these modulation curves exhibit sine-squared behavior, thus hindering widespread deployment of integrated-optic modulators in high-performance analog optical systems typically requiring that nonlinear distortions be 95 dB below the carrier.
A number of linearization techniques have been developed to suppress the nonlinear distortions produced by 10 modulators. In general, these techniques fall into two categories, namely, electronic compensation and optical methods of linearization. Electronic techniques, based on predistortion compensation or feedforward compensation, involve expensive high-speed electronic components and are limited by a bandwidth of a few GHz or less. Optical techniques include the dual-polarization technique, the use of two- and three-section directional couplers, parallel modulation schemes, and various cascaded schemes. Common to all these techniques is that the improvement in linearity is achieved at the expense of more complex device designs, particularly in the case of cascaded schemes involving multiple modulator structures. Many of these techniques employ multiple electrode sections and, as a result, require several separate modulating sources and/or several bias controls. The need for several modulating sources and bias controls seriously hampers the use of these linearized devices in high-speed applications where efficient and convenient matching of the electrode structure to the microwave source is required. In addition, complex devices are prone to thermal and temporal instabilities that make these schemes unattractive for practical applications.
Accordingly, there is a need for an integrated-optic modulator with highly linear performance, simple design, and reduced sensitivity to fabrication deviations. In addition, a modulator without high-speed electronic components for linearization would be advantageous. Furthermore, a modulator with a simple uniform electrode structure which can be conveniently matched to a single microwave driving source would be particularly advantageous. Also, a modulator which can be utilized at high speeds would be particularly advantageous.
BRIEF SUMMARY OF THE INVENTION
The subject invention pertains to electro-optic devices and, in particular, to an integrated-optic modulator with a linearized transfer curve. The subject invention is further concerned with realizing modulators with suppressed nonlinear distortions for analog optical links, for example fiber-optic communication links and cable television systems. In a specific embodiment, the subject invention relates to a coupled-waveguide modulator with a linearized transfer curve.
An object of the subject invention is to provide a means of linearizing the transfer curve of a Y-fed (1×2) directional coupler modulator. Another object is to provide a means of suppressing the third-order nonlinear distortion of a Y-fed directional coupler modulator. This can be achieved by properly adjusting the interaction length with respect to the coupling length of the device. Both a single-section and two-section (Db-type) embodiment of a Y-fed coupler modulator can be linearized with the two-section embodiment being least sensitive to fabrication imperfections.
A specific embodiment of the subject invention involves a single-section Y-fed coupler modulator having an interaction length of about 2.63 times the coupling length such that the modulator has a linearized transfer curve.
An additional embodiment of the subject invention involves a single-section Y-fed coupler modulator with suppressed nonlinear distortions, wherein the interaction length is about 2.86 times the coupling length.
A further embodiment of the subject invention involves a two-section Y-fed coupler modulator with a linearized transfer curve, wherein the total interaction length is divided in two sections, one section about 4.1 times the coupling length and the other section about 1.05 times the coupling length.
An additional embodiment of the subject invention involves a two-section Y-fed coupler modulator with suppressed nonlinear distortions, wherein one section is about 2.3 times the coupling length and the other section is about 1.02 times the coupling length.
In a preferred embodiment of the subject invention, a two-section Y-fed coupler modulator is formed in a substrate with a region where the electro-optic coefficient is but opposite in sign to that of the rest of the substrate. In specific embodiment, the electro-optic coefficient in this region can be equal in magnitude and opposite in sign to that of the rest of the substrate. Depending on the material of the substrate, this region can be, for example, a domain reversal in a ferroelectric material, such as lithium niobate, or a region produced by reversed poling of a polymer material with artificially created electro-optic properties. In a specific embodiment, a uniform electrode section can be used. The simplicity of this embodiment combined with significantly improved linear performance is especially advantageous for practical applications.


REFERENCES:
patent: 4795225 (1989-01-01), Sakai et al.
patent: 4887884 (1989-12-01), Hayden
patent: 5283685 (1994-02-01), Horsthuis
patent: 5757023 (1998-05-01), Koteles et al.
patent: 5970186 (1999-10-01), Kenney et al.

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