Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-09-02
2001-05-01
Sikder, Mohammad (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S003000, C385S004000, C385S005000, C385S008000, C385S009000, C385S014000, C385S015000
Reexamination Certificate
active
06226423
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide element and especially relates to an optical waveguide element used preferably for an optical waveguide device of a traveling-wave type which can perform a high speed modulation.
2. Description of Related Art
Recently, the optical waveguide device has been used as an external modulator according to an improvement of optical fiber communication system which permits high speed and large capacity signal transmissions. Since the external modulator mentioned above is used under a condition such that a high speed switching operation is required, the optical waveguide device used as the external modulator needs a high speed modulating property. To this end, there have been known some techniques such that an electrode, for applying the modulation signal, of the optical waveguide element which constructs the optical waveguide device is shaped particularly or that a buffer layer made of for example silicon dioxide is formed between the electrode and a substrate which constructs the optical waveguide element.
However, in the techniques mentioned above, there are drawbacks such that a producing cost is increased since producing steps of the electrode are complicated and that a positioning precision of the electrode is deteriorated when the buffer layer is formed. In order to eliminate the drawbacks mentioned above, the applicant disclosed, in Japanese Patent Laid-Open Publication No. 10-133159 (JP-A-10-133159), an optical waveguide element wherein a thickness of a portion of the substrate, which constructs the optical waveguide element, at least positioned correspondingly to the electrode is made thin so as to perform a high speed modulation.
However, in the case that the optical waveguide element mentioned above is used actually as the optical waveguide device in such a manner that an optical fiber is connected to the optical waveguide element and an electrical connector is further connected to the optical waveguide element so as to apply a modulation signal to the electrode, since a characteristic impedance of the optical waveguide device is relatively smaller than that of the modulation signal, an impedance matching therebetween is not sufficient. Therefore, there is a drawback such that a high frequency loss is increased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical fiber element which can perform an impedance matching of an optical waveguide device and decrease a high frequency loss of the optical fiber device which it is used in the optical fiber device.
According to the invention, an optical waveguide element including a substrate showing electrooptical effects and having a pair of major planes opposed with each other, an optical waveguide formed on one major plane of said substrate, and an electrode for applying a modulation signal to an optical wave transmitted through said optical waveguide, comprises; a thin thickness portion having a relatively thin thickness formed to a portion of said substrate at least positioned correspondingly to said electrode; and a buffer layer formed between said substrate and said electrode, wherein an impedance conformity with the modulation signal applied to said electrode is performed by controlling a thickness of said thin thickness portion.
The inventors investigated how to decrease a high frequency loss of the optical waveguide element and found the following facts.
It is well known that a thickness of the electrode is inversely proportional to a characteristic impedance of the optical waveguide element and that a thickness of the buffer layer is directly proportional to a characteristic impedance of the optical waveguide element. That is to say, if a thickness of the electrode is increased, a characteristic impedance is decreased correspondingly. Moreover, if a thickness of the buffer layer is increased, a characteristic impedance is increased correspondingly.
Under such an assumption, the inventors investigated eagerly a technique for improving a characteristic impedance of the optical waveguide element. As a result, the inventors found that, when the buffer layer made of for example SiO
2
was formed between the substrate and the electrode and the thin thickness portion was formed in the substrate at a portion corresponding to the electrode, a characteristic impedance of the optical wavelength element was varied in response to a thickness of the thin thickness portion. That is to say, the inventors found an unexpected phenomena such that a characteristic impedance of the optical waveguide element was decreased if a thickness of the thin thickness portion was increased and a characteristic impedance of the optical waveguide element was increased if a thickness of the thin thickness portion was decreased. The present invention is achieved on the basis of the facts that the inventors found as mentioned above.
According to the invention, since a characteristic impedance of the optical waveguide element can be matched correctly with that of the modulation signal i.e. an external high frequency power source, it is possible to achieve a lowering of high frequency loss.
REFERENCES:
patent: 4387343 (1983-06-01), Kondo
patent: 4685763 (1987-08-01), Tada et al.
patent: 5313535 (1994-05-01), Williams
patent: 5680497 (1997-10-01), Seino et al.
patent: 5748358 (1998-05-01), Sugamata et al.
patent: 5815610 (1998-09-01), Tokano et al.
patent: 0 413 568 A2 (1991-02-01), None
patent: 0 828 175 A2 (1998-03-01), None
patent: 10-133159 (1998-05-01), None
Database WPI, Week 199345, Derwent Publications Ltd., London, GB; JP 05 257104 A (Hikari Keisoku Gijutsu Kaihatsu KK) Oct. 8, 1993, abstract.
Hirai Takami
Imaeda Minoru
Kondo Jungo
Minakata Makoto
Burr & Brown
NGK Insulators Ltd.
Sikder Mohammad
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