Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-04-30
2001-05-01
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S007000, C385S037000, C359S285000, C359S237000
Reexamination Certificate
active
06226427
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical isolator, and more particularly to, an optical waveguide type integrated optical isolator.
BACKGROUND OF THE INVENTION
Semiconductor lasers are widely used as a light source for ultra-high speed and long distance optical fiber communication. However, due to a little reflecting-back light from an optical fiber, there occurs a noise in conventional semiconductor lasers. Because of this, an optical isolator needs to be always used as an optics component for removing the reflecting-back light to the semiconductor laser.
In general, conventional optical isolators comprise a magnetooptic crystal with Faraday effect and two polarizers which are disposed sandwiching the magnetooptic crystal and have polarization planes shifted 45° to each other. In the conventional optical isolators, light emitted from a semiconductor laser is transmitted through the first polarizer, entered into the magnetooptic crystal, where the polarization plane is rotated 45° in a predetermined direction, then transmitted through the second polarizer without being attenuated. On the other hand, reflecting-back light is transmitted through the second polarizer, entered into the magnetooptic crystal, where the polarization plane is further rotated 45° in the predetermined direction. Thereby, the polarization plane of the reflecting-back light is shifted by 90° to the polarization plane of the first polarizer, therefore the first polarizer can prevent the reflecting-back light from entering into the semiconductor laser.
However, in the conventional optical isolators, it is required to prepare at least four elements including a magnet and fabricate them, therefore they are very costly.
Moreover, it is, in fact, difficult for the conventional optical isolator to be monolithically integrated with the semiconductor laser.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an optical isolator whose production cost can be significantly reduced.
It is a further object of the invention to provide an optical isolator which can be monolithically integrated with a semiconductor laser.
According to the invention, an optical isolator, comprises:
a medium through which a light transmits and in which a dynamic diffraction grating that periodically repeats a refractive index distribution is formed;
wherein the dynamic diffraction grating causes a propagation loss difference between an incident light which is supplied into the medium and a returning light which is supplied into the medium in a direction reverse to the incident light.
According to another aspect of the invention, an optical isolator, comprises:
a cylindrical light-transmitting medium in which a periodical refractive index distribution is formed in the direction of a radius of the medium; and
means for rotating the cylindrical light-transmitting medium;
wherein an incident light is supplied to a circumferential surface of the cylindrical light-transmitting medium.
Furthermore, according to another aspect of the invention, an optical isolator, comprises:
an optical waveguide into which a light can be led and in which a diffraction grating that is formed as a wave traveling with a periodical refractive index distribution in a predetermined direction is formed;
wherein the diffraction grating causes a propagation loss difference between an incident light which is supplied into the optical waveguide and a returning light which is supplied into the optical waveguide in a direction reverse to the incident light.
Furthermore, according to another aspect of the invention, an optical isolator, comprises:
an optical waveguide composed of a material that can be monolithically with a semiconductor laser;
first and second electrodes which are formed both sides of the optical waveguide;
a terminal resistance which is connected to terminals of the first and second electrodes and is set to match with a characteristic impedance of a line with the first and second electrodes; and
a voltage source which applies a high-frequency voltage to opposite ends to the terminals of the first and second electrodes;
wherein an incident light is supplied into the optical waveguide.
Furthermore, according to another aspect of the invention, an optical isolator, comprises:
an optical waveguide with an acoustooptic characteristic;
a comb-like electrode which is formed on the optical waveguide; and
a voltage source which applies a high-frequency voltage to the comb-like electrode to induce an ultrasonic wave in the optical waveguide, whereby a diffraction grating that travels with a periodical refractive index variation in a predetermined direction is formed in the optical waveguide;
wherein an incident light is supplied into the optical waveguide.
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T. Shintaku; “Higher-Order-Mode-Converted Optical Waveguide Isolator”; Fifth Optoelectronics Conference (OEC '94) Technical Digest, Jul. 1994, Makuhari Messe, pp. 354-355.
Bovernick Rodney
Kim Ellen E.
McGuireWoods LLP
NEC Corporation
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