Optical waveguide isolator

Details Optical waveguide isolator Optical waveguide isolator

2000-03-16

2001-03-27

Ullah, Akm E. (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C257S011000

Reexamination Certificate

active

06208795

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical isolator, especially an optical wave guide capable of being integrated on a substrate together with a semiconductor laser and an optical wave guide.
2. Related Art Statement
In the optical communication system, an optical isolator is used to prevent reflected light and scattered light in the end face of the optical fiber from returning to a light source side. An optical isolator using a rotation of polarization plane according to the optical-magnet material, has been put to practical use as an optical isolator now. For example, the optical isolator of Farady rotation type is constituted by a polarizer, a Faraday rotator of optically transparent material and an analyzer, so that polarized components corresponding to the plane of polarization of polarizer out of advanced circular polarization of light to the forward direction, pass through the polarizer, the plane of polarization rotates by Faraday rotator by 45°, and emanate by passing the analyzer which is inclined at the polarizer by 45°. On the other hand, the return light propagated in the direction opposite to the forward direction is obstructed with a polarizer, since after passing the analyzer, the plane of polarization of the return light receives the rotation of 45° by the Faraday rotator and returns to the polarizer. The polarized beam splitter and the double refraction prism are utilized as polarizer and analyzer used by conventional optical waveguide isolator.
The above well known optical waveguide isolator was not able to be manufactured with a semiconductor optical element such as semiconductor lasers and optical modulators as one body, since the well known optical waveguide isolator does not have the semiconductor device structure. Therefore, the well known optical waveguide isolator must be made as another discrete structure with the substrate of the optical circuit to which the semiconductor device such as semiconductor lasers is integrated, so that the manufacturing step becomes complicated and the manufacturing cost becomes expensive, too. Particularly, since the phase matching is necessary, and a precise processing is necessary, so that the manufacturing process becomes complex. On the other hand, if the optical waveguide isolator can be formed on the substrate by using the same semiconductor manufacturing technology as semiconductor devices such as semiconductor lasers and photo diodes, it can manufactured with precise manufacturing step and thus the manufacturing cost can be made cheap greatly.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the above described disadvantages of the conventional.
It is another object of the present invention to provide an optical waveguide isolator capable of integrating it on the semiconductor substrate by using the semiconductor manufacturing technology.
It is another object of the present invention to provide an optical waveguide isolator capable of being manufactured without becoming necessary of phase matching and using complicated manufacturing step.
According to the present invention, there is provided an optical isolator comprising: a semiconductor optical amplifier structure including a semiconductor substrate of first conductivity type having a surface of a layer to be formed thereon, a first cladding layer of first conductivity type formed on the substrate, an active layer formed on the first cladding layer, a second cladding layer of the second conductivity type opposite to first conductivity type, formed on active layer, a first electrode formed on the surface of the semiconductor substrate opposite to the surface to be formed as a layer, and a second electrode formed on the second cladding layer; the first and the second cladding layers and the active layer form an optical waveguide in which the light wave propagates, the semiconductor optical amplifier structure further comprising a light absorptive magnetic material layer having light absorption function for the light wave propagating through the optical waveguide, the magnetic material layer is magnetized so as to have the magnetic-field component in the direction which corresponds to the direction where a magnetic vector of the light wave vibrates, the waveguide structure body has a nonreciprocity optical characteristic that effective refractive index changes into the light wave to which the optical waveguide is propagated according to the magneto-optical effect of the light absorptive magnetic material layer according to the direction of propagation, by the effective refractive index change in the nonreciprocity, the attenuation of the first light wave that the optical waveguide is propagated in the first direction caused when the said waveguide is propagated, becomes small more than the attenuation of the second light wave propagated in the second direction opposite to the first direction caused when the said waveguide is propagated.
The present invention is based on the recognition that the refractive index, that is, the equivalent refractive index of the light wave propagating in the optical waveguide can be changed in the entire waveguide according to the propagating direction of the light wave by using the magnet-optical effect. If the equivalent refractive index can be changed according to the propagating direction of the light wave, the attenuation amount of the light wave propagated to one direction and the attenuation amount of the light wave propagated to the opposite direction can be made different, and thus optical waveguide isolator can be achieved by using the difference of the attenuation amount in case of propagating the light wave in the optical waveguide.
The present invention is based on the above described recognition, to obtain nonreciprocal refractive index change by magnet-optical effect, an optical absorptive magnetic material layer is formed on the optical waveguide, the optical absorptive magnetic material layer is magnetized in the direction corresponding to vibrating direction of a magnetic vector of the light wave propagating the optical waveguide. The light wave propagates the optical absorptive magnetic material layer with the optical waveguide, so that the propagated light wave receives the magnet-optical effect of the optical absorptive magnetic material layer as the entire waveguide structure, and thus not only real part but also imaginary part of refractive index becomes nonreciprocal in the case of the optical absorption magnetic material layer, thereby being capable of making the amount of attenuation different in case of propagating the light wave in the optical waveguide in accordance with the propagating direction. In this case, the amount of the attenuation can differ only by appropriately setting magnetizable direction of the magnetic material layer according to the direction of propagation. As a result, a complicated structure for the phase matching and precise processing step become unnecessary.
Next, the nonreciprocity of the optical waveguide structure according to the present invention is explained theoretically due to the magnet-optical effect thereof. Here, the xyz coordinate system described later and shown in
FIG. 1
(the propagating direction of the light wave is assumed to be a z direction and the directions orthogonal to (the propagating direction of the light wave are assumed to be x and y directions) is assumed. In general, the character of the optical-magnet material is shown by the dielectric tensor, and when magnetic material formed on the waveguide is not magnetized, the permittivity &egr; is shown by the following expression.
ϵ
=
ϵ
0
⁢
⌈
ϵ
xx
0
0
0
ϵ
yy
0
0
0
ϵ
zz
⌉
(
1
)
Herein, &egr;
0
shows the permittivity in the vacuum. As is seen from the expression (1), when optical absorption magnetic material is not magnetized, the dielectric tensor has symmetric property, and thus does not exhibit the nonreciprocity of the light wave into w

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