JPH11281831A - Grating coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the grating coupler which can correct the shape of a projection beam or increase incidence coupling efficiency. SOLUTION: The grating coupler is constituted by stacking an optical waveguide 20 on a substrate 10, forming a grating consisting of plural transparent electrodes on the surface of the optical waveguide 20, and forming a common electrode 15 in the area which is on the border surface between the substrate 10 and optical waveguide 20 and faces the grating 22. In this case, respective elements 221 to 22n of the grating are so applied with a voltage that the radiation loss coefficient g(z) have a nearly Gaussian distribution in the propagation direction of waveguide light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grating coupler, and more particularly, to a grating in which an optical waveguide layer is provided on a substrate and the outside and the optical waveguide layer are optically coupled by a grating formed in contact with the optical waveguide layer. For couplers.

[0002]

2. Description of the Related Art In general, an optical waveguide is provided with a high-refractive-index optical waveguide layer on a substrate or a low-refractive-index optical buffer layer formed on the substrate, and the surface of the optical waveguide layer is air (low-refractive-index layer).
Then, the laser beam emitted from the semiconductor laser is input to and guided by the optical waveguide layer, and is output from the optical waveguide layer. For example, in a waveguide type acousto-optic deflector, an IDT (inter-d
A surface acoustic wave is generated from the digital-transducer (comb-shaped electrode) in the optical waveguide layer to deflect the laser beam. The coupling for inputting or outputting the laser beam to or from the optical waveguide layer is practically using a grating.

A conventional grating coupler is shown in FIG.
As shown in (A) and (B), for example, when used as an output means, the duty ratio a / Λ (−１: period , A: grating line width) is designed to be a constant 0.5 as a whole of the grating 55. 9A shows an example in which the grating 55 is formed on the surface of the optical waveguide layer 52, and FIG. 9B shows an example in which the grating 55 is formed at the interface between the substrate 51 and the optical waveguide layer 52.

[0004]

However, in the conventional grating coupler, when the duty ratio is constant throughout, that is, when the shape of the grating is constant, the radiation loss coefficient of the grating is constant in the propagation direction of the guided light. As a result, light having a larger power is output toward the upstream side of the grating coupled with the larger guided light power. As a result, as shown in FIG. 10, the energy distribution of the entire emitted light in the direction parallel to the propagation direction z, that is, the output-coupled radiation beam amplitude distribution g (z) becomes an exponential distribution. Such a distribution causes problems such as difficulty in designing the optical system and an increase in the minimum beam diameter of convergent light after passing through the optical system (out of focus) when the optical system is inserted immediately after emission. This leads to a decrease in the performance of the entire optical device when, for example, a waveguide type acousto-optic deflector provided with a grating coupler is used.

On the other hand, when a grating coupler is used as an input means, as shown in FIG. 11, the coupling efficiency depends on the amplitude distribution h of the incident beam on the surface of the grating 55.
(Z) and the radiation beam amplitude distribution g (z) shown in FIG.
Is proportional to the magnitude of the overlap integral value. Therefore, when a light beam having a Gaussian distribution h (z) is incident using a conventional grating coupler having a constant duty ratio as an output means, the overlap integral value of the distributions h (z) and g (z) becomes 1
, And even when the incident position is optimized, the overlap integral value is only 0.8, and no further coupling efficiency can be realized.

SUMMARY OF THE INVENTION An object of the present invention is to provide a grating coupler which can correct the shape of an output beam when used as an output means, and can improve the incident coupling efficiency when used as an input means.

[0007]

In order to achieve the above objects, a grating coupler according to the present invention comprises a grating formed of a transparent electrode and a grating in a region in contact with the optical waveguide layer and facing the grating or on the back surface of the substrate. An electrode is formed in a region opposite to the above, and an electric field is applied between the grating and the electrode such that the radiation loss coefficient of the grating has a predetermined distribution in the propagation direction of the guided light.

When an electric field is generated between the grating composed of the transparent electrode and the electrode facing the same, the refractive index of the portion of the optical waveguide layer in contact with the grating changes due to the electro-optic effect of the optical waveguide layer. The radiation loss coefficient of the grating corresponding to the portion where the refractive index has changed will also change. Therefore, by setting the voltage applied to each grating to a predetermined value, it is possible to change the radiation loss coefficient of diffracted light for each grating. The radiation beam amplitude distribution of the grating coupler depends on the radiation loss coefficient of each grating, and can be changed by the value of the voltage applied to each grating.

In the present invention, the radiation loss coefficient distribution of the grating is preferably substantially Gaussian distribution. Thus, when the grating coupler according to the present invention is used as the input means, the integral value of the overlap with the amplitude distribution of the incident beam having the Gaussian distribution is increased, and the incident coupling efficiency is improved. Further, when the grating coupler according to the present invention is used as an output unit, the amplitude distribution of the output beam becomes substantially Gaussian distribution, and the handling of the light beam in the optical system on the output side is facilitated. The minimum beam diameter can be made smaller.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the grating coupler according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows an optical waveguide device provided with a grating coupler according to the present invention, and an optical waveguide layer 2 laminated on a substrate 10.
A grating 21 for incidence and a grating 22 for emission are formed on the reference numeral 0. Further, the optical waveguide layer 20
An IDT 25 is formed on the upper side.
By generating a surface acoustic wave whose frequency continuously changes with a voltage applied to the optical waveguide 25, the guided light is deflected.

In the optical waveguide device having the above configuration, the grating coupler according to the present invention is applied to at least one of incident coupling and outgoing coupling.

(First Embodiment, see FIGS. 2 and 3)
In the embodiment, Zn is formed on a substrate 10 made of R-plane sapphire.
An optical waveguide layer 20 made of O is laminated, and a plurality of transparent electrodes are formed as emission gratings 22 on the surface of the optical waveguide layer 20. The transparent electrode is an interface between the substrate 10 and the optical waveguide layer 20 and faces the grating 22. The common electrode 15 is formed in the region. The common electrode 15 is made of ITO (Indium)
A transparent electrode film typified by Tin Oxide) may be formed by a photolithography method or the like.

Each element 22 _{1 of the} grating 22
To 22 _n are, as shown in FIG.
The power supply unit is connected to the power supply unit 31 so as to apply a different voltage to each of the 2 _{1 to} 22 _n , or as shown in FIG. 3
2 are connected. The common electrode 15 is connected to the ground.

When an electric field is generated between the grating 22 and the common electrode 15 by the power supply units 31 and 32, the refractive index of the portion of the optical waveguide layer 20 in contact with each of the elements 22 _{1 to} 22 _n changes, and the radiation loss coefficient Also change. The radiation beam amplitude distribution Each element 22 ₁ of the grating coupler
Because it is dependent on the radiation loss coefficient of 22 _n, by adjusting the voltage applied from the power supply unit 31, in this first embodiment, a substantially Gaussian distribution of the radiation beam amplitude distribution g (z) Was set as follows. Therefore, an output beam having an amplitude distribution substantially Gaussian distribution is output, and convergence is improved.

On the other hand, this grating coupler can be used for incident coupling. In this case, a high incidence coupling efficiency can be obtained by injecting a light beam having a Gaussian distribution.

As for the power supply section, each of the elements 22 ₁ to 22 ₁
Applying a different voltage every 22 _n (FIG. 3 (A)
), A precise Gaussian distribution g (z) can be obtained. In a configuration in which different voltages are applied to a plurality of adjacent elements (see FIG. 3B), the number of voltages to be set is small, the configuration is simplified, and the cost is reduced.

(Second Embodiment, see FIG. 4) In the second embodiment, the common electrode 15 is formed on the back surface of the substrate 10 in a region facing the grating 22. As in the first embodiment, the other configuration and usage, and that the radiation beam amplitude coefficient of the grating 22 has a substantially Gaussian distribution.

In the third embodiment, the grating 22 is formed at the interface between the substrate 10 and the optical waveguide layer 20, and the (transparent) common electrode 15 is formed on the optical waveguide layer 20.
And formed in a region facing the grating 22. As in the first embodiment, the other configuration and usage, and that the radiation beam amplitude coefficient of the grating 22 has a substantially Gaussian distribution.

(Fourth Embodiment, see FIG. 6) In the fourth embodiment, an S layer is formed on a silicon substrate 10 as an optical buffer layer.
An iO ₂ film 11 is formed, and an optical waveguide layer 20 made of ZnO is laminated on the SiO ₂ film 11. The grating 22 is formed on the surface of the optical waveguide layer 20. The common electrode 15 is formed at an interface between the SiO ₂ film 11 and the optical waveguide layer 20 and in a region facing the grating 22.
When the resistivity of the silicon substrate 10 is low, the substrate 1
0 itself can be used instead of the common electrode 15. As in the first embodiment, the other configuration and usage, and that the radiation beam amplitude coefficient of the grating 22 has a substantially Gaussian distribution.

(Fifth Embodiment, see FIG. 7) The fifth embodiment basically has the same configuration as that of the fourth embodiment, except that the grating 22 is formed of the SiO ₂ film 11 and the optical waveguide layer 2.
The common electrode 15 is formed on the surface of the optical waveguide layer 20 in a region facing the grating 22.

(Sixth Embodiment, See FIG. 8) In the sixth embodiment, the optical waveguide layer 2 is formed by diffusing Ti into the upper portion (the hatched portion in FIG. 8) of the substrate 10 made of LiNbO ₃ .
The grating 22 is formed on the surface of the optical waveguide layer 20. Further, the common electrode 15 is
And is formed in a region facing the grating 22. As in the first embodiment, the other configuration and usage, and that the radiation beam amplitude coefficient of the grating 22 has a substantially Gaussian distribution.

(Other Embodiments) The grating coupler according to the present invention is not limited to the above embodiments, but may be variously modified within the scope of the invention.

For example, in the first to fifth embodiments, R
A combination of a planar sapphire substrate or a silicon substrate and a ZnO thin-film optical waveguide layer is used, but a sapphire substrate with another plane orientation or a substrate of another material such as glass, and another optical waveguide layer material having an electro-optical effect. May be combined. In the sixth embodiment, a LiNbO ₃ substrate and Ti
Although the combination with the diffused optical waveguide layer is used, the substrate is L
Another substrate such as iTaO ₃ may be used, and the optical waveguide layer may be an optical waveguide layer manufactured by another method such as proton exchange.

The setting of each voltage value applied to each of the elements 22 _{1 to} 22 _n of the grating 22 is fixed at all times, changed at every time of use, or changed for incident light and outgoing light. A form in which the result of measuring the beam distribution is fed forward or fed back may be used.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an optical waveguide device including a grating coupler according to the present invention.

FIG. 2 is an explanatory diagram showing a grating coupler according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a voltage application circuit of the grating coupler.

FIG. 4 is a sectional view showing a grating coupler according to a second embodiment of the present invention.

FIG. 5 is a sectional view showing a grating coupler according to a third embodiment of the present invention.

FIG. 6 is a sectional view showing a grating coupler according to a fourth embodiment of the present invention.

FIG. 7 is a sectional view showing a grating coupler according to a fifth embodiment of the present invention.

FIG. 8 is an explanatory diagram showing a grating coupler according to a sixth embodiment of the present invention.

FIG. 9 is a cross-sectional view showing an optical waveguide device including a conventional grating coupler.

FIG. 10 is an explanatory diagram showing a case where a conventional grating coupler is used for emission coupling.

FIG. 11 is an explanatory diagram showing a case where a conventional grating coupler is used for incident coupling.

[Explanation of symbols]

10 ... substrate 15 ... common electrode 20 ... optical waveguide layer 22 ... grating 22 ₁ through 22 _n ... Elements 31, 32 power supply unit

Claims (3)

[Claims] An optical waveguide layer having an electro-optical effect is provided on a substrate, and a plurality of transparent couplers are provided in a grating coupler for optically coupling the outside and the optical waveguide layer by a grating formed in contact with the optical waveguide layer. A grating formed of an electrode, an electrode formed in a region in contact with the optical waveguide layer and facing the grating or in a region facing the grating on the back surface of the substrate, and a radiation loss coefficient of the grating is predetermined in a propagation direction of the guided light. Means for generating an electric field between the grating and the electrode so as to have the following distribution: 2. The grating coupler according to claim 1, wherein said electric field generating means applies a different voltage to each grating or to a plurality of adjacent gratings. 3. The grating coupler according to claim 1, wherein the radiation loss coefficient distribution of the grating is substantially Gaussian distribution.