JPH077134B2 - Waveguide type grating coupler - Google Patents

Description

The present invention relates to a waveguide grating coupler, and more particularly to a waveguide grating coupler used for an optical waveguide element that performs optical information processing using a waveguide.

[Conventional technology]

A thin film waveguide that propagates light can be used as a circuit for multi-wavelength optical demultiplexing or photosynthesis by using a lens or a diffraction grating, or as an optical spectrum analyzer by interfering guided light and surface acoustic waves. It is an important device for optical information processing.

FIG. 3 is a schematic side view showing an example of a conventional waveguide type grating coupler. Conventionally, when the guided light is extracted from the thin film waveguide 2 provided on the substrate 1 to the outside of the waveguide (here, in the air) as radiated light, as shown in FIG. 3, the thin film waveguide 2 is refracted differently from air. A grating coupler 4 having a structure in which a material having a refractive index is formed periodically is used. In this example, the guided light emitted from the semiconductor laser 5 coupled from the end face of the thin film waveguide 2 is guided to the waveguide 2 by the grating coupler 4.
It is emitted as radiated light in the radiation angle θ direction to the outside.

Next, the emission angle θ of the emitted light emitted from the grating coupler 4 will be described. In the portion of the thin film waveguide 2 facing the grating coupler 4, many spatial harmonics are generated from the guided light propagating in a predetermined waveguide mode. The propagation constant βh of the harmonic in the coupler 4 is β, the propagation constant of the guided light in the waveguide 2 is β, and the grating coupler 4 is
Of the grating structure of (grating period)
If p and the order of the harmonic are m, they are expressed by the equation (1).

βh = β−2mπ / p (m = 0, ± 1, ± 2, ...) (1) Here, the propagation constant βh of the harmonic in the grating coupler 4 is the guided light of the radiated light propagation constant β0. The component in the traveling direction and the component in the same direction of the propagation constant βh need to match. Now, assuming that the wavelength of the emitted light in air is λ0 and the propagation constant is β0, the equation (2) is established.

βh = β0 · sin θ (2π / λ0) · sin θ (2) The equivalent refractive index of the guided light propagating in the waveguide 2 is N (= β / β
0), β = N · 2π / λ0, so (1)
The equation can be transformed into equation (3).

(N-sin θ) / λ0 = m / p = k (3) Formula (3) is rearranged into formula (4) which is a sine wave function of the radiation angle θ.

sin θ = N−kλ 0 (4) That is, when the order m of the harmonics generated in the grating coupler 4 is properly selected and the selected m-th order harmonic is the radiated light to be guided outside the waveguide, this radiated light The radiation angle θ of is the equivalent refractive index N of the guided light propagating in the waveguide 2 in a predetermined guided mode, the wavelength λ0 of the guided light in air, the grating period p of the grating coupler 4 and the radiated light. It is a function of the structural constant k of the grating coupler 4 determined by the harmonic order m of.

[Problems to be solved by the invention]

In the above-mentioned conventional waveguide type grating coupler, as can be seen from the equation (4), the radiation angle θ changes as the wavelength λ0 of the emitted light changes.

Since the wavelength λ0 of laser light generally fluctuates with temperature in a semiconductor laser, the radiation angle θ also changes in this waveguide type grating coupler, and the intensity of laser light coupled to a circuit outside the waveguide changes. However, a practical problem occurs.

An object of the present invention is to provide a waveguide type grating coupler in which the variation of the emission angle of the emitted light is small even if the wavelength of the guided light propagating in the waveguide changes.

[Means for solving problems]

A waveguide type grating coupler of the present invention is a waveguide type waveguide coupler that guides guided light in a waveguide propagating in a predetermined waveguide mode to the outside of the waveguide as radiated light by means of periodical refractive index changing means provided in the waveguide. In the grating coupler, when the wavelength λ0 of the guided light in air changes, the amount of change in the equivalent refractive index N of the waveguide facing the periodic changing means includes the grating period of the periodic changing means. Means for making the amount of change of the product k · λ0 of the constant k and the wavelength λ0 substantially equal to each other are provided.

[Action]

In the above formula (4), since the equivalent refractive index N of the thin film waveguide hardly changes depending on the wavelength λ0 of the guided light, the conventional waveguide type grating coupler causes the variation of the radiation angle θ of the radiated light. Therefore, in the present invention, when the wavelength λ0 of the guided light propagating through the thin film waveguide increases, the structure is such that the equivalent refractive index N of the thin film waveguide increases, and the fluctuations of the two are canceled out. The change in the angle θ is reduced.

〔Example〕

Next, the present invention will be described with reference to FIGS. 1 and 2.

FIG. 1 is a schematic side view showing one embodiment of the waveguide type grating coupler of the present invention, and FIG. 2 is a view showing one characteristic example of the material used for the cladding layer 3 in FIG.

As shown in FIG. 1, in the present embodiment, in the structure of the conventional example including the substrate 1, the thin film waveguide 2 and the grating coupler 4 shown in FIG. 3, the thin film waveguide 2 and the grating coupler 4 are provided. In between, the cladding layer 3 is further arranged so as to face the grating coupler 4. The clad layer 3 is
In the change range of the wavelength λ0 of the guided light 3, the refractive index increases as the wavelength λ0 increases. Clad layer 3
As the material used for, a substance having a refractive index dispersion due to dipole absorption, such as GaAs (gallium arsenide) having exciton absorption, can be used as shown by the refractive index n and the absorption coefficient α in FIG. However, since the propagation loss of the guided light due to the cladding layer 3 becomes large near the absorption center, that is, the maximum value of the absorption coefficient α, the wavelength λ0 needs to be selected in a wavelength range outside the absorption center of the material of the cladding layer 3. There is.

With the configuration as described above, the equivalent refractive index N of the thin film waveguide 2 can have the same wavelength change as that of the cladding layer 3, so that the fluctuation of the equivalent refractive index N and the fluctuation of k · λ0 are canceled. The change in the radiation angle θ can be reduced. Especially (4)
If the cladding layer 3 having a refractive index change that matches the amount of change of the wavelength λ0 of the guided light is selected so that sin θ = N−kλ0 in the equation, the change of the radiation angle θ due to the change of the wavelength λ0 can be almost eliminated. it can.

〔The invention's effect〕

As described above, according to the present invention, when the wavelength λ0 of the guided light changes in the air, the change amount of the equivalent refractive index N of the waveguide facing the periodic changing means includes the grating period of the periodic changing means. Since the means for making the amount of change of the product k · λ0 of the structural constant k and the wavelength λ0 substantially equal to each other is provided, even if the wavelength of the guided light is changed, the fluctuation of the radiation angle of the radiated light outside the waveguide is reduced. Has the effect of

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side view showing an embodiment of a waveguide type grating coupler of the present invention, and FIG. 2 shows one characteristic of a material used for the cladding layer 3 in FIG. FIG. 3 and FIG. 3 are schematic side views showing an example of a conventional waveguide type grating coupler. 1 ... Substrate, 2 ... Thin film waveguide, 3 ... Clad layer, 4
…… Grating coupler, 5 …… Semiconductor laser.

Claims (1)

[Claims] 1. A waveguide type grating coupler for guiding guided light in a waveguide propagating in a predetermined guided mode to the outside of the waveguide as radiated light by means of periodic fluctuation means of a refractive index provided in the waveguide. When the wavelength λ0 of the guided light changes in the air, the equivalent refractive index N of the waveguide facing the periodically varying means is N.
The waveguide grating coupler is characterized in that it comprises means for making the amount of change of (1) substantially equal to the amount of change of the product k · λ0 of the structural constant k including the grating period of the periodic varying means and the wavelength λ0.