JP2586572B2 - Refractive index distributed optical coupler and method for producing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical coupler for inputting / outputting light to / from a thin-film optical waveguide, and more particularly, to a refractive index distribution in a propagation direction of light in the optical waveguide. And a method for producing the same.

[Prior art] Conventionally, a thin film having a higher refractive index than a substrate such as glass or SiO ₂ is provided on a substrate, or a region having a high refractive index is created by diffusing Ti or the like into a dielectric substrate made of LiNbO ₃ material. Optical couplers for confining and transferring light in this high refractive index region are already known. In order to guide light to the optical waveguide in such an optical coupler, a lens is provided on the optical waveguide 62 on the substrate 61 as shown in FIG.
End face coupling that focuses light at 63 etc. and enters from the end face, 7th
As shown in the figure, prism coupling using a prism 71 having a higher refractive index than the waveguide 62 such as a rutile prism, and light is emitted to the substrate side by gradually reducing the thickness of the waveguide 81 as shown in FIG. Taper coupling that utilizes this fact is used. On the other hand, as a method of producing the refractive index distribution of the waveguide, LiNb
There is known a method in which Ti having a function of increasing the refractive index is diffused into an O ₃ substrate, and then MgO, which reduces the refractive index, is diffused into the substrate on the Ti diffusion layer. This is one in which the refractive index distribution is formed in a direction perpendicular to the substrate.

[Problems to be solved by the invention] However, in the end face connection shown in FIG.
When light is focused on an optical waveguide of about 5 μm by a lens or the like, it is necessary to make the light spots coincide with each other.
The position must be adjusted with an accuracy of m, and a step of polishing the end face is required. In addition, the prism coupling shown in FIG. 7 requires an expensive prism, and it is difficult to couple with an optical fiber or the like. Also, as shown in Fig. 8, the cut-off by Takeda and Miyazaki (IEICE Technical Report MW87-112) is used to reduce the thickness of the waveguide in a tapered shape so that light cannot be confined in the waveguide. In a tapered waveguide that emits light toward the substrate side, the beam diameter of the emitted light is large and the efficiency for single mode coupling is low. On the other hand, LiNb
The method of creating a refractive index distribution by diffusing Mg after diffusing Ti on the surface of the O ₃ substrate not only complicates the process such as performing diffusion twice, but also in a direction where the refractive index distribution is perpendicular to the substrate. And a refractive index distribution that changes in the light propagation direction cannot be obtained.

The present invention has been made to solve the above-described problems, and includes an element or a compound having a function of decreasing the refractive index such as MgO of an element or a compound having a function of increasing the refractive index such as Ti. An object of the present invention is to provide a highly efficient optical coupler having a refractive index distribution in a light propagation direction in a simple process by performing thermal diffusion after vapor deposition at the same time.

[Means for Solving the Problems] In order to achieve this object, a gradient index optical coupler according to the present invention has a structure in which an optical waveguide is formed on a dielectric substrate. A high-refractive-index material layer and a low-refractive-index material layer, which are constituent materials of the optical waveguide, are arranged side by side so that the low-refractive-index material layer is located on the front side in the light propagation direction; At the boundary between the refractive index material layer and the low refractive index material layer, a diffusion layer is provided in which the equivalent refractive index of light propagating through the optical waveguide gradually decreases in the forward direction of the propagation direction and the light diffusion ratio gradually increases. Can be

In addition, a high-refractive-index material layer and a low-refractive-index material layer, which are constituent materials of an optical waveguide, are arranged on one surface of a dielectric substrate such that the low-refractive-index material layer is located on the front side in the light propagation direction. The layers are formed flush with each other, and the thicknesses of the material layers at the boundary between the high-refractive-index material layer and the low-refractive-index material layer are formed so as to gradually decrease as they approach each other. After that, by heat-treating the boundary between the two material layers, a diffusion layer is formed in which the equivalent refractive index of light gradually decreases in the forward direction of the propagation direction and the diffusion ratio of light gradually increases. .

[Operation] According to the refractive index distribution optical coupler of the present invention having such a configuration, when light propagates in the optical waveguide, the equivalent refractive index of the light in the optical waveguide gradually decreases in the forward direction. Light is no longer confined in the wave path and is emitted toward the dielectric substrate.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a main part of a vapor deposition apparatus for producing a gradient index optical coupler of the present invention. For example, on a dielectric substrate 1 such as Z-cut or LiNbO ₃ , a mask 2 having two holes for ejecting Ti vapor and for ejecting MgO vapor is arranged in parallel with a slight space therebetween. Through Ti3 and M
gO4 is ejected, and a Ti layer and an MgO layer are simultaneously vacuum-deposited on the substrate 1 so as to be flush with each other and overlap at the boundary. In addition,
FIG. 1 does not show a vacuum exhaust system, a chamber, a heater, a heating source such as an electron beam, or the like.

Shows the Ti3 and thickness distribution of the MgO ₄ on the LiNbO ₃ substrate 1 in Figure 2. In the portions 21 and 22 directly above the deposition source, Ti and MgO
Both have a uniform distribution. Part 23 just above the evaporation source
In, the film thickness is gradually reduced. Therefore, in the portion 23 on the substrate, the amount of Ti decreases toward the left and the amount of MgO increases. The film thickness of Ti and MgO is, for example, 400 mm and 800 mm, respectively.
Å. FIG. 3 shows the refractive index distribution after this was diffused at 1000 ° C. for 6 hours. That is, in the optical waveguide portion 21, the refractive index with respect to extraordinary light increases by about 0.004 due to the diffusion of Ti. On the other hand, in the portion 22 where MgO is diffused, the refractive index for extraordinary light decreases by about 0.004. In the region 23 where Ti and MgO overlap, the refractive index gradually decreases as the distance from the waveguide 21 increases. Note that the refractive index also changes in the depth direction, and has a substantially Gaussian distribution.

The manner of light propagation is shown in FIG. Optical waveguide part 21
In this case, the refractive index of the Ti diffusion portion 41 is larger than that of the substrate 1 and light is confined in the Ti diffusion portion 41 and guided to the left. Ti and MgO
In the simultaneous diffusion region 23, the refractive index of the waveguide layer 41 decreases toward the left. When the refractive index of the optical waveguide layer 41 decreases, the light distribution spreads over the substrate. Further, when the refractive index of the optical waveguide layer 41 decreases, light is not confined in the optical waveguide layer 41 and is emitted to the substrate 1. The light emitted to the substrate is coupled to, for example, an optical fiber 42 disposed at the left end. That is, region 23
Becomes a refractive index distribution optical coupler.

In this embodiment, the size, interval, and shape of the holes in the mask are not limited. Further, the positional relationship among the evaporation source, the mask, and the substrate is not limited. Also, Ti and MgO
The method of forming such films is not only the vacuum deposition method but also the sputtering method,
There is no particular limitation on the ion beam sputtering method or the like. The thickness and diffusion time of each of Ti and MgO are not limited. Substrate material also
Not limited to LiNbO _3. The shape of the film thickness distribution in the co-evaporation portion of Ti and MgO is not limited. The refractive index distribution of the refractive index distribution optical coupler can be controlled by the shape of the film thickness distribution. In this embodiment, the refractive index distribution changes linearly. For example, as shown in FIG.
The light emitted to the substrate can be condensed by changing the concave shape upward so as to be proportional to the square. The shape of the refractive index distribution is not limited. Further, the diffusion material is not limited to TI and MgO.

Although the slab waveguide has been described in the present embodiment, the same applies to a three-dimensional waveguide. In this case, after the Ti and MgO are deposited, the Ti and MgO may be diffused after etching into a desired optical waveguide shape.

[Effects of the Invention] As is clear from the above description, according to the present invention, the refractive index distribution light having a refractive index distribution in the light propagation direction in a simple step of diffusing after co-evaporating Ti and MgO. It is possible to produce a coupler. Further, by using Ti in which the refractive index of the substrate increases after diffusion and MgO in which the refractive index decreases, a large change in the refractive index can be obtained, and a highly efficient optical coupler can be manufactured.

[Brief description of the drawings]

FIGS. 1 to 4 show an embodiment of the present invention. FIG. 1 is an explanatory view of the simultaneous vapor deposition of Ti and MgO, which is an embodiment of the present invention, and FIG. FIG. 3 is a cross-sectional view showing a film thickness distribution of MgO, FIG. 3 is a view showing a refractive index distribution of an optical waveguide after diffusion, FIG. 4 is an explanatory view showing an operation principle of a refractive index distribution optical coupler, and FIG. FIG. 6 is a diagram illustrating a refractive index distribution which is a modification of the embodiment of FIG. 6, FIG. 6 is a diagram illustrating a conventional end face coupling, FIG. 7 is a diagram illustrating a conventional prism coupling, and FIG. It is. In the figure, 1 is a substrate, 2 is a mask, 3 is Ti, and 4 is MgO.

Claims (2)

(57) [Claims] An optical waveguide is formed on a dielectric substrate, and a high-refractive-index material layer and a low-refractive-index material layer, which are constituent materials of the optical waveguide, are provided on one surface of the dielectric substrate. The low-refractive-index material layers are arranged side by side so as to be located on the front side in the light propagation direction, and the boundary between the high-refractive-index material layer and the low-refractive-index material layer is located at the boundary between the light propagating through the optical waveguide. A refractive index distribution optical coupler characterized in that a diffusion layer is provided in which the equivalent refractive index gradually decreases in the forward direction of the propagation direction and the diffusion ratio of light gradually increases. 2. A high-refractive-index material layer and a low-refractive-index material layer, which are constituent materials of an optical waveguide, are disposed on one surface of a dielectric substrate so that the low-refractive-index material layer is located on the front side in the light propagation direction. And at the boundary between the high-refractive-index material layer and the low-refractive-index material layer, the thickness of those material layers is gradually reduced as they approach each other. Then, by heat-treating the boundary between the two material layers, a diffusion layer is formed in which the equivalent refractive index of light gradually decreases in the forward direction of the propagation direction and the diffusion ratio of light gradually increases. Of producing a gradient index optical coupler.