JPH04260006A - Optical branching and coupling device and production thereof - Google Patents

Abstract

PURPOSE:To obtain the manufacture of the optical branching and coupling device which is adjustable in optical branching ratio or coupling ratio by decreasing the refractive index of the end part of a coupling part between plural optical waveguides. CONSTITUTION:A substance 41 which provides effect to a decrease in the refractive index of a substrate 1 and has a larger diffusion coefficient than the material constituting optical waveguides 31 is used in the area of the end part of the coupling part between the optical waveguides 31. Consequently, a large refractive index difference is generated between the optical waveguides 31 and substrate 1, so the light in the optical waveguides 31 never leaks into the substrate 1 and the optical branching and coupling device which has desired coupling length is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical branching coupler having an optical waveguide and a method for manufacturing the same.

0002

2. Description of the Related Art One of the conventionally known optical branching couplers is a directional coupler, which is described in the book by Nishihara et al. (Optical Integrated Circuits, pages 264-267).

FIG. 5 is a perspective view showing the manufacturing process of the directional coupler. T for forming an optical waveguide
Substrate 1 on which i (titanium) pattern 3 is formed (see figure (a)
) is heat-treated for a predetermined time in a diffusion furnace set at a predetermined temperature, an optical waveguide 31 having a high refractive index by diffusing Ti is formed in the substrate 1 (see Fig. 1(b)). ).

[0004] The directional coupler manufactured in this way has two
The book optical waveguides 31 have regions that are close to each other while remaining parallel to each other. This close region is called a coupling portion 6, and light coupling between the optical waveguides 31 occurs in this portion. The distance between the two optical waveguides 31 in this coupling part 6 is approximately several μm, and the optical waveguide 3
1 has been widened.

[0005]

[Problems to be Solved by the Invention] In the above-mentioned directional coupler, in order to suppress the optical loss caused by bending of the optical waveguide 31 between the coupling part 6 and the light incidence part 7 shown in FIG. In addition, the radius of curvature is increased to gradually bend the optical waveguide 31. Therefore, since the spacing between the optical waveguides 31 has not yet become so large at the end of the coupling portion 6, the light in both optical waveguides 31 leaks into the substrate 1, causing optical coupling. Therefore, even if the design and fabrication are performed using a bond length (for example, L0) calculated by simulation etc. in order to obtain the desired bond ratio, the actual bond length will be larger than L0, and the designed bond ratio will not be achieved. The problem was that I couldn't get it.

SUMMARY OF THE INVENTION Accordingly, the present invention provides an optical branching/coupling device that solves the above-mentioned problems and can obtain branching ratios and coupling ratios as designed values, and a method for manufacturing the same.

[0007]

[Means for Solving the Problems] The present invention provides at least two
In an optical branching coupler in which optical waveguides are arranged parallel to each other and these optical waveguides are formed close to each other at a branching part or a coupling part, the optical waveguide that is the end of the branching part or coupling part It is characterized in that the refractive index of the substrates between them is lower than the refractive index of the substrates other than that portion.

[0008] The above-mentioned optical branching coupler has at least two optical waveguides that are partially parallel to each other to form a branching part or a coupling part, and a first material for increasing the refractive index is formed in the substrate. A step of doping the optical waveguide with a first
The second material is doped with a second material that has a diffusion coefficient larger than that of the second material and has the property of lowering the refractive index of the substrate.

[0009]

[Operation] According to the present invention, the refractive index difference between at least two optical waveguides at the end of the branching part or coupling part of the optical branching/coupling device and the substrate between the optical waveguides is This is larger than the refractive index difference between the optical waveguide and the substrate between the optical waveguides at the portions other than the end portions.

[0010] Furthermore, since the diffusion coefficient of the substance doped into the substrate in order to increase the refractive index difference described above is larger than that of the substance that increases the refractive index of the optical waveguide, branching/coupling parts are formed after forming the optical waveguide. Even if a diffusion treatment is performed to dope a material between the ends of the optical waveguide to lower the refractive index, the structure of the optical waveguide will not be affected.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a perspective view and a top view of an optical branching coupler according to an embodiment. As shown in FIGS. 1A and 2B, an optical waveguide 31 is formed on the surface of the substrate 1 by diffusion. A substance that lowers the refractive index of the substrate 1 (hereinafter referred to as a refractive index lowering substance 41) is provided on the substrate between the optical waveguides (hereinafter referred to as the joint part end 4) which is the end of the coupling part 6 of the optical waveguide 31. is doped. Therefore, the refractive index of the coupling part end 4 is lower than that of the other parts of the substrate, so that the difference in refractive index between the coupling part end 4 and the two optical waveguides 31 sandwiched therebetween becomes large.

In such a structure, for example, light incident on the light incidence section 7 propagates within the optical waveguide 31 having a high refractive index, and reaches the coupling section 6 where the distance between the two optical waveguides 31 becomes narrow. do. As mentioned above, these optical waveguides 3
1, the distance between them gradually narrows toward the coupling part 6, but the refractive index of the coupling part end 4 is greatly reduced by the refractive index lowering substance 41, resulting in a refractive index difference with the optical waveguide 31. is large, so that the light within the optical waveguide 31 does not leak into the coupling portion end 4. Therefore, unnecessary optical coupling at portions other than the coupling portion 6 can be prevented.

Next, a method of manufacturing the optical branching coupler according to the embodiment will be explained using FIG. First, a photoresist is applied onto a ferroelectric substrate 1, and a resist mask 2 having an opening in an optical waveguide forming region of the substrate 1 is formed using photolithography. On the resist mask 2 and the substrate 1 exposed through the opening, a metal film 30 made of a substance to have a high refractive index is formed using a vapor deposition method or a sputtering method.
(Illustrated in FIG. 3(a)). Next, the optical waveguide pattern 3 made of the metal described above is formed on the substrate 1 in the optical waveguide forming area by a lift-off method (as shown in FIG. 2(b)).
. Thereafter, by heating the substrate 1 in an atmosphere at a predetermined temperature for a predetermined time, the metal of the optical waveguide pattern 3 is diffused into the substrate 1, and an optical waveguide 31 is formed (as shown in FIG. 2(c)). A resist mask 21 having an opening is formed in the region of the coupling end 4 of the substrate 1 where the optical waveguide 31 is formed (as shown in FIG. 2(d)). Thereafter, a refractive index lowering material 41 is deposited on the resist mask 21 and on the substrate 1 exposed through the opening (as shown in FIG. 3(e)). Next, by using a lift-off method, the refractive index lowering substance 41 is left at the joint end portion 4 of the substrate 1, and a diffusion process is performed (as shown in FIG. 4(f)). Thereby, the difference in refractive index between the coupling part end 4 and the optical waveguide 31 can be made larger than that of other parts, so that the light inside the optical waveguide 31 does not leak into the coupling part end 4. Note that the refractive index lowering substance 41 used here is the optical waveguide 3
The diffusion coefficient of the substance must be larger than that of the substance that makes up 1. This is because the high refractive index material in the optical waveguide 31 should not be re-diffused by the diffusion treatment and the properties of the material should not be deteriorated and the properties of the device should not be affected.

By using the above-described manufacturing method, unnecessary optical coupling can be prevented from occurring without deteriorating the characteristics of the optical waveguide 31.

The above-mentioned embodiment will now be described in detail with reference to FIG. 3, together with the effects of its steps. First, the ferroelectric material lithium niobate (LiNbO3)
An optical waveguide pattern 3 made of a metal (titanium; Ti) film with a thickness of 70 nm is formed on the optical waveguide formation region of the substrate 1 using a lift-off method. The substrate 1 loaded with this waveguide pattern 3 is left in an atmosphere at 1050° C. for 6 hours and subjected to a diffusion treatment, thereby forming an optical waveguide 31. The manufactured optical waveguide 31 has a width of 7 μm, a waveguide spacing in the coupling portion 6 of 5 μm, and a length of the coupling portion of 12 mm (as shown in FIG. 3(b)). The calculated crosstalk in the joint 6 with such dimensions is -
It was 30dB. However, at the end of the joint 6
Since optical coupling is likely to occur in portions where the optical waveguides 31 are close to each other, it is necessary to further perform the steps described below in order to prevent deterioration of crosstalk.

First, after producing the optical waveguide 31 by the method described above, a resist material is applied to its surface to form a resist mask 21 having an opening at the joint end 4 of the substrate 1 (
Figure (b) shown). Magnesium oxide (MgO), which is a refractive index reducing substance 41, is deposited to a thickness of 10 nm on the resist mask 21 and on the joint end portion 4 exposed by the opening. Then, using the lift-off method, MgO
is patterned on the joint end portion 4 (as shown in FIG. 3(c)). Next, this substrate is left in a diffusion treatment furnace at 950° C. for 4 hours to perform a diffusion treatment (as shown in FIG. 4(d)). Through this step, when MgO diffuses into the bonding end 4 of the substrate 1, the refractive index of the diffused portion decreases. Note that since the diffusion coefficient of MgO is much larger than that of Ti, the diffusion treatment in an atmosphere of 950° C. has almost no effect on the distribution of Ti.

Through the above-described steps, the end portion 4 of the coupling portion is doped with MgO, its refractive index is lowered, and the difference in refractive index with the optical waveguide 31 is increased. Therefore, this joint end 4
In this case, light bleeding is reduced and light coupling becomes less likely to occur, so crosstalk can be obtained as designed.

Next, using FIG. 4, the refractive index distributions of an optical waveguide manufactured by a conventional manufacturing method and an optical waveguide manufactured by a manufacturing method according to the present invention will be compared. Figure (a)
and (b) show a top view of a conventional optical waveguide and its refractive index distribution, and (c) and (d) of the same figure show a top view of an optical waveguide produced by the manufacturing method according to the present invention and its refractive index distribution. shows. As shown in (a) and (b) of the same figure, the optical waveguides 31 at the ends of the coupling portion 6 are slightly closer to each other because the radius of curvature is increased to suppress optical loss. . Therefore, the light seeping out from both optical waveguides 31 is likely to couple with each other. However, when the manufacturing method according to the present invention is used, MgO is injected into the substrate 1 by diffusion into the substrate 1 between the curved optical waveguides 31 at the ends of the coupling portion 6 (the same Figures (c) and (d) shown)
. Therefore, the refractive index of the diffused portion of MgO is
) is lower than other parts of the board, as shown in
A large refractive index difference is produced between the optical waveguide 31 and the optical waveguide 31. Therefore, the light propagating within the optical waveguide 31 is less likely to leak into the substrate 1, and optical coupling does not occur.

As explained above, in the present invention, the dielectric (
Although the above description has been made regarding the LiNbO3 (LiNbO3) waveguide, it is fully possible to produce similar effects for semiconductor or glass waveguides by selecting and using a refractive index lowering material.

[0021] Note that Japanese Patent Laid-Open No. 61-241706 discloses a method using an ion exchange method to obtain a desired optical branching ratio/coupling ratio by changing the refractive index difference within the substrate.
No measures have been taken to prevent unnecessary optical coupling occurring at the coupling end of the optical waveguide.

[0022]

According to the present invention, by doping the substrate at the end of the coupling portion with a refractive index lowering substance, unnecessary coupling of light outside the coupling portion can be suppressed. Therefore,
An optical branching/coupling device having a coupling length as designed can be manufactured, and a desired branching ratio and coupling ratio can be obtained.

Furthermore, since a substance whose diffusion coefficient is larger than that of the material constituting the optical waveguide is used as the refractive index reducing substance, the characteristics of the optical waveguide may be affected by various conditions during the diffusion process of the refractive index reducing substance. There is no. Therefore, an optical branching coupler with good characteristics can be obtained.

Furthermore, by arranging electrodes and the like, it is possible to manufacture a switch having a high extinction ratio.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an optical branching coupler that has undergone a manufacturing method according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of an element according to steps showing a manufacturing method according to an embodiment of the present invention.

FIG. 3 is a top view of an element according to steps showing a manufacturing method according to an embodiment of the present invention.

FIG. 4 is a top view of a conventional optical branching coupler and a diagram showing the refractive index distribution of the optical branching coupler according to the present invention.

FIG. 5 is a perspective view of an optical branching coupler manufactured by a conventional manufacturing method.

[Explanation of symbols]

1...Dielectric substrate 2 and 21...Resist mask 3...Optical waveguide pattern 31...Optical waveguide 4…Connection end 41...Refractive index lowering substance 6...Joining part

Claims (2)

[Claims] 1. An optical branching/coupling device in which at least two optical waveguides are arranged parallel to each other and these optical waveguides are formed close to each other at a branching section or coupling section, wherein the branching section or coupling section An optical branching/coupling device characterized in that the refractive index of the substrate between the optical waveguides, which is the end portion of the portion, is lower than the refractive index of the substrate other than the portion. 2. A step of forming at least two optical waveguides that are partially parallel and close to each other to form a branching part or a coupling part by doping a first substance for increasing the refractive index into the substrate. and a second material having a diffusion coefficient larger than that of the first material and having a property of lowering the refractive index of the substrate, in the substrate between the optical waveguides that are the ends of the branching part or the coupling part. 1. A method for manufacturing an optical branching coupler, comprising the step of doping with a substance.