JP2669096B2 - Optical modulator and manufacturing method thereof - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator, and more particularly to an optical waveguide type optical modulator.

[Conventional technology]

Modulators are important key devices in optical communications.
Particularly, the waveguide type external modulator has an excellent feature that high-speed modulation can be performed without chirping. A Mach-Zehnder modulator (hereinafter referred to as an MZ modulator) that utilizes optical branching interference is known as one method of this waveguide-type external modulator. Ti
The operating principle of an MZ modulator using a diffused LiNbO ₃ waveguide will be described as an example.

FIG. 4 is a perspective view showing the MZ modulator. Single mode optical waveguide 12 by diffusing Ti on LiNbO ₃ substrate 11
Are formed. The optical waveguide 12 is divided into two arms by the Y branch, passes through the control electrode 13, and joins again by the Y branch. Further, between the optical waveguide 12 and the control electrode 13, SiO _{2 is used} as a buffer layer for preventing light absorption by the electrode.
A film 14 is formed.

FIG. 5 is a plan view of the MZ modulator. When no voltage is applied between the control electrodes 13 as shown in FIG. 5 (A), the 0th-order mode light 61 incident on the optical waveguide 12 has the same phase in the Y branch portion, that is, the in-phase light 62, The light is branched into two light beams 63 and merged in the same phase to become the output light 64 of the zero-order mode.

When a voltage corresponding to a half-wave voltage is applied between the control electrodes 13 as shown in FIG. 5 (B), the two in-phase light beams are in phase with each other after passing through the electrode portions due to the electro-optic effect of LiNbO ₃ . The light 65 and 66 are inverted and have opposite phases, and a first-order mode light 67 is generated at the merging portion. 1 in the single mode optical waveguide 12
The second mode light 67 is cut off and cannot be guided, so that it is emitted into the substrate. Therefore, no output light appears. Therefore, light can be modulated by applying a modulation signal voltage between the control electrodes 13.

The operation principle of the MZ modulator has been described above by taking the case of a Ti-diffused LiNbO ₃ optical waveguide as an example, but the same applies to other optical waveguides having an electro-optical effect such as semiconductor, and further, by applying stress, current injection, etc. If the waveguide refractive index can be changed to change the effective optical path length between the two arms, an MZ modulator can be constructed.

[Problems to be solved by the invention]

In order to miniaturize the waveguide type external modulator, the modulation efficiency is increased by increasing the overlap between the electric field distribution by the control electrode and the field distribution of the light propagating in the optical waveguide.
A method of shortening the electrode length is effective. That is, it is important to confine the optical field distribution of the guided light so that it is distributed near the control electrode. However, in this case, the confinement of light becomes stronger even at the merging portion of the branches, so that the distance required for the emission of the first-order mode becomes relatively long. Therefore, there is a problem that shortening the electrode length by improving the modulation efficiency does not necessarily lead to shortening the total element length.

[Means for solving the problem]

In the optical modulator of the present invention, the optical waveguide formed by diffusing the impurity having the action of increasing the refractive index on the device plate includes an incident optical waveguide, a branched optical waveguide, an arm portion, a converging optical waveguide, and an outgoing optical waveguide. In the optical modulator having a control electrode in the arm portion, another impurity having a function of decreasing the refractive index is introduced to make the refractive index of the output optical waveguide lower than the refractive index of the arm portion. Further, it is characterized in that the refractive index is fixed.

[Action]

In the present invention, by strongly confining the optical waveguide of the electrode portion of the MZ modulator in the vicinity of the electrode, the modulation efficiency is improved to shorten the electrode length, and the optical waveguide of the confluent portion has another function of reducing the refractive index. The introduction of impurities partially weakens the light confinement and shortens the distance required to emit the first-order mode light. In other words, by changing the impurity composition of the optical waveguide between the electrode portion and the merging portion, the optimum optical confinement condition is realized in each portion and the total length of the element is shortened.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

 FIG. 1 is a perspective view showing a first embodiment of the present invention.

An optical waveguide 12 is formed on a Z-Cut LiNbO ₃ substrate 11 under the following conditions. The optical waveguide 12 is composed of an incident optical waveguide 12a, a branch optical waveguide 12b, an arm portion 12c, a merging optical waveguide 12d, and an outgoing optical waveguide 12e. First, Ti as an impurity for increasing the refractive index is made of Ti before diffusion with an optical waveguide width W = 8 μm.
Diffusion is performed in an air atmosphere at 1050 ° C. for 8 hours at a film thickness d ₁ = 800 °. Mg as an impurity to further reduce the refractive index
O is merged into the outgoing optical waveguide 1 after merging, with a width W = 8 μm and M before diffusion.
Diffusion is performed in an oxygen atmosphere at 950 ° C. for 4 hours with a gO film thickness of d ₂ = 450 °.

Thereafter, an SiO ₂ film 14 is formed as a buffer layer to a thickness of 3000 μm, and a control electrode 13 is formed thereon.

The Ti film thickness d ₁ of the arm portion 12c before diffusion is set so that the propagation mode of the optical waveguide after diffusion has a confinement strength slightly weaker than the first-order mode cutoff. The exiting optical waveguide 12e after the merging is set to have a confinement strength sufficiently weaker than the first-order mode cutoff by doubly diffusing Ti that increases the refractive index and MgO that decreases the refractive index. . As a result, in the arm portion 12c, the overlap between the electric field distribution by the control electrode 13 and the field distribution of the propagating light is maximized, so that the modulation efficiency is improved and the electrode length can be reduced as compared with the conventional case. In the optical waveguide 12e, light confinement is weak so that the 0th-order mode light propagates, but the 1st-order mode light generated when a voltage is applied can be emitted to the outside of the optical waveguide at a sufficiently short distance.

By selectively double-diffusing Ti and MgO in this way, the optical confinement strength at the electrode portion and the merging portion can be optimized and the element total length of the MZ modulator can be shortened.

 FIG. 2 is a perspective view showing a second embodiment of the present invention.

In the present embodiment, in order to form the optical waveguide 12, first, Ti
With the optical waveguide width W = 8 μm and the Ti film thickness before diffusion as d ₁ , and then MgO is merged and before the branching, that is, the incident light waveguide 12a and the outgoing optical waveguide 12e have a width W = 8 μm.
m, to spread the MgO film thickness before diffusion as d _2. Thereafter, an SiO ₂ film 14 is formed as a buffer layer to a thickness of 3000 μm, and a control electrode 13 is formed thereon. This allows the modulator to be on the entrance side,
The output side also has a symmetrical structure, so that it is not necessary to use them separately.

 FIG. 3 is a perspective view showing a third embodiment of the present invention.

In the present embodiment, since the optical waveguide 12 is formed, Ti is diffused as the film thickness d ₁ before diffusion, and then the outgoing optical waveguide 12 after merging.
e and the MgO film thickness d ₂ of the incident optical waveguide 12a before branching, the optical waveguide 1
By providing the MgO film thickness taper portion 37 in the merging optical waveguide 12d and the branching optical waveguide 12b of 2 and gently changing the MgO film thickness before diffusion to diffuse, the mode conversion loss due to the abrupt change of the optical confinement strength We are trying to reduce it.

The case where the optical waveguide is formed by using Ti diffusion on the Z-Cut LiNbO ₃ substrate has been described above as an example, but the MZ modulator is formed by using impurity diffusion on another substrate orientation, or a semiconductor or other substrate material such as GsAs or InP. It is apparent from the principle of the MZ modulator that the method according to the present invention is effective even when the above is configured.

〔The invention's effect〕

As described above, according to the present invention, it is possible to control the optical confinement strengths of the electrode portion and the merging portion of the MZ modulator to the optimum strengths independent of each other, thereby shortening the total length of the element and reducing the size of the optical modulator. You can get a bowl.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a first embodiment of the present invention. FIG. 2 is a perspective view showing a second embodiment of the present invention. FIG. 3 is a perspective view showing a third embodiment of the present invention. FIG. 4 is a perspective view showing a conventional example. FIG. 5 is a plan view showing the operation principle of the Mach-Zehnder modulator. 11 …… Z-Cut LiNbO ₃ substrate, 12 …… optical waveguide, 13 …… control electrode, 14 …… SiO ₂ buffer layer, 12a …… incident optical waveguide before branching, 12c …… arm part, 12e …… Output optical waveguide after merging, 61 …… 0th-order mode incident light, 62,63 …… In-phase guided light, 64 …… 0th-order mode emitted light, 65,66 …… Reverse-phase guided light, 67… ... First-order mode light.

Claims (2)

(57) [Claims] 1. An optical waveguide formed by diffusing an impurity having an action of increasing a refractive index on a substrate comprises an incident optical waveguide, a branch optical waveguide, an arm portion, a converging optical waveguide, and an outgoing optical waveguide. In the optical modulator having a control electrode in its part, another impurity having a function of reducing the refractive index is introduced into the merging optical waveguide and the output optical waveguide, and the refractive index of the output optical waveguide is adjusted to the arm portion. The optical modulator is characterized in that the refractive index of the merging optical waveguide is decreased toward the output optical waveguide, and the reduced refractive index is kept constant in the output optical waveguide. 2. An incident optical waveguide, a branched optical waveguide, an arm portion, a converging optical waveguide, and an outgoing optical waveguide are formed by diffusing impurities having an action of increasing the refractive index on a substrate, and a control electrode is provided on the arm portion. In a method of manufacturing an optical modulator for forming a film, an impurity which has an action of increasing a refractive index is diffused to make an incident optical waveguide, a branch optical waveguide, an arm portion, a merging optical waveguide,
After forming the output optical waveguide, another impurity having a function of reducing the refractive index is diffused into the converging optical waveguide and the output optical waveguide, so that the output optical waveguide has a lower refractive index than the arm portion. In addition, the method for manufacturing an optical modulator is characterized in that the refractive index of the converging optical waveguide is decreased toward the output optical waveguide, and the decreased refractive index is kept constant in the output optical waveguide.