JPS6223847B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a waveguide optical modulator that can reduce modulation voltage and optical propagation loss.

1 and 2 are explanatory diagrams illustrating conventional branching path type optical modulators, respectively; FIG. 1 is a perspective view, and FIG.
The figure is a sectional view. In these figures, 1 is a substrate made of, for example, LiNbO ₃ crystal or glass, 2 is an optical waveguide, 3 is a branch point, 4 is a branch point,
5 and 6 are branch waveguides, 7 is an electrode, and 8 is a signal source. These optical modulators operate as follows. First, light incident from the optical waveguide 2 is branched at a branching point 3 as light having optical electric fields E ₁ and E ₂ , and then merged again at a merging point 4 . In the case of the optical modulator shown in FIG. 1, when a voltage is applied to the electrodes 7 sandwiching the branch waveguides 5 and 6, and in the case of the optical modulator shown in FIG. When a voltage is applied to the electrodes 7 placed on the branch waveguides 5 and 6, the output light I changes according to the following equation.

I=E ² ₁ + E ² ₂ + 2E ₁ E ₂ cos {kL (Δn(O) − Δn(E))} …(1) Here, k is the wave number of light (=2π/λ, λ: wavelength of light) , Δn(O) is the phase difference between E ₁ and E ₂ that exists between branch waveguides 5 and 6 when the modulation voltage is zero, Δn(E)
occurs between the branch waveguides 5 and 6 due to the applied voltage.
The phase difference between E ₁ and E ₂ and L is the electrode length. Branching point 3
Assuming that the incident light is split by 3 dB, E ₁ = E ₂
becomes. Further, if both the branch waveguides 5 and 6 have substantially the same shape, Δn(O)=0 and the modulation voltage V is given by the following equation.

V=λG/2n ³ rLΓ (2) where G is the electrode spacing, r is the electro-optic coefficient, n is the refractive index of the crystal, and Γ is the electric field correction coefficient. In order to reduce the modulation voltage V, since n, r, λ, and Γ are almost constant, G/L can be reduced. Incidentally, the electrode length L is limited by the size of the crystal substrate. Furthermore, increasing the electrode length L increases the length of the optical waveguide, which causes an increase in optical propagation loss. Therefore, it is considered better to reduce the modulation voltage by reducing the electrode spacing G.

However, in the optical modulator shown in FIG. 1, since the electrode 7 sandwiches the branch waveguides 5 and 6, the electrode must be installed between the branch waveguides 5 and 6. In this case, the distance between the branching waveguides 5 and 6 becomes larger, and the separation angle between the branching point 3 and the merging point 4 also becomes larger accordingly. This causes the light to not branch smoothly at the branch point 3 and the confluence point 4, resulting in increased light scattering.

On the other hand, in the optical modulator shown in FIG. 2, the direction perpendicular to the substrate 1 is used as the electric field direction, so the electrodes 7 are formed on the branch waveguides 5 and 6. If the groove 10 is formed between the branch waveguides 5 and 6 as in this structure, optical coupling between the branch waveguides 5 and 6 will not occur, so that the electrode spacing can be made sufficiently small. However, in this case, since the groove 10 is manufactured by ion milling or chemical etching, it is technically difficult to make it several μm wide. Therefore, the electrode spacing should be several microns.
It becomes very difficult to make the modulation voltage m, and in the end, it is difficult to achieve a significant reduction in the modulation voltage.

The present invention has been made in view of the actual situation in the prior art described above, and its purpose is to provide a waveguide that can prevent optical coupling between branch waveguides and reduce modulation voltage and optical propagation loss. An object of the present invention is to provide a shaped light modulator.

To achieve this objective, the present invention provides a method for dispersing the same or different type of diffusion material as the diffusion material used to form the optical waveguide on one of the two branch waveguides or on both of the branch waveguides and alternately. Diffusion of substances 2
It is configured to include a heavy diffusion section.

Hereinafter, the waveguide type optical modulator of the present invention will be explained based on the drawings. FIGS. 3 and 4 are perspective views illustrating optical waveguides constituting the present invention, and FIG. 5 is a sectional view illustrating an embodiment of the present invention. In these figures, the same reference numerals as those shown in FIGS. 1 and 2 indicate the same members. In these FIGS. 3 to 5, 11 is a double diffusion section. Now, if two optical waveguides with exactly the same shape and refractive index are close to each other in parallel, that is, if we consider these as branch waveguides 5 and 6, then the equation expressing the optical coupling between the two optical waveguides is as follows. η=10log1-p/p(dB)...(4)

Here, p is the optical output coupled from the input side optical waveguide to the coupling waveguide, δ _o is the refractive index difference between the branch waveguides, and η is the crosstalk. The above δ _o is the refractive index difference between the branch waveguides caused by double diffusion. Now, if L/λ=10000 and δ _o =0.001,
Crosstalk of 20dB or more can be obtained. This value of δ _o can be easily obtained by double diffusion. Therefore, the third
As shown in the figure, double diffusion portions 11 are formed on either one of the two branch waveguides, or alternately on both of the branch waveguides as shown in FIG. Therefore, the coupling of light between the two branch waveguides can be almost completely separated.

The double diffusion part 11 is formed by forming a pattern for double diffusion by photo-etching, such as a lift-off method, in accordance with the dimensions of the diffusion source pattern of the branch waveguide, and diffusing the first and second layer diffusion substances simultaneously. Alternatively, after forming a branched waveguide, a second layer of diffusing material is formed on the waveguide and then diffused. In this case, the first layer diffusion material and the second layer diffusion material may be of the same type or different types. In the optical modulator, for example, as shown in FIG. 5, a planar electrode is formed via a buffer layer 9 on branch waveguides 5 and 6 whose optical coupling portions are separated by a double diffusion portion 11. The structure is as follows.

As explained above, according to the waveguide type optical modulator of the present invention, since the double diffusion section is provided on the branch waveguide, it is possible to separate optical coupling between adjacent branch waveguides. This has the effect of making it possible to reduce the separation angle at the branching point and the merging point of the branching waveguide, thereby reducing light scattering at the branching point and the merging point.
Furthermore, since the branch waveguides can be placed close to each other, the electrode spacing can be reduced, which has the effect of reducing modulation voltage.

[Brief explanation of the drawing]

1 and 2 are explanatory diagrams illustrating conventional branch path modulators, respectively. FIG. 1 is a perspective view, FIG. 2 is a sectional view, and FIGS. 3 and 4 are illustrations of a conventional branch type modulator, respectively. FIG. 5 is a perspective view illustrating an example of an optical waveguide constituting a waveguide type optical modulator, and FIG. 5 is a sectional view showing an embodiment of the waveguide type optical modulator of the present invention. 2... Optical waveguide, 3... Branch point, 4... Merging point, 5, 6... Branch waveguide, 7... Electrode, 11...
...Double diffusion section.

Claims (1)

[Claims] 1. In a waveguide-type optical modulator equipped with an optical waveguide consisting of a branch point, two branch waveguides, and a confluence point, a waveguide-type optical modulator including a branch point, two branch waveguides, and a confluence point, wherein A waveguide-type optical modulator characterized in that a double diffusion section is provided on both sides and alternately with a diffusion material of the same kind or different kind as the diffusion material used to form the optical waveguide. 2.Equipped with an optical waveguide consisting of a branch point, two branch waveguides, and a confluence point, and on one of the branch waveguides or on both of the branch waveguides and alternately, the diffusion used for forming the optical waveguide is provided. A waveguide-type optical modulator comprising a double diffusion section formed by diffusing a diffusion substance of the same type or a different type as the substance, characterized in that a planar-type electrode is provided on the branch waveguide. modulator.