JPH03148625A - Optical device - Google Patents

Abstract

PURPOSE:To realize a fast, low-driving-voltage optical modulator by forming a material which reduces a refractive index by heat diffusion or embedding technique between Mach-Zehnder type optical waveguides formed on a substrate and coupling the waveguides roughly with each other. CONSTITUTION:On an LiNbO3 substrate 5, a pattern of Ti as a 1st diffused body is formed by a normal lift-off method. Namely, the substrate 5 is coated uniformly with photoresist, which is exposed by using a photomask and then developed to form a groove which is several mum wide in the same shape with the waveguide pattern. Further, Ti is vapor-deposited on the entire surface to hundreds of Angstrom and the resist is removed by a remover to form the waveguide pattern 3 of Ti. Then a low-refractive-index part 7 is formed between the optical waveguides 3 by diffusing MgO by the same method on the substrate 5 where the optical waveguides 3 are formed. Thus, the two optical waveguides which constitute the Mach-Zehnder optical waveguide 3 can be coupled roughly, so the gap between the two waveguides can be made narrow and the thickness D of a buffer layer is made large to realize band improvement.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high speed, low driving voltage optical modulation element.

(Prior art) Figure 6 shows a Matsuhatsu Enda optical waveguide with a conventional structure using Z-cut LiNbO and a substrate 5 (Kono et al.
It is a structural diagram of an example of the 3rd year IEICE Autumn National Conference C-195). In this figure, the central conductor radiation 2w of the coplanar nibe guide ((:P11) traveling wave electrode is 8 μm.
, the gap 2G with the ground conductor 2 is 34 μm, and the thickness D of the bubble layer 4 is 4000.

In this configuration, in order to increase the ratio between band and drive voltage, the buffer layer thickness must be increased, and when 2w is constant, 2
What is necessary is to increase w/2G, that is, to decrease the gap. However, since the two Matsuhatsu Enda optical waveguides are coupled to each other, the two optical waveguides have the characteristics of a directional coupler and cannot operate as a Matsuhatsu Enda optical waveguide.

Furthermore, if the gap between the center electrode l and the ground conductor 2 is extremely small, current concentration will occur at the conductor edge, and conversely, the 3d
There was a drawback that the 8-light modulation band Δf became narrow.

Figure 7 shows a Matsuhatsu Enda type optical waveguide (H,
Haga etal-; IEεεJournal o
f Quantum Electronics, vo
1. Lotto 22. No, 6. pp, 90
2-906゜1986) shows the structure of another example,
Unpaired coplanar strips are used as electrodes.

In this structure, since the grooves 6 are formed as a low refractive index layer by RIB (reactive ion etching), manufacturing is not easy. Furthermore, if the etching is performed very close to the optical waveguide, the scattering loss of the guided light due to the etched wall will increase.

In addition, since no buffer layer is used, the microwave propagation loss due to the electrodes is large, and when the center conductor 1 and the ground conductor 2 are brought close together, the microwave propagation loss increases rapidly.
The drawback was that it was not possible to increase the speed.

(Problems to be Solved by the Invention) An object of the present invention is to provide a high-speed, low-stationary dynamic voltage optical waveguide that solves the problems caused by coupling between Matsuhatsu Enda optical waveguides.

(Means for Solving the Problems) In the optical device of the present invention, a substance that reduces the refractive index of light is formed between Matsuhatsuenda-shaped optical waveguides by diffusion or the like.

That is, in a device that performs optical modulation, the present invention provides a first material on a substrate and a second material that reduces the refractive index between the waveguides of a Matsuhatsu-Enda-shaped optical waveguide formed by thermal diffusion or embedding technology. It is provided with a Matsuhatsu Enda-shaped optical waveguide formed by diffusion or embedding technology and with loose coupling between the waveguides, and an electrode for optical modulation.

This method differs from the conventional technology in the formation technology of the low refractive index layer, the thickness of the buffer layer, etc.

(Example) Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of the present invention, where l is a center electrode, 2 is a ground conductor, 3 is a Matsuhatsu Enda optical waveguide, 4 is a buffer layer, and 5 is a Z-cutLINbOs substrate. Here, 7 is a low refractive index layer formed by thermally diffusing SiO. The manufacturing method for this optical waveguide portion will be explained below.

First, a pattern of Ti as a first diffusion material is formed on the LiNb opening and the substrate 5 by a normal lift-off method. That is, for example, a photoresist is uniformly applied onto the LiNbOs substrate 5 using a spinner, the resist is exposed to light using a photomask, and then developed to form a groove with a width of several μm having the same shape as the waveguide pattern. Further T on the entire surface
After several hundred deposits of Ti, the resist is removed using a remover to form a Ti waveguide pattern. After that, the temperature was raised to about 1000℃ for several hours, and the Ti
It is diffused into the iNbOs substrate 5.

Next, the LiNbO which formed the optical waveguide 3 in this way
3. After uniformly coating the resist on the substrate 5 again, the resist in areas other than between the optical waveguides 30 is removed using the same method. Furthermore, for example, after MgO as a second diffusion substance is vapor-deposited over the entire surface, the resist is removed with a remover to form a thin film pattern of MgO.

After this, the LiNb opening and the substrate are heated to several hundred degrees Celsius (for example, 90 degrees Celsius).
As shown in Figure 1, M
The low refractive index portion 7 can be provided by diffusing gO between the optical waveguides 3 .

FIGS. 2(3), (b), and (C) show the resulting distribution of the refractive index difference 80. FIG. 2 (2) shows the distribution of the refractive index difference Δn when only Ti is thermally diffused, and X on the horizontal axis indicates distance. Further, the second drawing shows the distribution of the refractive index difference Δn when only MgO is thermally diffused, and it can be seen that the refractive index difference Δn in the portion where MgO is diffused is negative. Figure 2 (C) shows the distribution of the refractive index difference Δn when both Ti and MgO are diffused, and it is a superposition of Figure 2 (a) and Figure 2 (Company). The difference between the peak value of the refractive index difference Δn appearing in a portion and the minimum value appearing in an adjacent MgO portion is larger than in the case of FIG. 2(a) in which only Ti is thermally diffused.

In other words, as can be seen from FIG. 2(C), since the coupling between the two optical waveguides constituting the Matsuhatsu Enda optical waveguide 3 can be made loose, the gap between the two waveguides can be brought closer. Note that when the waveguide width is 6 μm, in the conventional configuration, the gap is limited to about 15 μm in order to avoid coupling between the waveguides. In other words, 2N/2G=0
．． It was 53.

FIGS. 3(a) and 3(6) show an optical 3 dB band and an electrical 3 dB band for 2N/2G, respectively.

Here, the driving voltage is kept constant. The thickness D of the buffer layer is used as a parameter.

As can be seen from these figures, by increasing the thickness D of the buffer layer, a ratio of 2N/2G exists that provides the maximum band. Therefore, electrical 3dB
Focusing on the bandwidth, the buffer layer is made thicker and 2w/2
By increasing G to about 1, that is, larger than before, a significant improvement in bandwidth can be achieved.

FIG. 4 is a diagram showing a second embodiment of the present invention.

If the present invention is applied to a shielded phase velocity matching optical waveguide (Kono et al., 1988 Institute of Electronics, Information and Communication Engineers Spring National Conference), which can match the phase velocities of microwave and light and achieve broadband by using a shielded conductor, It is clear that even higher speed and lower driving voltage operation becomes possible. Here, 8 is a shield conductor.

FIG.
Overlay type semiconductor optical waveguide that matches the phase velocity of microwave and light using
9) is a diagram showing a third embodiment applied to the above. In FIG. 5, 10 is an optical waveguide formed on the semiconductor substrate 9, 11 is an overlay for matching the phase velocity of microwave and light, and 12 is a low refractive index portion.

The present invention enables high speed and low drive voltage operation.

Note that the present invention uses x-cut LiNbO, substrates, and LiTa.
It is also effective when using O, etc. An optical switch is also possible.

(Effects of the Invention) As described above, in the optical device of the present invention, the gap between the Matsuhatsu Enda optical waveguides can be made closer, so the band can be expanded.

Furthermore, if the band is constant, the interaction length with the electrode can be made longer than in the conventional configuration, which has the advantage of reducing the driving voltage.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the first embodiment of the present invention, FIG. 2 is a diagram explaining the principle of the present invention, FIGS. 3(a) and (6) are diagrams showing the effects of the present invention, and FIG.
5 and 5 show the second and third embodiments of the present invention, FIG. 6 shows an example of a conventional optical waveguide, and FIG. 7 shows another example of a conventional optical waveguide. FIG. l... Center electrode 2-... Earth conductor 3-...
・Matsuhatsu Enda optical waveguide 4...Buffer layer 5...LtNbOs substrate row...Groove hole...Low refractive index part by Mg mouth diffusion 8...Shield conductor 9-...Semiconductor substrate 10.
...Semiconductor optical waveguide 11-...Overlay 12...
・Low refractive index section patent applicant Nippon Telegraph and Telephone Corporation Representative Patent Attorney Hidetoshi Sugimura Patent Attorney Oki Sugimura 21
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/X12W/2 (1; Fig. 3 1 ・-11 ゝ・＼ 1 2W/Z & Fig. 5, 71 //% 12-Reduction of weight β Fig. 6 Fig. 7 Low groove

Claims (1)

[Claims] 1. In an optical device that performs optical modulation, a second material for reducing the refractive index is placed between the waveguides of a Mach-Zehnder optical waveguide formed by thermal diffusion or embedding technology on a first material on a substrate. What is claimed is: 1. An optical device comprising a Mach-Zehnder type optical waveguide formed by thermal diffusion or embedding technology using a material described above, and having a Mach-Zehnder optical waveguide with loose coupling between the waveguides and an optical modulation electrode. 2. In the optical device according to claim 1,
so that the effective refractive index of the microwave propagating through the light modulation electrode approaches the effective refractive index of the light propagating through the optical waveguide,
An optical device characterized in that a buffer layer provided between the light modulation electrode and the substrate provided with the waveguide is thickened. 3. In the optical device according to claim 1,
An optical device characterized in that an overlay layer is provided so that the effective refractive index of microwaves propagating through the light modulation electrode approaches the effective refractive index of light propagating through the optical waveguide. 4. In the optical device according to claim 2,
Provided between the light modulation electrode and the substrate provided with the waveguide so that the effective refractive index of the microwave propagating through the light modulation electrode approaches the effective refractive index of the light propagating through the optical waveguide. 1. An optical device characterized in that the buffer layer is thickened and a shield conductor is provided near the light modulation electrode.