JPH0827471B2 - Method of manufacturing thin film type optical element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a thin film type optical element.

[Prior art]

Conventionally, a thin film type, that is, an optical element using an optical waveguide is used as an optical deflector, an optical modulator, a spectrum analyzer, a correlator,
Researches applied to optical switches and the like are being actively conducted.
Such a thin-film optical element changes the refractive index of the optical waveguide by an external action such as an acousto-optic (AO) effect or an electro-optic (EO) effect, and modulates or deflects the light propagating in the optical waveguide. It is a thing. As a substrate for forming the optical element, lithium niobate (hereinafter referred to as LiNbO ₃ ) crystal and lithium tantalate (hereinafter referred to as LiTaO ₃ ) having excellent piezoelectricity, acousto-optical effect, and electro-optical effect, and low light propagation loss. Crystal) is widely used. As a typical method for producing a thin film optical waveguide using such a crystal substrate, titanium (hereinafter referred to as Ti) is thermally diffused at a high temperature on the crystal substrate surface, thereby forming a thin film optical waveguide on the crystal substrate surface.
There is a method of forming an optical waveguide layer having a refractive index slightly higher than that of the substrate. However, the thin-film optical waveguide manufactured by this method is susceptible to optical damage, and has a drawback that only light of very low power can be introduced into the waveguide. Here, the term “optical damage” refers to “when the intensity of light input to the optical waveguide is increased, the intensity of light propagating in the optical waveguide and extracted to the outside is proportional to the input light intensity due to scattering. Phenomenon that does not increase. "

Further, an ion exchange method is known as another method for producing an optical waveguide that improves optical damage. This method is used in molten salts such as thallium nitrate (hereinafter referred to as TlNO ₃ ), silver nitrate (hereinafter referred to as AgNO ₃ ), potassium nitrate (hereinafter referred to as KNO ₃ ), or a weak acid such as benzoic acid (C ₆ H ₅ COOH). And LiNbO ₃ or
By subjecting the LiTaO ₃ crystal substrate to a low temperature heat treatment, lithium ions (Li ⁺ ) in the crystal substrate are converted into protons (H
⁺ ) And other ionic species, resulting in a large refractive index difference (Δh ~
An optical waveguide layer having 0.12) is formed. The thin film optical waveguide manufactured by the above ion exchange method has a good characteristic that the optical damage threshold value is improved several tens of times as compared with that of Ti diffusion.

By the way, when the optical deflector and the optical modulator are to be realized by utilizing the photoacoustic effect or the electro-optical effect, it is important to increase the efficiency of each of the above effects in the element formation. A typical example of utilizing the photoacoustic effect is a method of exciting a surface acoustic wave on the optical waveguide by applying a high-frequency electric field to a comb-shaped electrode manufactured by photolithography on the optical waveguide.
In this case, it is known that the interaction between the surface acoustic wave excited on the optical waveguide and the guided light propagating in the optical waveguide increases as the energy distribution of the guided light is confined near the substrate surface. (CSTsal, IEEE TRANSACTIONS O
N CIRCUITS AND SYSTEMS, VOL.CAS-26, 12, 1979] On the other hand, when the guided light is input to and output from the optical waveguide as described above, it is performed from a semiconductor laser, an optical fiber or the like via the end face of the optical waveguide. In this case, in order to increase the light coupling efficiency, the energy distribution of the guided light needs to spread in the thickness direction of the substrate in accordance with the light energy distribution of the optical fiber or the like.

As described above, since the required energy distribution of guided light is different between the optical coupling part for inputting / outputting guided light and the optical function part for modulating / deflecting guided light, the conventional thin-film optical element has a high-efficiency modulation. It was difficult to satisfy both the deflection and the high coupling efficiency at the same time. Further, as a solution to this problem, when the optical waveguide is formed by diffusion of titanium, a method of making the diffusion concentration of titanium different between the optical coupling portion and the optical function portion has been proposed. [Mitsukazu Kondo, Keiro Komatsu, Yoshinori Ota '84 Spring Conference Presentation, 31a-K-7 and 7th Toptical Meeting on Integrated and Guided
-Wave Optics TuA5-1] However, when forming an optical waveguide by ion implantation as described above, an effective means for solving the above problems has not been known.

[Outline of Invention]

An object of the present invention is to provide a method for manufacturing a thin film type optical element which has a sufficiently high threshold value for optical damage, has high coupling efficiency at the time of inputting / outputting guided light, and efficiently diffracts light. is there.

The above object of the present invention is to propagate a substrate, a thin film optical waveguide formed by implanting or thermally diffusing ions on the entire surface of the substrate, and propagating the optical waveguide by generating a surface acoustic wave on the substrate surface. A thin film type optical element comprising an optical coupling section for diffracting light, for inputting and outputting the propagating light from the end face of the optical waveguide, and an optical functional section having a means for diffracting the propagating light is formed. In the method, a step of injecting ions only into a portion other than the optical functional portion of the substrate surface, a step of injecting ions into the entire substrate surface, and a thermal diffusion of the injected ions are performed so that the optical coupling portion is A thin film type optical device comprising a process of forming an optical waveguide having a deeper diffusion depth of ions than that of the optical function part and a process of forming a means for diffracting propagating light in the optical function part. It is achieved by a method for manufacturing a child.

〔Example〕

FIG. 1 is a perspective view showing a first embodiment of a thin film type optical element according to the present invention utilizing the acousto-optic effect. 1 is x
Plate or y-plate LiNbO ₃ crystal substrate, 2 is an optical waveguide formed by proton exchange, 3 and 4 are polished optical waveguide end faces, 5 and 6 are cylindrical lenses, and 7 is a comb-shaped electrode.

The collimated light 8 from the He-Ne laser having a wavelength of 6328Å is condensed on the polished end face 3 of the optical waveguide by the cylindrical lens 5 in the thickness direction of the optical waveguide and is coupled into the optical waveguide. The guided light 9 coupled from the end face of the optical waveguide is diffracted by the surface acoustic wave 10 generated by applying RF power to the comb electrode 7, and the diffracted light is emitted from the end face 4 of the optical waveguide and collimated by the cylindrical lens 6. become. At this time, the width of the converged light beam (converging direction) at the end face 3 of the optical waveguide and the width of the guided light are substantially the same, and thus a high coupling efficiency of 80% was obtained.

Further, as shown in the figure, the optical waveguide 2 is injected with protons as it progresses from the optical coupling portion near the optical waveguide end faces 3 and 4 to the optical function portion where the surface acoustic wave 10 and the guided light 9 interact. The depth became gradually shallow, and guided light was confined near the substrate surface in the optical function part, and high diffraction efficiency was obtained.

FIG. 2 is a schematic cross-sectional view illustrating a method for manufacturing the thin film type optical element as shown in FIG.

First, as shown in FIG. 2 (a), the y plate or x
The y-plane or x-plane of the LiNbO ₃ crystal substrate 1 of the plate was polished to a flatness within a few Newton rings, then subjected to normal ultrasonic cleaning with acetone and then pure water, and dried by blowing nitrogen gas. . Next, a Ti thin film was vapor-deposited on the y-plane or the x-plane by electron beam evaporation to a thickness of 200Å, and was thermally diffused in an oxygen atmosphere at 965 ° C for 2.5 hours. As shown in FIG. The thermal diffusion layer 11 was formed. As the metal to be thermally diffused, V, Ni, Au, Ag, Co, Nb, Ge or the like may be used.

Next, as shown in FIG. 2 (c), a Cr thin film 12 was vapor-deposited on the optical function portion where the surface acoustic wave and the guided light interact, and used as a mask for the proton exchange treatment. Next, 2% of lithium benzoate was added to benzoic acid at a molar ratio of 2%, and the mixture was placed in a crucible of alumina. Into a crucible containing benzoic acid and lithium benzoate, put the LiNbO ₃ crystal substrate on which the mask is formed,
These were placed in a heating furnace and held at a temperature of 250 ° C. for 5 hours to carry out an ion exchange treatment. As a result, as shown in FIG. 2 (c), the Ti mask layer 11 was not masked. The proton exchange layer 13 was formed. In forming the proton exchange layer, in addition to a mixed solution of benzoic acid and lithium benzoate, a material having a dissociation degree of ^10-6 to 10 ^-3 in galonic acid and hydrogen of the carboxyl group of this carboxylic acid are replaced with lithium. Mixed with other materials such as palmitic acid [CH ₃ (CH ₂ ) ₁₄ COOH] and lithium palmitate [CH
₃ (CH ₂ ) ₁₄ COOLi] and stearic acid [CH ₃ (CH ₂ )
₁₆ COOH] and lithium stearate [CH ₃ (CH ₂ ) ₁₆ COOLi]
And a mixture with. Further, various samples were prepared by changing the molar ratio of the material substituted with lithium in the range of 1% to 10%. Ultrasonic cleaning was performed with ethanol, nitrogen gas was blown to dry it, and the mask was removed by etching.

Further, the substrate after the above proton exchange was carried out in a material in which 1% of lithium benzoate was added to benzoic acid in a molar ratio of 250%.
Proton exchange treatment was performed at 1 ° C. for 1 hour. As a result, the proton exchange layer 14 was formed as shown in FIG. In this proton exchange treatment, a mixture of palmitic acid and lithium palmitate used in the first proton exchange treatment, a mixture of stearic acid and lithium stearate, or the like can be used. After the above-mentioned proton exchange, ultrasonic cleaning was performed again with ethanol, and nitrogen gas was blown to dry it.

Next, the crystal substrate that has been subjected to the proton exchange treatment twice is put in a thermal furnace, and oxygen is introduced at a flow rate of 1.0 l / min through heated water in a moist oxygen atmosphere containing water vapor.
Annealing treatment was performed at 350 ° C. for 4 hours. As a result, as shown in FIG. 2 (e), the optical waveguide 15 is formed in which only the optical functional portion is proton-injected and the depth is shallow, and the thickness is increased toward the end faces 20 and 21 of the substrate. It was The proton distributions at the boundaries 18 and 19 between the optical function part and the non-optical function part were changed smoothly because the annealing treatment was performed, and it was confirmed in the waveguide experiment that the propagation loss in this part was small. .

The annealing conditions may be other than the above conditions, but the wave number of the absorption peak of the OH group in the optical function part is 3480 cm ^-1.
It is desirable to select it so that it exists in the range from to 3503 cm ^-1 .

Finally, the comb-shaped electrode 16 was formed on the optical waveguide 15 as shown in FIG. 2 (f) by using the usual photolithography method.

In the above embodiment, the optical waveguide was formed by Ti diffusion and thermal diffusion of protons, but Ti diffusion is not necessarily required, only proton injection or thermal diffusion, or proton injection or thermal diffusion and LiO external diffusion. By doing so, an optical waveguide may be formed.

FIG. 3 is a schematic diagram showing a reference example of an optical deflector utilizing the electro-optic (EO) effect. In FIG. 3, the same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

The laser light 8 is condensed on the polished end face 3 of the optical waveguide by the cylindrical lens 5 in the thickness direction of the optical waveguide and is coupled into the optical waveguide. The guided light 9 coupled from the end face of the optical waveguide is a comb-shaped electrode 17 for electro-optic (EO) effect.
The light is diffracted by the phase grating generated by applying a voltage to the light, is emitted from the other end face 4 of the optical waveguide, and is converted into parallel light by the cylindrical lens 6. The comb-shaped electrode manufactured here has an electrode width and a gap between electrodes of 2.2.
μm, cross width 3.8 mm, logarithm 350 pairs. It was also found that when a voltage of 5 V was applied to the mold electrode, a diffraction efficiency of 90% was obtained, and a high diffraction efficiency was obtained.

In the above examples, the LiNbO ₃ crystal substrate was used as the substrate, but even if a lithium tantalate (LiTaO ₃ ) crystal substrate is used, the thin-film optical element of the present invention can be formed by the same manufacturing method. I can. Further, the substrate is not limited to such a dielectric, and a semiconductor may be used for the substrate. Such a thin film optical element is shown below.

FIG. 4 is a perspective view showing a second embodiment of the thin film type optical element of the present invention. 4, the same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. Here, 31 is a gallium arsenide (GaAs) substrate, 32 is an aluminum gallium arsenide (AlGaAs) buffer layer, 33 is a gallium arsenide-aluminum arsenide (GaAs-AlGaAs) optical waveguide having an injection carrier distribution, and 34 is zinc oxide (ZnO). It is a thin film and has a tapered structure on one side. When RF power is applied to the comb-shaped electrode 7 provided on the ZnO thin film, surface acoustic waves 10 are excited on the thin film, pass through the tapered structure portion, and pass through the GaAs-Al.
The guided light 9 propagates on the GaAs optical waveguide 33 and the guided light 9 is the surface acoustic wave.
Diffracted by 10. On the optical waveguide layer 33, the optical coupling portions near the optical waveguide end faces 3 and 4 have a high carrier density and are deeply doped. Therefore, the effective refractive index of the optical waveguide is such that the surface acoustic wave and the guided light are mutually Low compared to the working optical function part. As a result, the energy distribution of the guided light is broadened in the optical coupling portion and exhibits high coupling efficiency, while the energy distribution of the guided light is concentrated on the surface in the optical function portion, and high diffraction efficiency is obtained.

〔The invention's effect〕

As described above, the thin-film type optical element manufactured by the method of the present invention has the optical damage caused by making the distribution of ions in the optical waveguide different in depth between the optical function section and the optical coupling section. While maintaining a sufficiently high threshold value, the coupling efficiency at the input / output of the guided light is increased and at the same time, the efficiency of diffraction by the surface acoustic wave is improved.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment in which a thin film type optical element according to the present invention is used in an optical deflector based on an acousto-optic effect, and FIG. 2 is a schematic cross section showing an example of a manufacturing process of the thin film type optical element of the present invention. 3 and 4 are schematic views showing a reference example of an optical deflector based on the electro-optical effect, and FIG. 4 is a schematic view showing another embodiment of the present invention. 1 ... LiNbO ₃ crystal substrate, 2 ... Optical waveguide, 3, 4 ... Polished optical waveguide end face, 5, 6 ... Cylindrical lens, 7, 17 ... Comb type electrode, 8 ... Laser light, 10 ... Surface acoustic wave.

Claims (1)

[Claims] 1. A substrate, a thin film optical waveguide formed by implanting or thermally diffusing ions on the entire surface of the substrate, and a light propagating through the optical waveguide by generating a surface acoustic wave on the surface of the substrate. It consists of diffracting means,
In the method for producing a thin film type optical element having an optical coupling part for inputting and outputting the propagating light from the end face of the optical waveguide and an optical functional part formed with a means for diffracting the propagating light, the optical function of the substrate surface is provided. The process of implanting ions only in the area other than the area, the process of implanting ions on the entire surface of the substrate, and the thermal diffusion of the implanted ions make it possible for the optical coupling part to have a greater ion diffusion depth than the optical function part. A method of manufacturing a thin film type optical element, comprising: a step of forming a deep optical waveguide; and a step of forming a means for diffracting propagating light in the optical function part.