JPH08271940A - Production of optical device - Google Patents

Abstract

PURPOSE: To provide a process for producing an optical device capable of uniformly forming good periodic fine polarization inversion structures with good reproducibility even on a relatively thick substrate which is easy to handle particularly in a process. CONSTITUTION: This process for producing the optical device comprises depositing and forming periodic electrodes 3 consisting of band-shaped electrodes 3a and frame-shaped electrodes 3b connected with both ends of the band-shaped electrodes 3a on the inner side on one main surface of the substrate 1 consisting of a nonlinear ferroelectric substance subjected to single polarization, arranging a common electrode 4 on the other main surface of the substrate 1 and impressing an electric field between the periodic electrodes 3 and the canon electrode 4 so as to generate polarization inversion in the substrate 1. The contour of the frame-shaped electrodes 3b is formed to a disk shape or a shape constituting a curved shape at the corners.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical device such as an optical second harmonic generating element or an optical modulator used in an optical information system or the like.

[0002]

2. Description of the Related Art In recent years, an optical second harmonic generating (hereinafter abbreviated as SHG) element has been used as a compact short-wave-mode coherent light source required for optical information systems such as semiconductor laser generating means used for optical recording media. Is attracting attention.

Generally, SHG elements are roughly classified into bulk type SHG elements and waveguide type SHG elements. Recently, lithium niobate (LiNbO3) has been used as a material for the SHG elements.
), Lithium tantalate (LiTaO3), KTiO
Inorganic oxides such as PO ₄ and potassium niobate (KNbO 3) and various organic nonlinear materials are preferably used because they have large nonlinear optical constants. Among these materials, LiNbO3 and LiTaO3 are particularly promising as a waveguide-type SHG element because their waveguide fabrication technology has been established and they have relatively large nonlinear optical constants.

In particular, the characteristic of the waveguide type SHG element is that the nonlinear optical coefficient d ₃₃ can be used by the quasi phase matching method described below. d ₃₃ is several times larger than other nonlinear optical coefficients (for example, d _{33 in} LiNbO 3
Is about 6 times larger than d ₃₁ ), and thus high conversion efficiency can be achieved. Further, by confining light, a high light intensity density can be realized over a long propagation distance.

In the conventional waveguide type SHG element, the waveguide is formed on the surface layer of the element body by a thermal diffusion method of a metal such as Ti. Then, when a fundamental wave (propagation constant β (ω)) of angular frequency ω is incident on one end of the waveguide, a wave of angular frequency 2ω (propagation constant β (2ω)) including the fundamental wave is emitted from the other end. .

However, since the relationship of 2β (ω) ≠ β (2ω) is generally satisfied, the phase matching condition is not satisfied, and the elementary waves of β (2ω) generated from arbitrary positions in the waveguide interfere with each other and cancel each other. Therefore, SHG conversion is not performed effectively.

Therefore, in order to effectively perform SHG conversion,
It is necessary to compensate for the difference in propagation constant between the fundamental wave and the harmonic with an optical periodic structure to achieve phase matching. This method is called quasi-phase matching, and in a ferroelectric crystal such as LiNbO3 or LiTaO3, the property that the positive / negative of the nonlinear optical coefficient corresponds to the polarity of the dielectric polarization is used, and the optical polarization The periodic structure is drawing attention.

An electric field application method shown in FIG. 9 has been proposed as one method of this polarization reversal. Here, 21 is a waveguide 22
A substrate of LiNbO3 or LiTaO3 containing 23,
Is a periodic electrode patterned on the substrate, 24 is a common electrode mounted on the entire back surface of the substrate, and 25 is a high-voltage power supply.

This method is carried out by using, for example, z-cut LiNbO.
The periodic electrode 23 is provided on the surface 21a (+ Z surface for LiNbO3, -Z surface for LiTaO3) on which the waveguide of the substrate 21 of 3 or LiTaO3 is formed, and the common electrode 24 is provided on the other main surface. This is a method of obtaining a periodically inverted structure of polarization necessary for quasi-phase matching by utilizing the phenomenon that an inverted polarization having the same pattern as that of the electrode 23 is obtained (for example, M. Yamada, N. Noda, and K. Watanabe: Integrated. P
See hotonics Research TuC2-1 (1992)).

Further, a method has been proposed in which a high voltage is applied to a substrate made of a non-linear ferroelectric material at a relatively low temperature of 150 ° C. or lower to perform polarization reversal (for example, Japanese Unexamined Patent Publication No. 4-335620). (See the official gazette). According to this method, the controllability of the inversion shape is excellent, and there is no change in the refractive index or crystallinity due to surface contamination or heat, and it is noted that a periodic fine polarization inversion structure can be easily obtained at low cost. ing.

[0011]

However, in the prior art, the outline (peripheral line) of the periodic electrode is generally rectangular, and when a voltage is applied to such periodic electrode as shown in FIG. According to the electric field intensity distribution of the component applied in the thickness direction of the substrate 11 made of a ferroelectric material, the electric field is concentrated at the portion where the inclination of the tangent line of the contour line of the periodic electrode changes abruptly, that is, the electric field intensity is It becomes very strong locally. Therefore, even if an electric field strength slightly higher than the electric field strength at which polarization reversal occurs is set on the inner side away from the electrode to which the electric field is applied relatively uniformly, the substrate 11 is destroyed at the site where the electric field strength is locally high. There was a problem that it was easy to do.

Further, if the thickness of the substrate is made thin in order to cause polarization reversal at a low electric field strength, the physical strength of the substrate becomes weak, and there is a problem that handling is significantly deteriorated in the process.

The present invention overcomes the above-mentioned problems, and in particular, an optical device capable of forming a good periodic fine polarization inversion structure uniformly and with good reproducibility even on a relatively thick substrate which is easy to handle in the process. It aims at providing the manufacturing method of.

[0014]

In order to achieve the above-mentioned object, a method of manufacturing an optical device according to the present invention comprises a strip electrode and a strip electrode on one main surface of a substrate made of a non-linear ferroelectric material having a single domain. A periodic electrode composed of a frame-shaped electrode in which both ends of the strip-shaped electrode are connected to the inside is adhered and formed, and a common electrode is arranged on the other main surface of the substrate, and the periodic electrode and the common electrode are arranged. A method for manufacturing an optical device in which a polarization inversion is caused in the substrate by applying an electric field between them, wherein the outline of the frame-shaped electrode has a circular shape or a shape in which corners are curved. Characterize.

In addition, a plurality of strip-shaped insulating films are arranged at a predetermined interval on one main surface of the substrate made of a single-domain non-linear ferroelectric material, and one main surface of the substrate including the insulating film is Covering with a conductor film having a circular shape or a curved outline with corners, arranging a common electrode on the other main surface of the substrate, and applying an electric field between the conductor film and the common electrode. A method for manufacturing an optical device in which polarization reversal is caused in the substrate by the method.

Further, strip-shaped periodic electrodes and strip-shaped insulating films are alternately arranged and deposited on one main surface of a plate-shaped conductor whose contour is circular or whose corners are curved. On the main surface of the substrate made of the non-linear ferroelectric material with one domain,
The plate-shaped conductor is placed on one main surface side, a common electrode is arranged on the other main surface of the substrate, and an electric field is applied between the plate-shaped conductor and the common electrode to form the substrate. The polarization reversal is caused.

Further, the insulating film is SiO ₂ or Si.
It is characterized by being made of ₃ N ₄ and having a thickness of 5000 Å or less.

Note that the circular shape means a shape formed only of a curve such as an ellipse as well as a circle, and the curved shape of a corner means a shape consisting of a curve and a straight line. Specifically, it means that the slope of the tangent of the contour line changes continuously.

[0019]

According to the present invention, since the periodic electrode has a circular contour or a curved corner, the concentration of the electric field generated at the edge (corner) of the electrode as in the prior art is minimized. It becomes possible to suppress the breakdown electric field and the coercive electric field margin to a large extent.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. The substrate 1 of the nonlinear ferroelectric substance shown in FIG. 1 is used for an SHG element that forms an optical waveguide after polarization inversion, but may be a constituent element of various optical devices such as an optical modulator.

First, as the substrate 1 made of a non-linear ferroelectric material, a single crystal of lithium niobate (LiNbO ₃ ) which can be expected to have a high output as an SHG element, and the main surfaces 1a and 1b are used.
Was cut out so as to have the Z plane, and the thickness was about 300 μm for easy handling. The main surface 1a on which the periodic electrode described later is provided is the + Z surface, and the main surface 1b is -Z.
As the surface, the inversion easily occurs from the main surface 1a.

Next, a conductive material such as aluminum or silver was patterned into a predetermined shape by a vapor deposition method, a sputtering method, or photolithography to form the first electrode body 3 as a periodic electrode. Here, the first electrode body 3 is composed of a strip-shaped electrode 3a and a frame-shaped electrode 3b in which both ends of the strip-shaped electrode 3a are connected to the inside, and for example, as shown in the figure, the outline of the frame-shaped electrode 3b. Is circular.
It should be noted that the frame-shaped electrode 3b may be one having a continuous contour line and a gradient of its tangent line continuously changing, that is, one having no corners or no corners (bent shape).

Further, a second electrode body 4 is formed on the other main surface 1b side of the substrate 1 on the side corresponding to the first electrode body 3 by a method similar to the method of depositing and forming the first electrode body 3. A film was formed as an electrode.

In the same manner as above, a plurality of substrates for comparison, each having a rectangular periodic electrode as the first electrode body 6 were prepared as shown in FIG.

Then, after that, the first electrode body 3 (6)
And a second high voltage power source 5 connected to the second electrode body 4
A pulse voltage of about 00 msec was applied between the first electrode body 3 (6) and the second electrode body 4.

As a result, the comparative substrate 11 shown in FIG.
As shown in FIG. 10, there is a portion where the electric field strength is locally strong, and when the applied electric field strength is about 17 kV / mm, dielectric breakdown occurs, and polarization reversal is rare and reproducibility is poor. there were. On the other hand, in the example according to the present invention, the electric field strength distribution as shown in FIG. 3 is shown, the difference between the central portion having low electric field strength and the peripheral portion having high electric field strength is small, and the electric field strength is about 22 kV.
The polarization reversal occurred before the value became / mm, and dielectric breakdown occurred when an electric field higher than that was applied. By this method, it was confirmed that the dielectric breakdown electric field was increased by about 5 kV / mm as compared with the conventional method, and polarization reversal could be performed with good reproducibility.

In this embodiment, the first electrode body 3
Area where is not deposited (area where polarization is not inverted) 2
In addition, an insulating film (SiO ₂ or Si ₃ N ₄ ) having a thermally and temporally stable property may be formed to a predetermined thickness by, for example, a sputtering method or a CVD method. This insulating film 2
The film thickness of can be changed according to the controlled polarization inversion ratio (a: b when the width of the polarization region is a, b). It has been found that the desired inversion cannot be easily obtained because it changes.

The electric field is preferably applied in vacuum or in insulating oil such as silicone oil. That is, it is advisable to carry out in an insulating medium such as vacuum so that no discharge occurs even if a high voltage is applied between the electrodes.

Next, the first electrode body 3 and the second metal electrode 4
A corresponding etching solution (for example, aluminum,
H ₂ PO ₄ : HNO ₃ : CH ₃ COOH: H ₂ O 1
6: 1: 2: 1 mixture). In this way, a fine and uniform periodically poled structure could be formed over a length of about 10 mm.

Next, another embodiment will be described. In the above-described embodiment, the first and second electrode bodies and the insulating film are formed by depositing on the substrate 1, and the domain-inverted structure is formed by applying the electric field to the integrated body. The body and / or the second electrode body may be prepared separately from the substrate, an insulating film may be formed on the first electrode body, and the substrate may be sandwiched between two metal electrodes to apply an electric field. Further, the first electrode body may be used as a common electrode, and the insulating film may be formed in a stripe shape on the first electrode body. An example of forming the domain-inverted structure in this way will be described below.

First, as shown in FIG. 4, the first electrode body A is used as a uniform electrode, and its manufacturing method will be described based on the sectional views shown in FIGS. 5 and 6. A conductor or low resistance,
Preferably, a solid electrode plate 14 is prepared. In this embodiment, an electrode plate having a thickness of about 0.5 mm was used, and an aluminum metal tip whose one surface was mirror-finished was used.
It may be a resin plated with metal.

Next, the main surface of the electrode plate 14 is thermally
The time-stable insulating film 12 has a thickness of 50 (strips).
Deposition was performed under 00 Å. Further, the resist is left only on the insulating film 12 by photolithography.

Next, an aluminum metal film 15 was formed on the electrode plate 14 and the resist by vapor deposition. The metal film 15 may be formed by a sputtering method, an electrolytic plating method, or the like. However, in the case of the electroplating method, since it is not laminated on the resist, the lift-off process becomes easy. Since it is preferable that the metal film 15 has good adhesion to the electrode plate 14, when aluminum is used as the electrode plate 14, the same aluminum, silver, or the like is most suitable, but it is not always a simple substance of metal. It does not need to be, and may be a strong one having good conductivity. In addition, the metal film 15
If the thickness is not made substantially equal to that of the stable insulating film 12, it becomes impossible to adhere to the polarization inversion material, so that current cannot flow.

Finally, the resist is removed to complete the first electrode body A.

As the second electrode body 16, the same one as the electrode plate 14 is prepared. Then, the substrate 1 is sandwiched between the first electrode body A and the second electrode body 16, and an electric field is applied by the high voltage power source 17 to invert the polarization of the substrate 1.

In the above embodiment, the waveguide formation (ion exchange method or metal thermal diffusion at a temperature below the Curie point) is performed after the polarization reversal without causing the electric moment to be scattered. The case where the waveguide is formed after the periodic polarization inversion is performed by applying the electric field has been described, but it is not always necessary to do so, and the SHG may be performed before applying the electric field.
A device having a waveguide formed therein may be used.

The second electrode can be used as a ground, and when it is directly formed on the element body, it may or may not be removed by etching or the like.

In addition to the circular shape, the first electrode body may have an elliptical shape as shown in FIG. 7, a shape as shown in FIG. 8 or the like. However, the contour line is continuous and the tangent slope is continuous. Any shape may be used as long as the shape changes, and the shape can be appropriately changed without departing from the scope of the present invention.

Further, in the above-mentioned embodiment, the example using the single crystal substrate of LiNbO ₃ is explained, but the single crystal substrate of lithium tantalate (LiTaO ₃ ) may be used,
In this case, if the domain-inverted structure is formed on the -Z plane, the domain-inverted structure is preferably obtained.

[0040]

According to the method for manufacturing an optical device element of the present invention, it is possible to secure a large margin between the breakdown electric field and the coercive electric field, and it is possible to form the polarization inversion with good reproducibility.

Further, if the periodic electrodes are formed on the strip-shaped insulating film like the common electrodes, the complicated fine patterning of the electrodes as in the conventional case can be eliminated.

If the periodic electrode, the insulating film, and the common electrode are formed separately from the substrate and the substrate is sandwiched between the electrodes, the element may be damaged by the peeling process of the electrodes and the insulating film. In addition, it becomes possible to manufacture a large number of optical devices continuously.

Further, the influence of the line resistance of the electrodes can be greatly improved, and it becomes possible to uniformly and easily form a fine domain-inverted structure in a wide region according to the area of the electrode. It can be formed uniformly over a long or wide area, and the characteristics of the SHG element and the like formed thereby can be improved.

If the present invention is applied to an optical modulator, D
It is possible to suppress the C drift and stabilize the operation.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating an embodiment according to the present invention.

FIG. 2 is a perspective view illustrating a comparative example.

FIG. 3 is a diagram showing an electric field strength distribution generated by the present invention.

FIG. 4 is a plan view of a first electrode body.

5 is a partial cross-sectional view taken along line VV in FIG.

FIG. 6 is a partial sectional view illustrating another embodiment.

FIG. 7 is a plan view showing a modified example of the first electrode body.

FIG. 8 is a plan view showing a modified example of the first electrode body.

FIG. 9 is a schematic perspective view showing a method of producing periodically inverted polarization by a conventional electric field application method.

FIG. 10 is a diagram showing a field intensity distribution generated by applying a conventional electric field.

[Explanation of symbols]

1 ... Substrate 12 ... Insulating film 3, A ... First electrode body 4, 16 ...
.Second electrode body

Claims (4)

[Claims] 1. A periodic electrode composed of a strip-shaped electrode and a frame-shaped electrode in which both ends of the strip-shaped electrode are connected to the inside is provided on one main surface of a substrate made of a non-linear ferroelectric material having a single domain. Of the optical device, in which the common electrode is formed on the other main surface of the substrate, and an electric field is applied between the periodic electrode and the common electrode to cause polarization reversal in the substrate. A method of manufacturing an optical device, characterized in that the outline of the frame-shaped electrode has a circular shape or a shape in which corners are curved. 2. A plurality of strip-shaped insulating films are arranged at a predetermined interval on one main surface of a substrate made of a single-domain non-linear ferroelectric material, and one main surface of the substrate including the insulating film is formed. Cover with a conductor film that is circular or has a curved corner.
A method of manufacturing an optical device, wherein a common electrode is arranged on the other main surface of the substrate, and an electric field is applied between the conductor film and the common electrode to cause polarization reversal in the substrate. 3. A strip-shaped periodic electrode and a strip-shaped insulating film are alternately arranged and deposited on one main surface of a plate-shaped conductor whose contour is circular or whose corners are curved. One main surface side of the plate-shaped conductor is placed on one main surface of a substrate made of a domain-divided nonlinear ferroelectric, and a common electrode is arranged on the other main surface of the substrate, A method of manufacturing an optical device in which polarization inversion is caused in the substrate by applying an electric field between a conductor and the common electrode. 4. The insulating film is SiO ₂ or Si ₃ N ₄
4. The method for manufacturing an optical device according to claim 2, wherein the optical device has a thickness of 5000 Å or less.