JPH04264534A - Production of wavelength converting element - Google Patents

Abstract

PURPOSE:To provide the producing method of a wavelength converting element having a short period and forming a deep inversion layer to produce the wavelength converting element with high efficiency and a short wavelength, since the conversion efficiency of the wavelength converting element depends on the depth of the inversion of polarization and the length of a converted wavelength depends on the converting period of the polarization. CONSTITUTION:Proton exchanging process is executed partially on the surface of an LiTaO3 crystalline substrate 1. Thereafter, electric field is applied to a proton exchanging layer 9. Simultaneously the substrate 1 is heated in the vicinity of the Curie temperature below the Curie temperature of the substrate. Thus, the periodic inversion layer of the polarization is formed by applying the electric field to the proton exchanging layer polarized at random.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of manufacturing a wavelength conversion element that applies a coherent light source and is used in the fields of optical information processing and optical applied measurement control. . [0002] Polarization inversion, which forcibly inverts the polarization of a dielectric material, is performed by forming a periodic polarization inversion layer on the dielectric material, and is used as an optical frequency modulator using surface acoustic waves or nonlinear polarization. It is used in wavelength conversion elements that utilize inversion. In particular, if it becomes possible to periodically invert the nonlinear polarization of a nonlinear optical material, a second harmonic generating element with extremely high conversion efficiency can be produced. By converting light from semiconductor lasers and other sources, it is possible to create compact, short-wavelength light sources, which can be applied to fields such as printing, optical information processing, and optical applied measurement and control, and are therefore being actively researched. A conventional method for manufacturing a wavelength conversion element using such polarization inversion includes, for example, a method of manufacturing a wavelength conversion element by forming a periodic polarization inversion layer on the surface of a LiNbO3 substrate. As shown in Figure 7, this is a LiNbO
3. A periodic Ti film 101 is formed on the surface of the substrate 100.
By diffusing into the substrate, the LiNbO3 substrate 100
In this method, a periodic domain-inverted layer 102 is formed, and an optical waveguide 103 is formed across the periodic domain-inverted layer 102 to manufacture a wavelength conversion element. [0004] Furthermore, as a method for manufacturing a wavelength conversion element using LiTaO3 crystal, a conventional method for manufacturing a wavelength conversion element will be described. For example, there is a method for manufacturing a wavelength conversion element by Mr. Kiyoshi Nakamura (Appl. Phys. Lett., 1990, No. 56, P1535). FIG. 7 is a process diagram of this conventional wavelength conversion element manufacturing method. 21 in Figure 7
2 is a LiTaO3 substrate, 22 is a proton exchange layer, and 23 is a polarization inversion layer. The manufacturing method is shown in FIG. 8(b) LiTaO3
The substrate is heat treated in benzoic acid at 590°C to form a proton exchange layer. (c) LiTaO3 substrate at Curie temperature (
When heat treated at 570°C to 590°C near
This is a method for manufacturing a wavelength conversion element in which a layer with inverted polarization is formed on the -C plane surface of an iTaO3 substrate. [0005] Problems to be Solved by the Invention: LiNbO3 crystals have the problem of optical damage, and because it is difficult to increase the optical power density, it is difficult to manufacture wavelength conversion elements with high conversion efficiency. Research is being conducted on manufacturing wavelength conversion elements by forming polarization inversion in LiTaO3 crystal, which has a crystal structure similar to LiNbO3 crystal, has high nonlinearity, and is also resistant to optical damage. Also, LiTa
O3 crystal has excellent optical properties, less contamination with impurities during crystal growth, and superior crystallinity compared to LiNbO3, so it has excellent properties in terms of optical damage and DC drift, and is used as an optical material for optical IC devices. It is a useful material. However, although the above method can form a slab-like inversion layer, there is a problem in that a periodic polarization inversion layer cannot be formed in the LiTaO3 crystal. Furthermore, in order to form a highly efficient wavelength conversion element using a periodic polarization inversion layer, there is a problem in that the polarization inversion layer must have a period of several μm and a depth greater than the wavelength of light. [0006] The present invention solves the above-mentioned problems.
Forms periodic polarization inversion by TaO3 crystal, several μm
It is an object of the present invention to provide a method for manufacturing a wavelength conversion element with high conversion efficiency by forming a deep polarization inversion layer with a short period. [Means for Solving the Problems] In order to solve the above problems, a striped electrode pattern is formed on the +C surface of a LiTaO3 substrate, which is a C plate (a substrate cut in a plane perpendicular to the C axis of the crystal). forming an electrode on the back surface of the LiTaO3 substrate, applying an electric field between the striped electrode pattern and the electrode formed on the back surface of the LiTaO3 substrate, and simultaneously converting the LiTaO3 substrate into LiT.
The method of manufacturing a wavelength conversion element includes a step of heating below the Curie temperature of aO3. [0008] Also, Li of the C plate (plane perpendicular to the C axis of the crystal)
A step of forming a stripe-shaped mask on the surface of the −C plane of the TaO3 substrate, and performing proton exchange treatment on the main body of the LiTaO3 substrate to exchange Li and H+ ions in the LiTaO3 substrate from the non-mask portion to form a proton exchange layer. a step of removing the striped mask formed on the surface of the LiTaO3 substrate; a step of forming electrodes on the front and back surfaces of the LiTaO3 substrate;
An electric field is applied between the front and back electrodes of the three substrates, and at the same time LiTaO
3, a polarization inversion treatment step of heating at a temperature below the Curie temperature. [Operation] According to the method described above, the present invention makes the polarization of the substrate random by heating the LiTaO3 crystal substrate near the Curie point, and by applying an electric field in this state, the polarization of the LiTaO3 crystal becomes periodic. invert it. With this, a wavelength conversion element can be manufactured. The present invention also provides LiTa by the method described above.
When the O3 crystal is partially subjected to proton exchange treatment, the Curie point of the proton exchange layer becomes lower than that of the substrate. Therefore, by heat treatment at a temperature higher than the Curie point of the proton exchange layer and lower than that of the substrate, the polarization of the proton exchange layer is reduced. You can only randomize. Then, an electric field is applied to reverse only the polarization of the proton exchange layer. In this way, the lateral spread of the proton exchange layer can be restricted by an electric field and heat treatment can be performed at the same time, so that the periodic polarization inversion layer can be formed deeper in the depth direction, and the lateral spread of the polarization inversion layer can be reduced. . Therefore, deep and short-period polarization inversion can be formed. As a result of the above, L
Periodic polarization inversion is formed by iTaO3 crystal, and several μ
By forming a deep polarization inversion layer with a short period of m, a wavelength conversion element with high conversion efficiency can be manufactured. Embodiment FIG. 1 shows a process diagram of a method for manufacturing a wavelength conversion element in a first embodiment. In Figure 1, 1
2 is a LiTaO3 substrate, 2 is a striped electrode, 3 is an electrode, and 4 is a polarization inversion layer. The manufacturing method of the wavelength conversion element of the first embodiment configured as described above will be explained below. A Ta film is formed on a LiTaO3 substrate 1 of FIG. 1(a)+C plate by a sputtering method. (
b) After applying a resist on the Ta film, stripes with a width of 2 μm are formed every 4 μm in a length of 10 mm in the Y propagation direction of the substrate by photolithography. This period was determined based on the phase matching condition of the fundamental wave with a wavelength of 0.8 μm and the second harmonic wave with a wavelength of 0.4 μm in LiTaO3. The phase matching condition is the length of the period at which the harmonic output is the largest, and is the distance uniquely determined by the refractive index specific to the material of the substrate and the wavelengths of the fundamental wave and harmonics. Next, dry etching is performed in a CF4 atmosphere to form striped electrodes 2. (c) A planar electrode was also provided on the back surface of the LiTaO3 substrate, and heat treatment was performed in a constant temperature bath while applying a DC electric field between the striped electrode and the planar electrode. Since the Curie point of the LiTaO3 substrate used was 610°C, the temperature of the heat treatment was 600°C. The heat treatment was performed in nitrogen to prevent oxidation of the electrode. The heat treatment here is a treatment in which the temperature of the LiTaO3 crystal is raised to near the Curie temperature and the polarization of the crystal aligned in a certain direction is partially reversed. (d) Remove the electrode. The periodically domain-inverted layer produced had a depth of 0.5 μm. A wavelength conversion element was fabricated by forming a proton exchange waveguide on the substrate surface perpendicular to the periodically poled layer. FIG. 2 shows a perspective view of the structure of the wavelength conversion element manufactured in Example 1. 1 is a LiTaO3 substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 6 is a fundamental wave with a wavelength of 800 nm, and 7 is a second harmonic wave with a wavelength of 400 nm. The proton exchange waveguide is 2 μm wide and 1 μm deep. wavelength 80
Light 6 from a semiconductor laser with a wavelength of 0 nm and an output of 40 mW was focused by a focusing optical system and made incident on the fabricated wavelength conversion element. The fundamental wave and second harmonic 7 emitted from the waveguide were collimated with a lens and measured with a power meter. As a result, the output of the second harmonic with a wavelength of 400 nm was 0.1 mW, which was more than 1000 times that in the case where phase matching by polarization inversion was not achieved, and high conversion efficiency was obtained. Furthermore, although the power of the fundamental light was increased to 200 mW, no fluctuation due to optical damage was observed. As a result, stable and highly efficient output was obtained. In this example, the polarization inversion layer was fabricated in the Y propagation direction, but a similar device can also be fabricated in the X propagation direction. Furthermore, although a LiTaO3 substrate was used as the substrate in this embodiment, a similar element can also be fabricated using a LiTaO3 substrate doped with MgO. FIG. 3 shows a process diagram of a method for manufacturing a wavelength conversion element in a second embodiment. In FIG. 3, 1 is a LiTaO3 substrate, 8 is a striped Ta mask, 9 is a proton exchange layer, 10 and 11 are platinum electrodes,
12 is a polarization inversion layer. The second
The method of manufacturing the wavelength conversion element of the embodiment will be described below. A Ta film is formed on the LiTaO3 substrate 1 of the FIG. 3(a)-C plate by sputtering. (b
) After applying resist on the Ta film, one stripe with a width of 2 μm is formed every 4 μm period by photolithography.
It is formed over a length of 0 mm in the Y propagation direction of the substrate. This period was determined based on the phase matching condition of the fundamental wave with a wavelength of 0.8 μm and the second harmonic wave with a wavelength of 0.4 μm in LiTaO3. Next, dry etching is performed in a CF4 atmosphere to form a striped mask pattern 8. (c) 260℃
pyrophosphoric acid for 20 minutes to form a proton exchange layer 9. Pyrophosphoric acid has a high degree of dissociation and a high boiling point, so a deep proton exchange layer can be formed in a short time by high temperature treatment, which is highly efficient in production. A uniform proton exchange layer with a large point drop can be formed. (d)Ta
Remove the mask with HF. (e) Platinum electrodes are provided on the front and back surfaces of the LiTaO3 substrate. (f) Heat treatment was performed in a constant temperature bath while applying a DC electric field between the front and back plane electrodes of the substrate. The Curie point of the LiTaO3 substrate used is 610.
℃, the heat treatment temperature was 600°C for 1 minute. At this time, the Curie point of the proton exchange layer was lower than that of the substrate, and the polarization inversion layer could be formed at a temperature between 460°C and 600°C. At temperatures below 450°C or above 600°C, it was impossible to form periodic inversion layers. This is because at temperatures below 460° C., the proton exchange layer does not reach the Curie point, so polarization reversal does not occur. Moreover, at 610° C. or higher, not only the proton exchange layer but also the substrate itself reaches the Curie point, so polarization inversion occurs over the entire surface of the substrate, and periodic polarization inversion disappears. The periodically domain-inverted layer 12 produced had a depth of 0.8 μm. A proton exchange waveguide was formed on the surface of the substrate so as to be perpendicular to the periodic polarization inversion layer, and a wavelength conversion element similar to that shown in FIG. 2 was fabricated. The proton exchange waveguide is 2 μm wide and 1 μm deep. Wavelength 800nm, output 40m
The light 6 from the W semiconductor laser was focused by a focusing optical system and made incident on the fabricated wavelength conversion element. The fundamental wave and second harmonics emitted from the waveguide were collimated with a lens and measured with a power meter. As a result, the output of the second harmonic of 7 with a wavelength of 400 nm was 0.2 mW, making it possible to perform conversion with twice the efficiency of the conventional method. In this example, a planar electrode was used to apply the electric field, but if a striped electrode 13 is used only in the area where the polarization inversion layer is to be formed, as shown in FIG. It is selectively applied only to the replacement part. As a result, a deeper polarization inversion layer 12 was formed, with a period of 4 μm and a depth of 1 μm, and a similar second polarization inversion layer 12 was formed.
When a harmonic generation experiment was conducted, an output of 0.3 mW was obtained, making conversion with extremely high efficiency possible. Furthermore, by selectively applying an electric field, it is possible to shorten the period of polarization inversion. Conventionally, only a period of 4 μm could be achieved, but by applying a local electric field, it is possible to shorten the period of polarization inversion with a period of 3 μm and a depth of 0.8 μm.
It is now possible to fabricate a polarization inversion layer with a wavelength of 0.
．． When the light from a 76 μm semiconductor laser was converted, the result was 0.
A second harmonic output of 2 mW was obtained, making it possible to shorten the wavelength. In this example, a plane electrode was used to apply the electric field, but as shown in FIG. 5, two comb-shaped electrodes 14 and 15 were arranged alternately, and one electrode was formed directly above the proton exchange layer. Then, a local electric field is selectively applied only to the proton exchange portion 9, and since the distance between the electrodes is very short, a high electric field is applied.As a result, the polarization inversion layer formed has a period of 4 μm and a depth of 1.5 μm. , which is about twice as much as when it is formed using a flat electrode. When a wavelength conversion element is fabricated using this inversion layer, when light from a semiconductor laser with a wavelength of 0.8 μm and 40 mW is incident, a second harmonic of 1 mW (wavelength 0
．． 4 μm), and a very high conversion efficiency of about 5 times that of a wavelength conversion element formed with a flat electrode was obtained. In this example, the polarization inversion layer was fabricated in the Y propagation direction, but a similar device can also be fabricated in the X propagation direction. Further, in this example, pyrophosphoric acid was used for ion exchange, but phosphoric acid, benzoic acid, nitric acid, hydrochloric acid, sulfuric acid, etc. can also be used. In this embodiment, a Ta film was used as the ionization-resistant mask, but any other acid-resistant film such as Ta2O5 or Pt may be used. Furthermore, although LiTaO3 is used as the substrate in this embodiment, a LiTaO3 substrate doped with MgO may also be used. As shown in FIG. 6, convex stripes with a height of 1 μm and a width of 2 μm are formed on the surface of the LiTaO3 crystal of the X plate in the Y-axis direction, and periodic electrodes 16, form 17. When this electrode was heated at 600°C for 10 minutes while applying a 2V electric field, C
The polarization pointing in the axial direction was periodically reversed in the -C direction. By this method, a periodic polarization inversion layer with a minimum period of 2 μm could be formed in convex stripes. Furthermore, by polishing both end faces of the stripe, a wavelength conversion element was fabricated using this as a waveguide. When 20 mW of light from a semiconductor laser with a wavelength of 680 nm was incident on the produced wavelength conversion element, an SHG output of 0.1 mW at a wavelength of 340 nm was generated, and a high output and short wavelength wavelength conversion element was fabricated. As a result, polarization inversion could not be produced in the past. A wavelength conversion element can be constructed using an X-plate or Y-plate LiTaO3 crystal. Furthermore, since the electric field is applied from both sides of the convex shape, the electric field density can be increased and polarization inversion can be formed with a low voltage. Furthermore, since polarization inversion is formed by applying an electric field from the side, a deep inversion layer can be formed with a short period, and a wavelength conversion element with high efficiency and a short wavelength can be constructed. Note that although an X-plate LiTaO3 crystal was used in this embodiment, a similar element can also be fabricated using a Y-plate LiTaO3 crystal; however, in that case, the convex stripes will propagate along the X axis. Effects of the Invention As explained above, by forming striped electrodes on LiTaO3 crystal and simultaneously applying an electric field and heat-treating it near the Curie point, LiTaO
3 The polarization of the crystal is periodically reversed. This makes it possible to form a periodic polarization inversion layer, and by combining it with an optical waveguide, a wavelength conversion element can be manufactured. As a result of the above, LiTaO3 crystal can form periodic polarization inversion, which has not been achieved before, and Li has excellent resistance to light damage.
A high-output wavelength conversion element can be manufactured using TaO3 crystal, and its practical effects are great. [0023] Furthermore, after partially subjecting the LiTaO3 crystal to proton exchange treatment, by applying an electric field and simultaneously heating it below the Curie temperature and near the Curie temperature,
The lateral spread of the proton exchange layer is restricted by an electric field,
Since the heat treatment can be performed simultaneously, the periodically domain-inverted layer can be formed deeper in the depth direction, and the lateral spread of the domain-inverted layer can be reduced. As a result of the above, deep and short-period polarization inversion can be formed using LiTaO3 crystal, which has not been realized in the past. When a wavelength conversion element is formed by this periodic polarization inversion, highly efficient conversion becomes possible, and a high-output wavelength conversion element can be manufactured using LiTaO3 crystal which has excellent resistance to light damage, which has great practical effects.

[Brief explanation of the drawing]

FIG. 1 is a process cross-sectional view of a method for manufacturing a wavelength conversion element according to an embodiment of the present invention.

FIG. 2 is a perspective view of the configuration of a wavelength conversion element.

FIG. 3 is a process cross-sectional view of a method for manufacturing a wavelength conversion element.

FIG. 4 is a perspective view of a method for manufacturing a wavelength conversion element.

FIG. 5 is a perspective view of a method for manufacturing a wavelength conversion element.

FIG. 6 is a perspective view of a method for manufacturing a wavelength conversion element.

FIG. 7 is a process cross-sectional view of a conventional wavelength conversion element manufacturing method.

FIG. 8 is a process cross-sectional view of a conventional wavelength conversion element manufacturing method.

[Explanation of symbols]

1 LiTaO3 substrate 2 Striped electrode 3 Electrode 4 Polarization inversion layer 5. Proton exchange optical waveguide 6 Fundamental wave 7 Second harmonic 8 Ta mask emission part 9. Proton exchange layer 10 Electrode 11 Electrode 12 Polarization inversion layer 13 Striped electrode 14 Comb-shaped electrode 15 Comb-shaped electrode 16 Electrode 17 Electrode 18 Convex stripes 21 LiTaO3 substrate 22 Proton exchange layer 23 Polarization inversion layer 31 LiTa3 substrate

Claims (5)

[Claims] 1. A step of forming a striped electrode pattern on the +C surface of a LiTaO3 substrate of a C plate (a substrate cut out perpendicular to the C axis of the crystal);
forming an electrode on the back surface of the substrate; applying an electric field between the striped electrode pattern and the electrode formed on the back surface of the LiTaO3 substrate;
A method for manufacturing a wavelength conversion element, comprising the step of heating a substrate at a temperature below the Curie temperature of LiTaO3. 2. A step of forming a stripe-shaped mask on the surface of the LiTaO3 substrate of the C plate;
A step of performing proton exchange treatment on the substrate body to exchange Li and H+ ions in the LiTaO3 substrate from the non-mask portion to form a proton exchange layer, and a step of removing the striped mask formed on the surface of the LiTaO3 substrate. ,
It is characterized by comprising a step of forming planar electrodes on the front and back surfaces of the LiTaO3 substrate, respectively, and a polarization inversion treatment step of applying an electric field between the planar electrodes and simultaneously heating the LiTaO3 substrate below the Curie temperature of LiTaO3. A method for manufacturing a wavelength conversion element. 3. A step of forming a stripe-shaped mask on the surface of the LiTaO3 substrate of the C plate;
A step of performing proton exchange treatment on the substrate body to exchange Li and H+ ions in the LiTaO3 substrate from the non-mask portion to form a proton exchange layer, and a step of removing the striped mask formed on the surface of the LiTaO3 substrate. a step of forming an electrode only on the upper surface of the proton exchange layer; a step of forming an electrode on the back surface of the LiTaO3 substrate; and a step of applying an electric field between the front and back electrodes of the LiTaO3 substrate and simultaneously converting the LiTaO3 substrate into a Curie of LiTaO3. 1. A method for manufacturing a wavelength conversion element, comprising a polarization inversion treatment step of heating at a temperature below temperature. 4. A step of forming a stripe-shaped mask on the surface of the LiTaO3 substrate of the C plate;
A step of performing proton exchange treatment on the substrate body to exchange Li and H+ ions in the LiTaO3 substrate from the non-mask portion to form a proton exchange layer, and a step of removing the striped mask formed on the surface of the LiTaO3 substrate. a step of forming a comb-shaped electrode directly above the proton exchange layer; a step of forming another comb-shaped electrode that does not intersect the comb-shaped electrode; applying an electric field between both the comb-shaped electrodes;
A method for manufacturing a wavelength conversion element, comprising a polarization inversion treatment step of heating an iTaO3 substrate below the Curie temperature of LiTaO3. 5. Li on an X plate (a substrate cut on a plane perpendicular to the X axis of the crystal) or a Y plate (a plane perpendicular to the Y axis of the crystal)
A step of forming convex stripes on the surface of the TaO3 substrate, a step of forming periodic electrodes on both sides of the stripe, applying an electric field between the electrodes, and simultaneously applying the LiT
A method for manufacturing a wavelength conversion element, comprising a polarization inversion treatment step of heating an aO3 substrate below the Curie temperature of LiTaO3.