JPH11282036A - Production of optical wavelength conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the mass productivity and to uniformly form periodic polarization inversion structures with respect to a method to produce an optical wavelength conversion element where an optical waveguide extended along one surface of a ferroelectric crystal substrate is formed on this substrate having a nonlinear optical effect and polarization inversion parts, where the direction of spontaneous polarization of the substrate is inverted, are periodically formed in this optical waveguide and a fundamental wave guided in the array direction of polarization inversion parts in the optical waveguide has the wavelength converted. SOLUTION: Plural voltage applying parts 13 consisting of an electrode 11 of a prescribed pattern corresponding to the pattern of polarization inversion parts to be formed and another electrode 12 facing this electrode 11 with a gap apart from the electrode 11 are formed on one surface of a ferroelectric crystal substrate 10 singly polarized, and grooves 15 which are extended between voltage applying parts 13 to isolate voltage applying parts 13 from each other on the surface are formed on a surface 10a of the substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type optical wavelength converter for converting a fundamental wave into a second harmonic or the like, and more particularly, to an optical waveguide using a ferroelectric crystal substrate as an optical waveguide substrate. The present invention relates to a method for manufacturing an optical wavelength conversion element having a periodically poled structure formed in a portion.

[0002]

2. Description of the Related Art A method of wavelength-converting a fundamental wave into a second harmonic using an optical wavelength conversion element provided with a region in which spontaneous polarization (domain) of a ferroelectric material having a nonlinear optical effect is periodically inverted. Has already been proposed by Bleombergen et al. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)).
In this method, the period 分 極 of the domain-inverted portion is given by: c = 2π / {β (2ω) -2β (ω)｝ where β (2ω) is the propagation constant of the second harmonic and β (ω) is the propagation of the fundamental wave. By setting the coherent length to be an integral multiple of the coherent length 定 数 c given by the constant, the fundamental wave and the second harmonic can be phase-matched (so-called quasi-phase matching).

For example, as shown in Japanese Patent Application Laid-Open No. 5-29207, an optical waveguide type optical wavelength conversion element having an optical waveguide made of a non-linear optical material and converting the wavelength of a fundamental wave guided therethrough is used. Attempts have been made to form a periodically poled structure as described above to achieve efficient phase matching.

[0004]

The optical waveguide type optical wavelength conversion element having the periodically poled structure thus formed can be formed, for example, by the method disclosed in Japanese Patent Application Laid-Open No. 9-218431. According to this method, an electrode having a predetermined pattern corresponding to a pattern of a domain-inverted portion to be formed and an electrode facing the electrode at an interval are formed on one surface of a monopolarized ferroelectric crystal substrate. Then, a voltage is applied to the substrate via these electrodes to form periodically repeated domain-inverted portions, and then an optical waveguide is formed so that these domain-inverted portions are arranged in the waveguide direction.

By the way, in order to manufacture this optical waveguide type optical wavelength conversion element with good mass productivity, a plurality of pairs of the above-mentioned electrode of the predetermined pattern and the electrode facing the same are collectively formed on one substrate. It is conceivable that a plurality of sets of periodically poled structures are formed on the substrate by using.

That is, as shown in FIGS. 4 and 5, an electrode having a predetermined pattern (for example, a comb-shaped electrode) corresponding to the pattern of the domain-inverted portion to be formed is formed on the surface of the monopolarized ferroelectric crystal substrate 1. 2 and a plurality of pairs of electrodes 3 facing the electrodes 2 at intervals. Next, when a voltage is applied from the power supply 4 to the substrate 1 via the electrodes 2 and 3, the spontaneous polarization is inverted between each electrode finger of the comb-shaped electrode 2 and the electrode 3, and a periodic polarization inversion structure is formed. If this voltage application is performed in a voltage application section composed of a set of each of the electrodes 2 and 3, a plurality of periodically poled structures are formed on the substrate 1. This voltage application may be performed simultaneously for each group or sequentially.

When a plurality of sets of periodically poled structures are formed as described above, an optical waveguide is formed for each of the periodically poled structures, and finally, the substrate is cut so as to separate them individually. Can be obtained all at once.

However, according to the study of the present inventor, when a plurality of waveguide-type optical wavelength conversion elements are manufactured as described above, polarization inversion occurs between each electrode finger of the comb-shaped electrode 2 and the electrode 3. There is a problem that it is difficult to form a periodic domain-inverted structure that is likely to occur in many non-existent portions and that is uniform (that is, the domain-inverted portions are formed in any of the predetermined portions in the same manner).

More specifically, according to the experiments of the present inventors,
Periodic reversibility when the MgO-doped LiNbO ₃ substrate 1 having a thickness of 400 μm is used and the substrate length, that is, the W dimension in FIG. 5 is set to 2, 4, 10, and 25 mm, respectively, is as shown in Table 1 below. became. In this case, the gap G between each electrode finger of the comb-shaped electrode 2 and the electrode 3 is 400 μm.
m. The substrate 1 is a 3 ° Y-cut substrate (Y axis is Y
A substrate cut in a plane perpendicular to an axis rotated by 3 ° toward the Z axis in the Z plane) was used. Further, in each case, the plurality of voltage applying sections composed of the electrodes 2 and 3 were arranged so as to be at the same distance from each other.

[0010]

[Table 1]

Here, "good" in the period reversibility in Table 1 means that the domain-inverted portions 5 are formed on the substrate 1 at predetermined intervals as shown in FIG. "Poor" indicates that the domain-inverted portion 5 was only partially formed as shown in FIG. Further, when there were a plurality of voltage applying units, the voltage was sequentially applied by using one of them.

The present invention has been made in view of the above circumstances, and provides a method of manufacturing an optical wavelength conversion element in which a plurality of periodically poled structures can be collectively and uniformly formed on one substrate. The purpose is to:

[0013]

According to the present invention, there is provided a method of manufacturing an optical wavelength conversion device, comprising the steps of: providing a voltage applying portion comprising an electrode having a predetermined pattern as described above and an electrode facing the electrode at an interval. In a method for manufacturing an optical wavelength conversion element in which a plurality of periodic domain-inverted structures are collectively formed on a substrate by applying a voltage to the substrate via these electrodes by forming a plurality of periodic polarization inversion structures on the surface, prior to applying a voltage, A groove is formed on the surface of the substrate so as to extend between each of the voltage applying portions, and a groove is formed on the surface to separate the voltage applying portions from each other.

In the method of manufacturing an optical wavelength conversion element according to the present invention, preferably, the two electrodes in the voltage applying section are formed apart from each other in a direction in which the direction of spontaneous polarization of the substrate is projected on the substrate surface. . As the electrodes of the predetermined pattern, a comb-shaped electrode in which the tip of each electrode finger faces the other electrode is preferably used.

Further, as the above substrate, a monopolarized ferroelectric crystal having a nonlinear optical effect is cut on a plane forming an angle θ (0 ° <θ <90 °) with respect to the direction of spontaneous polarization. The substrate formed is used. More specifically, as such a substrate, a substrate formed by cutting a ferroelectric crystal by a plane perpendicular to an axis rotated by 3 ° in the YZ plane toward the Z axis in the YZ plane, It is preferable to use a substrate formed by cutting the Z axis in a plane perpendicular to the axis rotated 87 ° in the ZX plane toward the X axis.

As the ferroelectric crystal serving as the substrate material, LiNb _x Ta _{1 -x} O ₃ (0 ≦ x ≦ 1) or a material doped with MgO, ZnO or Sc is preferably used.

On the other hand, the grooves as described above can be formed by engraving with a dicing saw or the like.

[0018]

According to the study of the present inventors, when a plurality of sets of periodically poled structures are formed on one substrate, it is difficult to uniformly form each periodically poled structure.
It is presumed to be due to the following.

That is, as the length of the substrate, that is, the dimension W in FIG. 5, is larger, more ions are gathered in the voltage application section. Therefore, immediately after the voltage is applied, the charge of the ions is compensated, and the effective voltage applied between the electrodes is reduced. I do. Between the electrodes, an inversion nucleus is generated immediately after the voltage is applied, and the domain-inverted portion grows. However, if the voltage is reduced for the above-described reason, no inversion nucleus is generated. On the other hand, when the applied voltage is increased, dielectric breakdown occurs and the weak part of the crystal is destroyed.
It is not possible to set an appropriate voltage value that enables uniform polarization inversion, and as a result, it becomes difficult to form a uniform periodic polarization inversion structure.

In the method of manufacturing an optical wavelength conversion element according to the present invention, a groove having a relatively short length is considered in the vicinity of the substrate surface by forming a groove on the substrate surface for isolating the voltage applying portions from each other. It is a form of multiple gatherings. Therefore,
Many ions are not collected at the voltage application section, and the effective voltage applied between the electrodes can be kept relatively high. Therefore, generation of inversion nuclei is promoted, and a uniform periodic polarization inversion structure can be formed.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIGS. 1 and 2 show steps of manufacturing an optical wavelength conversion element according to an embodiment of the present invention.

In this embodiment, LiNbO ₃ doped with 5 mol% of MgO, which is a ferroelectric substance having a nonlinear optical effect, is used.
A (MgO: LN) 3 ° Y-cut substrate 10 (substrate cut on a plane perpendicular to an axis rotated by 3 ° in the YZ plane toward the Z axis in the YZ plane: dimensions: 25 mm × 25 mm) was used. . First, an electrode pattern was formed on the surface 10a of the substrate 10 by photolithography, and Cr was vacuum-deposited and lift-off was performed to form a plurality of sets of comb-shaped electrodes 11 and plate electrodes 12. FIG. 3 shows the shapes of the comb electrode 11 and the flat plate electrode 12. The gap G between the two electrodes 11 and 12 is 400 μm.
m.

Next, a pair of electrodes is
A groove 15 was formed between each of the voltage application sections 13 composed of 11 and 12. Here, the thickness of the blade of the dicing saw 14 is 50 μm.
m, the depth of the groove 15 was 50 μm, 100 μm, and 150 μm, and the distance between the grooves 15 was 4 mm.

Next, a comb-shaped electrode 11 is supplied from a power source (not shown).
Then, a pulse voltage was applied to the substrate portion between them via the plate electrode 12. The voltage is 1 to 3 kV (2.5 to 7.
5 kV / cm) and the application time was 0.1 to 10 seconds.
In this example, the gap G between the electrodes 11 and 12 is 4 as described above.
Even if the gap G is changed, essentially the same result can be obtained by changing the voltage so that the applied electric field intensity is constant.

After the application of the voltage, the electrodes 11 and 12 were removed by etching, the substrate surface 10a was etched with a mixed solution of hydrofluoric acid and nitric acid, and then observed with an optical microscope. As is well known, in the etching with a mixed solution of hydrofluoric acid and nitric acid, the etching rates of the + Y direction and the -Y direction of MgO: LN are different.
The domain-inverted structure of “a” can be observed.

According to this observation, the depth of the groove 15 was 50 μm.
In any of m, 100 μm, and 150 μm, it was confirmed that the periodically poled structure was formed more uniformly than in the case where no groove was formed.

Next, a channel optical waveguide was formed on the MgO: LN substrate 10 as follows. First, a resist pattern was formed by ordinary photolithography, Ta was formed by sputtering, lift-off was performed, and an ion exchange mask for manufacturing an optical waveguide was formed. The width of this mask was 5 to 10 μm. The optical waveguide was prepared at a position within 10 μm from the tip of the comb-shaped electrode 11.

MgO: LN substrate 10 on which the above mask is formed
Was immersed in pyrophosphoric acid, ion-exchanged at 160 ° C. for 64 minutes, and then annealed in air for 1 hour to form a channel optical waveguide. Next, the −X face and the + X face of the substrate 10 including the end face of the channel optical waveguide are optically polished, and one of the end faces, which is the fundamental wave incident end face, is a single-layer SiOx for the fundamental wave wavelength = 950 nm. The second non-reflection coating made of SiOx for the second harmonic wavelength = 475 nm was applied to the other end surface as the second harmonic emission end surface. Such anti-reflection coating is disclosed in detail in Japanese Patent Application No. 9-207882 by the present applicant.

By the above processing, as shown in FIG. 8, the domain-inverted portions periodically arranged corresponding to each electrode finger of the comb-shaped electrode 11 are formed.
The optical wavelength conversion element 20 including the component 21 and the channel optical waveguide 22 extending in the domain-inverted portion 21 along the direction in which the domain-inverted portions 21 are arranged is completed.

When a laser beam 25 as a fundamental wave having a wavelength of 950 nm is incident on the channel optical waveguide 22 of the optical wavelength conversion element 20 from the one end face side, the wavelength is す な わ ち, that is, 475 nm, from the other end face side. The second harmonic wave 26 is emitted. At this time, a so-called quasi-phase matching is achieved by the action of the domain-inverted portions 21 arranged periodically.

When the performance of the optical wavelength conversion element 20 described above was examined, the conversion efficiency was 300% / Wcm ² or more, and one on one substrate was obtained by the method described in the above-mentioned JP-A-9-218431. It was confirmed that performance equal to or higher than that in the case of forming the periodically poled structure was obtained.

In the embodiment described above, 2
A plurality of periodically poled structures are formed in a single column on a 5 mm × 25 mm substrate 10, as schematically shown in FIG.
For example, grooves 15 are engraved vertically and horizontally on a 3-inch circular substrate 50, and a periodic polarization inversion structure is formed one by one in the region defined by these grooves 15, so that a plurality of periodic polarizations are formed on the substrate both vertically and horizontally. If the inverted structure is formed, mass productivity can be further improved.

In the embodiment described above, the 3 ° Y-cut substrate 10 is used. Alternatively, for example, an 87 ° cut substrate (the Z-axis may be perpendicular to the axis rotated 87 ° toward the X-axis in the ZX plane). According to the present invention, the same effects as described above can be obtained even when a substrate in which a crystal is cut on a certain surface) or an X-cut substrate or a Y-cut substrate which is well known in the art is used.

The substrate material is not limited to MgO: LN, but also other non-doped LN, ZnO or LN doped with Sc, further MgO: LT (MgO-doped LiTaO ₃ ), non-doped LT, ZnO
Alternatively, LT doped with Sc or the like can be used.

On the other hand, in order to apply a voltage to each of a plurality of voltage applying sections formed on one substrate, not only the voltage is applied sequentially as described above, but also the voltage is applied simultaneously at each voltage applying section. Is also good. However, considering that the possibility of dielectric breakdown is reduced, it is desirable to apply the voltages sequentially.

The grooves formed in the substrate can be formed by ion milling, RIE (reactive ion etching), or the like, in addition to being cut by a dicing saw as described above.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a state in which an optical wavelength conversion element is manufactured according to one embodiment of the present invention.

FIG. 2 is a plan view of a substrate in the process of manufacturing the optical wavelength conversion element.

FIG. 3 is a plan view of a voltage application electrode used for manufacturing the light wavelength conversion element.

FIG. 4 is a schematic view showing a state in which an optical wavelength conversion element is manufactured by a conventional method.

FIG. 5 is a plan view of a substrate used for manufacturing an optical wavelength conversion element by a conventional method.

FIG. 6 is a schematic view showing an example of a periodically poled structure formed by a conventional method.

FIG. 7 is a schematic view showing another example of a periodically poled structure formed by a conventional method.

FIG. 8 is a schematic perspective view of an optical wavelength conversion device manufactured by the method of the present invention.

FIG. 9 is a perspective view showing a state in which an optical wavelength conversion element is manufactured according to another embodiment of the present invention.

[Explanation of symbols]

 10 MgO: LN substrate 10 a Surface of substrate 11 Comb-shaped electrode 12 Plate electrode 13 Voltage application part 14 Dicing saw 15 Groove 20 Optical wavelength conversion element 21 Polarization inversion part 22 Channel optical waveguide 50 Circular substrate

Claims (8)

[Claims] An optical waveguide extending along one surface of a ferroelectric crystal substrate having a non-linear optical effect is formed, and a polarization inversion portion obtained by inverting the direction of spontaneous polarization of the substrate is formed in the optical waveguide. A method for producing an optical wavelength conversion element, which is formed periodically and converts the wavelength of a fundamental wave guided in the direction in which the domain-inverted portions are arranged in the optical waveguide, comprises the steps of: On the surface, while forming a plurality of voltage applying portions composed of an electrode of a predetermined pattern corresponding to the pattern of the domain-inverted portion to be formed, and another electrode facing the electrode at an interval, Extending between each of these voltage applying sections, forming a groove on the surface to isolate each voltage applying section from each other, and applying a voltage to the substrate via the electrode at each voltage applying section. Te, a method for manufacturing an optical wavelength conversion device, which comprises forming the polarization inversion unit cyclically repeated. 2. The two electrodes in the voltage applying section,
2. The method according to claim 1, wherein the directions of spontaneous polarization of the substrate are separated from each other in a direction projected onto the surface of the substrate. 3. The method of manufacturing an optical wavelength conversion element according to claim 1, wherein the electrodes of the predetermined pattern are comb-shaped electrodes, each electrode finger tip facing the other electrode side. 4. A monopolar ferroelectric crystal having a non-linear optical effect is cut as a plane at an angle θ (0 ° <θ <90 °) with respect to the direction of spontaneous polarization. The method for manufacturing an optical wavelength conversion element according to claim 1, wherein a substrate formed by using is used. 5. A substrate formed by cutting the ferroelectric crystal in a plane perpendicular to an axis whose Y axis is rotated by 3 ° in the YZ plane toward the Z axis side. Claim 4
A method for producing the optical wavelength conversion element according to the above. 6. A substrate formed by cutting the ferroelectric crystal on a plane perpendicular to an axis rotated by 87 ° in the ZX plane to the X axis side in the ZX plane is used. Claim 4
A method for producing the optical wavelength conversion element according to the above. 7. The method according to claim 7, wherein the ferroelectric crystal substrate is LiNb.
_x Ta _1-x O ₃ (0 ≦ x ≦ 1) or MgO, Z
7. The method of manufacturing an optical wavelength conversion device according to claim 1, wherein a substrate made of a material doped with nO or Sc is used. 8. The method for manufacturing an optical wavelength conversion element according to claim 1, wherein said groove is cut by a dicing saw.