JP2004101639A - Wavelength conversion element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion element utilizing a new structure which has less problems of unevenness of inversion and non-reproducibility of a process arising when a periodic polarizing structure is manufactured and which has a function as the wavelength converting element. <P>SOLUTION: The wavelength converting element is formed of a ferroelectric substrate which has a structure constituted by periodically arranging two kinds of polarizing regions 10 and 20 whose polarizing directions 11 and 21 are not parallel to each other. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength conversion element using a ferroelectric substance, and more particularly to a wavelength conversion element having a periodic polarization structure.
[0002]
[Prior art]
In the field of communication using optical fibers, large-capacity, high-speed data transmission is required. In particular, wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) are regarded as promising in that the transmission capacity of optical fibers can be significantly increased, and wavelength control techniques for controlling a plurality of carrier wavelengths with high accuracy and certain carriers are known. A wavelength conversion technique for converting a wavelength to another carrier wavelength becomes important.
[0003]
For example, in an existing optical communication network, optical transmission of a single wavelength using a 1.3 μm band as a carrier wavelength, in which loss of an optical fiber is small, is mainly used, and is generally laid to replace a telephone communication network in a city. I have. On the other hand, in a mainline optical communication network connecting cities, wavelength multiplexing optical transmission using a 1.5 μm band suitable for wavelength multiplexing transmission as a carrier wavelength is mainly used.
[0004]
When connecting both optical communication networks, since the carrier wavelengths are different from each other, it is necessary to convert the optical signal flowing through one network into an electrical signal once, and then convert it into an optical signal using a carrier wavelength compatible with the other network. is there. Then, the performance of optical communication is limited by the capability of electric signal processing.
[0005]
Therefore, if the carrier wavelength of one network can be directly converted to the carrier wavelength of the other network, electric signal processing will not be required and high performance of optical communication can be effectively maintained. Therefore, an optical mixing technique for converting the carrier wavelength is indispensable.
[0006]
In such wavelength conversion, a second harmonic generation (SHG), a sum frequency generation (SFG), a difference frequency generation (DFG), a parametric conversion, and the like due to the non-linear optical effect are used. Therefore, a material having a high non-linear optical effect is desired. .
[0007]
Related prior arts include Japanese Patent Application Laid-Open Nos. 10-213826 (Patent Document 1) and 2000-10130 (Patent Document 2). Related articles include Non-Patent Document 1, Non-Patent Document 2, and Non-Patent Document 2. 3 and Non-Patent Document 4.
[0008]
Non-Patent Document 1 describes optical mixing by difference frequency generation using a PPLN (periodically-poled lithium niobate) element having a waveguide structure. According to Non-Patent Document 2, a QPM (Quasi-Phase Matching) device using lithium niobate LiNbO ₃ is such that when high-intensity light is incident, an electric field is locally generated by free carrier absorption, and a nonlinear optical constant is generated. Is described about photorefractive damage in which fluctuates greatly. Non-Patent Documents 3 and 4 also relate to lithium niobate.
[0009]
When performing wavelength conversion in this manner, lithium niobate having a high nonlinear optical effect has been often used.
[0010]
As a most common method of using these materials as a wavelength conversion element, as shown in FIG. 8A, a polarization region 3 having a spontaneous polarization 1 and a polarization having a polarization direction 2 180 ° opposite to the spontaneous polarization 1 are used. There is a method of producing a periodically poled structure in which the regions 4 are periodically arranged and performing QPM. As a method of forming a periodically poled structure, a method of forming a periodic 180 ° polarization structure by applying an electric field is widely used (Non-Patent Documents 5 and 6). However, in the case of a 180 ° polarization structure, the polarization wall can take any direction (Non-Patent Document 7). This is because, in the case of a 180 ° polarization structure, only the plus or minus sign of the polarization direction (c-axis direction) of the original polarization region and the 180 ° polarization region changes, and Since the directions of the a-axis and the b-axis do not change except for the sign, it is possible to take an arbitrary polarization interface. For this reason, even when an ideal periodic domain-inverted structure as shown in the schematic diagram of FIG. 8A is manufactured, the domain-inverted regions 4 become non-uniform in the xy plane as shown in FIG. As shown in FIG. 8C, there are problems that the domain-inverted regions 4 become non-uniform in the depth direction and that the manufacturing process is not reproducible (Non-Patent Document 8).
[0011]
[Patent Document 1] JP-A-10-213826
[Patent Document 2] JP-A-2000-10130
[Non-Patent Document 1] Ming-Hsien CHOU, Kirishnan. R. Parameswaran, M .; M. Fejer, Igal Brener, "IEICE TRANSACTION OF ELECTRONICS". , (JUNE 2000) VOL. E83-C, NO. 6, p. 869
[0014]
[Non-Patent Document 2] C.I. Q. Xu, et. Okayama, Y .; Ogawa, "Journal of Applied Physics", 2000 VOL. 87, NO. 7, p. 3203
[0015]
[Non-Patent Document 3] Nao Kurimura, "Solid State Physics", 1994, Vol. 29, No. 1, p. 75
[0016]
[Non-Patent Document 4] Yasunori Furukawa and Masazumi Sato, "Journal of the Japan Society for Crystal Growth Studies", 1990, Vol. 17, No. 3 & 4, p. 277
[0017]
[Non-Patent Document 5] E. FIG. Myers, R.A. C. Eckardt, M .; M. Fejer, R.A. L. Byer, W.C. R. Bosenberg, J. et al. W. Pierce. Author, "Journal of Optical Society of America", B12, (1995) 2102,
[0018]
[Non-Patent Document 6] -P. Meyn, M .; E. FIG. Klein, D.A. Woll, R .; Wallemstein, D.M. Rytz, "Optics Letters". 24, (1999). P1154
[0019]
[Non-Patent Document 7] Wiesendanger, "Czechoslovakia Journal of Physics", B 23, (1973). P91
[0020]
[Non-Patent Document 8] Shuichi Seki, Takeshi Akutsu, Masayuki Maruyama, Keiki Nakajima, Nao Kurimura, Kenji Kitamura, Hideki Ishizuki, Takinori Equality, Jung Hoon RO, Myungsik CHA, "Religion Technical Report" OPE2002-40 , (2002) p. 19
[0021]
[Problems to be solved by the invention]
An object of the present invention is to reduce wavelength non-uniformity and non-reproducibility of processes, which are problems in producing a periodically polarized structure, and to use a wavelength conversion function that has a function as a wavelength conversion. It is to provide an element.
[0022]
[Means for Solving the Problems]
The present invention relates to a wavelength conversion element made of a ferroelectric substance, wherein the ferroelectric substance has a structure in which two different types of polarization regions are periodically arranged, and the polarization directions of the two different types of polarization regions are different from each other. A wavelength conversion element characterized by being parallel.
[0023]
In the present invention, it is preferable that the mutual polarization interface of the two different types of polarization regions is an interface limited to a specific direction in order to satisfy the continuity condition of the lattice.
[0024]
As described above, in the 180 ° polarization structure used as the conventional periodic polarization inversion structure, since the polarization wall can have an arbitrary structure, the produced polarization structure is not in-plane and in the depth direction. There have been problems such as uniformity and lack of reproducibility in the process. The present invention is a wavelength conversion element using a polarization structure in which a polarization wall is limited to a specific surface due to a continuous condition of a lattice.
[0025]
The wavelength conversion element is desirably an orthorhombic crystal, a tetragonal crystal, or a monoclinic crystal having a polarization wall having only a theoretically specific surface.
Among them, potassium niobate having a large nonlinear optical constant is preferable.
Examples of the polarization structure in which the polarization wall is limited to a specific plane depending on the continuous condition of the lattice include a 60 ° polarization wall, a 90 ° polarization wall, and a 120 ° polarization wall of potassium niobate.
[0026]
In performing actual wavelength conversion, it is desirable that reflection on the polarization wall does not occur. FIG. 2 shows an example. FIG. 2 is a schematic diagram of a 90 ° polarization structure. A polarization region 10 having a spontaneous polarization 11 and a polarization region 20 having a spontaneous polarization 21 are joined via a domain wall 15. Since the spontaneous polarization directions 11 and 21 make an angle of 90 °, they are called a 90 ° polarization structure. In the 90 ° polarization region 20 in which the spontaneous polarization direction differs from the original polarization region 10 by 90 °, only the b axis is common, and the a axis and the c axis are rotated by 90 ° with respect to each other. Therefore, the 90 ° polarization wall 15 can exist only in a direction parallel to the b-axis and at 45 ° from the a-axis and the c-axis, respectively, due to the continuity of the lattice. In order that the reflection does not occur on the domain wall 15, as shown in FIG. 2B and FIG. 2C, in the two polarized regions 10 and 20 via the polarized wall 15 like the 90 ° polarized structure. It is desirable that the major axis of the index ellipsoid with respect to a plane perpendicular to the wall in the region 20 be common. In this regard, 90 ° polarization is preferred.
[0027]
Hereinafter, this structure will be described using a 90 ° polarized structure of potassium niobate as an example.
[0028]
FIG. 1 shows the structure of the wavelength conversion element according to the present invention. FIG. 4 is a schematic diagram showing a process for creating a structure as shown in FIG. FIG. 4A is a schematic diagram of the potassium niobate single crystal substrate 33 before the polarization structure is produced. As shown in FIG. 2, the 90 ° polarization structure is formed only in a direction in which the polarization wall is inclined at 45 ° from the a-axis and the c-axis and is parallel to the b-axis due to the continuous condition of the lattice. Here, the c-axis direction is the direction of spontaneous polarization.
[0029]
In order to produce a periodic polarization structure, the substrate 33 is cut out perpendicular to the plane inclined at 45 ° from the a and c axes as shown in FIG. 4A, and cut out as shown in FIG. 4B. A periodic electrode 31 is formed on one side parallel to the b-axis, and a full-surface electrode 32 is formed on the opposite surface. By applying an electric field between the produced electrodes 31 and 32 by the power supply 34, the 90 ° polarized region 20 having the spontaneous polarization direction 21 could be produced periodically. At this time, the polarization wall 15 was limited to a theoretically determined plane, and no polarization wall inclined with respect to the cross-sectional direction (z｀-axis direction) or a polarization wall not parallel to the b-axis was not generated. By utilizing this physical property, it is easy to increase the thickness of the conventional 180-degree periodically poled structure, which is likely to be non-uniform in the cross-sectional direction as shown in FIG. Become.
[0030]
In the above description, the electrode shape does not depend on the material or material as long as the electrode has a pattern formed thereon. For example, by using photolithography for pattern formation, an electric field liquid such as LiCl is formed thereon through a metal electrode such as Al or Au formed by a lift-off method and an insulating layer such as a photoresist periodically formed by photolithography. These include liquid electrodes that have been brought into contact with each other, and electrodes made by combining two of these photoresists and metal electrodes.
[0031]
Further, the above-described pattern electrode 31 may be formed on one side or both sides.
[0032]
Next, the wavelength conversion characteristics of the manufactured wavelength conversion element will be described. First, a description will be given using a schematic diagram of the wavelength conversion element in FIG. The light propagation direction 40 is defined as the y ′ axis, and the principal axis directions of the refractive index ellipsoids in the plane parallel to the polarization wall 15 in each of the polarization regions 10 and 20 are defined as the x ′ and z ′ axes. Hereinafter, the second harmonic generation will be described as an example. When light of wavelength λ is incident from the direction of 40, a second harmonic of λ / 2 is generated due to the nonlinear optical effect. In order to perform such a wavelength conversion operation, a non-linear optical constant (d constant) in the direction of the wavelength conversion and a cycle for performing quasi-phase matching (QPM) are important. The non-linear optical constant in the case of using the 90 ° polarization structure is d'33, which is a d constant for obtaining polarized light in the z'-axis direction by inputting polarized light in the z'-axis direction. D'31, which is a d constant for obtaining converted light in the z 'axis direction by inputting polarized light in the' axis direction ', is d'31 = 9.1, and d constants in the 180 ° polarization structure d33 = 19.5, d31. Although it is slightly smaller than 11.3, it can hold a sufficiently large value as a wavelength conversion element. Furthermore, the period of the periodic polarization structure for generating the second harmonic from blue to infrared is about 10￣80 μm, which is not much different from the case of 180 ° polarization, and is a level that is sufficiently possible as a fabrication technique. .
[0033]
As described above, by using a polarization structure in which the polarization wall 15 can physically take only a specific surface among the polarization structures different from the 180 ° polarization, a highly accurate periodic polarization structure can be easily manufactured. Thus, a wavelength conversion element having excellent conversion efficiency can be realized.
[0034]
BEST MODE FOR CARRYING OUT THE INVENTION
(Example 1)
This will be described with reference to the schematic enlarged schematic diagram of FIG. Here, potassium niobate (KN) was used as the ferroelectric substrate, and a 2 mm-thick substrate 33 was cut out perpendicular to the plane inclined at 45 ° from the a and c axes as shown in FIG. On one of the cut surfaces, a periodic pattern having a period of 17 μm was formed using a photoresist in parallel with the b-axis, and gold was vapor-deposited thereon, thereby forming a periodic electrode 31. The entire surface electrode 32 was formed on the opposite surface by gold evaporation. A trapezoidal electric field of about 140 V / mm to 200 V / mm was applied between the produced electrodes 31 and 32 by the power supply 34. In this example, when the 90 ° polarization region 20 having the spontaneous polarization direction 21 is periodically formed, the width of the electrode portion of the periodic electrode 31 is set to be smaller than the width of the electrode portion in order to prevent the manufactured 90 ° polarization region 20 from spreading too much. The shape was such that the width of the insulating layer made of the resist became thinner by 5 μm.
[0035]
As a result, a periodic 90 ° polarization structure 20 could be produced as shown in the transmission microscope image shown in FIG. At this time, the polarization wall 15 was limited to a theoretically determined plane, and there was no occurrence of a polarization wall inclined obliquely to the cross-sectional direction or a polarization wall not parallel to the b-axis.
[0036]
Furthermore, in the case of a 90 ° polarization structure, the moving speed of the polarizing wall 15 is very slow, on the order of several minutes, compared to the moving speed (several milliseconds) of the 180 ° polarizing wall, and the controllability is very excellent. Thus, it was found that a periodic structure can be produced with good reproducibility without delicately controlling the electric field application time.
[0037]
When 1.55 μm light is incident on the manufactured device and the wavelength conversion operation is confirmed, the output of the wavelength-converted second harmonic can be confirmed.
[0038]
From the above results, it was possible to easily and accurately manufacture a wavelength conversion element using a periodic polarization structure using two different types of polarization structures other than the 180 ° polarization structure by the electric field application method.
[0039]
(Example 2)
A periodic 90 ° polarization structure 20 as shown in FIG. A waveguide structure as shown in FIGS. 6A, 6B and 6C is manufactured on the surface of the manufactured polarization structure. Hereinafter, a specific description will be given.
[0040]
FIG. 6A shows that a waveguide structure 52 is formed between the ion-implanted layer 51 and the surface of the substrate 4 by implanting He ions into a potassium niobate single crystal. FIG. 6B shows a slab-shaped waveguide obtained by thinly polishing this crystal. FIG. 6C shows a structure in which this crystal is polished thinly, and a ridge-type waveguide structure 52 is formed using a dicer or the like.
[0041]
When light of 1.55 μm is incident on the manufactured device having the waveguide structure and the wavelength conversion operation is confirmed, the output of the wavelength-converted second harmonic can be confirmed.
[0042]
(Example 3)
First, as shown in FIG. 4A, a potassium niobate substrate is cut out in the same manner as in Example 1 and He ions are implanted into the surface, so that the ion-implanted layer 51 and the substrate 4 are formed as shown in FIG. A waveguide structure 52 is formed between the substrate and the surface. Next, electrodes 31 and 32 are formed as shown in FIG. 7B, and an electric field is applied in the same manner as in the first embodiment to form a periodic 90 ° polarization region 20. The element manufactured in this manner is a wavelength conversion element having a waveguide structure as shown in FIG.
[0043]
When light of 1.55 μm is incident on the manufactured device having the waveguide structure and the wavelength conversion operation is confirmed, the output of the wavelength-converted second harmonic can be confirmed.
[0044]
From the above results, it was possible to easily and accurately manufacture a wavelength conversion element having a periodic polarization structure using two different types of polarization regions other than the 180 ° polarization structure by the electric field application method.
[0045]
In the above description, the 90 ° polarization structure of the KNbO ₃ crystal was mainly described as an example. However, other polarization structures (60 °, 120 °), orthorhombic and tetragonal KTiOPO ₄ , BaTiO ₃ , and RbTiOPO ₄ , LiB ₃ O _{5 and the} like.
[0046]
In addition, in a device having a waveguide structure, as a method of manufacturing a waveguide structure, a method of implanting He ions, a method of forming a slab by polishing, and a method of forming a ridge-shaped waveguide by using a dicer or the like are described as examples. Needless to say, the present invention can also be applied to other well-known waveguide structures 52 formed by a metal diffusion method such as a proton exchange method using an acid, a Ti diffusion method, and a Zn diffusion method as shown in FIG.
[0047]
In addition, although the description has been limited to a single crystal material, it goes without saying that a material grown epitaxially or on a substrate can be applied.
[0048]
【The invention's effect】
According to the present invention, in the conventional wavelength conversion element using the 180-degree periodic polarization structure, non-uniform production and non-reproducibility of the production have been a problem. Therefore, by using a structure having only a specific polarization wall, a periodic polarization structure with less non-uniformity can be manufactured with good reproducibility, and a wavelength conversion element with a good manufacturing accuracy can be realized. The practical and industrial effects of the present invention are enormous in terms of improving these device characteristics and improving the manufacturing accuracy.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a wavelength conversion element according to the present invention.
FIG. 2 is a diagram illustrating a 90 ° polarization structure.
(A) is a diagram showing a spontaneous polarization direction of a 90 ° polarization structure and a polarization wall that is theoretically determined.
FIG. 3B is a diagram illustrating a refractive index ellipsoid cut along a cross section parallel to a polarization wall in the polarization region 10.
(C) is a diagram showing a refractive index ellipsoid cut along a cross section parallel to the polarization wall in the polarization region 20.
FIG. 3 is an example of a wavelength conversion element according to the present invention having a waveguide structure manufactured by a proton exchange method or a metal diffusion method.
FIG. 4 is a schematic view illustrating an example of a method for manufacturing a wavelength conversion element according to the present invention. (A) is a schematic diagram of a substrate prepared for manufacturing a wavelength conversion element in the present invention.
(B) is a schematic view showing an electric field application method for producing a wavelength conversion element according to the present invention.
FIG. 5 is an example of an optical microscope photograph of the wavelength conversion element in the present invention.
FIG. 6 is an example showing a structure of a wavelength conversion element having a waveguide structure according to the present invention.
(A) This is an example in which a waveguide structure is manufactured by ion implantation.
(B) This is an example of a slab-like waveguide structure manufactured by thin polishing.
(C) This is an example of a ridge-type waveguide structure manufactured by finely cutting with a dicer or the like.
FIG. 7 is a schematic view showing a method for producing a periodically polarized structure in a wavelength conversion element according to the present invention on a substrate having a waveguide structure.
FIG. 8 is a schematic view of a wavelength conversion element using a conventional 180 ° periodic polarization structure. FIG. 8A shows an ideal wavelength conversion element structure.
(B) This is an example of a device in which non-uniformity occurs in a plane perpendicular to spontaneous polarization.
(C) This is an example of a device in which non-uniformity occurs in a direction parallel to spontaneous polarization.
[Explanation of symbols]
1: Spontaneous polarization direction, 2: Spontaneous polarization direction, 3: Polarized region 4: Polarized region, 10: Polarized region, 11: Spontaneous polarization direction,
15: polarization wall, 20: polarization region, 21: spontaneous polarization direction 31: periodic electrode, 32: whole surface electrode, 33: substrate,
34: power supply, 51: ion implantation layer, 52: waveguide

Claims (5)

A wavelength conversion element made of a ferroelectric, wherein the ferroelectric has a structure in which two different types of polarization regions are periodically arranged, and the polarization directions of the two different types of polarization regions are not parallel to each other. The wavelength conversion element characterized by the above-mentioned. 2. The wavelength conversion element according to claim 1, wherein a polarization interface between the two different types of polarization regions is an interface limited to a specific direction in order to satisfy a continuity condition of a lattice. 3. 3. The wavelength conversion element according to claim 1, wherein the polarization directions of the different polarization regions of the ferroelectric are 90 degrees, 60 degrees, or 120 degrees with each other. 4. 4. The wavelength conversion element according to claim 1, wherein the ferroelectric is one of orthorhombic and tetragonal. The wavelength conversion device according to claim 4, wherein the ferroelectric is potassium niobate.

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