JP2014222331A - Wavelength conversion element - Google Patents

Abstract

An object of the present invention is to provide a wavelength conversion element capable of improving wavelength conversion efficiency by effectively increasing the length of a waveguide. A wavelength conversion element according to the present invention includes a three-dimensional optical waveguide made of a second-order nonlinear optical material provided on a base substrate, and the three-dimensional optical waveguide includes a plurality of periodic polarization inversion structures. The linear waveguide includes a linear waveguide and a curved waveguide that is coupled with two linear waveguides of the plurality of linear waveguides and is not provided with the periodic polarization inversion structure. The linear waveguide has a periodic polarization inversion structure in the base substrate. The curved waveguide is provided so as to be parallel to the direction in which it can be formed, the length of the curved waveguide is adjusted so that the phases of the light guided through the two linear waveguides are matched, and the base substrate is zero based on the refractive index of the waveguide. Including a cladding region having a refractive index lower by .75% or more, the optical waveguide and the cladding region of the base substrate are joined by direct joining, and the radius of curvature of the curved waveguide is 0.2 mm or more and 5 mm or less. [Selection] Figure 2

Description

  The present invention relates to a wavelength conversion element, and more particularly to a wavelength conversion element using a bent waveguide.

Many for the generation and modulation of coherent light in the ultraviolet, visible, infrared, and terahertz range for applications such as optical signal wavelength conversion, optical modulation, optical measurement, optical processing, medicine, and biotechnology in optical communications Development of non-linear optical devices and electro-optical devices is underway. Various materials have been researched and developed as nonlinear optical media and electro-optical media used in such elements. An oxide-based compound substrate such as lithium niobate (LiNbO ₃ , hereinafter referred to as LN) is known as a promising material because it has a very high second-order nonlinear optical constant / electro-optic constant. As an example of an optical device using non-linearity with high LN, a wavelength conversion element using difference frequency generation by pseudo phase matching is known.

  In recent years, in order to increase the communication capacity of an optical communication system, a wavelength division multiplexing (WDM) communication system that multiplexes and transmits a plurality of lights having different wavelengths has been actively introduced. In such a WDM communication system, in order to effectively use a limited number of wavelengths, there is a demand for practical use of a wavelength conversion device that converts a signal wavelength into an arbitrary signal wavelength.

  Conventionally, as a wavelength conversion element for converting the wavelength of light, a device using a semiconductor optical amplifier, a device using four-wave mixing, and the like are known. However, these wavelength conversion elements have not been able to satisfy conditions such as high efficiency, high speed, wide band, low noise, and polarization independence required in an optical communication system.

FIG. 1 shows a configuration of a conventional quasi phase matching type wavelength conversion element using LN. As shown in FIG. 1, the signal light A having a relatively small light intensity and the excitation light B having a relatively large light intensity are combined by a multiplexer 1 and a nonlinear optical medium having a polarization inversion structure. The light enters the optical waveguide 2. In the optical waveguide 2, the signal light A is converted into converted light C having another wavelength by generation of a difference frequency wave due to a nonlinear optical effect. The converted light C is emitted from the optical waveguide 2 together with the excitation light B. The emitted converted light C and excitation light B are separated by the duplexer 3. If the wavelengths of the signal light A and the excitation light B are λ ₁ and λ ₃ , respectively, the wavelength λ ₂ of the converted light C satisfies the following formula (1).
1 / λ ₂ = 1 / λ ₃ -1 / λ ₁ formula (1)

The wavelength λ ₂ of the converted light C is a wavelength obtained by turning the wavelength λ _{1 of the} signal light around the wavelength that is twice the wavelength of the excitation light λ ₃ . For example, when the wavelength λ ₃ of the excitation light B is 0.78 μm, the signal light A having a wavelength λ ₁ = 1.54 μm is converted into converted light C that is a difference frequency light having a wavelength λ ₂ = 1.58 μm. be able to.

  The conversion band for the signal light A and the converted light C is as wide as ± 30 nm or more with respect to the wavelength of the excitation light. For example, a WDM signal bundled in a wavelength band C band used for wavelength division multiplexing (WDM) optical communication is L It is possible to perform batch conversion of wavelength groups such as to the band or from the L band to the C band.

  In the conventional method for manufacturing a wavelength conversion element using such quasi-phase matching, a wavelength conversion element is manufactured by manufacturing a proton exchange waveguide after forming a periodically poled structure on a nonlinear optical crystal substrate such as LN. Was making. On the other hand, in order to improve the optical confinement in the optical waveguide and realize highly efficient wavelength conversion using the bulk or near-bulk nonlinear effect, a wavelength conversion element with a ridge-type optical waveguide structure is proposed Has been.

  A method of manufacturing a general ridge type optical waveguide will be described. First, after a periodically poled structure is fabricated on an Mg-added LN substrate, it is bonded to a separately prepared LN substrate using an adhesive or the like. After the substrate thickness of the Mg-added LN substrate is reduced by surface grinding, a precision-grinding ridge-type waveguide using a dicing saw is manufactured (for example, see Non-Patent Document 1). As a waveguide manufacturing method, a dry etching process using lithography may be used.

  On the other hand, in addition to wavelength converters in the communication wavelength band, a laser light source in the visible range that has not been realized with a semiconductor laser has been put into practical use by using a quasi phase matching type wavelength conversion element. Currently, gas lasers such as He—Ne laser and Ar laser, solid-state lasers such as Nd: YAG laser, dye lasers and semiconductor lasers are known as lasers in practical use. In recent years, small, lightweight, and inexpensive semiconductor lasers have become widespread mainly in the visible and near-infrared wavelength bands. In particular, in the field of optical communication, 1.3 μm band and 1.5 μm band semiconductor lasers for signal light sources and 0.98 μm band and 1.48 μm band semiconductor lasers for pumping fiber amplifiers are widespread. Further, CD (0.78 μm band), DVD (0.65 μm band), and Blu-ray (0.4 μm band) semiconductor lasers are also widely used as light sources for pickup of optical recording medium readers.

  However, since there are wavelength bands that are difficult to realize with semiconductors, laser light source devices that combine highly efficient nonlinear optical media and semiconductor lasers with widely used wavelength bands have been developed. For example, it is difficult to realize a laser with a wavelength of 0.5 to 0.6 μm such as green, yellow green, and orange with a semiconductor, and a laser light source using second harmonic generation or sum frequency generation by a highly efficient nonlinear optical medium Has been put to practical use.

Japanese Patent No. 3848093

Tatsuo Kawaguchi et al., “Waveguide SHG Device Using LiNbO3 Epitaxial Growth and Ultraprecision Processing Technology”, Laser Research, September 2000, Vol. 28, No. 9, p.600-603 T. Umeki, Osamu Tadanaga, and Masaki Asobe, “Highly Efficient Wavelength Converter Using Direct-Bonded PPZnLN Ridge Waveguide”, IEEE Journal of Quantum Electronics, 2010, Vol.46, No.8, p.1206-1213 M. Ito, Shin kamei, Motohaya Ishii, Tomohiro Shibata, Munehisa Tamura and Yasuyuki Inoue, “Ultra Small 100 GHz 40 ch Athermal AWG Module Using 2.5% -ΔSilica-Based Waveguides”, ECOC2008, 2008, Vol.1, p. 91-92 Hiroshi Nishihara et al., “Optical Integrated Circuits”, 1st Edition, Ohmsha, 1985, p.47-49 K. Mitsunaga, K. Murakami, M. Masuda, and J. Koyama, “Optical LiNbO3 3-branched waveguide and its application to a 4-port optical switch”, Applied Optics, 1980, Vol. 19, No. 22, p.3837-3842

As described above, the wavelength conversion element is being studied for application in various fields, but there are problems to be overcome in order to realize a highly functional device. One of the problems is wavelength conversion efficiency. If the wavelength conversion efficiency increases, the output of the laser light source using the second harmonic generation or the sum frequency generation increases, and the performance becomes higher. Also, in parametric amplification, which is part of the difference frequency generation, a dramatic improvement in amplification gain can be realized by increasing the conversion efficiency. It is known that the following equation (2) holds for the conversion efficiency η of a waveguide type wavelength conversion element using a second-order nonlinear optical element.
η∝L ² / A _eff equation (2)

Here, L is the waveguide length, and A _eff is the effective cross-sectional area of the waveguide. That is, in order to increase the conversion efficiency of the waveguide type wavelength conversion element, it is only necessary to reduce the area of the waveguide and increase the waveguide length. When reducing the area of the waveguide, it is necessary to increase the confinement of light in the waveguide, so there is a large refractive index difference between the waveguide and the base substrate, or between the waveguide and the cladding. Necessary.

  However, under this strong confinement condition, the light single mode condition and the light propagation condition are easily affected by the waveguide size. Therefore, in order to sufficiently function as a wavelength conversion element, it is necessary to suppress a manufacturing error when manufacturing a waveguide, and there is a limit to downsizing the area. Although it is currently downsized to about 4 μm thickness × 5 μm width (see, for example, Non-Patent Document 2), it is very difficult to further reduce the size to a size of 2 μm thickness × 2 μm width or less.

  On the other hand, in the extension in the length direction of the waveguide, a uniform waveguide must be produced over a long distance, but the conversion efficiency η is the square of the waveguide length L as shown in Equation (2). Is highly expected. However, simply increasing the wafer size and increasing the length that can be cut out as a linear waveguide is problematic, but there are problems such as compatibility with process equipment and difficulty in reducing manufacturing errors over the entire wafer surface. . Further, there is an element length of about 50 mm at present, but a wavelength conversion element longer than that, that is, more than 100 mm, is a device in terms of shape and size as well as in terms of keeping the temperature inside the element uniform. It becomes difficult to use. Therefore, it has been studied to use a bent waveguide such as that used in a planar lightwave circuit on quartz or an LN modulator (see, for example, Non-Patent Documents 3 to 5).

  However, by applying a bent waveguide structure to a wavelength conversion element having a periodic domain-inverted structure, the invention has achieved an effective lengthening of the waveguide and an improvement in conversion efficiency due to the lengthening. The manufacturing method is not disclosed.

  In order to achieve such an object, the wavelength conversion element according to claim 1 includes a base substrate and a wavelength provided with a three-dimensional optical waveguide made of a second-order nonlinear optical material provided on the base substrate. The conversion element, wherein the three-dimensional optical waveguide includes a plurality of linear waveguides provided with a periodic polarization inversion structure and two linear waveguides among the plurality of linear waveguides. The curved waveguide is adjusted in length so that the phases of the light guided through the two linear waveguides are matched, and the base substrate is A clad region having a refractive index lower than the refractive index of the waveguide by 0.75% or more, wherein the optical waveguide and the clad region of the base substrate are joined by direct joining, and the radius of curvature of the curved waveguide is 0.2mm And characterized in that the upper 5mm or less.

The wavelength conversion element according to claim 2 is the wavelength conversion element according to claim 1, wherein the base substrate including the cladding region is formed of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O. ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or a material containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive, or a high melting point material including quartz and silicon oxide It is characterized by comprising.

  The wavelength conversion element according to claim 3 is the wavelength conversion element according to claim 1 or 2, wherein the base substrate is formed on the cladding region of the base substrate and on the three-dimensional optical waveguide. Is further provided.

  A wavelength conversion element according to a fourth aspect is the wavelength conversion element according to any one of the first to third aspects, wherein the curved waveguide is formed on an upper surface or a side surface of the curved waveguide or on the base substrate. The electrode is provided along.

The wavelength conversion element according to claim 5 is the wavelength conversion element according to any one of claims 1 to 4, wherein the nonlinear optical material is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x. O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or a material containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive.

  The wavelength conversion element according to claim 6 is the wavelength conversion element according to any one of claims 1 to 5, wherein each of the one or the plurality of curved waveguides includes two 90 ° waveguide portions. It is a curved waveguide.

  The wavelength conversion element according to claim 7 is the wavelength conversion element according to any one of claims 1 to 5, wherein each of the one or the plurality of curved waveguides includes a 60 ° curve waveguide and a 120 ° curve. It is one of waveguides.

  The wavelength conversion element according to claim 8 is the wavelength conversion element according to any one of claims 1 to 7, wherein the curved waveguide has a phase of light guided through each of the two linear waveguides. It includes a straight waveguide portion whose length is adjusted to match.

  As described above, according to the present invention, it is possible to increase the effective waveguide length while suppressing an increase in the element size, thereby realizing an improvement in wavelength conversion efficiency, which is significantly higher than the conventional one. A high-performance wavelength conversion element device can be provided.

It is a figure which shows the structure of the conventional wavelength conversion element of the quasi phase matching type | mold using LN. It is a top view which shows the wavelength conversion element which concerns on Example 1 of this invention. It is an enlarged view of the wavelength conversion element concerning Example 1 of the present invention. It is sectional drawing which shows the wavelength conversion element which concerns on Example 1 of this invention. It is sectional drawing which shows the other example of the wavelength conversion element which concerns on Example 1 of this invention. It is sectional drawing which shows the further another example of the wavelength conversion element which concerns on Example 1 of this invention. It is a figure which shows the preparation methods of the wavelength conversion element concerning this invention. It is a top view which shows the wavelength conversion element which concerns on Example 2 of this invention. It is sectional drawing which shows the wavelength conversion element which concerns on Example 2 of this invention. It is a top view which shows the wavelength conversion element which concerns on Example 3 of this invention. It is a top view which shows the wavelength conversion element which concerns on Example 4 of this invention. It is a top view which shows the wavelength conversion element which concerns on Example 5 of this invention. It is an enlarged view of the 60-degree curve waveguide part in the wavelength conversion element which concerns on Example 5 of this invention.

  Hereinafter, embodiments of the wavelength conversion element according to the present invention will be described in detail with reference to the drawings.

Example 1
FIG. 2 shows a top view of the wavelength conversion element according to Example 1 of the present invention. FIG. 2 shows the wavelength conversion element 100 in which the straight waveguides 101 and 103 and the curved waveguide 102 that couples the straight waveguides 101 and 103 are provided on the base substrate 110. The straight waveguides 101 and 103 can be ridge-type three-dimensional optical waveguides having a periodically poled structure having a width of 3 μm, a thickness of 3 μm, and a length of 50 mm, and the curved waveguide 102 has a width of 3 μm, A ridge-type three-dimensional optical waveguide having a thickness of 3 μm can be obtained.

  A three-dimensional optical waveguide means a waveguide that confines light vertically and horizontally and propagates light in the length direction, and a ridge-type waveguide is an example. The periodic domain-inverted structure is applied only to the straight waveguides 101 and 103 and is not applied to the curved waveguide 102. Light incident from the incident end of the straight waveguide 101 is emitted from the exit end of the straight waveguide 103 while being wavelength-converted by the straight waveguide 101 and the straight waveguide 103.

FIG. 3 shows an enlarged view of the curved waveguide 102 in the wavelength conversion element 100 according to the present invention. As shown in FIG. 3, the U-shaped curved waveguide 102 includes a straight waveguide portion 102 ₁ and 90 ° waveguide portions 102 ₂ and 102 ₃ . Linear waveguide 101 is linearly waveguide direction of the waveguide 101 is 90 ° change is connected to the straight waveguide portion 102 _1, straight waveguide portion 102 ₁ through a 90 ° waveguide portion 102 ₂ 90 ° waveguide portion 102 ₃ of the straight waveguide portion 102 ₁ of the waveguide direction is 90 ° changed via is connected to the linear waveguide 103. 90 ° waveguide portion 102 ₂ and 102 ₃ of the radius of curvature R is approximately 0.5 mm, the linear region _{S 0} of the straight waveguide portion 102 ₁ may be approximately 0.5 mm.

  As the three-dimensional waveguide, a substrate made of an LN crystal can be used. Since the Z plane of LN, which is a nonlinear optical material, has a three-fold symmetric crystal structure, there are not only one direction in which the periodic polarization inversion structure can be formed in the straight waveguide 101 and the straight waveguide 103, but three directions. To do. The straight waveguide 101 and the straight waveguide 103 are connected via the curved waveguide 102 as described above, so that the waveguide direction can form a periodically poled structure in the Z plane of the LN. It is provided so as to be parallel to one direction.

Since the U-curve portion of the curved waveguide 102 does not have a periodic polarization reversal structure, when propagating from the straight waveguide 101 to the straight waveguide 103, the phase matching between the light involved in wavelength conversion must be achieved. Effective conversion cannot be achieved. Therefore, for example, the straight waveguide portion 102 to adjust the phase by adjusting the _first length of the linear region S _0, the linear region S ₀ of the lengths of the guided light of linear waveguide 101 phase and linear waveguides 103 By making the length matched with the phase of the guided light, degradation of conversion efficiency is suppressed.

On the other hand, as shown in Non-Patent Documents 3 and 4, the radius of curvature R that can be bent without loss depends on the refractive index difference between the waveguide and the cladding region. According to the simulation by Non-Patent Document 3 and BPM (Beam Propagation Method), in the case of a buried waveguide, there is a relationship of the following formulas (3) to (7).
When the refractive index difference Δ = 0.75%, the minimum radius of curvature R _min ≈5 mm (3)
When the refractive index difference Δ = 1.5%, the minimum radius of curvature R _min ≈2 mm (4)
When the refractive index difference Δ = 2.5%, the minimum curvature radius R _min ≈1 mm (5)
When refractive index difference Δ = 5%, minimum radius of curvature R _min ≈0.8 mm Equation (6)
When the refractive index difference Δ = 10%, the minimum radius of curvature R _min ≈0.5 mm (7)

When the refractive index difference Δ between the waveguide and the cladding region is less than 0.75%, the radius of curvature R exceeds 5 mm, the substrate cannot be used effectively, and the element size increases. On the other hand, when the refractive index difference is high such that the minimum radius of curvature R _min is less than 0.2 mm, the confinement in the waveguide is too strong, and the core size must be made very small in order to achieve single mode. . In that case, as described above, the allowable manufacturing error becomes too small, and with the current technology, a uniform element cannot be manufactured, and conversely, conversion efficiency decreases.

In the wavelength conversion element 100 according to Example 1, the refractive index of the waveguide is about 2.1, and the refractive index difference Δ of the cladding region of the base substrate 110 is set to about 10%. Cladding region is constituted of SiO ₂ refractory was adjusted effective refractive index difference between the layers by adjusting the thickness. According to the cutback method in which the waveguide length is gradually shortened and the waveguide loss is examined, the loss due to bending at 90 ° is about −0.05 dB per piece, and is about −0.1 dB in total.

FIG. 4 is a sectional view of the wavelength conversion element 100 according to the first embodiment of the invention. FIG. 4 shows a wavelength conversion element 100 including a base substrate 110 and a three-dimensional optical waveguide 120 provided on the base substrate 110. For the three-dimensional optical waveguide 120 of the wavelength conversion element 100, Z-cut Zn-doped LN, which is a nonlinear optical medium, was used. The base substrate 110 is composed of a Z-cut Mg-added LN substrate 111 and a clad region 112 made of a high melting point SiO ₂ layer deposited on the Z-cut Mg-added LN substrate 111 by plasma CVD. The effective refractive index of the waveguide 120 and the cladding region 112 can be adjusted. Grooves 130 are formed around the three-dimensional optical waveguide 120, and effective light confinement is achieved by the upper and left air layers above the three-dimensional optical waveguide 120 and the cladding region 112 below the three-dimensional optical waveguide 120. The ridge structure can be realized.

As the cladding region 112, it is desirable to use a material such as high melting point SiO ₂ or quartz. The reason for using a high melting point material for the cladding region 112 is for temperature resistance. Part of the input / output light is absorbed by the waveguide and converted into heat. Since the present invention aims to realize a high-performance wavelength conversion element with high conversion efficiency, there is a possibility of very strong input and output. For this reason, when the cladding region 112 is made of low-melting glass or adhesive, since the melting point is low, the refractive index of the cladding region is easily affected by a temperature rise due to heat, and the characteristics are not stable. In addition, since the thickness of the cladding layer in the waveguide region may be adjusted by lithography and etching, a material such as high melting point SiO ₂ or quartz (quartz) that is compatible with these techniques is used as the cladding region 112. desirable.

  In addition, since the cladding region 112 according to the present invention is configured to have a high melting point, an adhesive effect that promotes bonding between a waveguide layer such as a low-melting glass or an adhesive and the base substrate during device fabrication. Is thin and does not serve as a bonding layer for bonding the two. In this respect, the optical waveguide element disclosed in Patent Document 1 is different from the optical waveguide element having a bonding layer having an adhesive effect.

FIG. 5 shows another example of the wavelength conversion element according to the first embodiment of the present invention. FIG. 5 shows a wavelength conversion element 200 including a base substrate 210, a three-dimensional optical waveguide 220, and an upper cladding region 230. The structure of the wavelength conversion element is not limited to the structure shown in FIG. 4, but as shown in FIG. 5, the entire base substrate 210 is a cladding region, and the cladding region of the base substrate 210 and the three-dimensional optical waveguide 220 are also included. Further, a structure in which an upper cladding region 230 is further provided may be employed. In the wavelength conversion element 200 shown in FIG. 5, a Z-cut quartz substrate is used as the base substrate 210, and Ta ₂ O ₅ deposited by sputtering is used as the upper cladding region 230.

FIG. 6 shows still another example of the wavelength conversion element according to the first embodiment of the present invention. FIG. 6 shows a wavelength conversion element 300 including a base substrate 310 including an LN substrate 311 and a lower cladding region 312, a three-dimensional optical waveguide 320, and an upper cladding region 330. As shown in FIG. 6, the upper cladding region 330 facing the lower cladding region 312 can be bonded by direct substrate bonding so that the three-dimensional optical waveguide 320 is sandwiched between the lower cladding region 312 and the upper cladding region 330. In the wavelength conversion element 300 shown in FIG. 6, a Z-cut Mg-added LN substrate is used as the LN substrate 311, a high melting point SiO ₂ layer deposited by plasma CVD is used as the lower cladding region 312, and the upper cladding region 330 is used. A Z-cut quartz substrate was used.

Next, a method for manufacturing a wavelength conversion element according to the present invention will be described with reference to FIGS. FIG. 7A shows an example using a Z-cut Mg-doped LN substrate in which SiO ₂ is deposited by plasma CVD as the base substrate, and FIG. 7B shows an example using a quartz substrate as the base substrate. In FIG. 7, a Z-cut Zn-doped LN substrate can be used as a waveguide substrate that is a nonlinear optical material, and a periodically poled structure is prepared in advance so that the phase matching condition is satisfied in the 1.5 μm band. Yes. The waveguide substrate is formed with a portion where the periodic polarization reversal structure constituting the straight waveguides 101 and 103 is formed and a portion where the periodic polarization reversal structure constituting the curved waveguide 102 is not formed. It is rare.

The thermal expansion coefficients of the waveguide substrate and the base substrate are substantially the same, and both the waveguide substrate and the base substrate can be a 3-inch wafer having both surfaces optically polished. The thickness of the waveguide substrate can be 300 μm, and the thickness of the base substrate can be approximately 500 μm. In addition to LN, KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn, or the like as the nonlinear optical medium of the waveguide substrate A material containing at least one selected from the group consisting of Sc and In as an additive can be used. In addition to LN, KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or a group consisting of Mg, Zn, Sc, and In is used as the base substrate. A material containing at least one selected from the above as an additive, quartz (quartz), and a high melting point material containing silicon oxide can be used.

After the surfaces of the prepared waveguide substrate and base substrate are made hydrophilic by normal acid cleaning or alkali cleaning, the waveguide substrate and base substrate are superposed in a clean atmosphere in which microparticles are not present as much as possible. The superposed waveguide substrate and base substrate are placed in an electric furnace and heat-treated at 400 ° C. for about 3 hours, thereby performing diffusion bonding between the waveguide substrate and the base substrate. The bonded substrate is free from microparticles and the like on the bonding surface, is void free, and does not crack when returned to room temperature. The bonding strength can also be increased by depositing an appropriate amount of high melting point SiO ₂ on the bonding surface of the waveguide substrate forming the waveguide so as to align the material of the bonding surface.

  Next, using a polishing apparatus in which the flatness of the polishing platen is controlled, polishing is performed until the thickness of the waveguide substrate portion of the bonded substrate becomes approximately 6 μm. By performing a polishing process after the polishing process, a mirror-polished polishing surface can be obtained with a desired substrate thickness.

  By measuring the parallelism of the substrate (difference between the maximum and minimum height) using an optical parallelism measuring instrument to obtain a submicron parallelism over almost the whole area except for the periphery of the 3-inch wafer, A thin film substrate suitable for manufacturing a wavelength conversion element can be manufactured. Since this thin film substrate is produced by directly bonding a waveguide substrate and a base substrate by diffusion bonding by heat treatment without using an adhesive, it has a uniform composition and film thickness over the entire area of a 3-inch wafer. Have

  Thereafter, as a means for producing the optical waveguide, a wavelength conversion waveguide is produced on the thin film substrate by using a dry etching process. A waveguide pattern is formed on the surface of the waveguide substrate portion of the thin film substrate by an ordinary photolithography process. Thereafter, the substrate is set in a dry etching apparatus, and the surface of the waveguide substrate of the thin film substrate is etched using Ar gas as an etching gas, thereby forming a ridge type three-dimensional optical waveguide as shown in FIG. A wavelength conversion element 100 as shown in FIG. 1 was produced.

  In order to obtain the characteristics as a wavelength conversion device, the signal output in the 1.56 μm band is input from the incident end of the linear waveguide 101 of the fabricated wavelength conversion element 100, and the first output from the output end of the linear waveguide 103 is obtained. The efficiency of wavelength conversion was evaluated by the second harmonic. In the wavelength conversion element according to Example 1 of the present invention, as a result of the small waveguide size of 3 μm width and 3 μm thickness, strong confinement due to the high refractive index difference, and the long wavelength conversion region of 100 mm in total, the normalized conversion efficiency is When light having a wavelength of 1555 nm was used as input light, a high value of 6000% / W was obtained. As described above, the loss due to the curve region was very small.

(Example 2)
Next, a wavelength conversion element according to Example 2 of the present invention will be described with reference to FIGS. FIG. 8 is a top view of the wavelength conversion element according to the second embodiment of the present invention. FIG. 8 shows a wavelength conversion element 400 in which straight waveguides 401 and 403 and a curved waveguide 402 that couples the straight waveguides 401 and 403 are provided on a base substrate 410. As shown in FIG. 8, a bottom electrode 441 is provided on the base substrate 410 via a buffer layer 444 described later, and an upper electrode 442 and side electrodes are provided on the curved waveguide 402 via the buffer layer 444. 443 is provided.

FIG. 9 shows a cross-sectional view of a wavelength conversion element 400 according to Example 2 of the present invention. 4 shows a base substrate 410 composed of an LN substrate 411 and a cladding region 412, a three-dimensional optical waveguide 420 provided on the base substrate 410, and a buffer layer provided on the cladding region 412 and the three-dimensional optical waveguide 420. 444, a bottom electrode 441 provided on the buffer layer 444 along the three-dimensional optical waveguide 420, an upper electrode 442 provided on the buffer layer 444 on the top surface of the three-dimensional optical waveguide 420, and the three-dimensional optical waveguide 420. The wavelength conversion element 400 provided with the side electrode 443 provided on one buffer layer 444 of the side surface is shown. Under each electrode, in order to avoid light loss due to the presence of metal in the waveguide region, SiO ₂ having a thickness of about 0.5 μm deposited by sputtering is formed as a buffer layer 444.

As described above, it is necessary to adjust the phase of the curved waveguide portion where the polarization inversion is not formed. Normally, this phase adjustment is performed by adjusting the length of the straight region S ₀ of the curved waveguide may be formed a slight shift and design by variation of the element produced. In order to correct this deviation, a bottom electrode 441, an upper electrode 442, and a side electrode 443 can be formed as shown in FIGS.

  When the upper electrode 442 and the side electrode 443 shown in FIGS. 8 and 9 are manufactured, the three-dimensional optical waveguide 420 is heated by passing a current between the upper electrode 442 and the side electrode 443, and a refractive index change due to temperature is generated. The phase shift can be corrected using the TO effect. Further, an EO effect is used in which an upper electrode 442 and a bottom electrode 441 shown in FIG. 8 are formed and an electric field is applied between the bottom electrode 441 and the upper electrode 442 to generate a refractive index change due to the electric field. Thus, the phase shift may be corrected. When the EO effect is used, the bottom electrode may be formed so as to be sandwiched between both sides of the three-dimensional waveguide 420 and the potentials of the bottom electrodes on both sides may be made uniform.

Example 3
Next, a wavelength conversion device according to Example 3 of the present invention will be described. FIG. 10 is a top view of the wavelength conversion element according to the third embodiment of the present invention. In FIG. 10, linear waveguides 501, 503, and 505, a curved waveguide 502 that couples linear waveguides 501 and 503, and a curved waveguide 504 that couples linear waveguides 503 and 505 are formed on base substrate 510. The provided wavelength conversion element 500 is shown.

  A wavelength conversion element 500 shown in FIG. 10 has a configuration in which a linear waveguide is further coupled to the wavelength conversion element 100 shown in FIG. 2. Light enters from the incident end of the linear waveguide 501, and It exits from the exit end. The lengths of the straight waveguides 501, 503, and 505 can be 50 mm. According to the wavelength conversion element 500 according to the third embodiment, the conversion efficiency is increased because the length is further increased by the length of the linear waveguide 505 than the wavelength conversion element 100 illustrated in the first embodiment, and the normalized conversion is performed. Efficiency was as high as 10,000% / W when light having a wavelength of 1555 nm was used as input light. The reason for not increasing by the square of the length is that not all regions contribute to the conversion due to the slightly produced non-uniformity.

Example 4
Next, a wavelength conversion element according to Example 4 of the present invention will be described. FIG. 11 shows a top view of a wavelength conversion element according to Example 4 of the present invention. FIG. 11 shows linear waveguides 602, 604, 606, 608 and 610, a curved waveguide 601 connected to the linear waveguide 602, a curved waveguide 603 coupling the linear waveguides 602 and 604, and a linear waveguide. A curved waveguide 605 that couples the waveguides 604 and 606, a curved waveguide 607 that couples the linear waveguides 606 and 608, and a curved waveguide 609 that couples the linear waveguides 608 and 610 are provided on the base substrate 620. A wavelength conversion element 600 is shown.

  In the wavelength conversion element 600 shown in FIG. 11, light is incident from the incident end of the curved waveguide 601 having an S-shape, and the linear waveguides 602, 604, 606, 608, 610 and these linear waveguides are coupled. The light exits from the exit end of the straight waveguide 610 via the U-shaped curved waveguides 603, 605, 607 and 609.

  A wavelength conversion element 600 shown in FIG. 11 has a configuration in which two linear waveguides are further coupled to the wavelength conversion element 500 shown in FIG. 10, and five linear waveguides are coupled. The wavelength conversion element 600 according to the fourth embodiment has a light input / output end at the center of the wavelength conversion element 600 and has a shape advantageous for coupling with an optical fiber or the like.

In the wavelength conversion element 600 shown in FIG. 11, the linear region S ₁ is 50 mm, the input / output portion S ₂ is approximately 0.5 mm, and the non-polarized inversion linear waveguides S ₃ , S ₄ , and S ₅ are approximately 1. The waveguide distance D was 0.5 mm, and the radius of curvature R was 0.5 mm. In the wavelength conversion element 600, the length of the straight waveguide portion of the curved waveguide is adjusted so as to achieve phase matching in the non-polarization inversion region. The normalized conversion efficiency of the wavelength conversion element 600 according to Example 4 was 20000% / W when light having a wavelength of 1555 nm was used as input light.

(Example 5)
Next, a wavelength conversion element according to Example 5 of the present invention will be described. FIG. 12 is a top view of the wavelength conversion element according to the fourth embodiment of the present invention. In FIG. 12, a plurality of linear waveguides 702 and a plurality of 120 ° curved waveguides 701 and 60 ° curved waveguides 703 that couple two of the plurality of linear waveguides 702 are formed on the base substrate 710. A wavelength conversion element 700 is shown. As shown in FIG. 12, in the wavelength conversion element 700, a plurality of linear waveguides 702 are coupled by a 120 ° curved waveguide 701 or a 60 ° curved waveguide 703, so that the entire waveguide is formed on the base substrate 710. It is formed in a regular triangular spiral shape.

  As described above, since the Z plane of LN, which is a nonlinear optical material, has a three-fold symmetric crystal structure, there are not only one direction but also three directions in which a periodically poled structure can be formed. Therefore, by connecting the straight waveguide 702 with a 60 ° curved waveguide 703 bent in the 60 ° direction as shown in FIG. 12 or a 120 ° curved waveguide 701 bent in the 120 ° direction, a spiral as shown in FIG. A type wavelength conversion element 700 can also be manufactured. In the wavelength conversion element 700 according to the fifth embodiment, the domain-inverted structure is formed along the direction corresponding to the three sides of the equilateral triangle. The spiral-type wavelength conversion element 700 according to the fifth embodiment has an advantage that the substrate can be used efficiently.

  According to the wavelength conversion element according to the present invention described above, a high melting point cladding region that produces an appropriate effective refractive index difference and a structure in which a straight waveguide is coupled by a curved waveguide having an appropriate radius of curvature are adopted. Accordingly, it is possible to realize a high-performance wavelength conversion element having a conversion efficiency remarkably improved with an element size substantially equal to that of the prior art.

Multiplexer 1
Optical waveguide 2
Demultiplexer 3
Wavelength conversion element 100, 200, 300, 400, 500, 600, 700
Linear waveguide 101, 103, 401, 403, 501, 503, 505, 602, 604, 606, 608, 610, 702
Curved waveguide 102, 402, 502, 504, 601, 603, 605, 607, 609
Base substrate 110, 210, 310, 410, 510, 620, 710
LN substrate 111, 311, 411
Clad region 112, 412
Three-dimensional optical waveguide 120, 220, 320, 420
Groove 130, 240, 340
Upper cladding region 230, 330
Lower cladding region 312
Bottom electrode 441
Upper electrode 442
Side electrode 443
Buffer layer 444
120 ° curved waveguide 701
60 ° curve waveguide 703

Claims (8)

A base substrate;
A wavelength conversion element comprising a three-dimensional optical waveguide made of a second-order nonlinear optical material provided on the base substrate,
The three-dimensional optical waveguide includes a plurality of linear waveguides having a periodically poled structure and a curve not having a periodically poled structure that couples two of the plurality of linear waveguides. Including a waveguide,
The curved waveguide is adjusted in length so that the phases of the light guided through the two linear waveguides are matched.
The base substrate includes a clad region having a refractive index lower than the refractive index of the waveguide by 0.75% or more, and the optical waveguide and the clad region of the base substrate are joined by direct joining, and the curved waveguide is provided. A wavelength conversion element having a curvature radius of a waveguide of 0.2 mm or more and 5 mm or less. The base substrate including the cladding region may be made of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn, Sc, and In 2. The wavelength conversion element according to claim 1, wherein the wavelength conversion element is made of a material containing at least one selected from the group consisting of additives, or a high melting point material containing quartz or silicon oxide.   The wavelength conversion element according to claim 1, wherein a cladding region is further provided on the cladding region and the three-dimensional optical waveguide of the base substrate.   4. The wavelength conversion element according to claim 1, wherein an electrode is provided along the curved waveguide on an upper surface, a side surface, or the base substrate of the curved waveguide. 5. The nonlinear optical material is selected from the group consisting of LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn, Sc, and In. The wavelength conversion element according to any one of claims 1 to 4, wherein the wavelength conversion element is made of a material containing at least one of these as an additive.   6. The wavelength conversion element according to claim 1, wherein each of the one or the plurality of curved waveguides is a curved waveguide including two 90 ° waveguide portions.   6. The wavelength conversion element according to claim 1, wherein each of the one or more curved waveguides is one of a 60 ° curved waveguide and a 120 ° curved waveguide.   8. The curved waveguide includes a straight waveguide portion whose length is adjusted so that phases of light guided through the two straight waveguides are matched with each other. Wavelength conversion element.

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