JP2011257558A - Optical element and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a low-loss optical waveguide having sufficiently effective refractive index difference and sufficient bonding strength without installation of an intermediate layer in an optical element formed by bonding two substrates having nearly equal refractive indexes.SOLUTION: The optical element formed by bonding two substrates having the nearly equal refractive indexes includes the optical waveguide within a waveguide substrate and also includes an unbonded layer along the optical waveguide on at least a direct bonding surface below the optical waveguide.

Description

  The present invention relates to an optical element and a method for manufacturing the optical element, and more specifically to an optical element formed on a bonded substrate and a method for manufacturing the optical element.

  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-optic media used in such elements, and oxide-based compound substrates such as lithium niobate (LiNbO ₃ ) have second-order nonlinear optical constants and electro-optics. It is known as a promising material with a very high constant. As an example of an optical device using the high nonlinearity of this lithium niobate, a wavelength conversion element using difference frequency generation by quasi phase matching is known.

  For example, since strong absorption lines such as various vibrations of environmental gases exist in the mid-infrared wavelength region of 2 μm to 5 μm, development of a small mid-infrared light source is desired. As such a light source in the mid-infrared region, a mid-infrared light source based on a difference frequency generation capable of using a technically mature excitation light source in the vicinity of 1 μm and signal light in the communication wavelength band is considered promising.

  In order to obtain high efficiency in wavelength conversion elements, optical waveguide devices are effective, and various waveguides have been researched and developed. However, in recent years, the crystal bulk characteristics can be used as they are, so that high optical damage resistance is achieved. Ridge-type optical waveguides with characteristics such as long-term reliability and ease of device design have been researched and developed.

  A ridge-type optical waveguide can be formed by thinning one substrate of the optical element formed by joining two substrates and then performing ridge processing. A direct bonding technique is known as a technique for firmly bonding substrates without using an adhesive or the like when bonding the substrates.

  In addition to features such as high light damage resistance, long-term reliability, and ease of device design, for example, in the mid-infrared light generation due to the difference frequency generation described above, the substrates should be firmly bonded without using an adhesive or the like. The direct bonding technique that can be used is also promising from the viewpoint of avoiding the mixing of impurities and absorption of adhesives and the like.

  An example in which lithium niobate or lithium tantalate, which is a ferroelectric crystal substrate, is directly bonded to the same substrate to generate mid-infrared light with high efficiency has been reported (see Non-Patent Document 1).

  In addition, by using direct bonding, various materials such as glass, semiconductors, ferroelectrics, and piezoelectric ceramics can be bonded with high precision, so that application to optical elements is expected. So far, optical elements in direct bonding substrates such as dielectric substrates, semiconductor substrates, glass substrates and the like have been proposed.

  However, an optical element composed of two identical substrates having the same refractive index cannot be used as an optical waveguide. Further, even when directly bonding the same kind of substrates having different refractive indexes, such as when directly bonding a lithium niobate substrate and an Mg-doped lithium niobate substrate, an Mg-doped lithium niobate substrate having a low refractive index is used as an optical waveguide. It was impossible to do. Furthermore, in the optical waveguide formed by directly joining substrates having substantially the same refractive index, the difference in refractive index between the two substrates is small and insufficient, so the size of the optical waveguide cannot be reduced, There was a problem that high conversion efficiency could not be obtained.

In response to this problem, some proposals have been made regarding an optical element formed by bonding two substrates through a thin film (see Patent Document 1). In an optical element using two substrates and one of which is a waveguide layer, the refractive index of the substrate that becomes the waveguide layer must be higher. Therefore, by arranging a thin film having a refractive index lower than that of the waveguide layer between the substrates, light can be guided regardless of the refractive index of the substrate. For example, Patent Document 1 shows that SiO ₂ is used as a thin film material.

  Thus, although the above-mentioned problem can be solved by providing the thin film between the two substrates, it is difficult to provide the thin film between the two substrates.

As disclosed in Patent Document 1, for example, when SiO ₂ is used as the thin film, it is difficult to control the surface roughness of the thin film layer, and the film surface roughness is large in the thin film using the sputtered film or the deposited film. Become. Such surface roughness is not suitable for direct bonding. For example, the surface roughness can be reduced by forming a thin film using a CVD (Chemical Vapor Deposition) apparatus or the like, but there is a problem that CVD is an expensive and large-scale apparatus. In addition, there is a problem that the adhesion and bonding strength between the thin film and the substrate are unevenly distributed depending on the thin film formation conditions, and when the bonded substrate is machined, sufficient strength cannot be obtained. . In addition, since the effective refractive index difference changes depending on the thickness of the thin film, it is necessary to control the thickness of the thin film with very high accuracy, and there is a problem from the viewpoint of cost and yield.

Furthermore, thin films such as SiO ₂ that are realized with ordinary vapor deposition equipment have an amorphous shape and strong absorption in the mid-infrared, so these materials must be bonded to realize a device for light generation in the mid-infrared region. There was a problem that it could not be used as an intermediate layer of a substrate. Further, as a material having little absorption of mid-infrared light, a fluoride-based material can be mentioned, but there is a problem that it is difficult to use from the viewpoint of reliability because it has deliquescence.

  Therefore, a method for providing an effective refractive index difference without using a thin film material for the intermediate layer has been desired.

  As a technique for providing an effective refractive index difference without using a thin film material in the intermediate layer, as shown in Non-Patent Document 2, a method of providing a void layer called an air gap has been proposed so far. If the air gap layer is provided by using the direct bonding method, absorption and reliability deterioration factors due to the material used as the intermediate layer can be removed.

FIG. 1 shows a schematic diagram of the structure of a conventional air gap waveguide shown in Non-Patent Document 2. After forming a groove called a gap 12 in the Mg-doped LiNbO ₃ substrate 11 which is a waveguide substrate, it is bonded to the LiNbO ₃ substrate 13 which is a base substrate. Thereafter, the ridge waveguide 14 is formed on the waveguide substrate 11 which has been thinned to a desired thickness.

  However, the existing air gap waveguide has the following problems.

  First, since the waveguide portion and the substrate portion are made of the same material, the existing air gap waveguide has a leakage loss. In other words, since the gap width needs to be larger than the width of the optical waveguide formed on the waveguide substrate, the core remaining layer is a core layer other than the ridge portion called a collar or ladder on the side surface of the optical waveguide. It is necessary to leave. As a result, there is a problem that light leaks from the side surface direction of the optical waveguide perpendicular to the light propagation direction of the optical waveguide, and loss increases. Non-Patent Document 3 shows that the loss of light may not be below a certain level depending on the polarization even if the depth of the gap is sufficiently deep.

  Furthermore, since the amount of leakage loss in the lateral direction of light greatly depends on the remaining amount of the core remaining layer, which is the core layer other than the ridge portion called the collar or ladder, on the side surface of the optical waveguide, the ridge waveguide The tolerance of processing is small, and it is necessary to control the remaining amount of the collar portion with high precision, which is problematic from the viewpoint of yield and productivity.

Also, in principle, it is necessary to leave the core remaining layer, which is the core layer other than the ridge portion called the collar or ladder, on the side surface of the optical waveguide, and the remaining amount of the core remaining layer that is the core layer other than the ridge portion On the other hand, since not only the amount of optical loss but also the effective refractive index of the optical waveguide depends, there is a problem that the characteristics deteriorate when performing wavelength conversion using a nonlinear optical medium such as a LiNbO ₃ substrate.

  In general, in order to obtain high conversion efficiency, it is necessary to match the phase between the excitation light and the converted light. For example, a quasi phase matching method using a polarization inversion structure is used. At this time, if the effective refractive index of the optical waveguide is changed, there are problems such that the center wavelength in wavelength conversion shifts from a desired value, and the wavelength conversion efficiency decreases.

  Moreover, since the width of the gap needs to be sufficiently larger than the width of the optical waveguide formed on the waveguide substrate, the number of optical waveguides that can be fabricated in one wafer is limited, and productivity / There was a problem in terms of yield. For example, if the width of the gap is about 10 times the width of the optical waveguide formed on the waveguide substrate, the area of the joint surface other than the gap is taken into consideration, and the gap is formed in the wafer when the gap is provided. The number of optical waveguides that can be used is about 1/10 of the case where no gap is provided.

  Further, as another method for providing an effective refractive index difference without using a thin film material in the intermediate layer, as disclosed in Patent Document 2, a method of providing a minute hole called an air hole between a waveguide substrate and a base substrate Has been proposed.

  However, the existing minute hole type (air hole type) waveguide has the following problems.

  Since the size of the holes needs to be sufficiently small with respect to the wavelength of light of 100 nanometers or less, it is necessary to use electron lithography or the like for the production, and a long and long element of several centimeters order is produced. Therefore, it is difficult to obtain a highly efficient wavelength conversion element. In addition, since it is necessary to use electronic lithography, there are also problems from the viewpoints of cost, productivity, and yield.

  Furthermore, if the holes are not randomly arranged, loss due to light diffraction may occur due to the periodicity of the holes, and thus the aperture ratio of the holes cannot be increased. That is, it is difficult to increase the effective refractive index difference. In addition, there is a problem that a complicated design is necessary to eliminate the periodicity of the arrangement of the holes.

  As a conclusion, to date, problems such as the strength of direct bonding and substrate cracking due to differences in crystal structure and thermal expansion coefficient, degradation of device characteristics and yield due to non-uniformity of the thickness of the intermediate layer, and further, the intermediate layer Therefore, there has been a demand for a method of providing an effective refractive index difference without using a thin film material in the intermediate layer, which can eliminate the loss of light and the deterioration factor of reliability due to absorption caused by the above materials. However, there has been no method that can produce a low-loss optical waveguide, can obtain a sufficient bonding strength, and can provide a sufficient effective refractive index difference with a simple yield and low cost.

Japanese Patent Laid-Open No. 06-222229 JP 2005-292245 A

O. Tadanaga, T. Yanagawa, Y. Nishida, H. Miyazawa, K. Magari, M. Asobe, and H. Suzuki, '' Efficient 3-um difference frequency generation using direct-bonded quasiphase-matched LiNbO3 ridge waveguides, ' 'Appl. Phys. Lett. 88, 061101 (2006). ELECTRONICS LETTERS 14th October 2004 Vol. 40 No. 21 Journal of the Institute of Electronics, Information and Communication Engineers C Vol. J89-C No. 10 pp.700-707

  The present invention is to solve the conventional problems as described above. According to the present invention, in an optical element formed by bonding two substrates having substantially the same refractive index, a sufficient effective refractive index difference can be provided without using a thin film material in the intermediate layer, and sufficient bonding strength can be provided. In addition, a low-loss optical waveguide can be easily formed with a high yield.

  One embodiment of the present invention is an optical element that includes an optically polished waveguide substrate and an optically polished base substrate that are bonded to each other via a bonding surface, and the optical element is included in the waveguide substrate. Has a ridge type optical waveguide, and has at least one non-joined region extending along the light propagation direction at the joint surface under the optical waveguide, and the core remaining layer does not exist in the core layer other than the ridge portion. The width of the non-joining region in the direction perpendicular to both the light propagation direction and the direction perpendicular to the joint surface is not more than the width of the optical waveguide.

  In one embodiment of the present invention, the waveguide substrate and the base substrate are bonded by direct bonding.

  In one embodiment of the present invention, the refractive index of the waveguide substrate is substantially equal to the refractive index of the base substrate, or the refractive index of the waveguide substrate is smaller than the refractive index of the base substrate.

In one embodiment of the present invention, the optical substrate comprises LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , and LiNbO ₃ , KNbO _3. , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or KTiOPO ₄ at least selected from the group consisting of Mg, Zn, Sc, or In. It contains one as an additive.

In one embodiment of the present invention, a first laser beam having a wavelength λ _{1 and} a second laser beam having a wavelength λ ₃ are input, and by generating a difference frequency between the first laser beam and the second laser beam, _{1 / λ 2 = 1 / λ} 3 outputs a wavelength lambda ₂ of the coherent light in the -1 / lambda ₁ relationship that the wavelength lambda ₂ of the coherent light is either in the range of 2μm ≦ λ ₂ ≦ 5μm It is characterized by.

In an embodiment of the present invention, at least _one of a first laser beam having a wavelength λ _{1 and} a second laser beam having a wavelength λ ₂ is input, and either sum frequency generation or second harmonic generation is performed. Thus, coherent light having a wavelength λ ₃ having a relationship of either 1 / λ ₃ = 1 / λ ₁ + 1 / λ ₂ or 1 / λ ₃ = 2 / λ ₁ is output.

  In one embodiment of the present invention, the optical waveguide has a tapered structure in the light input / output portion, and the number of non-bonding layers is changed in the tapered structure.

  One embodiment of the present invention is a method for manufacturing an optical element, which includes an optically polished waveguide substrate having a periodically poled structure and an optically polished base substrate bonded via a bonding surface. Then, a step of forming at least one concave portion extending in a polarization reversal direction by etching on the bonding surface of the waveguide substrate, and a diffusion by heat-treating the base substrate with the waveguide substrate through the bonding surface and performing heat treatment Bonding, bonding the base substrate and the waveguide substrate, polishing the surface of the bonded waveguide substrate using a polishing apparatus, polishing to obtain a thin film substrate, and etching And a step of creating a ridge type optical waveguide on the thin film substrate.

  In the optical element formed by bonding two substrates having substantially the same refractive index according to the present invention, a sufficient effective refractive index difference can be provided in the intermediate layer without using a thin film material, and sufficient bonding strength and A low-loss optical waveguide can be easily formed with a high yield.

It is the schematic which shows the structure of the conventional air gap waveguide. It is the schematic which shows the structure of the optical element concerning the 1st Example of this invention. It is a figure which shows the simulation result when not using the optical element concerning the 1st Example of this invention. It is a figure which shows the simulation result at the time of using the optical element concerning the 1st Example of this invention. It is a figure explaining the manufacturing process of the optical element concerning the 1st Example of this invention. It is a figure explaining the manufacturing process of the optical element concerning the 1st Example of this invention. It is a figure explaining mid-infrared light generation by difference frequency generation using the optical element concerning the 1st example of the present invention. It is the schematic which shows the structure of the optical element concerning the 2nd Example of this invention. It is a figure explaining the structure of the edge part of the optical element concerning the 2nd Example of this invention.

  Hereinafter, an optical element and a manufacturing method thereof according to the present invention will be described in detail with reference to the drawings.

  FIG. 2 is a view for explaining the structure of the optical element according to the first embodiment of the present invention.

  In the structure shown in FIG. 2, a waveguide substrate 21 made of Zn-doped lithium niobate having a domain-inverted structure and a base substrate 22 made of Mg-doped lithium niobate are directly joined at a direct joining surface 23, so that the waveguide substrate 21. A plurality of non-bonding layers 25 parallel to the optical waveguide are provided on the direct bonding surface under the ridge optical waveguide 24 formed by processing the above. Since the non-bonding layer formed on the lower side of the optical waveguide 24 looks like a rail for the optical waveguide, the non-bonding layer according to the present invention is referred to as the air rail 25, and the optical waveguide according to the present invention is referred to as the air rail waveguide. I will call it.

  Here, the refractive indexes of the waveguide substrate 21 and the base substrate 22 are 2.06 and 2.05, respectively, at a wavelength of 3.4 μm, and the refractive indexes of both substrates are substantially equal. The height of the air rail 25 is about 1 μm and the width is about 1 μm. The number of air rails 25 is four. The optical waveguide 24 has a height of about 8 μm and a width of 8 μm. The optical waveguide 24 has no core remaining layer in the core layer other than the ridge portion.

  By providing the air rail 25 on the waveguide substrate 21, the refractive index of the region including the air rail 25 can be reduced, and light can be confined in the optical waveguide 24.

  In this embodiment, the air rail structure 25 is provided on the waveguide substrate 21, but the air rail structure may be provided on the base substrate. In addition, if the air rail is provided at least under the optical waveguide 24, the effect of the present invention can be obtained. However, the air rail may be provided on the periphery or directly on the entire joining surface 23. In this case, there is an advantage that it becomes easy to align the positions of the optical waveguide 24 and the air rail structure 25.

  Next, the result of having calculated the effect of the present invention using a beam propagation method (BPM) is shown.

  FIG. 3 shows a case where an optical waveguide is formed after simple direct bonding without using the present invention. FIG. 3A shows the cross-sectional structure of the optical waveguide used in the calculation, in which a waveguide substrate 31 made of Zn-doped lithium niobate and a base substrate 32 made of Mg-doped lithium niobate are directly joined. A ridge optical waveguide is formed by processing the waveguide substrate 31. The upper and side portions of the waveguide substrate are air layers 33. FIG. 3B shows a calculation result using the beam propagation method. FIG. 3B shows that light leaks to the base substrate 32 because the refractive indexes of the waveguide substrate 31 and the base substrate 32 are substantially equal. Thus, in an optical waveguide obtained by bonding substrates having substantially the same refractive index, light loss increases. It is also clear that the core size of the optical waveguide cannot be reduced because the difference in refractive index is insufficient. For this reason, when such an optical element is used for wavelength conversion, high efficiency cannot be obtained.

  FIG. 4 shows a case according to the present embodiment. FIG. 4A shows a cross-sectional structure of the optical waveguide used for the calculation. In FIG. 4A, a waveguide substrate 41 made of Zn-doped lithium niobate and a base substrate 42 made of Mg-doped lithium niobate are directly bonded, and the bottom of the ridge optical waveguide formed by processing the waveguide substrate. A structure in which a non-bonding layer (air rail) 44 parallel to the optical waveguide is provided on the direct bonding surface is disclosed. There is an air layer 43 on the top and sides of the waveguide substrate. FIG. 4B shows a calculation result using the beam propagation method. Although the refractive index of the waveguide substrate 41 and the refractive index of the base substrate 42 are substantially equal, by providing the air rail 44, the refractive index of the region including the air rail 44 can be reduced. The light can be confined. In addition, since there is no core remaining layer in the core layer other than the ridge portion in the optical waveguide, confinement in the side surface direction of the optical waveguide is sufficient, and light leakage does not occur. Thereby, even in an optical waveguide obtained by bonding substrates having substantially the same refractive index, low-loss characteristics can be obtained. Since the difference in refractive index is sufficient, the core size of the optical waveguide can be reduced. When such an optical element is used for wavelength conversion, high efficiency can be obtained.

  When a 1.064 μm excitation light and a 1.55 μm signal light used for generating a difference frequency of 3.4 μm light were simulated using the same structure as described above, the refractive index was also determined for each wavelength. It was confirmed that the difference was sufficiently large and light could be confined in the optical waveguide. Therefore, a low-loss optical waveguide can be realized even for light of 1.064 μm and 1.55 μm.

  Conventionally, if there is a region where the optical waveguide is projected onto the bonding surface and there is a region bonded to the base substrate, the light confinement effect of the optical waveguide becomes insufficient, and propagation loss of the waveguide increases. That is, an area wider than this projection area is required as the non-joint area, including the area where the optical waveguide is projected onto the joint surface. However, we have found an air rail structure that can sufficiently reduce the propagation loss of the waveguide even when the region where the optical waveguide is projected onto the bonding surface is present in the region where it is bonded to the base substrate. Thereby, it is not necessary to leave a core remaining layer in the core layer other than the ridge portion, and light leakage from the side surface direction of the optical waveguide can be suppressed.

  Next, the shape of the air rail structure will be described. Four non-bonding layer (air rail) regions along the light propagation direction of the optical waveguide are provided on the direct bonding surface under the optical waveguide. In the present embodiment, each air rail has a depth of 1 μm, a width of 1 μm, and a distance between the air rails of 1 μm. Here, the width of the non-bonding layer (air rail) is defined as the width of one air rail.

  Conventionally, when a fine structure is provided on a waveguide substrate and a base substrate, if the shape of the fine structure is large, the light propagating through the waveguide is scattered by the fine structure and the loss of the waveguide increases. Must have a structure sufficiently small with respect to the wavelength of light propagating through the optical waveguide. However, in the present invention, by using an air rail structure that is a concave structure along the light propagation direction of the optical waveguide, the shape of the structure, for example, the air rail structure is 1 μm for a wavelength of 1.064 μm. Even when the wavelength of light propagating through the optical waveguide is not sufficiently small, the loss of light does not increase.

  It is sufficient to provide at least one air rail structure, but three or more are preferable. The area of the air rail structure depends on the bonding strength, the size of the optical waveguide, and the refractive index difference between the substrates.

  The effective refractive index can be controlled by changing the ratio of the area of the air rail structure to the area of the joint surface under the optical waveguide. For example, the effective refractive index felt by the optical waveguide can be controlled by changing the ratio between the width of the air rail and the width of the joint surface. If the ratio of the width of the air rail to the width of the joint surface is increased, the effective refractive index can be lowered, so that a more confined optical waveguide can be formed. To what extent the ratio of the width of the air rail to the width of the joint surface can be taken depends on the joint strength. If the ratio of the width of the air rail to the width of the joint surface is too large, the area of the joint surface becomes insufficient, and the joint strength that can withstand the processing cannot be obtained. The area of the air rail structure depends on the bonding strength and the bonding area under the optical waveguide, but is preferably 1/10 or more with respect to the bonding area under the optical waveguide, and more preferably with respect to the bonding area under the optical waveguide. 1/4 or more.

  Further, as another method of changing the area density of the air rail structure, a method of changing the number of air rail structures can be used. Since the effective refractive index can be controlled without using an ultrafine processing process, it is considered useful when the area density of the air rail structure is partially changed. For example, by using this structure, there is an advantage that the shape of the optical waveguide, which has been difficult in the past, can be easily and three-dimensionally controlled. Application to a mode converter or the like that changes the shape of the tapered structure or the waveguide mode becomes possible.

  Regarding the depth of the air rail, 1/10 or more of the wavelength of light propagating through the optical waveguide is preferable. The portion where the air rail structure is formed acts as a cladding layer having a different refractive index. If this cladding layer is not sufficiently thick, the electric field distribution of light propagating through the optical waveguide leaks to the substrate side, causing an increase in propagation loss of the optical waveguide. The thickness of the air rail (cladding layer) varies depending on the area density of the air rail structure, but it needs to be at least about 1/10 of the wavelength of light propagating through the waveguide, and more preferably through the optical waveguide. 1/5 or more of the wavelength of the light to be transmitted.

  In principle, there is no problem if the width of the air rail is small. However, the smaller the width of the air rail, the higher the aspect ratio with respect to the depth direction. Therefore, the minimum width of the air rail is limited by the processing limit. When the current technology is used, the air rail can be manufactured using normal photolithography if the width of the air rail is 0.3 μm or more. Therefore, the width of the air rail of 1 μm shown in this embodiment can be realized simply and at low cost by using ordinary photolithography.

  The manufacturing method will be described below.

FIG. 5 is a flowchart showing a process of manufacturing a thin film substrate for a wavelength conversion element used in this example. As shown in FIG. 5, in this embodiment, there are phase matching conditions between 1.5 μm light and 1.064 μm light as a waveguide substrate in advance and 3.4 μm band light generated in the crystal. A Z-cut Zn-doped LiNbO ₃ substrate 51 in which a periodically poled structure was formed so as to be satisfied was used. A Z-cut Mg-added LiNbO ₃ substrate was used as the base substrate 52. The thermal expansion coefficients of the waveguide substrate 51 and the base substrate 52 are substantially the same. The refractive index of the waveguide substrate 51 and the refractive index of the base substrate 52 are substantially equal, but the base substrate 52 is slightly smaller. The waveguide substrate 51 and the base substrate 52 are both 3-inch wafers whose surfaces are optically polished. The thickness of the waveguide substrate 51 is 300 μm, and the thickness of the base substrate 52 is 500 μm.

  First, the concave portion 53 is formed in the direct bonding surface of the waveguide substrate 51 having the periodically poled structure. Although there are various methods for forming the recess, dry etching is used in this embodiment. A resist was applied to the entire surface of the waveguide substrate 51, and a mask pattern corresponding to the air rail structure was formed by a normal photolithography process. Thereafter, the substrate was set in a dry etching apparatus, and an air rail structure was fabricated by etching the surface of the direct bonding surface of the waveguide substrate using Ar gas as an etching gas.

  Next, after making the surfaces of the waveguide substrate 51 and the base substrate 52 hydrophilic by normal acid cleaning or alkali cleaning, the two substrates 51 and 52 were superposed in a clean atmosphere in which microparticles were not present as much as possible. The superposed waveguide substrate 51 and base substrate 52 were placed in an electric furnace and heat treated at 400 ° C. for 3 hours to perform diffusion bonding. The bonded substrate was free of voids such as microparticles on the bonding surface, and no cracks were generated even when the substrate was returned to room temperature.

  Next, polishing was performed using a polishing apparatus in which the flatness of the polishing surface plate was controlled until the thickness of the waveguide substrate 51 of the bonded substrate reached 20 μm. A mirror-polished polished surface could be obtained by polishing after polishing. When the parallelism of the substrate (difference between the maximum height and the minimum height) was measured using an optical parallelism measuring instrument, submicron parallelism was obtained almost entirely except for the periphery of a 3-inch wafer. A thin film substrate 54 suitable for the production of the conversion element could be produced. Since the thin film substrate 54 is manufactured by directly bonding the waveguide substrate 51 and the base substrate 52 by diffusion bonding by heat treatment without using an adhesive, the thin film substrate 54 has a uniform composition and a uniform film thickness over the entire area of the 3-inch wafer. have.

  Thereafter, using the thin film substrate 54 produced, a wavelength conversion element was produced using a dry etching process as a means for producing an optical waveguide. That is, after a waveguide pattern is formed on the surface of the thin film substrate 54 (waveguide substrate 51) by a normal photolithography process, the substrate is set in a dry etching apparatus, and the Ar gas is used as an etching gas to form the thin film substrate 54 (waveguide). A ridge-type optical waveguide was produced by etching the surface of the substrate 51). Even if an air rail structure was provided, the strength of direct bonding was sufficient, so that etching was performed until the base substrate was dug until the side surface of the optical waveguide completely became an air layer.

  Since the side surface of the optical waveguide is completely an air layer, an optical waveguide with strong lateral confinement and low loss can be realized. Furthermore, by providing an air rail structure, a sufficient effective refractive index difference is provided between the base substrate and light leakage to the base substrate can be suppressed, resulting in a very low loss optical waveguide. Was able to be realized.

  In this embodiment, a dry etching process is used as a means for manufacturing the optical waveguide, but a machining technique such as dicing may be used. In general, the cost can be reduced by using machining rather than dry etching. However, when machining such as dicing is used, it is difficult to achieve machining accuracy in the depth direction in machining. Therefore, other than the ridge waveguide portion called the collar or ladder on the side surface of the optical waveguide, such as an air gap waveguide It was very difficult to fabricate a structure that needed to leave the remaining part. On the other hand, in the air rail waveguide according to the present invention, the side surface can be completely formed into an air layer, that is, the optical waveguide can be formed even if the optical waveguide is processed sufficiently deep up to the cladding layer. The following machining techniques can be used. Actually, when manufacturing was attempted, if the depth of the clad layer was processed sufficiently deep, the accuracy in the depth direction would become gradual, so that a high-performance wavelength conversion element could be formed even by using machining such as dicing. .

  As shown in FIG. 6, a plurality of optical waveguides 61 were produced in parallel with a thin-film substrate 62 which is a 3-inch wafer. Then, the thin film substrate 62 was cut into a strip shape for each of these optical waveguides 61, and both the end faces 61a of the optical waveguide 61 were optically polished to produce a wavelength conversion element 63 having a length of 50 mm.

  The number of optical waveguides that can be fabricated in the wafer will be described. In the air rail waveguide, it is sufficient that the air rail structure is formed at least in the lower portion of the optical waveguide, and therefore the distance between the optical waveguides is not limited due to the air rail structure. Therefore, the distance between the optical waveguides can be reduced within a range in which the electric field does not overlap with the adjacent optical waveguide due to the leakage of light. When the upper and side portions of the optical waveguide are air layers, the confinement is strong. Therefore, the oozing of the electric field from the side portion is small, and it is necessary to pay attention to the overlapping of the electric fields obtained by solving the light leaking to the base substrate. However, by using the air rail waveguide, the leakage of the electric field to the base substrate can be sufficiently reduced, so that the distance between the optical waveguides can be reduced. Thereby, a large number of optical waveguides can be formed in one wafer, and the yield and productivity can be improved.

FIG. 7 is a schematic diagram showing a configuration of mid-infrared light generation by difference frequency wave generation using the produced wavelength conversion element. As shown in the figure, the signal light A having a wavelength of 1.56 μm and the excitation light B having a wavelength of 1.064 μm are combined by a multiplexer 71 and incident on a wavelength conversion element 72. The light is converted into converted light C having a different wavelength by the generation of the difference frequency wave in the nonlinear optical effect in the optical waveguide, and is emitted from the waveguide together with the signal light A and the excitation light B. The emitted converted light C, signal light A, and excitation light B are separated by the duplexer 73. If the wavelengths of the signal light A and the pumping light B are λ ₁ and λ ₃ , respectively, the wavelength λ ₂ of the converted light C satisfies 1 / λ ₂ = 1 / λ ₃ −1 / λ ₁ . Therefore, the difference frequency light C having the wavelength λ ₂ = 3.4 μm can be generated.

  The normalized conversion efficiency of the fabricated device was as high as 100% / W.

When the mid-infrared light is generated by the difference frequency generation as in this embodiment, the wavelength of the mid-infrared light generated by the wavelength conversion becomes a long wave with respect to the wavelengths of the input excitation light and signal light. In the case of a long wave, generally, the light confinement effect is weakened, and the amount of light leakage from the optical waveguide tends to increase. In addition, thin films such as SiO ₂ that are realized with ordinary vapor deposition equipment have an amorphous shape and strong absorption in the mid-infrared, so these materials can be used as a bonding substrate in order to realize elements for light generation in the mid-infrared region. It cannot be used as an intermediate layer.

Therefore, the invention according to the present application is particularly effective for generation of mid-infrared light by difference frequency generation. That is, the wavelength conversion element according to the present embodiment inputs the laser light having the wavelength λ _{1 and} the laser light having the wavelength λ ₃ and satisfies the relationship 1 / λ ₂ = 1 / λ ₃ -1 / λ _{1. 2} is a wavelength conversion element that outputs coherent light, and is considered to be particularly effective when the wavelength λ ₂ is 2 μm to 5 μm.

  FIG. 8 is a schematic diagram of the shape of the optical waveguide of the optical element according to the second embodiment of the present invention. In FIG. 8, a waveguide substrate 81 made of Mg-doped lithium niobate and a base substrate 82 made of Zn-doped lithium niobate are directly joined at a direct joining surface 83 and processed by processing the waveguide substrate 81. A structure in which an air rail structure 85 parallel to the optical waveguide is provided on the direct bonding surface under the waveguide 84 is disclosed.

  The air rail structure 85 has a width of 0.5 μm and a height of 1 μm. The rectangular cross section of the ridge optical waveguide 84 has a width of 8 μm and a height of 8 μm.

  The difference between this embodiment and the first embodiment is that, at the end of the optical waveguide corresponding to the light input portion, the number of air rails gradually decreases toward the end and the width of the optical waveguide. , The height gradually widens toward the end.

  In a wavelength conversion element using a nonlinear optical medium, if the size of the waveguide layer of the optical waveguide is reduced, the power density of light can be increased and theoretically high efficiency can be obtained. However, when the size of the waveguide layer of the optical waveguide is reduced, there is a problem that it is difficult to connect to another optical element such as a fiber.

  In addition, in a wavelength conversion element using a nonlinear optical medium, the higher the input light energy, the higher the output energy of the converted light, so that the efficiency is increased by reducing the diameter of the waveguide layer of the optical waveguide, and the light The trade-off with I / O loss was a big problem.

  Conventionally, in an optical waveguide using direct bonding, a taper structure with a width of the optical waveguide could be realized, but it was difficult to realize a taper structure with a thickness. This is because in an optical waveguide using direct bonding, the thickness of the waveguide substrate of the optical waveguide is achieved by optical polishing or the like, so that the thickness in the plane is uniform. Even when wet etching, dry etching, or the like is used, it is difficult to realize optical waveguide shapes having different thicknesses with good controllability.

  However, if the structure according to the present embodiment is used, it is possible to provide a taper structure even in the thickness direction with a simple configuration of simply adjusting the “number” of air rails between the waveguide layer and the cladding layer. I found it.

  The taper in the thickness direction of the optical waveguide can also be realized by gradually changing the depth or width of the air rail structure at the end of the optical waveguide. However, the control of the number of air rails only needs to be reflected in the shape of the photomask used for photolithography at the time of structure fabrication, and it does not require complicated processing technology such as ultra-fine processing, and is realized simply and at low cost. can do.

  FIG. 9 is a top projection view for explaining the details of the shape of the end portion corresponding to the light input portion of the embodiment according to the present embodiment.

  Hereinafter, the shape of the end corresponding to the light input unit of the embodiment according to the present embodiment will be described.

  The width of the ridge optical waveguide 84 is 8 μm, but linearly changes so as to be 10 μm at the input end. The length of the input end having a tapered structure is about 80 μm.

  The number of air rails formed under the ridge optical waveguide 84 is eight, but one at the input end. Corresponding to the taper structure in the width direction, the number of air rails gradually changes from eight toward one at the input end. Specifically, the number is decreased one by one at intervals of about 10 μm.

  As a result, an optical waveguide having an effective optical waveguide core size of 10 μm wide and 9 μm high can be realized at the input end. When a 1.55 μm band single mode fiber was connected by a butt joint and the coupling loss of light was measured, a coupling efficiency higher by 1 dB or more was obtained compared to the case without a tapered structure. At that time, an increase in excess loss of the optical waveguide due to the provision of the tapered structure was not observed.

In the above embodiment, a waveguide substrate made of Zn-doped lithium niobate and a base substrate made of Mg-doped lithium niobate are used. As the optical substrate, LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) At least one selected from the group consisting of Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , and any of these added with Mg, Zn, Sc, or In What is contained as an additive can be used.

  In the above embodiment, coherent light is output by difference frequency generation. However, coherent light can be output by sum frequency generation or second harmonic generation.

  Furthermore, in the above embodiment, the number of air rails is adjusted at the end corresponding to the light input unit, but the number of air rails can also be adjusted at the end corresponding to the light output unit. it can.

11 MgO-doped LiNbO ₃ waveguide substrate 12 gap 13 LiNbO ₃ base substrate 14 ridge waveguide 21 waveguide substrate 22 base substrate 23 direct bonding surface 24 ridge optical waveguide 25 air rail 31 waveguide substrate 32 base substrate 33 air layer 41 Waveguide substrate 42 Base substrate 43 Air layer 44 Air rail 51 Waveguide substrate 52 Base substrate 53 Recess 54 Thin film substrate 61 Optical waveguide 61a End face 62 of optical waveguide Thin film substrate 63 Wavelength conversion element 71 Multiplexer 72 Wavelength conversion element 73 Demultiplexer 81 Waveguide substrate 82 Base substrate 83 Direct bonding surface 84 Ridge optical waveguide 85 Air rail

Claims (8)

An optical element composed of an optically polished waveguide substrate and an optically polished base substrate bonded via a bonding surface,
The optical element is
A ridge-type optical waveguide is provided in the waveguide substrate,
At least one non-joining region extending along a light propagation direction at the joining surface under the optical waveguide;
There is no remaining core layer in the core layer other than the ridge,
The optical element, wherein a width of the non-joining region in a direction perpendicular to both the light propagation direction and the direction perpendicular to the joining surface is equal to or less than the width of the optical waveguide.   The optical element according to claim 1, wherein the waveguide substrate and the base substrate are bonded by direct bonding.   3. The optical device according to claim 1, wherein a refractive index of the waveguide substrate is substantially equal to a refractive index of the base substrate, or a refractive index of the waveguide substrate is smaller than a refractive index of the base substrate. element. The optical substrate includes LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , and LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _{( x) At} least one selected from the group consisting of Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ with Mg, Zn, Sc, or In added as an additive The optical element according to claim 1, wherein the optical element is contained. By inputting a _first laser beam having a wavelength λ _{1 and} a second laser beam having a wavelength λ ₃ , 1 / λ ₂ = 1 by generating a difference frequency between the first laser beam and the second laser beam. / lambda ₃ outputs a wavelength lambda ₂ of the coherent light in the -1 / lambda ₁ relationship, the wavelength lambda ₂ of the coherent light, wherein, characterized in that either the range of 2μm ≦ λ ₂ ≦ 5μm Item 3. The optical element according to Item 1 or 2. At least _one of the first laser beam having the wavelength λ _{1 and} the second laser beam having the wavelength λ ₂ is input, and 1 / λ ₃ = 1 by either the sum frequency generation or the second harmonic generation. _3. The optical element according to claim 1, wherein coherent light having a wavelength of λ ₃ having a relationship of either / λ ₁ + 1 / λ ₂ or 1 / λ ₃ = 2 / λ ₁ is output. The optical waveguide has a tapered structure at the input / output portion of light,
The optical element according to claim 1, wherein the number of the non-bonding layers changes in the tapered structure. A method of manufacturing an optical element comprising a waveguide substrate having an optically polished periodic polarization reversal structure bonded through a bonding surface and an optically polished base substrate,
Forming at least one recess extending in a polarization reversal direction by etching on a joint surface of the waveguide substrate;
Overlaying the base substrate with the waveguide substrate via the bonding surface, performing diffusion bonding by heat treatment, and bonding the base substrate and the waveguide substrate;
Polishing the surface of the bonded waveguide substrate using a polishing apparatus, and then polishing to obtain a thin film substrate;
And a step of forming a ridge-type optical waveguide on the thin film substrate by etching.