Optical waveguide element

Abstract

One embodiment of the present disclosure provides an optical waveguide element. This optical waveguide element comprises: an optical waveguide having an under cladding and a core; and two structures for waveguide core structure control involving controlling the structure of a waveguide core formed in the optical waveguide along a light propagation direction. An interval between the core and one structure is different from an interval between the core and the other structure, and the two structures are asymmetric with respect to the core, and the width of the core is bilaterally symmetrical. The two structures for waveguide core structure control have the same layer structure as the optical waveguide. The core has a periodically poled structure. The optical waveguide is a ridge-type waveguide. The optical waveguide further comprises an over cladding, and the over cladding is an air cladding.

Description

Optical Waveguide Element

[0001] The present disclosure relates to an optical waveguide element and a method for manufacturing the same.

[0002] Currently, optical elements capable of generating and modulating coherent light in the ultraviolet, visible, near-infrared, and terahertz wavelength bands are being applied in a wide range of fields, including optical communications, optical measurement, optical processing, and optical quantum computing. Among these, optical elements that utilize nonlinear optical effects have been the subject of research and development due to their excellent wavelength conversion properties.

[0003] Various types of materials have been developed as optical materials with nonlinear optical effects. Among them, oxide-based compound substrates, such as lithium niobate (LiNbO3:LN) and lithium tantalate (LiTaO3:LT), have been researched and developed as promising materials because they have a high second-order nonlinear optical constant and are transparent over a wide wavelength range from visible to near-infrared. Among LN and LT, periodically poled lithium niobate (periodically poled LN:PPLN) and periodically poled lithium tantalate (periodically poled LT:PPLT), which have a periodically poled structure formed by taking advantage of their property of being able to spontaneously polarize at room temperature, are widely used. The above optical materials have high phase consistency due to their periodically poled structure. As a result, they are widely used because they have a high second-order nonlinear optical effect, etc. Known optical devices that utilize the high nonlinearity of PPLN and PPLT include wavelength conversion elements that utilize second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG).

[0004] To improve wavelength conversion efficiency, a technology that realizes quasi-phase matching with high accuracy is important (see Non-Patent Document 1). Quasi-phase matching technology is a method of achieving quasi-phase matching in the propagation direction of light by forming a structure in which the sign of the nonlinear susceptibility is periodically inverted. This quasi-phase matching is a method of achieving phase matching by providing a structure in which the sign of the nonlinear optical coefficient is periodically inverted in the propagation direction of light propagating through a nonlinear optical crystal, thereby compensating for the difference in wave vectors between the basic light (incident light, pump light) and the generated light (second harmonic) with the wave vector of the periodic structure.

[0005] The phase matching condition is Δk = k 2ω -2k ω = (2ω / c) (n 2ω -n ω ) can be expressed as k ω and k 2ω are the wave numbers of the fundamental wave light and the second harmonic wave light, respectively, and n ω and n 2ω are the refractive indices of the nonlinear optical material for the fundamental wave light and the second harmonic light, respectively, ω and 2ω are the frequencies of the fundamental wave light and the second harmonic light, respectively, and c is the speed of light. In ordinary materials, there is wavelength dispersion in the refractive index, so n 2ω = n ω This means that the speeds of the fundamental light and the second-harmonic light do not match within the medium. As a result, the intensity of the second-harmonic light within the medium changes periodically with the propagation distance of the light. To eliminate this periodicity, a technique called quasi-phase matching is used, in which the nonlinear optical coefficient is periodically modulated. Typically, a periodically poled structure is used, in which the crystal axis of the optical crystal is periodically reversed, in order to periodically modulate the nonlinear optical coefficient. This technique is currently an essential technology for realizing highly efficient waveguide-type wavelength conversion devices.

[0006] Wavelength conversion using periodically poled LN and LT utilizing quasi-phase matching began with the introduction of pulsed light into bulk LN and LT crystals, generating high localized optical confinement and achieving highly efficient wavelength conversion. Subsequently, a method for achieving high optical confinement was devised: creating a waveguide structure in the direction of light propagation. Initially, diffusion-type waveguides, such as titanium-diffused waveguides and proton-exchanged waveguides, were the mainstream. This was due to the difficulty of fabricating LN, a material that is difficult to process. However, these diffusion-type waveguides require the diffusion of impurities to form the waveguide during fabrication, which creates a refractive index difference, posing challenges in terms of optical damage resistance and long-term reliability. Furthermore, diffusion-type waveguide structures limited the optical power that could be input into the waveguide because high-power light incident on the waveguide could damage the structure due to the photorefractive effect.

[0007] As one method for solving this problem, research and development is being conducted on ridge-type optical waveguides. In particular, research and development is being conducted on direct-bonded ridge-type optical waveguide structures, which are fabricated by directly bonding an LN or LT substrate, which serves as the core of the waveguide, to a substrate made of another material, which serves as the cladding of the waveguide, to form a waveguide structure. The ridge-type optical waveguide structure enables high-power optical input, and is expected to have a wide range of applications, such as the generation of high-intensity optical modulation signals and laser processing technology.

[0008] M. Fejer et al., "Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances", IEEE Journal of Quantum Electronics, Vol.28, No.11, 1992

[0009] However, in the direct bonding method for forming ridge-type optical waveguides, the actual optical waveguide structure deviates from the target structure due to the processing accuracy limitations of various processes, and the target characteristics cannot be achieved. This is due to the fact that LN and LT are difficult-to-process materials. In particular, when LN and LT waveguide structures are formed using dry etching techniques used in semiconductor microfabrication, the deviation between the designed structure and the actually fabricated structure becomes significant.

[0010] An example of the discrepancy between the designed structure and the actually fabricated structure will be described with reference to FIG. 1. The optical waveguide element 100 shown in FIGS. 1(a) and 1(b) has a ridge-type optical waveguide, and includes an underclad 1, a waveguide core (hereinafter simply referred to as a waveguide core) 2, and an overclad 3. As shown in FIG. 1(c), the waveguide core has a periodically poled structure such as PPLN or PPLT. As shown in FIG. 1(b), even if the designed structure of the cross-sectional shape of the waveguide core 2 is symmetrical, the cross-sectional shape of the actually fabricated waveguide core 2 may be asymmetrical. FIG. 1(b) shows the width W of the left side of the center of the waveguide core 2 in a cross section perpendicular to the propagation direction of the optical waveguide. x1 But the width on the right side is W x2 , and (3) the temperature distribution of the substrate. Asymmetry in the cross-sectional shape of the waveguide core 2 is undesirable in terms of increasing the efficiency of the wavelength conversion function. This is because the mode refractive index of the waveguide core 2 having an asymmetric cross-sectional shape is different from the mode refractive index of the propagation mode of the designed waveguide structure, making it impossible to achieve quasi-phase matching. To make the cross-sectional structure of the waveguide core 2 formed by dry etching symmetrical, it is important to make uniform (1) the concentration distribution of reaction products, (2) the concentration distribution of reaction gases, and (3) the temperature distribution of the substrate. This is because these values ​​are important parameters that determine the etching rate and the selectivity during etching.

[0011] (1) Reaction Product Concentration: When the reaction product concentration is high, the etching rate and etching selectivity decrease. This is because, during etching of LN or LT, reaction products of the LN or LT vaporized by etching and the etching gas adhere to the waveguide sidewalls, adversely affecting the etching rate. As shown in FIG. 2(a), when the reaction product concentration is high, the cross-sectional shape of the ridge-shaped waveguide core 2 and underclad 1 formed by etching becomes trapezoidal. On the other hand, as shown in FIG. 2(b), when the reaction product concentration is low, the cross-sectional shape of the ridge-shaped waveguide core 2 and underclad 1 formed by etching becomes less trapezoidal than when the reaction product concentration is high. In FIGS. 2(a) and 2(b), as an example, the cross-sectional shape of the by-product (reaction product) 6 adhering to the waveguide sidewall is shown as a triangle. As shown in FIG. 2(b), the less the trapezoidal shape of the cross-sectional shape of the ridge-shaped waveguide core 2 and underclad 1, the closer the trapezoid becomes to a square or rectangle, and the larger the base angle θ becomes (approaching 90°). On the other hand, as shown in FIG. 2A, the greater the degree of trapezoidalization of the cross-sectional shape, the smaller the base angle θ of the trapezoid.

[0012] (2) Reactant Gas Concentration Distribution When the reactant gas concentration is high, the etching rate and the selectivity during etching improve. As shown in FIG. 3( a), when the reactant gas concentration is low, the cross-sectional shape of the ridge-shaped waveguide core 2 and underclad 1 formed by etching becomes trapezoidal. That is, the base angle θ of the trapezoid gradually becomes smaller. On the other hand, as shown in FIG. 3( b), when the reactant gas concentration is high, the cross-sectional shape of the ridge-shaped waveguide core 2 and underclad 1 formed by etching becomes less trapezoidal than when the reactant gas concentration is low. That is, the base angle θ becomes larger (approaching 90°).

[0013] (3) Substrate Temperature: A high substrate temperature increases the reaction rate between the reactive gas and the LN or LT substrate, improving the etching rate and the etching selectivity. As shown in Figure 3(a), a low substrate temperature causes the cross-sectional shape of the ridge-shaped waveguide core 2 and underclad 1 formed by etching to become trapezoidal. To make the cross-sectional shape of the waveguide core 2 symmetrical, these three parameters must be completely equal throughout the entire waveguide fabrication region within the processed substrate (i.e., wafer). However, it is practically impossible to make these parameters completely equal.

[0014] 4(a) and (b) show an example of the etching rate distribution of a wafer in an etching apparatus. The etching rate is high near the center of the wafer and low near the outer edge. Therefore, as shown in FIG. 4(c), a distribution occurs in the trapezoidal base angle θ observed in the cross section of the waveguide core 2 and underclad 1 formed by etching in the wafer. As a result, the cross-sectional structure of the waveguide of the waveguide core 2 and underclad 1 formed by etching near the outer edge of the wafer becomes asymmetric, which hinders the improvement of wavelength conversion efficiency. Note that the etching rate distribution of a wafer in an etching apparatus shown in FIGS. 4(a) and (b) is just an example, and multiple areas with high etching rates may occur within the wafer.

[0015] The present disclosure has been made in consideration of such problems, and its purpose is to provide an optical waveguide element having a waveguide core whose cross section perpendicular to the propagation direction of light is symmetrical.

[0016] To achieve this object, one embodiment of the present disclosure is an optical waveguide element including an optical waveguide having an underclad and a core, and two waveguide structure control structures formed in the direction of light propagation in the optical waveguide, wherein the distance from the core to the two waveguide structure control structures is not constant, the two waveguide structure control structures are asymmetric with respect to the core, and the width of the core is symmetric.

[0017] As described above, according to one embodiment of the present disclosure, it is possible to provide an optical waveguide element having a waveguide core whose cross section perpendicular to the propagation direction of light is symmetrical.

[0018]

[0023] Figures 1A and 1B show the configuration of a ridge-type optical waveguide, where (a) is a perspective view, (b) is a cross-sectional view, and (c) is a top view of a core of the optical waveguide.

[0024] Figures 1A and 1B are diagrams for explaining the cross-sectional structure of a core during the manufacturing process of a ridge-type optical waveguide, where (a) is a diagram showing the cross-sectional structure of a core when there is a large amount of reaction product, and (b) is a diagram showing the cross-sectional structure when there is a small amount of reaction product.

[0025] Figures 1A and 1B are diagrams for explaining the cross-sectional structure of a core during the manufacturing process of a ridge-type optical waveguide, where (a) is a diagram showing the cross-sectional structure of a core when the reaction gas concentration is low or the substrate temperature is low, and (b) is a diagram showing the cross-sectional structure of a core when the reaction gas concentration is high or the substrate temperature is high.

[0026] Figure 1A shows an example of the in-plane distribution of the etching rate of a processing substrate, (b) is a graph showing the etching rate at a position in the x-direction of (a), and (c) is a graph showing the relationship between the etching rate and the base angle of the trapezoid in the cross section of the core. 1A and 1B are diagrams showing the configuration of a ridge-type optical waveguide according to an embodiment of the present disclosure, where (a) is a perspective view, (b) is a cross-sectional view, and (c) is a top view of a core of the optical waveguide. (a) is a diagram explaining a manufacturing process of a waveguide core and a waveguide width control structure in an optical waveguide according to an embodiment of the present disclosure, (b) is a plan view of a photoresist pattern in which the spacing between the waveguide core and two waveguide core structure control structures is constant, and (c) is a plan view of the photoresist pattern after correcting the spacing between the waveguide core and the two waveguide core structure control structures. (b) are diagrams explaining a manufacturing method of an optical waveguide according to an embodiment of the present disclosure.

[0019] Hereinafter, embodiments of the present disclosure will be described in detail with reference to Figures 6 and 7. The same or similar reference symbols indicate the same or similar elements, and repeated description may be omitted. The materials and numerical values ​​in the following description are examples and are not intended to limit the technical scope of the present disclosure. The embodiments described below may be implemented using other materials and numerical values ​​without departing from the spirit and scope of the present disclosure.

[0020] Hereinafter, a method for manufacturing an optical waveguide element according to an embodiment of the present disclosure will be described using, as an example, a ridge-type optical waveguide in which a core layer and an undercladding layer made of a nonlinear optical material are directly bonded. The ridge-type optical waveguide included in the element may be, for example, a PPLN optical waveguide.

[0021] The nonlinear optical material used in this embodiment may be any material that has a nonlinear optical effect and can form a periodically poled structure. Examples include lithium niobate (LiNbO), lithium tantalate (LiTaO), and potassium titanyl phosphate (KTiOPO: KTP). Depending on the nonlinear optical material having a periodically poled structure, attention must be paid to the optical waveguide processing conditions below the Curie temperature at which the periodically poled structure is not lost.

[0022] FIG. 5 shows a cross-sectional structure of an optical waveguide element according to one embodiment of the present disclosure. The optical waveguide element 500 shown in FIGS. 5( a) and 5(b) has an undercladding layer 10, a core layer 20, and an overcladding layer 30. As shown in FIG. 5(c), the core layer 20 has a periodically poled structure such as PPLN or PPLT. The overcladding layer 30 is formed after etching the undercladding layer 10 and the core layer 20 to form an optical waveguide having an undercladding 1 and a waveguide core 2, and a waveguide core structure control structure 5, and functions as an overcladding 3. As shown in FIG. 5(b), in this embodiment, in a cross section perpendicular to the propagation direction of the optical waveguide, the width W x1 But the width on the right side is W x2 is equal to, i.e., symmetrical.

[0023] The optical waveguide element 500 has an optical waveguide structure in which light is confined and propagated in the waveguide core 2 by the underclad 1 and overclad 3. The underclad layer 10 and the core layer 20 are part of a bonded substrate bonded by direct bonding, and have high optical damage resistance. Therefore, an optical waveguide formed by processing the bonded substrate can input excitation light with a very high power density into the waveguide core 2. Note that the structure may be one in which etching is stopped halfway through the core layer 20, or one in which etching is completed up to the underclad layer 10. There are no particular restrictions on the refractive index of the overclad layer 30, and it may be glass deposited by chemical vapor deposition (CVD), flame hydrolysis deposition (FHD), or sputtering, or any other method as long as it has an overclad layer according to the optical waveguide structure design. There are no particular restrictions on the refractive index of the overclad layer 30, and the overclad layer 30 may be air (i.e., the overclad 3 may be an air-clad). An optical waveguide in which the overclad 3 is an air clad is called a ridge-type optical waveguide. An optical waveguide in which the side and top surfaces of the waveguide core 2 are covered with the overclad 3 is called a buried-type optical waveguide. The embodiments of the present disclosure can be applied to either the ridge-type or buried-type.

[0024] There is also no particular limitation on the size of the waveguide core 2, and it may be a relatively large core diameter (10 μm or more) or a small core diameter (10 μm or less). It may be an optical waveguide in which the core layer 20 is thinned using a smart cut method or the like to attempt to reduce the core size, and the diameter of the waveguide core 2 may be a very small core diameter (in nm units). There is also no particular limitation on the cross-sectional shape of the waveguide core 2, and it may be a square, rectangle, trapezoid, or any other shape that can be processed. In this embodiment, the cross-sectional shape of the waveguide core 2 may be formed symmetrically.

[0025] Next, a method for manufacturing an optical waveguide element according to an embodiment of the present disclosure will be described. The method for manufacturing an optical waveguide element according to this embodiment includes direct bonding in step 1, thinning in step 2, photolithography in step 3, dry etching in step 4, and resist film removal and overcladding formation in step 5.

[0026] In step 1, a substrate made of a nonlinear optical material that will become the undercladding layer 10 is directly bonded to a substrate that will become the core layer 20. The direct bonding in step 1 uses a direct bonding technique that does not use an adhesive, which leads to improved optical loss resistance when high-intensity light is used as input light. In step 1, by selecting the thermal expansion coefficients of the substrate for the undercladding layer 10 and the substrate for the core layer 20 that are as close as possible, it is possible to suppress substrate cracking in a heat treatment process in a later process. In the present disclosure, a substrate formed by directly bonding the core layer 20 and the undercladding layer 10 is also referred to as a bonded substrate. The material of the substrate that will become the undercladding layer 10 is not limited to a nonlinear optical material, and may also be a linear optical material.

[0027] In step 2, the substrate of the nonlinear optical material that will become the core layer 20 of the bonded substrate is thinned. The thickness of the core layer 20 after thinning is 0.5 μm to 20 μm. Techniques for thinning the substrate include a grinding / polishing process and a thinning process using smart cut. In this embodiment, there are no particular limitations on the thinning method, and either thinning by grinding / polishing or thinning by smart cut may be used.

[0028] In the thinning process by grinding and polishing, a device with a controlled flatness of the grinding and polishing surface plate is used to perform grinding and polishing until the desired thickness of the waveguide core 2 can be formed. By performing polishing after the grinding and polishing process, a mirror-like polished surface (optical end face) can be obtained. Finally, the parallelism of the substrate (the difference between the maximum and minimum heights of the substrate) is measured using an optical parallelism measuring device, and the parallelism of the entire substrate can be obtained.

[0029] The thinning process using Smart Cut consists of two main steps: ion implantation and thin-film delamination. In the ion implantation step, helium or hydrogen ions are implanted into the substrate that needs to be thinned to have a second-order nonlinear optical effect. The ions are implanted from the substrate surface with a controlled acceleration voltage and dose, and are trapped at a certain depth from the surface. The ions used are preferably hydrogen or helium, which are smaller than the atoms that make up the substrate. In the substrate delamination step, the ion-implanted substrate is heat-treated to delaminate it at the damaged layer within the substrate. If the nonlinear optical material has a periodically poled structure, the heat treatment temperature in the substrate delamination step must be below the Curie temperature of the second-order nonlinear optical crystal to prevent the patterned polarization direction from being disrupted.

[0030] In step 3, photolithography is performed using a photomask corresponding to the waveguide core 2 and a structure 5 for controlling the structure of the waveguide core 2 (hereinafter simply referred to as a waveguide core structure control structure) that is arranged to eliminate asymmetry in the cross-sectional structure of the waveguide core 2. The waveguide core structure control structure 5 is arranged adjacent to the waveguide core 2 to eliminate asymmetry in the cross-sectional structure of the waveguide core 2. The type of photoresist used in photolithography does not matter, and either a positive photoresist or a negative photoresist may be used. The type of developer also does not matter, and either an alkaline developer or an organic solvent developer may be used. The photomask used in photolithography is designed as described below. The etching rate and the selectivity during etching are controlled by the width of the waveguide core 2 and the distance between the waveguide core structure control structure 5.

[0031] The design of the photomask will be explained with reference to FIG. 6 . First, a photomask is designed so that the distance W0 between the waveguide core 2 and the two waveguide core structure control structures 5 is constant, and a patterned resist is formed on the surface of the core layer 20. As a result, a photoresist pattern 52 corresponding to the waveguide core 2 and photoresist patterns 55a and 55b corresponding to the two waveguide core structure control structures 5 are formed on the surface of the core layer 20, as shown in FIGS. 6( a) and 6(b). These photoresist patterns 52, 55a, and 55b are dry-etched to form the ridge-shaped undercladding 1 and waveguide core 2, as well as the two waveguide core structure control structures 5. The two waveguide core structure control structures 5 are symmetrical with respect to the waveguide core 2. Because a single bonded substrate is processed, the layer structure of the two waveguide core structure control structures 5 is the same as the layer structure of the optical waveguide.

[0032] Next, the cross-sectional structure of the waveguide core 2 actually fabricated by dry etching is observed with an electron microscope or an optical microscope, and the left-right asymmetry of the trapezoidal structure in the cross section is quantitatively evaluated. For example, the base angle of the trapezoidal structure is used as a quantitative evaluation index. The cross-sectional structure is observed multiple times in the propagation direction to obtain information on fluctuations in the left-right asymmetry relative to the propagation direction. Furthermore, based on the obtained quantitative evaluation value of the left-right asymmetry, the portion where the width of the waveguide core 2 becomes asymmetric is determined.

[0033] Next, for the left-right asymmetric portion of the waveguide core 2, the distance between the waveguide core 2 and the waveguide core structure control structure 5 is corrected, and the photomask design is updated. Note that if the distance between the waveguide core 2 and the waveguide core structure control structure 5 is narrow, the etching rate and selectivity decrease, and if the distance between the waveguide and the structure is wide, the etching rate and selectivity increase. Therefore, the smaller the base angle of the trapezoid in the cross-sectional structure of the waveguide core 2, the greater the distance from the waveguide core 2 to the waveguide core structure control structure 5 is corrected (the gap widens). The distance from the waveguide core 2 is corrected for each of the two waveguide core structure control structures 5.

[0034] In step 4, the resist formed on the surface of the core layer 20 is dry-etched using the updated photomask. As a result, a photoresist pattern 52 corresponding to the waveguide core 2 and photoresist patterns 55c and 55d corresponding to the two waveguide core structure controlling structures 5 are formed on the surface of the core layer 20, as shown in FIG. 6(c). Dry etching is performed on these photoresist patterns 52, 55c, 55d to create the ridge-shaped underclad 1 and waveguide core 2, and the two waveguide core structure controlling structures 5. Note that the two waveguide core structure controlling structures 5 are asymmetric with respect to the waveguide core 2. For ease of explanation, the photoresist pattern 55c shown in FIG. 6(c) shows an example in which the distance from a part of the photoresist pattern 55a to the photoresist pattern 52 is narrowed (W c1 <W c2 On the other hand, the photoresist pattern 55d shown in FIG. 6C shows an example in which the distance from the photoresist pattern 52 to a part of the photoresist pattern 55b is increased (W<W d1 <W d2 ).

[0035] There are no limitations on the equipment used for dry etching, and it may be a barrel-type plasma etcher, a CCP (Capacitively Coupled Plasma) etcher, a magnetron RIE (Reactive Ion Etching), an ECR (Electron Cyclotron Resonance) plasma etcher, an ICP (Induced Coupled Plasma) etcher, or other etchers. There are no limitations on the type of gas, and it may be any of CF-based gases, oxygen, argon, etc., simple substances, mixed gases of these, or other gases. There are no limitations on the method of chucking the substrate, and it may be any of mechanical chucking, electrostatic chucking, and other chucking methods. There are also no limitations on other etching conditions, such as the power and frequency of radio frequency (RF) during dry etching.

[0036] In step 5, after the photoresist pattern is peeled off, an overcladding layer 30 is deposited to cover the waveguide core 2 and the waveguide core structure control structure 5, as necessary. This results in the optical waveguide element 500 having the configuration shown in FIG. 5( a). In this embodiment, the optical waveguide is a ridge-type optical waveguide including an undercladding 1, a waveguide core 2, and an overcladding 3. The overcladding layer 30 corresponds to the overcladding 3. There are no limitations on the method for forming the overcladding layer 30, and it may be any of chemical vapor deposition (CVD), flame hydrolysis deposition (FHD), sputtering, and other film formation methods. Note that, as described above, if the overcladding 3 is an air cladding, step 5 is omitted. After the photoresist pattern is removed, the upper surface of the waveguide core 2 may be trimmed to adjust the effective refractive index, if necessary.

[0037] In this embodiment, an optical waveguide having a periodic polarization inversion structure for the purpose of quasi-phase matching is formed. The periodic polarization inversion structure is assumed to be formed on the core substrate before step 1: direct bonding, but the periodic polarization inversion structure may be formed at other times.

[0038] As an example, we will explain the process of creating a periodically poled structure on a z-cut LN substrate. In the poled inversion of ferroelectric materials, such as LN, the poled structure is formed using an electric field application method. Generally, high voltages are applied to poled ferroelectric materials, and in particular, a very high electric field (approximately 21 kV / mm) is used for LN. Specifically, a voltage pulse generated by an arbitrary waveform generator is amplified by a high-voltage amplifier and then applied to the ferroelectric material via a high-voltage cable. Metal or liquid electrodes are in contact with the surface of the z-cut LN substrate, and the ferroelectric material is spatially modulated by an electric field corresponding to the electrode pattern. A current monitor is at least required to determine the stopping point of the poled inversion process.

[0039] FIG. 7 shows a flowchart of steps 3 to 5 described above. In step S1, the characteristics of the processing equipment are acquired. As described above, in sub-step S11, photoresist patterns 52, 55a, and 55b corresponding to the waveguide core 2 and two waveguide core structure control structures 5 arranged with a constant interval W are formed on the surface of the core layer 20 of the wafer 4. Next, in sub-step S12, the photoresist pattern formed on the wafer is etched to create the ridge-shaped underclad 1 and waveguide core 2, and the two waveguide core structure control structures 5. Note that if known data indicating the characteristics of the processing equipment is available, it is not necessary to actually create the ridge-shaped underclad 1 and waveguide core 2, and the two waveguide core structure control structures 5. That is, in step 1, the characteristics of the processing equipment may be acquired by reading out the data.

[0040] Next, in step S2, a portion of the waveguide core 2 where the width becomes asymmetric is determined. As described above, the left-right asymmetry of the trapezoidal structure in the cross section of the waveguide core 2 is quantitatively evaluated. Furthermore, based on the obtained quantitative evaluation value of the left-right asymmetry, a portion of the waveguide core 2 where the width becomes asymmetric is determined. If known data indicating the characteristics of the processing device is read in step 1, in step S2, a portion of the waveguide core 2 where the width becomes asymmetric is determined based on the position of the waveguide core 2 in the wafer and the read data.

[0041] In step S3, as described above, the distances of the waveguide core structure controlling structures 5 from the waveguide core 2 in the photoresist patterns 55a and 55b in the photomask are corrected for the portions of the waveguide core 2 where the width is asymmetric. This results in an updated photomask including photoresist patterns 55c and 55d corrected from the photoresist patterns 55a and 55b, respectively.

[0042] Next, in step S4, photoresist patterns 52, 55c and 55d are formed on the surface of the core layer 20 of the wafer 4 using the updated photomask.

[0043] Furthermore, in step S5, the wafer 4 on which the photoresist pattern 52, 55c and 55d are formed is etched to form the waveguide core 2 and the two waveguide core structure control structures 5.

[0044] In step 6, it is determined whether the width of the waveguide core 2 has become symmetrical. As described above, the left-right asymmetry of the trapezoidal structure in the cross section of the waveguide core 2 is quantitatively evaluated. If the width of the waveguide core 2 has become symmetrical, proceed to step 7. If the width of the waveguide core 2 is not symmetrical, return to step S2. After returning to step S2, it is not necessary to repeat the quantitative evaluation of the left-right asymmetry of the trapezoidal structure again; based on the quantitative evaluation value of the left-right asymmetry in step 6, it is possible to determine the portion of the waveguide core 2 where the width becomes asymmetrical.

[0045] In step 7, as described above, the photoresist pattern is stripped and then an overcladding layer 30 is deposited, if desired.

[0046] According to the method described above, it is possible to provide an optical waveguide element having a waveguide core whose cross section perpendicular to the propagation direction of light is symmetrical.

[0047] According to an embodiment of the present disclosure, it is possible to provide an optical waveguide element having a waveguide core whose cross section perpendicular to the propagation direction of light is symmetrical.

[0048] REFERENCE SIGNS LIST 1 Underclad 2 Waveguide core 3 Overclad 4 Wafer (processing substrate) 5 Structure for controlling waveguide core structure 6 Reaction product 10 Underclad layer 20 Core layer 30 Overclad layer 52, 55a, 55b, 55c, 55d Photoresist pattern 100, 500 Optical waveguide element

Claims

1. An optical waveguide element comprising: an optical waveguide having an undercladding and a core; and two waveguide core structure control structures formed in the direction of light propagation in the optical waveguide, wherein the distance from the core to the two waveguide core structure control structures is not constant, the two waveguide core structure control structures are asymmetric with respect to the core, and the width of the core is symmetrical.

2. The optical waveguide element according to claim 1, wherein the two waveguide core structure control structures have the same layer structure as the optical waveguide.

3. The optical waveguide element according to claim 1, wherein said core has a periodically poled structure.

4. The optical waveguide element according to claim 1, wherein the optical waveguide is a ridge-type waveguide.

5. The optical waveguide element according to claim 1, wherein the optical waveguide further comprises an overclad.

6. The optical waveguide element according to claim 5, wherein the overclad is an air clad.

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