Optical waveguide element and method for manufacturing the same

Abstract

This optical waveguide element comprises: a first substrate (10); and a plurality of optical waveguides (40) which are formed on the substrate (10) and which comprises a non-linear optical material. The plurality of optical waveguides (40) include: a core (2a) of one main optical waveguide; and a core (2b) of at least one sub optical waveguide adjacent along the main optical waveguide. The core (2a) of the main optical waveguide has one or a plurality of regions arranged in a light propagation direction, and each of the one or plurality of regions in the core (2a) of the main optical waveguide has a periodic polarization inversion structure having a desired pitch. The core (2b) of the sub optical waveguide has a plurality of regions arranged in the light propagation direction. The plurality of regions in the core of the sub optical waveguide have periodic polarization inversion structures of mutually different pitches. The core (2a) of the main optical waveguide is trimmed on the basis of the optical characteristics of the sub optical waveguide, thereby making the effective refractive index with respect to the light propagation direction uniform.

Description

[Technical Field]

[0001] This disclosure relates to an optical waveguide element and a method for manufacturing the same. [Background technology]

[0002] Conventionally, optical elements capable of generating and modulating coherent light in the ultraviolet, visible, near-infrared, and terahertz wavelength ranges have been applied in a wide range of fields, including wavelength conversion and modulation of optical signals in optical communication systems, optical measurement, and optical processing. Among these, optical elements that utilize nonlinear optical effects are being researched and developed in particular due to their excellent properties in wavelength conversion and electro-optic effects.

[0003] Various types of optical materials have been developed that exhibit nonlinear optical effects and electro-optical effects, among which lithium niobate (LiNbO3:LN) and lithium tantalate are particularly noteworthy. Oxide compound substrates, such as (LiTaO3:LT), exhibit high second-order nonlinear optical constants. Because they possess high electro-optic constants and are transparent across a wide wavelength range, they are being researched and developed as promising materials. Among LNs and LTs, periodically polled lithium niobate (PPLN) and periodically polled lithium tantalate (PPLT), which have a periodically reversing polarization structure formed by taking advantage of their ability to spontaneously polarize at room temperature, are widely used. These optical materials are widely used because, due to their periodically reversing polarization structure, they have high phase consistency and, as a result, high second-order nonlinear optical effects. As optical devices utilizing the high nonlinearity of PPLNs and PPLTs, second harmonic generation (SHG) and difference frequency generation (Difference Wavelength conversion elements utilizing frequency generation (DFG) and sum frequency generation (SFG) are known.

[0004] In order to realize the above-described technology, it is essential to develop a highly efficient wavelength conversion element. For enhancing the wavelength conversion efficiency, quasi-phase matching technology is important (see Non-Patent Document 1). Quasi-phase matching technology is a method of achieving quasi-phase matching with respect to the light propagation direction 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 with respect to the light propagation direction of the light propagating in the nonlinear optical crystal, and compensating the difference in the wave vectors of the fundamental light (incident light, excitation light) and the generated light (second harmonic wave) with the wave vector of the periodic structure.

[0005] The phase matching condition can be expressed as Δk = k 2ω - 2k ω = (2ω / c)(n 2ω - n ω ). Here, k ω and k 2ω are the wave numbers of the fundamental light and the second harmonic light, respectively, n ω and n 2ω are the refractive indices of the nonlinear optical material with respect to the fundamental light and the second harmonic light, respectively , ω and 2ω are the frequencies of the fundamental light and the second harmonic light, respectively, and c is the speed of light. In ordinary materials, since there is wavelength dispersion in the refractive index, n 2ω ≠ n ω . That is, the speeds of the fundamental light and the second harmonic light do not match in the medium. Therefore, the intensity of the second harmonic light in the medium changes periodically with respect to the light propagation distance. To eliminate this periodicity, the method of periodically modulating the nonlinear optical coefficient is called quasi-phase matching. Usually, a periodically poled inversion structure that periodically inverts the crystal axis of the optical crystal is used to periodically modulate the nonlinear optical coefficient. This method has become an essential technology in realizing a highly efficient waveguide-type wavelength conversion element.

[0006] Until now, diffuse optical waveguides such as titanium diffuse optical waveguides and proton exchange optical waveguides have been the mainstream for periodic polarization reversal LN and LT waveguide structures that utilize pseudo-phase matching. This is because LN is a difficult material to process, making it difficult to fabricate anything other than diffuse optical waveguides. However, these diffuse optical waveguides have problems in terms of resistance to optical damage and long-term reliability because impurities are diffused during fabrication to form the optical waveguide, creating a refractive index difference. In addition, in diffuse optical waveguide structures, if high-power light is incident on the optical waveguide, the structure is damaged due to the photorefractive effect, thus limiting the optical power that can be input to the optical waveguide.

[0007] One approach to address this challenge is the research and development of ridge-type optical waveguides. In particular, the direct bonding method for forming ridge-type optical waveguides is expected to enable high-power optical input, leading to broader applications in areas such as the generation of high-intensity optical modulation signals and laser processing technology. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] M. Fejer et al., "Quasi-Phase-Matched Second Harmonic Generation : Tuning and Tolerances", IEEE Journal of Quantum Electronics, Vol.28, No.11, 1992 [Overview of the Initiative]

[0009] However, in the ridge-type optical waveguide formation method using direct bonding, the processing accuracy limits of various processes resulted in the fabricated optical waveguide structure deviating from the target structure, leading to the problem of not being able to obtain the target characteristics. As a trimming technique to bridge the gap between the target waveguide structure and the fabricated structure caused by the processing accuracy limits, there is a local etching technique that allows for localized processing of the waveguide. This local etching technique makes it possible to compensate for the distribution of the effective refractive index in the waveguide propagation direction caused by the processing accuracy limits. In order to determine the trimming position using local etching, it was necessary to measure both the width and thickness of the waveguide in advance, and this process required the use of multiple inspection devices. Therefore, the fabrication process itself was complicated. Furthermore, direct optical evaluation of phase matching was not performed, making it difficult to accurately determine the trimming position by local etching.

[0010] This disclosure has been made in view of the above problems and aims to provide an optical waveguide element having a waveguide structure that enables accurate determination of the trimming position using high-precision local processing technology, and a method for manufacturing the same.

[0011] To achieve this objective, an optical waveguide element according to one embodiment of the present disclosure comprises a first substrate and a plurality of optical waveguides formed on the substrate, wherein the plurality of optical waveguides includes a core of one principal optical waveguide and a core of at least one adjacent sub-optical waveguide along the principal optical waveguide, the core of the principal optical waveguide having one or more regions arranged in the direction of light propagation, each of the one or more regions in the core of the principal optical waveguide having a periodic polarization reversal structure having a desired pitch, the core of the sub-optical waveguide having a plurality of regions arranged in the direction of light propagation, the plurality of regions in the core of the sub-optical waveguide having a periodic polarization reversal structure having different pitches from each other, and the effective refractive index with respect to the direction of light propagation is made uniform in the direction of light propagation by trimming the core of the principal optical waveguide based on the optical properties of the sub-optical waveguides.

[0012] Furthermore, a method for manufacturing an optical waveguide element according to another embodiment of the present disclosure involves processing a second substrate, which is a core layer bonded to a first substrate that will be an undercladding, to form a plurality of ridge-type optical waveguides, wherein the plurality of optical waveguides include a core of one main optical waveguide and a core of at least one sub-optical waveguide adjacent to the main optical waveguide, the core of the main optical waveguide having one or more regions arranged in the direction of light propagation, each of the one or more regions in the core of the main optical waveguide having a periodic polarization reversal structure having a desired pitch, the core of the sub-optical waveguide having a plurality of regions arranged in the direction of light propagation, and the plurality of regions in the core of the sub-optical waveguide having a periodic polarization reversal structure having different pitches from each other, and also includes measuring the optical properties of the main optical waveguide and the optical properties of the sub-optical waveguides, and reprocessing the core of the main optical waveguide based on the optical properties of the sub-optical waveguides to make the effective refractive index of the main optical waveguide with respect to the direction of light propagation uniform. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 shows the cross-sectional structure of a ridge-type optical waveguide. [Figure 2] This figure illustrates a method for manufacturing an optical waveguide element according to one embodiment of the present disclosure. [Figure 3] This figure illustrates a substrate of a nonlinear optical material that forms the core layer of the main optical waveguide and the sub-waveguide in an optical waveguide element according to one embodiment of the present disclosure. [Figure 4] This figure illustrates the structure of the main optical waveguide and the sub-waveguide in an optical waveguide element according to one embodiment of the present disclosure. [Figure 5] This figure illustrates the SHG spectra of the main optical waveguide and the secondary optical waveguide in an optical waveguide element according to one embodiment of the present disclosure, where (a) and (c) are figures showing the SHG spectra of the secondary optical waveguide, and (b) is a figure showing the SHG spectrum of the main optical waveguide. [Figure 6] This figure illustrates a main optical waveguide and a sub-waveguide having an effective refractive index distribution in an optical waveguide element of one embodiment of the present disclosure, where (a) is a figure showing a core having a width distribution, and (b) is a figure showing the effective refractive index of the core having a width distribution. [Figure 7] It is a diagram for explaining the SHG spectra of the main optical waveguide and the sub - optical waveguide having the effective refractive index distribution in the optical waveguide device according to an embodiment of the present disclosure. (a) and (c) are diagrams showing the SHG spectra of the sub - optical waveguide, and (b) is a diagram showing the SHG spectrum of the main optical waveguide. [Figure 8] It is a flowchart showing a method for manufacturing an optical waveguide device according to another embodiment of the present disclosure.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and repeated descriptions 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 gist of the present disclosure.

[0015] Hereinafter, taking a ridge - type optical waveguide in which a core layer of a nonlinear optical material and an under - clad layer are joined by direct bonding as an example, a method for manufacturing an optical waveguide device according to an embodiment of the present disclosure will be described. The ridge - type optical waveguide included in the device can be, for example, a PPLN optical waveguide.

[0016] (Selection of Nonlinear Optical Material) The nonlinear optical material used in this embodiment may be any material that has a nonlinear optical effect and can form a periodically poled inversion structure. Examples include lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃), potassium titanyl phosphate (KTiOPO₄: KTP), etc. Depending on the nonlinear optical material having a periodically poled inversion structure, attention should be paid to the optical waveguide processing conditions below the Curie temperature at which the periodically poled inversion is not lost.

[0017] (Structure of Optical Waveguide) Figure 1 shows a cross-sectional structure of an optical waveguide. As shown in Figure 1, the structure consists of an underclad layer 1, an overclad layer 3, and a core layer 2, with light propagating inside the core layer 2. The underclad layer 1 and the core layer 2 are joined by direct bonding, which provides high resistance to optical damage, making it possible to input excitation light with a very high power density into the optical waveguide. Furthermore, there are no particular restrictions on the refractive index of the overclad layer 3, and chemical vapor deposition (CVA) is used. (on: CVD) and flame hydrolysis deposition (Flame Hydrolysis Deposition) The overcladding layer may be glass deposited by sputtering (FHD), or any other material, as long as it has an overcladding layer suitable for the optical waveguide structure design. There are no particular restrictions on the refractive index of the overcladding layer 3, and the overcladding layer 3 may be air (air cladding). An optical waveguide in which the overcladding layer 3 is air cladding is called a ridge-type optical waveguide. An optical waveguide in which the sides and top of the core layer 2 are covered with the overcladding layer 3 is called an embedded-type optical waveguide. Embodiments of this disclosure can be applied to either ridge-type or embedded-type waveguides.

[0018] Furthermore, there are no particular restrictions on core size; it can be a relatively large core diameter (10 μm or more) for multimode light propagation, or a small core diameter (10 μm or less) for single-mode light propagation. An optical waveguide in which the core layer 2 has been thinned using a smart-cut method or similar to achieve a smaller core diameter may also be very small (in nanometers). There are also no particular restrictions on core shape; it can be a square, rectangle, trapezoid, or any other shape that can be processed.

[0019] (Optical waveguide formation method) Next, with reference to Figure 2, a method for manufacturing an optical waveguide element according to one embodiment of the present disclosure will be described.

[0020] As shown in Figure 2, the manufacturing method of the optical waveguide element according to this embodiment includes direct bonding in step 1, thinning in step 2, optical waveguide formation and chipping in step 3, optical property measurement in step 4, reworking in step 5, repetition of steps 4 and 5 in step 6, and overcladding in step 7.

[0021] In step 1, a substrate 20 of a nonlinear optical material that will become the core layer 2 is fabricated, and the substrate 20 is directly bonded to a substrate 10 of a nonlinear optical material that will become the underclad layer 1. Direct bonding in step 1, by using a direct bonding technique that does not use adhesives, leads to improved light loss resistance when high-intensity light is used as input light. In step 1, by selecting substrates 10 of the underclad layer 1 and 20 of the core layer 2 with thermal expansion coefficients that are as close as possible, it is possible to suppress substrate cracking in the heat treatment process in subsequent processes. In this disclosure, the substrate formed by directly bonding substrate 10 and substrate 20 is also called a bonded substrate. Note that the material of the substrate 10 that will become the underclad layer 1 is not limited to a nonlinear optical material, but may also be a linear optical material.

[0022] In step 2, the substrate 20 of the nonlinear optical material that will become the core layer 2 of the bonded substrate is thinned. There are no particular restrictions on the thinning method, and grinding and polishing processes and smart cut methods are among the candidates.

[0023] In step 3, the cores 2a and 2b of the optical waveguide 40 are formed by processing the core layer 2, which is a thin film formed from the substrate 20. There are no particular restrictions on the optical waveguide formation method, and candidates include a dry etching process and cutting the optical waveguide 40 from the core layer 2 using a dicing saw. The processing method can be selected according to the required core shape. In this embodiment, the optical waveguide 40 includes one main optical waveguide (core 2a) and two secondary waveguides (core 2b) along the main optical waveguide. The number of secondary waveguides may be one or three or more. In step 3, the overcladding layer 3 is not formed. If an overcladding layer 3 of the optical waveguide 40 is required, i.e., when manufacturing an embedded optical waveguide, it is formed in a later step 7.

[0024] In step 3, the bonded substrate having the cores 2a and 2b of the fabricated optical waveguide 40 is chipped to produce an optical waveguide element 50. A dicing saw is one possible method for chipping, but there are no particular restrictions on the processing method. Furthermore, by optically polishing the end face or coating it with an anti-reflective film after chipping, it is possible to reduce optical loss when light enters or exits the end face of the optical waveguide element.

[0025] In step 4, the optical properties of the fabricated cores 2a and 2b are measured. It is determined whether core 2a has the target optical properties (target properties). If it is determined that core 2a has the target properties based on the measured optical properties, the process proceeds to step 7 as necessary. If it is determined that core 2a does not have the target properties, the process proceeds to reprocessing (trimming) in step 5.

[0026] In step 5, trimming is performed on the core 2a of the main optical waveguide. In this step, at least one of the width W and thickness H of the core 2a of the main optical waveguide can be corrected. As a method of correction, local structural modification using a local etching apparatus is a candidate.

[0027] In step 6, steps 4 and 5 are repeated until the main optical waveguide (core 2a) has a structure that exhibits the target characteristics. Through this process, the structure of the core 2a of the main optical waveguide that is finally obtained will have the target characteristics.

[0028] In step 7, the overcladding layer 3 is deposited as needed. In this embodiment, the optical waveguide 40 is a ridge-type optical waveguide including the undercladding layer 1 (substrate 10), the core layer 2 (cores 2a and 2b), and the overcladding layer 3. As described above, if the overcladding layer 3 is an air cladding layer, step 7 is omitted.

[0029] (Direct bonding method (process 1)) In step 1, a substrate 20 made of nonlinear optical material is fabricated to form the core layer 2, which will be bonded to a substrate 10 made of nonlinear optical material that will form the undercladding layer 1.

[0030] Figure 3 illustrates a substrate 20 of a nonlinear optical material that will form the core 2a of the main optical waveguide and the core 2b of the secondary optical waveguide. In step 1, a substrate 20 is fabricated that includes a region 2A to be fabricated as the core 2a of the main optical waveguide in a later step 3, and a region 2B adjacent to region 2A to be fabricated as the core 2b of the secondary optical waveguide. The length of the rectangular regions 2A and 2B in the X direction is equal to the length L of the main optical waveguide and secondary optical waveguide to be fabricated, and in this embodiment, L = 50 mm.

[0031] In region 2A, a periodic polarization reversal structure with a uniform or uniform pitch Λ is formed over a length L in the direction of light propagation (X direction). In region 2B, n adjacent regions are formed, arranged in the direction of light propagation (X direction). Each of the n regions has n different pitches Λ (where n is 2 or more). n A periodic polarization reversal structure having is formed. In this embodiment, n=5 This case is illustrated. The periodic polarization reversal structure formed in region 2A has a pitch Λ3, The periodic polarization reversal structures formed in the five regions of region 2B each have pitches Λ1 to Λ5. The magnitudes of the pitches of the periodic polarization reversal structures illustrated in Figure 3 are Λ4 > Λ2 > Λ3 > Λ1 > Λ5. The lengths in the X direction of the five regions in region 2B are equal at L / n (=10 mm). Such periodic polarization reversal structures in regions 2A and 2B can be fabricated, for example, by an electric field application method. For example, the pitch Λ3 can be set to a polarization reversal period corresponding to a wavelength of 775 nm. The remaining four pitches, Λ1, Λ2, Λ4, and Λ5, can be set as polarization reversal periods corresponding to wavelengths of 775nm±5nm and 775nm±10nm. Instead of polarization reversal periods corresponding to wavelengths of 775nm±5nm and 775nm±10nm, polarization reversal periods corresponding to wavelengths of 775nm±10nm and 775nm±20nm may be used. By setting polarization reversal periods corresponding to five linearly changing wavelengths in the five regions of region 2B, the measurement of optical properties described later becomes easier. In addition, a principal optical waveguide can be fabricated in which multiple pseudo-phase matching peaks can be obtained. In this case, one or more regions are provided in region 2A of length L, arranged in the direction of light propagation (X direction), and a periodic polarization reversal structure having a desired pitch is formed in each of the multiple regions.

[0032] Next, a bonded substrate is fabricated by joining substrate 10 and substrate 20 without using adhesive. Direct bonding technology is a technique for firmly joining substrates together.

[0033] (Thinning method (process 2)) In step 2, the substrate 20 of the bonded substrate is thinned according to the design values ​​for the thickness of the cores 2a and 2b of the target optical waveguide 40 to form the core layer 2. The thickness (Z direction) of the substrate 20 after thinning is 0.5 μm to 20 μm. Techniques for thinning the substrate 20 include grinding and polishing processes and thinning processes using smart cutting. In this embodiment, there are no particular restrictions on the thinning method, and either thinning by grinding and polishing or thinning by smart cutting may be used.

[0034] In the thin-film formation process using grinding and polishing, a device with controlled flatness of the grinding and polishing platen is used to perform grinding and polishing until an optical waveguide exists at a desired depth. After the grinding and polishing process is completed, a polishing process is performed to obtain a mirror-finish polished surface (optical end face). Finally, the parallelism of the substrate (the difference between the maximum and minimum heights of the substrate) is measured using an optical parallelism measuring instrument to obtain the overall parallelism of the substrate.

[0035] The thin-film deposition process using smartcut mainly consists of two steps: an ion implantation step and a thin-film delamination step. In the ion implantation step, helium or hydrogen ions are implanted into a substrate 20 that needs to be thinned to have a second-order nonlinear optical effect. The ions are implanted from the substrate surface under a controlled acceleration voltage and controlled dose, and are trapped to a certain depth from the surface. It is desirable to use ions smaller than the atoms that make up the substrate, such as hydrogen or helium. In the substrate delamination step, the substrate in which the ions were implanted is subjected to heat treatment, which delaminates the substrate along the damaged layer within the substrate. If the nonlinear optical material has a periodic polarization reversal structure, the heat treatment temperature in the substrate delamination step is kept below the Curie temperature of the second-order nonlinear optical crystal in order not to disrupt the patterned polarization direction.

[0036] The core layer thinned using the above method exhibits an in-plane film thickness distribution depending on the processing accuracy. In particular, the thinning process by grinding and polishing, which allows for the fabrication of a ridge-type optical waveguide 40 with a relatively large core layer 2 (cores 2a and 2b) exhibiting high optical damage resistance, has a relatively large processing limit in suppressing the film thickness distribution. Due to the film thickness distribution resulting from these processing accuracy limits, it is difficult to ultimately fabricate an optical waveguide with the desired structure.

[0037] (Method for forming the core layer of an optical waveguide (Step 3)) Next, the thinned substrate 20 is processed to form the core 2a of the main optical waveguide and the core 2b of the secondary optical waveguide. Figure 4 shows the core 2a formed by removing a part of region 2A, and the core 2b formed by removing a part of region 2B. The pitch D (Y direction) between the formed core 2a of the main optical waveguide and the core 2b of the secondary optical waveguide is from 5 μm to 100 μm. The smaller the pitch D, that is, the closer the cores 2a and 2b are, the greater the height and width distribution of the cores 2a and 2b will be at the same position in the X direction. The pitch D may be wider than 100 μm as long as the height and width distribution can occur.

[0038] Methods for forming the core 2a of the ridge-type main optical waveguide and the core 2b of the secondary optical waveguide include methods using a dry etching process and methods using machining, such as a dicing saw. In this embodiment, there are no particular restrictions on the method for forming the cores 2a and 2b of the ridge-type optical waveguide 40; it may be a dry etching method, a dicing saw method, or any other formation method.

[0039] In methods using dry etching, variations in the width of the optical waveguide may occur due to errors in fabricating the optical waveguide pattern during the photolithography process or errors in setting the etching rate during the dry etching process. In methods using a dicing saw, variations in the width of the optical waveguide may occur due to misalignment of the processed area.

[0040] (Optical property measurement (process 4, process 6)) Light is incident on the core 2a of the main optical waveguide and the core 2b of the secondary optical waveguide formed in step 3, and the optical properties of the light output from cores 2a and 2b, which have a periodic polarization reversal structure, are measured. For the main optical waveguide, the measured optical properties are evaluated to determine whether they meet the target characteristics. Figures 5(a) and (c) show the optical properties of light from the two secondary optical waveguides along the main optical waveguide when there is no distribution in film thickness and core width. Figure 5(b) shows the optical properties of light from the main optical waveguide when there is no distribution in film thickness and core width. The optical properties shown in Figures 5(a) to (c) indicate the wavelength and intensity (SHG intensity) of the second harmonic light generated by the periodic polarization reversal structure. The wavelengths of the five peaks of SHG intensity shown in Figures 5(a) and (c) correspond to Λ4, Λ2, Λ3, Λ1, and Λ5 of core 2b, in order from the short wavelength side. The wavelength of one of the SHG intensity peaks shown in Figure 5(b) corresponds to Λ3 in core 2a. When fabricating a principal optical waveguide that yields multiple pseudo-phase-matching peaks, the SHG intensity peaks in Figure 5(b) will be measured at multiple desired wavelengths.

[0041] By setting a uniform polarization reversal period in the main optical waveguide, it is possible to measure the second harmonic light of a desired wavelength, as shown in Figure 5(b). On the other hand, as described above, by setting the polarization reversal periods corresponding to five linearly changing wavelengths in the five regions of the secondary optical waveguide, it is possible to measure the second harmonic light of five wavelengths in a single measurement, as shown in Figures 5(a) and (c). The measurement resolution (L / n) of the formed secondary waveguide is 10 mm (L=50 mm, n=5). The resolution can be changed by changing the value of n. By setting n to 3 or more, a periodic polarization reversal structure can be constructed in n regions such that the peaks of the optical characteristics are aligned at equal intervals. By setting the wavelength interval to more than 5 times the full width at half maximum of the SHG intensity peak, high resolution can be obtained in the measurement of the sub-waveguide core. The period with pitch Λ1 shown in Figure 4(a) The optical properties of second-harmonic light in the polarization inversion structure are shown as the second optical property from the right in Figure 5(a). The peak wavelength of the optical property corresponding to pitch Λ1 is 776 nm, and SHG strong The full width at half maximum (FWHM) of the peak is approximately 0.2 nm. High resolution can be obtained if the interval between the wavelengths of adjacent optical characteristic peaks (the second optical characteristic from the right corresponding to pitch Λ3 and the first optical characteristic from the right corresponding to pitch Λ5) is 1 nm or more (5 times the FWHM of the peak, which is 0.2 nm) or more. Therefore, in Figure 3, the n regions in region 2B are three or more regions, and it is preferable that the polarization reversal periodic structure of the three or more regions is configured such that three or more peaks of the optical characteristics are arranged at equal intervals. Furthermore, it is preferable that the equal interval between the three or more peaks is 5 times or more the FWHM of the peaks.

[0042] Figure 6(a) shows core 2a formed by removing a portion of region 2A and core 2b formed by removing a portion of region 2B in step 2. Figure 6(a) differs from Figure 4 in that a distribution of core width occurs in the second region from the left of core 2a of the main optical waveguide and core 2b of the secondary optical waveguide. Figure 6(b) shows the effective refractive index of a core with a width distribution. As described above, when the pitch D between core 2a of the main optical waveguide and core 2b of the secondary optical waveguide is small, that is, when core 2a and core 2b are close together, a distribution of height and width occurs in the same position in the X direction (second region from the left) for core 2a and core 2b. Therefore, it can be estimated that the effective refractive index of core 2a of the main optical waveguide and core 2b of the secondary optical waveguide is generally as shown in Figure 6(b).

[0043] As shown in Figure 6(b), a difference arises between the effective refractive index of the regions in cores 2a and 2b where a width distribution occurs and the effective refractive index of the regions where no width distribution occurs. As a result, a shift occurs in the SHG intensity spectrum caused by the periodic polarization inversion structure in the regions where a width distribution occurs.

[0044] Figures 7(a) and (c) show the optical properties of light from two secondary optical waveguides along the main optical waveguide when there is a distribution in film thickness and core width. Figure 7(b) shows the optical properties of light from the main optical waveguide when there is a distribution in film thickness and core width (dashed line), and the optical properties of light from the main optical waveguide when the distribution is removed by trimming (solid line).

[0045] As can be seen from the comparison with Figures 5(a) and (c), the second high occurs in the periodic polarization reversal structure with pitch Λ2 set in the second region from the left in Figures 6(a) and (c). The optical properties of harmonic light are measured as the second disturbed peak from the right in Figures 7(a) and (c). A disturbed peak is, for example, a peak measured at a wavelength shifted to a shorter or longer wavelength from a given wavelength, or a peak with low intensity measured at a given wavelength.

[0046] From this, it can be inferred that the height and width distributions occurring at the X-direction position of core 2b corresponding to the disturbance in the optical properties of the measured sub-waveguide also occur at the same X-direction position of core 2a of the main waveguide. In particular, as shown in Figures 7(a) and (c), if disturbances occur in the peak of the SHG intensity in the corresponding regions of the two sub-waveguides on either side of the main waveguide, it is highly likely that a distribution of height and width occurs in the portion of the main waveguide corresponding to that region. Furthermore, by measuring the amount of shift in the peak of the SHG intensity in the sub-waveguide and whether the shift is on the longer wavelength side or the shorter wavelength side, it is possible to determine in which region of the sub-waveguide a distribution in the width of the core occurs, and similarly, in which region of the main waveguide a distribution in the width of the core occurs. The optical properties of the second harmonic light generated in the periodic polarization reversal structure with pitch Λ3 set in the second region from the left in Figure 6(b) are shown in Figure 7(b). As shown by the dashed line, the SHG intensity peak is measured at a wavelength shifted from the desired wavelength.

[0047] Furthermore, the error in the effective refractive index can be calculated in reverse based on the optical properties of the secondary optical waveguide. In addition, based on the calculated error in the effective refractive index, it is possible to determine the amount by which at least one of the width and height of the core 2b in the secondary optical waveguide should be reprocessed (trimming amount).

[0048] Furthermore, for the core 2a of the main optical waveguide that was reworked in step 5, light is incident on core 2a, and the optical properties of the light output from core 2a and core 2b, which have a periodic polarization reversal structure, are measured to evaluate the optical properties and determine whether they meet the target characteristics. For core 2b of the secondary optical waveguide, which is not subject to rework, it is not necessary to measure the optical properties again.

[0049] (Reprocessing (trimming) (Steps 5 and 6)) In step 5, the core 2a of the main waveguide is reworked based on the measurement results of the optical properties of the secondary waveguide in step 4. The X-direction position of the core 2a of the main waveguide to be reworked corresponds to the X-direction position of the core 2b corresponding to the disturbance in the optical properties of the secondary waveguide. More specifically, the portion of the main waveguide that corresponds to the location of the region in which a shift occurred in the measured SHG intensity peak among the n regions in the secondary waveguide is targeted for rework. Rework may be performed only if disturbance occurs in the SHG intensity peak in the regions at corresponding positions in the two secondary waveguides on either side of the main waveguide. For example, in each rework, the width or height of the core 2a is trimmed (removed) by a few nanometers to 10 or so nanometers. After rework, the process proceeds to step 6, and steps 4 and 5 are repeated until the optical properties of the main waveguide meet the target characteristics.

[0050] In this embodiment, a local etching method is used as the trimming method for the core 2a of the main optical waveguide, which allows for localized processing of only the targeted area with high precision. However, any method that can process the structure of the optical waveguide core with high precision may be used. In other words, any high-precision local processing technique with high spatial resolution and high etching control technology can be used.

[0051] (Overcladding (Step 7)) In step 4, after it is determined that the optical characteristics of the main optical waveguide meet the target characteristics, in step 7, an overcladding layer 3 is formed as needed.

[0052] Figure 8 is a flowchart of the method for manufacturing an optical waveguide element according to the embodiment of the present disclosure described above. Steps S1 to S7 correspond to steps 1 to 7.

[0053] In step S1, which corresponds to process 1, the substrate 10 and the substrate 20 are directly joined. Step S1 includes step S11 of manufacturing a substrate 20 having region 2A and region 2B adjacent to region 2A, and step S12 of directly joining the manufactured substrate 20 and the substrate 10 which will become the undercladding layer 1 to form a bonded substrate.

[0054] In step S2, which corresponds to process 2, the substrate 20 of the bonded substrate formed in S1 is thinned to create the core layer 2.

[0055] In step S3, which corresponds to process 3, an optical waveguide is formed and chipped. Step S3 includes step S31, which processes the core layer 2 of the bonded substrate to form the core 2a of the main optical waveguide and the core 2b of the secondary optical waveguide, and step S32, which chips the bonded substrate to generate the optical waveguide element 50.

[0056] In step S4, which corresponds to process 4, the optical properties of the optical waveguide are measured. Step S4 includes step S41, which measures the optical properties of the core 2b of the secondary optical waveguide; step S42, which measures the optical properties of the core 2a of the main optical waveguide; and step S43, which evaluates whether the core 2a of the main optical waveguide has the target properties. If the core 2a of the main optical waveguide has the target properties, the process proceeds to step S7. If the core 2a of the main optical waveguide does not have the target properties, the process proceeds to step S5.

[0057] In step S5, which corresponds to process 5, the core 2a of the principal optical waveguide is reworked. Step S5 includes step S51, which determines the portion of the core 2a of the principal optical waveguide to be reworked based on the optical properties of the core 2b of the secondary optical waveguide. In step S51, the region of the core 2b corresponding to the disturbance in the optical properties of the secondary optical waveguide is identified, and the portion of the core 2a of the principal optical waveguide corresponding to that region is determined to be the portion to be reworked. Step S5 further includes step S52, which trims the portion of the core 2a of the principal optical waveguide determined in S51. After step S52, the process returns to step S42, and the optical properties of the reworked core 2a of the principal optical waveguide are measured. As a result of the rework in process 5, there is a difference in at least one of the width and height between the shape of the reworked portion of the core 2a of the principal optical waveguide and the shape of the corresponding portion of the core 2b of the secondary optical waveguide. This difference corresponds to the amount of trimming.

[0058] If the core 2a of the main optical waveguide has the target characteristics in step S43, an overcladding layer 3 is formed in step 7, which corresponds to step 7, if necessary.

[0059] As described above, this disclosure makes it possible to provide an optical waveguide element having a waveguide structure that enables accurate determination of the trimming position using etching, and a method for manufacturing the same. [Industrial applicability]

[0060] The optical waveguide element and its manufacturing method disclosed herein are useful for realizing a highly efficient waveguide-type wavelength conversion element having a periodic polarization reversal structure. [Explanation of Symbols]

[0061] 1. Underclad layer 2 Core Layers 2a, 2b core 3. Overcladding layer 10, 20 circuit boards 40 Optical waveguide 50 chips

Claims

1. An optical waveguide element, circuit board and A plurality of optical waveguides of a nonlinear optical material formed on the substrate, Equipped with, The plurality of optical waveguides include the core of one main optical waveguide and the core of at least one secondary optical waveguide adjacent to the main optical waveguide. The core of the principal optical waveguide has one or more regions arranged in the direction of light propagation, Each of the one or more regions in the core of the principal optical waveguide has a periodic polarization reversal structure having a desired pitch. The core of the aforementioned secondary optical waveguide has a plurality of regions arranged in the direction of light propagation, The multiple regions in the core of the sub-optical waveguide have a periodic polarization reversal structure having different pitches from each other. The core of the principal optical waveguide is trimmed based on the optical properties of the secondary optical waveguide so that either its width or height differs from the width or height of the core of the secondary optical waveguide, and the effective refractive index in the direction of light propagation is uniform. Optical waveguide element.

2. The aforementioned sub-optical waveguides are formed one on each side of the main optical waveguide, or The aforementioned sub-optical waveguide is formed on one side of the main optical waveguide. The optical waveguide element according to claim 1.

3. The plurality of regions in the core of the sub-optical waveguide are three or more regions. The optical waveguide element according to claim 1, wherein the periodic polarization reversal structure having different pitches in the three or more regions is configured such that the three or more peaks of the optical properties of the sub-optical waveguide are aligned at equal intervals.

4. The optical waveguide element according to claim 3, wherein the equal spacing between the three or more peaks is five times or more the full width at half maximum of the peaks.

5. A method for manufacturing an optical waveguide element, The process involves processing a second substrate, which is a core layer bonded to a first substrate that serves as an undercladding, to form a ridge-type optical waveguide. The plurality of optical waveguides include the core of one main optical waveguide and the core of at least one secondary optical waveguide adjacent to the main optical waveguide. The core of the principal optical waveguide has one or more regions arranged in the direction of light propagation, Each of the one or more regions in the core of the principal optical waveguide has a periodic polarization reversal structure having a desired pitch. The core of the aforementioned secondary optical waveguide has a plurality of regions arranged in the direction of light propagation, The multiple regions in the core of the sub-optical waveguide have a periodic polarization reversal structure having different pitches from each other. That thing, To measure the optical characteristics of the main optical waveguide and the optical characteristics of the secondary optical waveguide, Based on the optical properties of the secondary optical waveguide, the core of the main optical waveguide is reprocessed to make the effective refractive index of the main optical waveguide uniform in the direction of light propagation. A manufacturing method that includes this.

6. The method further includes evaluating whether the main optical waveguide has the target characteristics. If the principal optical waveguide is evaluated as not having the target characteristics, the core of the principal optical waveguide is reprocessed based on the optical characteristics of the secondary optical waveguide. The manufacturing method according to claim 5.

7. Reworking the core of the main optical waveguide based on the optical characteristics of the secondary optical waveguide is This includes reworking the core of the portion of the principal optical waveguide adjacent to one of the multiple regions where disturbances in the intensity of second-harmonic light generated by the periodic polarization reversal structure having different pitches are measured. The manufacturing method according to claim 6.

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