Wavelength conversion element and method for manufacturing wavelength conversion element

Abstract

This wavelength conversion element has an optical waveguide including an under cladding layer and a core layer provided above the under cladding layer. The core layer has formed thereon a width adjustment region in which the width of the optical waveguide having length in a direction orthogonal to the waveguide direction is adjusted. A width that has been adjusted by the width-adjustment region corresponds to the product of the width of the width adjustment region and the thickness of the core layer at a position corresponding to said width in the waveguide direction.

Description

Wavelength conversion element, method for manufacturing a wavelength conversion element

[0001] This disclosure relates to a wavelength conversion element and a method for manufacturing a wavelength conversion element, and more particularly to a wavelength conversion element that forms an optical waveguide using etching, and a method for manufacturing a wavelength conversion element.

[0002] Optical elements capable of generating and modulating coherent light in the ultraviolet to terahertz wavelength range are being applied in fields such as 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 superior characteristics in wavelength conversion and electro-optic effects.

[0003] Oxide compound substrates, such as lithium niobate (LiNbO3: LN) and lithium tantalate (LiTaO3: LT), are promising materials being researched and developed as optical materials possessing nonlinear and electro-optic effects. They exhibit high second-order nonlinear optical constants, high electro-optic constants, and transparency across a wide wavelength range. Among LN and LT, 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 their periodic polarization reversal structure provides high phase consistency, resulting in high second-order nonlinear optical effects. As optical devices utilizing the high nonlinearity of PPLNs and PPLTs, wavelength conversion elements using second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG) are known.

[0004] As optical waveguide structures using PPLN, diffusion-type optical waveguides called titanium-diffused optical waveguides and proton-exchanged optical wavewaves have been the mainstream. This is because LN is a difficult-to-process material, which makes fabrication of optical waveguides other than diffusion-type ones difficult. However, such diffusion-type optical waveguides have problems from the viewpoints of optical damage resistance and long-term reliability, because impurities are diffused to form the optical waveguide during fabrication, which causes a difference in refractive index. In a diffusion-type optical waveguide structure, when high-power light is incident on the optical waveguide, the crystal structure is damaged due to the photorefractive effect, so there is a limit on the optical power that can be input to the optical waveguide.

[0005] As one of the methods for solving the above problems, research and development have been conducted on ridge-type optical waveguides (see Non-Patent Document 1). In particular, when a ridge-type optical waveguide formation method based on direct bonding is used, high-power optical input becomes possible, and it is expected that applications to generation of high-intensity optical modulation signals, laser processing technology, and the like will expand. However, even in the ridge-type optical waveguide formation method based on direct bonding, the actually fabricated optical waveguide structure may differ from the target structure due to the processing accuracy limits of various processes, and desired characteristics such as output efficiency may not be obtained. In order to solve this problem, it is disclosed that the thickness or film thickness of an actually fabricated ridge waveguide is measured, a digital twin of the ridge waveguide is created on a simulator, and characteristics are confirmed (Patent Document 1). Patent Document 1 discloses that when the digital twin cannot obtain desired characteristics, the ridge waveguide is processed again.

[0006] International Publication No. 2022 / 244274

[0007] S. Kurimura, Y. Kato, M. Maruyama, Y. Usui, and H. Nakajima,“ Quasi-Phase-Matched adhered ridge waveguide in LiNbO3,” Appl. Phys. Lett. 89(19), 191123(2006)

[0008] However, to improve the conversion efficiency of second harmonic generation in optical waveguides, it is required to keep the effective refractive index constant along the waveguide direction. For this reason, even if a digital twin is created using data on the width and height of the core layer, the refractive index of the core, and the refractive index of the cladding for the cross-section of the core layer, as disclosed in Patent Document 1, and the optical properties are predicted, it may not lead to an improvement in the efficiency of second harmonic generation.

[0009] This disclosure has been made in view of this point, and relates to a wavelength conversion element and a method for manufacturing a wavelength conversion element that can obtain a higher conversion efficiency for second harmonic generation regardless of the manufacturing precision of the wavelength conversion element.

[0010] To achieve the above objective, one embodiment of the wavelength conversion element of the present disclosure is a wavelength conversion element including an optical waveguide, wherein the optical waveguide includes a lower cladding layer and a core layer provided above the lower cladding layer, the core layer having a width-adjustable width adjustment region having a length in a direction perpendicular to the waveguide direction of the optical waveguide, the width of the width adjustment region, the width adjusted by the width adjustment region, corresponds to the product of the width and the thickness of the core layer at a position corresponding to the width in the waveguide direction.

[0011] Furthermore, a method for manufacturing a wavelength conversion element according to one embodiment of the present disclosure is a method for manufacturing a wavelength conversion element including an optical waveguide, comprising the steps of: forming a lower cladding layer and forming a core layer above the lower cladding layer to form the optical waveguide; and forming a width adjustment region in the core layer for adjusting the width which is the length in a direction intersecting the guidance direction of the optical waveguide, wherein the width adjusted by the width adjustment region corresponds to the product of the width and the thickness of the core layer at a position corresponding to the width in the guidance direction.

[0012] (a), (b), (c), (d), and (e) are diagrams illustrating an optical waveguide with a constant width and thickness of the core layer, and (f), (g), (h), (i), and (j) are diagrams illustrating an optical waveguide with a non-uniform width and thickness of the core layer. This is a perspective view of an optical waveguide of a wavelength conversion element according to one embodiment of the present disclosure. (a) is a top view of the core layer 1 shown in Figure 1, (b) is a vertical cross-sectional view along the arrow B-B shown in Figure 3(a), and (c) is a horizontal cross-sectional view along the arrow C-C shown in Figure 3(a). This is a cross-sectional view showing the cross-section shown in Figure 3(c) in more detail. (a) shows an example of trimming by local plasma etching, and (b) shows an example of trimming by dry etching using a mask. (a) to (c) are diagrams illustrating the relationship between the trimming amount and the angle of the core layer. (d) shows the relationship between the thickness of the core layer on the horizontal axis and the trimming amount, and (e) is a diagram showing the relationship between the trimming amount, the angle of the corner of the core layer, and the thickness of the core layer. This diagram illustrates an example of trimming multiple core layers in a single process.

[0013] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. The drawings are intended to explain the configuration, arrangement of parts, function, operation, effect, and technical concept of the present disclosure, and do not necessarily show the dimensions precisely. Such drawings do not limit the specific configuration of the wavelength conversion element of the present disclosure. The embodiments described below may be implemented using other materials and numerical values ​​without departing from the spirit of the present invention.

[0014] (Premise) Before describing specific embodiments of this disclosure, the manufacturing tolerances of the elements that are prerequisites for this disclosure will be explained. Figures 1(a) to 1(j) show the relationship between the core width and thickness of the waveguide and the effective refractive index. Figures 1(a) to 1(e) show an ideal example where the effective refractive index is uniform in the optical propagation direction, and Figures 1(f) to 1(j) show an example where the effective refractive index has a non-uniform distribution. Figures 1(a) and 1(f) are side views of the optical waveguide, Figures 1(b) and 1(g) are top views of the optical waveguide, Figures 1(c) and 1(h) are top views schematically showing the nonlinear response region of the optical waveguide, Figures 1(e) and 1(j) show the SHG (Second harmonic generation) of the optical waveguide, and Figures 1(e) and 1(j) show the yield of the wavelength conversion element. In both Figures 1(e) and 1(j), the horizontal axis represents the effective second harmonic generation (the output ratio obtained when 1W of light is injected), and the vertical axis represents the number of wavelength conversion elements.

[0015] As shown in Figures 1(a) and 1(b), in an optical waveguide where the core layer 301 and cladding layer 303 are uniform, a positive nonlinear response region 311 and a negative nonlinear response region 312 are uniformly formed, as shown in Figure 1(c). In a wavelength conversion element using such an optical waveguide, as shown in Figure 1(d), it can be seen that a wavelength conversion element that can obtain the second harmonic with high efficiency can be obtained. Furthermore, as shown in Figure 1(e), it can be seen that the yield of such a high-efficiency wavelength conversion element is high.

[0016] However, in optical waveguides actually manufactured, the widths of the core layer 301 and the cladding layer 303, and the thickness of the core layer 301, can be non-uniform, as shown in Figures 1(f) and 1(g). As a result, as shown in Figure 1(h), variations occur in the area and shape of the positive nonlinear response region 311 and the negative nonlinear response region 312. In wavelength conversion elements using such optical waveguides, it is clear that the output efficiency of the second harmonic is lower than that shown in Figure 1(d), as shown in Figure 1(i). Furthermore, as shown in Figure 1(j), the variation in the characteristics of the wavelength conversion element becomes larger.

[0017] The inventors of this invention focused on the correlation between the cross-sectional area of ​​the core layer 301 and the effective refractive index, and conceived the idea of ​​maintaining a constant cross-sectional area of ​​the core layer 301 along the waveguide direction to keep the effective refractive index of the core layer 301 constant, thereby preventing phase mismatch and improving the output efficiency of the optical waveguide.

[0018] (Wavelength Conversion Element) This embodiment describes a wavelength conversion element and a method for manufacturing a wavelength conversion element according to one embodiment of the present invention, using a ridge-type optical waveguide formed by a core layer and a lower cladding layer (undercladding layer) of a nonlinear optical material as an example. The ridge-type optical waveguide included in the wavelength conversion element may be, for example, a PPLN optical waveguide.

[0019] In this embodiment, the nonlinear optical material used for the core layer can be any material that is transparent at optical wavelengths of 400 nm to 2000 nm. The nonlinear optical material can be any material that has a nonlinear optical effect, and it may be a second-order or third-order or higher nonlinear optical effect. Examples include lithium niobate (LiNbO3), lithium tantalate (LiTaO3), beta-barium borite (β-BaB2O4: BBO), potassium titanyl phosphate (KTiOPO4: KTP), etc. The nonlinear optical material may have a periodic polarization reversal structure to increase the nonlinear optical effect. When using a nonlinear optical material with a periodic polarization reversal structure, optical waveguide processing conditions below the Curie temperature in which the periodic polarization reversal is not lost, and an optical waveguide structure that can achieve phase matching can be appropriately selected.

[0020] Figures 2, 3(a), 3(b), and 3(c) are diagrams illustrating the wavelength conversion element of this embodiment. Figure 2 is a perspective view of the optical waveguide 100 included in the wavelength conversion element. Figure 3(a) is a top view of the core layer 1 shown in Figure 2, Figure 3(b) is a vertical cross-sectional view along the arrow B-B shown in Figure 3(a), and Figure 3(c) is a horizontal cross-sectional view along the arrow C-C shown in Figure 3(a). Figure 4 is a more detailed view of the cross-section of the optical waveguide 100 of the wavelength conversion element shown in Figure 3(c).

[0021] As shown in Figure 2, the optical waveguide 100 includes an undercladding layer 101 and a core layer 1 provided above the undercladding layer 101. Here, "above" or "above" refers to the direction in which the Z axis of the X, Y, Z coordinates shown in the figure points. The undercladding layer 101 is a plane parallel to the X-Y plane. The waveguide direction in this embodiment is along the Y-axis direction in the figure.

[0022] The core layer 1 has width adjustment regions 1bb and 1cb formed therein, which adjust the width W (Figure 4), which is the length in the direction perpendicular to the waveguide direction of the optical waveguide 100. In this embodiment, the width adjustment regions 1bb and 1cb are formed such that the product of the width W of the core layer 1 and the film thickness at the location where the core layer 1 has this width W is constant across the waveguide direction. Therefore, the width W adjusted by the width adjustment regions 1bb and 1cb corresponds to the product of the width and thickness of the core layer 1. However, this correspondence is not one-to-one, and the width W can take on multiple values ​​depending on the film thickness at the location where the core layer 1 has the width W.

[0023] In other words, the width W of the core layer 1 is adjusted by trimming, which will be described later. As a result of trimming, width adjustment regions 1bb and 1cb are formed in the core layer 1 after the width W has been adjusted. Also, as shown in Figures 2, 3(b), and 3(c), the areas of the side surface of the core layer 1 excluding the width adjustment regions 1bb and 1cb become unadjusted regions 1ba and 1ca. The unadjusted region 1ba and the width adjustment region 1bb, and the unadjusted region 1ca and the width adjustment region 1cb, respectively, form the side surface of the core layer 1. Cross sections 1d and 1e form the front and rear surfaces of the core layer 1, respectively, and the top surface 1a is perpendicular to cross sections 1d and 1e and the side surface.

[0024] In this embodiment, the trimming is performed so that the effective refractive index corresponding to the cross-sectional area of ​​the core layer 1 is constant in the waveguide direction, as described above. That is, in this embodiment, the core layer is cut (etched) so that the cross-sectional area between sections 1d and 1e is constant, for a core layer whose width and thickness are not uniform along its optical waveguide direction (Y direction), as explained earlier in Figures 1(f) and 1(h). The amount of etching is determined by the thickness or width of the core layer and is not constant along the waveguide direction. Therefore, as shown in Figures 2, 3(a), and 3(b), width adjustment regions 1bb and 1cb, whose width is not uniform along the waveguide direction, are formed in the core layer after trimming. As shown in Figure 2, the distribution of width adjustment regions 1bb and 1cb has a wavy shape. As a result, it is possible to make the effective refractive index uniform along the optical propagation direction (Y direction).

[0025] The specific etching amount is determined, for example, by ensuring that the width of core layer 1 is inversely proportional to the thickness of core layer 1. In this way, the product of the width and thickness of core layer 1 in the waveguide direction becomes approximately constant, preventing phase mismatch and improving the conversion efficiency of the second harmonic generation in the optical waveguide.

[0026] The cross-sections of the core layer 1 having width adjustment regions 1bb and 1c, whose widths are not constant along the waveguide direction, are as shown in Figure 4. That is, as a result of trimming, the width adjustment regions 1bb and 1c are formed, and the side surfaces 1032 and 1052 of the width adjustment regions 1bb and 1c are formed. The angle between the side surface 1031 of the non-adjusted region 1ba and the underclad layer 101 is θ1, and the angle between the side surface 1032 and the underclad layer 101 is θ2, so the relationship is θ1 > θ2. Angle θ1 is the angle formed during the etching or dicing process to form the entire core layer 1. Also, angle θ2 is the angle formed when trimming the core layer 1, and the above angle relationship is observed. Depending on the differences in the above regions or angles, the core layer 1 comes to include a core layer portion 103 having side surfaces 1031 and 1051, and a core layer portion 105 located above the core layer portion 103 and having side surfaces 1032 and 1052.

[0027] As described above, the core layer of the wavelength conversion element in this embodiment can have its effective refractive index of the optical waveguide kept constant along the waveguide direction by keeping the cross-sectional area of ​​each part of the core layer constant along the waveguide direction. This embodiment makes it possible to obtain a wavelength conversion element with a higher conversion efficiency for second harmonic generation, regardless of the manufacturing precision of the wavelength conversion element. Furthermore, this embodiment can be realized by obtaining the thickness or width of the core layer and adjusting the trimming amount of the width adjustment region according to the thickness or width of the core layer. For this reason, a waveguide with the target structure can be obtained in a shorter time than simulating the optical properties of the core. In addition, the reduction in manufacturing throughput can be reduced.

[0028] (Manufacturing Method for Wavelength Conversion Element) Next, the manufacturing method for the wavelength conversion element of this embodiment will be described. The manufacturing method for wavelength conversion includes the steps of forming an underclad layer 101 and forming a core layer 1 above the underclad layer 101 to form an optical waveguide, and forming width adjustment regions 1bb and 1c in the core layer 1 to adjust the width of the core layer 1. The step of forming the width adjustment regions 1bb and 1c is performed so that the width of the core layer 1 adjusted by the width of the width adjustment regions 1bb and 1c corresponds to the product of this width and the thickness of the core layer at the position corresponding to this width in the waveguide direction.

[0029] Furthermore, in the manufacturing method of the wavelength conversion element, the thickness and width of the core layer are measured in advance along with the above-mentioned steps. The thickness of the core layer may be measured, for example, prior to the formation of core layer 1. The height distribution of the core layer (film thickness distribution) is measured, for example, using optical interference. Specifically, this can be done by irradiating the surface of a substrate on which the core layer film has been deposited with light and performing a non-contact evaluation of the film thickness of the multilayer film by analyzing the reflection spectrum of the light. The method of analyzing film thickness using interference with reflected light is a widely used method, and in this embodiment as well, the film thickness is measured using the optical interference method, which is commonly used. However, the film thickness measurement method may be a method other than one using optical interference, and any method is acceptable as long as it is non-invasive to the optical waveguide structure.

[0030] The width of the core layer 1 is measured after the formation of the core layer 1. The width of the core layer 1 is measured by directly observing the core layer 1 of the optical waveguide 100. Any observation method that is non-invasive to the optical waveguide structure is acceptable, and typical examples include methods using an optical microscope or electron microscopes such as a scanning electron microscope. In addition to the above methods, methods using a step meter or high-precision measurement methods using an atomic force microscope may also be used. In this case, there are no particular restrictions on the spacing of measurement points when measuring the width distribution of the core layer 1 of the optical waveguide, and it is acceptable as long as the number of measurement points is sufficient to obtain necessary and sufficient structural information for the structure trimming process, and the throughput of the optical waveguide formation process does not decrease significantly.

[0031] After the above measurements, this embodiment determines the thickness of the width adjustment regions 1bb and 1cb according to the width of the core layer, for example, and forms the width adjustment regions 1bb and 1cb. Figures 5(a) and 5(b) are diagrams illustrating the process of forming the width adjustment regions 1bb and 1cb. Figure 5(a) shows an example in which the width adjustment regions 1bb and 1cb are formed by local plasma etching, which generates plasma locally. Figure 5(b) shows an example in which the width adjustment regions 1bb and 1cb are formed by dry etching using a mask.

[0032] Localized plasma etching is performed, for example, by positioning a plasma gun on a substrate 501 with a gap of several micrometers, as shown in Figure 5(a). Electrodes are placed around the tip of the plasma gun, and the pressure inside the plasma gun is maintained at a lower pressure V1Pa than the atmospheric pressure V2Pa. As a result, the material gas is drawn into the plasma gas from the surroundings, and plasma 502 is generated between the tip of the plasma gun and the substrate 501 during this process. The core layer 1 is trimmed by a chemical reaction with the generated plasma, and is modified so that the corners of the core layer before trimming are removed. In this embodiment, the larger the trimming amount, the sharper the corners become, and the thinner the core layer 1 becomes. This trimming state will be described later. The localized etching apparatus shown in Figure 5(a) is disclosed, for example, in Japanese Patent Application Publication No. 2010-135351.

[0033] The trimming shown in Figure 5(b) is performed by known photolithography. A mask 401 is formed only in the area to be trimmed on the upper surface 1a of the core layer 1, and dry etching is performed. The amount of trimming is controlled by the thickness of the mask 401.

[0034] Figures 6(a) to 6(e) illustrate the relationship between the trimming amount and the core layer thickness in this embodiment. Figures 6(a) to 6(c) illustrate the relationship between the trimming amount and the angle θ at which the side surface and top surface of the core layer intersect, with the trimming amount increasing in the order of Figure 6(a), Figure 6(b), and Figure 6(c). Figures 6(a) to 6(c) show that in this embodiment, both the thickness and width of the core layer change due to etching, whether the core layer thickness is determined according to the core layer width or the core layer width is determined according to the core layer thickness. For this reason, in the manufacturing method of the wavelength conversion element of this embodiment, it is preferable to consider the amount of change due to etching in both the thickness and width of the core layer.

[0035] Figure 6(d) shows the trimming time on the horizontal axis and the trimming (etching) amount on the vertical axis. Figure 6(e) shows the trimming amount on the horizontal axis and the angle θ of the core layer corner (dotted line) and the core layer thickness d (solid line) on the vertical axis. As shown in Figures 6(a) to 6(e), the larger the trimming amount, the thinner the core layer becomes after trimming, and the angle θ of the corner becomes smaller. A smaller angle θ means that the width of the core layer is reduced, and the width of the width adjustment region becomes larger.

[0036] The method for manufacturing the wavelength conversion element described above involves forming the width adjustment region such that its thickness differs according to the width of the core layer at the corresponding position. This method can be achieved by obtaining the width of the core layer and adjusting the thickness of the width adjustment region according to the width of the core layer. However, this embodiment is not limited to this configuration, and the widths of the width adjustment regions 1bb and 1cb may be determined according to the thickness of the core layer. Even in such cases, the local etching shown in Figure 5(a) and the etching by the mask process shown in Figure 5(b) can be applied.

[0037] In this embodiment, the effective refractive index can be kept constant by keeping the area of ​​the cross-section perpendicular to the waveguide direction of the core layer of the optical waveguide constant. Therefore, by keeping the effective refractive index of the optical waveguide constant in the waveguide direction, the generation efficiency of the second harmonic can be increased. Furthermore, this embodiment can be realized by obtaining the width or thickness of the core layer. Therefore, a wavelength conversion element with an optical waveguide of the target structure can be realized more easily and quickly than, for example, obtaining the characteristics of the optical waveguide by simulation.

[0038] When multiple core layers are formed on a substrate, trimming one of the core layers will cause the surrounding core layers to be trimmed in a similar manner. In this embodiment, it is also possible to extract core layers that meet the specifications from among the multiple core layers trimmed in a single process and use them in subsequent processes. Such a process is illustrated in Figure 7. Figure 7 shows multiple core layers 601, 602, and 603. The core layers 601, 602, and 603 are formed, for example, at a pitch of 100 μm. The tip of the plasma gun used for local etching is, for example, about 1 mm, and the diameter from which the plasma is ejected is, for example, 500 μm. Multiple core layers formed relatively close together on the substrate are formed with similar manufacturing tolerances. Therefore, in this embodiment, multiple core layers can be trimmed similarly in a single process, and the cross-sectional area of ​​the core layers can be appropriately adjusted. Such a process can further reduce the decrease in manufacturing throughput.

[0039] 1, 301, 601, 602, 603 Core layer 1a Top surface 1ba, 1ca Non-adjustable region 1bb, 1cb Width adjustment region 1d, 1e Cross section 100 Optical waveguide 101, 303 Underclad layer 103, 105 Core layer portion 311, 312 Nonlinear response region 401 Mask 501 Substrate

Claims

1. A wavelength conversion element including an optical waveguide, wherein the optical waveguide includes a lower cladding layer and a core layer provided above the lower cladding layer, the core layer has a width adjustment region formed therein whose width is adjusted by a length in a direction perpendicular to the waveguide direction of the optical waveguide, and the width adjusted by the width adjustment region corresponds to the product of the width and the thickness of the core layer at a position corresponding to the width in the waveguide direction.

2. The wavelength conversion element according to claim 1, wherein the width of the width adjustment region is inversely proportional to the thickness of the core layer.

3. The wavelength conversion element according to claim 1, wherein the core layer includes a first core layer having a first side surface along the waveguide direction and a second core layer located above the first core layer and having a second side surface along the waveguide direction, and the angle of the first side surface with respect to the lower cladding layer is greater than the angle of the second side surface with respect to the lower cladding layer.

4. The wavelength conversion element according to claim 1, wherein the optical waveguide comprises a nonlinear optical material comprising at least one of lithium niobate (LiNbO3), lithium tantalate (LiTaO3), beta-barium borite (β-BaB2O4:BBO), and potassium titanyl phosphate (KTiOPO4:KTP).

5. A method for manufacturing a wavelength conversion element including an optical waveguide, comprising the steps of: forming a lower cladding layer and forming a core layer above the lower cladding layer to form the optical waveguide; and forming a width adjustment region in the core layer for adjusting the width, which is the length in a direction intersecting the guidance direction of the optical waveguide, wherein the width adjusted by the width adjustment region corresponds to the product of the width and the thickness of the core layer at a position corresponding to the width in the guidance direction.

6. The method for manufacturing a wavelength conversion element according to claim 5, wherein the step of forming the width adjustment region is performed by local plasma etching, which generates plasma locally.

7. The method for manufacturing a wavelength conversion element according to claim 5, wherein the step of forming the width adjustment region is performed by dry etching using a mask.

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