Method for manufacturing wavelength conversion elements

Abstract

Provided is a method for manufacturing a wavelength conversion element, including: a first step for forming an optical waveguide core substrate that has at least one periodic polarization-reversed region having a secondary nonlinear effect; a second step for joining the optical waveguide core substrate and a substrate that has a lower refractive index than the optical waveguide core substrate in at least a range of used optical wavelengths to form a joined substrate, and configuring the optical waveguide core substrate as a thin film to form an optical waveguide core layer; and a third step for processing the optical waveguide core layer of the joined substrate to form an optical waveguide core, wherein, in the third step, the polarization reversal period of a periodic polarization reversal structure of the formed optical waveguide core is adjusted at least locally by selecting the formation position of the optical waveguide core with respect to the at least one periodic polarization-reversed region.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a wavelength conversion element used in a wavelength conversion device.

Background Art

[0002] Wavelength conversion technology has attracted attention in applications where a wavelength range that cannot be directly output by a semiconductor laser is required, or where high-power light that cannot be obtained by a semiconductor laser is required even in a wavelength range that can be output. The wavelength conversion element used in the wavelength conversion device is realized by using an optical crystal having a second-order nonlinear effect. Representative optical crystals having a second-order nonlinear effect include, for example, LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), or KTiOPO4 (potassium titanyl phosphate). In particular, an optical waveguide using periodically poled lithium niobate (hereinafter referred to as PPLN) has attracted attention as an element capable of realizing high wavelength conversion efficiency by increasing the light intensity and using the quasi-phase matching technique. This PPLN is expected to be applied in a wide range of optical wavelength bands from the ultraviolet region to the terahertz region, which are used in various fields such as optical signal wavelength conversion, optical processing, medical treatment, and biotechnology in optical communication.

[0003] Furthermore, by using PPLN, it is possible to fabricate a parametric amplification element and an excitation light generation element that constitute a phase-sensitive amplifier (PSA) capable of low-noise optical amplification. Therefore, PPLN is being considered for application as a device that realizes high-gain, low-noise optical amplification characteristics and plays an important role in the next-generation optical fiber communication field. Also, in the field of quantum computing, an optical waveguide using PPLN can be inserted into a fiber ring resonator and used as a parametric oscillation element. Reports have been made demonstrating that an optical coherent imaging machine device is realized using this configuration and that large-capacity calculations can be performed at a higher speed than conventional computers.

[0004] A wavelength conversion element using a nonlinear optical waveguide having a periodic polarization reversal structure of an optical crystal having a second-order nonlinear effect such as LiNbO3 (hereinafter referred to as "nonlinear optical crystal") is described, for example, in Patent Document 1.

[0005] Patent Document 1 discloses an example of fabricating a ridge-type optical waveguide. Patent Document 1 describes fabricating a wavelength conversion element in a ridge-type optical waveguide by bonding a first substrate of a nonlinear optical crystal having a periodic polarization reversal structure with a second substrate having a refractive index smaller than that of the first substrate, in order to improve the light confinement effect.

[0006] In Patent Document 1, after the step of bonding the first substrate and the second substrate, the first substrate is polished until its thickness reaches 20 μm, and then the substrate is etched to create a ridge-type optical waveguide. By making the thickness of the nonlinear optical crystal film that forms the optical waveguide 20 μm, a high power density can be obtained in the optical waveguide.

[0007] Furthermore, Patent Document 1 describes using a nonlinear optical crystal of the same type as the first substrate as the second substrate to avoid deterioration of the adhesive and cracking due to temperature changes, and applying heat to the first and second substrates to diffuse bond them. In the technical fields utilizing these wavelength conversion technologies, it is important to realize wavelength conversion devices with higher wavelength conversion efficiency in order to further improve performance.

[0008] However, conventional wavelength conversion elements using ridge-type optical waveguides, such as those described in Patent Document 1, have the following problems.

[0009] (a) Issues with the process sequence, and the inability to adjust and control the QPM period in a later process step when fabricating the periodic polarization reversal into an optical waveguide shape. As shown in Patent Document 1 and other documents mentioned above, when fabricating a wavelength conversion element having an optical waveguide structure that satisfies the pseudo-phase matching condition, a nonlinear optical crystal with a large optical nonlinear constant (susceptibility) is often used as the material for the optical waveguide core. For example, materials such as LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), or KTiOPO4 (potassium titanate phosphate) are used.

[0010] In the case of nonlinear optical crystals as described above, in order to form a periodic polarization reversal structure, it is necessary to apply a very large electric field locally for polarization reversal. This electric field application is generally performed by applying a large voltage after forming a microstructure of metal electrodes on a nonlinear optical crystal substrate. Thus, the formation of a periodic polarization reversal structure requires a fine and complex fabrication process. For this reason, it is currently difficult to form electrodes on an optical waveguide core layer made of a nonlinear optical crystal after it has been processed into the shape of an optical waveguide core, in order to apply the electric field for polarization reversal.

[0011] The fabrication process for a wavelength conversion element consisting of an optical waveguide core having the periodic polarization reversal structure described above is generally as follows. (1) First, a periodic polarization reversal structure is formed in the material of the optical waveguide core layer. Specifically, a high electric field in a specific direction is applied to the entire surface of a flat optical waveguide core substrate formed from the optical waveguide core layer material to align the dielectric polarization domains across the entire substrate. Next, a photomask pattern matching the desired design value of the polarization reversal structure and an electrode pattern for polarization reversal are fabricated on the surface of the substrate using a photolithography process, and a reversal polarization structure is formed within the uniform dielectric polarization by applying a high electric field. After that, the photoresist and electrode film are removed to complete the optical waveguide core substrate with the periodic polarization reversal structure formed thereon. (2) Next, the optical waveguide core substrate on which the periodic polarization reversal structure is formed is bonded to a substrate having a lower refractive index than the optical waveguide core at the wavelength of light to be used. Specifically, the bonding surfaces of both substrates are polished to be flat and mirror-like, and the bonding surfaces of both substrates are bonded by thermal bonding or corona discharge. (3) The bonded substrates are processed using grinding and polishing equipment to create wafer shapes that can be used in subsequent photoprocessing steps such as patterning with photoresist. At this time, the thickness of the optical waveguide core layer is thinned to match the thickness of the optical waveguide core to be formed. (4) Next, the optical waveguide core layer is fabricated to form the optical waveguide core. Specifically, the optical waveguide core layer is patterned with the shape of the optical waveguide core using a photoresist or the like, and then the optical waveguide core is formed by dry etching, dicing, proton exchange, or the like. In this way, a wavelength conversion element consisting of an optical waveguide having a periodic polarization reversal structure is fabricated.

[0012] In the fabrication process for the wavelength conversion element described above, an optical waveguide core substrate with a pre-formed periodic polarization reversal structure is bonded to a low refractive index substrate, thinned, and then the optical waveguide core structure is created. Processing errors occur during the thinning of this optical waveguide core layer and during subsequent processing of the optical waveguide core. Due to the influence of these processing errors that occur during the processing of the optical waveguide core, the effective refractive indices n1, n2, and n3 of the optical waveguide core at each operating wavelength shown in (Equation 10) below do not have a single value, but rather become fluctuating values ​​due to the processing errors. Thus, processing errors caused by the process after the formation of the polarization reversal structure are a factor in the variation of optical properties such as the optical spectral distribution of the wavelength-converted light of the completed wavelength conversion element.

[0013] (b) The problem of not being able to process the film thickness distribution uniformly across the wafer surface In the manufacturing process of wavelength conversion elements as described above, there is a problem in that shape errors such as film thickness distribution are likely to occur during the processing of the optical waveguide core. The above manufacturing process includes a step of bonding (laminating) the optical waveguide core substrate to a substrate with a lower refractive index than the optical waveguide core substrate. In this process, two substrates with different coefficients of linear thermal expansion are bonded together, making the bonded substrate prone to warping. Therefore, the thickness of the bonded substrate, which should ideally have a uniform film thickness, becomes uneven due to the grinding and polishing processes used to thin the optical waveguide core layer.

[0014] In other words, errors in the film thickness of the optical waveguide core layer that occur during the manufacturing process described above cause fluctuations in the refractive index of the fabricated optical waveguide core, resulting in a certain degree of error in the phase matching conditions of the periodic polarization reversal structure of the optical waveguide. This leads to a decrease in wavelength conversion efficiency, such as the generation of optical difference frequency, and causes the optical properties, such as the center wavelength and light intensity of the wavelength-converted light generated from the wavelength conversion element, to deviate from the design values.

[0015] These film thickness errors are only discovered after the optical waveguide core layer has been processed. Therefore, when the optical waveguide core layer is processed for optical waveguide formation, the periodic polarization reversal structure of the resulting optical waveguide is already determined. As a result, it has been impossible to correct these errors to compensate for processing errors such as film thickness errors.

[0016] Furthermore, to achieve a desired effective refractive index of the optical waveguide core, it is possible to adjust the core width of the waveguide core to some extent by using a photomask pattern with a width varied in accordance with the film thickness variation, and performing photolithography and dry etching processes. However, in reality, processing errors occur during the formation of the optical waveguide core, so there are limitations to achieving a desired effective refractive index. In addition, variations in waveguide core width increase optical loss in the optical waveguide, leading to a decrease in the light intensity of control light and ultimately reducing the efficiency of the wavelength conversion device.

[0017] (c) Issues regarding film thickness accuracy due to grinding and polishing of the optical waveguide core layer Even in the thinning process of the optical waveguide core layer during the manufacturing process of the wavelength conversion element described above, processing errors inevitably occur. Therefore, the absolute value of the optical waveguide core layer itself varies slightly depending on the processing depth of each grinding and polishing process, resulting in variations in the thickness of the fabricated optical waveguide cores. In other words, even after going through the same grinding and polishing process, the same thickness is not guaranteed. Furthermore, at the submicron order, the average thickness also fluctuates, resulting in a certain degree of variation in the effective refractive index of the optical waveguide core. Consequently, the conditions for pseudo-phase matching of individual optical waveguide cores also vary, resulting in, for example, a variation in the center wavelength of the optical wavelength of the difference frequency generated light.

[0018] (d) Issues regarding the limitations of temperature control compensation Wavelength conversion elements, by using temperature control elements such as Peltier elements, can perform high-precision temperature control. This allows them to utilize the temperature dispersion of the effective refractive index of the optical waveguide to change the effective refractive index of the optical waveguide due to temperature changes, thereby adjusting the pseudo-phase matching conditions. For example, it is possible to control the central wavelength of difference frequency light generated by difference frequency generation to some extent.

[0019] However, while it is possible to correct the effective refractive index of the optical waveguide to some extent on average by temperature control to compensate for the film thickness error of the wavelength conversion element, correcting for localized film thickness distributions requires localized control of the optical waveguide core temperature, which complicates the temperature control elements and control circuits and necessitates detailed control, making it difficult.

[0020] Furthermore, in reality, the entire wavelength conversion element is not always at a perfectly uniform temperature. Temperature distribution occurs within the wavelength conversion element due to heat exchange with the temperature control element, the temperature difference between the ambient temperature and the wavelength conversion element, and the state of radiant heat from the surroundings of the wavelength conversion element and its mounting structure. Therefore, even if the effective refractive index of the optical waveguide core of the wavelength conversion element has a single, uniform value, a certain range of variation in the effective refractive index will occur.

[0021] In addition, since temperature control of the wavelength conversion device is also important as a control method for correcting changes in the ambient temperature during use, it is difficult to use temperature control only for correcting film thickness and processing errors. Further, in the temperature control of the wavelength conversion element, since heat diffusion due to direct heat conduction occurs, there is a limit to the local temperature control of the wavelength conversion element. From the above, there is also a limit to using temperature control for adjusting the quasi-phase matching conditions of the wavelength conversion element, for example, controlling the central wavelength of the difference frequency light generated by difference frequency generation.

[0022] As described above, in the manufacturing process of the wavelength conversion element as described above, there is a limit to improving the yield because there is a limit to correcting the error in the effective refractive index of the optical waveguide core caused by the film thickness variation of the optical waveguide core layer before the process of forming the optical waveguide core or compensating by temperature control.

[0023] Therefore, after the grinding and polishing processes of the thin film processing of the optical waveguide core layer and the occurrence of processing errors during the width processing of the optical waveguide core, a method for controlling the quasi-phase matching conditions of the optical waveguide core corresponding to the errors generated thereby is required.

[0024] Specifically, in the process of forming the optical waveguide core, a method is required that enables at least local control of the polarization inversion period of the periodically poled inversion structure possessed by the optical waveguide core.

Prior Art Documents

Patent Documents

[0025]

Patent Document 1

Summary of the Invention

[0026] The present disclosure is made to solve the above problems, and mainly aims to at least locally control the polarization inversion period of the periodically poled inversion structure possessed by the optical waveguide core in the process of forming the optical waveguide core.

[0027] One embodiment of the present disclosure, in order to achieve this objective, is characterized by including the following steps in a method for manufacturing a wavelength conversion element.

[0028] A method for manufacturing a wavelength conversion element, comprising: a first step of forming an optical waveguide core substrate having at least one periodic polarization reversal region having a second-order nonlinear effect; a second step of forming a bonded substrate by joining the optical waveguide core substrate with a substrate having a lower refractive index than the optical waveguide core substrate in at least the range of optical wavelengths used, and forming an optical waveguide core layer by thinning the optical waveguide core substrate; and a third step of processing the optical waveguide core layer of the bonded substrate to form an optical waveguide core. A method for manufacturing a wavelength conversion element, comprising a third step in which the polarization reversal period of a periodic polarization reversal structure having formed optical waveguide cores is adjusted at least locally by selecting the formation position of the optical waveguide core for at least one or more periodic polarization reversal regions. [Brief explanation of the drawing]

[0029] [Figure 1] This is a perspective view showing the basic configuration of a wavelength conversion element manufactured by the manufacturing method of this disclosure. [Figure 2] This figure shows an example of the configuration of the mounting structure of a wavelength conversion device with the wavelength conversion device elements shown as the basic configuration in Figure 1. [Figure 3] This figure shows the steps for manufacturing a wavelength conversion element used in each embodiment of this disclosure. [Figure 4] This is a schematic diagram illustrating the principle for adjusting the polarization reversal period of the periodic polarization reversal structure of an optical waveguide core by the manufacturing method of the first embodiment of this disclosure. [Figure 5] This is a schematic diagram illustrating the manufacturing method of the first embodiment of the present disclosure. [Figure 6] This is a schematic diagram illustrating the manufacturing method of a second embodiment of the present disclosure. [Figure 7] This is a schematic diagram illustrating another aspect of the manufacturing method of the second embodiment of the present disclosure. [Figure 8]This is a schematic diagram illustrating the second aspect of the manufacturing method according to the second embodiment of the present disclosure. [Figure 9] This is a schematic diagram illustrating the manufacturing method of the third embodiment of the present disclosure. [Figure 10] This is a schematic diagram illustrating another aspect of the manufacturing method of the third embodiment of the present disclosure. [Figure 11] This is a schematic diagram illustrating the manufacturing method of the fourth embodiment of the present disclosure. [Figure 12] This is a schematic diagram illustrating an example of the location for forming the optical waveguide core in the manufacturing method according to the fourth embodiment of the present disclosure. [Figure 13] This is a schematic diagram illustrating another example of the location for forming the optical waveguide core in the manufacturing method of the fourth embodiment of the present disclosure. [Figure 14] This is a schematic diagram illustrating another aspect of the manufacturing method of the fourth embodiment of the present disclosure. [Figure 15] This is a schematic diagram illustrating another aspect of the manufacturing method of the fourth embodiment of the present disclosure. [Figure 16] This is a schematic diagram illustrating a wavelength conversion element of a second embodiment of the present disclosure. [Modes for carrying out the invention]

[0030] In view of the above problems, the inventors conducted diligent studies and found that by optimizing the arrangement of the periodic polarization reversal region and the optical waveguide core, and the manufacturing process, the pseudo-phase matching conditions can be adjusted by at least locally selecting the polarization reversal period of the polarization reversal structure of the optical waveguide core, and as a result, the optical properties of the wavelength-converted generated light can be varied, thus completing the present invention. Embodiments of the present invention will be described in detail below with reference to the drawings. (Wavelength conversion device) Prior to describing each embodiment of the manufacturing method of this disclosure, a wavelength conversion device manufactured by the manufacturing method of this disclosure will be described. (Explanation of second-order nonlinear optical effects and phase matching conditions) Generally, when signal light [wavelength: λ1, frequency: ω1] and excitation light [wavelength: λ2, frequency: ω2] of different wavelengths are incident on a second-order nonlinear optical crystal, wavelength-converted light (also called idler light) [wavelength: λ3, frequency: ω3] is generated, which has wavelengths according to a relationship called the phase matching condition. First, let's consider the case of sum frequency generation, that is, ω3 = ω1 + ω2. Since the momentum of a photon is expressed as hk / (2π) where h is Planck's constant and k is wavenumber, if we let Δk be the wavenumber mismatch, k1 be the wavenumber of the signal light, k2 be the wavenumber of the excitation light, and k3 be the wavenumber of the wavelength-converted light, then the following relationship holds according to the law of conservation of momentum. hΔk / 2π = h(k3-k1-k2) / 2π ···(Equation 1) Therefore, Δk = k3 - k1 - k2 ... (Equation 2) If the length of the second-order nonlinear optical crystal through which light propagates is L and the propagation direction is the Z direction, the nonlinear polarization Pz(ω1+ω2) changes phase by exp[i(k1+k2)Z], but the phase of the resulting wavelength-converted sum-frequency light E(ω3) is exp(ik3·Z), so the following relationship (Equation 3) holds between the two. exp(ik3·Z)-exp[i(k1+k2)·Z] =exp[i(k3-k1-k2)·Z]=exp[iΔk·Z]···(Formula 3) From the above (Equation 3), a phase difference of Δk·L occurs between the sum-frequency light E(ω3) and the nonlinear polarization Pz(ω1+ω2).

[0031] When this phase difference exceeds π, the phase reverses, the direction of energy flow is reversed, and a process occurs in which the ω3 photon splits into ω1 and ω2 photons. In this way, the sum-frequency component of the light wave that was created begins to decrease. The distance at which the phase reverses. Lc = π / (|Δk|) ···(Equation 4) This is called the coherence length.

[0032] Furthermore, when this phase difference exceeds 2π (i.e., the propagation length of light exceeds twice the coherence length), the direction of energy flow returns to its original state, and it can be seen that the nonlinear polarization Pz increases or decreases with a period of twice the coherence length (increase and decrease alternate for each coherence length). Therefore, in order to increase the efficiency of generating wavelength-converted light, the coherence length at which attenuation begins must be longer than the propagation crystal length. In particular, the condition Δk=0, where wavenumber mismatch disappears, is called the phase matching condition and is a condition for generating wavelength-converted light.

[0033] In this case, when two light waves with frequencies ω1 and ω2 are input to a second-order nonlinear material as described above, and light with frequency ω3 (=ω1+ω2) is generated, this is called sum-frequency generation (SFG). On the other hand, when two light waves with frequencies ω1 and ω3 are input to a second-order nonlinear material, and light with frequency ω2 (=ω3―ω1) is generated, this is called difference-frequency generation (DFG).

[0034] Furthermore, the phenomenon in which a high-intensity light of frequency ω3 is incident and generates two light waves of frequencies ω1 and ω2 is called the optical parametric effect. Here, considering the case where all coupled light waves travel in the same direction, the wavenumber mismatch Δk is: Δk=2π(n3 / λ3-n1 / λ1-n2 / λ2) (Equation 5) Therefore, the phase matching condition is, n3 / λ3=n1 / λ1+n2 / λ2 (Formula 6) or, ω1n1+ω2n2=ω3n3 (Equation 7) This is the result.

[0035] In the above equation, n1, n2, and n3 are the refractive indices of the second-order nonlinear material through which light of wavelengths λ1, λ2, and λ3 (each frequency: ω1, ω2, ω3) propagates. Equation (7) means that the weighted average of n1 and n2, weighted by frequency, is equal to n3. In particular, in second-harmonic generation, when the polarization of the coupled fundamental photons is the same, the phase matching condition is satisfied when the refractive indices of the fundamental wave and the second harmonic are equal. However, in reality, since materials always have refractive index wavelength dispersion, the phase matching condition is not easily satisfied.

[0036] (Explanation of pseudo-phase matching) The above method eliminates wavenumber mismatch, i.e., sets Δk=0. However, there is an alternative method called quasi-phase-matched (QPM), which allows wavenumber mismatch and modulates the nonlinear susceptibility to cancel out the phase shift effect. This idea was proposed by Armstrong et al. in 1962 and is a technique that achieves pseudo-phase matching by periodically reversing the sign of the nonlinear susceptibility. As described above, since nonlinear polarization increases or decreases with a period of twice the coherence length, by setting the polarization reversal period to twice the coherence length (reversing the polarization at coherence length intervals), the nonlinear polarization waves generated from each point are added together without canceling each other out, producing an effect as if the amount of phase mismatch had been made zero. If the polarization reversal period of the periodic polarization reversal structure is Λ, then from the coherence length equation (Equation 4) Λ = 2·Lc ···(Equation 8) If we consider the case where all coupled light waves travel in the same direction, then from (Equation 4), the wavenumber mismatch is not zero, Δk=2π(n3 / λ3-n1 / λ1−n2 / λ2)=2π / Λ (Equation 9) Therefore, n3 / λ3-n2 / λ2-n1 / λ1-1 / Λ=0 (Equation 10) Thus, equation (Equation 10) is the phase matching condition for QPM. Here, n3 is the refractive index at wavelength λ3, n2 is the refractive index at wavelength λ2, and n1 is the refractive index at wavelength λ1.

[0037] This QPM method has the advantage of being able to use the material orientation that results in the maximum component of the nonlinear susceptibility, such as a second-order nonlinear crystal, and the ability to set the operating wavelength range by selecting the polarization reversal period. Furthermore, by creating an optical waveguide, light can be confined to a narrow region at high density and propagated over long distances, thus enabling highly efficient wavelength conversion.

[0038] Furthermore, as mentioned above, several methods are known for fabricating wavelength conversion elements using the QPM method. For example, one method involves creating a proton exchange waveguide using a periodic polarization reversal structure on a nonlinear optical crystal substrate. Another example is a method that similarly involves creating a periodic polarization reversal structure on a nonlinear optical crystal substrate and then fabricating a ridge-type optical waveguide using a photolithography process and a dry etching process.

[0039] For the optical waveguide core of a wavelength conversion element, it is desirable that the material used is an optical crystalline material with a second-order nonlinear effect. Furthermore, for the substrate to which the optical waveguide core material is bonded, it is desirable that the material used has a similar coefficient of linear expansion to the optical waveguide core material in order to reduce the effects of fracture caused by thermal stress due to temperature changes. Specifically, suitable materials for the optical waveguide core or the bonded substrate include LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), and LiNb (x) Ta (1-x) It is desirable that the material be O3 (0 ≤ x ≤ 1) (lithium tantalate with an unstoichiometric composition), or KTiOPO4 (potassium titanate phosphate), and furthermore, that it contains at least one of Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) as an additive.

[0040] (Structure of a wavelength conversion device) Figure 1 is a perspective view showing the basic configuration 10 of a wavelength conversion device according to one embodiment of the present disclosure. The basic configuration 10 corresponds to a wavelength conversion element manufactured by the manufacturing method of the present disclosure. The basic configuration 10 shown in Figure 1 is shown as an application to a wavelength conversion device that generates wavelength-converted light using the QPM method. Figure 1 shows only the basic components of the wavelength conversion device, including the wavelength conversion element 13, the multiplexer 14, and the demultiplexer 15. The wavelength conversion element 13 includes an optical waveguide core 11 and a substrate 12, with the optical waveguide core 11 mounted on the substrate 12. The optical waveguide core 12 is made of a nonlinear optical crystal having a periodic polarization reversal structure.

[0041] The operation of the wavelength conversion device will be explained using the basic configuration shown in Figure 1. As shown in Figure 1, low-intensity signal light 1a and high-intensity excitation light 1b are incident on the multiplexer 14 and combined. The signal light 1a, combined with the excitation light 1b, propagates toward the wavelength conversion element 13 and is incident on one end of the optical waveguide core 11. As the signal light 1a propagates through the optical waveguide core 11, it is converted into difference-frequency light 1c having a different wavelength from the signal light 1a, and is emitted from the other end of the optical waveguide core 11 together with the excitation light 1b. The difference-frequency light 1c and excitation light 1b emitted from the optical waveguide core 11 are incident on the demultiplexer 15 and separated from each other. The basic configuration 10 is a wavelength conversion device that receives signal light 1a as input and generates light with a different wavelength from the signal light 1a.

[0042] In the basic configuration 10 shown in Figure 1, the wavelength conversion element has a periodic polarization reversal structure in which the polarization direction of a ferroelectric crystal or a crystal lacking a center of symmetry is periodically reversed by 180°, and has an optical waveguide core that satisfies the quasi-phase matching (QPM) condition. In this case, SHG generation and optical parametric oscillation using the wavelength conversion element by the QPM method are utilized.

[0043] Specifically, as explained in the pseudo-phase matching method above, the polarization reversal period of the periodic polarization reversal structure, referred to as the QPM condition, is set to twice the coherence length Lc. In other words, the sign of the nonlinear optical constant d of the nonlinear optical crystal is reversed for each coherence length Lc.

[0044] In an optical waveguide core equipped with a polarization reversal structure with a polarization reversal period twice the coherent length Lc, the phase of the second harmonic is reversed, correcting the phase of the composite second harmonic from the coherent length Lc. As a result, the light intensity of the generated second harmonic is added together, increasing the amplitude (intensity) of the second harmonic and generating second-harmonic light. Furthermore, in the generation of sum frequency and difference frequency, as described above, by setting the polarization reversal period of the periodic polarization reversal structure to twice the coherent length Lc, the nonlinear polarization waves are added together without canceling each other out, and the nonlinear polarization waves are amplified.

[0045] The QPM method can utilize the material orientation that results in the maximum component of the nonlinear susceptibility, such as that of a second-order nonlinear crystal. Furthermore, the QPM method has the advantage of being able to set the operating wavelength range by selecting the inversion period, and by creating an optical waveguide, it can confine light at high density in a narrow region and propagate it over long distances.

[0046] The basic configuration 10 shown in Figure 1 is known to be housed together with a multiplexer and a demultiplexer in a metal housing equipped with input / output ports capable of inputting and outputting light, in order to prevent deterioration of its characteristics due to changes in the operating environment, thereby constituting an optical conversion device. Furthermore, the wavelength conversion efficiency of the wavelength conversion element is temperature-dependent, and it is necessary to control the temperature of the wavelength conversion element in order to maximize its wavelength conversion efficiency.

[0047] (Implementation structure of wavelength conversion device) Next, the implementation structure of the wavelength conversion device will be described. Figure 2 is a diagram illustrating an example of the implementation structure of the wavelength conversion device 20, which implements the basic configuration 10 of Figure 1. The wavelength conversion device 20 shown in Figure 2 includes, in addition to the basic configuration 10 in Figure 1, a metal housing bottom member 28, a lid member 29, and a temperature control element 26. The metal housing bottom member 28 and the lid member 29 constitute the metal housing of the wavelength conversion device. In Figure 2, the outline of the lid member 29 is shown by a dashed line, and the components housed inside the metal housing are shown through it. The lid member 29, which constitutes the metal housing, is provided with an optical input port 200 and an output port 201, and these ports are shown by dotted lines.

[0048] The wavelength conversion device 20 shown in Figure 2 further includes a support member 27 that supports a temperature control element 26. The support member 27 is a metal member for uniformly controlling the overall temperature of the wavelength conversion element 13, including the optical waveguide core 11 and the substrate 12. The temperature control element 26 is interposed between the support member 27 and the metal housing bottom member 28, and the temperature control element 26, the support member 27, and the metal housing bottom member 28 are bonded and fixed using a bonding member (not shown) that has excellent thermal conductivity and is less prone to changes in its fixed position. Note that the optical waveguide core 11, substrate 12, wavelength conversion element 13, multiplexer 14, demultiplexer 15, signal light 1a, and difference frequency light 1c are the same as those described in Figure 1, so their description is omitted here.

[0049] Furthermore, when a wavelength conversion element using a nonlinear optical crystal such as a ferroelectric crystal as the optical waveguide core material is used in a wavelength conversion device, a phenomenon called optical damage occurs, in which the refractive index of the optical waveguide core changes and its properties deteriorate when irradiated with light having a short wavelength. As a method to suppress the effects of this optical damage, it has been proposed to use the wavelength conversion element at high temperatures. For this reason, in the wavelength conversion device 20 shown in Figure 2, the temperature control element 26 is controlled to operate in an environment where condensation does not occur on the wavelength conversion element 13, and within a temperature range from near room temperature to a temperature range where the adhesive fixing the components does not deteriorate. Specifically, the temperature range is set to be between approximately 20°C and approximately 100°C.

[0050] (Manufacturing method for wavelength conversion elements) Next, the manufacturing method of the wavelength conversion element 13 described in Figures 1 and 2 will be explained. Figure 3 is a diagram showing the steps for manufacturing the optical waveguide core.

[0051] A high electric field in a specific direction is applied to the entire surface of a flat optical waveguide core substrate formed from a nonlinear optical crystal, which is a wavelength conversion material, to align the entire dielectric polarization domain. (Process 31) Subsequently, a metal electrode film with a pattern corresponding to the periodic polarization reversal structure to be formed is fabricated at a desired position on the optical waveguide core substrate using photolithography. The periodic polarization reversal structure is then formed by applying a high DC electric field, and the optical waveguide core substrate is fabricated by removing the metal electrode film and insulating film. (Process 32) Next, a bonding substrate is fabricated by bonding an optical waveguide core substrate, which has a periodic polarization reversal structure formed on a substrate with a lower refractive index than the optical waveguide core at the optical wavelength used, using a plasma discharge surface activation method or a thermal bonding method, and then grinding and polishing to the desired film thickness to create the desired core layer. (Process 33) A pattern of optical waveguide cores is formed on the surface of the optical waveguide core layer on the bonded substrate using a photoresist material. The core layer is then processed into an optical waveguide core with a desired ridge shape by a dry etching method under vacuum, such as using Ar plasma. Finally, resist residue and other debris are washed away from the surface of the optical waveguide core by methods such as piranha washing to form the optical waveguide core. (Process 34)

[0052] (First Embodiment) Figure 4 is a schematic diagram illustrating the principle of adjusting the polarization reversal period of the periodic polarization reversal structure of the optical waveguide core using the manufacturing method of the wavelength conversion element according to the first embodiment of this disclosure. Referring to Figure 4, the procedure for forming an optical waveguide core by process 34 in the manufacturing method of the first embodiment of this disclosure on a junction substrate having an optical waveguide core layer on which a periodic polarization reversal region having a polarization reversal structure of a certain period has been formed by processes 31 to 33 will be described.

[0053] Figure 4 shows a periodic polarization reversal region 41 formed in the optical waveguide core layer. The polarization reversal region 41 in Figure 4 has a periodic polarization reversal structure in which the polarization is periodically reversed in one dimension from left to right in the figure. In this specification, the boundary lines that form each polarization boundary of the polarization reversal region shown in Figure 4 will be referred to as "polarization boundary lines".

[0054] Conventionally, in process 34, a linear optical waveguide core is formed perpendicular to the polarization boundary line, as shown by the dashed line in Figure 4 for the formation position of the optical waveguide core 42. In contrast, in this first embodiment, in process 34, the optical waveguide core is formed in the polarization reversal region, as shown by the solid line in Figure 4 for the optical waveguide core formation position 43. That is, this first embodiment is characterized in that, in process 34, a linear optical waveguide core is formed at a constant angle θ from perpendicular to the polarization boundary line.

[0055] In this specification, when an optical waveguide is formed at a constant angle θ perpendicular to the polarization boundary, this "constant angle perpendicular to the polarization boundary" will be referred to as the "angle of intersection with respect to the polarization reversal region" or the "angle of intersection with respect to the polarization reversal structure." Therefore, in this specification, when an optical waveguide core is formed perpendicular to the polarization boundary, as in the conventional method, the angle of intersection with respect to the polarization reversal region (structure) of that optical waveguide core is expressed as 0 degrees. When an optical waveguide is formed at a constant angle θ perpendicular to the polarization boundary as described above, the angle of intersection with respect to the polarization reversal region (structure) of the optical waveguide core is expressed as θ.

[0056] By forming an optical waveguide at an intersection angle θ with respect to the polarization reversal region (structure) in this manner, the effect is achieved that the polarization reversal period is effectively extended by 1 / COS(θ) times compared to when it is formed at an intersection angle of 0 degrees. From this, it can be understood that in process 34, by adjusting the intersection angle of the optical waveguide core formed in the periodic polarization reversal region with respect to the polarization reversal region, it is possible to fabricate optical waveguide cores with periodic polarization reversal structures having different polarization reversal periods using periodic polarization reversal regions having the same polarization reversal period.

[0057] In principle, even if the intersection angle with respect to the polarization reversal region is 45 degrees or more, it is possible to artificially extend the polarization reversal period length. However, in practice, when the intersection angle with respect to the polarization reversal region is 45 degrees or more, the optical spectral distribution of the wavelength-converted light generation becomes blunted, i.e., the peak full width at half maximum increases. This is thought to be because the polarization boundary of the polarization reversal period becomes indistinct. In order to prevent the polarization boundary of the polarization reversal period from becoming indistinct, it is desirable for the intersection angle θ with respect to the polarization reversal region to be small, and practically speaking, it is desirable for it to be 30 degrees or less.

[0058] Next, with reference to Figure 5, the manufacturing method of the first embodiment of this disclosure will be described. Figure 5(a) shows a bonded substrate 50 in which a single periodic polarization reversal region 51 having a polarization reversal period L is formed in the core layer by processes 31 to 33 of Figure 3. In this example, the case in which optical waveguide cores are created in process 34 at the optical waveguide core formation positions indicated by lines 52 and 53 in Figure 5 is described. The periodic polarization reversal region 51 in Figure 5 has a periodic polarization reversal structure in which the polarization is reversed in one dimension from left to right in the drawing with a single polarization reversal period, similar to Figure 4.

[0059] The materials used for the optical waveguide core substrate or the substrate to be bonded are preferably LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb(x)Ta(1-x)O3 (0≦x≦1) (lithium tantalate with an unstoichiometric composition), or KTiOPO4 (potassium titanate phosphate), and further preferably materials containing at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) as an additive.

[0060] In this example, the lines indicate the locations where each waveguide core layer is formed when three optical waveguide cores are formed in the periodic polarization reversal region of an optical waveguide core layer on a single junction substrate: an optical waveguide core 52 formed at an intersection angle θ2 with respect to the polarization reversal region, an optical waveguide core 53 formed at an angle θ1, and an optical waveguide core 54 formed at an angle of 0 degrees.

[0061] As shown in Figure 5(b), the optical waveguide cores 52, 53, and 54 formed at the formation positions indicated by lines 52 to 54 in Figure 5 have different intersection angles with the polarization reversal region for the portion formed in the periodic polarization reversal region. This makes it possible to manufacture a wavelength conversion element having a periodic polarization reversal structure with a polarization reversal period different from the polarization reversal period L.

[0062] As is clear from this explanation, in the manufacturing method of this first embodiment, in process 34 of Figure 3, it is possible to form a wavelength conversion element having an optical waveguide core with a polarization reversal period L by selecting the intersection angle of the optical waveguide core formed in the optical waveguide core layer with respect to the polarization reversal region. Therefore, for example, it is possible to create a wavelength conversion element in which the polarization reversal period of the polarization reversal region is adjusted at the stage of process 34 in response to processing errors that occur in processes 31 to 33 of Figure 3. However, it is not limited to this, and it is also possible to create a wavelength conversion element in which the polarization reversal period can be discretely selected by fabricating a wavelength conversion element having multiple optical waveguide cores with different intersection angles with respect to the polarization reversal region and selecting one of them when mounting it in a wavelength conversion device.

[0063] (Second embodiment) FIG. 6 is a schematic diagram for explaining the manufacturing method of the second embodiment of the present disclosure. In the manufacturing method of the second embodiment of the present disclosure, as shown in FIG. 6, in the process 32 shown in FIG. 3, a plurality of periodically poled regions with different poling inversion periods of at least two or more are formed in an array in the direction of the poling boundary line on the optical waveguide core substrate, and it is characterized in that which periodically poled region with which poling inversion period is used to form the optical waveguide core can be selected in the process 34 which is a subsequent process.

[0064] In the process 32, for example, by forming electrodes corresponding to patterns of periodically poled regions with different poling inversion periods on the surface of the optical waveguide core substrate, a plurality of poled regions can be formed on one optical waveguide core substrate.

[0065] FIG. 6 shows a bonding substrate 60 in which three periodically poled regions 61, 62, 63 are formed in the optical waveguide core layer as an example for explaining the manufacturing method of the second embodiment. Each periodically poled region has a periodically poled structure in which poling inversion is performed one-dimensionally from left to right in the figure. In this example, the poling inversion periods of each periodically poled region are different, and the poling inversion period lengths of the periodically poled regions 61, 62, 63 are L1, L2, and L3, respectively, and the relationship of each period is set to L1 < L2 < L3.

[0066] Furthermore, in FIG. 6, on the bonding substrate 60, the positions where the optical waveguide core 64 formed passing over the periodically poled region 61, the optical waveguide core 65 formed passing over the periodically poled region 62, and the optical waveguide core 66 formed passing over the periodically poled region 62 are formed are shown as lines 64 to 66. Also in the second embodiment, as in the first embodiment, in the process 34 which is a subsequent process to the processes 31 to 33, a position of any one of the optical waveguide cores 64 to 66 is selected to form the optical waveguide core.

[0067] The materials used for the optical waveguide core substrate or the substrate to be bonded are preferably LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb(x)Ta(1-x)O3 (0≦x≦1) (lithium tantalate with an unstoichiometric composition), or KTiOPO4 (potassium titanate phosphate), and further preferably materials containing at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) as an additive.

[0068] In this second embodiment, in process 34, by selecting the position for forming the optical waveguide core, that is, by selecting the periodic polarization reversal region through which the optical waveguide core passes, it is possible to select which periodic polarization reversal region to use to form the optical waveguide core layer. As a result, it becomes possible to fabricate wavelength conversion elements with optical waveguide cores having periodic polarization reversal structures with different polarization reversal periods using a single substrate.

[0069] Therefore, for example, it became possible to create a wavelength conversion element in which the polarization reversal period of the polarization reversal region is adjusted at the stage of process 34 to correspond to the processing errors that occur in processes 31 to 33 in Figure 3.

[0070] Furthermore, when aligning the position of the optical waveguide core to select a desired periodic polarization reversal region, the position of the optical waveguide core can be aligned using, for example, an alignment marker. Moreover, it is also possible to create a wavelength conversion element that allows for discrete selection of polarization reversal periods by fabricating a wavelength conversion element having multiple optical waveguide cores 64 to 66 and then installing it in a wavelength conversion device, by selecting one of the cores.

[0071] As described above, by using the manufacturing method of this second embodiment, it is possible to increase the yield and obtain a wavelength conversion element having the desired optical properties.

[0072] In FIG. 6, an example is shown in which the number of periodically poled regions having different polarization inversion periods is three. However, the number of periodically poled regions may be at least two or more, as long as it can discretely adjust the adjustment range of the required polarization inversion period. In this case, a larger number of periodically poled regions is desirable because finer adjustment is possible. Also, the intervals between the polarization inversion period lengths of each periodically poled region do not need to be equal. For example, for a period range that requires finer adjustment, a plurality of periodically poled regions with short intervals between the period lengths of the polarization inversion periods are formed, and for other period ranges, a plurality of periodically poled regions with long intervals between the period lengths of the polarization inversion periods are formed, so that it is possible to practically adjust to a desired polarization inversion period.

[0073] Next, referring to FIG. 7, another aspect of the manufacturing method of the second embodiment of the present disclosure will be described. The difference between FIG. 7 and FIG. 6 is that in the example of FIG. 6, each optical waveguide core formed to select a periodically poled region is formed linearly and the formation positions of the input / output ends are different, whereas in the aspect of FIG. 7, the positions where the input / output ends of each optical waveguide core are formed are fixed.

[0074] Also in the aspect shown in FIG. 7, similar to FIG. 6, in process 32 shown in FIG. 3, a plurality of periodically poled regions having different polarization inversion periods of at least two or more are formed in an array in the direction of the polarization boundary line on the optical waveguide core substrate, and which polarization inversion period's periodically poled region is used to form the optical waveguide core is selected in process 34, which is a subsequent manufacturing process.

[0075] The bonding substrate 60 shown in FIG. 7 is the same as that in FIG. 6. Also in FIG. 7, three periodically poled regions 61, 62, 63 are formed in the optical waveguide core layer of the bonding substrate 60. Each periodically poled region has a periodically poled structure that is polarized and inverted one-dimensionally from left to right in the figure, and the polarization inversion periods of the periodically poled regions 61, 62, 63 are L1, L2, and L3, respectively, and the relationship between each period is set to L1 < L2 < L3.

[0076] As shown by lines 74, 75, and 76 in Figure 7, in this embodiment, regardless of which of the optical waveguide cores 74, 75, or 76 is formed in process 34, the positions of the input and output ends of the formed optical waveguide core are the same.

[0077] According to this manufacturing method, by selecting and determining the position for forming the optical waveguide core in the subsequent process 34, it becomes possible to realize a wavelength conversion element equipped with an optical waveguide core having a periodic polarization reversal structure with different polarization reversal periods, using the same optical waveguide chip shape where the positions of the input and output light are fixed. As a result, it becomes possible to adjust and control the optical properties of the wavelength conversion element to desired optical properties.

[0078] In the embodiment shown in Figure 7, the approach to setting the number of periodic polarization reversal regions and the interval of polarization reversal periods between multiple periodic polarization reversal regions is the same as in the second embodiment shown in Figure 6, so an explanation is omitted here.

[0079] Furthermore, another aspect of the second embodiment of this disclosure will be described with reference to Figure 8. The difference between the aspects of Figures 6 and 7 and Figure 8 is that in Figures 6 and 7, the optical waveguide core formed in process 34 is formed to pass through one periodic polarization reversal region, whereas in the example of Figure 8, it is formed to pass through multiple periodic polarization reversal regions.

[0080] In this embodiment of the manufacturing method, in process 32 shown in Figure 3, at least two or more periodic polarization reversal regions with different polarization reversal periods are formed and arranged in an array in the direction of the polarization boundary on the optical waveguide core substrate, and in a later manufacturing step, process 34, the polarization reversal period region to be used to form the optical waveguide core is selected.

[0081] As shown in Figure 8(a), in this example, in process 32, multiple periodic polarization reversal regions 81 and 82 with different polarization reversal periods are formed in the optical waveguide core layer of the junction substrate. Each periodic polarization reversal region has a periodic polarization reversal structure obtained by reversing the polarization in one dimension from left to right in the figure. The polarization reversal periods of periodic polarization reversal regions 81 and 82 are set to L1 and L2, respectively, and the relationship between the periods is set to L1 > L2.

[0082] As shown by lines 84 and 85 in Figure 8(a), this example illustrates a manufacturing method for forming an optical waveguide core 84 in process 34. By forming the optical waveguide core 84 as shown by line 84 in Figure 8(a), it is possible to form an optical waveguide core having a periodic polarization reversal structure with locally different polarization reversal periods, although some pulse-wave-like disturbances in the polarization reversal period occur at points where the two different periodic polarization reversal regions 81 and 82 are crossed, as shown in Figure 8(b).

[0083] In this way, by forming a single optical waveguide core so that it passes through multiple periodic polarization reversal regions having different polarization reversal periods, it becomes possible to adjust and control the local polarization reversal period of the periodic polarization reversal structure of the optical waveguide core.

[0084] In the example shown in Figure 8, for the sake of simplicity, an example is shown in which two periodic polarization reversal regions with different polarization reversal periods are formed. However, it is also possible to form three or more periodic polarization reversal regions and arrange them so that they intersect with each other. Furthermore, the number of periodic polarization reversal regions should be sufficient to allow for discrete adjustment of the required polarization reversal period adjustment range. In this case, a larger number of periodic polarization reversal regions is desirable because it allows for finer adjustments. Also, the intervals between the polarization reversal periods of each periodic polarization reversal region do not need to be equal.

[0085] Furthermore, in this example, the optical waveguide core 84 is formed to traverse from the periodic polarization reversal region 82 to 81, and then from 81 to 82. However, the number of traverses and the locations of the traverses can be appropriately set according to the required adjustment range of the polarization reversal period.

[0086] As described above, in this example, in process 34, the position for forming the optical waveguide core can be selected, and the periodic polarization reversal region through which the optical waveguide core passes can be locally selected to form the optical waveguide core layer. As a result, it becomes possible to fabricate a wavelength conversion element with an optical waveguide core having a periodic polarization reversal structure with locally different polarization reversal periods using a single substrate.

[0087] Therefore, for example, it became possible to fabricate a wavelength conversion element in which the polarization reversal period of the polarization reversal region is locally adjusted at the stage of process 34, in response to the processing errors that occur in processes 31 to 33 in Figure 3.

[0088] (Third embodiment) Figure 9 is a schematic diagram illustrating the manufacturing method of the third embodiment of the present disclosure. In the manufacturing method of the third embodiment of the present disclosure, in process 32 shown in Figure 3, at least four or more periodic polarization reversal regions, each having a polarization reversal structure with different polarization reversal periods, are formed on the optical waveguide core substrate in a two-dimensional array arrangement, not only in the direction of the polarization boundary line but also in a direction perpendicular to the polarization boundary line. In a later manufacturing step, process 34, the choice of which periodic polarization reversal region to use to form the optical waveguide core is made.

[0089] Fig. 9(a) shows, as an example for explaining the manufacturing method of the third embodiment, a bonded substrate 90 in which nine periodically poled inversion regions 911 to 913, 921 to 923, and 931 to 933 are two-dimensionally arranged in a 3×3 array, with three regions in the direction of the polarization boundary line and three regions in the direction perpendicular to the polarization boundary line in the optical waveguide core layer. Each periodically poled inversion region has a periodically poled inversion structure that is poled inversion one-dimensionally from left to right in the figure. In this example, each periodically poled inversion region is formed by any one of the poled inversion structures A, B, and C having three different poled inversion periods, as shown by the patterns A to C in the figure. The poled inversion structures A, B, and C have poled inversion period lengths of L1, L2, and L3, respectively, and the relationship between the periods is set to L1 < L2 < L3.

[0090] Furthermore, in Fig. 9(a), in the bonded substrate 90, the positions where the optical waveguide core 94 formed passing over the periodically poled inversion regions 911 to 913, the optical waveguide core 95 formed passing over the periodically poled inversion regions 921 to 923, and the optical waveguide core 96 formed passing over the periodically poled inversion regions 931 to 933 are formed are indicated by the lines 94 to 96. Also in the third embodiment, any one of the optical waveguide cores 94 to 96 is formed by the process 34 which is the process after the processes 31 to 33.

[0091] As the material used for the optical waveguide core substrate or the substrate to be bonded, it is desirable that it be LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb(x)Ta(1 - x)O3 (0 ≦ x ≦ 1) (lithium tantalate with non-stoichiometric composition), or KTiOPO4 (potassium titanyl phosphate), and furthermore, a material containing at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) as an additive.

[0092] In this example, the periodic polarization reversal regions 911 to 913, 921 to 923, and 931 to 933 selected by the optical waveguide formation positions 94 to 96 each contain regions composed of three polarization reversal structures A, B, and C with different polarization reversal periods, and the order of the regions consisting of the three polarization reversal structures A, B, and C from left to right in the diagram is different from one another.

[0093] Therefore, according to this embodiment, in process 34, by selecting the position where the optical waveguide core is formed, it is possible to select whether to form the optical waveguide core using polarization reversal regions 911 to 913, 921 to 923, or 931 to 933. As a result, it is possible to fabricate a wavelength conversion element on a single substrate that has an optical waveguide core with a polarization reversal structure having locally different polarization reversal periods, and an optical waveguide core having different local distributions of polarization reversal periods.

[0094] Figure 9(b) shows the local distribution of the polarization reversal period of the periodic polarization reversal region of the optical waveguide core formed at positions corresponding to lines 94 to 96 shown in Figure 9(a), using the same type of line. The optical waveguide core formed by selecting the position indicated by line 94 in Figure 9(a) has the local distribution of the polarization reversal period shown by line 94 in Figure 9(b). The local distribution of the polarization reversal period of optical waveguide cores 95 and 96 formed by selecting the positions indicated by lines 95 and 96 is shown similarly.

[0095] Therefore, for example, by arranging multiple patterns of periodic polarization reversal regions on the substrate in two dimensions, corresponding to local polarization reversal periods that need to be corrected in response to the expected film thickness distribution fluctuations, based on film thickness distribution data of the optical waveguide core resulting from processing errors in past manufacturing processes, it has become possible to fabricate a wavelength conversion element having an optical waveguide core with a local distribution of polarization reversal periods corresponding to the film thickness distribution fluctuation patterns expected to occur due to processing errors. Furthermore, in this example, regardless of whether the formation position of the optical waveguide core is selected from 94 to 96, the formed optical waveguide core contains one region each consisting of three types of polarization reversal structures A, B, and C. As a result, if there are no local changes in film thickness, and the effective refractive index of the optical waveguide core is the same for all polarization reversal periods 911 to 933, then optical waveguides that satisfy the same phase matching conditions can be obtained for each optical waveguide core 94 to 96.

[0096] Figure 9 shows a 3x3 two-dimensional array of periodic polarization reversal regions placed on the bonding substrate 90. However, the number of periodic polarization reversal regions placed on the bonding substrate 90 may be 2x2 (4 regions) or more, and the number of regions placed in the direction of the polarization boundary (up and down in the figure) and the direction perpendicular to the polarization boundary (left and right in the figure) may differ. The number of periodic polarization reversal regions placed on the bonding substrate 90 should be appropriately selected according to the film thickness distribution variation pattern assumed to be corrected. Figure 9 shows three types of polarization reversal structures with different polarization reversal period lengths, but there may be four or more types. In this case, by providing many types of polarization reversal structures with different polarization reversal period lengths, it becomes possible to finely adjust the local distribution of the polarization reversal period in accordance with the film thickness distribution pattern. Furthermore, the spacing of the polarization reversal period lengths between types of polarization reversal structures with different polarization reversal periods does not need to be equal. Furthermore, in Figure 9, the periodic polarization reversal regions selected by the formation position of the optical waveguide core were all composed of periodic polarization reversal regions consisting of three types of polarization reversal structures with different polarization reversal periods. However, the types of polarization reversal structures constituting the selected periodic polarization reversal region do not have to be the same. Alternatively, some of the types of polarization reversal periodic structures may be selected from those prepared in advance. In Figure 9, the positions for forming the optical waveguide core are shown as 94 to 96, but there may be two or more types, or even four or more types. Moreover, it is not limited to this, but it is also possible to create a wavelength conversion element that allows discrete selection of polarization reversal periods by fabricating a wavelength conversion element having multiple optical waveguide cores 94 to 96 and then selecting one of them when mounting it in a wavelength conversion device.

[0097] Next, another aspect of the manufacturing method of the third embodiment will be described using Figure 10. The difference between Figure 10 and Figure 9 is that in the embodiment of Figure 9, each optical waveguide core 94 to 96 formed to select the periodic polarization reversal region is formed linearly and the formation positions of the input and output ends are different, whereas in the embodiment of Figure 10, the positions where the input and output ends of each optical waveguide core are formed are fixed. The bonding substrate 90 shown in Figure 10 is the same as that shown in Figure 9, and parts with the same reference numerals are the same as in Figure 9, so their explanation here will be omitted.

[0098] As shown by lines 104, 105, and 106 in Figure 10, in this embodiment of the manufacturing method, regardless of whether optical waveguide cores 104, 105, or 106 are formed in process 34, the positions of the input and output ends of the formed optical waveguide cores are the same. According to this embodiment of the manufacturing method, similar to Figure 10, it is possible to realize, on a single substrate, an optical waveguide core equipped with a polarization reversal structure having locally different polarization reversal periods, and a wavelength conversion element equipped with an optical waveguide core having a different local distribution of polarization reversal periods, in the same optical waveguide chip shape where the positions of the input and output light are fixed. In Figure 10, the concept regarding the number of periodic polarization reversal regions arranged on the junction substrate 90, the number of types of polarization reversal structures with different periods used, and the arrangement order is the same as in the third embodiment of Figure 9, so an explanation is omitted here.

[0099] (Fourth embodiment) Figure 11 is a schematic diagram illustrating a manufacturing method according to a fourth embodiment of the present disclosure. In the fourth embodiment of the present disclosure, in process 32 shown in Figure 3, the periodic polarization reversal regions are formed in a two-dimensional array arrangement, similar to the third embodiment, and in process 34, the position for forming the optical waveguide core is selected to determine the polarization reversal region through which the optical waveguide core passes, thereby determining which periodic polarization reversal region will be used to manufacture the optical waveguide core. In the fourth embodiment, in process 34, the position for forming the optical waveguide core is determined to be a path where the intersection angle with respect to the polarization reversal region is not 0 degrees but a predetermined angle.

[0100] In the second and third embodiments, only the polarization reversal period length of the selected periodic polarization reversal region was directly utilized. In contrast, in the fourth embodiment, as described in the first embodiment, the effect of extending the polarization reversal period by 1 / COS(θ) times is utilized by changing the intersection angle θ of the optical waveguide core passing over the periodic polarization reversal region with respect to the polarization reversal region. Thus, in the fourth embodiment, not only is the polarization reversal period of each periodic polarization reversal region arranged in a two-dimensional array utilized, but it is also possible to fine-tune the values ​​of the polarization reversal periods between discrete periodic polarization reversal regions.

[0101] Figure 11 shows an example in which 24 polarization reversal regions are arranged on the bonded substrate 110 in a 6x4 two-dimensional array, each composed of one of three polarization reversal structures A, B, and C with different period reversal periods of period lengths L1, L2, and L3. In this fourth embodiment of the manufacturing method, the process 32 shown in Figure 3 is used to form the multiple periodic polarization reversal regions shown in Figure 11 on the optical waveguide core substrate. As is clear from the figure, the 24 polarization reversal regions in Figure 11 are arranged such that adjacent periodic polarization reversal regions have different polarization reversal periods. Furthermore, the three types of polarization reversal regions composed of polarization reversal structures A, B, and C are arranged in the same repeating pattern in the vertical direction (parallel to the polarization boundary line) and the horizontal direction (perpendicular to the polarization boundary line). These periodic polarization reversal regions are equipped with a periodic polarization reversal structure that is polarized in a one-dimensional manner from left to right in the figure.

[0102] Furthermore, in Figure 11, the positions where the optical waveguide cores 114 to 116 are formed in the optical waveguide core layer of the bonded substrate 110 during process 34 shown in Figure 3 are indicated by lines 114 to 116. In the manufacturing method of the fourth embodiment, as in the manufacturing methods of the first to third embodiments, one of the optical waveguide cores 114 to 116 is formed in process 34, which is a step after processes 31 to 33.

[0103] The materials used for the optical waveguide core substrate or the substrate to be bonded are preferably LiNbO3 (lithium niobate), KNbO3 (potassium niobate), LiTaO3 (lithium tantalate), LiNb(x)Ta(1-x)O3 (0≦x≦1) (lithium tantalate with an unstoichiometric composition), or KTiOPO4 (potassium titanate phosphate), and further preferably materials containing at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) as an additive.

[0104] The optical waveguide core 114 formed at the position indicated by line 114 in Figure 11 is formed to have a polarization reversal structure formed at the same polarization reversal region intersection angle, with a periodic polarization reversal region composed of periodic polarization reversal structures C having the same polarization reversal period L3. Therefore, in this case, when the polarization reversal region intersection angle is θ, it is possible to form an optical waveguide core having a polarization reversal structure with a polarization reversal period length of L3 / COS(θ). Furthermore, as shown by lines 115 and 116 in Figure 11, by selecting the formation position of the optical waveguide core by setting the polarization reversal region intersection angle of only a portion of the optical waveguide core passing through the periodic polarization reversal region to a predetermined angle θ, it is possible to form an optical waveguide core having a periodic polarization reversal structure with locally different polarization reversal periods.

[0105] Thus, in the fourth embodiment of this disclosure, by selecting the position for forming the optical waveguide core in process 34, it is possible to select a periodic polarization reversal region for forming the optical waveguide core, and to adjust the polarization reversal region intersection angle θ with the selected periodic polarization reversal region.

[0106] In the fourth embodiment of the present disclosure, in process 34, it becomes possible to more freely adjust the local distribution of the periodic poling inversion period length of the optical waveguide core. For example, in FIG. 12(a), as shown by lines 124 to 126, the formation positions of each of the lines 124 to 126 are set such that the optical waveguide core is formed so as to pass through a periodic poling inversion region configured with the same poling inversion period structure at an intersection angle with respect to a predetermined poling inversion region. For example, by selecting the formation position of line 124, the periodic poling inversion regions at the positions where the optical waveguide core is formed are all configured by a poling inversion structure A with a poling inversion period of L1. The optical waveguide core formed at the position shown by line 124 is formed at an intersection angle of a predetermined angle θ with respect to the poling inversion region. The optical waveguide cores formed at the positions shown by lines 125 and 126 are the same except that the poling inversion periods of the selected poling inversion regions are L2 and L3, respectively. Therefore, as shown by 124 to 126 in FIG. 12(b), the optical waveguide cores 124 to 126 formed at the positions of lines 124 to 126 will exhibit poling inversion periods that are constant and larger than the respective periods L1, L2, L3 (L1 < L2 < L3), namely, L4, L5, and L6.

[0107] Also, for example, as shown by lines 134 to 136 in FIG. 13(a), the formation position of the optical waveguide core may be selected so that the optical waveguide core passes over the periodic poling inversion region with the intersection angle with respect to the poling inversion region being as close to 0 degrees as possible. In this case, as shown in the figure, between the periodic poling inversion regions, the positions where the optical waveguide core is formed are selected so as to be connected by an S-shaped curve.

[0108] Figure 13(b) shows the polarization reversal periods of the optical waveguide cores 134 to 136 formed at the positions indicated by lines 134 to 136 in process 34. As shown in Figure 13(b) for lines 134 to 136, the optical waveguide cores 134 to 136 formed at the positions indicated by lines 134 to 136 in Figure 13(a) have a slight pulse-wave-like disturbance in the polarization reversal period at the S-shaped curves because the intersection angle with respect to the polarization reversal region is not 0 degrees. However, the polarization reversal periods are generally set to L1, L2, and L3. Thus, when connecting the positions where optical waveguide cores are formed between periodic polarization reversal regions with an S-shaped curve, it is possible to set the intersection angle with respect to the polarization reversal region to any angle other than 0 degrees.

[0109] In this embodiment, the periodic polarization reversal regions arranged on the bonding substrate 110 are shown in a 6x4 two-dimensional array, but the number of periodic polarization reversal regions arranged on the bonding substrate may be other. The number of periodic polarization reversal regions arranged on the bonding substrate 110 should be appropriately selected according to the film thickness distribution variation pattern assumed to be corrected. In addition, although three types of polarization reversal structures with different polarization reversal period lengths are shown, there may be four or more types. In this case, by providing many types of polarization reversal structures with different polarization reversal period lengths, it becomes possible to finely adjust the local distribution of the polarization reversal period in accordance with the film thickness distribution pattern. Furthermore, the intervals between the polarization reversal period lengths of the types of polarization reversal structures with different polarization reversal periods do not need to be equal.

[0110] Next, with reference to Figure 14, another embodiment of the manufacturing method of the fourth embodiment of this disclosure will be described. As shown in Figure 14(a), in this embodiment, 24 polarization reversal regions are formed on the junction substrate 140 in a 4 × 6 two-dimensional array, each composed of one of three polarization reversal structures A, B, and C with different polarization reversal periods of period lengths L1, L2, and L3, in process 32 of Figure 3.

[0111] The multiple periodic polarization reversal regions shown in Figure 14 are arranged such that adjacent periodic polarization reversal regions each have a different polarization reversal structure. In addition, in the vertical direction of the figure (parallel to the polarization boundary line), three types of polarization reversal regions composed of polarization reversal structures A, B, and C are repeated in the order A, B, and C, and in the horizontal direction of the figure (perpendicular to the polarization boundary line), a pattern is set up in which two arrangements, each shifted vertically by one, are repeated three times in the horizontal direction. When using this arrangement of polarization reversal regions, by selecting the formation position of a waveguide core that is bent in a zigzag pattern as shown by line 1441 in Figure 14(a), it is possible to form an optical waveguide core using a path that passes through polarization reversal regions with the same periodic polarization reversal period (in line 1441, the polarization reversal region composed of polarization reversal structure A) and whose intersection angle with respect to the polarization reversal region is approximately a predetermined angle.

[0112] In optical waveguide cores formed at such path locations, as shown by 1441 in Figure 14(b), some pulse-like disturbances in the polarization reversal period occur at the bent portion of the optical waveguide core, but generally, the polarization reversal period length is longer than L1 depending on the intersection angle with respect to the polarization reversal region. If the waveguide core formation position shown by line 1442 in Figure 14(a) is selected, the intersection angle with respect to the polarization reversal region can be made smaller than when line 1441 is selected. As shown in Figure 14(b), the polarization reversal period of the optical waveguide core formed at this path location has a period length smaller than 1441. The same applies to 1451, 1452, 1461, and 1462, except that the magnitude of the polarization reversal period length changes with respect to L2 and L3.

[0113] As described above, this embodiment not only utilizes the polarization reversal period of each polarization reversal region arranged in a two-dimensional array, but also makes it possible to fine-tune the values ​​of the polarization reversal periods between discrete periodic polarization reversal regions.

[0114] In this embodiment as well, as in the example described in FIG. 13, a path may be used in which the positions where the optical waveguide core is formed between the periodically poled inversion regions are connected by an S-shaped curve, and the formation position of the optical waveguide core is selected so as to pass over the periodically poled inversion region such that the intersection angle between the periodically poled inversion regions where the optical waveguide core is formed becomes 0 degrees as much as possible. Also, regarding the number of periodically poled inversion regions arranged on the bonding substrate and the number of types of periodically poled inversion structures having different poling inversion periods, and regarding the materials of the optical waveguide core substrate and the substrate to be bonded, the concepts are the same as those in the manufacturing method of the embodiment of FIG. 11, and thus the description here is omitted.

[0115] Furthermore, referring to FIG. 15, another aspect of the manufacturing method of the fourth embodiment of the present disclosure will be described. As shown in FIG. 15(a), in this aspect, on the bonding substrate 150, 88 poled inversion regions, each of which is constituted by any one of the poled inversion structures A, B, and C having different poling inversion periods of three types of period lengths L1, L2, and L3 (L1 < L2 < L3), are formed in a two-dimensional array of 8×11 on the optical waveguide core substrate in the process 32 of FIG. 3.

[0116] The plurality of periodically poled inversion regions shown in FIG. 15(a) are arranged such that the three types of periodically poled inversion regions are symmetric about the sixth column 151 from the left. Also, in this aspect, adjacent periodically poled inversion regions are arranged to have different poling inversion periods, and in the vertical direction (the direction parallel to the polarization boundary line) of the figure, they are arranged to have the same repeating pattern, and in the horizontal direction (the direction perpendicular to the polarization boundary line) of the figure, they are arranged to have the same repeating pattern from the column 151 in the horizontal direction. For example, when producing a bonded substrate through temperature changes such as plasma or thermal bonding, where the elastic modulus (Young's modulus) and the thermal expansion coefficient of the substrate material and the core material are different, warping is likely to occur symmetrically about the center of the substrate (wafer). Therefore, after grinding and polishing the substrate, an empirical tendency exists for the film thickness to change symmetrically about the center of the substrate (wafer). Thus, arranging it in an approximately centrosymmetric manner as shown in FIG. 15(a) is useful when manufacturing a wavelength conversion element using the bonded substrate. Furthermore, in Figure 15(a), the positions where the optical waveguide cores 154 to 156 are formed in the optical waveguide core layer of the bonded substrate 150 during process 34 shown in Figure 3 are indicated by lines 154 to 156. In this manufacturing method as well, similar to the manufacturing methods of each embodiment described above, one of the optical waveguide cores 154 to 156 is formed in process 34, which is a step after processes 31 to 33.

[0117] By selecting the positions indicated by lines 154 to 156 in Figure 15(a) to form the optical waveguide core, as shown by line 154 in Figure 15(b), the optical waveguide core formed by selecting line 154 will have a local distribution of polarization reversal periods with a downward convex shape in the central part. Similarly, the optical waveguide core formed by selecting line 155 will have a local distribution of polarization reversal periods with an upward convex shape in the central part, and the optical waveguide core formed by selecting line 156 will have a local distribution of polarization reversal periods with an upward convex shape only on the right side. In this example as well, the approach to the number of periodic polarization reversal regions to be placed on the bonded substrate, the number of types of periodic polarization reversal structures with different polarization reversal periods, and the materials of the optical waveguide core substrate and the substrate to be bonded are the same as in the manufacturing method of the embodiment in Figure 11, so the explanation is omitted here.

[0118] As described above, in the manufacturing method of the fourth embodiment of this disclosure, by selecting the arrangement of pre-formed periodic polarization reversal regions in the process of forming the bonded substrate, and by selecting the formation position of the optical waveguide core in the subsequent process of forming the optical waveguide, it becomes possible to select, adjust, and control any change in the polarization reversal period at the stage of the post-process of optical waveguide core processing. As a result, it becomes possible to fabricate a wavelength conversion element equipped with an optical waveguide core having a periodic polarization reversal structure with locally different polarization reversal periods using a single substrate.

[0119] Therefore, for example, it became possible to fabricate a wavelength conversion element in which the polarization reversal period of the polarization reversal region is locally adjusted at the stage of process 34, in response to the processing errors that occur in processes 31 to 33 in Figure 3. [Examples]

[0120] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples. (Example 1) As Example 1, an optical wavelength conversion element was fabricated using the manufacturing method of the first embodiment of this disclosure. In Example 1, the polarization reversal region shown in Figure 5(a) was formed on the optical waveguide core substrate by processes 31 and 32 in Figure 3. Specifically, the front and back surfaces of a Z-axis cut LiNbO3 substrate were immersed in a lithium chloride aqueous solution, and a voltage of DC 1kV or higher was applied to align the LiNbO3 polarization domains across the entire substrate. A photoresist pattern of several μm thickness with a 30 × 30 mm square periodic polarization reversal pattern was formed on one surface, and an Au metal film was deposited over the entire surface where the photoresist was formed. Subsequently, the front and back surfaces were immersed again in a lithium chloride aqueous solution, and the polarization was reversed by applying a voltage of DC 1kV or higher to fabricate a LiNbO3 substrate (optical waveguide core substrate) having a 30 × 30 mm square periodic polarization reversal region.

[0121] Subsequently, in process 33, a bonded substrate was fabricated. Specifically, the above-mentioned LiNbO3 substrate was bonded to a Z-axis cut LiTaO3 substrate, and then thinned by grinding and polishing to create a bonded substrate with an optical waveguide core layer having a partial 30 × 30 mm square periodic polarization reversal region.

[0122] Then, in process 34, an optical waveguide core pattern was formed using photoresist with a predetermined intersection angle with respect to the periodic polarization reversal region exemplified in Figure 5(a), and a ridge-shaped optical waveguide was fabricated by dry etching with Ar plasma. In Example 1, for comparison, optical waveguide cores were fabricated at the positions indicated by lines 52, 53, and 54 in Figure 5(a).

[0123] The optical properties of the optical waveguide were evaluated by optical connection using polarization-maintaining fibers with spherically processed tips. The transmission loss spectrum around 1550 nm and the emission spectrum of second-harmonic light (SHG light, (SHG: Second Harmonic Generation)) around 775 nm were evaluated using a tunable light source, an SC light source, and an optical spectrum analyzer. As a result, even for optical waveguides formed by the same periodic polarization reversal region, a comparison of optical waveguides with different crossing angles to the polarization reversal region showed that as the crossing angle increased, the SHG light wavelength also became longer. This result indicates that the wavelength-converted light can be controlled by selecting the crossing angle to the polarization reversal region in process 34 of forming the optical waveguide core. Therefore, it was shown that errors can be compensated by adjusting the polarization reversal period of the polarization reversal structure of the optical waveguide core.

[0124] (Example 2) Next, Example 2 will be described with reference to Figure 16. In Example 2, a wavelength conversion element was manufactured using the manufacturing method of the fourth embodiment. In processes 31 and 32 of Figure 3, 24 periodic polarization reversal regions were formed on the optical waveguide core substrate in a 6x4 two-dimensional array as shown in Figure 16. Specifically, similar to Example 1 described above, the front and back surfaces of a Z-axis cut LiNbO3 substrate were immersed in a lithium chloride aqueous solution, and a voltage of DC 1kV or higher was applied to align the polarization domains of LiNbO3 across the entire substrate surface.

[0125] Then, on one surface, with an in-plane size of 10 mm × 5 mm, as shown in Figure 16, 24 polarization reversal structures A, B, and C, each with different polarization reversal periods of three different period lengths L1, L2, and L3, were formed in a 6 × 4 two-dimensional array. In addition, a photoresist pattern several μm thick was formed at the position where the waveguide core 167 in Figure 16 was formed, with a pattern corresponding to one polarization reversal region having a period length L2 for comparison. An Au metal film was then deposited over the entire surface on which the photoresist was formed.

[0126] Subsequently, the front and back surfaces were again immersed in a lithium chloride aqueous solution, and polarization reversal was induced by applying a voltage of DC 1kV or higher, thereby fabricating a LiNbO3 substrate (optical waveguide core substrate) having a 40mm × 30mm square region with multiple periodic polarization reversal regions arranged as shown in Figure 16. However, for comparison purposes, a 40mm × 5mm region with a periodic polarization reversal of L2 was also fabricated on a part of the substrate, as shown in Figure 16. In this example, the polarization reversal periods L1, L2, and L3 were set to 16.9μm, 17.0μm, and 17.1μm, respectively.

[0127] Subsequently, in process 33, a substrate with an optical waveguide core layer (bonded substrate) with a thickness of approximately 6 μm was fabricated by bonding the LiNbO3 substrate with the Z-axis cut LiTaO3 substrate, grinding, and polishing to create a thin film. The arrangement of the 24 polarization reversal regions formed in the optical waveguide core layer of the bonded substrate in this embodiment is the same as the arrangement shown in Figure 11. In Figure 16, the orientation of the polarization reversal boundary lines of the polarization reversal structure of the periodic polarization reversal region formed on the bonded substrate 160 differs from that shown in Figure 11 in that it is formed at an angle with respect to each side of the substrate. Due to this difference, even when forming the optical waveguide core at a predetermined angle of intersection with respect to the periodic polarization reversal region, it is possible to form a linear optical waveguide core pattern.

[0128] Using the bonded substrate obtained in this way, in process 34, linear optical waveguide core patterns corresponding to the optical waveguide core formation positions 164, 165, and 166 shown in Figure 16 were formed using photoresist. Ridge-shaped optical waveguide cores were then fabricated at the positions of lines 164, 165, and 166 in Figure 15 by dry etching with Ar plasma. For comparison, a ridge-shaped optical waveguide core was also fabricated at the position of line 167.

[0129] The optical properties of the optical waveguide were evaluated in the same manner as in Example 1 above, by using a polarization-maintaining fiber with a spherical tip for optical connection, and evaluating the transmission loss spectrum around 1550 nm and the emission spectrum of second harmonic light (SHG light, (SHG: Second Harmonic Generation)) around 775 nm using a tunable light source, SC light source, optical spectrum analyzer, etc.

[0130] As a result, in Example 2, the SHG optical wavelength of the optical waveguide formed at position 165 in Figure 16 was longer than that of the optical waveguide SHG optical peak of the comparison optical waveguide formed at position 167 in Figure 16. This is because the polarization reversal period of the optical waveguide formed at an intersection angle with a predetermined periodic polarization reversal region becomes longer.

[0131] Furthermore, the SHG wavelength light from the optical waveguides formed at positions 164, 165, and 166 also showed a polarization reversal period that progressively increased in wavelength from 164 < 165 < 166. This result indicates that in the optical waveguide core formation process 34, the wavelength conversion light can be controlled by selecting the periodic polarization wavelength region for forming the optical waveguide core by selecting the position for forming the optical waveguide core, and further by adjusting the intersection angle of the optical waveguide with respect to the periodic polarization reversal region. Therefore, it is shown that errors can be compensated for by adjusting the polarization reversal period of the polarization reversal structure of the optical waveguide core.

[0132] As explained above, according to this disclosure, fluctuations in the optical properties of wavelength-converted light caused by factors such as the film thickness distribution of the optical waveguide core layer that occur in the step prior to the optical waveguide core formation can be compensated for at the optical waveguide core formation stage, thereby enabling a manufacturing method for wavelength conversion devices with excellent yield. Furthermore, since the polarization reversal period of the polarization reversal structure of the optical waveguide core can be selected and adjusted at least locally at the optical waveguide core formation stage, the manufacturing yield can be greatly improved, for example, when manufacturing an array-type wavelength conversion device that requires multiple identical wavelength conversion characteristics. In addition, by using the manufacturing method of this disclosure to form optical waveguides having multiple polarization reversal period structures with different polarization reversal periods, it is possible to provide a wavelength conversion device that can be used in a wider optical wavelength range. [Industrial applicability]

[0133] The present invention provides a method for manufacturing wavelength conversion elements that can significantly improve the manufacturing yield compared to conventional manufacturing methods.

Claims

1. A method for manufacturing a wavelength conversion element, comprising: a first step of forming an optical waveguide core substrate having at least one optical waveguide core forming region in which an optical waveguide core is formed including at least one periodic polarization reversal region having a second-order nonlinear effect; a second step of forming a bonded substrate by joining the optical waveguide core substrate with a substrate having a lower refractive index than the optical waveguide core substrate in at least the range of the optical wavelength used, and thinning the optical waveguide core substrate to form an optical waveguide core layer; and a third step of processing the optical waveguide core layer of the bonded substrate to form an optical waveguide core, wherein A method for manufacturing a wavelength conversion element, characterized in that, in the third step, the formation position of the single optical waveguide core is selected for at least one or more periodic polarization reversal regions in the optical waveguide core formation region, thereby adjusting at least locally the polarization reversal period of the periodic polarization reversal structure of the formed single optical waveguide core.

2. The method for manufacturing a wavelength conversion element according to claim 1, characterized in that, in the third step, the polarization reversal period of the periodic polarization reversal structure of the formed optical waveguide core is adjusted at least locally by selecting the intersection angle of the optical waveguide core with respect to the periodic polarization reversal region.

3. A method for manufacturing a wavelength conversion element according to claim 1, characterized in that, in the first step, at least two or more periodic polarization reversal regions with different polarization reversal periods are formed in an array in the direction of the polarization boundary line in the optical waveguide core forming region of the optical waveguide core substrate, and in the third step, the polarization reversal period of the periodic polarization reversal structure of the formed single optical waveguide core is adjusted at least locally by selecting a periodic polarization reversal region for forming the single optical waveguide core from the at least two or more periodic polarization reversal regions with different polarization reversal periods.

4. A method for manufacturing a wavelength conversion element according to claim 1, characterized in that, in the first step, at least four or more periodic polarization reversal regions with different polarization reversal periods are formed in a two-dimensional array in a direction perpendicular to and parallel to the polarization boundary line in the optical waveguide core forming region of the optical waveguide core substrate, and in the third step, the polarization reversal period of the periodic polarization reversal structure of the formed single optical waveguide core is adjusted at least locally by selecting a periodic polarization reversal region for forming the single optical waveguide core from the at least four or more periodic polarization reversal regions with different polarization reversal periods.

5. A method for manufacturing a wavelength conversion element according to claim 1, characterized in that, in the first step, at least four or more periodic polarization reversal regions with different polarization reversal periods are formed in a two-dimensional array in directions perpendicular and parallel to the polarization boundary line in the optical waveguide core forming region of the optical waveguide core substrate, and in the third step, at least one periodic polarization reversal region for forming the single optical waveguide core is selected from the at least four or more periodic polarization reversal regions with different polarization reversal periods, and the intersection angle of the single optical waveguide core with respect to the selected periodic polarization reversal region is selected to adjust the polarization reversal period of the periodic polarization reversal structure of the formed single optical waveguide core at least locally.

6. The optical waveguide core substrate and the substrate contain LiNbO 3 (Lithium niobate), KNbO 3 (Potassium niobate), LiTaO 3 (Lithium tantalate), LiNb (x) Ta (1-x) O 3 (0 ≤ x ≤ 1) (Lithium tantalate with non-stoichiometric composition), or KTiOPO 4 A method for manufacturing a wavelength conversion element according to any one of claims 1 to 5, characterized in that a material is used which contains potassium titanate phosphate, and further contains at least one selected from Mg (magnesium), Zn (zinc), Sc (scandium), or In (indium) as an additive.

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