Optical waveguide and production method for same

Abstract

The present disclosure provides a structure and a production method that make it possible to monitor an etching end point in production of an optical waveguide.　Specifically, provided is an optical waveguide comprising: a core (110); under-cladding (130) that is joined to one main surface of the core (110); and over-cladding (150) that is formed so as to cover the other main surface of the core (110) and one surface of the under-cladding (130) on which the core is formed. The under-cladding (130) is provided with one or more through holes (170) that pass through a cross-section of the under-cladding and expose part of the core after etching. A coolant for substrate cooling is supplied to the one or more through holes.

Description

Optical waveguide and method of manufacturing the same

[0001] The present disclosure relates to an optical waveguide, and more particularly to an optical waveguide capable of monitoring the etching endpoint of the optical waveguide, and a method for manufacturing the same.

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

[0003] Various types of materials have been developed as optical materials with nonlinear optical effects. Among these, oxide compound substrates, such as lithium niobate (LiNbO3:LN) and lithium tantalate (LiTaO3:LT), have been researched and developed as promising materials because they have a high second-order nonlinear optical constant and are transparent over a wide wavelength range from visible to near-infrared.

[0004] Among LN and LT, periodically poled lithium niobate (periodically poled LN: PPLN) and periodically poled lithium tantalate (periodically poled LT: PPLT) are widely used, as they have a periodically poled structure and are formed by taking advantage of the property that they can be spontaneously polarized at room temperature.

[0005] The above optical materials have a periodically poled structure, which provides high phase matching. As a result, they are widely used due to their high second-order nonlinear optical effect. Known optical devices that utilize the high nonlinearity of PPLN and PPLT include wavelength conversion elements that utilize second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG).

[0006] To improve wavelength conversion efficiency, a technology that realizes quasi-phase matching with high accuracy is important (see Non-Patent Document 1). Quasi-phase matching technology is a method of achieving quasi-phase matching in the propagation direction of light by forming a structure in which the sign of the nonlinear susceptibility is periodically inverted. This quasi-phase matching is a method of achieving phase matching by providing a structure in which the sign of the nonlinear optical coefficient is periodically inverted in the propagation direction of light propagating through a nonlinear optical crystal, thereby compensating for the difference in wave vectors between the basic light (incident light, pump light) and the generated light (second harmonic) with the wave vector of the periodic structure.

[0007] The phase matching condition can be expressed as the following equation 1.

[0008]

[0009] Here, k ω and k ２ω are the wave numbers of the fundamental wave light and the second harmonic wave light, respectively, and n ω and n ２ω are the refractive indices of the nonlinear optical material for the fundamental wave light and the second harmonic light, respectively, ω and 2ω are the frequencies of the fundamental wave light and the second harmonic light, respectively, and c is the speed of light.

[0010] In ordinary materials, the refractive index has wavelength dispersion, so n ２ω = n ω This means that the speeds of the fundamental light and the second-harmonic light do not match within the medium. As a result, the intensity of the second-harmonic light within the medium changes periodically with the propagation distance of the light. To eliminate this periodicity, a technique called quasi-phase matching is used, in which the nonlinear optical coefficient is periodically modulated to achieve phase matching.

[0011] Typically, a periodically poled structure is used to periodically invert the crystal axis of an optical crystal to periodically modulate the nonlinear optical coefficient. This technique is currently an essential technology for realizing highly efficient waveguide-type wavelength conversion devices.

[0012] Wavelength conversion using periodically poled LN and LT utilizing quasi-phase matching began with the technique of injecting pulsed light into bulk LN or LT crystals, generating high localized optical confinement and achieving highly efficient wavelength conversion. Later, a method of providing a waveguide structure in the direction of light propagation was devised to achieve high optical confinement.

[0013] Initially, the mainstream waveguide structure was diffusion-type optical waveguides, such as titanium-diffused optical waveguides and proton-exchanged optical waveguides. This was because LN is a difficult-to-process material, making it difficult to fabricate anything other than diffusion-type optical waveguides. However, these diffusion-type optical waveguides had issues in terms of optical damage resistance and long-term reliability because impurities were diffused to form the optical waveguide during fabrication, creating a refractive index difference. Furthermore, with diffusion-type optical waveguide structures, when high-power light was incident on the optical waveguide, the structure was damaged by the photorefractive effect, limiting the optical power that could be input to the optical waveguide.

[0014] One solution to this problem is the development of ridge-type optical waveguides. In particular, a direct-bonded ridge-type optical waveguide structure, fabricated by directly bonding an LN or LT substrate, which serves as the core of the waveguide, to a substrate made of another material, which serves as the cladding of the waveguide, enables high-power optical input, and is expected to have a wide range of applications, such as the generation of high-intensity optical modulation signals and laser processing technology.

[0015] M. Fejer et al., "Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances", IEEE Journal of Quantum Electronics, Vol.28, No.11, 1992.

[0016] Dry etching is used as a method for forming the above-mentioned direct-bonded ridge-type optical waveguide structure. In dry etching, it is important to etch all the way to the cladding without leaving any core, to prevent excessive etching (over-etching) after reaching the cladding, and to ensure that the design structure is not deviated from the intended structure. Conventionally, methods for detecting (monitoring) the etching end point (endpoint) have been used, such as optical interference, spectroscopy of plasma emission wavelengths, and mass spectrometry to detect gases during etching.

[0017] However, in direct-bonded LN ridge-type optical waveguides that use LN as the core, LT, which has a similar thermal expansion coefficient, is sometimes used as the cladding substrate. The endpoint monitoring method described above is ineffective for two substrates with similar material compositions. Therefore, since there is no effective endpoint monitoring method for conventional dry etching techniques, a method is used in which the etching rate is determined in advance and the etching time is calculated backward. However, this method is valid under the assumption that all conditions, such as equipment state, gas mixture ratio, etching power, and substrate temperature, are equal. The actual etching process has many variables, making it difficult to accurately monitor the endpoint.

[0018] To solve the above problem, a method has been proposed in which a vacuum break material is filled into the substrate as an endpoint monitor. However, in the formation of direct-bonded optical waveguides, the wafer flatness between dissimilar material substrates is directly related to the quality of the bonding, so the complex structure used in the conventional method is not suitable. Therefore, a device structure with an endpoint monitor mechanism that is simpler and applicable to direct-bonded ridge-type optical waveguides is needed.

[0019] The present disclosure has been made in consideration of these problems, and its purpose is to provide a new method of endpoint monitoring, in which fine holes penetrating the substrate that will become the cladding are drilled in advance, and as soon as core etching is completed, helium, which has been sprayed onto the backside of the substrate during the process for cooling, is leaked, and the endpoint is monitored by detecting the helium leak.

[0020] 1 is a diagram showing a cross-sectional structure of an optical waveguide in this embodiment; FIG. 2 is a diagram showing a manufacturing process of an optical waveguide in this embodiment, where (a) shows the formation of polarization inversion, (b) shows the formation of through holes, (c) shows direct bonding of an underclad substrate and a core substrate, (d) shows thinning, (e) shows waveguide formation, and (f) shows the formation of an overclad; FIG. 3 is a schematic cross-sectional view showing a method for forming polarization inversion in an optical waveguide in this embodiment; FIG. 4 is a flowchart showing a manufacturing method of an optical waveguide in this embodiment; FIG. 5 is a diagram showing an apparatus used for etching in this embodiment and a substrate cooling mechanism within the apparatus; FIG. 6 is a diagram showing a method for monitoring the endpoint in this embodiment, where (a) shows a cross-sectional view of an optical waveguide and a monitoring refrigerant before etching, (b) shows during etching, and (c) shows when the endpoint is reached.

[0021] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference symbols indicate the same or similar elements, and repeated description may be omitted. The materials and numerical values ​​in the following description are examples and are not intended to limit the technical scope of the present disclosure. The embodiments described below may be implemented using other materials and numerical values ​​without departing from the spirit of the present disclosure.

[0022] (Optical Waveguide) Hereinafter, an optical waveguide and a manufacturing method thereof according to an embodiment of the present disclosure will be described, taking as an example a ridge-type optical waveguide in which a core made of a nonlinear optical material and an underclad are directly bonded to each other. The ridge-type optical waveguide may be, for example, a PPLN optical waveguide.

[0023] The nonlinear optical material used in this embodiment may be any material that has a nonlinear optical effect and can form a periodically poled structure. Examples include lithium niobate (LiNbO), lithium tantalate (LiTaO), and potassium titanyl phosphate (KTiOPO: KTP). Depending on the nonlinear optical material having a periodically poled structure, attention must be paid to the optical waveguide processing conditions below the Curie temperature at which the periodically poled structure is not lost.

[0024] 1 is a diagram showing the cross-sectional structure of an optical waveguide 100 according to this embodiment. As shown in FIG. 1, the optical waveguide 100 is composed of a core 110 having a surface on which light is incident, an underclad 130 bonded to one main surface of the core 110, an overclad 150 formed to cover the other main surface of the core 110 and one surface of the underclad 130 on which the core 100 is formed, and one or more through holes 170 formed to penetrate the underclad 130 in a height direction (e.g., the Z direction in FIG. 1) and exposed after etching. The optical waveguide 100 also extends in a predetermined direction (e.g., the y direction in FIG. 1).

[0025] (Core) The core 110 can be bonded by direct bonding onto one side surface of the underclad 130 (for example, the upper surface in the Z direction in FIG. 1). After being incident on the core 110 (the X-Z plane in FIG. 1), light propagates inside the core 110 and travels along the extension direction of the optical waveguide 100. By directly bonding the core 110 to the underclad 130, the optical waveguide 100 has high optical damage resistance, making it possible to input pumping light with extremely high power density into the optical waveguide 100.

[0026] The size of the light incident surface of the core 110 is not particularly limited, and may be a core diameter for propagating light in multimode or a core diameter for propagating light in single mode. Regarding the core size, the optical waveguide may be one in which the core has been thinned by a smart cut method or the like to attempt to reduce the core size, and the core diameter may be a very small core diameter (in nm units).

[0027] Furthermore, there is no particular limitation on the shape of the core 110, and it may be square, rectangular, trapezoidal, or any other shape that can be processed.

[0028] (Underclad) The underclad 130 is directly bonded to one of the main surfaces of the core 110, and confines the light incident on the optical waveguide 100. The refractive index of the underclad 130 needs to be lower than the refractive index of the core 110, and any overclad may be used as long as it has an overclad according to the optical waveguide structural design.

[0029] The material of the substrate that becomes the undercladding 130 may generally be a nonlinear optical material, but is not limited to a nonlinear optical material and may be a linear optical material. In addition, the undercladding 130 may be, for example, glass deposited by chemical vapor deposition (CVD), flame hydrolysis deposition (FHD), or sputtering.

[0030] Furthermore, the underclad 130 does not have to be a single layer, but may be a multi-layer structure, and may have one or more through holes 170 penetrating the substrate that will become the underclad 130 described below.

[0031] (Overclad) The overclad 150 is formed to cover the other main surface of the core 110, the other main surface of the core 110, and one surface of the underclad 130 on which the core 100 is formed, and confines light incident on the optical waveguide 100. There is no particular limitation on the refractive index of the overclad 150, and the overclad 150 may be air (air clad).

[0032] An optical waveguide in which the overclad 150 is an air clad is called a ridge-type optical waveguide, and an optical waveguide in which the side and top surfaces of the core 110 are covered with the overclad 150, as shown in Figure 1, is called a buried-type optical waveguide. The embodiments of the present disclosure can be applied to either the ridge-type or the buried-type.

[0033] (Through Hole) One or more through holes 170 are formed to penetrate the underclad 130 from the lower surface to the upper surface, i.e., the cross section. Each of the one or more through holes 170 has a coolant intake port 170a on one side and a coolant outlet 170b on the other side. In the manufacturing method described below, the coolant outlet 170b is sealed by the core 110 before etching, but is exposed when the etching reaches an endpoint. In addition, in the case of an embedded optical waveguide, the coolant outlet 170b ultimately comes into contact with the overclad 150.

[0034] The one or more through-holes 170 are formed in the vicinity of the core 110 to be etched, thereby enabling highly accurate endpoint monitoring. As an example, the one or more through-holes 170 are preferably provided within 100 μm of the core 110.

[0035] The method for forming one or more through holes 170 is not limited, and any method capable of processing fine holes may be used, such as a method that combines photolithography and dry etching, a method of drilling holes by laser irradiation, or a method of physically drilling holes using a fine drill or the like.

[0036] Furthermore, the cross-sectional shape of one or more through holes 170 may be any shape, such as, but not limited to, a circle, an ellipse, a rectangle, a regular polygon, etc., as long as a leak of refrigerant (e.g., helium: He) can be detected.

[0037] (Method for Manufacturing Optical Waveguide) Hereinafter, a method for manufacturing the optical waveguide 100 capable of monitoring the endpoint will be described with reference to Fig. 2 to Fig. 5. A method 400 for manufacturing the optical waveguide 100 includes the steps of forming a polarization inversion structure in a ferroelectric substrate 310 (e.g., an LN substrate) that will become the core 110 (Fig. 2(a) and Fig. 3, step 403), forming one or more through holes 170 in the underclad 130 (Fig. 2(b), step 405), directly bonding the ferroelectric substrate 310 that will become the core 110 and a substrate 320 that will become the underclad 130 (Fig. 2(c), step 407), thinning the ferroelectric substrate 310 that will become the core 110 (Fig. 2(d), step 409), forming the bonded substrate 300 into a waveguide by etching (Fig. 2(e), step 411), and depositing the overclad 150 (Fig. 2(f), step 413).

[0038] (Polarization Reversal) Methods for fabricating a periodic polarization reversal structure include a Ti diffusion method, a LiO out-diffusion method, a SiO2 loading heat treatment method, and a voltage application method. In this embodiment, a method for fabricating a polarization reversal structure will be described using as an example a direct electric field application method in which desired electrodes are formed on the ferroelectric substrate 310 and a pulsed voltage is applied to locally reverse the polarization direction of the ferroelectric substrate 310.

[0039] The fabrication process of the polarization inversion 350 in the polarization inversion structure will be described in more detail with reference to FIGS. 2(a) and 3 . As shown in FIGS. 2(a) and 3 , a ferroelectric substrate 310, for example, a z-cut LN substrate, is first prepared. A metal electrode 330 is patterned into a desired comb-like or ladder-like shape on one of the main surfaces of the ferroelectric substrate 310, for example, the +z surface, by photolithography or etching. The metal electrode 330 can be made of a heat-resistant noble metal such as platinum, which is expensive, but an inexpensive material such as aluminum can also be used. In some cases, a liquid electrode may be used instead of a metal electrode.

[0040] After the metal electrode 330 is formed on one main surface of the LN substrate in this manner, a voltage having a very high electric field (for example, about 21 kV / mm) is applied to the LN substrate by generating a voltage pulse using an arbitrary waveform generator, amplifying it using a high-voltage amplifier, and then applying it to the LN substrate via a high-voltage cable.

[0041] As a result, the electric field is spatially modulated in the pattern shape of the metal electrode 330, and a polarization inversion structure 350 in the pattern shape of the electrode is formed on the LN substrate. Thereafter, the metal electrode 330 is removed by etching or the like.

[0042] By the above-described process, a polarization inversion structure is formed in the ferroelectric substrate 310 that will become the core 110 (step 403).

[0043] 2B, one or more through holes 170 for monitoring the endpoint are formed in the substrate 320 that will become the underclad 130 (step 405). The method for forming the one or more through holes 170 is not limited, and examples that can be used include a method that combines photolithography and dry etching, a method of drilling holes by laser irradiation, a method of physically drilling holes using a microdrill, or any other method that allows for micro-hole processing.

[0044] In this embodiment, by positioning the one or more through holes 170 near the core 110, there is an advantage that the etching endpoint can be monitored more accurately. As an example, it is desirable that the one or more through holes 170 are provided at a distance of 100 μm or less from the core 110. However, the positions of the one or more through holes 170 do not necessarily have to be near the core 110, and may be arbitrarily determined depending on the design of the optical waveguide 100, etc., as long as they are within a range in which substrate cracking of the underclad 130 does not occur.

[0045] 2(c), the substrate 320 (e.g., LT) that will become the underclad 130 is directly bonded to the ferroelectric substrate 310 that will become the core 110 having the polarization inversion 350 structure formed in the above step 403 (step 407). The direct bonding in step 407 is a technique in which two dissimilar material substrates (e.g., substrate 320 and substrate 310) with well-polished surfaces are brought into direct contact in a dust-free environment (e.g., a clean room) and bonded by an annealing treatment. The use of a direct bonding technique that does not use an adhesive leads to improved optical loss resistance when high-intensity light is used as input light.

[0046] In step 407, by selecting the thermal expansion coefficients of the substrate 320 that will become the underclad 130 and the ferroelectric substrate 310 that will become the core 110 as close as possible, it is possible to suppress cracking of the substrate in a heat treatment process in a later process. In the present disclosure, a substrate formed by directly bonding the substrate 320 that will become the underclad 130 and the ferroelectric substrate 310 that will become the core 110 is also referred to as a "bonded substrate" 300. Note that the material of the substrate 320 that will become the underclad 130 is not limited to a nonlinear optical material, and may be a linear optical material.

[0047] 2(d), the ferroelectric substrate 310, which will become the core 110 of the bonded substrate 300, is thinned (step 409). The thickness of the ferroelectric substrate 310 after thinning is, for example, 0.5 μm to 20 μm. In this embodiment, there are no particular limitations on the method of thinning, and thinning by either grinding and polishing or smart cutting may be used.

[0048] When thinning the substrate by grinding and polishing, a machine with a controlled flatness of the grinding and polishing platen is used to perform grinding and polishing until the optical waveguide is present at the desired depth. After the grinding and polishing process is completed, a polished mirror-like surface (optical end face) can be obtained. Finally, the parallelism of the substrate (the difference between the maximum and minimum heights of the substrate) can be measured using an optical parallelism measuring device to determine the parallelism of the entire substrate.

[0049] When thinning using Smart Cut, the thinning process mainly consists of two steps: an ion implantation step and a thin film peeling step. In the ion implantation step, He or hydrogen ions are implanted into the substrate that needs to be thinned to have a second-order nonlinear optical effect. The ions are implanted from the substrate surface under a controlled acceleration voltage and controlled dose, and are trapped at a certain depth from the surface. It is desirable to use ions such as hydrogen or He that are smaller than the atoms that make up the substrate.

[0050] In the substrate peeling process, the substrate into which ions have been implanted is subjected to a heat treatment to peel off the substrate at the damaged portion within the substrate. If the nonlinear optical material has a periodically poled structure, the heat treatment temperature in the substrate peeling process is set to a temperature below the Curie temperature of the second-order nonlinear optical crystal in order to prevent the patterned polarization direction from being disrupted.

[0051] (Forming a Waveguide) For forming a waveguide, microfabrication using photolithography and dry etching or mechanical polishing can be used. As shown in Fig. 2(e), the bonding substrate 300 is formed into an optical waveguide having a predetermined width using microfabrication using photolithography and dry etching (step 411).

[0052] Etching of the bonded substrate 300 is performed in an etching apparatus 500 equipped with a substrate cooling mechanism 510 shown in FIG. 5 to prevent substrate cracking and resist burning due to temperature increases during etching. He is typically used to increase thermal conductivity between the substrate holder 530 in the substrate cooling mechanism 510 and the bonded substrate 300. This is because He is lighter than air and etching gases, allowing for faster molecular motion and efficient transfer of thermal energy between the bonded substrate 300 and the substrate holder 530. This significantly increases thermal conductivity, enabling efficient cooling of the bonded substrate 300. Generally, an etching apparatus 500 incorporates a He leak detection mechanism to detect etching abnormalities such as substrate cracking. Therefore, it is not necessary to install a He leak detector (not shown) for endpoint monitoring in the present disclosure. Note that there are no limitations on other etching conditions, such as RF power and frequency, during dry etching in this embodiment.

[0053] In the microfabrication by photolithography and dry etching, the type of photoresist is not limited, and may be either a positive or negative photoresist. Also, the type of developer is not limited, and may be either an alkaline developer or an organic solvent developer.

[0054] There are no limitations on the equipment used for dry etching, and it may be a barrel-type plasma etcher, a CCP (Capacitively Coupled Plasma) etcher, a magnetron RIE (Reactive Ion Etching), an ECR (Electron Cyclotron Resonance) plasma etcher, an ICP (Induced Coupled Plasma) etcher, or any other etcher.

[0055] There is no limitation on the type of gas, and any of CF-based gases, oxygen, argon, etc., a mixture of these gases, and other gases may be used. There is no limitation on the method of chucking the substrate, and any of mechanical chucking, electrostatic chucking, and other chucking methods may be used.

[0056] (Overclad) In addition to the above steps 403 to 411, if necessary, the overclad 150 is deposited as shown in FIG. 2(f) (step 413). There are no limitations on the method for forming the overclad 150, and it may be any of chemical vapor deposition (CVD), flame hydrolysis deposition (FHD), sputtering, and other film formation methods. Note that, as described above, if the overclad 150 is an air clad (ridge-type optical waveguide), step 413 is omitted.

[0057] (Endpoint Monitoring Method) Figure 6 shows a method for monitoring the etching endpoint in the waveguide formation step (step 411). After thinning (step 409), the bonded substrate 300 in this embodiment includes a ferroelectric substrate 310 that will become the core 110, a substrate 320 that will become the underclad 130, and one or more through holes 170 formed in the substrate 320 that will become the underclad 130 (see Figure 6(a)). A photoresist for etching is applied to one main surface of the ferroelectric substrate 310 that will become the core 110, and the bonded substrate 300 is formed into a waveguide using dry etching, as shown in Figure 6(b).

[0058] As the etching progresses and reaches its endpoint (the point in time shown in FIG. 6(c)), the coolant outlets 170b of one or more through-holes 170 sealed in the ferroelectric substrate 310 that will become the core 100 are exposed, causing the He gas used to cool the substrate to leak. The etching process is completed when a He leak detector (not shown) installed in the etching apparatus 500 shown in FIG. 5 detects He gas.

[0059] The present disclosure provides a new method of endpoint monitoring, in which a substrate 320 that will become the underclad 130 is provided with a through-hole 170 in advance, and as soon as etching of the core 110 is completed, He that has been sprayed onto the back surface of the substrate during the process for cooling purposes is leaked, and the endpoint is monitored by detecting the He leak, providing a new substrate structure and manufacturing method.

[0060] Additional Considerations The foregoing description of embodiments of the present invention has been presented for purposes of illustration and is not intended to be exhaustive or to be limited to the precise form disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0061] Finally, the language used herein has been selected primarily for readability and instructional purposes, and may not have been selected to delineate or limit the subject matter of the invention. Accordingly, it is intended that the scope of the invention be limited not by this detailed description, but rather by the appended claims. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the claims.

[0062] REFERENCE SIGNS LIST 100 Optical waveguide 110 Core 130 Underclad 150 Overclad 170 Through hole 170a Coolant intake port 170b Coolant outlet 300 Bonding substrate 310 Ferroelectric substrate 320 Substrate to be underclad 330 Metal electrode 350 Polarization inversion 500 Etching device 510 Substrate cooling mechanism 530 Substrate holder

Claims

1. An optical waveguide, comprising: a core; and an underclad having one main surface of the core joined to a part of one main surface thereof, wherein the underclad includes one or more through holes penetrating between the one main surface and the other main surface, and the through holes are exposed on another part of the one main surface not joined to the one main surface of the core.

2. The optical waveguide according to claim 1, wherein the other main surface and side surfaces of the core and one surface of the underclad joined to the core are in contact with air.

3. The optical waveguide according to claim 1, further comprising an overclad formed to cover the other main surface of the core and one surface of the underclad on which the core is formed.

4. The optical waveguide according to any one of claims 1 to 3, wherein the one or more through holes are formed in the vicinity of a part of the one main surface of the underclad joined to the one main surface of the core.

5. A method for manufacturing an optical waveguide, comprising: forming one or more through holes through which a refrigerant for monitoring an etching endpoint passes in a substrate to be an underclad; directly joining the substrate to be the underclad and a substrate to be a core; and etching the substrate to be the core until the refrigerant ejected from the one or more through holes is detected, to form a waveguide.

6. The manufacturing method according to claim 5, further comprising a step of forming polarization inversion in a part of the substrate to be the core.

7. The manufacturing method according to claim 5 or 6, further comprising a step of depositing an overclad.

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