JP7062937B2 - Optical element and its manufacturing method - Google Patents

Description

The present invention relates to an optical element and a method for manufacturing the same, and more particularly to an optical element using a nonlinear optical effect or an electro-optical effect and a method for manufacturing the same, which are used in an optical communication system or an optical measurement system.

Optical elements applied to wavelength conversion, optical modulation, and optical measurement, optical processing, medical care, biotechnology, etc. of optical signals in optical communication systems generate coherent light in the ultraviolet-visible-infrared-terahertz region. Or can be modulated. As such optical elements, nonlinear optical devices and electro-optic devices are being developed.

Various materials have been researched and developed as nonlinear optical media and electro-optical media, and oxide-based compound substrates such as lithium niobate (LiNbO ₃ : LN) have extremely high second-order nonlinear optical constants and electro-optical constants. Known as a high and promising material. Lithium niobate (PPLN), which is periodically polarized and inverted, is known as an example of an optical device using the high non-linearity of LN. Further, wavelength conversion devices using second harmonic generation (SHG), differential frequency generation (DFG), or sum frequency generation (SFG) by PPLN are known.

For example, since strong absorption lines such as reference vibrations of various environmental gases exist in the wavelength range of mid-infrared light having a wavelength of 2-5 μm, the development of a compact mid-infrared light source is desired. As such a light source in the mid-infrared region, a technically mature excitation light source in the vicinity of 1 μm and a light source to which DFG can be used and signal light in the communication wavelength band are considered to be promising. Further, in the wavelength range of visible light having a wavelength of around 0.5 μm, there is a wavelength range that is difficult to realize with a semiconductor laser. Therefore, a wavelength conversion technique capable of generating visible light such as green light by SHG or SFG using an excitation light source of about 1 μm is promising.

Further, by using the wavelength conversion technique using DFG, the light in the wavelength band of 1.55 μm used for the optical fiber communication can be collectively converted into another wavelength band. Utilizing this, it is possible to apply optical routing in wavelength division multiplexing, wavelength collision avoidance in optical routing, etc., so wavelength conversion elements are one of the key devices for constructing large-capacity optical communication networks. It is considered.

In the wavelength conversion technique using DFG, signal distortion compensation can be performed by using the fact that the converted light becomes phase-coupled light with respect to the signal light. When the signal light is converted to phase-conjugated light at the midpoint of the transmission line, the signal distortion caused by the dispersion generated in the transmission line before conversion and the nonlinear optical effect in the fiber is propagated so as to cancel each other out in the transmission line after conversion. Can be made to. The wavelength conversion element also serves as a key device capable of reducing dispersion and nonlinear signal distortion.

When a wavelength conversion element having high wavelength conversion efficiency is used, an amplifier of signal light called optical parametric amplification can be configured by transferring energy from excitation light power to signal light. In particular, a phase-sensitive amplifier having amplification characteristics according to the phase relationship between the excitation light and the signal light is expected as a technique capable of low-noise optical amplification.

An optical waveguide type device is effective for improving the efficiency of a wavelength conversion element using PPLN. This is because the wavelength conversion efficiency is proportional to the power density of the light propagating in the nonlinear medium, and by forming the waveguide structure, the light can be confined in a limited region. For this reason, research and development have been conducted on various waveguides using non-linear media.

Conventionally, studies have been made using diffusion-type waveguides called Ti diffusion waveguides and proton exchange waveguides. However, since these waveguides diffuse impurities into the crystal during fabrication, there are problems from the viewpoint of light damage resistance and long-term reliability. In the diffusion type waveguide, when high-intensity light is incident on the waveguide, crystal damage occurs due to the photorefractive effect, so that the optical power that can be input to the waveguide is limited.

In recent years, research and development have been conducted on ridge-type optical waveguides having features such as high light damage resistance, long-term reliability, and easy device design because the bulk characteristics of crystals can be used as they are. A ridge-type optical waveguide can be formed by thinning one of the substrates of an optical element formed by joining two substrates and then performing ridge processing. It is also known that a ridge-type waveguide is produced by adhering two substrates using an adhesive, thinning one of the substrates, and then performing ridge processing (for example, non-). See Patent Document 1).

However, the method of laminating the substrates with an adhesive has a problem that the thin film is cracked when the temperature changes because the thermal expansion coefficients of the adhesive and the substrate are different. In addition, since the adhesive is deteriorated by the second harmonic light generated in the waveguide, there is also a problem that the waveguide loss increases during operation and the efficiency of wavelength conversion deteriorates. Furthermore, there is also a problem that the film thickness of the single crystal film becomes non-uniform due to the non-uniformity of the adhesive layer, and the phase matching wavelength of the wavelength conversion element shifts.

On the other hand, a direct joining technique is known as a technique for firmly joining substrates to each other without using an adhesive. The direct joining method is a method in which the surface of a substrate is first treated with a chemical, and then the substrates are overlapped with each other to be joined by an attractive force between the surfaces. The bonding is performed at room temperature, but since the bonding strength at this time is small, heat treatment at a high temperature is subsequently performed to improve the bonding strength. The direct joining technology, which can firmly bond substrates to each other without using an adhesive or the like, has features such as high light damage resistance, long-term reliability, and ease of device design. In addition, it is considered promising because it is possible to avoid contamination of impurities and absorption of adhesives and the like in the above-mentioned light generation in the mid-infrared region by DFG.

Furthermore, the direct joining method is expected not only for nonlinear optical devices but also for high-power optical modulator applications. Oxide-based compound substrates such as LN have large electro-optical constants in addition to the second-order nonlinear optical constants, and are widely used as optical modulators using the electro-optic effect (EO effect). However, conventionally, those using a Ti diffusion waveguide have been commercialized, but it has been difficult to input a high power of 100 mW or more. On the other hand, if the direct joining method is used, watt-class optical input becomes possible, so it can be expected to be applied to the generation of high-intensity optical modulation signals, laser processing technology, and the like.

The direct joining method requires heat treatment at a high temperature of about 400 ° C. Therefore, in addition to having good surface flatness, the substrates to be joined are also required to have similar coefficients of thermal expansion. Therefore, direct bonding formation between LNs to which LNs, lithium tantalate (LiTaO ₃ : LT), and additives such as Mg, Zn, Sc, In, and Fe are added is made of the same material substrate has been studied.

FIG. 1 shows the structure of a conventional ridge-type waveguide. The ridge-type waveguide has a core 2 formed on the base substrate 1 (underclad layer) according to the waveguide pattern, and has a step-type refractive index distribution. The core 2 has three side surfaces that are not in contact with the base substrate 1 and are in contact with the air layer. The ridge-type optical waveguide can operate even if the upper surface and the side surface of the core 2 are an air layer (refractive index = 1). However, as a practical problem, if the core is exposed, there is a concern that the characteristics may change with time due to the adhesion of dust and dirt floating in the air. Further, in order to form a film such as an AR coat on the end face of the optical waveguide, it is necessary to form an overclad layer that also serves as a protective film in order to obtain the required mechanical strength.

For example, when LN or LT crystals are used for the base substrate 1 and the core 2, LiNb _x Ta _1-x O ₃ (0 <x <1) having the same composition is the optimum material for the overclad layer. Therefore, it is desirable to grow epitaxially by the LPE (liquid phase epitaxial growth) method. However, in the LPE method, there is a problem that the group V element contained in the flux used for the liquid phase crystal growth of the core layer causes photodamage and deteriorates the element characteristics, and the formation of the overclad layer by the LPE method is practical. It has not been transformed.

On the other hand, there is also a method of using a different material as an overclad layer. When a dissimilar material is formed as an overclad layer, the material compatibility of the overclad layer material with the core and the underclad layer and the suitability of the optical properties become problems. That is, regarding the material compatibility, it is necessary that it does not chemically react with the core and the underclad layer to cause impurities and mechanical fragility. Further, regarding the optical characteristics, it is possible to control the waveguide light mode in the core by having an appropriate refractive index, that the light that leaks to the overclad layer does not absorb and scatter, and so on. The overclad layer of different materials can generally form an oxide material typified by SiO ₂ by an EB (electron beam) vapor deposition method, a sputtering method, or a plasma CDV (chemical vapor deposition) method. As an oxide having a refractive index close to that of LN, which is the core, TiO ₂ , Ta ₂ O ₅ , SiN _x , or the like can also be used.

S. Kurimura, Y. Kato, M. Maruyama, Y. Usui, and H. Nakajima, "Quassi-Phase-Matched adhered ridge waveguide in LiNbO3," Appl. Phys. Lett. 89 (19), 191123 (2006) Y. Zikuhara, E. Higurashi, N. tamura, T. Suga, "Sequential activation process of oxygen RIE and nitrogen radical for LiNbO3 and Si wafer bonding," ECS Trnsactions, 3 (6) 91-98 (2006)

However, the conventional overclad layer using a different material is an amorphous material, and there are some problems when it is used as an overclad layer.

First, the coefficient of thermal expansion differs greatly between LN, which is a substrate material, and an oxide material represented by SiO ₂ . The coefficient of thermal expansion of LN is 15.4 × 10 ^-6 , whereas that of SiO ₂ is 0.5-0.6 × 10 ^-6 , which are significantly different.

FIG. 2 shows a ridge-type waveguide provided with a conventional overcladed layer made of SiO ₂ . When the thick overcladed layer 3 is formed, compressive stress is generated in the overcladed layer 3 due to the difference in the coefficient of thermal expansion between the base substrate 1 and the core 2 (FIG. 2A), or the overcladed layer 3 is formed. A tensile stress is generated in (FIG. 2 (b)). This stress may cause warpage, fracture, and refractive index deviation of the waveguide.

Since the surface of the substrate of the ridge-type waveguide using the direct joining method is uneven due to the ridge structure, it is necessary to flatten the board in order to load electrodes on it. When depositing an oxide material typified by SiO ₂ , in order to perform flattening together, the overclad material is deposited sufficiently higher than the height of the ridge waveguide, and the overclad material reflects the ridge structure. A procedure for flattening the unevenness by polishing or the like is required. Secondly, as described above, it is difficult to flatten the substrate surface by this method because an overclad layer that covers the entire ridge-type waveguide cannot be deposited due to the difference in the coefficient of thermal expansion.

Further, in order to create an LN modulator using the EO effect, it is necessary to load a modulation electrode on the waveguide core. Since the metal absorbs the optical electric field, the electrodes cannot be directly loaded on the waveguide, but are loaded via an oxide overclad layer typified by SiO ₂ called a buffer layer. In the LN modulator using the Ti diffusion waveguide, SiO ₂ is used for the buffer layer, but this amorphous layer has DC drift (even if a constant DC bias voltage is applied, the operating point of the modulator changes over time. It is known to be one of the factors of the fluctuating phenomenon). Thirdly, even in the ridge type waveguide using the direct joining method, if the amorphous SiO ₂ is used for the overclad layer, the DC drift increases.

Fourth, from the viewpoint of optical characteristics, the oxide material represented by SiO ₂ is not transparent to light in the mid-infrared wavelength region, and there is a concern that the waveguide may be lost due to absorption. Therefore, the amorphous SiO ₂ overclad layer cannot be applied to the ridge waveguide of the wavelength conversion element for generating mid-infrared light. Amorphous SiO ₂ has similar optical property problems in wavelength conversion elements for generating visible light and ultraviolet light as well as mid-infrared light, and is suitable as an overclad layer. I couldn't always say that.

Fifth, in the overclad layer using different materials, the effective refractive index difference with the core is determined once the material to be used is determined. It has been extremely difficult to control the refractive index of the overclad layer to set an arbitrary effective refractive index difference.

As described above, the conventional method has a problem that it is difficult to realize an overcladed layer having good material compatibility and appropriate optical properties.

An object of the present invention is to provide an optical element provided with an overclad layer having good optical characteristics and a method for manufacturing the same.

In the present invention, in order to achieve such an object, one embodiment of an optical element is bonded to a first substrate made of a crystal substrate and the first substrate by a direct bonding method by thermal diffusion, and a ridge. A second substrate made of a single crystal nonlinear optical medium or an electro-optical medium containing additives, on which the core of the type waveguide is formed, is bonded to the upper part of the ridge type waveguide by a direct bonding method by thermal diffusion. A single crystal overclad layer comprising a nonlinear optical medium or an overclad layer made of an electro-optical medium containing an additive different from the additive of the second substrate, and the thickness of the overclad layer. The effective refractive index difference of the ridge type waveguide is set in the range of 0.2 μm to 0.6 μm.

According to the present invention, since the overclad layer of a single crystal whose thermal expansion coefficient value is close to that of the first and second substrates constituting the waveguide is provided, there is no stress strain due to the formation of the overcladed layer. The symmetry of the shape of the waveguide mode is improved, and the loss due to light absorption is suppressed over a wide wavelength range of mid-infrared light, visible light, and ultraviolet light. Further, since the same type of single crystal material is bonded by a direct bonding method by thermal diffusion, it is possible to input high light damage resistance, high long-term reliability, and high output optical power.

It is sectional drawing which shows the structure of the conventional ridge type waveguide. It is sectional drawing which shows the ridge type waveguide provided with the conventional overclad layer made of SiO ₂ . It is sectional drawing which shows the structure of the ridge type waveguide which concerns on 1st Embodiment of this invention. It is a figure which shows the manufacturing method of the wavelength conversion element which concerns on 1st Embodiment of this invention. It is a figure which shows the manufacturing method of the overclad layer which concerns on 1st Embodiment. It is a figure which shows the cutting method of the wavelength conversion element which concerns on 1st Embodiment. It is a figure which shows the effective refractive index of 0th order and 1st order mode with respect to the refractive index difference between a core and a base substrate. It is a figure which shows the effective refractive index of the 0th order and 1st order mode of the wavelength conversion element which concerns on 2nd Embodiment. It is sectional drawing which shows the structure of the optical phase modulator which concerns on 3rd Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First Embodiment)
FIG. 3 shows the structure of the ridge-type waveguide according to the first embodiment of the present invention. The ridge-type waveguide has a core layer 12 formed on the base substrate 11 (underclad layer) and has a core 12a formed according to the waveguide pattern, and has a step-type refractive index distribution. The base substrate 11 made of LiTaO ₃ and the core layer 12 made of a Z-cut Zn-added LN substrate having a periodic polarization inversion structure are bonded together by a direct bonding method by thermal diffusion, and the core layer 12 is processed by a dry etching process. To form a ridge type waveguide. The cross-sectional shape of the core 12a is a 5 × 5 μm ² square. On the core 12a and the core layer 12, the same LiTaO ₃ single crystal thin film as the material of the base substrate 11 is bonded as an overclad layer 13.

A process of manufacturing a wavelength conversion element having such a ridge-type waveguide will be described.

FIG. 4 shows a method for manufacturing a wavelength conversion element according to the first embodiment of the present invention. A Z-cut LiTaO ₃ substrate (first substrate) 21 as a base substrate and a Z-cut Zn-added LN substrate (second substrate) 22 which is a nonlinear optical medium and has a periodic polarization inversion structure are prepared. The first and second substrates are 3-inch wafers with both sides optically polished. The thickness of the first substrate 21 is 500 μm, and the thickness of the second substrate 22 is 300 μm. As the nonlinear optical medium, in addition to LN, LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), or at least one selected from the group consisting of Mg, Zn, Sc, and In. Can be used as an additive.

The periodic polarization inversion structure of the second substrate 22 is manufactured so that the phase matching condition is satisfied in the wavelength band of 1.5 μm. Much research has been done on techniques for producing periodic polarization inversion structures in LN crystals and the like, and several methods have been developed. Of these, a periodic polarization inversion structure is produced by an electric field application method in which good results can be obtained with good reproducibility. In this method, a periodic resist pattern is formed on the surface of an LN crystal by lithography, and a voltage pulse is applied using a metal thin film electrode, a liquid electrode, or the like to apply a voltage pulse to the portion where the resist pattern is not formed. The polarization is reversed to create a periodic polarization reversal structure.

The coefficient of thermal expansion in the in-plane direction of the second substrate 22 is 15.4 × 10 ^-6 , and the coefficient of thermal expansion in the in-plane direction of the first substrate 21 is 16.0 × 10 ^-6 , which are very high. It is a close value. Further, the refractive index of the first substrate 21 is smaller than the refractive index of the second substrate 22.

After cleaning the surfaces of the first and second substrates, these two substrates are superposed in a clean atmosphere in which microparticles are absent as much as possible. The stacked first and second substrates are placed in an electric furnace and heat-treated at 400 ° C. to perform diffusion bonding (first step). The joined substrate is void-free with no pinching of microparticles or the like on the joined surface, and cracks or the like do not occur even when the temperature is returned to room temperature.

Next, using a polishing device in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the second substrate 22 of the bonded substrate is 5 μm (second step). A mirror-polished surface can be obtained by performing a polishing process after the polishing process. When the parallelism of the substrate (difference between the maximum height and the minimum height) was measured using an optical parallelism measuring machine, submicron parallelism was obtained almost entirely except for the peripheral part of the 3-inch wafer. A thin film substrate can be manufactured. This thin film substrate (second substrate) is the core layer 12 in FIG. Since the first and second substrates are directly bonded by diffusion bonding by heat treatment, they have a uniform composition and film thickness over the entire area of the 3-inch wafer.

Then, a waveguide pattern is produced on the surface of the second substrate 22 by a normal photolithography process. By etching the surface of the second substrate 22 using a dry etching apparatus, as shown in FIG. 3, a core 12a having a periodic polarization inversion structure in the waveguide direction of light is formed, and the underclad layer is used. A ridge-type waveguide is formed together with a certain base substrate 11 (third step). Since the material of the second substrate on both sides of the core 12a is completely removed, the core 12a has an extremely thin joint surface with the base substrate 11 (first substrate 21). In the first embodiment, since the direct joining method is used, peeling does not occur even in a structure having an extremely thin joining surface, and sufficient joining strength can be maintained.

In this way, a square core 12a having a cross-sectional shape of 5 × 5 μm ² is formed. The voids removed by etching on both sides of the core 12a shown in FIG. 3 may be about 0.5 μm from the viewpoint of the mode field. However, since the LN material is difficult to process, it is preferable to provide voids equal to or larger than the size of the core in order to ensure the manufacturing accuracy of the ridge waveguide. In the first embodiment, the bonding strength of the overclad layer 13 described below can be increased by forming a void of 5 μm or more and leaving the other core layer 12.

As will be shown later in FIG. 6, the first and second substrates in which a plurality of ridge-type waveguides serving as wavelength conversion elements are formed on a 3-inch wafer are manufactured.

As a means for manufacturing the optical waveguide core, a machining technique such as dicing may be used in addition to the dry etching process.

FIG. 5 shows a method for producing an overcladed layer according to the first embodiment. Prepare a LiTaO ₃ substrate made of the same material as the base substrate 11 to be the underclad layer. An overcladed substrate is prepared by injecting He with a dose amount of 3 × 10 ¹⁶ over the entire surface at 200 keV on one surface of the LiTaO ₃ substrate.

The surface of the second substrate on which the ridge-type waveguide is formed by the manufacturing method of FIG. 4 and the surface of the overclad substrate 14 opposite to the surface on which He is driven are bonded by the above-mentioned direct joining method (the above-mentioned direct joining method). FIG. 5 (a), fourth step). After that, the heat treatment is performed in two steps. In the first heat treatment, the layer in which He is injected is peeled off by heating in a constant temperature bath at about 150 ° C. for 1 hour (FIG. 5 (b), first heat treatment step). As a result, the overcladed substrate remaining on the upper surface of the ridge-type waveguide after being peeled off becomes an overcladed layer 13 made of the same single crystal LiTaO ₃ as the underclad layer having a thickness of 0.7 μm. The second heat treatment is annealed at the same temperature and time as the direct bonding of the first and second substrates described above to firmly bond the core 12a and the core layer 12 to the overclad layer 13 (second heat treatment). Process).

When the warp of the entire substrate was measured with the overclad layer loaded, the warp amount of the entire 3-inch wafer was about 10 μm. This amount of warpage is similar to that of the bare LiTaO ₃ and LiNbO ₃ substrates. According to the first embodiment, since a substrate having the same coefficient of thermal expansion as the underclad layer can be used as the overclad layer, it can be seen that the stress strain associated with the loading of the overclad layer is extremely small.

FIG. 6 shows a method of cutting out the wavelength conversion element according to the first embodiment. As described above, a 3-inch wafer in which an overclad layer is loaded is produced on the first and second substrates on which a plurality of ridge-type waveguides are formed (FIG. 6A). A wavelength conversion element is completed by cutting out a desired number of ridge-type waveguides into strips with a desired length by dicing and optically polishing both end faces (FIG. 6 (b)).

According to the first embodiment, the distribution of the refractive index difference in the vertical direction of the optical waveguide becomes symmetric, so that the amount of mode overlap between the input light and the converted light is improved, and the wavelength conversion efficiency is a number. There is an improvement of about%. When a wavelength conversion element for generating mid-infrared light, visible light, and ultraviolet light was produced by changing the polarization inversion period of the ridge type waveguide using the method of the first embodiment, an overclad layer and an underclad layer were produced. Since the clad layer is the same single crystal material, no increase in loss due to light absorption was observed, and good wavelength conversion could be performed.

In addition, since the same type of single crystal material is bonded by the direct bonding method by thermal diffusion, it is possible to realize a wavelength conversion element that is highly resistant to light damage, has high long-term reliability, and can input high output optical power. Can be done.

(Second embodiment)
The structure of the ridge-type waveguide of the wavelength conversion element according to the second embodiment is the same as that of the first embodiment (FIG. 3). In the second embodiment, the base substrate 11 made of a Z-cut crystal substrate and the core layer 12 made of a Z-cut Zn-added LN substrate having a periodic polarization inversion structure are bonded by a direct bonding method using thermal diffusion. , The core layer 12 is processed by a dry etching process to form a ridge type waveguide. The cross-sectional shape of the core 12a is a 4 × 4 μm ² square. An overclad layer 13 made of a Z-cut Mg-doped LN is bonded onto the core 12a and the core layer 12. The second embodiment shows an example in which the effective refractive index of the waveguide can be controlled by the thickness of the overclad layer.

In the first embodiment, the same kind of material was used for the base substrate and the core layer. However, when the same kind of material substrates are joined, the difference in refractive index between the substrates cannot be made large. Therefore, the confinement of light is weak, the miniaturization of the optical waveguide is limited, and it becomes difficult to realize a highly efficient wavelength conversion device. Conventionally, the direct junction type waveguide by heat diffusion has a refractive index difference of about 0.5 to 0.7% between the core layer and the clad layer, and the cross-sectional shape of the core is 5 even if the waveguide is miniaturized. Only about × 5 μm ² can be realized. Unless the difference in refractive index between the core layer and the clad layer is at least 1% or more, it is difficult to further reduce the size of the waveguide.

The following two methods are known as methods for producing a waveguide by a direct joining method and capable of obtaining a large difference in refractive index. A method of joining dissimilar substrates by a surface-activated room temperature bonding method, and a method of forming an amorphous material such as glass between a core layer and a base substrate as a bonding layer to function as an underclad layer.

The surface-activated room temperature bonding method allows the bonding process to be performed at room temperature. By surface-treating the joint surface in vacuum, the atoms on the surface are put into an active state in which chemical bonds are easily formed. By using such a surface treatment, the temperature of the bonding at room temperature or the subsequent heat treatment can be significantly reduced. It has been reported that a silicon (Si) substrate and an LT substrate are bonded to each other by a surface-activated room temperature bonding method to form a bonded substrate having a large difference in refractive index (see Non-Patent Document 2).

However, the crystal substrates of LN and LT undergo a waveguide fabrication process such as dry etching, so that oxygen in the crystal is released and defects occur. If there is such a defect, the propagation loss of the optical waveguide increases and the light damage resistance also deteriorates. Therefore, after going through the waveguide fabrication process, annealing treatment is required to supplement the oxygen released from the crystal. However, the bonding between the Si substrate and the LT substrate by the surface-activated room temperature bonding method has a problem that the bonded substrate is damaged during this annealing treatment because the difference in the coefficient of thermal expansion between the two is large.

On the other hand, in the method of using an amorphous material as a bonding layer, an amorphous material having a refractive index smaller than that of the core layer and the base substrate functions as an underclad layer, thereby increasing the effective refractive index difference of the waveguide. Can be taken. Since the bonding is performed by using heat diffusion as in the normal direct bonding method, the problem of damage to the substrate during the annealing treatment as described above does not occur.

However, when an amorphous material is used as the bonding layer, the film thickness of the core layer may be non-uniform due to the non-uniformity of the film thickness of the bonding layer. As a result, there is a problem that the phase matching wavelength of the wavelength conversion element also becomes non-uniform over the entire element length. Further, since it is difficult to control the refractive index of the amorphous material itself, there is also a problem that the average value of the phase matching wavelength itself deviates from the design value. Further, when an amorphous material is used, the arrangement of the surface molecules on the bonding surface is random, and the number of effective bonds per unit area is smaller than that of direct bonding between crystals. For this reason, there is also a problem that the bonding strength is weak and long-term reliability is lacking. In addition, since the number of process steps is increased to form the bonding layer, the characteristics vary from process to process. From the above, it can be said that the production of the waveguide of the wavelength conversion element by the direct bonding method is suitable for bonding crystals having stable optical characteristics and heat treatment.

Therefore, when LN is used for the core layer, quartz can be considered as a crystal that can be used as a substrate that can be directly bonded and heat-treated. For quartz, processing technology has been established, and wafers with a good surface flatness are available. In addition, the coefficient of thermal expansion in the in-plane direction is 13.2 × 10 ^-6 , which is very close to the coefficient of thermal expansion in the in-plane direction of LN 15.4 × 10 ^-6 . From this, it is a crystal that can be directly bonded to LN and heat-treated.

When the core 2 made of a Z-cut Zn-added LN substrate having a periodic polarization inversion structure is formed on the base substrate 1 made of quartz by using the structure of the conventional ridge type waveguide shown in FIG. 1, the specific refractive index The difference is about 28%, which is very large. Therefore, the light confinement to the core is very strong, and even if the cross-sectional shape of the core is 5 × 5 μm ² or less, the waveguide is in multiple modes.

In order to realize a highly efficient wavelength conversion element, in addition to increasing the power density of light in the core, it is necessary to lengthen the interaction length of light in principle. In a wavelength conversion device, it is desirable that the light incident on the core excites only the ground mode of the optical waveguide. In the excited state in multiple modes, the overlap of the optical electric fields of the signal light and the excitation light is poor, the interaction between the signal light and the excitation light is reduced, and the efficiency of the nonlinear optical effect is deteriorated. Therefore, in the case of the wavelength conversion element of the ridge type waveguide shown in FIG. 1, it is necessary to make the cross-sectional shape of the core about 1 × 1 μm ² in order to excite only the ground mode. However, such a waveguide-sized device is not realistic in consideration of actual fabrication accuracy.

Therefore, it is necessary to appropriately adjust the effective refractive index difference between the overclad layer and the core layer by using the overcladed layer. However, as described above, an arbitrary refractive index difference can be obtained by using different materials. It was difficult to set up.

In the second embodiment, instead of using different kinds of amorphous materials for the overclad layer, the same kind of crystalline material can be used to provide an arbitrary effective refractive index difference. Specifically, as described with reference to FIG. 5, the thickness of the overclad layer remaining after peeling is strictly adjusted by adjusting the ion implantation energy into the overcladed substrate.

FIG. 7 shows the effective refractive index of the 0th and 1st order modes with respect to the difference in the refractive index between the core and the base substrate. When a material having an arbitrary refractive index is used for the base substrate in the wavelength conversion element of the ridge type waveguide shown in FIG. 1, the mode effective refractive index with respect to the refractive index difference (Δn) between the core and the base substrate is determined. It is the result of calculation. It can be seen that when Δn becomes large, the confinement of light in the core becomes strong and the effective refraction becomes small. As described above, it is sufficient if a material having an arbitrary refractive index can be used for the base substrate, but in reality, it is extremely difficult to make it compatible with the condition that the material has a similar coefficient of thermal expansion.

FIG. 8 shows the effective refractive index of the 0th and 1st order modes of the wavelength conversion element according to the second embodiment. In the wavelength conversion element of the ridge type waveguide shown in FIGS. 3 and 5, the cross-sectional shape of the core is assumed to be 4 × 4 μm ² . When the same kind of crystal material is used, the difference in refractive index between the core and the overcladed layer is as small as 1% or less, and the optical electric field leaks to the overcladed layer side simply by joining the overcladed layer. .. In the second embodiment, by controlling the thickness of the overclad layer, the amount of electric field leakage to the overcladed layer can be controlled to control the mode effective refractive index of the ridge-type waveguide.

In FIG. 8, the mode effective refractive index when the thickness of the overclad layer is 0.4 μm and in FIG. 7, the mode effective refractive index when Δn is about 3.8% are the same (n _eff = 2). .1010) It can be seen. This means that if the thickness of the overclad layer is about 0.4 μm, an optical waveguide corresponding to Δn = 3.8% can be effectively formed.

Similarly, the effective refractive index of the mode when the thickness of the overclad layer is 0.6 μm and the effective refractive index when Δn is about 1.9% are the same (n _eff = 2.1030). I understand. This means that if the thickness of the overclad layer is about 0.6 μm, an optical waveguide corresponding to Δn = 1.9% can be effectively formed, and the thickness of the overclad layer is 0.4. Any waveguide with Δn = 1.9-3.8% can be formed by using an arbitrary thickness between −0.6 μm. Further, when the thickness of the overclad layer is set to about 0.2 μm, an effective refractive index difference of about Δn = 6% can be provided, and when the thickness is set to about 1.2 μm, an effective refractive index difference of about Δn = 1.7% can be provided.

In this way, when substrates of the same type are joined together, the effective refractive index difference between the overclad layer and the core layer can be appropriately adjusted by using the overcladed layer, which is practical in manufacturing. It is an optical waveguide with sufficient light confinement in a large core size, and a wavelength conversion element with high conversion efficiency can be realized.

(Third embodiment)
FIG. 9 shows the structure of the optical phase modulator according to the third embodiment of the present invention. The structure of the ridge-type waveguide of the optical phase modulator according to the third embodiment is the same as that of the first embodiment (FIG. 3). In the third embodiment, the base substrate 11 made of LiTaO ₃ and the core layer 12 made of the Z-cut Zn-added LN substrate are bonded by a direct joining method using thermal diffusion, and the core layer 12 is subjected to a dry etching process. To form a ridge type waveguide. The cross-sectional shape of the core 12a is a 5 × 5 μm ² square. An overclad layer 13 made of a single crystal Z-cut quartz substrate is bonded onto the core 12a and the core layer 12. The overclad layer 13 is formed so as to have a thickness of 0.7 μm.

Further, an Au electrode is formed on the overclad layer 13 by photolithography and metal film vapor deposition. By applying a modulation signal and a bias voltage between the electrodes 15a on the core 12a and the electrodes 15b and 15c on the core layers 12 on both sides, optical phase modulation by an electro-optical effect (EO effect) is performed. A metal film is directly formed between the crystal thin film to be the overclad layer 13 and the metal film to be an electrode without providing a buffer layer such as an amorphous oxide material.

For comparison, an optical phase modulation (comparative example) was prepared in which SiO ₂ , which is an amorphous material, was used as an overclad layer having a thickness of 0.7 μm. As a result of evaluating the characteristics of DC drift, the amount of DC drift within a certain period of time can be reduced to 1/5 or less of the optical phase modulator of the comparative example in the third embodiment. In addition, the overclad layer made of a quartz substrate with a large difference in specific refractive index from the core layer prevents the optical electric field from leaking to the metal side, and optical modulation is performed even for watt-class high-power optical inputs without deterioration of characteristics. It can be carried out. Further, since the surface on which the metal film to be the electrode is formed becomes flat, the procedure for flattening is not required as compared with the overclad layer using an amorphous material having an uneven surface.

As described above, in the third embodiment, it is possible to realize an optical modulator having a small DC drift and capable of watt-class optical input. In the third embodiment, the cross-sectional shape of the core is 5 × 5 μm ² , but it is possible to design with an arbitrary waveguide size by appropriately changing the thickness of the overclad layer.

1,11 Base board 2,12a core 3,13 Overclad layer 12 Core layer 14 Overcladed board 15 Electrodes

Claims (5)

A first substrate made of a quartz substrate and
A second substrate made of a single crystal nonlinear optical medium or an electro-optic medium containing an additive, which is joined to the first substrate by a direct bonding method by thermal diffusion to form a core of a ridge type waveguide.
A nonlinear optical medium or electricity containing an additive different from the additive of the second substrate , which is a single crystal overclad layer bonded to the upper part of the ridge type waveguide by a direct bonding method by thermal diffusion. With an overclad layer made of an optical medium,
An optical element characterized in that the effective refractive index difference of the ridge type waveguide is set with the thickness of the overclad layer in the range of 0.2 μm to 0.6 μm. The optical element according to claim 1, wherein the core has a periodic polarization inversion structure in the waveguide direction of light and functions as an optical waveguide type wavelength conversion element using a second-order nonlinear optical effect. For the nonlinear optical medium or the electro-optic medium, at least one selected from the group consisting of Mg, Zn, Sc, and In is added to LiNbO ₃ , LiTaO ₃ , and LiNbxTa _1-x O ₃ (0 ≦ x ≦ 1). The optical element according to claim 1 or 2 , wherein the optical element is contained as a substance. It is a manufacturing method of optical elements.
A first step of joining a single crystal first substrate and a second substrate made of a single crystal nonlinear optical medium or an electro-optic medium containing additives by a direct bonding method by thermal diffusion.
The second step of polishing the second substrate to obtain a desired thickness, and
The third step of forming the core of the ridge-type waveguide on the second substrate,
An overclad substrate made of a single crystal nonlinear optical medium or an electro-optic medium in which an inert gas is injected into one surface, the surface opposite to the one surface, and the ridge-type lead of the second substrate. This is the fourth step of joining the surface on which the waveguide is formed by the direct joining method by thermal diffusion.
The first heat treatment step of peeling off the layer of the overclad substrate into which the inert gas is injected, and
A method for manufacturing an optical element, which comprises a fourth step including a second heat treatment step for strengthening the bonding between the overcladed substrate remaining after being peeled off and the second substrate. .. It is a manufacturing method of optical elements.
A first step of joining a first substrate made of a crystal substrate and a second substrate made of a single crystal nonlinear optical medium or an electro-optic medium containing an additive by a direct bonding method by thermal diffusion,
The second step of polishing the second substrate to obtain a desired thickness, and
The third step of forming the core of the ridge-type waveguide on the second substrate,
The surface of the single crystal overclad substrate in which the inert gas is injected into one surface, which is opposite to the one surface, and the surface of the second substrate on which the ridge-type waveguide is formed are formed. The effective refractive index of the ridge-type waveguide is formed by joining by a direct bonding method by thermal diffusion, peeling off the layer in which the inert gas is injected in the overclad substrate, and adjusting the thickness of the overclad substrate. A fourth step of setting the difference, wherein the overclad substrate comprises a nonlinear optical medium or an electro-optical medium containing an additive different from the additive of the second substrate. A method for manufacturing an optical element, which is characterized by being provided.

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