JP2004325536A - Nonlinear optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a nonlinear optical device such as an optical switch which is driven on a low voltage, operates at high response speed and brings about less insertion loss and crosstalk, an optical attenuator, an optical modulator and so on. <P>SOLUTION: Y branch 1×2 optical switch 201 as the optical device is constructed by laminating an epitaxial buffer layer 222 composed of KTa<SB>Y</SB>Nb<SB>1-Y</SB>O<SB>3</SB>(KTN) and an optical waveguide layer 223 composed of KTN on a substrate 221 composed of Nb doped SrTiO<SB>3</SB>single crystal semiconductor to be a lower electrode in this order. The substrate 221 is connected to ground 224. First and second upper electrodes 212, 213 are formed on the optical waveguide layer 223. These are connected to a power source not shown in the figure, via a driving circuit also not shown in the figure. A refractive index of the optical waveguide is varied by using an oxide paraelectric material with second order electro-optic effect and applying voltage to a specified portion of the optical waveguide. Consequently the high response speed becomes feasible. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０１８４】
なお、ＫＴＮに近い格子定数を有する材料としては、一般にＳｒＴｉＯ_３、ＭｇＯあるいはＰｔが用いられる。
【０１８５】
また、実施例および変形例では非線形光デバイスを電子ビーム蒸着法、イオンアシスト蒸着法、ＲＦマグネトロンスパッタ法あるいはイオンビームスパッタ法を用いて製造したが、この他にＤＣマグネトロンスパッタ法、イオンプレーティング法、分子線エピタキシ（ＭＢＥ）法、化学気相蒸着（ＣＶＤ）、ディップコーティング法等の各種の成膜法を用いて製造が可能であることは当然である。
【０１８６】
【発明の効果】
以上説明したように請求項１記載の発明によれば、２次の電気光学効果を有する酸化物常誘電体を使用し、光導波路の所定の部位に電圧を印加して光導波路の屈折率を変化させることにした。常誘電体は酸化物強誘電体と比べて比誘電率が極めて小さいので、高速応答の要求に応えることができる。
【０１８７】
請求項２記載の発明では、請求項１記載の非線形光デバイスで酸化物常誘電体は、一般式ＫＴａ_ＹＮｂ_１−ＹＯ_３（０．５≦Ｙ≦１．０）で表わされる物質であるので、光部品等で適用される室温を含む−４０℃から＋８５℃の温度範囲で、酸化物強誘電体あるいは酸化物常誘電体を選択的に形成することが可能となる。ＫＴＮでは、Ｔａ原子とＮｂ原子の組成比を０〜１．０の間で変更することにより、たとえば波長０．６３３μｍにおいて屈折率を２．０から２．３程度の広い範囲に渡って選択することができる。したがって、使用温度を考慮して常誘電体の状態を維持し得る組成比範囲内において、決定することができる。また、組成を変えることに対してエピタキシャル膜あるいは配向膜においては格子定数の変化は十分小さく、相互の結晶整合性も良好である。
【０１８８】
更に請求項４〜請求項７記載の発明に示すように、光導波路構造体およびバッファ層が、結晶構造を問わずエピタキシャル膜あるいは配向膜あるいは多結晶膜あるいはアモルファス膜でもよいので、結晶基板のみならず、安価なガラス基板や、有機ポリマ基板等の上にデバイスの各部品を形成することも可能になる。特に有機ポリマ基板上に成膜できると、非線形光デバイスを形成後、フレキシブルに曲げたり、変形したりできるので、さらなる高密度で適用範囲の広い安価な非線形光デバイスを提供することができる。
【０１８９】
また請求項１８あるいは請求項１９記載の発明によれば一対の電極による光導波路の分岐部への電圧印加により光経路の切り替えを制御するデジタル型光スイッチとしての制御が行われるので、一定の電圧で光経路が切り替わった後、それ以上の電圧を印加しても、その状態を保持し複数の動作点が発生しないこと、動作電圧のトレランスに優れており、偏波無依存化が可能であり、波長依存性も小さい等の特長がある。
【０１９０】
更に、請求項２４または請求項２５記載の発明によれば、同一基板上に、非線形光デバイスを複数配列し、複数の入射用光経路より、所望の入射用光経路から複数の出射用光経路へと、光経路を切り替えることにしたので、適切な間隔のポート間の切り替えを低電圧で駆動でき、電極長や曲がり光導波路長も必要に応じて短縮できるため、従来と同様のサイズの基板ウエハへ、より大規模な非線形光デバイスを形成することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施例におけるＹ分岐型の１×２光スイッチの平面図である。
【図２】図１に示した１×２光スイッチのＡ−Ａ断面図である。
【図３】本実施例の１×２光スイッチと同様の積層構造で、酸化物常誘電体の印加電圧に対する屈折率の変化量を測定した特性図である。
【図４】本発明の第２の実施例における光デバイスとしてＹ分岐型の１×８光スイッチの平面図である。
【図５】第２の実施例における第１の光スイッチ部を拡大して示した平面図である。
【図６】第２の実施例の変形例としての第１の光スイッチ部の要部断面図である。
【図７】本発明の第３の実施例における８×８光スイッチの構造の概要を示した平面図である。
【図８】第３の実施例の８×８光スイッチのＸ交差部を示した拡大平面図である。
【図９】第３の実施例の８×８光スイッチのＸ交差部を図８に示したＢ−Ｂ方向に切断した要部断面図である。
【図１０】本発明の第４の実施例における光デバイスとして方向性結合器型の２×２光スイッチの平面図である。
【図１１】図１０の２×２光スイッチのＣ−Ｃ方向の断面図である。
【図１２】本発明の第５の実施例における２×２光スイッチの構造の概要を示した平面図である。
【図１３】図１２の２×２光スイッチのＤ−Ｄ方向の断面図である。
【図１４】本発明の第６の実施例におけるマッハ・ツェンダ干渉型の１×２光減衰器の平面図である。
【図１５】第６の実施例の１×２光減衰器をＥ−Ｅ方向に切断した端面図である。
【図１６】第６の実施例の１×２光減衰器の結合長と光強度の関係を示した特性図である。
【図１７】光導波路層を構成する酸化物常誘電体の印加電圧に対する屈折率の変化を示した特性図である。
【図１８】本発明の第７の実施例における方向性結合型の光減衰器の平面図である。
【図１９】第７の実施例の方向性結合型の光減衰器をＦ−Ｆ方向に切断した端面図である。
【図２０】本発明の第８の実施例におけるマッハ・ツェンダ型の光変調器の平面図である。
【図２１】第８の実施例のマッハ・ツェンダ型の光変調器をＧ−Ｇ方向に切断した端面図である。
【図２２】本発明の第９の実施例における光減衰器と光スイッチを組み合わせた複合光デバイスの平面図である。
【図２３】デジタル型光スイッチのＹ分岐型光導波路の理想形状を示した要部説明図である。
【図２４】デジタル型光スイッチのＹ分岐型光導波路を実際の形状に製作した場合の要部説明図である。
【符号の説明】
２０１ １×２光スイッチ
２０８、２０８Ａ、４３６、４３７、５６１、５６４、５６５、５６８、６０６、６６１、６６８、６６４、６６５ 直線型光導波路
２０９、２０９Ａ、２１０、２１０Ａ、５６２、５６３、５６６、５６７、６６２、６６３、６６６、６６７ Ｓ字型光導波路
２１２、２１２Ａ、２１２Ｂ、２１３、２１３Ａ、２１３Ｂ、４３９、４５４、４５５、５０８、５０９、５７１、６７１ 上部電極
２２３、２２３Ｂ、４４４、４６４、５１４ 光導波路層
３０１ １×８光スイッチ
３２１〜３２７ 光スイッチ部
４０１ ８×８光スイッチ
４５２、４５３、５０６、５０７、６０７、６０８ 光導波路
４５１、７０６ 光スイッチ
５５１、７０３ 光減衰器
５７２、５７３、６７２、６７３ 接地電極
６０１ 方向性結合型の光減衰器
６１１、６１３ 制御電極
６１２、６１４ 方向性結合器
６７８ 信号源
７０１ 複合光デバイス[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical device such as an optical switch, an optical attenuator, and an optical modulator, and more particularly to a nonlinear optical device using a secondary electro-optic effect.
[0002]
[Prior art]
With the rapid growth of communication networks such as the Internet and the enrichment of multimedia contents, the development of optical communication technology using optical fibers is being actively carried out. For example, a SONET / SDH (Synchronous Optical Network / Synchronous Digital Hierarchy) network employs a wavelength division multiplexing communication system for multiplexing optical signals transmitted through optical fibers in order to allow a large number of identical information streams to flow simultaneously. WDM (Wavelength division multiplexing) is used.
[0003]
2. Description of the Related Art Conventionally, in an optical communication network, relatively simple optical communication of a point-to-point type connecting two fixed points has been mainstream. A wavelength division multiplexing communication system is adopted, and a repeater is arranged between two points having a transmission / reception function, and a highly integrated optical wavelength add-drop multiplexing (ADM (Add- There is a demand for an optical switch for performing drop multiplexing) and an optical switching device having an all-optical cross-connect switching function between a plurality of nodes in a large-capacity next-generation optical communication network called a mesh type or a ring type. I have. Under such a technical background, a matrix-type optical switch using a plurality of optical switches and optical gates has attracted attention. By using these optical switches, a flexible and high-capacity optical communication system can be constructed, such as arbitrarily switching the transmission path between multiple optical fibers or forming a detour route in the event of a failure. In this sense, it is considered to be essential as an optical switching device.
[0004]
By the way, optical switching devices are classified into a bulk type using a spatial beam of several tens μm (micrometer) to several mm (mm) and an optical waveguide medium medium using a spatial beam of several μm to several tens μm. Can be. Among them, the bulk type mechanically operates optical components such as a GRIN lens (rod lens), a small lens, a mirror, a prism, and an optical fiber having a refractive index distribution of several mm in a radial direction to switch an optical path. Do. A typical example is a MEMS (Micro Electro-Mechanical Structure) type optical switch. An optical switching device using such an optical component has a feature that it has little temperature dependency and relatively low loss. However, the switching speed is slow, and the vibration and impact characteristics are poor. In addition, it is difficult to reduce the size of the optical fiber, and it is difficult to manage the optical fiber.
[0005]
On the other hand, the medium optical waveguide type is not only small and inexpensive, but also excellent in high-speed switching and high integration. For this reason, various types have been proposed as matrix type optical switches of the optical waveguide type. The optical waveguide type optical switch mainly uses an oxide ferroelectric, a compound semiconductor, quartz, and an organic polymer as its material. Especially for oxide ferroelectrics, LiNbO ₃ And LiTaO ₃ Is typically used. By forming a ridge structure or a buried structure with these materials, various optical waveguide structures are formed, and an optical switch for electrically controlling the light propagation direction at the intersection of the entrance port, etc. The function of an optical gate that controls opening and closing of the light is controlled in the optical waveguide.
[0006]
Among them, the optical waveguide type optical switch using quartz has the same component system as the optical fiber, so that the core diameter can be made equal to the core diameter of the optical fiber. In a single mode fiber for a wavelength of 1550 nm, the core diameter is as small as about 10 μm. However, by forming a core of the same size with the optical waveguide type optical switch, the coupling efficiency between the optical fiber and the optical waveguide can be sufficiently increased, and the insertion loss can be reduced.
[0007]
However, in an optical waveguide type optical switch using quartz, a current flows through a heater electrode formed on the surface of the optical waveguide, and the light propagation direction is switched using a refractive index change due to a thermo-optic effect. Therefore, there is a problem that switching response is slow. For example, Toru Maruno, "Latest R & D Trends in Optical Switches", NTT R & D vol. 51 No. 11 2002. Shows that the response time ranges from 1 to 3 ms (millisecond). In this optical waveguide type optical switch, an electric current is applied to the heater electrode to heat the heater electrode. For this reason, it is reported that the operating power per unit switch is about 45 mW (milliwatt). For example, in a 16 × 16 matrix type optical switch, it is reported that when all these switches are energized, the total power consumption reaches 1.4 W. For this reason, there is a problem that the application field of the optical waveguide type optical switch using quartz is limited.
[0008]
Next, an optical waveguide type optical switch using an organic polymer using a Y-branch type structure will be considered. The optical waveguide type optical switch using an organic polymer has a thermo-optic effect that is at least one order of magnitude greater than that of a quartz-based optical switch, and has characteristics such as a small wavelength dependence. However, even when a polymer material is used, when optical switching is performed in the same manner as an optical waveguide type optical switch using quartz, a current flows through a heater electrode portion formed on the surface of the optical waveguide to cause refraction by the thermo-optic effect. The light propagation direction is switched using the rate change. Therefore, there is a problem that the response is similarly slow. According to the above-mentioned document, the response time is 3.4 ms to 6.0 ms, the power consumption reaches about 56 mW per unit switch, and the total power consumption is 1 × 8 Y-branch type optical switch. It is reported to reach 450 mW.
[0009]
Next, a bubble reflection type optical switch using liquid bubble reflection will be considered. In the bubble reflection type optical switch, a groove is formed in a crossed optical waveguide such as an X-crossing or a Y-crossing, and a refractive index adjusting liquid having the same refractive index as that of the core region of the optical waveguide is put in and out of the groove, thereby forming a groove portion. To control the total reflection or light transmission of the light propagation. Either a method of generating bubbles in the liquid using the technology of the ink jet printer or a method of moving the position of the bubbles by heating is used for taking in and out of the liquid. All of the methods perform optical switching using a total reflection condition, and thus have a feature that wavelength dependency is small. However, there is a problem that the response time is limited to about 10 ms even if the viscosity of the liquid is adjusted optimally because the mechanism of expansion, movement, and disappearance of bubbles is used.
[0010]
Next, an optical waveguide type optical switch using a compound semiconductor will be considered. In this optical waveguide type optical switch, a semiconductor amplifier (SOA) is manufactured, and switching is performed depending on whether the semiconductor amplifier is turned on to transmit light or is turned off to absorb light. This achieves a high-speed response characteristic of about nanosecond. However, especially in the case of a GaAs-based compound semiconductor, the core diameter is reduced to about 3 μm because the refractive index is 3 or more. For this reason, there is a problem that the coupling efficiency between the optical fiber and the optical waveguide portion is deteriorated, and the insertion loss is increased.
[0011]
Next, LiNbO, which is the most typical optical switching material and one of oxide ferroelectrics, ₃ Is considered. LiNbO ₃ In the case of (1), the first-order electro-optic effect is used. When a voltage is applied between the electrodes of the optical switch, the state of light changes at high speed due to a change in the refractive index due to the electro-optic effect, and the traveling direction of the light changes depending on which state is set. Thus, each optical switch can selectively output optical signal components from two input ports to two output ports. Therefore, by appropriately setting the traveling direction of light in each optical switch, an optical signal component from an input port can be sent to a desired output port.
[0012]
In addition, LiNbO ₃ In general, in an optical switch using, a Ti diffusion type optical waveguide or a proton exchange type optical waveguide is manufactured on a single crystal wafer. In this case, the core diameter can be made approximately equal to the core diameter of the optical fiber, and the coupling efficiency between the optical fiber and the optical waveguide is improved, so that the insertion loss can be reduced. However, this LiNbO ₃ The optical switch using the method employs a method of forming a planar electrode on the surface of an optical waveguide and applying a voltage. For this reason, the distance between the electrodes increases, and the electric field distribution in the cross section of the optical waveguide does not become an ideal state. For example, Nishihara, Haruna, Suhara, “Optical Integrated Circuit”, Chapter 7, p.217, Ohmsha. The electric field distribution vector does not become one direction inside the core as shown in FIG. As a result, in order to be polarization independent, it is necessary to increase the applied voltage to 40 volts or more. Further, it is necessary to increase the interaction length in the optical waveguide direction so as not to increase the driving voltage, and the length of the electrode needs to be 4 mm or more. Furthermore, since an optical waveguide is formed on a single crystal wafer by Ti diffusion or proton exchange, the effective refractive index of the optical waveguide cannot be sufficiently higher than the effective refractive index of the surrounding cladding layer, and the refractive index difference cannot be increased. . Accordingly, it is necessary to increase the radius of curvature of the S-shaped optical waveguide to 50 mm. Therefore, there is a problem that a large area is required for the optical waveguide type optical switch.
[0013]
There are several types of switching principles of the optical paths of these optical switches described above. That is, (i) a method of controlling an optical path by applying an electric field to a directional coupler in which two optical waveguides are arranged close to each other, and (ii) input light is separated into two by a directional coupler, and each path is separated. Mach-Zehnder method of switching the output end by controlling the interference state at the exit side directional coupler by providing a phase difference with the refractive index caused by the electric field between the light passing through, and (iii) at the X intersection A method of switching light paths by controlling interference between optical modes; (iv) in a Y-branch or an asymmetric X-crossing part, a lateral distribution of optical modes is generated by an electric field and controlled by a refractive index to control light; And (v) a method of switching the light path by performing total reflection or Bragg reflection by providing an electrode at the X intersection and controlling the refractive index using an electro-optic effect by an applied voltage, and the like. (T Patent Document 1 and the above-mentioned Nishihara if example, Haruna, Suhara al., "Optical integrated circuit", p321, Ohm Corporation. Reference).
[0014]
[Patent Document 1]
JP-A-7-318986 (paragraphs 0027 and 0028, FIG. 1)
[0015]
[Problems to be solved by the invention]
[0016]
By the way, an optical switch having only two states, ON and OFF, which can be taken with respect to a control voltage to be applied and having a function of taking the same state at a certain threshold voltage or more, is called a digital optical switch. Can be called. The digital optical switch has features such as excellent operating voltage tolerance, polarization independence, and small wavelength dependence. However, on the other hand, there are problems such as a higher driving voltage and a longer electrode length than other optical switches. Further, the Y-branch optical waveguide of the digital optical switch has a problem that a large applied voltage is required in relation to crosstalk. This will be described below.
[0017]
FIG. 23 shows an essential part of an ideal shape of a Y-branch optical waveguide of a digital optical switch. The Y-branch optical waveguide 102 of the digital optical switch 101 forms a Y-shaped branch 103. In this branch portion 103, the two optical waveguide portions intersect at an angle of less than 1 degree as shown in this figure, and the width d between the waveguides becomes gradually narrower as approaching the branch portion 103. Ideally, the distance would be zero.
[0018]
FIG. 24 shows a main part of the shape when the Y-branch optical waveguide of the digital optical switch is manufactured in an actual shape. In the patterning step of the Y-branch optical waveguide 102A by photolithography, it is difficult to form an ideal shape due to the limit of resolution. Normally, as in the digital optical switch 101A shown in FIG. 24, when the distance d between the waveguides of the branching portion 103A becomes about 1.5 μm, the tip of the acute angle portion is cut off, and the shape becomes blunt.
[0019]
Such a deviation from the ideal shape does not significantly affect the loss but has a very large effect on the crosstalk. When the opening angle of the Y-branch is 0.5 degrees and the refractive index of one of the branched waveguide portions is reduced to about 0.0008 by the electro-optic effect or the like, the crosstalk is set to 20 dB or less in the ideal shape of FIG. It is possible. When this is the shape shown in FIG. 24, the crosstalk is degraded to about 12 dB. As a result, in order to reduce the crosstalk to 20 dB, a larger change in the refractive index, that is, a larger applied voltage is required.
[0020]
Such a problem in manufacturing the optical waveguide can occur not only in the branch portion of the Y-branch type optical waveguide but also in the branch portion of the asymmetric X-cross type optical waveguide.
[0021]
As described above, LiNbO ₃ When using any of the following materials: compound semiconductor, quartz, polymer, and bubble reflection type, the size, drive voltage (or drive current or power consumption), switching speed, crosstalk, insertion loss, and temperature of the optical switch It is not possible to obtain a waveguide type optical switch that simultaneously satisfies the stability problem.
[0022]
In the foregoing, the conventional technology and the problem with the optical switching device have been described. Next, other optical devices, such as optical attenuators and optical modulators, which have attracted attention together with optical switches due to the development of communication networks, will be briefly described. First, the optical attenuator will be described.
[0023]
Conventionally, an optical attenuator using the same principle as the optical switch described above has been used as an optical attenuator. These can be classified into (i) a bulk type using a spatial beam of several tens μm to several mm, and (ii) an optical waveguide in a medium using a spatial beam of several μm to several tens μm.
[0024]
Among them, the bulk type (i) has a one-dimensional film thickness gradient between optical components such as a GRIN lens, a small lens, a mirror, a prism, and an optical fiber having a refractive index distribution of several mm in the radial direction. A method is disclosed in which the position of an ND (neutral density) filter on which a metal film is deposited is slid to adjust the attenuation factor (for example, Patent Document 2).
[0025]
In addition, a small shielding plate is inserted between the transmission fiber and the reception fiber by using MEMS (Micro Electro-Mechanical Structure) technology, and the position is controlled by MEMS to control the coupling loss between the fibers. A MEMS type optical attenuator is disclosed (for example, Patent Document 3). Although the temperature dependency is small and the loss is relatively low, the former has a slow switching speed of 1 second or more, and the latter has poor vibration / shock characteristics.
[0026]
On the other hand, the optical waveguide type (ii) does not involve mechanical movement, is excellent in small size, low cost, high speed switching, high integration, and the like, and various types are proposed. For example, LiNbO ₃ This is a waveguide type optical attenuator using a ferroelectric crystal substrate such as a crystal (for example, Patent Documents 4 to 6). However, these optical waveguide type optical attenuators have the same problem as the optical switch because they adjust the attenuation of the light amount based on the same principle as that of the optical switch. Taking the above cited document 5 as an example, the half-wave voltage reaches 11.4 V when the maximum attenuation rate is obtained. For this reason, an expensive high-speed high-voltage driving driver is required.
[0027]
Next, the optical modulator will be described. The optical modulator operates on basically the same principle as the optical switch and the optical attenuator described above. In particular, a high-speed response characteristic is required for an optical modulator in the optical communication field. For this reason, an optical waveguide type optical modulator has been mainly used as the optical modulator (for example, Patent Document 5 or Patent Document 7). In Patent Document 7, the half-wave voltage is 5.1 V, but it is better to drive at a lower voltage, and it is possible to drive at 3.3 V using a high-speed electronic device.
[0028]
These optical attenuators and optical modulators also have the same problem as the optical switching device because they have the same basic structure.
[0029]
[Patent Document 2]
JP-A-2000-56242 (paragraph 0024, FIG. 5)
[Patent Document 3]
JP-A-2003-66268 (paragraphs 0026 and 0027, FIG. 5)
[Patent Document 4]
JP 2001-249309 A (Paragraph 0023, FIG. 3)
[Patent Document 5]
JP-A-2000-227581 (paragraph 0013, FIG. 4)
[Patent Document 6]
JP-A-10-142569 (paragraph 0026, FIG. 1)
[Patent Document 7]
JP-A-6-300995 (17th paragraph, FIG. 1)
[0030]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a nonlinear optical device such as an optical switch, an optical attenuator, and an optical modulator that can perform low-voltage, high-speed response operation and has small insertion loss and crosstalk.
[0031]
[Means for Solving the Problems]
According to the first aspect of the present invention, (a) an optical waveguide made of an oxide paraelectric material having a second-order electro-optic effect, which guides light input from a predetermined incident port and guides the light to an output port, is provided on the substrate. (B) a pair of electrodes for applying a voltage to a predetermined portion of the optical waveguide; and (c) a refractive index of the optical waveguide by applying a voltage to the pair of electrodes. The nonlinear optical device is provided with a refractive index control means for changing the refractive index.
[0032]
The fact that the relative permittivity of an oxide ferroelectric is high and the capacitance thereof tends to be large, whereas the fact that the relative permittivity of a paraelectric is extremely smaller than that of a ferroelectric is used. Therefore, the present invention is suitable for a case where a faster response is required.
[0033]
According to a second aspect of the present invention, in the nonlinear optical device according to the first aspect, the oxide paraelectric substance has a general formula of KTa. _Y Nb _1-Y O ₃ It is characterized by being a substance represented by (0.5 ≦ Y ≦ 1.0).
[0034]
KTa _Y Nb _1-Y O ₃ (Hereinafter, referred to as KTN) indicates a completely solid solution phase or a solid solution phase even when the composition ratio of Ta atoms and Nb atoms is arbitrarily changed between 0 and 1.0, and indicates a single crystal film, an alignment film, and a polycrystalline film. Either a crystalline film or an amorphous film can be formed. Furthermore, the transition temperature (Curie temperature) between the ferroelectric and the paraelectric is -260 ° C. to + 435 ° C. by appropriately selecting the composition ratio of the Ta atom and the Nb atom between “0” and “1”. The oxide ferroelectric or oxide paraelectric can be selectively formed in a temperature range of −40 ° C. to + 85 ° C., including room temperature, applied to optical components and the like, since it can be obtained arbitrarily. It has the feature that it becomes possible. On the other hand, the materials described in Patent Literature 3 and the like are limited to use only as oxide ferroelectrics.
[0035]
In KTN, by changing the composition ratio between Ta atoms and Nb atoms between 0 and 1.0, for example, the refractive index is selected over a wide range of about 2.0 to 2.3 at a wavelength of 0.633 μm. be able to. Therefore, it can be determined within the composition ratio range that can maintain the state of the paraelectric substance in consideration of the use temperature. When the composition is changed, the change in the lattice constant is sufficiently small in the epitaxial film or the oriented film, and the mutual crystal matching is good.
[0036]
Here, the above-described solid solution phase will be described. When forming an epitaxial or single orientation film, it is necessary to satisfy the condition of lattice matching with the underlying surface. This matching condition varies depending on the material system, but it is generally required that the deviation of the lattice constant is 10% or less. When the oxide paraelectric thin film constituting the optical waveguide layer is composed of a plurality of materials, it must be applied under conditions that satisfy the solid solution phase. This is because, unless the solid-solution phase condition is satisfied, the respective material components are not completely mixed, and the respective material components are not uniform in the thin film, and are formed in a state of being precipitated as a micro-sized mass. is there. In that case, if the mass exists in the optical waveguide layer through which the optical signal propagates, the refractive index of each material is different, so that Fresnel reflection or disturbance in the propagation direction of the optical signal is induced, and the fundamental as the optical waveguide is generated. This is because it affects the function. As a material system satisfying the solid solution phase condition in the perovskite crystal structure, (KTaO ₃ -KNbO ₃ ), (BaTiO) ₃ -SrTiO ₃ ), (NaTaO) ₃ -NaNbO ₃ ), (AgTaO) ₃ -AgNbO ₃ ), (PbTiO ₃ -PbZrO ₃ ), (Pb _1-X La _X (Zr _1-Y Ti _Y ) _{1-X / 4} O ₃ )
However, there is a wide range such as (0 ≦ X ≦ 1, 0 ≦ Y ≦ 1).
[0037]
According to a third aspect of the present invention, the nonlinear optical device according to the first aspect includes a substrate having conductivity, and the substrate also serves as one of a pair of electrodes.
[0038]
According to a fourth aspect of the present invention, the nonlinear optical device according to the first aspect includes a substrate and a conductive film formed on a surface portion thereof, and the conductive film forms one of a pair of electrodes. Features.
[0039]
The third and fourth aspects of the present invention show aspects of the configuration of the electrode. In a nonlinear optical device, a conductive or semiconductive thin film provided on a single crystal substrate serving as a lower electrode is, for example, an epitaxial film, an alignment film, a polycrystalline film, or an amorphous film.
[0040]
The invention according to claim 5 is characterized in that the substrate of the nonlinear optical device according to claim 1 is a single crystal substrate.
[0041]
According to a sixth aspect of the present invention, the substrate of the nonlinear optical device according to the first aspect is a glass substrate.
[0042]
Since the optical waveguide structure and the buffer layer may be an epitaxial film, an alignment film, a polycrystalline film, or an amorphous film regardless of the crystal structure, for example, JP-A-2000-56343, JP-A-2000-56344, or The limitation of epitaxial or single orientation disclosed in JP-A-2000-305117 is eliminated. Therefore, it is possible to form each component of the device on not only a crystal substrate but also an inexpensive glass substrate, an organic polymer substrate, or the like. In particular, if a film can be formed on an organic polymer substrate, the nonlinear optical device can be flexibly bent or deformed after the formation, so that an inexpensive nonlinear optical device having a higher density and a wider application range can be provided.
[0043]
According to a seventh aspect of the present invention, in the nonlinear optical device according to the first aspect, the substrate is an organic polymer substrate. The organic polymer substrate includes an organic low molecular polymer substrate and an organic polymer polymer substrate.
[0044]
According to an eighth aspect of the present invention, in the nonlinear optical device according to the first aspect, the oxide paraelectric substance shows a solid solution phase and is constituted by a single crystal film.
[0045]
According to a ninth aspect of the present invention, there is provided the nonlinear optical device according to the first aspect, wherein the oxide paraelectric substance shows a solid solution phase and is composed of an alignment film.
[0046]
According to a tenth aspect of the present invention, in the nonlinear optical device according to the first aspect, the oxide paraelectric substance exhibits a solid solution phase and is formed of a polycrystalline film.
[0047]
According to an eleventh aspect of the present invention, in the nonlinear optical device according to the first aspect, the oxide paraelectric substance shows a solid solution phase and is formed of an amorphous film.
[0048]
According to a twelfth aspect of the present invention, in the nonlinear optical device according to the first aspect, the substrate is an organic polymer substrate, and the optical waveguide layer and the buffer layer are formed thereon.
[0049]
If light is input to the optical waveguide formed on the substrate when the conductive substrate has absorption within the applicable wavelength range, part of the incident optical electric field distribution in the optical waveguide also exists in the substrate, and The electric field amplitude causes absorption loss in the substrate. If the electric field amplitude in the substrate is replaced by a buffer layer whose refractive index is smaller than that of the optical waveguide layer material, the buffer layer functions as an isolation layer between this optical waveguide layer and the conductive substrate, and the absorption loss in the substrate is reduced. Thus, the propagation loss can be reduced.
[0050]
According to a thirteenth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide layer has a ridge structure provided in a layer adjacent to the lower side in a convex shape.
[0051]
According to a fourteenth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide layer has a ridge structure provided in a layer adjacent to the upper side in a convex shape.
[0052]
According to a fifteenth aspect of the present invention, in the non-linear optical device according to the first aspect, the cladding is made of an oxide between the optical waveguide layer and the upper electrode constituting one of the pair of electrodes, and has a smaller refractive index than the optical waveguide layer. It is characterized by having a layer.
[0053]
When a metal electrode as an upper electrode is provided on the optical waveguide layer and the vibration frequency of the light in the optical waveguide layer exceeds the plasma frequency of the metal electrode, the electric field amplitude into the metal electrode due to light propagation increases. The exuded component is strongly absorbed by the carrier in the metal, resulting in a propagation loss. However, if the area where this seeping occurs is replaced by a cladding layer having a smaller refractive index than the material of the optical waveguide layer, the cladding layer thus functions as an isolation layer between the optical waveguide layer and the metal electrode, and the upper electrode (metal electrode). ) Is eliminated, and the propagation loss can be reduced.
[0054]
According to a sixteenth aspect of the present invention, in the nonlinear optical device according to the first aspect, one of the electrodes is made of an oxide and has a cladding layer having a smaller refractive index than the optical waveguide layer.
[0055]
When a metal electrode as an upper electrode is provided on the optical waveguide layer and the vibration frequency of the light in the optical waveguide layer exceeds the plasma frequency of the metal electrode, the electric field amplitude into the metal electrode due to light propagation increases. The exuded component is strongly absorbed by the carrier in the metal, resulting in a propagation loss. However, if the area where this seeping occurs is replaced by a cladding layer having a smaller refractive index than the material of the optical waveguide layer, the cladding layer thus functions as an isolation layer between the optical waveguide layer and the metal electrode, and the upper electrode (metal electrode). ) Is eliminated, and the propagation loss can be reduced.
[0056]
According to a seventeenth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is configured such that the optical path is switched by applying a voltage to a branch portion that branches the incident optical path and the output optical path. Features.
[0057]
According to the eighteenth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is a Y-branch Y-shaped optical waveguide, and a voltage applied to a branch portion of the Y-branch optical waveguide by a pair of electrodes. It is characterized in that control as a digital optical switch that controls switching of an optical path by application is performed.
[0058]
The digital optical device is excellent in the tolerance of the operating voltage because the optical path is switched at a certain voltage, and even if a higher voltage is applied, the state is maintained and multiple operating points are not generated. It has features such as being polarization independent and having small wavelength dependence.
[0059]
In the nineteenth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is an X-branch type optical waveguide, and an optical path is formed by applying a voltage to a branch portion of the X-branch type optical waveguide by a pair of electrodes. This is characterized in that control is performed as a digital optical switch for controlling switching.
[0060]
The digital optical device is excellent in the tolerance of the operating voltage because the optical path is switched at a certain voltage, and even if a higher voltage is applied, the state is maintained and multiple operating points are not generated. It has features such as being polarization independent and having small wavelength dependence.
[0061]
According to a twentieth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is an X-branch type optical waveguide, and total reflection is performed by applying a voltage to a branch portion of the X-branch type optical waveguide by a pair of electrodes. It is characterized in that control as an optical switch for controlling switching of an optical path by a mold or transmission is performed.
[0062]
According to a twenty-first aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is an X-branch optical waveguide, and the optical path is formed by applying a voltage to a branch portion of the X-branch optical waveguide by a pair of electrodes. It is characterized in that control is performed as a Bragg reflection type optical switch for controlling the switching of.
[0063]
According to a twenty-second aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is an X-branch optical waveguide, and the optical waveguide is formed by applying a voltage to a branch portion of the X-branch optical waveguide by a pair of electrodes. It is characterized in that control is performed as a directional coupler type optical switch that controls switching of an optical path between the optical switches.
[0064]
According to a twenty-third aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is a Mach-Zehnder interference type optical waveguide, and switching of an optical path is controlled by applying a voltage to a branch portion of the optical waveguide by a pair of electrodes. This is characterized in that control as a Mach-Zehnder interference type optical switch is performed.
[0065]
According to a twenty-fourth aspect of the present invention, in the nonlinear optical device according to any one of the eighteenth to twenty-third aspects, a plurality of switches are arranged in a matrix.
[0066]
The nonlinear optical device of the present invention has a configuration in which the nonlinear optical devices are arranged in a matrix on the same substrate, and the optical path is changed from a plurality of input optical paths to a desired output optical path to a plurality of output optical paths. This is a matrix type nonlinear optical device characterized by switching. Since switching between ports at appropriate intervals can be driven at low voltage, and the electrode length and bent optical waveguide length can be shortened as necessary, a larger matrix type nonlinear light can be transferred to a substrate wafer of the same size as before. A device can be formed.
[0067]
According to a twenty-fifth aspect of the present invention, in the nonlinear optical device according to any one of the eighteenth to twenty-third aspects, a plurality of switches are arranged in a tree on the same substrate.
[0068]
The nonlinear optical device of the present invention arranges nonlinear optical devices in a tree shape on the same substrate, and forms an optical path from a plurality of input optical paths to a desired output optical path to a plurality of output optical paths. It is a tree-type nonlinear optical device characterized by switching. Since switching between ports at appropriate intervals can be driven at low voltage, and the electrode length and the bent optical waveguide length can be shortened as necessary, a larger tree-type nonlinear light can be transferred to a substrate wafer of the same size as before. A device can be formed.
[0069]
According to a twenty-sixth aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is a directional coupling type optical waveguide, and optical interference is controlled by applying a voltage to a branch portion of the optical waveguide by a pair of electrodes. It is characterized in that control as a directional coupler type optical attenuator is performed.
[0070]
According to a twenty-seventh aspect of the present invention, in the nonlinear optical device according to the first aspect, the optical waveguide is a Mach-Zehnder interference type optical waveguide, and Mach-Zehnder for controlling optical interference by applying a voltage to a branch portion of the optical waveguide by a pair of electrodes. It is characterized in that control as an interference type optical modulator is performed.
[0071]
BEST MODE FOR CARRYING OUT THE INVENTION
[0072]
【Example】
Hereinafter, the present invention will be described in detail with reference to examples.
[0073]
<First embodiment>
[0074]
FIG. 1 shows a 1 × 2 Y-branch optical switch as an optical device according to a first embodiment of the present invention. The 1 × 2 optical switch 201 has an input end face 202 and an output end face 203 arranged on the opposite side. One entrance port 204 is arranged on the entrance end face 202, and first and second exit ports 205 and 206 are arranged on the exit end face 203 at a predetermined interval. One linear optical waveguide 208 is arranged between the incident port 204 and the Y-shaped branch portion 207 shown in the drawing. Further, between the first and second emission ports 205 and 206 and the Y-shaped branch portion 207, first or second S-shaped optical waveguides 209 and 210 curved in an S-shape are disposed, respectively. These ends merge with one end of the linear optical waveguide 208 to form a Y-shaped branch 207.
[0075]
A first upper electrode 212 and a second upper electrode 213 are lined with the linear optical waveguide 208 above the layer where the linear optical waveguide 208 and the first and second S-shaped optical waveguides 209 and 210 exist. They are arranged in a symmetrical shape. The first upper electrode 212 is composed of a strip-shaped electrode portion arranged along the first S-shaped optical waveguide 209 by a predetermined length from the Y-shaped branch portion 207 and a wide electrode portion bent at a right angle to the strip-shaped electrode portion. It is configured. The second upper electrode 213 also includes a strip-shaped electrode portion disposed along the second S-shaped optical waveguide 210 by a predetermined length from the Y-shaped branch portion 207 and a wide electrode portion bent at a right angle to the strip-shaped electrode portion. Have been.
[0076]
FIG. 2 shows the 1 × 2 optical switch shown in FIG. 1 cut in the AA direction. The 1 × 2 optical switch 201 is composed of Nb-doped SrTiO serving as a lower electrode. ₃ KTa having a wavelength of 1.5 μm and a refractive index of 2.20 on a substrate 221 made of a (100) single crystal semiconductor. _Y Nb _1-Y O ₃ An epitaxial buffer layer 222 made of KTN (hereinafter referred to as KTN) and an optical waveguide layer 223 made of an epitaxial film or oriented film made of KTN having a composition having a refractive index of 2.25 at the same wavelength are laminated in this order. It has a structure. The substrate 221 is grounded 224. Further, on the optical waveguide layer 223, first and second upper electrodes 212 and 213 made of an aluminum thin film are formed. The first and second upper electrodes 212 and 213 are also connected to a power source (not shown) via a drive circuit connected to a control circuit (not shown).
[0077]
The following definition is made in this specification.
(A) "Orientation film" means a polycrystalline film in which one plane index and its higher order peak appear by the same X-ray diffraction pattern.
(B) "Polycrystalline film" refers to a film having a single phase but various surface indices mixed according to the same X-ray diffraction pattern. This means that the diffraction intensity corresponding to the crystal orientation is identified as 1% or more of the entire diffraction intensity.
(C) The term “epitaxial film” refers to an “alignment film” in which the in-plane crystal orientation of each crystal grain varies by X-ray pole figure, RHEED (reflection high-speed electron diffraction), LEED (slow electron diffraction), or the like. In-plane random alignment film "," In-plane selective alignment film "in which crystal grains oriented in two or more specific directions coexist, and" In-plane alignment film "completely oriented in one direction Say.
(C) “Amorphous film” refers to a film state in which there is no peak corresponding to the crystal orientation in a θ-2θ X-ray diffraction pattern. It means that the diffraction intensity corresponding to the crystal orientation is identified as 1% or less of the entire diffraction intensity.
[0078]
Nb-doped SrTiO constituting the substrate 221 shown in FIG. ₃ The lattice constant of the (100) single crystal is 3.905 °, and the lattice constant of the KTN single crystal film forming the buffer layer 222 is 3.90 °. Therefore, the lattice mismatch is as small as 0.13%, and good epitaxial growth is possible. In the optical waveguide layer 223, a linear optical waveguide 208 and first or second S-shaped optical waveguides 209 and 210 shown in FIG. 1 are formed. On the optical waveguide layer 223 made of an epitaxial film or an alignment film, first and second upper electrodes 212 and 213 made of an aluminum thin film for applying a voltage to the Y branch portion 207 (FIG. 1) are provided. .
[0079]
The 1 × 2 optical switch 201 of the first embodiment is manufactured as follows by applying the spin coating method.
[0080]
(1) Buffer layer formation
Ethoxy potassium KOC ₃ H ₅ , Pentaethoxy niobium Nb (OC ₃ H ₅ ) ₅ , And pentaethoxy tantalum Ta (OC ₃ H ₅ ) ₅ These were dissolved in ethanol and stirred at room temperature for 30 minutes. In the above steps, all alkoxides are affected by moisture. Therefore, these steps were performed in a nitrogen gas atmosphere. It should be noted that the composition ratio of Ta and Nb fluctuates in the state of the solution and in the process of the following heat solidification treatment. Therefore, pentaethoxyniobium Nb (OC) is finally formed so that a KTN buffer layer having a desired composition ratio of Ta and Nb is obtained. ₃ H ₅ ) ₅ , And pentaethoxy tantalum Ta (OC ₃ H ₅ ) ₅ Was adjusted to obtain a desired precursor solution for a KTN buffer layer.
[0081]
This buffer layer precursor solution is washed, etched and dried in advance with Nb-doped SrTiO. ₃ It was spin-coated on a (100) single crystal substrate and dried. Then, it heated by the heater in an oxygen atmosphere, hold | maintained at 300 degreeC for 1 hour, and cooled. By repeating this, solid phase epitaxial growth was performed, and a KTN epitaxial buffer layer having a composition with a refractive index of 2.20 was formed with a thickness of 1.2 μm.
[0082]
Next, the precursor solution for a KTN buffer layer obtained in the previous step was spin-coated on a KTN epitaxial buffer layer in a nitrogen atmosphere and dried. Thereafter, the resultant was heated with a heater in an oxygen atmosphere, kept at 300 ° C. for 1 hour, and then cooled. By repeating this, a KTN amorphous buffer layer was formed with a thickness of 0.23 μm.
[0083]
Next, a photoresist was spin-coated, prebaked, exposed, and developed. After the post-baking, a part of the KTN buffer layer was removed by wet etching with an aqueous HCl solution, and as shown in FIG. 2, etching was performed at a width of 5 μm and a depth of 0.23 μm in the depth direction of the optical waveguide to form a pattern on the epitaxial buffer layer. Was formed. Next, the resist material was peeled off by a remover.
[0084]
Next, a step of forming the optical waveguide layer 223 on the buffer layer 222 will be described.
[0085]
(2) Optical waveguide layer formation
A KTN optical waveguide layer precursor solution prepared in the same manner as the KTN buffer layer precursor solution was spin-coated and dried in a nitrogen atmosphere on the KTN amorphous buffer layer patterned by the etching described above. . Then, after heating with a heater in an oxygen atmosphere and maintaining at 300 ° C. for 1 hour, the KTN amorphous optical waveguide layer having a composition having a refractive index of 2.25 after epitaxial growth is cooled to a thickness of 0.23 μm by cooling. Formed. Further, by heating at 700 ° C. for 1 hour, solid phase epitaxial growth of the KTN amorphous buffer layer and the KTN amorphous optical waveguide layer was performed.
[0086]
Further, the precursor solution for the KTN optical waveguide layer was spin-coated on a surface thereof in a nitrogen atmosphere and dried. Then, it was heated at 300 ° C. by a heater in an oxygen atmosphere, further kept at 700 ° C. for 1 hour, and then cooled. By repeating this, solid-phase epitaxial growth was performed, and a KTN epitaxial optical waveguide layer having a composition with a refractive index of 2.25 was formed with a thickness of 1.4 μm. It is more desirable to repeat solid phase epitaxial growth with such a thin film thickness than in the case of solid phase epitaxial growth of a thick amorphous layer at a time in terms of crystal quality. Of course, the present invention is not limited to this, and the number of repetitions and the film thickness of each time can be selected as necessary. After an Al thin film having a thickness of 200 nm is formed on the KTN epitaxial optical waveguide layer of the Y branch portion 207 shown in FIG. 1 by a sputtering method, the same 5 μm width and 2000 μm length shown in FIG. The first and second upper electrodes 212 and 213 were formed to manufacture a 1 × 2 optical switch 201. The incident end face and the outgoing end face were polished.
[0087]
The crystallographic relationship between the stacked layers can be expressed as the following expression structure (1). Note that a slash "/" is provided between the layers. The same applies hereinafter.
Epitaxial or single orientation KTN (100) optical waveguide layer / KTN (100) buffer layer / Nb-SrTiO ₃ (100) substrate and in-plane orientation KTN [001] optical waveguide layer / KTN [001] buffer layer / Nb-SrTiO ₃ [001] Substrate ... (1)
The secondary electro-optic coefficient R of the KTN optical waveguide layer is represented by the following equation (2).
R = 2 × 10 ^-15 (M ² / V ² …… (2)
[0088]
When a laser beam having a wavelength of 1.5 μm is made incident on the incident port 204 of the 1 × 2 optical switch 201 obtained as described above, the first and second laser beams are output by the Y branching unit 207 with an optical power of 3 dB (50%). Laser light was output from the first and second emission ports 205 and 206 after being branched into two S-shaped optical waveguides 209 and 210. At this time, the Nb-SrTiO ₃ By applying a voltage to one of the electrodes (not shown) of the substrate 221 and one of the first and second upper electrodes 212 and 213, the refractive index between the electrodes changes.
[0089]
FIG. 3 shows the relationship between the applied voltage and the changing refractive index. This characteristic diagram is obtained by measuring the amount of change in the refractive index with respect to the applied voltage of the oxide paraelectric material constituting the optical waveguide layer in the same laminated structure as the 1 × 2 optical switch 201 of this embodiment. When the applied voltage is gradually increased from 0 V so that the first or second upper electrode 212 or 213 shown in FIG. 1 becomes a positive voltage, the refractive index acts in a decreasing direction, and a parabolic quadratic function is obtained. Behavior. It can be seen that for an applied voltage of about 20 V, the refractive index varies by -0.003.
[0090]
Now, suppose that the above-mentioned electrode of the substrate 221 shown in FIG. 2 is grounded, and 5 V is applied to the first or second upper electrode 212, 213 as an applied voltage. For example, when a voltage is applied to the first upper electrode 212, the refractive index of the first S-shaped optical waveguide 209 thereunder decreases, and the refractive index of the second S-shaped optical waveguide 210 does not decrease. As a result, the laser light having a wavelength of 1.5 μm incident from the incident port 204 is applied to the Y branch 207 at the second lower electrode 213 below the second upper electrode 213 where no voltage is applied and the refractive index is not reduced. Then, the light passes through only the S-shaped optical waveguide 210 and is emitted from the second emission port 206.
[0091]
Conversely, in order to emit laser light from the first emission port 205, a voltage of 5 V may be applied only to the second upper electrode 213. In the case of the 1 × 2 optical switch 201 of the present embodiment, the switching frequency is 100 MHz or more, the crosstalk is 20 dB or less, the insertion loss is 5 dB or less, and good characteristics independent of polarization can be secured. .
[0092]
<Second embodiment>
[0093]
FIG. 4 shows a Y-branch 1 × 8 optical switch as an optical device according to a second embodiment of the present invention. The 1 × 8 optical switch 301 has an incident end face 302 and an exit end face 303 located on the opposite side. One entrance port 304 is arranged on the entrance end face 302, and first to eighth exit ports 305 to 312 are arranged on the exit end face 303 at predetermined intervals. The input port 304 is connected to the input port side of the first optical switch section 321 having the same configuration as the 1 × 2 optical switch 201 of the first embodiment, and the first output port side is connected to the first optical switch section 321 having the same configuration. The input port side of the second optical switch unit 322 is connected. Further, the input port side of the third optical switch section 323 having the same configuration is connected to the first output port side of the second optical switch section 322. The first and second emission ports 305 and 306 are connected to the emission port side of the third optical switch unit 323. Similarly, the input port side of the fourth optical switch section 324 having the same configuration is connected to the second output port side of the second optical switch section 322. The third and fourth emission ports 307 and 308 are connected to the emission port side of the fourth optical switch unit 324. Further, the input port side of the fifth optical switch section 325 is connected to the second output port side of the first optical switch section 321. The input port side of the sixth optical switch section 326 is connected to the first output port side of the fifth optical switch section 325, and the input of the seventh optical switch section 327 is connected to the second output port side. The port side is connected. The fifth to eighth emission ports 309 to 312 are connected to the emission port sides of the sixth and seventh optical switch units 326 and 327, respectively.
[0094]
FIG. 5 shows the structure of only the first optical switch section taken out. Although the specific sizes of the second to seventh optical switch units 322 to 327 shown in FIG. 4 may be different from those of the first optical switch unit 321, their structures are substantially the same. The first optical switch section 321 has substantially the same structure as the 1 × 2 optical switch 201 of the first embodiment shown in FIG. Therefore, in the first optical switch section 321, the same portions as those of the 1 × 2 optical switch 201 are added with “A” to their reference numerals, and the description thereof is omitted as appropriate. FIG. 2 will be used as appropriate in the description of the cross-sectional structures of the first to seventh optical switch units 321 to 327.
[0095]
(1) Buffer layer formation
Similarly to the 1 × 2 optical switch 201 of the first embodiment, in the manufacture of the 1 × 8 optical switch 301, Nb-doped SrTiO that has been previously washed, etched and dried is used. ₃ On a (100) single crystal substrate, a KTN epitaxial buffer layer having a composition with a refractive index of 2.20 was formed with a thickness of 1.2 μm. Next, a KTN amorphous buffer layer having a composition with a refractive index of 2.20 was formed with a thickness of 0.23 μm. Next, a photoresist was spin-coated, prebaked, exposed, and developed. After the post-baking, a part of the KTN buffer layer was removed by wet etching with an aqueous HCl solution, and as shown in FIG. 2, a pattern was formed by performing etching with a width of 5 μm and a depth of 0.23 μm in the depth direction of the optical waveguide. Next, the resist material was peeled off by a remover.
[0096]
Next, a step of forming a KTN optical waveguide layer on the KTN buffer layer will be described.
[0097]
(2) Optical waveguide layer formation
A KTN epitaxial optical waveguide layer having a composition with a refractive index of 2.25 having a thickness of 1.6 μm was formed on the KTN amorphous buffer layer patterned by the etching described above, similarly to the KTN buffer layer. After forming an Al thin film having a thickness of 200 nm on the KTN epitaxial optical waveguide layer at the Y branch portion by a sputtering method, a first upper portion having a width of 5 μm and a length of 2000 μm as shown in FIG. 1 by a lift-off method. The electrode 212 and the second upper electrode 213 were formed for each of the first to seventh optical switches 321 to 327, and were formed for each 1 × 2 optical switch. Then, a 1 × 8 optical switch 301 composed of a total of seven stages of three stages using the 1 × 2 optical switch 201 shown in FIG. 1 as a basic unit was manufactured. One optical fiber (not shown) is disposed at the input port 304 of the input end face 302 of the 1 × 8 optical switch 301, and eight optical fibers are provided at 250 μm intervals at the first to eighth output ports 305 to 312 of the output end face 303. Of optical fibers (both not shown). Note that the input end face 302 and the output end face 303 were polished.
[0098]
The crystallographic relationship between the stacked layers can be expressed as the expression structure (1) as in the 1 × 2 optical switch 201 of the first embodiment. The secondary electro-optic coefficient R of the KTN optical waveguide layer is as shown in the above equation (2).
[0099]
As one 1 × 2 optical switch constituting the 1 × 8 optical switch, for example, the characteristics of the first optical switch unit 321 shown in FIG. 5 will be described. When a laser beam having a wavelength of 1.5 μm is incident on the incident port 204A of the upper electrode, the Y-branch 207A branches to the first and second S-shaped optical waveguides 209A and 210A with an optical power of 3 dB (50%). Then, laser light was output from the first and second emission ports 205A and 206A. At this time, the Nb-SrTiO ₃ When a voltage is applied between an electrode (not shown) of the substrate 221 and any one of the first and second upper electrodes 212A and 213A, the refractive index between the electrodes changes.
[0100]
The relationship between the applied voltage and the changing refractive index at this time has already been described with reference to FIG. That is, as the applied voltage is gradually increased from 0 V so that the first or second upper electrode 212A, 213A becomes a positive voltage, the refractive index acts in the direction of decreasing, and the parabolic quadratic function behaves. Is shown. It can be seen that for an applied voltage of about 20 V, the refractive index varies by -0.003.
[0101]
Conversely, if the applied voltage is gradually reduced from 0 V so that the first or second upper electrode 212A, 213A becomes a negative voltage, the refractive index acts in the direction of decreasing the same as when a positive voltage is applied. , It can be seen that for an applied voltage of about 20 V, the refractive index fluctuates by -0.003. By returning the applied voltage to 0 V, it is possible to adjust the refractive index by high-reproducible voltage control without hysteresis.
[0102]
Now, it is assumed that the above-mentioned electrode of the substrate 221 shown in FIG. 2 is grounded and 5 V is applied to the first or second upper electrode 212A, 213A as an applied voltage. For example, when a voltage is applied to the first upper electrode 212A, the refractive index of the first S-shaped optical waveguide 209A below it decreases, and the refractive index of the second S-shaped optical waveguide 210A does not decrease. As a result, the laser light having a wavelength of 1.5 μm incident from the incident port 204A is applied to the second branch 207A under the second upper electrode 213A to which no voltage is applied and the refractive index is not reduced in the Y branch 207A. Then, the light passes through only the S-shaped optical waveguide 210A and enters the second output port 206A to the incident port side of the fifth optical switch unit 325 shown in FIG.
[0103]
Conversely, in order for laser light to enter the second optical switch unit 322 from the first emission port 205A, a voltage of 5 V may be applied only to the second upper electrode 213A shown in FIG. The above is independently controlled by the first to seventh optical switch sections 321 to 327 each having a total of seven places, so that light is transmitted to any of the eight first to eighth emission ports 305 to 312. Can be switched.
[0104]
In the case of the 1 × 8 optical switch 301 of the second embodiment, the switching frequency was 100 MHz or more, the crosstalk was 20 dB or less, and the insertion loss was 7 dB or less, and good polarization-independent good characteristics could be secured.
[0105]
<Modification of Second Embodiment>
[0106]
FIG. 6 shows in principle the sectional structure of a first optical switch section as a modification of the 1 × 8 optical switch of the second embodiment of the present invention. The planar structure of the 1 × 8 optical switch of this modified example is substantially the same as the 1 × 8 optical switch 301 of the second embodiment shown in FIGS. Therefore, the same portions as those described above are denoted by reference numerals “B” after the numbers indicating these components, or “A” is replaced with “B”, and the description thereof will be omitted as appropriate. In addition, illustration of the second to seventh optical switch units 322B to 327B is omitted, and the drawing and description of the first optical switch unit 321B are appropriately used.
[0107]
As shown in FIG. 6, the first optical switch portion 321B of the Y-branch type 1 × 8 optical switch 301B of this modification is made of SrTiO 2 having insulating properties as a material of the substrate 221B. ₃ (100) A single crystal substrate is used. Therefore, in the embodiment shown in FIG. 2, the lower Nb-SrTiO ₃ The substrate 221 itself constitutes the lower electrode, which is grounded. In this modification, SrTiO 2 is used. ₃ The lower electrode layer 351 is formed on the single crystal substrate 221B, and is grounded. The lower electrode layer 351 is made of SrTiO ₃ It is a Pt thin film formed on a single crystal substrate 221B. On the lower electrode layer 351, an epitaxial buffer layer 222B made of KTN having a wavelength of 1.5 μm and a composition having a refractive index of 2.20, and an epitaxial film made of KTN having a composition having a refractive index of 2.23 at the same wavelength. Alternatively, an optical waveguide layer 223B made of an alignment film is formed in order, and a KTN epitaxial cladding layer 361 having a composition having a refractive index of 2.20 is further formed thereon.
[0108]
SrTiO ₃ The lattice constant of the (100) single crystal substrate 221B is 3.905 °, while the lattice constant of the Pt thin film forming the lower electrode layer 351 is 3.924 °. Further, since the KTN single crystal film constituting the epitaxial buffer layer 222B has a lattice constant of 3.90 °, the lattice mismatch is extremely small at 0.6% or less, and good epitaxial growth is possible. The above-described optical waveguide is formed in the optical waveguide layer 223B. An aluminum thin film for applying a voltage to the Y branch portion via the cladding layer 361 is formed on the optical waveguide layer 223B made of an epitaxial film or an alignment film. A first upper electrode 212B and a second upper electrode 213B are provided. The first upper electrode 212B and the second upper electrode 213B are connected to a power supply (not shown) via a drive circuit (not shown) connected to a control circuit (not shown), and the lower electrode layer 351 and the power supply are grounded. ing.
[0109]
Next, a method of manufacturing the 1 × 8 optical switch 301B of this modification will be described.
[0110]
(1) Buffer layer formation
SrTiO previously cleaned, etched and dried in the same manner as the 1 × 2 optical switch 201 of the first embodiment. ₃ A Pt epitaxial film having a thickness of 50 nm is formed as the lower electrode layer 351 on the (100) single crystal substrate 221B by sputtering. A KTN epitaxial buffer layer having a composition with a refractive index of 2.20 was formed thereon with a thickness of 2 μm. Next, a KTN amorphous buffer layer having a composition with a refractive index of 2.20 was formed with a thickness of 500 nm.
[0111]
Next, in the same manner as shown in FIG. 2, a part of the KTN buffer layer 222B is removed by wet etching with an aqueous HCl solution, and as shown in FIG. 2, etching of 5 μm in width and 230 nm in the depth direction of the optical waveguide is performed. A pattern was formed. Next, the resist material was peeled off by a remover.
[0112]
Next, the KTN optical waveguide layer formed on the KTN buffer layer will be described.
[0113]
(2) Optical waveguide layer formation
A KTN epitaxial optical waveguide layer having a composition with a refractive index of 2.23 having a thickness of 500 nm was formed on the KTN amorphous buffer layer patterned by etching, similarly to the KTN buffer layer. Thereafter, the KTN amorphous buffer layer and the KTN amorphous optical waveguide layer were subjected to solid-phase epitaxial growth to obtain a KTN epitaxial buffer layer and a KTN epitaxial optical waveguide layer 223B.
[0114]
Next, a KTN epitaxial cladding layer 361 having a composition with a refractive index of 2.20 was formed on the KTN epitaxial optical waveguide layer 223B to a thickness of 2 μm by solid phase epitaxial growth. After an Al thin film having a thickness of 200 nm is formed on the KTN epitaxial optical waveguide layer 223B at the Y branch portion by a sputtering method, a lift-off method is used to form an upper electrode having a width of 5 μm and a length of 3000 μm as shown in FIG. One upper electrode 212B and a second upper electrode 213B are formed. The above is formed for each of the first to seventh optical switch sections 321 to 327 (1 × 2 optical switch) to form a three-stage tree structure with the 1 × 2 optical switch shown in FIG. A 1 × 8 optical switch 301B composed of the individual pieces was manufactured. One optical fiber (not shown) is arranged on the first-stage incident end face of the 1 × 8 optical switch 301B, and eight optical fibers (not shown) are arranged at an interval of 250 μm on the third-stage exit end face. The incident end face and the outgoing end face were polished.
[0115]
The crystallographic relationship between the stacked layers can be expressed as the following expression structure (3).
Epitaxial or single-oriented KTN (100) cladding layer / epitaxial or single-oriented KTN (100) optical waveguide layer / KTN (100) buffer layer / Pt (100) epitaxial electrode layer / SrTiO ₃ (100) Substrate and in-plane orientation KTN [001] cladding layer / KTN [001] optical waveguide layer / KTN [001] buffer layer / Pt [001] layer / SrTiO ₃ [001] Substrate (3)
The secondary electro-optic coefficient R of the KTN optical waveguide layer is as in the above equation (2).
[0116]
As one 1 × 2 optical switch constituting the 1 × 8 optical switch, for example, attention is paid to a first optical switch section 321B (see the first optical switch section 321 in FIG. 5), and its characteristics will be described. When a laser beam having a wavelength of 1.5 μm is incident on the incident port 204B of the upper electrode, the Y-branch 207B branches to the first and second S-shaped optical waveguides 209B and 210B with an optical power of 3 dB (50%). Then, laser light was output from the first and second emission ports 205B and 206B. At this time, the Nb-SrTiO ₃ By applying a voltage between the electrode connected to the lower electrode layer 351 formed on the substrate 221B made of and any one of the first or second upper electrodes 212B and 213B, a voltage between the electrodes is increased. Changes.
[0117]
The relationship between the applied voltage and the changing refractive index at this time has already been described with reference to FIG. That is, when the applied voltage is gradually increased from 0 V so that the first or second upper electrode 212B, 213B becomes a positive voltage, the refractive index acts in a decreasing direction, and the parabolic quadratic function behaves. Is shown. It can be seen that for an applied voltage of about 20 V, the refractive index varies by -0.003.
[0118]
Conversely, when the applied voltage is gradually reduced from 0 V so that the first or second upper electrodes 212B and 213B become a negative voltage, the refractive index acts in the direction of decreasing as in the case where a positive voltage is applied. , It can be seen that for an applied voltage of about 20 V, the refractive index fluctuates by -0.003. By returning the applied voltage to 0 V, it is possible to adjust the refractive index by high-reproducible voltage control without hysteresis.
[0119]
Now, it is assumed that the lower electrode layer 351 shown in FIG. 6 is grounded, and 5 V is applied to the first or second upper electrode 212B, 213B as an applied voltage. As an example, when a voltage is applied to the first upper electrode 212B, refraction of the first S-shaped optical waveguide 209B thereunder (the reference character A of the corresponding component in FIG. 5 is replaced with B. The same applies hereinafter). The index decreases, and the refractive index of the second S-shaped optical waveguide 210B does not decrease. As a result, the laser light having a wavelength of 1.5 μm incident from the incident port 204B is applied to the Y-branch 207B at the second lower electrode 213B where no voltage is applied and the refractive index is not reduced. Then, the light passes through only the S-shaped optical waveguide 210B and enters the second optical output port 206B to the incident port side of the fifth optical switch unit 325 shown in FIG.
[0120]
Conversely, in order for laser light to enter the second optical switch unit 322 from the first emission port 205B, a voltage of 5 V may be applied only to the second upper electrode 213B shown in FIG. The above is independently controlled by the first to seventh optical switch sections 321 to 327 each having a total of seven places, so that light is transmitted to any of the eight first to eighth emission ports 305 to 312. Can be switched.
[0121]
The formation of the cladding layer 361 is performed when it is desired to further reduce the propagation loss in the optical waveguide. When the thickness of the cladding layer 361 is increased, low loss characteristics are obtained, and a desired effect is exhibited. However, when the thickness of the cladding layer 361 is large, the effective voltage applied to the optical waveguide layer 223B decreases. Therefore, the thickness of the cladding layer 361 is set in consideration of the switching voltage and the loss characteristics.
[0122]
In this modification, the switching frequency is 100 MHz or more, the crosstalk is 20 dB or less, the insertion loss is 6 dB or less, the loss is improved as compared with the case of the second embodiment, and the polarization independent good characteristics are obtained. Could be secured.
[0123]
<Third embodiment>
[0124]
FIG. 7 shows an outline of the structure of an 8 × 8 optical switch according to the third embodiment of the present invention. This 8 × 8 optical switch 401 is an 8 × 8 optical switch having a crossbar configuration in which 64 2 × 2 optical switches are combined, and has an input end face 402 and an output end face 403 arranged on the opposite side. I have. The incident end face 402 is divided into two groups of eight, and half of the ports are dummy, and the first to fourth incident ports 411 to 414 and the fifth to eighth incident ports 415 to 418 are arranged. I have. In addition, the output end face 403 is divided into two groups of eight, and the first to fourth output ports 421 to 424 and the fifth to eighth output ports 425 to 428 in which half of the ports are dummy are arranged. Have been.
[0125]
FIG. 8 is an enlarged view of one of a total of 64 X intersections where the 8 × 8 optical switches shown in FIG. 7 intersect in an X shape. The X intersection 431 includes first and second X incident ports 432 and 433 located on the incident end face 402 side shown in FIG. 7 and first and second X incident ports 432 and 433 located on the exit end face 403 side shown in FIG. X emission ports 434 and 435, a first linear optical waveguide 436 connecting the first X incidence port 432 and the second X emission port 435, a second X incidence port 433, and a first X emission port 434 are connected to each other, and the intermediate portions of the second linear optical waveguides 437 intersect at an X intersection position 438. Above the X-crossing position 438, an upper electrode 439 of a predetermined length, which is equidistant from the first linear optical waveguide 436 and the second linear-type optical waveguide 437, is located at the center position directly above the X crossing position 438. It is arranged to match the position.
[0126]
The 8 × 8 optical switch 401 shown in FIG. 7 has S-shaped or inverted S-shaped waveguides having linear first and second linear optical waveguides 436 and 437 in the middle thereof, each of which crosses X. Will be. Then, when the first and second X emission ports 434 and 435 are viewed from the first X incidence port 432 side as shown in FIG. 8, the respective X intersections are the same as those in the first embodiment. Of the Y-branch type optical switch. Similarly, when the first and second X emission ports 434 and 435 are viewed from the second X incidence port 433 side, a similar Y-branch type optical switch is obtained. Therefore, by controlling the voltage applied to the upper electrode 439, the signal light incident from the first to eighth incident ports 411 to 418 can be selected and output from the first to eighth exit ports 421 to 428.
[0127]
FIG. 9 illustrates a state in which one X intersection in the 8 × 8 optical switch is cut in the BB direction shown in FIG. 8 in principle. The 8 × 8 optical switch 401 uses an MgO (100) single crystal substrate having insulating properties as a material of the substrate 441. Therefore, a lower electrode layer 442 is formed thereon similarly to the modification of the second embodiment shown in FIG. 6, and this is grounded. The lower electrode layer 442 is an Ag epitaxial film. On top of this lower electrode layer 442, a KTN epitaxial buffer layer 443 having a composition of a refractive index of 2.20 at a wavelength of 1.5 μm and an epitaxial film or orientation of a KTN having a composition of a refractive index of 2.23 at the same wavelength. An optical waveguide layer 444 made of a film is formed in order, and further thereon, an upper electrode 439 made of an aluminum thin film for applying a voltage to the X branch portion 438 is arranged. The upper electrode 439 is connected to a power supply (not shown) via a drive circuit (not shown) connected to a control circuit (not shown), and the lower electrode layer 442 and the power supply are grounded.
[0128]
Next, a method of manufacturing an 8 × 8 optical switch 401 according to a modification of the third embodiment will be described with reference to FIGS.
[0129]
On the MgO (100) single crystal substrate 441 which has been washed, etched and dried in advance, an Ag epitaxial film having a thickness of 100 nm is laminated as the lower electrode layer 442 by RF magnetron sputtering. A KTN epitaxial buffer layer 443 having a composition with a refractive index of 2.20 was formed thereon with a thickness of 1500 nm by RF magnetron sputtering. A KTN epitaxial optical waveguide layer having a composition with a refractive index of 2.25 was formed thereon with a thickness of 1800 nm.
[0130]
Next, the KTN amorphous buffer layer and the KTN amorphous optical waveguide layer were removed by reactive ion etching using a resist pattern to form the X intersection portion 431 and expose the underlying electrode layer, leaving a part thereof. Then, after processing the optical waveguide having the crossbar structure as shown in FIG. 7 as a projection having a height of 300 nm and a width of 5 μm, the KTN amorphous buffer layer and the KTN amorphous optical waveguide layer are subjected to solid-phase epitaxial growth, and the KTN epitaxial buffer layer 443 and The KTN epitaxial optical waveguide layer 444 is used.
[0131]
On the KTN epitaxial optical waveguide layer at the center of the X intersection 431, a laminated thin film made of a 200 nm thick Al thin film and a 300 nm thick ITO (Indium Tin Oxide) is formed by sputtering. An upper electrode 439 having a width of 1 μm and a length of 3000 nm is formed by a lift-off method. These are formed for each 2 × 2 optical switch as the X intersection 431 shown in FIG. In this way, a total of 64 8 × 8 optical switches 401 using the 2 × 2 optical switches shown in FIG. 8 as a basic unit are manufactured. A total of four optical fibers (not shown) are arranged one by one at the first to fourth entrance ports 411 to 414 of the entrance end face 402 of the 8 × 8 optical switch 401 shown in FIG. A total of eight optical fibers (not shown) are arranged in each of the fourth emission ports 421 to 424 and the fifth to eighth emission ports 425 to 428. Note that the input end face 402 and the output end face 403 were polished.
[0132]
The crystallographic relationship between the stacked layers can be expressed as the following expression structure (4).
Epitaxial or single-oriented KTN (100) optical waveguide layer / KTN (100) buffer layer / Ag (100) epitaxial electrode layer / MgO (100) substrate, and in-plane orientation KTN [001] optical waveguide layer / KTN [001] Buffer layer / Ag [001] layer / MgO [001] substrate (4)
[0133]
As one 2.times.2 optical switch constituting the 8.times.8 optical switch 401, attention is paid to one X intersection 431 shown in FIG. When a laser beam having a wavelength of 1.5 μm is made incident on the first X incidence port 432 of the X intersection 431, the first and second X emission ports 434 and 435 are respectively emitted with an optical power of 3 dB (50%). Branched. At this time, by applying a voltage between the lower electrode layer 442 and the upper electrode 439, the refractive index between the electrodes changes. FIG. 3 shows the measurement of the change in the refractive index with respect to the applied voltage of the oxide paraelectric material constituting the optical waveguide layer in the same laminated structure as that of the optical switch of the present embodiment.
[0134]
When the applied voltage is gradually increased from 0 V so that the upper electrode 439 becomes a positive voltage, the refractive index acts in a decreasing direction, and exhibits a parabolic quadratic function. It can be seen that for an applied voltage of about 20 V, the refractive index varies by -0.003.
[0135]
Conversely, if the applied voltage is gradually decreased from 0 V so that the upper electrode 439 becomes a negative voltage, the refractive index acts in the direction of decreasing as in the case of applying the positive voltage, and the applied voltage is about 20 V. Thus, it can be seen that the refractive index changes by -0.003. By returning the applied voltage to 0 V, it is possible to adjust the refractive index by high-reproducible voltage control without hysteresis. Now, when the lower electrode layer 442 is grounded and 5 V is applied to the upper electrode 439, the refractive index of the optical waveguide below the upper electrode 439 decreases, and as a result, total reflection switch control is performed. That is, the laser light having a wavelength of 1.5 μm incident from the first X incidence port 432 is totally reflected by the X branch portion 438 and is emitted from the first X emission port 434. In a state where no voltage is applied to the upper electrode 439 and the refractive index does not decrease, the laser light incident from the first X incident port 432 goes straight through the X branch portion 438 and exits from the second X exit port 435. You.
[0136]
Next, a case where an optical signal having a wavelength of 1550 nm is incident on the first X incident port 432 and an optical signal having a wavelength of 1560 nm is incident on the second X incident port 433 will be described. When no voltage is applied to the upper electrode 439, the optical signal having a wavelength of 1550 nm at the first X incidence port 432 proceeds straight as it is at the X branching section 438 and is emitted from the second X emission port 435. The optical signal having a wavelength of 1560 nm at the second X incidence port 433 also proceeds straight as it is at the X branching section 438 and is emitted from the first X emission port 434.
[0137]
On the other hand, when a voltage is applied to the upper electrode 439 and the total reflection condition is satisfied, the optical signal having a wavelength of 1550 nm that has entered the first X incidence port 432 is totally reflected by the X branching section 438 and the first signal is emitted. The light is emitted from the X emission port 434. Also, an optical signal having a wavelength of 1560 nm is totally reflected by the X branching section 438 and emitted from the second X emission port 435. The above is independently controlled in each of the 8 × 8 optical switch sections including a total of 64 points, so that the signal light incident from the first to eighth incident ports 411 to 418 can be transmitted to the first to eighth exit ports 421. To 428.
[0138]
In the case of the 8 × 8 optical switch 401 of the present embodiment, the switching frequency is 100 MHz or more, the crosstalk is 20 dB or less, the insertion loss is 8 dB or less, and good characteristics independent of polarization can be secured. .
[0139]
<Fourth embodiment>
[0140]
FIG. 10 shows a directional coupler type 2 × 2 optical switch as an optical device according to the fourth embodiment of the present invention. The 2 × 2 optical switch 451 of this embodiment is a pair of optical waveguides which are parallel to each other by a predetermined length at a central portion and are close to each other, and which are separated from each other so as to draw a curve at both ends and have a line-symmetrical relationship with each other. Wave paths 452 and 453 are provided to form an X intersection as a whole. Above the first and second optical waveguides 452 and 453, there are formed a strip-shaped electrode portion arranged along the waveguide portion and a wide electrode portion bent at a right angle to the strip-shaped electrode portion. First and second upper electrodes 454 and 455 having a line-symmetric relationship with each other are arranged.
[0141]
FIG. 11 shows in principle the sectional structure of the 2 × 2 optical switch in the CC direction. The 2 × 2 optical switch 451 of the fourth embodiment uses a glass substrate 461. On this glass substrate 461, first, an amorphous SrRuO ₃ A thin film is formed with a thickness of 1000 nm by an ion beam sputtering method, and subsequently, a KTN amorphous buffer layer 463 having a refractive index of 2.15 is formed with a thickness of 1000 nm. Further thereon, a KTN amorphous optical waveguide layer 464 having a refractive index of 2.23 is formed with a thickness of 1500 nm. The KTN amorphous optical waveguide layer 464 is removed by ion etching except for a part thereof. In this way, the first and second optical waveguides 452 and 453 having the shape of the directional coupling switch shown in FIG. 10 are processed and formed as protrusions having a height of 200 nm and a width of 5.0 μm. Next, a KTN amorphous cladding layer 465 having a composition with a refractive index of 2.18 is formed on the KTN amorphous optical waveguide layer 464 to a thickness of 1200 nm by an ion beam sputtering method.
[0142]
On the KTN amorphous cladding layer 465 at the center of the X intersection, an Al electrode having a thickness of 200 nm is formed by an ion beam sputtering method, and first and second upper electrodes 454 and 455 are formed by a lift-off method. It is formed. Thus, a single mode X-crossing type 2 × 2 optical switch 451 similar to the X-crossing portion 431 (see FIG. 8) of the third embodiment can be manufactured.
[0143]
Note that the principle of the method of driving the 2 × 2 optical switch 451 of the fourth embodiment is almost the same as that of the first embodiment, and a description thereof will be omitted. Although the 2 × 2 optical switch 451 of the fourth embodiment uses a directional coupler type 2 × 2 optical switch, a similar optical switch is configured by applying a Bragg reflection type optical switch instead. It is also possible.
[0144]
<Fifth embodiment>
[0145]
FIG. 12 shows an outline of the structure of a 2 × 2 optical switch according to the fifth embodiment of the present invention. The 2 × 2 optical switch 501 of the present embodiment is a single mode Mach-Zehnder interference type 2 × 2 optical switch. The directional coupler type 2 × 2 optical switch 501 has a line symmetry relationship with two X crossing portions 502 and 503 and a mutually parallel waveguide portion 504 provided therebetween. The first and second optical waveguides 506 and 507 are linearly symmetrical with each other and are formed of a strip-shaped electrode portion disposed along each of the parallel waveguide portions 504 and a wide electrode portion bent at right angles thereto. The first and second upper electrodes 508 and 509 have the following shapes.
[0146]
FIG. 13 shows in principle the sectional structure of the 2 × 2 optical switch in the DD direction. The 2 × 2 optical switch 501 of the fifth embodiment uses a glass substrate 511. On this glass substrate 511, first, an amorphous SrRuO ₃ A thin film is formed with a thickness of 1000 nm by an ion beam sputtering method, and subsequently, a KTN amorphous buffer layer 513 having a refractive index of 2.27 is formed with a thickness of 1000 nm. Further thereon, a KTN amorphous optical waveguide layer 514 having a refractive index of 2.38 is formed with a thickness of 1500 nm. The KTN amorphous optical waveguide layer 514 is removed by ion etching except for a part thereof. In this manner, the first and second optical waveguides 506 and 507 having the shape of the directional coupling switch shown in FIG. 12 are processed and formed as convex portions having a height of 300 nm and a width of 5.0 μm. Next, a KTN amorphous cladding layer 515 having a composition with a refractive index of 2.18 is formed on the KTN amorphous optical waveguide layer 514 to a thickness of 1200 nm by an ion beam sputtering method.
[0147]
After forming an Al electrode to a thickness of 200 nm by ion beam sputtering on the KTN amorphous cladding layer 515 of the flat waveguide portion 504 (see FIG. 12) between the two intersections 502 and 503, the lift-off is performed. First and second upper electrodes 508 and 509 are formed by the off method. Thereby, a single mode Mach-Zehnder interference type 2 × 2 optical switch 501 can be manufactured. Note that the principle of the method of driving the 2 × 2 optical switch 501 of the fifth embodiment is almost the same as that of the first embodiment, and a description thereof will be omitted. In the 2 × 2 optical switch 501 of the fifth embodiment, a directional coupler type 2 × 2 optical switch is used. Instead, a similar optical switch is configured by applying a Bragg reflection type optical switch. It is also possible.
[0148]
<Sixth embodiment>
[0149]
FIG. 14 shows a single mode Mach-Zehnder interference type 1 × 2 optical attenuator according to the sixth embodiment of the present invention. This 1 × 2 optical attenuator 551 has an incident end face 552 and an output end face 553 arranged on the opposite side. One incident port 554 is arranged on the incident end face 552, and one exit port 555 is arranged on the exit end face 553. Between these ports 554 and 555, a linear optical waveguide 561 from the incident end face 552 side, and a pair of S-shaped optical waveguides 562 each having one end connected to one end of the linear optical waveguide 561 (Y branch), 563, parallel linear optical waveguides 564, 565 which are connected at one end to the other ends of the S-shaped optical waveguides 562, 563, and one ends at the other ends of the linear optical waveguides 564, 565, respectively. A pair of connected S-shaped optical waveguides 566 and 567 and a linear optical waveguide 568 having one end connected to the junction of the S-shaped optical waveguides 566 and 567 and the other end connected to the output port 555 are arranged. I have. On these optical waveguides 561 to 568, a predetermined distance is provided along a line symmetric position of the pair of linear optical waveguides 564 and 565 from a branch point of the linear optical waveguide 561 and the S-shaped optical waveguides 562 and 563. The upper electrode 571 composed of the provided strip-shaped electrode portion, the wide electrode portion bent in a direction perpendicular to the strip-shaped electrode portion, and the linear optical waveguides 564 and 565 are disposed in close proximity to the respective linear optical waveguides 564 and 565. First and second ground electrodes 572 and 573 are arranged. A groove is cut in the middle of the linear optical waveguide 568 so as to cross it, and a polarizer 574 is mounted on the groove. Depending on the application of the 1 × 2 optical attenuator 551, the polarizer 574 can be omitted, and the cost as an attenuator can be reduced.
[0150]
FIG. 15 shows an end face structure obtained by cutting the 1 × 2 optical attenuator of the sixth embodiment in the EE direction. This 1 × 2 optical attenuator 551 is formed by forming a KTN amorphous buffer layer having a refractive index of 2.18 as a buffer layer 582 with a thickness of 1000 nm on a polymer substrate 581 using an ion-assisted vapor deposition method. The KTN amorphous optical waveguide layer 583 having a ratio of 2.23 is formed with a thickness of 1500 nm. Next, the KTN amorphous optical waveguide layer 583 is removed by ion etching except for a part thereof, and a Mach-Zehnder interference type optical waveguide (portions of linear optical waveguides 564 and 565 in the figure) is 300 nm in height and 9 mm in width. It is processed and formed as a 0.0 μm convex portion.
[0151]
Next, a clad layer 584 is formed on the KTN amorphous optical waveguide layer 583, and an Al electrode is formed thereon with a thickness of 200 nm by ion-assisted vapor deposition. By forming the upper electrode 571 and the first and second ground electrodes 572 and 573, a single mode Mach-Zehnder interference type 1 × 2 optical attenuator can be manufactured.
[0152]
When a laser beam having a wavelength of 1.5 μm is made incident on the incident port 554 of the 1 × 2 optical attenuator 551, 3 dB is produced at the Y branch where the linear optical waveguide 561 and the pair of S-shaped optical waveguides 562 and 563 are connected. (50%), and split into two linear optical waveguides 564 and 565 (two output ports). At this time, by applying a voltage between the upper electrode 571 and the first and second ground electrodes 572 and 573, the refractive index between the electrodes changes.
[0153]
FIG. 16 shows the characteristics of the 1 × 2 optical attenuator of the sixth embodiment. In this figure, the horizontal axis indicates the coupling length L between the two optical waveguides, and the vertical axis indicates the light intensity of the laser light output from the emission port 555. When the coupling length L changes from “0” to “2Lc” which is twice the complete coupling length “Lc”, the interference effect increases or decreases accordingly, and as a result, the light of the laser beam output from the emission port 555 The intensity changes. In the 1 × 2 optical attenuator 551 of this embodiment, this change is performed by changing the refractive index of the optical waveguide layer.
[0154]
FIG. 17 shows the result of measuring the change in the refractive index with respect to the applied voltage of the oxide paraelectric material constituting the optical waveguide layer in the same structure as the 1 × 2 optical attenuator of the sixth embodiment. When the applied voltage is gradually increased from 0 V so that the upper electrode 571 shown in FIG. 14 becomes a positive voltage, the refractive index acts in a decreasing direction, and exhibits a parabolic quadratic behavior. The length k of the ground electrode in the light propagation direction is an even multiple of the complete coupling length Lc. When the applied voltage corresponds to the complete coupling length Lc, the maximum light decay rate is obtained, and the light intensity can be attenuated to 60 dB or less. Conversely, even when the applied voltage is gradually reduced from 0 V so that the upper electrode 571 becomes a negative voltage, the refractive index acts in the direction of decreasing similarly to the case where the positive voltage is applied.
[0155]
In the case of the 1 × 2 optical attenuator 551 of the seventh embodiment, the optical attenuation frequency was 100 MHz or more and the insertion loss was 5 dB or less, and good characteristics were obtained.
[0156]
<Seventh embodiment>
[0157]
FIG. 18 shows a directional coupling type optical attenuator according to a seventh embodiment of the present invention. The directional coupling type optical attenuator 601 has an incident end face 602 and an exit end face 603 arranged on the opposite side. One entrance port 604 is arranged on the entrance end face 602, and one exit port 605 is also arranged on the exit end face 603. One linear optical waveguide 606 is formed between the input port 604 and the output port 605. In addition, a first optical waveguide 607 is formed on the upper side in the figure in which the incident end face 602 side is curved in an S shape and the output end face 603 side is linear, and further on the lower side in FIG. A second optical waveguide 608 whose side is curved in an S shape and whose exit end face 603 side is linear is formed. Furthermore, a first directional coupler 612 on which three first control electrodes 611 are arranged so as to sandwich the linear optical waveguide 606 and the first optical waveguide 607 is provided on the incident end face 602 above these. It is configured on the near side. Next to this, three second control electrodes 613 are arranged so as to sandwich the linear optical waveguide 606 and the second optical waveguide 608, and a second directional coupler 614 is configured.
[0158]
FIG. 19 shows an end face structure of the directional coupling type optical attenuator cut in the FF direction in FIG. This directional coupling type optical attenuator 601 has a KTN unidirectionally oriented cladding layer 622 formed on an MgO substrate 621, and a linear optical waveguide 606 composed of a KTN unidirectionally oriented optical waveguide and a first optical waveguide 606. An optical waveguide 607 is formed. Three first control electrodes 611 are formed on the KTN uni-oriented cladding layer 622 via the buffer layer 624, and constitute a first directional coupler 612 (FIG. 18).
[0159]
Such a directional coupling type optical attenuator 601 is manufactured as follows. First, a KTN single-oriented cladding layer 622 having a refractive index of 2.16 is formed to a thickness of 2500 nm on the MgO substrate 621 by a sputtering method. Next, titanium (Ti) is thermally diffused through a part of the KTN single-orientation cladding layer to form a KTN single-orientation optical waveguide 623 having a refractive index of 2.19. For the thermal diffusion, a Ti film having a film thickness of about 500 nm is formed and then heated to a temperature of about 1000 ° C. so that Ti atoms are thermally diffused into the KTN unidirectional cladding layer 622 as shown in this figure. Then, the refractive index in that region increases, forming a core. An optical waveguide can also be formed by performing proton exchange instead of Ti.
[0160]
If it is desired to particularly reduce the waveguide propagation loss, another buffer layer may be formed. That is, a single-layer KTN buffer layer having a refractive index of 2.16 is formed to a thickness of 1100 nm by a sputtering method. Without the buffer layer, absorption loss occurs in the metal electrode in a part of the mode field propagating in the optical waveguide. By forming the buffer layer, the mode field can be formed with a low-loss medium.
[0161]
Next, the formation of the electrodes will be described. As the electrode, an Au electrode is formed as a first control electrode 611 with a thickness of 200 nm on the KTN unidirectional buffer layer 624 by an electron beam evaporation method. In consideration of film adhesion, an Au electrode may be formed after evaporating a metal such as Cr (chromium) or Ti (titanium) to a thickness of about 1 to 50 nm. The length of the electrode is twice the complete coupling length Lc, but is not limited to this, and may be any even number. By forming an electrode having the shape shown in FIG. 18, a directional-coupling optical attenuator including two single-mode stages can be manufactured.
[0162]
When a voltage is not applied to the first and second control electrodes 611 and 613 in a state where signal light having a wavelength of 1.5 μm is incident on the incident port 604 of the directional coupling type optical attenuator 601, almost 100 % Of the signal light is emitted from the emission port 605. When it is desired to attenuate the amount of signal light incident from the incident port 604, DC (direct current) is applied to the first and second control electrodes 611 and 613 disposed in the first and second directional couplers 612 and 614. ) A voltage is applied between adjacent waveguides forming the first and second directional couplers 612 and 614 (between the linear optical waveguide 606 and the first optical waveguide 607 or the second optical waveguide 608). ) Is applied with an external electric field. Then, the respective optical waveguides are in a phase mismatch state, and a part of the signal light moves from the linear optical waveguide 606 to the first optical waveguide 607 or the second optical waveguide 608. As a result, attenuation of the signal light emitted from the emission port 605 of the linear optical waveguide 606 is realized.
[0163]
As described above, in the present embodiment, the two directional couplers 612 and 614 are connected in cascade, so that the variable optical attenuation of about 15 to 0 dB is the limit in terms of manufacturing by itself, but in the case of this configuration, A variable optical attenuator having a large dynamic range of 40 to 60 dB can be realized. The number of directional couplers may be three or more.
[0164]
<Eighth embodiment>
[0165]
FIG. 20 shows a Mach-Zehnder type optical modulator according to an eighth embodiment of the present invention. This Mach-Zehnder type optical modulator 651 has an incident end face 652 and an exit end face 653 arranged on the opposite side. One entrance port 654 is arranged on the entrance end face 652, and an exit port 655 is arranged on the exit end face 653. Between them, a linear optical waveguide 661 from the incident end face 652 side, a pair of S-shaped optical waveguides 662 and 663 each having one end connected (Y-branched) to one end of the linear optical waveguide 661, Linear optical waveguides 664, 665 connected to the other ends of the U-shaped optical waveguides 662, 663 and one end thereof, respectively, and a pair of S-shaped one ends connected to the other ends of the linear optical waveguides 664, 665, respectively. Type optical waveguides 666 and 667, and a linear optical waveguide 668 having one end connected to the junction of the S-shaped optical waveguides 666 and 667 and the other end connected to the emission port 655. On each of these optical waveguides 661 to 668, a pair of S is formed along a line symmetrical position of the pair of linear optical waveguides 664 and 665 from the branch point of the linear optical waveguide 661 and the S-shaped optical waveguides 662 and 663. A first band-shaped electrode portion provided up to the junction of the U-shaped optical waveguides 666 and 667, a second band-shaped electrode portion bent in an L-shape at right angles from the junction, and the Y-branch portion described above. An upper electrode 671 composed of a wide electrode portion bent at a right angle in a direction opposite to the second strip-shaped electrode portion, and a linear optical waveguide 664, 665, which is disposed adjacent to and adjacent to each of the linear optical waveguides 664, 665, one by one. First and second ground electrodes 672 and 673 are arranged. In the middle of the linear optical waveguides 661 and 668, grooves are cut on the side close to the input port 654 and the side close to the output port 655 so as to cross them, and polarizers 674 and 675 are formed in these groove portions. Are attached. In this Mach-Zehnder type optical modulator 651, the polarizers 674 and 675 can be omitted depending on the application, and the cost as the optical modulator can be reduced.
[0166]
FIG. 21 shows an end face structure of the Mach-Zehnder type optical modulator cut in a direction GG in FIG. In the Mach-Zehnder type optical modulator 651, a KTN amorphous cladding layer 682 is formed on a glass substrate 681, and linear optical waveguides 664 and 665 made of a KTN amorphous optical waveguide are formed in a part thereof. On the KTN amorphous cladding layer 682, an upper electrode 671 and first and second ground electrodes 672 and 673 are formed via a buffer layer 684.
[0167]
The Mach-Zehnder type optical modulator 651 is manufactured as follows. First, a KTN amorphous cladding layer 682 having a refractive index of 2.16 is formed to a thickness of 2500 nm on a glass substrate 681 by a sputtering method. Next, a part of the KTN amorphous cladding layer 682 is thermally diffused with titanium (Ti) to form a KTN amorphous optical waveguide having a refractive index of 2.19. For thermal diffusion, a Ti film having a thickness of about 500 nm is deposited and then heated to a temperature of about 1000 ° C. As a result, Ti atoms are thermally diffused into the KTN amorphous cladding layer 682, the refractive index of the region is increased, and a core is formed as shown in FIG. An optical waveguide can also be formed by performing proton exchange instead of Ti.
[0168]
If it is desired to particularly reduce the waveguide propagation loss, another buffer layer may be formed. That is, a silicon dioxide (SiO 2) buffer layer having a refractive index of 1.46 is formed to a thickness of 1100 nm by a sputtering method. Without the buffer layer, absorption loss occurs in the metal electrode in a part of the mode field propagating in the optical waveguide. By forming the buffer layer, the mode field can be formed with a low-loss medium.
[0169]
Next, the formation of the electrodes will be described. The electrode is SiO ₂ An Au electrode is formed with a thickness of 200 nm on the amorphous buffer layer as an upper electrode 671 and first and second ground electrodes 672 and 673 by an electron beam evaporation method. In consideration of film adhesion, it is preferable to form an Au electrode after evaporating a metal such as Cr (chromium) or Ti (titanium) to a thickness of about 1 to 50 nm. As shown in FIG. 20, the upper electrode 671 has a length between two Y branch points constituting the optical waveguide, and is formed in parallel with the optical waveguide. By forming electrodes having the shape shown in FIG. 21, a Mach-Zehnder type optical modulator can be manufactured.
[0170]
The upper electrode 671 was connected between the upper electrode 671 and the first and second ground electrodes 672 and 673 with the signal light having a wavelength of 1.5 μm being incident on the incident port 654 of the optical modulator 651. A DC voltage is applied from a signal source 678 (FIG. 20). Then, only the phase of the light wave propagating through one of the pair of linear optical waveguides 664 and 665 is shifted by the voltage between the upper electrode 671 and the first and second ground electrodes 672 and 673. When light waves propagating through the two linear optical waveguides 664 and 665 are multiplexed at the Y-coupling point (merging point) and their phases are shifted by half a wavelength, they cancel each other out, and a signal is output from the output port 655. Not done. Conversely, if the phases are integer multiples of the wavelength, a signal is output from the output port 655 at the original incident signal light amount.
[0171]
Thus, the optical modulator 651 according to the eighth embodiment of the present invention modulates the intensity of light. FIG. 20 illustrates a Mach-Zehnder type optical waveguide as a preferable example of such a branch type. In the eighth embodiment, polarizers 674 and 675 are provided on both the entrance side and the exit side of the optical modulator 651. However, as described above, these polarizers 674 and 675 are not always required. Therefore, one or both of the polarizers 674 and 675 can be omitted. Even when the polarizers 674 and 675 are not provided at all, the function as the optical modulator 651 can be sufficiently achieved.
[0172]
In this embodiment, when a driving voltage was examined by applying a voltage from the signal source 678, the half-wave voltage Vπ was 2.8V. Conventional LiNbO ₃ Although the half-wavelength voltage is about 5.1 V in the waveguide type optical modulator using the above, an optical modulator with remarkably low voltage and low power consumption can be obtained. When the frequency response characteristics were examined, the frequency band was 18 GHz, which was ₃ The characteristics were as good as those of the optical modulator. Conventionally, LiNbO ₃ Although the optical modulator was configured using the single crystal substrate of the above, according to the film forming method on the glass substrate 681 as in the present embodiment, a low voltage and inexpensive optical modulator was realized as compared with the conventional one. can do.
[0173]
<Ninth embodiment>
[0174]
FIG. 22 shows a composite optical device combining an optical attenuator and an optical switch according to the ninth embodiment of the present invention. In the composite optical device 701, the left half of the figure provided with the input port 702 constitutes the optical attenuator 703, and the right half provided with the first and second exit ports 704, 705 constitutes the optical switch 706. I have. The optical attenuator 703 has the same configuration as the single mode Mach-Zehnder interference type 1 × 2 optical attenuator 551 shown in FIG. 14 in the sixth embodiment of the present invention. Therefore, each part of the optical attenuator 703 is denoted by the same reference numeral. The optical switch 706 has the same configuration as the directional coupler type 2 × 2 optical switch 451 as the optical device in the fourth embodiment of the present invention shown in FIG. Therefore, each part of the optical switch 706 is given the same reference numeral. The lower electrode layer 462 shown in FIG. 11 is exposed above the optical switch 706 for grounding.
[0175]
By the way, the KTN thin film can form a ferroelectric or paraelectric near room temperature by controlling the composition ratio of Ta atoms and Nb atoms. In the composite optical device 701 of the ninth embodiment, the optical attenuator 703 takes the same manufacturing steps as the sixth embodiment, and the optical switch 706 takes the same manufacturing steps as the fourth embodiment. By setting the ratio X to 0.5 or less, a ferroelectric substance can be obtained around room temperature, and a memory effect can be exhibited. Here, the memory effect is an effect in which even if the applied voltage is reduced to 0 volt from the state where the applied voltage is applied, there is a hysteresis in the change in the refractive index and the switch function is maintained. This effect is manifested only in ferroelectrics. In the case of an optical switch, in particular, there is a case where it is desired to maintain the switching state even when the applied voltage is reduced due to the memory effect. In this case, this effect is useful.
[0176]
Next, a specific method for manufacturing the composite optical device 701 will be described. The outline will be described first. Basically, a KTN paraelectric material forming the optical attenuator 703 and a KTN ferroelectric material forming the optical switch 706 are sequentially formed on a substrate using a mask (not shown). Finally, upper electrodes of the optical attenuator 703 and the optical switch 706 are formed.
[0177]
Specifically, first, a KTN paraelectric material constituting the optical attenuator 703 is formed by the following procedure. This will be described with reference to FIG. SrTiO ₃ A KTN epitaxial buffer layer having a refractive index of 2.19 and serving as a buffer layer 582 is formed to a thickness of 1000 nm on a (100) single crystal substrate by a sputtering method. Next, a KTN epitaxial optical waveguide layer having a refractive index of 2.23 is formed with a thickness of 1500 nm. Next, the KTN epitaxial optical waveguide layer is removed by ion etching while leaving a part thereof, and a Mach-Zehnder interference type optical waveguide layer similar to that shown in FIG. 15 is formed by a convex portion having a height of 300 nm and a width of 9.0 μm. Processed as Further, a KTN epitaxial cladding layer having a composition with a refractive index of 2.19 is formed thereon to a thickness of 1500 nm, and is laminated in this order.
[0178]
SrTiO ₃ Although the lattice constant of the (100) single crystal substrate 581 is 3.905 °, the lattice constant of the KTN single crystal film is 3.90 °, and the lattice mismatch is extremely small at 0.2% or less, and good epitaxial growth is possible. In the optical waveguide layer 583, linear optical waveguides 564 and 565 are formed.
[0179]
Subsequently, using another mask (not shown), the optical attenuator 703 (551) made of KTN paraelectric material is shielded, and an optical switch 706 (451) made of KTN ferroelectric material is formed in the following procedure. I do. This will be described with reference to FIG.
[0180]
SrTiO ₃ An Ag thin film having a thickness of 100 nm is formed as a lower electrode on a (100) single crystal substrate 461, and a refractive index at a wavelength of 1.5 μm is 2.21 and a composition ratio is X = 0.1. An epitaxial buffer layer made of KTN having a thickness of 900 nm and an optical waveguide layer made of an epitaxial film made of KTN or an alignment film having a composition ratio X of 0.22 and a refractive index of 2.25 at the same wavelength and having a thickness of 1500 nm are formed. Is done. Next, the KTN epitaxial optical waveguide layer is removed by ion etching while leaving a part thereof, and the optical waveguide layer 464 is processed and formed as a projection having a height of 300 nm and a width of 9.0 μm. Further, a KTN epitaxial cladding layer 465 having a composition with a refractive index of 2.21 is formed thereon to a thickness of 1500 nm, and the structure is formed in this order. The KTN epitaxial film having a composition ratio X of “0” to “0.5” has a lattice constant of 3.95 ° to 4.04 °, but the lower electrode Ag epitaxial thin film has a lattice constant of 4.086 ° and a lattice mismatch of 3 Very small and excellent epitaxial growth of 0.4% or less is possible. Here, a pair of optical waveguides 452 and 453 are also formed in the optical waveguide layer 464.
[0181]
Subsequently, first and second upper electrodes 454 and 455 are formed in the following procedure. First, an Al thin film having a thickness of 200 nm is formed on a KTN epitaxial buffer layer by a sputtering method using a mask for forming an upper electrode (not shown), and then the first and second Al films as shown in FIG. Upper electrodes 454 and 455 are formed. As described above, the optical attenuator 703 (551) made of KTN paraelectric and the optical switch 706 (451) made of KTN ferroelectric are formed on one substrate. At the boundary between the optical attenuator 703 (551) and the optical switch 706 (451), the refractive index of the optical waveguide at the interface is 2.23 and 2.24, respectively, but the Fresnel reflection at the interface is 2 × 10 ^-3 %, Which is negligible when disturbing the optical signal propagation due to the light reflection effect.
[0182]
In the embodiment and the modification described above, KTN, that is, KTaYNb _1-Y O ₃ The description has been made by taking the oxide paraelectric substance having the secondary electro-optic effect represented by the following example as an example, but the present invention is not limited to this. That is, as shown in the following Table 1, the general formula ABO ₃ The oxide thin film having a perovskite structure represented by the formula has a lattice constant between 3.9 ° and 4.04 °, and is a transparent and inexpensive substrate such as MgO or SrTiO from the visible region to the infrared region. ₃ There is an advantage that the lattice constant is close to the lattice constant of the single crystal substrate and lattice matching is easy. In addition, since the lattice constant of a metal film such as Pt, Ag, Al, or Au is close to the lattice constant, conditions suitable for various epitaxial or mono-oriented film oxide paraelectrics are prepared. The present invention can be similarly applied to these materials.
[0183]
[Table 1]

[0184]
In general, a material having a lattice constant close to KTN is SrTiO. ₃ , MgO or Pt is used.
[0185]
In the examples and the modified examples, the non-linear optical device was manufactured by using the electron beam evaporation method, the ion assist evaporation method, the RF magnetron sputtering method or the ion beam sputtering method. In addition, the DC magnetron sputtering method and the ion plating method were used. It can be naturally manufactured using various film forming methods such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and dip coating.
[0186]
【The invention's effect】
As described above, according to the first aspect of the present invention, an oxide paraelectric material having a secondary electro-optic effect is used, and a voltage is applied to a predetermined portion of the optical waveguide to reduce the refractive index of the optical waveguide. I decided to change it. Since the paraelectric substance has an extremely small relative dielectric constant as compared with the oxide ferroelectric substance, it can meet the demand for high-speed response.
[0187]
According to a second aspect of the present invention, in the nonlinear optical device according to the first aspect, the oxide paraelectric substance has a general formula of KTa. _Y Nb _1-Y O ₃ (0.5 ≦ Y ≦ 1.0), the oxide ferroelectric or oxide paraelectric in a temperature range of −40 ° C. to + 85 ° C. including room temperature applied to optical parts and the like. The body can be selectively formed. In KTN, by changing the composition ratio between Ta atoms and Nb atoms between 0 and 1.0, for example, the refractive index is selected over a wide range of about 2.0 to 2.3 at a wavelength of 0.633 μm. be able to. Therefore, it can be determined within the composition ratio range that can maintain the state of the paraelectric substance in consideration of the use temperature. In addition, when the composition is changed, the change in the lattice constant of the epitaxial film or the alignment film is sufficiently small, and the mutual crystal matching is good.
[0188]
Further, as described in claims 4 to 7, the optical waveguide structure and the buffer layer may be an epitaxial film, an alignment film, a polycrystalline film, or an amorphous film regardless of the crystal structure. Instead, it is also possible to form each component of the device on an inexpensive glass substrate, an organic polymer substrate, or the like. In particular, if a film can be formed on an organic polymer substrate, the nonlinear optical device can be flexibly bent or deformed after the formation, so that an inexpensive nonlinear optical device having a higher density and a wider application range can be provided.
[0189]
According to the invention described in claim 18 or claim 19, control as a digital type optical switch for controlling switching of an optical path is performed by applying a voltage to a branch portion of an optical waveguide by a pair of electrodes. After the optical path is switched, even if a higher voltage is applied, the state is maintained and multiple operating points are not generated, the operating voltage tolerance is excellent, and polarization independence is possible. It has features such as small wavelength dependence.
[0190]
Further, according to the invention as set forth in claim 24 or claim 25, a plurality of nonlinear optical devices are arranged on the same substrate, and a plurality of outgoing optical paths are shifted from a desired incoming optical path to a plurality of outgoing optical paths from the plurality of incoming optical paths. Since the optical path is switched, switching between ports at appropriate intervals can be driven at a low voltage, and the electrode length and bent optical waveguide length can be shortened as necessary. Larger nonlinear optical devices can be formed on a wafer.
[Brief description of the drawings]
FIG. 1 is a plan view of a Y-branch type 1 × 2 optical switch according to a first embodiment of the present invention.
FIG. 2 is a sectional view taken along the line AA of the 1 × 2 optical switch shown in FIG. 1;
FIG. 3 is a characteristic diagram in which the amount of change in the refractive index with respect to an applied voltage of an oxide paraelectric substance is measured in a laminated structure similar to the 1 × 2 optical switch of the present example.
FIG. 4 is a plan view of a Y-branch 1 × 8 optical switch as an optical device according to a second embodiment of the present invention.
FIG. 5 is an enlarged plan view illustrating a first optical switch unit according to a second embodiment.
FIG. 6 is a cross-sectional view of a main part of a first optical switch unit as a modification of the second embodiment.
FIG. 7 is a plan view showing the outline of the structure of an 8 × 8 optical switch according to a third embodiment of the present invention.
FIG. 8 is an enlarged plan view showing an X intersection of the 8 × 8 optical switch according to the third embodiment.
FIG. 9 is a cross-sectional view of a main part of the 8 × 8 optical switch according to the third embodiment, taken along the X-B direction shown in FIG.
FIG. 10 is a plan view of a directional coupler type 2 × 2 optical switch as an optical device according to a fourth embodiment of the present invention.
11 is a cross-sectional view of the 2 × 2 optical switch of FIG. 10 taken along the line CC.
FIG. 12 is a plan view showing the outline of the structure of a 2 × 2 optical switch according to a fifth embodiment of the present invention.
13 is a cross-sectional view of the 2 × 2 optical switch of FIG. 12 in the DD direction.
FIG. 14 is a plan view of a Mach-Zehnder interference type 1 × 2 optical attenuator according to a sixth embodiment of the present invention.
FIG. 15 is an end view of the 1 × 2 optical attenuator of the sixth embodiment cut in the EE direction.
FIG. 16 is a characteristic diagram showing a relationship between the coupling length and the light intensity of the 1 × 2 optical attenuator of the sixth embodiment.
FIG. 17 is a characteristic diagram showing a change in a refractive index with respect to an applied voltage of an oxide paraelectric material constituting an optical waveguide layer.
FIG. 18 is a plan view of a directional coupling type optical attenuator according to a seventh embodiment of the present invention.
FIG. 19 is an end view of the directional coupling-type optical attenuator of the seventh embodiment cut along the FF direction.
FIG. 20 is a plan view of a Mach-Zehnder type optical modulator according to an eighth embodiment of the present invention.
FIG. 21 is an end view of the Mach-Zehnder type optical modulator according to the eighth embodiment, which is cut in the GG direction.
FIG. 22 is a plan view of a composite optical device combining an optical attenuator and an optical switch according to a ninth embodiment of the present invention.
FIG. 23 is an explanatory view of a main part showing an ideal shape of a Y-branch optical waveguide of the digital optical switch.
FIG. 24 is an explanatory view of a main part when the Y-branch optical waveguide of the digital optical switch is manufactured in an actual shape.
[Explanation of symbols]
201 1 × 2 optical switch
208, 208A, 436, 437, 561, 564, 565, 568, 606, 661, 668, 664, 665 Linear optical waveguide
209, 209A, 210, 210A, 562, 563, 566, 567, 662, 663, 666, 667 S-shaped optical waveguide
212, 212A, 212B, 213, 213A, 213B, 439, 454, 455, 508, 509, 571, 671 Upper electrode
223, 223B, 444, 464, 514 Optical waveguide layer
301 1 × 8 optical switch
321-327 Optical switch section
401 8 × 8 optical switch
452, 453, 506, 507, 607, 608 Optical waveguide
451,706 Optical switch
551, 703 Optical attenuator
572, 573, 672, 673 Ground electrode
601 Directional coupling type optical attenuator
611, 613 Control electrode
612,614 Directional coupler
678 signal source
701 Composite optical device

Claims (27)

An optical waveguide layer formed of an oxide paraelectric having a secondary electro-optic effect, and formed on a substrate, an optical waveguide for guiding light input from a predetermined incident port and guiding the light to an output port;
A pair of electrodes for applying a voltage to a predetermined portion of the optical waveguide,
A non-linear optical device, comprising: a refractive index controller that changes a refractive index of the optical waveguide by applying a voltage to the pair of electrodes. 2. The nonlinear optical device according to claim 1, wherein the oxide paraelectric substance is a substance represented by a general formula KTa _Y Nb _1- YO ₃ (0.5 ≦ Y ≦ 1.0). 3. 2. The nonlinear optical device according to claim 1, further comprising a substrate having conductivity, wherein one of the pair of electrodes is also used by the substrate. 2. The nonlinear optical device according to claim 1, further comprising: the substrate and a conductive film formed on a surface portion thereof, wherein the conductive film forms one of the pair of electrodes. The nonlinear optical device according to claim 1, wherein the substrate is a single crystal substrate. The nonlinear optical device according to claim 1, wherein the substrate is a glass substrate. 2. The nonlinear optical device according to claim 1, wherein the substrate is an organic polymer substrate. 2. The nonlinear optical device according to claim 1, wherein the oxide paraelectric exhibits a solid solution phase and is formed of a single crystal film. 2. The nonlinear optical device according to claim 1, wherein the oxide paraelectric exhibits a solid solution phase and is composed of an alignment film. 2. The nonlinear optical device according to claim 1, wherein the oxide paraelectric exhibits a solid solution phase and is formed of a polycrystalline film. 2. The nonlinear optical device according to claim 1, wherein the oxide paraelectric exhibits a solid solution phase and is formed of an amorphous film. 2. The nonlinear optical device according to claim 1, wherein the substrate is an organic polymer substrate, and the optical waveguide layer and the buffer layer are formed thereon. 2. The nonlinear optical device according to claim 1, wherein the optical waveguide layer has a ridge structure provided in a layer adjacent to the lower side in a convex shape. 2. The nonlinear optical device according to claim 1, wherein the optical waveguide layer has a ridge structure provided on an upper adjacent layer in a convex shape. The nonlinear structure according to claim 1, further comprising a cladding layer made of an oxide and having a smaller refractive index than the optical waveguide layer, between the optical waveguide layer and an upper electrode constituting one of the pair of electrodes. Optical device. 2. The nonlinear optical device according to claim 1, wherein one of said electrodes is made of an oxide and has a cladding layer having a smaller refractive index than said optical waveguide layer. 2. The nonlinear optical device according to claim 1, wherein the optical path of the optical waveguide is switched by applying a voltage to a branch portion that branches the incident optical path and the output optical path. The optical waveguide is a Y-branch optical waveguide, and is controlled as a digital optical switch that controls switching of an optical path by applying a voltage to the Y-branch of the Y-branch optical waveguide using the pair of electrodes. The nonlinear optical device according to claim 1, wherein The optical waveguide is an X-branch optical waveguide, and a control as a digital optical switch that controls switching of an optical path is performed by applying a voltage to a branch portion of the X-branch optical waveguide by the pair of electrodes. The nonlinear optical device according to claim 1, wherein: The optical waveguide is an X-branch optical waveguide, and the optical switch as an optical switch that controls switching of an optical path by total reflection or transmission of light by applying a voltage to a branch portion of the X-branch optical waveguide by the pair of electrodes. 2. The nonlinear optical device according to claim 1, wherein control is performed. The optical waveguide is an X-branch optical waveguide, and a voltage is applied to a branch portion of the X-branch optical waveguide by the pair of electrodes to control a switching of an optical path as a Bragg reflection type optical switch. The nonlinear optical device according to claim 1, wherein: The optical waveguide is an X-branch optical waveguide, and a directional coupler type optical switch that controls switching of an optical path between the optical waveguides by applying a voltage to the branch portion of the X-branch optical waveguide by the pair of electrodes. The nonlinear optical device according to claim 1, wherein the control is performed as follows. The optical waveguide is a Mach-Zehnder interference type optical waveguide, and control is performed as a Mach-Zehnder interference type optical switch that controls switching of an optical path by applying a voltage to a branch portion of the optical waveguide by the pair of electrodes. The nonlinear optical device according to claim 1. 24. The nonlinear optical device according to claim 18, wherein a plurality of the switches are arranged in a matrix on the same substrate. 24. The nonlinear optical device according to claim 18, wherein a plurality of switches are arranged in a tree on the same substrate. The optical waveguide is a directional coupling type optical waveguide, and control is performed as a directional coupler type optical attenuator that controls optical interference by applying a voltage to a branch portion of the optical waveguide by the pair of electrodes. The nonlinear optical device according to claim 1, wherein: The optical waveguide is a Mach-Zehnder interference type optical waveguide, and control is performed as a Mach-Zehnder interference type optical modulator that controls optical interference by applying a voltage to a branch portion of the optical waveguide by the pair of electrodes. The nonlinear optical device according to claim 1.

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