WO2024075277A1 - Optical waveguide element, optical modulator using same, and optical transmission device - Google Patents

Optical waveguide element, optical modulator using same, and optical transmission device Download PDF

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WO2024075277A1
WO2024075277A1 PCT/JP2022/037614 JP2022037614W WO2024075277A1 WO 2024075277 A1 WO2024075277 A1 WO 2024075277A1 JP 2022037614 W JP2022037614 W JP 2022037614W WO 2024075277 A1 WO2024075277 A1 WO 2024075277A1
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optical waveguide
electrode layer
substrate
optical
electrode
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PCT/JP2022/037614
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French (fr)
Japanese (ja)
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淳司 新井
真悟 高野
宏佑 岡橋
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住友大阪セメント株式会社
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Priority to PCT/JP2022/037614 priority Critical patent/WO2024075277A1/en
Publication of WO2024075277A1 publication Critical patent/WO2024075277A1/en

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  • the present invention relates to an optical waveguide element and an optical modulator and optical transmitter using the same, and in particular to an optical waveguide element that includes a substrate on which an optical waveguide is formed and an electrode disposed on the substrate in close proximity to the optical waveguide, the electrode including an electrode layer and a base layer between the electrode layer and the substrate, and an optical modulator and optical transmitter using the same.
  • optical waveguide elements such as optical modulators that use substrates on which optical waveguides are formed are widely used.
  • an optical waveguide is formed on a substrate that has an electro-optic effect, such as lithium niobate (LN), and electrodes that apply an electric field to the optical waveguide are formed on the substrate.
  • LN lithium niobate
  • HB-CDMs high bandwidth coherent driver modulators
  • the adhesion between the LN substrate and Au is low, so a film of Au and a metal such as Ti or Nb is formed together as a base layer for the electrode, and then an Au electrode layer is formed on top of that.
  • the metal of the underlayer has a higher optical absorption rate than the Au electrode layer, and when the underlayer is placed near the optical waveguide, the light waves propagating through the optical waveguide are absorbed by the underlayer, resulting in a problem of increased optical loss.
  • the effect of optical absorption by the underlayer of the electrodes becomes more pronounced than ever before.
  • Patent Document 1 proposes that the electrode 2 be composed of a base layer m1 and an electrode layer M1, as shown in Figure 1, and that the base layer m1 be disposed farther away from the optical waveguide 10 than the electrode layer M1.
  • the optical waveguide 10 is a rib-type optical waveguide formed on the substrate 1.
  • an air layer is interposed between the electrode layer M1 and the optical waveguide 10, causing a problem that the electric field of the electrode layer M1 cannot be efficiently applied to the optical waveguide 10.
  • a buffer layer such as SiO2 is formed on the surface of the substrate 1, and an underlayer m1 and an electrode layer M1 constituting the electrode 2 are disposed on the buffer layer.
  • the buffer layer can suppress light absorption by the underlayer m1, but the buffer layer separates the optical waveguide from the electrode by the thickness of the buffer layer, and an electric field is applied through an air layer and the buffer layer, which reduces the electric field efficiency.
  • the problem that the present invention aims to solve is to provide an optical waveguide element that solves the problems described above and prevents a decrease in the efficiency of the electric field that the electrodes apply to the optical waveguide while suppressing the absorption of light waves propagating through the optical waveguide by the electrodes. Furthermore, the present invention aims to provide an optical modulator and an optical transmitter that use this optical waveguide element.
  • An optical waveguide element comprising a substrate on which an optical waveguide is formed, an electrode disposed on the substrate adjacent to the optical waveguide, the electrode comprising an electrode layer and an underlayer disposed between the electrode layer and the substrate, characterized in that the electrode layer is disposed between the optical waveguide and the underlayer so as to protrude beyond the underlayer, and the electrode layer is in direct contact with the substrate.
  • the optical waveguide element described in (1) above is characterized in that the light absorption rate of the metal in the base layer is higher than that of the metal in the electrode layer.
  • the optical waveguide element described in (1) above is characterized in that, when the electrode layer and the base layer are viewed in plan, the overlapping area between them is 50% or more of the area of the electrode layer.
  • the amount by which the electrode layer protrudes from the base layer toward the optical waveguide is equal to or greater than the mode diameter of the light wave propagating through the optical waveguide.
  • the optical waveguide element described in (1) above is characterized in that, when the base layer is viewed in plan, a plurality of projections and recesses are formed on the side of the base layer that is close to the optical waveguide.
  • the thickness of the underlayer is 200 nm or less.
  • the optical waveguide element described in (1) above is characterized in that a dielectric layer is provided that continuously covers the optical waveguide, between the optical waveguide and the electrode layer, and further covers at least a portion of the electrode layer.
  • An optical waveguide element comprising a substrate on which an optical waveguide is formed, an electrode disposed on the substrate adjacent to the optical waveguide, the electrode being a first electrode layer and a first underlayer disposed between the first electrode layer and the substrate, the first electrode layer being disposed between the optical waveguide and the first underlayer so as to extend beyond the first underlayer, and the first electrode layer being in direct contact with the substrate, a second underlayer being disposed on the substrate on the opposite side of the first underlayer to the optical waveguide, and a second electrode layer extending from the second underlayer to cover at least a portion of the first electrode layer.
  • An optical modulator comprising an optical waveguide element according to any one of (1) to (8) above, a housing for accommodating the optical waveguide element, and an optical fiber for inputting or outputting light waves to the optical waveguide.
  • the electrode is a modulation electrode for modulating the light wave propagating through the optical waveguide
  • the housing includes an electronic circuit for amplifying the modulation signal input to the modulation electrode.
  • An optical transmitter comprising the optical modulator described in (10) above, a light source for inputting a light wave to the optical modulator, and an electronic circuit for outputting a modulated signal to the optical modulator.
  • the present invention provides an optical waveguide element comprising a substrate on which an optical waveguide is formed, and an electrode arranged on the substrate adjacent to the optical waveguide, the electrode comprising an electrode layer and an underlayer arranged between the electrode layer and the substrate, the electrode layer being arranged between the optical waveguide and the underlayer so as to extend beyond the underlayer and being in direct contact with the substrate, so that by placing the underlayer containing a material with high light absorption efficiency away from the optical waveguide, it is possible to provide an optical waveguide element in which the absorption of light waves propagating through the optical waveguide by the electrode can be suppressed, and by arranging an electrode layer made of a material with low light absorption efficiency close to the optical waveguide, a decrease in the efficiency of the electric field applied by the electrode to the optical waveguide can be prevented. Furthermore, by using an optical waveguide element having such excellent characteristics, it is possible to provide an optical modulator or an optical transmitter that achieves the same effects.
  • FIG. 1 is a cross-sectional view showing an example of a conventional optical waveguide element.
  • FIG. 11 is a cross-sectional view showing another example of a conventional optical waveguide element.
  • 1 is a cross-sectional view showing a first embodiment of an optical waveguide element according to the present invention.
  • 4 is a cross-sectional view of the optical waveguide element taken along the arrow X-X' in FIG. 3 (as viewed from above in FIG. 3).
  • FIG. 5 is a cross-sectional view showing a second embodiment of the optical waveguide element of the present invention, corresponding to FIG.
  • FIG. 2 is a cross-sectional view showing an application example of the optical waveguide element of the present invention.
  • FIG. 1 is a cross-sectional view showing an example of a conventional optical waveguide element.
  • FIG. 11 is a cross-sectional view showing another example of a conventional optical waveguide element.
  • 1 is a cross-sectional view showing a first embodiment of an optical waveguide element according
  • FIG. 4 is a cross-sectional view showing a third embodiment of the optical waveguide element of the present invention.
  • 8 is a cross-sectional view showing an application example of the optical waveguide element shown in FIG. 7.
  • 9 is a cross-sectional view showing a further application example of the optical waveguide element shown in FIG. 8 .
  • 10 is a cross-sectional view showing an application example of the optical waveguide element shown in FIG. 9 .
  • FIG. 11 is a cross-sectional view showing a fourth embodiment of the optical waveguide element of the present invention.
  • 1 is a diagram illustrating an example of an optical transmitting device according to the present invention.
  • FIG. 3 is a cross-sectional view showing an example of the optical waveguide element of the present invention.
  • the optical waveguide element of the present invention comprises a substrate 1 on which an optical waveguide 10 is formed, and an electrode 2 arranged on the substrate in close proximity to the optical waveguide, the electrode comprising an electrode layer M1 and an underlayer m1 arranged between the electrode layer and the substrate, the electrode layer being arranged between the optical waveguide and the underlayer so as to protrude from the underlayer, and the electrode layer being in direct contact with the substrate.
  • the substrate 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect, specifically, a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a base material in which these substrate materials are doped with MgO or the like. It is also possible to form a film from these materials using a vapor phase growth method such as sputtering, deposition, or CVD. It is also possible to use a substrate in which a substrate having an electro-optic effect is bonded to another substrate and then the electro-optic substrate is processed into a thin film. Furthermore, semiconductor substrates and substrates made of organic materials such as EO polymers can also be used.
  • a substrate having an electro-optic effect specifically, a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a
  • the optical waveguide 10 it is possible to use an optical waveguide in which a high refractive index material such as Ti is thermally diffused into the substrate 1, an optical waveguide formed by the proton exchange method, or even a rib-type waveguide in which the portion of the substrate corresponding to the optical waveguide is made convex, such as by etching the substrate 1 other than the optical waveguide or by forming grooves on both sides of the optical waveguide. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to increase the refractive index by diffusing Ti or the like onto the substrate surface by the thermal diffusion method or the proton exchange method.
  • the size of the rib-type waveguide is a fine rib-type optical waveguide with a width and height of about 1 ⁇ m in order to increase the light confinement.
  • the thickness h of the substrate (thin plate) 1 on which the optical waveguide 10 is formed is set to 10 ⁇ m or less, more preferably 5 ⁇ m or less, and even more preferably 1 ⁇ m or less, in order to achieve speed matching between the microwave and light waves of the modulated signal.
  • the height of the rib-type optical waveguide is set to 80% or less of h, and is set to 4 ⁇ m or less, more preferably 3 ⁇ m or less, and even more preferably 0.8 ⁇ m or less or 0.4 ⁇ m or less, depending on the thickness of h.
  • a reinforcing substrate (not shown) is bonded to the underside of the substrate 1.
  • the substrate 1 and the reinforcing substrate are bonded and fixed by direct bonding or via an adhesive layer such as resin.
  • the reinforcing substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but is not limited to this.
  • an intermediate layer such as a metal oxide or metal may be included in the bonding portion.
  • the reinforcing substrate is preferably made of a material having a thermal expansion coefficient close to that of the substrate 1, such as a substrate containing an oxide layer of quartz or glass.
  • the same LN substrate as the substrate 1 a composite substrate in which a silicon oxide layer is formed on a silicon substrate abbreviated as SOI or LNOI, or a composite substrate in which a silicon oxide layer is formed on an LN substrate. If the refractive index of the reinforcing substrate is higher than that of the substrate 1, a layer with a lower refractive index than that of the substrate 1 is provided between the substrate 1 and the reinforcing substrate.
  • the electrode 2 formed on the substrate 1 is composed of an electrode layer M1 and an underlayer m1.
  • the electrode layer M1 is made of a metal such as Au or Cu.
  • the underlayer m1 is used to improve adhesion between the substrate 1 and the electrode 2 (electrode layer M1).
  • the electrode layer M1 is formed to cover the underlayer by electrolytic plating using an underlayer, electroless plating using a resist pattern, gas phase methods such as vapor deposition and sputtering, or a combination of these.
  • the material of the underlayer m1 must have a higher oxygen-metal binding energy, such as T, Nb, Ni, Cr, or Al, than the material used for the electrode layer.
  • metals with a binding energy of 300 kj ⁇ (mol-O) ⁇ 1 or more are preferred because they bind strongly with oxygen in the LN substrate.
  • the underlayer is formed by mixing Au into the above-mentioned metal material.
  • the film can be formed by sputtering, vapor deposition, or the like.
  • the thickness of the underlayer m1 is preferably set to 200 nm or less, and more preferably 20 nm or less.
  • the light absorption loss increases proportionally as the underlayer becomes thicker, so it is preferable to set the thickness of the underlayer thin.
  • the optical waveguide element of the present invention is characterized in that, as shown in FIG. 3, an electrode layer M1 is provided that protrudes from the base layer m1, and the electrode layer M1 is disposed so as to be in direct contact with the substrate. This configuration makes it possible to distance the base layer m1 from the optical waveguide 10 and suppress optical absorption loss.
  • the distance d between the end of the electrode layer M1 closest to the optical waveguide and the end of the base layer m1 closest to the optical waveguide is preferably set to be equal to or greater than the mode diameter of the light waves propagating through the optical waveguide.
  • the spacing between the electrode layers M1 (electrode 2) that sandwich the optical waveguide 10 is set to about 2 to 10 times the mode diameter of the light waves propagating through the optical waveguide.
  • FIG. 4 is a cross-sectional view taken along the line X-X' in FIG. 3, showing the optical waveguide element in plan view. Note that FIG. 4 shows only a portion of the optical waveguide element, and is not a drawing showing the entire optical waveguide element. The portion surrounded by a thick line shows the outer periphery of the electrode layer M1.
  • the relationship between the area A1 of the electrode layer M1 shown in FIG. 4 (A1 is the area within the thick frame including a1) and the area a1 of the base layer m1 in contact with the electrode layer M1 is a1 ⁇ 0.5 ⁇ A1. In other words, more than half (50%) of the bottom surface of the electrode layer M1 is in contact with the base layer m1. This allows the electrode layer to be firmly bonded to the substrate 1 via the base layer, which also helps prevent peeling of the electrode layer M1.
  • FIG. 5 shows an application example of FIG. 4, in which unevenness 20 is formed on the side of the base layer m1 close to the optical waveguide 10.
  • the unevenness is not limited to a regular comb-like configuration, and the size, depth (height), or spacing of the unevenness may be irregular.
  • an uneven shape it is possible to increase the bonding strength between the electrode layer M1 and the base layer m1.
  • an edge with unevenness has a lower optical absorption loss than a straight edge, so it can be said to be a more preferable shape.
  • Figure 6 shows an application example of the optical waveguide element of the present invention.
  • a metal layer M1 is arranged so as to encase the entire base layer m1.
  • the electrode on the side closer to the optical waveguide 10 is configured in the same way as in Figure 3, and on the opposite side away from the optical waveguide 10, the electrode layer M1 is arranged so as to protrude from the base layer m1, just like the side closer to the optical waveguide 10.
  • Figure 6(c) shows an example in which two groove structures are provided in the substrate to create a rib-type optical waveguide 10.
  • the base layer m1 protrudes beyond the electrode layer M1.
  • an optical waveguide in which Ti is thermally diffused is used instead of the rib-type optical waveguide 10.
  • Various modifications such as these can be adopted as necessary.
  • Figures 7 to 9 show two electrode layers (M1, M2) stacked on top of each other. Specifically, after forming the first electrode layer M1, the second electrode layer M2 is formed so as to cover at least a portion of the first electrode layer M1.
  • the thickness of the first electrode layer M1 is, for example, about 0.1 to 2 ⁇ m, and the thickness of the second electrode layer M2 is about 2 to 15 ⁇ m.
  • a second underlayer m2 can be disposed between the second electrode layer M2 and the substrate 1.
  • the first underlayer m1 and the second underlayer m2 can be made of the same material, or the material of the first underlayer m1 can be made of a material with lower light absorption loss than the material of the second underlayer.
  • m2 a material with high light absorption loss, it is possible to efficiently remove unnecessary light present in the substrate, and by making it thicker, it is possible to increase the bonding strength between the electrode layer M2 and the underlayer m2.
  • a third underlayer m3 is also disposed between the first electrode layer M1 and the second electrode layer M2.
  • the third underlayer m3 can be formed in the same process and using the same material as the second underlayer m2. Furthermore, when the third underlayer m3 is close to the optical waveguide 10, it is preferable to configure it using a material that has a lower optical absorption loss than the first underlayer m1 or the second underlayer m2.
  • the second electrode layer M2 is disposed so as to extend beyond the third base layer m3, similar to the relationship between the first base layer m1 and the first electrode layer M1. This configuration makes it possible to suppress light absorption loss due to the third base layer m3.
  • a dielectric layer that continuously covers the optical waveguide 10 and the area between the optical waveguide 10 and the electrode layer M1 (electrode 2), and further covers at least a part of the electrode layer M1.
  • a permanent resist or a metal oxide SiO2 , Al2O3 , etc.
  • Such a dielectric layer has the function of suppressing light scattering due to the roughness of the surface of the rib-type optical waveguide 10 and reducing light propagation loss.
  • this dielectric layer can also contribute to preventing peeling of the electrode layer M1.
  • the base layer m1 can be removed to a distance d by wet etching or the like, and a dielectric layer can be formed on top of it, so that the electrode layer M1 is pressed by its own weight, and the electrode layer M1 and the substrate 1 can be directly contacted.
  • Figure 11 shows an example of applying the structure of a Mach-Zehnder type optical waveguide, where the left and right electrodes are ground electrodes and the center electrode is a signal electrode.
  • the signal electrode has optical waveguides formed on both sides, so it must protrude beyond m1 on both sides, but it is sufficient for the ground electrode to protrude beyond m1 only on the optical waveguide side.
  • a dielectric layer may or may not be formed.
  • optical waveguide element of the present invention uses an example of HB-CDM, but the present invention is not limited to this, and can also be applied to optical phase modulators, optical modulators with polarization synthesis functionality, optical modulators that integrate more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide substrates made of other materials such as silicon, devices for sensor applications, etc.
  • the optical waveguide element has an optical waveguide 10 formed on a substrate 1 and electrodes (not shown) such as a modulation electrode that modulates the light waves propagating through the optical waveguide 10, and the substrate 1 is housed in a housing CA.
  • an optical modulator MD can be configured by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide.
  • the optical fiber F is optically coupled to the optical waveguide 10 in the optical waveguide element using an optical block with an optical lens, a lens barrel, a polarization multiplexer 6, or the like.
  • the optical fiber may be introduced into the housing through a through hole penetrating the side wall of the housing, and the optical fiber may be directly bonded to an optical component or substrate, or an optical fiber having a lens function at the end of the optical fiber may be optically coupled to the optical waveguide in the optical waveguide element.
  • a reinforcing member (not shown) may be overlapped and arranged along the end face of the substrate 1.
  • the optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal SOL that causes the optical modulator MD to perform a modulation operation to the optical modulator MD.
  • DSP digital signal processor
  • a driver circuit DRV is used to amplify the modulation signal.
  • the driver circuit DRV and digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.
  • the input light L1 to the optical modulator MD may be supplied from outside the optical transmitter OTA, but as shown in FIG. 12, a semiconductor laser (LD) can also be used as the light source.
  • the output light L2 modulated by the optical modulator MD is output to the outside via an optical fiber F.
  • an optical waveguide element that prevents a decrease in the efficiency of the electric field applied to the optical waveguide by the electrodes while suppressing the absorption of light waves propagating through the optical waveguide by the electrodes. Furthermore, it is possible to provide an optical modulator and an optical transmitter that use this optical waveguide element.
  • Substrate (thin plate, film) on which the optical waveguide is formed 2 electrode 4 dielectric layer 10, 11 optical waveguide m1 to m3 underlayer M1, M2 electrode layer F optical fiber LD light source CA housing MD optical modulator DRV driver circuit DSP digital signal processor OTA optical transmitter

Abstract

The objective of the present invention is to provide an optical waveguide element in which the absorption of light waves propagating in the optical waveguide by electrodes is suppressed while preventing a decrease in the efficiency of the electric field applied to the optical waveguide by the electrodes.　An optical waveguide element according to the present invention comprises a substrate 1 in which an optical waveguide 10 is formed and an electrode 2 placed on the substrate in close proximity to the optical waveguide, and is characterized such that in an optical waveguide element in which the electrode comprises an electrode layer M1 and a foundation layer m1 disposed between the electrode layer and the substrate, the electrode layer is disposed between the optical waveguide and the foundation layer and protruding from the foundation layer, and the electrode layer is in direct contact with the top of the substrate.

Description

光導波路素子及びそれを用いた光変調器並びに光送信装置Optical waveguide element, optical modulator using the same, and optical transmitter
　本発明は、光導波路素子及びそれを用いた光変調器並びに光送信装置に関し、特に、光導波路が形成された基板と、該基板上に該光導波路に近接して配置される電極を備え、該電極が電極層と該電極層と該基板との間に下地層を備えた光導波路素子及びそれを用いた光変調器並びに光送信装置に関する。 The present invention relates to an optical waveguide element and an optical modulator and optical transmitter using the same, and in particular to an optical waveguide element that includes a substrate on which an optical waveguide is formed and an electrode disposed on the substrate in close proximity to the optical waveguide, the electrode including an electrode layer and a base layer between the electrode layer and the substrate, and an optical modulator and optical transmitter using the same.
　光計測技術分野や光通信技術分野において、光変調器など、光導波路を形成した基板を用いた光導波路素子が多用されている。一般的な光導波路素子では、ニオブ酸リチウム（ＬＮ）などの電気光学効果を有する基板に光導波路を形成し、該光導波路に電界を印加する電極を基板上に形成している。 In the fields of optical measurement technology and optical communication technology, optical waveguide elements such as optical modulators that use substrates on which optical waveguides are formed are widely used. In a typical optical waveguide element, an optical waveguide is formed on a substrate that has an electro-optic effect, such as lithium niobate (LN), and electrodes that apply an electric field to the optical waveguide are formed on the substrate.
　近年では、広帯域幅コヒーレントドライバ変調器（ＨＢ－ＣＤＭ：High Bandwidth-Coherent Driver Modulator）が注目されており、高速化や小型化に対応した光導波路素子の開発が期待されている。このため、光導波路の幅も１μｍ以下に微細化し、電極もより光導波路に近づけて配置する必要がある。 In recent years, high bandwidth coherent driver modulators (HB-CDMs) have been attracting attention, and the development of optical waveguide elements that can handle higher speeds and smaller sizes is expected. For this reason, the width of the optical waveguide must be miniaturized to less than 1 μm, and the electrodes must be positioned closer to the optical waveguide.
　一般的に、ＬＮ基板上にＡｕなどの金属を電極として配置する際には、ＬＮ基板とＡｕとの密着性が低いため、電極の下地層として、ＴｉやＮｂなどの金属とＡｕとを一緒に成膜し、その上にＡｕの電極層を形成している。 Generally, when placing a metal such as Au on an LN substrate as an electrode, the adhesion between the LN substrate and Au is low, so a film of Au and a metal such as Ti or Nb is formed together as a base layer for the electrode, and then an Au electrode layer is formed on top of that.
　しかしながら、下地層の金属は、Ａｕの電極層と比較し、光吸収率が高く、光導波路の近傍に下地層が配置されると、光導波路を伝搬する光波が下地層に吸収され、光損失が大きくなるという問題を生じる。特に、光変調器の小型化に伴い電極と光導波路が近接するため、従来以上に電極の下地層への光吸収の影響が顕著となる。 However, the metal of the underlayer has a higher optical absorption rate than the Au electrode layer, and when the underlayer is placed near the optical waveguide, the light waves propagating through the optical waveguide are absorbed by the underlayer, resulting in a problem of increased optical loss. In particular, as the electrodes and optical waveguides become closer together with the miniaturization of optical modulators, the effect of optical absorption by the underlayer of the electrodes becomes more pronounced than ever before.
　この問題を解消するため、特許文献１では、図１に示すように、電極２を下地層ｍ１と電極層Ｍ１から構成し、下地層ｍ１を電極層Ｍ１よりも光導波路１０から離して配置することが提案されている。なお、光導波路１０は、基板１に形成したリブ型光導波路である。しかしながら、図１の場合には、電極層Ｍ１と光導波路１０との間に空気層が介在し、電極層Ｍ１の電界が、光導波路１０に効率よく印加できないという不具合を生じていた。 To solve this problem, Patent Document 1 proposes that the electrode 2 be composed of a base layer m1 and an electrode layer M1, as shown in Figure 1, and that the base layer m1 be disposed farther away from the optical waveguide 10 than the electrode layer M1. The optical waveguide 10 is a rib-type optical waveguide formed on the substrate 1. However, in the case of Figure 1, an air layer is interposed between the electrode layer M1 and the optical waveguide 10, causing a problem that the electric field of the electrode layer M1 cannot be efficiently applied to the optical waveguide 10.
　また、図２に示すように、基板１の表面に、ＳｉＯ２などのバッファ層を形成し、当該バッファ層の上に電極２を構成する下地層ｍ１と電極層Ｍ１とを配置している。バッファ層により下地層ｍ１による光吸収を抑制できるが、バッファ層の厚みだけ光導波路と電極が離れる上、空気層とバッファ層とを介して電界が印加されることとなるため、電界効率も低下することとなる。 2, a buffer layer such as SiO2 is formed on the surface of the substrate 1, and an underlayer m1 and an electrode layer M1 constituting the electrode 2 are disposed on the buffer layer. The buffer layer can suppress light absorption by the underlayer m1, but the buffer layer separates the optical waveguide from the electrode by the thickness of the buffer layer, and an electric field is applied through an air layer and the buffer layer, which reduces the electric field efficiency.
　これらの不具合を解消するため、印加する変調信号電圧を上げたり、電界が作用する光導波路の長さ（作用長）を長くすることが考えられるが、前者は変調器を駆動するドライバアンプの大出力化が必要になるため変調信号の広帯域化や低消費電力化の妨げとなり、後者は光導波路素子の小型化の妨げとなる。しかも、ＨＢ－ＣＤＭなどの複雑化した光導波路パターンで、作用長を長くするには、高度な電極形成技術が必要であり、電極形成不良による光学特性の劣化も発生している。 In order to resolve these problems, it is conceivable to increase the applied modulation signal voltage or to increase the length (action length) of the optical waveguide on which the electric field acts; however, the former would require a higher output from the driver amplifier that drives the modulator, which would hinder efforts to broaden the bandwidth of the modulation signal and reduce power consumption, while the latter would hinder efforts to miniaturize optical waveguide elements. Furthermore, in order to increase the action length in complex optical waveguide patterns such as HB-CDM, advanced electrode formation technology is required, and poor electrode formation can also cause degradation of optical characteristics.
特開２０１９－１７４６９８号公報（ＪＰ２０１９－１７４６９８Ａ）JP2019-174698A
　本発明が解決しようとする課題は、上述したような問題を解決し、電極による光導波路を伝搬する光波の吸収を抑制しながら、電極が光導波路に印加する電界の効率の低下を防止した光導波路素子を提供することである。さらには、その光導波路素子を用いた光変調器並びに光送信装置を提供することである。 The problem that the present invention aims to solve is to provide an optical waveguide element that solves the problems described above and prevents a decrease in the efficiency of the electric field that the electrodes apply to the optical waveguide while suppressing the absorption of light waves propagating through the optical waveguide by the electrodes. Furthermore, the present invention aims to provide an optical modulator and an optical transmitter that use this optical waveguide element.
　上記課題を解決するため、本発明の光導波路素子及びそれを用いた光変調器並びに光送信装置は、以下の技術的特徴を有する。
（１）　光導波路が形成された基板と、該基板上に該光導波路に近接して配置される電極を備え、該電極が電極層と該電極層と該基板との間に配置される下地層を備えた光導波路素子において、該光導波路と該下地層との間には、該電極層が該下地層からはみ出して配置され、かつ、該電極層が該基板上に直に接していることを特徴とする。 In order to solve the above problems, the optical waveguide element and the optical modulator and optical transmitter using the same according to the present invention have the following technical features.
(1) An optical waveguide element comprising a substrate on which an optical waveguide is formed, an electrode disposed on the substrate adjacent to the optical waveguide, the electrode comprising an electrode layer and an underlayer disposed between the electrode layer and the substrate, characterized in that the electrode layer is disposed between the optical waveguide and the underlayer so as to protrude beyond the underlayer, and the electrode layer is in direct contact with the substrate.
（２）　上記（１）に記載の光導波路素子において、該下地層の金属の光吸収率が該電極層の金属の光吸収率より高いことを特徴とする。 (2) The optical waveguide element described in (1) above is characterized in that the light absorption rate of the metal in the base layer is higher than that of the metal in the electrode layer.
（３）　上記（１）に記載の光導波路素子において、該電極層と該下地層とを平面視した場合には、両者の重なる面積は、該電極層の面積の５０％以上であることを特徴とする。 (3) The optical waveguide element described in (1) above is characterized in that, when the electrode layer and the base layer are viewed in plan, the overlapping area between them is 50% or more of the area of the electrode layer.
（４）　上記（１）に記載の光導波路素子において、該電極層が該下地層から該光導波路方向にはみ出す量は、該光導波路を伝搬する光波のモード径と同等以上であることを特徴とする。 (4) In the optical waveguide element described in (1) above, the amount by which the electrode layer protrudes from the base layer toward the optical waveguide is equal to or greater than the mode diameter of the light wave propagating through the optical waveguide.
（５）　上記（１）に記載の光導波路素子において、該下地層を平面視した場合には、該下地層の該光導波路に近い辺には、複数の凹凸が形成されていることを特徴とする。 (5) The optical waveguide element described in (1) above is characterized in that, when the base layer is viewed in plan, a plurality of projections and recesses are formed on the side of the base layer that is close to the optical waveguide.
（６）　上記（１）に記載の光導波路素子において、該下地層の厚さは、２００ｎｍ以下であることを特徴とする。 (6) In the optical waveguide element described in (1) above, the thickness of the underlayer is 200 nm or less.
（７）　上記（１）に記載の光導波路素子において、該光導波路及び該光導波路から該電極層の間、更に該電極層の少なくとも一部を連続的に覆う誘電体層を設けることを特徴とする。 (7) The optical waveguide element described in (1) above is characterized in that a dielectric layer is provided that continuously covers the optical waveguide, between the optical waveguide and the electrode layer, and further covers at least a portion of the electrode layer.
（８）　光導波路が形成された基板と、該基板上に該光導波路に近接して配置される電極を備え、該電極は第１電極層と該第１電極層と該基板との間に配置される第１下地層を備えた光導波路素子において、該光導波路と該第１下地層との間には、該第１電極層が該第１下地層からはみ出して配置され、かつ、該第１電極層が該基板上に直接接しており、該第１下地層の該光導波路とは反対側の該基板上に第２下地層が配置され、該第２下地層から該第１電極層の少なくとも一部を覆う第２電極層を有していることを特徴とする。 (8) An optical waveguide element comprising a substrate on which an optical waveguide is formed, an electrode disposed on the substrate adjacent to the optical waveguide, the electrode being a first electrode layer and a first underlayer disposed between the first electrode layer and the substrate, the first electrode layer being disposed between the optical waveguide and the first underlayer so as to extend beyond the first underlayer, and the first electrode layer being in direct contact with the substrate, a second underlayer being disposed on the substrate on the opposite side of the first underlayer to the optical waveguide, and a second electrode layer extending from the second underlayer to cover at least a portion of the first electrode layer.
（９）　上記（１）乃至（８）のいずれかに記載の光導波路素子と、該光導波路素子を収容する筐体と、該光導波路に光波を入力又は出力する光ファイバとを備えることを特徴とする光変調器である。 (9) An optical modulator comprising an optical waveguide element according to any one of (1) to (8) above, a housing for accommodating the optical waveguide element, and an optical fiber for inputting or outputting light waves to the optical waveguide.
（１０）　上記（９）に記載の光変調器において、該電極は、該光導波路を伝搬する光波を変調するための変調電極であり、該変調電極に入力する変調信号を増幅する電子回路を該筐体の内部に有することを特徴とする。 (10) In the optical modulator described in (9) above, the electrode is a modulation electrode for modulating the light wave propagating through the optical waveguide, and the housing includes an electronic circuit for amplifying the modulation signal input to the modulation electrode.
（１１）　上記（１０）に記載の光変調器と、該光変調器に光波を入力する光源と、該光変調器に変調信号を出力する電子回路とを有することを特徴とする光送信装置である。 (11) An optical transmitter comprising the optical modulator described in (10) above, a light source for inputting a light wave to the optical modulator, and an electronic circuit for outputting a modulated signal to the optical modulator.
　本発明は、光導波路が形成された基板と、該基板上に該光導波路に近接して配置される電極を備え、該電極が電極層と該電極層と該基板との間に配置される下地層を備えた光導波路素子において、該光導波路と該下地層との間には、該電極層が該下地層からはみ出して配置され、かつ、該電極層が該基板上に直に接しているため、光吸収効率の高い材料を含む下地層を光導波路から遠ざけることで、電極による光導波路を伝搬する光波の吸収を抑制できると共に、光吸収効率の低い材料で構成される電極層を光導波路の近くに配置することで、電極が光導波路に印加する電界の効率の低下を防止する光導波路素子を提供することが可能になる。
　さらに、このような優れた特性を備えた光導波路素子を用いることで、同様の効果を奏する光変調器や光送信装置も提供が可能となる。 The present invention provides an optical waveguide element comprising a substrate on which an optical waveguide is formed, and an electrode arranged on the substrate adjacent to the optical waveguide, the electrode comprising an electrode layer and an underlayer arranged between the electrode layer and the substrate, the electrode layer being arranged between the optical waveguide and the underlayer so as to extend beyond the underlayer and being in direct contact with the substrate, so that by placing the underlayer containing a material with high light absorption efficiency away from the optical waveguide, it is possible to provide an optical waveguide element in which the absorption of light waves propagating through the optical waveguide by the electrode can be suppressed, and by arranging an electrode layer made of a material with low light absorption efficiency close to the optical waveguide, a decrease in the efficiency of the electric field applied by the electrode to the optical waveguide can be prevented.
Furthermore, by using an optical waveguide element having such excellent characteristics, it is possible to provide an optical modulator or an optical transmitter that achieves the same effects.
従来の光導波路素子の一例を示す断面図である。FIG. 1 is a cross-sectional view showing an example of a conventional optical waveguide element. 従来の光導波路素子の他の例を示す断面図である。FIG. 11 is a cross-sectional view showing another example of a conventional optical waveguide element. 本発明の光導波路素子の第１実施例を示す断面図である。1 is a cross-sectional view showing a first embodiment of an optical waveguide element according to the present invention. 図３に示す矢印Ｘ－Ｘ’における光導波路素子の断面図（図３の上方から見た図）である。4 is a cross-sectional view of the optical waveguide element taken along the arrow X-X' in FIG. 3 (as viewed from above in FIG. 3). 本発明の光導波路素子の第２実施例を示す図であり、図４に対応する断面図である。FIG. 5 is a cross-sectional view showing a second embodiment of the optical waveguide element of the present invention, corresponding to FIG. 本発明の光導波路素子の応用例を示す断面図である。FIG. 2 is a cross-sectional view showing an application example of the optical waveguide element of the present invention. 本発明の光導波路素子の第３実施例を示す断面図である。FIG. 4 is a cross-sectional view showing a third embodiment of the optical waveguide element of the present invention. 図７に示す光導波路素子の応用例を示す断面図である。8 is a cross-sectional view showing an application example of the optical waveguide element shown in FIG. 7. 図８に示す光導波路素子のさらなる応用例を示す断面図である。9 is a cross-sectional view showing a further application example of the optical waveguide element shown in FIG. 8 . 図９に示す光導波路素子の応用例を示す断面図である。10 is a cross-sectional view showing an application example of the optical waveguide element shown in FIG. 9 . 本発明の光導波路素子の第４実施例を示す断面図である。FIG. 11 is a cross-sectional view showing a fourth embodiment of the optical waveguide element of the present invention. 本発明の光送信装置の一例を示す図である。1 is a diagram illustrating an example of an optical transmitting device according to the present invention.
　以下、本発明の光導波路素子及びそれを用いた光変調器並びに光送信装置について、好適例を用いて詳細に説明する。
　本発明の光導波路素子の一例を示す断面図を図３に示す。
　本発明の光導波路素子は、光導波路１０が形成された基板１と、該基板上に該光導波路に近接して配置される電極２を備え、該電極が電極層Ｍ１と該電極層と該基板との間に配置される下地層ｍ１を備えた光導波路素子において、該光導波路と該下地層との間には、該電極層が該下地層からはみ出して配置され、かつ、該電極層が該基板上に直に接していることを特徴とする。 Hereinafter, the optical waveguide element of the present invention and the optical modulator and optical transmitter using the same will be described in detail using preferred examples.
FIG. 3 is a cross-sectional view showing an example of the optical waveguide element of the present invention.
The optical waveguide element of the present invention comprises a substrate 1 on which an optical waveguide 10 is formed, and an electrode 2 arranged on the substrate in close proximity to the optical waveguide, the electrode comprising an electrode layer M1 and an underlayer m1 arranged between the electrode layer and the substrate, the electrode layer being arranged between the optical waveguide and the underlayer so as to protrude from the underlayer, and the electrode layer being in direct contact with the substrate.
　本発明の光導波路素子に使用される基板１としては、電気光学効果を有する基板が利用でき、具体的には、ニオブ酸リチウム（ＬＮ）やタンタル酸リチウム（ＬＴ）、ＰＬＺＴ（ジルコン酸チタン酸鉛ランタン）などの基板や、これらの基板材料にＭｇＯなどをドープした基材が使用可能である。また、これらの材料をスパッタ法、蒸着法、又はＣＶＤ法などの気相成長法を利用して膜形成することも可能である。また、電気光学効果を有する基板を別の基板に接合した後に、電気光学基板を薄膜加工した基板を用いることもできる。さらに、半導体基板やＥＯポリマーなどの有機材料の基板なども利用可能である。 The substrate 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect, specifically, a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a base material in which these substrate materials are doped with MgO or the like. It is also possible to form a film from these materials using a vapor phase growth method such as sputtering, deposition, or CVD. It is also possible to use a substrate in which a substrate having an electro-optic effect is bonded to another substrate and then the electro-optic substrate is processed into a thin film. Furthermore, semiconductor substrates and substrates made of organic materials such as EO polymers can also be used.
　光導波路１０としては、基板１にＴｉなどの高屈折率材料を熱拡散した光導波路や、プロトン交換法で形成した光導波路、さらには、光導波路以外の基板１をエッチングしたり、光導波路の両側に溝を形成するなど、基板に光導波路に対応する部分を凸状としたリブ型導波路を利用することが可能である。さらに、リブ型の光導波路に合わせて、Ｔｉなどを熱拡散法やプロトン交換法などで基板表面に拡散させることにより、屈折率をより高くすることも可能である。リブ型導波路のサイズとしては、光の閉じ込めを高めるため、１μｍ程度の幅や高さの微細なリブ型光導波路となっている。 As the optical waveguide 10, it is possible to use an optical waveguide in which a high refractive index material such as Ti is thermally diffused into the substrate 1, an optical waveguide formed by the proton exchange method, or even a rib-type waveguide in which the portion of the substrate corresponding to the optical waveguide is made convex, such as by etching the substrate 1 other than the optical waveguide or by forming grooves on both sides of the optical waveguide. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to increase the refractive index by diffusing Ti or the like onto the substrate surface by the thermal diffusion method or the proton exchange method. The size of the rib-type waveguide is a fine rib-type optical waveguide with a width and height of about 1 μm in order to increase the light confinement.
　光導波路１０を形成した基板（薄板）１の厚さｈは、変調信号のマイクロ波と光波との速度整合を図るため、１０μｍ以下、より好ましくは５μｍ以下、さらに好ましくは１μｍ以下に設定される。また、リブ型光導波路の高さは、ｈの８０％以下に設定され、ｈの厚さに応じ、４μｍ以下、より好ましくは３μｍ以下、さらに好ましくは０．８μｍ以下や０．４μｍ以下に設定される。 The thickness h of the substrate (thin plate) 1 on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to achieve speed matching between the microwave and light waves of the modulated signal. The height of the rib-type optical waveguide is set to 80% or less of h, and is set to 4 μm or less, more preferably 3 μm or less, and even more preferably 0.8 μm or less or 0.4 μm or less, depending on the thickness of h.
　光導波路を形成した基板１は、機械的強度を高めるため、基板１の下側に、補強基板（不図示）が接合されている。基板１と補強基板とは、直接接合又は樹脂等の接着層を介して接着固定される。直接接合する補強基板としては、光導波路や光導波路を形成した基板よりも屈折率が低いことが好ましいが、これに限定されるものではない。なお、直接接合の場合は、金属酸化物や金属等の中間層を接合部分に含んでも良い。また、補強基板は、基板１と熱膨張率が近い材料、例えば水晶やガラス等の酸化物層を含む基板が好適に利用される。さらに、基板１と同じＬＮ基板や、ＳＯＩ、ＬＮＯＩと略されるシリコン基板上に酸化ケイ素層を形成した複合基板やＬＮ基板上に酸化ケイ素層を形成した複合基板を利用することも可能である。基板１の屈折率よりも補強基板の屈折率が高い場合には、基板１と補強基板の間に基板１より低い屈折率の層が設けられる。 In order to increase the mechanical strength of the substrate 1 on which the optical waveguide is formed, a reinforcing substrate (not shown) is bonded to the underside of the substrate 1. The substrate 1 and the reinforcing substrate are bonded and fixed by direct bonding or via an adhesive layer such as resin. The reinforcing substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but is not limited to this. In the case of direct bonding, an intermediate layer such as a metal oxide or metal may be included in the bonding portion. In addition, the reinforcing substrate is preferably made of a material having a thermal expansion coefficient close to that of the substrate 1, such as a substrate containing an oxide layer of quartz or glass. Furthermore, it is possible to use the same LN substrate as the substrate 1, a composite substrate in which a silicon oxide layer is formed on a silicon substrate abbreviated as SOI or LNOI, or a composite substrate in which a silicon oxide layer is formed on an LN substrate. If the refractive index of the reinforcing substrate is higher than that of the substrate 1, a layer with a lower refractive index than that of the substrate 1 is provided between the substrate 1 and the reinforcing substrate.
　基板１上に形成される電極２は、電極層Ｍ１と下地層ｍ１とで構成される。電極層Ｍ１は、ＡｕやＣｕなどの金属が利用される。下地層ｍ１は、基板１と電極２（電極層Ｍ１）との密着性を高めるために使用される。電極層Ｍ１は、下地層を利用した電解メッキ法やレジストパターンを利用した無電解メッキ法、蒸着やスパッタなどの気相法、さらにはこれらを組み合わせにより、下地層を覆うように形成される。 The electrode 2 formed on the substrate 1 is composed of an electrode layer M1 and an underlayer m1. The electrode layer M1 is made of a metal such as Au or Cu. The underlayer m1 is used to improve adhesion between the substrate 1 and the electrode 2 (electrode layer M1). The electrode layer M1 is formed to cover the underlayer by electrolytic plating using an underlayer, electroless plating using a resist pattern, gas phase methods such as vapor deposition and sputtering, or a combination of these.
　下地層ｍ１の材料としては、Ｔ，Ｎｂ，Ｎｉ，Ｃｒ，又はＡｌ等の酸素の金属への結合エネルギーが電極層に使用する材料よりも大きいことが必要である。特に、３００ｋｊ・（ｍｏｌ－Ｏ）－１以上の金属の場合、ＬＮ基板中の酸素と強固に結合するため好ましい。また、下地層は、上述した金属材料にＡｕが混入した状態で成膜される。成膜方法はスパッタ法、蒸着法等が使用できる。 The material of the underlayer m1 must have a higher oxygen-metal binding energy, such as T, Nb, Ni, Cr, or Al, than the material used for the electrode layer. In particular, metals with a binding energy of 300 kj·(mol-O) −1 or more are preferred because they bind strongly with oxygen in the LN substrate. The underlayer is formed by mixing Au into the above-mentioned metal material. The film can be formed by sputtering, vapor deposition, or the like.
　下地層ｍ１の厚さは、２００ｎｍ以下、さらには２０ｎｍ以下に設定することが好ましい。下地層の厚さが薄い程、光吸収損失が低下する。特に、Ｔｉの場合は、下地層が厚くなるに従い光吸収損失が比例的に増加するため、下地層の厚さを薄く設定することが好ましい。下地層の厚さの下限値は特にないが、例えば、１ｎｍ以上に設定することが可能である。 The thickness of the underlayer m1 is preferably set to 200 nm or less, and more preferably 20 nm or less. The thinner the underlayer, the lower the light absorption loss. In particular, in the case of Ti, the light absorption loss increases proportionally as the underlayer becomes thicker, so it is preferable to set the thickness of the underlayer thin. There is no particular lower limit for the thickness of the underlayer, but it can be set to, for example, 1 nm or more.
　本発明の光導波路素子の特徴は、図３のように、下地層ｍ１からはみ出す電極層Ｍ１を設け、該電極層Ｍ１を基板に直接接するように配置することである。この構成により、光導波路１０から下地層ｍ１を遠ざけ、光吸収損失を抑制することが可能となる。 The optical waveguide element of the present invention is characterized in that, as shown in FIG. 3, an electrode layer M1 is provided that protrudes from the base layer m1, and the electrode layer M1 is disposed so as to be in direct contact with the substrate. This configuration makes it possible to distance the base layer m1 from the optical waveguide 10 and suppress optical absorption loss.
　図３に示すよう電極層Ｍ１の光導波路に近い端部と下地層ｍ１の光導波路に近い端部との距離ｄは、光導波路を伝搬する光波のモード径と同等以上、さらに離間して設定することが好ましい。なお、光導波路１０を挟む電極層Ｍ１（電極２）の間隔は、光導波路を伝搬する光波のモード径の２～１０倍程度に設定される。 As shown in FIG. 3, the distance d between the end of the electrode layer M1 closest to the optical waveguide and the end of the base layer m1 closest to the optical waveguide is preferably set to be equal to or greater than the mode diameter of the light waves propagating through the optical waveguide. The spacing between the electrode layers M1 (electrode 2) that sandwich the optical waveguide 10 is set to about 2 to 10 times the mode diameter of the light waves propagating through the optical waveguide.
　図４は、図３の矢印Ｘ－Ｘ’における断面図であり、光導波路素子を平面視した図である。なお、図４は、光導波路素子の一部のみを示しており、光導波路素子全体を示す図面ではない。また、太線で囲まれている部分は、電極層Ｍ１の外周を示している。 FIG. 4 is a cross-sectional view taken along the line X-X' in FIG. 3, showing the optical waveguide element in plan view. Note that FIG. 4 shows only a portion of the optical waveguide element, and is not a drawing showing the entire optical waveguide element. The portion surrounded by a thick line shows the outer periphery of the electrode layer M1.
　図４に示す電極層Ｍ１の面積Ａ１（Ａ１はａ１を含んだ太枠内の面積）と、該電極層Ｍ１と接触している下地層ｍ１の面積ａ１との関係は、ａ１≧０．５×Ａ１である。つまり、電極層Ｍ１の下面は、その半分（５０％）以上が下地層ｍ１と接している。これにより、電極層が下地層を介して強固に基板１に接合でき、電極層Ｍ１の剥離防止にも寄与する。 The relationship between the area A1 of the electrode layer M1 shown in FIG. 4 (A1 is the area within the thick frame including a1) and the area a1 of the base layer m1 in contact with the electrode layer M1 is a1 ≧ 0.5 × A1. In other words, more than half (50%) of the bottom surface of the electrode layer M1 is in contact with the base layer m1. This allows the electrode layer to be firmly bonded to the substrate 1 via the base layer, which also helps prevent peeling of the electrode layer M1.
　図５は、図４の応用例を示したものであり、下地層ｍ１の光導波路１０に近い辺に凹凸２０を形成している。図５のように、規則的な櫛状の構成に限らず、凹凸の大きさや深さ（高さ）又は間隔が不規則であっても良い。このような凹凸形状を採用することで、電極層Ｍ１と下地層ｍ１との接合強度を高めることが可能となる。また、凹凸を設けた辺は、直線的な辺よりも、光吸収損失が低いため、より好ましい形状ともいえる。 FIG. 5 shows an application example of FIG. 4, in which unevenness 20 is formed on the side of the base layer m1 close to the optical waveguide 10. As shown in FIG. 5, the unevenness is not limited to a regular comb-like configuration, and the size, depth (height), or spacing of the unevenness may be irregular. By adopting such an uneven shape, it is possible to increase the bonding strength between the electrode layer M1 and the base layer m1. Furthermore, an edge with unevenness has a lower optical absorption loss than a straight edge, so it can be said to be a more preferable shape.
　図６は、本発明の光導波路素子の応用例を示す図であり、図６（ａ）は下地層ｍ１全体を包み込むように金属層Ｍ１を配置したものである。光導波路１０に近い側の電極は、図３と同様に構成し、光導波路１０から離れた反対側では、光導波路１０に近い側と同様に、下地層ｍ１から電極層Ｍ１を張り出すように配置している。また、図６（ｃ）は基板に２本の溝構造を設けてリブ型光導波路１０とした例である。 Figure 6 shows an application example of the optical waveguide element of the present invention. In Figure 6(a), a metal layer M1 is arranged so as to encase the entire base layer m1. The electrode on the side closer to the optical waveguide 10 is configured in the same way as in Figure 3, and on the opposite side away from the optical waveguide 10, the electrode layer M1 is arranged so as to protrude from the base layer m1, just like the side closer to the optical waveguide 10. Also, Figure 6(c) shows an example in which two groove structures are provided in the substrate to create a rib-type optical waveguide 10.
　図６（ｂ）は、光導波路１０から離れた反対側では、下地層ｍ１が電極層Ｍ１より張り出している。また、図６（ｄ）は、リブ型光導波路１０の代わりに、Ｔｉを熱拡散した光導波路を用いている。このような種々の変更は、必要に応じて採用することが可能である。 In FIG. 6(b), on the opposite side away from the optical waveguide 10, the base layer m1 protrudes beyond the electrode layer M1. Also, in FIG. 6(d), an optical waveguide in which Ti is thermally diffused is used instead of the rib-type optical waveguide 10. Various modifications such as these can be adopted as necessary.
　図７乃至図９は、２つの電極層（Ｍ１，Ｍ２）を重ねて配置したものである。具体的には、第１電極層Ｍ１を形成した後、第１電極層Ｍ１の少なくとも一部に覆いかぶさるように第２電極層Ｍ２を形成している。第１電極層Ｍ１の厚さは、例えば、０．１～２μｍ程度であり、第２電極層Ｍ２の厚さは、２～１５μｍ程度である。 Figures 7 to 9 show two electrode layers (M1, M2) stacked on top of each other. Specifically, after forming the first electrode layer M1, the second electrode layer M2 is formed so as to cover at least a portion of the first electrode layer M1. The thickness of the first electrode layer M1 is, for example, about 0.1 to 2 μm, and the thickness of the second electrode layer M2 is about 2 to 15 μm.
　第２電極層Ｍ２を形成する際に、図７に示すように、第２電極層Ｍ２と基板１との間に第２下地層ｍ２を配置することができる。第１下地層ｍ１と第２下地層ｍ２とは同じ材料であっても良いし、第１下地層ｍ１の材料を、第２下地層の材料よりも光吸収損失の低い材料で構成することも可能である。また、ｍ２は例えば、光吸収損失が大きい材料とすることで基板中に存在する不要光を効率よく除去できたり、厚くすることで電極層Ｍ２と下地層ｍ２との接合強度を高めることもできる。 When forming the second electrode layer M2, as shown in FIG. 7, a second underlayer m2 can be disposed between the second electrode layer M2 and the substrate 1. The first underlayer m1 and the second underlayer m2 can be made of the same material, or the material of the first underlayer m1 can be made of a material with lower light absorption loss than the material of the second underlayer. In addition, for example, by making m2 a material with high light absorption loss, it is possible to efficiently remove unnecessary light present in the substrate, and by making it thicker, it is possible to increase the bonding strength between the electrode layer M2 and the underlayer m2.
　図８は、図７と異なり、第１電極層Ｍ１と第２電極層Ｍ２との間にも第３下地層ｍ３を配置している。第３下地層ｍ３は、第２下地層ｍ２と同じ材料で同一プロセスで形成することも可能である。また、第３下地層ｍ３が光導波路１０に近接する場合には、第１下地層ｍ１又は第２下地層ｍ２よりも光吸収損失の低い材料で構成することが好ましい。 In FIG. 8, unlike FIG. 7, a third underlayer m3 is also disposed between the first electrode layer M1 and the second electrode layer M2. The third underlayer m3 can be formed in the same process and using the same material as the second underlayer m2. Furthermore, when the third underlayer m3 is close to the optical waveguide 10, it is preferable to configure it using a material that has a lower optical absorption loss than the first underlayer m1 or the second underlayer m2.
　図９では、第１下地層ｍ１と第１電極層Ｍ１との関係と同様に、第３下地層ｍ３からはみ出すように第２電極層Ｍ２を配置している。この構成により、第３下地層ｍ３による光吸収損失を抑制することが可能となる。 In FIG. 9, the second electrode layer M2 is disposed so as to extend beyond the third base layer m3, similar to the relationship between the first base layer m1 and the first electrode layer M1. This configuration makes it possible to suppress light absorption loss due to the third base layer m3.
　図１０に示すように光導波路１０及び該光導波路１０から電極層Ｍ１（電極２）の間、更に該電極層Ｍ１の少なくとも一部を連続的に覆う誘電体層を設けることも可能である。誘電体層としては、永久レジストや金属酸化物（ＳｉＯ２、Ａｌ２Ｏ３等）を用いることができる。このような誘電体層は、リブ型光導波路１０の表面の粗さによる光散乱を抑制し、光伝搬損失を減少させる機能を有する。しかもこの誘電体層は、電極層Ｍ１の剥離防止にも寄与できる。
　また、電極層Ｍ１を第一下地層ｍ１からはみ出さずに形成した後、ウェットエッチング等で下地層ｍ１を距離ｄ程度まで除去し、その上から誘電体層を形成することで、電極層Ｍ１がそれの重さで押され、電極層Ｍ１と基板１とを直に接させることも可能である。 As shown in Fig. 10, it is also possible to provide a dielectric layer that continuously covers the optical waveguide 10 and the area between the optical waveguide 10 and the electrode layer M1 (electrode 2), and further covers at least a part of the electrode layer M1. As the dielectric layer, a permanent resist or a metal oxide ( SiO2 , Al2O3 , etc.) can be used. Such a dielectric layer has the function of suppressing light scattering due to the roughness of the surface of the rib-type optical waveguide 10 and reducing light propagation loss. Moreover, this dielectric layer can also contribute to preventing peeling of the electrode layer M1.
In addition, after forming the electrode layer M1 without protruding from the first base layer m1, the base layer m1 can be removed to a distance d by wet etching or the like, and a dielectric layer can be formed on top of it, so that the electrode layer M1 is pressed by its own weight, and the electrode layer M1 and the substrate 1 can be directly contacted.
　図１１は、マッハツェンダー型光導波路の構造を適用した例であり、左右の電極は接地電極、中央の電極は信号電極となる。この場合、信号電極はその両側に光導波路が形成されているので、ｍ１に対していずれの側もはみ出していなければならないが、接地電極は少なくとも光導波路側のみがｍ１に対してはみ出していれば良い。また、本構造において、誘電体層は形成されていても、されていなくても良い。 Figure 11 shows an example of applying the structure of a Mach-Zehnder type optical waveguide, where the left and right electrodes are ground electrodes and the center electrode is a signal electrode. In this case, the signal electrode has optical waveguides formed on both sides, so it must protrude beyond m1 on both sides, but it is sufficient for the ground electrode to protrude beyond m1 only on the optical waveguide side. Also, in this structure, a dielectric layer may or may not be formed.
　次に、本発明の光導波路素子を、光変調器や光送信装置に適用した例について説明する。以下では、ＨＢ－ＣＤＭの一例を用いて説明するが、本発明はこれに限らず、光位相変調器、偏波合成機能を備えた光変調器や、より多い又はより少ないマッハツェンダー型光導波路を集積した光変調器、シリコンなど他材料で構成した光導波路基板との接合デバイス、センサ用途のデバイスなどにも適用可能である。 Next, we will explain examples of applying the optical waveguide element of the present invention to an optical modulator or optical transmitter. The following explanation uses an example of HB-CDM, but the present invention is not limited to this, and can also be applied to optical phase modulators, optical modulators with polarization synthesis functionality, optical modulators that integrate more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide substrates made of other materials such as silicon, devices for sensor applications, etc.
　図１２に示すように、光導波路素子は、基板１に形成された光導波路１０と、該光導波路１０を伝搬する光波を変調する変調電極などの電極（不図示）とを有しており、当該基板１は筐体ＣＡ内に収容される。さらに、光導波路に光波を入出力する光ファイバ（Ｆ）を設けることで、光変調器ＭＤを構成することができる。図１２では、光ファイバＦは、光学レンズを備えた光学ブロックやレンズ鏡筒、偏波合波部６などを用いて光導波路素子内の光導波路１０と光学的に結合されている。これに限らず、光ファイバを筐体の側壁を貫通する貫通孔を介して筐体内に導入し、光学部品又は基板と、光ファイバとを直接接合したり、または光ファイバ端部にレンズ機能を有した光ファイバを光導波路素子内の光導波路と光学的に結合しても良い。また、光ファイバや光学ブロックとの接合を安定的に行うため、基板１の端面に沿って補強部材（不図示）を重ねて配置することも可能である。 As shown in FIG. 12, the optical waveguide element has an optical waveguide 10 formed on a substrate 1 and electrodes (not shown) such as a modulation electrode that modulates the light waves propagating through the optical waveguide 10, and the substrate 1 is housed in a housing CA. Furthermore, an optical modulator MD can be configured by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide. In FIG. 12, the optical fiber F is optically coupled to the optical waveguide 10 in the optical waveguide element using an optical block with an optical lens, a lens barrel, a polarization multiplexer 6, or the like. Not limited to this, the optical fiber may be introduced into the housing through a through hole penetrating the side wall of the housing, and the optical fiber may be directly bonded to an optical component or substrate, or an optical fiber having a lens function at the end of the optical fiber may be optically coupled to the optical waveguide in the optical waveguide element. In addition, in order to stably bond the optical fiber and the optical block, a reinforcing member (not shown) may be overlapped and arranged along the end face of the substrate 1.
　光変調器ＭＤに変調動作を行わせる変調信号ＳＯＬを出力する電子回路（デジタル信号プロセッサーＤＳＰ）を、光変調器ＭＤに接続することにより、光送信装置ＯＴＡを構成することが可能である。光導波路素子に印加する変調信号Ｓを得るためには、デジタル信号プロセッサーＤＳＰから出力される変調信号ＳＯＬを増幅する必要がある。このため、図１２では、ドライバ回路ＤＲＶを使用し、変調信号を増幅している。ドライバ回路ＤＲＶやデジタル信号プロセッサーＤＳＰは、筐体ＣＡの外部に配置することも可能であるが、筐体ＣＡ内に配置することも可能である。特に、ドライバ回路ＤＲＶを筐体内に配置することで、ドライバ回路からの変調信号の伝搬損失をより低減することが可能となる。 The optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal SOL that causes the optical modulator MD to perform a modulation operation to the optical modulator MD. In order to obtain the modulation signal S to be applied to the optical waveguide element, it is necessary to amplify the modulation signal SOL output from the digital signal processor DSP. For this reason, in FIG. 12, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.
　光変調器ＭＤへの入力光Ｌ１は、光送信装置ＯＴＡの外部から供給されても良いが、図１２に示すように半導体レーザー（ＬＤ）を光源とすることも可能である。光変調器ＭＤで変調された出力光Ｌ２は、光ファイバＦにより外部に出力される。 The input light L1 to the optical modulator MD may be supplied from outside the optical transmitter OTA, but as shown in FIG. 12, a semiconductor laser (LD) can also be used as the light source. The output light L2 modulated by the optical modulator MD is output to the outside via an optical fiber F.
　以上説明したように、本発明によれば、電極による光導波路を伝搬する光波の吸収を抑制しながら、電極が光導波路に印加する電界の効率の低下を防止した光導波路素子を提供することが可能となる。さらには、その光導波路素子を用いた光変調器並びに光送信装置を提供することができる。 As described above, according to the present invention, it is possible to provide an optical waveguide element that prevents a decrease in the efficiency of the electric field applied to the optical waveguide by the electrodes while suppressing the absorption of light waves propagating through the optical waveguide by the electrodes. Furthermore, it is possible to provide an optical modulator and an optical transmitter that use this optical waveguide element.
　１　光導波路を形成する基板（薄板，膜体）
　２　電極
　４　誘電体層
　１０，１１　光導波路
　ｍ１～ｍ３　下地層
　Ｍ１，Ｍ２　電極層
　Ｆ　光ファイバ
　ＬＤ　光源
　ＣＡ　筐体
　ＭＤ　光変調器
　ＤＲＶ　ドライバ回路
　ＤＳＰ　デジタル信号プロセッサー
　ＯＴＡ　光送信装置 1. Substrate (thin plate, film) on which the optical waveguide is formed
2 electrode 4 dielectric layer 10, 11 optical waveguide m1 to m3 underlayer M1, M2 electrode layer F optical fiber LD light source CA housing MD optical modulator DRV driver circuit DSP digital signal processor OTA optical transmitter

Claims (11)

  1. 　光導波路が形成された基板と、該基板上に該光導波路に近接して配置される電極を備え、該電極が電極層と該電極層と該基板との間に配置される下地層を備えた光導波路素子において、
    　該光導波路と該下地層との間には、該電極層が該下地層からはみ出して配置され、かつ、該電極層が該基板上に直に接していることを特徴とする光導波路素子。     An optical waveguide element comprising a substrate on which an optical waveguide is formed, and an electrode disposed on the substrate in the vicinity of the optical waveguide, the electrode comprising an electrode layer and a base layer disposed between the electrode layer and the substrate,
    an electrode layer is disposed between the optical waveguide and the underlayer so as to extend beyond the underlayer, and the electrode layer is in direct contact with the substrate;
  2. 　請求項１に記載の光導波路素子において、該下地層の金属の光吸収率が該電極層の金属の光吸収率より高いことを特徴とする光導波路素子。 The optical waveguide element according to claim 1, characterized in that the light absorption rate of the metal of the underlayer is higher than the light absorption rate of the metal of the electrode layer.
  3. 請求項１に記載の光導波路素子において、該電極層と該下地層とを平面視した場合には、両者の重なる面積は、該電極層の面積の５０％以上であることを特徴とする光導波路素子。 The optical waveguide element according to claim 1, characterized in that when the electrode layer and the underlayer are viewed in plan, the overlapping area between them is 50% or more of the area of the electrode layer.
  4. 　請求項１に記載の光導波路素子において、該電極層が該下地層から該光導波路方向にはみ出す量は、該光導波路を伝搬する光波のモード径と同等以上であることを特徴とする光導波路素子。 The optical waveguide element according to claim 1, characterized in that the amount by which the electrode layer protrudes from the base layer in the optical waveguide direction is equal to or greater than the mode diameter of the light wave propagating through the optical waveguide.
  5. 　請求項１に記載の光導波路素子において、該下地層を平面視した場合には、該下地層の該光導波路に近い辺には、複数の凹凸が形成されていることを特徴とする光導波路素子。 The optical waveguide element according to claim 1, characterized in that, when the base layer is viewed in plan, a plurality of projections and recesses are formed on the side of the base layer that is close to the optical waveguide.
  6. 　請求項１に記載の光導波路素子において、該下地層の厚さは、２００ｎｍ以下であることを特徴とする光導波路素子。 The optical waveguide element according to claim 1, characterized in that the thickness of the underlayer is 200 nm or less.
  7. 　請求項１に記載の光導波路素子において、該光導波路及び該光導波路から該電極層の間、更に該電極層の少なくとも一部を連続的に覆う誘電体層を設けることを特徴とする光導波路素子。 The optical waveguide element according to claim 1, further comprising a dielectric layer disposed between the optical waveguide and the electrode layer, and continuously covering at least a portion of the electrode layer.
  8. 　光導波路が形成された基板と、該基板上に該光導波路に近接して配置される電極を備え、該電極は第１電極層と該第１電極層と該基板との間に配置される第１下地層を備えた光導波路素子において、
    　該光導波路と該第１下地層との間には、該第１電極層が該第１下地層からはみ出して配置され、かつ、該第１電極層が該基板上に直接接しており、
    　該第１下地層の該光導波路とは反対側の該基板上に第２下地層が配置され、
    　該第２下地層から該第１電極層の少なくとも一部を覆う第２電極層を有していることを特徴とする光導波路素子。     An optical waveguide element comprising a substrate on which an optical waveguide is formed, and an electrode disposed on the substrate in the vicinity of the optical waveguide, the electrode comprising a first electrode layer and a first underlayer disposed between the first electrode layer and the substrate,
    the first electrode layer is disposed between the optical waveguide and the first underlayer so as to protrude from the first underlayer, and the first electrode layer is in direct contact with the substrate;
    a second underlayer is disposed on the substrate on a side of the first underlayer opposite to the optical waveguide;
    an optical waveguide element comprising a second electrode layer covering the second underlayer and at least a part of the first electrode layer;
  9. 　請求項１乃至８のいずれかに記載の光導波路素子と、該光導波路素子を収容する筐体と、
    　該光導波路に光波を入力又は出力する光ファイバとを備えることを特徴とする光変調器。     An optical waveguide element according to any one of claims 1 to 8, and a housing for accommodating the optical waveguide element;
    and an optical fiber for inputting or outputting a light wave to or from said optical waveguide.
  10. 　請求項９に記載の光変調器において、
    　該電極は、該光導波路を伝搬する光波を変調するための変調電極であり、
    　該変調電極に入力する変調信号を増幅する電子回路を該筐体の内部に有することを特徴とする光変調器。     10. The optical modulator according to claim 9,
    the electrode is a modulation electrode for modulating a light wave propagating through the optical waveguide,
    An optical modulator comprising: an electronic circuit disposed inside the housing for amplifying a modulation signal input to the modulation electrode.
  11. 　請求項１０に記載の光変調器と、
    　該光変調器に光波を入力する光源と、
    　該光変調器に変調信号を出力する電子回路とを有することを特徴とする光送信装置。     An optical modulator according to claim 10;
    a light source for inputting a light wave to the optical modulator;
    and an electronic circuit for outputting a modulated signal to the optical modulator.
PCT/JP2022/037614 2022-10-07 2022-10-07 Optical waveguide element, optical modulator using same, and optical transmission device WO2024075277A1 (en)

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Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH04217226A (en) * 1990-12-19 1992-08-07 Fujitsu Ltd Manufacture of light guide path type device
JPH04268531A (en) * 1991-02-25 1992-09-24 Fujitsu Ltd Signal electrode for optical waveguide device and its formation
US20030062551A1 (en) * 2001-10-02 2003-04-03 Jds Uniphase Corporation Electrode structure including encapsulated adhesion layer
JP2006039569A (en) * 2004-07-27 2006-02-09 Jds Uniphase Corp Low bias drift modulator having buffer layer
JP2010181454A (en) * 2009-02-03 2010-08-19 Fujitsu Ltd Optical waveguide device and method for manufacturing the same, light modulator, polarization mode dispersion compensator, and optical switch
JP2015197454A (en) * 2014-03-31 2015-11-09 住友大阪セメント株式会社 optical waveguide element
JP2019095698A (en) * 2017-11-27 2019-06-20 富士通オプティカルコンポーネンツ株式会社 Optical module and optical modulator
JP2020134874A (en) * 2019-02-25 2020-08-31 Tdk株式会社 Light modulator
JP2022056727A (en) * 2020-09-30 2022-04-11 住友大阪セメント株式会社 Optical waveguide element and optical modulator

Patent Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH04217226A (en) * 1990-12-19 1992-08-07 Fujitsu Ltd Manufacture of light guide path type device
JPH04268531A (en) * 1991-02-25 1992-09-24 Fujitsu Ltd Signal electrode for optical waveguide device and its formation
US20030062551A1 (en) * 2001-10-02 2003-04-03 Jds Uniphase Corporation Electrode structure including encapsulated adhesion layer
JP2006039569A (en) * 2004-07-27 2006-02-09 Jds Uniphase Corp Low bias drift modulator having buffer layer
JP2010181454A (en) * 2009-02-03 2010-08-19 Fujitsu Ltd Optical waveguide device and method for manufacturing the same, light modulator, polarization mode dispersion compensator, and optical switch
JP2015197454A (en) * 2014-03-31 2015-11-09 住友大阪セメント株式会社 optical waveguide element
JP2019095698A (en) * 2017-11-27 2019-06-20 富士通オプティカルコンポーネンツ株式会社 Optical module and optical modulator
JP2020134874A (en) * 2019-02-25 2020-08-31 Tdk株式会社 Light modulator
JP2022056727A (en) * 2020-09-30 2022-04-11 住友大阪セメント株式会社 Optical waveguide element and optical modulator

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