Optical waveguide element, optical modulation device using the same, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element which is provided with a dielectric layer covering an optical waveguide and in which occurrence of defects such as peeling and cracking of the dielectric layer is suppressed.　This optical waveguide element comprises an optical waveguide 2 formed on a substrate 1 and a dielectric layer IL covering the optical waveguide, and is characterized in that the optical waveguide 2 is a rib-type optical waveguide, and at least part of a side surface along the longitudinal direction of the rib-type optical waveguide has a slope shape formed by a curved surface (R6).

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element and an optical modulation device and an optical transmission device using the same, and more particularly to an optical waveguide element having an optical waveguide formed on a substrate and a dielectric layer covering the optical waveguide. [Background technology]

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements such as optical modulators using substrates with electro-optical effects are widely used. In particular, with the increase in the amount of information communication in recent years, there is a demand for faster and larger capacity optical communication between long distance cities and data centers. Furthermore, there is a space restriction in base stations, so there is a need to increase the speed and size of optical modulators.

[0003] In order to miniaturize optical modulators, the light confinement effect can be increased by narrowing the width of the optical waveguide, which results in a smaller bending radius of the optical waveguide and makes it possible to miniaturize the device. For example, lithium niobate (LN), which has an electro-optic effect, is used as an optical modulator for long distances because it has little distortion and little optical loss when converting an electrical signal into an optical signal. In the conventional optical waveguide of an LN optical modulator, the mode field diameter (MFD) is about 10 μm, and the bending radius of the optical waveguide is large, at several tens of mm, making it difficult to miniaturize the device.

[0004] In recent years, improvements in substrate polishing and bonding technologies have made it possible to make LN substrates thinner, and research and development is underway to reduce the MFD of optical waveguides to less than 3 μm, around 1 μm. As the MFD becomes smaller, the light confinement effect also increases, allowing the bending radius of the optical waveguide to be made smaller.

[0005] As the width and height of the optical waveguide become smaller, the roughness of the surface of the optical waveguide has a significant effect on the optical loss of the light waves propagating through the optical waveguide. For example, when forming a convex optical waveguide (called a rib-type optical waveguide), the etching speed and etching temperature tend to cause surface roughness due to minute irregularities on the side surfaces of the convex portions. In order to solve such problems, Patent Document 1 proposes providing a dielectric layer (insulating film) that covers the optical waveguide.

[0006] On the other hand, when using a fine optical waveguide having an MFD smaller than 10 μmφ, which is the MFD of an optical fiber, a large insertion loss occurs when the end (element end face) of the optical waveguide provided in the optical waveguide element is directly joined to an optical fiber.

[0007] In order to solve such problems, a spot size converter (SSC) is arranged at the end of the optical waveguide in Patent Document 2. As an example of the SSC, one proposed is one in which the SSC is configured by a block body (dielectric film) that covers the optical waveguide.

[0008] 1 shows an example of an optical waveguide element disclosed in Patent Document 3, which integrates multiple Mach-Zehnder type optical waveguides and has an SSC, and the optical waveguide element can also be used for a high bandwidth coherent driver modulator (HB-CDM) or the like. In an optical waveguide portion including a modulation section MP that applies a modulation signal to an optical waveguide 2 to modulate a light wave, a dielectric layer (insulating film) IL is disposed on the optical waveguide 2, as in Patent Document 1. In addition, in the region indicated by SSC, a block body (dielectric film) of the SSC is used, as in Patent Document 2. Note that Lin indicates incident light, and Lout indicates outgoing light.

[0009] Fig. 2 is an enlarged plan view of the portion enclosed by the dotted line A in Fig. 1, and shows an example of the configuration of the surrounding area including the SSC. Fig. 3 is a cross-sectional view taken along the dotted line CC' in Fig. 2, Fig. 4 is a cross-sectional view taken along the dotted line BB' in Fig. 2, and Fig. 5 is a cross-sectional view taken along the dotted line A-A' in Fig. 2.

[0010] 3 shows the same structure as an optical waveguide including a modulator MP in an optical waveguide element, with a rib-type optical waveguide 2 formed in a part of a substrate 1. A dielectric layer IL is disposed so as to cover the side and top surfaces of the optical waveguide 2. Since the substrate 1 and the optical waveguide 2 are extremely thin layers, a reinforcing substrate 3 is disposed on the bottom side of the substrate 1 to increase the mechanical strength.

[0011] As shown in Fig. 2, the width of the optical waveguide 2 and the substrate 1 have a tapered shape that gradually narrows toward the end of the substrate. For this reason, in Fig. 3, the rib-type optical waveguide 2 functions as the core part of the optical waveguide, but in Fig. 4, the rib-type optical waveguide 2 and the substrate 1 play the role of the core part. Furthermore, in Fig. 5, the dielectric layer IL also plays the role of the core part, and the MFD of the optical waveguide gradually expands. In this way, the MFD can be made smaller by narrowing the width of the dielectric layer, so that it is possible to control the MFD to the desired size and reduce the optical insertion loss with optical fibers, etc.

[0012] 3 to 5, the width of the dielectric layer IL narrows and the cross-sectional area (surface area) occupied by the rib-type optical waveguide 2 and the substrate 1 also decreases. In general, as the width of the dielectric layer IL narrows, the adhesion between the dielectric layer IL and the substrate 1 (optical waveguide 2) and between the dielectric layer IL and the reinforcing substrate 3 decreases, and phenomena such as peeling and cracking of the dielectric layer IL occur. This phenomenon is particularly noticeable in portions where the width of the substrate 1 (optical waveguide 2) and the dielectric layer IL are narrow, such as in SSC.

[0013] In the above explanation, examples have been described in which the substrate 1, the optical waveguide 2, or the dielectric layer IL gradually narrows in width toward the end of the optical waveguide. However, even if the width gradually increases, defects such as peeling can occur if the width itself is narrow. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Patent Application No. 2021-050409 (Application date: March 24, 2021) [Patent Document 2] Patent Application No. 2020-165004 (Application date: September 30, 2020) [Patent Document 3] PCT / JP2021 / 032007 (Application date: August 31, 2021) Summary of the Invention [Problem to be solved by the invention]

[0015] The problem to be solved by the present invention is to provide an optical waveguide element having a dielectric layer covering an optical waveguide, which suppresses the occurrence of defects such as peeling or cracking of the dielectric layer, and further to provide an optical modulation device and an optical transmission device using the optical waveguide element. [Means for solving the problem]

[0016] In order to solve the above problems, the optical waveguide element and the optical modulation device and optical transmission device using the same according to the present invention have the following technical features. (1) An optical waveguide element having an optical waveguide formed on a substrate and a dielectric layer covering the optical waveguide, the optical waveguide being a rib-type optical waveguide, A spot size conversion section including the rib type optical waveguide and the dielectric layer is formed at an end of the optical waveguide, and in the spot size conversion section, the width of the rib type optical waveguide decreases toward the end of the substrate, and the thickness of the rib type optical waveguide decreases. Accordingly, the dielectric layer functions as an optical waveguide, and At least a part of the side surface along the longitudinal direction of the rib type optical waveguide is formed into a slope shape formed by a curved surface.

[0017] (2) The optical waveguide element according to (1) above, wherein the cross-sectional shape of the rib-type optical waveguide perpendicular to the direction of propagation of the light waves is a trapezoid, a triangle, or a shape stacked in multiple stages, and at least a part of the side protruding laterally is formed by a curve.

[0020] ( 3 ) Above (1) Or (2) In the optical waveguide element described in the above, the refractive index of the dielectric layer is smaller than the refractive index of the rib-type optical waveguide. (4) In the optical waveguide element described in any one of (1) to (3) above, the substrate is composed of a thin plate on which the rib-type optical waveguide is formed and a reinforcing substrate bonded to the thin plate, and in the spot size conversion portion, the dielectric layer is in contact with the reinforcing substrate except for a portion covering the thin plate, and the slope shape is formed in the thin plate including the rib-type optical waveguide.

[0021] ( 5 ) (1) to ( 4 The optical waveguide element described in any one of the above is an optical modulation device, characterized in that the optical waveguide element is accommodated in a housing and includes an optical fiber that inputs or outputs a light wave to or from the optical waveguide.

[0022] ( 6 ) the above( 5 ) is characterized in that the optical waveguide element has a modulation electrode for modulating a light wave propagating through the optical waveguide, and has an electronic circuit inside the housing for amplifying a modulation signal input to the modulation electrode of the optical waveguide element.

[0023] ( 7 ) the above( 5 ) or ( 6 and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation. Effect of the Invention

[0024] The present invention provides an optical waveguide element having an optical waveguide formed on a substrate and a dielectric layer covering the optical waveguide, wherein the optical waveguide is a rib-type optical waveguide, and at least a portion of the side surface along the longitudinal direction of the rib-type optical waveguide has a sloping shape formed by a curved surface. This makes it possible to increase the contact area between the dielectric layer and the rib-type optical waveguide, thereby improving the adhesion between the two and suppressing defects such as peeling or cracking of the dielectric layer. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a plan view showing an example of an optical waveguide element having a dielectric layer covering an optical waveguide, as disclosed in Patent Document 3. [Diagram 2] FIG. 2 is an enlarged plan view of a portion enclosed by a dotted line in FIG. [Diagram 3] 3 is a cross-sectional view taken along dotted line CC' in FIG. 2. [Figure 4]3 is a cross-sectional view taken along dotted line BB' in FIG. 2. [Diagram 5] 3 is a cross-sectional view taken along dotted line AA' in FIG. 2. [Figure 6] FIG. 3 is a cross-sectional view taken along dotted line AA' in FIG. 2, showing a first embodiment according to the present invention. [Figure 7] FIG. 3 is a cross-sectional view taken along dotted line AA' in FIG. 2, showing a second embodiment of the present invention. [Figure 8] 3 is a cross-sectional view taken along dotted line BB' in FIG. 2, showing a third embodiment of the present invention. FIG. [Figure 9] FIG. 3 is a cross-sectional view taken along dotted line BB' in FIG. 2, showing a fourth embodiment of the present invention. [Figure 10] FIG. 5 is a cross-sectional view taken along the dotted line CC' in FIG. 2, showing a fifth embodiment of the present invention. [Figure 11] FIG. 13 is a diagram showing an application example (sixth embodiment) of the optical waveguide element of the present invention. [Figure 12] FIG. 13 is a diagram showing an application example (seventh embodiment) of the optical waveguide element of the present invention. [Figure 13] FIG. 13 is a diagram showing an application example (Eighth Example) of the optical waveguide element of the present invention. [Figure 14] 1 is a plan view illustrating an optical modulation device and an optical transmission device according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The optical waveguide element of the present invention will be described in detail below using preferred examples. As shown in Figures 6 to 13, the optical waveguide element of the present invention has an optical waveguide 2 formed on a substrate 1 and a dielectric layer IL covering the optical waveguide, and is characterized in that the optical waveguide 2 is a rib-type optical waveguide, and at least a part of the side surface along the longitudinal direction of the rib-type optical waveguide has a slope shape formed by curved surfaces (R1 to R9).

[0027] In the present invention, a "rib type optical waveguide" refers to a portion that has a convex cross-sectional shape and functions as an optical waveguide, as shown in Figures 6 to 13, and may include not only a portion 2 protruding from a substrate 1 as shown in Figure 10, but also the substrate 1 as well as the protruding portion 2 inside an SSC, as shown in Figures 8 and 9. Furthermore, there may be only the substrate 1, as shown in Figures 6 and 7. However, the "rib type optical waveguide" does not include a dielectric layer IL.

[0028] The material 1 having an electro-optic effect used in the optical waveguide element of the present invention may be a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a base material obtained by doping these substrate materials with magnesium. Vapor-grown films made of these materials may also be used. Also, various materials such as semiconductor materials and organic materials can be used as optical waveguides.

[0029] The optical waveguide 2 can be formed by etching the substrate 1 other than the optical waveguide, forming grooves on both sides of the optical waveguide, or by using a rib-type optical waveguide in which the portion of the substrate corresponding to the optical waveguide is made convex. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to increase the refractive index of the substrate surface by thermal diffusion of Ti or proton exchange.

[0030] The thickness of the substrate (thin plate) 1 on which the optical waveguide 2 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 4 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less or 0.4 μm or less. It is also possible to form a vapor-grown film on the reinforcing substrate 3 and process the film into the shape of the optical waveguide.

[0031] In order to increase the mechanical strength of the substrate on which the optical waveguide is formed, the substrate is bonded to a reinforcing substrate 3 by direct bonding or via an adhesive layer such as resin, as shown in Figures 3 to 10. As the reinforcing substrate 3 to be directly bonded, a substrate containing an oxide layer such as quartz or glass, which has a material with a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed and a thermal expansion coefficient close to that of the optical waveguide, is preferably used. Composite substrates, abbreviated as SOI or LNOI, in which a silicon oxide layer is formed on a silicon substrate or a silicon oxide layer is formed on an LN substrate, can also be used.

[0032] The optical waveguide 2 in Fig. 2 is covered with a dielectric layer (insulating film) IL shown in Patent Document 1. As shown in Fig. 10, the top and side surfaces of the rib type optical waveguide 2 are covered with the dielectric layer IL. The dielectric layer IL is preferably a dielectric material having a refractive index greater than 1, and is set to 0.5 to 0.75 times the refractive index of the optical waveguide 2. The thickness of the dielectric layer IL is not particularly limited, but it can be formed to a thickness of about 10 μm. In the optical waveguide portion (excluding SSC) including the modulation section MP that applies a modulation signal to the optical waveguide 2 to modulate the light wave, the optical waveguide 2 functions as a core section, and the dielectric layer functions as a cladding section.

[0033] The dielectric layer IL can be formed by sputtering or CVD using inorganic materials such as SiO2, but organic materials such as resins can also be used. As for resins, photoresists containing coupling agents (crosslinking agents) can be used, and so-called photosensitive insulating films (permanent resists) that harden as a result of a crosslinking reaction caused by heat can be used. Note that other materials such as polyamide resins, melamine resins, phenol resins, amino resins, and epoxy resins can also be used as resins.

[0034] In FIG. 2, the dielectric layer IL is disposed across the optical waveguide 2 (right side of the drawing) and the spot size conversion section SSC (left side of the drawing). The present invention is not limited to this example, and it is also possible to use different dielectric layers on the optical waveguide side and the SSC side. However, when the refractive indexes of the dielectric layers are different, a propagation loss of the light wave is likely to occur at the boundary between the dielectric layers, so it is desirable that the refractive index difference of the dielectric layers at the boundary is set to a predetermined value or less, for example, 0.5 or less. More preferably, the dielectric layer IL covering the optical waveguide is continuously inserted into the spot size conversion section as a part of the dielectric layer constituting the spot size conversion section from the modulation section side of the optical waveguide. Even more preferably, the dielectric layer of the optical waveguide on the modulation section side and a part of the dielectric layer constituting the spot size conversion section are simultaneously formed in the same manufacturing process.

[0035] In the spot size conversion portion SSC, the dielectric layer IL, together with the optical waveguide 2 and the substrate 1, functions as a part of the optical waveguide, in particular as a core portion of the optical waveguide.

[0036] The width of the dielectric layer IL constituting the SSC in Fig. 2 is tapered from the viewpoint of mode diameter conversion and optical confinement. The width is approximately 5 μm at the left end position in Fig. 2. On the other hand, in the modulation section, the width of the dielectric layer IL is 10 μm or more from the viewpoint of adhesion, etc., and therefore the horizontal width of the dielectric layer IL in the modulation section is wider than that of the SSC.

[0037] In FIG. 2, the width of the optical waveguide (convex portion 2) and the substrate 1 are gradually changed in a tapered shape, but this is not limited thereto, and the thickness of the optical waveguide 2 and the substrate 1 may be gradually made thinner or thicker, or a combination of both may be used.

[0038] The optical waveguide element of the present invention is characterized in that at least a part of the side surface along the longitudinal direction of the rib-type optical waveguide is formed into a slope shape formed by a curved surface (R1 to R9) as shown in Figures 6 to 13. By providing such a curved surface (a curved boundary line in a cross-sectional view), the contact area between the dielectric layer and the rib-type optical waveguide is increased, and the adhesion between them can be improved.

[0039] 6, the substrate 1 serves as a rib-type optical waveguide, and has a curved surface R1 formed on its side surface. The cross-sectional shape of the rib-type optical waveguide is substantially triangular. 7, the cross-sectional shape of the rib type optical waveguide (substrate 1) is substantially trapezoidal. A curved surface R2 is formed on the side (side edge) that protrudes in the horizontal direction of the trapezoid.

[0040] The cross-sectional shape of the rib-type optical waveguide in the present invention (the cross-sectional shape perpendicular to the propagation direction of the light waves) is a trapezoid, a triangle, or a shape stacked in multiple stages, and at least a part of the side extending laterally may be formed as a curve.

[0041] In the third embodiment of Fig. 8, the shape is formed by stacking trapezoids, and a curved surface R3 is provided on the side surface of the lower trapezoid. In the fourth embodiment of Fig. 9, curved surfaces R4 and R5 are formed on the side surfaces of the upper and lower trapezoids. Figs. 8 and 9 are cross sections taken along dotted line B-B' in Fig. 2, but such a shape stacked in multiple stages can also be formed, for example, on the substrate 1 of dotted line A-A' or the optical waveguide (convex portion 2) of dotted line C-C'.

[0042] In the fifth embodiment of FIG. 10, a curved surface R6 is formed on the trapezoidal side surface of a convex portion 2 (rib-type optical waveguide) formed on a substrate 1. Furthermore, as shown in Fig. 11, it is possible to form a curved surface R7 on a portion of the multiple steps, or as shown in Fig. 12, it is possible to make a portion of the same side surface flat (the boundary of the cross section is a straight line) and another portion a curved surface R8. It is also possible to make it a curved surface R9 that bulges outward as shown in Fig. 13.

[0043] A method for forming a curved surface as shown in Figures 6 to 13 can be performed by patterning a desired etching mask having a curved surface and performing a dry etching method such as reactive ion etching (RIE) or a wet etching method using a suitable etching solution.

[0044] The optical waveguide element of the present invention is provided with a modulation electrode that modulates the light wave propagating through the optical waveguide 2, and is housed in a housing CA as shown in Fig. 14. Furthermore, an optical modulation device MD can be configured by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide. In Fig. 14, the optical fiber F is optically coupled to the optical waveguide in the optical waveguide element using an optical lens 4. However, the optical fiber may be introduced into the housing through a through hole penetrating the side wall of the housing and directly joined to the optical waveguide element.

[0045] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal that causes the optical modulation device MD to perform a modulation operation to the optical modulation device MD. Since the modulation signal applied to the optical waveguide element needs to be amplified, a driver circuit DRV is used. 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. [Industrial Applicability]

[0046] As described above, according to the present invention, it is possible to provide an optical waveguide element having a dielectric layer covering an optical waveguide, which suppresses the occurrence of defects such as peeling or cracking of the dielectric layer, and further to provide an optical modulation device and an optical transmission device using the optical waveguide element. [Explanation of symbols]

[0047] 1. Substrate (thin plate, film) on which the optical waveguide is formed 2 Optical waveguide IL Dielectric layer MP Modulation Section SSC Spot size conversion section

Claims

1. An optical waveguide element having an optical waveguide formed on a substrate and a dielectric layer covering the optical waveguide, the optical waveguide is a rib-type optical waveguide, a spot size conversion portion including the rib-type optical waveguide and the dielectric layer is formed at an end of the optical waveguide; In the spot size conversion portion, the width of the rib-type optical waveguide decreases toward the end of the substrate, and the thickness of the rib-type optical waveguide decreases, and accordingly, the dielectric layer functions as an optical waveguide; The optical waveguide element is characterized in that at least a part of a side surface of the spot size conversion portion along the longitudinal direction of the rib-type optical waveguide is formed into a slope shape formed by a curved surface.

2. 2. The optical waveguide element according to claim 1, wherein the cross-sectional shape of the rib-type optical waveguide perpendicular to the propagation direction of the light waves is a trapezoid, a triangle, or a shape stacked in multiple stages, and at least a part of the side extending laterally is formed by a curve.

3. 3. The optical waveguide element according to claim 1, wherein the refractive index of said dielectric layer is smaller than the refractive index of said rib-type optical waveguide.

4. The optical waveguide element according to any one of claims 1 to 3, the substrate is composed of a thin plate on which the rib-type optical waveguide is formed and a reinforcing substrate bonded to the thin plate; the spot size conversion portion is in contact with the reinforcing substrate except for a portion of the dielectric layer covering the thin plate, The optical waveguide element is characterized in that the slope shape is formed on the thin plate including the rib-type optical waveguide.

5. The optical waveguide element according to any one of claims 1 to 4, The optical waveguide element is housed in a housing, An optical modulation device comprising an optical fiber for inputting or outputting a light wave to or from said optical waveguide.

6. 6. The optical modulation device according to claim 5, the optical waveguide element includes a modulation electrode for modulating an optical wave propagating through the optical waveguide; An optical modulation device comprising: an electronic circuit disposed inside the housing for amplifying a modulation signal input to a modulation electrode of the optical waveguide element.

7. An optical modulation device according to claim 5 or 6; and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation.

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