Patent Application
20240176071
The present invention relates to an optical waveguide device, and an optical modulation device and an optical transmission apparatus using the same, and particularly to an optical waveguide device including an optical waveguide member that includes a substrate on which an optical waveguide is formed, and an optical component that is fixed to an end surface of the optical waveguide member on which an input port or an output port of the optical waveguide is disposed and that allows transmission of input light into the input port or transmission of output light from the output port.
In the field of optical measurement technology or in the field of optical communication technology, optical waveguide devices such as an optical modulator using a substrate on which an optical waveguide is formed have been widely used. An optical modulation element (a lithium niobate (LN) chip) is formed by forming a control electrode for controlling a light wave propagating through the optical waveguide on the optical waveguide device using the substrate of LN or the like having an electro-optic effect. The LN chip is mounted inside a case of metal or the like, and an optical lens is adhesively fixed to an end surface of the optical waveguide device to input or output the light wave into or from the optical waveguide of the optical waveguide device.
By using the optical lens, an optical modulator having good coupling efficiency between the input light or the output light and the optical waveguide can be provided. Even in a case where a position at which the LN chip is mounted in the case deviates, a low-insertion loss configuration can be implemented by adjusting a position and an angle of the lens.
In recent years, in order to deal with size reduction and band widening of an optical modulation device in which the optical waveguide device is mounted inside the case, combining two polarization beams of signal light output from the optical waveguide device has been suggested as in Patent Literature No. 1. In addition, as illustrated in
In
The optical block (optical component) is not limited to the optical lens described above and may be an optical block on which another optical member such as a reflective member or a polarizer used in polarization combining or the like is integrally formed or held, or a spacer or the like for fixing the reflective member or a component of the reflective member. In addition, the optical component adhesively fixed to the substrate 1 and to the reinforcing block 10 is not limited to the optical block and includes a configuration in which an optical fiber fixed to a holding member having a sleeve (cylindrical) shape or to a V-groove substrate is directly bonded to the end surface of the substrate 1.
As illustrated in
Meanwhile, ferroelectric materials such as LN are used in the substrate constituting the optical waveguide device, and materials such as LN are also used in the reinforcing block to match a linear expansion coefficient between the reinforcing block and the substrate 1.
On the other hand, glass (organic glass, optical glass, and the like) and plastic are used as a material of the optical component. Thus, a difference in a coefficient of thermal expansion (linear expansion coefficient) may be 5×10−6/° C. or higher among the LN substrate constituting the optical waveguide member, the holding substrate or the reinforcing block, and the optical block (optical component).
As long as the optical waveguide of the substrate 1 is not adversely affected, the same material can be used in the reinforcing block and in the holding substrate 11 to match not only the linear expansion coefficient but also Vickers hardness. Thus, surface accuracy during substrate cutting can be improved.
In addition, regarding a size of the optical waveguide device, a chip width is approximately 0.5 to 3 mm, and a total thickness of the reinforcing block and the substrate of the optical waveguide device is approximately 1 to 2 mm. Particularly, in the case of polarization combining or of a folded structure in which input and output are performed on the same end surface as illustrated in
In a case where the joined members have different coefficients of thermal expansion, internal stress is generated in the joined part because of a change in a temperature of an environment. Thus, this results in problems such as a change in optical characteristics, a crack in the joined part, or detachment of the optical component. This tendency is noticeable particularly in a case where the coefficients of thermal expansion are different between directions parallel to the surfaces to be joined, such as a thickness direction (Y axis direction) and a width direction (X axis direction) of the optical waveguide device as illustrated in
As a method of eliminating such problems, the present applicant has presented a method of further decreasing the area of the joined part as disclosed in Patent Literature No. 2.
Furthermore, in order to finish the surfaces to be joined as specular surfaces, it is required to perform polishing processing on the end surfaces after cutting the substrate, and this also poses a problem of requiring more processing time.
An object to be solved by the present invention is to solve the above problem and to provide an optical waveguide device that can prevent detachment of an optical component caused by internal stress generated in a joined portion between the optical component and an optical waveguide member including a substrate on which an optical waveguide is formed and that can simplify a manufacturing process related to polishing processing of a surface to be joined. In addition, an optical modulation device and an optical transmission apparatus using the optical waveguide device are provided.
In order to solve the object, an optical waveguide device of the present invention, and an optical modulation device and an optical transmission apparatus using the same have the following technical features.
(1) An optical waveguide device includes an optical waveguide member that includes a substrate on which an optical waveguide is formed, and an optical component that is fixed to an end surface of the optical waveguide member on which an input port or an output port of the optical waveguide is disposed and that allows transmission of input light into the input port or transmission of output light from the output port, in which a rough part having higher roughness than the end surface of the optical waveguide member including the input port or the output port is provided in a part which is at least a part of the optical waveguide member and to which the optical component is fixed.
(2) In the optical waveguide device according to (1), a notched portion is provided in a part of the optical waveguide member, and the rough part is provided on at least a part of a surface on which the notched portion is formed.
(3) In the optical waveguide device according to (2), a depth of the notched portion is set within a range of 5 μm or higher and 300 μm or lower from the end surface including the input port or the output port.
(4) In the optical waveguide device according to any one of (1) to (3), the optical component has a different coefficient of thermal expansion from at least a part of a member constituting the end surface of the optical waveguide member in a direction parallel to the end surface of the optical waveguide member.
(5) In the optical waveguide device according to any one of (1) to (4), a spot size converter is provided in at least one of the input port or the output port of the optical waveguide.
(6) In the optical waveguide device according to any one of (1) to (5), an adhesive for joining the optical component is provided on at least a part of the rough part.
(7) In the optical waveguide device according to any one of (1) to (6), roughness Ra of the rough part is 50 nm or higher.
(8) In the optical waveguide device according to any one of (1) to (7), a distance from any of the input port or the output port to a position at which the rough part is formed is set within a range of 5 μm or higher and 300 μm or lower.
(9) In the optical waveguide device according to any one of (1) to (8), at least a part of the rough part protrudes to an optical component side with respect to the input port or the output port.
(10) In the optical waveguide device according to any one of (1) to (9), at least a part of a location in which a curved surface is formed in a region in which the rough part is formed has lower roughness than an other rough part.
(11) An optical modulation device includes the optical waveguide device according to any one of (1) to (10) including an electrode that modulates a light wave propagating through the optical waveguide, a case accommodating the optical waveguide device, and an optical fiber through which the light wave is input into the optical waveguide or output from the optical waveguide.
(12) In the optical modulation device according to (11), an electronic circuit that amplifies a modulation signal to be input into the optical waveguide device is provided inside the case.
(13) An optical transmission apparatus includes the optical modulation device according to (11) or (12), and an electronic circuit that outputs a modulation signal causing the optical modulation device to perform a modulation operation.
In the present invention, an optical waveguide device includes an optical waveguide member that includes a substrate on which an optical waveguide is formed, and an optical component that is fixed to an end surface of the optical waveguide member on which an input port or an output port of the optical waveguide is disposed and that allows transmission of input light into the input port or transmission of output light from the output port, in which a rough part having higher roughness than the end surface of the optical waveguide member including the input port or the output port is provided in a part which is at least a part of the optical waveguide member and to which the optical component is fixed. Thus, joining strength to an optical component side can be increased using the rough part, and detachment of the optical component can be prevented. In addition, since the rough part other than the optical waveguide part that is required to be formed as a specular surface in terms of performance can be used on a surface to be joined, a manufacturing process related to polishing processing of the surface to be joined can be simplified.
Hereinafter, an optical waveguide device of the present invention will be described in detail using preferred examples.
As illustrated in
In the following description, an example in which a holding substrate 11 and a reinforcing block 10 are combined with the substrate 1 as the optical waveguide member will be mainly described.
As a material of the substrate 1 used in the optical waveguide device of the present invention, a substrate of a ferroelectric material having an electro-optic effect, specifically lithium niobate (LN), lithium tantalate (LT), lead lanthanum zirconate titanate (PLZT), or the like, a vapor-phase growth film formed of these materials, or the like can be used. In addition, various materials such as semiconductor materials or organic materials can also be used as the substrate of the optical waveguide device.
A thickness of the substrate 1 on which the optical waveguide is formed may be set to 10 μm or lower and more preferably 5 μm or lower to improve modulation efficiency by strengthening confinement of a light wave propagating through the optical waveguide. In this case, the holding substrate 11 having a thickness of 0.2 to 1 mm is bonded via direct joining or through an adhesive to reinforce mechanical strength of the substrate 1. LN, quartz, Si, molten glass, alumina, and the like can be applied as a material of the holding substrate.
In the optical waveguide device of the present invention, the “substrate on which the optical waveguide is formed” does not simply mean only one substrate and includes a case where a vapor-phase growth film is formed on the holding substrate and where the film (thin film) is processed into the optical waveguide.
As a method of forming the optical waveguide on the substrate 1, a method of thermally diffusing a high-refractive index material such as Ti in the substrate or a method of forming a high-refractive index part using a proton exchange method can be used. In addition, a rib type optical waveguide obtained by forming a part corresponding to the optical waveguide to have a protruding shape in the substrate can be formed using a method of etching a substrate part other than the optical waveguide, a method of forming grooves on both sides of the optical waveguide, or the like. Furthermore, the rib type optical waveguide and the optical waveguide obtained using the thermal diffusion method or the like can be used together.
In addition, in the case of using the optical waveguide having a small width, such as the optical waveguide of which a mode field diameter (MFD) is 1 μm or lower, a spot size converter (SSC) that changes the MFD can be provided in the input port or the output port of the optical waveguide as a part of the optical waveguide. As will be described later, in a case where the SSC is present, the “rough part” of the present invention is formed at a position at which the light wave propagating through the SSC is not affected.
The reinforcing block 10 is disposed to be fixed to an upper portion of the substrate 1 on an end surface side on which the input port or the output port of the light wave is formed. A material having Vickers hardness similar to that of the reinforcing substrate 11 is used in the reinforcing block 10 to improve surface accuracy during substrate cutting. As long as the optical waveguide of the substrate 1 is not adversely affected, the same material as the holding substrate 11 can be used to match not only the Vickers hardness but also a linear expansion coefficient. An end surface (a surface on the same side as the end surface of the substrate 1) of the reinforcing block 10 is used as a surface to be joined to which the optical component such as an optical block adheres, as necessary.
The optical component includes the optical block that holds an optical lens, a reflective member, a polarizer, or the like, a holding member having a sleeve (cylindrical) shape or a V-groove substrate that holds an optical fiber near an end portion of the optical fiber, and the like. A glass material such as organic glass or optical glass or a plastic material is used as a material constituting the optical component.
An LN substrate has a coefficient of thermal expansion (linear expansion coefficient) of 4.0×10−6/° ° C. in a Z axis direction and of 14.0×10−6/° ° C. in an X axis (Y axis) direction. In a case where the optical component includes, for example, an optical glass material such as BK-7, the linear expansion coefficient is 7.1×10−6/° C. In a case where the optical component is bonded to the LN substrate, presence of an X axis or of a Y axis on a surface of the LN substrate to be joined results in a difference in the linear expansion coefficient of 5.0×10−6/° ° C. or higher between both of the LN substrate and the optical component, and the difference is noticeable. Consequently, a positional deviation of the optical component or peeling or detachment of the optical component occurs in accordance with a change in a temperature of the substrate or of an environment or the like.
In addition, a material having a linear expansion coefficient similar to that of the substrate used in the optical waveguide device is selected for a case of metal or the like that accommodates the optical waveguide device. While stainless steel is widely used in the case of the LN substrate, a linear expansion coefficient of stainless steel is 17.3×10−6/° C., which is significantly different from the linear expansion coefficient of the optical component. Thus, the optical component is not joined to the case and is only joined to the substrate 1 or to the reinforcing block 10 to be held. In a case where an element of quartz glass or the like having a low linear expansion coefficient is used in the holding substrate 11, a material such as Kovar having a low linear expansion coefficient may be used in the case.
As illustrated in
In addition, while a state where the “rough part” has entered a substrate 1 side with respect to the end surface B0 on which the input port or the output port is formed is illustrated in
Furthermore, a depth (a distance from the end surface B0 to the end surface B2 in
In
As illustrated in
In addition, in providing the curved surface, in a case where the curved surface has the same or higher surface roughness than the nearby rough part, concentration of stress on the curved surface is likely to occur because of the internal stress of the adhesive that has entered the part. Thus, this causes a crack or the like on the curved surface. In order to avoid this, roughness of the curved surface may be finished to be smoother than an other rough part (B1 to B6).
A shape of the “notched portion” is not limited to that in
An example of a method of forming the end surface including the “rough part” as illustrated in
In addition, in finishing the curved surface R in
An example of a method of forming the end surface including the “rough part” as illustrated in
While an example of applying the cutting blade perpendicular to the surface of the substrate 1 or the like is illustrated in
The SSC may be obtained by directly processing the optical waveguide by, for example, gradually changing a waveguide width of an end portion of the optical waveguide formed on the substrate 1 or by forming the SSC as a separate body on the end portion of the optical waveguide formed on the substrate 1. In the former configuration, the optical component is fixed to the end portion of the optical waveguide on which the SSC is formed, the holding substrate, or the reinforcing block. In the latter configuration, the optical component is fixed to the SSC as a separate body from the optical waveguide, the holding substrate, or the reinforcing block. In the configuration in which the SSC is formed, the SSC also constitutes a part of the optical waveguide member.
Furthermore, roughness of the rough part (D) may be 50 nm or higher in arithmetic average roughness Ra from the viewpoint of increasing joining strength of the adhesive. Roughness of the end surface of the substrate 1 on which the input port or the output port is formed is required to be set to 10 nm or lower.
In the region D in which the rough part can be formed in
In the optical waveguide device of the present invention, a modulation electrode that modulates the light wave propagating through the optical waveguide is provided in the substrate 1 and is accommodated inside a case CA as illustrated in
An optical transmission apparatus OTA can be configured by connecting, to the optical modulation device MD, an electronic circuit (digital signal processor DSP) that outputs a modulation signal causing the optical modulation device MD to perform a modulation operation. The modulation signal to be applied to the optical waveguide device is required to be amplified. Thus, a driver circuit DRV is used. The driver circuit DRV and the digital signal processor DSP can be disposed outside the case CA or can be disposed inside the case CA. Particularly, disposing the driver circuit DRV inside the case can further reduce a propagation loss of the modulation signal from the driver circuit.
As described above, according to the present invention, it is possible to provide an optical waveguide device that can prevent detachment of an optical component caused by internal stress generated in a joined portion between the optical component and an optical waveguide member including a substrate on which an optical waveguide is formed and that can simplify a manufacturing process related to polishing processing of surfaces to be joined. In addition, an optical modulation device and an optical transmission apparatus using the optical waveguide device can be provided.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-058578 | Mar 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/015879 | 3/30/2022 | WO |
