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

Abstract

An optical waveguide element includes a rib-type optical waveguide (10) formed of a material having an electro-optic effect, and a reinforcing substrate (3) that supports the optical waveguide. A spot size conversion section (2) that changes a mode field diameter of an optical wave propagating in the optical waveguide is provided at one end of the optical waveguide. The optical waveguide element includes a structure (S) that is disposed apart from the spot size conversion section across the spot size conversion section and is disposed on the reinforcing substrate (3), and an upper substrate (4) that is disposed on an upper side of the spot size conversion section (2) and the structure (S). A height (H) of the structure (S) is set to be equal to or greater than a maximum height of the spot size conversion section (2).

Description

Technical Field

[0001] The present invention relates to optical waveguide elements and optical modulation devices and optical transmission apparatuses using the same, and more particularly to optical waveguide elements having ribbed optical waveguides and reinforcing substrates supporting the optical waveguides. Background Technology

[0002] In the fields of optical measurement and optical communication technologies, optical waveguide components such as optical modulators that utilize substrates exhibiting electro-optic effects are widely used. In particular, with the increasing volume of information communication in recent years, there is a growing demand for high-speed and high-capacity optical communication for long-distance intercity or data center applications. Furthermore, the space constraints of base stations necessitate the high-speed and miniaturized development of optical modulators.

[0003] For miniaturization of optical modulators, narrowing the width of the optical waveguide increases the light containment effect, thereby reducing the bending radius of the waveguide and achieving miniaturization. For example, lithium niobate (LN), which exhibits electro-optic effects, is used as an optical modulator for long distances due to its minimal deformation and light loss when converting electrical signals to optical signals. In existing LN optical modulators, the mode field diameter (MFD) is around 10 μm, and the bending radius is as large as tens of millimeters, making miniaturization difficult.

[0004] In recent years, advancements in substrate polishing and bonding technologies have enabled the thinning of LN substrates, and the MFD (Mesh Deposition Factor) of optical waveguides has been researched and developed to be below 3μm and around 1μm. As the MFD decreases, the light containment effect increases, thus allowing for a smaller bending radius in the optical waveguide.

[0005] On the other hand, when using an MFD with a fiber optic cable, i.e. In the case of small MFDs with micro-waveguides, if the end of the waveguide (the end face of the element) is directly connected to the optical fiber, a large insertion loss will occur.

[0006] To eliminate this undesirable situation, it is considered to configure a spot size conversion section (spot size converter, SSC) at the end of the optical waveguide. A typical SSC is configured to expand the tapered shape of the optical waveguide portion in two or three dimensions. For reference, examples of tapered waveguides are shown in Patent Documents 1 to 3.

[0007] In tapered waveguides where the spot size increases with the size of the waveguide core, adjusting the refractive index of the core and cladding to achieve the desired spot size is challenging and can easily lead to multimode propagation. Therefore, the SSC (Self-Containing Waveguide Component) as an optical waveguide element has limitations in its usable designs. Furthermore, to achieve the required spot size, a relatively long tapered section needs to be formed, making miniaturization of the optical waveguide element difficult.

[0008] Moreover, the applicant has the right to... Figures 1 to 3 The SSC shown was studied, in which the front end of the ribbed optical waveguide 10 was formed into a tapered shape 12 with a narrower width, and a block portion 2, which serves as the core, was arranged to surround the tapered shape 12. The refractive index of the block portion 2 was set to be lower than that of the optical waveguide 10, and the block portion was surrounded by an organic material (5) with a refractive index about 0.01 to 0.03 lower than that of the block portion 2. This organic material can be made of a material formed by curing an adhesive, etc. When the SSC is covered by the block portion 2, it becomes the tapered shape 12 of the optical waveguide 10, thereby reducing the effective refractive index of the optical waveguide 10, weakening the light containment, and thus transferring the optical mode to the block portion 2, achieving a larger MFD than that of the optical waveguide 10.

[0009] Figure 2 It shows Figure 1 The sectional views at the dashed lines A-A' and B-B' in the diagram. Figure 3 It shows Figure 1 The cross-sectional view at the dashed line C-C'. Reference numeral 1 is a thin plate (film) of a material with electro-optic effect, such as lithium niobate, and the optical waveguide 10 is formed on the ribs remaining after partial removal of the thin plate. Reference numeral 3 is a reinforcing substrate supporting the thin plate 1 containing the optical waveguide 10.

[0010] Reference numeral 4 refers to the upper substrate that serves as a reinforcing member when connecting optical fibers or optical modules to the end face of the optical waveguide element. Reference numeral 5 refers to the organic cured material formed by curing the adhesive used to bond the reinforcing substrate 3 and the upper substrate 4. Additionally, reference numeral 11 indicates a portion of the thin plate 1 that remains after etching during the formation of the tapered portion 12 of the optical waveguide.

[0011] In patent documents 1 to 3 or Figures 1 to 3 The SSC shown not only contains a ribbed optical waveguide, but also a tapered portion, as described in Patent Document 3, which is thicker than the optical waveguide and protrudes significantly upwards from the surface of the reinforcing substrate 3. Figure 2 The block portion 2, etc. Thus, for the portion that protrudes significantly from the surface of the reinforcing substrate 3, it is difficult to attach the upper substrate 4 parallel to the surface of the reinforcing substrate 3 when attaching it due to the presence of this protrusion. Furthermore, when pressing the upper substrate 4 from above for bonding, pressure is concentrated on the protrusion, causing damage to it. Additionally, if the upper substrate 4 is not attached parallel to the surface, resulting in uneven adhesive layer thickness, uneven MFD (Medium Adhesive Deposition) or detachment of the upper substrate 4 due to thermal stress may occur.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2006-284961

[0015] Patent Document 2: Japanese Patent Application Publication No. 2007-264487

[0016] Patent Document 3: International Publication No. WO2012 / 042708 Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] The problem to be solved by the present invention is to address the issues described above by providing an optical waveguide element that, even when a spot size conversion section is provided at the end of the optical waveguide, allows the upper substrate to be bonded parallel to the reinforcing substrate, preventing damage to the spot size conversion section. Furthermore, by homogenizing the adhesive layer thickness, the MFD (Medium-to-Flat Diode) is stabilized, thereby homogenizing thermal stress and suppressing peeling of the upper substrate. Moreover, an optical modulation device and an optical transmission apparatus using this optical waveguide element are also provided.

[0019] Technical solutions for solving the problem

[0020] In order to solve the above-mentioned problems, the optical waveguide element of the present invention, the optical modulation device and the optical transmission device using the same have the following technical features.

[0021] (1) An optical waveguide element comprising: a ribbed optical waveguide formed of a material having an electro-optic effect; and a reinforcing substrate supporting the optical waveguide, characterized in that a spot size conversion section is provided at one end of the optical waveguide, the spot size conversion section changing the mode field diameter of a light wave propagating in the optical waveguide, the optical waveguide element comprising: a structure disposed separately from the spot size conversion section and disposed on the reinforcing substrate; and an upper substrate disposed above the spot size conversion section and the structure, the height of the structure being set to be greater than or equal to the maximum height of the spot size conversion section.

[0022] (2) The optical waveguide element according to (1) above is characterized in that the space formed by the spot size conversion part, the structure and the upper substrate is filled with an adhesive.

[0023] (3) The optical waveguide element according to (2) above is characterized in that a groove is formed on the surface of the structure opposite to the upper substrate to allow the adhesive to flow laterally.

[0024] (4) The optical waveguide element according to (2) or (3) above is characterized in that an adhesive outflow prevention unit for suppressing adhesive outflow is provided in the direction away from the light spot size conversion part of the optical waveguide.

[0025] (5) The optical waveguide element according to (4) above is characterized in that the surface of the optical waveguide from the adhesive outflow prevention unit to the side opposite to the spot size conversion part is covered with the same material as the adhesive outflow prevention unit.

[0026] (6) The optical waveguide element according to (4) or (5) above, characterized in that the refractive index of the material of the adhesive outflow prevention unit is lower than the refractive index of the material constituting the rib-shaped optical waveguide.

[0027] (7) An optical modulation device, characterized in that an optical waveguide element as described in any one of (1) to (6) is housed in a housing, the optical modulation device having an optical fiber for inputting or outputting optical waves to the optical waveguide.

[0028] (8) The optical modulation device according to (7) above, characterized in that the optical waveguide element has a modulation electrode for modulating the light wave propagating in the optical waveguide, and an electronic circuit for amplifying the modulation signal output to the modulation electrode of the optical waveguide element is provided inside the housing.

[0029] (9) An optical transmitting device, characterized in that it comprises: an optical modulation device as described in (7) or (8) above; and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

[0030] Invention Effects

[0031] This invention provides an optical waveguide element comprising: a ribbed optical waveguide formed of a material having an electro-optic effect; and a reinforcing substrate supporting the optical waveguide, wherein a spot size conversion section is provided at one end of the optical waveguide, the spot size conversion section changing the mode field diameter of a light wave propagating in the optical waveguide; the optical waveguide element comprising: a structure disposed separately from the spot size conversion section, separated from it, and disposed on the reinforcing substrate; and an upper substrate disposed above the spot size conversion section and the structure, the height of the structure being set to be greater than or equal to the maximum height of the spot size conversion section, thus the structure can support the upper substrate parallel to the reinforcing substrate, and can also suppress the contact between the upper substrate and the spot size conversion section. Attached Figure Description

[0032] Figure 1 This is a top view showing an example of a spot size conversion section used in optical waveguide components.

[0033] Figure 2 yes Figure 1 The sectional view at the dashed lines A-A' and B-B'.

[0034] Figure 3 yes Figure 1 The cross-sectional view at the dashed line C-C'.

[0035] Figure 4 This is a top view showing the first embodiment of the optical waveguide element of the present invention.

[0036] Figure 5 yes Figure 4 The sectional view at the dashed lines A-A' and B-B'.

[0037] Figure 6 This is a top view showing a second embodiment of the optical waveguide element of the present invention.

[0038] Figure 7 yes Figure 6 The sectional view at the dashed lines A-A' and B-B'.

[0039] Figure 8 This is a top view showing the third embodiment of the optical waveguide element of the present invention.

[0040] Figure 9 yes Figure 6 The cross-sectional view at the dashed line C-C'.

[0041] Figure 10 This is a top view illustrating the optical modulation device and optical transmission apparatus of the present invention. Detailed Implementation

[0042] Hereinafter, the optical waveguide element of the present invention will be described in detail using preferred examples.

[0043] like Figure 4 and Figure 5 As shown, the optical waveguide element of the present invention comprises: a ribbed optical waveguide 10 formed of a material having an electro-optic effect; and a reinforcing substrate 3 supporting the optical waveguide. The waveguide is characterized by having a spot size conversion section 2 at one end, which changes the mode field diameter of the light wave propagating in the optical waveguide. The optical waveguide element comprises: a structure S disposed separately from the spot size conversion section, separated from it, and disposed on the reinforcing substrate 3; and an upper substrate 4 disposed above the spot size conversion section 2 and the structure S, wherein the height of the structure S is set to be greater than or equal to the maximum height of the spot size conversion section 2.

[0044] The material 1 with electro-optic effect used in the optical waveguide element of the present invention can be a substrate such as lithium niobate (LN) or lithium tantalate (LT), PLZT (lead lanthanum zirconate titanate), or a vapor-grown film formed from these materials.

[0045] In addition, various materials such as semiconductor materials or organic materials can also be used as optical waveguides.

[0046] As a method for forming the optical waveguide 10, a rib-shaped optical waveguide can be formed on the substrate 1 other than the optical waveguide by etching or forming grooves on both sides of the optical waveguide, so that the portion corresponding to the optical waveguide is convex. Moreover, in accordance with the rib-shaped optical waveguide, Ti or the like can be diffused to the substrate surface by thermal diffusion or proton exchange, thereby further increasing the refractive index.

[0047] To achieve velocity matching between the microwave and light waves in the modulation signal, the thickness of the substrate (thin plate) 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. Furthermore, the height of the ribbed 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. Alternatively, a vapor-grown film may be formed on the reinforcing substrate 3, and this film may be processed into the shape of an optical waveguide.

[0048] To improve mechanical strength, the substrate with the optical waveguide is bonded and fixed to the reinforcing substrate 3 by direct bonding or an adhesive layer such as resin. As the reinforcing substrate 3 for direct bonding, a substrate containing an oxide layer of a material with a lower refractive index than the optical waveguide and the substrate with the optical waveguide, and a thermal expansion coefficient close to that of the optical waveguide, such as crystal or glass, is preferably used. Composite substrates formed by forming a silicon oxide layer on a silicon substrate (abbreviated as SOI or LNOI) or on an LN substrate can also be used.

[0049] Figure 4 and Figure 5 This is a diagram illustrating a first embodiment of the optical waveguide element of the present invention. Figure 4 It is a top view. Figure 5 yes Figure 4 Sectional view (a) at the dashed line A-A' and sectional view (b) at the dashed line B-B'.

[0050] exist Figure 4 and Figure 5 The text shows the use of... Figures 1 to 3 Examples of the same spot size conversion section (block 2) are provided, but the present invention is not limited to this, and the spot size conversion section may also be a cone-shaped section as shown in Patent Documents 1 to 3.

[0051] The optical waveguide element of the present invention is characterized in that, as Figure 4As shown in Figure 5, a spot size conversion section is disposed at one end of the optical waveguide, and a structure S is provided with the spot size conversion section (block 2) in a manner separated from it. This structure S has a refractive index lower than that of the optical waveguide 10, and is lower than the refractive index of the block 2; more preferably, it has a refractive index lower than that of the organic curing material 5. Alternatively, the refractive index of the structure S can be the same as that of the block 2, but in this case, a material with a low refractive index needs to be disposed between the block 2 and the structure S. On the other hand, if the refractive index is set to be lower than that of the block 2, it can also be the same as that of the organic curing material 5 or the reinforcing substrate 3. The structure S can be made of an ultraviolet (UV) curable resin. Furthermore, it can be a thermoplastic resin or a thermosetting resin, for example, including polyamide resins, melamine resins, phenolic resins, amino resins, epoxy resins, etc. As a low refractive index material, it can also include rubber materials or silicon oxide compounds. Additionally, the structure S can be, for example, a permanent photoresist. A photoresist made of thermosetting resin is applied to a reinforcing substrate 3. The photoresist is thermoset after patterning using a conventional photolithography process, thereby enabling the arrangement of the structure S.

[0052] An upper substrate 4 is disposed on the upper side of the spot size conversion section and the structure S. The upper substrate is made of a material having the same refractive index and coefficient of linear expansion as the reinforcing substrate 3. If the coefficients of linear expansion are the same, defects such as the upper substrate falling off due to thermal stress can be reduced, thereby obtaining an optical waveguide element with excellent heat resistance. The adhesive used to bond the upper substrate 4 to the reinforcing substrate 3 can be a UV-curable resin or an adhesive based on resins such as acrylic or epoxy.

[0053] The optical waveguide element of the present invention is characterized in that the height H of the structure S is higher than the maximum height of the spot size conversion section, thereby suppressing damage to the spot size conversion section (block 2) caused by the upper substrate 4.

[0054] Figure 6 and Figure 7 This diagram illustrates a second embodiment of the optical waveguide element according to the present invention. The second embodiment is characterized by a protrusion S1 provided on the upper surface of the structure S (the surface opposite to the upper substrate), and a groove formed between adjacent protrusions. This groove facilitates efficient drainage of excess adhesive between the reinforcing substrate 3 and the upper substrate 4. Therefore, it is possible to maintain a more uniform thickness between the reinforcing substrate 3 or the structure S and the upper substrate 4.

[0055] In particular, when the organic curing material 5 is used as part of the core of the spot size conversion section (SSC), by keeping the thickness of the layer formed by the organic curing material constant, not only can the size of the MFD of the light wave be stabilized, but also an SSC that is less prone to multimode generation can be achieved.

[0056] Figure 7 The thickness H of the structure S (up to the upper surface of the protrusion S1) shown also depends on the height of the spot size conversion section, but when the adhesive is used as part of the core, it is set to a range of 1μm to 3.5μm. Furthermore, the area occupied by the protrusion S1 on the entire upper surface of the structure S is set to a range of approximately 10% to 60%. This is because it is necessary to increase the mechanical strength of the protrusion to a certain extent, and if the gaps such as grooves become too narrow, problems such as air not being able to enter and escape will occur. If the area of ​​the upper surface of the protrusion S1 of the structure becomes larger, the friction at the contact surface between the structure and the cover increases when the upper substrate is bonded, the cover is fixed, and this adversely affects the workability of the process.

[0057] Figure 8 and Figure 9 This is a diagram illustrating a third embodiment of the optical waveguide element of the present invention. The third embodiment is characterized in that an adhesive outflow prevention unit EB for suppressing adhesive (5) outflow is arranged in the direction away from the spot size conversion section (block 2) of the optical waveguide 10.

[0058] If the adhesive (5) is spread within the optical waveguide element by covering the surface of the optical waveguide 10, the location of the photodetector and other components required to construct the optical modulator needs to be placed further away than the SSC section, thus causing problems such as the inability to miniaturize. In addition, since the adhesive adheres to the undesigned portion of the optical waveguide, some of the propagating light waves leak out, or the light cannot be contained in accordance with the bending of the optical waveguide, resulting in increased propagation loss.

[0059] The refractive index of the material constituting the adhesive outflow prevention unit EB needs to be lower than that of the material constituting the optical waveguide 10. In particular, the same material as the structure S can also be used, and the adhesive outflow prevention unit can be formed using the process of forming the structure S.

[0060] Furthermore, a cover portion CO, made of the same material as the adhesive outflow prevention unit EB, is formed on the surface of the optical waveguide 10 on the side from the adhesive outflow prevention unit EB to the spot size conversion section (block 2) or on the side opposite to the block 2. This cover portion serves to fill the rough surface of the optical waveguide 10, and by sequentially arranging the materials around the optical waveguide 10 as the cover layer CO, the adhesive outflow prevention unit EB, and the block 2, abrupt changes in refractive index are suppressed, thereby also contributing to the reduction of propagation loss.

[0061] The optical waveguide element of the present invention is provided with a modulation electrode for modulating the light wave propagating in the optical waveguide 10, and as follows: Figure 10As shown, it is housed within the housing 8. Furthermore, by providing an optical fiber (F) for inputting and outputting light waves to the optical waveguide, an optical modulation device MD can be constructed. Figure 10 In this configuration, the optical fiber is introduced into the housing through a through-hole in the side wall of the housing and directly coupled to the optical waveguide element. The optical waveguide element and the optical fiber can also be optically connected via a space optical system.

[0062] An optical transmitting device (OTA) can be constructed by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal that causes the optical modulator (MD) to modulate the signal. The modulation signal applied to the optical waveguide element needs to be amplified; therefore, a drive circuit (DRV) is used. The drive circuit (DRV) or the digital signal processor (DSP) can be configured outside the housing 8, or it can be configured inside the housing 8. In particular, by configuring the drive circuit (DRV) inside the housing, the transmission loss of the modulation signal from the drive circuit can be further reduced.

[0063] Industrial availability

[0064] As explained above, according to the present invention, an optical waveguide element is provided that, even when a spot size conversion section is provided at the end of the optical waveguide, the upper substrate can be bonded parallel to the reinforcing substrate to prevent damage to the spot size conversion section. By homogenizing the adhesive layer thickness, the MFD is stabilized, thereby homogenizing thermal stress and suppressing peeling of the upper substrate. Furthermore, an optical modulation device and an optical transmission apparatus using this optical waveguide element can also be provided.

[0065] Label Explanation

[0066] 1. Substrate (thin plate, film) forming the optical waveguide

[0067] 2. Blocks constituting the spot size conversion section

[0068] 3. Reinforced substrate

[0069] 4 upper base plate

[0070] 5. Adhesive

[0071] 10 Optical waveguide

[0072] S structure

[0073] S1 protrusion

[0074] EB adhesive outflow prevention unit

[0075] CO Covering

Claims

1. An optical waveguide element comprising: a ribbed optical waveguide formed of a material having an electro-optic effect; and a reinforcing substrate supporting the optical waveguide, characterized in that... One end of the optical waveguide is provided with a spot size conversion section, which changes the mode field diameter of the light wave propagating in the optical waveguide. The spot size conversion section is composed of a block portion as the core and an organic cured material surrounding the block portion, which has a lower refractive index than the material constituting the block portion. The optical waveguide element comprises: The structure is disposed separately from the spot size conversion section, separated from it, and is disposed on the reinforcing substrate; and The upper substrate is disposed on the upper side of the spot size conversion part and the structure. The organic cured material is formed by an adhesive filling the space formed by the block portion, the structure, and the upper substrate. The height of the structure is set to be greater than or equal to the maximum height of the spot size conversion section, and a groove is formed on the surface of the structure opposite to the upper substrate to allow the adhesive to flow away from the block portion.

2. The optical waveguide element according to claim 1, characterized in that, An adhesive outflow prevention unit for suppressing adhesive outflow is disposed in the direction away from the light spot size conversion section of the optical waveguide.

3. The optical waveguide element according to claim 2, characterized in that, The surface of the optical waveguide from the adhesive outflow prevention unit to the side opposite to the spot size conversion section is covered with the same material as the adhesive outflow prevention unit.

4. The optical waveguide element according to claim 2, characterized in that, The refractive index of the material of the adhesive outflow prevention unit is lower than that of the material constituting the rib-shaped optical waveguide.

5. An optical modulation device, characterized in that, The optical waveguide element according to any one of claims 1 to 4 is housed within a housing, and the optical modulation device includes an optical fiber for inputting or outputting optical waves to the optical waveguide.

6. The optical modulation device according to claim 5, characterized in that, The optical waveguide element includes a modulation electrode for modulating light waves propagating in the optical waveguide, and an electronic circuit inside the housing amplifies the modulation signal output to the modulation electrode of the optical waveguide element.

7. An optical transmitting device, characterized in that, have: The optical modulation device according to claim 5 or 6; and The electronic circuit outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

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