JP7468280B2 - Optical waveguide element, optical modulation device using the same, and optical transmission device - Google Patents

Description

The present invention relates to an optical waveguide element and an optical modulation device and optical transmission device using the same, and in particular to an optical waveguide element having a rib-type optical waveguide and a reinforcing substrate that supports the optical waveguide.

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 and communication traffic in recent years, there is a demand for faster and larger capacity optical communication between long distance cities and data centers. Furthermore, there are space limitations at base stations, making it necessary to increase the speed and size of optical modulators.

To miniaturize an optical modulator, the optical waveguide can be made narrower to increase the light confinement effect, which in turn reduces the bending radius of the optical waveguide and enables miniaturization. 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 conventional optical waveguides in LN optical modulators, 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 miniaturization difficult.

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 3 μm or less, 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.

On the other hand, when using a fine optical waveguide with an MFD smaller than 10 μmφ, which is the MFD of optical fiber, directly joining the end of the optical waveguide (element end face) provided in the optical waveguide element to the optical fiber generates a large insertion loss.

To solve this problem, it is possible to place a spot size converter (SSC) at the end of the optical waveguide. A typical SSC provides a tapered optical waveguide section that expands the optical waveguide in two or three dimensions. For reference, Patent Documents 1 to 3 show examples of tapered waveguides.

Tapered waveguides, in which the spot size increases as the core of the optical waveguide expands, are difficult to adjust the refractive index of the core and cladding to suit the spot size, and are prone to inducing multimodes, so there are limitations to the designs that can be used as SSCs for optical waveguide elements. Furthermore, a relatively long tapered section must be formed to convert to the required spot size, making it difficult to miniaturize optical waveguide elements.

Furthermore, the applicant is considering an SSC as shown in Figures 1 to 3, in which the tip of the rib-type optical waveguide 10 is tapered 12 with a narrowing width, and a block portion 2 is arranged to surround it, which serves as the core portion. The refractive index of the block portion 2 is set lower than that of the optical waveguide 10, and the block body is surrounded by an organic material (5) whose refractive index is about 0.01 to 0.03 lower than that of the block portion 2. This organic material can be made of a hardened adhesive, etc. This SSC has a tapered shape 12 of the optical waveguide 10 when covered with the block portion 2, which reduces the effective refractive index of the optical waveguide 10 and weakens the light confinement, allowing the optical mode to transfer to the block portion 2, realizing a larger MFD than the optical waveguide 10.

Figure 2 shows cross-sectional views taken along dotted lines A-A' and B-B' in Figure 1, and Figure 3 shows a cross-sectional view taken along dotted line C-C' in Figure 1. Reference numeral 1 denotes a thin plate (film) made of a material having an electro-optic effect, such as lithium niobate, and optical waveguide 10 is formed in the rib portion that remains after the thin plate is locally removed. Reference numeral 3 denotes a reinforcing substrate that supports thin plate 1 including optical waveguide 10.

Reference numeral 4 denotes an upper substrate that serves as a reinforcing member when connecting an optical fiber or an optical block to the end face of the optical waveguide element. Reference numeral 5 denotes an organic cured product formed by curing the adhesive that bonds the reinforcing substrate 3 and the upper substrate 4. Reference numeral 11 denotes a part of the thin plate 1, which is a portion that remains after etching when forming the tapered portion 12 of the optical waveguide.

In the SSCs shown in Patent Documents 1 to 3 and Figures 1 to 3, not only is there a rib-type optical waveguide, but there is also a tapered portion in Patent Document 3 and a block portion 2 in Figure 2 that are thicker than the optical waveguide and protrude upward from the surface of the reinforcing substrate 3. In this way, when the upper substrate 4 is attached, the presence of the protruding portion makes it difficult to attach the upper substrate parallel to the surface of the reinforcing substrate 3. Furthermore, when the upper substrate 4 is pressed from above for bonding, the pressing force is concentrated on the protruding portion, causing problems such as damage to the protruding portion. Furthermore, if the upper substrate 4 cannot be attached parallel to the surface and the thickness of the adhesive layer becomes uneven, problems such as uneven MFD and the upper substrate 4 coming off due to thermal stress may occur.

JP 2006-284961 A JP 2007-264487 A International Publication No. WO2012/042708

The problem that the present invention aims to solve is to provide an optical waveguide element that solves the problems described above, in which even when a spot size conversion section is provided at the end of the optical waveguide, the upper substrate is attached parallel to the reinforcing substrate, the spot size conversion section is prevented from being damaged, the MFD is stabilized by making the adhesive layer thickness uniform, and further, the thermal stress is made uniform, suppressing peeling of the upper substrate. Furthermore, the present invention aims to provide an optical modulation device and an optical transmission device that use this optical waveguide element.

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 including a rib-type optical waveguide formed of a material having an electro-optic effect and a reinforcing substrate supporting the optical waveguide, the optical waveguide having a spot size conversion section at one end thereof for changing a mode field diameter of a light wave propagating through the optical waveguide, the spot size conversion section being composed of a block section serving as a core section and an organic cured material surrounding the block section and having a lower refractive index than a material constituting the block section, the spot size conversion section being composed of structures disposed on the reinforcing substrate and spaced apart from the spot size conversion section so as to sandwich the spot size conversion section, the optical waveguide element further including an upper substrate disposed above the spot size conversion section and the structure, the organic cured material being formed of an adhesive filled in a space in which the block section, the structure, and the upper substrate are formed , the height of the structure being set to be equal to or greater than the maximum height of the spot size conversion section, and a groove being formed on a surface of the structure facing the upper substrate for allowing the adhesive to flow in a direction away from the block section .

(2) In the optical waveguide element described in (1) above, the surface of the structure facing the upper substrate is composed of a plurality of protrusions and the grooves formed around the protrusions, and the area of the surface occupied by the protrusions is set in the range of 10% to 60% .

(3) In the optical waveguide element according to (1) or (2) above, the height of the structures is set in the range of 1 μm to 3.5 μm .

(4) The optical waveguide element according to any one of (1) to (3) above , further comprising an adhesive outflow prevention means arranged in a direction from the spot size conversion portion to the optical waveguide to prevent the adhesive from outflowing.

(5) The optical waveguide element according to (4) above, further characterized in that a surface of the optical waveguide located on the opposite side of the adhesive outflow prevention means to the spot size conversion section has a covering section that is covered with the same material as the adhesive outflow prevention means.

(6) In the optical waveguide element described in either (4) or (5) above, the refractive index of the material of the adhesive outflow prevention means is lower than the refractive index of the material constituting the rib-shaped optical waveguide.

(7) The optical waveguide element described in any one of (1) to (6) above is an optical modulation device that is housed in a housing and includes an optical fiber that inputs or outputs light waves to the optical waveguide.

(8) In the optical modulation device described in (7) above, the optical waveguide element is provided with 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 of the optical waveguide element.

(9) An optical transmission device comprising the optical modulation device 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.

The present invention provides an optical waveguide element including a rib-type optical waveguide formed of a material having an electro-optic effect, and a reinforcing substrate supporting the optical waveguide, the optical waveguide having a spot size conversion section at one end thereof for changing a mode field diameter of a light wave propagating through the optical waveguide , the spot size conversion section being composed of a block section serving as a core section, and an organic cured material surrounding the block section and having a lower refractive index than a material constituting the block section, the spot size conversion section being arranged at a distance from the spot size conversion section and being arranged on the reinforcing substrate so as to sandwich the spot size conversion section, and further including an upper substrate arranged above the spot size conversion section and the structure, the organic cured material being formed of an adhesive which is filled in a space in which the block section, the structure, and the upper substrate are formed , the height of the structure being set to be equal to or greater than the maximum height of the spot size conversion section, and a groove for flowing the adhesive in a direction away from the block section being formed on a surface of the structure facing the upper substrate, the structure being capable of supporting the upper substrate parallel to the reinforcing substrate, and further making it possible to prevent the upper substrate from contacting the spot size conversion section.

FIG. 2 is a plan view showing an example of a spot size converter used in an optical waveguide element. 2 is a cross-sectional view taken along dotted lines A-A' and B-B' in FIG. 1. This is a cross-sectional view taken along dotted line C-C' in Figure 1. 1 is a plan view showing a first embodiment of an optical waveguide element according to the present invention; 5 is a cross-sectional view taken along dotted lines A-A' and B-B' in FIG. 4. FIG. 4 is a plan view showing a second embodiment of the optical waveguide element of the present invention. 7 is a cross-sectional view taken along dotted lines A-A' and B-B' in FIG. 6. FIG. 11 is a plan view showing a third embodiment of the optical waveguide element of the present invention. This is a cross-sectional view taken along dotted line C-C' in Figure 6. 1 is a plan view illustrating an optical modulation device and an optical transmission device according to the present invention;

The optical waveguide element of the present invention will be described in detail below using preferred examples.
As shown in Figures 4 and 5, the optical waveguide element of the present invention is an optical waveguide element comprising a rib-type optical waveguide 10 formed of a material having an electro-optical effect, and a reinforcing substrate 3 supporting the optical waveguide, wherein one end of the optical waveguide is provided with a spot size conversion section 2 that changes the mode field diameter of the light waves propagating through the optical waveguide, and a structure S is provided on the reinforcing substrate 3 and arranged at a distance from the spot size conversion section so as to sandwich the spot size conversion section, and wherein an upper substrate 4 is arranged above the spot size conversion section 2 and the structure S, and the height H of the structure S is set to be equal to or greater than the maximum height of the spot size conversion section 2.

The material 1 having an electro-optic effect used in the optical waveguide element of the present invention can be a substrate such as lithium niobate (LN), lithium tantalate (LT), or PLZT (lead lanthanum zirconate titanate), or a vapor-grown film made of these materials.
Additionally, various materials such as semiconductor materials and organic materials can also be used as optical waveguides.

The optical waveguide 10 can be formed by etching the substrate 1 other than the optical waveguide, or by forming grooves on both sides of the optical waveguide, and by using a rib-type optical waveguide in which the portion of the substrate corresponding to the optical waveguide is convex. 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 thermal diffusion or proton exchange.

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, 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.

The substrate on which the optical waveguide is formed is bonded to the reinforcing substrate 3 by direct bonding or via an adhesive layer such as resin to increase mechanical strength. 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.

Figures 4 and 5 are diagrams for explaining a first embodiment of the optical waveguide element of the present invention, where Figure 4 is a plan view, and Figure 5 is a cross-sectional view (a) taken along dotted line A-A' in Figure 4, and a cross-sectional view (b) taken along dotted line B-B'.

Figures 4 and 5 show an example using a spot size conversion unit (block body 2) similar to those in Figures 1 to 3, but the present invention is not limited to these, and a tapered spot size conversion unit as shown in Patent Documents 1 to 3 may also be used.

The optical waveguide element of the present invention is characterized in that, as shown in FIG. 4 or 5, a spot size conversion section is disposed at one end of the optical waveguide, and the structure S is provided so as to sandwich the spot size conversion section (block body 2). This structure S has a refractive index lower than that of the optical waveguide 10, and is equal to or lower than that of the block body 2, and more preferably has a refractive index lower than that of the organic cured material 5. The refractive index of the structure S may be the same as that of the block body 2, but in that case, a material having a low refractive index must be disposed between the block body 2 and the structure S. On the other hand, if the refractive index is lower than that of the block body 2, the refractive index may be the same as that of the organic cured material 5 or the reinforcing substrate 3. For the structure S, an ultraviolet (UV) curable resin can be used. Furthermore, the resin may be a thermoplastic resin or a thermosetting resin, and examples of the resin include polyamide resin, melamine resin, phenol resin, amino resin, epoxy resin, etc., and it is also possible to include a rubber material or a silicon oxide compound as a low refractive index material. For the structure S, for example, a permanent resist. The structure S can be arranged by applying a photoresist made of a thermosetting resin onto the reinforcing substrate 3, patterning it using a typical photolithography process, and then thermally curing it.

The upper substrate 4 is placed above the spot size conversion section and the structure S. The upper substrate is made of a material with the same refractive index and linear expansion coefficient as the reinforcing substrate 3. If the linear expansion coefficients match, it is possible to reduce defects such as the upper substrate coming off due to thermal stress, and an optical waveguide element with excellent heat resistance is obtained. The adhesive that bonds the upper substrate 4 and the reinforcing substrate 3 can be a UV-curable resin or an adhesive made of an acrylic or epoxy resin.

The optical waveguide element of the present invention is characterized in that the height H of the structure S is made greater than the maximum height of the spot size conversion section, thereby preventing damage to the spot size conversion section (block body 2) by the upper substrate 4.

Figures 6 and 7 are diagrams showing a second embodiment of the optical waveguide element of the present invention. The second embodiment is characterized in that a convex portion S1 is provided on the upper surface (the surface facing the upper substrate) of the structure S, and a groove is formed between adjacent convex portions. This groove contributes to efficiently discharging excess adhesive between the reinforcing substrate 3 and the upper substrate 4. Therefore, the distance between the reinforcing substrate 3 or structure S and the upper substrate 4 can be maintained at a more uniform thickness.

In particular, when the organic cured material 5 is used as part of the core of the spot size conversion section (SSC), the thickness of the layer formed by the organic cured material becomes constant, which not only stabilizes the size of the MFD of the light wave but also makes it possible to realize an SSC that is less likely to generate multimodes.

The thickness H of the structure S (up to the top surface of the protrusion S1) shown in FIG. 7 depends on the height of the spot size conversion section, but is set in the range of 1 μm to 3.5 μm when adhesive is used as part of the core section. The area of the protrusion S1 relative to the total area of the top surface of the structure S is set in the range of about 10% to 60%. This is because the mechanical strength of the protrusion needs to be increased to a certain extent, and if the gap in the groove or the like becomes too narrow, problems such as air getting in and being unable to be exhausted occur. If the area of the top surface of the protrusion S1 of the structure becomes large, the frictional force at the contact surface between the structure and the lid increases when the upper substrate is bonded, causing the lid to stick, which has a negative effect on process workability.

Figures 8 and 9 are diagrams showing a third embodiment of the optical waveguide element of the present invention. The third embodiment is characterized in that an adhesive outflow prevention means EB is arranged to prevent the adhesive (5) from outflowing in a direction away from the spot size conversion portion (block body 2) of the optical waveguide 10.

If the adhesive (5) spreads inside the optical waveguide element so as to cover the surface of the optical waveguide 10, it becomes necessary to locate the photodetector and other components required for constructing an optical modulator at a location farther away from the SSC section, which creates problems such as making it impossible to miniaturize the device. In addition, if the adhesive adheres to a part of the optical waveguide that is not designed, some of the propagating light waves may leak out, or the light may not be confined in a way that corresponds to the bending of the optical waveguide, resulting in large propagation losses.

The refractive index of the material constituting the adhesive outflow prevention means EB must be lower than the refractive index of the material constituting the optical waveguide 10, and in particular, it is possible to form the adhesive outflow prevention means using the same material as the structure S and utilizing the process for forming the structure S.

Furthermore, a covering portion CO made of the same material as the adhesive outflow prevention means EB is formed on the surface of the optical waveguide 10 on the side of the spot size conversion section (block body 2) or the side opposite the block body 2 from the adhesive outflow prevention means EB. This covering portion serves to fill in the rough surface of the optical waveguide 10, and by arranging the materials around the optical waveguide 10 in the order of covering layer CO, adhesive outflow prevention means EB, and block body 2, it also contributes to reducing propagation loss by suppressing sudden changes in refractive index.

The optical waveguide element of the present invention is provided with a modulation electrode that modulates the light waves propagating through the optical waveguide 10, and is housed in a housing 8 as shown in FIG. 10. Furthermore, an optical modulation device MD can be constructed by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide. In FIG. 10, the optical fiber is introduced into the housing through a through hole that penetrates the side wall of the housing, and is directly bonded to the optical waveguide element. The optical waveguide element and the optical fiber can also be optically connected via a spatial optical system.

The 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 8, but they can also be placed inside the housing 8. 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.

As described above, according to the present invention, even when a spot size conversion section is provided at the end of the optical waveguide, the upper substrate is attached parallel to the reinforcing substrate, preventing damage to the spot size conversion section, and the MFD is stabilized by making the adhesive layer thickness uniform. Furthermore, it is possible to provide an optical waveguide element in which thermal stress is uniformized and peeling of the upper substrate is suppressed. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using this optical waveguide element.

1. Substrate (thin plate, film) on which the optical waveguide is formed
2 Block body constituting spot size conversion section 3 Reinforcement substrate 4 Upper substrate 5 Adhesive 10 Optical waveguide S Structure S1 Projection EB Adhesive outflow prevention means CO Covering section

Claims (9)

An optical waveguide element including a rib-type optical waveguide formed of a material having an electro-optic effect and a reinforcing substrate supporting the optical waveguide,
a spot size conversion unit that changes a mode field diameter of a light wave propagating through the optical waveguide at one end of the optical waveguide;
the spot size conversion portion is composed of a block portion that serves as a core portion and an organic cured material that surrounds the block portion and has a refractive index lower than that of a material that constitutes the block portion;
a structure disposed on the reinforcing substrate and spaced apart from the spot size conversion portion so as to sandwich the spot size conversion portion;
Further, the spot size conversion unit and the structure have an upper substrate disposed above the structure,
the organic cured material is formed of an adhesive that fills a space in which the block portion, the structure, and the upper substrate are formed;
An optical waveguide element characterized in that the height of the structure is set to be equal to or greater than the maximum height of the spot size conversion portion, and a groove is formed on the surface of the structure facing the upper substrate to allow the adhesive to flow in a direction away from the block portion . 2. The optical waveguide element according to claim 1, wherein the surface of the structure facing the upper substrate is composed of a plurality of protrusions and the grooves formed around the protrusions, and the area of the surface occupied by the protrusions is set in the range of 10% to 60% . 3. The optical waveguide element according to claim 1 , wherein the height of said structures is set in the range of 1 [mu]m to 3.5 [mu]m . 4. The optical waveguide element according to claim 1 , further comprising an adhesive outflow prevention means arranged in a direction from the spot size conversion portion to the optical waveguide to suppress outflow of the adhesive. 5. The optical waveguide element according to claim 4, further comprising a covering portion on a surface of the optical waveguide located on the opposite side of the adhesive outflow prevention means from the spot size conversion portion, the covering portion being made of the same material as the adhesive outflow prevention means. The optical waveguide element according to claim 4 or 5, characterized in that the refractive index of the material of the adhesive outflow prevention means is lower than the refractive index of the material constituting the rib-shaped optical waveguide. The optical waveguide element according to any one of claims 1 to 6 is an optical modulation device, characterized in that the optical waveguide element is housed in a housing and has an optical fiber that inputs or outputs light waves to the optical waveguide. The optical modulation device according to claim 7, characterized in that the optical waveguide element has a modulation electrode for modulating the light wave propagating through the optical waveguide, and an electronic circuit for amplifying the modulation signal input to the modulation electrode of the optical waveguide element is provided inside the housing. An optical transmitter comprising the optical modulation device according to claim 7 or 8, and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

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