阅读说明：本技术 光波导元件 (Optical waveguide element ) 是由 加藤圭 宫崎德一 于 2019-07-22 设计创作，主要内容包括：本发明提供一种光波导元件,能够抑制以配置在基板上的对象物的热收缩为起因的传送特性的劣化。本发明的光波导元件具有形成有光波导(20)的基板(10)和配置在该基板上的对象物(31、32),其中,该光波导包括对在该光波导中传播的光波的模式进行转换并将光波分支的模式转换分支部(23),在俯视观察该基板时,以覆盖该模式转换分支部的一部分或全部的方式、或者以完全不覆盖该模式转换分支部的方式配置该对象物。在以覆盖该模式转换分支部的一部分的方式配置该对象物的情况下,以在光波的行进方向上不连续地覆盖规定值以上的长度的区间的方式配置该对象物。(The invention provides an optical waveguide element which can inhibit the deterioration of transmission characteristics caused by the thermal shrinkage of an object arranged on a substrate. An optical waveguide element comprises a substrate (10) on which an optical waveguide (20) is formed, and objects (31, 32) arranged on the substrate, wherein the optical waveguide comprises a mode conversion branching section (23) for converting the mode of an optical wave propagating through the optical waveguide and branching the optical wave, and the objects are arranged so as to cover a part or all of the mode conversion branching section or so as not to cover the mode conversion branching section at all when the substrate is viewed in a plan view. When the object is disposed so as to cover a part of the mode conversion branching section, the object is disposed so as to cover a section having a length equal to or greater than a predetermined value discontinuously in the traveling direction of the optical wave.)

1. An optical waveguide element having: a substrate on which an optical waveguide is formed; and an object disposed on the substrate, characterized in that,

the optical waveguide includes a mode conversion branching section that converts a mode of an optical wave propagating in the optical waveguide and branches the optical wave,

the object is disposed so as to cover a part or all of the mode converting branch portion or so as not to cover the mode converting branch portion at all when the substrate is viewed in plan view,

when the object is disposed so as to cover a part of the mode conversion branch portion, the object is disposed so as to cover a section having a length equal to or greater than a predetermined value discontinuously in the traveling direction of the optical wave.

2. The optical waveguide element according to claim 1,

the stated value is 40 μm and,

when the object is disposed so as to cover a plurality of discontinuous sections, a gap of 20 μm or more is provided between the sections.

3. The optical waveguide element according to claim 1 or 2,

the object is an electrode for controlling the light wave propagating through the optical waveguide, or a reinforcing member for reinforcing the connection between the optical fiber for inputting the light wave to the optical waveguide and the substrate.

4. An optical waveguide element according to any one of claims 1 to 3,

the substrate is a thin plate with a thickness of 20 μm or less, and is reinforced by bonding a reinforcing substrate.

Technical Field

The present invention relates to an optical waveguide element including a substrate on which an optical waveguide is formed and an object disposed on the substrate.

Background

In the field of optical communications, optical modulators are used which perform optical modulation on an input optical wave in accordance with transmission data or the like and output the optical modulation. Fig. 1 shows a structure of an optical waveguide element incorporated in a conventional optical modulator. The upper stage of fig. 1 is a plan view of a part of the optical waveguide element (a part on the light wave input side), and the lower stage is a cross-sectional view thereof.

The optical waveguide element is configured by forming an optical waveguide 20 including a mach-zehnder waveguide for performing optical modulation on the front surface side of a substrate 10 having an electro-optical effect. The optical waveguide 20 may be a nested waveguide in which a plurality of mach-zehnder waveguides are nested and incorporated. In fig. 1, a very thin substrate (i.e., a thin plate) is used as the substrate 10, and a reinforcing substrate 12 is bonded to the substrate with an adhesive 11 to form a multilayer structure. The optical waveguide element is not limited to such a multilayer structure, and may be a single-layer structure using a substrate having a sufficient thickness.

The optical waveguide 20 has a plurality of Y-branch waveguide sections 21 for distributing optical waves propagating through the optical waveguide to a uniform power ratio so as to arrange a plurality of mach-zehnder waveguides in parallel. Further, a mode converting portion 22 for converting the optical wave mode before branching the optical wave is disposed on the upstream side of each Y-branch waveguide portion 21 in the optical wave traveling direction. The mode converting section 22 has a structure (wedge structure) in which the optical waveguide width is gradually widened to branch 1 optical waveguide into 2 optical waveguides, for example.

An optical fiber (not shown) for light input is connected to the substrate 10, and a connection portion of the substrate 10 to the optical fiber is reinforced by disposing a reinforcing member 31 on the surface of the substrate 10. A control electrode 32 for controlling the light wave propagating through the optical waveguide 20 is also disposed on the surface of the substrate 10. The control electrode 32 includes a modulation electrode to which a high-frequency signal (modulation signal) is applied, a ground electrode surrounding the modulation electrode, a bias control electrode to which a DC voltage (bias) is applied, and the like.

A material such as LN (lithium niobate) used for the substrate 10 has an excellent piezoelectric effect, and therefore, when the substrate 10 is subjected to an external force or stress is generated in the substrate 10, the refractive index of the portion changes. Although the object (arrangement object) such as the reinforcing member 31 or the control electrode 32 arranged on the surface of the substrate 10 is thermally contracted by temperature variation, the degree of thermal contraction is different because the linear expansion coefficient and the young's modulus are different from each other. Therefore, stress due to the difference in thermal shrinkage occurs in each object, and as a result, the refractive index of the substrate 10 changes due to the stress of each object. In particular, at the end (boundary portion) of the arrangement region of the object, the stress change is likely to increase, which causes the refractive index distribution to occur in the substrate 10. The thermal stress of each object can be calculated by the following (formula 1).

Thermal stress E [ Pa ] ═ linear expansion coefficient [/K ] × temperature change [ K ] × young's modulus [ Pa ] … (formula 1)

If a refractive index distribution is generated by a change in the refractive index in the range of the Y-branch waveguide portion 21 or the mode converting portion 22 immediately before the Y-branch waveguide portion 21, the branching ratio (power ratio) of the propagating light in the Y-branch waveguide portion 21 is not uniform. As shown in fig. 1, conventionally, the end of the arrangement region of the reinforcing member 31 or the control electrode 32 is located in the range of the mode converting branch portion 23 including the Y-branch waveguide portion 21 and the mode converting portion 22.

With such a configuration, a refractive index distribution due to a change in stress is generated in the range of the mode converting branch portion 23, and therefore a difference occurs in the branching ratio of the propagating light of the Y-branch waveguide portion 21. As a result, there is a problem that deterioration of the On/Off extinction ratio of each mach-zehnder waveguide or a loss difference between branching waveguide portions (arm portions of the mach-zehnder waveguides) occurs, resulting in deterioration of transmission characteristics. Further, in an LN substrate or the like having an excellent piezoelectric effect, when an external force is received, polarization proportional to pressure is also generated at the same time, and the refractive index of the portion receiving the pressure changes more significantly.

The above-described problems are more readily apparent in the case of a multilayer structure in which substrates of different materials having large differences in linear expansion coefficient or young's modulus are bonded, particularly in the case of a structure in which an LN thin plate or the like is reinforced by a reinforcing substrate. This is because stress due to thermal shrinkage is likely to occur in a thin plate having an optical waveguide. Further, the above-described problem occurs even in the case of a single-layer structure using a substrate having a sufficient thickness, and therefore, it is expected that further improvement in transmission characteristics can be achieved by taking measures against this problem.

Various studies have been made on a structure for stabilizing the branching ratio in the Y-branched structure. For example, patent document 1 discloses a structure in which a 3-branch structure or a slab waveguide is disposed in a front stage of a Y-branch structure.

[ Prior Art document ]

[ patent document ]

[ patent document 1 ] Japanese patent laid-open publication No. 2016-

Disclosure of Invention

Drawings

Fig. 1 is a diagram showing a structure of an optical waveguide element incorporated in a conventional optical modulator.

Fig. 2 is a diagram showing the structure of an optical waveguide element according to a first embodiment of the present invention.

Fig. 3 is a diagram showing the structure of an optical waveguide element according to a second embodiment of the present invention.

Fig. 4 is a diagram showing the structure of an optical waveguide element according to a third embodiment of the present invention.

Fig. 5 is a diagram showing the structure of an optical waveguide element according to a fourth embodiment of the present invention.

Fig. 6 is a diagram showing an example in which an electrode is disposed so as to cover a part of a mode conversion branch portion.

Fig. 7 is a diagram showing an example of the structure of the mode converting branch portion.

[ Mark Specification ]

10 base plate

11 adhesive

12 reinforcing member

20 optical waveguide

21Y branched waveguide part

22 mode converting part

23. 24, 50 mode conversion branch part

31 reinforcing member

32. 51, 52 control electrode

Detailed Description

Preferred examples of the optical waveguide device of the present invention will be described in detail. The present invention is not limited to the examples shown below.

As shown in fig. 2 to 7, the optical waveguide element of the present invention includes a substrate (10) on which an optical waveguide (20) including mode converting branch portions (23, 24) that convert a mode of an optical wave propagating through the optical waveguide and branch the optical wave, and objects (31, 32) disposed on the substrate, and the objects are disposed so as to cover a part or all of the mode converting branch portions or so as not to cover the mode converting branch portions at all when the substrate is viewed in a plan view. When the object is disposed so as to cover a part of the mode converting branch portion, the object is disposed so as to discontinuously cover a section having a length equal to or greater than a predetermined value in the traveling direction of the optical wave.

In this way, by disposing the object (disposed article) on the substrate so as to cover a part or all of the mode converting branch portion, or so as not to cover the mode converting branch portion at all, the refractive index distribution due to the stress change caused by the thermal shrinkage of the object on the substrate is less likely to occur within the range of the mode converting branch portion. Therefore, it is possible to suppress the occurrence of a difference in the branching ratio of propagating light due to thermal contraction of the object on the substrate. As a result, the On/Off extinction ratio in each mach-zehnder waveguide can be reduced, and the occurrence of a loss difference between the branching waveguide sections can be reduced, thereby suppressing the deterioration of the transmission characteristics. The present invention is particularly suitable for an optical modulator using a substrate having an excellent piezoelectric effect such as an LN substrate, but is also effective for an optical modulator using another substrate (a substrate such as glass or crystal) in which a refractive index changes when pressure is applied.

The mode conversion branching section includes, for example, a mode conversion section (22) that converts the mode of the optical wave and a Y-branch waveguide section (21) that branches the mode-converted optical wave. Further, when there is a section in which the waveguide width changes immediately after the Y-branch waveguide portion, the section is also included in the mode conversion branch portion. That is, as an example, the mode converting branch portion of the present invention is constituted by the Y-branch waveguide portion and a section (a section in which the mode is unstable) in which the mode of the light wave before and after the Y-branch waveguide portion changes. When a change in refractive index occurs in such a mode conversion branch portion, the distribution of the optical wave propagating through the optical waveguide is shifted in the center position, or the distribution becomes asymmetric, or the optical wave is coupled into a mode other than a desired mode, and the distribution of the optical wave propagating through the optical waveguide becomes unstable. Therefore, in the present invention, by studying the arrangement of the object on the substrate, the change in the refractive index of the mode converting branch portion is suppressed, and the destabilization of the distribution of the light wave propagating through the optical waveguide is eliminated.

The mode conversion branching section can be implemented by various structures as in the several examples shown in fig. 7.

Fig. 7(a) shows an example of a mode converting branch portion having a Y-branch structure, and has a shape (wedge shape) in which the width gradually increases in the process of reaching the branch portion. In the case of such a configuration, the section S1 in which the width L2 of the entire 2 optical waveguide portions after branching is 3 times or less (L2 ≦ L1 × 3) the width L1 of the normal optical waveguide (the width of the portion not in the wedge shape) corresponds to the mode converting branch portion.

Fig. 7(b) is an example of a mode conversion branch portion based on an MMI (multi-mode interferometer) structure, and is a shape in which a predetermined section before the branch portion is widened to a width capable of multimode waveguide. In the case of such a configuration, the section S2 having a wider width corresponds to the mode conversion branch portion.

Fig. 7(c) shows an example of a mode conversion branch unit based on a directional coupler, and 2 optical waveguides are brought close to each other in a predetermined section. In the case of such a structure, as in the case of the Y-branch structure, the section S3 in which the width L2 of the entire portion in which the 2 optical waveguides are close to each other is 3 times or less (L2 ≦ L1 × 3) the width L1 of the normal optical waveguide corresponds to the mode converting branch portion.

The condition L2. ltoreq.L 1X 3 is one example of suitable conditions for the present invention, and is not limited to this condition.

Hereinafter, specific configurations of the optical modulator according to the present invention will be described with reference to examples.

Fig. 2 to 5 show the structures of optical waveguide elements of the first to fourth embodiments, respectively. The upper stage of each drawing is a plan view of a part of the optical waveguide element (a part on the light wave input side), and the lower stage is a cross-sectional view thereof. The basic structure of the optical modulator of each embodiment is the same as that of the conventional example described with reference to fig. 1.

[ first embodiment ]

As shown in fig. 2, the optical waveguide device of the first embodiment uses a very thin substrate (i.e., a thin plate) as a substrate 10, and has a multilayer structure reinforced by bonding a reinforcing substrate 12 to the substrate 10 with an adhesive 11. The substrate 10 is formed of a material having an excellent piezoelectric effect such as LN (lithium niobate). The thickness of the substrate 10 is about 20 μm, the thickness of the adhesive 11 is about 20 to 100 μm, and the thickness of the reinforcing substrate 12 is about 400 to 1000 μm.

The optical waveguide 20 formed on the substrate 10 includes mode conversion branching portions 23 and 24 that convert and branch the mode of the optical wave propagating through the optical waveguide. The mode converting branch portion 23 is the mode converting branch portion located most upstream in the optical wave traveling direction, and the mode converting branch portion 24 is the second mode converting branch portion from the upstream in the optical wave traveling direction. The structure may further include a mode conversion branch portion downstream of the first and second branch portions.

A reinforcing member 31 and a control electrode 32 are disposed on the surface of the substrate 10, the reinforcing member 31 is used to reinforce the connection between an optical fiber (not shown) for inputting a light wave into the optical waveguide 20 and the substrate 10, and the control electrode 32 is used to control the light wave propagating through the optical waveguide 20. The thermal expansion coefficient or young's modulus of these objects is greatly different from that of the substrate 10, and this causes stress to be generated in the substrate 10 due to thermal contraction, thereby changing the refractive index of the optical waveguide 20.

In the first embodiment, as shown in the lower sectional view of fig. 2, the reinforcing member 31 is disposed on the upstream side of the mode transition branch portion 23, and the control electrode 32 is disposed between the mode transition branch portion 23 and the mode transition branch portion 24 and on the downstream side of the mode transition branch portion 24. That is, the reinforcing member 31 and the control electrode 32 are disposed so as not to cover the mode converting branch portions 23 and 24 at all.

With such a configuration, in the range of the mode conversion branch portions 23 and 24, the refractive index distribution due to stress caused by thermal contraction of the object (the reinforcing member 31 and the control electrode 32) on the substrate 10 is less likely to occur. Therefore, when the optical wave is branched, the branching ratio of the propagating light is stabilized, and the deterioration of the On/Off extinction ratio in each mach-zehnder waveguide and the loss difference between the branching waveguide sections can be reduced, so that the deterioration of the transmission characteristics can be suppressed.

It is not required that the object is not present in the entire region in the width direction of the substrate (the direction orthogonal to the propagation direction of the optical wave) so as not to cover the mode conversion branching section at all. That is, even in a region where the position in the longitudinal direction of the substrate (the propagation direction of the optical wave) overlaps with the mode converting branch portion, the object may be present as long as the position in the width direction of the substrate is different from the mode converting branch portion.

[ second embodiment ]

In the second embodiment, as shown in the lower sectional view of fig. 3, the reinforcing member 31 is disposed so as to cover the entire mode converting branch portion 23. The control electrode 32 is disposed so as to cover the entire mode conversion branch portion 24.

With such a configuration, even in the range of the mode conversion branch portions 23 and 24, the refractive index distribution due to stress caused by thermal shrinkage of the object on the substrate 10 is less likely to occur. As a result, the branching ratio of the propagating light when branching the optical wave is stabilized, and deterioration of the transmission characteristic can be suppressed.

It is not required that the object be disposed so as to cover the entire mode converting branch portion, and the object be present over the entire region in the width direction of the substrate. That is, even in a region where the position in the longitudinal direction of the substrate overlaps with the mode converting branch portion, the object may not be present as long as the position in the width direction of the substrate is different from the mode converting branch portion.

[ third and fourth embodiments ]

The third embodiment shown in fig. 4 is a modification of the first embodiment shown in fig. 2. Furthermore, it is possible to provide a liquid crystal display device,

the fourth embodiment shown in fig. 5 is a modification of the second embodiment shown in fig. 3. The optical waveguide elements of the first and second embodiments have a multilayer structure, and the reinforcing substrate 12 is bonded to the back surface of the substrate 10 formed to be very thin, and is reinforced, but the optical waveguide elements of the third and fourth embodiments have a single-layer structure, and the substrate 10 is formed thick enough to obtain sufficient strength.

Even when the optical waveguide element has a single-layer structure, a refractive index distribution is generated due to stress caused by thermal contraction of an object on the substrate 10. Therefore, as shown in fig. 4 and 5, by studying the arrangement of the objects on the substrate 10, the branching ratio of the propagating light when the optical wave is branched is stabilized, and further improvement of the transmission characteristic can be achieved.

In the above embodiments, the objects on the substrate are arranged so as to cover the entire mode converting branch portion or so as not to cover the mode converting branch portion at all, but may be arranged so as to cover a part of the mode converting branch portion. However, the object needs to be disposed so as to cover a section having a length equal to or longer than a predetermined value discontinuously in the traveling direction of the optical wave. Specific conditions will be described with reference to fig. 6. Here, it is assumed that a general communication Band, i.e., a wavelength region of C-Band (1530 to 1565nm) or L-Band (1565 to 1625nm) is used.

In fig. 6, 2 control electrodes 51 and 52 are disposed so as to cover a part of the mode conversion branching portion 50. In this case, the widths w1 and w2 of the control electrodes 51 and 52 are preferably 40 μm or less. In this way, when the object is disposed so as to cover a part of the mode converting branch portion, the object needs to be disposed so as to cover a section of 40 μm or more discontinuously in the traveling direction of the optical wave. This is because, when the object is disposed so as to continuously cover a section of 40 μm or more in the traveling direction of the optical wave, a difference in optical loss (difference in branching ratio) occurs to such an extent that the performance of the optical modulator is affected.

The distance d between the control electrode 51 and the control electrode 52 is preferably 20 μm or more. In this way, when the object is disposed so as to cover a plurality of discontinuous sections, it is necessary to provide intervals of 20 μm or more between the sections. This is because, when the interval between the objects provided in the range of the mode converting branch portion is sufficiently wide, the influence of the stress change is small, but if the interval is narrowed to 20 μm or less, the stress change is continuously observed, and a difference in optical loss (difference in branching ratio) of such a degree as to affect the performance of the optical modulator is generated.

The above conditions (widths w1, w2, and interval d) are one of preferable examples when the object is disposed so as to cover a part of the mode converting branch portion, and the object is formed so as not to cover a section having a length equal to or greater than a predetermined value in the traveling direction of the optical wave. That is, the above-described conditions may be relaxed to such an extent that the influence on the performance of the optical modulator can be ignored (that is, to such an extent that the conditions are practically effective).

In the description so far, the case where the first and second mode converting branch portions from the upstream side in the traveling direction of the optical wave are the objects and the stress change due to the thermal contraction of the object on the substrate is made difficult to occur within the range has been described, but the same can be said for the mode converting branch portions located further downstream. However, it is preferable to apply the present invention in order from the mode converting branch portion on the upstream side where the power of the optical wave propagating through the optical waveguide is large, because more excellent effects can be obtained. In addition, when the present invention is applied to a plurality of mode conversion branching units, it is not necessary to make the arrangement of the object the same in all the mode conversion branching units, and a configuration may be adopted in which a certain mode conversion unit is covered with the object and the other mode conversion units are not covered with the object. In the case where the present invention is applied to the mode conversion branching portion on the downstream side, the control electrode is formed or not formed by patterning, and therefore, the conventional manufacturing process can be used.

The present invention has been described above based on the embodiments, but the present invention is not limited to the above, and design changes can be made as appropriate within a range not departing from the gist of the present invention. For example, although a reinforcing substrate is bonded to a substrate as a thin plate with an adhesive to form a multilayer structure in fig. 2 and 3, a substrate having a multilayer structure may be formed by performing crystal growth on a base substrate.

In addition, although the above description refers to the arrangement of the object in the mode conversion branching unit, the same can be said for the mode conversion multiplexing unit in the case where the optical waveguide element has the mode conversion multiplexing unit that converts and combines the modes of the respective optical waves propagating through the branching waveguide unit.

That is, when the optical waveguide formed on the substrate includes the mode-converting wave-combining portion, the object is preferably disposed so as to cover a part or all of the mode-converting wave-combining portion or so as not to cover the mode-converting wave-combining portion at all when the substrate is viewed in plan view. When the object is disposed so as to cover a part of the mode conversion/multiplexing unit, the object is preferably disposed so as to cover a section having a length equal to or longer than a predetermined value in the traveling direction of the optical wave in a discontinuous manner.

[ industrial applicability ]

According to the present invention, it is possible to provide an optical waveguide element capable of suppressing deterioration of transmission characteristics due to thermal shrinkage of an object arranged on a substrate.