阅读说明：本技术 光调制器 (Optical modulator ) 是由 冈桥宏佑 宫崎德一 本谷将之 于 2019-08-14 设计创作，主要内容包括：本发明为了提供通过降低由调制电极产生的基板的应力,能够防止对基板的损伤并且防止调制器的特性劣化的光调制器,光调制器(1)的特征在于,具备：基板(5),具有电光效应；光波导(10),形成于基板(5)；及调制电极(信号电极(S)及接地电极(G)),设置在基板(5)上,对在光波导(10)中传播的光波进行调制,在调制电极的底面的一部分和与调制电极的底面的一部分相对的基板(5)之间,配置有降低由调制电极产生的基板(5)的应力的树脂(8)。(In order to provide an optical modulator capable of preventing damage to a substrate and preventing deterioration of characteristics of the modulator by reducing stress of the substrate generated by a modulation electrode, an optical modulator (1) is characterized by comprising: a substrate (5) having an electro-optical effect; an optical waveguide (10) formed on the substrate (5); and modulation electrodes (a signal electrode (S) and a ground electrode (G)) which are provided on the substrate (5) and modulate the light wave propagating through the optical waveguide (10), wherein a resin (8) that reduces the stress of the substrate (5) caused by the modulation electrodes is disposed between a part of the bottom surface of the modulation electrodes and the substrate (5) facing the part of the bottom surface of the modulation electrodes.)

1. An optical modulator is provided with:

a substrate having an electro-optic effect;

an optical waveguide formed on the substrate; and

a modulation electrode provided on the substrate, for modulating the light wave propagating in the optical waveguide,

the light modulator is characterized in that it is,

a resin is disposed between a portion of the bottom surface of the modulator electrode and the substrate facing the portion of the bottom surface of the modulator electrode.

2. The light modulator of claim 1,

the modulation electrode is composed of a signal electrode and a ground electrode arranged along a part of the optical waveguide, and the resin is arranged between a part of a bottom surface of at least one of the signal electrode and the ground electrode and the substrate facing at least one of the signal electrode and the ground electrode.

3. The light modulator of claim 2,

wherein the width of the resin is set to be 1/3 or less of the width of the signal electrode when the resin is disposed between the signal electrode and the substrate,

when the resin is disposed between the ground electrode and the substrate, the width of the resin is set to be 1/2 or less of the width of the ground electrode.

4. The light modulator of any of claims 1 to 3,

the resin is disposed in the modulator electrode.

5. The light modulator of any of claims 1 to 4,

the resin is disposed so that a contact surface between the modulation electrode and the substrate is bilaterally symmetric with respect to the optical waveguide.

6. The light modulator of any of claims 1 to 5,

the resin is disposed such that a contact surface between the bottom surface of the modulation electrode and the substrate is disposed on the proximal end side of the optical waveguide.

7. The light modulator of any of claims 1 to 6,

the thickness of the resin is 1.0 [ mu ] m or more.

8. The light modulator of any of claims 1 to 7,

the resin is either a thermoplastic resin or a thermosetting resin.

9. The light modulator of any of claims 1 to 8,

the thickness of the substrate is 4.0 μm or less.

10. The light modulator of any of claims 1 to 9,

a rib portion protrudingly provided on the substrate is used as the optical waveguide.

11. The light modulator of any of claims 1 to 10,

the modulation electrode is made of metal, and the substrate is made of lithium niobate.

12. The light modulator of any of claims 1 to 11,

the optical waveguide is formed of a plurality of Mach-Zehnder portions.

Technical Field

The present invention relates to an optical modulator including a substrate having an electro-optical effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating an optical wave propagating through the optical waveguide.

Background

In recent years, lithium niobate (LiNbO) has been used in the fields of optical communication and optical measurement₃: hereinafter, referred to as LN), etc., and a metal modulation electrode for modulating a light wave propagating in the optical waveguide.

Patent document 1 discloses an optical modulator in which a gap is provided between a part of the bottom surface of a ground electrode and a corresponding part of a substrate. Patent document 2 discloses an optical modulator in which a ground electrode includes a first electrode portion and a second electrode portion disposed inside the first electrode portion.

In order to realize a wide bandwidth of the optical modulation frequency, it is important to realize velocity matching between the microwave and the optical wave as the modulation signal. Therefore, attempts have been made to reduce the driving power while matching the speed of the microwave and the optical wave by thinning the substrate.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 6-235891

Patent document 2: japanese laid-open patent publication No. 2010-181454

Disclosure of Invention

Problems to be solved by the invention

For example, the substrate is made of LN and the modulator electrode is made of metal, so that the linear expansion coefficients of the substrate and the modulator electrode are different. Therefore, due to the difference in linear expansion coefficient associated with the temperature change, an internal stress is generated in the substrate in the vicinity of the modulation electrode contact. In addition, a compressive stress from the modulator electrode disposed on the substrate occurs on the substrate, and particularly, the compressive stress cannot be ignored on a thin substrate. When such a stress such as an internal stress or a compressive stress is generated in the substrate, the substrate is damaged, and a crack or the like is generated in the substrate.

The substrate is made of a material having an electro-optical effect such as LN, and light modulation is performed by changing a refractive index by applying electricity. However, when stress is generated in the substrate, the refractive index of the substrate changes due to the photoelastic effect, and the propagation speed of the optical wave changes. As a result, for example, in an optical modulator having a mach-zehnder structure, there is a problem that a phase difference occurs at the time of combining waves in the mach-zehnder structure, and characteristics such as fluctuation of bias voltage are deteriorated.

As one of the methods for solving such a problem, for example, it is considered to form a buffer layer and relax the stress. However, the conventional buffer layer formed by sputtering deposition has a problem that the stress generated in the substrate cannot be sufficiently relaxed, particularly when the thickness of the substrate is small. In addition, SiO is generally used as the buffer layer₂Material with high iso-rigidity (SiO)₂Young's modulus of (2): 72 to 74 GPa). In the buffer layer made of such a highly rigid material, the influence of the modulation electrode on the stress of the substrate is greatly exhibited, and particularly, when the thickness of the substrate is thin, there is a problem that the stress generated in the substrate cannot be sufficiently relaxed.

The optical modulators disclosed in patent documents 1 and 2 have the effects of suppressing the occurrence of chirp, preventing the decrease in modulation efficiency, suppressing the drive voltage, and the like, but have less effect of reducing the stress of the substrate generated by the modulation electrode. Therefore, the optical modulators disclosed in patent documents 1 and 2 cannot solve the above-described problems discussed in the present invention.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical modulator capable of preventing damage to a substrate and preventing deterioration of characteristics of the modulator by reducing stress of the substrate generated by a modulation electrode.

Means for solving the problems

In order to solve the above problem, an optical modulator according to the present invention has the following features.

(1) In order to achieve the above object, an optical modulator according to the present invention includes: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and a modulation electrode provided on the substrate and modulating an optical wave propagating through the optical waveguide, wherein a resin is disposed between a part of a bottom surface of the modulation electrode and the substrate facing the part of the bottom surface of the modulation electrode.

According to this configuration, the resin disposed between the part of the bottom surface of the modulator electrode and the substrate facing the part of the bottom surface of the modulator electrode can reduce the stress on the substrate caused by the modulator electrode. The resin can ensure a larger film thickness than a conventional buffer layer formed by sputtering film formation, and can suppress the influence of the modulation electrode on the stress of the substrate. Further, the resin and SiO used in the buffer layer described above₂And the like are materials having lower rigidity than the other materials (Young's modulus of resin: approximately 1 to 2GPa), and even if there is a difference in linear expansion coefficient between the modulation electrode and the substrate, they function as buffer materials for relaxing stress caused by the difference in linear expansion coefficient. As a result, the resin is disposed, so that damage to the substrate can be prevented, and deterioration in the characteristics of the modulator can be prevented.

(2) The optical modulator according to the above (1), wherein the modulation electrode is composed of a signal electrode and a ground electrode arranged along a part of the optical waveguide, and the resin is arranged between a part of a bottom surface of at least one of the signal electrode and the ground electrode and the substrate facing at least one of the signal electrode and the ground electrode.

According to this configuration, the resin disposed between the part of the bottom surface of at least one of the signal electrode and the ground electrode and the substrate facing the part of the bottom surface of at least one of the signal electrode and the ground electrode can reduce the stress on the substrate caused by at least one of the signal electrode and the ground electrode, and can prevent damage to the substrate and deterioration in the characteristics of the modulator.

(3) The optical modulator according to the above (2), wherein in a case where the resin is disposed between the signal electrode and the substrate, a width of the resin is set to 1/3 or less of a width of the signal electrode, and in a case where the resin is disposed between the ground electrode and the substrate, a width of the resin is set to 1/2 or less of a width of the ground electrode.

According to this configuration, by setting the ratio of the width of the resin to the width of the modulation electrode as described above, it is possible to apply an effective electric field to the optical waveguide and suppress peeling of the modulation electrode from the substrate.

(4) The optical modulator according to any one of the above (1) to (3), wherein the resin is disposed in the modulation electrode.

According to this configuration, by forming the resin on the normal substrate and then forming the electrode so as to embed the resin, the resin can be easily and reliably disposed between the part of the bottom surface of the modulator electrode and the substrate facing the part of the bottom surface of the modulator electrode.

(5) The optical modulator according to any one of the above (1) to (4), wherein the resin is disposed so as to apply a bilaterally symmetric electric field to a pair of optical waveguides as modulation targets.

According to this configuration, for example, a bilaterally symmetric electric field can be applied to a pair of optical waveguides in a mach-zehnder type waveguide or the like, and it is possible to suppress the non-uniformity of the modulation efficiency that may occur due to the asymmetry of the electric field, the occurrence of chirp due to the asymmetry of the modulation efficiency, and the like.

(6) The optical modulator according to any one of the above (1) to (5), wherein the resin is disposed so that a contact surface between the bottom surface of the modulation electrode and the substrate is disposed on a proximal end side of the optical waveguide.

According to this configuration, the electric field can be efficiently concentrated on the optical waveguide, and the modulation efficiency of the optical wave in the optical waveguide can be improved.

(7) The optical modulator according to any one of the above (1) to (6), wherein a thickness of the resin is 1.0 μm or more.

According to this configuration, the resin capable of reliably reducing the thickness of the stress of the substrate generated by the modulator electrode can be disposed between the part of the bottom surface of the modulator electrode and the substrate facing the part of the bottom surface of the modulator electrode, and damage to the substrate and deterioration in the characteristics of the modulator can be more reliably prevented.

(8) The optical modulator according to any one of the above (1) to (7), wherein the resin is one of a thermoplastic resin and a thermosetting resin.

According to this configuration, by using a photoresist made of either a thermoplastic resin or a thermosetting resin, it is possible to reduce the stress of the substrate generated by the modulator electrode, prevent damage to the substrate, and prevent deterioration of the characteristics of the modulator. In particular, the resin can be formed on the substrate by a photolithography process, and the pattern shape, thickness, and the like of the resin can be controlled with high accuracy and ease.

(9) The optical modulator according to any one of the above (1) to (8), wherein a thickness of the substrate is 4.0 μm or less.

According to this configuration, even when the influence of the stress generated in the substrate increases as the substrate becomes thinner, the stress generated in the substrate by the modulator electrode can be reduced by the resin disposed between the part of the bottom surface of the modulator electrode and the substrate facing the part of the bottom surface of the modulator electrode, and damage to the substrate and deterioration in the characteristics of the modulator can be prevented.

(10) The optical modulator according to any one of the above (1) to (9), characterized in that a rib portion protrudingly provided on the substrate is used as the optical waveguide.

According to this configuration, even when the influence of the stress generated in the substrate along with the substrate formed of the rib waveguide becomes large, the stress of the substrate generated by the modulation electrode can be reduced by the resin disposed between the part of the bottom surface of the modulation electrode and the substrate facing the part of the bottom surface of the modulation electrode, and damage to the substrate can be prevented and deterioration of the characteristics of the modulator can be prevented.

(11) The optical modulator according to any one of the above (1) to (10), wherein the modulation electrode is made of metal, and the substrate is made of lithium niobate.

According to this configuration, even when stress is generated due to a difference in linear expansion coefficient between the substrate made of lithium niobate and the modulation electrode made of metal, the stress can be reduced, damage to the substrate can be prevented, and deterioration in the characteristics of the modulator can be prevented.

(12) The optical modulator according to any one of the above (1) to (11), wherein the optical waveguide is formed of a plurality of mach-zehnder portions.

According to this configuration, in the mach-zehnder type optical modulator in which a plurality of mach-zehnder type optical waveguides capable of generating optical signals corresponding to various modulation schemes are integrated, the stress of the substrate caused by the modulation electrode can be reduced by the resin disposed between the part of the bottom surface of the modulation electrode and the substrate facing the part of the bottom surface of the modulation electrode, and damage to the substrate and deterioration in the characteristics of the modulator can be prevented.

Effects of the invention

According to the present invention, in the optical modulator, by reducing the stress of the substrate generated by the modulation electrode, it is possible to prevent damage to the substrate and prevent deterioration of the characteristics of the modulator.

Drawings

Fig. 1 is a plan view for explaining an example of an optical waveguide formed on a substrate constituting an optical modulator in the embodiment of the present invention.

Fig. 2A is a diagram showing an example of a cross-sectional structure of an optical modulator according to an embodiment of the present invention, and is a cross-sectional view taken along line P-P in fig. 1.

Fig. 2B is a diagram showing an example of a cross-sectional structure of an optical modulator according to an embodiment of the present invention, and is a diagram showing an example of a cross-sectional structure of an optical modulator of another example in which an optical waveguide is formed in a substrate.

Fig. 3 is a plan view schematically showing an example of an optical modulator according to an embodiment of the present invention, and is a diagram schematically showing an arrangement pattern of a resin in the region R in fig. 1.

Fig. 4A is a plan view schematically showing an example of an optical modulator according to an embodiment of the present invention, and is a diagram showing a derivative example of the arrangement pattern of the resin in the region R in fig. 1.

Fig. 4B is a plan view schematically showing an example of an optical modulator according to an embodiment of the present invention, and is a diagram showing a derivative example of the arrangement pattern of the resin in the region R in fig. 1.

Fig. 4C is a plan view schematically showing an example of an optical modulator according to an embodiment of the present invention, and is a diagram showing a derivative example of the arrangement pattern of the resin in the region R in fig. 1.

Fig. 5 is a diagram showing a first example of a cross-sectional structure of an optical modulator according to an embodiment of the present invention.

Fig. 6 is a diagram showing a second example of the cross-sectional structure of the optical modulator according to the embodiment of the present invention.

Fig. 7 is a diagram showing a third example of the cross-sectional structure of the optical modulator according to the embodiment of the present invention.

Fig. 8 is a diagram showing a fourth example of the cross-sectional structure of the optical modulator according to the embodiment of the present invention.

Fig. 9 is a diagram showing a fifth example of the cross-sectional structure of the optical modulator according to the embodiment of the present invention.

Fig. 10 is a diagram showing a sixth example of the cross-sectional structure of the optical modulator according to the embodiment of the present invention.

Detailed Description

The following describes an optical modulator according to an embodiment of the present invention.

Fig. 1 is a plan view for explaining an example of an optical waveguide formed on a substrate, which constitutes an optical modulator in the embodiment of the present invention. In the drawings, the width direction of the light modulator is defined as the X axis, the length direction of the light modulator is defined as the Y axis, and the thickness direction of the light modulator is defined as the Z axis.

The optical modulator 1 shown in fig. 1 is an optical modulator 1 in which a plurality of mach-zehnder type optical waveguides are integrated, and is also called a nested type optical modulator. The optical modulator 1 in which a plurality of mach-zehnder type optical waveguides are integrated can generate optical signals corresponding to various modulation systems. In fig. 1, an optical modulator 1 in which a plurality of mach-zehnder type optical waveguides are integrated is shown as an example, but the present invention is not limited to this configuration, and for example, an optical modulator 1 having a single mach-zehnder type optical waveguide may be used.

As shown in fig. 1, an optical modulator 1 according to an embodiment of the present invention includes an optical waveguide 10 formed on a substrate 5 made of a material having an electro-optical effect. The optical modulator 1 shown in fig. 1 includes a first branch portion 2a that branches an incident waveguide into which an optical signal is introduced from the outside, a second branch portion 2b that further branches an optical waveguide 10 branched by the first branch portion 2a, and a third branch portion 2c that further branches an optical waveguide 10 branched by the second branch portion 2b, and is branched in 3 stages to form 8 parallel waveguides in total. The first to third branch portions 2a to 2c are realized by optical couplers or the like. The phase of the light wave propagating through each of the parallel waveguides is adjusted by, for example, applying an electric field 11 to each of the parallel waveguides using a metal modulation electrode (not shown in fig. 1) disposed in the vicinity of each of the parallel waveguides.

The light waves propagating through the parallel waveguides are combined by the first to third combining portions 3a to 3c corresponding to the first to third branching portions 2a to 2c, respectively, and then guided from the outgoing waveguide to the outside. Specifically, the optical modulator 1 shown in fig. 1 includes a third combining section 3c that combines parallel waveguides branched by the third branching section 2c, a second combining section 3b that combines optical waveguides 10 branched by the second branching section 2b, and a first combining section 3a that combines optical waveguides 10 branched by the first branching section 2a, and outputs an optical signal from an output waveguide through 3-stage combination. The first to third combining parts 3a to 3c are also realized by optical couplers or the like, as in the first to third branching parts 2a to 2 c.

Fig. 2A is a diagram showing an example of a cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention, and is a cross-sectional view taken along line P-P in fig. 1. Fig. 3 is a plan view schematically showing an example of the optical modulator 1 according to the embodiment of the present invention, and is a diagram schematically showing an arrangement pattern of the resin 8 in the region R in fig. 1.

As shown in the cross-sectional structure of fig. 2A, the light modulator 1 has a structure in which a substrate 5 is provided over a reinforcing substrate 7 and further a modulation electrode is provided over the substrate 5.

The substrate 5 is formed of a material having an electro-optical effect. The substrate 5 according to the embodiment of the present invention can be an extremely thin plate having a thickness of about 1.0 to 2.0 μm, for example, as compared with the conventional substrate having a thickness of about 8 to 10 μm. For example, LN can be used as the material having the electro-optical effect for the substrate 5, but lithium tantalate (LiTaO) can be used₃) Lead lanthanum zirconate titanate (PLZT), and the like. Fig. 2A shows, as an example, a cross-sectional structure of an optical modulator 1, and the optical modulator 1 is an LN modulator using an X-cut substrate 5 in which an optical waveguide 10 is arranged between modulation electrodes in an operation portion (modulation portion). However, the LN modulator may be one in which the X-cut substrate 5 of the optical waveguide 10 is disposed between the modulation electrodes in the working section, or may be one in which the Z-cut substrate 5 of the optical waveguide 10 is disposed below the modulation electrodes.

As shown in fig. 2A, a rib 6 is provided on the substrate 5. The rib 6 protrudes from the surface of the substrate 5, and functions to block light waves, and thus serves as the optical waveguide 10. Fig. 2A illustrates the optical modulator 1 having a rib-type substrate in which the rib 6 is formed on the substrate 5, as an example, but the configuration is not limited to this, and the optical modulator 1 may be formed with the optical waveguide 10 in the substrate 5 by, for example, thermal diffusion of metal as in fig. 2B.

The thickness of the rib substrate can be made extremely thin to be 1.0 to 2.0 μm compared with the thickness of the conventional substrate 5 of 8.0 to 10.0 μm, and the velocity matching between the microwave and the optical wave and the reduction of the drive power supply can be realized. However, in such an extremely thin substrate 5, the influence of the compressive stress from the modulator electrode disposed on the substrate 5 is largely exhibited, and there is a problem that the substrate 5 is damaged to cause cracks or the like, and the present invention can cope with this problem.

In the optical modulator 1 according to the embodiment of the present invention, for example, the maximum value of the thickness a of the substrate 5 including the rib 6 is 4.0 μm, the maximum value of the width B of the rib 6 is 4.0 μm, the maximum value of the height C of the rib 6 is 2.0 μm, and the ratio of the thickness a to the width B is 1: 1. since the smaller the rib, the substrate, and the like, the better in design, the minimum values of the thickness a, the width B, and the height C described above become limit values for minimization in the manufacturing process. From the viewpoint of blocking light, if the size is within the range in which the single mode condition of light is maintained, the smaller the size of each of the thickness a and the width B, the more light is blocked, which is preferable.

The modulation electrode includes a signal electrode S and a ground electrode G. The modulation electrode is formed by, for example, depositing Ti/Au on the substrate 5 and then patterning the electrode by a photolithography process. The modulation electrode is not particularly limited as long as it is made of an appropriate metal, and a method for forming the modulation electrode on the substrate 5 is also possible. The thickness of the modulation electrode is, for example, 20 μm or more.

The signal electrode S is an electrode for applying an electric field 11 to the optical waveguide 10, for example, as shown in fig. 5, and is disposed so as to extend parallel to the optical waveguide 10, for example. Although not shown, the signal electrode S is connected to a signal source and a terminating resistor, and a high-frequency electric signal is supplied from the signal source and terminated by the terminating resistor. The ground electrode G is an electrode connected to a reference potential point, and is disposed so as to extend parallel to the optical waveguide 10, for example, as in the case of the signal electrode S. The signal electrode S is provided spaced apart from the ground electrode G, and an electric field 11 is formed between the signal electrode S and the ground electrode G. The signal electrode S and the ground electrode G form a coplanar line, for example.

The reinforcing substrate 7 is a member that can stably support the substrate 5 and the modulator electrodes on the substrate 5 by supplementing the strength of the extremely thin substrate 5. The reinforcing substrate 7 is directly bonded to the back surface side of the substrate 5, or bonded to the back surface side of the substrate 5 with an adhesive or the like. In the case where the substrate 5 and the reinforcing substrate 7 are directly bonded, for example, a material having a lower dielectric constant than the material (for example, LN) of the substrate 5 can be used for the reinforcing substrate 7. On the other hand, when the substrate 5 and the reinforcing substrate 7 are bonded with an adhesive, the reinforcing substrate 7 may be made of the same material (for example, LN) as the substrate 5, for example. In this case, an adhesive having a lower dielectric constant and refractive index than the material (e.g., LN) of the substrate 5 is used, and the thickness of the adhesive layer between the substrate 5 and the reinforcing substrate 7 is, for example, 30 μm or more.

As shown in fig. 2A, the resin 8 is partially disposed between the modulator electrode and the substrate 5. As shown in the arrangement pattern of fig. 3, the resin 8 is arranged so as to extend between the modulation electrodes and the substrate 5, for example, along the propagation direction of the optical wave, that is, the direction in which the optical waveguide 10 extends. The resin 8 functions as a buffer material for relaxing stress between the modulator electrode and the substrate 5 due to its viscoelastic property. With SiO being generally used for buffer layers₂Etc. of a material having high rigidity (SiO)₂Young's modulus of (2): 72 to 74GPa), the resin 8 is a material having a lower rigidity (young's modulus of resin: approximately 1 to 2GPa) and can function as a buffer material for relaxing stress caused by a difference in linear expansion coefficient between the modulator electrode and the substrate 5.

In the example shown in fig. 2A, the resin 8 is disposed between the signal electrode S and the substrate 5 and between the ground electrode G and the substrate 5, but the resin 8 may be disposed only between the signal electrode S and the substrate 5 and between the ground electrode G and the substrate 5.

By partially disposing the resin 8 between the modulator electrode and the substrate 5, a contact surface between the modulator electrode and the resin 8, a contact surface between the modulator electrode and the substrate 5, and a contact surface between the substrate 5 and the resin 8 are generated, and the contact area between the modulator electrode and the substrate 5 can be reduced according to the amount of the disposed resin 8. This can reduce the stress generated in the substrate 5 by the modulator electrode.

The resin 8 is a resin such as a thermoplastic resin or a thermosetting resin, and examples thereof include a polyamide resin, a melamine resin, a phenol resin, an amino resin, and an epoxy resin.

The resin 8 is, for example, a permanent resist, and is a photoresist made of a thermosetting resin. In the manufacturing process of the optical waveguide element, the resin 8 is applied to the substrate 5 by spin coating, patterned by a general photolithography process, and then thermally cured, whereby the resin 8 can be disposed between the modulator electrodes and the substrate 5. Patterning by a photolithography process enables formation of a fine pattern shape with higher accuracy than conventional sputtering film formation, and is suitable for formation of a resin on the substrate 5 according to the embodiment of the present invention. Further, the buffer layer formed by the conventional sputtering film formation has a small film thickness, and when the resin 8 is applied by spin coating, the film thickness can be freely controlled as long as the film thickness is 1 μm or more, and is suitable for the resin formation on the substrate 5 according to the embodiment of the present invention. After the resin 8 is formed on the substrate 5, the resin 8 can be easily and reliably locally disposed between the modulator electrode and the substrate 5 by forming the electrode so that the resin 8 is embedded therein.

In fig. 3, an example in which the resin 8 is disposed so as to extend between the modulation electrode and the substrate 5 along the extending direction of the optical waveguide 10 is shown as the arrangement pattern of the resin 8, but for example, the arrangement pattern shown in fig. 4A to 4C may be adopted.

Fig. 4A to 4C are plan views schematically showing an example of the optical modulator 1 according to the embodiment of the present invention, and are views schematically showing a derivative example of the arrangement pattern of the resin 8 in the region R in fig. 1.

In the arrangement pattern shown in fig. 4A, a plurality of units of the resin 8 (4 units of the resin 8 in fig. 4A) are arranged along the extending direction of the optical waveguide 10. In the arrangement pattern shown in fig. 4B, a plurality of resins 8 are arranged in the width direction within 1 modulation electrode, and these resins 8 are arranged along the extending direction of the optical waveguide 10. In the arrangement pattern shown in fig. 4C, a plurality of resins 8 are arranged along the width direction in 1 modulation electrode, and these resins 8 are arranged intermittently along the extending direction of the optical waveguide 10.

The arrangement pattern of the resin 8 in the width direction and the extending direction of the optical waveguide 10 described in the present embodiment is merely an example, and any arrangement pattern may be adopted as long as the stress generated in the substrate 5 by the modulator electrode is alleviated, which is the object of the present invention.

On the other hand, the results of the peel test showed that AS1< AS2 ≦ AS3, the relationship among the adhesion strength AS1 between the modulator electrode and the resin 8, the adhesion strength AS2 between the modulator electrode and the substrate 5, and the adhesion strength AS3 between the resin 8 and the substrate 5. That is, the bonding strength AS1 between the modulator electrode and the resin 8 is smaller than the bonding strength AS2 between the modulator electrode and the substrate 5 and the bonding strength AS3 between the resin 8 and the substrate 5. Therefore, by using a pattern that reduces the contact area between the modulator electrode and the resin 8, it is possible to suppress the modulator electrode from peeling off from the substrate 5 due to weakening of the adhesion.

As described above, when the contact area between the modulator electrode and the substrate 5 is reduced, the stress generated in the substrate 5 can be reduced, and when the contact area between the modulator electrode and the resin 8 is reduced, the separation of the modulator electrode can be suppressed. However, the reduction in the contact area between the modulation electrode and the substrate 5 and the reduction in the contact area between the modulation electrode and the resin 8 are in an inverse relationship. That is, when the width of the resin 8 is increased, the contact area between the modulator electrode and the substrate 5 can be reduced, while the contact area between the modulator electrode and the resin 8 is increased. In addition, when the width of the resin 8 is reduced, the contact area between the modulation electrode and the resin 8 can be reduced, while the contact area between the modulation electrode and the substrate 5 can be increased.

The electric field 11 is emitted perpendicularly from the surface of the signal electrode S toward the ground electrode G, and is perpendicularly incident on the surface of the ground electrode G. In addition, when the dielectric constants are compared, for example, the dielectric constant (ε) of LN used as the material of the substrate 5₁₁＝43，ε₃₃28) is higher than the dielectric constant (e.g., 3 to 4) of the resin 8. Since the electric field 11 is concentrated on the material having a high dielectric constant, when the resin 8 is disposed, the electric field from the signal electrode S toward the ground electrode G is concentrated on the substrate 5 side, and the contact surface between the modulation electrode and the substrate 5 becomes the entrance and exit surface of the electric field 11 applied to the optical waveguide 10. If the width of the resin 8 is too large relative to the width of the modulation electrode, the electric field 11 may not be appropriately applied to the optical waveguide 10. Further, the contact surface between the signal electrode S from which the electric field 11 is emitted and the substrate 5 is preferably set larger than the contact surface between the ground electrode G from which the electric field 11 is incident and the substrate 5. In the present specification, the direction of the electric field 11 is a direction from the signal electrode S toward the ground electrode G, and therefore the contact surface between the signal electrode S and the substrate 5 is represented as the entrance surface of the electric field 11, and the contact surface between the ground electrode G and the substrate 5 is represented as the contact surface between the ground electrode G and the substrate 5The exit face of the electric field 11.

From the above viewpoint, in the embodiment of the present invention, the width of the resin 8 disposed between the signal electrode S and the substrate 5 is set to be 1/3 or less with respect to the width of the signal electrode S. The width of the resin 8 disposed between the ground electrode G and the substrate 5 is set to 1/2 or less with respect to the width of the ground electrode G. By setting the ratio of the width of the resin 8 to the width of the modulation electrode as described above, an effective electric field can be applied to the optical waveguide 10, and a structure in which the modulation electrode is suppressed from peeling from the substrate 5 can be realized. In the case where a plurality of resins 8 are arranged in the width direction with respect to 1 modulation electrode (for example, refer to fig. 8 showing the case where a plurality of resins 8 are arranged in 1 signal electrode S), the sum of the widths of the plurality of resins 8 is regarded as the width of the resin 8.

In the present specification, "the resin 8 is partially disposed between the modulation electrode and the substrate 5" means that the resin 8 is disposed between a part of the bottom surface of the modulation electrode and the substrate 5 facing the part of the bottom surface of the modulation electrode. More specifically, it means that, for example, the width of the resin 8 is set to be smaller than the width of the modulator electrode at the above ratio, and 3 contact surfaces, that is, the contact surface between the modulator electrode and the resin 8, the contact surface between the modulator electrode and the substrate 5, and the contact surface between the substrate 5 and the resin 8 are generated by the arrangement of the resin 8.

The position of the resin 8 disposed between the modulation electrode and the substrate 5 is not particularly limited, and the position of the resin 8 and the state of the electric field 11 formed between the signal electrode S and the ground electrode G at the position of the resin 8 will be described below by way of a few examples.

Fig. 5 is a diagram showing a first example of the cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention. The sectional structure of fig. 5 is the same as that shown in the sectional view of line P-P of fig. 1, but fig. 5 further shows the state of the electric field 11 formed between the signal electrode S and the ground electrode G and the optical waveguide 10.

Fig. 5 shows a cross-sectional structure of the optical modulator 1 in which the signal electrode S and the ground electrode G are provided on the X-cut substrate 5 in which the optical waveguide 10 is arranged between the modulation electrodes in the active portion of the LN modulator, and the rib 6 of the substrate 5 is used as the optical waveguide 10. An electric field 11 formed between the signal electrode S and the ground electrode G is applied to the optical waveguide 10 formed in the rib 6, and the electric field intensity is adjusted by controlling an electric signal supplied from a signal source, thereby appropriately modulating the light wave propagating in the optical waveguide 10.

When the electric field 11 is formed between the signal electrode S and the ground electrode G, the entrance and exit surfaces of the electric field 11 are narrowed to the contact surface between the modulator electrode and the substrate 5 due to the presence of the resin 8. For example, as shown in fig. 5, the resin 8 is disposed at the center in the width direction of the signal electrode S and the ground electrode G, and is formed at the end in the width direction of the signal electrode S, so that the signal electrode S contacts the substrate 5, and the ground electrode G contacts the substrate 5 at the end in the width direction of the ground electrode G. With this configuration, the entrance and exit surfaces of the electric field 11 are narrowed by the resin 8, and the entrance and exit surfaces of the electric field 11 can be arranged to be biased toward the proximal end side of the optical waveguide 10. As a result, the electric field 11 can be efficiently concentrated on the optical waveguide 10, and the modulation efficiency of the optical wave in the optical waveguide 10 can be improved.

Fig. 6 is a diagram showing a second example of the cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention. Fig. 6 shows a state of an electric field 11 formed between the signal electrode S and the ground electrode G and the optical waveguide 10, together with the cross-sectional structure of the optical modulator 1.

Fig. 6 shows a cross-sectional structure of the optical modulator 1 in which the signal electrode S and the ground electrode G are provided on the Z-cut substrate 5 in which the optical waveguide 10 is arranged under the modulation electrode in the LN modulator, and the rib 6 of the substrate 5 is used as the optical waveguide 10. As shown in fig. 6, in the case where the substrate 5 is Z-cut, the signal electrode S is provided on the rib 6, but a layer having no light absorption, such as a buffer layer or a transparent electrode layer, may be disposed between the optical waveguide 10 and the signal electrode S in order to suppress light absorption by the signal electrode S.

Fig. 6 shows, as an example, a case where the resin 8 is not disposed between the signal electrode S and the rib 6 of the substrate 5, and the resin 8 is disposed only between the ground electrode G and the substrate 5. The ground electrode G has a resin 8 disposed at the center in the width direction. In this case, the presence of the resin 8 causes the ground electrode G to come into contact with the substrate 5 at the end in the width direction of the ground electrode G. With this configuration, the exit surface of the electric field 11 can be reduced by the resin 8, and the exit surface of the electric field 11 can be disposed to be closer to the proximal end of the optical waveguide 10. As a result, the electric field 11 can be efficiently concentrated on the optical waveguide 10, and the modulation efficiency of the optical wave in the optical waveguide 10 can be improved.

Fig. 7 is a diagram showing a third example of the cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention. Fig. 7 shows the state of the electric field 11 formed between the signal electrode S and the ground electrodes G1 and G2, together with the cross-sectional structure of the optical modulator 1, and the optical waveguide 10.

Fig. 7 shows a cross-sectional structure of the optical modulator 1 in which the signal electrode S and the ground electrodes G1 and G2 are provided on the Z-cut substrate 5 in which the optical waveguide 10 is disposed under the modulation electrode in the LN modulator, and the rib 6 of the substrate 5 is used as the optical waveguide 10. As shown in fig. 7, in the case where the substrate 5 is Z-cut, the signal electrode S is provided on the rib 6, but a layer having no light absorption, such as a buffer layer or a transparent electrode layer, may be disposed between the optical waveguide 10 and the signal electrode S in order to suppress light absorption by the signal electrode S.

When comparing the cross-sectional structure of fig. 6 with the cross-sectional structure of fig. 7, the cross-sectional structure of fig. 6 has a configuration in which 1 ground electrode G is disposed near the center, while the cross-sectional structure of fig. 7 has a configuration in which 2 ground electrodes G2 are disposed near the center. The width-directional dimension of the ground electrode G near the center in fig. 6 may become larger depending on the width-directional dimension of the 2 optical waveguides 10. In contrast, in the cross-sectional structure of fig. 7, the ground electrode G near the center of fig. 6 is inserted into the slit, thereby being divided into 2 ground electrodes G2. Since the contact area between the 2 ground electrodes G2 of fig. 7 and the substrate 5 is smaller than the contact area between the ground electrode G of fig. 6 and the substrate 5, the structure shown in fig. 7 has a structure in which the stress generated in the substrate 5 by the modulator electrode is relaxed, compared to the structure shown in fig. 6.

Fig. 7 shows, as an example, a case where the resin 8 is not disposed between the signal electrode S and the rib 6 of the substrate 5, and the resin 8 is disposed only between the ground electrodes G1 and G2 and the substrate 5. The ground electrode G1 has a resin 8 disposed at the center in the width direction. On the other hand, the resin 8 is disposed at the end portion in the width direction and on the distal end side of the optical waveguide 10 in the ground electrode G2. With this configuration, the exit surface of the electric field 11 can be reduced by the resin 8, and the exit surface of the electric field 11 can be disposed to be closer to the proximal end of the optical waveguide 10. As a result, the electric field 11 can be efficiently concentrated on the optical waveguide 10, and the modulation efficiency of the optical wave in the optical waveguide 10 can be improved.

Fig. 8 is a diagram showing a fourth example of the cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention. Fig. 8 shows a state of an electric field 11 formed between the signal electrode S and the ground electrode G and the optical waveguide 10, together with the cross-sectional structure of the optical modulator 1.

Fig. 8 shows a cross-sectional structure of the optical modulator 1 in which the signal electrode S and the ground electrode G are provided on the Z-cut substrate 5 in which the optical waveguide 10 is disposed under the modulation electrode in the LN modulator, and the rib 6 of the substrate 5 is used as the optical waveguide 10. As shown in fig. 8, in the case where the substrate 5 is Z-cut, the signal electrode S is provided on the rib 6, but a layer having no light absorption, such as a buffer layer or a transparent electrode layer, may be disposed between the optical waveguide 10 and the signal electrode S in order to suppress light absorption by the signal electrode S.

Fig. 8 shows, as an example, a case where the resin 8 is disposed between the signal electrode S and the rib 6 of the substrate 5 and between the ground electrode G and the substrate 5. The signal electrode S has a resin 8 disposed at the end in the width direction. The ground electrode G has a resin 8 disposed at the center in the width direction. For example, as shown in fig. 8, the signal electrode S is in contact with the substrate 5 at the center in the width direction of the signal electrode S, and the ground electrode G is in contact with the substrate 5 at the end in the width direction of the ground electrode G. With this configuration, the entrance and exit surfaces of the electric field 11 are narrowed by the resin 8, so that the electric field 11 is concentrated on the optical waveguide 10 below the signal electrode S, and the exit surface of the electric field 11 is disposed to be biased toward the proximal end side of the optical waveguide 10. As a result, the electric field 11 can be efficiently concentrated on the optical waveguide 10, and the modulation efficiency of the optical wave in the optical waveguide 10 can be improved.

Fig. 9 is a diagram showing a fifth example of the cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention. The cross-sectional structure of fig. 9 is similar to that of fig. 5, but differs in that in the cross-sectional structure of fig. 5, the resin 8 is disposed in the modulator electrode, whereas in the cross-sectional structure of fig. 9, the resin 8 is disposed in the substrate 5.

As shown in fig. 9, even when the resin 8 is disposed in the substrate 5, the same operational effects as those of the cross-sectional structure of fig. 5 in which the resin 8 is disposed in the modulator electrode are obtained. In the cross-sectional structure of fig. 9, the entrance and exit surfaces of the electric field 11 can be disposed to be offset toward the proximal end side of the optical waveguide 10 by the disposition of the resin 8. As a result, the electric field 11 can be efficiently concentrated on the optical waveguide 10, and the modulation efficiency of the optical wave in the optical waveguide 10 can be improved.

Fig. 10 is a diagram showing a sixth example of the cross-sectional structure of the optical modulator 1 according to the embodiment of the present invention. The cross-sectional structure of fig. 10 is similar to that of fig. 5, but differs in that the resin 8 is disposed within the modulator electrode in the cross-sectional structure of fig. 5, whereas the resin 8 is disposed across both the modulator electrode and the substrate 5 in the cross-sectional structure of fig. 10.

As shown in fig. 10, even when the resin 8 is disposed across both the modulator electrode and the substrate 5, the same operational effects as those of the cross-sectional structure of fig. 5 in which the resin 8 is disposed inside the modulator electrode are exhibited. In the cross-sectional structure of fig. 10, the entrance and exit surfaces of the electric field 11 can be disposed to be offset toward the proximal end side of the optical waveguide 10 by the disposition of the resin 8. As a result, the electric field 11 can be efficiently concentrated on the optical waveguide 10, and the modulation efficiency of the optical wave in the optical waveguide 10 can be improved.

Here, the case where the resin 8 is disposed in the substrate 5 or across both the modulation electrode and the substrate 5 will be described while comparing the cross-sectional structure of fig. 5 with the cross-sectional structures of fig. 9 and 10, but the resin 8 may be disposed in the substrate 5 or across both the modulation electrode and the substrate 5 in the cross-sectional structures of fig. 6 to 8 or any other cross-sectional structures, for example.

In the cross-sectional structures of fig. 5 to 10, the resin 8 is disposed so that the contact surfaces between the signal electrode S, the ground electrode G, and the substrate 5 are bilaterally symmetric with respect to the optical waveguide 10. By disposing the resin 8 in this manner, an electric field can be efficiently applied to the optical waveguide 10. In the cross-sectional structures of fig. 5 to 10, the resin 8 is disposed so that the electric fields applied to a pair of parallel waveguides such as mach-zehnder waveguides are bilaterally symmetric. By disposing the resin 8 in this manner, a bilaterally symmetric electric field 11 can be applied to the pair of parallel waveguides. As a result, it is possible to suppress the modulation efficiency non-uniformity that may occur due to the asymmetry of the electric field 11, the occurrence of chirp (チャーピング) due to the modulation efficiency asymmetry, and the like.

The cross-sectional structures of fig. 5 to 10 can be applied to the arrangement patterns of fig. 3 and 4A to 4C or any arrangement pattern. The cross-sectional structures of fig. 5 to 10 have an effect of improving the modulation efficiency of the optical wave in the optical waveguide 10 by efficiently concentrating the electric field 11 in the optical waveguide 10, in addition to an effect of relaxing the stress generated in the substrate 5 by the modulation electrode. That is, according to the present invention, by locally disposing the resin 8 between the modulation electrode and the substrate 5, the stress generated in the substrate 5 by the modulation electrode can be reduced, and by appropriately designing the disposition position of the resin 8 having the stress relaxation effect, the electric field 11 can be efficiently concentrated in the optical waveguide 10, and the modulation efficiency of the light wave in the optical waveguide 10 can be improved.

In the present embodiment, a rib-shaped substrate in which the rib 6 is formed on the substrate 5 is described as an example. However, as described above, the present invention is not limited to the rib-type substrate, and is also applicable to a normal substrate in which the optical waveguide 10 is formed in the substrate 5 by thermal diffusion of metal (see fig. 2B), for example. In the same manner as in the case of the normal substrate shown in fig. 2B, the resin 8 can be arranged in the arrangement pattern shown in fig. 3 and fig. 4A to 4C or in an arbitrary arrangement pattern. In addition, in the normal substrate shown in fig. 2B, the resin 8 can be arranged at the position shown in fig. 9 and 10 in the same manner.

In the present embodiment, a coplanar line structure in which one ground electrode G is disposed on each of both sides of 1 signal electrode S is described as an example. However, the present invention is not limited to such a coplanar line structure, and for example, a coplanar line structure having a differential line in which one ground electrode G is disposed on each of both sides of 2 signal electrodes S arranged in parallel may be employed.

The present invention is not limited to the above-described embodiments and modifications, and various modifications, design changes, and the like within a range not departing from the technical spirit of the present invention are also included in the technical scope thereof.

Industrial applicability

The present invention provides an optical modulator capable of preventing damage to a substrate and preventing deterioration of characteristics of the modulator by reducing stress of the substrate generated by a modulation electrode, and is applicable to the fields of optical communication, optical measurement, and the like.

Description of the reference symbols

1 optical modulator

2a to 2c branched parts

3a to 3c synthesis part

5 base plate

6 Rib

7 reinforcing base plate

8 resin

10 optical waveguide

11 electric field

G. G1, G2 ground electrode

S signal electrode