JP7463722B2 - Optical waveguide element and optical modulator - Google Patents

Description

The present invention relates to an optical waveguide element and an optical modulator used in the fields of optical communications and optical measurement. In particular, the present invention relates to an optical waveguide element in which an optical waveguide and electrodes are provided on a substrate having an electro-optic effect, and an optical modulator in which the optical waveguide element is packaged.

In recent years, in the fields of optical communications and optical measurement, optical waveguide elements are used, in which an optical waveguide is formed on a substrate having an electro-optic effect, such as lithium niobate (LiNbO3: hereafter referred to as LN). In addition, optical modulators, in which electrodes are provided on an optical waveguide element, are used to modulate the light waves propagating in the optical waveguide formed in the optical waveguide element.

In addition, to achieve a broadband optical modulation frequency, it is important to achieve speed matching between the microwave and light waves, which are the modulation signals. For this reason, attempts are being made to reduce the thickness of the substrate, thereby achieving speed matching between the microwave and light waves and reducing the driving voltage.

The following Patent Document 1 discloses an organic waveguide type optical modulator that has a core layer made of an organic material as a medium for the electro-optic effect, and in which a stress relief layer is formed around the core layer between the core layer and the clad layer. The optical modulator disclosed in Patent Document 1 can relieve stress caused by the difference in thermal expansion coefficient between the materials of the core layer and the clad layer.

The following Patent Document 2 discloses an optical modulator that includes a substrate having an electro-optic effect, two optical waveguides formed on the substrate, and a base that holds the substrate, with an adhesive layer that bonds the substrate to the base positioned far from the two optical waveguides. The optical modulator disclosed in Patent Document 2 can reduce the difference in stress birefringence between the two optical waveguides that occurs due to the difference in thermal expansion coefficient between the base and the substrate.

JP 2006-243376 A JP 2010-181454 A

In an optical modulator having a thin plate rib type optical waveguide structure with a thickness of a few μm or less, a buffer layer with a thickness almost the same as that of the thin plate must be formed by sputtering or vacuum deposition to suppress light absorption by the electrodes. However, the wafer is thinner than conventional ones and is therefore sensitive to stress. In addition, while the wafer is made of, for example, LN having an electro-optic effect, the buffer layer is made of _SiO2 or the like.

The material of the wafer (substrate) and the material of the buffer layer have different coefficients of thermal expansion (linear expansion coefficient). As a result, when the buffer layer is formed in the wafer process or when the wafer or chip is heated, the difference in the coefficients of thermal expansion between the wafer (substrate) and the buffer layer creates stress (internal stress or residual stress) on the surface where the buffer layer and the wafer (substrate) come into contact. As a result, the stress on the substrate caused by the buffer layer can damage the substrate, causing problems such as cracks in the substrate.

The substrate is made of a material with an electro-optical effect, such as LN, and light modulation is performed by applying electricity to change the refractive index. However, if stress is generated in the substrate by the buffer layer, the refractive index of the substrate changes due to the photoelastic effect, causing a problem that the propagation speed of the light waves changes. As a result, for example, in optical waveguide elements with a Mach-Zehnder structure, a phase difference occurs when multiplexing in the Mach-Zehnder structure, causing a problem of characteristic degradation such as bias voltage fluctuation.

The technology disclosed in Patent Document 1 forms a stress relief layer around a core layer made of an organic material to relieve stress between the core layer and the cladding layer, but does not relieve stress on the substrate caused by a buffer layer.

In addition, the technology disclosed in Patent Document 2 is directed to an adhesive layer that bonds a substrate and a base, and is arranged at a position far from the two optical waveguides in order to reduce the difference in stress birefringence between the two optical waveguides, but does not alleviate the stress on the substrate caused by the buffer layer.

In order to solve the above problems, the present invention aims to reduce the effect of stress on the substrate caused by the buffer layer, thereby preventing damage to the substrate and deterioration of the substrate's characteristics that may be caused by the stress.

To solve the above problems, the optical waveguide element and optical modulator according to the present invention have the following technical features:

(1) In order to achieve the above object, the optical waveguide element of the present invention is an optical waveguide element comprising a substrate having an electro-optical effect, a rib-type optical waveguide having a rib portion formed on the substrate, and a buffer layer provided on the substrate, characterized in that a resin is disposed between the substrate and the buffer layer , and the resin is disposed over a part or the entire surface of an area on the buffer layer different from an area on which a modulating electrode is formed, and is not disposed over an area on the buffer layer on which the modulating electrode is formed .

With this configuration, the resin disposed between the substrate and the buffer layer can reduce the stress on the substrate caused by the buffer layer. The resin is a material with lower rigidity (Young's modulus of the resin: approximately 1 to ₂ GPa) than the material used for the buffer layer, such as SiO2, and acts as a buffer material that relieves the stress caused by the difference in thermal expansion coefficients, even if there is a difference in thermal expansion coefficient between the substrate and the buffer layer. As a result, the resin can prevent damage to the substrate and deterioration of the characteristics of the substrate.
Furthermore, with this configuration, the resin can be disposed in a region that does not prevent the application of an electric field from the modulation electrode to the optical waveguide and allows the electric field from the modulation electrode to be appropriately applied to the optical waveguide.
Furthermore, with this configuration, even if the effect of stress increases as the substrate is made thinner due to the rib-type optical waveguide structure, the resin disposed between the substrate and the buffer layer can reduce the stress on the substrate caused by the buffer layer, thereby preventing damage to the substrate and deterioration of the substrate's characteristics.

( 2 ) The optical waveguide element according to (1) above, wherein the resin has a thickness of 1.0 μm or more.

This configuration allows the resin to be disposed in a thickness that can reliably reduce the stress on the substrate caused by the buffer layer, preventing damage to the substrate and deterioration of its characteristics.

( 3 ) In the optical waveguide element according to (1) or (2) above, the resin is either a thermoplastic resin or a thermosetting resin.

With this configuration, the photoresist made of either a thermoplastic resin or a thermosetting resin can be used to reduce the stress on the substrate caused by the buffer layer, preventing damage to the substrate and deterioration of its characteristics. In particular, the resin can be formed on the substrate by a photolithography process, and the pattern shape and thickness of the resin can be easily controlled with high precision.

( 4 ) In the optical waveguide element according to any one of (1) to ( 3 ) above, the thickness of the substrate is 4.0 μm or less.

With this configuration, even if the impact of stress on the substrate increases as the substrate becomes thinner, the resin disposed between the substrate and the buffer layer can reduce the stress on the substrate caused by the buffer layer, preventing damage to the substrate and deterioration of the substrate's characteristics.

( 5 ) In the optical waveguide element according to any one of (1) to ( 4 ) above, the optical waveguide is formed by a plurality of Mach-Zehnder sections.

With this configuration, in an optical waveguide element having multiple Mach-Zehnder type optical waveguide structures capable of generating optical signals corresponding to various modulation methods, the resin disposed between the substrate and the buffer layer can reduce the stress on the substrate caused by the buffer layer, preventing damage to the substrate and deterioration of the characteristics of the substrate.

( 6 ) In order to achieve the above object, an optical modulator according to the present invention is characterized in that it uses, at least in part, an optical waveguide constituting the optical waveguide element described in any one of (1) to ( 5 ) above.

This configuration realizes an optical modulator that can reduce the stress on the substrate caused by the buffer layer by using the resin disposed between the substrate and the buffer layer, preventing damage to the substrate and deterioration of the substrate's characteristics.

In accordance with the present invention, in optical waveguide elements and optical modulators, the effect of stress on the substrate caused by the buffer layer can be reduced, thereby preventing damage to the substrate and deterioration of the characteristics of the substrate that may be caused by the stress.

FIG. 2 is a plan view for explaining an example of an optical waveguide formed on a substrate constituting an optical waveguide element in an embodiment of the present invention. FIG. 2 is a diagram showing a first example of a cross-sectional structure of an optical waveguide element according to an embodiment of the present invention, and is a cross-sectional view taken along line P-P in FIG. FIG. 2 is a plan view showing an example of an optical waveguide element according to an embodiment of the present invention, and is a diagram showing an example of a resin disposition pattern in a region R in FIG. 1 . 2A to 2D are plan views each showing a schematic example of an optical waveguide element according to an embodiment of the present invention, each showing a derivative example of a resin disposition pattern in region R in FIG. FIG. 13 is a diagram showing a second example of a cross-sectional structure of an optical waveguide element in an embodiment of the present invention, showing a state in which a modulation electrode is formed on a substrate. FIG. 13 is a diagram showing a third example of a cross-sectional structure of an optical waveguide element in an embodiment of the present invention, showing a state in which a modulation electrode is formed on a substrate. FIG. 13 is a diagram showing a fourth example of a cross-sectional structure of an optical waveguide element in an embodiment of the present invention, showing a state in which a modulation electrode is formed on a substrate. FIG. 13 is a diagram showing a fifth example of a cross-sectional structure of an optical waveguide element in an embodiment of the present invention, showing a state in which a modulation electrode is formed on a substrate. 1 is a plan view showing an example of a configuration of an optical modulator according to an embodiment of the present invention.

The following describes the optical waveguide element and optical modulator according to the embodiment of the present invention.

Figure 1 is a plan view illustrating an example of an optical waveguide 10 formed on a substrate 5 that constitutes an optical waveguide element 1 in an embodiment of the present invention. Note that in Figure 1, the optical waveguide element 1 is illustrated so that the width direction of the optical waveguide element 1 is the up-down direction on the paper, the length direction of the optical waveguide element 1 is the left-right direction on the paper, and the thickness direction of the optical waveguide element 1 is perpendicular to the paper.

The optical waveguide element 1 shown in FIG. 1 is an optical waveguide element 1 in which multiple Mach-Zehnder type optical waveguides are integrated. An optical waveguide in which multiple Mach-Zehnder type optical waveguides are combined is also called a nested optical waveguide. The optical waveguide element 1 in which multiple Mach-Zehnder type optical waveguides are integrated can generate optical signals corresponding to various modulation methods. Although FIG. 1 shows an optical waveguide element 1 in which multiple Mach-Zehnder type optical waveguides are integrated as an example, the present invention is not limited to this structure, and may be, for example, an optical waveguide element 1 having a single Mach-Zehnder type optical waveguide.

As shown in FIG. 1, the optical waveguide element 1 according to the embodiment of the present invention includes an optical waveguide 10 formed on a substrate 5 made of a material having an electro-optic effect. The optical waveguide element 1 shown in FIG. 1 includes a first branching section 2a that branches an input waveguide into which an optical signal is introduced from the outside, a second branching section 2b that further branches the optical waveguide 10 branched at the first branching section 2a, and a third branching section 2c that further branches the optical waveguide 10 branched at the second branching section 2b, forming a total of eight parallel waveguides through three stages of branching. The first to third branches 2a to 2c are realized by optical couplers or the like.

The phase of the light waves propagating through each parallel waveguide is adjusted, for example, in region D1. A metallic modulation electrode (not shown in FIG. 1) is formed in region D1, and the refractive index can be changed by the electric field applied from the modulation electrode to each parallel waveguide, thereby adjusting the propagation speed of the light waves.

The light waves propagating through each parallel waveguide are multiplexed in the first to third synthesis units 3a to 3c corresponding to the first to third branches 2a to 2c, respectively, and then output to the outside from the output waveguide. Specifically, the optical waveguide element 1 shown in FIG. 1 includes a third synthesis unit 3c that synthesizes the parallel waveguides branched at the third branch 2c, a second synthesis unit 3b that synthesizes the optical waveguides 10 branched at the second branch 2b, and a first synthesis unit 3a that synthesizes the optical waveguides 10 branched at the first branch 2a, and an optical signal is output from the output waveguide after three stages of synthesis. As with the first to third branches 2a to 2c, the first to third synthesis units 3a to 3c are also realized by optical couplers or the like.

The optical waveguide 10 of the optical waveguide element 1 shown in FIG. 1 is just an example, and the present invention is not limited to this configuration. For example, as in the optical waveguide element 202 of the optical modulator 200 described below with reference to FIG. 9, two optical signals may be output from the optical waveguide element 202 and polarized and combined by the polarized beam combiner 228.

A bias voltage is applied to the optical waveguide 10 to set the operating point. The bias voltage is applied to the phase-modulated light wave by, for example, a bias electrode formed in region D2.

Figure 2 is a diagram showing a first example of the cross-sectional structure of an optical waveguide element 1 in an embodiment of the present invention, and is a cross-sectional view taken along line segment P-P in Figure 1. Note that in Figure 2, the optical waveguide element 1 is illustrated so that the thickness direction of the optical waveguide element 1 is the up-down direction on the paper, the width direction of the optical waveguide element 1 is the left-right direction on the paper, and the longitudinal direction of the optical waveguide element 1 is perpendicular to the paper.

As shown in the cross-sectional structure of FIG. 2, the optical waveguide element 1 has a structure in which a substrate 5 is provided on a reinforcing substrate 7, and a buffer layer 9 is further provided on the substrate 5.

The substrate 5 is formed of a material having an electro-optic effect. Conventional substrates have a thickness of about 8 to 10 μm, whereas the substrate 5 in the embodiment of the present invention can be an extremely thin plate having a thickness of, for example, 2.0 μm or less, preferably 1.0 μm or less. By making the thickness of the substrate 5 extremely thin in this way (for example, about 1/10 of the conventional thickness), it is possible to realize a further reduction in the driving voltage. For example, LN can be used as a material having an electro-optic effect for the substrate 5, but lithium tantalate (LiTaO ₃ ), lead lanthanum zirconate titanate (PLZT), etc. may also be used.

A rib portion 6 is provided on the substrate 5. The rib portion 6 protrudes from the surface of the substrate 5 and is used as an optical waveguide 10 because it has the effect of confining light waves. In a conventional diffusion-type optical waveguide structure, the effect of confining light is weak, and leakage of propagating light from the optical waveguide 10 may occur at curved sections, etc. In contrast, when a rib-type optical waveguide structure is adopted, the effect of confining light is strengthened, and the optical waveguide 10 can be bent to form a folded structure, making it possible to shorten the length of the optical waveguide element 1. The height of the rib portion 6 is, for example, 2.0 μm or less from the surface of the substrate 5, and preferably 1.0 μm or less.

The dimensions of the rib-type substrate are described in more detail below. In the rib-type substrate of the embodiment of the present invention, for example, the maximum thickness A of the substrate 5 including the rib portion 6 is 4.0 μm, the maximum width B of the rib portion 6 is 4.0 μm, the maximum height C of the rib portion 6 is 2.0 μm, and the ratio of thickness A to width B is 1:1. Since the smaller the rib portion 6 and substrate 5 are designed, the better, the minimum values of the thickness A, width B, and height C are the limit values for minimization in the manufacturing process. Also, from the viewpoint of light confinement, as long as the dimensions are within a range in which the single mode condition of light is maintained, the smaller the dimensions of thickness A and width B, the more light is confined, which is preferable.

As an example, FIG. 2 shows an optical waveguide element 1 having a rib-type substrate on which a rib portion 6 is formed on a substrate 5. However, in the present invention, it is preferable to have a structure having a rib-type substrate on which a rib portion 6 is formed as an optical waveguide 10, but this is not limited thereto, and for example, an optical waveguide element 1 in which an optical waveguide 10 is formed in a substrate 5 by thermal diffusion of a metal may be used.

The reinforcing substrate 7 is a member that supplements the strength of the extremely thin substrate 5 and enables stable support of the substrate 5, the buffer layer 9, and even the electrodes formed on the substrate 5. The reinforcing substrate 7 is directly bonded to the back side of the substrate 5 by a direct bonding method, for example. The material of the reinforcing substrate 7 can be, for example, a material with a lower dielectric constant than the material of the substrate 5 (e.g., LN), or the same material as the substrate 5 (e.g., LN).

Direct bonding methods are broadly divided into two types: plasma activated bonding and FAB (Fast Atom Beam) bonding.

The plasma activation bonding method is a method in which the two surfaces to be bonded are hydrophilically treated with plasma or the like to improve their bondability, and then the two surfaces are superimposed on each other to perform direct bonding. When the plasma activation bonding method is used, the molecular chains on each surface of the substrate 5 and the reinforcing substrate 7 become entangled and compatible to form an interface layer (bonding layer).

On the other hand, the FAB method is a method in which a thin Si layer or a metal oxide layer is formed on each of the two surfaces to be bonded, and the two surfaces are activated by irradiating them with a neutron atomic beam at room temperature, and then the two surfaces are bonded together to perform direct bonding. When the FAB method is used, an adhesive layer such as a thin Si layer or a metal oxide layer is formed between the substrate 5 _and the reinforcing substrate 7. For _the adhesive layer 20, Si, _Al2O3 , _Ta2O5 , _TiO2 , _{Nb2O5 , Si3N4} , AlN, _{SiO2 ,} etc. are used.

A buffer layer 9 is provided on the substrate 5. The buffer layer 9 in the embodiment of the present invention has a thickness equivalent to that of the substrate 5, for example, 2.0 μm or less, preferably 1.0 μm or less. The material used for the buffer layer 9 is not particularly limited, but is preferably a material having a lower refractive index than LN and excellent light transmittance. The material used for the buffer layer 9 may be SiO ₂ , Al ₂ O ₃ , MgF ₃ , La ₂ O ₃ , ZnO, HfO ₂ , MgO, CaF ₂ , Y ₂ O _3, or the like, which are generally used for buffer layers 9.

While the thickness of conventional substrates was 8.0 to 10.0 μm, in the embodiment of the present invention, the thickness of the rib-type substrate can be made extremely thin, at 2.0 μm or less, as described above, making it possible to achieve speed matching between microwaves and light waves and further reduce the driving voltage. However, such an extremely thin substrate 5 is particularly sensitive to stress.

As described above, for example, LN is used for the substrate 5, whereas for example, SiO ₂ is used for the buffer layer 9, but the LN material for the substrate 5 and the SiO ₂ material for the buffer layer 9 have different thermal expansion coefficients. As a result, when the buffer layer 9 is formed or the wafer (substrate 5) or chip is heated, particularly in a wafer process involving temperature changes, the difference in thermal expansion coefficient between the substrate 5 and the buffer layer 9 generates stress (internal stress or residual stress) on the contact surface between the buffer layer 9 and the substrate 5.

As a result, the stress on the substrate 5 caused by the buffer layer 9 can damage the substrate 5, causing problems such as cracks in the substrate 5 and degradation of characteristics such as fluctuations in the bias voltage.

To address such problems, in the optical waveguide element 1 according to the embodiment of the present invention, as shown in FIG. 2, a resin 8 is disposed between the substrate 5 and the buffer layer 9. The resin 8 serves as a stress relief layer that relieves stress between the substrate 5 and the buffer layer 9 due to its viscoelastic properties. The resin 8 is a low-rigidity material (Young's modulus of the resin: approximately 1-2 GPa), and can serve as a buffer material that relieves stress caused by the difference in thermal expansion coefficient between the substrate 5 and the buffer layer 9. The resin 8 must be thick enough to relieve stress, and is preferably 1.0 μm or thicker, for example.

Resin 8 is a resin such as a thermoplastic resin or a thermosetting resin, and examples include polyamide-based resin, melamine-based resin, phenol-based resin, amino-based resin, epoxy-based resin, etc.

The resin 8 is, for example, a permanent resist, a photoresist made of a thermosetting resin. In the manufacturing process of the optical waveguide element, the resin 8 is applied onto the substrate 5 by spin coating, and then patterned by a typical photolithography process, followed by thermal curing, so that the resin 8 can be disposed on the substrate 5.

Patterning by photolithography process is capable of forming finer pattern shapes with higher accuracy than conventional sputtering deposition, and is suitable for forming resin on substrate 5 in the embodiment of the present invention. In addition, the buffer layer formed by conventional sputtering deposition is thin, but on the other hand, when resin 8 is applied by spin coating, the film thickness can be freely controlled as long as it is 1.0 μm or more, and is suitable for forming resin on substrate 5 in the embodiment of the present invention.

When the resin 8 is disposed on the substrate 5 and then the buffer layer 9 is formed thereon by sputtering or the like, the resin 8 formed on the substrate 5 in advance serves as a stress relaxation layer, and as a result, the stress on the substrate 5 caused by the buffer layer 9 is relaxed, and degradation of characteristics such as cracking and drift of the substrate can be prevented. In addition, when the wafer (substrate 5) is heated in the wafer process, the resin 8 relaxes the stress caused by the difference in thermal expansion coefficient between the substrate 5 and the buffer layer 9, so that cracking of the substrate during the wafer process can be prevented. Furthermore, when the buffer layer 9 is formed by sputtering or the like, the surface of the substrate 5 is exposed to plasma, but by disposing the resin 8 on the surface of the substrate 5, the area of the surface of the substrate 5 exposed to plasma can be reduced. As a result, the occurrence of oxygen deficiency in the LN material of the substrate 5 can be suppressed, and degradation of characteristics such as drift can be prevented.

As described below, it is desirable to dispose the resin 8 partially or entirely below an area on the buffer layer different from the area where the modulation electrodes (signal electrode S and ground electrode G) are formed. In other words, it is desirable to not dispose the resin 8 directly below the area on the buffer layer 9 where the modulation electrodes are formed. This allows the resin 8 to be disposed in a position that does not interfere with the application of an electric field from the modulation electrode to the optical waveguide 10, making it possible to properly apply an electric field from the modulation electrode to the optical waveguide 10.

Figure 3 is a plan view showing an example of an arrangement pattern of the resin 8 in region R of Figure 1 according to an embodiment of the present invention. In Figure 3, the optical waveguide element 1 is shown so that the longitudinal direction of the optical waveguide element 1 is the vertical direction of the paper, the width direction of the optical waveguide element 1 is the horizontal direction of the paper, and the thickness direction of the optical waveguide element 1 is perpendicular to the paper. The arrangement pattern shown in Figure 3 is a view of region R of Figure 1 viewed from above, and shows a schematic representation of the position of the resin 8 arranged under the buffer layer 9.

In the arrangement pattern shown in FIG. 3, the resin 8 is arranged along the longitudinal direction of the optical waveguide element 1 (extension direction of the optical waveguide 10), but is not arranged directly below the area on the buffer layer 9 where the modulation electrode is formed. With this arrangement pattern, the stress on the substrate 5 caused by the buffer layer 9 can be alleviated by the resin 8, and the resin 8 can be arranged in a position that does not interfere with the modulation action of the light wave without preventing the application of an electric field from the modulation electrode to the optical waveguide 10.

In addition to the resin 8 arrangement pattern shown in FIG. 3, it is also possible to adopt arrangement patterns such as those shown in FIGS. 4(a) to (d).

Figures 4(a) to (d) are plan views showing an example of an optical waveguide element 1 according to an embodiment of the present invention, and are diagrams showing schematic examples of the arrangement pattern of the resin 8 in region R in Figure 1. In Figures 4(a) to (d), the optical waveguide element 1 is shown so that the longitudinal direction of the optical waveguide element 1 is the vertical direction of the paper, the width direction of the optical waveguide element 1 is the horizontal direction of the paper, and the thickness direction of the optical waveguide element 1 is perpendicular to the paper. As with the arrangement pattern shown in Figure 3, the arrangement patterns shown in Figures 4(a) to (d) are views of region R in Figure 1 viewed from above, and are schematic diagrams showing the position of the resin 8 arranged under the buffer layer 9.

The arrangement pattern shown in FIG. 4(a) shows a state in which the resin 8 in the arrangement pattern shown in FIG. 3 is divided in the longitudinal direction of the optical waveguide element 1 (divided into four cells of resin 8 in the longitudinal direction). The arrangement pattern shown in FIG. 4(b) shows a state in which the resin 8 in the arrangement pattern shown in FIG. 4(a) is further divided in the width direction of the optical waveguide element 1 (divided into four cells of resin 8 in the width direction). The arrangement pattern shown in FIG. 4(c) shows a state in which the resin 8 in the arrangement pattern shown in FIG. 3 is divided in the width direction of the optical waveguide element 1 (divided into four cells of resin 8 in the width direction). The arrangement pattern shown in FIG. 4(d) shows a state in which the resin 8 is arranged in a lattice pattern.

In any of the arrangement patterns shown in Figures 4(a) to (d), the stress on the substrate 5 caused by the buffer layer 9 can be alleviated by the resin 8, and the resin 8 can be arranged in a position that does not interfere with the modulation of the light wave without preventing the application of an electric field from the modulation electrode to the optical waveguide 10. The arrangement patterns of the resin 8 shown in Figures 3 and 4(a) to (d) are merely examples, and any arrangement pattern can be adopted as long as it can achieve stress alleviation on the substrate 5 by the buffer layer 9.

Figures 2, 3, and 4(a) to (d) show a state in which resin 8 is disposed in regions on buffer layer 9 where no electrodes are formed (e.g., region R in Figure 1). On the other hand, there are regions on buffer layer 9 where electrodes are formed (e.g., regions D1 and D2 in Figure 1). In such regions, it is desirable to dispose resin 8 below the regions where no electrodes are formed. In particular, in the phase modulation section (region D1) that performs phase modulation of light waves, it is desirable to dispose resin 8 below the regions where no modulation electrodes are formed.

In addition, since the adhesion between the resin 8 and the buffer layer 9 is weaker than that between the substrate 5 and the buffer layer 9 and between the substrate 5 and the resin 8, any pattern that reduces the contact area between the resin 8 and the buffer layer 9 can be adopted. However, if the contact area between the resin 8 and the buffer layer 9 is reduced, the effect of alleviating the stress on the substrate 5 by the buffer layer 9 is reduced. For this reason, it is desirable to adopt an arrangement pattern of the resin 8 that can alleviate the stress on the substrate 5 by the buffer layer 9 and maintain adhesion to the extent that the buffer layer 9 does not peel off from the substrate 5. All of the arrangement patterns in Figures 4(a) to (d) are arrangement patterns that take into account the adhesion of the substrate 5, the resin 8, and the buffer layer 9. In these arrangement patterns, the resin 8 is arranged so that the highly adhesive portions where the substrate 5 and the buffer layer 9 contact each other are arranged in a balanced manner.

Below, we will explain the suitable placement position of the resin 8 in the phase modulation section where the modulation electrode is formed, giving some examples.

Figure 5 is a diagram showing a second example of the cross-sectional structure of the optical waveguide element 1 in an embodiment of the present invention, and shows a state in which a modulation electrode is formed on a substrate 5. Figure 5 is a cross-sectional view taken along line segment Q-Q in Figure 1. Note that in Figure 5, the optical waveguide element 1 is illustrated so that the thickness direction of the optical waveguide element 1 is the up-down direction on the paper, the width direction of the optical waveguide element 1 is the left-right direction on the paper, and the longitudinal direction of the optical waveguide element 1 is perpendicular to the paper.

Figure 5 shows the cross-sectional structure of an optical waveguide element 1 in which modulation electrodes (signal electrode S and ground electrode G) are formed on a substrate 5, and the rib portion 6 of the substrate 5 is used as an optical waveguide 10. The substrate 5 shown in Figure 5 has a structure in which the signal electrode S is arranged between the optical waveguides 10.

The signal electrode S and ground electrode G, which are modulation electrodes, are formed, for example, by depositing Ti/Au on the buffer layer 9 and then patterning the electrodes by a photolithography process. The modulation electrodes may be made of any suitable metal, and the method for forming the modulation electrodes on the buffer layer 9 is not particularly limited. The thickness of the modulation electrodes is, for example, 20 μm or more. Although not described or illustrated in this specification, when forming the modulation electrodes on the buffer layer 9, an antistatic conductive film layer made of Si or the like may be formed between the buffer layer 9 and the modulation electrodes.

The signal electrode S is an electrode for applying an electric field to the optical waveguide 10, and is arranged, for example, to extend parallel to the optical waveguide 10. Although not shown, the signal electrode S is connected to a signal source and a termination resistor, so that a high-frequency electrical signal is supplied from the signal source and terminated at the termination resistor.

The ground electrode G is an electrode connected to a reference potential point, and is arranged, for example, to extend parallel to the optical waveguide 10, similar to the signal electrode S. The signal electrode S and the ground electrode G are provided at a distance from each other, and an electric field is formed between the signal electrode S and the ground electrode G. The signal electrode S and the ground electrode G form, for example, a coplanar line.

The electric field formed between the signal electrode S and the ground electrode G is applied to the optical waveguide 10 formed in the rib portion 6. By controlling the electrical signal supplied from the signal source to adjust the electric field intensity, the light waves propagating in the optical waveguide 10 are appropriately modulated.

As shown in FIG. 5, in the phase modulation section, the resin 8 is not disposed directly below the signal electrode S and the ground electrode G. By taking into consideration the position of the resin 8 in this way, in the cross-sectional structure shown in FIG. 5, the resin 8 is disposed at both ends of FIG. 5 (positions far from the signal electrode G and outside the position where the ground electrode G is formed). This makes it possible to reduce the stress on the substrate 5 caused by the buffer layer 9.

Figure 6 is a diagram showing a third example of the cross-sectional structure of the optical waveguide element 1 in an embodiment of the present invention, and shows a state in which a modulation electrode is formed on a substrate 5. Figure 6 is a cross-sectional view taken along line segment Q-Q in Figure 1. Note that in Figure 6, the optical waveguide element 1 is illustrated so that the thickness direction of the optical waveguide element 1 is the up-down direction on the paper, the width direction of the optical waveguide element 1 is the left-right direction on the paper, and the longitudinal direction of the optical waveguide element 1 is perpendicular to the paper.

Figure 6 shows the cross-sectional structure of an optical waveguide element 1 in which modulation electrodes (signal electrode S and ground electrode G) are formed on a substrate 5, and the rib portion 6 of the substrate 5 is used as an optical waveguide 10. The substrate 5 shown in Figure 6 has a structure in which the signal electrode S is arranged between the optical waveguides 10.

When comparing the cross-sectional structure of FIG. 5 with the cross-sectional structure of FIG. 6, the cross-sectional structure of FIG. 5 has one signal electrode S arranged near the center, while the cross-sectional structure of FIG. 6 has a signal electrode S split into two arranged near the center. The width dimension of the signal electrode S near the center of FIG. 5 may become large depending on the width dimension of the two optical waveguides 10. In contrast, in the cross-sectional structure of FIG. 6, a slit is made in the signal electrode S near the center of FIG. 5 to separate it into two signal electrodes S.

In optical modulators, a differential output configuration is sometimes used for the output of the DSP (digital signal processor) or driver element in order to suppress the effects of external noise during line transmission and to enable operation at low voltage. As shown in the cross-sectional structure in Figure 6, it is possible to adopt a so-called "GSSG type" electrode structure that can utilize the differential electrical signals in such a differential output configuration.

The contact area between the two signal electrodes S and the buffer layer 9 in FIG. 6 is smaller than the contact area between the signal electrodes S and the substrate 5 in FIG. 5, so an area in which the resin 8 can be disposed between the two signal electrodes S can be secured. Thus, with the configuration shown in FIG. 6, the resin 8 can be disposed between the two optical waveguides 10 (at the center in the width direction) where the resin 8 could not be disposed in the configuration shown in FIG. 5. Therefore, the configuration shown in FIG. 6 is a configuration that can further reduce the stress on the substrate 5 caused by the buffer layer 9 compared to the configuration shown in FIG. 5.

Figure 7 is a diagram showing a fourth example of the cross-sectional structure of the optical waveguide element 1 in an embodiment of the present invention, and shows a state in which a modulation electrode is formed on a substrate 5. Figure 7 is a cross-sectional view taken along line segment Q-Q in Figure 1. Note that in Figure 7, the optical waveguide element 1 is illustrated so that the thickness direction of the optical waveguide element 1 is the up-down direction on the paper, the width direction of the optical waveguide element 1 is the left-right direction on the paper, and the longitudinal direction of the optical waveguide element 1 is perpendicular to the paper.

Figure 7 shows the cross-sectional structure of an optical waveguide element 1 in which modulation electrodes (signal electrode S and ground electrode G) are formed on a substrate 5, and the rib portion 6 of the substrate 5 is used as an optical waveguide 10. The substrate 5 shown in Figure 7 has a structure in which the signal electrode S is disposed on the optical waveguide 10.

As shown in FIG. 7, in the phase modulation section, the resin 8 is not disposed directly below the signal electrode S and the ground electrode G. By taking into consideration the position of the resin 8 in this way, in the cross-sectional structure shown in FIG. 7, the resin 8 is disposed between the signal electrode S and the ground electrode G. This makes it possible to reduce the stress on the substrate 5 caused by the buffer layer 9.

Figure 8 is a diagram showing a fifth example of the cross-sectional structure of the optical waveguide element 1 in an embodiment of the present invention, and shows a state in which a modulation electrode is formed on a substrate 5. Figure 8 is a cross-sectional view taken along line segment Q-Q in Figure 1. Note that in Figure 8, the optical waveguide element 1 is illustrated so that the thickness direction of the optical waveguide element 1 is the up-down direction on the paper, the width direction of the optical waveguide element 1 is the left-right direction on the paper, and the longitudinal direction of the optical waveguide element 1 is perpendicular to the paper.

Figure 8 shows the cross-sectional structure of an optical waveguide element 1 in which modulation electrodes (signal electrode S and ground electrode G) are formed on a substrate 5, and the rib portion 6 of the substrate 5 is used as an optical waveguide 10. The substrate 5 shown in Figure 8 has a structure in which the signal electrode S is disposed on the optical waveguide 10.

When comparing the cross-sectional structure of FIG. 7 with the cross-sectional structure of FIG. 8, the cross-sectional structure of FIG. 7 has one ground electrode G arranged near the center, while the cross-sectional structure of FIG. 8 has a ground electrode G divided into two arranged near the center. The width dimension of the ground electrode G near the center of FIG. 7 may become large depending on the width dimension of the two optical waveguides 10. In contrast, in the cross-sectional structure of FIG. 8, a slit is made in the ground electrode G near the center of FIG. 7 to divide it into two ground electrodes G.

The contact area between the two ground electrodes G and the buffer layer 9 in FIG. 8 is smaller than the contact area between the ground electrode G and the substrate 5 in FIG. 7, and an area in which the resin 8 can be disposed between the two ground electrodes G can be secured. Thus, according to the configuration shown in FIG. 8, the resin 8 can be disposed between the two optical waveguides 10 (at the center in the width direction) where the resin 8 could not be disposed in the configuration shown in FIG. 7. Therefore, the configuration shown in FIG. 8 is a configuration that can further reduce the stress on the substrate 5 caused by the buffer layer 9 compared to the configuration shown in FIG. 7.

As explained using the cross-sectional structures of Figures 5 to 8 as examples, the present invention can reduce the stress on the substrate 5 caused by the buffer layer 9 for both a substrate 5 having a structure in which a signal electrode S is arranged between optical waveguides 10, and a substrate 5 having a structure in which a signal electrode S is arranged on top of the optical waveguides 10.

The cross-sectional structures of Figures 5 to 8 can be applied to any arrangement pattern, including the arrangement patterns of Figures 3 and 4(a) to (d). In other words, in the present invention, as long as the position is such that an electric field can be efficiently applied to the optical waveguides 10, a resin 8 of any shape and size can be arranged on both a substrate 5 having a structure in which a signal electrode S is arranged between optical waveguides 10 and a substrate 5 having a structure in which a signal electrode S is arranged on the optical waveguides 10, and the stress on the substrate 5 caused by the buffer layer 9 can be reduced.

In this embodiment, a ribbed substrate in which a rib portion 6 is formed on a substrate 5 is described as an example. However, as described above, the present invention is not limited to ribbed substrates, and can also be applied to substrates in which an optical waveguide 10 is formed in the substrate 5 by thermal diffusion of metal, for example. Similarly, in substrates having a diffusion-type optical waveguide, it is possible to arrange the resin 8 in any arrangement pattern, including the arrangement patterns in Figures 3 and 4(a) to (d).

In addition, in this embodiment, a coplanar line structure in which one ground electrode G is arranged on each side of one 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 arranged on each side of two parallel signal electrodes S may be adopted.

The present invention can provide an optical modulator that uses, at least in part, the optical waveguide that constitutes the optical waveguide element described in this embodiment.

Figure 9 is a plan view showing an example of the configuration of an optical modulator 200 according to an embodiment of the present invention. The optical modulator 200 shown in Figure 9 includes an optical waveguide element 202, a housing 204 that houses the optical waveguide element 202, an input optical fiber 208 for inputting light to the optical waveguide element 202, and an output optical fiber 210 that guides the light output from the optical waveguide element 202 to the outside of the housing 204. Note that the configuration of the optical modulator 200 shown in Figure 9 is only one example, and the present invention is not limited to this configuration. It is possible to incorporate an optical waveguide element having the characteristics of the present invention into an optical modulator having an arbitrary configuration.

The optical modulator 200 shown in FIG. 9 has an input optical fiber 208 at one end in the longitudinal direction (left side of the drawing) and an output optical fiber 210 at the other end in the longitudinal direction (right side of the drawing). However, the input position and output position of the light in the optical modulator 200 can be set arbitrarily.

The optical waveguide element 202 has, for example, an optical waveguide 206 provided on a substrate, and a plurality of electrodes 212a to 212d formed on the substrate to modulate the light waves propagating in the optical waveguide 206. The optical waveguide element 202 has, for example, an optical waveguide 206 in which a plurality of Mach-Zehnder type optical waveguides are combined, as shown in FIG. 9.

The optical modulator 200 shown in FIG. 9 is configured, as an example, so that two light beams are output from the optical waveguide element 202, and the light that is polarization-combined by the polarization combining unit 228 is output to the outside of the housing 204 via the output optical fiber 210. However, the optical modulator 200 according to the present invention is not limited to such a configuration. For example, like the optical waveguide element 1 shown in FIG. 1 described above, it may be configured to include a first combining unit 3a and output one optical signal from the output waveguide.

Similarly to the optical waveguide element 1 described above, the optical waveguide element 202 has a configuration in which resin is arranged in an arbitrary arrangement pattern between the substrate and the buffer layer. With this configuration, the resin arranged between the substrate and the buffer layer reduces the stress on the substrate caused by the buffer layer.

The housing 204 is composed of a case to which the optical waveguide element 202 is fixed and a cover. The cover is arranged to cover the entire case, thereby hermetically sealing the inside of the housing 204. Electronic components such as a driver and a light receiving element (PD: Photo Detector) may also be housed inside the housing 204.

The case of the housing 204 is provided with a number of lead pins 240a to 240d, which are conductors for inputting high-frequency signals. The lead pins 240a to 240d are connected to one end of each of a number of electrodes 212a to 212d provided in the Mach-Zehnder type optical waveguide of the optical waveguide element 202 via the relay substrate 218. The other end of each of the multiple electrodes 212a to 212d is terminated by a termination substrate 250, which is an impedance element. Note that although detailed configuration is not shown in FIG. 9, the multiple electrodes 212a to 212d include a signal electrode S and a ground electrode G, and are capable of modulating the light waves propagating through the optical waveguide 206.

As described above, the present invention can provide an optical modulator including an optical waveguide element having a configuration in which resin is arranged in an arbitrary arrangement pattern between a substrate and a buffer layer.

The present invention is not limited to the above-described embodiments and modifications, but includes within its technical scope various modifications and design changes that do not deviate from the technical concept of the present invention.

The present invention provides an optical waveguide element and an optical modulator that can prevent damage to the substrate and deterioration of the substrate's characteristics that may be caused by the stress by reducing the effect of the stress on the substrate caused by the buffer layer, and is applicable to fields such as optical communications and optical measurement.

Reference Signs List 1, 202 Optical waveguide element 2a to 2c Branching section 3a to 3c Combining section 5 Substrate 6 Rib section 7 Reinforcing substrate 8 Resin 9 Buffer layer 10, 206 Optical waveguide 200 Optical modulator 204 Housing 208 Input optical fiber 210 Output optical fiber 212a, 212b, 212c, 212d Electrode 218 Relay substrate 228 Polarized wave combining section 240a, 240b, 240c, 240d Lead pin 250 Terminal substrate G Ground electrode S Signal electrode

Claims (6)

A substrate having an electro-optic effect;
a rib-type optical waveguide having a rib portion formed on the substrate;
a buffer layer provided on the substrate,
a resin is disposed between the substrate and the buffer layer ;
an optical waveguide element, characterized in that the resin is disposed partially or entirely below an area on the buffer layer different from an area where a modulation electrode is formed, and is not disposed directly below an area on the buffer layer where the modulation electrode is formed . 2. The optical waveguide element according to claim 1 , wherein the thickness of the resin is 1.0 [mu]m or more. 3. The optical waveguide element according to claim 1 , wherein the resin is one of a thermoplastic resin and a thermosetting resin. 4. The optical waveguide element according to claim 1, wherein the substrate has a thickness of 4.0 [mu]m or less. 5. The optical waveguide element according to claim 1, wherein the optical waveguide is formed by a plurality of Mach-Zehnder sections. 6. An optical modulator comprising at least a part of an optical waveguide constituting the optical waveguide element according to claim 1 .

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