Patent ID: 12204185

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, the optical waveguide element and the optical modulator according to the embodiments of the present invention will be described.

FIG.1is a plan view for explaining an example of the optical waveguide10formed on the substrate5constituting the optical waveguide element1in the embodiment of the present invention. InFIG.1, the optical waveguide element1is shown in such a manner that a width direction of the optical waveguide element1is a vertical direction of the paper surface, a longitudinal direction of the optical waveguide element1is a horizontal direction of the paper surface, and a thickness direction of the optical waveguide element1is a direction perpendicular to the paper surface.

The optical waveguide element1shown inFIG.1is an optical waveguide element1in which a plurality of Mach-Zehnder type optical waveguides are integrated. An optical waveguide in which a plurality of Mach-Zehnder type optical waveguides are combined is also called a nested optical waveguide. The optical waveguide element1in which a plurality of Mach-Zehnder type optical waveguides are integrated can generate optical signals corresponding to various modulation methods. As an example,FIG.1shows an optical waveguide element1in which a plurality of Mach-Zehnder-type optical waveguides are integrated, but the present invention is not limited to this configuration, and, for example, an optical waveguide element1having a single Mach-Zehnder-type optical waveguide may be used.

As shown inFIG.1, the optical waveguide element1according to the embodiment of the present invention is provided with an optical waveguide10formed on a substrate5made of a material having an electro-optical effect. The optical waveguide element1shown inFIG.1is provided with: a first branch portion2athat branches an incident waveguide into which an optical signal is introduced from the outside; a second branch portion2bthat further branches the optical waveguide10branched by the first branch portion2a; and a third branch portion2cthat further branches the optical waveguide10branched at the second branch portion2b, so that the optical waveguide1has formed therein total eight parallel waveguides through three-step branching. The first to third branch portions2ato2care realized by an optical coupler or the like.

The phase of the light wave propagating in each parallel waveguide is adjusted, for example, in a region D1. A metal modulation electrode (not shown inFIG.1) is formed in the region D1, so that a refractive index of the light wave can be changed by the electric field applied from the modulation electrode to each parallel waveguide, thereby to adjust the propagation speed of the light wave.

The light wave propagating in each parallel waveguide is combined in the first to third synthesis portions3ato3ccorresponding to each of the first to third branch portions2ato2c, and then outputted from an exit waveguide to an outside. To be specific, the optical waveguide element1shown inFIG.1is provided with: a third synthesis portion3cthat synthesizes a parallel waveguide branched at the third branch portion2c; a second synthesis portion3bthat synthesizes the optical waveguide10branched at the second branch portion2b; and a third synthesis portion3cthat synthesizes the optical waveguide10branched at the first branch portion2a, and an optical signal is outputted from an exit waveguide through three-step synthesis. Similar to the first to third branch portions2ato2c, the first to third synthesis portions3ato3care also realized by an optical coupler or the like.

The optical waveguide10of the optical waveguide element1shown inFIG.1is an example, and the present invention is not limited thereto. For example, as in an optical waveguide element202of an optical modulator200described later with reference toFIG.7, the present invention may be so configured that two optical signals are outputted from the optical waveguide element202and the polarization is synthesized by the polarization synthesis unit228.

Further, a bias voltage for setting an operating point is applied to the optical waveguide10. The bias voltage is applied to the phase-modulated light wave by, for example, a bias electrode formed in the region D2.

FIG.2is a diagram showing a first example of the cross-sectional structure of the optical waveguide element1according to the embodiment of the present invention, and is a cross-sectional view taken along the line PP ofFIG.1. InFIG.2, the optical waveguide element1is shown in such a manner that a thickness direction of the optical waveguide element1is a vertical direction of the paper surface, a width direction of the optical waveguide element1is a horizontal direction of the paper surface, and a longitudinal direction of the optical waveguide element1is a direction perpendicular to the paper surface.

As shown in the cross-sectional structure ofFIG.2, the optical waveguide element1has such a structure in which a lower buffer layer (second buffer layer)9bis provided on the reinforcing substrate7, the substrate5is provided on the lower surface buffer layer9b, and an upper buffer layer (first buffer layer)9ais provided on the substrate5.

The substrate5is made of a material having an electro-optical effect. While the conventional substrate has a thickness of about 8 to 10 μm, the substrate5in the embodiment of the present invention can use, for example, an extremely thin plate having a thickness of 2.0 μm or less, preferably 1.0 μm or less. By making the thickness of the substrate5extremely thin (for example, about 1/10 of the conventional thickness), it is possible to further reduce the drive voltage. For the substrate5, for example, LN can be used as a material having an electro-optical effect, but lithium tantalate (LiTaO3), lead lanthanate titanate (PLZT), or the like may be used.

A rib portion6is provided on the substrate5. The rib portion6is projected from the surface of the substrate5and has an action of confining light waves, and is therefore used as an optical waveguide10. In the conventional diffusion type optical waveguide structure, the action of confining light is weak, and the propagating light may leak from the optical waveguide10at a curved portion or the like. On the other hand, when the rib-type optical waveguide structure is adopted, the action of confining light is strengthened, the optical waveguide10can be bent to form a folded structure, so that it is possible to shorten the length of the optical waveguide element1. The height of the rib portion6is, for example, 2.0 μm or less, preferably 1.0 μm or less from the surface of the substrate5.

The dimensions of the rib-type substrate will be described in more detail below. In the rib-type substrate according to the embodiment of the present invention, for example, the maximum value of the thickness A of the substrate5including the rib portion6is 4.0 μm, the maximum value of the width B of the rib portion6is 4.0 μm, and the rib portion. The maximum value of the height C of the rib portion6is 2.0 μm, and the ratio of the thickness A to the width B is 1:1. Since the smaller the rib portion6and the substrate5in design, the more preferable the minimum values of the thickness A, the width B, and the height C are the limit values for minimization in the manufacturing process. Further, from the viewpoint of confining light, as long as the dimensions are within a range in which the single mode condition of light is maintained, the smaller the respective dimensions of the thickness A and the width B, the more preferable since the more the light is confined,

FIG.2shows, as an example, an optical waveguide element1having a rib-type substrate in which the rib portion6is formed on the substrate5. However, although, in the present invention, it is preferable to have a structure having a rib-type substrate in which the rib portion6is formed as the optical waveguide10, the structure is not limited to this, and an optical waveguide element1in which the optical waveguide10is formed inside of the substrate5by thermal diffusion of metal may be used.

The reinforcing substrate7is a member that supplements the strength of the extremely thin substrate5and can stably support the lower surface buffer layer9b, the substrate5, the upper surface buffer layer9a, and the electrodes formed on the substrate5. As will be described later, the reinforcing substrate7is directly bonded to the lower surface buffer layer9bby a direct bonding method. As the material of the reinforcing substrate7, for example, a material having a lower dielectric constant than the material of the substrate5(for example, LN) or the same material as the substrate5(for example, LN) can be used.

Further, an upper surface buffer layer9ais provided on the substrate5. The upper surface buffer layer9ain the embodiment of the present invention has a thickness the same as that of the substrate5, for example, 2.0 μm or less, preferably 1.0 μm or less. The material used for the upper surface buffer layer9ais not particularly limited, but is preferably a material having a lower refractive index than LN and excellent light transmission. The material used for the upper surface buffer layer9amay be a material generally used as a buffer layer. For example, SiO2, Al2O3, MgF3, La2O3, ZnO, HfO2, MgO, CaF2, Y2O3and the like may be used.

Whereas the thickness of the conventional substrate was 8.0 to 10.0 in the embodiment of the present invention, the thickness of the rib-type substrate can be made extremely thin to 2.0 μm or less as described above, so that it is possible to match the speed between microwaves and light waves and further reduce the drive voltage. However, such an extremely thin substrate5is particularly sensitive to stress.

Further, as described above, for example, LN is used for the substrate5, whereas SiO2is used for the upper surface buffer layer9aprovided on the substrate, but LN which is the material of the substrate5and SiO2which is the material of the upper surface buffer layer9adiffer from each other in thermal expansion coefficient. Therefore, stress (internal stress or residual stress) is generated on the surface where the upper surface buffer layer9aand the substrate5come into contact with each other, due to the difference in thermal expansion coefficient between the substrate5and the upper surface buffer layer9a, when the upper surface buffer layer9ais formed or when the wafer (substrate5) or the chip is heated, especially in a wafer process involving temperature changes.

As a result, there are such problems that the substrate5is deformed under influence of the stress due to the difference of the thermal expansion coefficient between the material of the upper surface buffer layer9aand the material of the substrate5, and that the deterioration of the properties occurs such as a fluctuation of bias voltage.

In order to cope with such a problem, in the optical waveguide element1according to the embodiment of the present invention, as shown inFIG.2, a lower surface buffer layer9bis provided between the reinforcing substrate7and the substrate5. The lower surface buffer layer9bin the embodiment of the present invention has substantially the same thickness as the upper surface buffer layer9a, for example, 2.0 μm or less, preferably 1.0 μm or less. Further, for the lower surface buffer layer9b, substantially the same material as that of the upper surface buffer layer9ais used.

Further, the fact that the upper surface buffer layer9aand the lower surface buffer layer9bhave substantially the same thickness means that the upper surface buffer layer9aand the lower surface buffer layer9bhave the same or substantially the same film thickness. To be specific in the present invention, the upper buffer layer9aand the lower buffer layer9bare defined to have substantially the same thickness, in the case that the difference in thickness between the upper surface buffer layer9aand the lower surface buffer layer9bis within ±20% with respect to the relative thickness of the upper surface buffer layer9aor the lower surface buffer layer9b, including an error due to process variation during manufacturing.

The fact that the upper surface buffer layer9aand the lower surface buffer layer9bare made of substantially the same material means that the upper surface buffer layer9aand the lower surface buffer layer9bare made of the same material or a material having substantially the same material. To be specific, in the present invention, the upper buffer layer9aand the lower buffer layer9bare defined to be made of substantially the same material, in the case that the difference in electrical resistivity between the upper surface buffer layer9aand the lower surface buffer layer9bis within ±20% with respect to the electrical resistivity of the upper surface buffer layer9aor the lower surface buffer layer9band the difference in the refractive index between the upper surface buffer layer9aand the lower surface buffer layer9bis within ±20% with respect to the refractive index of the upper surface buffer layer9aor the lower surface buffer layer9b, including an error due to process variation during manufacturing.

The method for measuring the film thickness, the electrical resistivity, and the refractive index is not particularly limited, and each parameter can be measured by a normal method. For example, with regard to the film thickness, it is possible to measure the film thickness using a general stylus type step system for a batch formed by charging a large number of wafers embedded with dummy wafers therein. Regarding the electrical resistivity, IV measurement (current/voltage measurement) using the mercury probe method can be performed on the batch including the dummy wafer, and the electrical resistivity can be calculated from the measurement result. Regarding the measurement of the refractive index, the refractive index can be measured by using a prism coupler (for example, measurement wavelength: 1550 nm) for the batch including the dummy wafer.

In the optical waveguide element1according to the embodiment of the present invention, the upper surface buffer layer9aand the lower surface buffer layer9bmade of substantially the same material and having substantially the same thickness are formed. Further, the upper surface buffer layer9ais formed so as to be in contact with the upper surface of the substrate5, and the lower surface buffer layer9bis formed so as to be in contact with the lower surface of the substrate5. By having the structure in which the upper surface buffer layer9aand the lower surface buffer layer9bsandwich the substrate5in this way, the stress similar to the stress generated on the upper surface of the substrate5by the upper surface buffer layer9acan be generated on the lower surface of the substrate5by the lower surface buffer layer9b, so that the stress balance on the upper surface and the lower surface of the substrate5can be made uniform. As a result, the bias of stress on the upper surface and the lower surface of the substrate5is alleviated, so that the deformation of the substrate5can be prevented, thereby making it possible to prevent the damage to the substrate5and the deterioration of the properties of the substrate.

It is known that the increase of the DC drift with elapse of time can be flattened by the added metal oxide and the DC drift properties can be improved over a long period of time, by adding a metal oxide such as indium or titanium to the buffer layer (upper surface buffer layer9a) provided on the substrate5(see Patent Documents 2 and 3). By applying this technique, the draft properties may be further improved by adding a metal oxide to both the upper surface buffer layer9aand the lower surface buffer layer9b.

To be more specific, both the upper buffer layer9aand the lower buffer layer9bmay be formed by: a mixture of silicon oxide and at least one oxide of one or more elements selected from the metal elements of groups 3-8, 1b and 2b of the periodic table and semiconductor elements other than silicon; or a transparent insulating film of an oxide of silicon and one or more elements selected from the metal element and the semiconductor element. To be specific, for the upper surface buffer layer9aand the lower surface buffer layer9b, for example, a material obtained by adding (doping) a metal oxide such as indium, titanium, zinc, tin, chromium, aluminum, or germanium to SiO2is used.

The elemental species of the additives added to the upper surface buffer layer9aand the lower surface buffer layer9bmay be the same or different between the upper surface buffer layer9aand the lower surface buffer layer9b. The upper surface buffer layer9aand the lower surface buffer layer9baccording to the present invention are made of substantially the same material. And, as described above, it would be sufficient if the difference in the refractive index between the upper surface buffer layer9aand the lower surface buffer layer9bis within ±20% with respect to the refractive index of the upper surface buffer layer9aor the lower surface buffer layer9b, including an error due to process variation during manufacturing. Additives added to the upper surface buffer layer9aand the lower surface buffer layer9bmay be different as long as this condition can be satisfied.

Further, it is known that the light absorption by the buffer layer can be suppressed and that the occurrence of positive DC drift can be prevented, by using a material having an appropriate electrical resistivity as the material of the buffer layer (upper surface buffer layer9a) provided on the substrate5(see, for example, Patent Document 3). By applying this technique, a material having an appropriate electrical resistivity may be used as the material of both the upper surface buffer layer9aand the lower surface buffer layer9b.

To be more specific, as the material of the upper surface buffer layer9aand the lower surface buffer layer9b, a material having an electrical resistivity of 108Ωcm or more and 1016Ωcm or less may be used. By setting the electrical resistivity of the material used for the upper surface buffer layer9aand the lower surface buffer layer9bto 108Ωcm or more, it becomes possible to prevent light absorption by the upper surface buffer layer9aand the lower surface buffer layer9b. Further, by setting the electrical resistivity of the material used for the upper surface buffer layer9aand the lower surface buffer layer9bto 1016Ωcm or less, it becomes possible to stably obtain a negative DC drift amount at the initial stage of the passage of time.

Further, as the material of the upper surface buffer layer9aand the lower surface buffer layer9b, a material having a refractive index lower than that of the material of the substrate5having an electro-optical effect (for example, LN) may be used. By using the upper surface buffer layer9aand the lower surface buffer layer9barranged above and below the substrate5as materials having a refractive index lower than that of the substrate5, the effect of confining the propagating light in the optical waveguide10formed on the substrate5is increased, so that the propagation loss can be efficiently reduced.

Further, by controlling the thicknesses of the upper surface buffer layer9aand the lower surface buffer layer9b, an appropriate DC drift characteristic may be obtained so that the light can be efficiently propagated in the optical waveguide10. To be specific, by setting the thickness of the upper surface buffer layer9aand the lower surface buffer layer9bin the range of 0.3 μm or more and 2.0 μm or less, an appropriate DC drift amount can be stably obtained.

Next, the cross-sectional structure in the phase modulation portion where the modulation electrode is formed will be described.

FIG.3is a diagram showing a second example of the cross-sectional structure of the optical waveguide element1according to the embodiment of the present invention, and is a diagram showing a state in which a modulation electrode is formed on the substrate5.FIG.3is a cross-sectional view taken along the line Q-Q ofFIG.1. InFIG.3, the optical waveguide element1is shown in such a manner that the thickness direction of the optical waveguide element1is the vertical direction of the paper surface, the width direction of the optical waveguide element1is the horizontal direction of the paper surface, and the longitudinal direction of the optical waveguide element1is the direction perpendicular to the paper surface.

FIG.3shows a cross-sectional structure of an optical waveguide element1in which modulation electrodes (signal electrode S and ground electrode G) are formed on the substrate5, and the rib portion6of the substrate5is used as the optical waveguide10. The substrate5shown inFIG.3has a structure in which the signal electrodes S are arranged between the optical waveguides10.

The signal electrode S and the ground electrode G, which are modulation electrodes, are formed by, for example, depositing Ti/Au on the upper surface buffer layer9aand then patterning the electrodes by a photolithography process. The modulation electrode may be made of an appropriate metal, and the method of forming the modulation electrode on the upper surface buffer layer9ais not particularly limited. The thickness of the modulation electrode is, for example, 20 μm or more. Although description and illustration are omitted in the present specification, when the modulation electrode is formed on the buffer layer9a, an antistatic conductive film layer made of Si or the like may be formed between the upper surface buffer layer9aand the modulation electrode.

The signal electrode S is an electrode for applying an electric field to the optical waveguide10, and is, for example, so arranged to extend in parallel with the optical waveguide10. Although not shown, the signal electrode S is connected to a signal source and a terminating resistor, so that 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, for example so arranged to extend in parallel with the optical waveguide10like the signal electrode S. The signal electrode S and the ground electrode G are provided apart from each other, so that 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 waveguide10formed in the rib portion6. By controlling the electric signal supplied from the signal source and adjusting the electric field strength, the light wave propagating in the optical waveguide10is appropriately modulated.

As shown inFIG.3, by adopting a structure in which the upper surface buffer layer9aand the lower surface buffer layer9bsandwich the substrate5, the stress balance on the upper surface and the lower surface of the substrate5can be made uniform. As a result, the bias of stress on the upper surface and the lower surface of the substrate5is alleviated, so that the deformation of the substrate5can be prevented, thereby making it possible to prevent the damage to the substrate5and the deterioration of the characteristics of the substrate5.

FIG.4is a diagram showing a third example of the cross-sectional structure of the optical waveguide element1according to the embodiment of the present invention, and is a diagram showing a state in which a modulation electrode is formed on the substrate5.FIG.4is a cross-sectional view taken along the line Q-Q ofFIG.1. InFIG.4, the optical waveguide element1is shown in such a manner that the thickness direction of the optical waveguide element1is the vertical direction of the paper surface, the width direction of the optical waveguide element1is the horizontal direction of the paper surface, and the longitudinal direction of the optical waveguide element1is the direction perpendicular to the paper surface.

FIG.4shows a cross-sectional structure of an optical waveguide element1in which modulation electrodes (signal electrode S and ground electrode G) are formed on the substrate5and the rib portion6of the substrate5is used as the optical waveguide10. The substrate5shown inFIG.4has a structure in which the signal electrode S is arranged on the optical waveguide10.

Similar toFIG.3described above, as shown inFIG.4, the structure is such that the upper surface buffer layer9aand the lower surface buffer layer9bsandwich the substrate5, so that the stress balance on the upper surface and the lower surface of the substrate5can be made uniform. As a result, the bias of stress on the upper surface and the lower surface of the substrate5is alleviated, so that the deformation of the substrate5can be prevented, thereby making it possible to prevent the damage to the substrate5and the deterioration of the characteristics of the substrate.

As described by taking the cross-sectional structure ofFIGS.3and4as an example, the present invention presents a signal on a substrate5having a structure in which a signal electrode S is arranged between the optical waveguides10and a signal on the optical waveguide10. The stress bias on the upper surface and the lower surface of the substrate5can be alleviated with respect to any of the substrates5having the structure in which the electrodes S are arranged. Further, the lower surface buffer layer9bcan be arranged over the entire lower surface of the substrate5regardless of the positions of the modulation electrodes (signal electrode S and ground electrode G) and the positions of the optical waveguide10.

Next, the manufacturing process of the optical waveguide element1according to the embodiment of the present invention will be described with reference toFIGS.5A to5F. Note thatFIGS.5A to5Fshow the manufacturing process of the optical waveguide element1having the cross-sectional structure ofFIG.3as an example.

In the first step, a layer (for example, SiO2) to be the lower surface buffer layer9bis formed with respect to the layer to be the substrate5(for example, the LN layer).FIG.5Ashows the state after the first step.

In the second step, the lower surface of the layer to be the lower surface buffer layer9band the upper surface of the reinforcing substrate7are directly bonded by the direct bonding method.FIG.5Bshows the state after the second step.

In the third step, a layer made of a material having an electro-optical effect to be a substrate5is processed so as to have an appropriate thickness.FIG.5Cshows the state after the third step.

In the fourth step, a portion other than the rib portion6is removed by, for example, dry etching to form a substrate5having the rib portion6.FIG.5Dshows the state after the fourth step.

In the fifth step, the upper surface buffer layer9ais formed on the substrate5by, for example, sputtering.FIG.5Eshows the state after the fifth step.

In the sixth step, for example, electrodes (for example, a signal electrode and a ground electrode) are formed on the upper surface buffer layer9a.FIG.5Fshows the state after the sixth step.

The direct bonding method used in the second step above is a suitable method for bonding dissimilar materials. Although the layer to be the buffer layer9band the reinforcing substrate7are made of different materials, they can be appropriately and surely bonded by using the direct bonding method.

The direct bonding method is roughly divided into two methods, which are a plasma activated bonding method and a FAB (Fast Atom Beam: high-speed atomic beam) method.

The plasma activated bonding method is a method in which two surfaces to be bonded by plasma or the like are treated with hydrophilicity to improve the bonding property, and then the two surfaces are overlapped to perform direct bonding. When the plasma activated bonding method is used, an interface layer (jponing layer) is formed in which the molecular chains of the layers to be the buffer layer9band the respective surfaces of the reinforcing substrate7are entangled with each other and are incompatible with each other.

On the other hand, in the FAB method, a thin Si layer or a metal oxide layer is formed on each of the two surfaces to be bonded, and each of the two surfaces is activated by irradiating each of the two surfaces with a neutron atom beam at room temperature, and then the two surfaces are activated. This is a method of directly bonding by pasting the surfaces together. When the FAB method is used, an adhesive layer such as a thin Si layer or a metal oxide layer is formed between the layer to be the buffer layer9band the reinforcing substrate7.

When the buffer layer9band the reinforcing substrate7are directly bonded using the FAB method, as shown inFIG.6, an extremely thin adhesive layer20having a thickness of about 10 to 500 nm is formed between the buffer layer9band the reinforcing substrate7. For the adhesive layer20, Si, Al2O3, Ta2O5, TiO2, Nb2O5, Si3N4, AlN, SiO2and the like are used.

FIG.6is a diagram showing a fourth example of the cross-sectional structure of the optical waveguide element according to the embodiment of the present invention, and is a cross-sectional view taken along the line P-P ofFIG.1. Note thatFIG.6illustrates a cross-sectional structure seen from the same viewpoint as inFIG.2. InFIG.6, the optical waveguide element1is shown in such a manner that the thickness direction of the optical waveguide element1is the vertical direction of the paper surface, the width direction of the optical waveguide element1is the horizontal direction of the paper surface, and the longitudinal direction of the optical waveguide element1is the direction perpendicular to the paper surface.

Some of the materials that can be used as the adhesive layer20have high light absorption. However, the lower surface buffer layer9bexists between the adhesive layer20and the substrate5, so that the lower surface buffer layer9bcan suppress the light absorption by the adhesive layer20. In other words, when the buffer layer9band the reinforcing substrate7are directly bonded using the FAB method, the lower surface buffer layer9bplays a role of alleviating stress bias on the upper surface and the lower surface of the substrate5, and, at the same time, suppressing absorption of propagating light by the adhesive layer20.

In the present embodiment, a rib-type substrate in which a rib portion6is formed on the substrate5is described as an example. However, as described above, the present invention is not limited to the rib-type substrate, but can be applied to, for example, a substrate in which the optical waveguide10is formed in the substrate5by thermal diffusion of a metal. Similarly, in the substrate having the diffusion type optical waveguide, the stress bias on the upper surface and the lower surface of the substrate5can be alleviated by having the structure in which the upper surface buffer layer9aand the lower surface buffer layer9bsandwich the substrate5.

Further, in the present 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 at least partly constituted by the optical waveguide constituting the optical waveguide element described

FIG.7is a plan view showing an example of the configuration of the optical modulator200according to the embodiment of the present invention. The optical modulator200shown inFIG.7is provided with: an optical waveguide element202; a housing204that accommodates the optical waveguide element202; an input optical fiber208that injects light on the optical waveguide element202; and an output optical fiber210that guides the output light to the outside of the housing204. The configuration of the optical modulator200shown inFIG.7is only an example, and the present invention is not limited to this configuration. It is possible to incorporate an optical waveguide element having the properties according to the present invention into an optical modulator having an arbitrary configuration.

The optical modulator200shown inFIG.7has an input optical fiber208at one end in the longitudinal direction (left side in the drawing) and an output optical fiber210at the other end in the longitudinal direction (right side in the drawing). The input position and output position of the light in the above optical module200can be set arbitrarily.

The optical waveguide element202has, for example, an optical waveguide206provided on the substrate and a plurality of electrodes212ato212dformed on the substrate to modulate the light wave propagating in the optical waveguide206. The optical waveguide element202has, for example, as shown inFIG.7, an optical waveguide206in which a plurality of Mach-Zehnder type optical waveguides are combined.

As an example, the optical modulator200shown inFIG.7is so configured that two lights are outputted from the optical waveguide element202, and polarized and synthesized by the polarization synthesis unit228, and the then outputted to the outside of the housing204through the output optical fiber210. However, the optical modulator200according to the present invention is not limited to such a configuration. For example, as in the optical waveguide element1shown inFIG.1described above, a configuration may be provided in which the first synthesis unit3ais provided and one optical signal is outputted from the exit waveguide.

Further, the optical waveguide element202, as well as the optical waveguide element1described above, has such a configuration that the upper surface buffer layer and the lower surface buffer layer, which are made of substantially the same material and have substantially the same thickness, are in contact with the upper surface and the lower surface of the substrate, respectively. By this configuration, the upper surface buffer layer and the lower surface buffer layer sandwich the substrate, so that the uniformization of the stress balance on the upper surface and the lower surface of the substrate is realized.

The housing204is composed of a case and a cover to which the optical waveguide element202is fixed. The cover is arranged to cover the entire case, whereby the inside of the housing204is hermetically sealed. An electronic component such as a driver or a light receiving element (PD: Photo Detector) may be housed in the housing204.

The case of the housing204is provided with a plurality of lead pins240ato240dwhich are conductors for inputting high frequency signals. The lead pins240ato240dare connected to one end of each of a plurality of electrodes212ato212dprovided in the Mach-Zehnder type optical waveguide of the optical waveguide element202through a relay substrate218. Further, the other ends of the plurality of electrodes212ato212dare terminated by a termination substrate250which is an impedance element. Although the detailed configuration is not shown inFIG.7, the plurality of electrodes212ato212dinclude the signal electrode S and the ground electrode G so that the light wave propagating in the optical waveguide206can be modulated.

As described above, according to the present invention, it is possible to provide the optical modulator including the optical waveguide element having such a configuration that the upper surface buffer layer and the lower surface buffer layer, which are composed of substantially the same material and have substantially the same thickness, are formed to be in contact with the upper surface and the lower surface of the substrate, respectively.

The present invention is not limited to the above-described embodiments and modifications, but includes various modifications and design changes within the technical scope thereof without departing from the technical idea of the present invention.

INDUSTRIAL APPLICABILITY

The present invention provides an optical waveguide element and an optical modulator that can prevent the damage to the substrate and the deterioration of the properties of the substrate that may occur due to the stress, by reducing the influence of stress on the substrate by the buffer layer, and therefore, is applicable to the optical communication field, the optical measurement field, and the like.

EXPLANATION OF REFERENCE NUMERALS

1,202Optical Waveguide Element2a-2cBranch Portion3a-3cSynthesis Portion5,102Substrate6Rib Portion7,101Reinforcing Substrate9aUpper Buffer Layer (First Buffer Layer)9bLower Buffer Layer (Second Buffer Layer)10,206Optical Waveguide20Adhesive Layer103Buffer Layer200Optical Modulator204Housing208Input Optical Fiber210Output Optical Fiber212a,212b,212c,212dElectrodes218Relay Substrate228Polarization Synthesis Portion240a,240b,240c,240dLead Pin250Termination BoardG Ground ElectrodeS Signal Electrode