Optical waveguide element, optical modulator using same, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element capable of shortening a distance between a side surface of an optical waveguide substrate and an electrode, and capable of suppressing the occurrence of chipping when cutting a wafer.　An optical waveguide element according to the present invention comprises: an optical waveguide substrate 1 on which an optical waveguide is formed; an upper layer UPL and a lower layer UNL that are disposed so as to sandwich the optical waveguide substrate 1; and a support substrate SS that is disposed on a side of the lower layer UNL opposite to the optical waveguide substrate 1. The optical waveguide element is characterized in that both the upper layer UPL and the lower layer UNL are composed of materials having a refractive index lower than a refractive index of the optical waveguide substrate 1, and a Young's modulus of the upper layer UPL is smaller than a Young's modulus of the lower layer UNL.

Description

Optical waveguide element, optical modulator using the same, and optical transmitter

[0001] The present invention relates to an optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, an upper layer and a lower layer arranged to sandwich the optical waveguide substrate, and a support substrate arranged on the lower layer opposite the optical waveguide substrate, and an optical modulator and an optical transmitter using the same.

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements using a substrate on which an optical waveguide is formed, such as optical modulators, are widely used. In a typical optical waveguide element, an optical waveguide is formed on a substrate having an electro-optic effect, such as lithium niobate (LN), and electrodes for applying an electric field to the optical waveguide are formed on the substrate.

[0003] In recent years, high-bandwidth coherent driver modulators (HB-CDMs) have been attracting attention, and the development of optical waveguide elements capable of achieving higher speeds and smaller sizes is anticipated. In the optical waveguide elements used in HB-CDMs, the distance between the electrodes on the optical waveguide substrate and the driver circuit element is shortened to reduce the propagation loss of high-frequency signals, thereby shortening the wiring length. For this reason, the electrodes are placed close to the side of the optical waveguide substrate. Furthermore, in order to obtain more chips from a single wafer, the cut end face of the optical waveguide substrate is often placed close to the electrodes on the optical waveguide substrate.

[0004] In this way, when cutting an optical waveguide substrate close to the electrodes on the optical waveguide substrate, there is a possibility that the end surface of the substrate may be chipped during cutting, and the electrodes may peel off or be cut off. ２ The optical waveguide substrate 1 is bonded via an intermediate layer IL such as SiO 2 . Although a rib-type optical waveguide is shown as the optical waveguide 10, it is also possible to form it by thermally diffusing a high refractive index material such as Ti into the optical waveguide substrate 1. An electrode EL for applying an electric field to the optical waveguide 10 is formed on the optical waveguide substrate 1. The intermediate layer IL is made of SiO 2 . ２If a cutting tool is used, the intermediate layer itself is likely to chip when it is cut, and as a result, the optical waveguide substrate 1 is also likely to chip.

[0005] As shown in Patent Document 1, a glass substrate with low dielectric loss may be used as the support substrate for the optical waveguide element in order to widen the bandwidth of the optical modulator. ２ The optical waveguide substrate 1 is bonded to the support substrate SS2 via the intermediate layer IL. Since such a glass-based substrate is a hard material that is difficult to cut, chipping of the support substrate SS2 itself occurs when the wafer is cut, and the electrodes are likely to peel off or break.

[0006] Patent Document 2 proposes a method for cutting a wafer in which a wide blade (cutter) is used to form a shallow groove, and then a narrow blade is used to cut the remaining part. This method makes it possible to suppress chipping, but as a result, the width required for cutting becomes larger, and the number of chips that can be obtained from one wafer also decreases.

[0007] Furthermore, in Patent Document 3, as shown in FIG. 3, a thin resin film RE is placed on a semiconductor substrate wafer WH so as to straddle a cut portion CL during the manufacture of semiconductor elements. The role of this thin resin film RE is to alleviate the stress applied to the encapsulating resin when the peripheral surface of the semiconductor elements is encapsulated with another resin after the wafer is diced. Furthermore, using such a thin resin film RE during dicing can also suppress chipping. However, to ensure a separate area for forming the thin resin film RE, it is necessary to increase the distance between the side surface (cutting position) of the optical waveguide substrate and the electrodes. This, as mentioned above, can cause propagation loss of high-frequency signals in HB-CDM and a reduction in the number of chips that can be obtained from a wafer. Furthermore, since a resin with high dielectric loss is placed near the high-frequency signal electrodes, degradation of high-frequency signal characteristics can also occur.

[0008] Chinese Patent Publication CN111061071B Japan Patent Publication No. 2013-235113 Japan Patent Publication No. 2014-116333

[0009] The object of the present invention is to provide an optical waveguide element that solves the above-mentioned problems, shortens the distance between the side surface of the optical waveguide substrate and the electrodes, and can suppress chipping during wafer cutting, and further provides an optical modulator and an optical transmitter that use the optical waveguide element.

[0010] In order to solve the above problems, the optical waveguide element of the present invention, and the optical modulator and optical transmitter using the same have the following technical features: (1) In an optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, an upper layer and a lower layer arranged to sandwich the optical waveguide substrate, and a support substrate arranged on the lower layer opposite the optical waveguide substrate, the upper layer and the lower layer are both made of a material having a refractive index lower than that of the optical waveguide substrate, and the Young's modulus of the upper layer is smaller than that of the lower layer.

[0011] (2) In the optical waveguide element described in (1) above, the upper layer and the lower layer contain an inert gas, and the content of the inert gas in each layer is higher in the upper layer than in the lower layer.

[0012] (3) The optical waveguide element according to (2) above, wherein the content of the inert gas in the upper layer is 1.0 atm % or more and 3.0 atm % or less.

[0013] (4) In the optical waveguide element according to any one of (1) to (3) above, the upper layer has a smaller thickness than the lower layer.

[0014] (5) In the optical waveguide element according to any one of (1) to (4) above, the upper layer is disposed at least in the peripheral portion of the optical waveguide substrate.

[0015] (6) In the optical waveguide element according to any one of (1) to (5) above, the upper layer is made of the same material as the lower layer.

[0016] (7) The optical waveguide element according to any one of (1) to (6) above, characterized in that an electrode layer is disposed on the upper or lower side of the optical waveguide substrate, and the shortest distance from the side of the optical waveguide substrate to the side of the electrode layer is set to 30 μm or less.

[0017] (8) An optical modulator comprising: the optical waveguide element according to any one of (1) to (7) above; a housing for accommodating the optical waveguide element; and an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

[0018] (9) In the optical modulator described in (8) above, the optical waveguide element is provided with a modulation electrode for modulating a light wave propagating through the optical waveguide, and an electronic circuit for amplifying a modulation signal input to the modulation electrode is provided inside the housing.

[0019] (10) An optical transmitter comprising the optical modulator according to (8) or (9), a light source for inputting a light wave to the optical modulator, and an electronic circuit for outputting a modulated signal to the optical modulator.

[0020] The present invention provides an optical waveguide element comprising an optical waveguide substrate having an optical waveguide formed thereon, and upper and lower layers arranged to sandwich the optical waveguide substrate, with a support substrate arranged on the opposite side of the lower layer from the optical waveguide substrate, in which the upper and lower layers are both made of materials having a refractive index lower than that of the optical waveguide substrate, and the Young's modulus of the upper layer is smaller than that of the lower layer, thereby suppressing chipping in the optical waveguide substrate when the wafer is cut, and as a result, it is possible to provide an optical waveguide element in which the distance between the side surface of the optical waveguide substrate and the electrodes is shortened. Furthermore, by using an optical waveguide element with such excellent characteristics, it is possible to provide an optical modulator and an optical transmitter that achieve similar effects.

[0021] FIG. 10 is a cross-sectional view showing an example of a conventional optical waveguide element. FIG. 11 is a cross-sectional view showing another example of a conventional optical waveguide element. FIG. 12 is a cross-sectional view showing an example of a semiconductor substrate wafer disclosed in Patent Document 3. FIG. 13 is a cross-sectional view showing an example of an optical waveguide element of the present invention. FIG. 14 is a cross-sectional view showing an example of an optical waveguide element of the present invention, in which an upper layer is arranged below an electrode layer. FIG. 15 is a cross-sectional view showing an example of an optical waveguide element of the present invention, in which an upper layer is arranged above an electrode layer. FIG. 16 is a plan view showing an example of an optical waveguide element using segmented electrodes shown in Patent Document 2. FIG. 17 is a cross-sectional view showing an example when the segmented electrode of FIG. 7 is applied to the optical waveguide element of the present invention. FIG. 18 is a cross-sectional view showing another example when the segmented electrode of FIG. 7 is applied to the optical waveguide element of the present invention. FIG. 19 is a plan view explaining the state near the side surface of the optical waveguide substrate of the optical waveguide element of the present invention. FIG. 19 is a diagram showing an example of an optical transmitting device of the present invention.

[0022] The optical waveguide element of the present invention, and the optical modulator and optical transmitter using the same will be described in detail below using preferred examples. A cross-sectional view showing an example of the optical waveguide element of the present invention is shown in Figure 4. The optical waveguide element of the present invention comprises an optical waveguide substrate 1 having an optical waveguide formed thereon, and an upper layer UPL and a lower layer UNL arranged to sandwich the optical waveguide substrate 1, with a support substrate SS arranged on the side of the lower layer UNL opposite the optical waveguide substrate 1. The upper layer UPL and the lower layer UNL are both made of materials having a refractive index lower than that of the optical waveguide substrate 1, and the Young's modulus of the upper layer UPL is smaller than that of the lower layer UNL.

[0023] The optical waveguide substrate 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect. Specifically, single crystal materials such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), as well as materials obtained by doping these substrate materials with MgO or the like, can be used. These materials can also be used to form films using vapor phase growth methods such as sputtering, evaporation, or CVD. Alternatively, a substrate having an electro-optic effect can be bonded to another substrate and then thin-film processed to form a thin film. Furthermore, semiconductor substrates and substrates made of organic materials such as EO polymers can also be used.

[0024] The optical waveguide 10 can be an optical waveguide in which a high refractive index material such as Ti is thermally diffused into the optical waveguide substrate 1, an optical waveguide formed by proton exchange, or a rib-type optical waveguide 10 in which the portion of the substrate corresponding to the optical waveguide is convex, as shown in Figures 5 or 6, by etching the substrate 1 other than the optical waveguide or forming grooves on both sides of the optical waveguide. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to further increase the refractive index by diffusing Ti or the like onto the surface of the substrate by thermal diffusion or proton exchange. The size of the rib-type optical waveguide is a finely structured optical waveguide with a width and height of about 1 μm or less to enhance light confinement.

[0025] The thickness (maximum thickness) of the optical waveguide substrate 1 on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to achieve velocity matching between the microwave and light waves of the modulation signal. Furthermore, the height of the rib-type optical waveguide 10 (the height of the portion protruding from the slab waveguide) is set to 80% or less of the maximum thickness of the optical waveguide substrate, specifically 4 μm or less, more preferably 3 μm or less, and even more preferably 0.8 μm or less or 0.4 μm or less.

[0026] In order to increase the mechanical strength of the optical waveguide substrate 1 on which the optical waveguide is formed, a support substrate SS is bonded to the underside of the optical waveguide substrate 1. The optical waveguide substrate 1 and the support substrate SS are bonded and fixed by direct bonding or via an adhesive layer such as resin. The support substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but is not limited to this. In the case of direct bonding, an intermediate layer such as a metal oxide or metal may be included in the bonding portion. The support substrate SS is made of a material having a thermal expansion coefficient close to that of the optical waveguide substrate 1, such as glass, quartz, fused quartz, synthetic quartz, alkali glass, alkali-free glass, lead glass, borosilicate glass, soda glass, sapphire, SiO such as alumina, etc. ２ System and Al ２ O ３A substrate including an oxide layer, such as a low-dielectric-constant substrate of the type, is preferably used. Furthermore, it is also possible to use the same LN substrate as the optical waveguide substrate 1, a composite substrate in which a silicon oxide layer is formed on a silicon substrate, abbreviated as SOI or LNOI, or a composite substrate in which a silicon oxide layer is formed on an LN substrate. If the refractive index of the support substrate SS is higher than that of the optical waveguide substrate 1, a layer having a lower refractive index than the optical waveguide substrate 1 is provided between the optical waveguide substrate 1 and the support substrate SS. Furthermore, as will be described later, the support substrate according to the present invention is not limited to one formed of a single substrate, but also includes a substrate formed by stacking and integrating multiple substrates.

[0027] In FIG. 4, as an example, a glass substrate is used as the support substrate SS, and an SiO ２ The optical waveguide substrate 1 is disposed on the bonding layer (intermediate layer) such as a bonding layer (intermediate layer). In the optical waveguide element of the present invention, the intermediate layer disposed below the optical waveguide substrate 1 and in contact with the optical waveguide substrate 1 is called the lower layer UNL. Furthermore, an upper layer UPL, which is a feature of the present invention, is disposed above the optical waveguide substrate 1.

[0028] In the optical waveguide element of the present invention, the upper layer UPL and the lower layer UNL sandwiching the optical waveguide substrate 1 function as cladding layers for the optical waveguide, and therefore use a dielectric material that has a lower refractive index and higher transparency than the optical waveguide substrate 1. Specifically, SiO ２ and Al ２ O ３ , MgF ２ , La ２ O ３ , ZnO, HfO ２ , MgO, CaF ２ , Y ２ O ３ For example, oxides and fluorides of metal elements from groups 1 to 17 of the periodic table are used.

[0029] Furthermore, the upper layer UPL is softer than the lower layer in order to absorb mechanical shocks during wafer cutting and to prevent chipping of the optical waveguide substrate 1, the lower layer (intermediate layer) UNL, and the support substrate SS. In other words, the Young's modulus of the upper layer UPL is smaller than that of the lower layer UNP. Specifically, the Young's modulus of the upper layer UPL is set to 90 GPa or less.

[0030] To adjust the Young's modulus of the upper and lower layers, the upper and lower layers may contain at least one oxide of a semiconductor element or an oxide of a group 3 to 8 of the periodic table. For example, metal oxides such as indium, titanium, zinc, tin, chromium, aluminum, and germanium are used. Furthermore, to adjust the refractive index, the upper layer UPL may be a multilayer structure made up of two or more materials. In this case, the Young's modulus of each layer is set so that the layer with the smallest Young's modulus among the multilayer structures has a smaller Young's modulus than the lower layer UNL.

[0031] In addition, inert gas can be incorporated into the upper and lower layers to adjust the Young's modulus. The upper and lower layers are formed by sputtering the material that will become the film, so the inert gas content in each layer can be changed by adjusting the concentration of the inert gas used in sputtering. The higher the inert gas content, the lower the density of the formed film, resulting in a softer film with a low Young's modulus. Therefore, the inert gas content in each layer is higher in the upper layer UPL than in the lower layer UNL. Sputtering deposition ensures a uniform composition within the film, allowing for reliable chipping prevention regardless of the location on the chip or wafer.

[0032] As the inert gas, a noble gas of Group 18 of the periodic table (He, Ne, Ar, Kr, Xe, Rn) or nitrogen is used. One type of inert gas may be used, or multiple inert gases may be mixed. For example, only Ar may be used, and the Ar content may be changed to adjust the Young's modulus. It is also possible to mix Ar and nitrogen, and change the content in a similar manner. Regarding the content of the inert gas, it is preferable to set the nitrogen content lower than the noble gas content. This is because noble gases do not react with other elements, but nitrogen forms nitrides. Therefore, considering the stability of characteristics after device fabrication, a lower nitrogen content is preferable.

[0033] The advantage of using an inert gas such as Ar in the upper layer is that even after the optical waveguide element (chip) is mounted in a housing, hermetically sealed, and formed into a device, the inert gas does not react with other functional layers such as electrodes, preventing deterioration of device characteristics. For example, even if an inert gas such as Ar is released into the housing after packaging, it does not affect other functional layers, such as oxidizing the electrodes, and therefore long-term stable device characteristics can be ensured.

[0034] In the lower layer UNL, since the lower layer functions as a bonding layer, the content of the inert gas is preferably set to 1.0 atm % or less so as not to affect the bonding strength and to form a hard film.

[0035] On the other hand, in order to make the upper layer UPL a soft film, the inert gas content is preferably 1.0 atm% or more. However, if the inert gas content is too high, the film will have many defects, and the device characteristics and reliability will be reduced due to moisture content, etc., so it is preferable to set the content to 3.0 atm% or less. The inert gas content in the film can be measured by RBS (Rutherford Backscattering Spectrometry) analysis.

[0036] The thickness h2 of the upper layer UPL can be set to be thinner than the thickness h1 of the lower layer UNL. The thickness h1 of the lower layer is preferably 1 μm or more, and more preferably 2 μm or more to suppress light absorption by the support substrate SS. The thickness h2 of the upper layer is 100 nm or more, and more preferably 200 nm or more to suppress chipping of the optical waveguide substrate 1.

[0037] The material constituting the upper layer UPL may be the same as the material constituting the lower layer UNL, or a different material may be used. When the same material is used, the upper layer UPL and the lower layer UNL sandwiching the optical waveguide substrate 1 have the same linear expansion coefficient, making it possible to provide an optical waveguide element that suppresses the occurrence of drift caused by temperature changes.

[0038] The upper layer UPL is SiO ２ The lower layer UNL is made of Al ２ O ３ This allows for the selection of the optimum material for the properties required for each layer.

[0039] 5 and 6 show an example in which an electrode layer EL that applies an electric field to the optical waveguide 10 is disposed above the optical waveguide substrate 1. In FIG. 5, the upper layer UPL is disposed below the electrode layer EL, but as shown in FIG. 6, it may be disposed so as to cover the electrode layer EL. When disposed below the electrode layer as shown in FIG. 5, the electrode relieves stress on the optical waveguide substrate and the optical waveguide. Furthermore, when disposed above the electrode layer as shown in FIG. 6, it is possible to increase the electric field efficiency of the electrode to the optical waveguide.

[0040] 7 is a plan view of an optical waveguide element using the segment electrodes SE shown in Patent Document 1. The segment electrodes SE are electrically connected to the transmission lines ME that propagate high-frequency signals using bridge electrodes BE. FIGS. 8 and 9 show examples in which such segment electrodes SE are applied to the optical waveguide element of the present invention. FIGS. 8 and 9 are enlarged cross-sectional views showing only the vicinity of the segment electrodes SE that sandwich the optical waveguide 10. As shown in FIG. 8, the segment electrodes SE, bridge electrodes, and transmission lines ME can be arranged above the upper layer UPL.

[0041] For example, using a glass substrate for the support substrate SS reduces the dielectric constant around the electrodes, but using segment electrodes makes it possible to adjust (increase) Nm (the effective refractive index of light). Also, as shown in Fig. 9, it is possible to configure the structure so that only the segment electrodes SE and bridge electrodes BE are arranged on the optical waveguide substrate 1, and the transmission line ME is arranged on the upper layer UPL. In Fig. 9, a portion of the transmission line ME arranged on the upper side of the upper layer UPL is arranged so as to straddle the optical waveguide substrate 1, but this role can also be performed by the bridge electrode BE.

[0042] 9, the upper layer UPL does not need to be disposed over the entire upper side of the optical waveguide substrate 1. Since it is necessary to dispose the upper layer UPL at least in the cut portion, the upper layer UPL is configured to be disposed at least in the peripheral portion of the optical waveguide substrate 1 at the time of chipping (the state after the wafer is cut).

[0043] As shown in Figure 10, electrode layers (EL1 to EL3) are disposed on the upper or lower side of the optical waveguide substrate 1, and the shortest distance (S1, S2) from the side of the optical waveguide substrate 1 (the left and bottom edges of the drawing) to the side of the electrode layer is set to 30 μm or less, more preferably 20 μm or less. The shorter the distance S1, etc., the less margin there is around the chip, and the more chips can be obtained from one wafer. Furthermore, the shorter the distance S1, the shorter the length of the wiring connecting from the outside to the electrode EL2 can be, which makes it possible to suppress degradation of high-frequency signals, for example.

[0044] Next, examples of application of the optical waveguide element of the present invention to an optical modulator or optical transmitter will be described. While the following description will use an example of HB-CDM, the present invention is not limited to this, and can also be applied to optical phase modulators, optical modulators with polarization combining functions, optical modulators integrating more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide substrates made of other materials such as silicon, devices for sensor applications, and the like.

[0045] As shown in FIG. 11 , the optical waveguide element includes an optical waveguide 10 formed on an optical waveguide substrate 1 and an electrode (not shown), such as a modulation electrode, that modulates the light wave propagating through the optical waveguide 10. The substrate 1 is housed within a housing CA. Furthermore, an optical modulator MD can be configured by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide. In FIG. 11 , the optical fiber (F) is introduced into the housing CA through a through-hole penetrating the sidewall, and the optical waveguide substrate 1 and the optical fiber are directly bonded. Alternatively, the light wave L1 incident from the optical fiber F and the light wave L2 emitted from the optical fiber F can be optically coupled to the optical waveguide 10 within the optical waveguide substrate 1 via an optical block equipped with an optical lens, a lens barrel, a polarization multiplexer, or the like. Furthermore, to ensure stable bonding with the optical fiber or optical block, a reinforcing member RI can be placed on top of the optical waveguide substrate 1 along the end face of the substrate 1.

[0046] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor, DSP) that outputs a modulation signal S0 that causes the optical modulator MD to perform modulation operations to the optical modulator MD. To obtain the modulation signal S to be applied to the optical waveguide element, the modulation signal S0 output from the digital signal processor DSP can be amplified. For this reason, in FIG. 11, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and digital signal processor DSP can be located outside the housing CA, but they can also be located inside the housing CA. In particular, locating the driver circuit DRV inside the housing can further reduce the propagation loss of the modulation signal from the driver circuit. If degradation of the modulation signal is minimal, a DRV is not necessary, and the optical modulator MD can be directly modulated by the DSP.

[0047] The input light L1 to the optical modulator MD may be supplied from outside the optical transmitter OTA, but it may also be incorporated integrally into the optical transmitter OTA using a semiconductor laser (not shown) as a light source. The output light L2 modulated by the optical modulator MD is output to the outside via an optical fiber F.

[0048] It is also possible to configure the optical modulator MD to be mounted in the optical transmission device OTA, or to configure the unitized optical transmission device OTA to be incorporated into a rack in the transmission / reception facility, so that electrical connection can be made simply by inserting the device into the connection plug. Furthermore, in the above-mentioned optical waveguide element, an example has been described in which the optical waveguide element is stored inside a housing to form an optical modulator, and the optical modulator is incorporated into the optical transmission device, but it is also possible to mount the optical waveguide element directly on the optical transmission / reception device, or to combine it with other functional elements in the optical transmission / reception device to form a subassembly.

[0049] As described above, according to the present invention, it is possible to provide an optical waveguide element that can reduce the distance between the side surface of the optical waveguide substrate and the electrodes and can suppress chipping during wafer cutting. Furthermore, it is possible to provide an optical modulator and an optical transmitter using the optical waveguide element.

[0050] REFERENCE SIGNS LIST 1 Optical waveguide substrate (thin plate, film body) 10 Optical waveguide UPL Upper layer UNL Lower layer SS Support substrate BL Adhesive layer EL Electrode layer M Upper electrode F Optical fiber CA Housing MD Optical modulator DRV Driver circuit DSP Digital signal processor OTA Optical transmitter

Claims

1. An optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, and upper and lower layers arranged to sandwich the optical waveguide substrate, with a support substrate arranged on the lower layer opposite the optical waveguide substrate, wherein the upper and lower layers are both made of materials having a refractive index lower than that of the optical waveguide substrate, and the Young's modulus of the upper layer is smaller than that of the lower layer.

2. An optical waveguide element according to claim 1, wherein the upper and lower layers contain an inert gas, and the content of the inert gas in each layer is greater in the upper layer than in the lower layer.

3. An optical waveguide element according to claim 2, wherein the content of the inert gas in the upper layer is 1.0 atm % or more and 3.0 atm % or less.

4. An optical waveguide element according to claim 1, wherein the thickness of said upper layer is thinner than the thickness of said lower layer.

5. An optical waveguide element according to claim 1, characterized in that the upper layer is disposed at least in the peripheral portion of the optical waveguide substrate.

6. An optical waveguide element according to claim 1, wherein the material constituting the upper layer is the same as the material constituting the lower layer.

7. An optical waveguide element according to claim 1, characterized in that an electrode layer is disposed on the upper or lower side of the optical waveguide substrate, and the shortest distance from the side of the optical waveguide substrate to the side of the electrode layer is set to 30 μm or less.

8. An optical modulator comprising: the optical waveguide element according to claim 1; a housing for accommodating said optical waveguide element; and an optical fiber for inputting or outputting a light wave to said optical waveguide.

9. An optical modulator according to claim 8, wherein the optical waveguide element is provided with a modulation electrode for modulating the light wave propagating through the optical waveguide, and the housing has an electronic circuit therein for amplifying the modulation signal input to the modulation electrode.

10. An optical transmitter comprising: the optical modulator according to claim 8; a light source for inputting a light wave to said optical modulator; and an electronic circuit for outputting a modulated signal to said optical modulator.

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