Patent Description:
In high-speed/large-capacity optical fiber communication systems, an optical modulator into which an optical modulation element serving as an optical waveguide element constituted of an optical waveguide formed on a substrate is incorporated is often used. Among these, since optical modulation elements, in which LiNbO<NUM> (hereinafter, which will also be referred to as LN) having an electro-optical effect is used as a substrate, have a small loss of light and can realize broadband optical modulation characteristics, they are widely used in high-speed/large-capacity optical fiber communication systems. In optical modulation elements using such a LN substrate, for example, a Mach-Zehnder optical waveguide and a signal electrode for applying a high-frequency electrical signal (modulation signal) to the optical waveguide are provided.

Particularly, regarding a modulation method in an optical fiber communication system, in response to the recent trend of increasing transmission capacity, multi-level modulation such as quadrature phase shift keying (QPSK) and dual polarization-quadrature phase shift keying (DP-QPSK) and a transmission format adopting polarization multiplexing in multi-level modulation have become mainstream, and these are not only used in key optical transmission networks but have also been introduced to metro networks.

Optical modulators performing QPSK modulation (QPSK optical modulator) and optical modulators performing DP-QPSK modulation (DP-QPSK optical modulator) include a plurality of Mach-Zehnder optical waveguides having a nested structure (so-called nest type), and each of the optical waveguides includes at least one signal electrode. In addition, in optical modulators using such Mach-Zehnder optical waveguides, generally, a bias electrode for compensating for variation in bias points due to so-called DC drift is also formed.

Such a signal electrode or a bias electrode (which will hereinafter be generically referred to simply as an electrode) is formed to extend to a part in the vicinity of an outer circumference of a LN substrate for connection to an electric circuit outside the substrate. For this reason, a plurality of optical waveguides and a plurality of electrodes intersect each other in a complicated manner on the substrate, and a plurality of intersecting parts at which the electrodes cross the optical waveguides thereabove is formed.

If the optical waveguides and the electrodes are formed to be in direct contact with each other in the foregoing intersecting parts, light propagated in the optical waveguides is absorbed by metal constituting the electrodes, and thus an optical loss (optical absorption loss) occurs in these intersecting parts. For example, this optical loss causes a difference between optical losses of two parallel waveguides constituting a Mach-Zehnder optical waveguide and may cause deterioration in extinction ratio of modulated light. Since required conditions for the extinction ratio become stricter as an increasing modulation speed is required for an optical modulator, it is expected that such deterioration in extinction ratio will become more apparent as the modulation speed increases in accordance with the increasing transmission capacity.

In addition, the foregoing intersecting parts may be extensively and generally formed not only in optical modulators using a Mach-Zehnder optical waveguide but also in directional couplers, optical modulators using optical waveguides constituting Y-branches, and/or optical waveguide elements such as optical switches. Further, the number of intersecting parts on a substrate will further increase as optical waveguide patterns and electrode patterns become more complicated due to optical waveguide elements being increasingly reduced in size, being multi-channeled, and/or being highly integrated. This will become a non-negligible cause of loss and may limit the performance of optical waveguide elements.

In the related art, regarding a technology of reducing an optical absorption loss due to electrode metal formed on an optical waveguide, a technology in which a buffer layer consisting of SiO<NUM> is provided on a surface of a substrate on which an optical waveguide is formed and electrode metal is formed above the buffer layer is known (for example, Patent Document <NUM>).

However, since SiO<NUM> has a higher rigidity than a LN substrate, when a SiO<NUM> layer is formed on a LN substrate, stress is applied not only to the substrate from the SiO<NUM> layer itself, but also stress is inflicted on the substrate via the SiO<NUM> layer from electrode metal formed thereabove. Further, such stress may also adversely affect optical characteristics or electrical characteristics of an optical waveguide element on account of a photoelastic effect of a LN substrate.

Particularly, in an optical waveguide element in which a thin LN substrate (for example, having a thickness equal to or smaller than <NUM>) is formed in order to further intensify interaction between a signal electric field and waveguide light in the substrate (that is, in order to enhance the efficiency of an electric field), stress inflicted on the substrate from a SiO<NUM> layer and electrode metal thereabove may have a non-negligible influence on optical characteristics and/or electrical characteristics and also cause occurrence of a local distortion due to a difference between linear expansion coefficients of the SiO<NUM> layer and the LN substrate, and thus it may become a factor prompting damage such as cracking or disconnection in the SiO<NUM> layer itself or the electrode thereabove at the time of manufacturing and/or over time.

[Patent Document <NUM>] <CIT>
<CIT> discloses an optical waveguide board which includes a substrate, an optical path changing unit being formed on the substrate used to change a direction of an optical path of incident light from a direction being vertical to a surface of the substrate to a direction being horizontal to the surface of the substrate and to condense a luminous flux and an optical waveguide being formed on the substrate to carry out multi-mode transmission of a luminous flux fed from the optical path changing unit wherein, based on a spread angle of the luminous flux formed by the optical path changing unit, mainly light components to be transmitted in a zero-order mode to a three-order mode only, out of various kinds of modes for the multi-mode transmission, is transmitted through the optical waveguide. <CIT> discloses a surface treatment method for forming a uniform film in which a coating is formed on a curved surface of a base material of the optical component by an atmospheric pressure plasma method. <CIT> discloses a semiconductor optical modulator including optical input and output portions; a plurality of Mach-Zehnder modulators including first and second waveguide arms having first and second modulation electrodes, respectively; an optical demultiplexer coupled between the optical input portion and the Mach-Zehnder modulator through an optical waveguide; an optical multiplexer coupled between the optical output portion and the Mach-Zehnder modulator; a plurality of electrical inputs including first, second, and common electrodes disposed between the first and second electrodes; and a plurality of differential transmission lines electrically connecting the electrical inputs to the Mach-Zehnder modulators. <CIT> relates to a Mach-Zehnder interferometer type optical modulator including optical waveguides. <CIT> relates to an electro-optic device, a method of manufacturing the electro-optic device, and an electronic apparatus. <CIT> relates to a velocity-matched traveling-wave electro-optical modulator using a benzocyclobutene buffer layer.

From the foregoing background, in an optical waveguide element, it is required to effectively reduce an optical absorption loss of waveguide light, which may occur due to electrode metal at an intersecting part between an optical waveguide and an electrode, without causing deterioration in optical characteristics and degradation of long-term reliability of the optical waveguide element.

The present invention is provided by the appended claims. The following disclosure serves a better understanding of the present invention.

According to the disclosure, in an optical waveguide element, it is possible to effectively reduce an optical absorption loss of waveguide light, which may occur due to electrode metal at an intersecting part between an optical waveguide and an electrode on a substrate, without causing deterioration in optical characteristics and degradation of long-term reliability of the optical waveguide element.

Regarding a measure to reduce stress applied to a substrate from a SiO<NUM> layer in a constitution in the related art described above, it is conceivable to adopt a constitution in which a SiO<NUM> layer is formed only in a substrate portion where an optical waveguide and an electrode intersect each other instead of providing a SiO<NUM> layer on the entire surface of a substrate and the electrode is formed above the SiO<NUM> layer.

However, as illustrated in <FIG>, when a portion of a SiO<NUM> layer <NUM> is formed on a substrate <NUM> at an intersecting part <NUM>, due to a local distortion caused by a steep change in shape at a stepped portion of the SiO<NUM> layer <NUM> or a difference between linear expansion coefficients of the SiO<NUM> layer <NUM> and the substrate <NUM>, a disconnection <NUM> or <NUM> may occur in an electrode <NUM> at a corner part in the vicinity of the SiO<NUM> layer <NUM>.

In <FIG>, an optical waveguide <NUM> extends in a Y direction in coordinate axes indicated at the upper right part in the diagram. In addition, an electrode (or a signal line) <NUM> constituted of a metal layer extends in a Z direction and intersects the optical waveguide <NUM> thereabove, thereby forming the intersecting part <NUM>.

In addition, when an electrode is constituted to be thicker, as illustrated in <FIG>, at an intersecting part <NUM>, due to stress from an electrode <NUM> (stress or the like accumulated inside the electrode <NUM> (metal layer) or on a boundary surface between the electrode <NUM> and the SiO<NUM> layer <NUM> at the time of forming the metal layer) and/or due to a local distortion caused by the difference between the linear expansion coefficients of the SiO<NUM> layer <NUM> and the substrate <NUM>, cracking <NUM> or <NUM> may occur at a corner part of the SiO<NUM> layer <NUM>.

In <FIG>, the optical waveguide <NUM> extends in the Y direction in the coordinate axes indicated at the upper right part in the diagram. In addition, the electrode <NUM> constituted of a metal layer extends in the Z direction and intersects the optical waveguide <NUM> thereabove, thereby forming the intersecting part <NUM>.

In <FIG> and <FIG>, the substrate <NUM> is a LN substrate thinned to have a thickness equal to or smaller than <NUM> (for example, <NUM>) and is fixed to a portion on a support substrate <NUM> with an adhesive layer <NUM> therebetween. The support substrate <NUM> is a glass substrate, a LN substrate, or a Si substrate, for example.

As in embodiments which will be described below, in an optical modulation element using a Mach-Zehnder optical waveguide, generally, an electrode such as a bias electrode in which a low-frequency signal is propagated is formed to have a thickness within a range of approximately <NUM> to <NUM>, and thus there is concern that breaking may occur in an electrode metal layer as in <FIG>. In addition, a high-frequency signal electrode in which a modulation signal is propagated is generally formed to have a thickness within a range of approximately <NUM> to <NUM>, and thus there is concern that cracking may occur in a SiO<NUM> layer as in <FIG>.

In an optical waveguide element according to the disclosure, occurrence of such disconnection or cracking at an intersecting part between an optical waveguide and an electrode intersecting the optical waveguide thereabove is prevented, and an optical absorption loss of waveguide light due to electrode metal at the intersecting part is effectively reduced without causing deterioration in optical characteristics and degradation of long-term reliability of the optical waveguide element.

Hereinafter, the embodiments of the disclosure will be described with reference to the drawings.

<FIG> is a view illustrating a constitution of an optical modulator <NUM> using an optical modulation element serving as an optical waveguide element according to a first embodiment of the disclosure. The optical modulator <NUM> has a casing <NUM>, an optical modulation element <NUM> accommodated inside the casing <NUM>, and a relay substrate <NUM>. The optical modulation element <NUM> is a DP-QPSK modulator, for example. Ultimately, in the casing <NUM>, a cover (plate body, not illustrated) is fixed to an opening thereof, and the inside thereof is sealed in an air-tight manner.

The optical modulator <NUM> also has signal pins 110a, 110b, 110c, and 110d for inputting a high-frequency electrical signal used for modulation of the optical modulation element <NUM>, and a feed through part <NUM> for introducing the signal pins 110a, 110b, 110c, and 110d into the casing <NUM>.

Moreover, the optical modulator <NUM> has an input optical fiber <NUM> for inputting light to the inside of the casing <NUM> and an output optical fiber <NUM> for guiding light modulated by the optical modulation element <NUM> to the outside of the casing <NUM> on the same surface of the casing <NUM>.

Here, each of the input optical fiber <NUM> and the output optical fiber <NUM> is fixed to the casing <NUM> with supports <NUM> and <NUM> (fixing members) therebetween. Light input from the input optical fiber <NUM> is collimated by a lens <NUM> installed inside the support <NUM> and is input to the optical modulation element <NUM> via a lens <NUM>. However, this is an example, and light can be input to the optical modulation element <NUM> in accordance with a technology in the related art, for example, by introducing the input optical fiber <NUM> into the casing <NUM> with the support <NUM> therebetween and connecting an end surface of the introduced input optical fiber <NUM> to an end surface of a substrate <NUM> of the optical modulation element <NUM>.

The optical modulator <NUM> also has an optical unit <NUM> for polarization-combining two rays of modulated light output from the optical modulation element <NUM>. Light after polarization combining output from the optical unit <NUM> is concentrated by a lens <NUM> installed inside the support <NUM> and is coupled to the output optical fiber <NUM>.

The relay substrate <NUM> relays a high-frequency electrical signal input from the signal pins 110a, 110b, 110c, and 110d to the optical modulation element <NUM> through a conductor pattern (not illustrated) formed on the relay substrate <NUM>. The conductor pattern on the relay substrate <NUM> is individually connected to pads (which will be described below) constituting one end of a signal electrode of the optical modulation element <NUM> through wire bonding, for example. In addition, the optical modulator <NUM> includes two terminators 112a and 112b having a predetermined impedance inside the casing <NUM>.

<FIG> is a view illustrating an example of a constitution of the optical modulation element <NUM> serving as an optical waveguide element accommodated inside the casing <NUM> of the optical modulator <NUM> illustrated in <FIG>. The optical modulation element <NUM> is constituted of optical waveguides (bold dotted lines in the diagram) formed on the substrate <NUM> which is constituted of LN, for example, and performs DP-QPSK modulation of <NUM>, for example. These optical waveguides can be formed through thermal diffusion of Ti on a surface of the substrate <NUM>.

The substrate <NUM> has a rectangular shape, for example, and has two sides 280a and 280b extending in a vertical direction in the diagram and facing each other on the left and the right in the diagram, and sides 280c and 280d extending in a lateral direction in the diagram and facing each other above and below in the diagram. In <FIG>, as indicated in the coordinate axes at the upper left part in the diagram, a normal direction toward a deeper side of the paper in <FIG> (from the front surface to the rear surface) will be referred to as an X direction, the rightward direction in the diagram will be referred to as the Y direction, and the downward direction in the diagram will be referred to as the Z direction.

The optical modulation element <NUM> includes an input waveguide <NUM> for receiving input light from the input optical fiber <NUM> (arrow directed in the rightward direction in the diagram) on a lower side in the diagram of the side 280b on the left side in the diagram in the substrate <NUM>, and a branching waveguide <NUM> for causing input light to branch into two rays of light having the same amount of light. In addition, the optical modulation element <NUM> includes so-called nested Mach-Zehnder optical waveguides 240a and 240b (portions individually surrounded by the one-dot dashed line in the diagram) which are two modulation parts for modulating each ray of light that branching off due to the branching waveguide <NUM>.

The nested Mach-Zehnder optical waveguides 240a and 240b respectively include two Mach-Zehnder optical waveguides 244a (a portion within the dotted line in the diagram) and 246a (a portion within the two-dot dashed line in the diagram) and two Mach-Zehnder optical waveguides 244b (a portion within the dotted line in the diagram) and 246b (a portion within the two-dot dashed line in the diagram) respectively provided in two waveguide portions forming pairs of parallel waveguides. Accordingly, the nested Mach-Zehnder optical waveguides 240a and 240b individually perform QPSK modulation of two rays of input light branching off due to the branching waveguide <NUM> and then output modulated light (output) to the left side in the diagram from output waveguides 248a and 248b respectively.

Thereafter, the two rays of output light are subjected to polarization combining by the optical unit <NUM> installed outside the substrate <NUM> and are united into one light beam. Hereinafter, optical waveguides, such as the input waveguide <NUM>, the branching waveguide <NUM>, the nested Mach-Zehnder optical waveguides 240a and 240b, and the Mach-Zehnder optical waveguides 244a, 246a, 244b, and 246b included therein, which are formed on the substrate <NUM> of the optical modulation element <NUM> will be generically referred to as an optical waveguide <NUM> and the like.

Signal electrodes 250a, 252a, 250b, and 252b for respectively causing the four Mach-Zehnder optical waveguides 244a, 246a, 244b, and 246b in total constituting the nested Mach-Zehnder optical waveguides 240a and 240b to perform modulation operation are provided on the substrate <NUM>. The left sides of the signal electrodes 250a and 252a in the diagram are bent and extend to the side 280c on the upper side in the diagram in the substrate <NUM> and are connected to pads 254a and 256a. In addition, the right sides of the signal electrodes 250a and 252a in the diagram extend to the side 280a on the right side in the diagram in the substrate <NUM> and are connected to pads 258a and 260a.

Similarly, the left sides of the signal electrodes 250b and 252b in the diagram extend to the side 280d on the lower side in the diagram in the substrate <NUM> and are connected to pads 254b and 256b, and the right sides of the signal electrodes 250b and 252b in the diagram extend to the side 280a on the right side in the diagram in the substrate <NUM> and are connected to pads 258b and 260b. The pads 258a, 260a, 258b, and 260b are connected to the relay substrate <NUM> described above through wire bonding or the like.

The signal electrodes 250a, 252b, 250b, and 252b constitute a coplanar transmission line having a predetermined impedance, for example, together with a ground conductor pattern (not illustrated) formed on the substrate <NUM> in accordance with the technology in the related art. For example, the ground conductor pattern is provided such that it is not formed on the optical waveguide <NUM> and the like. A plurality of regions formed to be divided by the optical waveguide <NUM> and the like in the ground conductor pattern can be connected to each other through wire bonding or the like.

The pads 254a, 256a, 254b, and 256b are connected to the terminators 112a and 112b described above. Accordingly, a high-frequency electrical signal input from the relay substrate <NUM> connected to the pads 258a, 260a, 258b, and 260b becomes a traveling wave and is propagated in the signal electrodes 250a, 252a, 250b, and 252b, thereby modulating optical waves propagated in the Mach-Zehnder optical waveguides 244a, 246a, 244b, and 246b.

Here, in order to further intensify interaction between an electric field formed inside the substrate <NUM> by the signal electrodes 250a, 252a, 250b, and 252b and waveguide light propagated in the Mach-Zehnder optical waveguides 244a, 246a, 244b, and 246b and to be able to perform high-speed modulation operation at a lower voltage, the substrate <NUM> is formed to have a thickness equal to or smaller than <NUM> and to preferably have a thickness equal to or smaller than <NUM>. The rear surface (a surface facing the surface illustrated in <FIG>) of the substrate <NUM> is adhered to a support substrate formed of a glass or the like with an adhesive layer therebetween (not illustrated in <FIG> but illustrated as an adhesive layer <NUM> and a support substrate <NUM> in <FIG> and the like, which will be described below).

The optical modulation element <NUM> is also provided with bias electrodes 262a, 264a, 262b, and 264b for compensating for variation in bias points due to so-called DC drift. The bias electrodes 262a and 262b are respectively constituted of two sets of an electrode pair and are respectively used for compensating for variation in the bias points in the Mach-Zehnder optical waveguides 244a, 246a, 244b, and 246b. In addition, the bias electrodes 264a and 264b are respectively used for compensating for variation in the bias points in the nested Mach-Zehnder optical waveguides 240a and 240b.

The bias electrodes 262a, 264a, 262b, and 264b also extend respectively to the sides 280c and 280d of the substrate <NUM> and are connected to a bias control circuit outside the casing with lead pins (not illustrated) provided therebetween on a side surface of the casing <NUM>, for example, in portions in the vicinity of the sides 280c and 280d. Accordingly, the bias electrodes 262a, 264a, 262b, and 264b are driven by the bias control circuit, and variation in the bias points in each of the corresponding Mach-Zehnder optical waveguides is compensated for. Hereinafter, the signal electrodes 250a, 252a, 250b, and 252b and the bias electrodes 262a, 264a, 262b, and 264b will be generically referred to as an electrode 250a and the like.

The bias electrodes 262a, 264a, 262b, and 264b are electrodes to which a direct current or a low-frequency electrical signal is applied and are formed to have a thickness within a range of <NUM> to <NUM>, for example. In contrast, the signal electrodes 250a, 252b, 250b, and 252b described above are formed within a range of <NUM> to <NUM>, for example, in order to reduce a conductor loss of a high-frequency electrical signal applied to the signal electrode.

The optical modulation element <NUM> constituted as described above includes many intersecting portions at which the electrode 250a and the like intersect (cross) the optical waveguide <NUM> and the like thereabove. As easily understood from the illustration of <FIG>, all the portions in which the bold dotted lines in the diagram indicating the optical waveguide <NUM> and the like and belt-shaped portions in the diagram indicating the electrode 250a and the like intersect each other in <FIG> are intersecting portions at which the electrode 250a and the like intersect the optical waveguide <NUM> and the like thereabove. In the present embodiment, the optical modulation element <NUM> includes <NUM> intersecting portions in total.

<FIG> is a partial detailed view of the part A of the optical modulation element <NUM> illustrated in <FIG>.

Hereinafter, with the part B, the part C, the part D, the part E, and the part F (intersecting portions) illustrated in <FIG> as examples, the constitutions of these intersecting portions will be described.

First, the constitution of the part B illustrated in <FIG> will be described as a first constitution example of the intersecting portion. <FIG> and <FIG> are partial detailed views illustrating the constitution of the part B in which a bias electrode 264b-<NUM> (a portion of the bias electrode 264b) intersects the input waveguide <NUM> thereabove. Here, <FIG> is a plan view of the part B, and <FIG> is a cross-sectional view taken in a direction of the arrow V-V in the part B illustrated in <FIG>.

The constitutions illustrated in <FIG> and <FIG> are examples of constitutions of portions in which the optical waveguide <NUM> and the like and the electrode 250a and the like in the optical modulation element <NUM> intersect each other and can also be used similarly for arbitrary portions, other than the part B, in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other.

In <FIG>, the bias electrode 264b-<NUM> extending in the vertical direction (Z direction) in the diagram intersects the input waveguide <NUM> extending in the lateral direction (Y direction) in the diagram and forms an intersecting part <NUM> (a portion surrounded by a rectangular shape of the one-dot dashed line in the diagram).

Particularly, as illustrated in <FIG>, in the present embodiment, a portion of a resin layer <NUM> is provided between the input waveguide <NUM> and the bias electrode 264b-<NUM> in a substrate portion including the intersecting part <NUM> in the substrate <NUM>. Further, corners of the resin layer <NUM> on the side of the bias electrode 264b-<NUM> are constituted to be curves <NUM>-<NUM> and <NUM>-<NUM> in a cross section (that is, for example, a cross section illustrated in <FIG>) in an extending direction of the bias electrode 264b-<NUM>. That is, the resin layer <NUM> is constituted such that a boundary line with respect to the bias electrode 264b-<NUM> leads to an end part of the resin layer <NUM> at the curves <NUM>-<NUM> and <NUM>-<NUM>.

Here, for example, the resin layer <NUM> can be a photoresist used in a patterning step of the electrode 250a and the like. In addition, portions of the curves <NUM>-<NUM> and <NUM>-<NUM> constituting corners on the side of the bias electrode 264b-<NUM> may be formed, for example, by causing the rate of temperature rise of the foregoing photoresist at the time of high-temperature treatment after patterning to become a rate faster (for example, <NUM>/min) than <NUM>/min (ordinary rate). Alternatively, portions of the curves <NUM>-<NUM> and <NUM>-<NUM> can be formed, for example, by performing plasma treatment (for example, ashing treatment) on a photoresist constituting the resin layer <NUM>.

In addition, as described above, since the bias electrode 264b-<NUM> is formed to have a thickness within a range of <NUM> to <NUM> which is relatively thin, the resin layer <NUM> is formed to have a thickness within a range of <NUM> to <NUM> which is approximately the same as the thickness of a SiO<NUM> layer in the technology in the related art.

In <FIG>, the substrate <NUM> is fixed to the support substrate <NUM> with the adhesive layer <NUM> therebetween. Here, the adhesive layer <NUM> is constituted of a thermosetting resin, for example, and the support substrate <NUM> is constituted of a glass substrate, a LN substrate, or a Si substrate, for example.

In the part B of the optical modulation element <NUM> having the foregoing constitution, the resin layer <NUM> is provided between the input waveguide <NUM> and the bias electrode 264b-<NUM> at the intersecting part <NUM>. Accordingly, occurrence of an absorption loss of waveguide light of the input waveguide <NUM> due to metal constituting the bias electrode 264b-<NUM> is prevented.

Particularly, a resin such as a photoresist, for example, constituting the resin layer <NUM> has a Young's modulus within a range of approximately <NUM> to <NUM> GPa which is smaller by one order of magnitude than the Young's modulus of SiO<NUM> within a range of <NUM> GPa to <NUM> GPa used between an electrode and an optical waveguide in the technology in the related art described above and has a lower rigidity than SiO<NUM>. For this reason, in the part B of the optical modulation element <NUM>, stress applied to the substrate <NUM> from the resin layer <NUM> itself is reduced compared to a constitution of the technology in the related art using a SiO<NUM> layer, and stress transferred from the bias electrode 264b-<NUM> to the substrate <NUM> is also reduced. In addition, due to the low rigidity of the resin layer <NUM> itself as described above, occurrence of a local distortion which may occur in the vicinity of the end part of the resin layer <NUM> due to the difference between the linear expansion coefficients of the resin layer <NUM> and the substrate <NUM> is also curbed.

Moreover, the corners of the resin layer <NUM> on the side of the bias electrode 264b-<NUM> are constituted to be the curves <NUM>-<NUM> and <NUM>-<NUM> in a cross section in the extending direction of the bias electrode 264b-<NUM> illustrated in <FIG>, and thus the continuity of the shape of the bias electrode 264b-<NUM> around the corners is enhanced (that is, a steep change in the shape is alleviated). For this reason, together with curbing a distortion due to the foregoing low rigidity of the resin layer <NUM>, occurrence of disconnection in the bias electrode 264b-<NUM> is curbed. Moreover, in addition to curbing the foregoing distortion, concentration of stress from the bias electrode 264b-<NUM> to the corner parts of the resin layer <NUM> is prevented by the curves <NUM>-<NUM> and <NUM>-<NUM>, and thus occurrence of cracking in the resin layer <NUM> is curbed.

As a result, in the optical modulation element <NUM>, when a constitution similar to that of the part B is also used at other intersecting parts between the electrode 250a and the like and the optical waveguide <NUM> and the like, it is possible to effectively reduce an optical absorption loss of waveguide light, which may occur due to metal constituting the electrode 250a and the like at the intersecting parts between the optical waveguide <NUM> and the like and the electrode 250a and the like on the substrate <NUM>, without causing deterioration in optical characteristics and degradation of long-term reliability of the optical modulation element <NUM>.

From the viewpoint of curbing occurrence of cracking at the corners of the resin layer <NUM> and occurrence of disconnection in the bias electrode 264b-<NUM> around the corners, it is desirable that distances L1 and L2 measured in the extending direction of the bias electrode 264b-<NUM> from starting points of the curves <NUM>-<NUM> and <NUM>-<NUM> constituting the foregoing corners to the end part of the resin layer <NUM> be longer than a thickness t1 of the resin layer <NUM>. That is, it is desirable that L1≥t1 and L2≥t1 be established.

Next, the constitution of the part C illustrated in <FIG> will be described as a second constitution example of the intersecting portion. <FIG> is a cross-sectional view taken in a direction of the arrow VI-VI in the part C in which the input waveguide <NUM> and the signal electrode 252b intersect each other.

The constitution illustrated in <FIG> is an example of a constitution of a portion in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other in the optical modulation element <NUM> and can also be used similarly for arbitrary portions, other than the part C, in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other.

In <FIG>, the signal electrode 252b extending in the lateral direction (Z direction) in the diagram intersects (crosses) the input waveguide <NUM> above the input waveguide <NUM> extending in the normal direction (Y direction) of this paper and forms an intersecting part <NUM> (a portion surrounded by a rectangular shape of the one-dot dashed line in the diagram).

In the constitution of the part C illustrated in <FIG>, similar to the constitution of the part B illustrated in <FIG>, a portion of a resin layer <NUM> is provided between the signal electrode 252b and the input waveguide <NUM> in a substrate portion including the intersecting part <NUM> in the substrate <NUM>.

Generally, stress transferred from the electrode 250a and the like to the substrate <NUM> increases as the thicknesses of the electrode 250a and the like become thicker. Therefore, it is desirable that the thickness t1 of a resin layer provided between the electrode 250a and the like and the optical waveguide <NUM> and the like become thicker as the electrode 250a and the like have a larger thickness. As described above, the signal electrode 252b is formed to have a thickness within a range of approximately <NUM> to <NUM> which is thicker by approximately one order of magnitude than the bias electrode 264b and the like formed to have a thickness within a range of approximately <NUM> to <NUM>. For this reason, the thickness t1 of the resin layer <NUM> illustrated in <FIG> is formed to be a thickness within a range of <NUM> to <NUM> which is thicker by approximately one order of magnitude than the thickness of a SiO<NUM> layer in the technology in the related art, and the resin layer <NUM> becomes an extremely thick layer compared to that in the technology in the related art.

In addition, similar to the resin layer <NUM> illustrated in <FIG>, corners of the resin layer <NUM> on a side of the signal electrode 252b are constituted to be curves <NUM>-<NUM> and <NUM>-<NUM> in a cross section (that is, for example, a cross section illustrated in <FIG>) of the resin layer <NUM> in the extending direction of the signal electrode 252b. That is, the resin layer <NUM> is constituted such that a boundary line with respect to the signal electrode 252b leads to an end part of the resin layer <NUM> at the curves <NUM>-<NUM> and <NUM>-<NUM>.

In addition, as the desirable form described above, the resin layer <NUM> is formed to have the distances L1 and L2 which are measured in the extending direction of the signal electrode 252b from the starting points to the end parts of the curves <NUM>-<NUM> and <NUM>-<NUM> and are larger than the thickness t1 of a resin layer <NUM> (that is, such that L1≥t1 and L2≥t1 are established). Accordingly, at the intersecting part <NUM>, similar to the intersecting part <NUM> illustrated in <FIG>, occurrence of disconnection in the signal electrode 252b in the vicinity of the corner parts of the resin layer <NUM> can be curbed by enhancing the continuity of the shape of the signal electrode 252b, and occurrence of cracking at the corner parts can be prevented by curbing concentration of stress from the signal electrode 252b to the corner parts of the resin layer <NUM>.

In addition, from the viewpoint of stabilization of light propagation characteristics of the optical waveguide <NUM> and the like, it is desirable that stress applied from the resin layer <NUM> to the optical waveguide <NUM> and the like be uniform. Therefore, on an upper surface of the resin layer <NUM> (that is, a surface which comes into contact with the signal electrode 252b), a range corresponding to an upper part of the input waveguide <NUM> (that is, a range of a width W in the diagram) is formed to be flat (that is, the thickness of the resin layer <NUM> in the range is uniform).

Further, particularly, in the resin layer <NUM>, as a desirable constitution, a length Lz measured in the extending direction of the signal electrode 252b has a value equal to or larger than three times the width W of the input waveguide <NUM> at the intersecting part <NUM>, that is, Lz≥3W is established. Accordingly, a contact area between the resin layer <NUM> and the substrate <NUM> increases at the intersecting part <NUM>, and thus stress transferred from the signal electrode 252b to the input waveguide <NUM> via the resin layer <NUM> is further reduced. For this reason, for example, it is possible to curb change in effective refractive index of the input waveguide <NUM> due to the foregoing stress on account of a photoelastic effect in the substrate <NUM> constituted of LN. As a result, for example, when a thick signal electrode 252b exceeding <NUM> is formed, it is possible to prevent deterioration or aggravation in optical characteristics of the optical modulation element <NUM> caused by stress from the thick signal electrode 252b.

Moreover, in the resin layer <NUM>, as a desirable constitution, the distances L1 and L2 measured in the extending direction of the signal electrode 252b from the starting points to the end parts of the curves <NUM>-<NUM> and <NUM>-<NUM> are formed to be not only larger than the thickness t1 of resin layer <NUM> but also larger than the width W of the input waveguide <NUM> at the intersecting part <NUM> (that is, such that L1≥W and L2≥W are established).

Accordingly, the corner parts of the resin layer <NUM> in which stress from the signal electrode 252b is likely to be concentrated are distant from the input waveguide <NUM>, and thus the foregoing stress applied to the input waveguide <NUM> via the resin layer <NUM> can be further reduced.

The resin layer <NUM> can be formed through crosslinking reaction by performing high-temperature heating, for example, using a photoresist including a crosslinking agent, for example. In a resin formed of a photoresist including a crosslinking agent, the degree of deformation can be further increased than that of an ordinary photoresist for fine processing through crosslinking reaction and high-temperature treatment (for example, <NUM>), and thus corner parts of a resin layer formed to have a thickness exceeding <NUM> as in the resin layer <NUM> can be curved extensively and easily. In a photoresist including a crosslinking agent, since contraction accompanied by crosslinking reaction cannot be avoided, although the photoresist is not suitable for fine processing requiring submicron accuracy, there is less change in physical property over time (degeneration) or generation of gas. Thus, in the resin layer <NUM> which does not require high processing accuracy compared to the optical waveguide <NUM> and the like, it is preferable to adopt the photoresist as a resin to be used inside an air-tight casing such as the casing <NUM> over a long term.

Similar to the resin layer <NUM>, the portions of the curves <NUM>-<NUM> and <NUM>-<NUM> included in the resin layer <NUM> can be formed by causing the rate of temperature rise at the time of high-temperature treatment after patterning to become a rate faster (for example, <NUM>/min) than <NUM>/min (ordinary rate) and/or performing plasma treatment (for example, ashing treatment) on the photoresist constituting the resin layer <NUM> in addition to performing the foregoing high-temperature treatment.

In the present embodiment, the signal electrodes 250a, 252b, 250b, and 252b are formed within a range of <NUM> to <NUM>, for example, but the embodiment is not limited thereto. In the cracking <NUM> and the cracking <NUM> at the corner parts of the SiO<NUM> layer illustrated in <FIG>, when the thickness of the electrode thereabove is thicker than <NUM>, the probability of occurrence of cracking may increase gradually. Therefore, when the thicknesses of the electrode 250a and the like are thicker than <NUM> at least at the intersecting parts between the electrode 250a and the like and the optical waveguide <NUM> and the like, due to the constitutions illustrated in <FIG> or <FIG> which will be described below, compared to the constitution in the related art using a SiO<NUM> layer, deterioration in optical characteristics and degradation of long-term reliability of the optical modulation element <NUM> can be effectively prevented, and an optical absorption loss of waveguide light due to the electrode 250a and the like can be reduced.

Next, the constitution of the part D illustrated in <FIG> will be described as a third constitution example of the intersecting portion. <FIG> is a cross-sectional view taken in a direction of the arrow VII-VII in the part D in which the input waveguide <NUM> and the signal electrode 250b intersect each other. The constitution illustrated in <FIG> is an example of a constitution of a portion in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other in the optical modulation element <NUM> and can also be used similarly for arbitrary portions, other than the part D, in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other.

In <FIG>, the signal electrode 250b extending in the lateral direction (Z direction) in the diagram intersects (crosses) the input waveguide <NUM> above the input waveguide <NUM> extending in the normal direction (Y direction) of this paper and forms an intersecting part <NUM> (a portion surrounded by a rectangular shape of the one-dot dashed line in the diagram).

Further, a portion of a plurality (for example, three) of resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> is provided between the signal electrode 250b and the input waveguide <NUM> in the substrate portion including the intersecting part <NUM> in the substrate <NUM>. Further, corners of each of the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> on a side of the signal electrode 250b are constituted to be curves. In addition, the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> are provided in a stacked manner to have stepped heights when measured from the surface of the substrate <NUM> in the extending direction of the signal electrode 250b. The desirable conditions for L1, L2, Lz, and the like described above may be applied to each of the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>.

In the constitution illustrated in <FIG>, the plurality of resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> is formed in a stepped shape in the extending direction of the signal electrode 250b. Thus, together with forming the curved corner parts of these resin layers, the continuity of the shape of the signal electrode 250b at the intersecting part <NUM> can be further enhanced (that is, the degree of change in shape can be further alleviated), and occurrence of disconnection in the signal electrode 250b at the intersecting part <NUM> can be better curbed. In addition, since the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> formed to have curved corner parts are formed in a stepped shape, stress from the signal electrode 250b is applied in a manner of being dispersed to each of the corner parts of the resin layers, and thus occurrence of cracking in these corner part is better curbed.

In addition, since a plurality of resin layers is used, the thicknesses of the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> in their entirety can be made desired thicknesses, and the layer thickness of each of the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> can be made equal to or smaller than a certain thickness. For this reason, for example, the positions and the shapes of the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> in their entirety can be set with high accuracy and the thicknesses in their entirety can have large values in accordance with the thickness of the signal electrode 250b by constituting each resin layer of an ordinary photoresist having submicron processing accuracy.

Next, the constitution of the part E illustrated in <FIG> will be described as a fourth constitution example of the intersecting portion. In the present constitution example, one resin layer is provided in a straddling manner between two intersecting parts. <FIG> is a cross-sectional view taken along VIII-VIII in the part E including two intersecting parts <NUM>-<NUM> and <NUM>-<NUM> at which the signal electrode 252b intersects each of two parallel waveguides 244b-<NUM> and 244b-<NUM> above the parallel waveguides 244b-<NUM> and 244b-<NUM> constituting the Mach-Zehnder optical waveguide 244b.

The constitution illustrated in <FIG> can be similarly applied not only to intersecting parts between parallel waveguides constituting a Mach-Zehnder optical waveguide and an electrode but also to arbitrary intersecting portions at which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other, that is, a plurality of adjacent intersecting parts.

In <FIG>, the signal electrode 252b extending in the lateral direction (Z direction) in the diagram intersects (crosses) the parallel waveguides 244b-<NUM> and 244b-<NUM> above the parallel waveguides 244b-<NUM> and 244b-<NUM> extending in the normal direction (Y direction) of this paper and forms each of the intersecting parts <NUM>-<NUM> and <NUM>-<NUM>.

Further, a resin layer <NUM> provided between the signal electrode 252b and the substrate <NUM> is formed to extend in a straddling manner between adjacent intersecting parts <NUM>-<NUM> and <NUM>-<NUM>. Accordingly, the resin layers <NUM> are provided between the signal electrode 252b and the parallel waveguides 244b-<NUM> and 244b-<NUM> at the intersecting parts <NUM>-<NUM> and <NUM>-<NUM>.

In addition, corners of the resin layer <NUM> on the side of the signal electrode 252b are constituted to be curves <NUM>-<NUM> and <NUM>-<NUM> in a cross section (that is, for example, a cross section illustrated in <FIG>) in the extending direction of the signal electrode 252b. That is, the resin layer <NUM> is constituted such that a boundary line with respect to the signal electrode 252b leads to an end part of the resin layer <NUM> at the curves <NUM>-<NUM> and <NUM>-<NUM>.

According to the foregoing constitution, since one resin layer <NUM> is provided in a straddling manner between the plurality of intersecting parts <NUM>-<NUM> and <NUM>-<NUM>, the number of resin layers to be formed on the substrate <NUM> can be reduced, and a manufacturing yield can be improved. In addition, as a result of providing one resin layer <NUM> in a straddling manner between the plurality of intersecting parts <NUM>-<NUM> and <NUM>-<NUM>, the area of a portion of the resin layer <NUM> in contact with the surface of the substrate <NUM> increases, and thus adhesion of the resin layer <NUM> with respect to the substrate <NUM> can be improved.

Moreover, when the resin layer <NUM> is formed in a straddling manner between the intersecting parts <NUM>-<NUM> and <NUM>-<NUM>, the thickness of a range having a width Fw in the resin layer <NUM> including the intersecting parts <NUM>-<NUM> and <NUM>-<NUM> can be easily formed to be a uniform thickness. For this reason, as in the constitution illustrated in <FIG>, when two adjacent intersecting parts <NUM>-<NUM> and <NUM>-<NUM> include the parallel waveguides 244b-<NUM> and 244b-<NUM>, conditions for stress or the like received by each of the two parallel waveguides 244b-<NUM> and 244b-<NUM> from the resin layer <NUM> (and/or via the resin layer <NUM>) are made uniform so that an additional difference between optical phases of the parallel waveguides 244b-<NUM> and 244b-<NUM> caused by nonuniformity of the stress or the like can be curbed and additional variation in operation points in the Mach-Zehnder optical waveguide 244b due to the provided resin layer <NUM> can be curbed.

Next, the constitution of the part F illustrated in <FIG> will be described as a fifth constitution example of the intersecting portion. In the present constitution example, in order to reduce an optical absorption loss due to the electrode 250a and the like, a SiO<NUM> layer similar to that in the technology in the related art is provided on the optical waveguide <NUM> and the like, and a resin layer is provided thereabove as a protective layer for preventing cracking of the SiO<NUM> layer and/or disconnection in the electrode as illustrated in <FIG> and <FIG>.

<FIG> is a cross-sectional view taken along IX-IX in the part F in which the signal electrode 252a intersects a parallel waveguide 244a-<NUM> (one of the Mach-Zehnder optical waveguide 244a) thereabove. The constitution illustrated in <FIG> is an example of a constitution of a portion in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other in the optical modulation element <NUM> and can also be used similarly for arbitrary portions, other than the part F, in which the optical waveguide <NUM> and the like and the electrode 250a and the like intersect each other.

In <FIG>, the signal electrode 252a extending in the lateral direction (Z direction) in the diagram intersects (crosses) the parallel waveguide 244a-<NUM> above the parallel waveguide 244a-<NUM> extending in the normal direction (Y direction) of this paper and forms an intersecting part <NUM> (a portion surrounded by a rectangular shape of the one-dot dashed line in the diagram).

Similar to the technology in the related art, a SiO<NUM> layer <NUM> is formed on the parallel waveguide 244a-<NUM> at the intersecting part <NUM>. However, differing from the technology in the related art, a resin layer <NUM> is provided between the SiO<NUM> layer <NUM> and the signal electrode 252a at the intersecting part <NUM>. Accordingly, similar to the intersecting part <NUM> illustrated in <FIG>, the intersecting part <NUM> has a constitution in which a portion of the resin layer <NUM> is provided between the signal electrode 252a and the parallel waveguide 244a-<NUM> in the substrate portion including the intersecting part <NUM>. Further, corners of the resin layer <NUM> on a side of the signal electrode 252a are constituted as curves <NUM>-<NUM> and <NUM>-<NUM>. Here, similar to the technology in the related art, the thickness of the SiO<NUM> layer is <NUM>, for example, which is a thickness sufficient for reducing an optical absorption loss in the parallel waveguide 244a-<NUM> due to the signal electrode 252a. In addition, the resin layer <NUM> is formed to have a thickness of <NUM>, for example.

According to the foregoing constitution, at the intersecting part <NUM>, the SiO<NUM> layer <NUM> provided on the parallel waveguide 244a-<NUM> is protected by the resin layer <NUM>, and thus cracking can be prevented from occurring at the corner parts of the SiO<NUM> layer <NUM> due to stress from the signal electrode 252a. In addition, similar to the intersecting part <NUM> illustrated in <FIG> and the intersecting part <NUM> illustrated in <FIG>, the curves <NUM>-<NUM> and <NUM>-<NUM> constituting the corners of the resin layer <NUM> can prevent cracking from occurring in the resin layer <NUM> due to stress from the signal electrode 252a and can prevent disconnection from occurring in a portion of the signal electrode 252a in the vicinity of the corner parts of the resin layer <NUM>.

Particularly, the constitution illustrated in <FIG> is preferably adopted when it is intended to curb disconnection in the electrode 250a and the like at the intersecting parts utilizing high electrical insulation properties, transparency, and temporal stability of the SiO<NUM> layer.

Next, a second embodiment of the disclosure will be described. The present embodiment relates to an optical modulation module <NUM> using the optical modulation element <NUM> included in the optical modulator <NUM> according to the first embodiment. <FIG> is a view illustrating a constitution of the optical modulation module <NUM> according to the present embodiment. In <FIG>, the same constituent elements as those in the optical modulator <NUM> according to the first embodiment illustrated in <FIG> will be described using the same reference signs as the reference signs indicated in <FIG>, and the foregoing description in <FIG> will be invoked by reference.

The optical modulation module <NUM> has a constitution similar to that of the optical modulator <NUM> illustrated in <FIG> but differs from the optical modulator <NUM> in including a circuit substrate <NUM> in place of the relay substrate <NUM>. The circuit substrate <NUM> includes a drive circuit <NUM>. The drive circuit <NUM> generates a high-frequency electrical signal for driving the optical modulation element <NUM> and outputs the generated high-frequency electrical signal to the optical modulation element <NUM>, for example, on the basis of a modulation signal supplied from the outside via the signal pins 110a, 110b, 110c, and 110d.

Similar to the optical modulator <NUM> according to the first embodiment described above, the optical modulation module <NUM> having the foregoing constitution includes the optical modulation element <NUM> having the constitution as illustrated in <FIG> at the intersecting portions between the optical waveguide <NUM> and the like and the electrode 250a and the like. For this reason, in the optical modulation module <NUM>, similar to the optical modulator <NUM>, favorable optical transmission can be performed by effectively reducing an optical absorption loss of waveguide light which may occur at the intersecting portions between the optical waveguide <NUM> and the like and the electrode 250a and the like on the substrate <NUM> and realizing favorable modulation characteristics without causing deterioration in optical characteristics and degradation of long-term reliability of the optical modulation element <NUM>.

Next, a third embodiment of the disclosure will be described. The present embodiment relates to an optical transmission device <NUM> equipped with the optical modulator <NUM> according to the first embodiment. <FIG> is a view illustrating a constitution of the optical transmission device <NUM> according to the present embodiment. This optical transmission device <NUM> has the optical modulator <NUM>, a light source <NUM> for causing light to be incident on the optical modulator <NUM>, a modulator driving part <NUM>, and a modulation signal generating part <NUM>. In place of the optical modulator <NUM> and the modulator driving part <NUM>, the optical modulation module <NUM> described above can be used.

The modulation signal generating part <NUM> is an electronic circuit for generating an electrical signal such that the optical modulator <NUM> performs modulation operation. The modulation signal generating part <NUM> generates a modulation signal which is a high-frequency signal for causing the optical modulator <NUM> to perform optical modulation operation in accordance with the modulation data and outputs the generated modulation signal to the modulator driving part <NUM> on the basis of transmission data applied from the outside.

The modulator driving part <NUM> amplifies a modulation signal input from the modulation signal generating part <NUM> and outputs four high-frequency electrical signals for driving the four signal electrodes 250a, 252a, 250b, and 252b of the optical modulation element <NUM> included in the optical modulator <NUM>.

The four high-frequency electrical signals are input to the signal pins 110a, 110b, 110c, and 110d of the optical modulator <NUM> to drive the optical modulation element <NUM>. Accordingly, light output from the light source <NUM> is subjected to DP-QPSK modulation, for example, by the optical modulator <NUM> and is output from the optical transmission device <NUM> as modulation light.

Particularly, since the optical transmission device <NUM> uses the optical modulator <NUM> including the optical modulation element <NUM> which may effectively reduce an optical absorption loss at the intersecting portions between the optical waveguide <NUM> and the like and the electrode 250a and the like, favorable modulation characteristics can be realized, and favorable optical transmission can be performed.

The disclosure is not limited to the constitutions of the foregoing embodiments and substitution constitutions thereof and can be performed in various forms within a range not departing from the gist thereof.

For example, in the first embodiment described above, in the optical modulation element <NUM>, regarding the part B which is an intersecting portion between the input waveguide <NUM> and the bias electrode 264b-<NUM>, the part C which is an intersecting portion between the input waveguide <NUM> and the signal electrode 252b, the part D which is an intersecting portion between the input waveguide <NUM> and the signal electrode 250b, the part E including two intersecting portions between the parallel waveguides 244b-<NUM> and 244b-<NUM> and the signal electrode 252b, and the part F which is an intersecting portion between the parallel waveguide 244a-<NUM> and the signal electrode 252a respectively have the constitutions illustrated in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG>, but the embodiment is not limited thereto.

The optical modulation element <NUM> serving as an optical waveguide element can have any of the constitutions illustrated in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> for a part or all of the intersecting portions between the optical waveguide <NUM> and the like and the electrode 250a and the like. Therefore, for example, any of the constitutions illustrated in <FIG> illustrating the intersecting parts including the signal electrode 250b, 252b, or 252a can be applied to any intersecting part including the bias electrode 264b and the like.

In addition, as will be easily understood by those skilled in the art, distinct constitutions of the part B, the part C, the part D, the part E, and the part F illustrated in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> may be combined and can be applied to any intersecting part between the optical waveguide <NUM> and the like and the electrode 250a and the like. For example, a plurality of resin layers stacked in a multistage manner can be formed at the intersecting portions between the optical waveguide <NUM> and the like and the electrode 250a and the like, in which a SiO<NUM> layer is formed, by combining the constitution in <FIG> and the constitution in <FIG>.

In addition, for example, a plurality of resin layers stacked in a multistage manner may be formed in a straddling manner between two intersecting parts by combining the constitution in <FIG> and the constitution in <FIG>. Alternatively, a plurality of resin layers stacked in a multistage manner may be formed in a straddling manner between two adjacent intersecting parts between the optical waveguide <NUM> and the like and the electrode 250a and the like, in which a SiO<NUM> layer is formed, by combining <FIG> and <FIG>.

In addition, regarding the constitution of the part D illustrated in <FIG>, a constitution consisting of three resin layers such as the resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> as a plurality of resin layers stacked in a multistage manner has been illustrated, but the constitution is not limited thereto. The number of resin layers stacked in a multistage manner can be two, four, or more.

In addition, regarding the constitution of the part F illustrated in <FIG>, a constitution in which the SiO<NUM> layer <NUM> is provided on the parallel waveguide 244a-<NUM> has been illustrated, but the constitution is not limited thereto. An insulating layer or a transparent insulating layer constituted of an arbitrary material having a refractive index and electrical insulation properties higher than those of the optical waveguide <NUM> and the like, such as SiN in addition to SiO<NUM>, can be formed above the optical waveguide <NUM> and the like (for example, a layer constituted of SiN or the like).

In addition, in the first embodiment described above, the intersecting part included in the part B or the part F is constituted to have the electrode 250a and the like and the optical waveguide <NUM> and the like orthogonal to each other, but the embodiment is not limited thereto. The constitutions of the intersecting parts in <FIG> described above can be applied to an intersecting part between the electrode 250a and the like and the optical waveguide <NUM> and the like formed by the electrode 250a and the like crossing the optical waveguide <NUM> and the like thereabove, that is, an intersecting part at which the electrode 250a and the like intersect the optical waveguide <NUM> and the like at an arbitrary angle (not parallel to each other).

In addition, in the embodiments described above, as an example of the optical waveguide element, the optical modulation element <NUM> using the substrate <NUM> formed of LN (LiNbO<NUM>) has been illustrated, but the embodiments are not limited thereto. The optical waveguide element can be an element which is constituted of a substrate made of an arbitrary material (InP, Si, or the like in addition to LN) and has an arbitrary function (an optical switch, an optical directional coupler, or the like in addition to optical modulation).

As described above, the foregoing optical modulator <NUM> according to the first embodiment includes the optical modulation element <NUM>. The optical modulation element <NUM> serving as an optical waveguide element has the optical waveguide <NUM> and the like which are formed on the substrate <NUM>, and the electrode 250a and the like which are electrodes for controlling optical waves propagated in the optical waveguide <NUM> and the like and have the intersecting part <NUM> and the like intersecting the optical waveguide <NUM> and the like thereabove. Further, a portion of the resin layer <NUM> and the like is provided between the optical waveguide <NUM> and the like and the electrode 250a and the like in a portion including the intersecting part <NUM> and the like in the substrate <NUM>, and corners of the resin layer <NUM> and the like on a side of the electrode 250a and the like are constituted to be curves in a cross section in the extending direction of the electrode 250a and the like.

According to this constitution, it is possible to effectively reduce an optical absorption loss of waveguide light which may occur due to electrode metal at the intersecting part <NUM> and the like between the optical waveguide <NUM> and the like and the electrode 250a and the like on the substrate <NUM> without causing deterioration in optical characteristics and degradation of long-term reliability of the optical modulation element <NUM>.

In addition, in the optical modulation element <NUM>, the distance measured in the extending direction of the electrode 250a and the like from the starting points to the end parts of the curves constituting the foregoing corners in the foregoing cross section in the resin layer <NUM> and the like has a value larger than the heights of the resin layer <NUM> and the like measured from the surface of the substrate <NUM>. According to this constitution, when the corners of the resin layer <NUM> and the like are constituted to be curves having a curvature equal to or greater than a certain value, the continuity of the shapes of the electrode 250a and the like at the intersecting part <NUM> and the like can be further enhanced, and stress applied to the corner parts of the resin layer from the electrode 250a and the like can be further dispersed. For this reason, disconnection in the electrode 250a and the like at the intersecting part <NUM> and the like and occurrence of cracking at the corner parts of the resin layer <NUM> and the like can be curbed more effectively. As a result, deterioration in optical characteristics and degradation of long-term reliability of the optical modulation element <NUM> can be prevented more effectively, and an optical absorption loss of waveguide light which may occur due to electrode metal at the intersecting part <NUM> and the like between the optical waveguide <NUM> and the like and the electrode 250a and the like on the substrate <NUM> can be reduced.

In addition, for example, the resin layer <NUM> of the optical modulation element <NUM> is provided over a distance equal to or longer than three times the width of the input waveguide <NUM> at the intersecting part <NUM> in the extending direction of the signal electrode 252b. According to this constitution, since intensive action of stress of the signal electrode 252b on the optical waveguide is curbed, for example, change in effective refractive index of the optical waveguide on account of a photoelastic effect in the substrate <NUM> constituted of LN can be curbed. As a result, for example, when a thick signal electrode 252b exceeding <NUM> is formed, it is possible to prevent deterioration or aggravation in optical characteristics of the optical modulation element <NUM> caused by stress from the thick signal electrode 252b.

In addition, for example, the plurality of resin layers <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> is provided between the input waveguide <NUM> and the signal electrode 250b at the intersecting part <NUM> of the optical modulation element <NUM> in a manner of being stacked in a stepped shape in the extending direction of the signal electrode 250b. According to this constitution, the continuity of the shapes of the intersecting part <NUM> or the signal electrode 250b therearound can be further enhanced, and occurrence of disconnection in the signal electrode 250b at the intersecting part <NUM> and occurrence of cracking at the corner parts of the resin layer <NUM>-<NUM> and the like can be further curbed.

In addition, for example, the SiO<NUM> layer <NUM> (insulating layer) is formed between the parallel waveguide 244a-<NUM> and the resin layer <NUM> at the intersecting part <NUM> of the optical modulation element <NUM>. According to this constitution, for example, occurrence of disconnection in the signal electrode 252a can be curbed and occurrence of cracking in the SiO<NUM> layer <NUM> and the resin layer <NUM> can be prevented utilizing high electrical insulation properties, transparency, and temporal stability of SiO<NUM>.

In addition, for example, a resin constituting the resin layer <NUM> of the optical modulation element <NUM> is a resin formed using a photoresist including a crosslinking agent. According to this constitution, compared to a case of using an ordinary photoresist for fine processing, curve portions can be easily and extensively formed at the corner parts of the resin layer <NUM>.

In addition, for example, the resin layer <NUM> of the optical modulation element <NUM> is formed in a manner of straddling between adjacent intersecting parts <NUM>-<NUM> and <NUM>-<NUM>. According to this constitution, the contact area between the resin layer <NUM> and the substrate <NUM> is increased, and thus adhesion of the resin layer <NUM> with respect to the substrate <NUM> can be improved. In addition, since the resin layer <NUM> can be easily formed with equal thickness in a range including two intersecting parts, for example, the thickness of the resin layer <NUM> can be formed to have the same thickness at the two intersecting parts <NUM>-<NUM> and <NUM>-<NUM> including two parallel waveguides 244b-<NUM> and 244b-<NUM> of the Mach-Zehnder optical waveguide 244b, and the conditions for stress or the like received by each of the two foregoing parallel waveguides from the resin layer <NUM> can be made uniform. As a result, additional variation in operation points in the Mach-Zehnder optical waveguide 244b can be curbed, and thus favorable modulation characteristics can be realized.

In addition, in the optical modulation element <NUM>, the electrode 250a and the like are formed to be thicker than <NUM> at least at the intersecting parts between the electrode 250a and the like and the optical waveguide <NUM> and the like. In addition, in the optical modulation element <NUM>, the substrate <NUM> has a thickness equal to or smaller than <NUM>. According to these constitutions, in a case of using a SiO<NUM> layer as in the technology in the related art, even when an electrode constitution and a substrate constitution in which the frequency of occurrence of disconnection in the electrode and occurrence of cracking at the corner parts in the SiO<NUM> layer is likely to be relatively high are employed, occurrence of disconnection and cracking can be effectively curbed.

In addition, the optical modulator according to the first embodiment includes any optical modulation element <NUM> serving as an optical waveguide element, the casing <NUM> for accommodating the optical modulation element <NUM>, the input optical fiber <NUM> for inputting light to the optical modulation element <NUM>, and the output optical fiber <NUM> for guiding light output by the optical modulation element <NUM> to the outside of the casing <NUM>.

In addition, the optical modulation module <NUM> according to the second embodiment includes the optical modulation element <NUM> serving as an optical waveguide element performing modulation of light, and the drive circuit <NUM> for driving the optical modulation element <NUM>.

In addition, the optical transmission device <NUM> according to the third embodiment includes the optical modulator <NUM> or the optical modulation module <NUM>, and the modulation signal generating part <NUM> that is an electronic circuit for generating an electrical signal such that the optical modulation element <NUM> performs modulation operation.

Claim 1:
An optical waveguide element, comprising:
an optical waveguide (<NUM>) formed in a substrate (<NUM>); and
an electrode (264b-<NUM>) controlling optical waves that are propagated in the optical waveguide (<NUM>) and having an intersecting part (<NUM>) intersecting the optical waveguide thereabove,
wherein a portion of a resin layer (<NUM>) is provided between the optical waveguide (<NUM>) and the electrode (264b-<NUM>) in a portion of the substrate (<NUM>) including the intersecting part (<NUM>),
the optical waveguide element being characterized in that
an upper surface of the resin layer (<NUM>) is in direct contact with a bottom surface of the electrode (264b-<NUM>) and a bottom surface of the resin layer opposite to the upper surface is in contact with a top surface of the optical waveguide (<NUM>), and
a corner of the resin layer (<NUM>) on a side of the electrode is constituted to be a curve in a cross section in an extending direction of the electrode (264b-<NUM>), wherein the curvature of the corner of the resin layer (<NUM>) is finite and non-zero, defined by a non infinite radius, that is distinct from a flat plane, and
in the resin layer (<NUM>), a distance measured in the extending direction of the electrode (264b-<NUM>) from a starting point to an end part of the curve constituting the corner in the cross section is longer than a height of the resin layer (<NUM>) measured from a surface of the substrate (<NUM>).