Patent Description:
In a commercial optical fiber communication system, an optical modulator incorporating an optical modulation element as an optical waveguide device including an optical waveguide formed on a substrate and a control electrode for controlling a light wave propagating in the optical waveguide is often used. Among the optical modulation elements, an optical modulation element using LiNbO3 (hereinafter, also referred to as LN) having an electro-optic effect for a substrate can achieve wide-band optical modulation characteristics with less optical loss, so that it is widely used in optical fiber communication systems for high-speed, large-capacity backbone optical transmission networks and metro networks.

As one measure for downsizing, widening the bandwidth, and saving power of such an optical modulation element, for example, an optical modulator using a rib-type optical waveguide or a ridge optical waveguide formed on the surface of a thin-film LN substrate(for example, a thickness of <NUM> or less) is being put into practical use (for example, <CIT>). The rib-type optical waveguide or the ridge optical waveguide is a protruding optical waveguide configured by forming a band-shaped protruding portion on the thinned LN substrate. As a result, the interaction between the waveguide light propagating in the convex waveguide and the signal electric field generated in the substrate by the control electrode is strengthened (that is, the electric field efficiency is increased), and the miniaturization, broadband and power saving of the optical modulation element are achieved.

One of the problems in such a protruding optical waveguide is the scattering loss of waveguide light due to the surface degradation of the protruding portion formed on the LN substrate. For example, a protruding optical waveguide is formed by etching the surface of an LN substrate, leaving a protruding portion (that is, a core portion that guides light) that becomes an optical waveguide. In this case, depending on the etching rate, the etching temperature, or the like, surface degradation due to minute irregularities may occur on the side surface of the protruding portion formed on the LN substrate. Then, due to the surface degradation of such a protruding portion, a scattering loss may occur in the light propagating through the protruding optical waveguide.

As a technique for reducing the scattering loss caused by the surface degradation of the convex waveguide, it is known that a clad layer made of lithium tantalum niobate is formed by a sol-gel method so as to cover the protruding optical waveguide on the LN substrate (<CIT>). In this configuration, the difference in refractive index between the protruding portion (core) of the convex waveguide and the clad is smaller, as compared with the case where the environmental atmosphere of the protruding optical waveguide is clad, and the scattering loss is effectively reduced.

However, in the configuration in the related art, the clad layer is formed in a portion other than the protruding optical waveguide on the LN substrate. Therefore, the control electrodes disposed at positions sandwiching the protruding optical waveguide after that are formed on the clad layer formed on the LN substrate, so that the strength of the electric field generated in the protruding optical waveguide is weaker as compared to the configuration in which the control electrodes are formed directly on the LN substrate. Along with this, the electric field efficiency of the optical waveguide device may decrease.

As a method of avoiding such a decrease in the electric field strength and maintaining the electric field efficiency, it is also conceivable to remove the area of the clad formed on the LN substrate other than the portion covering the protruding optical waveguide by patterning before formation of the control electrodes. However, in this method, for example, the clad layer is patterned by avoiding the portion in which the control electrodes are disposed, formed by sandwiching a protruding optical waveguide having a width of <NUM> at an clearance of <NUM>, which is technically difficult and can also affect the manufacturing yield.

<CIT><NUM> discloses an optical waveguide device according to the preamble of claim <NUM>.

<CIT> discloses a similar optical waveguide device, wherein contact parts are not covered with resin.

<CIT> discloses a similar optical waveguide device, and a Mach-Zehnder interferometric modulator having input light that is split between two EO waveguides.

From the above background, it is desired that the optical waveguide device effectively prevents the scattering loss of the waveguide light due to the surface roughness of the protruding optical waveguide while maintaining the high electric field efficiency.

The present invention provides an optical waveguide device according to claim <NUM>.

According to the present invention, in an optical waveguide device, it is possible to effectively prevent scattering loss of waveguide light due to surface roughness in a protruding optical waveguide while maintaining high electric field efficiency.

<FIG> is a diagram showing a configuration of an optical modulation element <NUM>, which is an optical waveguide device according to a first embodiment of the present invention. <FIG> is a partial detailed view of part A shown in <FIG>, and <FIG> is a partial detailed view of part B shown in <FIG>.

The optical modulation element <NUM> includes an optical waveguide <NUM> formed on the substrate <NUM>. The substrate <NUM> is, for example, a thinned X-cut LN substrate having an electro-optic effect, which is processed to a thickness of <NUM> or less (for example, <NUM>). The optical waveguide <NUM> is a protruding optical waveguide (for example, a rib-type optical waveguide or a ridge optical waveguide) including a band-shaped extending protruding portion formed on the surface of the thinned substrate <NUM>.

The protruding portion extending on the substrate <NUM> and configuring the optical waveguide <NUM> is covered with a dielectric layer <NUM> having a refractive index of more than <NUM>. The dielectric layer <NUM> can be formed of an inorganic material such as SiO2 by a sputtering method or a CVD method. In particular, in the optical modulation element <NUM> according to the present embodiment, the dielectric layer <NUM> is made of a resin. The resin configuring the dielectric layer <NUM> may be, for example, a photoresist containing a coupling agent (crosslinking agent), and may be a so-called photosensitive permanent film in which the crosslinking reaction proceeds by heat and is cured. However, the dielectric layer <NUM> is not limited to the photosensitive permanent film, and may be any resin such as a polyamide resin, a melamine resin, a phenol resin, an amino resin, and an epoxy resin having a predetermined refractive index.

The substrate <NUM> is, for example, rectangular and has two left and right sides 140a and 140b extending in the vertical direction and facing each other, and upper and lower sides 140c and 140d in the figure extending in the left and right direction and facing each other.

The input light (an arrow pointing the right side) input to the input waveguide <NUM> of the optical waveguide <NUM> on the lower side of the left side 140a of the substrate <NUM> is folded back by <NUM> degrees in the light propagation direction and is branched into two light beams, and the light beams are QPSK-modulated by two nested Mach-Zehnder type optical waveguides 108a and 108b, respectively. The two QPSK-modulated light beams are output from the upper side of the left side 140a of the substrate <NUM> via the output waveguides 126a and 126b on the left side, respectively (two arrows pointing the left side).

These two output light beams are output from the substrate <NUM>, polarized and combined, for example, by a polarization beam combiner into one optical beam, and transmitted to a transmission optical fiber as a DP-QPSK-modulated optical signal.

The nested Mach-Zehnder type optical waveguide 108a includes two Mach-Zehnder type optical waveguides 110a and 110b. Further, the nested Mach-Zehnder type optical waveguide 108b includes two Mach-Zehnder type optical waveguides 110c and 110d.

The Mach-Zehnder type optical waveguides 110a and 110b have two parallel waveguides 112a, 112b and 112c, 112d, respectively. Further, the Mach-Zehnder type optical waveguides 110c and 110d have two parallel waveguides 112e, 112f and <NUM>, <NUM>, respectively.

For QPSK modulation in the nested Mach-Zehnder type optical waveguide 108a, signal electrodes <NUM>-1a and <NUM>-1b (white rectangular portions) to which high-frequency electrical signals for modulation are input are disposed between the two parallel waveguides 112a and 112b of the Mach-Zehnder type optical waveguide 110a and between the two parallel waveguides 112c and 112d of the Mach-Zehnder type optical waveguide 110b, respectively.

Further, for QPSK modulation in the nested Mach-Zehnder type optical waveguide 108b, signal electrodes <NUM>-1c and <NUM>-1d into which high-frequency electrical signals for modulation are input are disposed between the two parallel waveguides 112e and 112f of the Mach-Zehnder type optical waveguide 110c, and between the two parallel waveguides <NUM> and <NUM> of the Mach-Zehnder type optical waveguide 110d, respectively.

The signal electrode <NUM>-1a configures a coplanar type transmission line together with the ground electrodes <NUM>-2a and <NUM>-2b (white portions) facing each other across the parallel waveguides 112a and 112b, respectively, and the signal electrode <NUM>-1b configures a coplanar type transmission line together with the ground electrodes <NUM>-2b and <NUM>-2c facing each other across the parallel waveguides 112c and 112d, respectively.

The signal electrode <NUM>-1c configures a coplanar type transmission line together with the ground electrodes <NUM>-2c and <NUM>-2d facing each other across the parallel waveguides 112e and 112f, respectively, and the signal electrode <NUM>-1d configures a coplanar type transmission line together with the ground electrodes <NUM>-2d and <NUM>-2e facing each other across the parallel waveguides 112e and 112f, respectively.

Hereinafter, the nested Mach-Zehnder type optical waveguides 108a and 108b are collectively referred to as nested Mach-Zehnder type optical waveguides <NUM>. Further, the Mach-Zehnder type optical waveguides 110a, 110b, 110c, 110d, 110e, 110f, <NUM>, <NUM> are collectively referred to as Mach-Zehnder type optical waveguides <NUM>. Further, the parallel waveguides 112a, 112b, 112c, 112d, 112e, 112f, <NUM>, <NUM> are collectively referred to as parallel waveguides <NUM>. Further, the signal electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d are collectively referred to as signal electrodes <NUM>-<NUM>. Further, the ground electrodes <NUM>-2a, <NUM>-2b, <NUM>-2c, <NUM>-2d, and <NUM>-2e are collectively referred to as ground electrodes <NUM>-<NUM>.

Further, the signal electrode <NUM>-<NUM> and the ground electrode <NUM>-<NUM> are collectively referred to as working electrodes <NUM>. The signal electrode <NUM>-<NUM> and the ground electrode <NUM>-<NUM>, which are the working electrodes <NUM>, control the light wave propagating in the optical waveguide <NUM>. Further, the signal electrode <NUM>-<NUM> and the ground electrode <NUM>-<NUM> are two working electrodes <NUM> that sandwich the parallel waveguide <NUM> of the optical waveguide <NUM> in the plane of the substrate <NUM>.

In the present embodiment, each of the signal electrode <NUM>-<NUM> and the ground electrode <NUM>-<NUM>, which are the working electrodes <NUM>, is a two stage electrode, and is configured to be stepped thick as the distance from the parallel waveguide <NUM> sandwiched by the working electrodes increases (see <FIG> below).

The right end portions of the signal electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d are connected to the signal wiring electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d (hatched strip portions), respectively. Further, the left end portions of the signal electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d are connected to the signal wiring electrodes <NUM>-1e, <NUM>-1f, <NUM>-<NUM>, and <NUM>-<NUM>, respectively.

The right ends of the ground electrodes <NUM>-2a, <NUM>-2b, <NUM>-2c, and <NUM>-2d are connected to the ground wiring electrodes <NUM>-2a, <NUM>-2b, <NUM>-2c, and <NUM>-2d, respectively. The left ends of the ground electrodes <NUM>-2a, <NUM>-2b, <NUM>-2c, and <NUM>-2d are connected to the ground wiring electrodes <NUM>-2f, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-2i, respectively.

The ground electrode <NUM>-2e is connected to the rectangular ground pattern, a portion including the upper edge extending to the right side of the rectangular ground pattern configures the ground wiring electrode <NUM>-2e, and a portion including the left edge extending to the lower side configures the ground wiring electrode <NUM>-2j.

Thus, the signal wiring electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d and the ground wiring electrodes <NUM>-2a, <NUM>-2b, <NUM>-2c, <NUM>-2d, and <NUM>-2e adjacent to these signal wiring electrodes configure a coplanar type transmission line. Similarly, the signal wiring electrodes <NUM>-1e, <NUM>-1f, <NUM>-<NUM>, and <NUM>-<NUM> and the ground wiring electrodes <NUM>-2f, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-2i, and <NUM>-2j adjacent to the signal wiring electrodes configure a coplanar type transmission line.

The signal wiring electrodes <NUM>-1e, <NUM>-1f, <NUM>-<NUM>, and <NUM>-<NUM> extending to the lower side 140d of the substrate <NUM> are terminated by a termination resistor having a predetermined impedance outside the substrate <NUM>.

Thus, the high-frequency electrical signal input from the signal wiring electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d extending to the right side 140b of the substrate <NUM> becomes a traveling wave to propagate through the signal electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, and <NUM>-1d, and modulates the light wave propagating through the Mach-Zehnder type optical waveguides 110a, 110b, 110c, and 110d, respectively.

Hereinafter, the signal wiring electrodes <NUM>-1a, <NUM>-1b, <NUM>-1c, <NUM>-1d, <NUM>-1e, <NUM>-1f, <NUM>-<NUM>, and <NUM>-<NUM> are collectively referred to as signal wiring electrodes <NUM>-<NUM>. Further, the ground wiring electrodes <NUM>-2a, <NUM>-2b, <NUM>-2c, <NUM>-2d, <NUM>-2e, <NUM>-2f, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-2i, and <NUM>-2j are collectively referred to as ground wiring electrodes <NUM>-<NUM>. Further, the signal wiring electrode <NUM>-<NUM> and the ground wiring electrode <NUM>-<NUM> are collectively referred to as wiring electrodes <NUM>. That is, the signal wiring electrode <NUM>-<NUM> and the ground wiring electrode <NUM>-<NUM> are wiring electrodes <NUM> connected to the working electrode <NUM>.

With reference to <FIG>, the B portion of the substrate <NUM> shown in <FIG> is provided with bias electrodes 130a, 130b, 130c, and 130d for adjusting the bias points of the Mach-Zehnder type optical waveguides 110a, 110b, 110c, and 110d, and bias electrodes 130e and 130f for adjusting the bias points of the nested Mach-Zehnder type optical waveguides 108a and 108b. The bias electrodes 130a, 130b, and 130e are connected to bias wiring electrodes 132a, 132b, and 132e extending to the upper side 140c of the substrate <NUM>, respectively. Further, the bias electrodes 130c, 130d, and 130f are connected to the bias wiring electrodes 132c, 132d, and 132f extending to the lower side 140d of the substrate <NUM>, respectively.

Hereinafter, the bias electrodes 130a, 130b, 130c, 130d, 130e, and 130f are collectively referred to as bias electrodes <NUM>. Bias wiring electrodes 132a, 132b, 132c, 132d, 132e, and 132f are collectively referred to as bias wiring electrodes <NUM>. In the present embodiment, the bias electrode <NUM> is also a two-stage electrode like the working electrode <NUM> described above (see <FIG> described later).

As described above, the optical modulation element <NUM> includes the substrate <NUM>, the optical waveguide <NUM> formed on the substrate <NUM>, two electrodes disposed at positions sandwiching a part of the parallel waveguide <NUM> included in the optical waveguide <NUM> from both sides in the plane of the substrate <NUM>, and a dielectric layer <NUM> covering a top of the optical waveguide <NUM>. Here, the above two electrodes are, for example, a working electrode <NUM> and/or a bias electrode <NUM> that sandwich the parallel waveguide <NUM>.

In particular, in the optical modulation element <NUM> of the present embodiment, the dielectric layer <NUM> extends to the extent including the edges of the two working electrodes <NUM> and the bias electrode <NUM>, facing the parallel waveguide <NUM>, in the width direction of the parallel waveguide <NUM>, and is disposed to partially cover each of the two working electrodes <NUM> and the bias electrode <NUM>.

<FIG> is a cross-sectional view taken along line IV-IV in <FIG>, showing the arrangement of the ground electrode <NUM>-2e, the signal electrode <NUM>-1d, and the ground electrode <NUM>-2d sandwiching the parallel waveguides <NUM> and <NUM>. The back surface (lower surface in <FIG>) of the substrate <NUM> is supported and reinforced by the supporting plate <NUM>. The supporting plate <NUM> is, for example, glass. Parallel waveguides <NUM> and <NUM> are formed on the upper surface of the substrate <NUM>, as protruding optical waveguides by the protruding portions 150a and 150b formed on the substrate <NUM>, respectively.

On the substrate <NUM>, a ground electrode <NUM>-2d and a signal electrode <NUM>-1d are formed at positions sandwiching the parallel waveguide <NUM> in the plane of the substrate <NUM>. Further, a signal electrode <NUM>-1d and a ground electrode <NUM>-2e are formed at positions sandwiching the parallel waveguide <NUM> in the plane of the substrate <NUM>.

A dielectric layer <NUM> covering the parallel waveguide <NUM> is disposed between the ground electrode <NUM>-2d and the signal electrode <NUM>-1d. Similarly, a dielectric layer <NUM> covering the parallel waveguide <NUM> is disposed between the signal electrode <NUM>-1d and the ground electrode <NUM>-2e.

The dielectric layer <NUM> between the ground electrode <NUM>-2d and the signal electrode <NUM>-1d extends to the extent including the edges 144a and 144b of the ground electrode <NUM>-2d and the signal electrode <NUM>-1d, facing the parallel waveguide <NUM>, in the width direction (left and right direction) of the parallel waveguide <NUM>, and is disposed to partially cover each of the ground electrode <NUM>-2d and the signal electrode <NUM>-1d.

Further, the dielectric layer <NUM> between the signal electrode <NUM>-1d and the ground electrode <NUM>-2e extends to the left and right to the extent including the edges 144c and 144d of the signal electrode <NUM>-1d and the ground electrode <NUM>-2e, facing the parallel waveguide <NUM>, in the width direction (left and right direction) of the parallel waveguide <NUM>, and is disposed to partially cover each of the signal electrode <NUM>-1d and the ground electrode <NUM>-2e.

Here, the ground electrode <NUM>-2d, the signal electrode <NUM>-1d, and the ground electrode <NUM>-2e include the first-stage electrodes 146a, 146b, 146c and the second-stage electrodes 148a, 148b, 148c, respectively. The above-described edges 144a, 144b, 144c, 144d are edges of the first-stage electrodes 146a, 146b, and 146c of the corresponding ground electrode <NUM>-2d, the signal electrode <NUM>-1d, and the ground electrode <NUM>-2e, respectively.

In particular, in the present embodiment, the dielectric layer <NUM> covering the parallel waveguide <NUM> is disposed to partially cover the first-stage electrode 146a of the ground electrode <NUM>-2d and the first-stage electrode 146b of the signal electrode <NUM>-1d, respectively. Similarly, the dielectric layer <NUM> covering the parallel waveguide <NUM> is disposed to partially cover the first-stage electrode 146b of the signal electrode <NUM>-1d and the first-stage electrode 146c of the ground electrode <NUM>-2e.

Thereby, for example, on the left and right sides of the dielectric layer <NUM> covering the parallel waveguide <NUM>, a gap of a width W16 is formed between the dielectric layer <NUM> and the second-stage electrode 148c of the ground electrode <NUM>-2e, and a gap of a width W17 is formed between the dielectric layer <NUM> and the second-stage electrode 148b of the signal electrode <NUM>-1d. Thereby, for example, between the signal electrode <NUM>-1d and the ground electrode <NUM>-2e, the electric field is concentrated on the dielectric layer <NUM> between the first-stage electrodes 146b and 146c, compared with the space between the second-stage electrodes 148b and 148c, and thus the electric field efficiency in the parallel waveguide <NUM> covered by the dielectric layer <NUM> is improved.

In the dielectric layer <NUM>, the other parallel waveguides 112a, 112b, 112c, 112d, 112e, and 112f are also configured in the same manner as described above. Here, the edges of the two working electrodes <NUM> that sandwich the parallel waveguide <NUM>, facing the parallel waveguide <NUM>, are collectively referred to as edges <NUM>. Further, the first-stage electrodes and the second-stage electrodes of the working electrode <NUM> are collectively referred to as first-stage electrodes <NUM> and the second-stage electrodes <NUM>, respectively.

That is, the dielectric layer <NUM> extends to an extent including the edges <NUM> of the two working electrodes <NUM> and the bias electrode <NUM>, facing the parallel waveguide <NUM>, in the width direction of the parallel waveguide <NUM>, and is disposed to partially cover each of the first-stage electrodes <NUM> of the two working electrodes <NUM>.

Here, the clearance between the edges <NUM> facing each other (for example, the distance W12 between the edges 144c and 144d in <FIG>) of the two working electrodes <NUM> sandwiching the parallel waveguide <NUM> is, for example, less than <NUM>.

In the present embodiment, the wiring electrode <NUM> is formed by extending a second-stage portion (that is, a thick portion) of the working electrode <NUM>, which is a two-stage electrode. <FIG> is a cross-sectional view taken along line V-V in <FIG>, showing a connection state between the signal electrode <NUM>-1d and the signal wiring electrode <NUM>-1d, as an example of a connection state between the working electrode <NUM> and the wiring electrode <NUM>. The signal electrode <NUM>-1d includes a first-stage electrode 146a and a second-stage electrode 148a, and the second-stage electrode 148a of the signal electrode <NUM>-1d extends to the right to configure the signal wiring electrode <NUM>-1d.

In the dielectric layer <NUM>, the bias electrode <NUM> shown in <FIG> is also disposed in the same arrangement as that in the working electrode <NUM> described above. That is, the two adjacent bias electrodes <NUM> are configured to sandwich the parallel waveguide <NUM> in the plane of the substrate <NUM>. Then, the dielectric layer <NUM> covering the parallel waveguide <NUM> extends to an extent including the edges of the two bias electrodes <NUM>, facing the parallel waveguide <NUM>, in the width direction of the parallel waveguide <NUM>, and is disposed to partially cover each of the two bias electrodes <NUM>.

<FIG> is a cross-sectional view taken along line VI-VI in <FIG>, showing the arrangement of the dielectric layer <NUM> in the bias electrode 130a as an example of the arrangement of the dielectric layer <NUM> in the bias electrode <NUM>. The bias electrode 130a is formed on the substrate <NUM> so as to sandwich each of the parallel waveguide 112a and 112b in the plane of the substrate <NUM>. The dielectric layer <NUM> covering the parallel waveguide 112a extends to the extent including edges 152a and 152b of the two bias electrodes 130a, facing the parallel waveguide 112a, on the both sides in the width direction (left and right direction) of the parallel waveguide 112a, and is disposed to partially cover each of the bias electrodes 130a. Further, in <FIG>, the bias wiring electrodes 132a are connected to the left and right bias electrodes 130a.

In the dielectric layer <NUM>, the other bias electrodes 130b, 130c, 130d, 130e, and 130f are also configured in the same manner as described above. Here, the edges of the two bias electrodes <NUM> that sandwich the parallel waveguide <NUM>, facing the parallel waveguide <NUM>, are collectively referred to as edges <NUM>. That is, the dielectric layer <NUM> extends to the extent including the edge <NUM> of each of the two bias electrode <NUM>, facing the parallel waveguide <NUM>, in the width direction of the parallel waveguide <NUM>, and is disposed to partially cover each of the two bias electrodes <NUM>.

In the optical modulation element <NUM> having the above configuration, the protruding portion extending on the substrate <NUM> and configuring the optical waveguide <NUM> is covered with the dielectric layer <NUM> which is a resin having a refractive index of more than <NUM>. As a result, in the optical modulation element <NUM>, the difference in refractive index between the protruding portion and the surrounding environment of the protruding portion becomes smaller compared with the state where the protruding portion is in contact with air, and scattering loss due to surface degradation in the side surface of the protruding portion generated in the process of forming the protruding portion is reduced.

The dielectric layer <NUM> can be formed, for example, by coating (spin coating) a photosensitive permanent film, which is a photoresist, on a substrate <NUM> with a spinner and patterning using ultraviolet rays.

In the optical modulation element <NUM>, the dielectric layer <NUM> extends to an extent including the edges facing each other of the two working electrodes <NUM> sandwiching the parallel waveguide <NUM> and is disposed to partially cover each of the electrodes. Therefore, even when the formation position of the dielectric layer <NUM> in the plane of the substrate <NUM> is deviated in the width direction of the parallel waveguide <NUM> in the manufacturing process (for example, the above patterning step), as long as the deviation amount is within a predetermined error range, the state in which a space between the two working electrodes <NUM> sandwiching the parallel waveguide <NUM> is charged with the dielectric layer <NUM> without a gap can be maintained. Therefore, in the optical modulation element <NUM>, the manufacturing variation of the capacity between the two working electrodes <NUM> sandwiching the parallel waveguide <NUM> is reduced, and the electric field applied to the parallel waveguide <NUM> from the two working electrodes <NUM> can be maintained high (that is, the electric field efficiency can be maintained high). The same applies to the bias electrode <NUM>.

As a result, in the optical modulation element <NUM>, it is possible to effectively prevent scattering loss of waveguide light due to surface roughness in a protruding optical waveguide while maintaining high electric field efficiency in the optical waveguide <NUM> (specifically, the portion of the parallel waveguide <NUM>).

The predetermined error range is, for example, a range within the smaller dimension of W14 and W15, in <FIG>, when the design values of the riding dimensions of the dielectric layer <NUM> covering the parallel waveguide <NUM> on the left and right working electrodes <NUM> (ground electrode <NUM>-2e and signal electrode <NUM>-1d) are W14 and W15. When the center of the width W10 of the upper surface of the dielectric layer <NUM> and the center of the distance W12 between the left and right working electrodes <NUM> coincide in design, the error range is (W10-W12)/<NUM>.

Further, the working electrodes <NUM> sandwiching the parallel waveguide <NUM> is generally formed as thick as about <NUM> in height from the surface of the substrate <NUM>, in an optical modulation element that uses a microwave electrical signal of several GHz as a high-frequency electrical signal for modulation. Therefore, the depth of the recess portion formed between the two working electrodes <NUM> is as deep as <NUM>. Filling such a deep recess portion with an inorganic dielectric such as silicon dioxide (SiO2) involves technical difficulties. Further, even when such a thick film is formed, the stress accumulated inside the inorganic dielectric layer in the film forming process is applied to the substrate <NUM> or the like after manufacturing, which may adversely affect the electrical and/or optical characteristics or long-term reliability of the optical modulation element.

As described above, in the optical modulation element <NUM>, since the dielectric layer <NUM> is made of resin, a thick layer of about <NUM> can be easily formed. Further, since the Young's modulus of the resin is generally smaller than that of the inorganic material, the stress applied from the dielectric layer <NUM> to the substrate <NUM> or the like after production is significantly reduced as compared with the case where the inorganic dielectric layer is used.

Further, the viscosity of the resin such as the photosensitive permanent film used for the dielectric layer <NUM> can be adjusted by adjusting the composition of the resin. By using the dielectric layer <NUM> which is a resin, it is possible to adjust the thickness of a film to be formed on the substrate <NUM> during spin coating, by adjusting the viscosity in addition to adjusting the rotation speed of the spinner during spin coating. Therefore, in the optical modulation element <NUM>, the capacity between the two working electrodes <NUM> can be set to a desired value, by adjusting the thickness of the dielectric layer <NUM> formed between the two working electrodes <NUM> sandwiching the parallel waveguide <NUM>, so that the impedance between the two working electrodes <NUM> can be adjusted, and the velocity matching between the light wave and the high-frequency electrical signal can be adjusted.

Further, the two working electrodes <NUM> sandwiching the parallel waveguide <NUM> are two-stage electrodes. Such a multi-stage configuration of the working electrode <NUM> is generally used to widen the frequency characteristic of the transmission line configured by the working electrode <NUM> while maintaining a high strength of the electric field generated in the parallel waveguide <NUM>. In particular, in the present embodiment, the dielectric layer <NUM> covering the parallel waveguide <NUM> is disposed to extend to the edge of the first-stage electrode closest to the parallel waveguide <NUM>, and partially cover the first-stage electrode. Therefore, the lines of electric force generated between the working electrodes <NUM> have a higher density in the dielectric layer <NUM> formed between the first-stage electrodes than in the space between the second-stage electrodes. As a result, in the present embodiment, the electric field is concentrated on the parallel waveguide <NUM> covered with the dielectric layer <NUM> between the first-stage electrodes, and the electric field efficiency is improved.

As described above, in the dielectric layer <NUM> covering the parallel waveguide <NUM>, by reducing a refractive index difference between the protruding portion on the substrate <NUM> which is an optical waveguide portion (so-called core) and the dielectric layer <NUM> which functions as a clad, the scattering loss on the surface of the protruding portion is reduced. <FIG> is a diagram showing the effect of reducing the scattering loss with respect to the refractive index of the material configuring the dielectric layer <NUM>. In <FIG>, the horizontal axis is the ratio (refractive index ratio) nclad/ncore of the refractive index nclad of the dielectric layer <NUM> with respect to the refractive index ncore of the protruding portion configuring the parallel waveguide <NUM>. The vertical axis indicates the optical loss (dB/cm) in the parallel waveguide <NUM>.

In <FIG>, a line <NUM> shows the scattering loss in the parallel waveguide <NUM>. <FIG> further shows lines <NUM> and <NUM>. The line <NUM> represents a waveguide loss that depends on the optical confinement effect in the parallel waveguide <NUM>. Further, the line <NUM> represents the total loss including the scattering loss shown by the line <NUM> and the waveguide loss shown by the line <NUM>.

From the line <NUM>, it can be seen that the closer the refractive index ratio nclad/ncore is to <NUM>, that is, as the difference in refractive index between the refractive index ncore of the protruding portion configuring the parallel waveguide <NUM> and the refractive index nclad of the dielectric layer <NUM> becomes small, the scattering loss in the parallel waveguide <NUM> is reduced.

On the other hand, from the line <NUM>, it can be seen that the waveguide loss of the parallel waveguide <NUM> increases as the refractive index ratio nclad/ncore approaches <NUM>. This increase in waveguide loss is due to the fact that the optical confinement effect in the parallel waveguide <NUM> decreases as the refractive index difference decreases, and when the refractive index ratio nclad/ncore exceeds <NUM>, the waveguide loss suddenly increases. As a result, as shown in line <NUM>, the total loss of the parallel waveguide <NUM> changes in a U shape with respect to the refractive index ratio nclad/ncore, and is a minimum value (about <NUM> dB/cm) at the refractive index ratio nclad/ncore = <NUM>.

From the line <NUM>, it can be seen that the refractive index ratio nclad/ncore is preferably <NUM> or more and <NUM> or less, assuming that the allowable increase in optical loss from the minimum value is <NUM> dB. That is, it is desirable that the refractive index nclad of the dielectric layer <NUM> is <NUM> times or more and <NUM> or less of the refractive index ncore of the protruding portion configuring the parallel waveguide <NUM>.

Next, modification examples of the optical modulation element <NUM> will be described. [First modification example].

<FIG> is a diagram showing the configuration of an optical modulation element <NUM>-<NUM> according to a first modification example of the optical modulation element <NUM>, and corresponding to <FIG> in the above-described embodiment. The dielectric layer <NUM>-<NUM> shown in <FIG> is used in place of the dielectric layer <NUM> in the optical modulation elements <NUM> shown in <FIG>. In <FIG>, for the same components as those shown in <FIG>, the same reference numerals as those shown in <FIG> are used, and the above description for <FIG> is used. Further, since the planar configuration of the optical modulation element <NUM>-<NUM> in which the dielectric layer <NUM>-<NUM> is used is the same as the planar configuration of the optical modulation element <NUM> shown in <FIG>, the above description of <FIG> is incorporated.

In <FIG>, the dielectric layer <NUM>-<NUM> has the same structure as the dielectric layer <NUM> shown in <FIG>, except that the upper surface is not flat and has irregularities. Specifically, in the dielectric layer <NUM>-<NUM> covering the parallel waveguide <NUM>, the height between the edge 144a and the edge 144b of the ground electrode <NUM>-2d and the signal electrode <NUM>-1d adjacent to the parallel waveguide <NUM> is lower than the height of the portion that partially covers the ground electrode <NUM>-2d and the signal electrode <NUM>-1d.

The dielectric layer <NUM>-<NUM> covering the other parallel waveguides <NUM> may also be configured in the same manner as described above. That is, in the dielectric layer <NUM>-<NUM> covering the parallel waveguide <NUM>, the height between the signal electrode <NUM>-<NUM> and the ground electrode <NUM>-<NUM> sandwiching the parallel waveguide <NUM> is lower than the height of the portion covering these electrodes.

Such an upper surface shape of the dielectric layer <NUM>-<NUM> can be achieved, for example, by adjusting the viscosity of the resin at the time of spin coating according to the composition of the resin configuring the dielectric layer <NUM>-<NUM>.

In general, a resin such as a photosensitive permanent film has viscosity before curing, so that even when the substrate <NUM> has irregularities due to protruding portions or electrodes configuring the optical waveguide <NUM>, the surface of the resin after spin coating is substantially flat. Further, in the patterning step of the spin-coated resin, when high-temperature treatment for drying or the like is performed, the surface of the spin-coated resin can be further flattened by high-temperature softening of the resin depending on the treatment temperature. Therefore, in general, the upper surface of the dielectric layer <NUM> between the electrodes sandwiching the parallel waveguide <NUM> is substantially flat as shown in <FIG>.

On the other hand, in the configuration shown in <FIG>, in the dielectric layer <NUM>-<NUM>, as described above, the height between the edge 144a and the edge 144b of the ground electrode <NUM>-2d and the signal electrode <NUM>-1d adjacent to the parallel waveguide <NUM> is lower than the height of the portion covering the ground electrode <NUM>-2d and the signal electrode <NUM>-1d. That is, the dielectric layer <NUM>-<NUM> is configured such that the thickness of the portion covering the parallel waveguide <NUM> is thinner than the thickness of the parallel waveguide <NUM> covering the dielectric layer <NUM> in the configuration shown in <FIG>. The configuration shown in <FIG> can be formed by adjusting the viscosity of the pre-cured resin used in the configuration of <FIG>. Further, the dielectric layer <NUM> can be formed by using a sputtering method or a CVD method because it is affected by the irregularities on the substrate <NUM> due to the protruding portions and electrodes configuring the optical waveguide <NUM>.

As a result, in the present modification example, the lines of electric force between the signal electrode <NUM>-1d and the ground electrode <NUM>-2d are concentrated on and pass through the thinly formed dielectric layer <NUM>-<NUM>, compared with the configuration of <FIG>, the electric field applied to the parallel waveguide <NUM> is further strengthened, and the electric field efficiency is further improved.

<FIG> is a diagram showing the configuration of an optical modulation element <NUM>-<NUM> according to a second modification example of the optical modulation element <NUM>, and corresponding to <FIG> in the above-described embodiment. The signal electrode <NUM>-3d and the ground electrodes <NUM>-4d and <NUM>-4e shown in <FIG> are used in place of the signal electrode <NUM>-1d and the ground electrodes <NUM>-2d and <NUM>-2e in the optical modulation element <NUM> shown in <FIG>. In <FIG>, for the same components as those shown in <FIG>, the same reference numerals as those shown in <FIG> are used, and the above description for <FIG> is incorporated.

Further, since the planar configuration of the optical modulation element <NUM>-<NUM> in which the signal electrode <NUM>-3d and the ground electrodes <NUM>-4d and <NUM>-4e shown in <FIG> are used is the same as the optical modulation element <NUM> shown in <FIG>, the above-described explanations of <FIG> are incorporated.

The signal electrode <NUM>-<NUM> and the ground electrode <NUM>-<NUM>, which are the working electrodes <NUM> shown in <FIG> and <FIG>, are configured as two stage electrodes as an example, but these working electrodes may be configured as one stage. The signal electrode <NUM>-3d and the ground electrodes <NUM>-4d and <NUM>-4e shown in <FIG> are configured as such one stage electrodes. As in the configuration shown in <FIG>, the dielectric layer <NUM> covering the parallel waveguide <NUM> extends to the extent including the edges of the ground electrode <NUM>-4d and the signal electrode <NUM>-3d sandwiching the parallel waveguide <NUM>, facing the parallel waveguide <NUM>, in the width direction (left and right direction) of the parallel waveguide <NUM>, and is disposed to partially cover each of the ground electrode <NUM>-4d and the signal electrode <NUM>-3d.

The present embodiment is an optical modulator using any one of the above-described optical modulation elements. <FIG> is a diagram showing the configuration of an optical modulator <NUM> according to the second embodiment. The optical modulator <NUM> includes a case <NUM>, an optical modulation element <NUM> housed in the case <NUM>, and a relay substrate <NUM>. The optical modulation element <NUM> is any one of the above-described optical modulation elements <NUM>, <NUM>-<NUM>, and <NUM>-<NUM>. Finally, a cover (not shown), which is a plate body, is fixed to the opening of the case <NUM>, and the inside of the case <NUM> is hermetically sealed.

The optical modulator <NUM> has signal pins <NUM> for inputting a high-frequency electrical signal used for modulation of the optical modulation element <NUM>, and signal pins <NUM> for inputting an electrical signal used for adjusting the operating point of the optical modulation element <NUM>.

Further, the optical modulator <NUM> has an input optical fiber <NUM> for inputting light into the case <NUM> and an output optical fiber <NUM> for guiding the light modulated by the optical modulation element <NUM> to the outside of the case <NUM>, on the same surface of the case <NUM> (in the present embodiment, the surface on the left side).

Here, the input optical fiber <NUM> and the output optical fiber <NUM> are fixed to the case <NUM> via the supports <NUM> and <NUM> which are fixing members, respectively. The light input from the input optical fiber <NUM> is collimated by the lens <NUM> disposed in the support <NUM> and then input to the optical modulation element <NUM> via the lens <NUM>. However, this is only an example, and the light may be input to the optical modulation element <NUM>, based on the related art, for example, by introducing the input optical fiber <NUM> into the case <NUM> via the support <NUM>, and connecting the end surface of the introduced input optical fiber <NUM> to the end surface of the substrate <NUM> of the optical modulation element <NUM>.

The light output from the optical modulation element <NUM> is coupled to the output optical fiber <NUM> via the optical unit <NUM> and the lens <NUM> disposed on the support <NUM>. The optical unit <NUM> may include a polarization beam combiner that combines two modulated light output from the optical modulation element <NUM> into a single beam.

The relay substrate <NUM> relays the high-frequency electrical signal input from the signal pins <NUM> and the electrical signal for adjusting an operating point (bias point) input from the signal pins <NUM> to the optical modulation element <NUM>, according to a conductor pattern (not shown) formed on the relay substrate <NUM>. The conductor pattern on the relay substrate <NUM> is connected to each pad configuring one end of the electrode of the optical modulation element <NUM> by, for example, wire bonding or the like. Further, the optical modulator <NUM> includes a terminator <NUM> having a predetermined impedance in the case <NUM>.

Since the optical modulator <NUM> having the above configuration is configured by using the optical modulation element <NUM> which is any of the optical modulation elements <NUM>, <NUM>-<NUM>, <NUM>-<NUM> having a configuration capable of effectively preventing the scattering loss of the waveguide light due to the surface roughness in the protruding optical waveguide, it is possible to provide a modulation operation with low optical loss and a low drive voltage.

Next, a third embodiment of the present invention will be described. The present embodiment is an optical modulation module <NUM> using the optical modulation element according to any one of the above-described embodiments or modification examples. <FIG> is a diagram showing the configuration of an optical modulation module <NUM> according to the present embodiment. In <FIG>, for the same components as in the optical modulator <NUM> according to the second embodiment shown in <FIG>, the same reference numerals as those shown in <FIG> are used, and the above description for <FIG> is incorporated.

The optical modulation module <NUM> has the same configuration as the optical modulator <NUM> shown in <FIG>, but differs from the optical modulator <NUM> in that it includes a circuit substrate <NUM> instead 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> based on, for example, a modulation signal supplied from the outside via the signal pins <NUM>, and outputs the generated high-frequency electrical signal to the optical modulation element <NUM>.

Since the optical modulation module <NUM> is configured by using the optical modulation element <NUM> which is any of the optical modulation elements <NUM>, <NUM>-<NUM>, <NUM>-<NUM> having a configuration capable of effectively preventing the scattering loss of the waveguide light due to the surface roughness in the protruding optical waveguide, it is possible to provide a modulation operation with low optical loss and a low drive voltage, similarly to the optical modulator <NUM> according to the second embodiment described above.

Next, a fourth embodiment of the present invention will be described. The present embodiment is an optical transmission apparatus <NUM> equipped with the optical modulator <NUM> according to the second embodiment. <FIG> is a diagram showing a configuration of an optical transmission apparatus <NUM> according to the present embodiment. The optical transmission apparatus <NUM> includes an optical modulator <NUM>, a light source <NUM> that input light to the optical modulator <NUM>, a modulator drive unit <NUM>, and a modulation signal generation unit <NUM>. The above-described optical modulation module <NUM> can also be used instead of the optical modulator <NUM> and the modulator drive unit <NUM>.

The modulation signal generation unit <NUM> is an electronic circuit that generates an electrical signal for causing the optical modulator <NUM> to perform a modulation operation, which generates, based on transmission data given from the outside, a modulation signal which is a high-frequency signal for causing the optical modulator <NUM> to perform an optical modulation operation according to the modulation data, and outputs the modulation signal to the modulator drive unit <NUM>.

The modulator drive unit <NUM> amplifies the modulation signal input from the modulation signal generation unit <NUM>, and outputs a high-frequency electrical signal for driving a signal electrode such as the optical modulation element <NUM> included in the optical modulator <NUM>. As described above, instead of the optical modulator <NUM> and the modulator drive unit <NUM>, for example, the optical modulation module <NUM> provided with a drive circuit <NUM> including a circuit corresponding to the modulator drive unit <NUM> inside the case <NUM> can also be used.

The high-frequency electrical signal is input to the signal pins <NUM> of the optical modulator <NUM> to drive the optical modulation element <NUM> and the like. Thus, the light output from the light source <NUM> is modulated by the optical modulator <NUM>, becomes modulated light, and is output from the optical transmission apparatus <NUM>.

Since the optical transmission apparatus <NUM> having the above configuration is configured by using the optical modulation element <NUM> which is any of the optical modulation elements <NUM>, <NUM>-<NUM>, <NUM>-<NUM> having a configuration capable of effectively preventing the scattering loss of the waveguide light due to the surface roughness in the protruding optical waveguide, it is possible to achieve good light transmission by a modulation operation with low optical loss and a low drive voltage, similarly to the optical modulator <NUM> according to the second embodiment and the optical modulation module <NUM> according to the third embodiment described above.

The present invention is not limited to the configuration of the above embodiment and its alternative configuration, and can be implemented in various embodiments without departing from the appended claims.

For example, the optical waveguide <NUM> is a convex waveguide in the above-described embodiment, but may be a planar waveguide formed on the surface of the substrate <NUM>, for example, a Ti diffused waveguide. Even in this case, the scattering loss due to the roughness of the surface of the substrate <NUM> can be reduced by the dielectric layer <NUM>.

Further, although the working electrode <NUM> of the optical modulation element <NUM> is configured in two stages in the above-described embodiment, it may be configured in a stepped shape having three or more stages.

Further, in the configuration shown in <FIG>, the dielectric layer <NUM> is disposed to partially cover the first-stage electrodes 146a, 146b, and 146c of the signal electrode <NUM>-1d and the ground electrodes <NUM>-2d and <NUM>-2e, but the range in which the dielectric layer <NUM> covers these working electrodes <NUM> is not limited to the first-stage electrodes <NUM>. As long as the dielectric layer <NUM> extends to the edge <NUM> of the working electrode <NUM>, facing (or adjacent to) the parallel waveguide <NUM>, the dielectric layer <NUM> may extend to the second-stage electrodes <NUM> of the working electrodes <NUM> (alternatively, when these electrodes has three or more stages, the dielectric layer <NUM> may extend to the second or higher staircase portion).

However, from the viewpoint of concentrating the electric field on the parallel waveguide <NUM>, it is preferable that the dielectric layer <NUM> partially covers only the first-stage electrodes <NUM> of the working electrodes <NUM>, and the clearances between the respective second-stage electrodes <NUM> and the dielectric layer <NUM> (for example, the clearances W16 and W17 in <FIG>) are empty.

Further, it is assumed that in the optical modulation element <NUM>, with respect to not only the working electrode <NUM> but also the bias electrode <NUM>, the dielectric layer <NUM> covering the parallel waveguide <NUM> extends to the edges <NUM> of the two bias electrodes <NUM> sandwiching the parallel waveguide <NUM> to partially cover the bias electrode <NUM>. However, such a configuration of the dielectric layer <NUM> may be implemented for either the working electrode <NUM> or the bias electrode <NUM>. Thereby, in the working electrode <NUM> or the bias electrode <NUM>, it is possible to effectively prevent the scattering loss of the waveguide light due to the surface roughness in the optical waveguide while maintaining the high electric field efficiency.

The present invention is not limited to the configuration of the above embodiment, and can be implemented in various embodiments without departing from the appended claims.

For example, in the optical modulation element <NUM> shown in <FIG>, there is a portion of the substrate <NUM> on which no electrode and optical waveguide <NUM> are formed, but all or a part of such a portion may be covered with a ground pattern, according to the related art.

As described above, the optical modulation element <NUM>, which is the optical waveguide device according to the above-described embodiment, has a substrate <NUM> and an optical waveguide <NUM> formed on the substrate <NUM>. Here, the optical waveguide <NUM> includes, for example, a parallel waveguide <NUM> as a part. The optical modulation element <NUM> includes two electrodes (two working electrodes <NUM> or two bias electrode <NUM>) disposed at positions sandwiching the parallel waveguide <NUM> from both sides in the plane of the substrate <NUM>, and a dielectric layer <NUM> covering a top of the optical waveguide. Then, in the optical modulation element <NUM>, the dielectric layer <NUM> extends to an extent including the edges of the two electrodes, facing the parallel waveguide <NUM>, in the width direction of the parallel waveguide <NUM>, and is disposed to partially cover each of the two electrodes.

According to this configuration, even when the formation position of the dielectric layer <NUM> covering the optical waveguide <NUM> is deviated, the dielectric layer <NUM> can be formed without a gap between the above two electrodes (that is, the two working electrodes <NUM> or the two bias electrodes <NUM>) sandwiching the parallel waveguide <NUM>. According to the above configuration, it is possible to effectively prevent the scattering loss of the waveguide light due to the surface roughness of the optical waveguide <NUM> while maintaining the high electric field efficiency.

Further, in the optical modulation element <NUM>, the optical waveguide <NUM> is a protruding optical waveguide including a protruding portion extending on the substrate <NUM>. According to this configuration, in a protruding optical waveguide in which roughness is likely to occur on the side surface, it is possible to effectively prevent scattering loss of waveguide light due to the surface roughness of the optical waveguide <NUM>, while maintaining high electric field efficiency.

Further, the refractive index of the dielectric layer <NUM> is <NUM> times or more and <NUM> times or less the refractive index of the core portion of the optical waveguide <NUM> through which light propagates. According to this configuration, the trade-off between the decrease in the scattering loss and the increase in the waveguide loss in the optical waveguide <NUM> can be balanced, and the optical propagation loss of the optical waveguide <NUM> can be maintained low.

Further, the dielectric layer <NUM> is, for example, a resin. According to this configuration, since the dielectric layer <NUM> can be formed as thick as about <NUM> on the optical waveguide <NUM>, for example, the dielectric layer <NUM> can be easily formed between two electrodes sandwiching the parallel waveguide <NUM> which is a part of the optical waveguide <NUM>.

Further, the height of the dielectric layer <NUM> between the two electrodes sandwiching the parallel waveguide <NUM>, which is an optical waveguide <NUM>, is lower than the height of the portion covering these electrodes. According to this configuration, since the electric field generated by the above two electrodes can be concentrated on the optical waveguide <NUM> covered with the dielectric layer <NUM>, the electric field efficiency is increased and the operating voltage of the optical modulation element <NUM> can be reduced.

Further, the two working electrodes <NUM> sandwiching the parallel waveguide <NUM>, which is the optical waveguide <NUM>, are configured as, for example, two stage electrodes so as to be stepped thick as a distance from the parallel waveguide <NUM> increases. Then, the dielectric layer <NUM> extends to an extent including the edge of the first-stage electrode <NUM> of each of the two working electrodes closest to the parallel waveguide <NUM>, in the width direction of the parallel waveguide <NUM>, and is disposed to partially cover each of the two electrodes. According to this configuration, the electric field is concentrated on the parallel waveguide <NUM> covered with the dielectric layer <NUM> between the first-stage electrodes <NUM>, and as a result of improving the electric field efficiency, the operating voltage of the optical modulation element <NUM> is reduced.

Further, the dielectric layer <NUM> is disposed to partially cover each of the first-stage electrodes <NUM> of the two working electrodes <NUM> that sandwich the parallel waveguide <NUM>. According to this configuration, the electric field concentration on the parallel waveguide <NUM> can be further increased, and the operating voltage of the optical modulation element <NUM> can be further reduced.

Further, the clearance between the edges <NUM> of the two working electrodes <NUM> sandwiching the parallel waveguide <NUM> is less than <NUM>. According to this configuration, in a configuration in which the working electrodes <NUM> are disposed at clearance of less than <NUM>, the dielectric layer <NUM> can be appropriately disposed without deviation in the parallel waveguide <NUM> disposed between the adjacent working electrodes <NUM>. As a result, the scattering loss in the parallel waveguide <NUM> can be reduced, and the optical modulation element <NUM> with low loss can be realized.

Further, the optical waveguide <NUM> configures two parallel waveguides <NUM> of each Mach-Zehnder type optical waveguides <NUM>. According to this configuration, in the Mach-Zehnder type optical waveguide <NUM> in which the difference in optical loss in the two parallel waveguides <NUM> is easily to affect the optical characteristics, the scattering loss due to the surface roughness in these two parallel waveguides <NUM> is reduced to achieve good optical characteristics.

Further, the optical modulator <NUM> according to the second embodiment described above includes an optical modulation element <NUM> which is an optical modulation element <NUM>, <NUM>-<NUM> or <NUM>-<NUM> that modulates light, a case <NUM> that houses the optical modulation element <NUM>, an input optical fiber <NUM> that inputs light to the optical modulation element <NUM>, and an output optical fiber <NUM> that guides the light output by the optical modulation element <NUM> to the outside of the case <NUM>.

Further, the optical modulation module <NUM> according to the third embodiment described above includes an optical modulation element <NUM>, a case <NUM> that houses the optical modulation element <NUM>, an input optical fiber <NUM> that inputs light to the optical modulation element <NUM>, an output optical fiber <NUM> that guides the light output by the optical modulation element <NUM> to the outside of the case <NUM>, and a drive circuit <NUM> that drives the optical modulation element.

Further, the optical transmission apparatus <NUM> according to the fourth embodiment described above includes the optical modulator <NUM> according to the second embodiment or the optical modulation module <NUM> according to the third embodiment, and a modulation signal generation unit <NUM> which is an electronic circuit for generating an electrical signal for causing the optical modulation element <NUM> to perform a modulation operation.

According to these configurations, it is possible to achieve an optical modulator <NUM>, an optical modulation module <NUM>, or an optical transmission apparatus <NUM> having low power consumption and low loss.

In an optical waveguide device, it is possible to effectively prevent scattering loss of waveguide light due to the surface roughness of a protruding optical waveguide, while maintaining high electric field efficiency.

Claim 1:
An optical waveguide device comprising:
a substrate (<NUM>);
an optical waveguide (<NUM>) formed on the substrate;
two electrodes (<NUM>) disposed at positions sandwiching the optical waveguide (<NUM>) from both sides in a plane of the substrate; and
a dielectric layer (<NUM>, <NUM>-<NUM>) covering a top of the optical waveguide, wherein
the dielectric layer extends to an extent including edges of the two electrodes, facing the optical waveguide, in a width direction of the optical waveguide, and is disposed to partially cover the upper part of each of the two electrodes,
characterized in that:
the two electrodes (<NUM>) are configured to be stepped thick as a distance from the optical waveguide increases;
the dielectric layer (<NUM>, <NUM>-<NUM>) extends to an extent including an edge of a first stage (144a-144d) of each of the two electrodes closest to the optical waveguide, in the width direction of the optical waveguide, and is disposed to partially cover the upper part (W14, W15) of the first stage of each of the two electrodes.