OPTICAL WAVEGUIDE ELEMENT, OPTICAL MODULATOR, OPTICAL MODULATION MODULE, AND OPTICAL TRANSMITTER

Abstract

To effectively prevent a scattering loss of waveguide light caused by surface roughness in a convex optical waveguide while keeping electric field efficiency high, in an optical waveguide device.SOLUTION: An optical waveguide device comprises a substrate, an optical waveguide formed on the substrate, two electrodes placed at positions sandwiching the optical waveguide from both sides in a surface of the substrate, and a dielectric layer covering the optical waveguide. The dielectric layer extends over a range including edges facing the optical waveguide, of the two electrodes in a width direction of the optical waveguide, so as to cover the respective two electrodes partially.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element, an optical modulator, an optical modulation module, and an optical transmission device. [Background technology]

[0002] In commercial optical fiber communication systems, optical modulators incorporating an optical modulation element as an optical waveguide element, which is composed of an optical waveguide formed on a substrate and a control electrode that controls the light wave propagating through the optical waveguide, are widely used. Among them, optical modulation elements using LiNbO3 (hereinafter also referred to as LN) substrates that have electro-optical effects can realize low optical loss and wideband optical modulation characteristics, and are therefore widely used in optical fiber communication systems of high-speed and large-capacity trunk optical transmission networks and metro networks.

[0003] As one measure for miniaturizing, broadening the bandwidth, and reducing power consumption of such optical modulation elements, optical modulators using a rib-type optical waveguide or a ridge-type optical waveguide formed on the surface of a thinned LN substrate (e.g., a thickness of 20 μm or less) are being put to practical use (e.g., Patent Document 1). A rib-type optical waveguide or a ridge-type optical waveguide is a convex optical waveguide configured by forming a strip-shaped convex portion on the thinned LN substrate. This strengthens the interaction between the guided light propagating through the convex waveguide and the signal electric field generated in the substrate by the control electrode (i.e., the electric field efficiency is increased), and the miniaturization, broadening of the bandwidth, and reduction in power consumption of the optical modulation element are realized.

[0004] One of the problems with such a convex optical waveguide is the scattering loss of guided light caused by the surface roughness of the convex portion formed on the LN substrate. For example, a convex optical waveguide is formed by etching the surface of the LN substrate while leaving the convex portion that becomes the optical waveguide (i.e., the portion that becomes the core that guides light). In this case, depending on the etching speed, etching temperature, etc., surface roughness due to minute irregularities may occur on the side surface of the convex portion formed on the LN substrate. Such surface roughness of the convex portion may cause scattering loss in the light propagating through the convex optical waveguide.

[0005] As a technique for reducing scattering loss caused by surface roughness of a convex waveguide, a cladding layer made of lithium tantalum niobate is formed by a sol-gel method so as to cover a convex optical waveguide on an LN substrate (Patent Document 2). In this configuration, the refractive index difference between the convex part (core) of the convex waveguide and the cladding is smaller than in a configuration in which the environmental atmosphere of the convex optical waveguide is used as the cladding, and scattering loss is effectively reduced.

[0006] However, in the conventional configuration, the cladding layer is formed on the LN substrate in areas other than the convex optical waveguide. Therefore, the control electrodes that are subsequently arranged on either side of the convex optical waveguide are formed on the cladding layer formed on the LN substrate, and the strength of the electric field generated in the convex optical waveguide is weaker than in a configuration in which the control electrode is formed directly on the LN substrate. This may result in a decrease in the electric field efficiency in the optical waveguide element.

[0007] As a method for avoiding such a decrease in electric field strength and maintaining the electric field efficiency, it is possible to remove the area of ​​the cladding formed on the LN substrate other than the part covering the convex optical waveguide by patterning before forming the control electrode. However, this method involves patterning the cladding layer to avoid the arrangement part of the control electrode formed by sandwiching the convex optical waveguide with a width of 2 μm at an interval of 4 μm, which is technically difficult and may affect the manufacturing yield. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2018 / 031916 [Patent Document 2] Publication No. 11-64664 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above, it is desirable to effectively prevent scattering loss of guided light caused by surface roughness in a convex optical waveguide while maintaining high electric field efficiency in the optical waveguide element. [Means for solving the problem]

[0010] One aspect of the present invention is a semiconductor device comprising: a substrate; an optical waveguide formed on the substrate; and a pair of semiconductor devices disposed in positions sandwiching the optical waveguide from both sides within the plane of the substrate. and controls the light waves propagating through the optical waveguide. Two electrodes and a layer covering the optical waveguide Invitation A dielectric layer, the optical waveguide is a convex optical waveguide formed by a convex portion extending on the substrate, The dielectric layer is A photosensitive permanent film having a refractive index greater than 1, The optical waveguide element extends in the width direction of the optical waveguide to an extent including the edges of the two electrodes facing the optical waveguide, and is disposed so as to partially cover each of the two electrodes. According to another aspect of the present invention, the refractive index of the dielectric layer is 0.5 to 0.75 times the refractive index of a core portion of the optical waveguide through which light propagates. According to another aspect of the present invention, the height of the dielectric layer between the two electrodes is lower than the height of the portion covering the electrodes. According to another aspect of the present invention, the two electrodes extend along the optical waveguide while maintaining a cross-sectional shape that becomes thicker in a stepped manner with increasing distance from the optical waveguide, and the dielectric layer is arranged to extend in the width direction of the optical waveguide to an extent that includes the edge of the first step of each of the two electrodes that is closest to the optical waveguide, and to partially cover each of the two electrodes. According to another aspect of the present invention, the dielectric layer is disposed so as to partially cover the first stages of the two electrodes, respectively. According to another aspect of the invention, the spacing between the edges of the two electrodes is less than 10 μm. According to another aspect of the present invention, the optical waveguide comprises two parallel waveguides of a Mach-Zehnder type optical waveguide. Another aspect of the present invention is an optical modulator comprising: an optical waveguide element according to any one of claims 1 to 9 which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light to the optical waveguide element; and an optical fiber for guiding light output by the optical waveguide element to the outside of the housing. Another aspect of the present invention is an optical modulation module comprising: an optical waveguide element according to any one of claims 1 to 9 which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light to the optical waveguide element; an optical fiber for guiding light output by the optical waveguide element to the outside of the housing; and a drive circuit for driving the optical waveguide element. Yet another aspect of the present invention is an optical transmission device comprising the above optical modulator or the above optical modulation module, and an electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation. Effect of the Invention

[0011] According to the present invention, in an optical waveguide element, it is possible to effectively prevent scattering loss of guided light caused by surface roughness in a convex optical waveguide while maintaining high electric field efficiency. [Brief description of the drawings]

[0012] [Figure 1] 1 is a diagram showing a configuration of a light modulation element according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a partial detailed view of part A shown in FIG. [Diagram 3] FIG. 2 is a partial detailed view of part B shown in FIG. [Figure 4] 4 is a cross-sectional view of part A shown in FIG. 2 taken along the line IV-IV. [Diagram 5] 3 is a cross-sectional view of part A shown in FIG. 2 taken along the arrow VV. [Figure 6] 6 is a cross-sectional view of part B shown in FIG. 3 taken along the line VI-VI. [Figure 7] 2 is a diagram showing the change in scattering loss of an optical waveguide with respect to the refractive index of a material of a dielectric layer in the optical modulation element shown in FIG. [Figure 8] 2 is a first modified example of the light modulation element shown in FIG. [Figure 9] 13 is a second modified example of the light modulation element shown in FIG. [Figure 10] FIG. 4 is a diagram showing a configuration of an optical modulator according to a second embodiment of the present invention. [Figure 11] 13 is a diagram showing a configuration of an optical modulation module according to a third embodiment of the present invention. FIG. [Figure 12] FIG. 13 is a diagram illustrating a configuration of an optical transmitting device according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [First embodiment] Fig. 1 is a diagram showing the configuration of an optical modulation element 100 which is an optical waveguide element according to a first embodiment of the present invention. Fig. 2 is a partial detailed view of part A shown in Fig. 1, and Fig. 3 is a partial detailed view of part B shown in Fig. 1.

[0014] The optical modulation element 100 is composed of an optical waveguide 104 formed on a substrate 102. The substrate 102 is, for example, an X-cut LN substrate having an electro-optic effect that has been processed and thinned to a thickness of 20 μm or less (e.g., 2 μm). The optical waveguide 104 is a convex optical waveguide (e.g., a rib-type optical waveguide or a ridge-type optical waveguide) that is composed of a convex portion extending in a strip shape formed on the surface of the thinned substrate 102.

[0015] The convex portion extending on the substrate 102 and constituting the optical waveguide 104 is covered with a dielectric layer 160 having a refractive index greater than 1. The dielectric layer 160 can be formed of an inorganic material such as SiO2 by a sputtering method or a CVD method, but in particular, in the optical modulation element 100 according to this embodiment, the dielectric layer 160 is made of a resin. The resin constituting the dielectric layer 160 can be, for example, a photoresist containing a coupling agent (crosslinking agent), and can be a so-called photosensitive permanent film that hardens as a crosslinking reaction progresses due to heat. However, the dielectric layer 160 is not limited to a photosensitive permanent film, and can be any resin having a predetermined refractive index, such as a polyamide resin, a melamine resin, a phenol resin, an amino resin, or an epoxy resin.

[0016] The substrate 102 is, for example, rectangular and has two opposing left and right sides 140a, 140b extending in the vertical direction in the figure, and upper and lower sides 140c, 140d extending in the horizontal direction in the figure.

[0017] An input light (arrow pointing to the right in the figure) that is incident on the input waveguide 106 of the optical waveguide 104 at the lower side of the left side 140a of the substrate 102 in the figure is turned back by 180 degrees, and then split into two lights, which are QPSK modulated by two nested Mach-Zehnder optical waveguides 108a and 108b, respectively. The two QPSK modulated lights are output from the upper side of the left side 140a of the substrate 102 in the figure via output waveguides 126a and 126b on the left in the figure, respectively (two arrows pointing to the left in the figure).

[0018] After these two output lights are emitted from the substrate 102, they are combined into one optical beam by, for example, a polarization combiner, and then sent to a transmission optical fiber as a DP-QPSK modulated optical signal.

[0019] The nested Mach-Zehnder optical waveguide 108a includes two Mach-Zehnder optical waveguides 110a and 110b, and the nested Mach-Zehnder optical waveguide 108b includes two Mach-Zehnder optical waveguides 110c and 110d.

[0020] The Mach-Zehnder optical waveguides 110a and 110b each have two parallel waveguides 112a, 112b and 112c, 112d, respectively. Also, the Mach-Zehnder optical waveguides 110c and 110d each have two parallel waveguides 112e, 112f and 112g, 112h, respectively.

[0021] For QPSK modulation in the nested Mach-Zehnder optical waveguide 108a, signal electrodes 114-1a and 114-1b, to which high-frequency electrical signals for modulation are input, are disposed between the two parallel waveguides 112a and 112b of the Mach-Zehnder optical waveguide 110a and between the two parallel waveguides 112c and 112d of the Mach-Zehnder optical waveguide 110b (white rectangular portions in the figure).

[0022] For QPSK modulation in the nested Mach-Zehnder optical waveguide 108b, signal electrodes 114-1c and 114-1d to which high-frequency electrical signals for modulation are input are disposed between the two parallel waveguides 112e and 112f of the Mach-Zehnder optical waveguide 110c and between the two parallel waveguides 112g and 112h of the Mach-Zehnder optical waveguide 110d, respectively.

[0023] The signal electrode 114-1a forms a coplanar transmission line together with ground electrodes 114-2a and 114-2b (shown as white outlined portions in the figure) that face each other across the parallel waveguides 112a and 112b, respectively, and the signal electrode 114-1b forms a coplanar transmission line together with ground electrodes 114-2b and 114-2c that face each other across the parallel waveguides 112c and 112d, respectively.

[0024] The signal electrode 114-1c forms a coplanar transmission line together with the ground electrodes 114-2c and 114-2d that face each other across the parallel waveguides 112e and 112f, respectively, and the signal electrode 114-1d forms a coplanar transmission line together with the ground electrodes 114-2d and 114-2e that face each other across the parallel waveguides 112e and 112f, respectively.

[0025] Hereinafter, the nested Mach-Zehnder optical waveguides 108a and 108b are collectively referred to as the nested Mach-Zehnder optical waveguide 108. The Mach-Zehnder optical waveguides 110a, 110b, 110c, 110d, 110e, 110f, 110g, and 110h are collectively referred to as the Mach-Zehnder optical waveguide 110. The parallel waveguides 112a, 112b, 112c, 112d, 112e, 112f, 112g, and 112h are collectively referred to as the parallel waveguide 112. The signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d are collectively referred to as the signal electrode 114-1. The ground electrodes 114-2a, 114-2b, 114-2c, 114-2d, and 114-2e are also collectively referred to as ground electrodes 114-2.

[0026] Moreover, the signal electrode 114-1 and the ground electrode 114-2 are collectively referred to as the working electrodes 114. The signal electrode 114-1 and the ground electrode 114-2, which are the working electrodes 114, control the light waves propagating through the optical waveguide 104. Moreover, the signal electrode 114-1 and the ground electrode 114-2 are two working electrodes 114 that sandwich the parallel waveguide 112 of the optical waveguide 104 within the plane of the substrate 102.

[0027] In this embodiment, each of the signal electrode 114-1 and the ground electrode 114-2, which are the working electrode 114, is a two-stage electrode that is configured to become thicker in a stepped manner with increasing distance from the parallel waveguide 112 that they sandwich (see FIG. 4, which will be described later).

[0028] The right ends of the signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d are connected to the signal wiring electrodes 118-1a, 118-1b, 118-1c, and 118-1d (hatched strip-shaped portions), respectively. The left ends of the signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d are connected to the signal wiring electrodes 118-1e, 118-1f, 118-1g, and 118-1h, respectively.

[0029] The right ends of the ground electrodes 114-2a, 114-2b, 114-2c, and 114-2d in the figure are connected to the ground wiring electrodes 118-2a, 118-2b, 118-2c, and 118-2d, respectively. Also, the left ends of the ground electrodes 114-2a, 114-2b, 114-2c, and 114-2d in the figure are connected to the ground wiring electrodes 118-2f, 118-2g, 118-2h, and 118-2i, respectively.

[0030] The ground electrode 114-2e is connected to a rectangular ground pattern, and the portion of the rectangular ground pattern including the upper edge extending to the right in the figure constitutes the ground wiring electrode 118-2e, and the portion including the left edge extending downward in the figure constitutes the ground wiring electrode 118-2j.

[0031] Thus, the signal wiring electrodes 118-1a, 118-1b, 118-1c, and 118-1d, together with the ground wiring electrodes 118-2a, 118-2b, 118-2c, 118-2d, and 118-2e adjacent to these signal wiring electrodes, form a coplanar transmission line. Similarly, the signal wiring electrodes 118-1e, 118-1f, 118-1g, and 118-1h, together with the ground wiring electrodes 118-2f, 118-2g, 118-2h, 118-2i, and 118-2j adjacent to these signal wiring electrodes, form a coplanar transmission line.

[0032] Signal wiring electrodes 118-1e, 118-1f, 118-1g, and 118-1h extending to the side 140d of the substrate 102 on the lower side in the figure are terminated outside the substrate 102 by a terminating resistor having a predetermined impedance.

[0033] As a result, high-frequency electrical signals input from the signal wiring electrodes 118-1a, 118-1b, 118-1c, and 118-1d extending to the right-hand side 140b of the substrate 102 in the figure become traveling waves and propagate through the signal electrodes 114-1a, 114-1b, 114-1c, and 114-1d, and modulate the optical waves propagating through the Mach-Zehnder optical waveguides 110a, 110b, 110c, and 110d, respectively.

[0034] Hereinafter, the signal wiring electrodes 118-1a, 118-1b, 118-1c, 118-1d, 118-1e, 118-1f, 118-1g, and 118-1h are collectively referred to as signal wiring electrodes 118-1. The ground wiring electrodes 118-2a, 118-2b, 118-2c, 118-2d, 118-2e, 118-2f, 118-2g, 118-2h, 118-2i, and 118-2j are collectively referred to as ground wiring electrodes 118-2. The signal wiring electrodes 118-1 and the ground wiring electrodes 118-2 are collectively referred to as wiring electrodes 118. That is, the signal wiring electrodes 118-1 and the ground wiring electrodes 118-2 are wiring electrodes 118 connected to the working electrode 114.

[0035] 3, bias electrodes 130a, 130b, 130c, 130d for adjusting the bias points of the Mach-Zehnder optical waveguides 110a, 110b, 110c, 110d, and bias electrodes 130e, 130f for adjusting the bias points of the nested Mach-Zehnder optical waveguides 108a, 108b are provided in the B portion of the substrate 102 shown in FIG. 1. The bias electrodes 130a, 130b, 130e are connected to bias wiring electrodes 132a, 132b, 132e that extend to the illustrated upper side 140c of the substrate 102, respectively. The bias electrodes 130c, 130d, 130f are connected to bias wiring electrodes 132c, 132d, 132f that extend to the illustrated lower side 140d of the substrate 102, respectively.

[0036] Hereinafter, the bias electrodes 130a, 130b, 130c, 130d, 130e, and 130f are also collectively referred to as bias electrodes 130. The bias wiring electrodes 132a, 132b, 132c, 132d, 132e, and 132f are also collectively referred to as bias wiring electrodes 132. In this embodiment, the bias electrode 130 is also a two-stage electrode like the above-mentioned working electrode 114 (see FIG. 6 described later).

[0037] As described above, the optical modulation element 100 includes the substrate 102, the optical waveguide 104 formed on the substrate 102, two electrodes disposed at positions sandwiching a part of the parallel waveguide 112 included in the optical waveguide 104 from both sides within the plane of the substrate 102, and a dielectric layer 160 covering the optical waveguide 104. Here, the two electrodes refer to, for example, the working electrode 114 and / or the bias electrode 130 sandwiching the parallel waveguide 112.

[0038] In particular, in the optical modulation element 100 of this embodiment, the dielectric layer 160 extends in the width direction of the parallel waveguide 112 to a range including the edges of the two working electrodes 114 and the bias electrode 130 facing the parallel waveguide 112, and is arranged so as to partially cover each of the two working electrodes 114 and the bias electrode 130.

[0039] 4 is a cross-sectional view taken along the line IV-IV in FIG. 2, showing the arrangement of the ground electrode 114-2e, the signal electrode 114-1d, and the ground electrode 114-2d sandwiching the parallel waveguides 112h and 112g. The back surface (the surface on the lower side in the figure) of the substrate 102 is supported and reinforced by a support plate 142. The support plate 142 is made of, for example, glass. On the upper surface of the substrate 102 in the figure, the parallel waveguides 112g and 112h are configured as convex optical waveguides by convex portions 150a and 150b formed on the substrate 102, respectively.

[0040] A ground electrode 114-2d and a signal electrode 114-1d are formed on the substrate 102 at positions on either side of the parallel waveguide 112g within the plane of the substrate 102. A signal electrode 114-1d and a ground electrode 114-2e are also formed on either side of the parallel waveguide 112h within the plane of the substrate 102.

[0041] A dielectric layer 160 that covers the parallel waveguide 112g is disposed between the ground electrode 114-2d and the signal electrode 114-1d. Similarly, a dielectric layer 160 that covers the parallel waveguide 112h is disposed between the signal electrode 114-1d and the ground electrode 114-2e.

[0042] The dielectric layer 160 between the ground electrode 114-2d and the signal electrode 114-1d extends to the left and right in the width direction (left and right direction in the figure) of the parallel waveguide 112g to a range that includes the edges 144a and 144b of the ground electrode 114-2d and the signal electrode 114-1d that face the parallel waveguide 112g, and is arranged so as to partially cover each of the ground electrode 114-2d and the signal electrode 114-1d.

[0043] In addition, the dielectric layer 160 between the signal electrode 114-1d and the ground electrode 114-2e extends to the left and right in the width direction (left and right direction in the figure) of the parallel waveguide 112h to a range that includes the edges 144c and 144d of the signal electrode 114-1d and the ground electrode 114-2e that face the parallel waveguide 112h, and is arranged so as to partially cover each of the signal electrode 114-1d and the ground electrode 114-2e.

[0044] Here, the ground electrode 114-2d, the signal electrode 114-1d, and the ground electrode 114-2e are respectively composed of first-stage electrodes 146a, 146b, and 146c and second-stage electrodes 148a, 148b, and 148c. The above-mentioned edges 144a, 144b, 144c, and 144d are edges of the first-stage electrodes 146a, 146b, and 146c of the corresponding ground electrode 114-2d, the signal electrode 114-1d, and the ground electrode 114-2e, respectively.

[0045] In particular, in this embodiment, the dielectric layer 160 covering the parallel waveguide 112g is arranged to partially cover the first-stage electrode 146a of the ground electrode 114-2d and the first-stage electrode 146b of the signal electrode 114-1d. Similarly, the dielectric layer 160 covering the parallel waveguide 112h is arranged to partially cover the first-stage electrode 146b of the signal electrode 114-1d and the first-stage electrode 146c of the ground electrode 114-2e.

[0046] As a result, for example, on the left and right sides of the dielectric layer 160 covering the parallel waveguide 112h, gaps of widths W16 and W17 are formed between the ground electrode 114-2e and the second-stage electrode 148c and between the signal electrode 114-1d and the second-stage electrode 148b on the left and right sides of the dielectric layer 160 covering the parallel waveguide 112h. As a result, for example, between the signal electrode 114-1d and the ground electrode 114-2e, the electric field is concentrated in the dielectric layer 160 between the first-stage electrodes 146b and 146c compared to the space between the second-stage electrodes 148b and 148c, and the electric field efficiency in the parallel waveguide 112h covered by the dielectric layer 160 is improved.

[0047] The dielectric layer 160 is configured in the same manner as above in the other parallel waveguides 112a, 112b, 112c, 112d, 112e, and 112f. Here, the edges of the two working electrodes 114 sandwiching the parallel waveguide 112 and facing the parallel waveguide 112 are collectively referred to as edges 144. Also, the first-stage electrode and the second-stage electrode of the working electrode 114 are collectively referred to as first-stage electrode 146 and second-stage electrode 148, respectively.

[0048] That is, the dielectric layer 160 extends in the width direction of the parallel waveguide 112 to an extent including the two working electrodes 114 and the edge 144 of the bias electrode 130 facing the parallel waveguide 112, and is arranged so as to partially cover each of the first stage electrodes 146 of the two working electrodes 114.

[0049] Here, the distance between the opposing edges 144 of the two working electrodes 114 sandwiching the parallel waveguide 112 (for example, the distance W12 between the edges 144c and 144d in FIG. 4) is, for example, less than 10 μm.

[0050] In this embodiment, the wiring electrode 118 is formed by extending the second stage portion (i.e., the thicker portion) of the working electrode 114, which is a two-stage electrode. Fig. 5 is a cross-sectional view taken along the line VV in Fig. 2, showing a connection state between the signal electrode 114-1d and the signal wiring electrode 118-1d as an example of a connection state between the working electrode 114 and the wiring electrode 118. The signal electrode 114-1d is composed of a first stage electrode 146a and a second stage electrode 148a, and the second stage electrode 148a of the signal electrode 114-1d extends to the right in the figure to form the signal wiring electrode 118-1d.

[0051] 3, the dielectric layer 160 is disposed in the same manner as in the working electrode 114 described above. That is, two adjacent bias electrodes 130 are configured to sandwich the parallel waveguide 112 within the plane of the substrate 102. The dielectric layer 160 covering the parallel waveguide 112 extends in the width direction of the parallel waveguide 112 to a range including the edges of the two bias electrodes 130 facing the parallel waveguide 112, and is disposed to partially cover each of the two bias electrodes 130.

[0052] 6 is a cross-sectional view taken along the line VI-VI in FIG. 3, showing the arrangement of the dielectric layer 160 in the bias electrode 130a as an example of the arrangement of the dielectric layer 160 in the bias electrode 130. The bias electrodes 130a are formed on the substrate 102 so as to sandwich the parallel waveguides 112a and 112b in the plane of the substrate 102. The dielectric layer 160 covering the parallel waveguide 112a extends to a range including the edges 152a and 152b of the two bias electrodes 130a facing the parallel waveguide 112a on both sides of the width direction (left and right direction in the figure) of the parallel waveguide 112a, and is arranged so as to partially cover each of the bias electrodes 130a. In addition, in FIG. 6, bias wiring electrodes 132a are connected to the bias electrodes 130a on the left and right sides in the figure.

[0053] The dielectric layer 160 is configured in the same manner as described above for the other bias electrodes 130b, 130c, 130d, 130e, and 130f. Here, the edges of the two bias electrodes 130 sandwiching the parallel waveguide 112 facing the parallel waveguide 112 are collectively referred to as the edge 152. That is, the dielectric layer 160 is arranged so as to extend in the width direction of the parallel waveguide 112 to a range including the edges 152 of the two bias electrodes 130 facing the parallel waveguide 112, and to partially cover each of the two bias electrodes 130.

[0054] In the optical modulation element 100 having the above configuration, the convex portion extending on the substrate 102 and constituting the optical waveguide 104 is covered with the dielectric layer 160 which is a resin having a refractive index greater than 1. As a result, in the optical modulation element 100, the refractive index difference between the convex portion and the environment surrounding the convex portion is smaller than in a state in which the convex portion is in contact with air, and scattering loss caused by surface roughness on the side surface of the convex portion generated in the process of forming the convex portion is reduced.

[0055] The dielectric layer 160 can be formed, for example, by coating (spin-coating) a photosensitive permanent film, such as a photoresist, on the substrate 102 with a spinner, and patterning the film using ultraviolet light.

[0056] In the light modulation element 100, the dielectric layer 160 extends to a range including the opposing edges of the two working electrodes 114 sandwiching the parallel waveguide 112, and is disposed so as to partially cover each of these electrodes. Therefore, even if the formation position of the dielectric layer 160 in the plane of the substrate 102 is shifted in the width direction of the parallel waveguide 112 during the manufacturing process (for example, the above-mentioned patterning process), as long as the amount of shift is within a predetermined error range, the dielectric layer 160 can be kept filled without gaps between the two working electrodes 114 sandwiching the parallel waveguide 112. Therefore, in the light modulation element 100, the manufacturing variation of the electrostatic capacitance between the two working electrodes 114 sandwiching the parallel waveguide 112 can be reduced, and the electric field applied from the two working electrodes 114 to the parallel waveguide 112 can be kept high (i.e., the electric field efficiency can be kept high). The same is true for the bias electrode 130.

[0057] As a result, in the optical modulation element 100, the electric field efficiency in the optical waveguide 104 (specifically, the portion of the parallel waveguide 112) can be maintained high, while effectively preventing scattering loss of guided light caused by surface roughness in the convex optical waveguide.

[0058] In addition, the above-mentioned predetermined error range is within the smaller of W14 and W15 when the design values ​​of the jumping-over dimensions of the dielectric layer 160 covering the parallel waveguide 112h onto the left and right working electrodes 114 (ground electrode 114-2e and signal electrode 114-1d) in Fig. 4 are W14 and W15. In addition, when the center of the width W10 of the upper surface of the dielectric layer 160 and the center of the distance W12 between the left and right working electrodes 114 coincide with each other in design, the above-mentioned error range is (W10-W12) / 2.

[0059] In addition, in an optical modulation element in which a microwave electric signal of several GHz is used as a high-frequency electric signal for modulation, the working electrodes 114 sandwiching the parallel waveguide 112 are generally formed thick, with a height of about 1 μm from the surface of the substrate 102. Therefore, the depth of the recess formed between these two working electrodes 114 is also deep, with a depth of 1 μm. Filling such a deep recess with an inorganic dielectric such as silicon dioxide (SiO2) is technically difficult. Even if such a thick film can be formed, the stress accumulated inside the inorganic dielectric layer during the film formation process will be applied to the substrate 102 etc. after manufacture, which may adversely affect the electrical and / or optical characteristics and long-term reliability of the optical modulation element.

[0060] As described above, in the light modulation element 100, the dielectric layer 160 is made of resin, so that a layer as thick as about 10 μm can be easily formed. In addition, since the Young's modulus of a resin is generally smaller than that of an inorganic material, the stress applied from the dielectric layer 160 to the substrate 102 and the like after manufacturing is significantly reduced compared to the case where an inorganic dielectric layer is used.

[0061] In addition, the viscosity of the resin such as the photosensitive permanent film used for the dielectric layer 160 can be adjusted by adjusting the composition. The thickness of the resin dielectric layer 160 formed on the substrate 102 during spin coating can be adjusted by adjusting the viscosity in addition to adjusting the rotation speed of the spinner during spin coating. Therefore, in the optical modulation element 100, the thickness of the dielectric layer 160 formed between the two working electrodes 114 sandwiching the parallel waveguide 112 can be adjusted to a desired value of the capacitance between these two working electrodes 114, so that the impedance between these two working electrodes 114 and the speed matching between the light wave and the high-frequency electrical signal can be adjusted.

[0062] Moreover, the two working electrodes 114 sandwiching the parallel waveguide 112 are two-stage electrodes. Such a multi-stage configuration of the working electrodes 114 is generally used to broaden the frequency characteristics of the transmission line formed by these working electrodes 114 while maintaining the intensity of the electric field generated in the parallel waveguide 112 high. In particular, in this embodiment, the dielectric layer 160 covering the parallel waveguide 112 is arranged so as to extend to the edge of the first-stage electrode closest to the parallel waveguide 112 and partially cover the first-stage electrode. Therefore, the density of the electric field lines generated between the working electrodes 114 is higher in the dielectric layer 160 formed between the first-stage electrodes than in the space between the second-stage electrodes. As a result, in this embodiment, the electric field is concentrated in the parallel waveguide 112 covered by the dielectric layer 160 between the first-stage electrodes, improving the electric field efficiency.

[0063] As described above, the dielectric layer 160 covering the parallel waveguide 112 reduces the difference in refractive index between the convex portion on the substrate 102, which is the optical waveguide portion (so-called core), and the dielectric layer 160, which functions as a cladding, thereby reducing the scattering loss on the surface of the convex portion. Fig. 7 is a diagram showing the scattering loss reduction effect with respect to the refractive index of the material constituting the dielectric layer 160. In Fig. 7, the horizontal axis represents the refractive index n core The refractive index n of the dielectric layer 160 clad Ratio of refractive index (n) clad / n core The vertical axis represents the optical loss (dB / cm) in the parallel waveguide 112.

[0064] 7, line 200 indicates the scattering loss in the parallel waveguide 112. Further, line 202 and line 204 are shown in Fig. 7. Line 202 indicates the waveguide loss depending on the optical confinement effect in the parallel waveguide 112. Furthermore, line 204 indicates the total loss including the scattering loss indicated by line 200 and the waveguide loss indicated by line 202.

[0065] From line 200, the refractive index ratio n clad / n core The closer to 1, that is, the refractive index ncore and the refractive index n of the dielectric layer 160 clad It can be seen that the smaller the difference in refractive index between the parallel waveguide 112 and the parallel waveguide 112, the more the scattering loss in the parallel waveguide 112 is reduced.

[0066] On the other hand, looking at line 202, the refractive index ratio n clad / n core It can be seen that the waveguide loss of the parallel waveguide 112 increases as the refractive index ratio n approaches 1. This increase in the waveguide loss is due to the decrease in the optical confinement effect in the parallel waveguide 112 as the refractive index difference decreases. clad / n core As a result, the total loss of the parallel waveguide 112 increases rapidly as the refractive index ratio n clad / n core The refractive index ratio n clad / n core = 0.65, it shows a minimum value (about 2 dB / cm).

[0067] From the line 204, if the allowable increase in optical loss from the minimum value is 0.5 dB, the refractive index ratio n clad / n core It can be seen that the refractive index n of the dielectric layer 160 is preferably 0.5 or more and 0.75 or less. clad is the refractive index n of the convex portion constituting the parallel waveguide 112. core It is desirable that the value be 0.5 or more and 0.75 or less.

[0068] Next, a modification of the light modulation element 100 will be described. [First Modification] FIG. 8 is a diagram showing the configuration of a light modulation element 100-1 according to a first modified example of the light modulation element 100, and corresponds to FIG. 4 in the above-described embodiment. The dielectric layer 160-1 shown in FIG. 8 is used in place of the dielectric layer 160 in the light modulation element 100 shown in FIGS. 1 to 6. In FIG. 8, the same components as those shown in FIG. 4 are designated by the same reference numerals as those in FIG. 4, and the above-described explanation of FIG. 4 is incorporated herein. In addition, the planar configuration of the light modulation element 100-1 using the dielectric layer 160-1 is similar to that of the light modulation element 100 shown in FIGS. 1 to 3, and therefore the above-described explanation of FIGS. 1 to 3 is incorporated herein.

[0069] 8, the dielectric layer 160-1 has a similar structure to the dielectric layer 160 shown in Fig. 4, but is different in that its upper surface is not flat but has projections and recesses. Specifically, the height of the dielectric layer 160-1 covering the parallel waveguide 112g between the edges 144a and 144b of the ground electrode 114-2d and the signal electrode 114-1d adjacent to the parallel waveguide 112g is lower than the height of the portion completely covering the ground electrode 114-2d and the signal electrode 114-1d.

[0070] The dielectric layer 160-1 covering the other parallel waveguide 112 may be configured in a similar manner to the above. That is, the height of the dielectric layer 160-1 covering the parallel waveguide 112 between the signal electrode 114-1 and the ground electrode 114-2 that sandwich the parallel waveguide 112 is lower than the height of the portion covering these electrodes.

[0071] Such a shape of the top surface of the dielectric layer 160-1 can be realized, for example, by adjusting the viscosity of the resin constituting the dielectric layer 160-1 during spin coating depending on the composition of the resin.

[0072] Generally, resins such as photosensitive permanent films have viscosity before curing, so even if there are irregularities on the substrate 102 due to the protrusions and electrodes that constitute the optical waveguide 104, the surface of the resin after spin coating is almost flat. Furthermore, when high-temperature treatment for drying or the like is performed in the patterning process of the spin-coated resin, the surface can be further flattened by high-temperature softening of the resin depending on the treatment temperature. Therefore, generally, the top surface of the dielectric layer 160 between the electrodes sandwiching the parallel waveguide 112 is almost flat as shown in FIG. 4.

[0073] On the other hand, in the configuration shown in FIG. 8, as described above, the height of the dielectric layer 160-1 between the edge 144a and the edge 144b of the ground electrode 114-2d and the signal electrode 114-1d adjacent to the parallel waveguide 112g is lower than the height of the part covering these ground electrode 114-2d and the signal electrode 114-1d. That is, the thickness of the part covering the parallel waveguide 112 of the dielectric layer 160-1 is configured to be thinner than the thickness of the parallel waveguide 112 covering the dielectric layer 160 in the configuration shown in FIG. 4. The configuration shown in FIG. 8 can be formed by adjusting the viscosity of the resin before hardening used in the configuration shown in FIG. 4. Furthermore, the dielectric layer 160 can be formed by using a sputtering method or a CVD method, since it is affected by the unevenness on the substrate 102 due to the convex parts and electrodes constituting the optical waveguide 104.

[0074] As a result, in this modified example, the electric field lines between the signal electrode 114-1 d and the ground electrode 114-2 d pass through the thin dielectric layer 160-1 in a concentrated manner, and the electric field applied to the parallel waveguide 112 is further strengthened compared to the configuration in FIG. 4, thereby further improving the electric field efficiency.

[0075] [Second modified example] Fig. 9 is a diagram showing the configuration of an optical modulation element 100-2 according to a second modified example of the optical modulation element 100, and corresponds to Fig. 4 in the above-mentioned embodiment. The signal electrode 114-3d and the ground electrodes 114-4d and 114-4e shown in Fig. 9 are used in place of the signal electrode 114-1d and the ground electrodes 114-2d and 114-2e in the optical modulation element 100 shown in Figs. 1 to 4. In Fig. 9, the same components as those shown in Fig. 4 are designated by the same reference numerals as those in Fig. 4, and the above description of Fig. 4 is incorporated herein.

[0076] In addition, the planar configuration of the light modulation element 100-2 in which the signal electrode 114-3d and the ground electrodes 114-4d and 114-4e shown in Figure 9 are used is similar to that of the light modulation element 100 shown in Figures 1 to 3, so the explanation of Figures 1 to 3 above is cited.

[0077] The signal electrode 114-1 and the ground electrode 114-2, which are the working electrode 114 shown in Fig. 4 and Fig. 8, are configured as two-stage electrodes as an example, but these working electrodes may be configured as one stage. The signal electrode 114-3d and the ground electrodes 114-4d and 114-4e shown in Fig. 9 are configured as such one-stage electrodes. The dielectric layer 160 covering the parallel waveguide 112g is arranged to extend to a range including the edges of the ground electrode 114-4d and the signal electrode 114-3d sandwiching the parallel waveguide 112g in the width direction (left-right direction in the figure) of the parallel waveguide 112g, as in the configuration shown in Fig. 4, and to partially cover each of the ground electrode 114-4d and the signal electrode 114-3d.

[0078] [Second embodiment] Next, a second embodiment of the present invention will be described. This embodiment is an optical modulator using any one of the optical modulation elements described above. FIG. 10 is a diagram showing the configuration of an optical modulator 400 according to the second embodiment. The optical modulator 400 has a housing 402, an optical modulation element 404 housed in the housing 402, and a relay substrate 406. The optical modulation element 404 is any one of the optical modulation elements 100, 100-1, and 100-2 described above. A plate-shaped cover (not shown) is finally fixed to the opening of the housing 402, and the inside is hermetically sealed.

[0079] The optical modulator 400 also has a signal pin 408 for inputting a high frequency electrical signal used to modulate the optical modulation element 404 , and a signal pin 410 for inputting an electrical signal used to adjust the operating point of the optical modulation element 404 .

[0080] Furthermore, the optical modulator 400 has an input optical fiber 414 for inputting light into the housing 402, and an output optical fiber 420 for guiding the light modulated by the optical modulation element 404 to the outside of the housing 402, on the same surface of the housing 402 (the left surface in the figure in this embodiment).

[0081] Here, the input optical fiber 414 and the output optical fiber 420 are fixed to the housing 402 via supports 422 and 424, which are fixing members. The light input from the input optical fiber 414 is collimated by a lens 430 arranged in the support 422, and then input to the light modulation element 404 via a lens 434. However, this is just one example, and the input of light to the light modulation element 404 can also be performed according to the conventional technology, for example, by introducing the input optical fiber 414 into the housing 402 via the support 422 and connecting the end face of the introduced input optical fiber 414 to the end face of the substrate 102 of the light modulation element 404.

[0082] The light output from the light modulation element 404 is coupled to an output optical fiber 420 via an optical unit 416 and a lens 418 disposed on a support 424. The optical unit 416 may include a polarization combiner that combines the two modulated lights output from the light modulation element 404 into one beam.

[0083] The relay board 406 relays a high-frequency electric signal input from a signal pin 408 and an electric signal for adjusting an operating point (bias point) and the like input from a signal pin 410 to the optical modulation element 404 by a conductor pattern (not shown) formed on the relay board 406. The conductor patterns on the relay board 406 are connected to pads constituting one ends of electrodes of the optical modulation element 404, for example, by wire bonding. The optical modulator 400 also includes a terminator 412 having a predetermined impedance in the housing 402.

[0084] The optical modulator 400 having the above-mentioned configuration is constructed using an optical modulation element 404 which is any one of the optical modulation elements 100, 100-1, and 100-2 having a configuration capable of effectively preventing scattering loss of guided light caused by surface roughness in the convex optical waveguide, and therefore can provide modulation operation with little optical loss and low driving voltage.

[0085] [Third embodiment] Next, a third embodiment of the present invention will be described. This embodiment is an optical modulation module 500 using an optical modulation element according to any of the above-mentioned embodiments or modifications. Fig. 11 is a diagram showing the configuration of the optical modulation module 500 according to this embodiment. In Fig. 11, the same components as those in the optical modulator 400 according to the second embodiment shown in Fig. 10 are indicated by the same reference numerals as those shown in Fig. 10, and the above description of Fig. 10 is used.

[0086] 8, the optical modulation module 500 differs from the optical modulator 400 in that it includes a circuit board 506 instead of the relay board 406. The circuit board 506 includes a driving circuit 508. The driving circuit 508 generates a high-frequency electrical signal for driving the optical modulation element 404 based on, for example, a modulation signal supplied from the outside via a signal pin 408, and outputs the generated high-frequency electrical signal to the optical modulation element 404.

[0087] The optical modulation module 500 having the above configuration is configured using an optical modulation element 404 which is any one of the optical modulation elements 100, 100-1, and 100-2 having a configuration capable of effectively preventing scattering loss of guided light due to surface roughness in the convex optical waveguide, and therefore can provide modulation operation with low optical loss and low drive voltage, similar to the optical modulator 400 according to the second embodiment described above.

[0088] [Fourth embodiment] Next, a fourth embodiment of the present invention will be described. This embodiment is an optical transmission device 600 equipped with the optical modulator 400 according to the second embodiment. Fig. 12 is a diagram showing the configuration of the optical transmission device 600 according to this embodiment. This optical transmission device 600 has the optical modulator 400, a light source 604 that inputs light to the optical modulator 400, a modulator driving unit 606, and a modulation signal generating unit 608. Note that the optical modulation module 500 described above can also be used instead of the optical modulator 400 and the modulator driving unit 606.

[0089] The modulation signal generating unit 608 is an electronic circuit that generates an electrical signal for causing the optical modulator 400 to perform a modulation operation. Based on transmission data provided from outside, the modulation signal generating unit 608 generates a modulation signal, which is a high-frequency signal for causing the optical modulator 400 to perform an optical modulation operation in accordance with the modulation data, and outputs the modulation signal to the modulator driving unit 606.

[0090] The modulator driving unit 606 amplifies the modulation signal input from the modulation signal generating unit 608, and outputs a high-frequency electric signal for driving signal electrodes of the optical modulation element 100 and the like included in the optical modulator 400. As described above, instead of the optical modulator 400 and the modulator driving unit 606, it is also possible to use an optical modulation module 500 including, for example, a driving circuit 508 including a circuit equivalent to the modulator driving unit 606 inside the housing 402.

[0091] The high-frequency electrical signal is input to a signal pin 408 of the optical modulator 400 to drive the optical modulation element 100, etc. As a result, the light output from the light source 604 is modulated by the optical modulator 400, and is output from the optical transmitter 600 as modulated light.

[0092] The optical transmitting device 600 having the above-mentioned configuration is configured using the optical modulation element 404, which is any one of the optical modulation elements 100, 100-1, and 100-2 having a configuration that can effectively prevent scattering loss of guided light due to surface roughness in the convex optical waveguide, and therefore can achieve good optical transmission through modulation operation with low optical loss and low driving voltage similar to the optical modulator 400 of the second embodiment and the optical modulation module 500 of the third embodiment described above.

[0093] The present invention is not limited to the configurations of the above-described embodiment and alternative configurations thereof, and can be embodied in various forms without departing from the spirit and scope of the present invention.

[0094] For example, although the optical waveguide 104 is a convex waveguide in the above-described embodiment, it may be a planar waveguide, such as a Ti-diffused waveguide, formed on the surface of the substrate 102. In this case as well, the scattering loss associated with the roughness of the surface of the substrate 102 can be reduced by the dielectric layer 160.

[0095] Furthermore, the working electrode 114 of the light modulation element 100 is configured with two steps in the above embodiment, but may be configured in a stepped shape with three or more steps.

[0096] 4, the dielectric layer 160 is arranged to partially cover the first stage electrodes 146a, 146b, and 146c of the signal electrode 114-1d and the ground electrodes 114-2d and 114-2e, but the range that the dielectric layer 160 covers is not limited to the first stage electrodes 146. The dielectric layer 160 may extend to the second stage electrodes 148 of the working electrodes 114 (or, in the case where these electrodes are configured in three or more stages, extend to the step portions of the second or more stages) as long as it extends to the edge 144 of the working electrodes 114 that faces (or is adjacent to) the parallel waveguide 112.

[0097] However, from the viewpoint of concentrating the electric field in the parallel waveguide 112, it is preferable that the dielectric layer 160 only partially covers the first-stage electrode 146 of the working electrode 114, and that there is a gap (for example, gaps W16 and W17 in FIG. 4) between the first-stage electrode 146 and the second-stage electrode 148.

[0098] In the optical modulation element 100, the dielectric layer 160 covering the parallel waveguide 112 of not only the working electrode 114 but also the bias electrode 130 extends to the edges 152 of the two bias electrodes 130 sandwiching the parallel waveguide 112 to partially cover the bias electrode 130. However, such a configuration of the dielectric layer 160 may be realized for either the working electrode 114 or the bias electrode 130. This makes it possible to effectively prevent scattering loss of guided light caused by surface roughness in the optical waveguide while maintaining high electric field efficiency in the working electrode 114 or the bias electrode 130.

[0099] The present invention is not limited to the configurations of the above-described embodiments, and can be embodied in various forms without departing from the spirit and scope of the present invention.

[0100] For example, in the optical modulation element 100 shown in FIG. 1, there are portions of the substrate 102 where no electrodes or optical waveguides 104 are formed, and all or part of such portions may be covered with a ground pattern in accordance with conventional technology.

[0101] As described above, the optical modulation element 100, which is an optical waveguide element according to the embodiment described above, includes the substrate 102 and the optical waveguide 104 formed on the substrate 102. Here, the optical waveguide 104 includes, for example, the parallel waveguide 112 as a part thereof. The optical modulation element 100 includes two electrodes (two working electrodes 114 or two bias electrodes 130) arranged at positions sandwiching the parallel waveguide 112 from both sides within the plane of the substrate 102, and a dielectric layer 160 covering the optical waveguide. In the optical modulation element 100, the dielectric layer 160 is configured to extend in the width direction of the parallel waveguide 112 to a range including the edges of each of the two electrodes facing the parallel waveguide 112, and to partially cover each of the two electrodes.

[0102] According to this configuration, even if the formation position of the dielectric layer 160 covering the optical waveguide 104 is shifted, the dielectric layer 160 can be formed without any gap between the two electrodes (i.e., the two working electrodes 114 or the two bias electrodes 130) sandwiching the parallel waveguide 112. Therefore, according to the above configuration, it is possible to effectively prevent scattering loss of guided light caused by the surface roughness of the optical waveguide 104 while maintaining high electric field efficiency.

[0103] Moreover, in the optical modulation element 100, the optical waveguide 104 is a convex optical waveguide formed by a convex portion extending on the substrate 102. With this configuration, in a convex optical waveguide in which roughness is likely to occur on the side surface, it is possible to effectively prevent scattering loss of guided light caused by the surface roughness of the optical waveguide 104 while maintaining high electric field efficiency.

[0104] Moreover, the refractive index of the dielectric layer 160 is 0.5 to 0.75 times the refractive index of the core portion, through which light propagates, of the optical waveguide 104. With this configuration, the trade-off between the reduction in scattering loss and the increase in waveguide loss in the optical waveguide 104 can be balanced, and the optical propagation loss of the optical waveguide 104 can be kept low.

[0105] Moreover, the dielectric layer 160 is, for example, a resin. According to this configuration, the dielectric layer 160 can be formed on the optical waveguide 104 to a thickness of about 10 μm, so that the dielectric layer 160 can be easily formed, for example, between two electrodes sandwiching the parallel waveguide 112, which is a part of the optical waveguide 104.

[0106] Furthermore, the height of the dielectric layer 160 between the two electrodes sandwiching the parallel waveguide 112, which is the optical waveguide 104, is lower than the height of the portion covering these electrodes. With this configuration, the electric field generated by the two electrodes can be concentrated in the optical waveguide 104 covered with the dielectric layer 160, so that the electric field efficiency can be increased and the operating voltage of the optical modulation element 100 can be reduced.

[0107] The two working electrodes 114 sandwiching the parallel waveguide 112, which is the optical waveguide 104, are configured as, for example, two-stage electrodes so that the thickness increases stepwise with increasing distance from the parallel waveguide 112. The dielectric layer 160 extends in the width direction of the parallel waveguide 112 to a range including the edge of the first-stage electrode 146 of each of the two working electrodes that is closest to the parallel waveguide 112, and is disposed so as to partially cover each of the two electrodes. With this configuration, the electric field is concentrated on the parallel waveguide 112 covered by the dielectric layer 160 between the first-stage electrodes 146, improving the electric field efficiency, and as a result, the operating voltage of the optical modulation element 100 is reduced.

[0108] Moreover, the dielectric layer 160 is arranged so as to partially cover each of the first-stage electrodes 146 of the two working electrodes 114 sandwiching the parallel waveguide 112. With this configuration, the electric field concentration on the parallel waveguide 112 can be further increased, and the operating voltage of the optical modulation element 100 can be further reduced.

[0109] Moreover, the distance between the edges 144 of the two working electrodes 114 sandwiching the parallel waveguide 112 is less than 10 μm. With this configuration, in a configuration in which the working electrodes 114 are arranged at intervals of less than 10 μm, the dielectric layer 160 can be appropriately arranged without misalignment on the parallel waveguide 112 arranged between the adjacent working electrodes 114. As a result, the scattering loss in the parallel waveguide 112 can be reduced, and a low-loss optical modulation element 100 can be realized.

[0110] Moreover, the optical waveguide 104 constitutes two parallel waveguides 112 of the Mach-Zehnder optical waveguide 110. With this configuration, in the Mach-Zehnder optical waveguide 110 in which the difference in optical loss in the two parallel waveguides 112 is likely to affect the optical characteristics, it is possible to reduce the scattering loss caused by the surface roughness in both of these parallel waveguides 112 and realize good optical characteristics.

[0111] Moreover, the optical modulator 400 according to the second embodiment described above includes an optical modulation element 404 which is the optical modulation element 100, 100-1, or 100-2 that modulates light, a housing 402 that houses the optical modulation element 404, an input optical fiber 414 that inputs light to the optical modulation element 404, and an output optical fiber 420 that guides the light output by the optical modulation element 100 to the outside of the housing 402.

[0112] In addition, the optical modulation module 500 according to the third embodiment described above includes an optical modulation element 404, a housing 402 that houses the optical modulation element 404, an input optical fiber 414 that inputs light to the optical modulation element 404, an output optical fiber 420 that guides the light output by the optical modulation element 404 to the outside of the housing 402, and a driving circuit 508 that drives the optical modulation element.

[0113] In addition, the optical transmitting device 600 according to the fourth embodiment described above includes the optical modulator 400 according to the second embodiment or the optical modulation module 500 according to the third embodiment, and a modulation signal generating unit 608 which is an electronic circuit that generates an electrical signal for causing the optical modulation element 404 to perform a modulation operation.

[0114] According to these configurations, it is possible to realize the optical modulator 400, the optical modulation module 500, or the optical transmission device 600 that consumes little power and has low loss. [Explanation of symbols]

[0115] 100, 100-1, 100-2, 404... Optical modulation element, 102... Substrate, 104... Optical waveguide, 106... Input waveguide, 108a, 108b... Nested Mach-Zehnder optical waveguide, 110, 110a, 110b, 110c, 110d... Mach-Zehnder optical waveguide Waveguide, 112, 112a, 112b, 112c, 112d, 112e, 112f, 112g, 112h...Parallel waveguide, 114...Working electrode, 114-1, 114-1a, 114-1b, 114-1c, 114-1d, 114-3d...Signal electrode, 114-2, 114-2a, 114-2b, 114-2c, 114-2d, 114-2e, 114-4d, 114-4e...Ground electrode, 118...Wiring electrode, 118-1, 118-1a, 118-1b, 118-1c, 118-1d, 118-1e, 118-1f, 118-1g, 118-1h...Signal wiring electrode, 118-2, 118-2a, 118-2b, 118-2c, 118-2d, 118-2e, 118-2f, 118-2g, 118-2h, 118-2i, 118-2j...Ground wiring electrode, 126a, 126 b...output waveguide, 130, 130a, 130b, 130c, 130d, 130e, 130f...bias electrode, 132, 132a, 132b, 132c, 132d, 132e, 132f...bias wiring electrode, 140a, 140b, 140c, 140d...side, 142...support plate, 144, 144a, 144b, 144c, 144, 152, 152a, 152b, 152c, 152d...edge, 146, 146a, 146b, 146c...first stage electrode, 148, 148a, 148b, 148c...second stage electrode, 15 0, 150a, 150b...convex portion, 160, 160-1...dielectric layer, 200, 202, 204...line, 400...optical modulator, 402...housing, 406...relay board, 408, 410...signal pin, 412...terminator, 414...input optical fiber, 416...optical unit, 418, 430, 434...lens, 420...output optical fiber, 422, 424...support, 500...optical modulation module, 506...circuit board, 508...driver circuit, 600...optical transmitting device, 604...light source, 606...modulator driver, 608...modulation signal generator.

Claims

1. A substrate; an optical waveguide formed on the substrate; two electrodes disposed on both sides of the optical waveguide within the plane of the substrate to control a light wave propagating through the optical waveguide; a dielectric layer covering the optical waveguide; having the optical waveguide is a convex optical waveguide formed by a convex portion extending on the substrate, the dielectric layer is a photosensitive permanent film having a refractive index greater than 1, and is disposed so as to extend in a width direction of the optical waveguide to a range including edges of the two electrodes facing the optical waveguide, and to partially cover each of the two electrodes; Optical waveguide element.

2. The refractive index of the dielectric layer is 0.5 to 0.75 times the refractive index of a core portion of the optical waveguide through which light propagates.

2. The optical waveguide element according to claim 1.

3. The height of the dielectric layer between the two electrodes is lower than the height of a portion covering the electrodes.

3. The optical waveguide element according to claim 1 or 2.

4. the two electrodes extend along the optical waveguide while maintaining a cross-sectional shape that becomes thicker in a stepped manner with increasing distance from the optical waveguide; the dielectric layer extends in a width direction of the optical waveguide to a range including an edge of a first stage of each of the two electrodes that is closest to the optical waveguide, and is disposed so as to partially cover each of the two electrodes; 4. The optical waveguide element according to claim 1.

5. The dielectric layer is disposed so as to partially cover the first stages of the two electrodes, 5. The optical waveguide element according to claim 4.

6. The spacing between the edges of the two electrodes is less than 10 μm.

6. The optical waveguide element according to claim 1.

7. The optical waveguide constitutes two parallel waveguides of a Mach-Zehnder type optical waveguide.

7. The optical waveguide element according to claim 1.

8. The optical waveguide element according to claim 1 , which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light into the optical waveguide element; an optical fiber for guiding light output from the optical waveguide element to the outside of the housing; An optical modulator comprising:

9. The optical waveguide element according to claim 1 , which is an optical modulation element for modulating light; a housing for accommodating the optical waveguide element; an optical fiber for inputting light into the optical waveguide element; an optical fiber for guiding light output from the optical waveguide element to the outside of the housing; A drive circuit for driving the optical waveguide element; An optical modulation module comprising:

10. An optical modulator according to claim 8 or an optical modulation module according to claim 9; an electronic circuit for generating an electrical signal for causing the optical waveguide element to perform a modulation operation; An optical transmitting device comprising:

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